New Insight in Sodium Intercalation in MoS2 at Different Stages and the Optimized Cycle Performance for Sodium Ion Batteries

Due to the high abundance and low cost of the sodium (Na), Na-ion batteries are an alternative to lithium-ion (Li-ion) batteries for large-scale energy storage, such as electric vehicles and stationary storage for renewable energies. Similar to the successful lithium intercalation chemistry, Na-ion batteries have recalled much attention recently and a significant achievement have been made. However, the large size of the ionic diameter and ionization potential of the Na ions make it difficult in structural stability and ion diffusion, usually resulting in different reaction mechanisms¹.

         Transition metal dichalcogenides MS₂ (M=Mo, W) and their intercalated compounds belong to a large class of the so-called 2D layered solids. The layers of these materials consist of three interconnected, hexagonally coordinated atomic sheets (S-Mo-S)². The weak interlayer van der Waals force permits the possible intercalation of some foreign atoms between the layers, such as alkali metal. Li-ion intercalated MoS₂ (Li_xMoS₂) has been widely explored and known as intercalation (x<0.5), 2H-1T phase transition (0.5<x<1) and conversion (x>1)³. However, few studies have been focused on the structural change of the host when Na ions are inserted into the MoS₂ lattice. Moreover, the electrochemical performance of MoS₂for Na-ion insertion needs to be improved.

         In this work, in situ and ex situ techniques were combined to monitor the structural changes of MoS₂ at atomic levels at various intercalation stages and recognize the reaction products. By optimizing the binder and the cut-off potential, we improve the cycling performance of MoS₂ for Na-ion battery by a large margin. When discharged to 0.2 V, the cell shows an initial discharge capacity of 241 mAh g^-1 and maintains at 165 mAh g^-1 after 70 cycles. However, intercalation of too many Na ions in the host leads to the decomposition of MoS₂ into metallic Mo and Na_xS₂ (x£2). The performance of the cell becomes deteriorated in the subsequent cycles due to large volume expansion and the electrolyte reduction at low potentials (0.01 V) though a reversible capacity up to 440 mAh g^-1 can be obtained.

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