In Situ Synthesis Of LiFePO₄/CNTs Composites For Enhanced Electrochemical Performance

The olivine LiFePO₄ has been considered as one of the most promising cathode material for lithium ion batteries?LIB?because of its flat voltage profile,long life span,low cost,nontoxicity,particularly suitable thermal stability and strong overcharge tolerance.Unfortunately,the intrinsic low Li+ transport velocity and poor electronic conductivity limit the commercial use of LiFePO₄ in LIB for EVs and HEVs.In order to improve the electrochemical performance,especially at high current rate,carbon nanotubes?CNTs?with excellent electrical conductivity and unique one-dimensional morphology have been added in situ to form CNT conductive networks which coated on or embedded in LiFePO₄.Herein,the LiFePO₄/CNTs composites have been successfully prepared via different methods,including liquid-phase method,hydrothermal method and ball-milling method.LiFePO₄ particles embedded with CNTs were prepared in situ by a liquid-phase method with the assistance of P123 and glucose.The influence of the amount of P123 and glucose on the structure and properties of LiFePO₄ has been investigated.From the results,the P123 plays an important role in distribution of CNTs,and could be beneficial to form porous microstructure.When combine P123 with glucose,the CNTs and amorphous carbon decomposed from glucose could construct efficient conductive networks which can improve the conductivity of LiFePO₄.The electrochemical results show that the LiFePO₄/CNTs composites with 0.3g P123 and 0.3g glucose exhibit the best electrochemical performance with a discharge capacity of 150 mAh g-1 at 0.1C and capacity retention of 57%and 45.7%at 2C and 5C,respectively.Mesoporous LiFePO₄ microspheres embedded homogeneously with three-dimensional?3D?CNT conductive networks were successfully prepared via an effective and controllable hydrothermal approach with the assistance of polyethylene glycol?PEG?.The concentration of reactants and the hydrothermal treatment time were optimized firstly.The concentration of 0.01mol/65mL and hydrothermal treatment time of 8h were choosed to prepare mesoporous LiFePO₄/CNTs microspheres.The results demonstrated that porous structure of these microspheres can be well modulated by varying molecular weight of PEG and the formation mechanism of the porous structure has been revealed.The LiFePO₄/CNTs composite prepared using PEG400,which possessed the best mesoporous structure with the pore size centered at 40 nm,and 3D CNT conductive network embedded in the microspheres homogeneously,exhibited the best electrochemical performance with high capacity retention of 95.7%even at 5 C after 100 cycles.To meet the need of industrial manufacture,the ball-milling method,which is simple,easy to control,high repeatability and productivity,was adopted to prepare LiFePO₄/CNTs composites.Among various dispersant,PVP could promote the dispersion of CNTs markedly,which is beneficial to construct homogeneous CNT conductive networks.Moreover,the milling time and the time of adding CNTs could modulate the morphology of LiFePO₄ particles and the length of CNTs,and the sintering temperature and time could affect the crystalline degree of LiFePO₄ and the content of Fe3P,thus influencing the electrochemical properties of LiFePO₄.The sample sintered at 750? for 5 h exhibits the best electrochemical performance with a discharge capacity of 123?76 mAh g-1 at 10C and 50C,respectively.