Abstract
It is well known that many layered transition metal oxides can transform into a spinel structure upon repeated battery cycling, but a phase transition in the opposite direction is rare. Recently, the transformation from spinel Mn3O4 to layered MnO2 was observed during the operation of a Mg battery in aqueous conditions, resulting in high performance Mg batteries. We hereby use ab initio calculations to unveil the mechanism by which crystal water plays a critical role in this unique transformation. Once inserted into the spinel form, a water molecule donates an electron, offering a key structural and thermodynamic driving force to initiate the transformation process. These crystal water molecules then get favorably clustered into a planar form in the layered structure and act as a stabilizing agent for birnessite. Kinetically, the inserted crystal water dramatically promotes the necessary rearrangement of Mn during the transition by lowering the activation barrier by >2 eV. The present structural, thermodynamic and kinetic understanding of the crystal water-driven phase transition provides novel insights to further the design of related low dimensional hydrated materials for multi-valent cathodes.
Highlights
Layered transition metal (TM) oxides have been widely investigated as electrode materials for both lithium and post-lithium ion batteries, since they potentially have a higher energy density than other materials including sul des, uorides and polyanionic compounds.[1,2,3] Birnessite-type manganese oxides, or birnessites, are naturally abundant compounds with a layered structure and the general chemical formula AMn2O4$(H2O)m, where A can be various cations such as Na+, K+, Mg2+ or Ca2+.4 Each layer of the birnessite is composed of edge shared MnO6 octahedra and the interlayer space is lled with pillaring cations and/or water molecules
The main driving force for this spinel-to-layered structure conversion is suggested to be the insertion of crystal water on the basis of experimental data which showed that the phase transition occurs only when spinel Mn3O4 is cycled in aqueous electrolytes[11,12,19] and that a higher water content in the compound resulted in a more efficient transformation into birnessite,[20] which makes this spinel-to-layered transformation even more peculiar
Speaking, since the spinel structure shares the same close-packed oxygen framework as the layered structure,[24] the phase transition between the two structures is fairly feasible without changing the oxygen stacking sequence
Summary
Layered transition metal (TM) oxides have been widely investigated as electrode materials for both lithium and post-lithium ion batteries, since they potentially have a higher energy density than other materials including sul des, uorides and polyanionic compounds.[1,2,3] Birnessite-type manganese oxides, or birnessites, are naturally abundant compounds with a layered structure and the general chemical formula AMn2O4$(H2O)m, where A can be various cations such as Na+, K+, Mg2+ or Ca2+.4 Each layer of the birnessite is composed of edge shared MnO6 octahedra and the interlayer space is lled with pillaring cations and/or water molecules. Once inserted into the spinel form, a water molecule donates an electron, offering a key structural and thermodynamic driving force to initiate the transformation process.
Sign-in/Register to view highlights & summary Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
