Electrochemically induced cleavage cracking at twin boundary of sodium layered oxide cathodes

Twinning defects often present in crystalline materials when are subject to mechanical stimuli and are mostly affecting their physicochemical properties. The twinning formation and twin-assisted cracking upon cycling in sodium layered oxides (SLOs) are poorly understood. Combining atomic-resolution imaging, spectroscopy and first principles calculations, we reveal that growth twinning is unexpectedly common in the SLO materials and the twin boundaries show distinct structural and chemical characters from those identified in lithium layered oxides. A unique O-P-O twinning plane was identified in the O3 type SLO materials. We discover that twin-assisted Na diffusion cause large volume variations and trigger cleavage fracture during electrochemical cycling. The present findings not only establish a robust correlation between growth twinning and cleavage cracking in SLOs, but also offer general implications for the development of high-performing intercalation electrode materials by regulating crystallographic defects.