The transition from fossil fuels to renewable energy sources requires economic, high-performance electrochemical energy storage. Hightemperature sodium-metal chloride batteries combine long cycle and calendar life, with high specific energy, no self-discharge, and minimum maintenance requirements, while employing abundant raw materials. However, largescale deployment in mobility and stationary storage applications is currently hindered by high production cost of the complex, commercial tubular cells and limited rate capability. The present study introduces sodium-metal chloride cells with a simple, planar architecture that provide high specific power while maintaining the inherent high specific energy. Rational cathode design, considering critical transport processes and the effect of cathode composition on the cell resistance, enables the development of high-performance cells with average discharge power of 1022 W kg −1 and discharge energy per cycle of 258 Wh kg −1 on cathode composite level, shown over 140 cycles at an areal capacity of 50 mAh cm −2 . This corresponds to a 3.2C discharge over 80% of full charge. Compared to the best performing planar sodium-metal chloride cells with similar cycling stability and mass loading in the literature, the presented performance represents an increase in specific power by more than a factor of four, while also raising the specific energy by 74%.
Na–NiCl
2
thermal batteries have been developed for applications such as electric energy backup, energy storage, and automotive application. A typical Na–NiCl
2
battery consists of a molten sodium anode, a solid‐state electrolyte (β″‐alumina), and a secondary liquid electrolyte (NaAlCl
4
) in the cathode side, with NiCl
2
as the active cathode materials. These systems, operating at high temperatures (about 270–350 °C), provide a battery completely independent of ambient temperature with high specific energy and high specific power. Special characteristics such as relatively low cost, long deep cycle life, long‐term cold‐storage life, abuse resistant, zero electrical self‐discharge, maintenance free, and safety place this technology as the best choice where environmentally strong conditions are present and other secondary battery systems fail.
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.
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