| Rechargeable lithium ion battery(LIB)has high energy density and long service life,so it is considered to be a promising energy storage and conversion device,which can be used in electric vehicle(EV)and smart grid.However,due to the limitations of energy density and safety characteristics,graphite anode materials are difficult to meet the requirements of the latest LIB large-capacity and large-scale applications.Spinel lithium titanate(Li4Ti5O12,LTO)is considered to be the most promising substitute for graphite anode because of its excellent safety properties and long service life.Different from the graphite anode electrode which will expand for 10 vol%during charging,the volume change of spinel LTO during charging and discharging can be ignored,which ensures high cycle stability and long service life.In addition,LTO has a flat charge-discharge platform(1.55 V vs.Li/Li+),which greatly enhances its safety.However,the electronic conductivity of LTO is relatively low,which leads to large polarization loss and poor rate performance.In addition,the theoretical capacity of LTO is only 175 mAh·g-1,which is relatively low.In this paper,we focus on the modification of LTO from two aspects:improving the electronic conductivity and increasing the capacity of LTO.In this paper,lithium titanate was carbon modified by cotton fiber as carbon source.Taking advantage of the hydrophilicity of hydroxyl,the metal ions can be adsorbed on the surface of the cotton fiber.After calcination at high temperature,cotton fiber was carbonized and distributed on the surface of lithium titanate anode materials and between particles.When carbonized at high temperature,the surface of the fiber can be used as a nucleation point,which helps to reduce the particle size of LTO,and carbonized into a carbon layer,forming a conductive network on the surface of lithium titanate and between the particles.Based on the improvement of surface conductivity by carbon modification,the composite modification was carried out by doping Mg2+,and the bulk phase conductivity and surface conductivity were improved at the same time.LMg0.2TO/C composite was prepared,and the electrochemical properties of the composite were analyzed.It is found that the performance of LTO modified by doping and carbon has been further improved.At 0.57C rate,the discharge specific capacity can be maintained at 168.6 mAh·g-1.At the rate of 10C,the specific capacity can reach 107.9 mAh·g-1,which is 124.8%higher than that of LTO.On the basis of Mg2+doping,Al3+was introduced for co-doping.The addition of Al3+can significantly reduce the size of LTO particles,in which the sample Mg0.2Al0.05 particles are the smallest and uniform,the particle size is about 200 nm.In the low rate cycle test,the initial discharge specific capacity of Mg2+ doped lithium titanate anode material decreases slightly with the addition of Al3+,but the appropriate amount of Al3+doping shows the advantage in the process of high rate charge and discharge.The discharge specific capacity of sample Mg0.2Al0.05 is 96.4 mAh·g-1 after 100 cycles at 10C rate.The most convenient and effective way to increase the specific capacity of LTO anode materials was by adding a high capacity phase.In order to improve the specific capacity of LTO anode materials,recombination by adding a second phase with high specific capacity is the most intuitive method.The specific capacity of lithium titanate anode materials was improved by adding a high capacity phase,and the advantages of the two anode materials were used to make up for each other's shortcomings by blending and coating.By means of blending,the morphology and electrochemical properties of the electrode sheets before and after charge and discharge of composites with different ratios of LTO and SiOx were compared.It was found that due to the synergistic effect between LTO and SiOx,The mass specific capacity and rate properties of SiOx-LTO/C blends were improved,and the composites had the best properties when LTO:SiOx was 7:3.LTO is used as a stable matrix and SiOx as a bridge between spinel particles to reduce the contact resistance between LTO particles.The change of SiOx volume is hindered by LTO phase.In order to further improve the coulombic efficiency of LTO and SiOx composites,a kind of SiOx/LTO/C composites with structurally stable LTO as outer layer and high specific capacity phase SiOx as core was proposed and prepared.The outer layer of LTO can effectively alleviate the volume expansion of internal SiOx.The initial Coulomb efficiency of the prepared SiOx/LTO/C composites can reach 96.7%under the current density of 0.1 A·g-1,and the discharge specific capacity can be maintained at 245.8 mAh·g-1 after 100 cycles.This shows that the respective properties of LTO and SiOx can be better brought into play through the design of the structure,and the superposition effect can further improve the properties of the materials. |
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