Influence of the Cr content on the electrochemical behavior of the LiCryMn2-yO4 (0 <= y <= 1) compounds - II. Cyclovoltammetric study of bulk and superficial processes

In the 3.8-4.3 V range, deintercalation of Li from the title compounds, occ urs in one or two well resolved steps (depending on y) which are single-pha se phenomena for y > 0. For 0 < y <less than or equal to> 0.5, the first st ep capacity is constant, whereas the second decreases rapidly. For larger y values, the only remaining step is the first, whose capacity decreases lin early with y, down to zero for y = 1. An interpretation based on Li-Li repu lsive interactions which play a role when y < 0.5 and disappear when y > 0. 5, is proposed. In the 4.3-5.2 V range, deintercalation of Li occurs in two steps (at similar to4.55 and similar to4.92 V for 0 < y <less than or equa l to> 0.12 and at similar to4.75 V and similar to4.92 V for 0.12 < y <less than or equal to> 0.75) which are single-phase phenomena. For y less than o r equal to 0.75, the cyclable capacity increases with y while the irreversi ble capacity remains low. On the other hand, for y > 0.75 an important capa city loss is observed at the second cycle, and a catalytic effect of the Cr content on the electrolyte oxidation is observed. Above 5.5 V, the kinetic s of electronic transfer, associated to the electrolyte oxidation, remains about three times slower than expected for a standard monoelectronic transf er. This oxidation begins similar to0.2 V below, and its kinetics is faster for y = 0.62 compared to y = 0.25. The onset voltage of this reaction is a lways higher than the end of deintercalation voltage. LiCryMn2-yO4 material s remain remarkably stable against oxidation when they are subjected to hig h end-of-charge voltages. (C) 2001 The Electrochemical Society.