Unstable Li metal/solid polymer electrolyte (SPE) interface, or solid electrolyte interface (SEI), limits applications of SPEs in lithium metal batteries (LMBs). Arising from ultrahigh Li reducibility, parasitic reactions (for instance, reaction of Li with PEO to form C₂H₄, Li₂O, and H₂) inevitably take place at Li/PEO interface, harming the electrochemical performances of LMBs. Additionally, during the battery operation, the Li/SPE interface continuously thickens due to the repeated reactions between the Li metal and fresh SPE, leading to uneven surface morphology and large electrochemical impedance. These events occurring at the Li/PEO interface will lead to capacity loss and inferior battery lifespan. SEI is an ionically conducting and electronically insulating nanolayer which spontaneously forms on anodes as a result of reductive decomposition of an electrolyte in lithium batteries. The SEI layer plays a critical role in Li plating/stripping during charge/discharge processes and, hence, is a key factor in determining the morphology of Li electrochemical deposits, power capability, shelf life and cycle life, and safety of lithium batteries. The issue of an unstable Li/PEO-based SPE interface can also be addressed by engineering an interface rich in components with high Li ions conductivity and high stability against Li metal. Although the importance of the SEI layer is well-recognized, the structure, chemistry, and kinetics of the SEI layer formation and evolution are not well-understood, which is negatively impacting battery technology.

Herein, we develop an LMB with superior electrochemical performance and a long lifespan enabled by an engineered Li/SPE interface. The SPE is made based on PEO-LiTFSI and a cyclic phosphazene compound as additive. The electrochemical degradation of the phosphazene compound during electrochemical cycling of the LMB can lead to an SEI layer rich in lithium nitride (Li₃N), lithium phosphide (Li₃P), and lithium phosphide (Li₃PO₄). Using cryo-TEM coupled with EDS and EELS characterization, detailed structure and chemistry of the SEI layer at Li/SPE interface is explored. The interface between the Li metal anode and the control SPE seems unstable, leading to significant fluctuations in overpotential followed by a sudden drop and finally a short-circuit. In contrast, a long-term stability could be observed for the symmetric Li/Li cell made using the engineered SPE, indicating the stable nature of the interface. A significant capacity loss is observed for the Li/LFP cell made using the control PEO-LiTFSI SPE. This can be correlated to the poor Li ions conductivity and instability against Li metal of the conventional SEI layer formed in the control PEO-LiTFSI SPE. On the other hand, and owing to the SEI layer rich in Li₃N, Li₃P, and Li₃PO₄ with high Li ions conductivity and stability against Li metal, a stable and long lifespan LMB made of the engineered PEO-LiTFSI-Phosphazene SPE is observed. The critical role of Li₃N, Li₃P and Li₃PO₄ in improving the electrochemical performance will be investigated by density functional theory (DFT) calculations.

Reference

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