Concentrated potassium acetate as a water-in-salt electrolyte provides a wide potential window from −1.7 to 1.5 V vs Ag/AgCl KCl sat. . It facilitates the reversible operation of KTi 2 (PO 4 ) 3 , an anode of potassium-ion batteries, that otherwise only functions in nonaqueous electrolytes.

Sodium-ion batteries operating at ambient temperature hold great promise for use in grid energy storage owing to their significant cost advantages. However, challenges remain in the development of suitable electrode materials to enable long lifespan and high rate capability. Here we report a sodium super-ionic conductor structured electrode, sodium vanadium titanium phosphate, which delivers a high specific capacity of 147 mA h g−1 at a rate of 0.1 C and excellent capacity retentions at high rates. A symmetric sodium-ion full cell demonstrates a superior rate capability with a specific capacity of about 49 mA h g−1 at 20 C rate and ultralong lifetime over 10,000 cycles. Furthermore, in situ synchrotron diffraction and X-ray absorption spectroscopy measurement are carried out to unravel the underlying sodium storage mechanism and charge compensation behaviour. Our results suggest the potential application of symmetric batteries for electrochemical energy storage given the superior rate capability and long cycle life.

Since the discovery of Ti 3 C 2 T x in 2011, the family of two-dimensional transition metal carbides, carbonitrides, and nitrides (collectively known as MXenes) has quickly attracted the attention of those developing energy storage applications such as electrodes for supercapacitors with acidic aqueous electrolytes. The excellent performance of these MXenes is attributed to a pseudocapacitive energy storage mechanism, based on the nonrectangular shape of cyclic voltammetry curves and changes in the titanium oxidation state detected by in situ X-ray absorption spectroscopy. However, the pseudocapacitive mechanism is not well understood and no dimensional changes due to proton insertion have been reported. In this work, in situ X-ray diffraction and density functional theory are used to investigate the charge storage mechanism of Ti 3 C 2 T x in 1 m H 2 SO 4 . Results reveal that a 0.5 Å expansion and shrinkage of the c-lattice parameter of Ti 3 C 2 T x occur during cycling in a 0.9 V voltage window, showing that the charge storage mechanism is intercalation pseudocapacitance with implication for MXene use in energy storage and electrochemical actuators.

The Li[Li 0.23 Co 0.3 Mn 0.47 ]O 2 cathode material was prepared by a sol-gel method. Combinative X-ray diffraction (XRD) and Raman scattering studies showed that the material was a solid solution rather than a composite of nano Li 2 MnO 3 and LiCoO 2 . The material had a high discharge capacity of 250 mAh g -1 in the voltage window of 2.0-4.8 V. However, the capacity retention was poor. The material showed different electrochemical mechanisms in the first charge and subsequent cycles. Galvanostatic intermittent titration technique (GITT) study showed that the Li + diffusion coefficients during the first charge were as small as 10 -19 cm 2 s -1 because of the high kinetic barriers associated with the concurrent Li + extraction, oxygen loss, and structural rearrangement. The Li + diffusion coefficients increased to 10 -14 cm 2 s -1 after the first charge. However, they were still much smaller than those of typical layered materials such as LiCoO 2 and Li(Ni 1/3 Co 1/3 Mn 1/3 )O 2 . Electrochemical impedance spectroscopy (EIS) study showed that the large interface impedance at high potential seriously hindered the electrode performance of the material. A lower charge cutoff voltage of 4.6 V was the most suitable for this material considering that the correponding reversible capacity (∼200 mAh g -1 ) was attractive for high energy density lithium ion batteries.

Thanks to low costs and the abundance of the resources, sodium‐ion (SIBs) and potassium‐ion batteries (PIBs) have emerged as leading candidates for next‐generation energy storage devices. So far, only few materials can serve as the host for both Na+ and K+ ions. Herein, a cubic phase CuSe with crystal‐pillar‐like morphology (CPL‐CuSe) assembled by the nanosheets are synthesized and its dual functionality in SIBs and PIBs is comprehensively studied. The electrochemical measurements demonstrate that CPL‐CuSe enables fast Na+ and K+ storage as well as the sufficiently long duration. Specifically, the anode delivers a specific capacity of 295 mA h g−1 at current density of 10 A g−1 in SIBs, while 280 mA h g−1 at 5 A g−1 in PIBs, as well as the high capacity retention of nearly 100% over 1200 cycles and 340 cycles, respectively. Remarkably, CPL‐CuSe exhibits a high initial coulombic efficiency of 91.0% (SIBs) and 92.4% (PIBs), superior to most existing selenide anodes. A combination of in situ X‐ray diffraction and ex situ transmission electron microscopy tests fundamentally reveal the structural transition and phase evolution of CuSe, which shows a reversible conversion reaction for both cells, while the intermediate products are different due to the sluggish K+ insertion reaction.