Synthesis And Electrochemical Performance Study Of Anatase TiO₂ As Anode Material For Sodium Ion Batteries

Renewable energy with obvious intermittency and instability is an effective way to solve the current energy and environmental issues.In order to integrate these renewable energies into the electrical grid,a large-scale energy storage system(ESS)is vital to peak shift operation.Recently,because of the natural abundance,availability,and low cost of Na,Na-ion batteries(SIBs)have been investigated for large-scale energy storage applications as promising alternatives to lithium-ion batteries(LIBs).However,the larger ion radius(1.02?)of sodium than that of lithium(0.76?)affects the phase stability,reaction kinetics,and interphase formation.Thus it is challenging to design a suitable host structure that can accommodate Na ions and allow sufficient Na-ions diffusion.Due to the reasonable cost,stability,nontoxicity,suitable operation voltage,and desirable calculated activation energy for Na⁺insertion,anatase TiO₂ is deemed as a potential anode material for SIBs.However,its intrinsically electronic and ionic conductivities are low,the sodium storage mechanism is still under debate,and there is an urgent need for facile performance improvement method.It will greatly affect the development of high-performance,low cost and environmental-friendly durable TiO₂ anode for SIBs,thus restricting its practical application in SIBs.In order to overcome these obstacles,several studies were carried out to improve its electrochemical performance,clarify the unclear electrochemical phenomena,and further achieve the economic/environmental goals.The detailed research contents are as follows:(i)To solve the issues of nanostructured TiO₂ material used for SIBs anode,spindle?shaped anatase TiO₂ secondary particles were successfully fabricated via the oriented attachment of primary TiO₂ nanocrystals.By adjusting the concentration of tetrabutyl titanate,the size of the TiO₂ nanocrystals and particles could be controlled,resulting in the pore evolution.Pores for the random aggregation of secondary particles gradually transformed to nanopores originating from the oriented attachment of the primary nanocrystals,resulting in an excellent micro/nanostructure that increased the performance of a sodium-ion battery.The mesoporous TiO₂ microparticle anode,with its unique combination of nanocrystals and uniform nanopores,displayed super durability(95 m Ah g^-1 after 11,000 cycles at 1 C),and excellent rate performance(265 and 77 m Ah g^-1 at 0.1and 20 C,respectively).This excellent performance can be attributed to the stability of the material and its high ionic conductivity,resulting from the stable architecture with a mesoporous microstructure and without the random aggregation of secondary particles.A fundamental understanding of the pore structure and controllable pore construction has been proven to be effective in increasing the rate capability and durability of nanostructured electrode materials.(ii)Interestingly,this excellent micro/nanostructure suffered from a slow capacity activation behavior at 1 C or other high rates,which lasted for near 1000 cycles.To understand the slow behavior of anatase TiO₂as SIBs anode during cycling,the Na-ions storage mechanism of the prepared mesoporous TiO₂ microparticle was studied deeply.Based on the comprehension of the mechanism,the behavior was demonstrated to be related with the gradual formation of amorphous phase resulting from the phase transition during discharge.And the level of phase transition was determined by the discharge rates and cycle numbers,which strongly affected the electrochemical performance of anatase TiO₂.Via a quick formation process of amorphous phase in the initial cycles,the capacity activation was accelerated,and high initial capacity of 163 m Ah g^-1 was immediately achieved and there was no decay after 500 cycles at 1 C.Particularly,anatase TiO₂displayed surprisingly unique properties in the fast charge(even at 20 C,6.7 A g^-1)mode,delivering a 179 m Ah g^-1charge capacity and thus meeting the requirements for fast energy storage.This study is significant for the comprehensive understanding of the controversial sodium storage mechanisms and unclear special behaviors occurring in anatase TiO₂,thus greatly contributing to better guidance on the computational studies and experiment technologies for further performance promotion.(iii)Gnerrally,to promote the electrochemical performance,most strategies were focused on improving its intrinsically low electrical conductivity via designing and modifying the active material in the processes of synthesis.However,these strategies were generally complex in synthetic routes,sensitive to experiment conditions,and the prepared electrode suffered from a low initial coulombic efficiency of less than 50%.Here,for the first time,we developed a facile and scalable strategy of utilizing sodium alginate(SA)as the aqueous binder to fabricate high-performance,low cost and environmental-friendly durable TiO₂ anode for SIBs.Compared to the conventional polyvinylidene difluoride(PVDF)binder,electrodes using SA as the binder exhibited significant promotion of electrochemical performances.The initial coulombic efficiency was as high as 62%at 0.1 C.A remarkable capacity of 180 m Ah g^-1 was achieved with no decay after 500 cycles at 1 C.Even at 10 C(3.4 A g^-1),it remained 82 m Ah g^-1 after 3600 cycles with approximate 100%coulombic efficiency.The apparent diffusion coefficient of TiO₂ electrode with SA binder increased by two orders of magnitude along with the less charge transfer resistance and the decreasing resistance from the SEI layer.When the coin cells were assembled with SA as binder,less electrolyte decomposition and side reaction,high electrochemistry reaction activity,effective suppression of polarization,and good electrode morphology can be observed,which were ascribed to SA binder with rich carboxylic groups,high Young's modulus,and good electrochemical stability.