WHY TRANSITION-METAL (DI) OXIDES ARE THE MOST ATTRACTIVE MATERIALS FOR BATTERIES

Recent developments of materials for rechargeable lithium batteries are highlighted. The reactions using advanced batteries consist of lithium ion insertion into and extraction from a solid matrix without the destruction of core structures, (called topotactic reaction) enable us to study systematically battery materials. By applying a hard-sphere model the optimum chemical composition and element in terms of volumetric capacity in Ah.cm-3 are indicated to be square MeO2 or LiMeO2 (Me = transition metal elements). The calculated values, assuming one electron transfer per a transition metal ion, are in the range 1.15-1.5 Ah.cm-3 for both square MeO2 and LiMeO2 using available structural data, which is alone merely attainable using transition metal dioxides. The approximate operating voltages for the reaction Li + square MeO2 reversible LiMeO2 are pictured against the number of d-electrons. The order of operating voltages of transition metal (di) oxides is approximately; 3d>4d>5d and d(n)<d(n+1) (n = 0 to 6) distributed in the voltage between 0.5 and 4.5 V versus a lithium electrode. From these results, we discuss why transition metal (di)oxides are the most attractive materials for advanced lithium batteries. The specific problems in developing the insertion materials based on metal (di)oxides further are also discussed.