Synthesis And Characterizations Of Li₃V₂(PO₄)₃ Cathode Materials For Lithium Secondary Batteries

Rechargeable lithium batteries are considered to be the most advanced energy storage system. Most of commercial lithium batteries utilize LiCoO2 as the cathode material, but the high cost and toxicity properties of LiCoO2 prohibit its intensive use in large-scale applications. Recently, Li3V2(PO4)3 has attracted great attention due to its low cost, structure stability, high recharge potentials, and high theoretical specific capacity (197 mAh ? g-1) ,expected to be replaced by the lithium cobalt oxide cathode materials.However, a separation of the MO6 octahedra by the polyanions also reduces the electric conductivity of the materials, which is a negative factor for the material electrochemical properties. In recent years, many efforts have been done to improve the electric conductivity of these phosphates through carbon coating or cation doping.In this work, we prepared a series of Li3V2(PO4)3 compounds using H2 reduction and CTR methods,which are both based on a solid-state reaction. In addition, we prepared the Li3V2(PO4)3 with the Cu in it .The physical and electrochemical properties of the materials were studied by XRD, SEM, TEM,FTIR , electrochemical charge-discharge cycling and so on.We use Li2CO3, (NH4)2HPO4 and NH4VO3 as the starting materials. Then mixed with a certain of sucrose (C12H22O11) powder as the carbon source for CTR reaction. The amount of sucrose was adjusted to 0.2, 0.3 and 0.4 mol for 1 mol of Li3V2(PO4)3. The final products obtained by these sucrose amounts were labeled as LVP-CTR1, LVP-CTR2 and LVP-CTR3, respectively. All of the XRD patterns could be indexed on the single-phase of monoclinic Li3V2(PO4)3 with a space group P21/n.There are no traces of carbon in XRD patterns of the LVP-CTR samples. This may be attributed to either the amorphous nature of the residual carbon in the products, or the amount of residual carbon was too small to be detected. The SEM and the TEM images show that the sample LVP-H2 was consisted of big clusters of agglomerated particles with a size of 1μm or so. By contrast, the sample LVP-CTR3 dosen't show significant particle agglomeration. The material has a small mean particle size about 100 nm. There was small dark particles with a light grey uniformly carbon-coated layer in the the sample LVP-CTR3.The material prepared by H2 reduction process (LVP-H2) showed a low electric conductivity of 9×10-8 S?cm-1, which is in good agreement with previous report. The materials prepared by CTR process exhibited much higher electric conductivities than LVP-H2. The electric conductivity was prominently enhanced by five orders to 10-3 S?cm-1, even for LVP-CTR1, which has only 0.9 % of carbon. The high electric conductivities of LVP-CTR indicates that the residual carbon should be coated on the particle surface uniformly, which can provide good electric contact between the Li3V2(PO4)3 particles. From the impedance spectroscopy of the Li3V2(PO4)3 samples prepared by different processes. It is seen that the Li3V2(PO4)3 sample prepared by H2 reduction process (LVP-H2) exhibited much larger charge transfer resistance than those prepared by CTR process. Moreover, the LVP-CTR sample with more residual carbon showed a smaller charge transfer resistance. The charge transfer resistance provides direct information for Li+ insertion/extraction at the electrolyte-electrode interface. It is well-known that a small charge transfer resistance is a positive factor for the material to obtain good electrochemical performance.Electrochemical charge-discharge experiment was carried out in the potential region of 3.0– 4.8 V. The LVP-CTR sample with more residual carbon, which also had a higher electric conductivity and a lower charge transfer resistance, exhibited larger charge/discharge capacities. The LVP-H2 sample exhibited the smallest capacities in all of the Li3V2(PO4)3 samples. It is interesting to see that the charge capacity of LVP-H2 for the three plateaus before 4.30 V was similar as those of LVP-CTR samples. However, LVP-H2 showed an almost neglectable charge capacity at the forth plateau, which is responsible for its small total capacity. The small capacity of the forth plateau for LVP-H2 indicates that the extraction of Li(3) from LVP-H2 was more difficult than its extraction from LVP-CTR.The discharge capacities vs. cycle numbers of the Li3V2(PO4)3 samples at a current density of 20 mAg-1 shows that LVP-CTR3 sample that containing the most residual carbon displays the best cycling performance and the reversible discharge capacity decreased with the lowering of coated carbon in LVP-CTR. The material prepared by H2 reduction process exhibited the worst electrochemical performance in all of the samples. We measured the vanadium content dissolved in electrolyte after fifty cycles .The figure clearly shows that the vanadium dissolution in LVP-H2 is ten times higher than that in the LVP-CTR samples. It could be concluded that the carbon layer coated on the particle surface prevented the vanadium dissolution from Li3V2(PO4)3.The cycle performances of the two samples measured at different temperature are shown that the LVP-CTR3 sample had a better electrochemical performance both at 25°C and -20°C compared with LVP-H2 sample. When the temperature returned to 25°C from -20°C, LVP-CTR3 sample still remained a good capacity (109.9mAhg-1), which was important for the battery in the practical process. When the temperature fell to -20°C, the interface film resistance and the charge transfer impedance of the two samples both increased greatly. It was the reason why the electrochemical performance was poor at a low temperature. Besides, compared with the LVP-CTR3 sample, the LVP-H2 sample had a bigger charge transfer impedance at low temperature. It indicated that the charge transfer of LVP-H2 sample was more difficult at low temperature, which caused losses of capacity.In addition, we added the metal copper powder in the raw materials and prepared successfully Cu-added Li3V2(PO4)3 cathode material ,in which the account of the metal copper powder was 1% of the product at weight. XRD results showed that the Cu-added Li3V2(PO4)3 did not change the monoclinic structure of Materials, SEM pointed out that the Cu-added Li3V2(PO4)3 has more homogeneous particles, but there is no metal powder particles. There are two reasons contributed to this phenomenon. On the one hand, the added amount of copper compared is so small, on the other hand, the copper powder as nucleating agent has been wrapped up by the Li3V2(PO4)3 particles. Electrochemical testing results showed that the Cu-added Li3V2(PO4)3 samples has a higher charge-discharge capacity (charge capacity: 168.2 mAh.g-1, the discharge capacity of 160.3 mAh.g-1) and better stability of the cycle. Adding the powder material greatly reduces the charge transfer resistance, so the electrochemical properties of the material has been greatly improved.