Preparation And Electrochemical Behavior Of Spinel Lithium Titanate/Carbon Nanotube Composites

As a kind of secondary battery,lithium ion battery(LIBs)has attracted much attention because of its high energy density,excellent cycling performance,good thermal stability and service life.The traditional industrial graphite anode Li⁺has poor rate performance of embedding/desorption,and it is easy to form lithium dendrites on the surface of graphite anode under high power conditions.Spinel type lithium titanate(LTO),as a"zero strain material",has excellent structural stability during Li⁺insertion/removal process.There is no problem of lithium dendrite and SEI film formation.It has a long cycle life and good safety,and stands out for its remarkable characteristics.However,the low electron/ion conductivity,poor rate performance and low theoretical capacity(175 m Ah·g^-1)of LTO greatly limit its application in lithium-ion batteries.In view of the shortcomings of LTO,three modification strategies are proposed,including the construction of nanostructures,carbon nanotubes(CNTs)modification and introduction of titanium dioxide(TiO₂)to synthesize dual-phase nanocomposite structures.The specific research contents are as follows.LTO curds with a core-shell structure were synthesized in situ on oxidized carbon nanotubes,and the curds were composed of ultrathin nanosheets,which greatly shortened the transport path of lithium ions.The surface of the oxidized carbon nanotubes is rich in oxygen-containing functional groups,which can provide more nucleation sites for LTO and suppress the size of LTO curds during the reaction.The oxidized carbon nanotubes also formed a conductive network between the LTO curds,improving the electrical conductivity and cycling stability.LTO/CNTs curds with different core-shell structures were prepared by controlling the size of the curds by controlling the functional group density of the oxidized carbon nanotubes.The synthesized LTO-1h CNTs showed a high reversible capacity of 167.3 m Ah·g^-1 at 1 C,which still reached 139.3 m Ah·g^-1 at 30 C.At different rates of 0.5-30 C,they showed excellent cyclic stability.The volume retention rate of LTO-1h CNTs was 80.5%,which was significantly higher than that of LTO(55.9%),compared with the specific volume at the low rate of 0.5 C,showing excellent rate performance and cycle stability.In this paper,flower spheres of LTO-CNTs with different core-shell structures were prepared by controlling the functional group density of oxidized carbon nanotubes for the first time to control the size of flower spheres.It provides a new idea for preparing other metal oxide flower spheres with core-shell structure.LTO-TiO₂/CNTs two-dimensional nanosheets were synthesized by glucose assisted hydrothermal one-step for the first time.The LTO-TiO₂ two-phase nanocomposite structure increased grain boundary density and provided more electrons and ion channels,thus improving the storage performance of lithium.LTO-TiO₂ nanosheets were transformed into highly conductive matrix under the synergistic effect of oxidized carbon nanotubes and carbon coating,which improved the diffusion coefficient of lithium ion and the electronic conductivity of the material.As an additive,glucose not only acts as a carbon source and auxiliary TiO₂ phase formation,but also acts as a connecting agent between carbon nanotubes and LTO nanosheets to reduce the size of nanosheets.LTO-TiO₂/CNTs showed a high reversible capacity of 174.4 m Ah·g^-1 at 1 C and 151.3 m Ah·g^-1 at 30 C.The capacity retention rate of LTO-TiO₂/CNTs was 83.4%compared with the specific capacity at a low rate of 0.5 C,and the capacity attenuation was small at different rates of 0.5-30 C,showing excellent capacity performance.We also synthesized LTO-TiO₂/CNTs nanosheets with different proportions by adjusting glucose addition without changing Li/Ti feeding ratio,and explored the mechanism of glucose action on TiO₂ phase in LTO-TiO₂/CNTs.It provides a research basis for exploring the effect of additives on the phase composition of LTO-TiO₂.