SOME ASPECTS OF LIMN2O4 ELECTROCHEMISTRY IN THE 4 VOLT RANGE

LiMn2O4 is a hopping semiconductor (sigma = 2 X 10(-6) Omega(-1) cm(-1 )) as is lambda-MnO2 obtained by full delithiation. A semiconductor-me tal transition is not observed by Li+ extraction because the critical Mn-Mn distance for this transition (2.743 Angstrom) is shorter than th ose observed for lambda-MnO2 and LiMn2O4 (2.803 and 2.913 Angstrom, re spectively). However, on Li+ removal the charge-transfer resistance of the spinel decreases, perhaps as a consequence of enhanced Li+ crossi ng of the electrolyte/ cathode interface. A full charge/discharge cycl e does not allow one to reinsert the same amount of Li+ extracted as a consequence of a structural alteration. The abrupt voltage drop at th e end of the cycle coincides with a unit cell parameter of similar to 8.24 Angstrom, i.e., the same value of the original spinel. Spinels pr epared at 700-750 degrees C have electrochemical features different fr om those of spinels made at 800-900 degrees C. In particular, Li+ is m ore easily extracted from the former in a process which seems to creat e Li+ ordering for x = 2/3 and x = 1/3 ion LixMn2O4. The self-discharg e characteristics of fully oxidized spinels in different electrolytes have been examined.