Preparation And Electrochemical Performance Of Cobalt-doped Lithium Titanate As Anode Material For Lithium-ion Batteries

China is a country with large energy consumption,but with the gradual depletion of traditional non-renewable fossil fuels such as oil and coal,it is facing the dual pressure of rapid and sustained growth in energy demand and the resulting environmental problems.The development of environment-friendly new energy is a strategic choice for China's energy development,and it is of great significance to solving the two major problems of energy shortage and environmental pollution.Renewable energy in nature,such as hydropower,wind energy and solar energy.Without exception,it is constrained by the natural environment and it is difficult to apply it on a large scale.The chemical power source can conveniently and effectively realize the storage and conversion of electrical energy and chemical energy.The energy generated by many new energy sources is stored as chemical energy,and power resources are provided when needed.Lithium-ion batteries have become a new generation of ideal energy conversion and storage devices due to their long life,high power density,and environmental protection.As a lithium-ion battery electrode material,lithium titanate has excellent"zero strain" characteristics,which makes it have a long cycle life.At the same time,a high lithium insertion voltage?1.55 V vs.Li/Li⁺?avoids the formation of lithium dendrite and SEI films,ensuring the safety of the battery.However,the lower theoretical specific capacity(175 mAhg^-1)and the higher lithium intercalation voltage together limit the energy density of the battery.In addition,poor electronic conductivity and ionic conductivity limit the rate performance of lithium titanate,can not meet the needs of rapid charge and discharge,limiting its application in high power density batteries.This article starts with the improvement of the lithium titanate rate performance.Firstly,the effects of preparation methods on the micro-morphology and electrochemical performance of lithium titanate were compared.Pure-phase lithium titanate was synthesized by low-temperature solid-phase sintering method and hydrothermal method respectively.The system was used to study the synthesis of raw materials,reaction temperature and annealing conditions for the microscopic shape.Appearance and electrochemical performance.Explore the best process and method for synthesis of pure phase lithium titanate.The study found that the sample morphology is relatively single,at different temperatures are granular and secondary agglomerate into a spherical shape.The electrochemical performance of the hydrothermal method is better than that of the solid phase sintering method.With LiOH-H₂O and tetrabutyl titanate as raw materials,the morphology of lithium titanate prepared by hydrothermal changes with temperature,and the samples obtained at 120?,150?,and 180? are respectively flaky,granular,and regular.Cube structure,but the size of the gap.The best parameters are:the molar ratio of lithium to titanium is 4.2:5,hydrothermal at 180? for 24 hours,and sintering at 700? for 6 hours.The obtained sample had a first discharge specific capacity of 148.3 mAhg'1 at 0.1 C rate,and the sample had a stable charging and discharging platform.After 10 cycles,the specific discharge capacity was maintained at 132.5 mAhg^-1,and the capacity retention rate was 89.3%.The sample prepared by the solid phase sintering method is more uniform and the agglomeration phenomenon is significantly reduced.In order to further increase the capacity of lithium titanate,CoCl₂·6H₂O was used as the cobalt source,and the samples prepared by hydrothermal method were doped with cobalt.The influence of the amount of impurities on the lithium titanate capacity was studied.Cobalt doped samples have good crystallinity and are distributed in three dimensions.The average thickness of the two layers is about 20.5 nm.The electrochemical performance of lithium titanate after cobalt doping is significantly improved.The initial discharge capacity reached 351.36 mAhg-1 and remained at 210.55 mAhg^-1 after 50 cycles.The discharge specific capacity reaches 172.57 mAhg^-1 at 10 C rate,which is 3 times that of the undoped sample.Electrochemical impedance spectroscopy showed that the internal resistance of cobalt-doped lithium titanate battery decreased from 10.74???to 5.06???.At the same time,the charge transfer resistance of the sample decreased from 72.02???to 39.05 Q,and the lithium ion diffusion rate increased,and the rate performance was improved.Significantly enhanced.Based on the density functional theory,the first-principles calculation of the electronic properties of lithium titanate before and after cobalt doping is performed.It is hoped that the reasons for the improvement of its electrochemical performance can be further revealed theoretically.After Co doping,the lattice parameter and cell volume of lithium titanate increase slightly.Co doping does not change the lattice type of lithium titanate,but the crystal in the doped local region is distorted and its symmetry is decreased.The total energy is negative,indicating that Co-doped lithium titanate has sufficient thermodynamic stability.The difference in total energy and the change in bond lengths in the doped local regions prove that the Li-site doping is prior to the Ti-site doping and the structure is more stable.The impurity level of Li3.5Co0.5Ti5O12 obtained by Li site doping can cut the bandgap of lithium titanate more evenly,which leads to better electron transport than Ti-doping.When doping at the Ti site,the Co atom can provide extra electrons and increase the lithium titanate conductivity.At the same time,Ti site doping can destroy the symmetry of Ti-O tetrahedron and reduce the activation energy of Li+ diffusion.In short,after the doping of Co,the electron transport in the crystal and the diffusion process of Li+ appear obvious directionality,and it is easier to form a directional electron/ion transport channel,thereby improving the electrochemical performance of lithium titanate.The above studies indicate that cobalt doping can significantly improve lithium titanate capacity,and electron/ion conductivity,and facilitate lithium titanate anode materials for commercial applications.