| Rechargeable lithium batteries with high energy density are the key research and development directions in the chemical power sources field. Elemental selenium has been proposed as new generation and promising cathode materials for high performance rechargeable lithium batteries due to its remarkable advantages, such as a high theoretical volume ratio capacity equal with sulfur, electron conductibility 20 orders of magnitude greater than that of sulfur. Currently, the study about lithium-selenium batteries is in the initial stage, and the key materials relevant to Li-Se battery such as electrolyte, cathode material and binder have been important research directions. The key research purpose of this paper is to develop high performance selenium-based composite cathode material. Additionally, Fe3O4-based composite material as LIBs anode material was also studied.The stable crystal structure of elementary selenium is trigonal with long chain sorial molecule structure. So one-dimension trigonal selenium crystal with large length-diameter ratio is easily obtained by “down to up” liquid synthesis method. When being used as cathode, this liner selenium crystal will provide low ratio capacity because of poor selenium utilization efficiency. Therefore, we firstly prepare porous carbon with high surface area and large porous volume, and then heat the solid mixture of selenium and carbon. Amorphous selenium or microcrystalline selenium particles will form in the carbon pores due to the molten state selenium inflowing into pore with the help of the capillary force. That is to say, by controlling the size and morphology of active selenium to make selenium enough react with Li+, and further to improve the ratio capacity of selenium and the other electrochemical performance.Two kinds of porous carbon materials with different pore structures and their according selenium/porous carbon nanocomposite materials were designed and prepared in this paper. The physical performances of all materials prepared in this paper were characterized by XRD, BET, Raman and so on. The electrochemical performances of two selenium/porous carbon nanocomposites as anode of rechargeable lithium batteries were studied detailedly.A hierarchical porous carbon(HPC) was prepared by template method with F127 and nano-Fe3O4 particles as template agents and resin as carbon precursor. Se/HPC with the Se content of 50% was obtained by infiltrating selenium into the pores of HPC through heat-melting technique. Comparing with the ether-based electrolyte, Se(50%)/HPC composite electrode exhibited better cyclic stability and rate performance in the carbonate-based electrolyte(Li PF6-EC-DMC). A stable capacity of 420 m Ah g-1 was kept in the initial 60 cycles at the current density of 150 m A g-1. In rate test, when the current density was returned from 900 m A g-1to 150 m A g-1, the capacity was restored from 200 m Ah g-1 to 410 m Ah g-1. According to the EIS spectra of Se(50%)/HPC electrode, the capacity decay during the later cycles was mainly attributed to the aggregation of active material Se or Li2 Se locating in the 3D-interconnected big mesoporores, which was testified by the improving cyclic stability(no decay for 100 cycles) of Se(33%)/HPC electrode obtained by removing Se locating in the mesopores through heating volatilization at 350 ℃.A porous carbon material with high surface area, large porous volume and half-graphitized structure was obtained by carbonizing pomole peel at high temperature and further activating the hard carbon with KOH. Then, active material selenium was infiltrated into the pores(<4 nm) of PPDC by ball-milling and heat-melting the mixture of selenium and porous carbon to form Se/PPDC nanocomposite. The electrochemical test results indicated that Se/PPDC has good cyclic stability and superior C-rate performance. During the initial 35 discharge/charge cycles, Se/PPDC delivered a stable and close to theoretical value(675 m Ah g-1) capacity of 650 m Ah g-1 with approximate 99% Columbic efficiency. After 100 cycles, 61.5% of the initial stable capacity was still reserved. Besides, in the C-rate test, when the current density was elevated to 900 m A g-1 after 45 cycles, a steady capacity of 440 m Ah g-1 was obtained, and when the current density was returned to 150 m A g-1, the electrode still could offer a high capacity of 630 m Ah g-1. The formation of SEI film in the first cycle process was testified by XPS test. So the superior performance of electrode is attributed to the special composite structure of Se/PPDC and the stable SEI film.As a new class Lithium-ion anode material, transition metal oxides due to high theoretical ratio capacity are very suitable for electric vehicle(EV) and high power storage battery. A new foam carbon material was prepared firstly using nano-Ca CO3 as template agent, and then nanocomposite Fe3O4/FC with Fe3O4 nanoparticles(about 20 nm) homogenously distributing in pores of FC matrix was obtained through immersion and calcination process. As anode of LIBs, Fe3O4/FC composite exhibited high ratio capacity, superior cyclic stability and C-rate performance. A steady capacity of 1100、700 and 250 m Ah g-1 without obvious decay for 100 cycles was obtained at the current density of 0.1 C(1 C=1000 m A g-1)、0.5 C and 5 C, respectively. It is concluded that the introduce of FC matrix is very favorable to improve the electrochemical performance of Fe3O4. FC matrix not only inhibits effectively the structure destroy caused by the volume expansion and the aggregation of active materials, but also accelerates the conduction of electron and diffusion of Li+. By virtue of the low-cost raw materials, simple and feasible preparation process and superior electrochemical performance, Fe3O4/FC will be a promising lithium-ion battery anode material. |
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