Intermix-free Liquid Electrolyte-based High-energy Na-ion Bipolar Batteries

Abstract

Electrolyte intermixing in bipolar Na-ion batteries is a significant challenge, particularly with liquid electrolytes, which can lead to short-circuiting and performance loss. Solid-state or gel electrolytes can mitigate this issue; however, interfacial resistance, limited utilization of the active materials, and cell dead mass have adversely affected performance. Thus, it is of paramount importance to mitigate liquid-electrolyte intermixing and increase the areal loading to achieve higher energy density at the cell level. Here, we report an electrolyte intermix-free high-energy Na-ion bipolar battery. To demonstrate the operation of bipolar cells, thermoplastic polyolefin wax is used to seal the cell and prevent intermixing of the liquid electrolytes. The bipolar electrode, i.e., Na 3 V 2 (PO 4 ) 3 , is utilized due to the reversible oxidation/reduction of V 2+ /V 3+ and V 3+ /V 4+ states. A high material loading of approximately 25 mg/cm² is used to increase the energy density and demonstrate the industrial feasibility of the Na-ion bipolar battery. The liquid electrolyte-tosolid content is optimized to ~1 µL/mg, yielding an energy density of ~180 Wh/kg. The bipolar battery achieves a high round-trip efficiency of over 90% and retains more than 94% of its capacity after 100 cycles. We believe that the present findings are critical and may pave the way forward for the development of high-energy Na-ion bipolar batteries.