Metal-incorporated interphases formed between inorganic solid electrolytes and Li metal: beneficial or detrimental?

Abstract

Most inorganic solid electrolytes (ISEs) undergo reduction when in contact with Li metal to form a new interphase. Traditionally, it was suggested that such interphases should ideally be metal-free, since metals with high electronic conductivity may contribute to the generation of mixed conducting interphases, thereby leading to continuous reduction of the ISE and significant consumption of Li metal. However, recent studies challenge this viewpoint, showing that certain metal-incorporated interphases (MIIs) can suppress Li intrusion and improve lithium reversibility compared to metal-free interphases. In this perspective, we aim to address the ongoing debate surrounding the role of metal incorporation at the interphase formed between ISEs and Li metal. We first propose a protocol for critically assessing the MII based on Li asymmetric cells. Throughout the comparative analysis, we discover that the use of conventional Li symmetric cells yields misleading results when evaluating MIIs. In contrast, our proposed Li asymmetric cells can more accurately reflect the impact of MIIs on Li reversibility. Furthermore, we observe that when the metal volume fraction in the MII is sufficiently low, the MII significantly enhances lithium reversibility compared to its metal-free counterparts. We explain this phenomenon through percolation theory. Lastly, we provide insights into future design strategies for MIIs, aiming to offer guidance for the progress of Li/ISE interface design.