Capacitive energy storage
1. Background and state-of-the-art
The global insatiable demand for ubiquitous and ever smaller electronic devices calls for the development of advanced energy storage (ES) materials and associated technologies. Currently, the two main types of technologies for ES are batteries and capacitors. Batteries typically exhibit higher energy densities.1–5 However, batteries usually have a limited electrical power output, due to the slow movement of the charge carriers involved. Capacitors, on the other hand, have the ability to release the stored charge extremely rapidly6,7, creating high currents and power densities for a short time. This capability is essential for various practical applications, such as medical devices, pulsed power systems, hybrid electric vehicles, etc. Adding into the equation the environmental and safety issues related to batteries, e.g. Li-battery overheating8, solid-state technologies in the form of capacitors are becoming increasingly interesting and promising for the use in the above-mentioned and other industrial areas. Compared to their counterparts, dielectric capacitors based on antiferroelectric (AFE) materials display higher energy densities and higher power/charge release densities. The origin of this is visualized in Figure 1, where the polarization-electric field (P-E) hysteresis loops for the different types of dielectric materials are shown.
The recoverable energy density (Wreco) is represented by the blue shaded areas and the grey shaded areas represent the energy loss (Wloss). The sum of Wreco and Wloss is the stored energy density (Wst). Device efficiency (η) is defined as η = Wreco / Wst. AFE materials are prime candidates for ES applications, owing to their very high Wreco values. This is due to the presence of phase transitions between ordered and disordered states that can be triggered by electric field, strain and/or temperature, leading to an increase/decrease of the order parameter (polarization)9. Their η can be further improved by engineering them in such a way as to introduce relaxor-like behavior.10–12
Figure 1: Examples of typical P-E loops for different types of materials. The blue and grey shaded areas represent Wreco and Wloss, respectively.13
Despite the clear advantages of AFE for ES, numerous issues regarding the understanding of the mechanisms, as well as technical matters, remain to be solved. Many research groups around the world are currently working on improving our understanding of AFE materials and finding ways to further improve the properties important for ES. As environmental concerns are becoming ever more prominent, the need to move away from Pb-based materials is increasing. Therefore, a large fraction of the aforementioned groups is studying various Pb-free materials.
One of the highly promising Pb-free candidates for ES are AgNbO3-based (AN-based) materials. The highest Wreco value achieved in La-modified AN ceramics (= 7.0 J cm-3)14 is comparable to the values obtained in state-of-the-art Pb-based systems, where the highest value has been reported15 for (Pb, La)(Zr, Sn)O3 based ceramics with Ba substitution (= 12.8 J cm-3).
The strong and increasing interest in the development of advanced AN-based materials is also reflected in the very high number of related peer-reviewed publications in the past 5 years. Together with the increasing demand for interim ES and device miniaturization, this attests to the timeliness of research dealing with Ag(Nb,Ta)O3 (ANT) thin films.
2. Objectives, originality, and impact on new research approaches
Before AFE materials can be used in actual ES applications, several critical issues must be addressed.
Numerous studies have been focused on improving the ES performance of AN-based ceramics, mainly using chemical modifications and microstructure engineering.
A significantly smaller amount of research has been devoted to studying AN-based thin films. Thin films, particularly epitaxial ones, can exhibit excellent crystalline quality with a low concentration of planar and point defects, thereby drastically improving the DBS. Consequently, a higher Pmax can be obtained. With epitaxial strain, the AFE-FE phase transition can be shifted to higher fields, which are reachable in thin films, due to the higher DBS. Doping (with rare-earth elements) and nanograin/nanodomain engineering can reduce the remnant polarization and hysteresis.
Very recent studies have shown AFE behavior in AN-based thin films. Nevertheless, based on the properties of AN-based ceramics, which are comparable to state-of-the-art Pb-based materials, a breakthrough is to be expected in the field of AN-based thin films. The use of thin films enables the exploitation of additional degrees of freedom. The application of biaxial epitaxial strain acts to mechanically confine the material, thereby changing/shifting the phase transitions. However, while epitaxial thin films can generally be synthesized with a low defect density, owing to relaxation mechanisms at the interface, which include the formation of dislocations and other defects, their quality can be lowered. This is especially notable in very thin films, just above the critical thickness, where the contribution of the interface is substantial.
Furthermore, substrate clamping presents a serious issue for realizing the full potential of the films under electric field stimulation. Nonlinear behavior is often elusive in thin films. We will use 2D materials at the substrate-film interface to achieve dislocation-free relaxation and alleviate the effects of substrate clamping on the ES properties of the thin films.
Limitations in the development of AN-based materials are also related to the high volatility of Ag. Fabricating high-quality AN-based ceramics is challenging due to the fact that the most commonly used starting material Ag2O decomposes at ~ 200 °C16, which can lead to the formation of Ag precipitates in the form of micro- or nanoparticles17, which deteriorate the electrical properties18. This issue can be alleviated by sintering in oxygen atmosphere. It should be noted, that in contrast to the undesired effect caused by the precipitation of pure Ag particles, a small amount of Ag vacancies in the AN matrix has actually been shown to be beneficial for the ES properties19. Therefore, in the project we will also clarify the issues related to Ag2O decomposition and the effect of Ag vacancies. The know-how obtained our past work on PMN-PT thin films (where the high volatility of Pb also presented a challenge) will be used to tackle these issues.
Our innovative approach is built on the following strategies:
a. Leveraging epitaxial strain
Single-crystalline-like epitaxial thin films offer important advantages over their polycrystalline counterparts, owing to their high density, low defect concentration and the possibility to tune their properties via epitaxial strain. Higher DBS and higher polarization values can be obtained in such films. Note, that for achieving such properties in practice, the optimization of the synthesis procedure is crucial. We will determine the complete set of pulsed-laser deposition (PLD) parameters to control the growth of epitaxial high-quality ANT thin films with a defined composition. Special attention will be given to potential Ag loss. The objective for this part of the study is to obtain a thorough understanding of the effects of epitaxial strain on the phase transitions and film properties.
b. Exploiting an alternative strain relaxation mechanism and declamp the response of the films
In strained films, nonlinear behavior can be inhibited by the substrate clamping. We will overcome this issue by introducing a 2D material (graphene) to the substrate-film interface. The idea is to exploit the principle of remote epitaxy, as the atomic potential fields can penetrate a monolayer or bilayer of graphene. Graphene abruptly relaxes the strain by interface displacements on graphene’s slippery surface. The effect is more prominent in highly mismatched systems. The principle of strain relaxation is shown in Figure 2.
Figure 2: Strain relaxation in conventional epitaxy via the formation of dislocations (top row) and strain relaxation in remote epitaxy (bottom row).46
c. Using this innovative synergistic approach to deliver films with beyond state-of-the-art ES performance by combining strain engineering with doping
By designing relaxor-like AFE materials via increased local chemical disorder using rare-earth doping, increased ES efficiency can be realized. Furthermore, this enables high ES performance at lower electric fields. Through understanding and tuning the combined chemical and mechanical pressure, ANT thin films with beyond state-of-the-art energy storage characteristics (Wreco > 70 J cm-3, η > 90 %) are expected to result from this project.
3. Unique methodology
Our unique methodology makes use of synergistic effects of different approaches, summarized in Figure 3. The desired ferroic phases will be achieved via strain engineering, their response will be declamped using graphene and the yield will be increased using dopants.
Figure 3: A schematic overview of the methodology to achieve higher energy storage performance.
PLD targets will be prepared in-house. The process will be optimized in order to prevent the decomposition of ANT and to ensure a good overall stoichiometry and density of the PLD targets with and without rare-earth dopants.
ANT films with different dopants will be prepared. The film thickness will be varied between ~ 10 nm and ~ 300 nm in order to achieve the desired properties. The difference of the thermal expansion coefficients of the substrates and thin films will also be taken into consideration. As we have shown in our past work on Pb-based films,20,21 we can engineer polar defects in the films via tuning the target composition and introduce an imprint, without the need of post-processing. The imprint will have beneficial effects on the energy storage properties.
A selection of in situ (RHEED, MOSS) and ex situ (AFM, XRD-RSM, XRR, SEM-EDXS, STEM with iDPC detector, dielectric and ferroelectric measurements) analysis techniques will be used in order to gain a unique insight into the structural and functional properties of the films and multilayers/superlattices.
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