graphite has a theoretical capacity of 279 mAh g −1 . [3,17] In 2015, Ji and Hu have separately demonstrated the electrochemical intercalation of K ions into graphite in potassium hexafluorophosphate (KPF 6 )/ ethylene carbonate (EC)-diethyl carbonate (DEC) electrolytes. [4,18] However, their works did not achieve a highly reversible K + insertion/extraction process in KPF 6 /EC-DEC electrolyte because the formed solid electrolyte interphase (SEI) becomes fragile and unstable due to the large volume variation (≈60%) during K + insertion/extraction. [19,20] Compared to the conventional lowconcentration electrolyte (LCE), adopting a high-concentration electrolyte (HCE, e.g., >3 m) is a promising strategy to solve the above problem because it possesses some unusual physicochemical and electrochemical properties due to the unique solvation structure of ions, which make it different from an LCE. [21][22][23][24] In 2007, Jeong adopted the concentrated lithium bisperfluoroethylsulfonyl imide LiN(SO 2 C 2 F 5 ) 2 / propylene carbonate (PC) to realize a reversible graphite anode for lithium-ion batteries. [24] Recently, Komaba and Lu have reported the highly reversible graphite anode for PIBs at concentrated potassium bis(fluorosulfonyl)imide (KFSI)/ dimethoxyethane (DME) and KFSI/ethyl methyl carbonate electrolytes, respectively. [25,26] Despite these progresses, the issues of high viscosity, low ionic conductivity, and the increased cost of the HCE still hinder its practical applications. To overcome these disadvantages in using HCE, several groups have added a low-polarity cosolvent to dilute an HCE by forming a localized high-concentration electrolyte (LHCE). It is believed that the introduced cosolvent does not participate in the solvation process. Zhang's group used the bis(2,2,2,-tri-fluoroethyl) ether to dilute the concentrated lithium bis(fluorosulfonyl) imide (LiFSI)/dimethyl carbonate and improve the coulombic efficiency (CE) of lithium metal anodes without dendrite formation. [27] They also diluted concentrated LiFSI in sulfone with a fluorinated ether for high-voltage (4.9 V) lithium metal batteries. [28] Wang's group used the same cosolvent in the concentrated LiFSI/DME to increase both coulombic efficiencies of S cathode and Li anode for Li-S batteries. [29] However, none of the LHCE reported to date has been applied in PIBs, its stability and compatibility with PIBs remain in question.Herein, our work reports for the first time that a highly reversible K + insertion/extraction into graphite interlayer can Reversible intercalation of potassium-ion (K + ) into graphite makes it a promising anode material for rechargeable potassium-ion batteries (PIBs). However, the current graphite anodes in PIBs often suffer from poor cyclic stability with low coulombic efficiency. A stable solid electrolyte interphase (SEI) is necessary for stabilizing the large interlayer expansion during K + insertion. Herein, a localized high-concentration electrolyte (LHCE) is designed by adding a highly fluorinated ether into the con...

Rechargeable potassium batteries, including the potassium–oxygen (K–O2) battery, are deemed as promising low-cost energy storage solutions. Nevertheless, the chemical stability of the K metal anode remains problematic and hinders their development. In the K–O2 battery, the electrolyte and dissolved oxygen tend to be reduced on the K metal anode, which consumes the active material continuously. Herein, an artificial protective layer is engineered on the K metal anode via a one-step method to mitigate side reactions induced by the solvent and reactive oxygen species. The chemical reaction between K and SbF3 leads to an inorganic composite layer that consists of KF, Sb, and KSb x F y on the surface. This in situ synthesized layer effectively prevents K anode corrosion while maintaining good K+ ionic conductivity in K–O2 batteries. Protection from O2 and moisture also ensures battery safety. Improved anode life span and cycling performance (>30 days) are further demonstrated. This work introduces a novel strategy to stabilize the K anode for rechargeable potassium metal batteries.

In the past 20 years, research in metal−O 2 batteries has been one of the most exciting interdisciplinary fields of electrochemistry, energy storage, materials chemistry, and surface science. The mechanisms of oxygen reduction and evolution play a key role in understanding and controlling these batteries. With intensive efforts from many prominent research groups, it becomes clear that the instability of superoxide in the presence of Li ions (Li + ) and Na ions (Na + ) is the fundamental root cause for the poor stability, reversibility, and energy efficiency in aprotic Li−O 2 and Na−O 2 batteries. Stabilizing superoxide with large K ions (K + ) provides a simple but elegant solution. Superoxide-based K−O 2 batteries, invented in 2013, adopt the one-electron redox process of O 2 /potassium superoxide (KO 2 ). Despite being the youngest metal−O 2 technology, K−O 2 is the most promising rechargeable metal−air battery with the combined advantages of low costs, high energy efficiencies, abundant elements, and good energy densities. However, the development of the K−O 2 battery has been overshadowed by Li−O 2 and Na−O 2 batteries because one might think K−O 2 is just an analogous extension. Moreover, due to the lower specific energy and the high reactivity of K metal, K− O 2 is often underestimated and deemed unsuitable for practical applications. The objective of this Perspective is to highlight the unique advantages of K−O 2 chemistry and to clarify the misconceptions prompted by the name "superoxide" and the judgment bias based on the claimed theoretical specific energies. We will also discuss the current challenges and our perspectives on how to overcome them.

Discovering new K-ion solid-state electrolytes is crucial for the emerging K-batteries to improve energy density, cycle life, and safety. Here, we present a combined experimental and theoretical study of antiperovskite K 3 OI as a K-ion solid-state electrolyte. A solid−solid phase transition at approximately 240 °C induces an increase in ionic conductivity by 2 orders of magnitude. Anion disorder in the I−O sublattice is found to be a potential mechanism for the observed phase transition. The Ba-doped K 3 OI sample K 2.9 Ba 0.05 OI achieves 3.5 mS cm −1 after the phase transition with a low activation energy of 0.36 eV. Stable cycling of K/ K 2.9 Ba 0.05 OI/K symmetric cells are observed with a low overpotential of 50 mV at 0.5 mA/cm 2 at 270 °C. This study not only reports K 3 OI as a promising K-ion solid-state electrolyte that is compatible with reactive K metal but also improves the understanding of alkali antiperovskite solid-state electrolytes in general.

Antiperovskites of composition M 3 AB (M = Li, Na, K; A = O; B = Cl, Br, I, NO 2 , etc.) have recently been investigated as solid-state electrolytes for all-solid-state batteries. Inspired by the impressive ionic conductivities of Li 3 OCl 0.5 Br 0.5 and Na 3 OBH 4 as high as 10 −3 S/cm at room temperature, many variants of antiperovskite-based Li-ion and Na-ion conductors have been reported, and K-ion antiperovskites are emerging. These materials exhibit low melting points and thus have the advantages of easy processing into films and intimate contacts with electrodes. However, there are also issues in interpreting the stellar materials and reproducing their high ionic conductivities. Therefore, we think a critical review can be useful for summarizing the current results, pointing out the potential issues, and discussing best practices for future research. In this critical review, we first overview the reported compositions, structural stabilities, and ionic conductivities of antiperovskites. We then discuss the different conduction mechanisms that have been proposed, including the partial melting of cations and the paddlewheel mechanism for cluster anions. We close by reviewing the use of antiperovskites in batteries and suggest some practices for the community to consider.