Insights into the Crystallinity of Layer-Structured Transition Metal Dichalcogenides on Potassium Ion Battery Performance: A Case Study of Molybdenum Disulfide.

Layer-structured transition metal dichalcogenides (LS-TMDs) are being heavily studied in K-ion batteries (KIBs) owing to their structural uniqueness and interesting electrochemical mechanisms. Synthetic methods are designed primarily focusing on high capacities. The achieved performance is often the collective results of several contributing factors. It is important to decouple the factors and understand their functions individually. This work presents a study focusing on an individual factor, crystallinity, by taking MoS2 as a demonstrator. The performance of low and high-crystallized MoS2 is compared to show the function of crystallinity is dependent on the electrochemical mechanism. Lower crystallinity can alleviate diffusional limitation in 0.5-3.0 V, where intercalation reaction takes charge in storing K-ions. Higher crystallinity can ensure the structural stability of the MoS2 layers and promote surface charge storage in 0.01-3.0 V, where conversion reaction mainly contributes. The low-crystallized MoS2 exhibits an intercalation capacity (118 mAh g-1 ), good cyclability (85% over 100 cycles), and great rate capability (41 mAh g-1 at 2 A g-1 ), and the high-crystallized MoS2 delivers a high capacity of 330 mAh g-1 at 1 A g-1 and retains 161 mAh g-1 at 20 A g-1 , being one of the best among the reported LS-TMDs in KIBs.