A Lithium Superionic Conductor Softened by Nonmetal-Chlorine Chemical Bonds

All-solid-state lithium batteries (ASSLBs) offer improved energy density and safety over traditional liquid-electrolyte systems. However, their practical use is limited by the rigidity of inorganic lithium superionic conductors, which require impractically high stack pressures (>50 MPa) to maintain close solid-solid contacts during cycling. We introduce the idea of incorporating nonmetal-chlorine chemical bonds into the conductive network to make rigid conductors more flexible. Because nonmetal-chlorine chemical bonds (e.g., P-Cl, Si-Cl) exhibit low bond dissociation energies, they can undergo facile rotation and torsion, thereby facilitating Li⁺ migration and framework deformability. A liquid SiCl₄ activation method is developed to introduce these chemical bonds, yielding a soft superionic conductor, Li₃P_0.58Si_1.25Zr_1.78Cl_10.86O_3.58. This material shows a high room-temperature Li⁺ conductivity of 4.55 mS cm^-1 and a low Young's modulus of 2.09 GPa. This combination enables over 3000 cycles of ultrahigh-nickel cathode LiNi_0.92Co_0.05Mn_0.03O₂ at a high current density of 3 mA cm^-2, and even allows for stable operation of ASSLBs with no capacity decay after 300 cycles under a low stack pressure of 5 MPa, much lower than the usual 50 MPa needed for most inorganic superionic conductors. Additionally, this chemical-bond-tuning method works with various nonmetal centers (P, Si, C, S), providing a flexible strategy for designing deformable superionic conductors suitable for low-pressure ASSLBs.