Synthesis and Cathodic Properties of LiCo_1-yRh_yO₂ (0 ≤ y ≤ 0.2) and LiRhO₂

Compounds of the formula LiCo_1-yRh_yO₂ (0.0 ≤ y ≤ 0.20 and y = 1.0) have been synthesized by high temperature solid-state reaction and characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy, and infrared spectroscopy. Single-phase materials were obtained for y ≤ 0.10 and y = 1.0 as observed by XRD. However, IR and electrochemical data indicate that the true solid solubility may be y ≤ 0.05. Their cathodic behavior in coin cells with Li metal as anode and a liquid electrolyte has been examined. Cyclic voltammograms and galvanostatic charge/discharge curves show that for y = 0.05, the phase transformation occurring at x = 0.5 in Li_1-xCoO₂ is suppressed and the deintercalation voltage is decreased. The compositions with y = 0.05 and 0.10 show improved cathodic behavior and discharge capacity retention compared to LiCoO₂ (y = 0) at 0.14C rate in the voltage range 2.7-4.3 V up to 18 cycles. Electrochemical studies on LiRhO₂ show that phase transitions occur on charge/ discharge cycling (varying x in Li_1-xRhO₂) in the voltage window 3.2-3.7 V and these are reversible. This compound shows an initial charge capacity of 162 mAh/g when charged to 4.0 V (vs. Li metal), corresponding to x = 0.85 in Li_1-xRhO₂. After an initial loss of 40 mAh/g in capacity, the discharge capacity retention corresponds to 90% after 18 cycles with respect to the initial discharge capacity in the voltage window, 2.7-4.0 V.