Design methodology of a promising category of metal phosphate electrodes for quasi-solid-state proton batteries

A proton battery is a low-cost and safe alternative to a Li-ion battery. However, limited electrode options, especially cathodes, stably perform in acidic electrolytes and many of them show large voltage decreases along with the depth of discharge. Herein, an all-phosphate configuration is proposed to open up an avenue for a promising category of metal phosphate electrodes. Fe^2+/3+ and Mn^2+/3+ are investigated, for the first time, as example redox couples of phosphates for anode and cathode, respectively. The dynamic electronic and structural variations during the proton (de)-insertion process are captured using in-situ X-ray absorption spectroscopy (XAS). In-situ distribution of relaxation times (DRT) analysis reveals that the insufficient charge transfer hinders the performance, which is optimized by forming a phosphate-carbon composite. An NH₄MnPO₄·H₂O-carbon composite presents a capacity of 121 mAh g^-1 with flat voltage profiles and excellent high-rate performance at 150 A g^-1. A full quasi-solid-state proton battery demonstrates smooth operation for 5000 cycles.