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Mass dissolution is one main problems for cathodes in aqueous electrolytes due to the strong polarity of water molecules. In principle, mass dissolution is a thermodynamically favorable process as determined by the Gibbs free energy. However, in real situations, dissolution kinetics, which include viscosity, dissolving mass mobility, and interface properties, are also a critical factor influencing the dissolution rate. Both thermodynamic and kinetic dissolving factors can be regulated by the ratio of salt to solvent in the electrolyte. In this study, concentration‐controlled cathode dissolution is investigated in a susceptible Na3V2(PO4)3 cathode whose time‐, cycle‐, and state‐of‐charge‐dependent dissolubility are evaluated by multiple electrochemical and chemical methods. It is verified that the super‐highly concentrated water‐in‐salt electrolyte has a high viscosity, low vanadium ion diffusion, low polarity of solvated water, and scarce solute−water dissolving surfaces. These factors significantly lower the thermodynamic‐controlled solubility and the dissolving kinetics via time and physical space local mass interfacial confinement, thereby inducing a new mechanism of interface concentrated‐confinement which improves the cycling stability in real aqueous rechargeable sodium‐ion batteries. [ABSTRACT FROM AUTHOR]

Recently, increasing attention has been paid to the newly emerging area of sodium-ion batteries, as a promising supplement for lithium-ion batteries. Although many cathode materials have been proposed, most of these have limitations for practical applications, such as a low capacity or a poor cycling performance. Here, sodium-deficient O3-Na0.9[Ni0.4Mn x Ti0.6− x]O2 (where x 5 0.1-0.6, abbreviated as NNMT-9415, 9424, 9433, 9442, 9451, and 9460, respectively) materials are introduced, which can be used as the cathode in sodium-ion batteries. Among these materials, the electrochemical behavior of materials with x 5 0.4 and 0.3 is the highest with a higher capacity and better cycling property than the other materials. These can deliver an initial capacity of about 120 mA h g21 in a voltage range of 2.5-4.2 V with a negligible capacity loss even after 100 cycles. The rate capabilities of 82% and 64% at 1 C and 2 C current rates, respectively, are also satisfactory. The good cycling performance and high capacity make these two materials potential candidates as the cathode material for sodium-ion batteries. [ABSTRACT FROM AUTHOR]

Na‐ion batteries have experienced rapid development over the past decade and received significant attention from the academic and industrial communities. Although a large amount of effort has been made on material innovations, accessible design strategies on peculiar structural chemistry remain elusive. An approach to in situ construction of new Na‐based cathode materials by substitution in alkali sites is proposed to realize long‐term cycling stability and high‐energy density in low‐cost Na‐ion cathodes. A new compound, [K0.444(1)Na1.414(1)][Mn3/4Fe5/4](CN)6, is obtained through a rational control of K+ content from electrochemical reaction. Results demonstrate that the remaining K+ (≈0.444 mol per unit) in the host matrix can stabilize the intrinsic K‐based structure during reversible Na+ extraction/insertion process without the structural evolution to the Na‐based structure after cycles. Thereby, the as‐prepared cathode shows the remarkably enhanced structural stability with the capacity retention of >78% after 1800 cycles, and a higher average operation voltage of ≈3.65 V versus Na+/Na, directly contrasting the non‐alkali‐site‐substitution cathode materials. This provides new insights into alkali‐site‐substitution constructing advanced Na‐ion cathode materials. [ABSTRACT FROM AUTHOR]