Lithium manganese oxide (LMO, LiMn₂O₄) of spinel structure is very promising as a cathode material for secondary lithium ion batteries. This compound has several advantages like: low cost and easy preparation, non-toxicity, high discharge potential (4V vs. lithium metal), a satisfactory practical capacity (120 mAh/g), high-energy density and low self-discharge. One of the drawbacks of lithium manganese oxide is its modest electronic conductivity. There are several ways of enhance it: i) introducing metal particles onto LiMn₂O₄ internal surfaces, ii) coating the spinel particles by conducting polymers, iii) the most popular - using carbon either as thin layers or mixing as-synthesized LiMn₂O₄ with carbon species.

In our studies we used graphene oxide (GO) as a carbon species. The pristine nanocrystalline LiMn₂O₄ powder was synthesized by modified sol-gel method [1-3]. Graphene oxide was prepared by a modified Hummers method [4,5]. The wet low temperature chemical method was used to modify the LMO grains using graphene oxide.

The structure and morphology of the synthesised powders were characterized by: X-ray powder diffraction (XRD), Raman spectroscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). The electrochemical charge-discharge tests were performed in three electrode cells with LiMn₂O₄/n-GO as working and lithium as a reference and counter electrode. A lithium hexafluorophosphate LiPF₆ in a mixture of ethylene and dimethyl carbonates (1:1) was used as an electrolyte. The working and counter electrode was detached by Celgard 2400 separator. Every cell was cycled using constant current mode in potential range between 3.5 V and 4.5 V Charge – discharge current rates for LMO/n-GO tests varied from 1 C to 30 C, where 1 C corresponds to current density of 148 mA/g.

Acknowledgments

This work was supported by The National Centre for Research and Development through the research grant PBS1 (contract no. PBS1/A1/4/2012).

References

[1] B. Hamankiewicz, M. Michalska, M. Krajewski, D. Ziolkowska, L. Lipińska, M. Kamińska, A. Czerwinski, The effect of electrode thickness on electrochemical performance of LiMn₂O₄ cathode synthesized by modified sol-gel method, Solid State Ionics 262 (2014) 9-13.

[2] M. Michalska, L. Lipińska, M. Mirkowska, M. Aksienionek, R. Diduszko, M. Wasiucionek, Solid State Ionics 188 (2011) 160.

[3] Monika Michalska, Ludwika Lipińska, Ryszard Diduszko, Marta Mazurkiewicz, Artur Małolepszy, Leszek Stobinski, Krzysztof J. Kurzydłowski, Physica Status Solidi C 8 No. 7–8 (2011) 2538.

[4]. W. S. Hummers, R. E. Offeman, J. Am. Chem. Soc., 80 (6) (1958) 1339.

[5] Ł Majchrzycki, M. Michalska, M. Walkowiak, Z. Wiliński, L. Lipińska, Polish Journal of Chemical Technology 15 3 (2013) 15.

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