Atomistic Observation of Desodiation-Induced Phase Transition in Sodium Tungsten Bronze

The phase instability in layered-structure Na 0.5 WO 3.25 induced by the extraction of Na ions was investigated by applying transmission electron microscopy. Real-time atomic-scale observation reveals the phase transition pathway: Na 0.5 WO 3.25 (triclinic) → Na x WO 3 (cubic) → WO 3 (monoclinic) with specific orientation relationships. The dynamic evolution of Na 0.5 WO 3.25 /Na x WO 3 phase boundaries shows that Na 0.5 WO 3.25 will cleave along the (100) T and (010) T and recrystallize as (101) C and (010) C of Na x WO 3, respectively. The phase transition pathway can be well-explained according to the structural characteristics in the three phases. By better understanding of the phase instability induced by the extraction of Na ions in possible layered-structure cathode materials, this work provides a reference for the design of sophisticated strategies toward high-performance Na-ion batteries.