Anne Berger, Benjamin Strehle, Tanja Zünd, David Schreiner, Hubert A. Gasteiger “Technical University Of Munich, Tum School Of Natural Sciences, Department Of Chemistry And Catalysis Research Center, Chair Of Technical Electrochemistry, D-85748 Garching, Germany” For Anne Berger, Benjamin Strehle, Tanja Zünd And Hubert A. Gasteiger And Institute For Machine Tools And Industrial Management, Technical University Of Munich, D-85748 Garching, Germany For David Schreiner
Introduction
With the current need to shift from fossil fuels to renewable energies, alternatives to internal combustion engines for transportation are needed. For short range transportation, battery electric vehicles powered by lithium-ion batteries (LIBs) have reached a wide acceptance, while for long range transportation or for heavy duty applications (i.e. trucks, ships, trains), fuel cell electric vehicles based on proton exchange membrane fuel cells (PEMFCs) are foreseen to play a major role.[1-2] Mercury intrusion porosimetry (MIP) can be a powerful technique to characterize battery and fuel cell materials, which can aid materials optimization and lead to solutions for current issues in terms of energy density and durability.
This article is designed to be a technical guide with practical strategies to use mercury intrusion porosimetry for the characterization and subsequent optimization of battery and fuel cell materials. Increasing the volumetric energy density and thus the range of battery electric vehicles is one of the major goals of current R&D efforts. One approach is to compress the battery electrodes into a thinner layer, which among other advantages[3] results in the same energy stored in a reduced volume. However, there are limitations to the compression: one is determined by the densest possible packing of the approximate spherical active material particles (in the μm range) and the other is set by their internal porosity (in the nm range). MIP is a powerful method to quantify and separate the porosities of these different pore size regimes. This guide will highlight how to properly examine electrode porosities and the advantages of characterizing the morphology of the active material powder prior to electrode fabrication. In fuel cell applications, the mass transport of reactants and products is dominated by diffusion processes, which are governed by the porous networks in the electrodes and the so-called gas diffusion medium. Here, MIP is already an established method and differences in total porosity and pore size distribution have been linked with the achieved cell performance.[4-5] This article illustrates the aspects that have to be considered in order to avoid measurement artifacts.
Using Mercury Intrusion Porosimetry in Battery Research
Cathode electrodes commonly consist of the cathode active material (CAM) to which small amounts of conductive carbon and a polymer binder are added. Figure 1 shows a sketch of the electrode structure: the large particles in blue represent the cathode active material (CAM) particles, while the small black spheres represent the conductive carbon; the polymer binder is illustrated in gray. While some CAMs do not have any internal porosity, many do have an elaborate porous network inside the particles and thus a significant internal porosity. Therefore, we would like to distinguish between the inter-particular volume between the CAM particles in the electrode structure and the intra-particular volume inside the CAM particles. The sketch in Figure 1 shows these intra-particular pores in a very simplified and enlarged representation. As the intra-particular pore volume is contained in pore size domains that are significantly smaller than the pore size domains that contain the inter-particular pore volume, the two regimes can be clearly separated by mercury intrusion porosimetry, so that a detailed analysis of the porosity values in the two regimes is possible. For a typical LIB cathode, the electrode is composed of the CAM, a carbon black (CB, serving as conductive carbon) additive, and a polymeric binder (in this case PVdF). The components are included in the sketch in Figure 1 (not to scale).
Figure 1. Sketch of the cathode electrode structure obtained with a porous cathode active material. The CAM particles depicted in blue are perforated with an extensive network of pores. Carbon black additive is marked in black and PVdF binder in gray. The electrode structure is supported on an aluminum foil that serves as a current collector. The representation of particle and pore sizes is not to scale.
In the first step, using MIP data, we would like to summarize three different methods to calculate the electrode porosity.
Figure 2 shows the pore size distribution for electrodes containing two different CAMs, one with and one without intra-particular porosity, at three different nominal compression ratios.[6] For the overall electrode porosity analysis, we would like to focus exemplarily on an electrode prepared with an NCA (LiNixCoyAlzO2, with x + y + z = 1) CAM. The electrodes were compressed using several calendering steps. The nominal porosity (ε1) was determined by measuring the coating thickness dcoating and the coating mass mcoating for a defined area Acoating, for a given composition (weight fractions ωi and skeletal density ρi of all components i):
This nominal porosity ε1 can be compared with the porosity determined in two ways from mercury intrusion porosimetry. As, in the following, the weight fraction of the aluminum current collector ωAl needs to be separated from the weight of the coating, the following conversion can be used:
With the absolute pore volume Vpore in mL, the mass of the coating mcoating, the mass of the sample msample, and the weight fraction of the coating wcoating = mcoating / msample, the porosity of the coating can be calculated by using the bulk sample density ρbulk,sample, which is determined by the instrument at very low pressures (e.g., the filling pressure) by comparing the sample volume with the calibrated volume of the penetrometer, using the following equation:
The porosities determined by this method agree well with the nominal porosities obtained via Eq. 1. For example, the NCA electrode porosities determined with MIP via Eq. 3 yield 45%, 40%, and 31% for the three investigated calendering steps, which are in good agreement with the nominal porosities of 47%, 42%, and 32% that are obtained via Eq. 1.
A different way to determine the electrode porosities relies on prior knowledge of the skeletal density of the individual components. The skeletal density can be easily determined via gas pycnometry. Instead of the skeletal density, the crystallographic density can be used for cathode active materials as an approximation, as it is very close to the skeletal density. The crystallographic density can be extracted by X-ray diffraction (XRD) measurements. The porosity can then be calculated as:
with ρi being the skeletal density or the crystallographic density for the CAM as an approximation.
Table 1. Weight fraction and skeletal/crystallographic densities of the materials that are incorporated in the NCA (LiNixCoyAlzO2, with x + y + z = 1) and the Li- and Mn-rich (LMR-NCM; Li1+δ[NixCoyMnz]1−δO2, with x + y + z = 1 and δ ≈ 0.1–0.2) electrodes.[6]
| ω [%] | ρ [g/cm3] | |
|---|---|---|
| LMR-NCM | 92.5 | 4.35 |
| NCA | 92.5 | 4.65 |
| CB | 4.0 | 2.0 |
| PVdF | 3.5 | 1.76 |
When using Eq. 4 and the coating parameters from Table 1, the porosities obtained for the three differently calendered NCA electrodes are 45%, 41%, and 34%. These values agree well with the above obtained nominal porosities (47%, 42%, 32%) and the porosities determined with Eq. 3 (45%, 40%, 31%). A summary of the porosity values of the differently calendered NCA electrodes, determined by either Eq. 1, Eq. 3, or Eq. 4 can be found in Table 2.
Table 2. Summary of the porosity values for the differently calendered NCA electrodes as determined by the three different methods described above.
| ε1 (Eq. 1) | ε2 (Eq. 3) | ε3 (Eq. 4) | |
|---|---|---|---|
| Light calendering | 47% | 45% | 45% |
| Medium calendering | 42% | 40% | 41% |
| Strong calendering | 32% | 31% | 34% |
After having established three different methods to quantify the overall porosity of the electrodes, mercury intrusion porosimetry allows a detailed analysis of the pore volume in different pore size regimes. When looking at Figure 2, three distinct pore regimes are visible, as indicated with the numbers ①–③. The pore size boundaries of the different pore regimes are defined depending on the material. Regime ③ shows the intra-particular volume at pore sizes between 3.6 nm (lower limit of MIP) and 240 nm. Regime ② marks the inter-particular volume between the CAM particles in the electrode structure. Regime ①, however, is more difficult to interpret, as it stems from a measurement artifact.
To achieve the required sample pore volume for sufficient stem usage (25–90%), it is necessary to add several layers of the electrode material to the penetrometer (in our case 15–25 pieces of ≈2–2.5 cm² double-side coated electrode pieces per penetrometer). However, the volume created between these layers is intruded with mercury and is clearly not due to the porosity of the electrode. The large error bars in regime ① support the conclusion that it is not a representative material property. Fortunately, in this example, the pore size region of interest is clearly separated from regime ①.
How to deal with the effect of intruded mercury volume when the regime of volume between the layers overlaps with the region of interest is elaborated later in this article when discussing fuel cell gas diffusion layers.
Regime ② represents the inter-particular porosity, where the two here examined cathode active materials behave differently upon calendering. The so-called lithium- and manganese-rich nickel-cobalt-manganese oxide (LMR-NCM) has a substantial intra-particular porosity, quite different from the non-porous NCA CAM. While pore volume and the pore diameter of the latter does not change significantly upon compression, the pore volume and pore diameter of the porous LMR-NCM material decreases substantially with stronger calendering.
When looking at the intra-particular pore size distribution (regime ③) of the uncalendered materials, the LMR-NCM has two peaks: one at 40–70 nm, which is attributed to the internal porosity of the CAM, and one at 130 nm, which correlates to pores within the conductive carbon. At the latter pore size of the conductive carbon, a small peak is visible for the NCA CAM electrode, but no other internal porosity attributable to internal CAM porosity is visible. The internal porosity for the LMR-NCM particles can be calculated by using the intruded volume of mercury at this specific pore region vpore.
By using the electrode specifications given in Table 1 and with a volume of intruded mercury of 80 mm³/g, the intra-particular porosity amounts to 24% (referenced to the CAM particle volume). As the area under the curve scales with the volume in this pore region in this representation (log differential volume versus the logarithm of the pore size), the pore volume of the LMR-NCM material is not decreased upon calendering, but even increases in regime ③ (intra-particular volume). This increase is unexpected and suggests a structural change in the CAM that allows more pores to be accessible.
However, as the intra-particular volume is not reduced during calendering, the limit of decreasing the porosity by compressing is set by the internal porosity of the active material. The theoretical limit of the compaction of the inter-particular volume is fixed by the close-packing of equal spheres unless these spherical particles were to break. Given that the intra-particular porosity of the active material is already at 24% and, as an approximation, that the lowest porosity for single-diameter spherical particles is 26%, a nominal compaction of 31% is physically not possible unless a deformation or particle crack occurs. This conclusion explains the observation of severe embossing of the aluminum foil at this high compression.[6]
A detailed interpretation of the MIP results for LMR-NCM and NCA battery electrodes can be found in reference [6].
Figure 2. Pore size distributions (log. differential volume versus the pore size on a logarithmic scale) of uncalendered and calendered electrodes (the error bars represent the standard deviation of 3 repeat measurements): a) prepared with an LMR-NCM CAM that exhibits an intra-particular porosity; b) prepared with an NCA CAM that has no intra-particular porosity. Three regimes can be separated: ① filling in between sheets, ② filling of inter-particular pores, and ③ filling of intra-particular pores. The figure is adapted from reference [6].
As described in the previous paragraph, the intra-particular porosity can already give an estimation of the maximum achievable degree of compaction of an electrode. To optimize the cathode active material already in the early stages of its development in terms of its theoretically achievable energy density in an electrode, determining the intra-particular porosity of the CAM in powder form is a promising screening technique.[7]
Figure 3 illustrates the MIP results of two different LMR-NCM powders. As with the electrodes, three different regimes can be separated. Region ① represents a continuous increase of intruded mercury volume into larger pores (≥ 4 μm), which can be attributed to particle rearrangement and compaction. When increasing the pressure to regime ②, the volume between the particles is filled. Regime ③ represents the intra-particular volume (< 300 nm).
One potential difficulty with measuring CAM powders is already visible in regime ①. In this measurement, the powder was added loosely to the penetrometer. When increasing the pressure, the particles rearrange to a denser packing. Depending on the properties of the material, the compaction of the particles upon compression might already require the entire volume of mercury in the stem. Consequently, a further evaluation of the region of interest is not possible anymore.
As a solution, one can either increase the mercury filling pressure, which causes some of the particle rearrangement to already take place during mercury filling without using up stem volume. In this measurement, a filling to 1 psi (69 mbar) was sufficient, but a proper filling pressure needs to be adjusted for the powder properties. Another solution to avoid stem depletion during regime ① is to compress the powder to a pellet prior to the measurement.
In regime ②, mercury fills the volume in between the CAM particles. The pore diameter is a function of the particle size and the packing structure. Giesche et al. have reported a rule of thumb that the pore diameter in between densely packed and approximately spherical particles is ~25–50% of the particle diameter, which is also satisfied in our example (D50 values of ~6 μm).[8]
Finally, regime ③ is the region of interest where the intra-particular volume is filled. It can be observed that the porous material (“P-woCo-6” in reference [7]) possesses internal porosity within the particles, while the dense material (“D-woCo-6” in reference [7]) does not show pore volume at smaller pores. By analogy with the electrode coatings discussed above, the internal porosity can be assessed with Eq. 5.
However, as only the active material is analyzed, i.e., without any binder or conductive carbon, the formula simplifies to:
With a crystallographic density of ρLMR–NCM = 4.35 g/cm³ and a determined pore volume of vpore* = 44 mm³/g (referenced to LMR-NCM volume and weight), the internal porosity amounts to ~16% for the inherently porous CAM.
In the next step, we would like to estimate from the measurement of the porous LMR-NCM CAM powder the achievable LMR-NCM electrode density, which scales linearly with the energy density. To convert εintra* (on the powder level) to εintra (on the electrode level), using the electrode components and composition given in Table 1, the following equation (adapted from Eq. 5) can be applied:
