High-frequency EIS: a powerful tool for the future of mobility

11.03.2024 г.

Артикул

The market for electric vehicles (EVs) is growing rapidly due to environmental and economic factors. As EVs become more mainstream, developments in battery technology will be critical to support the energy storage needs of this growing industry. Solid-state batteries (SSBs) offer a promising alternative to conventional lithium-ion battery technology. Electrochemical characterization of SSBs can be difficult, but by using electrochemical impedance spectroscopy (EIS) at high frequencies (up to 10 MHz), rapid processes are more easily captured.

Battery charging status interface on electric vehicle

Introduction

Electric vehicles offer zero direct emissions and lower fueling costs compared to vehicles powered by fossil fuels. Global EV sales reached 13.6 million units in 2023 and these numbers are projected to rise significantly in the near future [1,2].

The batteries that power EVs must store more energy while also being safer, smaller, lighter, and less expensive than current technology allows. Improvements in energy density are particularly important, as battery packs are one of the heaviest and most expensive components in EVs. Improving battery performance will dictate the pace at which automakers can produce EVs that rival combustion engine vehicles in terms of driving range and purchase price.

As discussed in a previous blog post, solid-state batteries (SSBs) are a potentially superior alternative to Li-ion batteries (LIBs). SSBs could help advance the large-scale adoption of EVs by providing higher energy density using a solid electrolyte material rather than a flammable liquid electrolyte. The inherent toughness of solid electrolytes helps improve safety compared to lithium-ion batteries by greatly reducing the risk of fire from short circuits. In addition, solid electrolytes are typically both chemically and thermally more stable than liquid electrolytes, reducing degradation and dendrite formation over time.

Despite still being in the research and development phase (aside from some exceptions [3]), SSB technology holds great promise for enhancing battery performance. This includes allowing for higher voltages, longer battery life, and faster charging capabilities. Significant challenges remain, however, in developing solid electrolytes that can conduct ions as effectively as liquids do at room temperature.

Though all-solid-state battery systems have great potential, they encounter contact issues at the interfaces between the cathode and electrolyte composite (Figure 1, right). These «solid-solid» interfaces pose challenges to the efficient flow of ions and electrons within the battery.

To address this limitation, researchers have proposed hybrid solid/liquid electrolyte (SE/LE) systems. By incorporating a liquid electrolyte component, these systems aim to enhance the cathode performance and mitigate the contact issues described above [4].

Characterization techniques for solid-state batteries

SSB characterization presents new electrochemical challenges to researchers. This is due to the use of novel materials in SSBs compared to those found in conventional LIBs.

In liquid cells, electrochemical impedance spectroscopy (EIS) measurements are often limited to below 100 kHz (see Application Notes at the end of this article). However, the time constants associated with fundamental processes in solid electrolyte batteries (e.g., intragrain Li-ion diffusion within the bulk of the grains and intergrain diffusion occurring at the grain boundaries) occur on drastically faster timescales [5].

Figure 2 shows the impedimetric profile generated with the simulation tool available in the NOVA software from Metrohm Autolab, based on the data published by Fuchs et al. [6]. The experimental setup consisted of a mixed solid/ionic liquid electrolyte (SE/ILE) with symmetrical lithium metal electrodes.

The Nyquist plot in this configuration displays four semicircles. These were generated through a modeling approach that incorporates five distinct time constants using a proportional weighting method.

In the lower frequency range, three time constants are identified. One is associated with the electrochemical reaction (RC_{EC Reaction}) at the lithium metal anode. The other two, which are combined (RC_{SLEI + SEI}), represent the ionic transfer across the SE/ILE phase boundary, taking into account both the solid-liquid electrolyte interphase (SLEI) and the solid electrolyte interphase (SEI) [6].

At intermediate frequencies, the small semicircle represents ion mobility between the grain boundaries of the solid electrolyte (RC_{Grain boundaries}). At higher frequencies, the semicircle corresponds to ion mobility within the bulk of the solid electrolyte grains (RC_Bulk). The uncompensated resistance of the liquid electrolyte is negligible, given its presence is limited to an extremely thin interlayer [7].

Comparing the two curves in Figure 2, it is clear that an analysis limited to 1 MHz would be insufficient to fully characterize this cell. The semicircle representing ion mobility within the bulk appears only at higher frequencies.

Proper instrumentation for SSB research

Traditional potentiostats/galvanostats (PGSTATs) used for EIS typically have a maximum usable frequency range of 1 MHz or less. While sufficient for characterizing most liquid cells, this upper limit is not adequate for resolving the impedance signatures of transport mechanisms in solid electrolytes. Solid electrolytes of practical importance are frequently polycrystalline or polymeric, and bulk and grain boundary conductivities must be taken into account [6].

State-of-the-art PGSTATs with a frequency response analyzer (FRA) have been developed to perform EIS tests up to 10 MHz (one order of magnitude higher than standard PGSTATs). Such PGSTATs have become essential tools in SSB research and development.

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Practical aspects when measuring EIS at high frequencies

Proper experimental setups and hardware capable of high-frequency ranges above 1 MHz are necessary to fully understand ion transport mechanisms in novel solid-state materials [7].

To ensure accurate EIS results beyond 1 MHz, it is crucial to emphasize the significance of employing short, well-connected wires. This is a standard feature included with VIONIC, addressing potential stray impedance contributions from cables and connectors. These contributions may compromise the integrity of a measurement at such high frequencies (see Application Notes at the end of this article).

Conclusion

EIS has emerged as an essential tool in battery research, valued for its high precision and short execution time.

Consolidated EIS methods reaching up to 100 kHz are generally suitable for standard lithium-ion batteries, but they fall short in capturing rapid processes such as ion diffusion in the bulk or at the grain boundaries of solid electrolyte.

As the bulk conductivity is a critical parameter for evaluating SSBs or «hybrid» SE/LE batteries, the choice of a PGSTAT able to reach an EIS frequency up to 10 MHz is crucial for this kind of application.

If you have more questions, please reach out to your nearest Metrohm Autolab support office for help and further recommendations. Feel free to contact us for a demonstration!