Anisotropic Strain Evolution of Layered LiNi_0.5Co_0.2Mn_0.3O₂ Cathode in Formation Cycle

The formation process plays a key role in developing long life and high energy density materials, during which the phase transition, strain evolution, and structural self-adaption interact rationally with each other to produce a qualified electrode for Li-ion batteries (LIBs). Taking the layered LiNi_0.5Co_0.2Mn_0.3O₂ cathode as an example, the substantial structural changes, both in the bulk and at the surface are thoroughly documented during the formation cycle. Upon initial charging, different phases emerge continuously, resulting in lattice mismatches, inhomogeneous strain, and microstructural distortions. The irreversible loss of lattice oxygen, combined with the spontaneous cation mixing and the formation of rock-salt phase at the surface, contributes to the initial capacity loss. As a self-adjustment process, this initial capacity loss facilitates the development of a robust Li_xNi_0.5Co_0.2Mn_0.3O₂ electrode by relieving internal strain and stabilizing the rippled layered structure, thereby ensuring highly reversible electrochemical behavior in subsequent cycles. These findings lay a solid foundation for the design and optimization of layered cathodes for rechargeable batteries.