Preparation And Crystallization Behavior Of Lithium-iron-phosphate Glasses

Lithium ion batteries have been concerned as a new alternative energy resource since the oil crisis. LiFePO₄ has a high specific capacity and good cycling performance. It is thermally stable in the fully-charged state, environmentally benign and has a low cost. Lithium iron phosphate has a great potential as cathodes for the next generation of rechargeable lithium ion batteries. LiFePO₄ prepared by traditional methods has a relatively low tap density. The crystallization of lithium-iron-phosphate glass could produce LiFePO₄ electrode materials with a high density. Up to now, only a few lithium-iron-phosphate glasses have been studied.In the present work, melts of Li2_O-Fe₂O₃-P₂O₅ ternary system were prepared and annealed or water quenched. Based on the analysis by XRD and polar optical microscope, the glass formation region of Li2_O-Fe₂O₃-P₂O₅ ternary system was determined. It provides an important guideline for the future work on the lithium-iron-phosphate glasses.The density and chemical durability of glass were measured based on the Archimedes principle and the weight loss of glass particles after being boiled in water, respectively. The structure, glass transition temperature (Tg) and crystallization behavior of the glass were analyzed by FTIR, DTA and XRD. Studies on the glasses with compositions inside the glass formation region show both Li₂O and Fe₂O₃ can increase the density of the ternary glass. It has been found that Fe₂O₃ is beneficial for increasing the water attack resistance and reducing the crystallization tendency of the glasses. However, the increase in amount of Li₂O has generally negative effects.Glasses were prepared by substituting Li₂O with Na₂O, K₂O, MgO, CaO, BaO, ZnO and substituting P₂O₅ with SiO2, B₂O₃, substituting Fe₂O₃ with Al₂O3 based on the ternary lithium-iron-phosphate glass.The studies about the lithium-iron-phosphate glasses with Na₂O, K₂O, MgO, CaO, BaO, ZnO show that the decreases in Tg with the initial substitutions are mainly caused by the diminution of the aggregation effect of Li⁺ on the phosphate glass structure due to the decreases in Li⁺ concentrations upon the substitutions. The initial increases in glass density are due to the larger molecule weights of the substitutes. The mixed-alkali and depressing effects as well as the slower mobilities of the substitute ions are responsible for the initial increases in the chemical durability. With increasing the substitutions, Tg could increases as a result of the increase of substitute ions with larger radii and slower mobilities, and the density, chemical durability could decrease due to the increasing breakage of the glass network. With increasing the amount of ZnO, the glass density and chemical durability increase, while the transition temperature decreases. It is shown that the ZnO substitutions could enter the glass network by forming [ZnO₄] tetrahedral or cross-linked the phosphate chains by forming P-O-Zn bridges. However, as more Li₂O is replaced by ZnO, Zn²⁺ ions tend to occupy interstitial sites due to the lack of coordination oxygen.The results show that the density, Tg, crystallization temperature (Tc) and chemical stability increased with the substitution of P₂O5 with SiO2. However, the substitution can not be more than 10%, otherwise, glasses can not be formed. The Tg, density and the chemical stability increases when Fe₂O3 was substituted by Al₂O₃ 10%. Further increasing the amount of Al₂O₃, the opposite trend was observed. With the initial substitutions of B₂O₃, the density, chemical stability of glass is increased due to the formation of [BO₄]. With the further increase in B₂O₃, the formation of [BO₃] slowed down the increases in the density and deteriorating the chemical stability.Based on the DTA curves which were obtained at different heating rates, the activation energy for crystallization and the temperature for heat treatments were determined. It was found that after the heat treatments, the primary crystalline phase changes from LiFeP₂O₇ into NaFeP₂O₇ and KFeP₂O₇ with the increasing substitutions of Li₂O with Na₂O and K₂O. In the cases of alkali-earth metal oxide subsititutions, only LiFeP₂O₇ crystallized after the heat treatments. However, besides the LiFeP₂O₇ phase, crystallization of Fe₇(PO₄)₆ increased with the addition of Al₂O₃. In contrast, increasing crystallization of LiFePO₄ was identified in the heat-treated lithium-iron-phosphate glasses with increasing B₂O₃ substitutions.