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HighlightsThe formation mechanism of solid electrolyte interface (SEI) is analyzed based on charge distributions at the electrode/electrolyte interface and molecular orbital theory.The factors affecting the formation of SEI are generalized from four aspects: Zn anode, electrolyte, current density and temperature.The design strategies for SEI layer are proposed from regulating temperature, electric and magnetic fields.Due to its high theoretical capacity (820 mAh g−1), low standard electrode potential (− 0.76 V vs. SHE), excellent stability in aqueous solutions, low cost, environmental friendliness and intrinsically high safety, zinc (Zn)-based batteries have attracted much attention in developing new energy storage devices. In Zn battery system, the battery performance is significantly affected by the solid electrolyte interface (SEI), which is controlled by electrode and electrolyte, and attracts dendrite growth, electrochemical stability window range, metallic Zn anode corrosion and passivation, and electrolyte mutations. Therefore, the design of SEI is decisive for the overall performance of Zn battery systems. This paper summarizes the formation mechanism, the types and characteristics, and the characterization techniques associated with SEI. Meanwhile, we analyze the influence of SEI on battery performance, and put forward the design strategies of SEI. Finally, the future research of SEI in Zn battery system is prospected to seize the nature of SEI, improve the battery performance and promote the large-scale application.

Aqueous graphite-based dual ion batteries have unique superiorities in stationary energy storage systems due to their non-transition metal configuration and safety properties. However, there is an absence of thorough study of the interactions between anions and water molecules and between anions and electrode materials, which is essential to achieve high output voltage. Here we reveal the four-stage intercalation process and energy conversion in a graphite cathode of anions with different configurations. The difference between the intercalation energy and hydration energy of bis(trifluoromethane)sulfonimide makes the best use of the electrochemical stability window of its electrolyte and delivers a high intercalation potential, while BF 4 − and CF 3 SO 3 − do not exhibit a satisfactory potential because the graphite intercalation potential of BF 4 − is inferior and the graphite intercalation potential of CF 3 SO 3 − exceeds the voltage window of its electrolyte. An aqueous dual ion battery based on the intercalation behaviors of bis(trifluoromethane)sulfonimide anions into a graphite cathode exhibits a high voltage of 2.2 V together with a specific energy of 242.74 Wh kg −1 . This work provides clear guidance for the voltage plateau manipulation of anion intercalation into two-dimensional materials. The interactions between water molecules, electrode materials and anions are essential yet challenging for aqueous dual ion batteries. Here, the authors demonstrate the voltage manipulation of dual ion batteries through matching intercalation energy and solvation energy of different anions.

One of the major obstacles hindering the application of zinc metal batteries is the contradictory demands from the Zn metal anode and cathodes. At the anode side, water induces serious corrosion and dendrite growth, remarkably suppressing the reversibility of Zn plating/stripping. At the cathode side, water is essential because many cathode materials require both H + and Zn 2+ insertion/extraction to achieve a high capacity and long lifespan. Herein, an asymmetric design of inorganic solid-state electrolyte combined with hydrogel electrolyte is presented to simultaneously meet the as-mentioned contrary requirements. The inorganic solid-state electrolyte is toward the Zn anode to realize a dendrite-free and corrosion-free highly reversible Zn plating/stripping, and the hydrogel electrolyte enables consequent H + and Zn 2+ insertion/extraction at the cathode side for high performance. Therefore, there is no hydrogen and dendrite growth detected in cells with a super high-areal-capacity up to 10 mAh·cm −2 (Zn//Zn), ~5.5 mAh·cm −2 (Zn//MnO 2 ) and ~7.2 mAh·cm −2 (Zn//V 2 O 5 ). These Zn//MnO 2 and Zn//V 2 O 5 batteries show remarkable cycling stability over 1000 cycles with 92.4% and over 400 cycles with 90.5% initial capacity retained, respectively. While Zinc anodes are thermodynamically unstable in aqueous solutions, the protons (H+) from the water favor the cathodes of Zinc batteries. Here, the authors address this contradiction by designing an asymmetric electrolyte composed of an inorganic solid-state electrolyte and a hydrogel electrolyte.

The quest for advanced energy storage devices with cheaper, safer, more resource‐abundant storage has triggered intense research into zinc ion batteries (ZIBs). Among them, organic materials as cathode materials for ZIBs have attracted great interest due to their flexible structure designability, high theoretical capacity, environmental friendliness, and sustainability. Although numerous organic electrode materials have been studied and different redox mechanisms have been proposed in the past decade, their electrochemical performance still needs further improvement, and the mechanisms require further exploration. This paper provides a systematical overview of three types of organic materials (bipolar‐type conductive polymer, n‐type conjugated carbonyl compounds, and p‐type material) on the energy storage mechanisms and distinct characteristics. We then focus on discussing the design strategies to improve electrochemical performance. Furthermore, the challenges and future research directions are discussed to provide a foundation for further developing organic‐based ZIBs. As cathode materials for zinc‐ion batteries, organic materials have attracted great interests due to their flexible structure designability, high theoretical capacity, environmental friendliness, and sustainability. A systematical overview of three types of organic materials is presented to summarize their structures and performances, propose the strategies for their performance enhancement, expound the current challenges and perspectives.

HighlightsThe porous structure and interconnected conductive pathways accommodate a large amount of iodine, entrap polyiodides and guarantee its efficient utilization. While the Fe single atom catalyst efficiently catalyzes the iodine/polyiodide conversion.With “confinement-catalysis” host, the ZnǀǀI2 battery delivers a high capacity of 188.2 mAh g−1 at 0.3 A g−1, excellent rate capability with a capacity of 139.6 mAh g−1 at 15 A g−1 and ultra-long cyclic stability over 50,000 cycles with 80.5% initial capacity retained under high iodine loading of 76.72 wt%.Rechargeable aqueous zinc iodine (ZnǀǀI2) batteries have been promising energy storage technologies due to low-cost position and constitutional safety of zinc anode, iodine cathode and aqueous electrolytes. Whereas, on one hand, the low-fraction utilization of electrochemically inert host causes severe shuttle of soluble polyiodides, deficient iodine utilization and sluggish reaction kinetics. On the other hand, the usage of high mass polar electrocatalysts occupies mass and volume of electrode materials and sacrifices device-level energy density. Here, we propose a “confinement-catalysis” host composed of Fe single atom catalyst embedding inside ordered mesoporous carbon host, which can effectively confine and catalytically convert I2/I− couple and polyiodide intermediates. Consequently, the cathode enables the high capacity of 188.2 mAh g−1 at 0.3 A g−1, excellent rate capability with a capacity of 139.6 mAh g−1 delivered at high current density of 15 A g−1 and ultra-long cyclic stability over 50,000 cycles with 80.5% initial capacity retained under high iodine loading of 76.72 wt%. Furthermore, the electrocatalytic host can also accelerate the I+↔I2 conversion. The greatly improved electrochemical performance originates from the modulation of physicochemical confinement and the decrease of energy barrier for reversible I−/I2 and I2/I+ couples, and polyiodide intermediates conversions.

HighlightsThe working principle of the asymmetric electrolyte and the long-term-seated contradictory issues were analyzed.The characterization methods for the interfaces of anolyte/catholyte and electrolyte/electrode were summarized for revealing the fundamental mechanism of asymmetric electrolytes.The future research directions for asymmetric electrolyte systems were proposed.With the rapid development of portable electronics and electric road vehicles, high-energy-density batteries have been becoming front-burner issues. Traditionally, homogeneous electrolyte cannot simultaneously meet diametrically opposed demands of high-potential cathode and low-potential anode, which are essential for high-voltage batteries. Meanwhile, homogeneous electrolyte is difficult to achieve bi- or multi-functions to meet different requirements of electrodes. In comparison, the asymmetric electrolyte with bi- or multi-layer disparate components can satisfy distinct requirements by playing different roles of each electrolyte layer and meanwhile compensates weakness of individual electrolyte. Consequently, the asymmetric electrolyte can not only suppress by-product sedimentation and continuous electrolyte decomposition at the anode while preserving active substances at the cathode for high-voltage batteries with long cyclic lifespan. In this review, we comprehensively divide asymmetric electrolytes into three categories: decoupled liquid-state electrolytes, bi-phase solid/liquid electrolytes and decoupled asymmetric solid-state electrolytes. The design principles, reaction mechanism and mutual compatibility are also studied, respectively. Finally, we provide a comprehensive vision for the simplification of structure to reduce costs and increase device energy density, and the optimization of solvation structure at anolyte/catholyte interface to realize fast ion transport kinetics.

Highlights There is a novel “hydrolysis-driven synthesis” approach for the preparation of a dual-surface functionalized micron-sized Si anode with a SiO x /C layer. The functionalized inner pores and dual-functional SiO x /C layer synergistically alleviate volume change of Si lithiation, minimize stress concentration and improve electrochemical reaction kinetics. The optimized micron-Si anode performs impressive lifespan, excellent high rate capacity and outstanding stack cell volumetric energy density. Micro-silicon (Si) anode that features high theoretical capacity and fine tap density is ideal for energy-dense lithium-ion batteries. However, the substantial localized mechanical strain caused by the large volume expansion often results in electrode disintegration and capacity loss. Herein, a microporous Si anode with the SiO x /C layer functionalized all-surface and high tap density (~ 0.65 g cm⁻ 3 ) is developed by the hydrolysis-driven strategy that avoids the common use of corrosive etchants and toxic siloxane reagents. The functionalized inner pore with superior structural stability can effectively alleviate the volume change and enhance the electrolyte contact. Simultaneously, the outer particle surface forms a continuous network that prevents electrolyte parasitic decomposition, disperses the interface stress of Si matrix and facilitates electron/ion transport. As a result, the micron-sized Si anode shows only ~ 9.94 GPa average stress at full lithiation state and delivers an impressive capacity of 901.1 mAh g⁻ 1 after 500 cycles at 1 A g⁻ 1 . It also performs excellent rate performance of 1123.0 mAh g⁻ 1 at 5 A g⁻ 1 and 850.4 at 8 A g⁻ 1 , far exceeding most of reported literatures. Furthermore, when paired with a commercial LiNi 0.8 Co 0.1 Mn 0.1 O 2 , the pouch cell demonstrates high capacity and desirable cyclic performance.

Zinc-ion batteries (ZIBs) are receiving increasing research attention due to their high energy density, resource abundance, low-cost, intrinsic high-safety properties, and the appropriate plating/stripping voltage. Gel-state electrolytes possess merits of having a wide electrochemical window, good flexibility, superior water retainability, and excellent compatibility with aqueous electrolytes, which makes them potential candidates for flexible batteries. However, the practical applications of ZIBs with gel-state electrolytes still have some issues of water content easily dropping, poor mechanical stability, and the interface problem. Therefore, the application of hydrogel-based, self-healing gel, gel polymer, thermos-reversible, and other additional functions of gel electrolytes in ZIBs are discussed in this review. Following that, the design of multi-functional gel-state electrolytes for ZIBs is proposed. Finally, the prospect and the challenges of this type of battery are described.

HighlightsThe strong interaction between Ti3C2Sx and FeN4 species induces the central metal Fe(II) in FeN4 species with intermediate spin state transferred to high spin state, in which the latter is favorable to initiate the reduction of oxygen.This strong interaction induces a remarkable Fe 3d electron delocalization with d band center upshift, boosting oxygen-containing groups adsorption on FeN4 species and oxygen reduction reaction kinetics.The resulting FeN4–Ti3C2Sx with FeN4 moieties in high spin state exhibits high half-wave potential of 0.89 V vs. RHE and high limiting current density of 6.5 mA cm−2, enabling wearable zinc–air battery showing a good discharge performance with a maximum power density of 133.6 mW cm−2.Transition metal–nitrogen–carbon materials (M–N–Cs), particularly Fe–N–Cs, have been found to be electroactive for accelerating oxygen reduction reaction (ORR) kinetics. Although substantial efforts have been devoted to design Fe–N–Cs with increased active species content, surface area, and electronic conductivity, their performance is still far from satisfactory. Hitherto, there is limited research about regulation on the electronic spin states of Fe centers for Fe–N–Cs electrocatalysts to improve their catalytic performance. Here, we introduce Ti3C2 MXene with sulfur terminals to regulate the electronic configuration of FeN4 species and dramatically enhance catalytic activity toward ORR. The MXene with sulfur terminals induce the spin-state transition of FeN4 species and Fe 3d electron delocalization with d band center upshift, enabling the Fe(II) ions to bind oxygen in the end-on adsorption mode favorable to initiate the reduction of oxygen and boosting oxygen-containing groups adsorption on FeN4 species and ORR kinetics. The resulting FeN4–Ti3C2Sx exhibits comparable catalytic performance to those of commercial Pt-C. The developed wearable ZABs using FeN4–Ti3C2Sx also exhibit fast kinetics and excellent stability. This study confirms that regulation of the electronic structure of active species via coupling with their support can be a major contributor to enhance their catalytic activity.