Abstract
Lithium ion batteries are essential for energy storage in many applications, such as portable electronic devices and electric vehicles. However, charge-discharge rates and cycle life of the current batteries are insufficient to fulfill the targeted requirements of projected future devices, due to the severe limitations of conventional graphite anodes. Niobium tungsten oxides have recently been attracting attention as a new anode material for fast (dis)charging, owing to their stable host structure for lithium intercalation. It was originally believed that nanoscaling and nanostructuring would have a negligible effect on its electrochemical performance. However, the influence of the particle and grain size of niobium tungsten oxide electrodes on its electrochemical behavior is still an open question. This work reports on an investigation of the dependence of the lithiation process on the grain size of Nb18W16O93 anodes down to 60 nm. The results demonstrate that downscaling below 100 nm significantly enhances the lithiation dynamics of niobium tungsten oxide. Furthermore, it suggests that the grain boundaries of Nb18W16O93 have significant influence to the fast lithiation process. It provides a new perspective on the impact of downscaling grains to improve the electrochemical performance of Nb18W16O93 anodes for realizing fast (dis)charging in future energy storage devices.
Highlights
Energy storage has been attracting a lot of attention over the past decade, as it is essential for realizing global energy sustainability [1,2]
Lithium ion batteries (LIBs) are becoming the bottleneck in tech nology development, as the current specific capacities of electrodes, charge-discharge rates and cycle life of the battery are insufficient to fulfill the targeted requirements of projected future devices [7,8,9]
New anode materials with higher voltages of the chemical reaction and highercharge rates are needed for next-generation LIBs [13]
Summary
Energy storage has been attracting a lot of attention over the past decade, as it is essential for realizing global energy sustainability [1,2]. The lithium diffusion coefficients of LixNb18W16O93 (1.1 × 10− 13 m2 s− 1) and LixNb16W5O55 (1.7 × 10− 13 m2 s− 1) are several orders of magnitude higher than Li4+xTi5O12 (10− 16 m2 s− 1) and LixTiO2 (10− 15 m2 s− 1), which indicates that lithium transport in niobium tungsten oxides is remarkably faster than in tita nium oxides [34] Both the tetragonal NbWO phases (Nb16W5O55[34], Nb14W3O44[35]) as well as the distorted tetragonal tungsten bronze NbWO phases (Nb8W7O47[36], Nb18W16O93 [33,34]) exhibit exceptional electrochemical performance with charge-discharge rate up to 20 C over hundreds of cycles. The lithiation process, during which lithium ions are inserted into the structure, has been proposed to follow the order from first the largest channel (pentagonal), the intermediate channel (square) and the smallest channel (triangular), as shown in Fig. 1 (b) During this discharge procedure the original Nb18W16O93 phase transforms consecutively into Li13Nb18W16O93, Li21Nb18W16O93 and Li35Nb18W16O93, by subsequently filling each type of channel [33]. Comparison of the electrochemical performance of Nb18W16O93 with different grain sizes indicates that smaller Nb18W16O93 grains exhibited significantly enhanced cycling and rate performance, which leads to the conclusion that the grain size is a key parameter for achieving enhanced lithiation dynamics in down scaled Nb18W16O93 electrodes
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