Self-adjusted oxygen-partial-pressure approach to the improved electrochemical performance of electrode Li[Li0.14Mn0.47Ni0.25Co0.14]O2 for lithium-ion batteries

Abstract

We initiated a self-adjusted oxygen-partial-pressure approach to prepare high-performance Li₂MnO₃–LiMO₂ cathode material. Four different lithium resources, lithium acetate, lithium hydrate, lithium carbonate, and lithium nitrate were used to create the local oxygen partial pressure over the samples. Since the melting points or decomposition temperatures for these lithium resources decrease in a sequence, Li₂CO₃ ≈ LiOH > LiNO₃ > CH₃COOLi, the oxygen partial pressure of the four crucibles that contain these lithium salts increases in a sequence, S4 ≈ S3 < S2 < S1 ≈ air in muffle furnace (S1: CH₃COOLi·2H₂O, S2: LiNO₃, S3: LiOH·H₂O, and S4: Li₂CO₃). Regardless of the lithium resources, the decomposed gases reduced the local oxygen partial pressures, leading to an incomplete oxidation of Mn ions in the final product Li[Li_0.14Mn_0.47Ni_0.25Co_0.14]O₂. That is, some of the Mn³⁺ ions existed in the final product Li[Li_0.14Mn_0.47Ni_0.25Co_0.14]O₂, and the amount of Mn³⁺ ions was closely related to the oxygen partial pressure. The lower oxygen partial pressure gave rise to a larger amount of Mn³⁺ in the final products, as confirmed by X-ray photoelectron spectroscopy. Electrochemical tests showed that the products prepared using lithium carbonate exhibited the best electrochemical performance: the initial discharge capacity was 279.4 mA h g⁻¹ at a current density of 20 mA g⁻¹, which remained as high as 187.2 mA h g⁻¹ even at a much higher current density of 500 mA g⁻¹. Such excellent electrochemical performance could be ascribed to the presence of Mn³⁺ that decreased the surface layer resistance and charge transfer resistance, and that further increased the conductivity and Li⁺ ion diffusion coefficient.