Study On Modification Of Li₃V₂(PO₄)₃ Cathode Materials For Li-Ion Batteries

Cathode material is an important compositive part of Li-ion battery, however, its lower energy-density and power-density compared with anode material restrict the development of Li-ion battery. So research and exploration of cathode material with excellent performance for Li-ion battery has been a key to the development of Li-ion battery. Monoclinic Li₃V₂(PO₄)₃ has received great attention because of its stable framework, relative high operative voltage, higher lithium ion transport ability, higher theoretic capcity and high safety. However, the main drawback of pristine Li₃V₂(PO₄)₃ is its intrinsic low electronic conductivity and Li+ diffusion coefficient, which results in poor electrochemical performance, especially poor rate performance. We have carried through the research of modification on Li₃V₂(PO₄)₃/C by metal oxide coating and cation doping to improve its comprehensive electroncchemical performances.In this paper, the cathode material Li₃V₂(PO₄)₃/C was prepared by sol-gel method, and the researches of modification by metal oxide coating, such as MgO, Al₂O₃, ZrO₂ and cation doping, such as Mn²⁺, Y³⁺, Nd³⁺ in vandium site are carried out. Microstructure, electrochemical performance and kinetic characteristics of the cathode materials have been investigated systematically by means of XRD, FESEM, TEM-EDS analyses and electrochemical test method, such as galvanostatic charge-discharge, CV and EIS.The results show that the coating with a small amount of metal oxide does not affect the structure of Li₃V₂(PO₄)₃/C. The researches of HRTEM-EDS show that the metal oxides are coated on the surface of Li₃V₂(PO₄)₃/C. The researches of FESEM and particle analyses show that the particles of the cathode materials Li₃V₂(PO₄)₃/C coated by metal oxide become smaller and more homogeneous. Through galvanostatic charge-discharge test, we can see that the cycling stability and rate performance of the cathode material Li₃V₂(PO₄)₃/C coated by metal oxide are significantly improved. For example, at discharge current density of 40 mA/g, the discharge capacity of 4.5 mol% MgO, 4.5 mol% Al2O3 and 3.3 mol% ZrO₂ coated Li₃V₂(PO₄)₃/C decline from initial194.4 mAh/g, 165.1 mAh/g and 165.8 mAh/g to 137.5 mAh/g, 124.0 mAh/g and 110.5 mAh/g after 100 cycles, respectively, and the capacity retention are 70.7%, 75.1% and 66.6%, respectively. However, the discharge capacity of uncoated Li₃V₂(PO₄)₃/C declines from initial 164.2 mAh/g to 72.7 mAh/g, the capacity retention is only 44.3%. At higher discharge current density, the cathode materials Li₃V₂(PO₄)₃/C coated by metal oxide maintain still high initial discharge capacity and excellent cycling performance. For example, at discharge current density of 197 mA/g, the discharge capacity of 4.5 mol% MgO and 4.5 mol% Al₂O₃ coated Li₃V₂(PO₄)₃/C decline from initial 157.8 mAh/g and 150.3 mAh/g to 138.0 mAh/g and 105.0 mAh/g after 100 cycles, respectively. However, the discharge capacity of uncoated Li₃V₂(PO₄)₃/C declines from 153.4 mAh/g to 64.4 mAh/g. At 118.2 mA/g discharge current density, the discharge capacity of 3.3 mol% ZrO₂ coated Li₃V₂(PO₄)₃/C is 138.3 mA/g after 30 cycles, it does not almost decline compared with the initial discharge capacity 140.8 mAh/g. Especially for 4.5 mol% MgO coated Li₃V₂(PO₄)₃/C, the initial discharge capacity still reaches 144.6 mAh/g when the current density is increased up to 394 mA/g. In adition, the maxmium discharge capacity of 4.5mol% MgO coated Li₃V₂(PO₄)₃/C is enhanced significantly and is 194.4 mAh/g at 40 mA/g current density, which is close to its theoretical capacity of 197 mAh/g. The EIS indicates that the modification of metal oxide decreases the charge transfer resistance and increases the exchange current density, which enhances the electrocatalytic activity, and this is favorable for the insertion/extraction of Li⁺. The investigation of CV implies that the modification of metal oxide enhances the reversibility of electrode reaction.The modification research of cation doping shows that doping with a small amount of Mn²⁺, Y³⁺, Nd³⁺ does not affect the structure of Li₃V₂(PO₄)₃/C. The XPS analysis indicates that valences state of V, Mn and Y are +3, +2 and +3 in Li₃V_1.94Mn_0.09(PO₄)₃/C and Li₃V_1.94Y_0.06(PO₄)₃/C, respectively. The citric acid in raw materials was decomposed into carbon during calcination, and residual carbon exists in Li₃V_1.94Mn_0.09(PO4), and enhances the electronic conductivity of Li₃V₂(PO₄)₃. The study of charge/discharge curves indicates that there is a tendency from multiphase reaction to single-phase reaction in the charge-discharge process for the sample of doping with Mn²⁺. The reaction mechanisms are not changed for the samples of doping with Y³⁺and Nd³⁺. Galvanostatic charge-discharge test indicates that the cycling stability and rate performance of Li₃V₂(PO₄)₃/C doping cation are improved. Such as for cycling stability, at the discharge current density of 40 mA/g, the discharge capacity of Li₃V_1.94Mn_0.09(PO₄)₃/C and Li₃V_1.94Y_0.06(PO₄)₃/C decline from 158.8 mAh/g and 160.3 mAh/g to 120.5 mAh/g and 100.6 mAh/g after 100 cycles, resectively. However, the discharge capacity of the samples undoped Mn²⁺ and Y³⁺ decline from 164.2 mAh/g and 152.5 mAh/g to 72.6 mAh/g and 66.2 mAh/g, respectively. The discharge capacity of Li₃V1.98Nd_0.02(PO₄)₃/C declines from 164.1mAh/g to 145.3 mAh/g after 50 cycles at the discharge current density of 40 mA/g. However, the discharge capacity of the sample undoped Nd³⁺declines from 152.5 mAh/g to109.3 mAh/g. For rate performance, at the discharge current density of 197 mA/g, the discharge capacity of Li₃V_1.94Mn_0.09(PO₄)₃/C declines from 146.4 mAh/g to 107.5 mAh/g after 100 cycles , however, the discharge capacity of the sample undoped Mn²⁺declines from 153.4 mAh/g to 64.4 mAh/g. At the discharge current density of 98.5 mA/g, the discharge capacity of Li₃V_1.94Y_0.06(PO₄)₃/C declines from 164.6 mAh/g to 89.6 mAh/g after 100 cycles, the discharge capacity of the sample undoped Y³⁺ declines from 139.6 mAh/g to 53.0mAh/g, and the discharge capacity of Li₃V_1.94Nd_0.02(PO₄)₃/C declines from 142.1 mAh/g to 124.3 mAh/g after 30 cycles, the discharge capacity of the sample undoped Nd³⁺ declines from 141.4 mAh/g to 71.7 mAh/g. The EIS measurement indicates that the doping with appropriate amount of Mn²⁺ and Y³⁺ decreases the charge transfer resistance and increases the exchange current density, which enhances the electrocatalyzing activity, and this is favorable for the insertion/extraction of Li+. The investigation of CV implies that the doping with appropriate amount of Y³⁺and Nd³⁺ reduces the polaraization of electrode and enhances the reversibility of electrode reaction.