Introduction
The constant development of portable electronic devices brings the need for new and improved energy storage systems. Lithium-ion batteries have been widely used for commercial applications since their introduction in 1992.[1] Nevertheless, lithium-ion batteries suffer from many limitations. In addition to inherent limitations to energy density,[2] safety issues due to the liquid electrolyte (leakage/bursting) are also concerning, especially when it comes to applications in electric vehicles. The all-solid-state battery (ASSB), utilizing a solid electrolyte (SE), tackles these disadvantages by eliminating the flammability risks and providing a barrier to Li dendrite growth that plagues liquid electrolyte systems,[2–6] allowing the use of Li metal anodes to achieve the maximum energy density. These battery systems can provide reasonable power/energy density with highly conductive solid-state electrolytes developed over the past decades.[2–4,7] Nevertheless, for commercial production of ASSBs, scalable synthesis and processing routes for the SE are crucial. Liquid-phase synthesis has thus gained interest in recent years,[8–13] as well as solvent-based processing of the electrolyte into separators,[14–17] or finished cathode composites.[3,5] This is either done as a slurry process (where the electrolyte is not dissolved) for mixing or as an infiltration/coating process (where the electrolyte is dissolved), with the benefit of allowing for intimate contact of the materials.[18–22] For slurry processing, as opposed to dry mixing of components,[23–27] a binder is generally required, and the performance of the cathode is often reduced (reductions in conductivity alone are often observed after exposure to seemingly inert solvents).[17,28] However, the system is a complex mixture, and something that is often missing is a thorough investigation of the stability of various solvents against individual components such as the SE. For liquid processing, the stability of the SE in the solvent is obviously of utmost importance.
Lithium thiophosphate solid electrolytes offer excellent conductivities, and their mechanical softness makes them promising candidates for easier processing into batteries.[29–31] Li3PS4 is a well-studied conductor, and is often a precursor in solution processing of members of the more highly conductive argyrodite family Li6PS5X (X=Cl, Br, I). The argyrodites, and the related Li3PS4 or Li2S-P2S5 systems, are often processed in solvents, but the chemical and structural effects of the various solvents on the electrolyte alone are rarely methodically studied. Li3PS4 and glass-(ceramics) have been slurry-processed into cathodes using a large variety of solvents. These include the fabrication of composite electrodes with anisole[17,32–34] or n-decane.[35] In addition, the formation of separators via slurry processing was reported using xylene[36] and toluene.[15] The fabrications of active material coatings based on a suspension or solution of electrolyte precursors or presynthesized electrolyte are widely reported. For suspension processes, solvents like acetonitrile (ACN),[37] ethyl propionate,[38] or tetrahydrofuran (THF)[39] were used, whereas N-methylformamide (NMF),[40,41] NMF/n-hexane,[42] and diethylene glycol dimethyl ether[43] were preferred for dissolution coatings. Thin separator films can be fabricated when using toluene (although exposure of the pristine electrolyte to toluene was shown to reduce conductivity for Li3PS4 glass-ceramic, but not significantly for Li10GeP2S12).[15] For making slurries of Li3PS4 glass, a more thorough work on investigating the stability against a variety of solvents was undertaken; the influence of decane, 1,2-dichloroethane, anisole, 1,4-dioxane, propylene carbonate, propanenitrile, 2-butanone, and diethylene glycol dimethyl ether on the conductivity was studied.[33] For those solvents with donor number higher than 14, the conductivity was adversely affected due to decomposition of the SE caused by a nucleophilic attack. Of note, 75:25 mixtures of Li2S:P2S5 have been shown to be chemically stable against solvents such as THF, xylene, toluene, and heptane, but in some cases, crystallinity or structure was drastically altered.[17,44]
Argyrodites have also been used with xylenes, mineral spirit, and dibromomethane in slurry processes.[28,45–50] The formation of a SE suspension was reported to slightly affect the conductivity,[28,45,46] while still improving cell performance compared with dry mixing[28] or noncoated active material.[47] Unlike Li3PS4,[10,51] argyrodites can be dissolved in alcohols for infiltration processes by using ethanol or mixtures of ethanol with ethyl acetate, ethyl propionate, THF, or ACN.[16,18–22,51–58] As a result of infiltration processing (in alcohols), higher capacities,[20,50] high reversible capacities,[19,22] and an improvement in cycling stability and rate capability[22] were achieved, even though ethanol exposure seems to reduce the conductivity of the argyrodite, with recovery of full conductivity not occurring until heat treatment to high temperatures at which other composite components may not be stable.[19,54,56,57] For argyrodites treated in ethanol, the grain boundary resistance was reported to increase due to surface residues originating from the solvent.[45,56,57] Furthermore, partial decompositions of the argyrodite, indicated by the presence of Li2S and LiCl after recovery, were reported in a mixture of ethanol and ACN.[21] Nevertheless, ethanol seems to be a compatible solvent for solution syntheses of argyrodites, but compatibility with the argyrodite and other cathode components, e.g., binders or dispersants, seems to be strongly affected by the time of exposure and the conditions of the necessary subsequent heat treatment.[20,21,56] To the best of our knowledge, methanol was only reported for a 0.4LiI-0.6Li4SnS4 coating of cathode active material (CAM) via a dissolution process which was stated to exhibit a positive impact on the contact of electrolyte/active material.[59] There is clearly a growing interest in solution processing of electrolytes for cathode production, but a comprehensive knowledge here of the chemical stability of electrolytes against solvents is needed outside of the convoluted context of full cathode composites; this knowledge can then be utilized more generally for solvent selections of varied processes.
In this article, we selected the lithium superionic argyrodite Li6PS5Cl as SE due to its high ionic conductivity in the 10−3 S cm−1 range,[60–64] the well-understood syntheses, and the increasing interest in solvent-based synthesis and processing. In this article, we investigate structural and chemical changes of the electrolyte resulting from exposure to a selection of five organic solvents, based on the prevalence of specific solvents in the literature for synthesis and solution processing of thiophosphates, and to ensure a broad range of solvent properties. Solid-state battery cells were assembled to investigate whether effects of the solvent treatments on electrolyte properties were mirrored in cell performance and ionic transport, using NCM-622 as a commonly used and well-studied active material and untreated Li6PS5Cl as a separator.
Results and Discussion
Li6PS5Cl was chosen as an ideal electrolyte to study as it has shown good performance in solid-state batteries in recent literature and is already explored for solution processing.[21,22,54,57,65–67] Li6PS5Cl was produced according to well-established high-temperature synthesis. To study the effects of solvent on the electrolyte alone rather than on the entire system with active material, the electrolytes were first treated with the solvent and then dry-mixed with the CAMs to produce cathode composites.
For the processing of the solid electrolytes, a reasonable selection of various solvents is crucial. In this article, we chose for stability tests the solvents ACN, THF, toluene, ethanol (EtOH) and methanol (MeOH). This selection was based on the frequently used solvents in the literature for processing. Based on the different functional groups, the different solvents cover the range from aprotic/polar (ACN and THF), aprotic/nonpolar (toluene) to protic (EtOH, MeOH), as well as a variety of different physical and chemical properties such as boiling points, flash point, vapor pressure, and dielectric constants (see Table S1, Supporting Information). Powders of Li6PS5Cl were subjected to 2 days of solvent interaction to ensure enough time was given to observe reactions and to well exceed potential contact times in relevant industrial processes.
Chemical, Structural, and Microstructural Stability
Figure 1 shows the resulting solvent–electrolyte mixtures with a mass loading of 10 wt% of Li6PS5Cl. Varying solubilities and colors were observed. Treatment with ACN resulted in a blue suspension and a region of the previously white powder was colored yellow suggesting reacting polysulfidic species.[68] The blue color itself stems from the S3− radical anion (see Raman spectrum, Figure S1, Supporting Information).[69,70] Because the argyrodite should not contain SS bonds, observation of polysulfides in large amounts strongly suggests redox processes and degradation. Using THF as solvent resulted in a suspension that was colored light yellow. A colorless suspension was achieved by treatment with toluene, which suggests no or little side reactions of the sulfur-containing species with the solvent. The split image of the suspension in THF is intended to show the change in color of the solution due to the fast precipitation of the powder particles (within one minute), showing that stable suspensions are not possible. This occurs also in the case of toluene. Furthermore, with both EtOH and MeOH, a yellow solution of Li6PS5Cl was obtained. The changing color of the liquid indicates underlying reactions; this is already a widely observed phenomenon.[17,33] There is much research into colors of polysulfide solutions and dependence on polysulfide chain length, concentration, and solvent;[71] in short, the color suggests their presence but cannot be used conclusively here.
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To assess the structural impacts of the solvent treatment, the materials were characterized by X-ray diffraction (XRD) and Raman spectroscopy before and after treatment. Figure 2a shows the diffraction pattern of the pristine Li6PS5Cl (red) with only minor traces of LiCl and Li3PO4, present from the material synthesis. After solvent treatment, the solvent was removed under vacuum at 25 °C for ACN, THF, and toluene. The complete removal of EtOH and MeOH required that the temperature be raised to 100 °C, although previous reports show that lower temperatures were sufficient.[44] Nevertheless, the required temperatures for the removal of the alcohols would not be expected to cause decomposition as Li6PS5Cl is generally synthesized or recovered at higher temperatures (see Figure S3, Supporting Information). Despite the aforementioned apparent reactions taking place, the diffraction patterns shown in Figure 2a suggest that the crystal structure after treatment in ACN, THF, and toluene either remained relatively unchanged or was recovered. In contrast, EtOH (yellow) and MeOH (pink) seem to decompose Li6PS5Cl into the precursors, seen by the reflections of lithium sulfide and lithium chloride. Figure 2b shows the Raman spectra of the pristine argyrodite and after solvent treatment and solvent removal. The clearly visible PS43− signal in each spectrum after solvent removal for ACN, toluene, and THF is in agreement with the results of XRD. These signals are located in the range of 426–427 cm−1 as expected.[72] As indicated by the sharp reduction or disappearance of the PS43− peak, the use of EtOH and MeOH results in the partial decomposition of Li6PS5Cl (although for EtOH, the argyrodite seems to be reformed after heat treatments).[21] The formation of polysulfides can be observed for both solvents approximately in the broad range of 476–506 cm−1; however, the signals in the Raman after MeOH treatment are barely visible. A plot is shown in Figure S2, Supporting Information. In addition, the use of EtOH seems to result in the formation of an additional species at 390 cm−1. Although literature suggests that the argyrodite structure can be recovered from ethanol at 80 °C,[56,57] the EtOH and MeOH treated electrolytes were excluded from further studies on the solid-state battery performance as they no longer reflect Li6PS5Cl under these processing conditions; higher temperatures are required when using EtOH to reobtain the argyrodite structure (Figure S3, Supporting Information). To recover full conductivity, a potential slurry drying at 550 °C is unrealistic considering the possible instability of other materials in the cathode at higher temperatures. X-ray photoelectron spectroscopy (XPS) measurements substantiate these findings (see Figure S4, Supporting Information). The spectra of the pristine Li6PS5Cl show the typical signals for the argyrodite.[24,27,73] Treatment of Li6PS5Cl with THF shows PO43− species.[74–76] Since other species, such as P−[S]n−P, are also present at these binding energies, deconvolution is not possible for the other materials; however, XRD confirms the presence of Li3PO4 originated from the synthesis. The spectra of the material treated with ACN and toluene show no significant changes compared to the pristine sample. The detrimental impact of EtOH and MeOH becomes evident, which is in accordance with the data of the X-ray diffraction.
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To reveal how the solvent treatment may have affected the microstructure of Li6PS5Cl, scanning electron microscopy (SEM) was performed on the pristine solid electrolyte, as well as the materials treated with ACN, toluene, and THF, all of which leave the phase itself intact. Scanning electron micrographs are shown in Figure S5, Supporting Information. The SEM image of the pristine Li6PS5Cl exhibits a broad particle size distribution compared to the solvent-treated materials. The influence of ACN on the microstructure results in an apparent increase in smaller particles. In contrast, more medium-sized particles and larger particles are formed by the influence of THF. Toluene treatment gave the most unusual appearance, of the particles seemingly bound together in a matrix. In general, the handling consistency of the powders also changed noticeably after the treatment with some solvents. Although the material treated with ACN exhibited a loose powdery consistency, the material treated with THF seemed to have a much finer powder consistency, but when pressed, the powder seemed to stick together readily, making it difficult to handle and process. Using toluene resulted in a similar behavior.
At this point, we feel a short discussion on the source of the argyrodite is needed. Li6PS5Cl can be synthesized by a high-temperature reaction of mixtures of P4S10, Li2S, and LiCl as was done here,[61,64] by ball milling the same precursors (followed by short heat treatments),[77–79] or by solution chemistry from Li2S, LiX, and Li3PS4 (which itself can be ball milled or made via solution) in a solvent such as ethanol.[51,80] It can also be bought commercially from a few suppliers, where the synthesis route is not always clear. It is striking that, although Raman and XRD analyses suggest the materials to be structurally identical, very different behaviors can be observed—for example, some reports mention dissolution of the argyrodite in ethanol leading to brown-colored solutions,[19,56,57] whereas others give yellow/clear solutions.[51,67,80] This could be indicative of some impurities or nonhomogeneity between samples not noted in more limited characterization, which in turn itself may present different properties and reactivities during any solvent processing.
Investigation of Transport Properties
To reveal the influence of the solvent treatment on the conductivity of the SE Li6PS5Cl, temperature-dependent impedance spectroscopy was performed. In Figure 3a, the Nyquist plots of the pristine Li6PS5Cl (red) as well as the spectra of the materials treated with ACN (green), toluene (blue), and THF (orange) at 25 °C with their corresponding fits are shown. For fitting these spectra, a fit model consisting of two resistors and constant phase elements (R/CPEs) and a CPE element was used; representatives of the data and fitting were shown for the toluene-treated material in Figure 3b. In addition, the Nyquist plots of the conductivity measurements at −40 °C for all materials can be found in Figure S6, Supporting Information. The impedance data show that treating the SE with any type of solvent results in an increase in the resistance.[15,22,28,81] In all Nyquist plots, the first (R/CPE) element is assigned to the total resistance (Rtotal) of the SE as grain and grain boundary contributions cannot be deconvoluted for Li6PS5Cl at the investigated temperatures.[82] The corresponding capacitances of these processes show increasing capacitances with the solvent treatment from 10−11 F (pristine) to 10−10 F (ACN, toluene) and 10−9 F (THF), suggesting a significant impact of the organic solvent on the grain boundary resistance and the total conductivity. Low-temperature measurements would be needed,[82] however, due to the handling characteristics of the powders from THF and toluene, it was not possible to prepare free-standing pellets. The second contribution of the resistance can be assigned to the sample–electrode interface roughness based on the averaged modeled capacitances in the 10−6 F range.[83,84]
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Figure 3c shows the Arrhenius behavior of the ionic conductivity of the different materials and Figure 3d shows the total conductivity at 25 °C and the extracted activation energies of the pristine material as well as the solvent-influenced solid electrolyte. A clear drop of the total conductivity can be observed after treatment with all of the solvents; from a pristine conductivity of 1.7 mS cm−1, ACN treatment decreased the conductivity to 0.75 mS cm−1 and THF resulted in a reduction to 0.46 mS cm−1. The highest and most surprising impact in terms of conductivity resulted from treatment with toluene, a reduction to 0.066 mS cm−1. This drastic change can potentially be correlated with the fact that toluene showed the most unusual morphological change under SEM.
Activation energies were moderately impacted. From 0.40 eV for the pristine material, a minor increase to 0.42 eV can be observed in the case of ACN. In addition, the activation energy further increased due to the treatment with THF and toluene to 0.44 eV in both cases. However, considering the general uncertainty of measured activation barriers,[82] we cannot draw any conclusion as to a change in the activation barriers here, although the solvent treatment clearly detrimentally influences the ionic conductivity. The trend of the Arrhenius prefactor supports the persistence of the argyrodite structure by XRD and Raman spectroscopy (Figure S7, Supporting Information). The prefactor remains constant in the range of 108–109 K S cm−1.
Interfacial Resistances in Solid-State Batteries
Clearly, the nature of the solvent affects the microstructure and ionic transport properties of Li6PS5Cl. To test if the solvent processing also affects solid-state battery performance, In/LiIn│Li6PS5Cl│NCM-622:Li6PS5Cl solid-state cells were assembled and cycled. As mentioned earlier, the separator consists of untreated Li6PS5Cl whereas the electrolyte in the cathode composite is Li6PS5Cl after solvent treatment (with the pristine untreated electrolyte as control). Electrochemical impedance spectra were recorded before cycling and after each charge and discharge to identify the underlying interfacial processes in a frequency range of 7 MHz–100 mHz. Figure 4 shows the Nyquist plots and the corresponding fits of the impedance data after charging in the first cycle. To support the quality of the employed fits, Bode plots of cycles 1, 5, and 10 are shown representatively in Figure S8, Supporting Information. In the measured impedance spectra, the presence of four processes can be identified corresponding to the bulk resistance of the SE RSE,bulk, the grain resistance of the SE in the composite RSE,grain, the resistance at the interface/interphase between the SE and CAM RSE/CAM, as well as the interface between the electrolyte and the anode RSE/anode as recently discussed in literature.[23]
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These previously found processes can be distinguished based on their frequency ranges and capacitances (see Table S2, Supporting Information).[23–25] One process is located in the high-frequency (HF) region. In the middle-frequency (MF) range, two processes can be found in the 103 Hz and 100 Hz region. The low-frequency (LF) part shows one semicircle in the approximately hertz range. In the case of the cell with only untreated Li6PS5Cl, it was necessary to fit the obtained spectrum with a pre-resistance and three (R/CPE) elements[25] to ensure a reasonable fit.
Recently, these different processes were assigned to the possible interfaces in a solid-state battery.[23–25] The process in the HF range of all impedance spectra stems from the bulk resistance of the SE (RSE,bulk). This is applicable in all spectra a–d, independent of whether the fit was performed with a preresistance or a (R/CPE) element. The assignment to the RSE,bulk is corroborated based on the resulting capacitances of the solvent-treated Li6PS5Cl.[24,85]
The second observable semicircle shows the resulting capacitances of this process to be strongly influenced by the solvent treatment. Although the process of the cell made with pristine Li6PS5Cl exhibits a lower average capacitance, an increase can be observed after using an organic solvent. The dimension of the capacitance suggests grain boundary contributions,[84] and it is therefore reasonable to assume that this process stems from the grain boundaries of the SE in the composite cathode rather than the separator, as the separator was untreated. However, pure grain boundary capacitance should be of a different order of magnitude and the stated capacitances of these processes seem to reflect surface layer contributions.[84] Nevertheless, it is reasonable to assume that the grain surface is influenced by the organic solvents, which is well in line with the observed microstructural changes. Thus, an influence on the grain boundary contribution is the consequence.
The capacitances of the third process in the MF region exhibit only minor variations; these are therefore negligible. Similar to previous reports, this process can be assigned to the solid electrolyte/CAM interface.[23–25] Although the variations in the capacitances are small, a significant increase in the resistance of this process can be found after solvent treatment. Although the overall cell resistance of cycle one in case of the pristine Li6PS5Cl can be determined as ≈58 Ω, the treatment with ACN results in an increase to ≈92 Ω. Remarkably, the previous treatment of the SE in the composite cathode with toluene and THF induces a stark increase to ≈330 and ≈347 Ω, respectively.
Finally, the fourth semicircle in the LF region of 101–10−3 Hz can be attributed to the solid electrolyte/anode resistance (RSE/anode). This interface contribution is often reported to be in the lower frequency region and is known to exhibit capacitances in the millifarad range,[23,25] which are also found here for all cells.
Clearly, solvent treatment of the SE Li6PS5Cl leads to strong differences in its grain resistance in the composite, RSE,grain, and even moreso in the resistance at the interface/interphase between the SE and CAM RSE/CAM. This can potentially be explained in the case of toluene by the unusual microstructure revealed by SEM; it seems that some component has been partially dissolved and has coated smaller particles. In the case of THF, there is precedent in the literature that for Li3PS4 (which may be a degradation product of the argyrodite or present as an amorphous impurity), reaction with THF can lead to the creation of amorphous Li3PS4 with the potential to coat particles.[17]
Cycling of Solvent-Treated Electrolyte Composites
Although solvent treatment of the electrolyte has varied affects on ionic conductivity, the influence on the long-term battery performance also needs elucidation. Figure 5 shows the charge and discharge curves (dashed and solid, respectively) of the cells assembled with the pristine Li6PS5Cl (red) and those treated with solvents ACN (green), toluene (blue), and THF (orange). Remarkably, all cells exhibited a high first charge capacity; the use of a solvent seems to have a smaller impact on the charge capacity of the first cycle, despite the changes to the overall cell resistance. The solvent treatment results in a change of the capacity, relative to pristine at 166, to 163 mAh g−1 (ACN), 169 mAh g−1 (toluene), and 160 mAh g−1 (THF). Comparing this to the first discharge capacities of the solid-state batteries, the trend slightly changes (pristine 143 mAh g−1, ACN 140 mAh g−1, toluene 119 mAh g−1, and THF 119 mAh g−1), showing that solvent treatment can detrimentally affect the performance in the discharge. When considering the different resistances of the solid electrolyte–CAM interface, the largest overpotentials are found in the cells with the largest resistances, i.e., after treatment with THF or toluene (Figure S9, Supporting Information). Remarkably, the cells assembled with the pristine and ACN-treated electrolyte show similar overpotentials; this is also found by toluene and THF.
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To explore stability during cyclization, the trend of the charge/discharge curves after the 5th, 10th, 20th, 30th, 40th, and 50th cycle was compared. The Coulombic efficiencies of all cell types are shown in Figure S10, Supporting Information. The cell assembled with the pristine electrolyte shows the smallest fading behavior when comparing the charge capacities of the first and the 50th cycle. ACN results in a slightly higher decrease in capacities from the first to 50th cycle; however, it is similar to the perforrmance of the cell with the pristine electrolyte. For the following cycles, the capacities remain in the same region indicating a stable cycling behavior. In contrast, the toluene-treated sample exhibits the strongest fading of the charge capacitities. THF treatment resulted in a moderate stability of the cell; a relatively high fading in the charge capacities can be observed. Therefore, the charge capacities seem to be more affected than the discharge, where the fading is approximately in the same range.
Figure 6 shows the development of the charge (dark color) and discharge capacities (light color) of the cell with the pristine material (red) and with the ACN- (green), toluene- (blue), and THF-treated (orange) materials. The cell with pristine Li6PS5Cl exhibits the highest capacities over 50 cycles, but does show some capacity fading after 20–30 cycles as previously observed.[73,86] Remarkably, the treatment with ACN results in lower capacities, but shows very stable behavior up to 50 cycles, even with a steady slight increase in capacity. Toluene and THF resulted in the lowest attainable capacities. Nevertheless, the discharge capacities of the 50th cycle are 127 mAh g−1 (pristine), 118 mAh g−1 (ACN), 102 mAh g−1 (THF), and 97 mAh g−1 (toluene).
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In addition to the charge/discharge behavior, the development of the overall cell resistance was investigated and shown for cycle 1–10 as well as 20, 30, 40, and 50 of the prepared solid-state batteries depending on the solvent-treated SE (Figure S11, Supporting Information). As is often observed, the overall cell resistance increases with increasing cycle number, which is usually related to contact loss during cycling[23,85] as well as electrochemically induced side reactions.[24] This increase in this resistance is observable for every Li6PS5Cl, treated or untreated, independent of the chemical/physical properties of the solvent. The resistances of the pristine and the ACN-treated Li6PS5Cl remained below 170 Ω, whereas treatment with toluene and THF resulted in a tremendous increase from cycle 1–50.
Clearly, the nature of the solvent affects the solid-state battery performance, which likely stems directly from the changes to conductivity and microstructure of the solid electrolyte. Even solvents that do not seem to strongly affect structure or composition of the electrolyte seem to have unexpected effects on transport. Solvents may generally have to be tested with the individual SE to match specific processing conditions for solid-state battery applications.
Conclusion
In summary, we provide a systematic study of the influence of various organic solvents on the phase, microstructure, and conductivity of the superionic argyrodite Li6PS5Cl (synthesized via classical high-temperature technique) as well as the resulting effects on performance in cathodes with NCM as CAM. X-ray diffraction, Raman spectroscopy, and XPS suggest that the electrolyte may be stable against the solvents ACN, toluene, and THF, in contrast to the alcohols EtOH and MeOH, where a clear decomposition can be observed. Impedance spectroscopy shows the detrimental influence of the solvents on the total ionic conductivity. Toluene, despite having little impact on structure, decreased the conductivity more than 20-fold. In addition, the changing microstructures after solvent treatments and the changing consistency may influence the solid-state battery performance. In/LiIn│Li6PS5Cl│NCM-622:Li6PS5Cl cells revealed that the predominant increase in resistance stems from the SE/CAM interface. The worst cycling performance was seen in the cells using THF- and toluene-treated solid electrolytes, whereas ACN treatment led to stable cycling similar to the pristine Li6PS5Cl.
This work shows the importance of the selection of the solvent for processing cathode composites for solid-state batteries. Although typically optimization of solid-state battery comprises the mixing, particle size distribution, and protective coatings, alongside the search for faster solid ion conductors, the careful choice of a solvent for a slurry processing needs to not only be considered, but fully tested.
Experimental Section
Solid Electrolyte Synthesis
The synthesis of Li6PS5Cl as well as the solvent treatment was conducted in an Ar filled glovebox (MBraun, O2 < 0.1 ppm, H2O < 0.1 ppm). Li6PS5Cl was prepared using a typical solid-state synthesis approach.[61,64] Stoichiometric amounts of the precursors lithium sulfide (Li2S, 5 equiv., 28.0 (m)M, Alfa Aesar, 99.9%), diphosphorus pentasulfide (P4S10, 0.5 equiv., 5.59 (m)M, Merck, 99%), and lithium chloride (LiCl, 2 equiv., 11.3 (m)M, anhydrous, Alfa Aesar, 99%) were hand ground in an agate mortar for 15 minutes. After obtaining 3 g of a homogenous powder, the precursor mixture was pressed into pellets and filled into a quartz glass ampule (10 mm diameter). To prevent side reactions, the ampule was coated with carbon and preheated under dynamic vacuum at 800 °C for 2 h. After adding the precursors, the ampule was sealed under vacuum and heated in a furnace for 2 weeks at 550 °C. The obtained Li6PS5Cl pellets were then hand ground for the subsequent analysis and solvent treatments.
Solvent Treatment
ACN (Sigma Aldrich, anhydrous, 99.8%), toluene (Sigma Aldrich, anhydrous, 99.8%, prior to use: distilled over CaH2), THF (Sigma Aldrich, anhydrous, ≥99.9%, inhibitor-free), EtOH (ACROS Organics, extra dry, absolute, ≥99.5%), and MeOH (Sigma Aldrich, ACS spectrophotometric grade, 99.9%) were added to Li6PS5Cl, respectively, to obtain a 10 wt% suspension of the solid electrolyte. All solvents were purchased in an anhydrous state and stored in a glovebox. To ensure more than enough contact time of the SE with the solvent, 2 days of exposure were chosen. Afterward, the solvent was removed under vacuum at room temperature using a conventional Schlenk line. For proper drying, the materials were kept under vacuum at room temperature for one hour to obtain the solvent-treated Li6PS5Cl. In case of the MeOH- and EtOH-treated Li6PS5Cl, a temperature of 100 °C was needed during this drying process to fully remove the solvent.
Raman Spectroscopy
For Raman spectroscopic analysis, the obtained materials were placed on a microscopy glass slide and sealed with Kapton tape to prevent reactions with ambient air and humidity. The measurement was performed by focusing through the glass slide. A Bruker Senterra Raman microscope equipped with a 532 nm excitation laser was used for the measurements. Furthermore, a laser power of 2 mW was used. An integration time and a coaddition of 5 and 10 s was used, respectively. Raman spectra in the range of 47–1548 cm−1 were recorded with a spectral resolution of 3–5 cm−1. The obtained data were processed using the OPUS 7.5 software.
X-Ray Powder Diffraction
For investigating the changes of the crystal structure, all materials were placed on a (911)–oriented silicon sample holder and covered with Kapton foil. Diffractograms were obtained on a PANalytical Empyrean powder diffractometer in Bragg–Brentano geometry in the 2θ range of 10°–90° (step size 0.013, integration time/step 150 s). The Pawley fits to obtain the lattice parameters were conducted using the TOPAS-Academic V6 software.[87]
X-Ray Photoelectron Spectroscopy
For the XPS measurements, the samples were prepared in a glovebox and tranferred to the analysis chamber using an argon flushed transfer shuttle. The measurement was performed with a PHI 5000 Versaprobe Scanning ESCA Microprobe from Physical Electronics with a monochromatized Al Kα X-ray source (beam diameter 200 μm, X-ray power of 50 W). The data collection was conducted with a step size of 0.2 eV, 50 ms per step, and 23.5 eV analyzer pass energy. Measurements were performed within a vacuum of 10−6 Pa on charge neutralized samples which was achieved with slow electrons and argon ions. For the raw data analysis, the CasaXPS software (version 2.3.23, Casa Software Ltd) was used. The spectra were analyzed using a Gaussian–Lorentzian line shape (GL30), a Shirley background, and a charge correction on the PS43− signal at 161.7 eV.[24,27,73]
Scanning Electron Microscopy
With a Merlin SEM from Carl Zeiss AG, the changes of the microstructure caused by the solvent treatment were investigated. To transfer the sample under air-tight conditions to the microscope, the Leica EM VCT500 transfer module (Leica Microsystems GmbH) was used. For imaging the structural information, a secondary electron detector with a pixel average scanning mode was utilized.
Electrochemical Impedance Spectroscopy
Press cells such as those reported in previous studies [25] were used to measure the conductivity of the obtained materials. 70 mg of each powder was pressed into pellets manually. After applying ≈380 MPa for 3 min with an automatic press (Specac Atlas Autotouch 25 T), the press cell was fixed with 70 MPa[23,24,82] in an Al frame. Electrochemical impedance spectra were measured with an impedance potentiostat SP-300 (Biologic) in a frequency range of 7 MHz to 100 mHz (amplitude 10 mV) in a temperature range of −40 to 40 °C using a climate chamber (Weiss Klimatechnik).
Assembly of ASSBs
To investigate the influence of solvent-treated Li6PS5Cl on the battery performance, ASSBs for cell cycling were assembled. A solid-state battery cell setup as previously reported was used.[25] The battery composition of In/LiIn│Li6PS5Cl│NCM-622:Li6PS5Cl was chosen to monitor the battery cyclability and the developing impedance. For the composite cathode, the as-prepared Li6PS5Cl was used as well as the ones which were treated with ACN, toluene, and THF, respectively. The separator was composed of Li6PS5Cl that had not been solvent treated. For the composite cathode, Ni0.6Co0.2Mn0.2O2 (NCM–622) was used as an active material and dried at 250 °C over night using a Büchi oven B–585. NCM-622 and the Li6PS5Cl (treated and untreated for reference) were weighted in a 70:30 ratio and hand ground for 15 min. For the cell assembly, 60 mg of the untreated Li6PS5Cl was filled into the press cell as separator. After hand pressing, 12 mg of the composite cathode was distributed homogeneously on the separator. This amount of NCM related to an area capacity of 2.14 mAh cm−2, based on a theoretical specific capacity of 200 mAh gCAM−1.[88] The homogeneous distribution of the composite mixture became more difficult for the electrolytes treated with toluene and THF due to the sticky consistency; this affected the addressed active material. The cell arrangement was pressed manually. Furthermore, a pressure of three tons was applied for 3 min (Specac Atlas Autotouch 25 T). As anode a lithium (200 μm thickness, 4 mm diameter) and indium foil (chemPUR, 100 μm thickness, 9 mm diameter, 99.999%) were used to ensure a stable alloy during cycling (32.3 at% Li[89]). After finishing the cell assembly, the battery was resting for ≈2 h to achieve microstructural relaxation. Before starting the electrochemical measurements, the press cells were fixed in an Al frame. For the long-term cell cycling, the cells were charged to 3.7 V and discharged to 2.0 V versus In/LiIn at room temperature by applying a current density of 0.214 mA cm−2, corresponding to a C-rate of 0.1 C. Electrochemical impedance spectroscopy measurements were performed at room temperature by adopting the parameters stated earlier.
Acknowledgements
The research was supported by the Federal Ministry of Education and Research (BMBF) within the project FESTBATT under grant number 03XP0177A and 03XP0176D.
Conflict of Interest
The authors declare no conflict of interest.
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Copyright John Wiley & Sons, Inc. 2021
Abstract
With growing interest in solution‐based processing of electrolytes for all‐solid‐state batteries comes the need to more deeply understand potential detrimental effects of the solvent on electrolyte materials, as well as effects on the cell performance that may not have been evident by structural characterization alone. Herein, the superionic solid electrolyte Li6PS5Cl is treated with five organic solvents selected for a range of different physical and chemical properties. The electrolytes treated with solvents that do not lead to obvious degradation are used in cathode composites of solid‐state batteries In/LiIn│Li6PS5Cl│NCM‐622:Li6PS5Cl. After treatment in some solvents, the solid electrolyte remains seemingly unaffected, but a strong influence on the solid‐state battery performance is observed, revealing underlying effects that warrant deeper study.
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1 Center for Materials Research (LaMa), Justus-Liebig-University Giessen, Giessen, Germany
2 Institute for Inorganic and Analytical Chemistry, University of Münster, Münster, Germany