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).

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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

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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.

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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.

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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.

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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.

Dept. of AEI IESCE

Seminar Report 2011-2012 Nanowire batteries for next generation electronics

WHAT CAN THE NANOWIRE BATTERY REPLACE?

The silicon nanowire battery is expected to replace the lithium-ion battery. For

example, with the lithium ion battery, a fully charged laptop can last for approximately two

hours. Using the nanowire battery, a laptop’s battery life will last ten times longer; hence the

battery life will be extended to twenty

hours!

The lithium-ion battery’s anode,

usually made with carbon material, has a

limited quantity of lithium it can store.

Stanford researchers discovered that if the

carbon is replaced by silicon nanowires,

the battery can store ten times more

lithium allowing the battery to have a

much longer battery life. 

When the battery is charged, Dr. Yi Cui and his staff realized that as the battery

absorbs positively charged lithium components, the silicon swells. When the battery is in use,

the lithium atoms are released from the silicon allowing the silicon to shrink. This growing

and shrinking can be compromising to the battery performance. For this

reason,nanotechnology is implemented meaning lithium is placed in tiny nanowires allowing

for the silicon to resist breakage.

A nanowire battery is a lithium-ion battery invented by a team led by Dr. Yi

Cui at Stanford University in 2007. The team's invention consists of a stainless

steel anode covered in silicon nanowires, to replace the traditional graphite anode. Silicon,

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Seminar Report 2011-2012 Nanowire batteries for next generation electronics

which stores ten times more lithium than graphite, allows a far greater energy density on the

anode, thus reducing the mass of the battery. The large surface area further allows for fast

charging and discharging.

DESIGN

Traditional silicon anodes were researched and dismissed due to the tendency of

silicon to crack and become unusable because it swelled with lithium during operation. The

nano-wires do not suffer from this flaw. According to Dr. Cui, the battery reached 10x

density on the first charge  and plateaued to 8x density on subsequent charges. In order to

take advantage of this anode advancement, an equivalent  cathode advancement is required to

achieve the increased storage density.

Commercialization is expected to occur in 2012  with the batteries costing the same or

less per watt hour than conventional lithium-ion batteries. The next milestone, life cycle

testing, should be completed and the team expects to achieve at least one thousand charge

cycles from nano-wire batteries. In September 2010, Dr. Yi Cui's team demonstrated that 250

charge cycles are possible before the charge capacity drops below 80 percent of its initial

storage capacity. The team expects to reach 3,000 charge cycles by 2012. Reaching this goal

would make nano-wire batteries viable for use in electric vehicles. A prototype for use in

cellular phones and other electronic devices was expected to be delivered by the first quarter

of 2011.

Dept. of AEI IESCE

Seminar Report 2011-2012 Nanowire batteries for next generation electronics

POTENTIAL PROBLEM

The very high surface area of the nanowires, which allows high charging rates, also

has a downside: heterogeneous side reactions.These will occur as the nanowires on the

negative electrode are brought below around +0.8 V, where the electrolyte becomes

thermodynamically unstable and starts getting reduced .The result will be a film made from

decomposition products that coats the surfaces of the nanowires.This coating, called a "solid

electrolyte interphase (SEI)," is present in all Li-ion batteries that use conventional

electrolytes and low voltage electrodes such as graphite or silicon. Typically, the active

particles on the negative electrode side (graphite) are around 10 microns in diameter. While

such large sizes extract a penalty by lowering the surface area and power, that size is

necessary in order to reduce the amount of SEI formed (which is proportional to the surface

area). Even so, 5-10% of all of the Li in a Li-ion battery ends up incorporated into the SEI,

leading to an irreversible capacity loss (ICL) of that amount. (The source of the Li in a cell is

mainly the positive electrode, such as LiFePO4.) Fortunately, the SEI formation reactions are

self-limiting, and after the first cycle ICL can be very small.

On the other hand, a nanowire might have a couple of orders of magnitude more

surface area per unit volume than a 10 micron particle, which would result in a couple of

orders of magnitude more SEI formed—except that there is not enough Li in the positive

electrode to make this much SEI. The result of this loss of accessible Li would be a drastic

loss of capacity after the first cycle. Nanowire cells can nevertheless cycle hundreds of times

in half-cells. In a half cell, an electrode made from a piece of Li metal would be cycled

against the nanowires. Since in a half cell there is a nearly unlimited supply of Li, capacity

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Seminar Report 2011-2012 Nanowire batteries for next generation electronics

need never decline. Such half cells, however, would have no commercial value. There are

tricks that can be employed to reduce ICL—for example, by pre-forming the SEI before

assembling the cell. However, this process is not done commercially because of the high cost

of adding such a processing step.

NANOWIRE

A nanowire is a nanostructure, with the diameter of the order of a nanometer

(10−9 meters). Alternatively, nanowires can be defined as structures that have a thickness or

diameter constrained to tens of nanometers or less and an unconstrained length. At these

scales, quantum mechanical effects are important — which coined the term "quantum wires".

Many different types of nanowires exist, including metallic (e.g., Ni, Pt, Au), semiconducting

(e.g., Si, InP,GaN, etc.), and insulating (e.g., SiO2, TiO2). Molecular nanowires are composed

of repeating molecular units either organic (e.g. DNA) or inorganic (e.g. Mo6S9-xIx). The

nanowires could be used, in the near future, to link tiny components into extremely

small circuits. Usingnanotechnology, such components could be created out of chemical

compounds.

Overview

Typical nanowires exhibit aspect ratios (length-to-width ratio) of 1000 or more. As

such they are often referred to as one-dimensional (1-D) materials. Nanowires have many

interesting properties that are not seen in bulk or 3-D materials. This is because electrons in

nanowires are quantum confined laterally and thus occupy energy levels that are different

from the traditional continuum of energy levels or bands found in bulk materials.

Peculiar features of this quantum confinement exhibited by certain nanowires

manifest themselves in discrete values of the electrical conductance. Such discrete values

arise from a quantum mechanical restraint on the number of electrons that can travel through Dept. of AEI IESCE

Seminar Report 2011-2012 Nanowire batteries for next generation electronics

the wire at the nanometer scale. These discrete values are often referred to as the quantum of

conductance and are integer values of

They are inverse of the well-known resistance unit h/e2, which is roughly equal to

25812.8 ohms, and referred to as the von Klitzing constant RK (after Klaus von Klitzing, the

discoverer of exact quantization). Since 1990, a fixed conventional value RK-90 is accepted.

Examples of nanowires include inorganic molecular nanowires (Mo6S9-xIx,

Li2Mo6Se6), which can have a diameter of 0.9 nm and be hundreds of micrometers long.

Other important examples are based on semiconductors such as InP, Si, GaN, etc., dielectrics

(e.g. SiO2,TiO2), or metals (e.g. Ni, Pt).

There are many applications where nanowires may become important in electronic,

opto-electronic and nanoelectromechanical devices, as additives in advanced composites, for

metallic interconnects in nanoscale quantum devices, as field-emitters and as leads for

biomolecular nanosensors.

Dept. of AEI IESCE

Seminar Report 2011-2012 Nanowire batteries for next generation electronics

SYNTHESIS OF NANOWIRES

There are two basic approaches of synthesizing nanowires: top-down and bottom-up

approach. In a top-down approach a large piece of material is cut down to small pieces

through different means such as lithography and electrophoresis. Whereas in a bottom-up

approach the nanowire is synthesized by the combination of constituent ad-atoms. Most of

the synthesis techniques are based on bottom-up approach.Nanowire structures are grown

through several common laboratory techniques including suspension, deposition

(electrochemical or otherwise), and VLS growth.

Dept. of AEI IESCE

Seminar Report 2011-2012 Nanowire batteries for next generation electronics

USES OF NANOWIRES

Nanowires still belong to the experimental world of laboratories. However, they may

complement or replace carbon nanotubes in some applications. Some early experiments have

shown how they can be used to build the next generation of computing devices.To create

active electronic elements, the first key step was to chemically dope a semiconductor

nanowire. This has already been done to individual nanowires to create p-type and n-type

semiconductors.

The next step was to find a way to create a p-n junction, one of the simplest electronic

devices. This was achieved in two ways. The first way was to physically cross a p-type wire

over an n-type wire. The second method involved chemically doping a single wire with

different dopants along the length. This method created a p-n junction with only one wire.

After p-n junctions were built with nanowires, the next logical step was to build logic

gates. By connecting several p-n junctions together, researchers have been able to create the

basis of all logic circuits: the AND, OR, and NOT gates have all been built from

semiconductor nanowire crossings. It is possible that semiconductor nanowire crossings will

be important to the future of digital computing. Though there are other uses for nanowires

beyond these, the only ones that actually take advantage of physics in the nanometer regime

are electronic.

Nanowires are being studied for use as photon ballistic waveguides as interconnects

in quantum dot/quantum effect well photon logic arrays. Photons travel inside the tube,

electrons travel on the outside shell.When two nanowires acting as photon waveguides cross

each other the juncture acts as a quantum dot.

Conducting nanowires offer the possibility of connecting molecular-scale entities in a

molecular computer. Dispersions of conducting nanowires in different polymers are being

investigated for use as transparent electrodes for flexible flat-screen displays.

Dept. of AEI IESCE

Seminar Report 2011-2012 Nanowire batteries for next generation electronics

Because of their high Young's moduli, their use in mechanically enhancing

composites is being investigated. Because nanowires appear in bundles, they may be used as

tribological additives to improve friction characteristics and reliability of electronic

transducers and actuators. Because of their high aspect ratio, nanowires are also uniquely

suited to dielectrophoretic manipulation.

Dept. of AEI IESCE