Preparation And Low-temperature Electrochemical Properties Of Graphene Modified LiFePO₄/C Cathode Materials For Lithium-ion Batteries

LiFePO₄/C/graphene composites were successfully prepared via a suspension mixing method to improve the low temperature electrochemical performances.The contents of graphene are 0 wt.%,3 wt.%,5 wt.% and 7 wt.%.The chemical composition and microstructure of these composites were detected by XRD,Raman,SEM,HRTEM and so on.The cathode films were made by coating method and then assembled into button-lithium batteries.The electrochemical performances of all the samples at different low temperatures?0 °C,-10 °C and-20 °C?were conducted by galvanostatic charge/discharge test,cyclic voltammetry?CV?,electrochemical impedance spectroscopy?EIS?and potential step chronoamperometry?PSCA?.Galvanostatic electrochemical measurements indicate that surface modification with graphene is an effective way to enhance the discharge capacity and cycling stability of LiFePO₄ cathode materials.Especially,5 wt.% graphene modified LiFePO₄/C delivers the best high rate electrochemical performances in the voltage range of 2.0-4.2 V at low temperatures.At-20 °C,the samples with various amounts of graphene?0,3,5 and 7 wt.%?show initial discharge capacity of 82.5,91.5,98.2,89.1 mA·h·g-1 at 1 C.The 5 wt.% graphene modified sample shows the best rate performance and reversible electrochemical performances.At 1 C at 0 °C,all the samples remain capacity retention of 83.0 %,92.2 %,94.1 % and 89.2 % after 200 cycles,respectively.As the operation temperature is lowered to-20 °C at 1 C,the discharge capacity of the sample with 5 wt.% graphene is about 31.1 mA·h·g-1 higher than that of the pristine sample.EIS test indicates that the charge transfer resistance?Rct?is most sensitive to the low temperature.The Rct of pristine sample increases from 182.3 ? to 595.9 ? as the temperature decreases from 0 °C to-20 °C.However,the Rct of the sample with 5 wt.% graphene only increases from 70.4 ? to 307.8 ? under the same conditions.Meanwhile,the samples with graphene exhibit higher diffusion coefficients of Li+,which is also benefitial to improving the charge-discharge properties of electrodes at low temperatures.