Hierarchical TiO2-x nanoarchitectures on Ti foils as binder-free anodes for hybrid Li-ion capacitors
Graphical abstract
Introduction
Supercapacitor (SC), also called electrochemical capacitor, is one of widely used in electrochemical energy storage (EES) devices due to its advantages of high power density, fast charge/discharge efficiency, long cycle life and low cost. However, the low energy density limits its further widespread use [1], [2]. In contrast, the Li-ion battery (LIB) displays high energy density with lithium de/intercalation, but relatively lower power density and shorter cycle life [3], [4]. Therefore, a new type EES device of hybrid Li-ion capacitor (LIC) combining the high-power SC and high-energy LIB has drawn more attention for energy sources in hybrid electric vehicles (HEVs) and electric vehicles (EVs) in future [5], [6]. In general, LICs employ a SC-type cathode with carbonaceous materials (e. g. active carbon, carbon nanotube, graphene) and a LIB-type anode (such as TiO2, Li4Ti5O12) in non-aqueous electrolyte [7], [8], [9], [10], [11]. Thanks to the electron adsorption/desorption and Li-ion insertion/extraction storage mechanisms, LICs exhibit higher power density than LIBs and higher energy density than SCs.
Recently, titanium-based compounds were extensively explored as anode materials for high-performance LICs. Among them, TiO2 has great development prospect due to the advantages of abundance, stability, low-cost and safety. There are many methods to prepare TiO2, such as sol–gel, hydrothermal method, anodic oxidation and so on. However, the poor electrical conductivity and low ion diffusion coefficient of TiO2 seriously affect the rate capability and power capacity of LICs. Therefore, many strategies are developed to overcome the shortcomings of TiO2. Generally, to use 0D to 3D TiO2 nanostructures has been considered as an efficient way to enlarge the surface area and shorten the Li-ion diffusion paths [12], [13], [14]. Compared to disordered nanostructures, the oriented and free-standing 1D TiO2 nanostructures (e. g. nanowire, nanotube, nanorod) on conductive substrates can effectively shorten ion diffusion path and improve the electrical conductivity in binder (polyvinylidene difluoride, PVDF) free system [15]. In addition, the phases of TiO2 (anatase, rutile and bronze (B) phases) also play an important role in materials properties and electrochemical performance. Recently, many studies have suggested the benefits of artificially create biphasic boundaries (anatase/B phase or anatase/rutile phases) for improving capacity and rate capability [16], [17]. Furthermore, doping with other conductive materials is also an useful protocol to further improve the electrochemical performance [18], [19], [20]. Thus, the structure design and phase control are important for TiO2 as an anode material in LICs.
Additionally, many researches showed that having a TiCl4 treatment on TiO2 film can adjust the conduction band edge, enlarge the surface area, increase light harvesting and electron diffusion coefficient for solar cells using TiO2 as electron transport layer [21], [22], [23]. However, the effect of TiCl4 treatment on TiO2 in EES devices has not been clarified. Herein, we successfully fabricated titanate nanowires/nanosheets (NWS) on Ti-foil by a simple hydrothermal method combined with an ion-exchange reaction. After annealed in Ar/H2 atmosphere, the TiO2-x NWS with oxygen vacancies (OVs)-doping was obtained. By treating with TiCl4 solution, the TiO2-x nanocrystallines finally formed on the TiO2-x NWS surface. The purpose to designing this kind of hierarchical TiO2-x NWS/nanocrystallines (NWSC) nanoarchitectures is to build the regular and well-organized structure, which may effectively improve the electronic conductivity and structural stability. Meanwhile, attaching the TiO2 nanocrystallines on the TiO2 NWS can increase more active sites and contact areas for ion and electron transportation. As will be shown, the as-prepared TiO2-x electrode exhibits a high initial discharge capacity of 562.3 mAh g−1 and excellent cycling stability (more than 228.8 mAh g−1 after 1000 cycles at 1 A g−1). Thus, the hybrid LIC assembled with a TiO2-x NWSC anode and an activated carbon (AC) cathode presents a maximum energy density as high as 44.2 W h kg−1 and delivers an energy density of 3.1 W h kg−1 even at a high power density of 7.5 kW kg−1, which exhibits fascinating potential in high-energy and high-power EES devices.
Section snippets
Materials
All the chemicals were the analytical grade and were used without further purification. Activated carbon (surface area >2100 m2 g−1, and the particle size ∼10 μm) was acted as the cathode material. Ti-foils (99.7%, 0.032 mm thick, Alfa Aesar) provide Ti source for TiO2-x hierarchical structure and served as substrates at the same time. Teflon rings with external diameter of 35 mm and the inner diameter of 25 mm were used as spacers for Ti-foils during hydrothermal process.
Hierarchical TiO2-x structure synthesis
As illustrated in Fig.
Anode material: TiO2-x NWSC nanoarchitectures
The formation mechanism of TiO2 NWAs on Ti-foil has been documented in many literatures [24], [25]. For controllable fabrication, the hydrothermal process of Ti-foil in NaOH solution mainly including pure Ti dissolution, sodium titanate diffusion and sodium titanate precipitation crystallization was used more often. At first of the hydrothermal reaction, a thin and compact sodium titanate nanocrystallines were formed on Ti-foil. With hydrothermal reaction time increase, sodium titanate
Conclusions
In summary, the hierarchical TiO2-x NWS structure was successfully fabricated on Ti-foil by an improved hydrothermal method and an ion-exchange reaction. With six pieces of pretreated Ti-foil in teflon-lined stainless steel autoclave, the suitable concentration gradient between Ti-foil and NaOH solution was conducive to the formation of hierarchical structure. After a TiCl4 treatment, the TiO2-x NWSC binder-free electrode achieve superior electrochemical performance of high specific capacity
Acknowledgement
This work was supported by the Natural Science Foundation of Shaanxi University of Science and Technology (2016BJ-49) and the Scientific Research Program Funded by Shaanxi Provincial Education Department (17JK0109).
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