Rapid deposition of WS2 platelet thin films as additive-free anode for sodium ion batteries with superior volumetric capacity

One of the key issues hampering the practical application of powerful and thin sodium-ion batteries (SIBs) is low volumetric capacity. The purpose of this study is to report an innovative sputtering strategy to directly construct the vertically-aligned WS2 columnar platelet films with low-tortuosity pores (VA/LT-WS2) on an Al foil as an additive-free anode. Profiting from a 6 ​nm carbon overlayer on the top of the thickness-controllable film electrode (VA/LT-WS2/C), the vertical feature of WS2 and carbon layer enable high efficiency of ion and electron transportation pathways, respectively. The 2.8 ​μm VA/LT-WS2/C film anode exhibits a high volumetric capacity of 1,339 mAh·cm-3 at 100 ​mA·g-1, of which 30.2% is recorded when the current density gets a 50-fold increase from 100 to 5,000 ​mA·g-1. Its gravimetric capacity of 366.8 mAh·g-1 is comparable to the reported WS2 based anode materials for SIBs. Moreover, it is interesting to note that the discontinuous carbon layer (6 ​nm thickness) protects the presence of solid electrolyte interphase (SEI) layer from cracking caused by the volume expansion of WS2 anode, and less than 16.7% volumetric capacity fading is presented after 300 cycles. It outperforms most of the recently reported outstanding SIBs anode materials. The SIB full cell exhibited a reversible volumetric capacity of 554 mAh·cm-3 at 50 ​mA·g-1 when the additive-free anode was applied in the VA/LT-WS2/C//Na2V3(PO4)3/C full cell, demonstrating the potential of the physical sputtering VA/LT-WS2/C for further battery development.