Defect Spinel Li[sub 8n/n+4]Mn[sub 8/n+4]O[sub 4] Cathode Materials for Solid-State Lithium-Polymer Batteries

Abstract

We have used a self-reaction process, called the JMC (Japanese Metal and Chemical Co., Ltd.) method, to prepare a series of defect spinels Li8n/n+4Mn8/n+4O4,Li8n/n+4Mn8/n+4O4, Li4Mn5O12−δ(n=0.8),Li4Mn5O12−δ(n=0.8), Li2Mn3O7−δ(n=0.65),Li2Mn3O7−δ(n=0.65), and Li2Mn4O9−δ(n=0.5).Li2Mn4O9−δ(n=0.5). We found that it is easy to oxidize a defect spinel with a higher lithium content (Li/Mn ratio of 0.8) during synthesis. At the same time, however, the defect spinel easily becomes oxygen deficient. By contrast, a defect spinel with a smaller lithium content, especially Li/Mn of 0.5, is difficult to fully oxidize. The defect spinels deliver at initial capacity of 160 mAh/g both in the liquid-electrolyte and solid-state polymer-electrolyte-based cells. Li2Mn3O7−δLi2Mn3O7−δ shows the best battery performance; the capacity loss rate is 0.18% per cycle for a lithium-polymer cell during the first 100 cycles at 65°C, and the cell gives a specific energy of 360 Wh/kg based on the pure oxide. All compounds are thermally stable up to 200°C when they are in contact with polymer electrolytes, but undergo thermal runaway over 200°C. The exothermic reaction proceeds via a redox reaction among Mn4+,Mn4+, Mn3+,Mn3+, and the polymer electrolyte. © 2000 The Electrochemical Society. All rights reserved.