Li 3 VO 4Synthesis And Modification Of Anode Materials For Lithium Ion Batteries

Lithium ion battery,it is also called “rocking battery”,is a kind of energy store device that lithium ions shuttle back and forth between the internal cathode and anode lattices and making the electrical energy and chemical energy transform to each other at the same time.In the last few years,lithium ion battery has received widespread attention and widely used for its good features of small size,high power density,no memory effect,long cycle life,high voltage and low self-discharge.Lithium ion batteries are mainly composed of positive electrode,diaphragm,electrolyte and cathode,and the positive and negative electrode materials are the important factors to determine the performance of lithium ion batteries.In recent years,during the process of exploring new anode materials for lithium ion batteries,lithium-transition metal compounds,such as Li₄Ti₅O₁₂,Li_1+xV_1-xO₂,Li₃VO₄ and other materials,due to the multiple electronic reaction characteristics of Ti and V metal elements that making the compounds can hold up to more than one Li embedded or prolapsed,and keep the structure unchanged,which is considered to be a practical potential can replace graphite anode materials.In this thesis,we focus on the new anode materials for lithium ion batteries,Li₃VO₄.We system study its structure,conductivity,electrochemical properties and morphology through a series of characterizations and testing under different doping situations.Using the high temperature solid phase method,we successfully synthesized carbon coated Li_3-xV_1+xO₄（-0.15≤x≤0.15）series of materials as anode materials for lithium ion batteries.By using XRD and Rietveld refinement,SEM,electrochemical test,electrical conductivity test methods,we carefully studied the co-doping of Li and V for Li_3-xV_1+xO₄ material synthesis,structure and electrochemical properties.XRD and Rietveld refinement results show that all the synthesized samples are single phase,cell volume of carbon coated Li_3-xV_1+xO₄（-0.15≤x≤0.15）materials decreases monotonically with increasing amount of doping x.The electrochemical performance test results show that the Li_2.95V_1.05O₄ and Li_3.10V_0.9O₄ samples showed better cycling stability.At 0.2 C discharge condition after 25 cycles,the charging and discharging capacity under the doping amount of x=-0.10,-0.05,0,0.05,0.10 are about 479 mAh g-1,354 mAh g-1,447 mAh g-1,478 mAh g-1and 446 mAh g-1,respectively,which are 108.07%,94.43%,84.43%,102.44% and 93.48% charge/discharge capacity of the respective samples of the second cycle.The test results of carbon coated Li_3-xV_1+xO₄（x=-0.10,0 and 0.05）samples under different rates showed that Li_2.95V_1.05O₄ exhibited excellent rate performance.When the discharge rate increased to 20 C,The charge/discharge capacity of Li₃VO₄,Li_2.95V_1.05O₄ and Li3.10V0.9O4 samples are about 60 mAh g-1,420 mAh g-1and 260 mAh g-1,respectively.The optimum doping amount for the Li_3-xV_1+xO₄（-0.15≤x≤0.15）series sample is x= 0.05.The experimental results show that low content of doping V in Li site in the Li_3-xV_1+xO₄ structure distribution of particles and improve the electronic conductivity and ionic activity to improve the structural stability and structure in Li_3-xV_1+xO₄ Li and V elements were low content of doping can increase the structural stability of Li_3-xV_1+xO₄,improve the particle distribution,the electronic conductivity and ionic activity,and then improve the electrochemical properties of the material.In addition,a series of Li₃V_1-xP_xO₄/C（x= 0,0.025,0.050,0.10,0.125,0.150）samples were successfully prepared as anode materials for lithium ion batteries using high temperature solid state method.By using XRD and Rietveld refinement,SEM,electrochemical test,conductivity test and other methods,we carefully studied the affections of P doping in V site of Li₃V_1-xP_xO₄/C materials including its synthesis,structure and electrochemical properties.XRD and Rietveld refinement results showed that all the synthesized Li₃V_1-xP_xO₄/C（x= 0,0.025,0.050,0.075,0.10,0.125,0.150）compounds are single phase.Sample analysis results show that there is amorphous carbon in our final product.With the increase of P doping amount,the electronic conductivity of Li₃V_1-xP_xO₄/C compounds increases monotonously.The charge-discharge test of the Li₃V_1-xP_xO₄/C（x= 0,0.025,0.050,0.075,0.10,0.125,0.150）compounds showed that the doping amount of x=0.125 compounds show the highest reversible charge discharge capacity,but has poor cycle stability.With the doping amount increased,the sample of x=0.15 holds the lowest maximum reversible capacity,but the cycle performance is the highest,reaching 113.48% of the second cycle.The rate capability test results of Li₃V_1-xP_xO₄/C compound under different test conditions show that the Li₃V_0.875P_0.125O₄/C compounds exhibited the most excellent rate performance.During the high rate discharge conditions in 30 C,the charge and discharge specific capacity of the samples doping amount of x= 0,0.025,0.050,0.075,0.10,0.125,0.150 were about 244.5 mAh g-1,222.7 mAh g-1,181.2 mAh g-1,207.9 mAh g-1,187.6 mAh g-1,264.8 mAh g-1,252.4 mAh g-1,respectively.Our results show that the inert P incorporating into the active V position leads to the decrease of electrochemical activity of Li₃V_1-xP_xO₄/C,which affects the electrochemical reversible capacity and change its electrochemical platform,and the appropriate content（x=0.125）of the P element doping in V site can increase the structure stability of Li₃V_1-xP_xO₄/C compounds,improve the distribution of particles,the electronic conductivity and ionic activity,and then improve the electrochemical performance of the material.