Seminar Report 2011-2012 Nanowire batteries for next generation electronics ABSTRACT The scaling of electronic devices also requires the evolution of high energy density power sources. By using nanowires, high charge storage materials, which otherwise have mechanical breakage problems due to large structure transformations and volume changes, can be adopted as electrode materials. High power operation can also be possible due to the short lithium insertion distances in the nanowires. We have studied Si and Ge nanowires and demonstrated charge storage capacities several times higher than the graphite anodes used in existing battery technology.LiMn2O4 nanorod cathodes were found to show much higher power rates than commercial powders. Detailed morphology and structure characterization have shown that these improvements are attributed to facile strain relaxation, good electronic contact and conduction, and short Li insertion distances in the nanowire battery electrode. We also developed a Langmuir-Blodgett assembly technique to produce nanowire pillars as battery electrodes, which opens up the possibility for the fabrication of on-chip battery power sources. Dept. of AEI IESCE
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Seminar Report 2011-2012 Nanowire batteries for next generation electronics
ABSTRACT
The scaling of electronic devices also requires the evolution of high energy density
power sources. By using nanowires, high charge storage materials, which otherwise have
mechanical breakage problems due to large structure transformations and volume changes,
can be adopted as electrode materials. High power operation can also be possible due to the
short lithium insertion distances in the nanowires. We have studied Si and Ge nanowires and
demonstrated charge storage capacities several times higher than the graphite anodes used in
existing battery technology.LiMn2O4 nanorod cathodes were found to show much higher
power rates than commercial powders.
Detailed morphology and structure characterization have shown that these
improvements are attributed to facile strain relaxation, good electronic contact and
conduction, and short Li insertion distances in the nanowire battery electrode. We also
developed a Langmuir-Blodgett assembly technique to produce nanowire pillars as battery
electrodes, which opens up the possibility for the fabrication of on-chip battery power
sources.
Dept. of AEI IESCE
Seminar Report 2011-2012 Nanowire batteries for next generation electronics
INTRODUCTION
The scaling of electronic devices such as transistors,memories and hard-disks has
induced a revolution in the electronics industry. For portable electronics such as cell phones
and laptops, it is necessary to have corresponding power sources to evolve as well. Li-ion
rechargeable batteries have been the dominating power source. To match the future scaling of
electronics, higher energy density (and specific energy) rechargeable batteries are desirable.
The existing Liion battery technology consisting of a graphite anode (370 mAh/g) and
LiCoO2 cathode (170 mAh/g) has limited charge storage capacity and energy density,
making it necessary to explore new electrode materials. There are several high storage
capacity materials suitable for making a higher energy density anode. For example, Si and Ge
can alloy with large amounts of lithium to give theoretical capacities of 4200 mAh/g and
1600 mAh/g, respectively. However, one common problem of high charge storage materials
is that the alloying process results in large structural transformations and volume changes. In
bulk materials, these large volume changes can cause the electrode to crack and pulverize
(Fig. 1a).
Dept. of AEI IESCE
Seminar Report 2011-2012 Nanowire batteries for next generation electronics
Figure 1. Schematic of morphological changes that occur in Si duringelectrochemical cycling. A) Si films and particles tend to pulverize during cycling, resulting in poor transport of electrons, as indicated by the arrow. B) Facile strain relaxation in the NWs allows them to
increase in diameter and length without breaking.
Often times, this leads to material losing electronic contact to the current collector
over time, which results in poor cycle life. Using the material in a nanowire (NW)
morphology has several advantages. First, the small NW diameter allows for better
accommodation of the large volume changes without the initiation of fracture that can occur
in bulk or micron-sized materials. Second, NWs have direct 1D electronic pathways allowing
for efficient charge transport. One drawback to using nanoparticles, for example, is that they
must be assembled into a composite containing conducting carbon and binders to maintain
good electronic conduction throughout. Electronic charge carriers must move through small
interparticle contact areas in these electrodes, but in nanowire electrodes the carriers can
move efficiently down the length of each wire.
Finally, several nanowire synthesis methods allow for vertically oriented growth on
various types of substrates. Thus, nanowires can be grown directly on the metallic current
collector. This is a clear advantage as every NW is connected to the currentcarrying
electrode, and moreoever the need for binders or conducting additives, which add extra
weight, is eliminated. We have explored the use of nanowires of high capacity materials for
battery electrodes (1). By using Si and Ge nanowires grown using the vapor-liquid-solid
growth as an example, we showed that NWs do not break when undergoing large structural
and volume changes. Both Si and GeNWs can provide a charge storage capacity close to their
theoretical capacities, with SiNWs displaying a capacity 10 times higher than in graphite
anodes. We also demonstrated a Langmuir- Blodgett assembly technique to produce SiNW
pillars as battery electrodes, which opens up the possibility for the fabrication of on-chip
battery power sources.
Dept. of AEI IESCE
Seminar Report 2011-2012 Nanowire batteries for next generation electronics
RESULTS
Li insertion into the SiNWs was found to exhibit higher capacities than other forms
of Si or graphite. Fig. 2a shows the first and second cycles at the C/20 rate (20 hours to
charge or discharge). The voltage profile observed was consistent with previous studies on Si
anodes, with a long flat plateau during the first charge, during which amorphous LixSi is
formed from crystalline Si. Subsequent discharge and charge cycles have different voltage
profiles, characteristic of amorphous silicon. The discharge capacity was ~3100 mAh/g with
little fading over 10 cycles and a coulombic efficiency >90% (Fig. 2b). As a comparison, our
data are shown along with those reported for thin films containing 12 nm Si nanocrystals
(NCs) and graphite carbon in Fig. 2b. This improved capacity and cycle life in the SiNWs
indicates the advantage of their small diameter. The SiNWs displayed a good power
performance as well. Fig. 2c shows the charge and discharge curves observed at 10 hr (C/10),
5 hr (C/5), 2 hr (C/2), and 1 hr (1C) rates. Even at the 1C rate, the capacities remained >2100
mAh/g, which is ~ 6 times of graphite. Under constant capacity charging (1043 mAh/g, >3
times of graphite), we have been able to achieve 145 cycles with 95% capacity retention,
which shows promise for commercialization(Fig. 2d).
Figure 2a. The voltage profiles for the first and second galvanostati
Dept. of AEI IESCE
Seminar Report 2011-2012 Nanowire batteries for next generation electronics
cycles of the SiNWs
Figure 2b. Capacity vs. cycle number for the SiNWs at the C/20 ratecompared to Si nanocrystals and graphite.
Figure 2d. Cycling of SiNWs using a fixed charge of 0.12 mAh (1043 mAh/g).
Dept. of AEI IESCE
Seminar Report 2011-2012 Nanowire batteries for next generation electronics
To understand the high capacity and good cyclability ofour SiNW electrodes, we
studied the structural morphology changes. Fig. 3a shows scanning electron microscopy
(SEM) images of SiNWs before and after lithiation.
Fig. 3b shows the change in the diameter distribution of the NWs. The diameter of
the SiNWs expands by 50% after Li insertion. An important observation is that despite the
large volume change,the SiNWs remain intact and have not broken into smaller particles. The
SiNWs also change their length during the volume change (Fig. 3c). To evaluate this, 25 nm
Ni was evaporated onto the SiNWs before cycling to serve as an inert backbone. Afterwards,
the morphology of the SiNWs suggested that length expansion of the NW was impeded by
connection to the Ni, instead leaving the NW in a helical shape around the Ni. With both a
diameter and length increase, the SiNW volume change after Li insertion appears to be about
300%. The detailed transmission electron microscopy (TEM) studies (Fig. 3d) showed that
the SiNWs change from a single crystalline to an amorphous structure during lithiation in the
first cycle and remain amorphous thereafter.
Figure 3a. Scanning electron microscopy images of SiNWs before and after cycling.
Dept. of AEI IESCE
Seminar Report 2011-2012 Nanowire batteries for next generation electronics
Figure 3b. Size distribution of NWs before and after charge to 10mV (bin size 10 nm). The average diameter of the NWs increased from 89 to 141 nm.
Figure 3c.Transmission electron microscope image of a pristine SiNW with a partial Ni coating before and after Li cycling.
Dept. of AEI IESCE
Seminar Report 2011-2012 Nanowire batteries for next generation electronics
Fig. 3d. Transmission electron microscopy images of SiNW becoming amorphous during lithiation
We have developed a CMOS compatible process to fabricate SiNW battery electrodes
for on-chip power sources. As shown in Fig. 4, the Langmuir-Blodgett method is used to
assemble SiO2 nanoparticles on a Si wafer. We have obtained monolayers of particles
covering a 4” wafer surface. The nanoparticles, which function as an etch mask, can have
diameters controllably tuned using reactive ion etching (RIE). RIE was also used to produce
vertical nanopillars with controllable diameters and spacings from 50nm to 1000nm. We have
exploited these NW pillars as well-defined anodes for lithium batteries.
Dept. of AEI IESCE
Seminar Report 2011-2012 Nanowire batteries for next generation electronics
Figure 4. Schematic representation of the fabrication of Sinanopillars with corresponding scanning electron microscope
images.
Finally, the general advantages of NW battery electrodes shown in this paper have
also been demonstrated in other materials. We have also demonstrated high capacity GeNW
anodes (2) and high rate LiMn2O4 nanorods cathodes (3). The LiMn2O4 nanorods were
found to display high charge storage capacities at high power operation with good
reversibility and cyclability (Fig. 5). The nanorods performed significantly better than
commercially available powders with particle sizes around 10 μm at the higher rates.
Dept. of AEI IESCE
Seminar Report 2011-2012 Nanowire batteries for next generation electronics
Figure 5. Scanning electron microscope image of LiMn2O4 nanorods. Evaluation of the
nanorods at high power rates showed better capacity retention compared to commericial
powders (particle size ~ 10 μm).
CONCLUSIONS
Dept. of AEI IESCE
Seminar Report 2011-2012 Nanowire batteries for next generation electronics
We have found that Si and Ge nanowires can perform as suitable high capacity
anodes for Li-ion batteries. The good structural properties of the NWs allows for large
volume changes to occur without pulverization. We have also found that LiMn2O4 nanorods
can display better power operation than bulk commercial powders. Having shown these
systems as examples, we believe that nanowire battery electrodes have the the potential to
greatly improve the energy and power delivered to the class of next generation electronics.
Dept. of AEI IESCE
Seminar Report 2011-2012 Nanowire batteries for next generation electronics
ADVANTAGES OF THE NANOWIRE BATTERY
Many advantages arise with the use of the silicon nanowire battery. This battery can
be used with laptops, iPods, cell phones, digital cameras and video cameras. Although the
technology has evolved tremendously, companies with traveling businessmen, the movie
industry, delivery trucks and perhaps even hospitals can benefit from this battery.
Of course, computers are essential to all businesses. A laptop is used by many
business people as they save many company documents that they may need outside of the
office. Business people who have to make a presentation or people who are frequent fliers
will benefit from a longer battery life. It can be risky to rely on laptops as it is very difficult
to find a wall plug when needed. In the short run, this battery can create a competitive
advantage for companies who manufacture laptops with this new battery. Also, business
people will always be able to remain in touch with their companies with cell phone that hold
a longer battery life as well. Consumers’ interest in laptops that hold a charge for twenty
hours will encourage people to replace their laptops causing sales to increase. In the long run,
this battery will probably become the standard battery of a laptop.
Another industry that can benefit from the silicon nanowire battery is the movie
industry or filming companies as the battery holds the charge of video cameras as well. While
filming a movie, people will no longer have to worry about tripping over wires or moving too
far away from the wall plug. Also, people who film parties or weddings will no longer have
to worry about being obtrusive to the guests at the party.
In the future, this battery can work in the favour of delivery trucks. If the battery is
going to work on electric cars, there is a possibility of it being beneficial to electric trucks as
well. Using the silicon nanowire battery, delivery trucks will be able to drive a must longer
distance without needing to fill up for gas or charge their truck. This can save companies a lot
of money as many companies have numerous trucks on the road simultaneously.
Further, the battery might be used for medical equipment in the future. Hospitals that
have machines running on electricity will be able to perform surgeries when there is a power
failure. With the silicon nanowire battery, machines will be able to last for hours without
having to be recharged.
Dept. of AEI IESCE
Seminar Report 2011-2012 Nanowire batteries for next generation electronics
DISADVANTAGES OF THE NANOWIRE BATTERY
As with every new product, there are several disadvantages to the new silicon
nanowire battery; car companies will have to design new cars, products operating on lithium-
ion batteries will become obsolete.
First, if electric cars become the way of the future, car companies will have to invest a
lot of money into product design. Although electric cars exist today, car companies will have
to design a variety of new models to incorporate the new nanowire battery. Since consumer’s
interest in cars that run on gas will severely decrease, it will be necessary for companies to
introduce many cars with the nanowire battery.
Second, products made with a lithium-ion battery will be considered obsolete. Once
the silicon nanowire batteries become popular, like other technologies, consumers will not
want to purchase them. This is a disadvantage for technology companies as they will have to
redesign their products incorporating the nanowire battery. This can be become expensive
and many small to medium size businesses may not be able to afford this. Technology is ever
changing and companies have to be able to keep up with the times to stay afloat.
Third, at first, most companies will want to create products using the silicon nanowire
battery to test it in the market. Since it will be a on trial basis, cell phone companies or Apple
may only make a limited number of cell phones or iPods using the new battery. If advertised
well, many consumers will want to purchase the product once it is on sale. The products with
the nanowire battery will probably sell out quickly causing many companies to have a
waiting list or back-orders. Unfortunately, this causes companies to lose out on potential
profits. Companies will have to devote a lot of resources to creating these new products and
try to not run out of stock too quickly.
Dept. of AEI IESCE
Seminar Report 2011-2012 Nanowire batteries for next generation electronics
THE SILICON NANOWIRE BATTERY
Have you ever been in the middle of an important phone call on your cell phone and a
couple minutes into the call your phone battery just died? A new information technology will
change rechargeable batteries for our gadgets as we know it.Stanford researchers discovered
a way to create the new silicon nanowire battery which is a rechargeable battery that can hold
ten times more power than the batteries used today.
Dr. Yi Cui, Assistant Professor of Material Science and Engineering
at Stanford University invented this revolutionary development. In an interview, he explained
that silicon nanowires have been around for quite some time but they have never been applied
to batteries before. He has filed for a patent and hopes to partner up with a battery
manufacturing company to bring the new silicon nanowire battery into the market soon.