Solvothermal Synthesis of LiFePO4 with an Energy Saving Technique

Lithium iron phosphate (LiFePO 4 ) 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 LiFePO 4 , 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 cm 2 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 LiFePO 4 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 LiFePO 4 . 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 LiFePO 4 ’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 LiFePO 4 solvothermal process. In this work, we have applied a solvothermal process using ethylene glycol(EG) as solvent to investigate LiFePO 4 crystallization. The synthesis of LiFePO 4 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 LiFePO 4 in the reaction solution was controlled to be 0.2M. LiOH·H 2 O, FeSO 4 ·7H 2 O and H 3 PO 4 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 LiFePO 4 phase appeared associated with little Fe 3 (PO 4 ) 2 ·H 2 O and Li 3 PO 4 when oven temperature was set at 160 ˚C for 1min. It indicates that the LiFePO 4 nuclei could be formed as low as 89˚C (real temperature) in EG during solvothermal process. Then we tried to synthesize LiFePO 4 samples in 85 and 80 ˚C for 10 hours. The XRD results show that LiFePO 4 crystal phase can be formed even at as low as 80 ˚C in EG while in water the lowest temperature for LiFePO 4 crystallization is 105 ˚C. The decrease of LiFePO 4 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 LiFePO 4 crystallization. And EG has a smaller solubility compared with water which leads to a higher degree of supersaturation at the same raw material concentration. LiFePO 4 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 LiFePO 4 crystallization in solvothermal synthesis process. Also it provides the probability of synthesis materials for lithium ion batteries on an energy saving mode. Figure 1