Multiscale Design of Na(Li1/3Mn2/3)O2 Cathode Material Operated by Anionic Redox Reactions for Sodium-Ion Batteries

Abstract

To discover new high-energy-density cathode materials for sodium-ion batteries (SIBs), multiscale design is highly considered to be an essential manner because cathode materials occur in complicated physical and chemical reactions from atomic scale to macro scale. Considering the intrinsically larger ionic radius of Na+ than that of Li+, cathode materials for SIBs are operated by electrochemical reactions accompanied by severe structural changes and phase transformations in comparison to those for lithium-ion batteries (LIBs) owing to insertion and extraction of Na+, which leads to cyclic degradation during charge/discharge. That is, the Na-migration affect qualitative and quantitative variations of cathode materials in atomic scale, which are the results of electrochemical performance for the overall system. In addition, the intrinsically lower redox window (by ≈0.3 V) of the cathode for SIBs compared to the LIBs result in low-energy-density properties. Therefore, multiscale-based analysis and design approach in light of the two intrinsic features of Na+ are critical for the rational design of cathode materials for SIBs. Using the multiscale-based framework from first-principles calculations including thermodynamic mixing enthalpies associated with phase stabilities, kinetic properties, mechanical constants, chemomechanical strain, and qualitative and quantitative electronic structures in the perspective of atomic scale not only to phase separation kinetic simulations consisting of homogeneous chemical potentials originating from homogeneous free energy coupled with the thermodynamic values of atomic calculations, but also to chemomechanical stress obtained by the fitted mechanical constants and chemomechanical strain using polynomial functions, we rationally design and experimentally realize Na(Li1/3Mn2/3)O2 as a high-energy-density cathode material (≈4.2 V versus Na/Na+ with a high charge capacity of 190 mAh g-1) operated by the new reaction paradigm (anionic redox: O2-/O-) beyond the conventional reaction mechanism (cationic redox: Mn+/M(n+1)+, M: transition metals) for sodium-ion batteries (SIBs), which is fundamentally inspired by in-depth understandings of Li2MnO3 redox reactions for LIBs. Furthermore, this rationally designed Na(Li1/3Mn2/3)O2 is an example of a new class of promising cathode materials, Na(Li1/3M2/3)O2 (M: transition metals featuring stabilized M4+), for further advances in SIBs. To overcome the drawbacks of phase change and separation, and structural collapse related to cyclic performance in the newly discovered Na(Li1/3Mn2/3)O2 material, we further design high-energy-density cathode, Na(Li1/3Mn1/2Cr1/6)O2, utilizing the Cr 3d-electron and labile O 2p-electron for electrochemical reactions with reduced phase change, slow phase separation kinetics, and lower chemomechanical strain and stress compared to those of Na(Li1/3Mn2/3)O2. Additionally, to provide rational insights into the use of anionic redox reactions for future sodium-ion batteries, Na(Li1/3Mn1/2Cr1/6)O2 is regarded as an exciting example of superior Na(Li1/3M2/3(1-y)Mcy)O2 analogues (M and Mc: transition metals with stabilized M4+ species and with cationic redox active Mc4+ species) with good cyclic performance. The rationally designed and experimentally validated cathode materials by the multiscale-based design approach open an exciting direction to break the energy density limit of cathode materials for SIBs. Furthermore, the present multiscale-based design approach could be applied to various energy-related systems such as batteries, capacitors, fuel cells, solar cells, and even catalyst in order to improve their performance and rationally design new materials.