Synthesis And Properties Of A Novel Cathode Material Li₂FeP₂O₇for Lithium-ion Battery

Abstract:Cathode material plays a key role on electrochemical properties of lithium ion batteries. The new pyrophosphate structure material Li2FeP2O7, with specific capacity as high as110mAh·g-1, has discharge plaut of3.5V vs Li, which is the highest potential in all Fe-based phosphate materials. Compared with LiFePO4, the pyrophosphate could deliver one-lithium theoretical specific capacity without any technical effort such as nanosizing or carbon coating, exhibiting excellent thermal and chemical stability. With the gradual development of technology, it could realize two electron discharge-charge reactions at high potential. Choosing appropriate method and optimizing the synthesis process are effective ways to improve the electron and ionic conductivities. It has quite possibilities that material Li2FeP2O7could become new high power liuthum ion battery.In this paper, the cathode materials Li2FeP2O7were successfully synthesized via solid-state balling method, as well as metal-doped materials were prepared. The structure, constitution and morphology property were characterized by many research methods, such as X-ray diffraction (XRD), X-ray Photoelectron Spectroscopy (XPS), Electron Scanning microscopy (SEM), Transmission Electron Microscopy (TEM), Fourier Transform Infrared Spectrometer (FTIR), etc. Meanwhile, the electrochemical performance was tested through the constant current charge-discharge, cyclic voltammetry (CV) and Electrochemical Impedance Spectroscopy (EIS) technology. The effects of various experimental conditions were investigated on physical properties and electrochemical performance through these tests.Firstly, the method of solid-state balling was improved and optimized. The effects of each optimal parameter such as sintering temperature, sintering time, carbon amount, lithium sources on material purity and electrochemical property were discussed. The synthesis process of thermal reaction mechanism was principal studied, it infers that the optimal sintering temperature of solid-state method is650℃. Furthermore, the influence of different lithium source (LiH2PO4、Li2CO3、 LiOH、LiF) on physical and chemical performance were discussed. The results show that the sample synthesized by LiH2PO4has highest purity, best crystallizing, smallest particle size, delivering the initial discharge capacity of110.3mAh-g"1at the rate of0.025C. Furthermore, after various high discharge rate, a discharge capacity of103.8mAh·g-1could be still achieved after110cycles, showing minimize polarization and excellent electrochemical proformance, which is more appropriate for liuthium ion battery than other lithium sources.Then, pure phase Li2FeP2O7and Fe site doped, Li2Fe1-xMxP2O7/C (M=Ni, Zn, Ti, Nb) were successfully synthesized via solid state balling method. It is found that some suitable amount of metal ion doping have little influence on material crystalline structure. On the contrast, it will ameliorate the electron conductivity and improve the electrochemical performance. Particularly, the nickel doping has great effect on electrochemical properties of material Li2FeP2O7, a charge/discharge rate of0.025C offers a discharge capacity of110.2mAh·g-1for sample Li2Fe0.98Ni0.02P2O7/C. Moreover, the cyclic voltammetry and electrochemical impedance spectroscopy measurements demonstrate that the nickel-doped materials were enhanced not only lithium ion diffusion coefficient, but also high rate capability and cycle performance. The capacity retention of sample could still keep90.1%after100cycles at various high discharge rates, indicative of a remarkably improved electrochemical performance as compared with the pristine Li2FeP2O7.There are49figures,8tables and103references in this paper...