Role of Local and Electronic Structural Changes with Partially Anion substitution Lithium Manganese Spinel Oxides on Their Electrochemical Properties: X-ray Absorption Spectroscopy Study

摘要

The electronic and local structures of partially anion-substituted lithium manganese spinel oxides as positive electrodes for lithium-ion batteries were investigated using X-ray absorption spectroscopy (XAS). LiMn _(1.8)Li_(0.1)Ni_(0.1)O_(4-η)F _η (η = 0, 0.018, 0.036, 0.055, 0.073, 0.110, 0.180) were synthesized by the reaction between LiMn_(1.8)Li_(0.1)Ni _(0.1)O_4 and NH_4HF_2. The shift of the absorption edge energy in the XANES spectra represented the valence change of Mn ion with the substitution of the low valent cation as Li~+, Ni ~(2+), or F~- anion. The local structural change at each compound with the amount of a Jahn-Teller Mn~(3+) ion could be observed by EXAFS spectra. The discharge capacity of the tested electrode was in the order of LiMn_2O_4 > LiMn_(1.8)Li _(0.1)Ni_(0.1)O_(4-η)F_η (η = 0.036) > LiMn_(1.8)Li_(0.1)Ni_(0.1)O_4 while the cycleability was in the order of LiMn_(1.8)Li _(0.1)Ni_(0.1)O_(4-η)F_η (η = 0.036) ≈ LiMn_(1.8)Li_(0.1)Ni_(0.1)O_4 > LiMn_2O_4. It was clarified that LiMn_(1.8)Li _(0.1)Ni_(0.1)O_(4-η)F_η has a good cycleability because of the anion doping effect and simultaneously shows acceptable rechargeable capacity because of the large amount of the Jahn-Teller Mn~(3+) ions in the pristine material.