Potassium metal battery is a promising alternative to Li-ion battery for large-scale energy storage due to the abundant potassium resources and high energy density. However, it suffers from rapid capacity fading and safety issues due to the uncontrolled dendrite growth. Herein, we design a fluorine-free ultra-low concentration electrolyte (ULCE) with the super bulky [BPh₄]⁻ anions for stable potassium metal battery. In this special electrolyte, the migration rate of K⁺ in the electrolyte is about six times faster than that of the [BPh₄]⁻ anions because of the super bulky structure of the [BPh₄]⁻ anions, thus resulting in a high K⁺ transference number of 0.76. This high transference number can effectively make up for the deficiency of K⁺ in ULCE for ensuring the normal operation of the potassium metal battery. In addition, the improved transference number can also promote the uniform distribution of K⁺ flux on the surface of the K metal anode, resulting in uniform K deposition. As a result, this electrolyte achieves a high K plating/stripping Coulombic efficiency of 92.6% over 200 cycles and a stable discharging/charging for 100 cycles under the full battery configuration (K used as the anode and perylene-3,4,9,10-tetracarboxylic dianhydride used as the cathode).

The application of redox mediators has been considered as a promising strategy to boost the performance of aprotic Li-O₂ batteries. However, the issues brought with redox mediators, especially on the Li anode side have been overlooked. Here, we propose a facile approach of preparing a gel polymer membrane that not only allow uniform Li plating/stripping with large current densities over extended cycling but also inhibit the diffusion of redox mediators and avoid redox shuttling, self-discharge, and internal short-circuiting. More importantly, the gel polymer membrane prevents the penetration of O₂ and superoxide intermediates from the Li anode. Therefore, it ensures the successful application of both lithium anode and redox mediators in Li-O₂ batteries to achieve the desired high capacity and rate performance. Meanwhile, it helps understand the benefit and problems of added redox mediators and reactive oxygen species so that the performance of such Li-O₂ batteries can be truly evaluated.