When portable device designers develop the latest gadgets with the most advanced features, much thought goes into the battery technology that powers these small, yet complex products. For over a decade, lithium-ion cobalt-oxide has been the technology of choice for mobile device manufacturers because it offers high energy density, which translates into more run-time for the end users. During the past two years there have been several recalls of lithium-ion cobalt oxide battery packs, including cells from a major Japanese manufacturer.  It is becoming increasingly clear that engineering controls and safety devices have not been sufficient to prevent safety recalls.  Today's newest phosphate based lithium-ion chemistry may have the ability to make lithium-ion batteries safer than ever before.

Battery Options: What You Should Know
The latest lithium-ion battery technologies represent the pinnacle of a development process that has progressed from the humble lead-acid battery through Nickel-Cadmium (NiCd) and Nickel-Metal Hydride (NiMH) batteries. When compared to these older battery chemistries, lithium-ion technology offers several advantages, including: higher energy density, longer cycle life, no memory effect and materials that are more environmentally friendly.

In 2006 lithium ion batteries were forecasted to reach a production rate of tens of millions of cells per month, with a high annual growth rate.  Most lithium ion cells use lithium cobalt oxide and are targeted at small portable electronic devices, where energy density is critical but safety, thermal stability and cost have not been determining factors to date.  Other technologies like lithium manganese oxide have been shown to be more thermally stable and are less expensive, but have failed to replace lithium cobalt oxide in these applications.  In light of recent safety recalls, lithium cobalt oxide's safety issues represent a hidden cost in small format batteries, while new applications such as large format batteries raise safety requirements that rule out its use.

Identifying New Chemistries
New phosphate-based lithium ion cathode materials have been identified, such as Valence Technology, Inc.'s Saphion® lithium-ion technology. Due to the high available material specific capacity of this chemistry compared to cobalt and manganese oxides, Saphion® lithium-ion cathode material is currently regarded as a very promising cathode material because of its enhanced thermal stability, for small as well as large platform applications. Its low cost, non-toxicity, excellent thermal stability, safety characteristics and very good electrochemical performance add to an already long list of desirable criteria required for a viable cathode material.

With Saphion phosphate cathode material, the strong covalent bonding between the oxygen and the phosphorus forms a strong polyanion unit in the phosphate ion that allows for greater stabilization of the structure compared to layered oxides, e.g. lithium-ion cobalt oxide, wherein the oxygen is more weakly bound.  The large phosphate polyanion also enlarges the free volume of the host's interstitial space available for lithium. The phosphorus-oxygen-metal bonding helps to stabilize the redox energies of the metal cation and the structure, allowing a relatively fast ion migration. Consequently, oxygen atoms are a lot harder to extract from Saphion phosphate cathode materials. Under abuse conditions, there is a much less likelihood of oxygen liberation from the structure due to phosphate decomposition.  Only under extended and extensive heating (typically > 800ºC) can decomposition to a Nasicon related phase, in part, (without oxygen release) occur. This is important to note because it further illustrates the ability of Saphion phosphate cathode materials to remain stable even in the harshest conditions, thus avoiding any uncontrollable thermal excursions.

Upon removal of lithium, lithiated cobalt oxide undergoes a nonlinear expansion of the unit cell. This is particularly important for battery safety in that it affects the structural integrity of the material, and hence its safety. Removing all of the lithium available in a Saphion phosphate cathode material causes no structural modification. In fact, the structures of the fully lithiated and de-lithiated phases are very similar. This confirms that the thermal stability of the Saphion phosphate cathode material even fully depleted of lithium is still far better than the partially de-lithiated lithium-ion cobalt oxide.

With higher energy density batteries available, safety is of paramount concern for consumer batteries and more advanced safety technology is required. Insight into the behavior and thermal stability of the cathode in the charged state is essential in determining the overall safety of the final cell. With this in mind, there are methods available for evaluating the safety of cells under abuse conditions. Creating an over-charge condition, for example, may lead to a thermal runaway (or excessive heat), which can cause a combustion reaction in the battery because of the presence of flammable solvents and vapor mixtures in the cell. This situation would make the battery unsafe for consumer use.

Power Safety for the Future
Fundamental properties of Saphion phosphate cathode material make for an intrinsically safer cathode material for lithium ion applications. When fully charged, no excess lithium is left in the cathode (unlike lithium-ion cobalt oxide where 50% still remains). Lithium cobalt oxide material has a high resilience to oxygen loss, which can result in a significant exothermic event upon heating.  Consumers are becoming more reliant on their portable devices than ever before but do not stop to think about inherent risks associated with the batteries powering them. This may change in view of recent publicity on the recall of portable device batteries.  In view of recent events and changes in consumer attitude, it will become the burden of the designer to recognize and incorporate safety along with the most stable power sources available to ensure the optimal use of such portable devices. Lithium-ion batteries continue to be the technology of choice for small electronics but, as illustrated, their continued evolution and the introduction of new large format applications require a closer look at the technology inside the battery and the important differentiators among the choices for cathode materials.

M.Y. Saidi, with Valence Technology, Inc., holder of several patents related to the phosphate cathode material in Saphion® lithium-ion technology.