An In Situ Cross‐Linked Nonaqueous Polymer Electrolyte for Zinc‐Metal Polymer Batteries and Hybrid Supercapacitors

This work reports the facile synthesis of nonaqueous zinc‐ion conducting polymer electrolyte (ZIP) membranes using an ultraviolet (UV)‐light‐induced photopolymerization technique, with room temperature (RT) ionic conductivity values in the order of 10^?3 S cm^?1. The ZIP membranes demonstrate excellent physicochemical and electrochemical properties, including an electrochemical stability window of >2.4 V versus Zn|Zn²⁺ and dendrite‐free plating/stripping processes in symmetric Zn||Zn cells. Besides, a UV‐polymerization‐assisted in situ process is developed to produce ZIP (abbreviated i‐ZIP), which is adopted for the first time to fabricate a nonaqueous zinc‐metal polymer battery (ZMPB; VOPO₄|i‐ZIP|Zn) and zinc‐metal hybrid polymer supercapacitor (ZMPS; activated carbon|i‐ZIP|Zn) cells. The VOPO₄ cathode employed in ZMPB possesses a layered morphology, exhibiting a high average operating voltage of ≈1.2 V. As compared to the conventional polymer cell assembling approach using the ex situ process, the in situ process is simple and it enhances the overall electrochemical performance, which enables the widespread intrusion of ZMPBs and ZMPSs into the application domain. Indeed, considering the promising aspects of the proposed ZIP and its easy processability, this work opens up a new direction for the emergence of the zinc‐based energy storage technologies.     

Hybrid Aqueous/Nonaqueous Water‐in‐Bisalt Electrolyte Enables Safe Dual Ion Batteries

Dual ion batteries (DIBs) have recently attracted ever‐increasing attention owing to the potential advantages of low material cost and good environmental friendliness. However, the potential safety hazards, cost, and environmental concerns mainly resulted from the commonly used nonaqueous organic solvents severely hinder the practical application of DIBs. Herein, a hybrid aqueous/nonaqueous water‐in‐bisalt electrolyte with both broad electrochemical stability window and excellent safety performance is developed. The lithium‐based DIB assembled using KS6 graphite and niobium pentoxide as the active materials in the cathode and anode exhibits good comprehensive performance including capacity, cycling stability, rate performance, and medium discharge voltage. Initial capacities of ≈47.6 and 29.6 mAh g^?1 retention after 300 cycles can be delivered with a medium discharge voltage of around 2.2 V in the voltage window of 0–3.2 V at the current density of 200 mA g^?1. Good rate performance for the battery can be indicated by 29.7 mAh g^?1 discharge capacity retention at 400 mA g^?1. It is noteworthy that the coulombic efficiency of the battery can reach as high as 93.9%, which is comparable to that of the corresponding DIBs using nonaqueous organic electrolytes.     

A Novel Potassium‐Ion‐Based Dual‐Ion Battery

In this work, combining both advantages of potassium‐ion batteries and dual‐ion batteries, a novel potassium‐ion‐based dual‐ion battery (named as K‐DIB) system is developed based on a potassium‐ion electrolyte, using metal foil (Sn, Pb, K, or Na) as anode and expanded graphite as cathode. When using Sn foil as the anode, the K‐DIB presents a high reversible capacity of 66 mAh g^?1 at a current density of 50 mA g^?1 over the voltage window of 3.0–5.0 V, and exhibits excellent long‐term cycling performance with 93% capacity retention for 300 cycles. Moreover, as the Sn foil simultaneously acts as the anode material and the current collector, dead load and dead volume of the battery can be greatly reduced, thus the energy density of the K‐DIB is further improved. It delivers a high energy density of 155 Wh kg^?1 at a power density of 116 W kg^?1, which is comparable with commercial lithium‐ion batteries. Thus, with the advantages of environmentally friendly, cost effective, and high energy density, this K‐DIB shows attractive potential for future energy storage application.     

2D Metal–Organic Frameworks (MOFs) for High‐Performance BatCap Hybrid Devices

Two identical layered metal–organic frameworks (MOFs) (CoFRS and NiFRS) are constructed by using flexible 1,10‐bis(1,2,4‐triazol‐1‐yl)decane as pillars and 1,4‐benzenedicarboxylic acid as rigid linkers. The single‐crystal structure analysis indicates that the as‐synthesized MOFs possess fluctuant 2D networks with large interlayer lattices. Serving as active electrode elements in supercapacitors, both MOFs deliver excellent rate capabilities, high capacities, and longstanding endurances. Moreover, the new intermediates in two electrodes before and after long‐lifespan cycling are also examined, which cannot be identified as metal hydroxides in the peer reports. After assembled into battery‐supercapacitor (BatCap) hybrid devices, the NiFRS//activated carbon (AC) device displays better electrochemical results in terms of gravimetric capacitance and cycling performance than CoFRS//AC devices, and a higher energy‐density value of 28.7 Wh kg^?1 compared to other peer references with MOFs‐based electrodes. Furthermore, the possible factors to support the distinct performances are discussed and analyzed.     

Nonaqueous Hybrid Lithium‐Ion and Sodium‐Ion Capacitors

Hybrid metal‐ion capacitors (MICs) (M stands for Li or Na) are designed to deliver high energy density, rapid energy delivery, and long lifespan. The devices are composed of a battery anode and a supercapacitor cathode, and thus become a tradeoff between batteries and supercapacitors. In the past two decades, tremendous efforts have been put into the search for suitable electrode materials to overcome the kinetic imbalance between the battery‐type anode and the capacitor‐type cathode. Recently, some transition‐metal compounds have been found to show pseudocapacitive characteristics in a nonaqueous electrolyte, which makes them interesting high‐rate candidates for hybrid MIC anodes. Here, the material design strategies in Li‐ion and Na‐ion capacitors are summarized, with a focus on pseudocapacitive oxide anodes (Nb₂O₅, MoO₃, etc.), which provide a new opportunity to obtain a higher power density of the hybrid devices. The application of Mxene as an anode material of MICs is also discussed. A perspective to the future research of MICs toward practical applications is proposed to close.     

High‐Safety Nonaqueous Electrolytes and Interphases for Sodium‐Ion Batteries

Rapidly developed Na‐ion batteries are highly attractive for grid energy storage. Nevertheless, the safety issues of Na‐ion batteries are still a bottleneck for large‐scale applications. Similar to Li‐ion batteries (LIBs), the safety of Na‐ion batteries is considered to be tightly associated with the electrolyte and electrode/electrolyte interphase. Although the knowledge obtained from LIBs is helpful, designing safe electrolytes and obtaining stable interphases in Na‐ion batteries is still a huge challenge. Therefore, it is of significance to investigate the key factors and develop new strategies for the development of high‐safety Na‐ion batteries. This comprehensive review introduces the recent efforts from nonaqueous electrolytes and interphase aspects of Na‐ion batteries, proposes their design strategies and requirements for improving safety characteristics, and discusses the potential issues for practical applications. The insight to formulate safe electrolytes and design the stable interphase for Na‐ion batteries with high safety is intended to be provided herein.     

Zn2+ Pre‐Intercalation Stabilizes the Tunnel Structure of MnO2 Nanowires and Enables Zinc‐Ion Hybrid Supercapacitor of Battery‐Level Energy Density

Although there has been tremendous progress in exploring new configurations of zinc‐ion hybrid supercapacitors (Zn‐HSCs) recently, the much lower energy density, especially the much lower areal energy density compared with that of the rechargeable battery, is still the bottleneck, which is impeding their wide applications in wearable devices. Herein, the pre‐intercalation of Zn²⁺ which gives rise to a highly stable tunnel structure of Zn_xMnO₂ in nanowire form that are grown on flexible carbon cloth with a disruptively large mass loading of 12 mg cm^?2 is reported. More interestingly, the Zn_xMnO₂ nanowires of tunnel structure enable an ultrahigh areal energy density and power density, when they are employed as the cathode in Zn‐HSCs. The achieved areal capacitance of up to 1745.8 mF cm^?2 at 2 mA cm^?2, and the remarkable areal energy density of 969.9 µWh cm^?2 are comparable favorably with those of Zn‐ion batteries. When integrated into a quasi‐solid‐state device, they also endow outstanding mechanical flexibility. The truly battery‐level Zn‐HSCs are timely in filling up of the battery‐supercapacitor gap, and promise applications in the new generation flexible and wearable devices.     

Etching‐Assisted Crumpled Graphene Wrapped Spiky Iron Oxide Particles for High‐Performance Li‐Ion Hybrid Supercapacitor

From graphene oxide wrapped iron oxide particles with etching/reduction process, high‐performance anode and cathode materials of lithium‐ion hybrid supercapacitors are obtained in the same process with different etching conditions, which consist of partially etched crumpled graphene (CG) wrapped spiky iron oxide particles (CG@SF) for a battery‐type anode, and fully etched CG for a capacitive‐type cathode. The CG is formed along the shape of spikily etched particles, resulting in high specific surface area and electrical conductivity, thus the CG‐based cathode exhibits remarkable capacitive performance of 210 F g^?1 and excellent rate capabilities. The CG@SF can also be ideal anode materials owing to spiky and porous morphology of the particles and tightly attached crumpled graphene onto the spiky particles, which provides structural stability and low contact resistance during repetitive lithiation/delithiation processes. The CG@SF anode shows a particularly high capacitive performance of 1420 mAh g^?1 after 270 cycles, continuously increases capacity beyond the 270th cycle, and also maintains a high capacity of 170 mAh g^?1 at extremely high speeds of 100 C. The full‐cell exhibits a higher energy density up to 121 Wh kg^?1 and maintains high energy density of 60.1 Wh kg^?1 at 18.0 kW kg^?1. This system could thus be a practical energy storage system to fill the gap between batteries and supercapacitors.     

CuO Nanoplates for High‐Performance Potassium‐Ion Batteries

Potassium‐ion batteries (KIBs) are promising alternatives to lithium‐ion batteries because of the abundance and low cost of K. However, an important challenge faced by KIBs is the search for high‐capacity materials that can hold large‐diameter K ions. Herein, copper oxide (CuO) nanoplates are synthesized as high‐performance anode materials for KIBs. CuO nanoplates with a thickness of ≈20 nm afford a large electrode–electrolyte contact interface and short K⁺ ion diffusion distance. As a consequence, a reversible capacity of 342.5 mAh g^?1 is delivered by the as‐prepared CuO nanoplate electrode at 0.2 A g^?1. Even after 100 cycles at a high current density of 1.0 A g^?1, the capacity of the electrode remains over 206 mAh g^?1, which is among the best values for KIB anodes reported in the literature. Moreover, a conversion reaction occurs at the CuO anode. Cu nanoparticles form during the first potassiation process and reoxidize to Cu₂O during the depotassiation process. Thereafter, the conversion reaction proceeds between the as‐formed Cu₂O and Cu, yielding a reversible theoretical capacity of 374 mAh g^?1. Considering their low cost, easy preparation, and environmental benignity, CuO nanoplates are promising KIB anode materials.     

Emerging Nonaqueous Aluminum‐Ion Batteries: Challenges,Status, and Perspectives

Aluminum‐ion batteries (AIBs) are regarded as viable alternatives to lithium‐ion technology because of their high volumetric capacity, their low cost, and the rich abundance of aluminum. However, several serious drawbacks of aqueous systems (passive film formation, hydrogen evolution, anode corrosion, etc.) hinder the large‐scale application of these systems. Thus, nonaqueous AIBs show incomparable advantages for progress in large‐scale electrical energy storage. However, nonaqueous aluminum battery systems are still nascent, and various technical and scientific obstacles to designing AIBs with high capacity and long cycling life have not been resolved until now. Moreover, the aluminum cell is a complex device whose energy density is determined by various parameters, most of which are often ignored, resulting in failure to achieve the maximum performance of the cell. The purpose here is to discuss how to further develop reliable nonaqueous AIBs. First, the current status of nonaqueous AIBs is reviewed based on statistical data from the literature. The influence of parameters on energy density is analyzed, and the current situation and existing problems are summarized. Furthermore, possible solutions and concerns regarding the construction of reliable nonaqueous AIBs are comprehensively discussed. Finally, future research directions and prospects in the aluminum battery field are proposed.     

A Surface‐Functionalized Ionovoltaic Device for Probing Ion‐Specific Adsorption at the Solid–Liquid Interface

Aqueous ion–solid interfacial interactions at an electric double layer (EDL) are studied in various research fields. However, details of the interactions at the EDL are still not fully understood due to complexity induced from the specific conditions of the solid and liquid parts. Several technical tools for ion–solid interfacial probing are experimentally and practically proposed, but they still show limitations in applicability due to the complicated measurements. Recently, an energy conversion device based on ion dynamics (called ionovoltaic device) was also introduced as another monitoring tool for the EDL, showing applicability as a novel probing method for interfacial interactions. Herein, a monitoring technique for specific ion adsorption (Cu²⁺ and Pb²⁺ in the range of 5 × 10^?6–1000 × 10^?6m ) in the solid–liquid interface based on the ionovoltaic device is newly demonstrated. The specific ion adsorption and the corresponding interfacial potentials profiles are also investigated to elucidate a working mechanism of the device. The results give the insight of molecular‐level ion adsorption through macroscopic water‐motion‐induced electricity generation. The simple and cost‐effective detection of the device provides an innovative route for monitoring specific adsorption and expandability as a monitoring tool for various solid–liquid interfacial phenomena that are unrevealed.     

Few‐Layered Tin Sulfide Nanosheets Supported on Reduced Graphene Oxide as a High‐Performance Anode for Potassium‐Ion Batteries

Anodes involving conversion and alloying reaction mechanisms are attractive for potassium‐ion batteries (PIBs) due to their high theoretical capacities. However, serious volume change and metal aggregation upon potassiation/depotassiation usually cause poor electrochemical performance. Herein, few‐layered SnS₂ nanosheets supported on reduced graphene oxide (SnS₂@rGO) are fabricated and investigated as anode material for PIBs, showing high specific capacity (448 mAh g^?1 at 0.05 A g^?1), high rate capability (247 mAh g^?1 at 1 A g^?1), and improved cycle performance (73% capacity retention after 300 cycles). In this composite electrode, SnS₂ nanosheets undergo sequential conversion (SnS₂ to Sn) and alloying (Sn to K₄Sn₂₃, KSn) reactions during potassiation/depotassiation, giving rise to a high specific capacity. Meanwhile, the hybrid ultrathin nanosheets enable fast K storage kinetics and excellent structure integrity because of fast electron/ionic transportation, surface capacitive‐dominated charge storage mechanism, and effective accommodation for volume variation. This work demonstrates that K storage performance of alloy and conversion‐based anodes can be remarkably promoted by subtle structure engineering.     

Rechargeable Calcium–Sulfur Batteries Enabled by an Efficient Borate‐Based Electrolyte

Rechargeable metal–sulfur batteries show great promise for energy storage applications because of their potentially high energy and low cost. The multivalent‐metal based electrochemical system exhibits the particular advantage of the feasibility of dendrite‐free metal anode. Calcium (Ca) represents a promising anode material owing to the low reductive potential, high capacity, and abundant natural resources. However, calcium–sulfur (Ca–S) battery technology is in an early R&D stage, facing the fundamental challenge to develop a suitable electrolyte enabling reversible electrochemical Ca deposition, and at the same time, sulfur redox reactions in the system. Herein, a study of a room‐temperature Ca–S battery by employing a stable and efficient calcium tetrakis(hexafluoroisopropyloxy) borate Ca[B(hfip)₄]₂ electrolyte is presented. The Ca–S batteries exhibit a cell voltage of ≈ 2.1 V (close to its thermodynamic value) and good reversibility. The mechanistic studies hint at a redox chemistry of sulfur with polysulfide/sulfide species involved in the Ca‐based system.     

Red Phosphorus Nanoparticle@3D Interconnected Carbon Nanosheet Framework Composite for Potassium‐Ion Battery Anodes

Red phosphorus (P) has been recognized as a promising storage material for Li and Na. However, it has not been reported for K storage and the reaction mechanism remains unknown. Herein, a novel nanocomposite anode material is designed and synthesized by anchoring red P nanoparticles on a 3D carbon nanosheet framework for K‐ion batteries (KIBs). The red P@CN composite demonstrates a superior electrochemical performance with a high reversible capacity of 655 mA h g^?1 at 100 mA g^?1 and a good rate capability remaining 323.7 mA h g^?1 at 2000 mA g^?1, which outperform reported anode materials for KIBs. The transmission electron microscopy and theoretical calculation results suggest a one‐electron reaction mechanism ofP + K⁺ + e^?→ KP, corresponding to a theoretical capacity of 843 mA h g^?1,which is the highest value for anode materials investigated for KIBs. The study not only sheds light on the rational design of high performance red P anodes for KIBs but also offers a fundamental understanding of the potassium storage mechanism of red P.     

Layered Oxide Cathode for Potassium‐Ion Battery: Recent Progress and Prospective

Rechargeable potassium‐ion batteries (KIBs) have demonstrated great potential as alternative technologies to the currently used lithium‐ion batteries on account of the competitive price and low redox potential of potassium which is advantageous to applications in the smart grid. As the critical component determining the energy density, appropriate cathode materials are of vital need for the realization of KIBs. Layered oxide cathodes are promising candidates for KIBs due to high reversible capacity, appropriate operating potential, and most importantly, facile and easily scalable synthesis. In light of this trend, the recent advancements and progress in layered oxides research for KIBs cathodes, covering material design, structural evolution, and electrochemical performance are comprehensively reviewed. The structure–performance correlation and some effective optimization strategies are also discussed. Furthermore, challenges and prospects of these layered cathodes are included, with the purpose of providing fresh impetus for future development of these materials for advanced energy storage systems.     

Nonaqueous Sodium‐Ion Full Cells: Status,Strategies, and Prospects

With ever‐increasing efforts focused on basic research of sodium‐ion batteries (SIBs) and growing energy demand, sodium‐ion full cells (SIFCs), as unique bridging technology between sodium‐ion half‐cells (SIHCs) and commercial batteries, have attracted more and more interest and attention. To promote the development of SIFCs in a better way, it is essential to gain a systematic and profound insight into their key issues and research status. This Review mainly focuses on the interface issues, major challenges, and recent progresses in SIFCs based on diversified electrolytes (i.e., nonaqueous liquid electrolytes, quasi‐solid‐state electrolytes, and all‐solid‐state electrolytes) and summarizes the modification strategies to improve their electrochemical performance, including interface modification, cathode/anode matching, capacity ratio, electrolyte optimization, and sodium compensation. Outlooks and perspectives on the future research directions to build better SIFCs are also provided.