Lithium can be inserted reversibly within most carbonaceous materials. The physical mechanism for this insertion depends on the carbon type. Lithium intercalates in layered carbons such as graphite, and it adsorbs on the surfaces of single carbon layers in nongraphitizable hard carbons. Lithium also appears to reversibly bind near hydrogen atoms in carbonaceous materials containing substantial hydrogen, which are made by heating organic precursors to temperatures near 700°C. Each of these three classes of materials appears suitable for use in advanced lithium batteries.

containing Zr. This supposition is also borne out by the relatively poor corrosion resistance exhibited by the bulk lithium zirconate specimen.The one nitride included in this test program (A1N + 3% Y203) was equal in corrosion resistance to the best of the oxides. This nitride showed no change in physical appearance and no significant weight loss.The introduction of foreign elements by the capsule material used in these compatibility tests can probably be discounted as a factor affecting results. Metallographic examination of selected capsules after test revealed no evidence of microstructural change or surface attack. Furthermore, comparison of results obtained from steel capsules showed no obvious differences from results of duplicate tests in molybdenum capsules. The method of removing lithium from the test specimens, however, did affect the test results. In cases where specimens had been penetrated by lithium, dissolution of the lithium in water often led to disintegration of the specimen. Removal of the lithium by vacuum distillation was less destructive to the specimens and to any reaction products and therefore facilitated the examination of the less compatible materials. Even in the case of distillation, however, the reaction products can be materially changed by the decrease in lithium activity at the distillation temperature. Thus, the detailed analysis of reaction products is difficult at best, and failure of a specimen to survive water dissolution may in the end be the most definitive test result. Conclusions1. Based on exposure tests in lithium at 400~ beryllia, magnesia, yttria, yttrium aluminum garnet, and aluminum nitride appear to be suitable insulators for service in a Downs cell for lithium electroreduction.2. Aluminum nitride is a particularly promising candidate because of its excellent electrical and thermal properties and good fabricability.3. Yttrium aluminum garnet in single-crystal form is compatible with molten lithium. Performance of polycrystalline YAG is presently being evaluated. The poor performance of alumina suggests that the composition must be closely controlled to avoid any residual alumina in the sintered YAG body.

Li/graphite and Li/petroleum coke cells using a 1M LiAsF6 in a 50:50 mixture of propylene carbonate (PC) and ethylene carbonate (EC) electrolyte exhibit irreversible reactions only on the first discharge. These irreversible reactions are associated with electrolyte decomposition and cause the formation of a passivating film or solid electrolyte interphase on the surface of the carbon. The amount of electrolyte decomposition is proportional to the specific surface area of the carbon electrode. When all the available surface area is coated with the film of decomposition products, further decomposition reactions stop. In subsequent cycles, these cells exhibit excellent reversibility and can be cycled without capacity loss.

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