Tuning MoS2 Interlayer Spacing for Mg2+ Insertion: Navigating the Capacity–Stability Trade-Off in Mg-Ion Batteries

The strong electrostatic interaction between divalent Mg2+ and host lattices causes sluggish kinetics and severely limits the utilization of redox-active capacity in magnesium-ion battery (MIB) electrodes. Many strategies have been explored to lower the migration barrier of the active ion, among which in-situ interlayer expansion has emerged as an effective route to enhance solid-state Mg2+ diffusion. However, the structural instability accompanying this kinetic enhancement remains poorly understood. Here, we use molybdenum disulfide (MoS2), a two-dimensional transition metal dichalcogenide whose tunable interlayer spacing and compatibility with Mg electrolytes make it a promising MIB electrode, as a model system to probe this trade-off. We systematically investigate how organic cations of varied molecular sizes affect the electrochemical behavior of expanded MoS2 (e-MoS2). Larger cations promote faster initial activation and a higher maximum capacity, but at the same time trigger pronounced lattice destabilization, leading to sulfur loss and structural degradation. Combined X-ray diffraction, X-ray absorption and photoelectron spectroscopies, and electron microscopy analyses indicate that this degradation is driven by a partial conversion reaction that ruptures Mo–S bonds and depletes redox-active sulfur from the lattice. Thus, while interlayer expansion enhances ion-transport kinetics, it also accelerates irreversible degradation, a fundamental trade-off in this system. These insights provide essential design principles for multivalent batteries, underscoring the importance of tuning the size of pillaring ions to balance insertion kinetics and the structural integrity of layered sulfide electrodes.