We report a low-cost water-in-salt electrolyte (WiSE), of 30 m ZnCl2, which enables a dendrite-free Zn metal anode to possess a high coulombic efficiency.

Zn batteries potentially offer the highest energy density among aqueous batteries that are inherently safe, inexpensive, and sustainable. However, most cathode materials in Zn batteries suffer from capacity fading, particularly at a low current rate. Herein, it is shown that the ZnCl 2 "water-in-salt" electrolyte (WiSE) addresses this capacity fading problem to a large extent by facilitating unprecedented performance of a Zn battery cathode of Ca 0.20 V 2 O 5 •0.80H 2 O. Upon increasing the concentration of aqueous ZnCl 2 electrolytes from 1 m to 30 m, the capacity of Ca 0.20 V 2 O 5 •0.80H 2 O rises from 296 mAh g −1 to 496 mAh g −1 ; its absolute working potential increases by 0.4 V, and most importantly, at a low current rate of 50 mA g −1 , that is, C/10; its capacity retention increases from 8.4% to 51.1% over 100 cycles. Ex situ characterization results point to the formation of a new ready-to-dissolve phase on the electrode in the dilute electrolyte. The results demonstrate that the Zn-based WiSE may provide the underpinning platform for the applications of Zn batteries for stationary grid-level storage.

Potassium-ion batteries (KIBs) are a promising sustainable energy storage technology due to the high abundance and low cost of potassium. Carbon anode materials for KIBs have seen great successes, but the development of cathode materials is yet to catch up. In this study, poly(anthraquinonyl sulfide) (PAQS) is evaluated as a cathode material for KIBs. It exhibits a high reversible capacity of 200 mAh/g, which is the highest value for a potassium storage cathode material. The cell shows two slopes averaged at 2.1 and 1.6 V vs. K + /K. It shows a good cycling performance with the capacity retention of 75% after 50 cycles at a rate of C/10. These preliminary results indicate that PAQS is a promising cathode material for KIBs.

We report that crystalline 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA), an organic solid, is highly amenable to host divalent metal ions, i.e., Mg and Ca, in aqueous electrolytes, where the van der Waals structure is intrinsically superior in hosting charge-dense ions. We observe that the divalent nature of Mg causes unique squeezing deformation of the electrode structure, where it contracts and expands in different crystallographic directions when hosting the inserted Mg-ions. This phenomenon is revealed experimentally by ex situ X-ray diffraction and transmission electron microscopy, and is investigated theoretically by first-principles calculations. Interestingly, hosting one Mg ion requires the coordination from three PTCDA molecules in adjacent columns of stacked molecules, which rotates the columns, thus reducing the (011) spacing but increasing the (021) spacing. We demonstrate that a PTCDA Mg-ion electrode delivers a reversible capacity of 125 mA h g, which may include a minor contribution of hydronium storage, a good rate capability by retaining 75 mA h g at 500 mA g (or 3.7 C), and a stable cycle life. We also report Ca storage in PTCDA, where a reversible capacity of over 80 mA h g is delivered.

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.

Dual-ion batteries operate on two intercalants: anions for the cathode and cations for the anode. This battery was initially known as a dual-graphite battery, where both electrodes are graphitic carbon. The primary challenge of dual-graphite batteries is the very high operation potential of the cathode, often requiring an upper cutoff potential above 5 V vs Li+/Li. Such a potential readily oxidizes alkyl and alkylene carbonate-based electrolytes. The anode side, in fact, can employ any anode of most metal-ion batteries, although, to date, the focus has still been the Li–graphite anode. Recent progress has significantly advanced the technology readiness level for this battery. Additives or ionic liquid electrolytes help mitigate cathode irreversibility; nongraphite anodes, such as aluminum, allow new carbonate electrolytes that lack the necessity of ethylene carbonate; nongraphite cathodes, including metal–organic frameworks and polycyclic aromatic hydrocarbons have exhibited a remarkable potential. This Perspective highlights the challenges, summarizes the recent progress, and attempts to point out future directions in the field.

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 have demonstrated, for the first time, a polycyclic aromatic hydrocarbon (PAH), crystalline and readily available coronene, exhibits highly reversible anion-storage properties. Conventional graphite anion-insertion electrodes operate at potentials >4.5 V vs Li + /Li, requiring electrolyte additives or the use of ionic liquids as electrolytes. The coronene electrode shows flat plateaus at 4.2 V (charge) and 4.0 V (discharge) in a standard alkyl carbonate electrolyte and delivers a reversible discharge capacity of ∼40 mA h g −1 . Ex situ characterization reveals that coronene retains its crystalline structure and chemical bonding upon initial PF 6 − incorporation. Coronene−PF 6 electrodes show impressive cycling stability: 92% capacity retention after 960 cycles. The discovery of the reversible anion-storage properties of coronene may open new avenues toward dualion batteries based on PAHs as electrodes.