Uniform High Ionic Conducting Lithium Sulfide Protection Layer for Stable Lithium Metal Anode

Abstract

DOI: 10.1002/aenm.201900858 dendrites,[6] guided lithium plating,[7] and nanostructured electrode design.[8] Among all the methods, the focus on solidelectrolyte interphase (SEI) between anode materials and electrolyte is one of the most critical issues. During LMB operation, the SEI that primarily originated from electrolyte decomposition, is easily cracked. This will locally enhance ion flux and promote nonuniform lithium depositing/stripping,[9] resulting in Li dendrites that can trigger internal short circuit and compromise battery safety. Repeated breakdown and repair of SEI during cycling create a vicious cycle which alternates between “uneven stripping/plating and SEI fracture,” brings about continuous loss of active materials and limited battery cycle life. Therefore, an ideal SEI should continuously passivate the anode and prevent the parasitic reactions between reactive anode and electrolyte to address the aforementioned problems in principle.[3] Previous studies have demonstrated several effective artificial SEI to protect lithium metal anode such as polymer,[10] inorganic conductive compounds,[11,12] electrolyte additives,[13,14] and carbonbased materials.[7,15] However, the evolution of SEI during cycling and key mechanisms such as impact of SEI quality on its stability need to be further explored.[16] Herein, we demonstrate a “simultaneous homogeneous and high ionic conductivity” strategy by developing a method of forming a uniform lithium sulfide (Li2S) protective layer for suppressing dendrite growth and stabilizing the lithium metal anode. Although Li2S interfacial layers through soluble electrolyte additives have been studied before,[14,17–20] the work here demonstrates that the elevated temperature (170 °C) and gas phase reaction are critical for the synthesis of a homogenous Li2S coating, which importantly can be used as SEI in carbonate electrolyte system. We reveal the evolution of thus formed Li2S artificial SEI component distribution during battery operation: the uniform and high ionic conductivity protective layer turns into a layered SEI that preserves protective function, rather than into a disordered, broken SEI mainly made up of parasitic reaction products. Simulation results also confirm the critical importance of compositional homogeneity and high ionic conductivity in stabilizing SEI. With this strategy, stable cycles in both high capacity symmetric cells and Li–LiFePO4 full cells were realized. We believe that this practical fabrication method, fundamental design strategy, and understanding on Artificial solid-electrolyte interphase (SEI) is one of the key approaches in addressing the low reversibility and dendritic growth problems of lithium metal anode, yet its current effect is still insufficient due to insufficient stability. Here, a new principle of “simultaneous high ionic conductivity and homogeneity” is proposed for stabilizing SEI and lithium metal anodes. Fabricated by a facile, environmentally friendly, and low-cost lithium solidsulfur vapor reaction at elevated temperature, a designed lithium sulfide protective layer successfully maintains its protection function during cycling, which is confirmed by both simulations and experiments. Stable dendritefree cycling of lithium metal anode is realized even at a high areal capacity of 5 mAh cm−2, and prototype Li–Li4Ti5O12 cell with limited lithium also achieves 900 stable cycles. These findings give new insight into the ideal SEI composition and structure and provide new design strategies for stable lithium metal batteries.