Elsevier

Ceramics International

Volume 37, Issue 7, September 2011, Pages 2215-2220
Ceramics International

Electrochemical properties of LiCoyMn2−yO4 synthesized using a combustion method in a voltage range of 3.5–5.0 V

https://doi.org/10.1016/j.ceramint.2011.03.071Get rights and content

Abstract

LiCoyMn2−yO4 (y = 0.00, 0.04 and 0.08) were synthesized using a combustion method, and the electrochemical properties were examined in the voltage range of 3.5–5.0 V. The XRD patterns of the synthesized samples were similar, and the samples had a spinel phase structure. The first charge capacity curves exhibited an inflection in the voltage range of 4.2–5.0 V, where it is believed that additional, previously unreported phase transition occurs. The voltage vs. x curves for the first to fifth cycle exhibited two distinct voltage plateaus, corresponding well to a two-phase reaction and a one-phase reaction, respectively, as reported previously. For the voltage range of 3.5–5.0 V, the first discharge capacity increased and the cycling performance improved as y increased. Among these samples, LiCo0.08Mn1.92O4 had the largest first discharge capacity of 132.5 mA h/g at 600 μA/cm2, and its cycling efficiency was 91.1% at the 15th cycle in the voltage range of 3.5–5.0 V.

Introduction

Transition metal oxides such as LiCoO2 [1], [2], [3], LiNiO2 [4], [5] and LiMn2O4 [6], [7], [8], [9], [10], [11], [12] have been investigated for their use as cathode materials in lithium secondary batteries. LiCoO2 has a large diffusivity and a high operating voltage, and can be easily prepared. Nevertheless, it contains an expensive element, Co. LiNiO2 has a large discharge capacity [13] and is relatively excellent from the view points of economics and environment. However, its preparation is very difficult compared with those of LiCoO2 and LiMn2O4. LiMn2O4 does not have a good cycling performance, but it is very cheap and does not bring about environmental pollution.

LiMn2O4 is usually synthesized through a solid-state reaction method which uses mechanical mixing followed by high-temperature sintering. LiMn2O4 compounds are made from stoichiometric amounts of Li salts such as LiOH, LiNO3, and Li2CO3, mixed with manganese oxides [chemical manganese dioxides (CMD) or electrochemical manganese dioxides (EMD)] [14], [15], [16], [17], [18], [19].

The solid-state reaction method has numerous disadvantages: difficulty in a homogeneous formation of phase, formation of particles with non-uniform size and shape, and difficulty in the formation of a compound with stoichiometric composition. On the other hand, the homogeneous mixing of the starting materials can be accomplished using the combustion method because in this method nitrates as starting materials and urea as a fuel are mixed in distilled water by a magnetic stirrer. This may lead to good crystallinity and a homogeneous particle size when the sample is synthesized.

Liu et al. [20] reported that partial substitution of a small quantity of Co for the Mn can significantly improve the cycling behavior of LiMn2O4. Shen et al. [21] reported that chemical substitution of Co+3 for Mn+3 in LiMn2O4 improved the cathodic properties and the efficiency in maintaining electrochemical capacity over a large number of cycles without sacrificing the initial reversible capacity and performance at temperature below room temperature.

In this work, we chose Co as a substituting element for Mn in LiMn2O4 to improve the electrochemical properties of LiMn2O4. We synthesized LiCoyMn2−yO4 (y = 0.00, 0.04 and 0.08) via the combustion method using nitrates and urea as starting materials and examined their electrochemical properties in a voltage range of 3.5–5.0 V. The upper limit of the voltage range, 5.0 V, is quite a higher voltage than that usually used in research on the electrochemical properties of LiMn2O4.

Section snippets

Experimental

LiCoyMn2−yO4 (y = 0.00, 0.04 and 0.08) were synthesized using a combustion method. Starting materials were LiNO3, Mn(NO3)2·6H2O, Co(NO3)2·6H2O, and NH2CONH2 (urea) with purities of 98%. The starting materials in the desired compositions were mixed homogeneously by a magnetic stirrer. The mixed samples were in light reddish brown color. These mixed samples were preheated at 400 °C for 4 h, and then calcined two times at 750 °C for 24 h. The heating rate was 100 °C/h and the cooling rate was 50 °C/h.

Results and discussion

The XRD patterns of LiCoyMn2−yO4 (y = 0.00, 0.04 and 0.08) synthesized via the combustion method were obtained. They were similar, but the diffraction angles increased as y increased, indicating that the lattice parameter decreased as y increased. Samples were identified as being in the spinel phase with a space group of Fd3¯m. The lattice parameters of these samples were larger than that of LiMn2O4 synthesized using the solid-state method [22].

The SEM photographs of LiMn2O4 calcined for 24 h

Conclusions

LiCoyMn2−yO4 (y = 0.00, 0.04 and 0.08) were synthesized using the combustion method by preheating at 400 °C for 4 h and then calcining two times at 750 °C for 24 h. The XRD patterns of LiCoyMn2−yO4 (y = 0.00, 0.04 and 0.08) synthesized via the combustion method were similar, and the samples had a spinel phase structure. The first charge capacity curves exhibited an inflection in the voltage range of 4.2–5.0 V, where it is believed that additional, previously unreported phase transition occurs. For the

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