The growing interest in room-temperature sodium metal rechargeable batteries (SMBs) for large-scale energy storage applications stems from their potential to serve as a cost-effective alternative to lithium-based batteries. This is attributed to the natural abundance of sodium and its low redox potential (-2.71 vs. SHE).^[1] However, the use of metallic sodium as an anode entails drawbacks such as the generation of a low conductive solid electrolyte interface (SEI), together with dendrites formation during the sodium plating/striping process, which implies short-circuit and safety hazards.^[2] Therefore, the development of advanced electrolytes that enable the use of metallic sodium as an anode is crucial for the overall good performance battery.

On the other hand, the commercialization of SMBs also requires the development of practical cathodes that provide satisfactory rate performance, long cycling life, and high areal capacities. During several years, inorganic active materials such as layered transition metal oxide (LTMO), polyanionic compounds, or Prussian blue analogs (PBA) have been widely studied as cathodes in sodium-based batteries. However, these inorganic compounds present low insertion kinetic, their theoretical capacity is commonly limited to one-electron transfer, and exhibit severe volume alterations during intercalation–deintercalation cycling leading to poor cycling stability. On the contrary, redox-active organic compounds can offer multielectron reactions, present high theoretical capacity, and are able to better accommodate volume changes.^[3,4] However, the low electrical conductivity and the partial dissolution of organic materials in the electrolyte often limit the areal capacity and the long-term cyclability of the battery.

In this communication, it is described the use of a highly concentrated electrolyte (> 7M) based on liquid ammonia, whose formula is NaI·3.3NH₃,^[5] which can effectively stabilize the metallic sodium delivering a coulombic efficiency of 99.8 % during the plating/stripping process, even at very high currents. This electrolyte is utilized in a sodium metal battery (SMB) where the metallic sodium anode is combined with a hybrid anthraquinone-based conjugated microporous polymer as cathode.^[6] Due to the intrinsic textural properties of this compound it is possible to prepare thick electrodes maintaining excellent electrochemical performance.

All in all, the battery depicted here shows an outstanding areal capacity of 7 mAh·cm^-2, never reported before for SMBs. In addition, this battery has demonstrated excellent rate capability up to 250C (37.3 A g^-1, near 70% capacity retention) and stable cycling (80% capacity retention after 4000 cycles with coulombic efficiency close to 100 %). In summary, the combination of a hybrid conjugated porous polymer with an ammoniate-based electrolyte results in a sodium metal battery characterized by high energy and power density, excellent areal capacity and long-term cyclability. This combination proves useful in designing practical Sodium Metal Batteries (SMBs).