Batteries

1. Background and state-of-the-art

Thin film lithium-ion batteries are an important energy storage system for a variety of stand-alone devices including sensors, detectors, medical implants and wearable electronics. Recent advances in electronic and microelectromechanical systems have driven the energy consumption of these devices to a few mW, so microbatteries can provide the energy needed for these kinds of practical applications and can even be integrated on a microchip, supporting commonly used capacitors in terms of energy storage¹. A typical thin film battery consists of two electrodes, separated by solid electrolyte, but in a thin film format. Using a stable electrolyte in a solid form not only enhances battery safety, but also improves the energy density and rate capability of the battery.

Pulsed laser deposition (PLD) enables the preparation of epitaxial thin films of lithium-ion-conducting ceramics on various substrates, where the choice of substrate allows for controlled orientation of the thin films. PLD-prepared thin films serve as a model system for the in-depth understanding of the interplay between crystal structure and the electrochemical properties of ion-conducting ceramics. The ability to grow epitaxial films on single-crystal substrates enables precise structural control on an atomic scale. By fine-tuning deposition parameters, the crystal orientation of thin films can be manipulated, facilitating a deeper understanding of the relationship between structural and functional properties. Furthermore, PLD enables precise interface engineering, as well-defined interfaces improve ionic conductivity, reduce interfacial resistance, and enhance battery performance. This capability is particularly advantageous for thin film microbatteries, where the anisotropic nature of functional materials can be harnessed to enhance device performance through manipulation of crystallographic orientation. 

Figure 1: Schematic of thin film solid-state battery fabrication and possible applications.

Positive electrodes are the main active source of lithium ions, so the overall specific capacity of the full cell depends more on the cathode material. One of the most promising oxide cathodes is a complex LiNiO₂-LiMnO₂-LiCoO₂ (NMC) solid solution. The advantages of NMC materials is a combination of high specific capacity, thermal stability and increased safety². On the other hand metallic lithium negative electrodes are typically used, which limits the thermal stability of the battery, so replacing lithium metal with a stable oxide anode is preferable when it comes to safety concerns of electronic devices. Li₄Ti₅O₁₂ (LTO) is a well-studied oxide anode used in lithium-ion batteries. The main advantage of using LTO in a solid-state thin film battery is near-zero volume change during battery operation, which makes it mechanically stable and long-lasting anode. The most challenging component of a thin film microbattery is a solid electrolyte, which should conduct ions and be stable in contact with both electrodes. Li₃xLa_{2/3-x□1/3-2x}TiO₃ (LLTO) and Li₇La₃Zr₂O₁₂ (LLZO) are oxide-type solid electrolytes where the main ion conduction mechanism is explained by lithium-ion hopping through vacancies that can reach conductivity values around 10^-3 S/cm at room temperature³, comparable to the conductivity of commercial liquid electrolytes. All-oxide solid-state thin film microbatteries, utilizing advanced materials and precise deposition techniques like PLD, offer a promising solution for compact, high-performance energy storage, with optimized safety, stability, and energy density, making them ideal for next-generation electronic and wearable applications.

2. Objectives, originality and impact on new research approaches

The main objective of our research is to gain a comprehensive understanding of the electrochemical behavior of thin films of various ceramic materials, as well as the interfacial processes occurring within solid-state battery (SSB) stacks—particularly at the solid-state electrolyte/electrode interface. In bulk-format SSBs, a range of mechanical degradation mechanisms—including inter- and intra-particle cracking, interfacial fracture and delamination, alongside (electro)chemical degradation processes, such as the formation of secondary phases at interfaces, significantly complicate the identification of dominant failure pathways following extended electrochemical cycling. Additionally, discerning the influence of the intrinsic properties of the active material/solid-state electrolyte interface on the overall performance of bulk SSBs remains a significant challenge, which complicates the evaluation of novel ceramic materials as suitable candidates for integration into SSB systems.

Figure 2: Thin film solid-state battery lamella for Scanning Transmission Electron Microscopy (STEM) analysis. The interface between the thin film electrode and the solid electrolyte, observed with STEM.

Since the atomic scale growth control provided by PLD enables growth of epitaxial heterostructures with very smooth interfaces, we can thereby create phase-pure thin-film SSB stacks with good interfacial contact and much better mechanical stability. As the majority of currently produced SSB systems feature amorphous layers, an epitaxially grown thin-film SSB with a clearly defined out-of-plane orientation also unlocks the possibility to research Li ion movement in specific crystallographical directions.

The versatility of PLD, particularly in terms of highly tunable pressure and temperature conditions, enables the synthesis of widely studied and commercialized battery materials under previously unexplored growth regimes. This, in turn, also gives us the opportunity to explore novel morphologies and electrochemical behaviors in these materials.

3. Unique methodology

Since the field of thin-film Li-ion SSBs is still in its early stages, standardized protocols for electrochemical testing have yet to be fully established. To balance ease of assembly with measurement repeatability and consistency, we employ pouch cell configurations—commonly used in bulk Li-ion battery research—for electrochemical testing of Li half-cells incorporating PLD-grown electrode materials. For more sensitive thin-film solid-state battery stacks, dedicated measurement setups are utilized, often tailored to the specific characterization technique that is being deployed, such as electrochemical impedance spectroscopy (EIS), or galvanostatic cycling (GC). Due to the low amount of active material in a thin-film Li-ion SSB, special care must also be taken to prevent the material from ambient degradation. Most longer-duration structural and electrochemical characterization techniques must therefore be conducted under high vacuum or in an inert atmosphere, making vacuum transfer systems and inert gas glove boxes essential tools in our daily research workflow.

REFERENCES

¹ T. Wu, W. Dai, M. Ke, Q. Huang, and L. Lu, Advanced Science, 2021, vol. 8, no. 19.
² J. M. Kim and H. Chung, Electrochimica Acta, 2004, 49 (6), 937–944.
³ Y. Inaguma, C. Liquan, M. Ito, and T. Nakamura, Solid State Commun., 1993, 86, 689.