Contact loss in all solid-state Li-ion batteries via deposition of impurities

Solid-state batteries promise high energy storage with enhanced safety but suffer from contact loss at the lithium (Li) metal anode-solid electrolyte interface as Li is stripped into the ionically-conducting solid electrolyte. Contrary to the prevailing view that the contact loss arises from void formation in the Li metal at this interface, we demonstrate, in cells with garnet electrolytes, that it instead arises from the deposition of insulating impurities from within the electrode onto the interface. These impurities form a porous layer, and an imposed stack pressure is required for Li to creep through the pores of this layer. This proposed mechanism is supported by our experimental observations that no voids are present on the interface and by our finding that contact loss persists at stack pressures that are sufficiently high to collapse any voids in the Li. Theoretical models developed using the impurity deposition mechanism are in excellent agreement with measurements. This finding has significant implications for improving the design of all solid-state batteries including the development of anode-free cells for which impurities within the electrode are expected to be substantially reduced.