Phase Evolution Of LiFePO₄ Working At Low Temperature And Its Modifications

LiFeP04 is one of the most prospective cathode materials. However, severe capacity decay at low temperatures significantly restricts its applications in energy storage aspects. Capacity decay has long been considered close related to the reaction in discharge. As with LiFePO4, unlike other cathode materials, the phase evolution is different, in which Li+ intercalates via two phase mechanism. Therefore, phase boundary emerged in the process would hinder the diffusion of Li+. Phase evolution under low temperature should be clarified as to explore limiting factors of discharge which is essential to improve the low temperature performance of LiFePO4.In this dissertation, LiFePO4 prepared by solid-state route is examined by TEM. The ex-situ TEM technique clearly presents the two phase evolution of discharge of LiFePO4 at -40℃. Combined with XRD, the constitution of cathode is deduced with the proceeding of discharge in detail. Limiting factors of low temperature performance are proposed, and are improved for each factor which bring a 3-time increase in capacity retention at -40℃ when compared with commercial LiFePO4. Detailed works carried out are listed as follows:1. We have successfully tracked the two phase evolution of discharging at -40℃ whose boundary is identified as a distinctive distorted section different from Li-rich and Li-lean in construction with 2-3 nanometers in width. An intermediated metastable phase is detected in the phase evolution. Different from phase evolution at room temperature, phase evolution at low temperature is identified as the transition from Li-lean to metastable phase, and from metastable phase to Li-rich subsequently. From refining XRD patterns, it can be found that FePO4 exists in the entire discharge process with approximately the same fraction (27.01%) which can be ascribed as one factor of capacity loss at low temperature. The estimated extent of reaction of active FePO4 was 77.10% which indicates the premature ending of the electrochemical reaction. To improve the performance at -40℃, possible solutions are raised with regard to the factors mentioned above, such as improving electronic conductivity, structure designing, reducing particle size, cation doping, etc.2. The influence of ball-milling time, ingredients and sintering techniques are investigated for improving low temperature performance. LiFePO4 prepared by two-step route with a mean diameter of 50～80 nm could effectively shorten the diffusion path of Li+, delivering a capacity retention of 50% which increased by 2 times compared with commercial LiFePO4.3. To improve electronic conductivity of carbon coating in LiFePO4, the influence of carbon content and extent of graphitization are investigated, among which in-situ graphitizing technique can effectively increase the conductivity of the material, reaching 2.15e-1 S cm-1. However, with the dramatic increase in conductivity, the is-situ graphitized sample presents little enhancement in low temperature performance, delivering a capacity retention of 57% which indicates that it is of little effect to low temperature performance by further improving its conductivity when whose electronic conductivity is fairly high.4. The influence of divalent cation doping of Mg, Zn, Ni, Mn is investigated, among which Mn10% delivers the best performance at -40℃ with capacity retention of 65.2%. It is found that Mg doping does effectively improve the Li+ diffusion property of the material, though, whose phase evolution process does not change by Mg doping, thus improving low temperature performance only to some extent. Zn, Ni and Mn doping can effectively shorten the transition from Li-lean to metastable phase, thus significantly improving the low temperature performance of the material. Further more, Mn10% doping effectively extends the solution reaction period which is very essential to discharge at low temperature. As a result, Mn10% presents the best low temperature performance.