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UWO team discovers new conductive phase during carbon-coating process of lithium iron phosphate; potential to improve performance

Researchers at The University of Western Ontario (UWO) have discovered a new conductive phase during the carbon-coating process of lithium iron phosphate (LFP) cathode material that can improve the electrochemical performance of lithium-ion batteries.

As described in an open-access paper in Nature Communications, the phase content is size-, temperature-, and annealing-atmosphere-dependent. The formation of this phase is related to the reducing capability of the carbon coating process. The researchers suggest that the finding can help to guide the control of the phase composition of carbon-coated lithium iron phosphate and to tune its quality during the manufacturing process.

41467_2018_3324_Fig1_HTML
Surface conductive phase formation during carbon coating process. (a) Upper panel: SEM images (left side), BSE images (middle and right); lower panel: EDS mapping of surface conductive phase formation on LiFePO4. (b) Schematic representation of new phase formation on LiFePO4. Scale bar, 1 mm (SEM image), 200 µm (BSE), and 50 µm (right side BSE and lower panel EDS images). Lie et al. Click to enlarge.

Lithium iron phosphate (LFP) is a widely used cathode material in lithium-ion batteries for EV applications. Nevertheless, there are still some underlying scientific challenges that are interesting, such as the interactions at the C/LFP interface, and the uniformity of the quality of LFP.

The researchers, led by Xueliang (Andy) Sun, professor of mechanical and materials engineering, have worked on the carbon coating process of LFP in collaboration with Johnson Matthey (previous Phostech) for a long time.

Surface carbon coating is often used to increase the electrical conductivity of LFP and has proven to be an effective strategy. However, an in-depth understanding of surface chemistry change during the carbon coating process remains elusive.

Carbon coating usually involves a strong reducing environment and often requires high temperature. This increases the reaction kinetics between the surface of LFP and the supplied carbon. As a result, secondary phases will form on the surface, thereby altering the electronic/ionic conductivity of LFP. Because the carbon coating process on LFP is a multi-component system at small scale, it is very difficult to define the presence of secondary phase, to map the distribution of secondary phase, to record the change of secondary phase and the control of secondary phase. Therefore, understanding formation mechanism for emergence of secondary phases would be highly beneficial, especially for LFP manufacturers.

—Liu et al.

Carbon coating at high temperature triggers the surface phase change on the LFP. The researchers found a new conductive phase, Fe2P, in LFP/C materials with the help of advanced characterization. Such a phase was highly dependent on size, temperature and atmosphere, which enabled the control of the quality and quantity of the conductive phase.

In addition, the conductive phase forms a conductive pathway to improve the electrochemical reaction. The authors proposed a mechanism from a thermodynamic point of view that was in good agreement with experimental results.

41467_2018_3324_Fig5_HTML
Schematic illustration of Fe2P phase formation during carbon coating process at reducing condition with regards to oxygen partial chemical potential. Liu et al. Click to enlarge.

Such a phase change phenomenon can be extended to other insulating materials with high temperature carbon coating process.

—Liu et al.

To understand the phase change phenomenon, the team worked together with Prof. T.K, Sham using synchrotron mapping techniques that can give the phase distributions on the surface of carbon coated LFP.

The UWO researchers believe their paper demonstrates that interface chemistry change at the active materials/coating layer plays a critical role in the electrochemical performance of electrode.

Resources

  • Yulong Liu, Jian Liu, Jiajun Wang, Mohammad Norouzi Banis, Biwei Xiao, Andrew Lushington, Wei Xiao, Ruying Li, Tsun-Kong Sham, Guoxian Liang & Xueliang Sun (2018) “Formation of size-dependent and conductive phase on lithium iron phosphate during carbon coating.” Nature Communications 9, Article number: 929 doi: 10.1038/s41467-018-03324-7

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