Study Of TiO₂ Anode Materials With Fast Charging And Discharging Characteristics For Lithium Ion Batteries

Lithium ion batteries are widely studied to fullfill the requiement of the developing electronics and electric vehicles,and the elelctrode materials with high specific energy and power is highly demanded.Due to the high risk of the commercialized graphite when being charged and discharged under high rates,the development of alternative anode materials with high power capabilities and safety is requied.TiO₂ which owns high specific capacity and safy is a potential anode material.However,its application in fast charging/charging lithium ion batteries is restricted by its essential low ionic and electronic conduction.In this work,two strategies are tried to enhance the rate performance of TiO₂: 1.To enhance the integrate ionic and electronic conductivity through synthesizing TiO₂/nanocarbon composite with the rational design of the composite and electrode structure,2.To enhance the ionic and electronic conductivity of TiO₂ by doping and controlling the phase structure.Ti glycolate/MWNTs hybrid precusor was synthesized through the polyol method,and mesoporous TiO₂/MWNTs is obtained after the hydrolysis of the hybrid precusor in pure water.In comparison with the pistine MWNTs,the carboxylated MWNTs is favorable for continuous deposition of Ti glycoate,as the carboxylated groups could support the nucleate site through hydrogen bonding,and continuous TiO₂ coating layer is maintained after hydrolysis.The influence of coating morphology on the electrochemical performance is studied,mesoporous TiO₂/MWNTs with uniform coating morphology is confirmed to deliver better rate performance due to the direct electron transformation between TiO₂ and MWNTs.The method based on the hydrolysis of hybrid precusor is general to prepare other mesoporous metal oxide/nanocarbon hybrid.Colloid particles are formed by dissolving Ti glycolate in dilute sulfuric solution under the protron attack.By the hydrolysis of the colloid particles,mesoporous TiO₂ is obtained through the oxolation between the intermediate Ti-OH?Ti-O-Ti?.SWNTs with-OH groups is used for hybrid,as TiO₂ could be deposited onto SWNTs surface driven by the oxolation between C-OH and Ti-OH,such interfacial bonding is favorable for electronic transformation.By the method of synthesizing mesoporous TiO₂,mesoporous TiO₂/SWNTs with discrete coating morphology is obtained.Clearly,the synthsic process is simpled,and TiO₂ productivity is enhanced,it is also general to prepare other mesoporous metal oxide/nanocarbon hybrid.After vacuum filtration,mesoporous TiO₂/SWNTs free-standing electrode with an under-layer of MWNTs current collector is constructed with the assistance of winding between the uncovered SWNTs.Through such unique design of composite structure,interfacial interaction and electrode structure,the rate performance of mesoporous TiO₂ is highly improved.In order to get a free-standing electrode with much higher strength,flexibility as well as the excellent fast charge/discharge capabilities,low-defect graphene synthesized by the interaction method is used for constructing mesoporous TiO₂/graphene free-standing electrode which has a graphene layer at the bottom serving as the current collector.Low-defect graphene has excellent electronic conductivity and mechanical strength,by using the low-defect graphene instead of the graphene oxide,the series problems caused by the reduction process will be avoided.In order to provide nucleation sites for TiO₂ without destroying the high conductive structure of graphene,the-OH groups are introduced onto graphene through anchoring sodium lignin sulfonate via the ?-? interaction.The flexible free-standing electrode delivers excellent rate performance when being tested in coin cells,while being packed in flexible cells,it also gives good structural stability and rate capability undered bent deformation.Amorphous TiO₂ is favorable for getting higher fast charge/discharge capabilities than anatase TiO₂,due to its higher Li⁺ diffusion cofficient.However,the lithiation induce phase transition of amorphous TiO₂ to rocksalt Li TiO₂ that is confirmed to slow the Li⁺ diffusion by the means of electrochemical test.By using the EXAFS technique,the mechanism for structural evolution and decresed rate capabilities are confirmed from the vision of atomic level.There are too mcuh long Ti-O bonds existed in amorphous TiO₂,such Ti-O bonds are easily broken under Li⁺/Ti4+ repulsive force due to its low bond energy.Upon the diffusion of Ti and O atoms,rocksalt coordination structure form with the Li⁺ insertion and disappear with the Li⁺ deinsertion,with the onging insertion of Li⁺,the irreversible long-range ordered rocksalt will be formed and it serves as a nail to immobilize the residual amorphous region.The interaction of Ti O6 octahedral in amorphous structure are corner shared and edge shared to a small extent,and Li⁺ diffuse within the wide tetrahedral interstices.The Ti O6 and Li O6 octahedral in rocksalt structure are edge shared to a big extent,the Li⁺ transfer between neighbored octahedrals through the tetraheral interstices,the diffusion space for Li⁺ is relatively small,and replusion force from neghbored Ti4+ is bigger,that all slow the diffusion of Li⁺.Though there are rocksalt coordination structure formed after Li⁺ inserted into amorphous TiO₂,its influence on Li⁺ diffusion is much smaller than long range ordered rocksalt TiO₂,as a result,the Li⁺ diffusion in amorphous TiO₂ is much easier.The rate performance of amorphous TiO₂ could be enhanced by stabiliztion of Ti-O bond upon Zr doping.Taking advantage of the coordiation structure evolution induced by lithiation in TiO₂,Ti³⁺ self-doping could be formed in amorphous TiO₂ under fast Li⁺ insertion and deinsertion,meanwhile,the formation of long rang ordered rocksalt structure could also be inhibited,therefore,the fast charge/discharge capabilities of amorphous TiO₂ is greatly improved by this method.