Stabilizing High-Voltage Cathodes via Ball-Mill Coating with Flame-Made Nanopowder Electrolytes.

LiMnNiO (LMNO) spinel has recently been the subject of intense research as a cathode material because it is cheap, cobalt-free, and has a high discharge voltage (4.7 V). However, the decomposition of conventional liquid electrolytes on the cathode surface at this high oxidation state and the dissolution of Mn have hindered its practical utility. We report here that simply ball-mill coating LMNO using flame-made nanopowder (NPs, 5-20 wt %, e.g., LiAlO, LATSP, LLZO) electrolytes generates coated composites that mitigate these well-recognized issues. As-synthesized composite cathodes maintain a single P432 cubic spinel phase. Transmission electron microscopy (TEM) and X-ray photoelectron spectra (XPS) show island-type NP coatings on LMNO surfaces. Different NPs show various effects on LMNO composite cathode performance compared to pristine LMNO (120 mAh g, 93% capacity retention after 50 cycles at C/3, ∼67 mAh g at 8C, and ∼540 Wh kg energy density). For example, the LMNO + 20 wt % LiAlO composite cathodes exhibit Li diffusivities improved by two orders of magnitude over pristine LMNO and discharge capacities up to ∼136 mAh g after 100 cycles at C/3 (98% retention), while 10 wt % LiAlO shows ∼110 mAh g at 10C and an average discharge energy density of ∼640 Wh kg. Detailed postmortem analyses on cycled composite electrodes demonstrate that NP coatings form protective layers. In addition, preliminary studies suggest potential utility in all-solid-state batteries (ASSBs).