Magnetic Properties Of LiFePO₄/C Positive Materials And Their Effects On The Electrochemical Properties

Polyanion-type lithium ion positive material lithium iron phosphate has excellent thermal stability,safety and electrochemical properties in the field of power cells.However,iron-containing magnetic impurities and other oxides are inevitably present during the preparation of lithium iron phosphate materials.In order to improve the electrochemical performance of lithium iron phosphate,to suppress and detect magnetic impurities such as ?-Fe2O3 and Fe2P has become a hot issue.In this paper,LiFePO4/C particles were prepared by solid state reaction and hydrothermal method.The magnetic impurities produced in the process of material preparation and their influences on electrochemical performances were mainly studied,and the main contents are described as follows:Firstly,pure LiFePO4 and LiFe,-xMnxPO4/C(x=0.0,0.1,0.3,0.5)were synthesized by a high energy ball milling-promoted solid state reaction.The experimental results indicate that,the structures of all the undoped and doped samples are pure olivine;pure LiFeP04,LiFe1-xMnxPO4/C(x=0.0?0.1?0.3?0.5)and commercialized LiFePO4/C have all weak ferromagnetism at 100K,200K,300K,but their magnetic moments are different;There exists Fe3+ in six samples,which corresponds to y-Fe2O3;The electrochemical properties of the samples are associated with impure y-Fe2O3 introduced during preparation;Magnetic and XPS measurements can sensitively probe ?-Fe2O3 impurity which can be used as the effective technique for monitoring quality of LiFePO4 cathode materials.Secondly,by high temperature solid state reaction,five different samples(AH16,AH40,AH60,AH120 and AH160)were prepared with different gas flow rate of Ar+5%H2(16 ml/min.,40 ml/min.,60 ml/min.,120 ml/min.and 160 ml/min.).The experimental results indicate that,as the gas flow rate is less than 40ml/min.,the calcined sample does not have miscellaneous phases;as the gas flow rate reaches 60ml/min.,miscellaneous phases begin to appear.It is found that,the main phase of the material is an olivine structure,and the impurities of the samples are mainly Fe2P,FeP,Li3PO4.The magnetic testing results indicate that under 300K,AH 16 has the least magnetic moment,and AH160 has the largest magnetic moment.Through comparative analysis,it is found that the attenuation degree of the electrochemical specific capacity of LiFePO4 is related to the content of Fe2P impurities during annealing.The less impurities are,the weaker the magnetism is,the larger the initial capacity is.Lastly,the LiFePO4 samples were synthesized by hydrothermometry at 443K.LiFePO4/C materials were prepared using different carbon sources.The LiFePO4/C materials prepared with glucose,saccharose,ascorbic acid were named as LFP-1,LFP-2 and LFP-3.The experimental results show that the LFP-3 sample with ascorbic acid as the carbon source has the weakest magnetic properties,the highest specific capacity,and the best cycle stability.