Preparation and properties of LiCoyMnxNi1−x−yO2 as a cathode for lithium ion batteries
Introduction
Layered LiNiO2, which has been used as a cathode material, has a relatively low cost and high capacity when used in lithium ion batteries [1], [2], [3], [4], [5], [6], [7], [8], [9]; however, there are two important problems for such a cathode. One problem is the difficulty in the preparation of electroactive LiNiO2 because its discharge capacity significantly depends on the Li/Ni ratio in LixNiO2, partial cation mixing and the oxidation state of nickel. The other problem is the poor cycle life of the LiNiO2 electrode when it is charged to a high voltage (4.3 V vs. Li+/Li) for withdrawing a higher capacity.
We solved these problems by the substitution of Ni with Mn. LizMn0.1Ni0.9O2 prepared in O2 shows a constant discharge capacity of ca. 160 mA h/g over the wide range of z from 0.98 to 1.10 [10], [11]. Capacity fading of this compound is lower than that of LiNiO2. Furthermore, electroactive LiMnxNi1−xO2 can be prepared in air for x≧0.2 though its capacity decreases by about 10%.
A convenient way to overcome the relatively lower capacity of LiMnxNi1−xO2 prepared in air would be the Co doping of LiMnxNi1−xO2 because the presence of cobalt stabilizes the structure in a strictly two-dimensional fashion. We have found that the Co doping of LiMn0.2Ni0.8O2 increases the discharge capacity when such compounds are prepared in air. In this paper, we report the optimum conditions for the synthesis of the electroactive LizCoyMnxNi1−x−yO2, its electrochemical properties and the valence of Mn in it.
Section snippets
Experimental
A manganese compound, γ-MnOOH (Tohso), was used as the manganese source. It is a very fine, needle-like particle with a 0.1–0.2 μm diameter. The cobalt source was Co3O4. LiOH·H2O (2.06 g), Ni(OH)2 (3.48 g), γ-MnOOH (0.88 g) and Co3O4 (0.20 g) were mixed (y=0.05, and x=0.2) and ground using a mortar and pestle. The mixture was then pressed at 800 kg/cm2. The obtained disks were heated in air at 700–900°C for 20 h.
Crystallographic characterization of the samples was carried out using a Rigaku
Preparation of LiCoyMnxNi1−x−yO2 in air
We have already reported that the electroactive LiMn0.2Ni0.8O2 with a discharge capacity of 140 mA h/g can be prepared in air containing CO2 and moisture [10], [11]. Cobalt is doped into the LiMn0.2Ni0.8O2 system in order to increase the capacity because the doping of cobalt into the lithium–nickel oxide matrix stabilizes its layered structure [13].
Fig. 2 shows the XRD patterns of the products obtained by heating the LiOH·H2O–Ni(OH)2–γ-MnOOH–Co3O4 mixture at 700–900°C. The Li2MnO3 phase is
Conclusion
Electroactive LiCoyMn0.2Ni0.8−yO2 compounds with discharge capacities greater than 155 mA h/g were successfully prepared in air. The capacity was the same as that of LiMn0.2Ni0.8O2 in oxygen. Excess lithium and the Co doping level 0.05≦y≦0.2 did not significantly affect the discharge capacity of LizCoyMn0.2Ni0.8−yO2.
The doping of Co into LiMn0.2Ni0.8O2 accelerated the oxidation of the transition metal ion and suppressed partial cation mixing. The valence of the manganese ion in LiCo0.05Mn0.2Ni
Acknowledgements
The authors would like to thank Mr. T. Konishi of the Asahi Chemical Analytical Research Laboratory for the measurement of the HRXRF spectra and Mr. S. Ikeda of the Saga University Instrumental Analysis Center for the use of their instruments. The present work was partly supported by a grant-in-aid for scientific research from the Ministry of Education, Science and Culture, Japan.
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