Influence of Ion Size on Structure and Redox Chemistry in Na-Rich and Li-Rich Disordered Rocksalt Battery Cathodes.
basic_science · Level V
Where this comes from
- Record sourced from PubMed, PMID 40444362.
- Also identified by DOI 10.1002/adma.202419878 and PMC identifier 12355563.
- Licence recorded as CC BY.
- The licence permits redistribution, so the abstract is shown in full and the full text is available from the publisher.
Abstract
Li-rich disordered rocksalts are promising next-generation cathode materials for Li-ion batteries. Recent reports have shown it is also possible to obtain Na-rich disordered rocksalts, however, it is currently poorly understood how the knowledge of the structural and redox chemistry translates from the Li-rich to the Na-rich analogs. Here, the properties of Li<sub>2</sub>MnO<sub>2</sub>F and Na<sub>2</sub>MnO<sub>2</sub>F are compared, which have different ion sizes (Li<sup>+</sup> = 0.76 vs Na<sup>+</sup> = 1.02 Å) but the same disordered rocksalt structure and stoichiometry. It is found that Na<sub>2</sub>MnO<sub>2</sub>F exhibits lower voltage Mn- and O-redox couples, opening access to a wider compositional range within the same voltage limits. Furthermore, the intercalation mechanism switches from predominantly single-phase solid solution behavior in Li<sub>2</sub>MnO<sub>2</sub>F to a two-phase transition in Na<sub>2</sub>MnO<sub>2</sub>F, accompanied by a greater decrease in the average Mn─O/F bond length. Li<sub>2</sub>MnO<sub>2</sub>F retains its long-range disordered rocksalt structure throughout the first cycle. In contrast, Na<sub>2</sub>MnO<sub>2</sub>F becomes completely amorphous during charge and develops a local structure characteristic of a post-spinel. This amorphization is partially reversible on discharge. The results show how the ion intercalation behavior of disordered rocksalts differs dramatically when changing from Li- to Na-ions and offers routes to control the electrochemical properties of these high-energy-density cathodes.