Publication date: 10th April 2024
Sodium layered oxide positive electrodes afford promising gravimetric energy densities (exceeding 170 mAh g-1 for P2-type materials when the upper cut-off is 4.5 V) but are plagued by irreversible phase transitions at high states-of-charge (>4 V). Such unwanted phase transitions are associated with large anisotropic volume changes and oxygen release, both of which limit cyclability.[1][2] The phases participating in these reactions are line compounds that exist over a limited range of sodium content and manifest as ordered arrangements of sodium ions and vacancies. Understanding and manipulating these sodium-vacancy ordered phases is crucial for improving high voltage stability.
In this study, we establish the effect of transition metal disordering on the sodium-vacancy ordered phases. Specifically, in P2-type Na2/3-2xNi1/3-xMn2/3+xO2 (NNM), we observe a transition from a staircase-like voltage profile to a smooth one for small changes in composition (x < 0.1), which indicates the disruption of the sodium-vacancy ordered phases. We hypothesize that electrostatic repulsion, induced by transition metal disordering, causes such behavior. Alternatively, there could exist a competition between different long-range cation orderings as the manganese-to-nickel ratio increases (such as honeycomb, row, and ribbon ordering). [3] Neutron diffraction will be conducted as future work to determine which of these scenarios occurs.
The insights from this study will show the effect of cation disordering on the shape of the voltage profile and could be extended to other positive electrodes featuring phase transitions (including Li4Ti5O12, LiFePO4, and LiMn1.5Ni0.5O4). Voltage profile modification can be important for making a battery chemistry compatible with systems electronics, or with a given electrolyte. Furthermore, in the case of NNM, the softening of phase transitions by inducing electrostatic disorder in the transition metal layer can potentially improve capacity retention by delaying the formation of high voltage phases which are associated with irreversible capacity loss.
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