Structural transformation on cycling layered Li(Mn1-yCoy)O-2 cathode materials

Layered Li-x(Mn1-yCoy)O-2 can deliver a discharge capacity on cycling of 20 0 mA h g(-1) at 0.1 mA cm(-2) Structural transformations occurring in these solid solutions during electrochemical cycling of lithium have been invest igated by combining the results from powder neutron diffraction with those from cyclic voltammetry and incremental capacity measurements. The layered structure of the solid solutions converts to spinel on cycling. The rate of conversion depends on the cobalt content. There is evidence that conversio n to spinel commences even within the first cycle of the solid solution wit h y = 0.1 whereas compounds with the composition y = 0.3 show almost no con version to spinel within three cycles and only some conversion after 30 cyc les. A two-phase model composed of layered and cubic spinels does not signi ficantly improve the fit to the neutron data for the y = 0.1 composition wi thin the first cycle indicating that there is little evidence of long range order in any spinel that is formed. Clear diffraction evidence of an order ed spinel phase for y = 0.1 is observed after four cycles. Conversion to sp inel does not appear to have a deleterious effect on the capacity retention on cycling. (C) 1999 Elsevier Science Ltd. All rights reserved.