Hydrothermal Synthesis Of LiFeP0₄ Cathode For Lithium-ion And Its Properties

LiFeP04 is one of the most promising power battery due to its high and flat voltage plateau(3.4V),high theorectical capacity,safety,low cost and environmental protection.In this paper,olivine structure LiFeP04 was synthesized by hydrothermal synthesis.X-ray diffraction(XRD),scanning electron microscopy(SEM),transmission electron microscopy(TEM),galvanostatic charge and discharge tests,and electrochemical impedance tests were used to systematically analyze the effect of hydrothermal synthesis temperature,time,pH,sintering and carbon coating on the crystal phase,morphology,particle size,and electrochemical performance of LiFeP04.The main points can be summarized as follows:1.In this paper,pure phase LiFeP04 was synthesized at 120? by hydrothermal route.With the increase of reaction temperature,the crystallinity of LiFeP04 crystal increases,and the preferential orientation of crystal growth along(020)crystal surface increases.The diamond shape changes into a rod shape and the particle size decreases.The sample synthesized at 200? showes the best electrochemical performance due to the best preferential orientation along the(020)crystal plane and smallest particle size.2.The fixed hydrothermal synthesis temperature was 200?.The effect of reaction time(3h?24h)on the performance of LiFePO4 was investigated.When reaction time is 3h,the Fe2P2O7 parasitic diffraction peaks can be observed from XRD pattern,and the particles were diamond-shaped and crystal defects were serious can be observed from SEM image.As the reaction time prolonged,the content of Fe2P2O7 impurities decreased,and the LiFePO4 particles changed from diamonds to rods.The sample showes the best capacity,cycling and rate performance when the reaction time is 12h,because the content of Fe2P2O7 is the least,the particle size is the smallest.3.Fixed hydrothermal temperature 200?,reaction time 12h,to explore the effect of reaction pH(5.2?5.8)on the performance of LiFePO4.When pH=5.2,the sample had a parasitic diffraction peak of Fe2P2O7,and the particles were irregular diamonds with large particle size.As the pH value of reaction solution increased,the content of Fe2P2O7 was reduced.When the pH increased to 5.8,the pure phase of LiFeP04 was synthesized.The particles changed from rhombohedral to rod-like and the particke size decreased.The electrochemical performance of LiFeP04 synthesized at pH=5.8 was the best,but the capacity decayed faster than that of sample containning Fe2P2O7 at of 5C rate.The reason is that the Fe2P2O7 phase increases the Li+ diffusion coefficient,which makes sample containning Fe2P2O7 phase have the advantage at high rate charge and discharge.4.Sintering improves the crystallinity of LiFeP04 sample,increases the preferential orientation of the crystal along the(020)crystal plane,and reduces the particle size.Sintering reduces the resistance of LiFeP04 electrode by 1/2,increases the Li+ diffusion coefficient by an order of magnitude,and improves the electrochemical performance of the LiFeP04 material.When the carbon content increases from 1.65%wt to 6%wt,the preferential orientation of growth of LiFeP04 crystals along the(020)crystal plane decreases,the solid electrolyte interface resistance Rsf increases,and the electrochemical performance of LiFeP04 material decreases.The carbon content of 1.65%wt showes the best electrochemical performance.