Lattice and electronic structure variations in critical lithium doped nickel oxide thin film for superior anode electrochromism
Graphical abstract
Introduction
The growing demand for intelligent technology has stimulated the development of energy storage and conservation devices, such as lithium-ion battery for electric vehicles [1], solar cells for energy generation [2], supercapacitor for long time power supply [3] and so on. Among all these studies, electrochromic devices have attracted world-wide attention over the past decades due to their potential applications in energy saving and energy storage [[4], [5], [6], [7], [8], [9]]. It is highly important for these devices to possess characteristics of high energy and power density, charge capacity, transmittance contrast as well as working stability, which requires superior materials as electrodes. Among all the potential candidate materials, nickel oxide (NiO), as one kind of wide-band-gap p-type semiconductor and promising energy storage material, plays an important role in the design of electrochromic devices as an anode material [[10], [11], [12], [13], [14]]. Much effort has been made to improve the physical properties of NiO/NiO-based thin film devices [[15], [16], [17], [18], [19]], and chemical doping that demonstrated to be most effective has been widely used. For instance, pure lithium doping [[20], [21], [22], [23]] or lithium-based two/three element doping like aluminum [24], zirconium [25] and magnesium [26,27], are very representative ways due to the increasing demand for electrochromic devices.
Basically, electronic structures of electrochromic nickel oxide thin films change through storage and release of electrons and ions induced by different voltage. This processing is accompanied by the change in visible light transmittance [[28], [29], [30], [31], [32]]. Strategies for improving performance, especially doping, would affects properties like lattice structure, vacancies and electronic structure, and contributes to the change in performance like capacity, color, optical modulation, etc [[33], [34], [35], [36], [37]]. Also, nickel oxide could be thought as a kind of photonic crystals, which could change the color electrically and reversibly by alterations in photonic lattice [38].
The typically reversible electrochromic reaction of the (lithium doped) NiO films in non-aqueous Lithium based electrolyte could be presented as follows [39,40]:Liα0Ni1-x1O (colored) + α1Li+ +α1e− ↔ Li(α0+α1)Ni1-x2O (bleached)Li(α0+α1)Ni1-x2O (bleached) ↔ Liα2Ni1-x3O (colored) + α3Li+ +α3e-
Here α0+α1 = α2+α3, α0 equals zero for NiO thin films and larger than zero if Li+ is doped. And the valence states of nickel ions change as the oxidation-reduction reactions proceeding, and accompanied by the ions injection or extraction [41,42]. However, microstructure change of nickel oxide could hardly be observed in these processing. Also, the study on doping content that positively contributes to ions storage and optical performance still need to be enriched, especially in microstructure. Furthermore, the transport of lithium ions in electrochromic devices is closely related to the principle of lithium batteries. Therefore, an insight of electrochromic process combined with structure change may not only become the key for improving anode ion storage material, but also open up a new idea for lithium-ion and related ion transport problems.
Here, we demonstrate the tunable appropriate lithium-doped nickel oxide thin films (Li-NiO) show a marked improvement of the electrochromic performance compared with undoped NiO. To run the coloring and bleaching process under the safety voltage (±1.5 V for 30 s) respectively, the transmittance contrast of Li-NiO thin film could reach 66.5% which is much higher than most of the traditional anode materials. Also, the charge capacitance of the Li-NiO thin film (13 mF cm−2) more than twice of the NiO thin film which present a bright prospect in energy saving application. Even more interesting is the micromechanics of the Li-doped NiO film under the reversible electrochromic reaction. The lattice structure changes of the thin films in bleached and colored states were analyzed through X-ray Diffraction (XRD) measurement. An obvious reversible lattice distortion can be observed which is rarely appear in the NiO thin film. The further transmission electron microscopy (TEM), source atomic emission spectrometer (ICP) and X-ray photoelectron spectroscope (XPS) measurements confirmed the fundamental reason on the enhanced performance of the Li-NiO. Density functional theory (DFT) based first-principles calculations reveal the physical origin of the electrochromic process, i.e. defects states induced by Li dopant effective reduces the band gap of NiO that contributed to the light absorption and coloring/bleaching processes. In addition, the improvement and the electrochromic process of Li-NiO thin film are elaborated and characterized through intelligible models.
Section snippets
Deposition of thin films
NiO and Li-NiO thin films were prepared by reactive direct current (DC) magnetron sputtering. The targets were 6-cm-diameter pure nickel (99.9%) and Li2O-Ni alloy target (Atom ratio RLi/Ni = 1), respectively. The substrates were In2O3: Sn coated (ITO) glass, and Table 1 is the list of process parameters. In the sputtering process, the slant angle of the target is 15°, distance from substrate to the target is 25 cm, and the substrates holder were kept rotating by a constant speed at about 12°/s.
Theoretical calculations
The DFT based first-principles calculations were performed by using projector-augmented wave (PAW) [43] as implemented in the VASP code [44,45]. The energy cutoff for the plane-wave basis was set to 550 eV. For the exchange-correlation functional, we used the screened hybrid functional of Heyd, Scuseria, and Ernzerhof (HSE06), in which 75% of Perdew-Burke-Ernzerhof (PBE) exchange is combined with 25% of non-local Hartree-Fock exchange, and the screening parameter that separates the exchange
Preparation of NiO and Li-NiO thin films
The films were prepared by reactive magnetron sputtering. We used pure nickel (99.99%) target to deposit NiO thin films and Li2O-Ni (Atom ratio RLi/Ni = 1) alloy target for Li-NiO thin films. Samples were well-designed by and easy and one-step depositing process adjusting conditions in order to ensure the good uniformity of doping and controllable deposited parameters. As Fig. 1a shows, we deposited the Li-NiO thin films on In2O3: Sn coated (ITO) glass under the atmosphere consist of different
Conclusion
For the summary, we deposited a kind of lithium doped NiO thin film with high energy capacity and good electrochromic performance. The physical mechanism reason for improving the properties by appropriately lithium doping was discussed and theoretical models for the coloring and bleaching of Li-NiO thin film were built. The Li-NiO thin film shows good ability to color deeply (16.8%) and increase the transmittance contrast (66.8%). And compared with NiO thin film, the switch window of the Li-NiO
Acknowledgements
This work is supported by the National Natural Science Foundation of China (KZ73095501 and KZ73086401),the Science Challenge Project (TZ2018004), the National Natural Science Foundation of China (11674042), and the Fundamental Research Funds for the Central Universities (KG12099501).
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