Aqueous K-ion batteries have attracted increasing attention. Yet, their capacity and durability are facing severe challenges. Construction of available electrolyte plays a vital role to overcome these challenges. Herein, a polymer-free CH3COOK (KAc) gel electrolyte is developed with a ultrahigh salt-water mole ratio, which can be high as 1: 1.16 (KAc: H2O). With a glassy carbon electrode, the electrochemical stability window of the gel is up to 4 V (-1.9 ~ 2.1 V vs Ag/AgCl). At room temperature, the ion conductivity is measured to be 10.9 mS cm−1, which increases to 23.5 mS cm−1 at 90 ℃ and is still 3.4 mS cm−1 at -20 ℃, indicating good temperature adaptability. Surprisingly, unlike other high-concentered electrolytes, anti-deliquescence ability of the gel is also rather good. With Raman analysis, it is found that there are almost no free water and cross-link structure among K+, Ac− and water should be formed in the gel, which is further demonstrated by density function theory (DFT) computation. Using this gel electrolyte, FeSe2 can deliver a high and reversible capacity of 250 mAh g−1 at 0.5 A g−1 as anode. Most importantly, stable solid electrolyte interphase (SEI) layer is found to be formed. It is believed that the quasi-solid state of the electrolyte together with the SEI layer can effectively suppress the reaction activity and the solvent ability of water, and hence inhibit mass loss of active materials, even the materials that store charges through phase conversion reactions.
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