Application Of Chemical Dispersants In LiFePO₄Cathodes

LiFePO4batteries have become one of the best alternative power for the new energy vehicles and large-scale energy storage power station because of its safety and superior cycle performance. However, the agglomeration of nanoparticles in the LiFePO4cathode is greatly related to the performances of the cathode, and the traditional mechanical dispersion method is hardly to prepare the homogeneous cathode slurry. In this work, we study on the dispersants which can change the nanoparticles surface properties and prevent the particles from agglomerating in the cathode through electrostatic repulsion or steric effects, thus improving the performance of LiFePO4cathode. And the dispersing effects of the slurry and electrochemical properties of the cathode are also researched.After comparing with the structure and properties of various dispersants, we choose three kinds of dispersants:poly (ethylene glycol) p-isooctyl-phenyl ether Triton X-100(MTritonX-100=652)、polyacrylic acid PAA(MW=130,000, solid powder) and PAA(Mw=5000, aqueous solution) as research objects. Electrochemical stability and dispersing effects of the three dispersants are studied, as well as their influences on LiFePO4cathode. The main results of the experiment are presented as follows:1. Triton X-100is used to pre-disperse the nano-carbon black (SP) in organic N-methyl-pyrrolidone (NMP) solvent. Triton X-100can absorb onto SP particles and prevent them from agglomerating due to steric effects. The particle size obviously decreased after pre-dispersed. Also the impedance and the electrochemical performance of the LiFePO4cathode are significantly improved by employing the pre-dispersed SP slurry as the conductive agent. The capacity retention of LiFePO4with pre-dispersed SP cathode is93.9%after200cycles at1C. It can be seen from the cyclic voltammetry curves that the LiFePO4with pre-dispersed SP cathode has a smaller polarization than the LiFePO4cathode.2. PAA (Mw=130,000, solid powder) is used to pre-disperse SP in NMP solvent. The SP particle size become smaller after pre-disperse, and the viscosity of the SP slurry is lower, and the fluidity and stability are also impoved. This pre-dispersed SP slurry as a conductive agent is added to the LiFePO4cathode results that more homogeneous SP distribution among the dried composite electrode to form better electronic conductive network of the active material particles. The results exhibiting that the electrochemical performance of LiFePO4cathode has obviously improved. For94.3%capacity retention after200cycles at1C and discharge capacity of135.1mAh g-1at4C.3. Dispersion property of aqueous LiFePO4slurry and the corresponding cell performance of LiFePO4cathode with anionic dispersant PAA (Mw=5000, aqueous solution) are investigated. The results show that the low molecular weight PAA can absorb onto the surface of the LiFePO4particles to change its surface hydrophobic nature, and to enhance its surface electric potential. Thus it can prevent the particles from agglomerating due to its steric effects, and the particle size reduced. Dispersed uniformly aqueous LiFePO4slurry has better liquidity and processing performances.The surface of cathode is more smoother after drying, and easy processing; The lithium ion diffusion coefficient in the LiFePO4cathode with PAA is larger because the smaller LiFePO4particle size; indicate that the kinetics behaviors of LiFePO4cathode with PAA are better. That enhanced the reversibility of electrochemical reaction of LiFePO4cathode, and the electrochemical performance of LiFePO4cathode in the water system is also improved. Figures43, tables10, references85...