Potassium ion batteries (PIBs) are recognized as one promising candidate for future energy storage devices due to their merits of cost‐effectiveness, high‐voltage, and high‐power operation. Many efforts have been devoted to the development of electrode materials and the progress has been well summarized in recent review papers. However, in addition to electrode materials, electrolytes also play a key role in determining the cell performance. Here, the research progress of electrolytes in PIBs is summarized, including organic liquid electrolytes, ionic liquid electrolytes, solid‐state electrolytes and aqueous electrolytes, and the engineering of the electrode/electrolyte interfaces is also thoroughly discussed. This Progress Report provides a comprehensive guidance on the design of electrolyte systems for development of high performance PIBs.

By contrast, sodium is widely distributed and accessible worldwide, offering a low-cost and inexhaustible resource, and has similar electrochemical properties as lithium, making Na-ion batteries (NIBs) the most promising alternative to LIBs, especially for large-scale stationary applications. [4][5][6][7] Cathode materials, mainly including oxides [8][9][10][11] and polyanionic compounds, [12][13][14][15] have been reported to deliver encouraging electrochemical performances for practical applications. Although extensive investigations into anode materials, such as carbonaceous materials, [16][17][18][19] alloys, [20][21][22] oxides, [23][24][25] and organic compounds, [26][27][28] have been performed, most show insufficient performance to match the cathodes well, thus greatly impeding the commercialization of NIBs. Presently, hard carbons (HCs), turbostratic carbon with nanosized parallel/random-aligned graphene sheets with abundant nanopores and defects, are the most intriguing anodes for NIBs due to their good performance. [18,19,29,30] Typically, the sodium-storage behavior in HCs shows two distinct voltage regions: one slope above 0.1 V and one flat plateau below 0.1 V. Although many investigations into Na storage in HCs have been conducted, [18,[31][32][33][34][35][36][37][38][39][40][41][42] the specific Na-insertion mechanisms in the two voltage regions are still controversial.Stevens and Dahn first investigated the sodium-insertion behavior into HCs derived from pyrolytic glucose. [31][32][33] They conducted in situ small-angle X-ray scattering (SAXS)/wideangle X-ray scattering measurements at different sodiation/desodiation states and observed a (002) peak shift of the graphitic interlayer spacing in the high-potential sloping region and a change in the electron density of voids in the low-potential plateau, revealing Na + -ion intercalation inside the turbostratic graphene layers (sloping region) and nanopore filling (plateau region), respectively. Subsequently, similar results were obtained by Komaba et al. using ex situ X-ray diffraction (XRD) and SAXS. [34] The Raman peak of the G-band displayed an obvious shift in the sloping region, while no shift in the plateau region was observed, further manifesting that sodium intercalated between the graphene layers and then filled into nanopores. Then, they performed solid-state 23 Na nuclear magnetic resonance (NMR) analysis to study the state of Na-ion insertion in HCs and corroborated the storage mechanisms. [35] Conversely, Liu and co-workers concluded that Na + ions can intercalate between the graphene layers of HCs with a Hard carbons (HCs) are the most promising candidate anode materials for emerging Na-ion batteries (NIBs). HCs are composed of misaligned graphene sheets with plentiful nanopores and defects, imparting a complex correlation between its structure and sodium-storage behavior. The currently debated mechanism of Na + -ion insertion in HCs hinders the development of high-performance NIBs. In this article, ingenious and reliable strategies a...

Bismuth has emerged as a promising anode material for sodium‐ion batteries (SIBs), owing to its high capacity and suitable operating potential. However, large volume changes during alloying/dealloying processes lead to poor cycling performance. Herein, bismuth nanoparticle@carbon (Bi@C) composite is prepared via a facile annealing method using a commercial coordination compound precursor of bismuth citrate. The composite has a uniform structure with Bi nanoparticles embedded within a carbon framework. The nanosized structure ensures a fast kinetics and efficient alleviation of stress/strain caused by the volume change, and the resilient and conductive carbon matrix provides an interconnected electron transportation pathway. The Bi@C composite delivers outstanding sodium‐storage performance with an ultralong cycle life of 30 000 cycles at a high current density of 8 A g−1 and an excellent rate capability of 71% capacity retention at an ultrahigh current rate of 60 A g−1. Even at a high mass loading of 11.5 mg cm−2, a stable reversible capacity of 280 mA h g−1 can be obtained after 200 cycles. More importantly, full SIBs by pairing with a Na3V2(PO4)3 cathode demonstrates superior performance. Combining the facile synthesis and the commercial precursor, the exceptional performance makes the Bi@C composite very promising for practical large‐scale applications.

Potassium−sulfur (K−S) batteries are a promising alternative to lithium ion batteries for large-area energy storage applications, owing to their high capacity and inexpensiveness, but they have been seldom investigated. Here we report room-temperature K−S batteries utilizing a microporous carbon-confined small-molecule sulfur composite cathode. The synergetic effects of the strong confinement of microporous carbon matrix and the small-molecule sulfur structure can effectually eliminate the formation of soluble polysulfides and ensure a reversible capacity of 1198.3 mA h g −1 and retain 72.5% after 150 cycles with a Coulombic efficiency of ∼97%. The potassium-storage mechanism was investigated by X-ray photoelectron spectroscopy analysis and theoretical calculations. The results suggest that K 2 S is the final potassiation product along with the reaction of 2K + S ↔ K 2 S, giving a theoretical capacity of 1675 mA h g −1 . Our findings not only provide an effective strategy to fabricate high-performance room-temperature K−S batteries but also offer a basic comprehension of the potassium storage mechanism of sulfur cathode materials.

Red phosphorus (P) has been recognized as a promising storage material for Li and Na. However, it has not been reported for K storage and the reaction mechanism remains unknown. Herein, a novel nanocomposite anode material is designed and synthesized by anchoring red P nanoparticles on a 3D carbon nanosheet framework for K-ion batteries (KIBs). The red P@CN composite demonstrates a superior electrochemical performance with a high reversible capacity of 655 mA h g at 100 mA g and a good rate capability remaining 323.7 mA h g at 2000 mA g , which outperform reported anode materials for KIBs. The transmission electron microscopy and theoretical calculation results suggest a one-electron reaction mechanism ofP + K + e → KP, corresponding to a theoretical capacity of 843 mA h g ,which is the highest value for anode materials investigated for KIBs. The study not only sheds light on the rational design of high performance red P anodes for KIBs but also offers a fundamental understanding of the potassium storage mechanism of red P.