Improving long-term capacity retention of NMC811 via lithium aluminate coatings using mixed-metal alkoxides

Abstract

Ni-rich LiNi_xMn_yCo_zO₂ (NMC) materials such as NMC811 (80% Ni) are promising Li-ion battery cathodes due to their high energy density and low cost. However, their reactive surfaces pose a practical challenge for their handling and storage, due to their fast degradation under operation or storage in ambient air. To tackle these problems, lithium aluminum alkoxides are used here for the first time to coat NMC811 with lithium aluminate (Li_1−xAl_1+x/3O₂). This method allows the controlled deposition of lithium aluminates on the surface of NMC811 by hydrolysis of the alkoxide precursor with transition metal TM(O)–OH surface groups. Of the series of alkoxides investigated, Li[Al(O^tBu)₄] was found to have the best coating/deposition properties in terms of solubility, thermal decomposition, phases formed and reactivity with NMC811. The effects of different NMC811 particle morphologies and surface treatments on the solution deposition of lithium aluminate were carefully evaluated by testing two different substrates: pristine polycrystalline NMC811 (PC-NMC811) and Al₂O₃-coated single-crystal NMC811 (SC-NMC811). A 17% increase in capacity retention at C/2 was seen for Li_1−xAl_1+x/3O₂/Al₂O₃ coated SC-NMC811 compared to Al₂O₃ coated SC-NMC811 after 100 charge–discharge cycles. Furthermore, coating PC-NMC811 that was degraded by soaking in water led to an impressive recovery of 50% of its C/20 capacity retention after 300 charge–discharge cycles while also showing higher specific capacities (25 mA h g⁻¹ more on the 2nd cycle) due to surface Li enrichment and regeneration of degraded surfaces. This coating method is a significant step forward in the scalable processing of new-generation battery cathodes with higher energy density and longer useful life.