Electrochemical Properties of Hydrothermally Obtained LiCo[sub 1−x]Fe[sub x]O[sub 2] as a Positive Electrode Material for Rechargeable Lithium Batteries

Abstract

Lithium cobalt oxide (LiCoO 2 ) with ␣-NaFeO 2 -type (layered rock-salt) structure is now extensively used as a positive electrode material in commercial lithium-ion batteries. In this structure, LiO 6 and CoO 6 octahedra share their corners and stack alternatively along the c axis direction, which allows two-dimensional diffusion of Li ions during electrochemical deintercalation and intercalation. Because the Co in LiCoO 2 is a relatively rare metal, dopants such as Ni, 1 Cr, 2 Mn, 3 B, 4 Al 5 have been used to reduce material costs. Although Fe doping 6,7 is among the effective methods to accomplish this aim, the incorporation of Fe has been limited to 10% per formula unit (LiCo 0.9 Fe 0.1 O 2 ) by solid-state reaction at high temperature (800ЊC 6,7 ). Although Alcantara et al. 7 claimed that the solubility limit of Fe in LiCoO 2 was 20%, a small nonindexable peak near the 101 peak (2 ϭ 36Њ) was observed for LiCo 0.8 Fe 0.2 O 2 . This limitation originates from sample contamination by the formation of cubic ␣-LiFeO 2 with a cation-disordered structure, as for the case of Fe-doped LiNiO 2 , 8 because LiFeO 2 , isostructural with LiCoO 2 (layered-LiFeO 2 ), is a metastable form and can be obtained by a softchemical synthetic route such as ion-exchange and hydrothermal reactions. A soft-chemistry method is necessary to form rhombohedral Fe-doped LiCoO 2 with high Fe content.Charge-discharge voltages and capacities of Li/LiCoO 2 cells are sensitive to the starting materials and LiCoO 2 preparation conditions. 9-12 Some structural models for Li-extracted LiCoO 2 have been reported. 9,10 Concerned about Fe doping effects into LiNiO 2 , Reimers et al. 8 showed that a single-phase layered structure could be obtained up to LiNi 0.8 Fe 0.2 O 2 and that the charge and discharge capacities decreased with increasing Fe content. Recently, Prado et al. 13 reported that a homogeneous solid solution could be obtained up to LiNi 0.7 Fe 0.3 O 2 , a rhombohedral symmetry was maintained up to Li 0.4 Ni 0.9 Fe 0.1 O 2 during electrochemical oxidation, and some of the trivalent iron was oxidized to the tetravalent state after electrochemical charging of a Li/LiNi 0.9 Fe 0.1 O 2 cell. Although Alcantata et al. examined the cation distribution and electrochemical properties of Fe-doped LiCoO 2 , 7 no systematic study of changes of crystal structure and the valence state of 3d metals after charge-discharge cycle tests has been reported for Li/LiCo 1Ϫx Fe x O 2 cells.We have successfully obtained metastable layered LiFeO 2 14 and layered LiMnO 2 15 with an ␣-NaMnO 2 structure similar to ␣-NaFeO 2 using a mixed-alkaline hydrothermal reaction below 300ЊC. Up to 25% Fe-doped LiCoO 2 could be prepared by hydrothermal reaction at 220ЊC. 16 Structural changes and valency of Co and Fe during charge-discharge tests of Li/LiCo 1Ϫx Fe x O 2 cells were studied to understand the Fe doping effect on the LiCoO 2 positive electrode. ExperimentalIron-doped LiCoO 2 samples (LiCo 1Ϫx Fe x O 2 nominal x ϭ 0.00 (sample A), 0.05, 0.10, 0.15, and 0.25) were ...

Layered LiCo1−x Mg x O2 (x = 0.0, 0.1, 0.2, 0.3 and 0.5) cathode materials for lithium-ion rechargeable batteries

“…The high cost and toxicity of Co, however, makes the use of LiCoO 2 restricted as a commercial cathode. To reduce the cost and to improve the cell voltage and specific energy, other transition metals such as Cr [7], Mn [8], Fe [9][10][11], Ni [11][12][13] and Rh [14] are used to partially substitute Co in LiCoO 2 . Non-transition metals such as Al, Ca and Mg, which have fixed oxidation states, are also used to particularly substitute Co. Ceder et al [15] have predicted, from first principles and also shown by experiment, that substitution of Al increases the potential as well as the performance of LiCoO 2 .…”

Nuclear and Magnetic Structure of Layered LiFe1−xCoxO2 (0≤x≤1) Determined by High-Resolution Neutron Diffraction

“…This LiFeO phase was obtained using a hydrothermal reaction and it was found that the Li//LiFeO cell exhibited good charge and discharge reversibility between 1.5 and 3.0 V vs Li/Li>. (12) and by AlcaH ntara using a solid-state high-temperature reaction at 8003C (13). Using an indirect approach by Na>/Li> ionexchange, we succeeded in the preparation of the solid solution LiFe \V Co V O in the whole composition range (04x41) and reported its structural, physicochemical, and electrochemical characterization (14).…”

Improved Electrochemical Investigation of Combustion Synthesized Cd-doped LiCoO₂ Powders

“…Such modifications led to either improved capacity retention characteristics by suppressing the surface reactions of LiCoO 2 particles or higher achievable specific capacity by increasing the upper limit of the intercalation voltage through the formation of stronger M-O bonds and to stabilize the layered structure in its fully delithiated state [6]. Intensive investigations have been carried out on doped LiCo 1-y M y O 2 oxides (M = Ti, Cr, Mn, Ni, Fe, Cu, Fe, Bi, Zn, Mo) [7][8][9][10][11][12][13][14], which show interesting structural and electrochemical properties. Doping with transition metals such as cadmium has gained increasing interest for the following reasons: (i) the low cost and low toxicity, (ii) the fact that cadmium substitution for transition metal oxides will increase the capacity, (iii) the cadmium doping stabilizes the layered structure and extends the cyclability and enhances the capability of the electrochemical cells.…”