Microstructure Design And Electrochemical Energy Storage Performance Of MXene-based Composites

With the development of emerging industries,high elelctrochemical performance such as new energy vehicles,high energy density,excellent cycling life,and intrinsic safety becomes the central pursuits for electrochemical energy storage devices,which play a crucial role in achieving the goal of carbon neutrality and emission peak.The performance of traditional lithium-ion batteries(LIBs)based on graphite negative electrode has reached the upper limit,which can hardly meet the demand of high energy density.Problems including flammable electrolyte decomposition and the growth of lithium dendrite lead to serious safety risks.Therefore,developing the electrode materials with both excellent lithium storage capacity and high safety is the key to achieve high-performance lithium ion battery.On the other hand,Zn-air batteries(ZABs)have become one of the most promising metal-air battery technologies owing to their superior theoretical energy density(1086 Wh kg-1),intrinsic security,and environmental friendliness.However,the practical application of ZABs is seriously impeded by the sluggish kinetics of the oxygen reduction reaction(ORR)and oxygen evolution reaction(OER)for the air electrode,leading to large battery polarization,low power density,and poor rate capability.The formation of Zn dendrite further threatens the cycling stability and service life of ZABs.The high-performance LIB anodes and ZAB catalysts with optimized electrochemical/catalytic activity and structural stability can be synthesized through the modulation of chemical composition,atomic coordination and electronic structure,which could accelerate the electrochemical reaction kinetics.Two-dimensional transition metal carbides/nitrides(MXenes)possess multiple advantages including metal-like conductivity,unique layer structure,rich surface redox sites,and excellent hydrophilic and mechanical flexibility,which have shown great application potential in the field of electrochemical energy storage.Herein,based on the intrinsic structural characteristics of MXene-based composites,we design a series of advanced LIB electrodes and ZAB catalysts/anodes via coupling hierarchical structure design,interface optimization,and defect structure regulation strategies,and deeply investigate the electrochemical reaction mechanism.The main research contents are as follows.(1)The Ti3C2/Si composites consisting of Ti3C2 MXene nanosheets and Si nanoparticles anchored on MXene are fabricated through orthosilicate hydrolysis and low-temperature reduction.The Si nanoparticles anchored on Ti3C2 through Si-O-Ti bonds could effectively shorten the Li+ transport route and provide sufficient alloying reaction sites.The homogeneously dispersed Si nanoparticles suppress the self-stacking effect of MXene nanosheets,exposing their electrochemically active surface to contact with electrolyte.Importantly,Ti3C2 MXene not only provides fast transfer channels for Li+ and electrons,but also relieves volume expansion of Si during cycling to improve structural stability.The strong interface coupling between MXene and Si promotes the interfacial charge/ions transport,accelerating the electrochemical reaction kinetics.Moreover,Ti3C2/Si electrode displays the"battery-capacitive" dual-model energy storage mechanism.MXene exhibits the excellent pseudocapacitive performance due to its abundant surface redox sites,which provide important capacity contribution.The Ti3C2/Si-based LIB delivers the highly improved reversible capacity,rate capability and cycling stability,providing the specific capacity of 1849 mAh g-1 at 100 mA g-1,even retaining 956 mAh g-1 after 800 cycles at 1 A g-1.(2)The Ti3C2 MXene lithium-storage electrodes with different vacancy defect concentration were prepared by regulating the etchant concentration,and their electrochemical properties were systematically compared to explore the internal connection between the Ti vacancy(VTi)defect and the initial coulombic efficiency(ICE)and cycle stability.As a |energy trap",VTi defect would irreversibly capture Li+,and also induces the electrolyte decomposition to form a thick and uneven solid electrolyte interlayer(SEI),which increases impedance and promotes the growth of lithium dendrite,resulting in large irreversible lithium consumption.For defects passivation,we proposed a simple method of selective growth of ultrasmall Al2O3 nanoclusters on MXenes defects sites to form Ti3C2@Al2O3 composites.The Al2O3 nanoclusters selectively covering on VTi sites can effectively prevent the defects irreversibly capturing Li+,which dramatically improves Li+diffusion kinetics.Besides,Al2O3 nanoclusters can relieve the electrolyte decomposition induced by defects to form the thin and homogeneous SEI layer,which greatly improves the electrodes stability and boosts the uniform lithium deposition without catastrophic dendrites formation.Attributed to the excellent passivation effect of Al2O3 nanoclusters,LIBs based on Ti3C2@Al2O3 electrodes exhibit excellent electrochemical performance,including improved ICE of 76.6%at a current density of 100 mA g-1 and better cycling stability(285.5 mAh g-1 after 500 cycles at 1 A g-1).(3)The Ti3C2@SrTiO3 composite was prepared using a controllable in-situ phase transformation strategy.The cubic phase SrTiO3 nanoparticles are uniformly anchored on MXene matrix,providing rich catalytic active sites and preventing MXene nanosheets from self-stacking.The electrochemically active surface of MXene nanosheets are sufficiently exposed,which promotes rapid charge transfer and ion transport.The O vacancies introduced in SrTiO3 during the transformation reaction could remarkably enhance the oxygen intermediates’adsorption ability,greatly lowering the energy barrier for the ORR/OER process.Furthermore,the presence of Ti vacancies in the Ti3C2 MXene can effectively boost the electron transfer in Ti3C2@SrTiO3 heterostructures,further modulating the electronic structure of active sites to optimize oxygen intermediates’ adsorption behavior,endowing MXene with an important synergetic effect.In addition,the interface bonding between MXene and SrTiO3 can significantly improve the interfacial charge/ion transfer kinetics and structural stability,which guarantees the excellent catalyst durability.Owing to the synergetic effect of Ti/O double vacancies,Ti3C2@SrTiO3 exhibits excellent ORR/OER bifunctional catalytic activity.The ZAB with Ti3C2@SrTiO3 delivers an extraordinary open-circuit voltage of 1.44 V,and an essentially improved power density of 122 mW cm-2,showing outstanding cycling stability after 1000 cycles under the current density of 10 mA cm-2.(4)An optimized electrode-electrolyte integrated MXene/Zn-LDH-array@PVA structure is developed via an electrochemical Zn deposition,in situ LDH growth,polymer infiltration,and cross-linking route,which are applied to high-performance flexible ZABs.The highly orientated hydrophilic CoNi-LDH arrays sufficiently cross-link with PVA polymer chains,which not only decreases the crystallinity degree of PVA polymer to improve the flexibility of local chain segmental motion,but also provides fast ionic diffusion channels to reduce the ionic transport barrier,endowing LDH-array@PVA GPE with significantly improved ionic conductivity(55.3 mS cm-1),water retention capability,and mechanical flexibility.Moreover,the optimized anode-GPE integrated interface of MXene/ZnLDH-array@PVA demonstrates excellent interfacial compatibility and stability,which guarantees sufficient interfacial contact and effectively reduces the interfacial impedance,promoting fast and homogeneous interfacial ionic flux to enhance the uniform zinc deposition without dendrite formation.Ti3C2 MXene/Zn with a layered structure achieves rapid electron transport and relatively uniform charge distribution to promote the stable Zn deposition.The optimized ionic transfer kinetics and stable anode-GPE integrated interface bring the MXene/Zn-LDH-array@PVA-based flexible ZAB a long working life(more than 50 h)with a lowered charge/discharge polarization(voltage gap of 0.85 V),and a high power density of 92.3 mW cm-2.The integrated ZABs could work stably without performance degradation under bending conditions,showing great application potential.