Improving structural and thermal stability of LiNi_0.8Co_0.15Al_0.05O₂ by a fast-ionic-conductive LiAlSiO₄ surface coating for Li-ion batteries

In this study, we prepared Ni-rich Li[Ni_0.8Co_0.15Al_0.05]O₂ (NCA) as a cathode material for lithium-ion batteries (LIBs) through co-precipitation in a Taylor flow–assisted continuous reactor, and then using a wet-chemical process to coat the NCA with LiAlSiO₄ (LASO). In situ X-ray diffraction, scanning electron microscopy, and transmission electron microscopy are used to characterize the structures and morphologies of the pristine and LASO-coated NCA materials. Relative to the pristine NCA, the LASO-coated NCA exhibited greater electrical conductivity and higher diffusivity of Li⁺ ions and, thereby, improved cycling stability. Among our samples, the NCA coated with 1 wt.% LASO displays optimal electrochemical performance. The initial discharge capacity of pristine and 1 wt% coated LASO samples are, 198.9 and 194.1 mAh g⁻¹ with columbic efficiencies of 87.6 and 90.2%, respectively. The capacity retention for LASO-coated material at 1 C (200 mA g^–1) after 100 cycles is 91.2% at room temperature and 68.1% at 55 °C; for the pristine NCA, they are 73.2 and 37.4%, respectively. The coating effectively decreases anisotropic mechanical stress and, thus, prevents the formation of micro-cracks on the secondary particle surface. Electrochemical impedance spectroscopy and cyclic voltammetry reveal that the improvement in the electrochemical performance originates from lower surface impedance and higher Li⁺ ion diffusivity. Furthermore, the thermal properties, measured using a multiple-module micro-calorimeter, reveals that the coated electrodes exhibited markedly lower heat-generated flux when cycled; post-mortem analysis after long-term cycling reveals that the amorphous LASO coating markedly inhibits any morphological changes to the structure, acts as relatively stable protective barriers, and provides pathways for rapid Li⁺ ion diffusion. Thus, our LASO-modified NCA cathode materials appear to be promising candidates for application in high-energy-density LIBs.