Degrees of freedom for energy storage material

Abstract Building a bridge between properties and structures has always been the key focus of any materials research. Nowadays, energy storage materials, especially lithium‐ion batteries, are crucial both in daily life and for the research community. Therefore, there is an urgent need to discover the functionality origin of battery performances to improve and design better material systems. Functionality originates from local symmetry and field. Local symmetry can be described by four fundamental degrees of freedom: lattice, charge, orbital, and spin. On the basis of this, detailed descriptions of the battery's properties in terms of lattice, charge, orbital, and spin are presented from the perspective of frontier transmission electron microscopy in this review. Besides, frontier in situ methods are introduced to record the dynamic structural evolution process during the battery cycle. Future discussion from the perspectives of both materials and characterizations is provided at the end of this review.