Correlation of phase (in)stability and lattice misfits for high-power-density Na cathodes

Abstract

The correlation between phase (in)stability and lattice misfit is herein investigated to provide an in-depth understanding of the phase transitions using the intriguing concept denoted as the “directional lattice misfit” in achieving high-power-density cathodes based on Na_x[Ni_1/4Mn_3/4]O₂ (NM13) and Na_x[Ni_1/3Mn_2/3]O₂ (NM12) Na oxide models. The binary Ni–Mn cathodes are similarly characterized using various experimental analysis methods; however, their electrochemical charge/discharge curves reveal completely different pathways. Upon cycling, the former exhibits a smooth profile regarded as a solid–solution reaction, whereas the latter exhibits a stair-like structure, thereby suggesting multiple phase transitions during Ni redox reactions below 4.0 V. Interestingly, the cycling retention of NM13 is lower than that of NM12 at 0.1C, which is not governed by the general concept that the phase stability increases the electrochemical cycling performance. In contrast, NM13 exhibits distinctly higher cycling retention compared with NM12 in higher current density modes. Using first-principle calculations, the intriguing reverse trend depending on the materials is theoretically understood by the “directional lattice misfit” concept of an increase in the thermodynamic phase instability induced by phase transitions upon desodiation. The formation energies suggest that the biphasic and monophasic reactions underpin the different charge/discharge profiles of NM13 and NM12, thereby indicating that thermodynamic instability leads to the generation of a two-type lattice misfit, depending on the crystallographic directions. Based on the correlation between thermodynamics and the lattice misfit, the structural effect accounts for the cycling retention relying on the current density mode and it can potentially provide a universal design strategy for high-power-density sodium and lithium-ion batteries.