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Progress in Materials Science 50 (2005) 192Ball-milling in liquid mediaApplications to the preparation of anodic

materials for lithium-ion batteries

Raphael Janot, Daniel Guerard *

Laboratoire de Chimie du Solide Mineral, UHP Nancy I, UMR 7555, BP 239,54506 Vandoeuvre-les-Nancy Cedex, France

Received 30 July 2002; received in revised form 30 May 2003; accepted 20 July 2003Abstract

Due to the intense development of portable electronic devices and the aim of the electric

vehicle, the preparation of batteries with higher and higher electrochemical performances is of

great interest. The best anodic material for lithium batteries should be the metal itself, but due

to the formation of lithium dendrites provoking short-circuits while the battery is cycled, one

uses graphite, since the lithium intercalation presents a high reversibility and occurs at very

low potentials versus Li/Li0.

In order to improve the anode performance, there are two possible ways, starting from

either:

small particles of graphite obtained by grinding. In this case, the chargedischarge rate canbe improved, as well as the amount of intercalated lithium, due to the lower particles size

and to the presence of dangling carbon bonds, respectively. Unfortunately, ball-milling

leads to disordered or amorphous carbons exhibiting a high hysteresis when electrochem-

ically cycled,

graphite intercalation compounds with a high lithium content such as LiC2 prepared at 300C under 50 kbars. This synthesis requires a heavy apparatus and the amount of preparedpowders at once is quite small: several hundreds milligrams. Moreover, the compound is

not stable and decomposes (into a mixture of LiC6 and free lithium) as the pressure is re-

leased and the LiC2 compound is not formed back by electrochemical reaction.* Corresponding author. Tel.: +33-383-684633/912107; fax: +33-383-684634/912447.

E-mail address: daniel.guerard@lcsm.uhp-nancy.fr (D. Guerard).

0079-6425/$ - see front matter 2003 Elsevier Ltd. All rights reserved.doi:10.1016/S0079-6425(03)00050-1

mail to: daniel.guerard@lcsm.uhp-nancy.fr

2 R. Janot, D. Guerard / Progress in Materials Science 50 (2005) 192By modeling and some experimental measurements, it was shown that the pressure and

temperature temporarily induced by the shocks occurring during the ball-milling can reach

values close to those used to prepare the LiC2 compound. Therefore, the grinding of graphite

lithium mixtures was studied in order to obtain superdense graphitelithium compounds

stable under ambient conditions. Due to its high ductility, lithium agglomerates readily on the

milling tools and, thus, the obtained powder, roughly LiC6 in spite of a large excess of metal

(the starting mixture is Li + 2C), is partly amorphous. This is the reason why we have added

small quantities of a liquid, inert towards lithium: the n-dodecane (C12H26). The presence ofthis liquid allows a good lubrication and dispersion of lithium, which does not paste on the

tools.

With appropriate milling conditions, the formation of a superdense compound with a LiC3stoichiometry is possible. This compound is stable at room temperature under normal pres-

sure. The electrochemical properties of the LiC3 compound were investigated as anode in

lithium-ion batteries. The primary capacity is very high (close to 1 Ah/g) and the voltage

profile is low (all the lithium removal occurs below 300 mV). Unfortunately, the superdense

LiC3 compound is not formed back by electrochemical mean. The reversible capacity corre-

sponds to the formation of the LiC6 compound and consequently is around 370 mAh/g. This

compound constitutes a good candidate as anodic material since its first deintercalation

corresponds to a high capacity and is partly reversible with the same behavior as graphite.

In the same manner, graphite powders were milled together with dodecane. The powder

obtained after separation from the liquid (by simple evaporation) is made of well crystallized

thin particles of graphite (typically 20 nm thick, with a geometrical anisotropy of about 100).

This graphite leads to a smaller consumption of lithium (used for the so-called solid electrolyte

interface formation) than with the starting graphite powder, in spite of a much larger specific

surface area.

The previous syntheses were done with an inert liquid. In the case of graphite milled within

water in a stainless steel vial, one observes the formation of graphite particles covered by small

crystals of maghemite (c Fe2O3), these last coming from the oxidation of the vial by water.This reactive ball-milling was used in two different directions:

easy preparation of pure nanosized maghemite crystals. The in situ production of hydrogenduring the milling is relevant, because it favors a spinel type structure. For a milling of 48 h,

the average diameter of the particles is 15 nm. The technique is remarkable due to the nar-

row size distribution of the obtained particles, which is very interesting for magnetic appli-

cations,

preparation of anode materials by depositing maghemite nanograins at the surface ofgraphite particles ground together with water. In this manner, the respective properties

of graphite and transition metal oxide are added: it was shown recently that such oxides

may present higher reversible capacities than graphite.

The electrochemical performances of these graphitemaghemite composites were tested. The

nanometric size of the maghemite grains, and consequently their great reactivity, allows a

partial reversibility of the reaction between iron and maghemite. The corresponding capacity

reaches 440 mAh/g and makes such composites good candidates for anodes in lithium-ion

batteries. However, the reversible capacity decreases during the electrochemical cycling due to

the coalescence of the maghemite nanoparticles.

2003 Elsevier Ltd. All rights reserved.

R. Janot, D. Guerard / Progress in Materials Science 50 (2005) 192 3Contents1. Introduction ............................................................................................................................................... 51.1. The ball-milling............................................................................................................................ 5

1.1.1. Background..................................................................................................................... 5

1.1.2. The diverse milling systems...................................................................................... 6

1.1.3. Main milling parameters ........................................................................................... 9

1.1.4. Conclusion .................................................................................................................... 11

1.2. The graphite and its intercalation compounds............................................................... 11

1.2.1. The graphite ................................................................................................................. 11

1.2.2. The intercalation phenomenon ............................................................................. 13

1.2.3. The graphite intercalation compounds .............................................................. 14

1.2.4. The graphitelithium compounds ........................................................................ 15

1.3. The lithium batteries................................................................................................................ 17

1.3.1. Introduction.................................................................................................................. 17

1.3.2. Lithium batteries with a polymer electrolyte................................................... 17

1.3.3. Lithium-ion batteries: the use of insertion compounds as anodes.......... 19

1.3.4. Conclusion .................................................................................................................... 21

1.4. The maghemite (Fe2O3) ....................................................................................................... 211.4.1. Structural aspect......................................................................................................... 21

1.4.2. Synthesis ........................................................................................................................ 222. Intercalation of lithium into graphite by ball-milling............................................................... 222.1. Material used in this study.................................................................................................... 23

2.1.1. The mill and its devices............................................................................................ 23

2.2. Reagents....................................................................................................................................... 24

2.3. Synthesis conditions................................................................................................................. 25

2.4. Optimization of the milling conditions ............................................................................. 25

2.4.1. Nature of lithium ....................................................................................................... 25

2.4.2. Lithium/carbon ratio................................................................................................. 26

2.4.3. Charge ratio.................................................................................................................. 27

2.4.4. Balls size......................................................................................................................... 29

2.4.5. Conclusion .................................................................................................................... 29

2.5. Ball-milling of a mixture of Li + 2C powders................................................................. 29

2.5.1. X-rays diffraction........................................................................................................ 30

2.5.2. TEM study.................................................................................................................... 32

2.5.3. Density measurements.............................................................................................. 34

2.5.4. Conclusion .................................................................................................................... 353. Synthesis by ball-milling in a liquid medium............................................................................... 353.1. Choice of the liquid.................................................................................................................. 35

3.2. Synthesis....................................................................................................................................... 36

3.3. Actual composition of the powders: chemical analyses.............................................. 37

3.4. Characterization by X-rays diffraction.............................................................................. 37

3.5. TEM investigations .................................................................................................................. 38

3.5.1. Electron energy loss spectroscopy........................................................................ 39

3.6. Density measurements by pycnometry.............................................................................. 40

4 R. Janot, D. Guerard / Progress in Materials Science 50 (2005) 1923.7. 7Li NMR characterization..................................................................................................... 41

3.7.1. Generalities ................................................................................................................... 41

3.7.2. Experimental results.................................................................................................. 43

3.7.3. Where is the doublet at 260 ppm coming from?............................................ 46

3.7.4. Spinlattice relaxation time.................................................................................... 48

3.7.5. Structure ........................................................................................................................ 48

3.7.6. Comparison with the high pressure superdense phases............................... 51

3.7.7. Conclusion .................................................................................................................... 53

3.8. Characterization by electronic paramagnetic resonance............................................. 53

3.9. High pressure investigations ................................................................................................. 55

3.10. Conclusion................................................................................................................................... 574. Preparation of highly anisometric graphite particles................................................................ 574.1. Milling within dodecane......................................................................................................... 58

4.2. X-rays diffraction ...................................................................................................................... 59

4.3. SEM observations..................................................................................................................... 59

4.4. TEM observations .................................................................................................................... 61

4.5. Conclusion................................................................................................................................... 625. Synthesis of maghemite nanoparticles ........................................................................................... 625.1. Synthesis by milling within water ....................................................................................... 63

5.2. XRD characterization ............................................................................................................. 64

5.3. Chemical analysis...................................................................................................................... 65

5.4. TEM observations .................................................................................................................... 65

5.4.1. Electron energy loss spectroscopy........................................................................ 66

5.5. Mossbauer spectroscopy......................................................................................................... 685.6. Magnetic susceptibility measurements............................................................................... 70

5.7. Conclusion................................................................................................................................... 716. Graphitemaghemite composites ..................................................................................................... 716.1. X-rays diffraction ...................................................................................................................... 71

6.2. SEM observations..................................................................................................................... 72

6.3. TEM studies................................................................................................................................ 747. Electrochemical performances........................................................................................................... 747.1. Introduction................................................................................................................................ 74

7.1.1. Carbonaceous materials........................................................................................... 76

7.2. Experimental............................................................................................................................... 76

7.3. Highly anisometric graphite.................................................................................................. 77

7.4. LiC3 Li powder prepared by ball-milling ..................................................................... 787.4.1. Comparison with LiC synthesized under high pressure.............................. 80

7.4.2. Conclusion .................................................................................................................... 82

7.5. Graphitemaghemite composites......................................................................................... 82

7.6. Conclusion................................................................................................................................... 838. Conclusion ............................................................................................................................................... 85

R. Janot, D. Guerard / Progress in Materials Science 50 (2005) 192 5Acknowledgements ........................................................................................................................................ 86

References ......................................................................................................................................................... 871. Introduction

1.1. The ball-milling

The ball-milling was directly oriented to industrial applications: in 1966, Benja-

min (INCO Co.) tried to improve the mechanical properties of nickel-based super-

alloys by high energy mechanical grinding. This technique allows to ameliorate both

the high temperature (10001300 C) properties by homogeneous dispersion of fineoxides particles in the alloy and the medium temperature range (8001000 C) byprecipitation of an intermetallic c phase of Ni3 (Al, Ti) type. The first publicationdedicated to this new technique appeared in 1970 and this solid-state process was

called Mechanical Alloying by Benjamin [1]. This method was then extended to

the synthesis of a large variety of materials as alloys with Cu, Al, Mg, supercon-

ductors, ceramics. . .Several terms are used to call this technique: Mechanical Alloying when there is

a chemical reaction between different powders, Mechanical Grinding or Me-

chanical Milling when the only goal is to modify the texture and/or the structure ofa material (no chemical reaction is involved in the process). In this paper, we will use

mainly the term of Ball-Milling, which does not presuppose any kind of behavior

and which tends to be more and more used.1.1.1. Background

The ball-milling is generally used as a mechanical co-grinding of powders, initially

different in nature, up to the preparation of a new powder, homogeneous in com-

position. The milling is done in cylindrical containers called vials and containing

balls. The nature of the milling tools can be as diverse as steel, agate, tungsten

carbide. . . The vials are generally filled under an inert atmosphere to avoid sidereactions, since the particles are fractured during the milling process and, therefore,new highly reactive surfaces can react with the surrounding gases.

Two opposite phenomena are induced by the milling process: on one hand, the

particles split as a result of the important internal strain created by the high pressure

applied to the grains and, on the other hand, the highly divided particles tend to

agglomerate due to the high reactivity of their surfaces, in order to minimize the

surface energy [2,3]. When the initial particle size is relatively high (typically more

than 10 lm), the milling starts by a rapid splitting and, then, there is successivelysplitting, crushing and coalescence of the particles (cf. Fig. 1), leading to an ho-mogeneous powder, whose mean particle size depends on the nature of the powders

and on the milling conditions (duration, rotation rate, charge ratio: ratio between

balls and powders weights. . .).

Fig. 1. Schematic representation of the ball-milling process.

6 R. Janot, D. Guerard / Progress in Materials Science 50 (2005) 192Harder and fragile the milled powder is, finer the obtained particles are [4]. This

last point is particularly important in our case since we are dealing with very ductile

materials: the alkali metals. In order to palliate this fact and to prepare fine particles,there are two solutions. The first one consists in the addition of a lubricant, inert

towards the material, in the milling container [5,6]. It is generally liquid but can

however act as a source of pollution. The other technique is a cryo-milling in liquid

nitrogen [7,8], the low temperature hardens the materials, which become fragile and

can be pulverized. This technique is quite difficult to handle and is rarely used.

1.1.2. The diverse milling systems

The most common mills are of planetary, vibrant, attritors, cannon-ball types.

The planetary or vibrant mills are the most used in laboratories. Their capacity is ofseveral grams, whereas the industry deals with attritors or cannon-ball mills, whose

capacity can be as high as several tons of powders.

1.1.2.1. The planetary mills. The vials rotate at a rotation speed x on a tray movingin the opposite way at a speed X (cf. Fig. 2). The values for X and x are connected inthe commercial mills.Numerous authors have modelized the shocks occurring during

the milling process (both between balls and between vial wall and balls) in order to

quantify the energy transferred to the powder.

The shocks are defined by the speed and the incidence angle of the collidingmaterials. The tangential shocks involve important friction phenomena, whereas

frontal shocks induce compression equivalent to a microwelding. LeBrun et al. [9]

consider three different milling regimes as a function of the ratio between x and X(the rotation speeds of the vial and the tray, respectively). If this ratio is small, the

regime is qualified as chaotic: as soon as the ball strikes the wall, it is immediately

pushed back. Inversely, when the ratio is high, the ball remains on the wall and the

regime is a friction mode. In between, the motion of the ball is decomposed in two

parts: a friction followed by an impact. This model is established for one ball, ne-glecting the gravity forces.

1.1.2.2. The vibrant mills. Those mills are working with small vials (several tens cm3).

They are moved linearly by one, two or three-axial translations, whose frequency is

Fig. 2. Planetary type mill.

Fig. 3. Vibrant type mill.

R. Janot, D. Guerard / Progress in Materials Science 50 (2005) 192 7several tens Hertz (cf. Fig. 3). Generally, the powder and balls occupy only 1/3 of the

volume, unless the balls speed is too low to lead to an efficient milling. This kind of

mill involves mainly frontal shocks.

Maurice and Courtney [10] have shown that, with a vibrant mill SPEX 8000, the

balls reach a speed as high as several m/s and that the shocks lead to very high instant

pressures (typically 40 kbars with stainless steel balls of 6 mm in diameter). Several

other studies to know the kinematics of the balls during the milling were done, par-

ticularly by Davis et al. [11], who recorded the balls motion in a transparent vial.

1.1.2.3. Attritors. The principle of this mill differs from the previous ones: the vial isstatic and one or more stirrers are placed inside the vial and their rotation (several

hundreds rpm) mix the powders and balls together (cf. Fig. 4). The friction and shear

are then preponderant [12].

The efficiency of such mills is relatively low: only a small part of the powder lies at

the place where an efficient mixing occurs: the powder tends to fall, by gravity, at the

bottom of the vial and concentrates near the vial wall, a place where the milling is

very limited. Thus, the recent models of attritors present a curved bottom in order to

favor the movement of the balls and powders towards more active parts of the vial.

1.1.2.4. Cannon-ball mills. The vial rotates horizontally around its revolution axis (cf.Fig. 5). Part of the powder and balls is drag by the wall and falls down by gravity.

Moreover, part of the balls which arising slip down on the others. The combination

between those two movements induces shock and friction phenomena on the

Fig. 4. Attritor.

Fig. 5. Cannon-ball type mill.

8 R. Janot, D. Guerard / Progress in Materials Science 50 (2005) 192powder. The relative impact of those movements depends mainly on the rotation

speed and diameter of the vial [13].

This mill is largely used in the industry, since the vial can reach several meters,

both in length and diameter, and makes possible the treatment at once of several

tons of powders. The milling duration is generally long (several hundreds hours) but,

taking into account the mass of milled powders, this system remains one of the most

interesting at the industrial scale. One should also note that some vials contain as-

perity, which greatly ameliorates the milling efficiency.

1.1.2.5. Other milling systems.

Vibrating plateaus with a flat or hemispherical bottom are also used. Due to thevibrations, a large ball bounces on the plateau and, therefore, crushes a part of the

powder.

The use of rods instead of balls increases the shearing part of the milling [14].

R. Janot, D. Guerard / Progress in Materials Science 50 (2005) 192 91.1.3. Main milling parameters

Numerous parameters influence the kinetics of the milling as well as the micro-

structure of the prepared powders. Some of those parameters depend on the type of

mill, others on the contrary can be defined whatever the mill is. Following is a non-

exhaustive list of the milling parameters, classified approximately by inverse order of

importance:

milling duration, charge ratio (ratio between balls and powders weights), size and nature of the balls, rotation speed (for planetary, attritor or cannon-ball types) or frequency and am-

plitude of the vibrations (for vibrant type),

filling rate of the vial (ratio between the balls and powder volumes and the totalvolume of the vial).

1.1.3.1. Influence of the milling duration. This parameter should generally be as low as

possible: a longer milling leads to energy consumption and increases the risk of

pollution due to the abrasion of the milling tools. The needed milling time for thepreparation of a material depends obviously on the material nature, but also on the

other milling conditions. It is generally admitted that, for a given material, longer

the milling is, smaller the particle size is. However this size tends to a limit value due

to an agglomeration process, as seen previously.

1.1.3.2. Influence of the charge ratio. In a first approximation, one can consider the

frequency of the shocks to be proportional to the charge ratio: as this ratio is high, as

the particles are split and more their size is decreased. The increase of the charge

ratio allows, on the other hand, to diminish the milling duration since some equiv-

alence between those two parameters exist as shown in Fig. 6 [15].

However, in some cases, a too large intensity of the milling can involve an

amorphization of the material and one has to decrease the milling duration as it mustbe done with graphite, which leads rapidly to amorphous carbons as the milling is

too violent. This particular point will be discussed later.

1.1.3.3. Influence of the size and nature of the balls. It is obvious that, larger the ballsare, more the plastic deformations and the constraints applied to the powders are

increased. If all the other milling conditions are identical, the particles become as

small as the milling balls are larger. However, the grains size tends to a minimal

value, as calculated from the following equation (HallPetch law):r r0 A=pd 1with A and r0 constants, r the limit of elasticity for the given material. As a con-sequence, when one wants fine particles, one has to impose large constraints.

The nature of the balls is diverse: as a function of the density, there are agate

d 2:3, alumina d 4:0, zircona d 5:7, stainless steel d 7:8 and tungsten

Fig. 6. Correlation between milling duration and charge ratio (after Ref. [15]).

10 R. Janot, D. Guerard / Progress in Materials Science 50 (2005) 192carbide d 16:4. The most generally used balls are made of stainless steel (extrahard, type Z200C12). One of the major problems with mechanical alloying consists

in the pollution coming from the erosion of the milling materials, which is almost

impossible to avoid. This contamination is directly connected to the hardness ratio

between balls and samples. When the millings are done with balls and vials intungsten carbide, there is no pollution at all [16], but the price of such milling tools is

high and, tungsten carbide being fragile, it is rarely used. Schaffer and Forrester have

shown that denser the density is, more important the collision energy is [17]. It is also

interesting to note that the most energetic millings are generally done with mixtures

of balls of different diameters.1.1.3.4. Influence of the rotation speed of the vials. During the milling, the shocks

create heat due to the plastic deformation of the powder particles. That heat is

dissipated into the vial and the local temperature increases proportionally to thecollisions violence, thus, to the rotation speed. The local and instantaneous tem-

perature increase is difficult to quantify, but Eckert et al. [18] has shown that this

local warming can reach 400 C with highly energetic millings.This temperature increase is rapidly transmitted to the whole vial and its content.

Maurice and Courtney have shown [10] that a particle is cooled down before a new

impact, so one can consider the temperature to be homogeneous inside the vial. The

equilibrium temperature depends on the type of milling, but it can reach 100 C witha vibrant type mill. In fact, the milling temperature is a very important parameter,since the behavior of a powder depends strongly on the temperature. The tem-

perature increase involves a larger ductility of the milled material, thus, a higher

difficulty to decrease its particles size. Moreover, the recrystallization of an amorphous

R. Janot, D. Guerard / Progress in Materials Science 50 (2005) 192 11phase can occurred during the milling process if the temperature increase is im-

portant [10].

1.1.4. Conclusion

The ball-milling allows the preparation of numerous and diverse powders. The

result depends on different choices, based on compromises, since several milling

parameters are working in opposite directions. In fact, the mechanical alloying isgenerally a simple step in the elaboration of materials and, often, the resulting

powders are compacted or sintered in order to improve their mechanical properties.

One of the large interests of the ball-milling is the easiness to prepare either nano-

crystalline or amorphous powders, to synthesize alloys with metals whose melting

points are very different, which is quite difficult by classical melting methods.

The mechanical alloying presents however several limits:

its efficiency is very limited if one phase is too ductile, due to an important stickingon the milling tools,

the pollution connected to the abrasion of the milling tools can be large and,therefore, one has to decrease as much as possible the milling duration by optimi-

zation of the other parameters governing the synthesis (charge ratio, size and na-

ture of the balls, rotation speed).

The choice of the mill is also drastic and depends on the powder quantity to

prepare (especially at the industrial scale). It depends also on the type of desiredshocks: vibrant type mills (SPEX) involve mainly frontal shocks, whereas the

planetary mills induce both head on and friction shocks. In the present work, we are

dealing with graphite, whose tendency to cleave is favored by friction and, thus, we

have used a planetary mill, which also helps for the intercalation of lithium.

1.2. The graphite and its intercalation compounds

1.2.1. The graphite

1.2.1.1. Structure. The lamellar structure of graphite is applied for centuries as pencil.

The graphite is a two-dimensional material and presents a large anisotropy of bonds.The sp2 hybridization induces a covalent r type bond with three other carbon atomsin the plane, which involves the formation of aromatic macromolecules in a hex-

agonal net called graphene layer. The carboncarbon in-plane distance is 1.42 A,shorter than that in diamond (1.54 A), an other allotropic variety of carbon char-acterized by a sp3 hybridization. The r bonds of graphite present, thus, a higherelectronic density, which explains their high cohesion.

The graphene layers are superimposed parallel to each other and the cohesion of

the material (between the graphene planes) is ensured by Van der Waals type bonds,quite weak, which explains the easiness of the cleavage of graphite and the large

distance between successive graphene planes (3.35 A). There are two types of

stacking: ABAB. . . (cf. Fig. 7), hexagonal graphite (2H) or ABCABC. . . (cf. Fig. 8),rhombohedral graphite (3R).

Fig. 7. Hexagonal graphite.

Fig. 8. Rhombohedral graphite.

12 R. Janot, D. Guerard / Progress in Materials Science 50 (2005) 192The hexagonal graphite, also called a graphite, is the most common form. Itsstructure, firstly described by Bernal in 1924 [19], was confirmed by Mauguin in 1926

[20]. The unit cell belongs to the space group P63/mmc with the following parame-

ters: a 2:46 A and c 2 Ic 6:70 A.

R. Janot, D. Guerard / Progress in Materials Science 50 (2005) 192 13Rhombohedral graphite, also called b graphite, is characterized by a unit cellR 3m with the following parameters: a 3:635 A, a 39:5 as shown by Lipson andStokes in 1942 [21]. Often associated to the hexagonal form, this phase does not exist

alone. Its presence is favored by milling [22], ultrasonic treatment [23] up to 40%, but

disappears by heating above 2000 C [24] or by intercalation/deintercalation ofvarious species between the graphene planes.1.2.1.2. Physical properties. Graphite is a black, soft and easy to cleave material, its

crystallographic density is 2.27, lower than that of diamond (3.54), which explains

why artificial diamond can be synthesized from graphite under high pressure and

temperature [25]. The carbon atoms have four valence electrons and the sp2 hy-

bridization deals with three electrons shared with the neighbors carbons. The fourth

electron is delocalized and this delocalization of those p electrons in the planes isresponsible of the fair electrical conductivity (rk 2 104 S/cm). In fact, the in-plane conductivity of graphite occurs by holes and the number of carrier is quitesmall (1018/cm3): the high value of the conductivity is due to very high carrier mo-

bility. Graphite behaves as a semi-metal in the plane with a small overlap between

conduction and valence bands. The transverse conductivity is 103104 smaller than

that of the in-plane one: the electronic exchanges are more difficult in that direction

connected to the high interplanar distance (3.35 A). Like diamond, graphite presentsgood heat conduction, mainly due to the phonons. The weak interactions perpen-

dicular to the planes leads to a large heat conduction anisotropy.1.2.2. The intercalation phenomenon

Graphite reacts with a large variety of species and leads to a wide family ofgraphite intercalation compounds (GICs). This phenomenon is connected to:

the lamellar structure of graphite and the weak bonding between the planes, whichfavors their splitting,

the electronic structure of graphite: the planes form aromatic macromolecules,which can accommodate electron donor as well as electron acceptor species.

By intercalation, the graphene planes remain geometrically unchanged, the in-tercalated species forming planar layers made, in some cases, of several planes (e.g.

with transition metal halides, there is generally three intercalated planes: halide

metalhalide). Therefore, the GIC remains with a lamellar structure.

The intercalation process results of two simultaneous reactions: oxido-reduction

and intercalation of a charged species, whose charge compensates that of the host

structure. With electron donors (typically the alkali metals), the graphite is re-

duced and there is intercalation of the metal cations. The electron acceptor species

are intercalated as anions into the oxidized graphite network. For the intercala-tion phenomenon, graphite has an amphoterous behavior. The name donor or

acceptor is also given to the GIC itself, instead of GIC with donor (or acceptor)

species.

14 R. Janot, D. Guerard / Progress in Materials Science 50 (2005) 1921.2.3. The graphite intercalation compounds

Since 1977, International Conferences were devoted in part or totally to the GICs:

La Napoule (France) 1977 [26], Nijmegen (The Netherlands) 1977 [27], Province-

town (USA) 1980 [28], Trieste (Italy) 1981 [29], Pont-a-Mousson (France) 1983 [30],Tsukuba (Japan) 1985 [31], Jerusalem (Israel) 1987 [32], Berlin (Germany) 1989 [33],

Orleans (France) 1991 [34], Louvain-la-Neuve (Belgium) 1993 [35], Vancouver(Canada) 1995 [36], Arcachon (France) 1997 [37], Okazaki (Japan) 1999 [38],

Moscow (Russia) 2001 [39]. The corresponding proceedings gather a large part of the

knowledge on GICs and other intercalation compounds.

The intercalation phenomenon was firstly discovered in 1926 by Fredenhagen and

Cadenbach [40]. The intercalation compounds are now well-known, due to the de-

velopment of the lithium-ion batteries based on the formation of the LiC6 compound

at the negative electrode, this last being firstly described in the sixties [41,42].

An intercalation compound is defined by several criteria:

its chemical formula, generally written as a function of one intercalated atom ormolecule (e.g. LiC6, KC8),

its stage, which is the number of graphene planes lying between two successive in-tercalated layers. In a first stage compound, all intervals between the graphene

layers are occupied by the intercalate,

its interplanar distance: that separating two carbon planes surrounding the inter-calated layer. This distance (di) depends on the size of the intercalated species andvaries, for the compounds with alkali metals from 3.70 A (Li compounds) to 5.95A (Cs compounds),

its repeat distance along the c axis Ic, which is the value directly determined byX-ray diffraction.

Those different values are linked by the following equation:Ic di n 1 3:35 with Ic and di in A 2The structure of the intercalation compounds is tightly connected to that of

graphite: as described just above, along the c axis, the structure corresponds to thatof graphite with some dilatation, due to the presence of the intercalated species. Thein-plane structure is also derived from that of graphite, especially in the case of the

compounds with alkali metals, where the intercalated atoms lie in epitaxy between

the center of graphite hexagons. This involves the graphite planes to be stacked in

AAA and, as a consequence, the rhombohedral phase disappears by intercalation/

deintercalation. This explains also why the formula of the intercalation compounds

is simple like KC8 or LiC6. The structure of those compounds was qualified re-

spectively of octal or hexal. In fact, one carbon hexagon corresponds to two

carbon atoms and a composition like LiC6 means that one site in three possible isoccupied by the lithium atoms, as it appears in Fig. 9(a). The structure of the GICs

can be also expressed as a superstructure of that of graphite and the in-plane cell is

noted as (p3p3, R30), which means that the a parameter is p3 times longer than

Fig. 9. In-plane structure of (a) LiC6 (first stage) and (b) LiC18 (second stage).

R. Janot, D. Guerard / Progress in Materials Science 50 (2005) 192 15that of graphite and that the graphite and compound cells are rotated by an angle of

30.This fact involves that the graphite h k 0 reflections are present in the XRD spectra

of the GICs with electron donors. In the particular case of LiC6, an other phe-nomenon complicates the interpretation of the X-rays analysis, since the a and cparameters are roughly in the ratio

p3=2, which involves a systematic superimpo-

sition of the 0 0 l and l 0 0 reflections [43].

The stacking of the intercalated layer can complicate the structure. For instance,

in LiC6, whose composition involves the occupation of one site in three possible, all

lithium atoms are exactly superimposed. The resulting unit cell is hexagonal and

belongs to the P6/mmm space group with a 4:30 A, c Ic 3:70 A [44,45]. On thecontrary, in BaC6 [46] or EuC6 [47], the metal atoms occupy regularly two sites inthree and lead to a hexagonal unit cell, space group P63/mmc, with a 4:30 A andc 2 Ic (respectively 10.51 and 9.75 A).1.2.4. The graphitelithium compounds

The first graphitelithium intercalation compounds were prepared by action oflithium vapor on graphite. But, the lithium pressure being low (around 104 Torr at

400 C, the maximal temperature beyond which the formation of lithium carbidebecomes important [42]), the kinetic of the intercalation reaction is slow. Synthesis

via a solid way was then set up: lithium and graphite powders were compressed

together at a pressure of around 5 kbars [45]. The reaction is done at room tem-

perature with a hand press placed in a glove box with a purified argon atmosphere.

The lithium intercalation occurs at room temperature and is almost complete after

one week, but a moderate heating at 200 C (under vacuum or argon atmosphere) ofthe pellet allows this intercalation to be finished after 24 h.

The intercalation by compression, contrarily to the lithium vapor intercalation,

allows the synthesis of compounds with stages other than one (LiC6) by varying the

16 R. Janot, D. Guerard / Progress in Materials Science 50 (2005) 192Li/C ratio. Stages 14 can be prepared by this method, which has proved a variation

of the composition for the stages higher than one: LiC12 to LiC18 for stage 2, LiC18 to

LiC30 for stage 3 for instance. All those compounds present the same interplanar

distance of 3.70 A, independently of the exact stoichiometry.Those experiments were re-investigated and improved under high pressure (50

kbars) and at a temperature of 300 C by a Russian team [48,49]. Their techniqueleads to new phases, especially a first stage compound called superdense phase. Its

composition is LiC2, which corresponds to the occupation of all possible sites for

the lithium atoms (cf. Fig. 10(a)). The lithiumlithium distances are very small (2.46A) compared to that of the Li2 molecule (2.67 A). This compound, whose inter-planar distance is the same as the classical LiC6 phase, presents, in spite of the

presence of two carbon atoms, a lithium density higher than that of metallic lithium

itself: this fact induces a negative potential versus Li0 in an electrochemical cell, as

shown by Yazami et al. [50]. The LiC2 compound is not stable and decomposesslowly into LiC6 and free lithium as the pressure is released. Several phases can

be characterized during the decomposition process and some examples are given in

Fig. 10(b and c).

The mechanical alloying leads, as it was already seen, to local and instantaneous

conditions close to those required by the high pressure technique. This is the reason

of our investigations for the synthesis of new superdense graphitelithium phases

using this method.

The electrochemical intercalation of lithium is now daily used since the reversibleintercalation of lithium into graphite allows the energy storage: the anode of most of

the lithium-ion batteries is made of graphite. Metallic lithium should be preferred

since its capacity of 3.8 Ah/g is roughly 10 times higher than that of LiC6 (372 mAh/

g). However, lithium forms dendrites during the cycling process, provoking short-

circuits between cathode and anode. The high reactivity of electrolytes towards

lithium and their possible co-intercalation with lithium into graphite was an obstacle

[51], which was overcome with the apparition of new electrolytes mainly based on

ethylene-carbonate (EC) and its derivatives [5254].Fig. 10. (a) LiC2 compound obtained under high pressure, (b and c) phases observed during the de-

composition process at ambient pressure.

R. Janot, D. Guerard / Progress in Materials Science 50 (2005) 192 171.3. The lithium batteries

1.3.1. Introduction

The use of lithium as negative electrode in electrochemical systems was investi-

gated in the 1950s and a French patent on lithium batteries was firstly deposited by

Herbert and Ulam in 1957. The interests for lithium are:

its metallic character responsible of a good electronic conductivity for the elec-trode,

its low molecular weight (6.94 g/mol) allowing to reach an electrochemical capac-ity as high as 3860 mAh/g,

its very low standard potential ()3.04 V versus NHE in aqueous solutions).

This last property is at the origin of the high potential for lithium batteries and,therefore, of their high energy densities. However, the strong reducing character of

lithium is responsible of the formation of a passivating layer on the metal surface,

due to the electrolyte decomposition. This film was extensively studied by means of

transmission electronic microscopy [55] and impedance spectroscopy [56].

The first primary lithium batteries, commercialized in the 1970s, were constituted

of metallic lithium and fluorinated carbon (CFx) as negative and positive electrodes,

respectively. This battery has an operating voltage close to 3 V. This system is still

commercialized today (mainly for watches), but it is often replaced by the Li/MnO2system. When higher energy densities are required, as in military or spatial appli-

cations, SOCl2 or SO2Cl2 are used as positive electrodes.

For secondary batteries, the use of metallic lithium as negative electrode is un-

fortunately not possible due to an important lack of safety. In fact, the lithium

electro-deposition is not uniformous and leads, during the electrochemical cycling, to

the formation of dendrites provoking short-circuits. To avoid this problem, two

main technologies were set up: the polymer electrolyte technology [57,58] and the

lithium-ion technology [58,59]. The first technique consists to replace the liquidelectrolyte by a solid polymer, this last requires however a temperature above 60 Cto present a sufficient ionic conductivity. The second solution is the replacement of

metallic lithium by an insertion compound, often a carbonaceous material. Both

technologies can be combined to obtain a plastic lithium-ion battery, called

PLiON [60]. The lithium-ion batteries are perfectly suitable for portable devices,

since their commercialization by Sony in 1991 [54], whereas the solid polymer bat-

teries seem very promising as energy sources for future electric vehicles.

1.3.2. Lithium batteries with a polymer electrolyte

The first technique allowing to avoid the formation of lithium dendrites consists

to replace the liquid electrolyte by a polymer (cf. Fig. 11). The possible application of

polymer as battery electrolytes was demonstrated by Armand et al. in the 1970s[57,58]. These authors claimed that the crystalline complexes formed from alkali

metal salts and polyethylene oxide (PEO) were capable of demonstrating significant

ionic conductivity. This work inspired intense research and development on the

Fig. 11. Schematic representation of solid polymer lithium batteries.

18 R. Janot, D. Guerard / Progress in Materials Science 50 (2005) 192synthesis of new polymer electrolytes. Many works are currently aiming to find

polymers with a high ionic conductivity (around 103 S cm1 at room temperature).

To date, several types of polymer electrolytes have been developed, such as those

based on PEO, polyacrylonitrile, polymethyl metacrylate and polyvinylidene fluo-

ride. The polymers can be sort into two categories: the pure solid polymer electro-

lytes and the gelled polymer electrolytes.The first category is made of lithium salts (e.g. LiClO4, LiBF4, LiPF6, LiCF3SO3)

dissolved in high molecular weight polyether (e.g. PEO), which act as solid solvents.

Such electrolytes commonly exhibit ionic conductivities ranging from 108 to 104

S cm1 at temperatures above 50 C, which excludes practical applications at am-bient temperature. This obstacle originates from the high degree of crystallinity,

which is not favorable for ionic conduction, and the low solubility of lithium salts.

The most striking advancement in the ionic conductivity of polymer electrolytes has

been obtained through the incorporation of substantial amounts of plasticizer and/orsolvent to the polymer matrix. This leads to the second category of polymer elec-

trolytes: the so-called gelled polymer electrolytes [61].

The gelled polymer electrolytes are characterized by a high ambient ionic con-

ductivity. For instance, the use of polyethylene glycol (PEG) as plasticizer within

PEO leads to a strong enhancement of conductivity. The ionic conductivity can

reach 103 S cm1 at 25 C, with decreasing molecular weight of PEG and increasingPEG content [62]. The conductivity enhancement is mainly attributed to the re-

duction of the crystallinity and the lowering of the glass-transition temperature ofthe polymers. If the gelled polymer electrolytes have higher ambient ionic conduc-

tivities, their mechanical properties are poorer, when compared with pure solid

polymer electrolytes. In order to improve the mechanical properties, components

which can be cross-linked may be added to the gel electrolyte formulation. The

R. Janot, D. Guerard / Progress in Materials Science 50 (2005) 192 19chemical cross-linking is a process associated with the covalent bonding of polymer

chains by means of a chemical reaction to form junction points.1.3.3. Lithium-ion batteries: the use of insertion compounds as anodes

The second way to avoid the dendritic formation of lithium consists to replace

metallic lithium at the negative electrode by a compound able to reversibly inter-

calate lithium ions at low potentials [58,59]. The lithium ions are, therefore, suc-

cessively intercalated/deintercalated from the negative and positive electrodes, giving

the name of rocking-chair to those batteries (cf. Fig. 12). If the lack of safety isresolved by this technique, this system requires the association of materials with a

strong reducting character as anode and a strong oxidant character as cathode to

obtain batteries with high energy densities.

Among many materials for positive electrodes, lamellar oxides as LiCoO2 and

LiNiO2 [6365] and spinel as LiMn2O4 [66] are very attractive. LiCoO2 is the most

widely used in commercial batteries, due to its low cost, its easy synthesis and its

good electrochemical performances. The capacity is of the order of 140 mAh/g with

an average voltage of 4 V. But, due to the high toxicity of Co, the development ofnew cathodic materials is of a great interest. Polyanionic materials with the olivine

structure are very promising (e.g. LiFePO4 [67]) and represent today a good alter-

native to LiCoO2.

For the anode, various materials were investigated. It appeared that carbonaceous

materials are the most relevant, since reversible lithium insertion into those materials

occurs at very low potentials. As a matter of fact, lithium is reversibly intercalated

into graphite below 0.1 V versus Li/Li and leads to the formation of the LiC6compound, corresponding to a capacity of 372 mAh/g [52]. This process is howeverFig. 12. Schematic representation of lithium-ion batteries.

20 R. Janot, D. Guerard / Progress in Materials Science 50 (2005) 192associated with the formation of a passivating layer at the carbon surface, due to the

decomposition of the electrolyte. Many studies were carried out in order to char-

acterize this layer and to better understand its formation. This film, often called

Solid electrolyte interface [68], enables the co-intercalation of solvent molecules

between the graphene sheets and, therefore, strongly limits the graphite exfoliation

during electrochemical cycling. This carbon passivation, during the first electro-chemical reduction, is responsible of a lithium loss and an irreversible capacity (cf.

Fig. 13).

Carbonaceous materials with higher capacities than graphite can be prepared:

either by a chemical mean (e.g. pyrolysis of an organic precursor [69]), either by a

physical mean (e.g. ball-milling of graphite [70]). If much more lithium can be re-

versibly inserted into those disorganized carbons than into graphite, leading to ca-

pacities of the order of 450500 mAh/g [71], this is unfortunately associated with a

large hysteresis of the charge/discharge process, decreasing strongly the energydensities of the corresponding batteries. Moreover, these carbons have a large spe-

cific surface area, which is responsible of a large irreversible capacity, during the first

electrochemical reduction, due to the formation of an important passivating layer.

An other category of anodic materials is related to the electrochemical formation

of lithium-based metallic alloys (mainly with Sn). This technique, very interesting

from the point of view of the capacity, is however not used for practical applications,

due to the important volume variation of the particles, leading to their fracture and,

finally to the loss of the electrical conductivity between the grains [72,73]. Thisproblem was partially resolved by the Fuji Co Ltd. [74]. Their idea was to electro-

chemically reduce amorphous tin oxides by lithium. This reaction leads to the for-

mation of Sn particles dispersed in a Li2O matrix. This matrix allows to compensate

the important volume variation, due to the formation/dissociation of the LixSn

alloys.0 100 200 300 400 500

0.0

0.5

1.0

1.5

2.0

2.5

3.0

SEI

C reversibleC irrev

E (V

)

Capacity (mAh / g)

1st charge 1st discharge

Fig. 13. Typical first galvanostatic cycle of graphite as anode in Li-ion batteries.

R. Janot, D. Guerard / Progress in Materials Science 50 (2005) 192 211.3.4. Conclusion

Sony was the first society, in 1991, to commercialize lithium-ion batteries using

graphite and LiCoO2 as negative and positive electrodes, respectively. These bat-

teries are assembled at the discharged state (lithium-free graphite). The first charge is

a very important step, since it corresponds to the formation of the passivating layer

onto the carbon surface and, therefore, it is responsible of the electrochemical per-formances during the following cycles. At the present, graphite (either natural, either

synthetic) is still the best anodic material for lithium-ion batteries, since lithium

intercalation/deintercalation processes occur below 0.1 V. Moreover, the cyclability

and durability of graphite are excellent when appropriates electrolytes are used.1.4. The maghemite (c Fe2O3)

The maghemite is an iron III oxide (c Fe2O3) largely used as material for magneticstorage. Its structure is of spinel type slightly derived from that of magnetite (Fe3O4)

and this oxide presents a ferromagnetic behavior.

The purpose for the preparation of maghemite nanoparticles is mainly due to

their potential applications: magnetism, catalysis, electrochemistry. . . This last pointis developed in this paper because transition metals oxides (especially with Fe, Co,

Ni), with a nanometric particles size, are good candidates to replace graphite as

anodes for lithium-ion batteries [75]. For instance, CoO particles with a diameter inthe 1020 nm range present a reversible capacity around 700 mAh/g and an excellent

cyclability. This is due to the reversibility of the following reaction:CoO 2Li () Li2O Co 3From the thermodynamic point of view, the reaction is displaced deeply to theright. During the first charge, the CoO reduction (at 0.8 V versus Li/Li) leads to the

formation of cobalt nanoparticles (2 nm in diameter), whose high reactivity allows

the oxidation and, thus, ensures the reversibility of reaction (3). It was interesting to

investigate such kind of behavior on the maghemite nanoparticles.1.4.1. Structural aspect

The basic structural unit for all Fe oxides is an octahedron, in which each Fe atomis surrounded by six O ions. The O ions form layers which are approximately hex-

agonally close-packed (hcp), as in hematite, or cubic close-packed (ccp), as in ma-

ghemite. In both hcp and ccp structures, tetrahedral interstices also exist between

three O in one plane and an other O atom in the plane above. The hcp form is termed

a phase, whereas the corresponding ccp form is termed c phase. The a phase is morestable than the c phase.

Hematite (a Fe2O3) consists of layers of FeO6 octahedra, which are connected byedge and face sharing (as in corundum a Al2O3) and stacked perpendicular to the cdirection. Two-thirds of the octahedral interstices are filled with Fe3. The face

sharing is accomplished by a slight distortion of the octahedral which causes a

regular displacement of the Fe ions.

22 R. Janot, D. Guerard / Progress in Materials Science 50 (2005) 192Maghemite (c Fe2O3) has the composition of hematite, but the structure ofmagnetite. It can be considered as a fully oxidized magnetite. In the cubic structure

of both magnetite and maghemite, 1/3 of the interstices are tetrahedrally coordinated

with oxygen and 2/3 are octahedrally coordinated. In magnetite, all these positions

are filled with Fe. Magnetite is an inverse spinel: the tetrahedral positions are

completely occupied by Fe3, the octahedral ones by equal amounts of Fe3 andFe2. In maghemite, only 5/6 of the total available positions are filled and only by

Fe3, the rest are vacant (h): Fe2:67h0:33O4. Maghemite can have different symme-

tries depending on the degree of ordering of the vacancies. Completely ordered

maghemite has a tetragonal symmetry, otherwise it is cubic.1.4.2. Synthesis

In fact, the stabilization of c Fe2O3 is favored by a small particles size: above 50nm, the stable form of iron III oxide is a Fe2O3, whereas the c form is stabilizedbelow 10 nm. This phenomenon is considered as a phase transition assisted by the

particles size. The development of new synthesis methods for maghemite nanopar-

ticles is currently very active. The most used method was set up by Massart [76] andconsists in two steps: firstly, a co-precipitation of ferrous and ferric cations in al-

kaline solutions leading to magnetite (Fe3O4) nanoparticles, which are then oxidized

in air at 200250 C. With appropriate conditions, mainly the choice of the pH, oneobtains 10 nm maghemite particles. More recently, other techniques, which are quite

complicated and/or require expansive apparatus, were discovered. For instance, very

finely divided and monodisperse maghemite is prepared by microwave plasma from

FeCl3 or Fe3(CO)12 [77] or by electrochemical techniques allowing the formation of

iron clusters, which are then softly oxidized [78].The ball-milling constitutes a process, leading easily to nanosized particles [79].

Phase transitions are also induced by mechanical alloying e.g. with transition metal

oxides like TiO2, Y2O3, WO3. . . [80,81]. In the same manner, hematite (a Fe2O3) canlead to maghemite (c Fe2O3) using special milling techniques. For instance, when adispersion of hematite in methanol is placed between two discs rotating alternatively

in opposite directions, maghemite is obtained. The important shearing effect created

by such device provokes a sliding of the oxygen planes followed by a cationic re-

organization. However the transformation is not fully achieved, even after 55 days:only 57% of hematite are transformed into maghemite and the particles size is larger

than 50 nm [82]. More recently, a similar reaction was described using a planetary

type mill. The results are relevant (particles size between 4 and 20 nm after 48 h), but

the reaction remains incomplete even for long milling durations [83].2. Intercalation of lithium into graphite by ball-milling

The pressure and temperature induced to the particles by the shocks occurring

during the milling are of a great importance to understand the mechanism for

the ball-milling synthesis. Unfortunately, those values are difficult to determine since

R. Janot, D. Guerard / Progress in Materials Science 50 (2005) 192 23the phenomena are short and very localized. They can be nevertheless estimated

indirectly: for instance, in the case of amorphous alloys, it is possible to know if the

recrystallization temperature was reached or not, as it can be proved by micro-

structural investigations or enthalpy measurements [18].

Several authors [10,11] have shown that the highly energetic milling leads to in-

stantaneous and local temperature and pressure (40 kbars, 300 C). Those values areclose to that used for the high pressure technique (50 kbars, 300 C). The interca-lation of lithium into graphite under high pressure leads to superdense phases, but

requires a special and complex material for a low rate of several hundreds milli-

grams at once. It is therefore interesting to develop the intercalation of lithium by

ball-milling, since this technique is easier to handle and the quantity of prepared

powders can be much higher: several grams instead of several hundreds of milli-

grams.2.1. Material used in this study

2.1.1. The mill and its devices

The planetary type mill, model Retsch PM400, and the vial are represented in Fig.

14.

The maximal rotation speed is 400 rpm and the vial rotates at the same speed as

the tray, but in the opposite direction. The vials are made of special hard steel

(Z200C12, 12% of chromium, 2% of carbon in weight). The volume of 270 cm3 al-

lows the synthesis of 5 g of powders at once. A rubber ring placed on the cover

ensures the tightness of the vial, which is filled under the inert atmosphere of a glove

box with purified argon circulation. Three different sizes of balls are used as it ap-pears in Table 1.

It seems important to examine the abrasion of the milling tools. For instance,

when 5 g of graphite powder are milled with 200 g of balls at a rotation speed of 200

rpm, the contamination, as measured by microprobe, is as follows:Fig. 14. Planetary mill (left) and 270 cm3 stainless steel vial (right).

Table 1

Characteristics of the stainless steel balls used in this work

Diameter (mm) Mass (g) Steel

5 0.6 11C6

10 4 100C6

20 32 Z200C12

24 R. Janot, D. Guerard / Progress in Materials Science 50 (2005) 192 when stainless steel balls (softer than the vial material) with 5 or 10 mm diameterare used, the pollution remains low: the iron content in the graphite powder is be-

low 0.1% after 24 h and 1% after 96 h,

with balls of 20 mm, made of the same steel as the vial, the iron contents are re-spectively 1% and 3% in the same conditions. Therefore, the use of such 20 mm

balls should be avoided for long millings.2.2. Reagents

Ceylon type graphite, commercialized by Alfa, was generally used. Its purity is

99.5% in carbon. The spheroid particles present a mean size below 40 lm. Before itsintroduction in the glove box, the graphite is outgassed under vacuum at 800 C for12 h. The XRD transmission pattern is given in Fig. 15 (kMo 0:70926 A).

The graphite is well organized as pointed out by the narrowness of the reflections,

which proves the good stacking of the graphene planes along the c axis, since theinterplanar distance is 3.35 A. An enlargement of the 1822 (in 2h) domain showsup the presence of rhombohedral phase, coming probably from the grinding done by

the manufacturer: the milling increases the amount of the rhombohedral phase. This

rhombohedral phase is revealed by the reflections (1 0 1) and (1 0 2) at respectivelyFig. 15. XRD pattern of initial Alfa graphite (k 0:70926 A).

R. Janot, D. Guerard / Progress in Materials Science 50 (2005) 192 2519.6 and 20.8. The rhombohedral phase content can be estimated around 78%. Infact, pure hexagonal graphite is quite rare and one has to heat natural graphite at

2000 C to entirely remove the rhombohedral phase [84].The lithium is either in granules (16 mm, Alfa, purity 99.4%), or in powder

(mean size 170 lm). This last was prepared by the CEA (Commissariat a energieatomique, France) using a method derived from that set up by Hansley [85]: itsoxygen and nitrogen content is lower than 0.1% in weight.

2.3. Synthesis conditions

The vial with the balls is introduced in the glove box. The argon purification is

obtained by passing the gas through molecular sieves for the water retention and

titaniumzirconium chips whose heating at 800 C allows eliminating both oxygenand nitrogen.

The reactants are weighted in the glove box within an accuracy of 0.1 mg and

placed in the vial, which is tightly closed by screwing the cover on the vial body. The

vial is then removed from the glove box and placed on the tray of the mill. After

milling, the vial is opened in the glove box and the separation between the powder

and the balls is simply done with a sieve.

2.4. Optimization of the milling conditions

As it was previously seen, there are numerous parameters in the ball-milling. The

goal was to obtain a homogeneous powder and to avoid the agglomeration on the

balls, which makes difficult the separation of the reaction products. The optimization

was done mainly on the following parameters:

size of the lithium (granules or powder), lithium/carbon ratio (from Li+ 2C to Li + 6C), charge ratio (from 20 to 80), balls diameter (5, 10 or 20 mm), milling duration (from 12 to 24 h).

In this manner, we have chosen the best conditions for the preparation of the

first stage graphitelithium compounds and, then, the synthesis for the superdense

phases.

2.4.1. Nature of lithium

The use of lithium granules was rapidly shown as inadequate: Fig. 16 represents

the XRD patterns of the resulting products of a mixture with a C/Li ratio of 2

prepared from lithium granules and powders respectively. All other conditions are

the same for the synthesis:

5 g of reactants (graphite: 3.87 g, lithium: 1.13 g), 200 g of balls (diameter 5 mm),

Fig. 16. XRD of a Li + 2C mixture: with lithium powder (top), with lithium granules (bottom).

26 R. Janot, D. Guerard / Progress in Materials Science 50 (2005) 192 charge ratio: 40, rotation of 200 rpm.

With granules of lithium, there is no real intercalation of lithium and the size of

the granules is almost unchanged, which involves some amorphization of the

graphite matrix: the 1 0 0 and 1 0 1 reflections of graphite are weakened. The graphite

0 0 2 distance is still close to 3.35 A, showing up some organization of the graphite,but the presence of a shoulder at lower angles proves either the apparition of tur-

bostratic carbon or that of some high stage graphitelithium compounds.

With lithium powder (mean size of 170 lm), the intercalation occurs after 12 h ofmilling as it appears on the XRD diagram shown in Fig. 16, where the reflections ofthe normal first stage compound appear clearly. The excess of metallic lithium,

visible at 2h 16:4 (1 1 0 reflection) is in good agreement with the relative intensitiesof the classical graphitelithium first stage compound: no formation of a superdense

phase can be detected at this point. When the milling duration is increased up to 24

h, there is no real change, which means that the lithium excess acts as a lubricant.

From those experiments, it appears clearly that the granules have to be avoided for

the ball-milling synthesis.2.4.2. Lithium/carbon ratio

Due to its high ductility, lithium tends to agglomerate on the balls during the

milling process. This agglomeration influences largely the shocks efficiency, the

R. Janot, D. Guerard / Progress in Materials Science 50 (2005) 192 27constraints applied to the powders, and, thus the microstructure of the obtained

phases. The C/Li ratio is, then, a very important parameter. The milling conditions

are similar to those previously announced and lithium powder is preferred to

granules. The following XRD patterns are relative to milling duration of 24 h.

Li+ 6C: The synthesized powder is made of a mixture of first and second stage

compounds, whose main reflections are respectively 3.70 and 3.52 A, in the ap-proximate ratio 2/1. Lithium excess seems necessary to reach a pure first stage

compound, which compensates the loss of metal by agglomeration on the milling

tools.

Li+ 4C: When the amount of lithium is higher (Li + 4C), the XRD diagram

corresponds to a pure LiC6 compound. All reflections can be indexed in the hex-

agonal P6/mmm unit cell of the first stage compound (with a 4:30 A and c 3:70A). However, one should note that the compound is not well crystallized: thebackground is quite high and the width at half height of the reflections is relativelylarge, especially for the 0 0 l reflections. This points out the presence of stacking

faults due to the violence of the shocks. No excess of lithium can be detected, which

can easily be explained by the low electron number for lithium atom. Moreover, the

lithium forms probably very small particles unable to diffract the X-rays.

Li+ 2C: When the ratio reaches Li + 2C, the powder is yellow, typically the color

of LiC6. This initial composition allows to obtain the pure first stage after 12 h of

milling, instead of 24 h for a C/Li ratio of 4. The relative intensities of the observed

reflections correspond to that calculated for LiC6. One should note that this com-pound is better crystallized than when the starting composition is Li + 4C. This

difference results of the role of lubricant plaid by the excess of lithium, which

weakens the violence of the shocks. Thus, the carbon matrix is less disorganized.

As a result of these previous experiments, an initial Li + 2C composition seems to

be the best compromise for the ball-milling synthesis: the resulting phase is LiC6 and

the excess of lithium acts as a lubricant, which avoids a too large degradation of the

graphite network.

2.4.3. Charge ratio

Normally, the intensity of the shocks is directly proportional to the charge ratio:

higher is this ratio, higher is the frequency of the shocks and, thus, the milling is

more energetic. In our case, we have determined the influence of the charge ratio on

the microstructure of the compounds starting from 5 g of a graphite +Li mixture.

We have varied the mass of the balls for a charge ratio between 20 and 80. One has to

note here that this charge ratio is connected with the filling rate of the vial, which

value ranges from 5% to 20%. One can admit that this variation does not influencedrastically the reaction since the free volume in the vial remains quite high even with

a charge ratio of 80 and one can consider that the speed of the balls is sufficient to

achieve an efficient milling.

The XRD data shown in Fig. 17 correspond to the following conditions:

mixture composition Li + 2C, 5 mm balls,

Fig. 17. XRD of a Li + 2C mixture: with a charge ratio of 20 (top) with a charge ratio of 80 (bottom).

28 R. Janot, D. Guerard / Progress in Materials Science 50 (2005) 192 rotation speed: 200 rpm, 24 h of milling.

With a charge ratio of 20 (100 g of balls), the powder is a mixture of first and

second stage compounds, in good agreement with the brown color of the powder. A

shoulder appears on the right (short distances) of the main peak. One can estimate

the content of this mixture: 70% of LiC6, 30% of LiC12. A longer milling is necessary

to obtain a pure first stage, but pollution by metals (in particular iron) due to the

abrasion of the milling tools appears. This result agrees well with some equivalence

between the milling duration and the charge ratio [15]. When the charge ratio is 40,the XRD diagram of a pure and well crystallized first stage compound is observed.

With higher charge ratios (60 or 80), the first stage compound is less and less crys-

tallized as the charge ratio is increased, in particular, the relative intensity of the 1 1 0

reflection versus 0 0 1 indicates that the graphite planes are more fractured than in

the presence of a smaller charge ratio.

R. Janot, D. Guerard / Progress in Materials Science 50 (2005) 192 29Therefore, the choice for the most appropriated charge ratio results of a com-

promise between a long milling duration (which leads to a contamination by the

milling tools) connected to a low ratio and the risk of an important amorphization

when large charge ratios are used. The value taken for the synthesis of first stage

graphitelithium compounds is 40 in the following.2.4.4. Balls size

Three different ball sizes are available: 5, 10 and 20 mm in diameter. All the other

milling conditions are similar and correspond to that previously used with a charge

ratio of 40 (200 g of balls), 200 rpm and 24 h of milling for 5 g of a Li + 2C powders.

With 5 mm balls, we have seen previously that the powder is a pure and well crys-

tallized first stage compound (cf. Fig. 16). With a ball size of 10 mm, the compound

is also a pure first stage, but contains obviously more defects. The milling done with

balls of 20 mm is totally inefficient: one obtains mainly graphite. This inefficiency canbe related to the low balls number: 6 and their tendency to remain on the vial wall

under the effect of the centrifuge forces and do not participate properly to the

milling. Moreover, due to the large size of the balls, there are large dead zones

(especially between the vial and its cover) in which the powders are not crushed. On

the other hand, those balls are made of the same hard steel as the milling container,

which involves a large abrasion and, therefore, the contamination by iron reaches

around 0.5 at.%.2.4.5. Conclusion

The study of the relative influence of the different parameters leads to the fol-

lowing optimized conditions for the ball-milling synthesis of first stage graphite

lithium compounds:

5 g of a mixture of graphite (mean size: 40 lm) and lithium (170 lm) powders, lithium/carbon ratio: 1/2, charge ratio: 40 (200 g of balls), ball size: 5 mm in diameter, rotation speed: 200 rpm, milling duration: 12 h.2.5. Ball-milling of a mixture of Li + 2C powders

The milling conditions are those determined just above and the following inves-

tigations are done on the resulting powder:

X-rays diffraction, in order to determine the optimal rotation duration leading toa well crystallized first stage graphitelithium compound,

density measurements, which allow to calculate easily the amount of non-interca-lated lithium and, consequently, the real stoichiometry of the intercalation com-

pound,

30 R. Janot, D. Guerard / Progress in Materials Science 50 (2005) 192 electronic microscopy (both SEM and TEM), for the determination of the sizeand texture of the crystallites.

2.5.1. X-rays diffraction

Numerous studies of the effect of ball-milling on graphite (without the presence of

lithium) point out the tendency of graphite to be rapidly disorganized and to lead to

amorphous carbons [8693]. Later in this paper, as introduction to the preparation

of highly anisometric graphite (HAG), it will be presented, in particular from the

work done by Salver-Disma [94,95], that the structure and microtexture of ball-

milled graphites depend strongly of the mill type. It appears that the planetary typemill is adequate for this work since it favors the friction and, as a result, the cleavage

of graphite. Moreover, in the case of co-milling of graphite and lithium powders, the

presence of the later in large quantity (its initial volume represents 55% of the total

volume for a Li + 2C composition) decreases the violence of the shocks and, thus, the

graphite amorphization.

The product of the milling of a Li + 2C mixture is not really a fine powder, it looks

more like chips: as already explained, the ball-milling consists in both breaking and

agglomeration of particles. Due to the large lithium excess (42% in volume if oneconsiders the graphite intercalation compound to be LiC6), the milling leads to an

important agglomeration and results in a large distribution of the particles size. This

agglomeration problem is quite drastic and it is very difficult to get back the whole

amount of powders due to the tendency of the particles to paste on the milling tools.

This is the reason why we have set up a new synthesis method by addition of a liquid

in the milling container (acting as a lubricant), as it will be developed in Part 3.

XRD characterization is performed by introducing the powders prepared by ball-

milling in a tube made of Lindemann glass (inside diameter 1 mm, wall thickness0.01 mm). The tube is then placed on an INEL diffractometer equipped with a quartz

monochromator and a scintillation detector. The wavelength is that of molybdenum:

kKa 0:70926 A and the data are analyzed with the Diffrac-AT software. Fig. 18shows the XRD diagrams of powders prepared with a Li + 2C initial composition

and milling duration of 6, 12, 24, 48 and 96 h.

First of all, the maximal intensities are decreasing as the milling time is increased:

its value of 550 counts per minute (for a milling of 6 h) becomes less than 100 cpm

(for 96 h). This is the consequence of the amorphization of the carbon matrix. Aftera milling for 6 h, the powder consists in a mixture of 1st and 2nd stage compounds,

the reflections are quite sharp and the miller indices, indicated in Fig. 18, are based

on the hexagonal lattices whose parameters are: a 4:30 A and, respectively,c 3:70 (for LiC6) and c 7:05 A for LiC12. A pure and well-crystallized first stagecompound is obtained after a milling of 12 h. The free lithium excess is visible at

2h 16:4 (2.49 A).For longer milling duration, the reflections remain sharp and they are not widen

by a large decrease of the particles size. However, the background is increased, signof the appearance of some disorder. Even after 96 h of milling, the reflections present

relative intensities compatible with a pure LiC6 first stage compound. Traces of

Li2C2 lithium carbide are still not visible, in spite of a long and energetic milling. On

Fig. 18. XRD of Li + 2C powders after various milling durations.

R. Janot, D. Guerard / Progress in Materials Science 50 (2005) 192 31the same manner, there are no traces of lithium nitride or oxide, which proves the

excellent tightness of the vials. The 96 h ball-milled powder is made of large and very

anisotropic particles, susceptible to be slightly out of the rotation center of thegoniometer and, therefore, the diffraction can be given by two parallel particles,

responsible for a twinning of the reflections.

The XRD pattern of the powder obtained after a milling for 12 h is typical of that

given by a pure first stage LiC6. However, the differences between the XRD diagrams

of LiC6 and of superdense phases obtained under high pressure (50 kbars, 300 C), as

Fig. 19. Two-dimensional structures of LiC6 (a) and compounds coming from the decomposition of LiC2(b).

32 R. Janot, D. Guerard / Progress in Materials Science 50 (2005) 192well as their derivatives formed by decomposition of LiC2 (Li9C24 or Li7C24), are

quite small [96]. Moreover, the powdered state of the ball-milled compounds does

not allow a separate study of the different reflections families (0 0 l, h k 0 or h k l)

[97,98] and, thus, it is difficult to determine easily the two-dimensional structure of

the intercalated layer by XRD.

2.5.1.1. Some crystallographic particularities of the graphitelithium compounds. The

structural study of the LiC6 compound is made difficult for the following reason: the

hexagonal structure with a 4:30 A and c 3:70 A (Fig. 19(a)) and a=c p3=2[43] involves an identical norm for the reciprocal vectors a, b and c and, thus, asystematic superimposition of the h 0 0 and 0 0 h reflections. With a powder, those

reflections cannot be distinguished by XRD. Moreover, the reticular distance of 1.85A gathers nothing less than three reflections with the following Miller indices: 2 0 0,0 0 2 and 1 1 1, the last one being the most intense.

The lithium density in the intercalated layers can be deduced from the comparison

of the relative intensity for the reflections observed at respectively 3.70 and 1.85 A.This ratio increases as the lithium content is increased, as shown in Fig. 20: from 33%

in LiC6 x 1 to 75% in LiC2 x 3. The experimental ratio for powders preparedby mechanical alloying (Fig. 18) is around 30%, in good agreement with the for-

mation of LiC6. The synthesis of superdense phases by ball-milling seems excluded.

One can conclude of this study that the ball-milling of Li + 2C powders mixture

leads, after 12 h, to a first stage LiC6 compound, but the presence of large quantities

of free lithium induces an important agglomeration on the balls. It results that the

product presents a large distribution of particles size. The XRD diagrams exhibits

well crystallized compounds, even after a milling duration of 96 h. No trace of

lithium carbide, nitride or oxide appears and the formation of superdense phasesseems to be excluded.

2.5.2. TEM study

Under the inert atmosphere of the glove box, the synthesized powder is placed on

a copper grid covered by a thin (100 A) amorphous carbon film. The grid is then puton a tight sample holder before its introduction into the microscope column (Philips

20

30

40

50

60

70

80

90

100

0 0.5 1 1.5 2 2.5 3 3.5x in LixC6

I 1,85

A /

I 3,7

0 A (

%)

Fig. 20. Relative intensity of the reflections at 1.85 A versus that at 3.70 A, as a function of the lithiumcontent.

R. Janot, D. Guerard / Progress in Materials Science 50 (2005) 192 33CM20, 200 kV). In spite of the tightness of the sample holder, there appears some

oxidation, as it will be seen later on. This is due to the high reactivity of lithium andto the small size of the particles. This problem does not influence the results con-

cerning the particles size and their shape since the oxidation does not involve any

exfoliation of the carbon matrix.

The larger particles (several lm in diameter) are highly anisometric since theirthickness does not exceed 100 nm. The electronic diffraction pattern shown in Fig.

21(a) exhibits the six equivalent 1 0 0 reflection of graphite. At this stage, it cannot be

concluded that those spots correspond or not to the 1 1 0 reflection of LiC6: it should

be associated with an other set of six spots (1 0 0 reflection of the LiC6 hexagonal unitcell) closer to the direct beam. The absence of the 1 0 0 reflection can be explained byFig. 21. Electronic diffraction on graphitelithium particles: (a) 5 lm, (b) 200 nm.

Table 2

Reflections observed by electron diffraction on small particles (cf. Fig. 21(b))

d (A) 3.33 2.66 2.07 1.64 1.39 1.24 1.16

h k l G 0 0 2 (2 spots) Li2O 1 1 1 G 1 0 0 Li2O 2 2 0 Li2O 3 1 1 G 1 1 0 G 1 1 2

34 R. Janot, D. Guerard / Progress in Materials Science 50 (2005) 192its low intensity (20% of the 1 1 0 reflection). One has also to note the absence of

lithium oxide, which appears clearly in the diffraction pattern of a smaller particle

(cf. Fig. 21(b)). In the small particles, there is a visible oxidation of the LiC6 com-

pound, revealed by the presence of Li2O reflections (cf. Table 2).2.5.3. Density measurements

The lithium density is much smaller (0.53) than that of graphite (2.27) or

graphitelithium intercalation compounds (cf. Fig. 22), thus, the determination of

the powder density should allow the estimation of the composition of the interca-

lated phase. The densities are calculated on the basis of an interplanar distance of3.70 A, which corresponds to that of the normal LiC6 compound [44] as well as ofthe superdense phases [96].

The experimental densities were firstly determined by uniaxial compression real-

ized with a small hand press in the glove box. However, this technique appears in-

adequate since the pellets are fragile, due to the partial amorphization of graphite,

especially when the milling time is long. Moreover, the applied pressure remains

quite low (5 kbars) and the compactness is not sufficient. For instance, with graphite

whose theoretical density is 2.27, the experimental value is only 2.10. This is thereason why we managed further density measurements by liquid pycnometry within

n-dodecane (C12H26, Acros Organics, 99%).0.5

1

1.5

2

2.5

3

0 0.5 1 1.5 2 2.5 3x in LixC6

Den

sity

Calculated density of the compounds

density of mixture whose global composition is LiC2

Fig. 22. Calculated densities of several graphitelithium intercalation compounds and of a powder with a

Li + 2C initial composition as a function of the amount of intercalated lithium.

R. Jano