Excessive emission of greenhouse gases has serious adverse effects on global climate. How to reduce carbon emissions has become a global problem. Supercapacitors have advantages of long cycle life, high power density and relatively low carbon emissions. Developing supercapacitor energy storage is an effective measure to build the reliable future energy system. In recent years, MXene materials have achievedgreat popularity in the field of supercapacitor energy storage applications due to their excellent hydrophilicity, electrical conductivity, high electrochemical stability, and surface chemical tunability. However, the serious self-stacking problem of MXene limits its performance in energy storage. Developing advanced MXene materials is critical for next generation high-performance electrochemical energy storage devices. This paper reviews the research progress of MXene material in the field of supercapacitor energy storage. Firstly, the structure and energy storage properties of MXene are introduced, followed by analysis of the energy storage mechanism of MXene. Secondly, nanoengineering of structure design to improve the performance of MXene electrode is depicted. Thirdly, structure-performance relationship of MXene composite materials and its latest research progress in application of supercapacitor are summarized. Finally, research and development trends of MXene as an electrode for supercapacitor are broadly prospected.

The microreactor was constructed by using the block copolymer (P123)/sodium dodecyl sulfate (SDS) hybrid emulsion. Horseshoe-shaped hollow porous carbon was prepared by hydrothermal carbonization of xylose. The results showed that hydrothermal reaction of xylose occurred at interface between microreactor and solution. Hydrophilicity of PEO (hydrophilic block in P123) decreased at hydrothermal temperature of 160 ℃. Hybrid emulsion was swelled and destroyed gradually because PEO ran into the interior of emulsion. Furthermore, the morphology of microreactor could be regulated by the mass ratio of P123/SDS and the opening angle, and cavity diameter could be controlled by the hydrothermal time. Owing to the open cavity, the capacity of charges and ions was magnified and transport distance was reduced. In addition, specific capacitance and energy density of porous carbons were improved and showed positive correlation with cavity diameter. The horseshoe-shaped hollow porous carbon with largest opening angle (63°), cavity diameter (80 nm) and optimal supercapacitor performance was obtained at a P123/SDS mass ratio of 1.25 : 1 by hydrothermal method for 12 h. In a three-electrode system, the product showed a high specific capacitance of 292 F·g^-1 at a current density of 1 A·g^-1. In a two-electrode system, the product showed an excellent energy density (6.44 Wh·kg^-1), specific capacitance of 185 F·g^-1 at a current density of 0.2 A·g^-1 and outstanding cycling stability (94.83%) after 5000 cycles at a current density of 5 A·g^-1.

Lithium-sulfur batteries (LSBs) have attracted wide attention due to their high energy density, abundant raw material reserves and environmental friendliness. However, the shuttle effect of polysulfides, the large volume expansion during the reaction, and the poor electron conductivity of sulfur greatly limit their practical development. In this work, a ZIF-8 derived flower-like two-dimensional (2D) porous carbon nanosheet/sulfur composite (ZCN-SnS₂-S) combined with SnS₂ nanoparticles is designed as the cathode for LSBs. The unique 2D flower-shaped porous structure not only effectively alleviates the volume expansion during the reaction process, but also provides a fast channel for Li⁺ and electron transport. The presence of heteroatom N further promotes the adsorption of polysulfide. In particular, the polar SnS₂ enhances the chemical adsorption on polysulfides, resulting in excellent electrochemical performance. The ZCN-SnS₂-S electrode exhibits high reversible specific capacity of 948 mAh·g^-1 after 100 cycles at 0.2C (1C=1675 mA·g^-1), demonstrating the capacity retention rate of 83.7%. Even at a high current density of 2C for 300 cycles, it still has a reversible specific capacity of 546 mAh·g^-1.

Metallic Li is one of the ideal anodes for high energy density lithium-ion battery due to its high theoretical specific capacity, low reduction potential as well as abundant reserves. However, the application of Li anodes suffer from serious incompatibility with traditional organic liquid electrolyte. Herein, a gel complex electrolyte (GCE) with satisfactory compatibility with metallic Li anode was constructed via in situ polymerization. The double lithium salt system introduced into the electrolyte can cooperate with the polymer component, which broadens electrochemical window of the electrolyte to 5.26 V compared to 3.92 V of commercial electrolyte, and obtains a high ionic conductivity of 1×10^-3 S·cm^-1 at 30 ℃ as well. Results of morphology characterization and elemental analysis of Li anode surface show that GCE exhibits obvious protective effect on lithium metal under the condition of double lithium salt system, and volume effect and dendrite growth of Li anode are obviously inhibited. At the same time, the lithium metal full battery, assembled with commercial lithium iron phosphate (LiFePO₄) cathode material, exhibits excellent cycling stability and rate performance. The capacity retention rate of the battery reaches 92.95 % after 200 cycles at a constant current of 0.2C (1C = 0.67 mA·cm^-2) at 25 ℃. This study indicates that the GCE can effectively improve the safety, stability and comprehensive electrochemical performance of lithium-metal battery, which is expected to provide a strategy for universal quasi-solid electrolyte design.

Cycle stability and specific capacity of cathode materials for sodium ion batteries play an important role in achieving their wide application. Based on the strategy of introducing specific heteroelements to optimize the structural stability and specific capacity of cathode materials, O3-Na_0.9Ni_0.5-xMn_0.3Ti_0.2Sb_xO₂ (NMTSb_x, x=0, 0.02, 0.04, 0.06) was prepared by a simple solid-state reaction method, and effects of Sb doping amount on the sodium storage properties of Na_0.9Ni_0.5Mn_0.3Ti_0.2O₂ cathode materials were investigated. The characterization results show that the electrostatic repulsion force between oxygen atoms in the transition metal layer is reduced after Sb doping, while the lattice spacing is expanded, which is conducive to deintercalation of Na⁺. Meanwhile, the strong electron delocalization caused by Sb doping decreases energy of the whole system, leading to a stable structure, more conducive to cyclic charging and discharging. The electrochemical test shows that initial discharge specific capacity of undoped NMTSb₀ is 122.8 mAh·g⁻¹ at 1C(240 mA·g⁻¹), and the capacity retention rate is only 41.5% after 200 cycles. But initial discharge specific capacity of doped NMTSb_0.04 is 135.2 mAh·g⁻¹ at 1C, and the capacity retention rate is up to 70% after 200 cycles. This study shows that Sb doped O3 type Na_0.9Ni_0.5Mn_0.3Ti_0.2O₂ cathode material can significantly improve initial discharge specific capacity and capacity retention rate of sodium ion batteries. Our results suggest that Sb doping strategy might be a useful approach for preparation of high stable sodium ion batteries..

Suffering from strong electrostatic interactions between divalent Zn²⁺ and host framework, molybdenum disulfide exhibits slow reaction kinetics as cathode for aqueous zinc-ion batteries. The narrow layer spacing of MoS₂ is difficulty in accommodating large size insertion of hydrated Zn²⁺, resulting in a lower discharge specific capacity. Here, NH₄⁺ expanded MoS₂-N was prepared by a simple ammonia-assisted hydrothermal. The result showed that the ammonia promoted hydrolysis of thioacetamide to provide reduced S^2- and generated a large amount of NH₄⁺ as intercalating particles. These particles expanded the layer spacing of pristine MoS₂ from 0.62 nm to 0.92 nm, greatly reducing the Zn²⁺ inserting energy barrier (with its charge transfer resistance of MoS₂-N only 35 Ω), and increased the discharge specific capacity to 149.9 mAh·g⁻¹ at the current density of 0.1 A·g^-1, 2 times that of MoS₂ electrode without NH₄⁺ expansion. Consequently, it exhibited a stable discharge capacity of about 110 mAh·g^-1 at the current density of 1.0 A·g^-1 with nearly 100% Coulombic efficiency after 200 cycles. The approach of ammonia-assisted layer expansion proposed in this study enriches the modification strategy to enhance the electrochemical performance of MoS₂ and provides a new idea for subsequent cathode development.

Lithium-sulfur (Li-S) batteries play a crucial role in the development of next-generation electrochemical energy storage technology due to its high energy density and low cost. However, their practical application is still hindered by the sluggish kinetics and low reversibility of the conversion reactions, which contribute to relatively low practical capacity, Coulombic inefficiency, and cycling instability. In this regard, the rational design of conductive, adsorptive and catalytic functional materials presents a critical pathway to stabilize and promote sulfur electrochemistry. Benefiting from the unique atomic and electronic structures of boron, boron-based materials exhibit multifarious and tunable physical, chemical and electrochemical properties, and have received extensive research attentions in Li-S batteries. This paper reviews the recent research progress of boron-based materials, including borophene, boron atom-doped carbon, metal borides and non-metal borides in Li-S batteries, concludes the remaining problems and proposes the future developing perspective.

Magnesium metal batteries (MMBs) have attracted increasing attention due to the high volume specific capacity (3833 mAh/cm³) and high safety of Mg metal anode. Nevertheless, the high polarization effect induced by Mg²⁺ inhibits its diffusion in solid phase and therefore limits the specific capacity of MMBs. Li⁺/Mg²⁺ dual-salt electrolyte has been proposed to circumvent the sluggish diffusion of Mg²⁺ in solid phase, which enables Li⁺ to replace Mg²⁺ to drive the cathode reaction. In this work, the electrochemical performance of CoS₂ as conversion cathode of MMBs is studied based on different Li⁺/Mg²⁺ dual-salt electrolytes, and the effects of Li-salt concentration and discharge-charge voltage range on cycling stability are analyzed. The strategy of Li-salt additive remarkably promotes the conversion kinetics of CoS₂ based MMBs. By developing charge voltage to 2.75 V, the cycling stability of Mg-CoS₂ cell in LiCl-APC electrolyte is significantly enhanced. Its specific capacity can be maintained at 275 mAh/g after 150 cycles, which is much higher than that (33 mAh/g) under the protocol of 2.0 V charge voltage. It is found that the capacity degradation of MMBs is related to the irreversible reaction of CoS₂ and generation of Co₃S₄ at the charge potential of 2.0 V. And the dissolution of Co and S elements from active species aggravates the irreversible loss of capacity. This study provides an electrochemical activation solution to the development of transition metal sulfides in conversion type MMBs.

Sulfurized pyrolyzed poly(acrylonitrile) (S@pPAN) composite as cathode material of Li-S battery realizes a solid-solid conversion reaction mechanism without dissolution of polysulfides. However, its surface and interface characteristics influence the electrochemical performance significantly, and there are also obvious volume changes during electrochemical cycling. In this study, single-walled carbon nanotubes (SWCNT) and sodium carboxymethyl cellulose (CMC) were used as binder for S@pPAN cathode to regulate the surface of S@pPAN and alleviate volume changes during charging and discharging. At a current density of 2C, capacity retention rate of the batteries after 140 cycles was 84.7%, and a high specific capacity of 1147 mAh∙g^-1 can still be maintained at a high current density of 7C. The ultimate tensile strength for the film of the composite binder increases by 41 times after adding SWCNT, and the composite binder guarantees a more stable electrode interface during operation, thereby effectively improves the cycle stability of the as sembled lithium-sulfur batteries.

Metal cyanamides/carbodiimides [M_x(NCN)_y], as oxygen/chalcogenide-like compounds, are a new class of inorganic functional materials. The quasi-linear [NCN]^2- anion endows their open and porous crystal structure, unique electronic structure and unique physicochemical properties. They have shown potential applications in many fields including solid-state luminescence, photo/electrocatalysis, and electrochemical energy storage, becoming a research hotspot in recent years. This review outlines the research history of metal cyanamides, introduces the crystal structures and physicochemical properties, summarizes their synthetic methods and strategies, and discusses the applications for electrochemical energy storage, focusing on the electrochemical performance and charge storage mechanism as new-type negative electrode materials for lithium/sodium ion batteries.

Na-ion batteries, which currently use flammable and explosive organic electrolytes, now urgently need to develop high performance sodium ion solid electrolyte to realize more safe and practical application. Na₃Zr₂Si₂PO₁₂ is one of the most promising solid sodium electrolytes for its wide electrochemical window, high mechanical strength, superior air stability and high ionic conductivity. But its inhomogeneous mixing of the ceramic particles with the binders causing much more pores in the green bodies makes it difficult to obtain high-density and high-conductivity ceramic electrolytes after sintering. Herein, the spray drying method was used to enable Na₃Zr₂Si₂PO₁₂ particles uniformly coated with binders and granulated into spherical secondary ones. The as-prepared normal distributed particles can effectively contact each other and reduce porosity of ceramic green body. After sintering, Na₃Zr₂Si₂PO₁₂ ceramic pellets via the spray drying show relative density of 97.5% and ionic conductivity of 6.96×10^-4 S∙cm^-1 at room temperature. In contrast, the relative density and room-temperature ionic-conductivity of Na₃Zr₂Si₂PO₁₂ ceramic pellets prepared without the spray-drying are only 88.1% and 4.94×10^-4 S∙cm^-1, respectively.

Transition metal selenides (TMSs) with the merits of their versatile material species, ample abundance and high theoretical specific capacity have been regarded as attractive anode materials for sodium-ion batteries (SIBs). However, the large volume changes during the electrochemical reactions which result in limited cycle performance hinder their commercialization. Herein, the hollow structure composed of CoSe₂ and carbon skeleton (denoted as H-CoSe₂/C), derived from the metal organic framework material ZIF-67 using tannic acid as the etching agent, was used as the anode for SIBs. Owing to the unique hollow structure which can alleviate the volume change of the material, H-CoSe₂/C exhibited excellent sodium ions storage performances in terms of cycling stability. Compared with the solid counterpart, the reversible specific capacity of the H-CoSe₂/C electrode remains 383.4 mAh·g^-1 after 350 cycles at a current density of 50 mA·g^-1 with the capacity retention of 83.6%. Even at 500 mA·g^-1, the capacity retention can still reach 72.2% after 350 cycles. This work manifests that hollow structure can provide enough space to alleviate the problem of volume changes for TMSs during the sodiation/desodiation process, thus the cycle performances can be improved.

Lithium (Li) metal is one of the most attractive anode materials for the development of high energy density batteries due to its high theoretical specific capacity and low electrochemical potential. However, during the repeated deposition/stripping of Li metal anode, irregular Li dendrite growth inevitably takes place, which seriously affects the cycle life and safety of Li metal batteries. In this study, a simple and mild strategy was developed to in-situ modify the carbon nanotubes with bismuth (Bi) nanoparticles, followed by coating the as-prepared materials on the surface of commercial copper foil as current collector for Li metal anode. It is demonstrated that the in-situ modified Bi nanoparticles promotes the uniform Li deposition, thereby inhibiting the growth of Li dendrites and improving the electrochemical performance of Li metal batteries. Under the current density of 1 mA·cm^-2, Coulombic efficiency of Li|Cu cell based on the Bi@CNT/Cu current collector maintains 98% after 300 cycles. Meanwhile, the symmetric cell based on the Li@Bi@CNT/Cu anode can maintain the stable cycling for 1000 h. When it is applied in LiFePO₄ (LFP) full cell, the Bi@CNT/Cu current collector also exhibits excellent electrochemical performance, which can retain the stable cycling for 700 cycles at the rate of 1C (170 mA·g^-1). This study provides a new strategy for suppressing dendrite growth of Li metal anodes.