Unraveling the Role of Composite Li3PO4/ZrO2 Coatings Prepared by Dry Milling on High Voltage Spinel Cathodes for Lithium-Ion Batteries: Insights into Lattice Strain, Thermal Behavior, Material Compatibility, and Electrochemical Performance

In the present investigation, three composite coatings for high voltage spinels were prepared from Li3PO4:ZrO2 (with varying loadings of zirconia, ZrO2, and lithium phosphate, Li3PO4) using ball milling techniques. The coatings were prepared on high voltage spinel LNMO (LiNi0.5Mn1.5O4) using mechanical mixing, and the samples were heated at 600 °C to achieve a uniform, crystalline coating. A coating referred to as LZ75 (75 wt % zirconia and 25 wt % Li3PO4) exhibited the highest relative weight stability and a lower heat of reaction than other ratios of ZrO2 and Li3PO4 prepared for these custom coatings on LNMO. X-ray photoelectron spectroscopy indicated the presence of zirconia and phosphorous on the LNMO surfaces. Band gap studies indicated a decrease in the direct and indirect band gaps for the coatings with a higher ZrO2 content, which could enhance conductivity in the material. In contrast to this, the Urbach energy (Eu) yielded a reverse trend and followed the order (LNMO) < (LZ25-coated LNMO) < (LZ50-coated LNMO) < (LZ75-coated LNMO). Electron microscopy studies indicated a change in morphology for the coated LNMO, especially for the LZ75-coated LNMO. Additional studies evaluated bare and coated LNMO for their stability in the presence of LiPF6 electrolyte. Scanning electron microscopy and X-ray diffraction analyses of the LNMO/LiPF6 interfaces for the coated and bare cathode materials indicated that the coating tends to suppress Mn dissolution in comparison to the uncoated LNMO. Additionally, more phases were seen in the LNMO/LiPF6 interface than for the coated LNMO materials. Electrochemical performance depicted an enhanced capacity retention of up to 88.5% for the LZ75-coated LNMO, compared to 69.2% for pristine LNMO after 100 cycles at 1 C (3–5 V vs Li/Li+). The lithium-ion diffusion coefficient (DLi+) also exhibited a significant increase in the coated cathodes along with a decrease in the charge transfer resistance and surface film resistance.