Unraveling the formation mechanism of NaCoPO4 polymorphs

Phosphate compounds are one of the attractive classes of cathodes for their potential application in rechargeable Na-ion batteries (NIBs) due to their high insertion voltages and stabilities. Unlike olivine-LiMPO4 (M ​= ​Fe, Mn, Co, Ni) cathodes, maricite-NaMPO4 phases are electrochemically inactive due to lack of sodium diffusion channels. Interestingly, cobalt bearing compounds exhibit rich polymorphism (α-, β-, ABW- and Red-NaCoPO4). However, the preparation of NaCoPO4 single phases is challenging due to their closer enthalpies of formation. In this study, we have attempted to understand the formation mechanism of NaCoPO4 polymorphs during solid state reaction using two different cobalt precursors (Co(OH)2 and CoCO3). The temperature dependent in-situ powder X-ray Diffraction (PXRD) studies showed the formation of β-NaCoPO4 at a much lower temperature compared to the one reported in the literature, whilst we have discovered a new high temperature polymorph γ-NaCoPO4. The choice of precursors was also found to be important for the preparation of ABW-NaCoPO4, i.e., the rapid decomposition of Co(OH)2 has resulted in its formation whilst the high stability of CoCO3 prohibited the same. The order of the formation of the NaCoPO4 polymorphs during heating was found to be ABW→ β → α → γ. During normal cooling, γ-NaCoPO4 transformed primarily into α-polymorph whilst quenching in liquid N2 resulted in the formation of β-NaCoPO4. The as-synthesized β- and ABW-NaCoPO4 phases exhibited high sodium (de)-intercalation voltages (~4.2 and 4.5 ​V vs. Na+/Na0, respectively), albeit with limited reversible capacities. This study is expected to open new avenues to harvest metastable polymorphs based on other transition metals (Fe, Mn and Ni).