Studies On Vanadium-based Anode Materials For Lithium-ion Battery

With the development of automobile industry, the problems of unrenewable energy depletion, such as oil depletion, are deteriorating. Meanwhile, the air pollution and greenhouse effect caused by the use of unrenewable energy also become the worldwide challenge. Due to the requirements of energy conservation and environmental protection, electric vehicles (EVs) get appealing, and lithium ion battery (LIB) is the key part of the development of EVs. To meet the demands for high power and energy density, it is extremely important to develop the anodes with superior performance to graphite in these years. Compared with graphite, the layered Li1.1V0.9O2 attracts more and more attention due to its low intercalation voltage (0.1 V vs Li/Li+), high volume energy density (1360 mAh/cm3, vs 790 mAh/cm3 of graphite) and good safety. However, commercial application of Li1.1V0.9O2 is hindered by several drawbacks, such as large irreversible capacity, poor cycling performance and rate capability derived from the large volume change during charge/discharge process. How to improve the cycling stability and rate performance of vanadium-based material is the key point to develop high-performance anode.Based on the above research background, we are devoted to improve the cycle stability and rate performance of vanadium-based anode materials in this dissertation. The layered Li1.1V0.9O2 was first prepared under different calcination atmosphere and then the effects of calcination atmosphere on the structure and electrochemical performance were investigated. Meanwhile, the modification of Li1.1V0.9O2 by doping was carried out, which Na and Cr doped Li1.1V0.9O2 materials have been successfully prepared. The effects of dopants on the structure and electrochemical performance were systematically investigated. The cycling performance has been enhanced by improving the structural stability. With the aim to promote the application of Li1.1V0.9O2, a Li1.1V0.9O2/graphite composite has been prepared by an in-situ method in this dissertation, and it exhibits significantly enhanced volume energy density, cycling stability and rate performance. Moreover, this dissertation explores a novel vanadium-based anode material, which displays high specific reversible capacity, improved rate capability and good cyclic stability。 In addition, the electrochemical mechanism of this anode material has been intensively investigated. Finally, the influence of nano-materials on the safety of LIB is also included in this dissertation. The effects of particle size on the safety of LIB was investigated through thermal stability tests, and we predict what kind of electrode materials can be designed as nanostructure for high-performance and safe LIBs. A systematic investigation was performed on the vanadium-based anode materials including the preparation, electrochemical characterization and mechanism research. It is significant for preparation of vanadium-based anode materials with high performance. The specific contents of this dissertation are given as follow:In Chapter 3, layered Li1.1V0.9O2 materials have been prepared by solid state method under different calcination atmosphere. The influence of calcination atmosphere on the structure and electrochemical performance was investigated. From the results of XRD and Rietveld analysis, the impurity of LiVO2 is detected in the samples prepared under Ar and N2, while Li1.1V0.9O2 prepared under H2/Ar atmosphere is free from impurity with high crystallization. It is also found that the growth of (101) and (104) lattice plane are restrained under Ar and N2, leading to the decrease of layer space and particle size, which hinders the insertion/extraction of lithium ion and decreases the specific capacity and cycle stability. The initial discharge capacity is 345,351 and 318 mAh/g for the samples prepared under Ar, N2 and H2/Ar atmosphere, with corresponding coulomb efficiency of 65%,55% and 50%, respectively. Moreover, the specific capacity declines more quickly for the samples prepared under Ar and N2 atmosphere. From In-situ XRD results of layered Li1.1V0.9O2 prepared under H2/Ar atmosphere, it is found that there is a new phase (Li2VO2), co-existing with the as-prepared phase during the electrochemical process.In Chapter 4, the modification of Li1.1V0.9O2 by doping was performed. The solid solution Li1.1(V, Cr)0.9O2 with Cr substituting for V has been successfully prepared. From the results of XRD and Rietveld analysis, it is found that the lattice constants of the a-axis increase and those of the c-axis decrease with the increase in chromium content in Li1.1Vo.9-xCrxO2, leading to a decrease in c/a ratio, which may be propitious to alleviate the distorted coordination environment during Li insertion/extraction process and improve the structural stability of Li1.1(V, Cr)o.902. According to the results of galvanostatic charge/discharge experiments, it is found that the capacity decreases with the increase of Cr substitution amount for V and the formation of more inactive LiCrO2. The initial discharge capacities of various samples are 345,331,280 and 198 mAh/g for samples with 0%,5%,10% and 15% Cr substituting for V, with coulombic efficiencies of 65%,68%,67% and 43%, respectively. However, the cyclic stability can be effectively improved by a suitable amount of Cr substitution for V. After 20 cycles, the reversible capacity remains 201 mAh/g for sample with 10% Cr substituting for V, whereas only 120 mAh/g for sample without doping. The effect of Cr doping on improving the cyclic stability was investigated by ex-situ XRD and in-situ XANES. It is found that the structure of Cr is stable during charge/discharge cycles and the structural stability can be improved by the inactive LiCrO2.In addition, the volume change and particle fracture are effectively alleviated, illustrated by the morphologies of the fully lithiated samples. What is more, the effects of Na doping on the structure and electrochemical performance were also investigated. As depicted in XRD and Rietveld analysis results, it is found that the solid solutions (Li, Na)1.1V0.9O2 by Na substitution for Li and V have been successfully prepared without impurity and the layer space increases obviously. However, the insertion/extraction of Li ion is hindered due to Na doping, resulting in the decrease of reversible capacity and the deterioration of cyclic stability. The initial discharge and charge capacity is 340 and 151 mAh/g for (Li, Na)1.1V0.9O2 with 3% Na doping, and a charge capacity of only 51 mAh/g remains after 20 cycles.In Chapter 5, Graphite-anchored lithium vanadium oxide has been successfully synthesized via a "one-pot" in-situ method. From the results of XRD, it is found that the Li1.1V0.9O2/graphite composite is free from impurity at the temperature of 850 ℃, while the by-product V8C7 forms at 1100 ℃. The graphite-anchored lithium vanadium oxide shows better electrochemical performance than the composite prepared by mixing graphite and lithium vanadium oxide by reducing the agglomeration of Li1.1V0.9O2 and increasing the contact of Li1.1V0.9O2 with graphite. The charge capacity of Li1.1V0.9O2/graphite composite is 232 mAh/g after 50 cycles at 0.1 C, with a corresponding capacity retention of 96%, whereas the charge capacity is 208 mAh/g for composite prepared by mixing graphite and lithium vanadium oxide, with a corresponding capacity retention rate of 83%. Meanwhile, the charge capacity for Li1.1V0.9O2/graphite composite is 219 mAh/g at 0.2 C and 191 mAh/g at 0.5 C, while 194 and 136 mAh/g for composite prepared by mixing graphite and Li1.1V0.9O2, respectively, displaying better rate capability for Li1.1V0.9O2/graphite composite. From the results of the electrochemical impedance spectra of the prepared samples at open circuit potentials and the corresponding equivalent circuit, it is found that the resistance (Rct) (obtained by fitting the equivalent circuit), which determines the charge transfer process of the lithium ion insertion/extraction reaction, is 20.7 and 23 Ω for Li1.1V0.9O2/graphite composite and composite prepared by mixing graphite and Li1.1V0.9O2, respectively. From the above results and discussion, it can be found that the composite prepared via a "one-pot" in-situ method has lower electrochemical resistance, which accounts for better reversible capacity and cyclic performance as anode of lithium ion battery. Considering both volumetric capacity and cyclic stability, a mass ratio of graphite to Li1.1V0.9O2 of 1:1 is selected through optimizing the graphite content ratio to Li1.1V0.9O2. Moreover, after carbon coating treatment, Li1.1V0.9O2/graphite composite shows a reversible capacity of 282 mAh/g and negligible capacity decay after 50 cycles.In Chapter 6, the Na3VO4 has been prepared by a solid state method. As shown in XRD patterns, the as-prepared Na3VO4 is isostructural with the y-form Na3AsO4 and contains four formula units per unit cell. From the SEM and HR-TEM images of the as-prepared Na3VO4, micro-sized particles with aggregation of numerous nanocrystallines which is surrounded by amorphous phase are observed for the as-prepared Na3VO4. Through galvanostatic charge/discharge measurements, the discharge capacity is 1501 mAh/g for Na3VO4 at the current density of 40 mA/g, with a reversible efficiency of 43%. The low reversible efficiency of Na3VO4 mainly results from the generation of irreversible phase of Na2O. In the following charge/discharge measurements, the discharge capacity can reach up to 594,545 and 533 mAh/g, with the reversible efficiency of 92%,97% and 98%, respectively. Its initial specific capacity is 430 mAh/g and remains 427 mAh/g with a capacity retention of 99% after 100 cycles at the current density of 80 mA/g, showing superior cyclic stability. In addition, the specific capacity is 370,204,93 and 21 mAh/g at the current density of 120,400,1000 and 5000 mA/g, respectively, representing good rate capability. The collected XRD results of the electrodes on OCV and after Li insertion/extraction show the broadened and weakened diffraction peaks for Na3VO4. Moreover, from the results collected from In-situ XANES of V K-edge spectra, it is found that the V5+ in Na3VO4 is reduced to V3+ after Li insertion and V3+ returns completely to V5+ in extraction process. Therefore, it can be concluded that NaLi2VO3 is formed after Li insertion, and NaVO3 is generated after Li extraction. The as-prepared Na3VO4 displays small specific surface area comfirmed by the BET analysis, which means negligible surface charge storage. Therefore, it can be concluded that the large charge storage partly stems from intercalation pseudo capacitance, delivering the fast charge storage.Safety problem is an old but still a hot topic in the field of LIBs, especially in the large-scale batteries for electric vehicle and stationary power source applications. Recently, the nano-scale materials indeed improve the power density and cycling stability of LIBs. However, the nano-scale materials usually have large surface area and high reaction reactivity, which may have some negative effects on the thermal stability of LIBs. In Chapter 7, we have compared the thermal stability among several electrode materials with different particle size (micro and nano-scale) through the DSC tests. By comparison, the exothermic peak at around 100 ℃ for thermal degradation of the SEI layer is not observed for lithium titanate (LTO) due to the high Li-ion intercalation potential (1.5 V versus Li/Li+) over the conventional SEI formation potential (lower than 1.0 V). Furthermore, only a slight difference in the exothermic peak and total thermal runaway values are found between lithiated micro-LTO and nano-LTO due to the strong covalent bonding characteristic between Ti and O. For lithium iron phosphate (LFP), no heat signal before 300 ° C appears due to the strong covalent P-O bonds in the tetrahedral (PO4)3- anion, which effectively inhibits oxygen release from delithiated micro-LFP and nano-LFP. However, as a result of oxygen released from Lii_xCoO2, the lower exothermic peak and larger thermal runaway for nano-LCO is caused by the promotion of oxygen released from the delithiated nano-LCO with larger high reactive surface area. From the above results and discussion, it is concluded that the thermal stability (the exothermic peak temperature and heat flow) is critically associated with its oxidation/reduction activity. That is, the higher the oxidation or reduction activities of the electrode materials are, the lower their thermal stability, and that the difference in thermal stability between the nano- and micro-particles will also be larger according to the material’s oxidation/reduction activity. From the view of safety, the materials with much higher or lower intercalation potential than 3.3 V vs Li/Li+ is not recommended to design into nano-scale, only these with low oxidation or reduction activity with a thermodynamic potential close to 3.3 V vs Li/Li+ is allowed. The surface modification may provide a useful approach to improve the thermal stability.