Sodium-Ion Intercalation Mechanism in MXene Nanosheets

Abstract

MXene, a family of layered compounds consisting of nanosheets, is emerging as an electrode material for various electrochemical energy storage devices including supercapacitors, lithium-ion batteries, and sodium-ion batteries. However, the mechanism of its electrochemical reaction is not yet fully understood. Herein, using solid-state (23)Na magic angle spinning NMR and density functional theory calculation, we reveal that MXene Ti3C2Tx in a nonaqueous Na(+) electrolyte exhibits reversible Na(+) intercalation/deintercalation into the interlayer space. Detailed analyses demonstrate that Ti3C2Tx undergoes expansion of the interlayer distance during the first sodiation, whereby desolvated Na(+) is intercalated/deintercalated reversibly. The interlayer distance is maintained during the whole sodiation/desodiation process due to the pillaring effect of trapped Na(+) and the swelling effect of penetrated solvent molecules between the Ti3C2Tx sheets. Since Na(+) intercalation/deintercalation during the electrochemical reaction is not accompanied by any substantial structural change, Ti3C2Tx shows good capacity retention over 100 cycles as well as excellent rate capability.

Review—Two-Dimensional Layered Materials for Energy Storage Applications

“…Furthermore, only few MXene materials have been used for SIB anodes, such as Ti 3 C 2 T x as shown in Figures 3i-3k. 78 This study further reports that a small fraction of Na + remains in the structure of MXenes event after full desodiation, resulting in the increase in interlayers distance from 9.7 Å to 12.5 Å. 78 This is because the trapped Na + , in conjunction with penetrated solvent molecule lead to expansion of interlayer distance during the initial cycles of sodiation-desodiation by pillaring and swelling mechanisms.…”

“…78 This study further reports that a small fraction of Na + remains in the structure of MXenes event after full desodiation, resulting in the increase in interlayers distance from 9.7 Å to 12.5 Å. 78 This is because the trapped Na + , in conjunction with penetrated solvent molecule lead to expansion of interlayer distance during the initial cycles of sodiation-desodiation by pillaring and swelling mechanisms. Further structural change in layered MXenes was not observed upon subsequent sodiation-desodiation cycles, resulting in good capacity retention.…”

Emerging nanostructured electrode materials for water electrolysis and rechargeable beyond Li-ion batteries

“…This high performance was attributed to the increased number of intercalation sites and large interlayer spacing of 9.8 Å which allow for the double-layer intercalation mechanism [153]. Solid-state NMR and XRD analyses demonstrated that the interlayer distance expands in the first sodiation process by 2.8 Å [154]. It was found that such significant interlayer expansion is caused by both Na + ion intercalation and electrolyte solvent penetration.…”

“…The mechanism of sodium intercalation into Ti 3 C 2 T x electrode was investigated using aberration-corrected scanning transmission electron microscopy [153] and solid-state 23 Na magic angle spinning NMR spectroscopy [154]. The atomic scale study by STEM showed that double layer of Na + ions can accommodate within the Ti 3 C 2 T x interlayer space upon deep discharge down to 0.0 V vs. Na/Na + ( Figure 14(e) and (f)).…”

“…A fraction of the electrochemically intercalated Na + ions is trapped in Ti 3 C 2 T x electrode and serves as pillars, while the penetrated solvent molecules swell the interlayer space. As a result of the presence of these two species, the interlayer distance of ≈12.5 Å is constant during further sodiation/desodiation, leading to high cycle stability and fast Na + ion diffusion in the expanded interlayer space [154]. The initial capacity of 270 mAh g −1 was attributed to Na + ion intercalation, adsorption of Na + ions on material surface, and electrolyte decomposition leading to the formation of SEI layer, suppressing electrolyte decomposition in the subsequent cycles.…”