Ultrathin NixCoy-silicate nanosheets natively anchored on CNTs films for flexible lithium ion batteries
Graphical abstract
The NixCoy-silicate@CNTs composite film is ultralight but able to withstand a large loading without breaking. The flexible lithium ion battery equipped with this material can give stable power output even when it is bent to 180°.
Introduction
With the rapid advancement of flexible electronic devices, such as electronic skin and flexible screens, and so on, the development of high-performance flexible energy storage technologies has become a hot topic in the current research field [1], [2], [3], [4], [5], [6], [7], [8]. At present, lithium ion batteries (LIBs), known for their low cost, long cycle life, and high energy density, are still the most widely used and mature among the many energy storage systems [9], [10], [11], [12], [13], [14], [15], [16], [3], [17], [18]. For LIBs, the key issue for obtaining flexibility is the design and fabrication of electrode materials with balanced mechanical properties and electrochemical performance [19], [20], [21].
Conventionally, a commercial LIB electrode is usually fabricated by uniformly coating a slurry of active materials, conducting additives (e.g., carbon black), and binders (e.g., polyvinylidene fluoride) onto a metal foil current collector. When used for flexible LIBs or flexible electronic equipment, it has to face complicated external environments such as bending, twisting, and other mechanical deformation. In this case, the weak binding between the active material and the metal collector, together with the volume variation occurred on the electrode material during the repeated charging and discharging, cooperatively lead to the detachment of active materials from the current collector, which will directly lead to the overall performance degradation of the LIB as well as the whole electronic system. At the same time, additional inactive materials will also reduce the energy density of the battery system [22], [23].
One commonly adopted strategy for solving these issues is to design a flexible electrode with active materials directly loaded on a mechanically flexible and electrically conductive substrate, therefore avoiding the use of conductive additives or binders and eliminating the complex coating processes. For a long time, carbon nanotubes (CNTs), carbon nanofibers (CNFs), and other forms of carbonaceous materials have been assembled into flexible electrodes for LIBs because of their excellent mechanical and conductivity properties [2], [21], [24], [25], [26], [27]. However, if these carbonaceous materials are used alone as the electrode, they tend to perform poorly due to their intrinsically low specific capacity [19], [23]. Therefore, high-capacity active materials are integrated with these flexible substrates, presenting a possible solution to overcome the low-capacity drawback of the traditional carbon-based flexible materials.
In recent years, metal silicate nanomaterials have received extensive attention due to their high capacity, good rate capability, high safety, and abundant reserves on the earth [28], [29], [30], [31], [32], [33], [34]. These materials typically have a layered silicate structure, providing a large interlayer space for the transportation and storage of lithium ions, which ensures the high utilization of the active species during LIB operations. In addition, their stable structure, especially nickel silicate, can also accommodate the volume change caused by the electrochemical charging and discharging process to a certain extent. These advantages make nickel silicate an ideal material for the storage of alkaline metal ions, including Li+ ion and Na+ ion [4]. However, the small number of redox couples and poor conductivity of conventional silicate materials usually limit their capability to store Li ion at high capacities or high rates [34], [35]. Therefore, improving the electronic conductivity and introducing more redox reactions are considered to be an effective way to improve the electrochemical performance of nickel silicate anode materials.
As for improving the silicate materials’ conductivity, one conventional method for is to combine them with highly conductive carbon materials, such as graphite [36], CNTs [37], and graphene [38]. Besides, the shape and size of the electrode material are also important factors affecting the final electrochemical performance [39]. As for providing more redox couples for higher capacity of Li ion storage, introducing a secondary metal species to the silicate system can have been proven an effective solution, which may enhance the intrinsic electronic conductivity of the silicate material as well [33], [35]. Apart from this, the design of high-performance electrode materials, an efficient interconnected 3D network is often necessary. Especially, constructing a porous network of 1-dimensional CNTs can provide not only excellent electron transport characteristics but also abundant buffering space for the volume variation of the active materials [40].
Based on the above considerations, we herein present a dual-metal NixCoy-silicate@CNTs flexible film electrode. On this material, the NixCoy-silicate nanosheets are bridged and interconnected by CNTs, providing suitable channels for the rapid transportation of lithium ions and electrons and ensuring the flexibility of the whole material. Benefiting from these merits, the NixCoy-silicate@CNTs films possess excellent flexibility, high rate performance, and decent cycling stability simultaneously.
Section snippets
Starting materials
Absolute ethyl alcohol (C2H5OH, GR, Tianjin Jiangtian Chemical Technology Co., Ltd, China) and tetraethyl orthosilicate (C8H20O4Si, TEOS, AR, Tianjin Guangfu Fine Chemical Research Institute, China) were selected as the precursors for fabricating flexible SiOx/CNTs film. Ferrocene (C10H10Fe, 1.9 wt%, AR, Tianjin Guangfu Fine Chemical Research Institute, China) and thiophene (C4H4S, 1.0 wt%, AR, Tianjin Guangfu Fine Chemical Research Institute, China) were used as the catalyst and promoter,
Materials synthesis and characterization
The flexible NC/SC hybrid film was fabricated through a silica-mediated conversion process [41]. First, continuous and free-standing SiOx/CNT hybrid films were prepared by the modified chemical vapor deposition (CVD) method in a vertical tube furnace (Fig. 1a) [42]. Our equipment is able to continuously produce paper-like thin films in about half an hour and ensure an industry scale production under a continuous supply of precursors. The obtained paper-like SiOx/CNT hybrid film (16 × 12 cm,
Conclusions
In conclusion, through SiOx-mediated transformation, we have achieved uniform formation of silicate nanocrystalline sheets on highly graphitized and few-walled CNT networks, which are controllable in terms of nanocrystalline aggregation, resulting in a flexible and free-standing composite film for energy storage applications. By combining the advantages of high loading density of silicates, high electrical conductivity, and good flexibility of CNTs, the NC/SC composite film achieves remarkable
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
This work was supported by the National Natural Science Foundation of China (No. 51072130, 51502045, and 21905202), and the Australian Research Council (ARC) through Discovery Early Career Researcher Award (DECRA, No. DE170100871) program.
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