Electrochemical performance of overlithiated Li_1+xNi_0.8Co_0.2O₂: structural and oxidation state studies

Abstract

Pure, layered compounds of overlithiated Li_1+xNi_0.8Co_0.2O₂ (x = 0.05 and 0.1) were successfully prepared by a modified combustion method. XRD studies showed that cell parameters of the material decreased with increasing the lithium content. SEM revealed that the morphology of particles changed from rounded polyhedral-like crystallites to sharp-edged polyhedral crystals with more doped lithium. EDX showed that the stoichiometries of Ni and Co agrees with calculated synthesized values. Electrochemical studies revealed the overlithiated samples have improved capacities as well as cycling behavior. The sample with x = 0.05 shows the best performance with a specific capacity of 113.29 mA∙h∙g^{-<?Pub Caret1?>1} and the best capacity retention of 92.2% over 10 cycles. XPS results showed that the binding energy of Li 1s is decreased for the Li doped samples with the smallest value for the x = 0.05 sample, implying that Li⁺ ions can be extracted more easily from Li_1.05Ni_0.8Co_0.2O₂ than the other stoichiometries accounting for the improved performance of the material. Considerations of core level XPS peaks for transition metals reveal the existence in several oxidation states. However, the percentage of the+3 oxidation state of transition metals for the when x = 0.1 is the highest and the availability for charge transition from the+3 to+4 state of the transition metal during deintercalation is more readily available.

Keywords

overlithiation / LiNi_0.8Co_0.2O₂ / interstitial doped / Li_1.05Ni_0.8Co_0.2O₂ / Li_1.1Ni_0.8Co_0.2O₂

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Acknowledgements

The authors would like to acknowledge the Faculty of Applied Science Universiti Teknologi MARA Malaysia for funding support for travel grant and also University of Malaya for research support with internal grant UMRG (RG220-12AFR). Grateful thanks are also accorded to the Institute of Science for funding of the research.

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2014 Higher Education Press and Springer-Verlag Berlin Heidelberg