INDUSTRY TRACK: Polyoxometalates as an Alternative Coating Strategy for Layered Ni-Rich Oxide Cathodes in Liquid- and Solid-State Batteries

INDUSTRY TRACK: With the widespread use of lithium-ion batteries (LIBs), layered oxides like LiNi_xCo_yMn_1−x−yO₂ (NCM) have become crucial cathode active materials (CAMs). Although NCMs offer better cycling performance than LiNiO₂, electro-chemo-mechanical degradation during operation leads to capacity fading. A major factor are interfacial side reactions with the electrolyte, causing metal dissolution, lithium depletion, and gas evolution. To mitigate these issues, surface coatings are applied to act as an artificial cathode electrolyte interphase (CEI), physically separating the CAM from the electrolyte. However, an effective coating must also have high permittivity to minimize the additional resistance introduced by the CEI during cycling. Therefore, polyoxometalates (POMs) were strategically applied for surface coating of a Ni-rich CAM to achieve a uniform and consistent coverage. POMs are compounds with large and diverse structures, formed by the linkage of d⁰ metal-centered polyhedra with oxygen atoms located at the vertices. They exhibit flexibility and a polymer-like behavior, making them suitable as building blocks for extended materials and nanostructures. Firstly, the well-known Lindqvist polyoxoniobate (PON) [Nb₆O₁₉]⁸⁻ was used to coat the surface of NCM-851005. The PON was intentionally prepared with the formula Li₆[H₂Nb₆O₁₉], with an equal molar proportion of Li and Nb in order to form LiNbO₃ upon annealing, which was confirmed by XRD analysis. The coating procedure consisted in mixing NCM and PON in a selected solvent (acetonitrile or ethanol) through ultrasonication and heating the resulting powder at 400 °C under oxygen atmosphere. Microscopy images revealed that the NCM morphology remained unchanged, while a thin film covered the primary particles. The electrochemical performance was tested in solid-state (SSB) and lithium batteries (LIBs) cells. In LIBs, the materials were cycled in half-cells at 25 °C for 100 cycles, in the potential range between 3.0 and 4.3 V vs. Li⁺/Li. The initial specific discharge capacity was ~228 mAh/g, similar to that of uncoated NCM (230 mAh/g), with capacity retentions of 93 and 71% for the coated and uncoated NCM, respectively. For SSBs, cells were tested using argyrodite Li₆PS₅Cl (LPSCl) as solid electrolyte at 45 °C in a potential range of 2.28-3.68 V vs. In/InLi. The Nb-coated NCM delivered an initial specific capacity of 176 mAh/g with 88% retention after 100 cycles, in comparison to 173 mAh/g and 36% for the bare NCM. A second POM, the polyoxovanadate (POV) H₃TBA₃(V₁₀O₂₈), chosen for its solubility in non-aqueous solvents, was also applied for NCM coating using similar procedures. In this case, analysis indicated that the final coating consists of a mixture of phases, predominantly V₂O₅ and Li₃VO₄. Evaluation in LIBs demonstrated superior capacity retention of 87% for the coated sample after 100 cycles, compared to 70% for the bare NCM. Similarly, in thiophosphate-based SSBs, the capacity retention increased from 69% to 81% (after 50 cycles). Accordingly, the application of polyoxometalates for CAM coating appears to be a promising strategy to enhance the stability of layered Ni-rich oxide cathodes.