Tuning phase structures of in situ polymerized elastomeric electrolytes via monomer structure engineering for achieving high stability in solid-state lithium metal batteries

To achieve high-performance solid-state lithium (Li) metal batteries (LMBs), both the Li-ion conductivity and mechanical robustness of the polymeric electrolytes are crucial. Herein, we develop a series of elastomeric electrolytes with different phase structures to optimize their Li-ion conductivity and mechanical properties. The phase structures are controlled by modifying the side alkyl chain length of the acrylate monomer during the in situ polymerization process. Specifically, when using short alkyl chain monomers such as methyl and ethyl, a homogeneous phase of the electrolyte was generated after the in situ polymerization. On the other hand, longer-chain monomers like butyl and hexyl result in bicontinuous and macrophase-separated structures, respectively. Among these different structures, the elastomeric electrolyte based on butyl acrylate (BA) and succinonitrile (SN), denoted as BA-SPE, exhibits excellent ionic conductivity (>1 mS cm⁻¹) and high mechanical extensibility due to its bicontinuous structure. The BA-SPE also demonstrates stable operation when used in a Li/LiNi_0.8Co_0.1Mn_0.1O₂ (NCM811) full cell. To further enhance the performance of the BA-SPE, we introduced a dual salt system comprising lithium bis(trifluoromethane sulfonyl)imide (LiTFSI) and lithium difluoro(oxalato)borate (LiDFOB). This modification resulted in superior full cell performance, with 87% capacity retention after 100 cycles under a current density of 0.5 mA cm⁻², using a 40 µm-Li anode and high-loading NCM811 cathode (>10 mg cm⁻²).