Pursuing rechargeable metal-ion batteries with greater energy density is attracting great attention due to increasing demand for energy storage, where alloying anodes can provide very high capacity. [1][2][3][4][5][6][7] This is particularly true since sodium and potassium ion battery technologies offer limited capacity and stability using classic carbon-based anodes compared to lithium ions. [8][9][10][11] However, alloying anodes are notorious for their severe capacity fading, which has hindered their practical applications. The failure mechanism of alloying anodes has always been ascribed to the large volumetric change (~300%) and/or the fragile solid electrolyte interphase (SEI). [12][13][14][15] This interpretation is popular because the pulverization of the alloy-based electrodes can be observed during the reactions. As a result, many strategies have been developed to overcome this issue. These strategies include nano-structural controlling, carbon modification, and improving electrical conductivity. Thus, many nanostructured alloys including particles, 16 fibers/tubes, 5,17 film/membrane, 18,19 and hierarchical material 20,21 are being explored to stabilize alloying anodes. Characteristic, conductive and/or protective materials such as carbon and artificial solid electrolyte interphase (SEI) have been also used to improve alloying anode capacities stabilities. [22][23][24][25] Herein we show that an unprecedented high capacity (>650 mAh g -1 ) and stability (>500 cycles) can be achieved in alloying anodes by simply tuning the electrolyte composition, without the need for nanostructural control, carbon modification, and/or SEI engineering. We confirm that the cation solvation structure (e.g., Na + , K + ), particularly the type and location of the anions present in the metal salt and solvents, plays a critical role in affecting the alloying anode performance. In addition, we present a new anionic model showing that the anion corrosion plays at least an equally important role in alloying anode stability as the volume variation and fragile SEI models. Moreover, we present a new reaction model for alloying anode to make the ASSOCIATED CONTENT Supporting Information. Experimental and simulation section. Figures S1-S17 and Table S1 are included.

The attractive features of KIBs include abundant potassium sources, and K + /K redox potential that is close to Li + / Li (−2.93 V vs −3.04 V) and even lower than −2.71 V of Na + /Na. [2] Therefore, KIBs have the potential to provide greater energy density at a lower cost. However, designing high capacity, stable, and safe electrodes for KIBs remains challenging. This is because the ionic radius of potassium (K +) is large, which can cause a low specific capacity and degrade stability of the electrode during the K + (de-)intercalation or reaction. [3] Although many kinds of cathodes with a large crystalline channel or durable conjugated structure (e.g., metal layered oxide, [4] polyanionic compound, [5] Prussian blue analogs, [6] organic materials [7]) are being hitherto developed, most reported capacities have been less than 150 mAh g-1. This low capacity of the cathodes presents a serious bottleneck for KIB development. [8] Alternatively, various high capacity anodes such as alloys, [9] transition-metal oxides/sulfides, [10] and MXene-based materials [11] bring new opportunities to design high-energy-density KIBs. Particularly, the alloying anodes can exhibit significantly higher capacity than classic carbon-based anodes (i.e., 280 mAh g-1 of graphite). [12] For example, the potassium storage capacity of metallic antimony (Sb) can reach as high as 660 mAh g-1 , which is close to the theoretical capacity of metal K (i.e., 687 mAh g-1). [13] But, the capacity suffers a severe capacity decay caused by the large volumetric change of Sb (≈300%). [9a] Although a variety of strategies including structural design (e.g., nanoparticles, [14] hollow spheres, [15] fibers, [9a] films, [13] and hierarchical structures [16]), carbon modification (e.g., Sb@NC, [17] Sb@CNFs, [13] Sb@Go, [18] Sb@CSN, [19] and Sb/C [20]), and SEI engineering [19] are being developed to stabilize the Sb anode, the capacity and stability still have a large room to be improved. Herein, we present a new approach of electrolyte engineering to stabilize bulk Sb anodes. Extremely high capacity of 628 mAh g-1 at the current density of 100 mA g-1 and a good rate capability of 305 mAh g-1 at the hash current density of 3 A g-1 can be achieved by varying the electrolyte composition (e.g., anions, solvent, and concentration). To our knowledge, this is the best electrochemical performance reported so far for Sb alloying anode in KIBs. Amazingly, this high capacity which remained stable for over 200 cycles, was achieved without the need for Alloying anodes exhibit very high capacity when used in potassium-ion batteries, but their severe capacity fading hinders their practical applications. The failure mechanism has traditionally been attributed to the large volumetric change and/or their fragile solid electrolyte interphase. Herein, it is reported that an antimony (Sb) alloying anode, even in bulk form, can be stabilized readily by electrolyte engineering. The Sb anode delivers an extremely high capacity of 628 and 305 mAh g-1 at current densities of 100 and 3...

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