Alkali Metal-Modified P2 NaxMnO2: Crystal Structure and Application in Sodium-Ion Batteries

Divya Sehrawat; Aditya Rawal; Soshan Cheong; Maxim Avdeev; Chris D Ling; Justin A Kimpton; Neeraj Sharma

doi:10.1021/acs.inorgchem.0c01078

Alkali Metal-Modified P2 Na_xMnO₂: Crystal Structure and Application in Sodium-Ion Batteries

Inorg Chem. 2020 Sep 8;59(17):12143-12155. doi: 10.1021/acs.inorgchem.0c01078. Epub 2020 Aug 18.

Authors

Divya Sehrawat¹, Aditya Rawal², Soshan Cheong², Maxim Avdeev^{3

4}, Chris D Ling⁴, Justin A Kimpton⁵, Neeraj Sharma¹

Affiliations

¹ School of Chemistry, UNSW Sydney, Sydney, NSW 2052, Australia.
² Mark Wainwright Analytical Centre, UNSW Sydney, Sydney, NSW 2052, Australia.
³ Australian Nuclear Science and Technology Organisation (ANSTO), Locked Bag 2001, Kirrawee DC, NSW 2232, Australia.
⁴ School of Chemistry, The University of Sydney, Sydney 2006, Australia.
⁵ Australian Synchrotron, ANSTO, 800 Blackburn Road, Clayton, VIC 3168, Australia.

PMID: 32808781
DOI: 10.1021/acs.inorgchem.0c01078

Abstract

Sodium-ion batteries (NIBs) are an emerging alternative to lithium-ion batteries because of the abundance of sodium resources and their potentially lower cost. Here we report the Na_0.7MnO₂ solid state synthesized at 1000 °C that shows two distinct phases; one adopts hexagonal P2-type P6₃/mmc space group symmetry, and the other adopts orthorhombic Pbma space group symmetry. The phase ratio of P2 to the orthorhombic phase is 55.0(5):45.0(4). A single-phase P2 structure is found to form at 1000 °C after modification with alkali metals Rb and Cs, while the K-modified form produces an additional minor impurity. The modification is the addition of the alkali elements during synthesis that do not appear to be doped into the crystal structure. As a cathode for NIBs, parent Na_0.7MnO₂ shows a second charge/discharge capacity of 143/134 mAh g^-1, K-modified Na_0.7MnO₂ a capacity of 184/178 mAh g^-1, Rb-modified Na_0.9MnO₂ a capacity of 159/150 mAh g^-1, and Cs-modified Na_0.7MnO₂ a capacity of 171/163 mAh g^-1 between 1.5 and 4.2 V at a current density of 15 mA g^-1. The parent Na_0.7MnO₂ is compared with alkali metal (K, Rb, and Cs)-modified Na_xMnO₂ in terms of surface morphology using scanning transmission electron microscopy coupled with energy-dispersive X-ray spectroscopy, scanning electron microscopy, ²³Na solid-state nuclear magnetic resonance, and X-ray photoelectron spectroscopy and in terms of electrochemical performance and structural electrochemical evolution using in situ or operando synchrotron X-ray diffraction.