Enhanced high rate capability of Li intercalation in planar and edge defect-rich MoS2 nanosheets

Abstract

(i) Edge and planar defect-rich and (ii) defect-suppressed MoS₂ nanosheets are fabricated by controlled annealing of wet-chemically processed precursors. Wrinkles, folds, bends, and tears lead to the introduction of severe defects in MoS₂ nanosheets. These defects are suppressed and highly crystalline MoS₂ nanosheets are obtained upon high-temperature annealing. The influence of defects on the electrochemical properties, particularly rate capability and cycling stability, in the Li intercalation regime (1 V to 3 V vs. Li/Li⁺) and conversion regime (10 mV to 3 V vs. Li/Li⁺) are investigated. In the intercalation regime, the initial Li intake (x in Li_xMoS₂) for defect-rich nanosheets is larger (x ≈ 1.6) as compared to that in defect-suppressed MoS₂ (x ≈ 1.2). Although the reversible initial capacity of all the anodes is nearly the same (x ≈ 0.9) at 0.05C rate, defect-rich MoS₂ exhibits high rate capability (>40 mA h g⁻¹ at 40C or 26.8 A g⁻¹). When cycled at 10C (6.7 A g⁻¹) for 1000 cycles, 75% capacity retention is observed. High rate capability can be attributed to the defect-rich nature of MoS₂, providing faster access to lithium intercalation by a shortened diffusion length facilitated by Li adsorption at the defect sites. The defect-rich nanosheets exhibit a power density of ∼20% more than that of defect-suppressed nanosheets. For the first time, MoS₂/Li cells with a high power density of 10–40 kW kg⁻¹ in the intercalation regime have been realized. In the conversion regime, defect-rich and defect-suppressed MoS₂ exhibit initial lithiation capacities of ∼1000 and ∼840 mA h g⁻¹, respectively. Defect-rich MoS₂ had a capacity of ∼800 mA h g⁻¹ at 0.1C (67 mA g⁻¹), whereas defect-suppressed MoS₂ had a capacity of only ∼80 mA h g⁻¹ at the same current rate. Capacity retention of 78% was observed for defect-rich MoS₂ with a reversible capacity of 591 mA h g⁻¹ when cycled at 0.1C (67 mA g⁻¹) for 100 cycles. Despite having a lower energy density in the intercalation regime, the power density of defect-rich MoS₂ in the intercalation regime is significantly larger (by three orders of magnitude) as compared to that of defect-suppressed MoS₂ in the conversion regime. Defect-rich MoS₂ nanosheets are promising for high-rate-capability applications when operated in the intercalation regime.