Influence of LiFePO4 Electrode Structure on Electrochemical Property

１．Introduction Well known as safety, non-toxicity and remarkable thermal stability, LiFePO 4 (LFP) is one of a promising candidate for the cathode material of large size lithium ion batteries. However, LFP is also known as the material with poor electrochemical property at low temperature. In order to improve the low temperature properties of LFP, several attempts were made to control the particle size and the morphology of LFP particle to increase the diffusion in solid and the surface reaction area. 1 ） On the other hand, along with the increase of LFP’s specific surface area, the problems arised for the manufacturing of the electrode. The problems are the increase of the amount of binder needed and the LFP agglomeration which leads to the increase of the surface roughness of the electrode. In this research, we have studied the influence of the LFP’s primary and secondary particle size and the morphology of the secondary particle to improve the electrochemical properties. ２．Experimental LFP was synthesized by a hydrothermal process. Spherical secondary particle with the size ranging from 10 to 30 μm in diameter was observed from the spray-drying of the LFP slurry. The electrode slurry mixing ratio was LFP:AB:PVdF=90:5:5 wt.%. After the slurry was coated on Al foil current conductor, it was vacuum dried under @120℃ for 12h、and pressed by a load of 200kgf. The cross-section image of the obtained electrode was observed by SEM. The particle size of the secondary particle was observed with particle size distribution measurements. The electrochemical properties were measured with pouch cell (anode：Li 4 Ti 5 O 12 electrode, separator：porous polypropylene, electrolyte：1M LiPF 6 /EC:DEC=1:1 +VC2wt%) ３．Result The relationship between LFP’s secondary particle size and Output – Direct Current Resistance (DCR) was shown in Fig. 1. The smaller the secondary particle size, the DCR decreased. The porous structure of the LFP’s electrode was thought to have a correlation to the electrochemical characteristics. Fig.2 shows the cross-sectional SEM images of the electrode which showed the different structure for the electrode manufactured with different particle sized LFP. The bigger the secondary particle size, the bigger the vacant spaces present inside the electrode and the small secondary particle was enclosed in the large secondary particle. The non-uniform structure was observed for the electrode made with large secondary particle compared with the uniform structure made with the small secondary particle. The main factor of DCR increase for the large secondary particle size was thought to relate to the non-uniformity of secondary particle structure and electrode structure. The relationship between the electrode structure, the secondary particle shape and the electrochemical properties will be studied further. Ref. 1) A. Yamada, S. C. Chung and K.Hinokuma , J. Electrochem. Soc., 144, p. A224 (2001) Figure 1