Report to Submit

8/3/2019 Report to Submit

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Nanoscoop Page 1

1. Introduction

A very fast depletion of world oil reserves and their related hazards have propelled

great advancements in hybrid and all electric automobiles. Batteries designed for electric

vehicles should be able to provide high energy and power densities. The concept of battery

electric vehicles is to charge batteries on board vehicles for propulsion using the electric grid.

Battery electric cars are becoming more and more attractive with the advancement of new

battery technology (Lithium Ion) that have higher power and energy density (i.e. greater

possible acceleration and more range with less batteries) and higher oil prices. An entirely

new type of nano material developed at Rensselaer Polytechnic Institute could enable the

next generation of high-power rechargeable lithium Li-ion batteries for electric automobiles,

as well as batteries for laptop computers, mobile phones, and other portable devices. The

Rensselaer research team, led by Professor Nikhil Koratkar, demonstr/ated how a Nanoscoop

electrode could be charged and discharged at a rate 40 to 60 times faster than conventional

battery anodes, while maintaining a comparable energy density. This stellar performance,

which was achieved over 100 continuous charge/discharge cycles, has the team confident that

their new technology holds significant potential for the design and realization of high-power,

high-capacity Li-ion rechargeable batteries. "Charging my laptop or cell phone in a few

minutes, rather than an hour, sounds pretty good to me," said Koratkar, a professor in the

Department of Mechanical, Aerospace, and Nuclear Engineering at Rensselaer. By using our

nanoscoops as the anode architecture for Li-ion rechargeable batteries, this is a very real

prospect. Moreover, this technology could potentially be ramped up to suit the demanding

needs of batteries for electric automobiles. According to the Koratkar the main limitation of

nanoscoops is their nanoscale size; our nanoscoops can soak and release Li at high rates far

more effectively than the macroscale anodes used in today's Li-ion batteries. This means ournanoscoop may be the solution to a critical problem facing auto companies and other battery

manufacturers. A limitation of the nanoscoop architecture is the relatively low total mass of

the electrode, Koratkar said. To solve this, the team's next steps are to try growing longer

scoops with greater mass, or develop a method for stacking layers of nanoscoops on top of

each other. Another possibility the team is exploring includes growing the nanoscoops on

large flexible substrates that can be rolled or shaped to fit along the contours or chassis of the

automobile.


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2. Lithium Ion Batteries

Lithium ion batteries are key components for providing electricity in portable

entertainments, telecommunications and computing devices in the 21st century.

A lithium-ion battery (sometimes Li-ion battery or LIB) is a family of rechargeable

battery types in which lithium ions move from the negative electrode to the positive electrode

during discharge, and back when charging. Chemistry, performance, cost, and safety

characteristics vary across LIB types. Unlike lithium primary batteries (which are

disposable), lithium-ion electromechanical cell use an intercalated lithium compound as the

electrode material instead of metallic lithium.

Lithium-ion batteries are common in

consumer electronics. They are one of the

most popular types of rechargeable battery

for portable electronics, with one of the best

energy density, no memory effect, and a

slow loss of charge when not in use. Beyond

consumer electronics, LIBs are also growingin popularity for military, electric vehicle

and aerospace applications. Research is

yielding a stream of improvements to

traditional LIB technology, focusing on energy density, durability, cost, and intrinsic safety.

a). Construction

The three primary functional components of a lithium-ion battery are the anode,

cathode, and electrolyte. The anode of a conventional lithium-ion cell is made from carbon,

the cathode is a metal oxide, and the electrolyte is a lithium salt in an organic solvent.

The most commercially popular anode material is graphite. The cathode is generally

one of three materials: a layered oxide (such as lithium cobalt oxide), a poly anion (such as

lithium iron phosphate), or a spinel (such as lithium manganese oxide).

The electrolyte is typically a mixture of organic carbonates such as ethylene carbonate

or diethyl carbonate containing complexes of lithium ions. These non- aqueous electrolytes

Figure 1 Li - ion Battery


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generally use non-coordinating anion salts such as lithium hexafluorophosphate (LiF6),

lithium hexafluoroarsenate monohydrate (LiAsF6), lithium perchlorate (LiClO4), lithium tetra

fluoroborate (LiBF4), and lithium triflate (LiCF3SO3).

Depending on materials choices, the voltage, capacity, life, and safety of a lithium ion

battery can change dramatically. Recently, novel architecture using nano technology have

been employed to improve performance. Pure lithium is very reactive. It reacts vigorously

with water to form lithium hydroxide and hydrogen gas. Thus, a non-aqueous electrolyte is

typically used, and a sealed container rigidly excludes water from the battery pack.

Lithium ion batteries are more expensive than other batteries. Batteries but operate

over a wider temperature range with higher energy densities, while being smaller and lighter.

They are fragile and so need a protective circuit to limit peak voltages.

b). Charging Method of Li-ion Battery

During discharge, lithium

ions Li+

carry the current from the

negative to the positive electrode, through

the non-aqueous electrolyte and separator

diaphragm. During charging, an external

electrical power source (the charging

circuit) applies a higher voltage (but of the

same polarity) than that produced by the

battery, forcing the current to pass in the

reverse direction. The lithium ions then

migrate from the positive to the negative electrode, where they become embedded in the

porous electrode material in a process known as intercalation.

Depending on materials choices, the voltage, capacity, life, and safety of a lithium-ion

battery can change dramatically. Recently, novel architecture using nanotechnologies have

been employed to improve performance. Pure lithium is very reactive. It reacts vigorously

with water to form lithium hydroxide and hydrogen gas. Thus, a non-aqueous electrolyte is

typically used, and a sealed container rigidly excludes water from the battery pack.

Lithium-ion batteries have also been in the news lately. Thats because these batteries have

the ability to burst into flames occasionally. It's not very common just two or three battery

packs per million have a problem. The Li+ ions are not the only species that move. Inside the

Figure 2 Charging & Discharging of Li-ion

Battery


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battery all charge carriers (ions, electrons) move due to diffusion and migration causing a

potential difference. Inside the electrodes charge is carried by the movement of electrons (and

holes), whereas in the electrolyte charge is carried by the Li+ and CLO4- ions. Electrons

leave the anode via its current collector and enter the cathode via its current collector.

At the same time, at the anode-electrolyte and cathode-electrolyte interfaces Li+ ions

are extracted from the anode and inserted into the cathode, respectively. The ClO4- ions, on

the other hand, remain inside the electrolyte. The extraction and insertion of Li+ions take

place at the same rate, so that global electro neutrality of the three domains is preserved

C). Electro chemistry

The three participants in the electrochemical reactions in a lithium-ion battery are the

anode, cathode, and electrolyte. Both the anode and cathode are materials into which, and

from which, lithium can migrate. During insertion (or intercalation) lithium moves into the

electrode. During the reverse process, extraction (or deintercalation), lithium moves back

out. When a lithium-based cell is discharging, the lithium is extracted from the anode and

inserted into the cathode. When the cell is charging, the reverse occurs. Useful work can only

be extracted if electron flow through a closed external circuit. The following equations are in

units of moles, making it possible to use the coefficient x. The positive electrode half-

reaction (with charging being forwards) is:

The negative electrode half-reaction is:

The overall reaction has its limits. Over discharge supersaturates Lithium cobalt

oxide, leading to the production of lithium oxide, possibly by the following irreversible

reaction:

Overcharge up to 5.2 Volts leads to the synthesis of cobalt (IV) oxide, as evidenced

by X-ray diffraction.


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In a lithium-ion battery the lithium ions are transported to and from the cathode or

anode, with the transition metal, cobalt (Co), in LixCoO2 being oxidized

from Co3+

to Co4+

during charging, and reduced from Co4+

to Co3+

during discharge.

d). Advantages

1. Lithium-ion batteries are popular because they have a number of important

advantages over competing technologies:

2. They are generally much lighter than other types of rechargeable batteries of the same

size. The electrodes of a lithium-ion battery are made of lightweight lithium and carbon.

3. Lithium is also a highly reactive element, meaning that a lot of energy can be stored

in its atomic bonds. This translates into a very high energy density for lithium-ion

batteries.

4. A typical lithium-ion battery can store 150 watt-hours of electricity in 1 kilogram of

battery.

5. They hold their charge.

6. A lithium-ion battery pack loses only about 5 percent of its charge per month,

compared to a 20 percent loss per month for NiMH batteries.

7. They have no memory effect, which means that you do not have to completelydischarge them before recharging.

8. Lithium-ion batteries can handle hundreds of charge/discharge cycles.

9. That is not to say that lithium-ion batteries are flawless

e). Disadvantages

They have a few disadvantages as well:

1. They start degrading as soon as they leave the factory. They will only last two or three

years from the date of manufacture whether you use them or not.

2. They are extremely sensitive to high temperatures. Heat causes lithium-ion battery

packs to degrade much faster than they normally would.

3. If you completely discharge a lithium-ion battery, it is ruined.

4. A lithium-ion battery pack must have an on-board computer to manage the battery.

This makes them even more expensive than they already are.

5. There is a small chance that, if a lithium-ion battery pack fails, it will burst into flame.


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f). Increase the efficiency of battery

Lithium-ion batteries show poor performance for high power applications involving

ultrafast charging/discharging rates. Batteries designed for electric vehicles should be able to

provide high energy and power densities. Lithium (Li)-ion batteries are known to deliver very

high energy densities in comparison to other battery systems.1However, they suffer from low

power densities. In contrast, super capacitors provide very high power densities due to their

surface-based reactions[2], to replace a traditional combustion engine; it is highly desirable to

combine the advantages of Li-ion batteries and super capacitors into one single battery

system. Earlier reports have shown the development of high rate cathode materials.[5], This

has also led to an equal effort in the development of high-rate capable anode architectures.

Silicon (Si) has been envisioned as a promising anode material because of its high theoreticalcapacity of ~4200mAh/g based on the stoichiometry of the alloy Li22Si5. The main limitation

of this high capacity is an accompanying volumetric expansion of~ 400% for crystalline Si

(or ~280% for amorphous Si) which results in pulverization and delamination of the electrode

structure.[2] Pulverization results in more capacity losses due to increased solid electrolyte

interphone solid electrode Interphase (SEI) formation while delamination results in loss of

electrical contact with the substrate. At higher charge/discharge rates (C-rates), these failure

mechanisms are severely exacerbated and thus it is important to design architectures that

perform well at fast C-rates to enable high power Li-ion rechargeable batteries.

3. NanoScoopElectric cars currently use super capacitors to perform power-intensive functions,

including starting the vehicle and rapid acceleration, in conjunction with conventional

batteries that deliver high energy density for

normal cruise driving and other operations. The

researchers believe that nanoscoops may now

enable these two separate systems to be

combined into a single, more efficient battery

unit. According to the team at Rensselaer, the

reason that contemporary batteries take so long

to charge is that they are purposefully

programmed to do so. This is because the

anode structure of a Li-ion battery physically grows, with discharging having the opposite

Figure 3 Nanoscoop


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effect, causing an amount of stress that will cause the battery to fail if done too quickly.

Nanoscoop technology effectively relieves the need to protect against battery failure by

intentional slowing down the charging. New nano engineered batteries developed at

Rensselaer exhibit remarkable power density, charging more than 40 times faster than today's

lithium-ion batteries. The new material, dubbed a "nanoscoop" because its shape resembles a

cone with a scoop of ice cream on top, can withstand extremely high rates of charge and

discharge that would cause conventional electrodes used in today's Li-ion batteries to rapidly

deteriorate and fail. The nanoscoop's success lies in its unique material composition,

structure, and size. Batteries for all-electric vehicles must deliver high power densities in

addition to high energy densities, Koatkar said. These vehicles today use super capacitors to

perform power-intensive functions, such as starting the vehicle and rapid acceleration, in

conjunction with conventional batteries that deliver high energy density for normal cruise

driving and other operations. Koratkar said the invention of nanoscoops may enable these two

separate systems to be combined into a single, more efficient battery unit. The anode

structure of a Li-ion battery physically grows and shrinks as the battery charges or

discharges. When charging, the addition of Li ions increases the volume of the anode, while

discharging has the opposite effect. These volume changes result in a build up of stress in the

anode. Too great a stress that builds up too quickly, as in the case of a battery charging or

discharging at high speeds, can cause the battery to fail prematurely. This is why most

batteries in today's portable electronic devices like cell phones and laptops charge very

slowly.

The slow charge rate is intentional and designed to protect the battery from stress-

induced damage. The anode structure of a Li-ion battery physically grows and shrinks as the

battery charges or discharges. When charging, the addition of Li ions increases the volume of

the anode, while discharging has the opposite effect. These volume changes result in a build

up of stress in the anode. A stress that builds up too quickly, as in the case of a battery

charging or discharging at high speed that can cause the battery to fail prematurely. That is

why most batteries in today's portable electronic devices like cell phones and laptops charge

very slowly. So, the slow charge rate is intentional and designed to protect the battery from

stress-induced damage. The Rensselaer team's nanoscoop, however, was engineered to

withstand this build up of stress. Made from a carbon (C) nano rod base topped with a thin

layer of nano scale aluminium (Al) and a "scoop" of nanoscale silicon (Si), the structures are

flexible and able to quickly accept and discharge Li ions at extremely fast rates without

sustaining significant damage. The segmented structure of the nanoscoop allows the strain to


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be gradually transferred from the C base to the Al layer and finally to the Si scoop. This

natural strain gradation provides for a less abrupt transition in stress across the material

interfaces, leading to improved structural integrity of the electrode. The nanoscale size of the

scoop is also vital since nanostructures are less prone to cracking than bulk materials,

according to Koratkar. Silicon (Si) has been envisioned as a promising anode material

because of its high theoretical capacity of~4200mAh/g based on the stoichiometry of the

alloy Li22Si5.The main limitation of this high capacity is an accompanying volumetric

expansion of ~400% for crystalline Si (or ~280% for amorphous Si) which results in

pulverization and delamination of the electrode structure. One interesting approach to dealing

with the stresses from Li-Si alloying has been the use of nano structured Si instead of bulk Si.

In addition to providing shorter Li-conduction distances, it is widely established that nano

structured Si has superior resistance to fracture because cracks do not reach their critical size

for propagation. Carbon-coated Si nano tubes have also been tested at rates as high as ~5C

(15 A/g). A very thin Si films (~40-50 nm thick) show good cycle ability at high C-rates

(>12C). However stress-induced cracking in thicker films limits the scalability of such

architectures. Deposited using a layer-by-layer self-assembly technique were found to deliver

high power and energy densities.

a).Composition of Carbon and Silicon

The compositions of carbon and silicon have been studied as anode materials since

carbon(C) forms a stable SEI while Si provides enhanced capacity. However there is a big

difference in the volumetric strains developed in C (~10%) and Si (~280% for amorphous Si)

due to their different Li uptake capacities. The interface formed between materials that

experience drastically different strain conditions have

high chances to fracture and is unable to accommodate

the rapid volume changes that occur under high rate

cycling conditions. By introducing materials between Si

and C that have intermediate volumetric strains, a natural

strain gradation can be developed in the multilayer

architecture.

b.) Use of aluminium

Aluminium (Al) is an intermediate layer between

C and Si to generate a functionally strain-gradedFigure 4 C-Al-Si Nanoscoops


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nanoscoop architecture that can provide average capacities of~412 mAh/g with a power

output of~100 kW/kg continuously over 100

cycles. Even when the power output is as high as

~250 kW/kg, the average capacity over 100

cycles is still ~90mAh/g. In all gravimetric

density calculations, the total mass of the

electrode (including C, Al, and Si) has been

considered. The choice of Alas the intermediate

layer in the strain-graded composite is justified by

the fact that in the lithiated condition it undergoes

~94% volumetric expansion based on the

stoichiometry of the alloy LiAl. Thus the strain in

Al is intermediate between that in C and Si.

Besides it is an inexpensive metal and can be easily deposited by physical vapour deposition

techniques. Al was initially proposed as the insertion anode material as soon as the Li

dendrite problem was identified in Li-ion batteries. However large volumetric strains resulted

in rapid capacity loss and thus Al was not a

preferred insertion material. In fact recent

reports show that even nano structured Al has

a rapid capacity loss at low rates of 0.5C.

Thus a thin layer of Al was chosen, enough to

just provide a strain gradient from carbon to

silicon in the lithiated state.

In this figure 6, the architecture

consists of an array of C nano rods (~170 nm

long) each with an intermediate layer of Al

(~13 nm thick) and finally capped by a scoop

of Si (~40 nm thick). Stainless steel (SS) is used as the conducting substrate. This figure

shows that C - Al Si strain graded anode architecture. The entire composite nanostructure

array deposited by dc magnetron sputtering with oblique angel deposition (OAD) and

required no patterning orlithography steps. This technique can efficiently generate an array

of composite nanoscoops on a large area in an inexpensive manner. They says in this figure,

Silicon wafer is used as the substrate for the cross-section Images (in Figure 5) since it is easy

Figure 6 C-Al-Si Nanoscoop structure

deposited on SS & SEM image show the

top view of the C-Al-Si Nanoscoop

Figure 5 XPS depth profile of the C-Al-Si

Nanoscoop


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to cleave. However, the morphology remains the

same even on stainless steel as the deposition

technique yields the same structure independent of

the type of substrate used. This is justified by the

fact that the top view SEM image of the C-Al-Si

nanoscoops on stainless steel (shown in Figure 7)

looks identical to those on Si wafer. The as

deposited C-Al-Si nanoscoop structure on stainless

steel was analyzed for depth profile with X-ray

photoelectron spectroscopy (XPS).This profile is in

agreement with the structure of the composite

nanoscoops. The XPS scans (not shown here) also confirmed the presence of surface oxide in

Si and Al with the majority being elemental Si, Al, and C. X-ray diffraction (XRD) pattern

from the as-deposited C-Al-Si sample shows the presence of amorphous C and Si along with

polycrystalline Aland lower intensity Fe peaks originating from the stainless steel.

In this figure7, under lithiated state C, Al, and Si would expand by ~10%, ~94%, and ~280%,

respectively. Such a strain gradation from C to ward Si would provide for a less abrupt

transition across the material interfaces. Besides, the C nano rod tips offer nano structured

Heavy interfaces. As a result the Si and the Al layers can relax their built-up strain by

undergoing out-of-plane displacements as opposed to a flat interface where strain relaxation

can occur only by delamination. All of the above factors contribute to highly stress resistant

interfaces between the C, Al, and Si in our nanoscoop architecture.

In the figure show the charge & discharge

voltage profiles between 0.05 and 2 V at

Current density of ~1.28 A/g(1C),~12.8

A/g , the electrode achieves a first

discharge capacity as high as

~1230.9mAh/g obtained from a weighted

average of the theoretical capacities of

carbon(372mAh/g based on LiC6),

aluminium (993 mAh/g based on LiAl),

and silicon (4200)mAh/g based on(Li22Si5). The voltage profile at 128 A/g (100C) shows capacitance behaviour and thus it is the

Figure 7 Strain graded multilayer

nanostructure

Figure 8. Charge & Discharge Profile


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limit of the operating rates. It is unlikely that even partial insertion of Lip occurs at C-rates of

the order of 100C, However, at rates as high as ~51.2 A/g(40C), partial insertion and removal

of Li+ occurs as can be seen from the plateau in the voltage profile.

This figure shows the differential

capacity curve as a function of voltage for the

first and 10th cycles. In the first cycle, the

discharge differential capacity curve shows

three main peaks at ~0.23, ~0.14, and~0.06 V.

The peak at 0.23 V could be attributed to the

formation of an amorphous LiSi phase. The

peak at 0.14 V could correspond to Li

intercalation in Al. The peak at 60 mV is

reported to correspond to the transformation of

amorphous LiSi to a rapid crystallization of

Li15Si4. During the charge cycle, there are two

peaks at 0.3 and 0.48 V which could potentially

correspond to delithiation into amorphous LiSi phases. For the 10th discharge cycle, the

peak at 0.14 V seems to disappear. This could indicate a potential shift in the Al. The peak at

60 mV is reported to correspond to the

transformation of amorphous LiSi to a rapid

crystallization of Li15Si4. During the charge

cycle, there are two peaks at 0.3 and 0.48 V

which could potentially correspond to

delithiation into amorphous LiSi phases. For

the 10th discharge cycle, the peak at 0.14 V

seems to disappear. This could indicate a

potential shift in the Al lithiation mechanism.

The charge cycle also shows a small peak

between 0.1 and 0.2 V. For the lithiation case,

the peaks of C appear to be shadowed by the Si

lithiation peaks in Lithiation mechanism the charge cycle also shows a small peak between

0.1 and 0.2 V. For the lithiation case, the peaks of C appear to be shadowed by the Si

lithiation peaks.

Figure 9 Differential Capacity Curve

Figure 10 Capacity as a function of cylce

index for C-Al-Si


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Figure 10 shows the performance of the C-Al-Si electrode against Li metal over 100

cycles at constant current densities of ~51.2 A/g

(40C), ~76.8 A/g (60C), and ~128 A/g (100C).

The potential window used for these constant

current charge/discharge tests was 0.05-2 V.

These C-rates were calculated based on the

theoretical capacity of the composite as

indicated earlier, i.e., 1C = 1.28 A/g. Even at a

very high C-rate of 40C (i.e., current density of

~51.2 A/g), the average capacity obtained over

100 cycles for the C-Al-Si system is ~412

mAh/g with a capacity fade of only ~0.2% per

cycle. When the current density is increased to

~76.8 A/g (i.e., C-rate of60C), the average

capacity over 100 cycles is ~330mAh/g with a

capacity retention of ~90% after 100 cycles.

Capacity as a function of cycle index shown for

C-Al-Si electrodes at ~51.2, ~76.8, and ~128 A/g over 100 cycles. The empty symbols

represent discharge capacity while the filled symbols represent charge capacity in each case.

Comparison at ~51.2 A/g current density of the charge/discharge capacity versus cycle

number for the C-Al-Si electrode versus an electrode comprised of only C nano rods. The

length and diameter of the C nano rods in the control sample are identical to those of the C

nano rods in the C-Al-Si multilayer structure.

The length and diameter of the C nano rods in the control sample are identical to those

of the C nano rods in the C-Al-Si multilayer structure. The charge/discharge capacities of the

C nano rods were compared to the C-Al-Si nanostructures at an accelerated current density

of~51.2 A/g. The average capacity of the C nano rods was ~140mAh/g. The ideal capacity of

~372mAh/g for C is practically observed only at low charge/discharge rates. At ultrahigh

rates, only partial lithiation occurs even for carbon, which explains the reduced capacity.

However carbon offers excellent stability with cycle number as is evident from Figure 11.

Figure 11 C-Al-Si electrode versus an

electrode comprised of only C Nano

rods


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c.) Compression Si-C Vs Si-Al-C

The significance of the intermediate Al layer is illustrated by comparing the discharge

capacity retention in the C-Al-Si system with a C-Si system (Figure m1). The C-Si system

consists of 170 nm long C nano rods with just a 40 nm thick Si nanoscoop and nointermediate Al layer. It can be seen that at the same C-rate of 60C, at the end of 100 cycles,

there is ~90%capacity retention in the C-Al-Si system while just ~60%capacity retention in

the C-Si system. This shows that including Al as an intermediate layer between C and Si

significantly improves the capacity retention.

Figure 12

Figure 13

Figure 12. Discharge capacity retention over the 100 cycle for the C-Al-Si architecture as

compared with the C-Si System. Inclusion of Al as an intermediate layer between C and Si

improves the capacity retention from ~ 60% to ~90% at the end of 100 cycles. Figure 13.

Comparison of discharge capacity between C-Al-Si System and a Si nanoscoop on a Cr nanorod

in the capacity Cr nanorod operated at ~ 63A/g.


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They are also tested (Figure 13) ~40 nm thick Si scoops deposited on 170 nm long

chromium (Cr) nano rods. It may be noted that the Cr nano rods do not alloy with Li and so

there is no strain gradation at the Si-Cr interface. For this case also, the capacity loss is severe

(~50% after 100 cycles) in comparison to the strain-graded C-Al-Si architecture. Comparison

of discharge capacity retention between C-Al-Si system (current density of ~77 A/g) and a Si

nanoscoop on a Carbon nano rod operated at ~63 A/g. Since Cr does not alloy with Li, there

is no volume expansion in Cr resulting in no strain gradation. There is a constant degradation

in the capacity all the way to 50% by the 100th cycle in the case of no strain gradation. These

two cases illustrate how the capacity degradation can be greatly reduced by using a C-Al-Si

strain-graded architecture.

Figure 14 Show the top view SEM image of the C-Al-Si composite nanoscoops on the

stainless steel substrate after high rate cycling at ~51.2 A/g (40C) until the 30th discharge

cycle. In this Image, the composite nanoscoops seem to be in an expanded state. This is

confirmed by the loss of spacing between the nanostructures in this state. In order to get a

clearer perspective of the volumetric change in these nanostructures.

Figure 15, show the cross-section SEM images of as-deposited C-Al-Si, sample

discharged to the first cycle at 1C (~1.28A/g) and sample discharged to the 30th cycle at

40C(~51.2 A/g). The as-deposited composite nanostructures roughly measure to be ~228 nm

in height and ~78 nm in diameter. After the first discharge (Li insertion) at 1C, the

nanostructures swell up to ~357 nm in height and ~136 nm in diameter. Approximating to a

cylindrical geometry, this volume increase corresponds to ~376%. Given that this discharge

was performed at a slow rate of ~ 1.28 A/g(1C), it is expected to see a drastic increase in

volume.

After the 30th discharge at 40C, the nanostructures measure to be ~314 nm in height

and ~86 nm in diameter. On the basis of the same cylinder geometry approximation, this

Figure 14 Top View SEM C-Al-SiFigure 15 Cross Section SEM image for

C-Al-Si Nanoscoop Structure on


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volume increase corresponds to ~67%. It should be noted that the calculated percentages of

volume increase is approx. C-Al-Si is not in cylindrical geometry; these give a comparison

between the volume changes at low and high rates. The fact that the volume increase is lower

at high rates is not surprising given that the capacity decreases at higher rates. However, it is

interesting to note that even at rapid rates such as ~51.2 A/g (40C), there exists charge

storage in the bulk which results in a volume increase. From the cross-section SEM image for

C-Al-Si at 40C (Figure 17), we can see that the C nano rod (dark contrast region) measures

~180 nm, while the Al and Si scoop together (light contrast region) measures ~135 nm. The

as-deposited C-Al-Si structure measures about 228 nm totally with C ~170 nm, Al and Si

together ~58 nm. This shows that after lithiation at 40C, the C nano rod itself expands

minimally while most of the expansion can be seen in the Al and Si region. Note that even

though there is a large volume increase, the C-Al-Si electrode is only partially lithiated at

40C. If the electrode was lithiating fully, the measured capacity at 40C should have been

~1280 and not ~412 mAh/g (Figure 12). Similarly the volume expansion that we physically a

large volume increase, the C-Al-Si electrode is only partially lithiated at 40C. If the electrode

was lithiating fully, the measured capacity at 40C should have been ~1280 and not ~412

mAh/g (Figure 12). Similarly the volume expansion that we physically measure from our

cross-sectional SEM imaging is ~67% at 40C as opposed to ~376% at 1C. However because

the capacity of Si is so high, even partial lithiation of Si has a dramatic impact on the capacity

performance (Figure 11) when compared with the control sample (C nanorod electrode). This

Figure 16 Profile After the first

discharge cycle at a rate 1.28 A/gFigure 17 Profile After the 30th discharge

cycle at a rate of 51.2 A/g


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is the reason why the C-Al-Si electrode can generate high energy density and high power

density simultaneously. We also observe that the expanded structure in the low rate case

(Figure 15) maintains the same aspect ratio as the as deposited nanostructure while the

expanded structure in the high rate case (Figure 16) shows a higher aspect ratio. We believe

that this difference in isotropic (low rate) versus anisotropic (high rate) volume change

indicates key differences in the Li+ flux in different directions. In the high rate case, there is

probably a higher Li+ flux in the vertical direction than the lateral direction due to presence

of a high local electric field. A similar cross section image was taken from a C-Al-Si sample

discharged to the first cycle at 40C. The volume increase was much lower for this case. Thus

it is clear that it takes several cycles to see large volume changes in the structure. This means

that within the first ~30 cycles, the capacity keeps increasing due to more bulk inclusion of

Li. This gradual increase in the Li uptake within the first ~30 cycles could be responsible for

the observed increase in specificcapacity with cycle number (Figure 10). Figure 17 shows the

XPS depth profile after cycling the C-Al-Si nanostructures at a rate of ~51.2 A/g (40C) up to

the 30th discharge cycle. XPS was alternated with Ar+ sputtering cycles each lasting 30 s.

The sputter rate could not be calibrated to obtain the depth since the composite nano rods

consisted of three materials with different sputtering rates. This is the reason why we report

data only in terms of sputtering time and not in terms of depth. However, the XPS depth

profile (Figure 18) shows clear peaks for the three different regions (Si, Al, and C) as a

function of sputter time. Overall, the first ~25-30 min of sputtering showed a major presence

of LiF and Li2CO3 with a minor presence of poly (ethylene oxide) (PEO)-type oligomers with

the structure of -( CH2CH2O)n- and alkyl

fluorocarbons all of which form the

composition of the solid electrolyte inter

phase (SEI). These SEI compounds were

identified from high-resolution scans of C

(1s), O (1s), F (1s), and Li (1s). The peak

assignmentswere made based on published

literature and the online NIST XPS database.

The SEI composition reported here is

in agreement with the XPS analysis of SEI on

silicon nano wires reported earlier. The Li

(1s) signal shows a peak at ~57.25eV beyond

Figure 18 X ray Photoelectron

Spectroscopy depth profile after cyclingthe C-Al-Si


17/19

Nanoscoop Page 17

~23 min of sputter time, which could possibly pertain to LiC6 based on the binding energy of

57.1eV obtained from the NIST database. Other alloy states such as Li-Si or Li-Al would be

difficult to identify due to the lack of information on their binding energies. The C curve on

the XPS depth profile shows an increasing atomic concentration with sputtering time as the

base of the composite nanostructure is carbon which is attached to the stainless steel

substrate. Si and Al curves show their presence somewhere between 10 and 20 min of

sputtering time. This is in agreement with the position of Si and Al on the top of the C

nanorod. Importantly the Li curve shows an overall decreasing trend starting from the Si, Al,

and finally into the C regions. If the Li was being inserted into C and not Si, then there would

be Li due to SEI only in Si and Li due to SEI plus alloying in the C region. Thus one would

expect a clear increasing trend of Li concentration with depth as one move from the Si into

the carbon region. However, we see the opposite trend, with the Li concentration decreasing

markedly as we transition from the Si into the Al and C regions. This means we now have a

case of SEI plus alloy in both Si and C and since the capacity of Si is higher than C, the

overall Li concentration is observed to be higher in Si than in C. This is also consistent with

the cross-sectional SEM results for volume expansion shown previously in Figure 15-17.

In summary, we report a novel functionally strain-graded C-Al-Si nanoscoop anode

architecture that can achieve average capacities of ~ 412 mAh/g with a power density of ~100

kW/kgelectrode (current density of ~ 51.2 A/g) continuously through 100 cycles. We also

show that the C-Al-Si composite can yield power densities as high as ~250 kW/kg electrode

(current density of ~128 A/g) continuously over 100 cycles with an average capacity of ~90

mAh/g. The C-Al-Si architecture has the potential for mass scalability by increased

deposition time as well as the possibility of stacking C-Al-Si nanostructure films on

intermediate carbon thin film supports. When the mechanism of charge generation involves

alloying with the host material and the demand for current is high, the electrode architecture

is put through large strain rates accompanied by rapid volume changes. In such a situation, a

functionally strain-graded structure could potentially undergo rapid volume changes with

reduced possibility of interfacial cracking or delamination. By building a strain graded

structure, it is possible to eliminate interfaces between materials that have a large strain

difference during lithiation. Low strain difference between adjacent materials in the

composite leads to highly efficient alloy-de alloy reactions preserving the overall structural

integrity of the electrode. To further improve the strain gradient, we could potentially insert

materials such as Sb (strain of ~147%) or As (strain of ~201%) between Al and Si. This will


18/19

Nanoscoop Page 18

also help in increasing the area mass density while still improving the performance. Such

strain-graded multilayer anode architectures show significant potential for the design of high

power and high capacity Li-ion rechargeable batteries.

4. FUTURE PROSPECTUSTo further improve the strain gradient, we could potentially insert materials such as

Sb (strain of ~147%) or as (strain of ~ 201%) between Al and Si. This will also help in

increasing the area mass density while still improving the performance. Such strain-graded

multilayer anode architectures show significant potential for the design of high power and

high capacity Li-ion rechargeable batteries.


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Nanoscoop Page 19

5. Reference

1. Rahul Krishnan, Toh-Ming Lu, and Nikhil Koratkar Functionally Strain-Graded

Nanoscoops for High Power Nano Latter December 30, 2010.

2. Pushparaj, V. L.; Shaijumon, M. M.; Kumar, A.; Murugesan, S.; Ci, L.; Vajtai, R.;

Linhardt, R. J.; Nalamasu, O.; Ajayan, P. M. Proc. Natl. Acad. Sci. U.S.A. 2007, 104,

13574.

3. Maranchi, J. P.; Hepps, A. F.; Evans, A. G.; Nuhfer, N. T.; Kumta, P. N. J.

Electrochem. Soc. 2006, 153, A1246A1253.

4. Kang, K.; Meng, Y. S.; Breger, J.; Grey, C. P.; Ceder, G. Science 2006, 311, 977980

5. Chuan Cai and Ying Wang Novel Nanocomposite Materials for Advanced Li-Ion

Rechargeable BatteriesMaterials2009, 2, 1205-1238; doi:10.3390/ma2031205

6. Shulei Chou University of Wollongong Nano/composite materials for lithium-ion

batteries and supercapacitors P. Hd Thesis 2010.

7. Wu, X.-L.; Jiang, L.-Y.; Cao, F.-F.; Guo, Y.-G.; Wan, L.-J. Adv. Mater. 2009, 21,

27102714.

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