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Study On Olivine-type LiFePO4/C Composite Cathodic Materials

Posted on:2006-09-03Degree:DoctorType:Dissertation
Country:ChinaCandidate:C H MiFull Text:PDF
GTID:1101360182973087Subject:Materials science
Abstract/Summary:PDF Full Text Request
Olivine-structured LiFePO4 is gaining particular interest as a potential candidate cathode material for rechargeable lithium batteries from both economic and environmental points of view. In this paper, LiFePO4/C, Li(Mn,Fe)PO4/C, and LiFePO4/(Ag+C) composite cathode materials have been synthesized. The micro-structures and morphologies of these composites were investigated by XRD, TEM, SEM, EDS, and Raman observations. The electrochemical performances have been evaluated by galvanostatic charge-discharge, cyclic voltammetry (CV) and electrochemical impedance spectra (E1S). The effects of the synthesis parameters on the physico-electrochemical properties of carbon-coated LiFePO4 prepared by "in-situ synthesis" using Fe3+ compounds as iron source have been discussed in detail.LiFePO4/C composite, in which amorphous nano-carbon webs are wrapping and connecting LiFePO4 particles, has been synthesized by solid-state reaction using Fe2+ compounds as iron source and polypropylene as conductive carbon source. The particle size of LiFePO4/C synthesized at 600°C is 100-200 nm (sample C), which is much smaller than that of LiFePO4 sintered by solid-state reaction at 600℃, about 5-10 μm (sample A) and that of LiFePO4+(carbon black) after milling, 1-5 μm (sample B). The initial discharge capacities of cathode A, B, and C at 0.1 C are 116.4, 143.6, and 159.8 mA h·g-1 respectively. It was found that, the homogeneous distribution of nano-sized carbon webs would be effective in enhancing the electronic conductivity of LiFePO4 and hindering the particle growth, consequently improving its electrochemical performance.A novel synthesis method for in-situ carbon-coated LiFePO4 has been developed, using inexpensive Fe3+ compounds as the iron source and polypropylene as the reductive agent and the conductive carbon source. Polypropylene pyrolyzes during the formation of LiFePO4 synchronously, as a result, carbon-coated LiFePO4 composites are synthesized in-situ by a one-step solid-state reaction. The advantages of this novel method are the direct synthesis of carbon-coated LiFePO4 powders with neither presintering nor postdeposition treatments. It was found that, hydrogen and carbon generated from pyrolysis of polypropylene play the key roles in the synthesis processing: (1) hydrogen and carbon as the reductive agents for the reduction of Fe3+ to Fe2+;(2) carbon as the electronic conductor for the enhancement of LiFePO4 conductivity;(3) carbon as the obstructer film for the hinderment of LiFePO4 particle growth.The carbon-coated LiFePO4 composite cathode prepared by the in-situ synthesis was effective in enhancing the electrochemical properties. Its initial discharge capacity of 164 mA h·g-1 at 30℃ and 0.1 C is close to the theoretical value of 170 mA h℃g-1. The first capacities of carbon-coated LiFePO4 are 154.5 mAh·g-1 (0.3 C) and 150.5 mAh·g-1 (0.5 C). The capacities are still maintained at 145 mA h·g-1 (0.3 C) and 135.7 mA h·g-1 (0.5 C) after 150 cycles. The corresponding capacity retaintion ratios are 93.9% and 96.8%, respectively. Galvanostatic charge-discharge tests at high-temperature (55℃) showed that the reversible discharge capacity of 55℃ at 0.05-1.5 C are larger than that of 30℃. The capacity at 1 C and 1.5 C rates was improved from 121 and 105 m A h·g-1 at 30℃ to 136 and 123 mA h·g-1 at 55℃.respectively. Electrochemical impedance spectra indicate that overall impedance of carbon-coated LiFePO4 at 55°C is lower than that at 30 °C.The influences of synthetic conditions such as sintering temperature, sintering time, and presintering-regrinding treatment on the physico-electrochemical properties of carbon coated LiFePO4 prepared by the in-situ synthesis" have been investigated. It was found that increasing the sintering temperature leads to higher crystallinity, but to a larger particle size. In the range of 5OO8OO°C, 700°C is the optimum synthetic temperature for the carbon-coated LiFePO4 with both small particle sizes and perfect crystal, which are two key factors to enhance the electrochemcial performance. An additional presintering-regrinding treatment is effective to synthesize the carbon-coated LiFePC>4 with fine and homogeneous particle sizes, consequently improving the electrochemical properties. Extending sintering time from 5 h to 40 h improves the crystallinity of LiFePO4 slightly, and the particle growth is also negligible. The Iq/Ig ratios in Raman shift of the coated carbon decreases from 1.19 to 0.74 as the sintering temperature increases from 500°C to 800°C, indicating that the higher the sintering temperature, the higher the graphitization degree of carbon pyrolyzed from polypropylene, while the Id/Ig values remain invariant (0.96) with extending time from 5 h to 40 h at 700°C.Fine particles LiFeP(V(Ag+C) composites were synthesized by sol-gel and solid-state reaction combined with chemically reductive reaction. The electrochemcial properties of both LiFePO4/(Ag+C) composites are better than those of LiFePCVC. The addition of Ag conductor is effective to improve the electrochemical properties of LiFePO4.Carbon-coated Li(Mn,Fe)PO4 solid-solution composites were synthesized by solid-state reaction. Electrochemical tests of LiMnyFeiyPO4/C (y=0.2, 0.6) cathodes indicate that there is a new pair of higher charge-discharge voltage plateaus, which are related to the redox reactions of Mn3+/Mn2+ couple. Though the initial capacity of LiMno^FecuPCVC (143 mAh-g"1) at 0.1 C is lower than those of LiFePO4/C (164 mAh-g"1) and LiMno.2Feo gPCVC (162.5 mAh-g"1), its average discharge voltage reaches 3.65 V. It was also found, the coexistence of Mn with Fe at Ac sites would improve the redox kinetics of Fe3+/Fe2+ couple in LiMno.6Feo.4P04/C cathode.
Keywords/Search Tags:lithium-ion batteries, composite cathode material, LiFePO4, in-situ carbon-coating, one-step solid-state reaction, nano-carbon webs, Li(Mn,Fe)PO4/C, solid-solution composite, electrochemical performance
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