SPES, 6Li MAS NMR, and Ni3+ EPR evidence for the formation of Co2+-containing spinel phases in LiCoO2 cycled electrode materials

Abstract

Step potential electrochemical spectroscopy (SPES), 6Li magic angle spinning nuclear magnetic resonance (MAS NMR) and Ni3+ electron paramagnetic resonance (EPR) probes were used to examine the effect of lithium extraction and reinsertion in the structure of pseudo-spinel and layered modifications of LiCoO2 used as cathode materials in 4 V lithium-ion batteries. Current relaxations at each step of the SPES experiment show a similar lithium ion diffusivity in both structural modifications during cell charge. The different electrochemistry of pseudo-spinel and layered LiCoO2 can be attributed to the structural instability of the delithiated pseudo-spinel phase as compared to the layered modification. EPR studies of cycled electrodes based on pseudo-spinel LiCoO2 give direct evidence for the appearance of Co2+ in tetra coordination. For layered LiCoO2, the 6Li NMR line broadening and the spin lattice relaxation of Ni3+ spin probes reveal the internal stresses induced in the solid matrix by lithium extraction and reinsertion reactions.