Planar Sodium‐Nickel Chloride Batteries with High Areal Capacity for Sustainable Energy Storage.

High‐temperature sodium‐nickel chloride (Na‐NiCl2) batteries are a promising solution for stationary energy storage, but the complex tubular geometry of the solid electrolyte represents a challenge for manufacturing. A planar electrolyte and cell design is more compatible with automated mass production. However, the planar cell design also faces a series of challenges, such as the management of molten phases during cycling. As a result, cycling of planar high‐temperature cells until now focused on moderate areal capacities and current densities. In this work, planar cells capable of integrating cost‐efficient nickel/iron electrodes at a substantially enhanced areal capacity of 150 mAh cm−2 is presented. Due to a low cell resistance during operation at 300 °C, these cells deliver a specific discharge energy of 300 Wh kg−1 at high discharge current densities of 80 mA cm−2 (C/2, 10%–100% state‐of‐charge). This results represent the first demonstration of planar Na‐NiCl2 cells at a commercially relevant combination of areal capacity, cycling rate, and energy efficiency. It is further identified the secondary molten NaAlCl4 electrolyte to contribute to the cell capacity during cycling. Mitigating electrochemical decomposition of NaAlCl4 will play an important role in further enhancing both cycling rates and cycle life of high temperature Na‐NiCl2 batteries. [ABSTRACT FROM AUTHOR]