A robust gradient solid electrolyte interphase enables fast Zn dissolution and deposition dynamics

The construction of a robust solid-electrolyte interphase (SEI) on zinc anode is an effective approach for tackling the high thermodynamic instability and side reactions of Zn-metal anode (ZMA), particularly at high current densities and high utilization ratios. Herein, a ternary aqueous electrolyte with N,N-dimethyltrifluoroacetamide (DMTFA), dimethylformamide (DMF), and H2O is developed to build a stable SEI. DMTFA is introduced as a functional solvent, which is preferentially decomposed to form a gradient rigid-soft coupling SEI layer. Meanwhile, DMF is added as a co-solvent to suppress the water activity by forming an intermolecular hydrogen bond, thus protecting the as-formed SEI against corrosion. It is found that a 31 nm-thick SEI film with a -CF3-rich-organic outer layer and a gradient zinc salts-rich-inorganic (e.g., ZnF2, Zn3N2, ZnSO3, ZnS, ZnO) inner layer delivers excellent structural integrity to block the direct contact of water and ZMA. Moreover, the as-formed SEI exhibits a high compression modulus (23.5 GPa), which is strong enough for extreme stress, such as dendrite puncture. Scanning electrochemical microscopy reveals the electron-insulating feature of the SEI, which can promote the uniform spherical zinc deposition underneath it. Consequently, AZIBs with the gradient SEI achieve ultra-long cycling stability of 4100 h in harsh conditions of 20 mA cm-2 and 20 mA h cm-2, super-high cumulative capacity of 41 A h cm-2, excellent reversibility with average coulombic efficiency of 99.8%, and an 11 000-cycle lifespan for Zn||NaV3O8 cell. A robust gradient rigid-soft coupling SEI layer on the Zn surface results in the ultra-long cycling stability and the high zinc utilization rate of AZIBs.