Stabilization of NCM811 cathode interface through macromolecular compound protective film formed by 2,5-bis(2,2,2-trifluoroethoxy)-benzoic acid additive in lithium metal batteries

Abstract

Lithium metal batteries (LMBs) offer substantial promise for next-generation energy storage owing to lithium metal's low reduction potential (−3.045 V vs. the standard hydrogen electrode) and its high specific capacity of 3860 mA h g⁻¹. Among various cathode materials in LMBs, LiNi_0.8Co_0.1Mn_0.1O₂ (NCM811) is extensively employed because of its notably high specific capacity (over 200 mA h g⁻¹) and comparatively lower cost. However, structural stress, nickel ions migration, and uneven Li⁺ deposition in NCM811 particles lead to cracking, irreversible decomposition of active substances, and the growth of mossy Li dendrites, causing severe capacity decline and low Coulomb efficiency in LMBs. In this study, we introduce an effective ethoxyl additive, 2,5-bis(2,2,2-trifluoroethoxy)-benzoic acid (2,5BTBA), directly into the carbonate electrolyte. This additive forms a dense and conductive macromolecular protective film on the NCM811 cathode and lithium metal anode during initial cycles, preventing electrode contact with the electrolyte. Consequently, it safeguards the cathode's structural integrity and enables dense lithium deposition. Adding 3 wt% 2,5BTBA, the Li/NCM811 battery retains a high capacity of 150.60 mA h g⁻¹ and 89.41% retention after 700 cycles at 0.5C, maintaining an average Coulomb efficiency of 99.13%. This study presents an efficient and straightforward strategy to enhance the capacity retention of LMBs.