Solid-state batteries: A promising technology thriving under pressure

Solid-state batteries (SSBs) are currently a hot research topic in the field of electrochemical energy storage. Many believe that solid-state battery technology is the successor of lithium-ion—especially in the context of electric vehicles. The technology has the potential to revolutionize energy storage in several ways. SSBs boast a high energy density, have extended lifespans and fast-charging capabilities, and are safer than traditional Li-ion batteries.  

Solid-state batteries are intrinsically different from Li-ion batteries. Both their fabrication methods and testing conditions remain to be fully standardized, from the research laboratory environment to the production line. Notably, Japan, China, and the European Union have set ambitious targets to commercialize the technology by 2030 [1]. This blog article discusses the general differences between SSBs and Li-ion batteries, challenges that remain to be overcome for commercial production of SSBs, and the use of Electrochemical Impedance Spectroscopy (EIS) for testing different battery parameters.

Why could solid-state be the future of batteries?

State-of-the-art lithium-ion batteries (LIBs) are usually composed of two insertion electrodes (anode and cathode) with a liquid electrolyte in between (Figure 1, left). This liquid electrolyte is an ionically conductive medium which allows lithium ions to shuttle between the anode and cathode where it is intercalated, allowing for storing (charge) or dissipating energy (discharge). The anode and cathode are electronically separated by a nonconductive membrane. This measure prevents direct contact between the electrodes and avoids short circuits. 

On the other hand, the electrolyte in solid-state batteries (SSBs) is solid and serves as a separator between the anode and the cathode (Figure 1, right). This means that the anode and cathode materials must be in contact with the solid electrolyte which will facilitate the diffusion of lithium ions. This difference in the nature of the electrolyte comes with many promises regarding performance and safety.

What are the main advantages of SSBs vs. LIBs?

In this section, four major topics are discussed: safety, energy density, voltage, and charge rate.

While this seems promising, some challenges remain to be overcome – from the discovery and testing of new materials to the scaling up of production at levels equivalent to the current Li-ion industry [2]. Some of these challenges are discussed in more detail in the following sections.

Challenge #1: Lack of standard testing and assembly protocols

As solid-state batteries are a more recent addition to the repertoire of most academic laboratories, there are hardly any standardized equipment or procedures to reliably benchmark new materials or fabrication procedures. 

Homemade setups in which the components (anode composite, solid electrolyte, cathode composite) are layered successively and compressed into a pellet/cylinder are still the most common practice. Although there are doubts about the scalability of this format, it remains simple and straightforward. 

Off-the-shelf setups for the fabrication and testing of these cells are beginning to appear on the market and should lead to more reproducible and comparable results between laboratories.

Challenge #2: Fabrication pressure

During SSB assembly, it is necessary to form and maintain good contact between the different solid materials: solid electrolyte, electrodes, and possibly carbon additives [3]. Good intermixing and packing is critical. Many mixing methods are suitable, from simple manual co-grinding in mortar and pestle, to ball-milling, etc.

Once mixed, pressure is key – specifically the fabrication pressure (between 100–1000 MPa), which is significantly higher than operating pressure. The separator layer (purely solid electrolyte) is typically formed first by applying ~100 MPa to form a solid base. Then, the electrode composites are added in a similar fashion. 

The electrodes and solid electrolyte are typically brittle, can easily fracture, and form porous and inactive surfaces. Therefore, the pressure is critical—in particular the maximum pressure and pressure profile during both pressing and releasing. 

Challenge #3: Operating pressure 

After fabrication, pressure continues to play a critical role during cycling. Most cathode materials (e.g., LiCoO₂) will expand and contract upon lithiation (charge) and delithiation (discharge), resulting in delamination and/or cracking (Figure 2). Both of these situations create dead surfaces, increasing the internal resistance of the battery. 

Too little pressure is not enough to maintain sufficient contact. However, too much pressure can lead to rising overpotential or short circuits. Controlled pressure helps to alleviate these so-called «chemo-mechanical» issues to a certain degree [4]. The exact amount of pressure for a SSB to thrive is still an open question and depends on the chemistry and cell, and later, on stack designs.

Challenge #4: Needs for meeting the testing conditions

At the laboratory level, when testing new materials or configurations (beyond usual cycling), one of the most informative techniques regarding the battery state is Electrochemical Impedance Spectroscopy (EIS). With EIS, diverse phenomena within each component (e.g., electrode materials, electrolyte) or at the interfaces can be separated and investigated. 
 

Check out some of our related Application Notes to learn more about EIS and its applications for batteries.

Electrochemical impedance Spectroscopy (EIS) Part 1 – Basic Principles

Electrochemical Impedance Spectroscopy (EIS) Part 2 – Experimental Setup

EIS is used on batteries to understand dynamic physical properties, such as the conductivity of the electrolytes, electron transfer in the bulk, capacitances at phase boundaries, and more [5]. It is expected that these parameters can be measured during the operation of the battery and be analyzed to provide information about its state of health (SoH) or state of charge (SoC).

One peculiarity of SSBs is that properties of the bulk of the solid electrolytes can be observed only at a very high frequency (>1–5 MHz). This presents a challenge for the measurement of these properties. Very few potentiostats/galvanostats can measure beyond a few hundred kHz (like VIONIC powered by INTELLO), while bulk properties of SSBs are accessible only from 1 MHz up to 10 MHz.

EIS was successfully applied to decipher pressure effects coming from boundaries between grains and the grains themselves in solid electrolytes (Figure 3). This makes EIS an ideal tool to investigate increased porosity – cracking which affects the bulk materials as well as their interfaces. For example, positive pressure effects during cycling or operation were monitored by EIS and attributed to increased conductivity between grains, while the bulk conductivity of the grains remain unchanged. This means that SSBs benefit from applied/controlled pressure during operation, which should guide the design of future cells and packs. 

Examples in the work of Vadhva et al. [6] show the power of EIS for solid-state batteries. They use EIS to research temperature, composition, and assembly-pressure effects on SSBs. This could be used in battery management systems to assess the SoH and SoC of individual cells.

EIS up to 10 MHz: the challenges

Measuring EIS at such high frequencies requires not only a carefully chosen instrument, but also the right setup to ensure the highest data quality: namely short cables and a limited number of junctions between the potentiostat and the cell. A four-point contact or Kelvin-type measurement is essential to ensure high-quality results. The following Application Note explains this in more detail.
 

The importance of using four-terminal sensing for EIS measurements on low-impedance systems

This is yet another reason to standardize the way cells for SSBs are assembled and tested to ensure complete transparency of results and their interpretation.

Outlook and conclusion

Solid-state batteries have a bright future ahead. They should provide a safer, faster-charging, more volume-efficient energy storage solution for many applications. 

With the rising interest in SSB research, it is imperative to standardize and properly report fabrication and testing parameters for solid-state cells, especially when it comes to the pressure during assembly and use (or testing). 

Among the tools available to researchers, EIS at high frequency can help monitor various effects at an early stage of the development of new materials. Such practices should increase the reproducibility of results among different labs. This will hopefully help speed up the industrial adoption of research breakthroughs into practical cells to see them available on the market by 2030. 

Interested in battery research on a higher level? Contact us for a VIONIC powered by INTELLO demonstration!

References

[1] The Roadmap. Battery 2030+. https://battery2030.eu/research/roadmap/ (accessed 2023-10-09).

[2] Janek, J.; Zeier, W. G. Challenges in Speeding up Solid-State Battery Development. Nat. Energy 2023, 8 (3), 230–240. DOI:10.1038/s41560-023-01208-9

[3] Bielefeld, A.; Weber, D. A.; Janek, J. Modeling Effective Ionic Conductivity and Binder Influence in Composite Cathodes for All-Solid-State Batteries. ACS Appl. Mater. Interfaces 2020, 12 (11), 12821–12833. DOI:10.1021/acsami.9b22788

[4] Lewis, J. A.; Tippens, J.; Cortes, F. J. Q.; et al. Chemo-Mechanical Challenges in Solid-State Batteries. Trends Chem. 2019, 1–14. DOI:10.1016/j.trechm.2019.06.013

[5] Wang, S.; Zhang, J.; Gharbi, O.; et al. Electrochemical Impedance Spectroscopy. Nat. Rev. Methods Primer 2021, 1 (1), 41. DOI:10.1038/s43586-021-00039-w

[6] Vadhva, P.; Hu, J.; Johnson, M. J.; et al. Electrochemical Impedance Spectroscopy for All-Solid-State Batteries: Theory, Methods and Future Outlook. ChemElectroChem 2021, 8 (11), 1930–1947. DOI:10.1002/celc.202100108

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