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The lithium rechargeable cell with lithium metal anodes has a very high energy density, and is an attractive candidate as a post lithium-ion cell. When the lithium-ion cell was invented in 1992, the use of lithium metal was suddenly discontinued. However, the lithium metal anode has the very high theoretical capacity of 3860mAh/g, in contrast to the value of 372mAh/g for a LiC6 carbon anode. Recently, the lithium anode is again being studied for application in Li/air and Li/S cells. It will be interesting to reconsider the possible practical use of the lithium metal anode based on past studies tens of years prior to 1992. The most significant problems of the lithium metal anode are short cycling life and safety. When I was working for NTT in 1990, the AA-size Li/amorphous-V2O5 system was studied, and the best cycle life of the cell with a 3 times excess lithium in the cell was 300 cycles. The cycling life depends on the chargedischarge current. The cycling life was 70 cycles, a very short cycle life, when the charge-discharge current is low. The safety of the cell also became worse when the charge-discharge current is low. The cell sometimes caught fire during the nail penetration test after the low rate cycling. During the cycling of lithium metal, so-called dead lithium is accumulated on the lithium sheet anode. The dead lithium is a small quantity of lithium metal which is disconnected from the lithium sheet anode, and cannot be used as the active material. However, the dead lithium reacts with the electrolyte at an elevated temperature causing heat generation, which sometimes results in the cell burning. The amount of the dead lithium is very high when the cell is cycled at a low charge-discharge current. Therefore, if the dead lithium is not created during cycling, both problems of cycling life and safety can be solved. Charging of the lithium metal is lithium plating. The plated lithium on the lithium sheet is not flat but rod-like or particle-like, which is formed by non-uniform plating on the lithium sheet. The charge current density is different on different parts of the lithium sheet. The non-uniform plating is probably caused by the non-uniform thickness of the SEI (Solid Electrolyte Interphase, a protective film formed on the lithium anode which prevents further reduction of the electrolyte) on the lithium sheet. Discharging of lithium metal is dissolution of the lithium. The dissolution is also not flat but forms many pits on the lithium metal sheet. This non-uniform dissolution is also caused by the SEI. If an electrolyte which is not reduced at the lithium deposition potential can be realized (not easy to develop), SEI formation on the lithium anode can be avoided. If such an electrolyte is used in the Li metal anode cells, all the problems of the lithium metal anode (cycling life and safety) will then disappear. A possible candidate will be an electrolyte with an ionic liquid as the solvent. Jun-ichi YAMAKI Professor Emeritus of Kyushu University Contract Professor of Kyoto University Electrochemistry, 80(10)

New or Old Battery Using Lithium Metal Anode