Comparative electrochemical sodium insertion/extraction behavior in layered NaxVS2 and NaxTiS2

This study investigates the electrochemical sodium insertion/extraction of NaxVS2, and NaxTiS2 in the voltage range where either intercalation (0.2 ≤ x ≤ 1) or displacement-conversion reaction (x > 1) occurs. Both NaxVS2 and NaxTiS2 showed good reversible capacities, as high as ∼160 mAh/g at an average voltage of ∼1.9 V vs. Na in the region for the intercalation reaction (0.2 ≤ x ≤ 1). When sodium (Na) insertion was forced further to the x > 1 composition, NaxVS2 exhibited the direct displacement-conversion reaction at 0.3 V vs. Na without further Na intercalation, which contrasted with the wider lithium intercalation range of 0 < x ≤ 2 for LixVS2. The displacement-conversion reaction for NaxVS2 (x > 1) was reversible with a specific capacity of above 200 mAh/g up to 15 cycles, but the displacement reaction for NaxTiS2 (x > 1) was not observed.