High-Voltage Anode-Free Sodium Metal Batteries with Polymer Electrolyte Membranes

With the growing demands of electronic vehicles, portable electronics and large-scale energy storage, lithium metal batteries have been intensively studied due to the superior energy density provided by the lithium anode. However, the scarcity of lithium reserve significantly increases the cost of lithium metal batteries. Among alternatives, high-voltage anode-free sodium (Na) metal batteries (AFSMBs) are attracting enormous interest since they can attain high energy density in a low-cost way. One challenge that hinders the development of AFSMBs is that the highly reactive Na metal deposited on the current collector reacts with liquid electrolyte easily, especially when carbonate electrolytes are employed. As a result, the growth of Na dendrites and the formation of “dead Na” are exacerbated and thus shorten the lifespan of AFSMBs. Herein, we proposed some poly(vinylidene fluoride)-based polymer electrolyte membranes (PEMs) for stable and dendrite-free AFSMBs in carbonate electrolytes. The electrochemical properties of the PEMs are optimized by tuning the solvent, Na salt, and polymer matrix. The optimal PEMs have the merits of high ionic conductivity (>5×10^-4 S cm^-1), great electrochemical stability with Na (the Na|PEM|Na cell runs steadily for over 4,000 hours at a capacity of 0.1 mAh cm^-2 with a low voltage hysteresis), good electrolyte resistance and decent mechanical strength; therefore, when they are applied on the current collectors in AFSMBs, Na metal can be uniformly deposited on the current collectors and the deposited Na metal is effectively separated from carbonate-based electrolyte, leading to the reduction of parasitic reactions and the elongation of the cycle life of the batteries. With the PEMs, high-voltage AFSMBs with a 4.0V-class layered oxide cathode and carbonate electrolyte are assembled and tested. These batteries can run for ~60 cycles due to the stabilized interface between deposited Na and the corrosive carbonate electrolyte. In contrast, batteries without a PEM fail quickly in just a few cycles due to the poor Na deposition and much side reactions. In short, with the support from the chemically/thermally stable and mechanically strong PEMs, it is promising to fabricate long-cycle-life, high-energy-density, and low-cost AFSMBs.