Several MAX Phases And Their Derivatives:Molten Salt Synthesis And Electrochemical Properties

MAX phases are a family of layered ternary carbides/nitrides,where "M" is an early transition metal,"A" is an A-group element,and "X" is C and/or N.Owing to the fact that MX layers are chemically stable while A-group atoms are relatively weakly bonded,A-group atom layers can be selectively etched to prepare the two-dimensional derivatives referred to as MXenes.Both MAX phases and MXenes are prospective in the field of Li-ion batteries.It is generally believed that the conversion of electric energy to chemical energy in MAX phases is realized by means of Li-ion intercalation or alloying reaction with the A element.While MXenes are typical intercalation pseudocapacitive materials,which store electric energy through Li-ion intercalation and surface redox reaction.However,the following issues hinder their practical application.On the one hand,the MAX phases have limitations of high synthesis temperature(above 1300℃),large grain size,few reaction interfaces,and unclear Li-storage mechanism.On the other hand,the electrochemical performance of MXenes relies on two processes:surface adsorption and Li-ion intercalation.Large grain size results in low proportion of Li-ion adsorption contribution to capacity and long Li-ion diffusion distance,which deteriorates the rate performance of MXenes.In order to address these issues,submicron powders of typical MAX phases,including Ti2AlC,Ti3AlC2,and Ti2SnC,were synthesized by molten salt method at relatively lower temperatures.Comprehensive investigations were carried out on the formation mechanism of MAX phases in molten salt and the Li storage properties of both MAX phases and MXenes.In addition,it is found that the initial coulombic efficiency(ICE)of MXene does not correlate with its specific surface area,which confirms that the low ICE of MXene is dominated by the irreversible bonding between Li ions and MXene.Based on this understanding,the full cells with high ICE were prepared by electrode prelithiation method.This method was further extended to flexible electrodes.The main research contents and results are as follows:(1)Three types of submicron MAX phase powders,including Ti2AlC,Ti3AlC2,and Ti2SnC,were synthesized by molten salt method at temperature no more than 1100℃.The investigation on the reaction mechanism demonstrates that the formation of MAX phase in molten salts depends on the diffusion of metal elements to carbon.Therefore,the key point to synthesize small-sized MAX phase is using small-sized carbon source.(2)These three types of submicron MAX phase powders exhibit low intrinsic Li storage capacity.The contribution of the conductive agent,namely carbon black,to the capacity is much higher than that of the MAX phase in the electrode.In contrast,submicron Ti2CTx derived from submicron Ti2AlC,processes excellent rate performance and cycling stability.At a high current density of 10 A g-1,submicron Ti2CTx can release a specific capacity of～155.2 mAh g-1 in 56 seconds,reaching 57%of its reversible capacity(～270 mAh g-1,corresponding to a chemical formula of Ti2CTxLi1.4).In-depth electrochemical analyses demonstrate that the reduction in lateral size of Ti2CTx MXene particulates remarkably facilitates Li ions in(de)lithiation process in the low potential region.(3)A series of MXenes,with various specific surface areas,were prepared,whose ICEs do not correlate with specific surface area.This result unambiguously indicates that the main reason for the low ICE of MXene is the irreversible bonding between Li+ion and MXene rather than forming solid electrolyte interface on its surface caused by the electrolyte decomposition.On the basis of this conclusion,the full cells,with high ICE,were prepared by prelithiating MXene-based electrode,which laid a good foundation for the practical application of MXenes in the full cells.(4)Flexible anode electrodes,with high rate capability and cycling stability,were prepared by constructing a composite containing SWCNTs and submicron Ti2CTx.This strategy was further extended to the flexible cathode electrodes composing of SWCNTs and LiFePO4.The full cells exhibit high ICE using flexible cathode and prelithiated anode.Following this conception,a flexible catalysis containing SWCNTs and Ti3C2Tx@Pt was prepared to realize a stable hydrogen evolution reaction,which sheds light on the advantages of MXenes as emerging two-dimensional materials in the field of energy storage and conversion.