The solid electrolyte interface (SEI) plays a critical role in determining the performance, stability, and longevity of batteries. This review comprehensively compares the construction strategies of the SEI in Li and Mg batteries, focusing on the differences and similarities in their formation, composition, and functionality. The SEI in Li batteries is well-studied, with established strategies that leverage organic and inorganic components to enhance ion diffusion and mitigate side reactions. In contrast, the development of the SEI in Mg batteries is still in its initial stages, facing significant challenges such as severe passivation and slower ion kinetics due to the divalent nature of magnesium ions. This review highlights various approaches to engineering SEIs in both battery systems, including electrolyte optimization, additives, and surface modifications. Furthermore, it discusses the impact of these strategies on electrochemical performance, cycle life, and safety. The comparison provides insights into the underlying mechanisms, challenges, and future directions for SEI research.

Cost‐effective magnesium ion batteries (MIBs) offer a promising new pathway for next‐generation large‐scale energy storage, and yet its development is largely hindered by the severe polarization, limited rate capability, and poor cycling stability, where the challenges are largely rooted in the sluggish kinetics of Mg ²⁺ storage. Here, we report a metastable phase evolution strategy that enables high‐performance Mg ²⁺ storage by leveraging Ti‐modulated VS ₄ (T‐VS ₄ ), demonstrating a structurally soft and yet dynamically adaptive lattice, which are among the preconditions for the metastable phase formation. Metastable Mg _x T‐VS ₄ is formed during the initial Mg ²⁺ intercalation, significantly facilitating the subsequent Mg ²⁺ migration, favoring multi‐electron redox reaction, and enhancing the charge transfer kinetics. Impressively, the T‐VS ₄ cathode demonstrates exceptional Mg ²⁺ storage performance with a high specific capacity (205.4 mAh g ⁻¹ at 50 mA g ⁻¹ ), excellent rate capability (up to 1000 mA g ⁻¹ ), and long‐term cycling stability (over 3000 cycles). This work exemplifies a new metastable phase engineering approach as the design paradigm for breaking kinetic limitations in MIBs, offering a novel avenue toward next‐generation energy storage systems.