Synthesis and Electrochemical Performance of Olivine NaFePO4/Grahene Composite As Cathode Materials for Sodium Ion Batteries
Authors: Li Dan , Liu Hua Kun , Guo Zaiping 
Published: February 2020

Lithium ion batteries have been attracting widespread interests over recent decades to meet the urgent needs for applications in both fully electric vehicles and hybrid electric vehicles.1, 2 However, the lithium resources are unevenly distributed and limited,3 resulting the drastically increase of the cost for lithium ion batteries in the large-scale applications. Sodium ion batteries are considered as options for large scale energy storage due to the abundance of sodium and low cost. In terms of the cathode materials for the sodium ion batteries, phosphate compounds have been paid more attentions because of the high redox potential and good structural and thermal stability. The material with highest theoretical specific capacity (154 mA h g-1) is olivine NaFePO4 among the phosphate polyanion cathode materials. However, the thermodynamically stable structure of NaFePO4 is not olivine, but maricite. The maricite shows one-dimensional, edge-sharing FeO6 octahedra and no carionic channels, giving rise to a different connectivity of the Fe and Na octahedra compared to olivine LiFePO4 which blocks Na-ion migration pathways, resulting in a structure that is not amenable to Na+ (de)insertion.4, 5

There are two methods to obtain olivine NaFePO4, chemical delithiation and then electrochemical sodiation of olivine LiFePO4, and electrochemical delithiation and sodiation of olivine LiFePO4. In terms of the chemical delithiation, the oxidizing agents are NO2BF4 in acetonitrile, and bromine in dissolved in water, which are used to obtain the olivine FePO4. For the electrochemical sodiation, the Na insertion is realized by using FePO4 as the cathode and metallic Na foil as anode. Therefore, there are few paper reported on the olivine phase NaFePO4, due to the difficulty to get it directly by standard routes.
Compared to the LiFePO4, the diffusion coefficient for Na-ion in NaFePO4 is 1-2 orders of magnitude lower than that of lithium-ion in LiFePO4, and the contact and charge transfer resistance in NaFePO4 is 10 times higher than that of in LiFePO4.6 To circumvent these problems, the strategy of using nanosized structures can be applied to reduce the diffusion distance of Na ions. Moreover, introducing a conductive coating layer, decorating the nanoparticles on certain substrate and embedding the nanostructures in a conductive matrix can be used to increase the electric conductivity and prevent the nanostructures from aggregation. Due to the excellent electric conductivity and mechanical properties, graphene is widely used in the synthesis of electrode materials to hinder the growth and aggregation of nanostructures, offer easy pathways for electrons, and provide a good conductive matrix for active materials.
In this work, a composite cathode material for sodium ion battery applications, olivine NaFePO4/graphene, is prepared by chemical delithiation and then electrochemical sodiation of olivine LiFePO4 nanoparticles in situ grown on graphene sheet. The nanosized LiFePO4 particles were homogeneously distributed on the graphene to inhibit the restacking of the neighbouring graphene sheets. The NaFePO4/graphene composite obtained by chemical delithiation and electrochemical Na ions insertion exhibits improved cycling performance and rate capability, indicating the important role of graphene, which not only facilitates the formation of nanosized and uniformly distributed LiFePO4 precursor, but also increases the electrical conductivity of the composite material. The electrochemical performance of this NaFePO4/graphene was evaluated and showed promising results.

Acknowledgements

This work is supported by an Australian Research Council (ARC) Discovery project (DP1094261).

References

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