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NANOMATERIAL APPLICATIONS IN LITHIUM BATTERIES FOR EV’S

Christian DiCenso

12/5/2014

Abstract

With growing use of hybrid and electric vehicles, battery technology struggles to synchronize

with power demands. Automotive companies have shifted to use of Lithium-ion batteries (LIBs)

in recent years but energy density in these batteries still limit use to less mileage than their petrol

counterparts. Since commercial introduction, extensive research has been made in LIB

technology to reduce this disparity. Lithium metal oxide and phosphate cathodes (LiFePO4,

LiCoO2, LiMn2O4) are currently the most widespread. Current research incorporates silicon and

carbon nanostructures and nanofabrication techniques to increase efficiency in these cathode

types. Completely new lithium-based battery types have emerged as well. Lithium-Sulfur has a

higher theoretical energy density than any other current LIB type but cycle time is still highly

limited. Nanomaterials are being employed to increase lifespan and reduce crystallization in Li-S

cells and make them reliable for commercial use.

I. Introduction

The goal of this paper is to introduce new technologies that improve lithium rechargeable

batteries for use in electric vehicles. EV’s are an exciting, fast-growing market, however, battery

technology has always been a limiting factor in its growth. Lithium rechargeable batteries were

first introduced commercially by Sony in 1991 [1] and have been researched intensively since.

Lithium batteries are the most likely candidates for next generation EV power sources, although

there are still many hurdles. However, the introduction of nanoscale materials and fabrication

techniques in battery design promises to close the gap between power demands and battery

capacity. In this paper, several new battery technologies incorporating nanomaterials and

nanofabrication will be discussed.

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II. The Battery

Since the introduction of the early galvanic cell to today’s various offerings, all batteries operate

on the same underlying principles. There are three components of any battery: 1) the anode or

negative electrode that donates electrons, 2) the cathode or positive electrode that accepts

electrons from the anode and 3) an electrolyte that allows positive ions from the anode and

cathode to move freely between one another to equalize charge [1]. Figure 1 shows a standard

battery layout. Electrons move from the anode to the cathode through the applied circuit,

meanwhile, positive ions make their way from the anode to the cathode through the electrolyte as

electrons collect on the cathode and create a negative charge.

Figure 1 – Schematic of a typical battery [1]

The types of batteries useful for EV’s and other electronic devices are secondary batteries. These

batteries are rechargeable through energy input from a charging device that replaces depleted

electrons in the anode. Secondary batteries include Lithium ion, NiCd and NiMH. These have all

seen use in EV’s. Primary batteries, like the disposable alkaline type from most retail stores, are

one-time use and would be far too expensive and cumbersome for EV use.

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Figure 2 shows a timeline of the most common secondary battery types. The lead-acid

battery has been in use since the early 20th century and is found in virtually all modern gasoline

vehicles as an engine starter battery. The introduction of NiCd and NiMH batteries improved

upon lead acid in weight, size and energy capacity. These three battery types still use an aqueous

electrolyte. Lithium batteries make use of a nonaqueous electrolyte and use lithium in the anode

and cathode [3].

Figure 2 – Timeline of secondary battery types [2]

Below in Figure 3, the specific energy of NiCd, NiMH and Li-ion is shown. Li-ion has the

greatest potential specific energy and energy density of the three, making it optimal for EV use.

Figure 3 – Specific energy vs. energy density of common secondary battery types

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III. Lithium Cells

Lithium cells use intercalation compounds in the anode and cathode. These compounds allow for

the movement of lithium between electrodes without significant structural change over many

cycles. The most common cathode and anode materials in use today are LiCoO2 and graphite,

respectively [5]. Shown below in Figure 4 is a diagram for a lithium cell. Upon charge, lithium

ions and electrons move to the anode from the cathode; upon discharge lithium is removed from

the anode back to the cathode and electric energy is carried through a connected circuit.

Figure 4 – Lithium Cell; intercalation compound anode, graphitic cathode [4]

Anodes

Silicon anodes show promise for their good cycle capability and theoretical capacity, over 4000

mAh/g [5]. Compared to common graphite anodes, that’s an increase of over 900%. This is yet

to be experienced in a laboratory setting however. One of the challenges with silicon is its

significant expansion under lithiation. McDowell et al. gathered research on several crystalline

and amorphous silicon experiments. In one study, crystalline Si nanostructures were etched from

single crystal wafers and created using nanowires. These structures were then lithiated and

monitored with electron microscopy. Figure 5 shows the volumetric expansion of one of the

fabricated silicon crystals undergoing lithiation.

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Figure 5 – Silicon crystal expansion during lithiation process

These crystals did not only undergo volume changes, but also changed shape along several

planes. One of the largest issues with silicon structures is while they have high theoretical

capacity, volumetric expansion can cause fracture in the structure. This severely hinders battery

performance by reducing lithiation sites and thereby decreasing cycle life. Another characteristic

of silicon electrodes is poor solid electrolyte interphase (SEI) [6]. SEI is a thin film that forms on

the anode. It is a byproduct of the reduction reaction between the solvent and salt in the

electrolyte, and its presence allows for longer cycle life of the battery [7].

Ye et al. used ALD to fabricate 25:1 height/diameter ratio silicon micropillars coated in

metal oxide (Al2O3 and TiO2) to enhance these characteristics. The lengths of these pillars are the

longest in any such study and help reduce the substrate confinement effect. The metal oxide

coating helps boost fracture resistance of the underlying silicon and have several other desirable

properties for lithiation, including heat resistance and low electrical conductivity. Using SEM,

the number of lithium atoms per silicon was found to be 8.8 Li per Si, vastly improved over the

theoretical value of 3.75 (Li15Si4).

Graphene composite anodes are another area of interest. Graphene is a single layer of

carbon, mechanically extracted from bulk graphite that features a hexagonal lattice structure [8].

In another attempt to improve Si-based anodes, Chou et al. [9] mixed nanosize Si and graphene

together in a 1:1 ratio by weight to produce a composite electrode. Figure 6 shows the improved

performance of the Si/graphene composite over graphene or Si alone, however the experimental

capacity is still less than the theoretical Si capacity. The Si/graphene composite also exhibited

good cycle performance over the 30 cycles, being much more stable than nano-Si.

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Figure 6 – Cycle performance for Si/graphene

Graphene has also been used with other electrode types to increase performance. Zhang et al.

combined graphene with SnO2 nanoparticles to curtail volume expansion [10]. SnO2 has a high

theoretical capacity, ~800 mAh/g, but exhibits large volume expansion under lithiation, similar

to Si. Graphene has a much lower expansion rate under lithiation than SnO2 and also shows great

lithiation properties. Previous literature shows that incorporating graphene with SnO2

nanostructures result in actual reversible capacity over 800 mAh/g. Zhang and team went a step

further and coated the Gr-SnO2 structure with amorphous carbon to further boost performance

(Gr-SnO2-C). Compared to Gr-SnO2 and SnO2-C, Gr-SnO2-C achieved higher capacity and

much longer cycle life, 757 mAh/g over 150 cycles. Shown in Figure 7, cycle performance is

superior in the Gr-SnO2-C over previous materials, which ended at 100 cycles and had lower

capacities at each.

Figure 7 – Graphene/SnO2 electrode performance

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Another carbon nanostructure, the carbon nanotube, has also been researched significantly for

increasing anode performance. As reported by Lahiri and Choi [11], early studies pairing CNT’s

with Sn2Sb were favorable but cyclability was not much better than LiCoO2. They found that

when CNT’s were mixed with Si, results improved. Two types of carbon nanotubes have been

researched as electrode materials: single walled (SWCNT) and multiwalled (MWCNT). Porous

Si films on SWCNT’s have achieved capacities of over 2000 mAh/g for 40 cycles. When Si

nanoparticles were used with MWCNT’s, 900 mAh/g of capacity was maintained for over 100

cycles.

Using alternative techniques for Si/carbon bonding, these numbers can be further

improved. Mechanical mixing is still the most commonly used method for combining Si or other

anode materials with graphene or CNT’s. Alternative methods of carbon/silicon mixing such as

ball milling or using nanoporous silica structures has proved successful in further increasing

electrode capacity [12].

Cathodes

Similar to anode design, engineers desire to increase lithium capacity in cathodes as well. The

leading commercial cathode material, LiCoO2 still leaves much to be desired in terms of lithium

storage capacity [13]. LiCoO2 forms a weak bond with oxygen that easily allows oxidation at

high temperatures. A recent material of interest in current research is a lithium/olivine structure

in LiFePO4 and LiMnPO4. This structure has a stronger bond with oxygen and helps prevent

oxygen reacting with electrolyte material, extending battery life [14]. Lithium iron phosphate

electrodes have shown high energy density, long life and are lower cost as well [15]. In Figure 8,

LiFePO4 is on the left, LiMnPO4 on the right. The grey dots are lithium atoms in the crystal

structure. Olivines are good intercalation compounds because of their rigid crystal structure that

exhibits very little volume expansion.

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Figure 8 – Lithiated olivine cathode materials, LiFePO4 (left), LiMnPO4 (right) [16]

Xin et al. synthesized nano LiFePO4 in the 60-100nm range. They were then attached to a

nanoporous carbon matrix shown in Figure 9. Charge/discharge time was excellent as was cycle

life, losing only 3% over 700 cycles.

Figure 9 - Nanoporous carbon matrix with nano LiFePO4 [17]

Another study has shown that LiFePO4 is also capable of very rapid charge and discharge rates,

roughly 30s, on par with supercapacitor performance, in nano LiFePO4 on similar nanoporous

carbon structures [18].

Spinels and inverse spinels have also garnered a great degree of attention. Some varieties

are already in use for niche applications [19]. Recent research on LiMVO4, where M can be

either Ni or Co, has shown good performance [20]. A study by Kitajou et al. tested the response

of these cathode materials with favorable results. Synthesized at high pressure, crystals of Mn

and Co-doped LiNiVO4 were tested. Figure 10 shows the voltage vs. capacity response for the

Mn-doped LiNiVO4 inverse spinel. Charge/discharge capacity increased with higher

concentrations of Mn. Initial charge and discharge capacity was 73.9 mAh/g and 23.4 mAh/g,

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respectively. While these novel inverse spinel structures still have much less capacity than the

current leader, LiCoO2, at 100 mAh/g for Mn.3, this is still the highest recorded capacity for an

inverse spinel.

Figure 10 – Performance with Mn-doped LiNiVO4 inverse spinel

Whereas the previous technologies are more traditional in scope, still based off the carbon/metal

oxide model originally commercialized by Sony, new research in lithium-sulfur batteries are a

vast departure. Huge energy capacities are required to meet the needs of EV’s and make them

competitive with gasoline automobiles and to reduce “range anxiety” that exists for EV drivers

today [21, 22]. Lithium sulfur batteries have the theoretical capacity to fill the need.

Experimental lithium cells use a lithium metal anode and a sulfur-carbon anode. While

research is promising for energy capacity, the greatest challenge is short cycle life, due to the

formation of lithium-blocking polysulfides [23]. Many micro- and nanoscale solutions have been

proposed, as shown in Figure 11, however none of the listed has been successful in keeping

capacity loss to <10% over 100 cycles [24].

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Figure 11 - Hierarchical designs of carbon-based sulfur composites: (a) microporous carbon spheres, (b) spherical ordered mesoporous carbon nanoparticles, (c) porous hollow carbon, (d) graphene oxide sheets, (e) porous carbon nanofibers, and (f) hollow carbon nanofibers [24]

Novel CNT structures combined with sulfur have shown great performance. Dörfler et al. have

experienced much improved cycle performance and good capacity retention using CNT’s

deposited and grown on a metallic substrate by CVD. LiNO3 was also added to combat

polysulfide formation in the electrolyte. Results show a marked increase, with capacity of 800-

900 mAh/g for over 20 cycles.

Jeddi et al. have shown even better performance using silica infused polymer electrodes

to contain polysulfide growth [26]. By polymerizing the electrolyte, the problem was much

improved over an aqueous solution. Compared to a liquid electrolyte, the composite polymer

electrolyte (CPE) demonstrated capacity of ~1200 mAh/g over 100 cycles, shown in Figure 12.

Figure 12 – CPE vs. liquid electrolyte Li-S battery

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IV. Conclusion

The current amount of research in lithium batteries is vast and growing day by day. With an

ever-increasing market of EV’s and consumer electronics, it can only grow more. New

technologies listed in this paper are the forefront; further development in these areas is necessary

to replace the mature LiCoO2/graphite design. Nanoscale materials are the key to engineering

batteries with next-generation energy capacities. Carbon nanotubes and graphene show great

promise in cathode and anode design. Lithium iron phosphates and spinel structures are very

stable platforms on which to build, being much safer and cheaper than LiCoO2. Lithium sulfur is

the newest technology, and researchers have much yet to learn, however nanomaterials and

nanofabrication processes will be essential in minimizing polysulfide growth to attain energy

capacities to be successful in powering EV’s.

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