Construction of Polyionic Liquid Coating Layer Enables High Cyclic Stability of 4.5 V LiCoO2 Cathode by In Situ Polymerization

The pursuit of high-energy-density portable electronic applications has intensified the development of high-voltage LiCoO₂ (LCO) cathodes. However, the LCO cathode undergoes severe side reactions with the electrolyte at elevated cutoff voltages, leading to significant interface degradation and structural collapse. Modifying the cathode/electrolyte interface is a very good strategy to restrain interfacial side reactions and stabilize the structure. Herein, we proposed a high-voltage-stable polyionic liquid (PIL) as an artificial solid electrolyte interface, achieved by in situ bulk polymerization of a diallyldimethylammonium bis(trifluoromethanesulfonyl)imide (DMDA) monomer to form a polymer-containing cross-linked structure on the LCO cathode surface. The polymerized DMDA (PDMDA) coating layer forms a thin, dense cathode-electrolyte interphase (CEI) that effectively isolates the LCO from direct contact with the electrolyte and suppresses side reactions. Consequently, the PDMDA-modified LCO cathode retains 80% of its initial capacity over 500 cycles at 1C within a voltage range of 3 to 4.5 V, significantly outperforming the bare-LCO cathode. Additionally, the PDMDA layer also enhances the thermal stability of the LCO cathode, offering substantial value for safe lithium battery applications. This PDMDA modification strategy provides a promising pathway for practical deployment of high-voltage LCO cathodes.