Designing Metastable P3-type Layered Negative Electrodes with High Na Vacancy Concentration for High-Power Sodium-Ion Batteries

Titanium-based layered oxides exhibit promising characteristics for negative electrode applications for sodium-ion batteries (SIBs) owing to their low operating potential and stable redox behavior. However, challenges persist in synthesizing single-phase Na_xCr_xTi_1–xO₂ materials with reduced sodium content (x < 0.58) due to structural instability. In this study, an unconventional approach utilizing potassium analogues to design high Na vacancy concentration compounds is proposed. By exploiting the larger ionic size of K⁺ ions, a P3-type K_0.5Cr_0.5Ti_0.5O₂ layered material with lower alkali ion concentrations (x = 0.50) is stabilized. Subsequently, through a facile room-temperature K⁺/Na⁺ ion-exchange process, sodium-deficient metastable P3-type Na_0.5Cr_0.5Ti_0.5O₂ is successfully synthesized. A similar K⁺/Li⁺ ion-exchange process is also applied to synthesize O3-type Li_0.5Cr_0.5Ti_0.5O₂. X-ray diffraction combined with electron microscopy reveals the formation of metastable single-phase P3-type Na_0.5Cr_0.5Ti_0.5O₂ and metastable O3-type Li_0.5Cr_0.5Ti_0.5O₂ with a unique layer arrangement. The high Na vacancy concentration in P3-type Na_0.5Cr_0.5Ti_0.5O₂ results in an increased initial capacity of 125 mA h g⁻¹ at 10 mA g⁻¹. Additionally, Na_0.5Cr_0.5Ti_0.5O₂ exhibits Na⁺/vacancy disordering and high electronic conductivity, enabling a high-rate charge capability without sodium plating. This work provides new insights into the design of metastable layered materials for durable and safe sodium battery applications with fast-charge capability.