Unraveling the formation mechanism of NaCoPO$_{4}$ 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-LiMPO$_4$ (M ​= ​Fe, Mn, Co, Ni) cathodes, maricite-NaMPO$_4$ phases are electrochemically inactive due to lack of sodium diffusion channels. Interestingly, cobalt bearing compounds exhibit rich polymorphism (α-, β-, ABW- and Red-NaCoPO$_4$). However, the preparation of NaCoPO$_4$ single phases is challenging due to their closer enthalpies of formation. In this study, we have attempted to understand the formation mechanism of NaCoPO$_4$ polymorphs during solid state reaction using two different cobalt precursors (Co(OH)$_2$ and CoCO$_3$). The temperature dependent in-situ powder X-ray Diffraction (PXRD) studies showed the formation of β-NaCoPO$_4$ at a much lower temperature compared to the one reported in the literature, whilst we have discovered a new high temperature polymorph γ-NaCoPO$_4$ The choice of precursors was also found to be important for the preparation of ABW-NaCoPO$_4$, i.e., the rapid decomposition of Co(OH)2 has resulted in its formation whilst the high stability of CoCO$_3$ prohibited the same. The order of the formation of the NaCoPO$_4$ polymorphs during heating was found to be ABW$→ β → α → γ$. During normal cooling, γ-NaCoPO$_4$ transformed primarily into α-polymorph whilst quenching in liquid N$_2$ resulted in the formation of β-NaCoPO$_4$. The as-synthesized β- and ABW-NaCoPO$_4$ phases exhibited high sodium (de)-intercalation voltages (~4.2 and 4.5 ​V vs. Na$^+$/Na$^0$, 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)