Sodium-ion and potassium-ion batteries have been increasingly acknowledged as sustainable and cost-effective alternatives to lithium-ion batteries, due to the plentiful availability of their constituent elements. Layered oxide materials, particularly, hold great promise as positive electrodes for sodium-ion batteries (SIBs). In this study, we overview the synthesis and electrode performance of P’2/P2-Na_2/3MnO₂, P2/P3-Na_2/3[Ni_1/3Mn_2/3]O₂, beta-NaMnO₂, and their doped samples. Furthermore, we recently leveraged a specially constructed database for O3-type Na-ion batteries to predict the electrochemical properties of Ni-Mn-Fe-Ti-based layered oxides.
In the frame of systematic study on P2-Na_2/3[Mn_1-xMe_x]O₂ (Me = transition metal) materials, single-phase products of Sc-substituted P’2-Na_2/3Mn_1-xSc_xO₂ were synthesized,. Electrochemical tests revealed superior cycle stability and rate performance in Sc-substituted materials than non-substituted ones. Further, we recently succeeded in synthesis of high-crystalline P2/P3-type Na_0.76[Ni_0.38Mn_0.62]O₂ possessing similar particle size and proved the better rate capability of the P3 electrode than the P2 case. To compensate sodium deficiency in P2 and P3 in Na-ion cells, we also proposed an application of Na₂CO₃ as a sacrificial electrode additive to balance the sodium content (= state of charge) in sodium-free negative electrode and sodium deficient Na_2/3[Fe_1/2Mn_1/2]O₂ positive electrode.
We further made an original database of compositions and electrochemical properties for O3-type layered oxides, which were previously published as well as unpublished in our paper. Leveraging this data, we constructed a predictive model that determines electrochemical properties based on their compositions. This model enabled us to pinpoint promising compositions of Ni-Mn-Fe-Ti-based layered oxides. We proceeded to experimentally synthesize these promising compositions and examined their electrochemical properties. Our investigations verified that these compositions showcased notable electrochemical properties, including an initial energy density of 549 Wh kg^–1_{(active materials)}.
Overall, our findings show that optimizing the composition of layered oxide materials with suitable crystal structure and particle morphology can enhance the performance of SIBs. The strategies developed in this study will facilitate the future development of high-energy-density SIBs, making them more accessible and commercially viable for various applications.

Saaya Sekine

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