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Calcium-ion batteries (CIBs) have been considered promising multivalent ion battery systems due to the natural abundance and low redox potential of calcium. The practical realization is largely hampered by the lack of reliable electrode materials. Selenium is first explored as a new conversion-type electrode for both non-aqueous and aqueous CIBs. The selenium provides a specific capacity of 476 mAh g–1 with an average voltage of 2.2 V versus Ca/Ca2+ at a current density of 50 mA g–1, offering a higher energy density than other reported cathode materials. Long-term cyclic stability at a large current density of 500 mA g–1 has been achieved in non-aqueous electrolytes through encapsulating the selenium in mesoporous carbon. The spectroscopy analysis and density functional theory calculations suggest multi-step conversion processes involving CaSe4 and Ca2Se5 polyselenides intermediates before reaching the final CaSe phase, exhibiting a distinct reaction pathway from those in other metal-Se batteries. Furthermore, the application of selenium is extended to the aqueous electrolyte after expanding the electrochemical window. A 1.1 V class aqueous CIB is demonstrated by coupling with a Cu-based Prussian blue electrode. The discovery of reversible Ca–Se chemistry opens new opportunities for emerging CIBs. © 2022 Wiley-VCH GmbH

The recent revival of research on Na and K ion batteries has two benefits. It not only provides alternate energy storage technologies to Li ion batteries with potential cost advantages but also enhances our understanding of charge storage through systematic studies on alkali-metal ion batteries with increasing insertion ion sizes. Using MoS2 as a model material, the structure evolution upon the uptake of Li, Na, and K ions are compared through in situ TEM. Despite their larger size, insertion of K ions shows both the better electrochemical and structural stability. To understand this paradoxical and counter-intuitive phenomenon, in situ XRD is carried out to examine the phase transitions of MoS2 upon ion insertion, while ex situ TEM is further applied to closely examine the structures at the nanoscale. Complementary DFT calculations are performed to understand the kinetic/thermodynamic origins of the unusual stability. The result reveal that the less electrovalent K–S bond favors the intercalation process, resulting in preservation of the layered structure for stable cycling. This study provides a structural insight to design stable electrodes for the K-ion batteries.

There has been increasing evidence that ether-based electrolytes offer more stable anode performance in sodium ion batteries, even for microsized alloy electrodes which suffer huge volume change upon charge/discharge cycling. It is speculated that ether-based electrolytes may give rise to more robust solid electrolyte interphases (SEIs), but the detailed mechanism remains unknown, due to the structural complexity and the extremely vulnerable nature of SEIs. In this work, we unveil the characteristic SEI structure at Sn electrode surfaces in both carbonate- and ether-based electrolytes. We adopt cryogenic transmission electron microscopy to probe the pristine SEI structure, in combination with X-ray photoelectron spectroscopy and density functional theory calculations. An ultrathin SEI forms in the ether-based electrolyte, with amorphous particles dispersed in the polymer-like matrix. This unique nanostructure exhibits superior mechanical elasticity and renders anomalous stability against the large volume change of alloy electrodes, as evidenced by both electrochemistry measurement and atomic force microscopy. Our work unravels the causes for the superiority of ether-based electrolytes in sodium-ion batteries, and we expect the potential of such an optimized SEI to enable the application of high-capacity anodes such as microsized alloys. © 2019 The Royal Society of Chemistry.

Transitional metal dichalcogenides represent one important type of anodes for emerging K-ion batteries. K ions are stored through both intercalation and conversion reactions, but the detailed phase transition is not clear. It is believed that deep potassiation would trigger the conversion reaction, which induces the fracture of particles and leads to fast capacity degradation. By utilizing MoS2 as a model material, the competition between intercalation and conversion is revealed, which shows a rate-dependent behavior. The crystal structure of several newly discovered intermediate phases including K0.5MoS2 and K1.0MoS2 is disclosed by complementary experimental and calculational approaches. It shows that intercalation takes place even discharge down to 0 V, differing from the cases in Li-ion and Na-ion batteries. The intercalated compound preserves the layered structure of MoS2, which avoids the structural collapse and maintains the integrity of the electrode for stable cyclic performance. This finding opens up a new opportunity in the exploration of high capacity anode among layered transitional metal dichalcogenide families. © 2019 American Chemical Society.

Li-ion capacitors (LICs) are emerging as complementary energy storage devices to Li-ion batteries to satisfy some specific applications where high power density and long cycle life are required. Due to the wide usage of LICs, LICs with promising energy density are urgently needed; however, at this stage, the achievement of this type of LICs is the main challenge. In this study, we increased the energy density of LICs via both material optimization and charge storage mechanism exploration. Moreover, porous carbon with a high surface area of over 2800 m2 g-1 was fabricated from alkali lignin via a traditional KOH activation method assisted by self-activation. A wide voltage window of 1.0-4.8 V was applied where the synergistic storage of anions and cations was achieved. This shows that a deep discharge down to 1.0 V is necessary for the complete desorption of anions, which also triggers the adsorption of cations (Li+), resulting in increased capacity. However, a compromise must be made in the energy efficiency due to intensified battery polarization upon deep discharging. Furthermore, considering the natural abundance of sodium and potassium over lithium, Na- and K-ion capacitors have been investigated for sustainable development using the as-prepared carbon materials. © 2019 The Royal Society of Chemistry.