Controllable Gradient Structures Toward Enhancing Ion Transport Kinetics in Lithium Metal Anodes: A Review

The slow kinetics of lithium ions in the desolvation process, solid‐electrolyte interphase (SEI), and Li nucleation leads to interfacial instability and lithium dendrites in lithium metal anodes. It was demonstrated that gradient structures with controllable graded physicochemical properties overcome the limitations of conventional homogeneous structures in simultaneously achieving rapid kinetics and interfacial stability. Herein, this review summarizes different gradient structures and emphasizes their key roles in elevating ion kinetics. First, it was revealed that, in the absence of gradient structures, the interfaces exhibited sluggish transport kinetics due to a single electrolyte concentration profile, spontaneously formed a random SEI, and featured defects with highly concentrated nucleation sites. Second, to elucidate the evolution of gradient structures at interfaces, monitoring methods for dynamic gradients were established from both temporal and spatial dimensions. More fundamentally, it was revealed that the core design principles by which gradient structures enhance kinetics lie in achieving directional regulation of Li ions tailored to the distinct intrinsic characteristics of different interfaces. Specifically, the solvation‐gradient structures accelerated desolvation in the electrolyte; the SEI‐gradient structures stabilized rapid lithium‐ion transport at the liquid–solid interface; and the host‐parameter‐gradient structures promoted uniform nucleation and deposition at the solid interface.