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

Abstract

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_1.8Li_0.1Ni_0.1O_4−ηF_η (η = 0, 0.018, 0.036, 0.055, 0.073, 0.110, 0.180) were synthesized by the reaction between LiMn_1.8Li_0.1Ni_0.1O₄ and NH₄HF₂. 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⁺, Ni²⁺, or F⁻ anion. The local structural change at each compound with the amount of a Jahn–Teller Mn³⁺ ion could be observed by EXAFS spectra. The discharge capacity of the tested electrode was in the order of LiMn₂O₄ > LiMn_1.8Li_0.1Ni_0.1O_4−ηF_η (η = 0.036) > LiMn_1.8Li_0.1Ni_0.1O₄ while the cycleability was in the order of LiMn_1.8Li_0.1Ni_0.1O_4−ηF_η (η = 0.036) ≈ LiMn_1.8Li_0.1Ni_0.1O₄ > LiMn₂O₄. It was clarified that LiMn_1.8Li_0.1Ni_0.1O_4−ηF_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 Mn³⁺ ions in the pristine material.