Electrochemical Properties of High-Voltage Spinel Positive Electrodes Prepared with Non-Fluorine PAN-Based Binders

Abstract

For the application of lithium batteries with high energy density, high voltage spinel oxides are considered as promising candidates for a positive electrode of rechargeable lithium batteries. However, it is needed to reexamine the battery components including, electrolyte, conductive carbon, separator, and binder. In this study, we focused on the polyacrylonitrile (PAN) possessing high oxidation resistive properties, and synthesized non-fluorine PAN-based copolymer prepared by graft polymerization of PAN and an adhesive polymer. Electrochemical properties of LiNi1/2Mn3/2O4composite electrodes prepared with PAN-based copolymer are examined and compared with those of PVdF used as the binder. LiNi1/2Mn3/2O4 was prepared by solid-state reaction from stoichiometric amounts of Li2CO3, NiCO3･2Ni(OH)2･4H2O, and MnCO3 at 1000 °C for 12 h followed by heating at 700 °C for 48 h in air.[1] Thus obtained sample was used as the positive electrode active material. PVdF (TA6010, Solvay) and non-fluorine PAN-based copolymer were used as binders. The electrode active material (AM) and acetylene black (AB, HS-100, Denka) were mixed with the binders, and NMP was added to adjust the viscosity of slurry. The slurry was uniformly pasted on to aluminum foil, and then dried in a vacuum. Compositions of electrodes were AM : AB : PVdF = 80 : 10 : 10 and AM : AB : PAN-based polymer = 80 : 10 : 5 in weight ratios. Electrochemical measurement was carried out using a two-electrode cell (TJ-AC, Tomcell Japan) consisting of the composite electrode, separator (polyolefin microporous membrane), electrolyte solution (LiPF6dissolved in EC : DMC = 3 : 7 by volume) and Li foil as a negative electrode. Electrochemical properties of the LiNi1/2Mn3/2O4 composite electrodes prepared with different binders are compared in Li cells at 50 oC in Figure 1a. Reversible capacity of the composite electrode with non-fluorine PAN-based polymer reaches >130 mAh g-1 after the 30 cycles test, which is much better than that of PVdF. Moreover, Coulombic efficiency for charge/discharge cycles at 50 oC is effectively improved by using non-fluorine PAN-based polymer as the binder as shown in Figure 1b. A study on X-ray photoelectron spectroscopy reveals that the PAN-based binder has better coatability for active materials and conductive carbons compared with PVdF. Electrolyte decomposition at elevated temperatures is proposed to be suppressed by difference in coatability as the binder. Together with these results, we will further discuss mechanisms of improved electrochemical properties by using non-fluorine PAN-based binders and possibility of rechargeable lithium batteries with the high-voltage positive electrode.