Tuning The Layered Structure Of MXenes And High-Rate Alkali Metal Ion Storage Performance Investigation

MXenes materials are widely applied in the field of electrochemical energy storage because of their typical accordion-like layer structure,modified surface terminal,excellent metal conductivity,and adjustable interlayer spacing.However,MXenes materials exhibit self-stacking,volume expansion,and slow ion diffusion rate during ion inserted/extracted processes,which seriously affects the cycle stability and rate capability in alkali metal ion batteries.This topic takes MXenes（Ti₃C₂,Ti₂C,and Ti₃CN）as the research objective,starting from the point of view that structure determines performance.Significant research mainline has been proposed to enlarge interlayer spacing and strengthen the structural stability.The MXenes was modified by designing the experimental schemes of layer-by-layer self-assembly of different two-dimensional materials,regulating interlayer spacing of organic rigid molecules,and expanding interlayer spacing of organic electrode materials composite intercalation.The effects of interlayer structure stability and interlayer spacing are studied ion diffusion kinetics and rate capability of Li⁺/Na⁺.The research content are displayed as follows:（1）GO-PABA-Ti₃C₂ layered composite materials were constructed by para-aminobenzoic acid（PABA）molecular welding.Owing to the strong chemical bonds（HN-C=O）formed by PABA molecules welded with GO and Ti₃C₂,the stability of GO-PABA-Ti₃C₂ layered structure is enhanced,and the volume expansion is reduced during Li⁺inserted/extracted processes.Meanwhile,PABA molecule acts as a spacer between GO and Ti₃C₂ to inhibit the self-stacking of GO and Ti₃C₂ and expose more active sites.The GO-PABA-Ti₃C₂ exhibits excellent Li⁺storage performance with a high specific capacity of 493.0 m Ah g^-1 at 0.1 A g^-1 after 230 cycles and outstanding coulombic efficiency of 99.0%at 1.0 A g^-1 after 700 cycles.Ex situ XRD measurements demonstrate that GO-PABA-Ti₃C₂ possesses a stable layered structure during charge/discharge processes,resulting in excellent cycle stability and long cycle life.（2）The Ti₂C-n BR composites with different interlayer spacing were controllable and adjusted by organic molecular welding that dehydration condensation reaction of organic molecules and carboxy-functionalized Ti₂C（p-Phenylenediamine:PPDA,3,3′-Diaminobenzidine:DABZ）.The layers spacing of Ti₂C increased from 1.16 nm to 1.35 nm,1.38 nm,and 1.36 nm according to the number of benzene rings inserted,and the Na⁺diffusion kinetics and storage performance are investigated.The rigid molecules between Ti₂C layers enlarge the interlayer spacing,alleviate the volume expansion,and self-stacking during Na⁺inserted/extracted processes.Meanwhile,it strengthens the structural stability of Ti₂C layers,improves the ion diffusion rate,and enhances the Na⁺rate capability.The research declares that Ti₂C-PPDA with 1.38 nm interlayer spacing exhibits a high reversible capacity of 100.2 m Ah g^-1at 0.1 A g^-1 after 960 cycles and a superior rate capability with 80.5 m Ah g^-1at 5.0 A g^-1.（3）Ti₃CN-PTCDA was prepared by perylene-3,4,9,10-tetracarboxylic dianhydride（PTCDA）intercalation to expand the interlayer spacing of Ti₃CN.The interlayer spacing of Ti₃CN expanded from 1.44 nm to 1.58 nm.PTCDA molecules display straining and pillaring effects during Na⁺inserted/extracted processes,which improved the stability of the interlayer structure,inhibited the self-stacking and volume expansion of Ti₃CN,and provided more electrochemical active sites.Ti₃CN-PTCDA exhibited a high specific capacity of 127.9 m Ah g^-1 at 0.1 A g^-1,and still maintained a specific capacity of 51.8 m Ah g^-1 at 5.0 A g^-1.Compared with pure Ti₃CN,Ti₃CN-PTCDA exhibits excellent cyclic stability and rate performance.In summary,the methods of two-dimensional layer-by-layer self-assembly,organic rigid molecules control interlayer spacing,and organic electrode materials composite intercalation are designed to expand the interlayer of MXenes by molecular welding.It is demonstrated that the stability of layer structure and the regulation of layer spacing can effectively improve the Li⁺/Na⁺ion diffusion kinetics and rate capability of MXenes electrode materials.In addition,it provides important guidance for the design of other two-dimensional material structures and the development of high-rate capability.