Highly Stable Triclinic Polymorph in Nanoscale Na2-2xCo1+XP2O7/C for High-Voltage Na-Ion Battery Cathode

Abstract

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 Na2-2xCo1+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. Na1.8Co1.1P2O7 nanoparticles with C (Na1.8Co1.1P2O7-NPs/C) exhibited the discharge capacity of 90 mA h g-1 at the 30th 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.