In Na-ion batteries (NIBs), the net energy density decreases because of the intrinsically low practical capacity and limited Na ion migration compared to that of Li-ion batteries (LIB). So, it is essential to use a high-voltage cathode to compensate these problems [1]. Pyrophosphates are one of the candidates that can be used as high-voltage cathodes [2]. In pyrophosphate-based materials, it is well-known that the operating voltage can be increased if the transition metals (TMs) such as Mn, Co, or Ni are used instead of Fe under electrochemical TM²⁺_/TM³⁺ redox reactions [3,4]. Particularly, Na₂CoP₂O₇ with triclinic polymorph exhibits an average operating voltage of ~4.3 V (vs. Na/Na⁺), which is one of the NIB cathode materials that electrochemically react with Na ions at high-voltage ranges [5]. Unfortunately, however, it is hard to synthesize a pure triclinic polymorph because of its metastability in synthesizing process. So, preparing a highly stable and pure triclinic-polymorph Na₂CoP₂O₇ is challenging work for high-voltage cathode in NIBs.

In this work, we analyze an appropriate elemental ratio of Na/Co in Na_2-2xCo_1+xP₂O₇ to prepare a stable triclinic polymorph, and determine suitable synthetic conditions in each bulk and nano compound. The formation of rose or blue phase Na₂CoP₂O₇ with regard to their particle size is schematically illustrated in Fig. 1. Also, the electrochemical performance is measured for the high-voltage NIB cathode. Na_1.8Co_1.1P₂O₇ nanoparticles with C (Na_1.8Co_1.1P₂O₇-NPs/C) exhibited the discharge capacity of 90 mA h g^-1 at the 30^th cycle. High operating voltage and good specific capacity could lead to the significant increase in energy density to ~400 Wh kg^-1, which is the highest value among polyanionic NIB cathode materials.

Fig. 1. Schematic illustration for the formation of rose or blue phase Na₂CoP₂O₇ with regard to their particle size.

References

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[2] P. Barpanda, S.-i. Nishimura, A. Yamada, Adv. Energy Mater. 2 (2012) 841-859.

[3] G. Hautier, A. Jain, S. P. Ong, B. Kang, C. Moore, R. Doe, G. Ceder, Chem. Mater. 23 (2011) 3495-3508.

[4] M. Tamaru, S. C. Chung, D. Shimizu, S.-i. Nishimura, A. Yamada, Chem. Mater. 25 (2013) 2538-2543.

[5] H. Kim, C. S. Park, J. W. Choi, Y. Jung, Angew. Chem. Int. Ed. 55 (2016) 6662-6666.