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

Project Information

CITRES

Grant agreement ID: 817190

DOI

10.3030/817190

Project closed

EC signature date 26 March 2019

Start date 1 April 2019

End date 31 December 2024

Funded under

• EXCELLENT SCIENCE - European Research Council (ERC)

Total cost

€ 1 996 519,00

EU contribution

€ 1 996 519,00

Coordinated by

SILICON AUSTRIA LABS GMBH
Austria

Periodic Reporting for period 4 - CITRES (Chemistry and interface tailored lead-free relaxor thin films for energy storage capacitors)

Reporting period: 2023-10-01 to 2024-12-31

Summary of the context and overall objectives of the project

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 ended on December 31st, 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 energy storage 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.

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 on defects and on the metal-dielectric interface.

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 (Pr) while at the same time maintaining high saturation polarisation (Ps).
-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 use of a comprehensive multiscale theoretical and experimental framework for 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.

Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far

The activities performed in CITRES 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.
• 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.
• 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 and relating polar order to macroscopic electric properties.
• Sample preparation routes for STEM analyses of thin films have been established. HRTEM analyses on binary BaTiO3 thin films have been carried out.
• The understanding from combined DFT/MD modelling, Raman measurements and TEM/STEM measurements allowed to defined guidelines for chemical substitution of relaxors in order to achieve high recoverable energy density.
• Literature studies and trial-and-error procedures allowed to identify interface architectures for both bottom and top electrodes in metal-insulator-metal (MIM) capacitor structures, which allow to minimize the leakage currents and thus to enhance the efficiency of the capacitors.
• Following the above guidelines, a pool of thin film capacitors, with various compositions, have been realized and characterized for their energy storage properties. The resulting Virtual Device Models confirm that these thin films are suitable as energy storage elements for energy autonomous IoT devices.

The project obtained the following main achievements:
• Development of the spherical averaging method for DFT-based Raman spectra calculation.
• Clarification of difference in the origin of the relaxor behavior in homovalent- and heterovalent-substituted BaTiO3 using an integrated experimental-computational approach. Publication of research advancement.
• Development and consolidation of modelling procedures (DFT, Effective Hamiltonian and MD) for substituted BaTiO3 compositions. The methodology allows a direct link between local atomic scale polar order and both structural and electrical properties.
• Definition of high-energy density relaxor compositions and realization in thin film form as MIM capacitors. Characterization of their unprecedented energy storage properties, including thermal and cyclic stability, and calculation of Virtual Device Models.

Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far)

The following results are available from CITRES:
• Simulation of Raman spectra starting from realistic atomic arrangements and calculation of macroscopic electric/dielectric properties.
• Understanding the influence of substitution on local polar order and macroscopic electrical properties.
• Definition of chemical compositions and processing routes to achieve relaxor thin films with high recoverable energy density.
• Definition of MIM device architectures to maximize energy storage efficiency.

Progress beyond the state of the art:
• A unique methodological setup involving analytical methods (Raman, TEM, AFM) and modelling methods (DFT, MD) across several length scales is established and applied to studying structure-property relationships related to local chemical/polar order in relaxors.
• 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 - impacted the fundamental understanding of relaxor perovskites and enabled property-driven design of relaxor materials, as demonstrated by the Virtual Device Models.
• Designing capacitors with high power density and high energy density, CITRES opened 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 allows footprint reduction and miniaturisation in IoT systems.

Scheme of the CITRES project