The status of room‐temperature potassium‐ion batteries is reviewed in light of recent concerns regarding the rising cost of lithium and the fact that room‐temperature sodium‐ion batteries have yet to be commercialised thus far. Initial reports of potassium‐ion cells appear promising given the infancy of the research area. This review presents not only an overview of the current potassium‐ion battery literature, but also attempts to provide context by describing previous developments in lithium‐ion and sodium‐ion batteries and the electrochemical reaction mechanisms discovered thus far. Perspectives and directions on the techniques available to characterize newly developed battery materials are also provided based on our experience and knowledge from the literature. It is hoped that through this review, the potential of potassium‐ion batteries as a competitive energy‐storage technology will be realised, and the accessibility and available knowledge of the techniques required to develop the technology will be made apparent.

Prussian blue analogues (PBAs) are reported to be efficient sodium storage materials because of the unique advantages of their metal–organic framework structure. However, the issues of low specific capacity and poor reversibility, caused by phase transitions during charge/discharge cycling, have thus far limited the applicability of these materials. Herein, a new approach is presented to substantially improve the electrochemical properties of PBAs by introducing high entropy into the crystal structure. To achieve this, five different metal species are introduced, sharing the same nitrogen‐coordinated site, thereby increasing the configurational entropy of the system beyond 1.5R. By careful selection of the elements, high‐entropy PBA (HE‐PBA) presents a quasi‐zero‐strain reaction mechanism, resulting in increased cycling stability and rate capability. The key to such improvement lies in the high entropy and associated effects as well as the presence of several active redox centers. The gassing behavior of PBAs is also reported. Evolution of dimeric cyanogen due to oxidation of the cyanide ligands is detected, which can be attributed to the structural degradation of HE‐PBA during battery operation. By optimizing the electrochemical window, a Coulombic efficiency of nearly 100% is retained after cycling for more than 3000 cycles.

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Improving the interfacial stability between cathode active material (CAM) and solid electrolyte (SE) is a vital step toward the development of high‐performance solid‐state batteries (SSBs). One of the challenges plaguing this field is an economical and scalable approach to fabricate high‐quality protective coatings on the CAM particles. A new wet‐coating strategy based on preformed nanoparticles is presented herein. Nonagglomerated nanoparticles of the coating material (≤5 nm, exemplified for ZrO2) are prepared by solvothermal synthesis, and after surface functionalization, applied to a layered Ni‐rich oxide CAM, LiNi0.85Co0.10Mn0.05O2 (NCM85), producing a uniform surface layer with a unique structure. Remarkably, when used in pelletized SSBs with argyrodite Li6PS5Cl as SE, the coated NCM85 is found to exhibit superior lithium‐storage properties (qdis ≈ 204 mAh gNCM85−1 at 0.1 C rate and 45 °C) and good rate capability. The key to the observed improvement lies in the homogeneity of coating, suppressing interfacial side reactions while simultaneously limiting gas evolution during operation. Moreover, this strategy is proven to have a similar effect in liquid electrolyte‐based Li‐ion batteries and can potentially be used for the application of other, even more favorable, nanoparticle coatings.

Solid-state batteries (SSBs) have been touted as the next major milestone for electrochemical energy storage, improving safety and enabling higher energy densities. LiNiO 2 (LNO) has long been considered a promising cathode material; however, its commercial implementation is complicated by stability issues, including reactivity toward the electrolyte components. To address this, a detailed study probing the electrochemical behavior of LNO in pellet-stack SSB cells, in combination with argyrodite Li 6 PS 5 Cl solid electrolyte (SE) and Li 4 Ti 5 O 12 anode, is for the first time presented herein. In this configuration, LNO delivers a specific capacity of 105 mAh/g LNO after 60 cycles (0.2C, 45 °C), which was improved considerably to 153 mAh/g LNO by applying a LiNbO 3 coating to the material. Using complementary operando and ex situ characterization techniques, contributions to the initial capacity loss and capacity fading could be resolved and attributed to decomposition of the argyrodite SE and to volume changes and gas evolution in LNO.