Nanoreactor-Structured Defective MoS2: Suppressing Intercalation-Induced Phase Transitions and Enhancing Reversibility for Potassium-Ion Batteries

Abstract

Conversion-type electrode materials hold significant promise for potassium-ion batteries (PIBs) due to their high theoretical capacities, yet their practical deployment is hindered by sluggish kinetics and irreversible structural degradation. To overcome these limitations, we propose a rationally engineered nanoreactor architecture that stabilizes defect-rich MoS2 via interlayer incorporation of a carbon monolayer, followed by encapsulation within a nitrogen-doped carbon shell, forming a MoSSe@NC heterostructure. This tailored structure synergistically accelerates both K+ diffusion kinetics and electron transfer, enabling unprecedented rate performance (107 mAh g−1 at 10 A g−1) and ultralong cyclability (86.5% capacity retention after 1200 cycles at 3 A g−1). Mechanistic insights reveal a distinctive “adsorption-conversion” pathway, where sulfur vacancies on exposed S–Mo–S basal planes act as preferential K+ adsorption sites, effectively suppressing parasitic phase transitions during intercalation. In situ X-ray diffraction and transmission electron microscopy corroborate the structural reversibility of the conversion reaction, with the carbon matrix dynamically accommodating strain while preserving electrode integrity. This work not only advances the understanding of defect-driven interfacial chemistry in conversion-type materials but also provides a versatile strategy for designing high-performance anodes in next-generation PIBs through heterostructure engineering.

1. Introduction

Molybdenum disulfide (MoS2), a layered transition metal dichalcogenide, has attracted considerable attention as a promising anode material for potassium-ion batteries (PIBs) due to its high theoretical capacity (~670 mAh g−1) and tunable interlayer spacing [1–4]. The van der Waals gaps between the S–Mo–S layers create natural pathways for K+ intercalation, promoting the electrochemical reaction. In addition, MoS2 exhibits a conversion reaction mechanism that enables multi-electron transfer, which is essential for improving capacity. However, the practical application of MoS2 in PIBs is hindered by several key challenges.

One of the main issues is the first-order phase transition that occurs during K+ intercalation, wherein MoS2 transitions from the K-deficient 2H phase to the K-rich 1T phase [5, 6]. This phase transition introduces significant kinetic barriers, as ion diffusion is coupled with phase boundary migration, thereby limiting the rate performance of the material (Fig. 1a). As a result, capacity retention of MoS2 at current densities above 2 A g−1 typically drops below 50%, restricting its application in high-power scenarios. Another challenge arises during deep discharge (<1.0 V), when the conversion reaction produces metallic Mo and K2S. The irreversible formation of K2S during discharge leads to persistent structural defects that prevent reversible conversion upon charging [7]. Incomplete reconversion of K2S exacerbates capacity loss, while residual K2S promotes oxidative dissolution at higher voltages (>2.0 V), resulting in the formation of soluble polysulfides. This polysulfide dissolution induces a shuttle effect similar to that in the potassium–sulfur system, which accelerates degradation and results in rapid Coulombic efficiency decay (Fig. 1a). Consequently, addressing the interrelated issues of sluggish kinetics, irreversible phase transitions, and interfacial instability has become essential.

To overcome the limitations related to phase transitions and structural instability, recent studies have increasingly emphasized the structural optimization of MoS2, aiming to enhance its electrochemical performance and improve long-term cycling stability. A key strategy involves the controlled expansion of interlayer spacing and the introduction of defects to facilitate ion transport and buffer volume changes.