Innovations in Battery Technology for Portable Devices

Abstract This white paper reviews several significant recent trends in the battery industry. While the Lithium-ion (Li-ion) market is quickly overtaking other rechargeable battery technologies, in popularity, there have been a few additions to the selection of Li-ion batteries available. Most noteworthy are: the replacement of Cobalt-based cathodes with alternatives; the availability of high current cells to the general market; and the rapid adoption of Lithium polymer cells. After an overview of the basics of pack and cell construction and chemistry including choices and trade-offs, this paper will review recent innovations in cells and what these innovations mean to designers of portable and mobile devices.

Background What can be seen by looking at this graph of market data from Fredonia is that many applications are adopting Li-ion technology because of its many advantages. Sealed Lead Acid (SLA) is the biggest battery market today and is predicted to grow in the future, but the Li-ion market (shown in red) has already overtaken the Nickel market and is predicted to be almost as big as the SLA market by 2017. Li-ion is clearly the solution of choice for many applications.

Based on weight and size, Li-ion technology offers very high energy density as compared to older chemistries. Li-ion cells store and deliver more energy than other rechargeable batteries and have better cycle life. There is a wide variety of Li-ion cells. Li-ion is best thought of as a family of chemistries that share the working lithium ion shuttling between anode and cathode. Several variations of Li-ion technology are on the market with various assembly methods and cathode and anode chemistries. Li-ion battery cells operate at higher

voltages than other rechargeables, typically about 3.6 volts, versus 1.2 volts for Ni-Cd or NiMH and 2V for SLA.

The voltage output of the battery pack is increased by adding cells that are wired in series. The capacity is increased by adding cells in parallel. The higher voltage of Li-ion means a single cell can often be used rather than the multiple cells required when using older technology batteries.

Another key difference is that Li-ion has a self-discharge rate that is three times less than Nickel, so the shelf life is longer. Li-ion battery packs are not a simple configuration of cells. They are carefully engineered products with many safety features. The main components of a battery pack are shown below.

The cells are the fundamental building blocks that store the energy. The chemistry chosen for the anode, cathode and electrolyte determine the cell’s voltage, capacity and current capability. Other components of the cell are built to enhance and enable the desired features of the chemistry combination. For example, the current collectors will be sized in order to have high energy density or high power delivery.

The external cell packaging can be a variety of sizes and a couple shapes: cylindrical or prismatic or brick shaped.

1

Innovations in Battery Technology for Portable Devices

The Li-ion market has already overtaken the Nickel market.

Internal Construction of Lithium-Ion Cell

At the cell level, safety mechanisms include:

• The Separator, which is perhaps the most fundamental safety feature, protecting against shorts between the anode and cathode material. Multi-layer separators and ceramic coatings on electrode surfaces provide an extra level of security and safety.

• The PTC, which provides the function of both a current fuse and a thermal fuse. When over-current flows through the PTC, self-heating increases the resistance and breaks the circuit.

• The Current Interrupt Device, which works with the pressure release valve to release or vent excess gas if internal pressure reaches an unsafe level during charging.

• A Pressure Release Scribe or center vent tube to guide any pressure build-up at the bottom of the cell up to the pressure release valve located at the top of the cell. These eliminate ruptures in the can due to excess pressure that could be trapped in the lower portion of the can.

Cell size has a lot to do with the run time of a battery. However, in comparing cylindrical versus prismatic, keep in mind that the thinner the cell is, the larger percentage of packaging material there will be.

2

Innovations in Battery Technology for Portable Devices

CYLINDRICAL:

• Advantage: Stability of design over time and standard sizes (18650 and 26650). It is important to note that not all cell manufacturers follow these rules; there are some who try to stretch these a little to gain more run time out of a classified cell size. The best thing to do is to design around the largest available size to keep options open.

• Advantage: Cost is lower since this is where the volume is and where R&D dollars have historically gone.

• Advantage: Higher watt-hour output. Currently 18650s are available in over 3Ah per cell

• Disadvantage: Inter-cell packing inefficiencies.

LITHIUM POLYMER:

• Advantage: Thin and great packing density possible for some products.

• Drawback: Swelling issues and associated mechanical problems.

• Drawback: Higher costs because more difficult to manufacture.

• Drawback: Less energy density.

• Drawback: Form factor changes quickly with market. It is important to avoid obsolescence by using common footprints.

PRISMATIC:

• Advantage: Thin and great packing density possible for some products.

• Drawback: Swelling issues and associated mechanical problems.

• Drawback: Higher costs because more difficult to manufacture.

• Drawback: Less energy density.

• Drawback: Form factor change quickly with market. It is important to avoid obsolescence by using common footprints.

This is a cross section of an 18650 cell, 18 mm in diameter and 65 mm long, with its safety features highlighted. Lithium ions shuttle between the anode and cathode, corresponding to the current through the external circuit.

Battery TrendsIndustries like medical and military are increasingly relying on portable and mobile devices. The diverse requirements of applications in these industries are pushing the boundaries of the capabilities of traditional batteries. Military applications typically require low temperature operation. Medical applications require a high state of readiness and quality state of charge indicators. Oil exploration requires high temperature operation. In reality, these industries are niche markets for batteries.

New chemistry development is slow and driven by huge volume applications. The first applications to demand high quality and good energy density rechargeable batteries were consumer electronics like laptops, cell phones, PDAs, etc. The drive is for high energy density in relatively small form factors. Cycle life is not a huge concern since these products are not expected to last. However, in the emerging electric vehicle market, energy density is important because that gives the vehicle a long range, but cycle life and safety are taking on increased importance. No one wants to replace the vehicle’s battery every two years. Also, there is a huge amount of energy stored in the car’s battery so it is important to emphasize safety.

In the future, it is likely that a decreased reliance on fossil fuel will require an evolution in battery technology since all the energy generated by wind farms and the like will need storage and grid stabilization.

Cathode Chemistry Evolution The anode material in a cell is typically graphite or coke, simple carbon-based products. The future direction for the anode is toward silicon-based material, another common product so it is relatively easily obtained. Cathode materials, on the other hand, represent a challenge in availability.

To achieve maximum runtime, cell phones, digital cameras and laptops historically have used Cobalt-based Lithium-ion because alternatives had lower energy density. This cathode material has several drawbacks. Until recently, almost all Li-ion cells were based on lithium cobalt oxide (LiCoO2), a material patented in the early eighties. Its strengths are that it is relatively easily processed and has good cycle life. However, it is not readily available. As shown in the pie chart, 64% of known cobalt deposits are in the Congo and Zambia; land which the Chinese have been buying up.

3

Innovations in Battery Technology for Portable Devices

Spot prices have huge price fluctuations. Finally, this cathode is more chemically volatile than other potential cathode materials. For these reasons the industry is moving away from it. While 99% of Li-ion batteries shipped used a cobalt based cathode in 2006, only 54% of Li-ion batteries used a LiCoO2 cathode in 2009; the trend has continued.

To find a workable compromise between high energy density, operational safety and good current delivery, manufacturers of Lithium-ion batteries are turning to mixtures. Alternate transition metals are substituted for Co with an effort to change the structure and performance very little. Most of these mixtures use Nickel, Manganese, and sometimes Aluminum. The benefits of the new chemistries include an increase in safety, as well as lower cost.

Lithium Iron Phosphate and Lithium Manganese Oxide (Spinel) are also being introduced as alternatives. However, they have a more radical change in performance and lend themselves to high current applications, so they will be addressed further on in this paper.

43% of known cobalt deposits are in the Congo and Zambia; land which the Chinese have been buying up.

Spot prices have huge price fluctuations.

4

Innovations in Battery Technology for Portable Devices

Below are discharge curves of several different alternatives compared to Cobalt (the dark blue). These are in high volume production today and are available from a variety of manufacturers. Both the capacity and voltage is dependent on the cathode chemistry, even in the same cell.

As a side note, the Iron phosphate yields an incredibly flat discharge profile, which makes it very challenging to measure the state of charge.

The trend in batteries is toward diversification rather than unification. Even in the mixed materials being developed, there is a trade-off and choices to be made.

• Increased material stability is tied to higher costs with an increase in Cobalt content.

• An increase in Nickel content yields an increase in capacity, but a decrease in safety.

• Additional Manganese gives up capacity but increases safety.

The positive side to all this diversification is that other industries can take advantage of the choices.

Below are some materials that may be available in the future. The blue area shows the electrolyte window. A voltage above this window will decompose the electrolyte. Aside from current capability, it is better to be in the upper right of this graph. The Cobalt-based material is in green. Researchers

are working on a number of Vanadium- and Titanium-based materials. The primary objective today is to improve cycle life for the electric vehicle market, but other industries can ride on the coat tails and take advantage of these innovations.

Anode DevelopmentsWhen compared to the rate of progress in cathode materials, anode development has been much less dramatic. Silicon is one material which has received much attention as an anode material due to its exceptional theoretical capacity compared to that of carbonaceous anodes. Unfortunately silicon has one major drawback – it swells by as much as 270% during the discharge cycle! To counter this, silicon based electrodes are sometimes constructed with substantial void volumes, which in turn reduces the effective volumetric energy density. Nevertheless, Si based cells are appearing on cell vendor roadmaps and offer an incremental increase in capacity.

Rate Capability Improvements Current and power delivery are important factors for many applications such as power tools, surgical instruments and other products containing a motor. The charge and discharge current of a battery is measured in C-rate and is used for estimating or designating the expected effective run time of a battery under variable charge and discharge conditions.

Most portable batteries are rated at 1C. This means that a 1000mAh battery would provide 1000mA for one hour if discharged at 1C rate. The same battery discharged at 0.5C would provide 500mA for two hours. Theoretically, the capacity should be the same as with a slower discharge since the identical amount of energy is dispensed, only over

This graph shows the voltage of the cathode chemistry against the potential of the Li-ion in the anode (in other words, the voltage) versus the capacity yielded for those chemistries.

5

Innovations in Battery Technology for Portable Devices

a shorter time. However, due to internal energy losses and a voltage drop that causes the battery to reach the low-end voltage cut-off sooner, the capacity reading may be lower. For Lead acid batteries, this effect is extreme, but for Li-ion it is minimal.

Temperature and current can both affect a battery’s performance. As shown in this graph, the capacity (and runtime) of this battery drops 65% when the temperature is varied 45 degrees to 5 degrees C for a current of 2.4 (1C). This effect can be greater or smaller depending on only slight variations in cell chemistry so for a specific application, different vendors’ cells may perform differently.

To design a cell that can accommodate high discharge and charge rates, the path length and resistance must be reduced for the transport of ions and electrons.

Shortening the path length can be done by changing the physical morphology of the battery’s active material and/or changing the material’s chemical structure. One approach to addressing the problem physically is to decrease the particle size of the materials to as small as nano-scale. Cobalt oxide has a layered atomic crystal structure. The ions shuttle in and out of the 2 dimensional layers. Alternatives, such as Manganese spinel or Iron phosphate olivine structures are 3-D and give good mobility and power delivery, but poor energy density. Typically, a cell made of a pure Manganese cathode provides only about half the capacity of Cobalt.

In addition to these changes, the resistance of the cells can be lowered by using thin materials, increasing the amount of current collectors and increasing the electrolyte concentration and reducing its viscosity with solvents. Many of these changes suggest that Li-polymer cells, which can be very thin, lend themselves to design for high rates.

Lithium Polymer Cells and Batteries Lithium Polymer (Li-polymer) is a physical structure of the cell, not a different material. The primary advantage of Li-polymer batteries is the variety of form factors available.

Manufacturers of Bluetooth® devices were the first to recognize the advantage of Li-polymer batteries. The availability of very thin batteries soon enabled the Motorola RAZR™ phone with great market success simply because it was so thin. Apple was the next company to recognize the appeal of very thin products. Most of the new Apple products use Polymer batteries. Apple uses the thin feature to differentiate in the highly commoditized notebook market.

This product, enabled by Li- Polymer is a digital X-ray plate, thin enough to fit in conventional film X-ray cassettes.

This is the performance of a high drain cell using Iron phosphate material. It delivers full capacity at 40A, which corresponds to a C rate of about 15. There are cells capable of 30C or more.

Effect of Temperature and Discharge Rate on Capacity(2400mAh Rated Cell)

6

Innovations in Battery Technology for Portable Devices

The images above illustrate two different types of construction. A prismatic Li-ion with a metal can and jelly roll construction is on the left and a thin, layered Polymer is on the right. For Li-ion, the materials are rolled while the Li-polymer can be in jelly roll format, or thin and layered like a deck of cards. Li-ion uses a discrete porous polymer membrane placed between the electrodes. Once assembled, the cell is backfilled with electrolyte solution. Some Li-polymers use polymer gel containing the electrolyte solution, which is coated onto the electrode surface. The structure may then be laminated before packaging.

The biggest difference is the packaging: Li-ion is packaged in metal cans while Li-polymer is packaged in a flexible “coffee bag” material- Al with polymer coating. Other features to note are that the prismatic Li-ion cell has a pressure vent and the terminals are on the metal can, where the positive and negative terminals on the Polymer cell are tabs protruding from the cell.

Conclusion The portfolio of cell options available for battery packs is constantly evolving. This was a review of some recent additions, but there are many more changes to come in the near future as more applications come to rely on portable power. Designers of portable devices can rely on the expertise of battery technology integrators like Electrochem Solutions, Inc.to understand and implement these cell innovations.

About Electrochem Solutions, Inc.

Electrochem, founded in 1979, is a world leader in the design and manufacture of customized total power solutions. A subsidiary of Greatbatch, Inc., Electrochem was born from the lithium battery invented for the implantable pacemaker by founder, Wilson Greatbatch. Today, Electrochem is known for providing safe and reliable products which are used across a range of critical applications in the portable medical, energy, military, and environmental markets.

For additional information on Electrochem, visit www.electrochemsolutions.com.

About Greatbatch, Inc.

Greatbatch, Inc. (NYSE: GB) provides top-quality technologies to industries that depend on reliable, long-lasting performance through its brands - Greatbatch Medical, Electrochem and QiG Group. Greatbatch Medical develops and manufactures critical medical device technologies for the cardiac, neurology, vascular and orthopaedic markets. Electrochem designs and manufactures custom battery technologies for high-end niche applications in the portable medical, energy, military and other markets. The QiG Group empowers the design and development of new medical devices for the company’s core markets.

For additional information on Greatbatch, visit www.greatbatch.com.

Prismatic Polymer