Lithium iron phosphate (LiFePO₄) with stable olivine structure is considered as a promising cathode material for rechargeable lithium batteries, due to its nontoxicity, high specific electrochemical capacity, excellent cycling stability and low cost. The LiFePO₄, however, suffers from low power capacity attributed to its low electronic conductivity (about 10^-9 S cm^-1) and low ionic conductivity(about 1.8×10^-14 cm² S^-1). Various approaches have been investigated to improve its electronic conductivity, such as carbon coating, higher valent cation doping, etc. Because of lithium ion’s one dimensional migration channel parallel to the b axis reducing particle sizes and crystal morphology control have been confirmed as the effective ways to improve LiFePO₄ performances. Compared with conventional solid-phase synthesis method, hydrothermal/ solvothermal synthesis methods have shown advantages in controlling particle size and crystal orientation in synthesizing many materials including LiFePO₄. hydrothermal/solvothermal synthesis technique is more complicated and more sensitive to reaction parameters compared with solid-phase method. Reaction parameters like feeding sequence, temperature and time have great influences on product LiFePO₄’s crystallinity, morphology and especially electrochemical performances. As investigated ethylene glycol(EG) as a solvent or co-solvent have significant positive influences (for smaller crystal size and better electrochemical performance) on LiFePO₄solvothermal process.

In this work, we have applied a solvothermal process using ethylene glycol(EG) as solvent to investigate LiFePO₄ crystallization. The synthesis of LiFePO₄ was carried out in a 50ml Teflon vessel, which was sealed in a stainless-steel autoclave. The molar ration of Li:Fe:P in the precursor solution was 2.7:1:1, and the concentration of LiFePO₄ in the reaction solution was controlled to be 0.2M. LiOH·H₂O, FeSO₄·7H₂O and H₃PO₄ were chosen as Li, Fe, P sources, and EG was chosen as solvent for this solvothermal synthesis. The typical feeding sequence was chosen:. The reactor was heated in an air oven setting at 120, 140, 160, 180˚C for 1min. The heating and cooling rates were measured and collected. Fig. 1 shows the comparison of the temperature in and out of Teflon when air oven is set as 160 ˚C for 1min. The temperature of air in oven increased faster than the slurry in reactors. When oven was set at 160˚C for 1min,the highest temperature of the slurry is 89˚C instead. The result samples were characterized by XRD shown in Fig. 2. Crystallized LiFePO₄ phase appeared associated with little Fe₃(PO₄)₂·H₂O and Li₃PO₄ when oven temperature was set at 160 ˚C for 1min. It indicates that the LiFePO₄ nuclei could be formed as low as 89˚C (real temperature) in EG during solvothermal process. Then we tried to synthesize LiFePO₄ samples in 85 and 80 ˚C for 10 hours. The XRD results show that LiFePO₄ crystal phase can be formed even at as low as 80 ˚C in EG while in water the lowest temperature for LiFePO₄crystallization is 105 ˚C.

The decrease of LiFePO₄ crystallization temperature is attributed to EG’s unique properties. There are 2 hydroxyl groups in EG’s chemical formula which have a strong hydrogen bonding ability. The two hydrogen bonds are not formed in one plane which results in a loose bonding structure and produces a low energy intermediate during crystallization reaction. It indicates a catalytic effect on LiFePO₄ crystallization. And EG has a smaller solubility compared with water which leads to a higher degree of supersaturation at the same raw material concentration. LiFePO₄ nucleis with slow crystal growth from diffusing solvents are because of the lower viscosity of EG. The results of this work provide deep insights into LiFePO₄ crystallization in solvothermal synthesis process. Also it provides the probability of synthesis materials for lithium ion batteries on an energy saving mode.