Synthesis, structural and electrochemical properties of sodium nickel phosphate for energy storage devices

Electrochemical energy production and storage at large scale and low cost, is a critical bottleneck in renewable energy systems. Oxides and lithium transition metal phosphates have been researched for over two decades and many technologies based on them exist. Much less work has been done investigating the use of sodium phosphates for energy storage. In this work, the synthesis of sodium nickel phosphate at different temperatures is performed and its performance evaluated for supercapacitor applications. The electronic properties of polycrystalline NaNiPO₄ polymorphs, triphylite and maricite, t- and m-NaNiPO₄ are calculated by means of first-principle calculations based on spin-polarized Density Functional Theory (DFT). The structure and morphology of the polymorphs were characterized and validated experimentally and it is shown that the sodium nickel phosphate (NaNiPO₄) exists in two different forms (triphylite and maricite), depending on the synthetic temperature (300-550 °C). The as-prepared and triphylite forms of NaNiPO₄vs. activated carbon in 2 M NaOH exhibit the maximum specific capacitance of 125 F g^-1 and 85 F g^-1 respectively, at 1 A g^-1; both having excellent cycling stability with retention of 99% capacity up to 2000 cycles. The maricite form showed 70 F g^-1 with a significant drop in capacity after just 50 cycles. These results reveal that the synthesized triphylite showed a high performance energy density of 44 Wh kg^-1 which is attributed to the hierarchical structure of the porous NaNiPO₄ nanosheets. At a higher temperature (>400 °C) the maricite form of NaNiPO₄ possesses a nanoplate-like (coarse and blocky) structure with a large skewing at the intermediate frequency that is not tolerant of cycling. Computed results for the sodium nickel phosphate polymorphs and the electrochemical experimental results are in good agreement.