Synthesis and Electrochemical Performance of Olivine NaFePO4/Grahene Composite As Cathode Materials for Sodium Ion Batteries

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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