Electrochemical and Magnetic Properties of Li1-XCo1-XFexPO4 High Voltage Cathodes

Li_1-xCo_1-xFe_xPO₄ particles with x<0.10 were prepared by the sol gel method. The gel precursors were thermally decomposed at 350°C and then annealing at 550°C in air. They all were single phase of orthorhombic olivine compounds and the lattice parameters almost obeyed the Vegard’s law. The Moessbauer spectrum at 300K showed a paramagnetic doublet with d=0.44mm/sec and De=0.76mm/sec, and these parameters are characteristic to high spin-state Fe³⁺ in FePO₄. The antiferromagnetic Neel temperature shifted higher with an increase in X, due to strong exchange coupling via Fe³⁺. The effective magnetic moment derived form the Curie-Weiss law was raised with the Fe³⁺ substitution. It was confirmed from these experimental facts that they are the solid solution compounds between LiCoPO₄ and FePO_4. The electrochemical cycling of these cathode materials was evaluated with Li coin cell at the range from 3.5 to 5.0V under the CC-CV mode. They exhibited the charge/discharge plateau approximately at 4.9/4.7V. The initial discharge capacity increased (from 125 to 140mAh/g) with x up to 0.05 and then decreased (to 130mAh/g). On the other hand, the capacity retention after 20th cycles was significantly improved with the substitution. Hence, a partial substitution with Fe³⁺ may enhance the electrochemical activity of LiCoPO₄ high voltage cathode compound. Now, detail studies on the charge-discharge mechanism, such as the structural variation and the electronic state change of transition metals, are being carried out. These results will be presented and discussed at the conference. This work has been carried out under the support of Advanced Low Carbon Technology Research and Development Program (ALCA).