Unlocking Stable Multi‐Electron Cycling in NMC811 Thin‐Films between 1.5 – 4.7 V

Abstract Among cathode materials, LiNi_0.8Mn_0.1Co_0.1O₂ (NMC811) is the most discussed for high performance Li‐ion batteries, thanks to its capacity of ≈200 mAh g^‐1 and low Co content. Here, it is demonstrated that NMC811 can reversibly accommodate more than one Li‐ion per formula unit when coupled with a solid‐state electrolyte, thus significantly increasing its capacity. Sputtered Li‐rich NMC811 cathodes are tested with lithium–phosphorus–oxynitride as a solid‐state electrolyte in a thin‐film architecture, which is a simplified 2D model with direct access to the cathode‐electrolyte interface. The solid‐state electrolyte helps to stabilize the interface and prevents capacity fading, voltage decay, and interface resistance growth, thus allowing cycling at extended voltage ranges of 1.5–4.7 V. While the liquid electrolyte cells suffer from rapid capacity decay, the Li‐rich NMC811 cells with the solid‐state electrolyte can cycle at a fast rate and an initial capacity of 149 mAh g^‐1 from 1.5 to 4.3 V for 1000 cycles. The all‐solid‐state thin‐film cells with a lithium metal anode yield a discharge capacity of up to 350 mAh g^‐1 at C/10 because of multi‐electron cycling with a coulombic efficiency of 90.1%. The results demonstrate how solid‐state electrolytes that are stable against NMC811 cathodes can unlock the full potential of this Li‐rich and Ni‐rich cathode class.