High-throughput battery materials testing based on test cell arrays and dispense/jet printed electrodes

Rechargeable batteries are of crucial importance for the transition towards a sustainable energy economy. Major applications of secondary batteries are for stationary storage of electricity from renewable sources, and batteries for electrical vehicles, leading to the new, demanding requirements. Here, both the discovery of new materials and the study and optimization of new and known materials can be accelerated significantly by high-throughput (HT) testing technologies (Muster et al. 2011).

Combinatorial and HT methods comprise rapid synthesis, high-throughput testing and high capacity information processing. In this context, “combinatorial” refers to a change in the nature of the parameters (different materials or components of a recipe) while HT means the systematic variation of parameters in a wide parameter space of a particular system with a given composition (Maier et al. 2007), both promising an acceleration of research. In addition, the case of testing several identical cells in parallel increases the statistical validation. However, in the known art, the implementation of combinatorial and HT methods in battery research may be difficult for several reasons. Huge variation of the electrochemical behavior of materials can occur even for very small variations of the material composition, in fact, impurities and additives of extremely low concentration can influence the electrochemical behavior greatly.

Moreover, the fabrication of battery electrodes are characterized by complex technologies like mixing which are not easily suited for the preparation of continuous material libraries. For example, synthesizing electrodes with continuous composition spreads were performed with alloy electrodes that were sputtered or evaporated simultaneously from two or more spatially separated sources (Alcock et al. 2011; Fleischauer et al. 2003; Jun et al. 2014; MacEachern et al. 2015; Whitacre et al. 2003). Unfortunately, this technology is not relevant for many interesting and industry relevant electrode materials.

HT methods were also used to screen polymer and liquid electrolytes for batteries (Cartier et al. 2015; Cekic-Laskovic et al. 2014; Su et al. 2014). Impressive results have been obtained recently with the help of commercial robotic liquid screening platforms with an automated mixing system (Cartier et al. 2015; Su et al. 2014), leading to detailed electrolyte libraries with different amounts of various organic solvents and various lithium salts and evaluating the ionic conductivity but not the performances of those electrolytes in battery cells was shown.

Although the research needs in the field of secondary batteries are high, and the large amount of possible electrode and electrolyte materials opens an almost infinite number of different cell configurations, actually a rather small number of publications report HT methods for the investigation of complete electrochemical cells, perhaps due to the complexity of the total setup, where test cell arrays with liquid electrolyte (Clemmons 2016; Cekic-Laskovic et al. 2014; Takada et al. 2004) are even more challenging to realize than cells with solid state thin film electrolyte (Alcock et al. 2011; Fleischauer et al. 2003; Whitacre et al. 2003). Thus, until now the great majority of battery experiments is conducted based on single cell preparation. Coin cells and pouch packaged cells are used for two-electrode measurements while three-electrode measurements are carried out with sophisticated mechanical constructions like T-cells (Garcia et al. 2016; Loveridge et al. 2016) or El-cells (Steinhauer et al. 2017).

This paper proposes a breakthrough in electrochemical testing methodology, by much wider use of arrays of battery cells for high throughput tests, which allow the parallelization of potentiometry, amperometry, and electrochemical impedance spectroscopy (EIS) experiments. This will speed up testing in order to evaluate electrochemical properties and correlate electrode/electrolyte variations for a large number of rationally modified cells. Our cells will be low cost, because hundreds of them can be fabricated on the same substrate with the help of Fraunhofer IZM´s glass panel fabrication line. Moreover, in contrast to the already existing in-house proprietary technologies of ILIKA (Alcock et al. 2011) and Wildcat Technologies (Takada et al. 2004; Zhu et al. 2016), we provide a complete platform for producing electrochemical test cell arrays, including HT devices intended for use as fully finished or as work-in-progress (partially finished) goods, to any interested lab.

Said technology is based on IZMs micro battery technology (Hahn et al. 2016, 2017; Hoeppner et al. 2015; Hoeppner 2015; Ferch et al. 2016) and comprises the following key components:

The main building blocks that are required for that approach are shown in Fig. 1a. They can be divided into two main parts: the high throughput battery cell array fabrication and the high throughput electrochemical characterization and data evaluation. We are using a multichannel battery test station (Basytec) but other concepts like multiplexed potentiostats and impedance analyzers can also be applied.

Due to the single–use nature of the test-cell arrays and the standardized assembling technology, failures arising from impurities and manual handling will be significantly reduced in comparison to reusable single cells (T-cells/Swagelok cells, or El-cells). Cells of the area-array-substrate concept can be made in various sizes and variants, allowing diverse measurements including 3-electrode electrochemical characterization, optical diagnosis and gas evolution. Despite their single-use nature, such test-cells are very cost-efficient: nearly 10,000 test cells of size 5 × 5 mm² can be fabricated simultaneously on one glass substrate. The test cells can also be used for highly corrosive electrolytes, since the active materials are only in contact with the glass housing and the tested stable thin-film metallization. Materials can be identical across an array to document consistency from cell to cell, leading to higher precision and statistical significance; it can also help to evaluate the differences in electrochemical performance for a plurality of experiments ran in parallel with varying electrolyte, electrode composition or content.

Electrodes for conventional state-of-the-art test cells are fabricated similar to large scale manufacturing by slurry casting or doctor blading. A paste that is fabricated by mixing the powder components and binder into a solvent to make a slurry; the slurry mixture is pasted onto a metal foil; evaporating the solvent and then pressing (calendering) to finalize the electrode.

In order to deposit the patterned micro electrodes mooted here onto a substrate a process must be used that is suitable for the deposition of such high viscosity materials in small dimensions. Screen printing would be possible but paste additives are required to control the rheology; those additives can deteriorate the electrochemical performance. Thus dispensing is the most straight forward way to fabricate arrays of micro electrodes, where the viscosity required by the process can be achieved by variation of the solvent content and no other additives are needed (Ferch et al. 2016). As an alternative method we report here for the first time the use of high speed jetting technology for the deposition of battery electrode pastes. The jetting technology was developed for depositing high viscosity solder or silver pastes for the fabrication of solder bumps and interconnection lines (Becker et al. 2014; Gu et al. 2016): here, fluid is ejected rapidly through a nozzle, using said fluid´s momentum to break free from the nozzle. It is mostly used for small scale production; the main advantages over dispense printing is the much higher speed; moreover, there is no need for accurate control of the nozzle height because it is a contactless process.

The fundamentals of jet deposition are rather complex, in particular for battery electrode pastes, which are liquid–solid two-phase fluids that are composed of several components (ceramic and carbon particles, polymer binders and solvents). The many process parameters will not only influence deposition accuracy; but in many cases it is hard to find a parameter window that does not result in clogging of the nozzle orifice or even the damage of the electrode paste components under the high internal forces of the ejection system.

Therefore we conducted a design of experiments leading to a working jetting process for several anode, cathode and separator materials.

The dispensing and the jetting approach are suitable to evaluate materials in a form close to the battery production since the same electrode paste composition and particle sizes that are relevant for large scale production can be employed.

Both processes can support the combinatorial and high throughput approach because changing cartridges for different or modified material compositions is straight forward thus allowing to change material composition during the fabrication of a test cell array. This so called post-synthesis array transfer (PoSAT, Roberts et al. 2007) has the advantage of providing a robust composite structure, optimized for high electronic and ionic conductivities and a good contact with the current collector as in a real battery cell.

Reasonable test array confiurations are as follows:

Pipetting of liquid electrolyte into such test cell arrays is straight forward only in case of low vapor pressure fluids. Unfortunately conventional electrolytes of state of the art lithium-ion batteries use high vapor pressure organic solvents. For this type of tests we designed a particular set-up that uses the cell sealing plate for electrolyte filling as shown in Fig. 1b, c. For that purpose a fluidic manifold is integrated into the sealing lid of the test cell array that allow to supply the same electrolyte into each row of the array while the electrodes are varied over the columns. The manifold channel is dried after electrolyte supply to avoid ionic short circuit between the individual test cells.

Another advantage is the possibility to integrate MPREs during the micro-fabrication process or the dispense-print process with nearly no extra cost. Several types of pseudo-reference electrodes have been proven to exhibit long-term reference-potential stability in relevant electrochemical systems. They are essential for performing precise and reproducible electrochemical measurements (Bonnaud et al. 2016; Ives 1961; La Mantia et al. 2013; Ruch et al. 2009; Weingarth et al. 2012a, b; Zhou and Notten 2004). Lithium (Zhou and Notten 2004) and partially lithiated lithium titanate (LTO) (La Mantia et al. 2013) can be used as reference electrodes for lithium-ion batteries with organic electrolytes. Metallic lithium can be deposited by electroplating with room-temperature ionic liquid (RTIL) electrolyte. Porous carbonaceous materials and platinum have also been tested as reference electrodes in ionic liquid electrolytes (Bonnaud et al. 2016). Porous carbon electrodes can be fabricated by electrophoretic deposition, dispensing or spray deposition. Those carbonaceous materials had a low potential drift over days as well as a high tolerance for impurities (Maminska et al. 2006; Saheb et al. 2006; Weingarth et al. 2012a, b; Widmaier et al. 2016). We here investigated the use of LTO MPREs for lithium-ion cells; since LTO is one of the anode materials, it requires no additional fabrication effort.

A high-throughput testing platform has been developed based on Fraunhofer IZM’s glass panel micro-patterning technology. Glass-based micro-test-cells and test-cell arrays have been batch-fabricated and assembled to achieve full electrochemical test-cells successfully. Fabrication of electrode pastes and electrodes by conventional/industry relevant methods and sequential variation of material parameters is possible. Micro dispensing and jetting were investigated as means for the standardized application of different electrode materials and SiO₂-based separators. The reproducibility of the process was increased during the project significantly for the whole range of investigated materials and can be adapted for any material; PVDF and CMC-SBR binders were used. Jetting is a high speed deposition alternative for feature sizes above ca. 0.6–1 mm but showed reproducible results only for some electrode materials (graphite, LTO). Electrode mass variation of the micro-cells was 5–10% which is sufficient for many basic investigations and practical battery material optimizations, but still have to be improved for the optimization of production-relevant battery systems. By deposition of micro-structured metals and LTO micro-electrodes in electrochemical test-cells, micro-pseudo-reference electrodes of high stability were applied with high geometric and structural accuracy as provided by the micro-system technology processing. The application of CV and EIS with such micro-reference electrodes was demonstrated. Such test setup gives rise to an improved reliability and reproducibility of electrochemical experiments. The economic fabrication of our test arrays by batch processing allows their single-use in electrochemical experiments, preventing contamination issues due to repeated use as in conventional laboratory test cells. Clean room processing prevents particulate contamination and high grade purified material can be used cost-efficient for all experiments since only very small material volumes are required. For implementation of the substrate integrated cell arrays, a reusable electrical interface to multichannel electrochemical testing system was developed. Thus, the presented test-cell arrays can be efficiently deployed for screening tests of battery materials at the level of full-cells and are expected to speed up electrochemical materials research significantly. As a next step, standardized arrays and array interfaces have to be developed that are optimized for specific materials research tasks. They can include only the array substrates, substrates with electrodes or full-cells including electrolyte that will be made available. In addition, the glass substrate option allows these cells to be optically accessible which can be exploited for various in situ measurement methods.