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

Abstract 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 x Cr x Ti 1– x O 2 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 0.5 Cr 0.5 Ti 0.5 O 2 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 0.5 Cr 0.5 Ti 0.5 O 2 is successfully synthesized. A similar K + /Li + ion‐exchange process is also applied to synthesize O3‐type Li 0.5 Cr 0.5 Ti 0.5 O 2 . X‐ray diffraction combined with electron microscopy reveals the formation of metastable single‐phase P3‐type Na 0.5 Cr 0.5 Ti 0.5 O 2 and metastable O3‐type Li 0.5 Cr 0.5 Ti 0.5 O 2 with a unique layer arrangement. The high Na vacancy concentration in P3‐type Na 0.5 Cr 0.5 Ti 0.5 O 2 results in an increased initial capacity of 125 mA h g −1 at 10 mA g −1 . Additionally, Na 0.5 Cr 0.5 Ti 0.5 O 2 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.