Recent progress and prospective on layered anode materials for potassium-ion batteries

Potassium-ion batteries(PIBs),also known as“novel post-lithium-ion batteries,”have promising energy storage and utilization prospects due to their abundant and inexpensive raw materials.Appropriate anode materials are critical for realizing high-performance PIBs because they are an important component determining the energy and power densities.Two-dimensional(2D)layered anode materials with increased interlayer distances,specific surface areas,and more active sites are promising candidates for PIBs,which have a high reversible capacity in the energetic pathway.In this review,we briefly summarize K+storage behaviors in 2D layered carbon,transition metal chalcogenides,and MXene materials and provide some suggestions on how to select and optimize appropriate 2D anode materials to achieve ideal electrochemical performance.

MXene-based materials for electrochemical energy storage

Rechargeable batteries and supercapacitors are widely investigated as the most important electrochemical energy storage devices nowadays due to the booming energy demand for electric vehicles and hand-held electronics. The large surface-area-to-volume ratio and internal surface areas endow two-dimensional（2D） materials with high mobility and high energy density; therefore, 2D materials are very promising candidates for Li ion batteries and supercapacitors with comprehensive investigations. In 2011, a new kind of 2D transition metal carbides, nitrides and carbonitrides, MXene, were successfully obtained from MAX phases. Since then about 20 different kinds of MXene have been prepared. Other precursors besides MAX phases and even other methods such as chemical vapor deposition（CVD） were also applied to prepare MXene, opening new doors for the preparation of new MXene. Their 2D nature and good electronic properties ensure the inherent advantages as electrode materials for electrochemical energy storage. In this review, we summarize the recent progress in the development of MXene with emphasis on the applications to electrochemical energy storage. Also, future perspective and challenges of MXene-based materials are briefly discussed regrading electrochemical energy storage.

Recent progress in 2D materials for flexible supercapacitors

High performance supercapacitors coupled with mechanical flexibility are needed to drive flexible and wearable electronics that have anesthetic appeal and multi-functionality. Two dimensional（2D） materials have attracted attention owing to their unique physicochemical and electrochemical properties, in addition to their ability to form hetero-structures with other nanomaterials further improving mechanical and electrochemical properties. After a brief introduction of supercapacitors and 2D materials, recent progress on flexible supercapacitors using 2D materials is reviewed. Here we provide insights into the structure–property relationships of flexible electrodes, in particular free-standing films. We also present our perspectives on the development of flexible supercapacitors.

Synthesis and electrochemical behavior of monolayer-Ti_(3)C_(2)T_(x) for capacitive deionization

MXene materials have got great attention from researchers of environmental treatment for the great electrochemical performance.Monolayer-Ti_(3)C_(2)T_(x)(T_(x) is the surface terminal groups such as-O,-OH and/or-F species),as a typical structural MXene,always shows better chemical-physical characteristics than multilayer-Ti_(3)C_(2)T_(x).Thus,we prepared monolayer-Ti_(3)C_(2)T_(x) electrode by HF etching method and absolute ethyl alcohol intercalationdelamination treatment for capacitive deionization(CDI).The prepared monolay-Ti_(3)C_(2)T_(x) shows a higher specific surface area(235.6 m^(2)/g)and a thinner thickness(0.8 nm).Moreover,a series of systematic investigation demonstrated that monolayer-Ti_(3)C_(2)T_(x) has obvious promotional phenomenon on electrochemical properties(e.g.,mass specific capacitance increased from 52.1 F/g to 144.7 F/g).The NaCl adsorption capacity of monolayer-Ti_(3)C_(2)T_(x),is 30.7 mg/g in 1000 mg/L NaCl solution at 1.2 V.We concluded that the electro-sorption mechanism could be expressed as double electric layer and monolayer coverage by a good fitting of Langmuir isotherms and the pseudo-second-order kinetics equation.This work would provide a new strategy for the application of monolayer-Ti_(3)C_(2)T_(x) material in wastewater treatment in the future.

Cationic intermediates assisted self-assembly two-dimensional Ti_(3)C_(2)T_(x)/rGO hybrid nanoflakes for advanced lithium-ion capacitors

Two-dimensional(2 D)material MXenes have been intensively concerned in energy-storage field due to these unique properties of metallic-like conductivity,good hydrophilicity and high volumetric capacity.However,the self-restocking of ultra-thin 2 D materials seriously hinders these performances,which significantly inhibits the full exploitation of MXenes in the field of energy storage.To solve this issue,a strategy to prepare delaminated Ti_(3)C_(2)T_(x)(MXene)nanoflakes/reduced graphene oxide(r GO)composites is proposed using the electrostatic self-assembly between positively charged Ti_(3)C_(2)T_(x) with tetrabutylammonium ion(TBA+)modification and negatively charged graphene.The nanoflakes of Ti_(3)C_(2)T_(x)/rGO are well dispersed and arranged in a face-to-face structure to effectively alleviate the self-restacking and provide more electroactive sites for accessible of electrolyte ions.The prepared delaminated Ti_(3)C_(2)T_(x)/r GO anode shows a high reversible capacity up to 1394 m Ah g^(-1) at a current density of 50 m A g^(-1).Moreover,a lithium-ion capacitor(LIC)was assembled with delaminated Ti_(3)C_(2)T_(x)/r GO anode and activated carbon(AC)cathode which can exhibit a specific capacity of 70.7 F g^(-1) at a current density of 0.1 A g^(-1) and deliver an ultrahigh energy density of 114 Wh kg^(-1) at a relatively high power density of 3125 W kg^(-1).These good electrochemical performances demonstrate the potential of delaminated Ti_(3)C_(2)T_(x)/r GO as an anode material for lithium-ion capacitors.