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1. Introduction
A very fast depletion of world oil reserves and their related hazards have propelled
great advancements in hybrid and all electric auto- mobiles. 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
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~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
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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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5. Reference
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Nanoscoops for High Power Nano Latter December 30, 2010.
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