Preparation,Characterization And Lithium Storage Properties Of Columbite MNb₂O₆ Oxides

With the continuous improvement of energy storage requirements,the development of rechargeable electrochemical energy storage devices with high-energy/power density and long cycle life has always been a research hotspot.Lithium-ion capacitors（LICs）,as a new type of device that utilizes capacitive positive electrodes and battery negative electrodes,can combine high energy and power density,making them the next generation of highly promising electrochemical energy storage devices.However,due to the different mechanisms of lithium storage between positive and negative electrodes,there will always be a kinetics imbalance,resulting in a significant decrease in the overall performance of LICs.This paper mainly focuses on the niobite MNb₂O₆ oxides,and uses Nb₂AlC MAX as the niobium source to synthesize a series of 3D accordion like MNb₂O₆ oxides constructed by micro/nano particles through a general strategy,to solve the problems of mismatching positive and negative dynamics of LICs and limited cycle life.The specific research results are as follows:（1）The multilayer accordion shaped monoclinic Cu Nb₂O₆（M-CNO）framework was successfully synthesized by the subsequent solvent heat treatment and annealing at 800°C with multilayer Nb₂CT_x MXenes as the precursor,and the orthorhombic Cu Nb₂O₆（O-CNO）can be obtained by simply grinding M-CNO.It is worth noting that by grinding,the high-temperature annealing（>1000°C,insulation for 12 h）process required to generate O-CNO is avoided,greatly saving energy consumption.In addition,for the first time,we have reasonably proposed the lithium storage mechanism of O-CNO by combining in-situ characterization technology.During the initial lithiation process,Li Nb₃O₈ and Nb O₂ formed by the decomposition of O-CNO are electrically active phases for reversible lithium storage,while the newly generated Cu acts as a conductive to provide additional transmission paths for electron transport.Thanks to its unique lithium storage mechanism and structural advantages,the O-CNO electrodes obtained have a high specific capacity of 150.3 m Ah g^-1 at 2.0 A g^-1 and long cycle stability.When assembled with activated carbon（AC）positive electrodes to form LICs,O-CNO//AC LICs achieved a high energy density of 138.9 Wh kg^-1 at a power density of 200 W kg^-1 and excellent cycling stability.（2）A series of multilayer columbite MNb₂O₆（M=Cd,Co,Zn,Mn,Mg,Ca）oxides constructed by single crystal micro/nano particles were prepared using Nb₂AlC MAX as the niobium source through a simple three-step synthesis strategy.Density functional theory（DFT）,ultraviolet photoelectron spectroscopy（UPS）and ex-site XRD were used to analyze the band structure,Li⁺diffusion barrier,functional value and the volume change during charging and discharging process of six MNb₂O₆ samples.Specifically,the Cd Nb₂O₆ sample exhibits excellent electron/ion transport properties and minimal（≈0.32%）volume change during Li⁺（de）intercalation.Therefore,it exhibits a specific capacity of 102.8 m Ah g^-1 at a maximum current density of 10 A g^-1,and a capacity retention up to 80.3%after 20000 cycles.In addition,the assembled Cd Nb₂O₆//AC LICs shows the highest energy density and power density of150.3 Wh kg^-1 and 10 k W kg^-1,separately.At the same time,after 10000 cycles,the capacity retention rate is 75.8%,which is equivalent to a capacity attenuation of only 0.0024%per cycle.（3）We found that the process of stress induced transformation from M-CNO to O-CNO through grinding can be effectively suppressed by doping Ni²⁺,Co²⁺,and Zn²⁺instead of Cu ²⁺.Specifically,when the proportion of doped atoms is greater than or equal to 0.2,i.e.,the sample with a stoichiometric ratio of Ni_0.2Cu_0.8Nb₂O₆（28NCNO）,the M-CNO crystal phase can be more stable,neither grinding nor high-temperature annealing can achieve the transition from M-CNO to O-CNO.In addition,as the doping content of Ni atoms continues to increase to 0.5（55NCNO）,the crystal structure of the product tends to be more inclined towards orthorhombic Ni Nb₂O₆（NNO）.Interestingly,the 55NCNO sample exhibits a clear intercalation mechanism.When 55NCNO is used as the negative electrode and AC is used as the positive electrode to assemble LICs,the highest power density displayed is 10.75 k W kg^-1,corresponding to a high energy density of 49.5 Wh kg^-1.After 3000 cycles,there is still 80.4%capacity retention rate,equivalent to a capacity decay of 0.0065%per cycle.