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Chemistry and interface tailored lead-free relaxor thin films for energy storage capacitors

Periodic Reporting for period 2 - CITRES (Chemistry and interface tailored lead-free relaxor thin films for energy storage capacitors)

Reporting period: 2020-10-01 to 2022-03-31

The project CITRES “Chemistry and Interface Tailored lead-free Relaxor thin films for Energy Storage capacitors” is an ERC Consolidator Grant (GA. 817190), which started in April 2019 and ends in March 2024.

The goal of CITRES is to design novel dielectric capacitors for energy storage applications with high power and energy density based on relaxor thin films.

Energy storage units for energy autonomous sensor systems for the Internet of Things (IoT) must possess high power and energy density to allow quick charge/recharge and long-term energy supply. Current energy storage devices cannot meet those demands: Batteries have large capacity but long charging/discharging times due to slow chemical reactions and ion diffusion. Ceramic dielectric capacitors – being based on ionic and electronic polarisation mechanisms – can deliver and take up power quickly, but store much less energy due to low dielectric breakdown strength (DBS), high losses, and leakage currents. For this reason, they are also not suitable to withstand high voltages, which prevents their use as switching capacitors for power electronics (e.g. in electric vehicles).

CITRES aims to overcome those issues by using relaxor compositions in form of thin films, because:
-Thin film processing allows obtaining low porosity and defects, thus enhancing the DBS and allowing high-voltage operation.
-Slim polarisation hysteresis loops, intrinsic to relaxors, allow reducing the losses. If the dielectric permittivity is maintained high, high energy density can be achieved. At the same time, leakage currents must be minimised. These aspects are controlled by the amount, type and local distribution of chemical substituents in the relaxor lattice, whereas the leakage current depends also on the chemistry of the electrode metal.

In CITRES, we will identify the influence of substituents on electric polarisation (slim loop and permittivity) from atomic to macroscopic scale by combining multiscale atomistic modelling with advanced structural, chemical and electrical characterization methods on several length scales both in the relaxor material bulk and at interfaces with various electrodes. This will allow for the first time the design of energy storage properties of relaxor thin films by chemical substitution and electrode selection.

The specific overall objectives of CITRES, necessary to reach its goals, are:
-To design a lead-free perovskite composition that enables minimising the remanent polarisation while at the same time maintaining high Pmax.
-To produce relaxor-based capacitors using thin film technology and design an optimised interface between electrode and dielectric in order to minimise leakage currents.

The ground-breaking nature of CITRES resides in the design and realisation of relaxor thin film-based dielectric capacitors with better energy storage performances than supercapacitors and batteries, thus enabling energy autonomy for IoT sensor systems and applications in the power electronics field.
The activities performed in CITRES during the period covered by the report are summarised as follows:
• A laboratory for the deposition of lead-free relaxor thin films has been installed and is now fully functioning.
• A reliable procedure to fabricate thin films based on binary BaTiO3 solid solutions, Bi0.5Na0.5TiO3 and their mixtures has been established.
• Modelling procedures have been setup to study the influence of chemical substitution on the local polar order in BaTiO3 solid solutions. This includes effective Hamiltonian approaches to calculate dielectric properties (including energy density) from simulated atomic structures.
• Conventional Raman spectroscopy measurements aided by phonon calculations have identified the presence of charge-compensating defects in the lattice of heterovalent-substituted BaTiO3. Consequently, atomistic simulations pinpointed the effect of those atomic arrangements on the local polar order.
• Sample preparation routes for STEM analyses of thin films have been established. HRTEM analyses on binary BaTiO3 thin films have been carried out.

The project obtained the following main achievements in the first reporting period:
• A model based on Density Functional Theory (DFT) to fully calculate the spectral signature in polycrystalline materials has been developed, allowing the identification of lattice defects or superstructures. The work has been published (M. N. Popov et al., npj Computational Materials, 2020).
• Atomistic simulations combined with Raman spectroscopy have shown that heterovalent substitution is more effective than homovalent one in disrupting the ferroelectric behaviour in BaTiO3 solid solutions. A full correlation was obtained between substitution-induced defects and local polar order. The work is now accepted for publication (V. Veerapandiyan et al., Advanced Electronic Materials, 2022).
• A thin film deposition procedure to obtain thin films of BaTiO3 solid solutions with controlled microstructure has been established.
The following results are expected until the end of CITRES:
-Analysis of local chemical arrangements in substituted relaxor perovskites via STEM methods and determination of local electrical polarisation.
-Analysis of local polar order at atomic scale under dynamical E-field excitation via Raman spectroscopy.
-Simulation of Raman spectra starting from realistic atomic arrangements and calculation of macroscopic electric/dielectric properties.
-Investigation of leakage currents at electrode/dielectric interface by proximity methods, and definition of buffer layers or electrodes able to minimise leakage currents.
-Definition of a chemical composition and a processing route for relaxor thin films with high energy density.

Progress beyond the state of the art:
-A unique methodological setup involving analytical methods (STEM, Raman, AFM) and modelling methods (DFT, Monte Carlo, effective Hamiltonian, Molecular Dynamics) across several length scales is established and applied to studying structure-property relationships of local chemical/polar order in relaxors.
-Designing capacitors with high power density and high energy density, CITRES will open up new possibilities in IoT devices, in which a single miniaturised device can support both fast charge/discharge times and long-term energy supply. This will allow footprint reduction and further miniaturisation capabilities in IoT systems. Furthermore, understanding the link between the local chemical environment and the macroscopic electrical/dielectric properties of capacitors - as enabled by the unique analytical and modelling framework of CITRES - will impact the fundamental understanding of relaxor perovskites and will enable property-driven design of relaxor materials also for high power applications.
Scheme of the CITRES project