Characterization and Optimization of the Carbon Nanocoatings Onto LiFePO4 Particles for Lithium-Ion Batteries

Lithium iron phosphate (LiFePO₄, LFP), among the various materials under development for use as cathodes in lithium-ion batteries, has been so far studied and considered as the most promising candidate for this application. Olivine-structured LiFePO₄ has been first reported by Pahdi et. al. in 1997 [1] and has gained significant attention because not only exhibits good electrochemical performance (a high theoretical capacity of 170 mAh×g¹ and a high discharge potential) but also it has other good features as: good cycle performance, low price, environmental benignity, high thermal and chemical stability [2]. However, low electronic conductivity and low Li⁺ diffusion rate limits the application of this material for high-power battery. To overcome these limitations it is necessary to synthesize nanosized LiFePO₄ particles coated with carbon layer. A variety methods of preparation carbon coating were studied and existed because it is straightforward technology improving the conductivity of LiFePO₄. It is still the challenge to prepare homogenous, highly conductive thin layer of carbon that does not block the lithium ion transfer between LiFePO₄ and electrolyte [3].

The results of simple and environmental-friendly method of the carbon coatings on low-conductive cathode material powders have been developed, optimized and shown in this work. The carbon nanocoatings were prepared by wet impregnation process of precursor derived from hydrophilic polymer based on poly(N-vinylformamide) modified by pyromellitic acid [4] onto LiFePO₄ particles.

The existence of nano-LiFePO₄ was confirmed with X-ray powder diffraction (XRD). Transmission electron microscopy (TEM) was used to determine particle size and morphology of carbon nanocoated LFP material. The optimal calcination temperature of C/LFP composite precursors  were investigated from thermal gravimetry analyses (TGA/DTG/STDA). The properties of carbon nanocoatings were determined by Raman spectroscopy and electrical conductivity measurements. The process of formation C/LiFePO₄ nanocomposite significantly enhances the electrical conductivity of the material and improves its capacity retention and electrochemical performance.

                                       

References

[1] A.K. Padhi, K.S. Nanjundaswamy, J.B. Goodenough, J. Electrochem. Soc. 144 (1997) 1188

[2] Y.-D.Cho, G. Ting-Kuo Fey, H.-M. Kao, J. Power Sources 189 (2009) 256–262

[3] A. Örnekn, E. Bulut, M. Özacar, Ceramics International40(2014)15727–15736

[4] M. Molenda, R. Dziembaj, A. Kochanowski, E. Bortel, M. Drozdek, Z. Piwowarska, PAT.216549, Patent appl.: US 2011/015112