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Building safer Li-lon batteries

Li Ion iron phosphate's fundamental properties hold promise as an intrinsically safe cathode material for small and large platform applications.

When portable-device designers develop the latest systems with the most advanced features, much thought goes into the battery technology that powers these complex products. For over a decade, Lilon cobalt oxide has been the battery chem­istry of choice because it offers high energy density, which translates into more runtime. But more recently, thermal runaway, or battery overheating, has posed problems for users.

Today's newest Lilon technology holds the promise to make these batteries safer than ever. This technology repre­sents the pinnacle of a development process that has pro­gressed from the humble lead-acid battery through NiCd and NiMH. When compared to these older chemistries, Li­lon offers higher energy density, longer cycle life, and no memory effect.

Recently, rechargeable Lilon cells have reached an estab­lished commercial status with a production rate of several million units per month. Currently, Lilon cobalt oxide is the most commercially available variation of Lilon. Safety and thermal stability tests for lithium-manganese oxide and lithium-cobalt oxide have shown the former to be more sta­ble. And despite Lilon cobalt oxide's wide commercial acceptance, its cost, low abundance in the earth's crust, and environmental impact remain critical concerns.

New materials incorporating phosphate ions have been investigated, including Lilon iron phosphate. Due to the high availability of this chemistry compared to cobalt and manganese ions, Lilon iron phosphate is regarded as a very promising cathode material for small and large platform applications because of its enhanced thermal stability. Its low cost, non-toxicity, high abundance of iron, excellent thermal stability, safety characteristics, and good electrochemical per­formance add to an already long list of desirable criteria required for a viable cathode material.

With Lilon iron phosphate, the strong covalent bonding between the oxygen and phosphate form a strong polyanion unit that allows for greater stabilization of the staicture com­pared to layered oxides. The large polyanion also enlarges the free volume of the host's interstitial space available for lithium. The P-O-Metal bonding helps stabilize the redox energies of the metal cation and the structure, allowing a relatively fast ion migration. Consequently, oxygen atoms are harder to extract from Lilon iron phosphates.

Under normal abuse conditions, there's less likelihood of phosphate decomposition that may result in oxygen liberation from the structure. Only under extended and extensive heating (typically more than 800°C) can decomposition oc­cur in part (without oxygen release) to a Nasicon-related phase. That's important to note because it further illustrates the ability of Lilon iron phosphates 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. That's particularly important for battery safety because it affects the structural integrity of the material and hence its safety. Removing all the lithium available in Lilon iron phosphate causes no struc­tural modification. In fact, the structure of the fully lithiated and de-lithiated phases are similar, which confirms that the thennal stability of the Lilon iron phosphate even fully de­pleted of lithium is still better than the partially de-lithiated Lilon 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 that in mind, several methods are available for evaluating the safety of cells under abuse conditions. Creating an over-charge condition, for example, may lead to 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.

Fundamental properties of Lilon iron phosphate, the material in saphion-enabled batteries, make for an intrinsi­cally safe cathode material for current Lilon applications. When fully charged, no excess lithium remains in the cath­ode (unlike Lilon cobalt oxide, where 50% still remains). This material has high resilience to oxygen loss, which would otherwise result in a significant exothermic event upon heat­ing. Polymer technology provides a system with no free elec­trolyte, unlike the commercially available liquid-based bat­teries, further adding to its safety characteristics.

M.Y. Soldi holds patents related to the phosphate cathode mate­rial in saphion iit/uum-ion technology. Valence Technology, based in Austin, TX, can be reached at. (512) 527-292J or www.valence.com.

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