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Unraveling the formation mechanism of NaCoPO4 polymorphs

Journal Article · · Journal of Solid State Chemistry
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  1. Jawaharlal Nehru Centre for Advanced Scientific Research, Jakkur, Bangalore (India)
  2. Argonne National Lab. (ANL), Argonne, IL (United States)
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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 gamma-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 →$$\gamma$$. During normal cooling, gamma-NaCoPO4 transformed primarily into alpha-polymorph whilst quenching in liquid N-2 resulted in the formation of β-NaCoPO4 The as-synthesized beta- and ABW-NaCoPO4 phases exhibited high sodium (de)-intercalation voltages (similar to 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).

Research Organization:
Argonne National Laboratory (ANL), Argonne, IL (United States)
Sponsoring Organization:
Government of India - Department of Science and Technology; USDOE
Grant/Contract Number:
AC02-06CH11357
OSTI ID:
1840566
Journal Information:
Journal of Solid State Chemistry, Journal Name: Journal of Solid State Chemistry Vol. 293; ISSN 0022-4596
Publisher:
ElsevierCopyright Statement
Country of Publication:
United States
Language:
English

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Related Subjects

25 ENERGY STORAGE
Na-ion batteries
NaCoPO4
Polymorphs