Solvothermal Synthesis of LiFePO4 with an Energy Saving Technique
Authors: Huang Xiankun , Ke Shaoyong , He Xiangming , Jiang Changyin , Liu Yongzhong 
Published: February 2020

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.

Figure 1