Revolutionizing P2-type layered sodium cathodes: Unveiling the role of transition metal layer vacancies on structure and performance

Mounting environmental concerns and the threat of global warming necessitate a substantial reduction in fossil fuel consumption. Renewable energy sources stand as the most feasible alternatives to traditional power generation, underscoring the critical role of energy storage systems (ESSs). Sodium, being one of the most plentiful elements in the Earth’s crust, offers a significant cost advantage when used in ESSs. Nevertheless, the energy density of sodium-ion batteries (SIBs) is lower than that of lithium-ion batteries (LIBs) due to the lower electrochemical potential of sodium relative to the standard hydrogen electrode (SHE). Consequently, SIB cathode materials must exhibit high capacity to mitigate the lower energy density and achieve parity with the energy density of LIBs.

A key area of interest is the impact of vacancies in transition metal (TM) layers on oxygen redox reactions. These vacancies facilitate the redox process by creating a Na-O-V (V: vacancy) configuration, which spontaneously generates unpaired electrons in the oxygen orbital. For effective charge compensation, the redox activity of manganese (Mn) and oxygen (O) is sufficient to attain high capacity. However, further refinement is necessary to enhance the operating voltage.

To address this issue, the team led by Professor Seung-Taek Myung at Sejong University has designed Na_0.6[Ni_0.3Ru_0.3Mn_0.4]O₂ (NRM) and vacancy introduced Na_0.7[Ni_0.2V_Ni0.1Ru_0.3Mn_0.4]O₂ (V-NRM) compounds, for which the average oxidation state of Mn was controlled to be 4+. The V-NRM compound exhibited a slight enhancement in capacity and rate performance, achieving approximately 184 mAh g⁻¹ compared to 163 mAh g⁻¹ for NRM. Notably, V-NRM displayed an additional short voltage plateau at approximately 3.9V during charging and 3.8V during discharging. Both compounds underwent a straightforward phase transition from P2 to OP4 during de/sodiation, as confirmed by operando X-ray diffraction (o-XRD) studies.

X-ray absorption near edge spectroscopy (XANES) data elucidated the redox reactions involving Ni, Ru, Mn, and O species within the voltage range of 1.5–4.2 V. The additional voltage plateau observed at around 3.9 V is attributed to the oxidation of lattice oxygen. Seung-Taek Myung's team further investigated the distinct oxygen behavior in both electrodes using operando differential electrochemical mass spectrometry (o-DEMS). They found that the evolution of oxygen facilitated the redox reaction by generating unpaired electrons in the O 2p orbital for the NRM electrode.

Based on these findings, Seung-Taek Myung's team concluded that the enhanced capacity of the V-NRM cathode is closely linked to pre-existing vacancies in the TM layers. These vacancies promote the formation of lone-pair electrons in the O 2p orbital in the bulk, which supports the early-stage oxidation of oxygen. This mechanism ultimately results in an additional capacity of approximately 21 mAh g⁻¹, corresponding to about 0.09 mol Na per formula unit.