| Li3V2(PO4)3, as one of important representatives for poly anionic lithium ion batteries cathode materials, has high theoretical specific capacity, high charge-discharge potential and stable structure; thereby, it is currently considered as a kind of cathode material with great promising. In this thesis, Li3V2(PO4)3 is used as a main research object, in order to improve its shortcomings of low electronic conductivity and Li-ion diffusion rate, several modification approaches are adopted to enhance the electrochemical properties of Li3V2(PO4)3. The detailed research contents are as follows:1、A core-shell structured Li3V2(PO4)3@C cathode material was prepared by a freeze-drying assisted sol-gel method. The sol of precursor which obtained after sol-gel process is momentarily frozen by liquid nitrogen; then the frozen sol is dried in the vacuum freeze dryer, during which the microstructure of precursor is well-maintained. At last, after sintered at N2 atmosphere, a core-shell structured Li3V2(PO4)3@C with uniform carbon coating is obtained. The obvious porous structure and a complete carbon coated layer are formed on the surface of Li3V2(PO4)3@C particles, which increase the contact area between electrode and electrolyte and improve the electrical conductivity of Li3V2(PO4)3. Therefore, this material exhibits excellent rate stability:it is demonstrated that a discharge capacity of 116.9 mAh~g-1 at 1C between 3.0 and 4.3V vs. Li/Li+ could be obtained. Even at the discharge rates of IOC and 30C, this material can still deliver a discharge capacity of 108 and 87.5 mAh·g-1, respectively.2、The uniform nitrogen doping carbon coated Li3V2(PO4)3 composite is synthesized via an in-situ preparation process. Citric acid and urea are used as carbon and nitrogen sources, respectively; the uniform nitrogen doping carbon coated Li3V2(PO4)3 is prepared by a freeze-drying assisted sol-gel method. After N doping, the electric conductivity, electrochemical activity and defect degree of surface carbon layer are obviously improved. As a result, the electrochemical properties of Li3V2(PO4)3 further enhanced. This Li3V2(PO4)3/C+N shows perfect rate and cycling stability:when discharge current density is increased from 0.5C to 50C, its capacity of 119.5 mAh·g-1 decreased to 111 mAh·g-1 and the capacity loss is only 7%. Furthermore, the structure of Li3V2(PO4)3/C+N is quite stable:after 500 cycles under high current densities, the crystal structure and the surface carbon layer of Li3V2(PO4)3/C+N are also well-maintained.3、Preparation and electrochemical performances of sulfur-doping carbon coated Li3V2(PO4)3 cathode materials were researched. Citric acid and benzyl disulfide are used as carbon and sulfur sources, respectively; after the facile sol-gel process, the modification approach of sulfur doped carbon coating is initially applied on Li3V2(PO4)3 cathode materials. After the formation of C-S bond in carbon coated layer, especially the thiophene type C-S bond, the electroneutrality of carbon matrix are broken and the number of charge carrier increased. Therefore, the electric conductivity of carbon layer improved. In addtition, S-doping also increased the defect degree and the active site of carbon layer. As a result, the transfer rate of electron and Li ion of carbon layer are obviously increased. Therefore, the electrochemical properties of LVPC-S are also further improved. Compared with the pure carbon coated Li3V2(PO4)3, sulfur doping carbon coated Li3V2(PO4)3 composites demonstrate better cycle and rate stability in the potential range of 3-4.3V and 3-4.8V.4、Boron-doped carbon coating is applied in lithium ion batteries electrode materials and the cause of its modification effect is analysed. After a facile sol-gel process, boron doped carbon coating approach is initially applied to modify electrode materials of LIBs. In this thesis, various B doped carbon coating Li3V2(PO4)3 samples with different B doping contents are synthesized, and there are four B doping types (B4C, BC3, BC2O and BCO2) appeared in all Li3V2(PO4)3/C+B samples. With the increase of B doping content, the electrochemical properties of various Li3V2(PO4)3/C+B samples meliorate initially and then deteriorate; moreover, the doping ratio of BC3 type climb up and then decline. It is illustrated that BC3 type makes an important effect on the electric conductivity and electrochemical activity of surface coated carbon layer and on the electrochemical properties of LVPC-B. Compared with bare carbon coating Li3V2(PO4)3, the electrochemical performances of moderate boron doped carbon coating Li3V2(PO4)3 further improved:when cycled at 20C in the potential range of 3-4.3V, the initial specific capacity of LVPC-B50 is 118.4 mAhg·g-1, and there are nearly no capacity loss appeared after 200 cycles. Furthermore, boron doped carbon coating approach also is demonstrated obvious modification effects on Li4Ti5012 anode materials.5、Combining surface N doped carbon coating with bulk phase K+ doping to improve the electrochemical performances of Li3V2(PO4)3 in wide potential range of 3-4.8 V. After the in situ fabrication process, Li3V2(PO4)3 cathode material is successfully modified by surface N doped carbon coating and bulk phase K+ doping. On one hand, N doped carbon coating is able to improve the electron transfer rate on the electrode/electrolyte interface and enhance the electrochemical reaction rate; on the other hand, bulk phase K+ doping can stable the crystal structure, limit the structural damage during Li+ reinsertion process and improve the intrinsic conductivity of Li3V2(PO4)3. After collaborative modification of N doped carbon coating and K+ doping, the electron conductivity and Li+ diffusion rate of Li3V2(PO4)3 are improved obviously. Therefore, compared with single modified Li3V2(PO4)3 samples, the collaborative modified sample reveals better electrochemical properties in the potential range of 3-4.8V. |
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