Operando High-Temperature X-Ray Absorption Spectroscopy of LiNiO2 Cathode Material during Synthesis

Abstract

The increasing demand for higher energy density in batteries for automotive applications has led to a trend toward increasing the Ni content in layered oxide cathode materials to increase their reversible capacity. However, LiNiO2, which has the highest Ni content, shows a faster capacity drop during charge-discharge cycling, which is attributed to its inherent structural instability in both the pristine and delithiated states. Therefore, to address the former issue and to better understand the appropriate synthesis conditions for LiNiO2, we have studied the changes in the local structure of LiNiO2 during the synthesis process using operando analysis techniques. Recently, high temperature in situ X-ray diffraction during LiNiO2 synthesis was reported by Bianchini [1] and Weber [2], which confirmed the loss of lithium and oxygen and the migration of Ni to Li layers above 700 °C. To further understand the mechanism of Li/Ni mixing and oxygen loss, operando high-temperature X-ray absorption spectroscopy (XAS) was performed during the synthesis of LiNiO2 to clarify the electrical structure of Ni at high temperatures. Pre-heated Li x NiO2 (x = 0.76, 1.01, 1.26, and 1.51) precursors were prepared by heating LiOH·H2O and Ni(OH)2 at 480 °C for 24 h in air. The precursor was pelletized and set in a quartz sample holder. Operando high-temperature Ni K-edge XAS spectra of the precursor were obtained at BL9A and BL12C in Photon Factory (PF), High Energy Accelerator Research Organization (KEK) by a transmission mode. The operando spectra of all samples except Li/Ni = 0.76 revealed a clear edge shift to the lower energy side corresponding to nickel reduction above 700 °C. The Li/Ni = 0.76 sample has a lithium-deficient composition and Ni is not oxidized to Ni3+ below 700 °C, so the Li/Ni = 0.76 sample has almost no Ni K-edge energy shift above 700 °C. These results suggest that Ni in LiNiO2 is reduced above 700 °C even in an oxygen atmosphere, regardless of whether the Li/Ni ratio is greater than 1. The Ni reduction above 700 °C is probably the cause of the Li/Ni mixture.Reference[1] M. Bianchini, J. Janek, et al., Angew. Chem., Int. Ed., 58, 10434 (2019).[2] R. Weber, J. R. Dahn, et al., J. Electrochem. Soc., 167, 100501 (2020).