The reaction of charged cathodes with nonaqueous solvents and electrolytes- II. LiMn2O4 charged to 4.2 V

The thermal decomposition of LiMn2O4 charged to 4.2 V was studied using a c ombination of accelerating rate calorimetry and X-ray diffraction. The elec trolyte and solvent were removed from the surface of the electrode using a rinsing procedure. Then electrode samples were either heated dry with solve nt added or with electrolyte added. The dry electrode was determined to con vert from the lambda -MnO2 structure to the beta -MnO2 structure upon expos ure to elevated temperatures (below 300 degreesC). In the presence of solve nt, there is solvent oxidation and reduction to MnO. The reaction of LixMn( 2)O(4) with xM LiPF6/EC:DEC (0 < x < 1.5) was also studied. The reactivity increases as the salt concentration increases. These experiments point the way to safer lithium-ion cells using LiMn2O4, since electrodes in electroly tes with lower concentrations of LiPF6 than, for example, 1 M, demonstrate improved thermal behavior. (C) 2001 The Electrochemical Society.