Layered Ti₃C₂ Nanocomposites And Their Electrochemical Properties For Lithium-ion Batteries

Layered MXenes are novel two-dimensional?2D?materials that are fabricated through exfoliation from the MAX phases by chemical etching.The formula of MXenes is M_n+1X_n,where n=1,2,3,M is early transition metal,and X is C or N.Because of the2D layered structure,which is like graphene,abundant surface chemical composition,excellent mechanical properties and good electrical and thermal conductivity,MXenes have great application potential in the field of energy storage.Ti₃C₂ is a typical representative of 2D layered MXenes materials.In this paper,the selection of the Ti₃AlC₂precursor was firstly studied to determine the appropriate precursor powders and etching technique,and then the electrochemical behavior of Ti₃C₂ as the anode material of lithium ion batteries?LIBs?was deeply analyzed.Novel anode materials for LIBs were fabricated based on Ti₃C₂.The effects of morphology and structure of the as-prepared nanocomposites on the electrochemical properties were explored.The effect of the powder structure on the etching performance was explored using Ti₃AlC₂ powders prepared by combustion synthesis and non-pressure sintering.The results showed that the powders prepared by non-pressure sintering method were easier to be etched.Utilizing HF as the etching agent,accordion-like multilayer Ti₃C₂ was fabricated.The obtained Ti₃C₂ has the comparable chemical diffusion coefficient of lithium ions to that of artificial graphite.The electrochemical properties of Ti₃C₂ were improved greatly by vacuum heat treatment,through the systematic evaluation of its thermal stability in different atmosphere.In air,the as-prepared Ti₃C₂ could not be oxidized up to 430 ^oC and the rutile-TiO₂ would remain as the oxidation product at 1200^oC.After vacuum heat treatment at 400 ^oC,the Ti₃C₂ showed much higher reversible capacity due to the removal of OH groups,and exhibited excellent rate capability.Besides,the formation of TiO₂ nanoparticles at 700 ^oC further increased the first coulombic efficiency?62%?and capacity retention after 100 cycles?97%?.Fe³⁺was successfully adsorbed onto the surface of alkalized Ti₃C₂ by electrostatic attraction,thus forming Fe?III?nanocomplex?the obtained composite material is named as FNC@Ti₃C₂?.After 100 cycles at 1C,the specific capacity of FNC@Ti₃C₂ electrode was 143 mAh·g^-1,which was 64%higher than that of initial Ti₃C₂.Fe₃O₄@Ti₃C₂nanocomposite samples with three different oxide proportions synthesized by in-situ growth method.SEM and TEM results showed that the size of Fe₃O₄ nanoparticles increased with the increase of Fe²⁺proportion,and the electrical conductivity of nanocomposites measured through 4-point probes method decreased with the increase of Fe₃O₄ content.The nanocomposites exhibited excellent electrochemical performance in Li-ion storage when used as the anode materials for LIBs,which benefited from the combination of the high capacity of Fe₃O₄ and favorable electrical conductivity of Ti₃C₂.Using glucose as carbon source,a uniform continuous amorphous carbon layer,with a thickness of about 1 nm,was coated on the surface of the Fe₃O₄@Ti₃C₂ nanocomposites,to further protect Fe₃O₄ particles that were in direct contact with electrolytes.The presence of amorphous carbon layer on the surface enhanced the cycling stability and rate performance of the nanocomposites.Si@m-Ti₃C₂ nanocomposites were fabricated by combining silicon nanoparticles with multilayer Ti₃C₂?m-Ti₃C₂?through physical assembly method.Thanks to the high conductivity and 2D layered structure of Ti₃C₂,Si nanoparticles have good electrical contact with m-Ti₃C₂,and the nanocomposites exhibited significantly enhanced capacity retention.Using LiF+HCl solution as the etching agent,Ti₃C₂ nanosheet dispersion solution with a concentration of 1.5 mg/mL was prepared.AFM results showed that the lateral dimension of Ti₃C₂ nanosheets was several microns,while the thickness is only a few nanometers.The physical and electrochemical properties of few-layer Ti₃C₂?f-Ti₃C₂?obtained by freeze-drying were compared with Ti₃C₂ film obtained by vacuum filtration.The restacking of Ti₃C₂ film seriously affected the infiltration of electrolyte and the diffusivity of electrolyte ions between layers,weakening the electrochemical properties of Ti₃C₂ nanosheets.Si nanoparticles were immersed into PDDA solution for electropositive modification and enhancement of their activity.Under the action of electrostatic attraction,modified silicon?P-Si?was uniformly distributed on the Ti₃C₂ nanosheets.P-Si@f-Ti₃C₂nanocomposites were formed after freeze-drying.The three-dimensional conductive network structure ensured the rapid diffusion of lithium ions in the nanocomposites and alleviated the volume expansion of silicon nanoparticles in charging and discharging process.