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In lithium-ion batteries (LIBs) comprising spinel cathode materials, the dissolution of transition metals (TMs) in the cathodes causes severe cyclic degradation. We investigate the origin and mechanism of surface TM dissolution in high-voltage spinel oxide (LiNi0.5Mn1.5O4) nanoparticles to find a practical method for its mitigation. Atomic structures of the LiNi0.5Mn1.5O4 surfaces are developed, and the electronic structures are investigated by first-principles calculations. The results indicate that titanium is a promising dopant for forming a more stable surface structure by reinforcing metal–oxygen bonds in LiNi0.5Mn1.5O4. Experimentally synthesized LiNi0.5Mn1.5O4 with titanium surface doping exhibits improved electrochemical performance by suppressing undesirable TM dissolution during cycles. The theoretical prediction and experimental validation presented here suggest a viable method to suppress TM dissolution in LiNi0.5Mn1.5O4.

2D materials have shown great promise to advance next-generation lithium-ion battery technology. Specifically, tin-based chalcogenides have attracted widespread attention because lithium insertion can introduce phase transformations via three types of reactions-intercalation, conversion, and alloying-but the corresponding structural changes throughout these processes, and whether they are reversible, are not fully understood. Here, the first real-time and atomic-scale observation of reversible phase transformations is reported during the lithiation and delithiation of SnSe2 single crystals, using in situ high-resolution transmission electron microscopy complemented by first-principles calculations. Lithiation proceeds sequentially through intercalation, conversion, and alloying reactions (SnSe2 → Lix SnSe2 → Li2 Se + Sn → Li2 Se + Li17 Sn4 ) in a manner that maintains structural and crystallographic integrity, whereas delithiation forms numerous well-aligned SnSe2 nanodomains via a homogeneous deconversion process, but gradually loses the coherent orientation in subsequent cycling. Furthermore, alloying and dealloying reactions cause dramatic structural reorganization and thereby consequently reduce structural stability and electrochemical cyclability, which implies that deep discharge for Sn chalcogenide electrodes should be avoided. Overall, the findings elucidate atomistic lithiation and delithiation mechanisms in SnSe2 with potential implications for the broader class of 2D metal chalcogenides.