Electrochemical Investigations of Cobalt-Doped LiMn2O4 as Cathode Material for Lithium-Ion Batteries

A wide range (y = 0.05—0.33) of Co-doped LiCo5Mn2_504 spinels were synthesized and electrochemically characterized. These Co-doped spinels showed improved specific capacity and capacity retention over pure spinels. Electrochemical impedance spectroscopy and the linear polarization resistance technique were used to determine the transport and electrochemical kinetic parameters of Co-doped spinels. The presence of Co in the spinel inhibits the passivation process occurring on the surface of the cathode. Also, Co increases the exchange current density and facilitates the charge-transfer reaction of the active material. The lower self-discharge observed for Co-doped spinels was attributed to their low surface areas. The cumulative capacity loss estimated for a pure spinel resulting from self-discharge in the first 30 h was 3 and 6 times larger than those estimated for Co-doped spinels with y = 0.05 and y = 0.16 in LiCo9Mn2_004, respectively. Introduclion Layered LiCoO2 is currently used as a cathode material for production of commercial, high-energy-density lithium-ion batteries.12 Because of the high cost of Co and its toxicity, the three-dimensional LiMn2O4 spinel phase was studied extensively to substitute LiCoO2 as the cathode material for Li-ion batteries.3-0 The Li insertion—deinsertion processes occur in the spinel phase in two composition ranges resulting in two voltage plateaus at 4 and 3 V vs the Li/Lit reference electrode.9"° Attempts to substitute LiMn2O4 for LiCoO2 in commercial Li-ion batteries have not been successful due to the lower specific capacity of the spinel and the fast capacity fade upon cycling. 13 Efforts have been made to improve cycle life by controlling the capacity fading in LiMn2O4. 35,10-17 The capacity fading has been attributed to spinel dissolution,1' the Jahn—Teller effect,'2 and lattice instability13 at high oxidation levels. Baochen et al.15 found that cobalt-doped LiCo0Mn2,04 improved cycling performance. According to Guohua et al.,'2 LiCo116Mn,11604 showed good cycle performance with an energy density of 370 Wh/kg at the 300th cycle. Recently, Sanchez and Tirado1t reported a new cobalt-substituted Li20-yMnO2 (y = 4) spinel phase reversible in a 3.3—2.3 V potential window. The capacity for the first cycle was 165 mAh/g. However, the initial capacity decreased sharply during the first five cycles, reaching values of about 110 and 85 mAh/g after 100 and 200 cycles, respectively. In this study, a wide range (y = 0.05—0.33) of Co-doped LiCo9Mn2.O4 spinels was synthesized and electrochemically characterized. Electrochemical impedance spectroscopy (EIS) and the linear polarization resistance technique (LP) -were used to determine the transport and electrochemical kinetic parameters of pure and Co-doped spinels. The capacity fade rates of LiMn2O4 and LiCo5Mn25O4 were studied and the active material loss during cycling was determined. Also, the effect of Co content on the selfdischarge of pure and Co-doped LiMn2O4 was investigated. Experimental LiMn2O4 and Co-doped LiMn2O4 cathodes were prepared by a solid-state reaction between lithium carbonate (Aldrich, 99%), manganese carbonate (Aldrich, 99.9%), and cobalt oxalate hydrate (Aldrich, 99%). The mixture was preheated at 600 °C for 6 h and then heated at 750 °C for 3 days in air with intermittent grinding, followed by slow cooling (about 2 °C/min) to cool the sample to ambient temperature. LiCoMn25O4 compounds with y = 0, 0.05, 0.06, 0.08, 0.16, and 0.33 were prepared. * Electrochemical Society Student Member. * * Electrochemical Society Active Member. The active material was mixed with carbon black and Teflon binder in a ratio of LiCo0Mn22O4:carbon black: Teflon = 100:10:5 by weight and pressed into a thin film. Scanning electron microscopy (SEM) and X-ray diffraction (XRD) studies were carried out to determine the particle size and the structure of the synthesized cathode materials. Electrochemical characterization of the cathode materials was carried out using Swagelok three-electrode cells presented in Fig. 1. The anOde and reference electrodes were disks of lithium foil, and a sheet of glass fiber acted as the separator. The electrolyte contained 1 M LiPF6 dissolved in a mixture of propylene carbonate (PC), ethylene carbonate (EC), and dimethyl carbonate (DMC) in a ratio 1:1:3. The T-cell was assembled in a glove box filled with argon. The cathode had a diameter of 1.25 cm (an area of 1.23 cm2) and a thickness of 70 p.m with a total mass of 20 mg. It was heated in a minifurnace at 300 °C for 2 h in the glove box before assembling. The cathode and anode separation was approximately 3 mm. After the cell was assembled, it was left in the dry box for half an hour, enabling the electrolyte to disperse into the porous structure of the cathode. The cells were tested with a Bitrode (Bitrode Co., MI, USA) cycler with cutoff potentials of 3 and 4.3 V. Charge/discharge curves were obtained galvanostatically using a current density of 0.1 mA/cm2. Cyclic voltammograms were obtained using a scan rate of 0.06 mV/s over a potential range of 3.0 to 4.3 V vs Li/Lit reference electrode. EIS experiments were carried out at different states of charge on both pure spinel and Co-doped spinel electrodes with y = 0.160 in LiCo0Mn2_0O4. The impedance data generally covered the frequency range from 0.002 Hz to 100 kHz with an ac voltage signal of 5 mV, which ensured the electrode system to be under minimum perturbation. The Co-doped spinels were characterized by XRD using a Rigaku 405S5 diffractometer with