scarce resources, uneven distribution, and arduous recycling of lithium. Sodium-ion batteries (SIBs) and potassium-ion batteries (PIBs) operating with similar mechanism to that of LIBs are considered as affordable alternatives, [2] as a result of the desirable performances as well as much abundant resources of sodium and potassium. [3] The performances of the alkali metal-ion batteries depend much on the cathode and anode materials. Various types of cathode materials based on the reversible insertion/ extraction of alkali metal ions including transition metal oxides, fluorides, phosphates, hexacyanoferrates, and sulfates have been developed, and plenty of them exhibit desirable energy density and cycling performances. [4] Progress on the research for anode materials is relatively slow, however, as compared with their cathode counterparts. [5] Based on the reaction mechanisms, the anodes generally fall into three categories: insertion based, conversion based, and alloying based. [6] The conversion-based materials exhibit high theoretical specific capacities derived from the conversion reactions during the uptake of alkali metal ions. [7] Due to the large volume variations during charge/discharge, however, the conversion-based anodes exhibit rapid capacity fading. The alloyingbased materials deliver high specific capacity by the alloying reaction, but the material pulverization derived from repeated volume changes results in poor reversibility. [8] Insertion-based materials include titanium-based oxides and carbonaceous materials. Although the small volume change, high rate capability, and good cycling stability of titanium-based oxides are desirable, their high working voltages and low specific capacities are detrimental to the power density of the full cells. [9] Carbonaceous materials, including graphite, carbon nanotubes (CNTs), graphene, soft carbon (SC), hard carbon (HC), etc., are promising anode candidates for alkali metal-ion batteries. [10] Graphite has been developed as a practical anode for commercial LIBs. They have steady discharge curves and low operation potential (≈0.1 V vs Li + /Li), and the formation of stable graphite intercalation compounds (GICs) LiC 6 delivers a moderate theoretical intercalation capacity of 372 mAh g −1 . [11] While the intercalation capacities of graphite anodes for SIBs and PIBs are not satisfactory, delivering 35 mAh g −1 for SIBs with NaC 64Hard carbon (HC) is recognized as a promising anode material with outstanding electrochemical performance for alkali metal-ion batteries including lithium-ion batteries (LIBs), as well as their analogs sodium-ion batteries (SIBs) and potassium-ion batteries (PIBs). Herein, a comprehensive review of the recent research is presented to interpret the challenges and opportunities for the applications of HC anodes. The ion storage mechanisms, materials design, and electrolyte optimizations for alkali metal-ion batteries are illustrated in-depth. HC is particularly promising as an anode material for SIBs. The solid-electrolyte interph...

photocopying process took nearly a century from 1843 until the early 1940s, while the detailed crystal structure of PB was first confirmed as cubic by Ludi and co-workers in 1977, which is now widely accepted. [6] Remarkably, the past four decades have witnessed the exploration of PB in more and more new and totally different, but very promising application areas, reaching from rechargeable batteries [7] to catalysis [8] and biosensors, [9] from optically switchable films in electrochromic devices (smart windows) [10] to a helpful nanomaterial for cancer therapy. [11] Due to their excellent redox activity, low cost, and highly reversible phase transitions during the insertion/extraction process of certain cations, PB and PBAs have also been widely investigated as promising active materials for energy storage devices, especially for commercial sodium-ion batteries (SIBs) beyond other batteries system (potassium-ion batteries, [12,13] lithium-ion batteries (LIBs), [14] lithium-sulfur batteries (LI-S), [15] lithium-air batteries, [16] zinc-air batteries, [17] solid-state batteries, [18] etc.) in large-scale stationary energy storage systems in the near future. [19,20] The chemical formulas of PBAs could be represented asHere, A represents a single alkali metal or alkaline earth metal, or a mixture of these metals, while M 1 and M 2 typically are transition metals bonded by CN − bonds to form a 3D open structure with the capability to host element(s) A inside the crystal structure. □ represents the vacancy that is caused by the loss of an M 2 (CN) 6 group and the occupation by coordination water and interstitial water, the species and ionic radii of which are shown in Figure 2a. [21] With the different species and various ratios of A/M 1 /M 2 , the number of family members could reach more than 100, sharing different crystal phases, including monoclinic, [22,23] rhombohedral, [24,25] cubic, [26,27] tetragonal, [28] hexagonal, [29] etc. According to the amount of redox-active sites for battery application, PB and PBAs could be divided into dual-electron transfer type (DE-PBAs: M 1 and M 2 = Mn, Fe, Co) and single-electron transfer type (SE-PBAs: M 1 = Zn, Ni and M 2 = Fe, Co, Mn) with theoretical specific capacity of 170 and 85 mAh g −1 , respectively. [21] Taking the high average voltage and capacity of the DE-PBAs into consideration, they are promising and competitive, even to the level of LiFePO 4 (a well-known cathode material for the LIBs), for high energy density devices (≈450 Wh kg −1 on the material level). On the other hand, the negligible structural distortion and high conductivity of SE-PBAs make them desirable choices for fast-charging and long-life devices. [20,30] Prussian blue analogues (PBAs) have attracted wide attention for their application in the energy storage and conversion field due to their low cost, facile synthesis, and appreciable electrochemical performance. At the present stage, most research on PBAs is focused on their material-level optimization, whereas their properties in practical b...

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As an anode material for sodium-ion batteries (SIBs), hard carbon (HC) presents high specific capacity and favorable cycling performance. However, high cost and low initial Coulombic efficiency (ICE) of HC seriously limit its future commercialization for SIBs. A typical biowaste, mangosteen shell was selected as a precursor to prepare low-cost and high-performance HC via a facile one-step carbonization method, and the influence of different heat treatments on the morphologies, microstructures, and electrochemical performances was investigated systematically. The microstructure evolution studied using X-ray diffraction, Raman, Brunauer–Emmett–Teller, and high-resolution transmission electron microscopy, along with electrochemical measurements, reveals the optimal carbonization condition of the mangosteen shell: HC carbonized at 1500 °C for 2 h delivers the highest reversible capacity of ∼330 mA h g –1 at a current density of 20 mA g –1 , a capacity retention of ∼98% after 100 cycles, and an ICE of ∼83%. Additionally, the sodium-ion storage behavior of HC is deeply analyzed using galvanostatic intermittent titration and cyclic voltammetry technologies.

Vanadium‐based materials are fascinating potential cathodes for high energy density Zn‐ion batteries (ZIBs), due to their high capacity arising from multi‐electron redox chemistry. Most vanadium‐based materials suffer from poor rate capability, however, owing to their low conductivity and large dimension. Here, we propose the application of V2C MXene (V2CTx), a conductive 2D nanomaterial, for achieving high energy density ZIBs with superior rate capability. Through an initial charging activation, the valence of surface vanadium in V2CTx cathode is raised significantly from V2+/V3+ to V4+/V5+, forming a nanoscale vanadium oxide (VOx) coating that effectively undergoes multi‐electron reactions, whereas the inner V‐C‐V 2D multi‐layers of V2CTx are intentionally preserved, providing abundant nanochannels with intrinsic high conductivity. Owing to the synergistic effects between the outer high‐valence VOx and inner conductive V‐C‐V, the activated V2CTx presents an ultrahigh rate performance, reaching 358 mAh g−1 at 30 A g−1, together with remarkable energy and power density (318 Wh kg−1/22.5 kW kg−1). The structural advantages of activated V2CTx are maintained after 2000 cycles, offering excellent stability with nearly 100% Coulombic efficiency. This work provides key insights into the design of high‐performance cathode materials for advanced ZIBs.

Prussian blue analogs with an open framework are ideal cathodes for Na‐ion batteries. A superior high‐rate and highly stable monoclinic nickel hexacyanoferrate (NiHCF‐3) is synthesized via a facile one‐step crystallization‐controlled co‐precipitation method. It gives a high specific capacity of 85.7 mAh g−1, nearly to its theoretical value. It also exhibits an excellent rate capability with a high capacity retention ratio of 78% at 50 C and a stable cycling performance over 1200 cycles. Through the ex situ X‐ray diffraction and pair distribution function measurements, it is found that the monoclinic structure with distorted framework is greatly related to the high Na content. The electronic structure studies by density functional theory (DFT) calculation demonstrate that NiHCF‐3 deformation promotes the framework conductivity and improves the electrochemical activity of Fe, which results in an ultrahigh‐rate performance of monoclinic phase. Furthermore, the high‐quality monoclinic (NiHCF‐3) exhibits excellent compatibility with both hard carbon and NaTi2(PO4)3 anodes in full cells, which shows great prospects for the application in the large‐scale energy storage systems.

Prussian blue analogs (PBAs) are promising cathode materials for sodium‐ion batteries (SIBs) due to their low‐cost, similar energy density comparable with that of LiFePO4 in lithium‐ion batteries, and long cycle life. Nevertheless, crystal water (≈10 wt%) in PBAs from aqueous synthesis environments can bring significant side effects in real SIBs, especially for calendar life and high temperature storage performance. Therefore, it is of great importance to eliminate crystal water in PBAs for future commercial applications. Herein, a facile heat‐treatment method is reported in order to remove water from Fe‐based PBAs. Although the heat‐treated sample can be easily rehydrated in air, it still exhibits a stable cycling performance over 2000 times under controlled charge cut‐off voltage. In situ synchrotron high‐temperature powder X‐ray diffraction demonstrates that the as‐prepared sample is maintained at a new trigonal phase after dehydration. Moreover, the redox reaction of low‐spin Fe2+/Fe3+ is activated and the high‐temperature storage performance of as‐prepared sample is significantly improved after removal of water.

An effective strategy is developed to synthesize high‐nuclearity Cu clusters, [Cu53(RCOO)10(C≡CtBu)20Cl2H18]+ (Cu53), which is the largest CuI/Cu0 cluster reported to date. Cu powder and Ph2SiH2 are employed as the reducing agents in the synthesis. As revealed by single‐crystal diffraction, Cu53 is arranged as a four‐concentric‐shell Cu3@Cu10Cl2@Cu20@Cu20 structure, possessing an atomic arrangement of concentric M12 icosahedral and M20 dodecahedral shells which popularly occurs in Au/Ag nanoclusters. Surprisingly, Cu53 can be dissolved in diethyl ether and spin coated to form uniform nanoclusters film on organolead halide perovskite. The cluster film can subsequently be converted into high‐quality CuI film via in situ iodination at room temperature. The as‐fabricated CuI film is an excellent hole‐transport layer for fabricating highly stable CuI‐based perovskite solar cells (PSCs) with 14.3 % of efficiency.