Hard carbon microspheres (HCS) exhibit a highly reversible capacity of 262 mAh g−1 for K‐ion batteries. They present a superior rate capability for K‐ions to Na‐ions, where at 2C, HCS/K cells deliver 190 mAh g−1 in contrast to 97 mAh g−1 from HCS/Na cells. It is determined that the K‐ion diffusion coefficient of HCS is higher than that of Na‐ions.

There exist tremendous needs for sustainable storage solutions for intermittent renewable energy sources, such as solar and wind energy. Thus, systems based on Earthabundant elements deserve much attention. Potassium-ion batteries represent a promising candidate thanks to the abundance of potassium resources. As for the choices of anodes, graphite exhibits encouraging potassium-ion storage properties; however, it suffers limited rate capability and poor cycling stability. Here, we systematically investigated and compared non-graphitic carbons as K-ion anodes with sodium carboxymethyl cellulose as the binder. Compared to hard This article is protected by copyright. All rights reserved.2 carbon and soft carbon, a composite hard-soft carbon with 20 wt% soft carbon distributed in the matrix phase of hard carbon micron-spheres exhibits highly amenable performance: high capacity, high rate capability, and very stable long-term cycling. In contrast, pure hard carbon suffers limited rate capability, while the capacity of pure soft carbon fades more rapidly.

Aqueous rechargeable batteries are promising solutions for large-scale energy storage. Such batteries have the merit of low cost, innate safety, and environmental friendliness. To date, most known aqueous ion batteries employ metal cation charge carriers. Here, we report the first "rocking-chair" NH -ion battery of the full-cell configuration by employing an ammonium Prussian white analogue, (NH ) Ni[Fe(CN) ] , as the cathode, an organic solid, 3,4,9,10-perylenetetracarboxylic diimide (PTCDI), as the anode, and 1.0 m aqueous (NH ) SO as the electrolyte. This novel aqueous ammonium-ion battery demonstrates encouraging electrochemical performance: an average operation voltage of ca. 1.0 V, an attractive energy density of ca. 43 Wh kg based on both electrodes' active mass, and excellent cycle life over 1000 cycles with 67 % capacity retention. Importantly, the topochemistry results of NH in these electrodes point to a new paradigm of NH -based energy storage.

This study reveals the transport behavior of lattice water during proton (de)insertion in the structure of the hexagonal WO·0.6HO electrode. By monitoring the mass evolution of this electrode material via electrochemical quartz crystal microbalance, we discovered (1) WO·0.6HO incorporates additional lattice water when immersing in the electrolyte at open circuit voltage and during initial cycling; (2) The reductive proton insertion in the WO hydrate is a three-tier process, where in the first stage 0.25 H is inserted per formula unit of WO while simultaneously 0.25 lattice water is expelled; then in the second stage 0.30 naked H is inserted, followed by the third stage with 0.17 HO inserted per formula unit. Ex situ XRD reveals that protonation of the WO hydrate causes consecutive anisotropic structural changes: it first contracts along the c-axis but later expands along the ab planes. Furthermore, WO·0.6HO exhibits impressive cycle life over 20 000 cycles, together with appreciable capacity and promising rate performance.

We demonstrate for the first time that hydronium ions can be reversibly stored in an electrode of crystalline 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA). PTCDA exhibits a capacity of 85 mAh g at 1 A g after an initial conditioning process. Ex situ X-ray diffraction revealed reversible and significant structure dilation upon reduction of PTCDA in an acidic electrolyte, which can only be ascribed to hydronium-ion intercalation. The lattice expansion upon hydronium storage was theoretically explored by first-principles density functional theory (DFT) calculations, which confirmed the hydronium storage in PTCDA.

To fill the gap between batteries and supercapacitors requires integration of the following features in a single system: energy density well above that of supercapacitors, cycle life much longer than Li-ion batteries, and low cost. Along this line, we report a novel nonaqueous potassium-ion hybrid capacitor (KIC) that employs an anode of KTiO (KTO) microscaffolds constructed by nanorods and a cathode of N-doped nanoporous graphenic carbon (NGC). KTiO microscaffolds are studied for potential applications as the anode material in potassium-ion storage for the first time. This material exhibits an excellent capacity retention of 85% after 1000 cycles. In addition, the NGC//KTO KIC delivers a high energy density of 58.2 Wh kg based on the active mass in both electrodes, high power density of 7200 W kg, and outstanding cycling stability over 5000 cycles. The usage of K ions as the anode charge carrier instead of Li ions and the amenable performance of this device suggest that hybrid capacitor devices may welcome a new era of beyond lithium.

The rate capability of hard carbon has long been underestimated in prior studies that used carbon/Na two-electrode half-cells. Through a three-electrode cell setup, we discover that it is the overpotential of the sodium counter electrode that drives the half-cells to the lower cutoff potential prematurely during hard carbon sodiation, particularly at high current rates, which prevents the hard carbon anode from being fully sodiated.

We prepared Prussian white analogues-K x MFe(CN) 6 •mH 2 O (M = Fe, Co, Ni, or Cu). These compounds are investigated as cathode materials for potassium-ion batteries with a non-aqueous electrolyte. Each of these materials exhibits different K-ion storage properties with reversible capacity values ranging from 35 to 110 mAh g-1 at high potentials above 3.2 V vs. K + /K. Among them, FeFe-Prussian white, K 1.68 Fe 1.09 Fe(CN) 6 •2.1H 2 O, exhibits the highest reversible capacity of 110 mAh g-1 and stable cycling performance in contrast to rapid capacity fading of other analogues. Our results demonstrate a remarkable potential of this family of cathode materials for the emerging potassium-ion batteries.