Ultrathin salt-free polymer-in-ceramic electrolyte for solid-state sodium batteries

The practical energy density of solid-state batteries remains limited, partly because of the lack of a general method to fabricate thin membranes for solid-state electrolytes with high ionic conductivity and low area-specific resistance (ASR). Herein, we use an ultrahigh concentration of a ceramic ion conductor (Na₃SbS₄) to build an ion-conduction “highway”, and a polymer (polyethylene oxide, 2 ​wt%) as a flexible host to prepare a polymer-in-ceramic ion-conducting membrane of approximately 40 ​μm. Without the use of any salt (e.g., NaPF₆), the resulting membrane exhibits a threefold increase in electronic ASR and a twofold decrease in ionic ASR compared with a pure ceramic counterpart. The activation energy for sodium-ion transport is only 190 ​meV in the membrane, similar to that in pure ceramic, suggesting ion transport predominantly occurs through a percolated network of ion-conducting ceramic particles. The salt-free design also provides an opportunity to suppress dendritic metal electrodeposits, according to a recently refined chemomechanical model of metal deposition. Our work suggests that salt is not always necessary in composite solid-state electrolytes, which broadens the choice of polymers to allow the optimization of other desired attributes, such as mechanical strength, chemical/electrochemical stability, and cost. © 2021 The Authors.