Layered transition metal oxides as cathode materials for sodium-ion batteries

Abstract

Lithium-ion batteries (LIBs) have dominated the global market for electronic vehicles and devices because of their long cycle lives, large specific capacity, and high operating voltage. However, limited and unevenly distributed lithium resources around the world results in relatively high cost LIBs. Sodium-ion batteries (NIBs) have emerged as an appealing alternative to LIBs for energy storage solutions in smart electricity grids, where cost plays more important role than energy density. Thus, it is important to develop electrode materials for commercial purposes that are suitable for the reversible insertion/extraction of a large amount of Na+ during charge/discharge. This project examines “attempted” chemical doping of P2 NaxMnO2 and P2 Na0.7Mn0.8Mg0.2O2 layered materials with alkali metals (K, Rb and Cs). STEM-EDS mapping analysis indicates that K is not homogeneously distributed across the P2 Na0.7MnO2 and P2 Na0.7Mn0.8Mg0.2O2 particles unlike Na, Mn, Mg and O. Rietveld analysis also shows there is no significant change in the c-axis (or stacking axis) even after the doping with a larger ionic radii alkali metals in comparison to Na. Since there is no chemical doping, the term “modified” is used to describe the active materials, e.g. K-modified P2 Na0.7MnO2 and K-modified P2 Na0.7Mn0.8Mg0.2O2 (similar for Rb and Cs samples). 23Na solid-sate NMR shows changes in Na-environment after chemical modification. Electrochemically the K/Rb/Cs-modified P2 Na0.7Mn0.8Mg0.2O2 do not show improvements in NIB performance compared to the parent P2 Na0.7Mn0.8Mg0.2O2. While alkali metal (K, Rb and Cs) modification of P2 NaxMnO2 seems to improve the electrochemical performance. A key technique to understand electrode function inside a battery is in situ SXRD at the Powder Diffraction Beamline at the Australian synchrotron. To extract crystallographic information and to see the phase transitions occurring at higher current rate for multiple cycles, operando XRD experiments were performed at the Materials Science and Powder Diffraction Beamline at ALBA Synchrotron for the synthesised K modified P2 Na0.7Mn0.8Mg0.2O2 and P2 Na0.87MnO2 samples. With an understanding of how structural transitions affect the electrochemical performance of a battery, a better electrode material suitable for reversible insertion/extraction of a large amount of Na+ during charge/discharge cycling can be developed.