New Nb₁₂O₂₉ M-Nb-O Series Materials As Anode Electrode Materials For Lithium Ion Batteries

Due to the advanced performances of higher energy output,longer storage life and lower maintenance than other secondary batteries,lithium-ion batteries(LIBs)are considered as very promising power sources for electric vehicles(EVs).The electrode material is an important part of the LIBs,and the performance of the anode material will directly affect the performance of the LIBs.With the development of EVs,as the demand for high performance LIBs are increased,the demand for high performance anode materials are increased.Among the numerous alternative anode materials,M-Nb-O compounds have attracted much research interest due to the high theoretical capacity,safe working potentials and durable cyclic stability.However,there are two problems need to be solved.On the one hand,there are little M-Nb-0 compounds have been discovered.On the other hand,the poor electronic conductivity and the insufficient Li+-ion diffusion coefficient limit its application in LIBs.Therefore,two M-Nb-O compounds,FeNb11O29 and GaNb11O29,were selected as experimental objects.And three kinds of technologies,including doping,oxygen vacancy modification and electrospinning technology,were adopted to improve their electrochemical performance.(1)The recently explored FeNb11O29 is an advanced anode material for lithium-ion batteries due to its large specific capacity and high safety.However,its poor electronic conductivity significantly limits its rate capability.To tackle this issue,a Cr3+ doping is employed,and CrxFe1-xNb11O29(x ? 0 and 0.2)materials were fabricated using a solid-state reaction.X-ray diffraction analyses combined with Rietveld refinements demonstrate that the Cr3+ doping does not destroy the orthorhombic shear ReO3 crystal structure(Amma space group)of FeNb11O29 or obviously change the unit cell volume.Scanning electron microscopy images combined with specific surface area tests reveal the similar particle size after the Cr3+ doping.Due to the free 3d electrons in the Cr3+ ions,the electronic conductivity of Cr0.2Fe0.8Nb11O29 is enhanced by three orders of magnitude comparing with FeNb11O29.Consequently,Cr0.2Fe0.8Nb11O29 exhibits improved electrochemical properties.At 0.1 C,it delivers a large reversible capacity of 254 mAh g-1.At 10C,it still provides a large capacity of 123 mAh g-1 with large retention of 86.9%after 500 cycles.In contrast,FeNb11O29 shows a small capacity of 57 mAh g-1 and small retention of 41.6%.These results reveal that Cr0.2Fe0.8Nb11O29 can be a promising anode material for high-performance lithium-ion batteries.(2)The recently explored FeNb11O29 is an advanced anode material for lithium-ion batteries due to its high specific capacity and safety.However,it suffers from its poor rate capability.To tackle this issue,a crystal-structure modification is employed.Defective FeNb11O29(FeNb11O27.9)is fabricated using a one-step solid-state reaction method in N2.FeNb11O27.9 has the same orthorhombic shear ReO3 crystal structure(Amma space group)as FeNb11O29 but a larger unit-cell volume and 3.8%O2-vacancies(vs.all O2-ions),which improve the Li+-ion diffusion coefficient by a factor of 88.3%.The contained Nb4+ ions with free Ad electrons significantly increase the electronic conductivity by three orders of magnitude.Consequently,FeNb11O27.9 shows an improved pseudocapacitive behaviour and electrochemical properties.In comparison with FeNb11O29,FeNb11O27.9 exhibits a higher reversible capacity of 270 mAh g-1 with a higher first-cycle Coulombic efficiency of 90.6%at 0.1 C.At 10 C,FeNb11O27.9 still retains a high capacity of 145 mAh g-1 with low capacity loss of 6.9%after 200 cycles,in contrast to the values of 99 mAh g-1 and 11.1%obtained for FeNb11O29.(3)M-Nb-O compounds have been considered as promising anode materials for lithium-ion batteries due to their high capacities,safety and cyclic stability.However,very limited M-Nb-0 anode materials have been developed.Herein,GaNb11O29 with a shear ReO3 crystal structure and a high theoretical capacity of 379 mAh g-1 is explored as a new member in the M-Nb-O family.GaNb11O29 nanowebs(GaNb11O29-N)are synthesized based on a facile single-spinneret electrospinning technique,which are constructed by interconnected GaNb11O29 nanowires with an average diameter of?250 nm and a large specific surface area of 10.26 m2 g-1.This intriguing architecture affords good structural stability,restricted self-aggregation,a large electrochemical-reaction area and fast electron/Li+-ion transport,leading to the significant pseudocapacitive behavior and outstanding electrochemical performance of GaNb11O29-N.At 0.1 C,it shows a high specific capacity of 264 mAh g-1 with a safe working potential of 1.69 V(vs.Li/Li+)and an ultra-high first-cycle Coulombic efficiency of 96.5%.At 10 C,it exhibits a superior rate capability with a high capacity of 175 mAh g-1 and a durable cyclic stability with large capacity retention of 87.4%after 1000 cycles.These impressive results indicate that GaNb11O29-N is a high-performance anode material for lithium-ion batteries.