Combining ternary, ionic liquid-based, polymer electrolytes with a single-ion conducting polymer-based interlayer for lithium metal batteries
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Abstract
Among the many approaches to improve the performance of lithium-metal batteries, ternary polyethylene oxide/ionic liquid/lithium salt electrolytes offer several advantages such as low flammability, high conductivity
Keywords
INTRODUCTION
With the increasing dependence on energy storage devices, lithium-metal batteries have attracted much attention due to their advantages of potentially higher gravimetric and volumetric energy density as compared to the widely used Li-ion batteries. However, the use of lithium metal as an anode triggers a variety of challenges such as intense dendrite growth, which can induce cell short circuits and severe safety issues as well as fierce interface side reactions, evolution of dead Li from dendrites, and increased resistance and heat generation[1]. Especially, the combination of Li anodes and the organic liquid electrolytes that are commonly used in Li-ion batteries is problematic due to the intrinsic volatility and flammability of solvents and the high reactivity between the electrolyte and lithium metal[2]. To overcome the challenges related to the use of Li anodes, solid polymer electrolytes (SPEs) appear as a good choice as alternative electrolytes since they are inherently low-volatile, low-flammable and, therefore, less prone to leakage or fire. This property has allowed the commercialization of solid lithium metal polymer batteries for electric vehicles using polyethylene oxide (PEO)-based electrolytes[3]. Besides being compatible with lithium metal, PEO contains repeating oxygen units that can dissolve a variety of conducting lithium salts and solvate the lithium cation to ensure the Li transportation via local polymer chain segmental mobility[4]. However, the conductivity of PEO-based SPEs is limited to ca. 10-4 S cm-1 at 40 °C, which is far lower than those of commonly used liquid electrolytes (10-3-10-2 S cm-1)[5].
Various plasticizers, including molecular solvents (propylene carbonate, ethylene carbonate, and glymes or longer and less volatile polyethylene glycol dimethyl ethers (PEGDMEs), and ionic liquids (ILs)[6-12], have been employed in SPEs to improve their ionic conductivity, since they can decrease the melting temperature and glass transition temperature. Among them, aprotic ILs with the advantages of non-volatility and low flammability, high thermal stability, and high ionic conductivity can act not only as plasticizers but also as supporting electrolytes to form high-performance ternary PEO/IL/lithium salt electrolytes. This idea was first proposed by Shin et al. in 2003 using the PEO/N-methyl-N-propyl pyrrolidinium bis(trifluoromethanesulfonyl)imide (PYR13TFSI)/bis(trifluoromethanesulfonyl)imide lithium salt (LiTFSI) system to improve the conductivity up to ~10-4 S cm-1 at 20 °C and then used it in lithium metal polymer electrolyte batteries[6]. Rupp et al. prepared an ultraviolet (UV) crosslinked ternary polymer electrolyte (PEO/N-methyl-N-butyl pyrrolidinium (PYR14) TFSI)/LiTFSI with a molar ratio of 10/1/1, OPEO/LiTFSI/PYR14TFSI) using benzophenone as a photoinitiator to improve the mechanical stability without sacrificing conductivity[13]. Afterward, ternary polymer electrolytes were combined with composite cathode tapes to enable the cycling of solid lithium metal polymer batteries[14-16]. Although the ternary SPEs (TSPEs) are superior in terms of ionic conductivity and safety compared with other types of SPEs, their demerits are concerning. For instance, their low Li transference numbers lead to significant salt concentration gradients and even full Li depletion at the interface during charge. In addition, their low mechanical strength provides insufficient lithium metal confinement. Accordingly, protrusion growth might occur, and lead to related issues (extension of surface area of the Li anode and, in the worst case, short circuits).
On the other hand, interface engineering on the surface of the Li anode is an effective strategy to improve the performance of lithium-metal batteries. The solid electrolyte interphase (SEI) layer plays a crucial role in this regard, since it can prevent the continuous decomposition of the electrolyte. The ideal SEI is merely conductive to Li+ cations, while blocking electrons and other electrolyte species. However, real SEIs grown in contact with the electrolyte might lack mechanical stability, homogeneous ionic transport, as well as the ability to fully suppress electrolyte decomposition, which commonly leads to dendrite growth and limited lifespans of the battery cells. This issue can be addressed by applying an artificial SEI (art-SEI) layer onto the lithium metal surface because they not only act as a physical barrier to reduce the continuous electrolyte reaction with the lithium metal, but potentially possess other advantages such as mechanical strength, high conductivity, and good chemical stability. A variety of art-SEI layers has been reported, including LiF[17],
Herein, we show the limit of a classical TSPE and propose the use of an art-SEI made from two kinds of ionomers: Poly(lithium sulfonyl(trifluoromethanesulfonyl)imide methacrylate) (PMTFSILi) and poly(lithium sulfonyl(trifluoromethanesulfonyl)imide styrene) (PSTFSILi) to overcome its disadvantages and benefit from its advantages such as high conductivity and low flammability. On the one hand, the
EXPERIMENTAL SECTION
Materials
Poly(ethylene oxide) (PEO, average Mv~1,000,000 (nominal), powder, Merck), acetonitrile (99.9%, Merck), LiTFSI (99.95%, Merck), 1-butyl-1-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide (PYR14TFSI, 99.9%, Solvionic), benzophenone (99%, Merck), PSTFSILi (Mn = 27,592 g mol-1, Mw = 190,562 g mol-1,
Preparation of the solid polymer electrolyte
An amount of 0.6 g of PEO (dried at 60 °C for 48 h under dynamic vacuum) was dissolved in 9.4 g of acetonitrile (dried on molecular sieves to < 20 ppm H2O) by stirring for 12 h inside an Ar-filled glovebox (H2O and O2 < 0.1 ppm). Then, 0.5760 g of PYR14TFSI, 0.3915 g of LiTFSI (corresponding to a molar ratio PEO/PYR14TFSI/ LiTFSI of 10/1/1), and 0.03 g benzophenone (i.e., 5% of the polymer weight) were added and mixed by further stirring for 3 h. The obtained viscous solution was cast onto a polydimethylsiloxane-coated Mylar foil and dried at RT in the glovebox for three days then at 60 °C for 12 h. Finally, the thin film was crosslinked by UV for 12 min on each side with a Bio-Link-BLX (VILBER) at a wavelength of 312 nm.
Preparation of art-SEI
The coated Li electrodes were fabricated by immersing the Li anodes into a PC solution with
Cells assembly and measurements.
Two types of Li electrodes were used. Some were made from a Li foil of 50 μm thickness and had diameters of 12 mm and were used as stripped electrodes in single direction plating experiments. The others were made by scratching the surface of 1 mm thick Li chips with a polypropylene tweezer in a glovebox to completely remove the native passivation layer on the Li surface. They had diameters of 12 and 14 mm. They were then manually roll-pressed and finally pressed with a hydraulic press (between Mylar foils in both cases) to a thickness of ca. 200 µm. The crosslinked polymer membranes were cut into 18 mm disks and used as electrolytes and separators with a thickness of 200 µm (± 25 µm).
All the electrochemical characterizations were carried out in 2,032 type coin cells and conducted at 80 °C using a Neware BTS4000 battery test system. For the experiments involving a single galvanostatic plating of Li, the deposits were made onto 14 mm polished electrodes, using a 12 mm unpolished Li foil as the counter electrode at 0.1 and 0.25 mA cm-2. The cycling of symmetric Li||Li cells was done with polished Li anodes at 0.1 mA cm-2 with cycled capacities of 0.1 and 1 mAh cm-2. The LFP electrodes were prepared by casting a slurry of LFP, PVDF, and carbon black with a weight ratio of 8:1:1, using NMP as a solvent, onto an Al current collector. The films were dried at 80 °C for 12 h, cut into 12 mm round disks, pressed at 3 tons with a hydraulic press and then vacuum-dried for 12 h at 120 °C. The electrochemical performance of LFP||Li coin cells was measured via galvanostatic cycling within the voltage range of 2.5-3.8 V at 0.2 and 0. 1C for LFP active material mass loadings of 2.6 and 5.6 mg cm-2, respectively (the 1 C rate was calculated considering a specific capacity of 170 mAh g-1 for LFP). Electrochemical impedance spectroscopy (EIS) was performed within the frequency range of 100 kHz~0.01Hz with an amplitude of 5 mV using a VMP2 multichannel potentiostat (Biologic).
Materials characterization
To characterize the Li electrodes after the electrochemical tests, the cells were dismantled and the Li electrodes/polymer electrolyte assembly was immersed in DME for 24 h to allow for peeling off the TSPE from the Li electrodes.
The ex-situ scanning electron microscopy (SEM)/EDX analysis of the bare Li and coated Li was performed via a ZEISS Crossbeam XB340 instrument, equipped with an energy dispersive X-ray (EDX) spectrometer. The cross-sectional bare Li and coated Li were prepared using a Capella focused ion beam (FIB) with a gallium ion source, utilizing a milling current of 1.5 nA. The cross-section images were captured following FIB milling, with the smart SEM software employed for tilt correction to compensate for image distortion arising from the 54° tilt from the optical axis.
X-ray photoelectron spectroscopy (XPS) measurements on Li anodes were conducted with an AXIS Supra+ (Kratos) with a focused 20-500 μm diameter beam of monochromatic X-rays and a 15 kV filament voltage source energy. The Al Kα radiation with an energy of 1,486.3 eV and an angle of 0° of emission was used for the measurements. Adventitious carbon in the C 1s spectra at 284.8 eV was used as a charge reference to correct the binding energies.
Electrochemical measurements of the polymer electrolyte
Symmetric SS|TSPE|SS coin cells (SS: stainless steel electrodes) were assembled to determine the ionic conductivity via EIS within the temperature range from 30 to 100 °C, with increments of 10 °C. The ionic conductivity (σ) of TSPE was calculated via:
Where l, R, and A represent the thickness, resistance, and area of the TSPE employed in the coin cells, respectively.
Viscoelastic properties measurement
The viscoelastic properties of the electrolyte were measured with an Anton Paar MCR502 rheometer in oscillating mode, at 1 Hz with a constant strain of 0.1% and applying a constant normal force of 0.25 N during a cooling ramp from 100 to -20 °C at -3 °C min-1. Temperature was controlled using a plate and hood Peltier systems. A parallel-plate (diameter 25 mm) geometry was chosen, with a gap corresponding to the membrane thickness, varying from 246 µm at 80 °C to 238 µm at 25 °C.
RESULTS AND DISCUSSION
Results obtained with lithium metal electrodes strongly depend on the lithium metal used, with large differences usually obtained with lithium carbonate-coated Li chips (commonly used to assemble “half-cells” in Li-ion studies) or uncoated Li foils (processed under dry air and thus covered with a passivation layer made mostly of Li2O). In fact, Li foils from different providers or stored in multiple conditions in terms of storage time, and atmosphere (dry room or glovebox under Ar, presence of atmospheric impurities) also exhibit very distinct cycling behaviors. It is thus critical to establish the “reference” behavior of the TSPE in the conditions of the laboratory. Several strategies have been implemented to “erase” the history of the Li foil[32-34] or at least improve the consistency of the results. Indeed, the Li foil might age and the passivation layer at its surface grows over time because of contaminants (e.g., alkyl carbonate vapors, N2 ingress into the glovebox, etc.). Here, we started using Li foils “as received and stored in the glove-box”, but we found issues with the reproducibility of the results. Especially, when coating art-SEIs onto the Li foil, the electrochemical results were not consistent since the variations of the surface reactivity of the Li foil led to different coating layers. Good reproducibility was then reached by using polished Li electrodes, as it avoided the effect of surface changes over time on the formation of the art-SEI. In practice, for better surface control, lithium metal treatment should be done immediately after foil extrusion or roll pressing to better control the process.
Therefore, the polished Li electrodes were adopted to assemble coin cells for electrochemical performance measurements in this study. They were prepared by manually rolling the completely scratched/polished Li chips with a thickness of 1 mm to a ca. 200 µm thickness between two pieces of Mylar foil and then cut with a puncher of 14 mm or 12 mm depending on their application. The surface of the obtained Li electrodes is relatively shiny and smooth, as shown in Supplementary Figure 1A. The flat Li anode obtained after pressing it between two pieces of Mylar foil, as shown in Supplementary Figure 1B, is noted as bare Li. For coating art-SEIs, two kinds of Li+ ionomers, PSTFSILi and PMTFSILi (molecular formulas given in Supplementary Figure 1C and D), were used, since this combination leads to clear improvements in liquid (DME-based) and polymer (PEO-based, single-ion), ether-based electrolytes[31]. The bare Li electrodes were dipped into a 5 wt.% PMTFSILi + 5 wt.% PSTFSILi solution in PC for 10 min and dried at RT under vacuum for 12 h and referred to in the following as coated Li. An example of such an electrode is shown in Supplementary Figure 1E.
As for TSPE membranes, differences might arise from the starting materials. For instance, the molecular weight distribution of the starting PEO can influence the amount of soluble polymer chains after crosslinking, (e.g., via the proportion of short chains that are statistically less likely to form radicals and bind with the rest of the polymer network) and thereby the mechanical properties and ionic mobility. Besides, experimental conditions such as the wavelength and power of the UV curing apparatus and curing time, or the battery cells used can all affect the results obtained. Rupp et al. showed, for instance, that, in their conditions, a maximum of insoluble fraction was reached at intermediate curing time, as longer curing time favored chain fragmentation[13].
Preparation of ternary solid polymer electrolyte
The preparation of the TSPE is illustrated in Figure 1. It was done by first mixing all the components in acetonitrile. The clear and viscous solution obtained was then cast onto a Mylar foil in a glovebox and the acetonitrile evaporated at RT, leading to a self-standing membrane that was then crosslinked via UV light. As can be seen, the final membranes are fully transparent without any sign of crystallinity.
Figure 1. The preparation process of the TSPE.
The ionic conductivity (σ) of TSPE is a crucial property that greatly affects the electrochemical performance of the cells. Figure 2A shows the variation of conductivity for TSPE vs. temperature. The conductivity at
Figure 2. (A) Ionic conductivity of the TSPE in a temperature range of 30 to 100 °C; (B) Evolution of the viscoelastic parameters of the TSPE with temperature.
Figure 2B shows the evolution of the viscoelastic parameters of the TSPE with temperature. As can be seen, the loss factor stays below 1 over the whole temperature range, confirming the solid-like behavior expected in the case of a crosslinked polymer. Upon cooling, an increase of the storage modulus is observed, from
Lithium cycling
Cycling results have been previously reported with similar TSPEs, mostly at 40 °C, with a 20:2:2 non-crosslinked electrolyte[35] and LFP||Li cells reached 180 cycles. The best results reported in terms of lithium metal cycling were obtained with a higher IL content for a 20:2:4 composition, with 1,000 cycles reached at 0.078 mA cm-2 in 1 h steps in symmetric Li||Li cells[14]. Figure 3A shows that, at 80 °C, the 20:2:2 TSPE allows reaching well above 1,250 cycles at 0.1 mA cm-2. However, one can wonder about the stability of the
Figure 3. Voltage profiles of Li||Li cells using bare Li and coated Li electrodes cycled at 0.1 mA cm-2. (A) with 1 h steps and (B) with 10 h steps.
For comparison, cells with the art-SEI Li were cycled in the same conditions and maintained a relatively stable overvoltage over cycling, indicating a more stable interface. The improvement is not obvious when cycling a low amount of lithium metal [Figure 3A], since both cells can cycle for thousands of cycles. However, Figure 3B confirms that the decrease of overvoltage indeed corresponds to an increase of surface area of the lithium metal electrode, since the cell with art-SEIs only shows obvious voltage noise and overvoltage decay after 720 h. This demonstrates the significant protective function of the art-SEI layer to limit dendrite growth and postpone the occurrence of short circuits.
Resistance to short-circuit
To characterize the resistance to dendrite growth, continuous Li electrodeposition was done at two different current densities in Li|TSPE|Li cells using polished Li foils, coated or not, as plated electrodes, and the results are shown in Figure 4 (the stripped Li electrode with a thickness of 50 μm provides a capacity of
Figure 4. Voltage profiles of Li||Li cells using various lithium metal anodes with different current densities: (A) 0.1 mA cm-2,
It must be emphasized that these results strongly depend on the surface state of the Li foil used, since Li foils from different producers might have distinct surface chemistry and it was previously shown that, for a given Li foil, the storage in a dry room results in a much different surface chemistry compared to the storage in a glovebox under Ar[39]. In fact, Passerini et al. reported the full plating of a 50 µm foil onto another at
In stark contrast, at 0.1 mA cm-2, the art-SEI coated cell does not exhibit any short-circuit and achieves the complete electrodeposition of Li until the 50 µm foil is fully consumed (ca. 9.5 mAh cm-2). At a higher current density of 0.25 mA cm-2, the coated Li cells allow depositing 6.8 mAh cm-2 of Li before some voltage noise occurs after 27 h and 8.3 mAh cm-2 before reaching the cut-off voltage. This indicates that the art-SEI is beneficial to improving the homogeneity of Li deposition and limiting dendrite growth, thus limiting the appearance of short circuits.
Furthermore, the interface between TSPE and Li anodes was explored through visual observation and SEM imaging after dismantling the bare Li and coated Li cells. The Li anodes and TSPEs were stuck to each other. Thus, to separate them, DME was used to swell and soften the TSPE for 24 h. As can be seen in Supplementary Figure 2A, the plated Li anode of the bare Li cell still exhibits a relatively smooth appearance with no obvious black Li protrusion on its surface. However, dendrites have clearly grown into the TSPE, resulting in a short circuit, as confirmed by the aspect of the peeled TSPE that exhibits black dots
Evolution of the interface upon cycling
It is expected that the Li anode coated with the single-ion polymers evolves in contact with the TSPE to form the final art-SEI. In particular, the liquid fraction of the TSPE is likely to penetrate the coating, at least where the polymers did not react with the lithium surface, in the outer SEI layer. Therefore, the art-SEI is expected to contain both fixed anions (therefore limiting Li+ depletion and limiting the growth of protrusions) from the single-ion polymers and mobile anions from the TSPE. In addition, reaction products originating from the TSPE and coating, are likely incorporated in the inner layer and participate in blocking electrolyte species. Furthermore, the whole art-SEI likely participates in the mechanical confinement of lithium metal. Although it is difficult to attribute the improvement of performance to a single factor, the respective evolutions of the interfaces of bare and coated lithium are compared in the following.
First, cells were kept at open circuit voltage for 24 h at 80 °C to form stabilized interfaces and then cycled at 0.1 mA cm-2 with 10 h steps. EIS measurements were conducted on cells using bare Li and coated Li electrodes, and the results are reported in Figure 5; the corresponding resistance values are shown in Supplementary Table 1 and the equivalent circuit used for fitting the curves is shown in
Figure 5. Impedance evolution for Li|TSPE|Li cells using (A) bare Li and (B) coated Li cycled at 0.1 mA cm-2 with 10 h steps at 80 °C.
Besides, to follow the change in lithium metal surface morphology, top-view and cross-sectional SEM images were acquired after five cycles at 0.1 mA cm-2 with 10 h steps for bare Li and coated Li anodes. As can be seen from the top-view SEM images [Supplementary Figure 5], dendrite growth is observed on the surface of bare Li (highlighted by the red oval) while the coated Li has no obvious dendrite growth. Also, coated Li exhibits a smoother surface as compared to bare Li. Cross-sectional images of the electrodes were also acquired via FIB-SEM. As shown in Figure 6A, bare Li exhibits an interfacial layer with a thickness of ca. 1.1 μm, which is relatively thick and indicates a strong accumulation of electrolyte and Li degradation products. Also, cavities have been formed at the interface between the SEI and the bare Li anode, likely due to inhomogeneous Li deposition or stripping. The reduced contact between the electrode and the SEI is likely to induce further inhomogeneities upon further cycling. In contrast, as can be seen from Figure 6B, a thinner layer with a thickness of ca. 0.4 μm is observed on coated Li, demonstrating that the art-SEI layer has remained after cycling. Also, the coated Li exhibits an uninterrupted contact area between the SEI layer without any cavities. This is another indication that the art-SEI improves the homogeneity of the Li deposition and prevents the formation of a thick electrolyte-derived SEI with poor contact with Li leading to a lower SEI thickness and limiting dendrite growth.
Figure 6. Cross-sectional SEM images and corresponding EDX images of (A) bare Li and (B) coated Li after 5 cycles of plating/stripping at 0.1 mA cm-2 with a cycling capacity of 1 mAh cm-2.
In addition, Figure 6 shows that the bare Li anode exhibits a relatively smooth cross-section while some columnar damages are observed on the cross-section of coated Li, although the same milling current was used for both electrodes. This obvious difference is likely due to their different porosities. That is, bare Li has a porous outer layer caused by repeated inhomogeneous plating/stripping. In fact, the larger view of the electrode shown in Supplementary Figure 6 shows that the porosity of the electrode is not limited to the cavities at the interface but extends under the surface. On the contrary, coated Li is covered by a thinner interfacial layer compared to bare Li indicating a more limited reactivity as its art-SEI leads to homogeneous deposition beneath this layer, thus maintaining the dense nature of the electrode. Therefore, the cross-section of the coated Li anode exhibits more FIB damage compared to that of bare Li which is more porous and easily etched. EDX images show that not only are C, O, N, F, and S signals present on the surface and cross-section of the SEIs for both electrodes, but they also clearly appear on the cross-section of the Li beneath the SEI, in the case of the bare Li, whereas the signals are very weak in the case of coated Li. This confirms that the cycled Li, in the case of bare Li, is porous and that the porosity is filled with electrolytes and/or electrolyte degradation products. On the other hand, the art-SEI seems to allow a denser and more homogenous deposit with little, if any, electrolyte-derived species within the Li electrode, which results in a strong contrast between the SEI and the Li for the coated Li electrode.
To explore the surface chemistry evolution of cycled Li anodes, XPS spectra were acquired for pristine and cycled bare Li and coated Li (100 h at 0.1 mA cm-2 with 10 h steps). In the case of cycled electrodes, the electrodes plated first were used. Supplementary Table 2 shows the atomic concentrations of elements for these samples. Pristine bare Li has the largest proportion of Li (34.0%) with low O and C fractions (32.0% and 26.4%, respectively), indicating a relatively clean Li surface (the Li electrodes used are freshly polished). The Si element (7.6%) is likely from the polydimethylsiloxane-coated Mylar foil used to press the polished Li to avoid adhesion. In comparison, for the coated Li, the fraction of C increases to 41.5% while that of O and Li elements decreases to 24.8% and 18.0%. In addition, the F, S, and N elements appear with fractions of 4.9%, 2.7%, and 2.0%, originating from the deposited layer (i.e., the polymers as well as their reaction products and those of PC). The small amount of Cl is probably an impurity from the synthesis process of polymers. After cycling, the proportion of C element increases to 54.0% (bare Li) and 65.6% (coated Li) while that of O and Li elements decreases to 18.5% and 21.5%, respectively, for bare Li and 17.1%, and 11.1%, respectively, for coated Li. Also, bare Li has 4.0% of F and a small proportion of N and S elements below 1% originating from the reduction products of the bis(trifluoromethanesulfonyl)imide (TFSI) anions from the TSPE. Similarly, fractions of F (3.4%), N (0.5%), and S (0.3%) can also be observed on coated Li after cycling and are smaller than before cycling. This is probably because, in the outer layer of the SEI, the TSPE penetrates the art-SEI without being reduced. Upon rinsing with DME, the IL fraction is washed away, and the S and N elements decrease, leaving behind insoluble reduction products (the proportion of F for bare Li and coated Li after cycling is 4.0% and 3.4%, respectively, rather than less than 1% as N and S elements).
In detail, as shown in the C1s spectra [Figure 7A and Supplementary Table 3], four peaks attributed to
Figure 7. XPS spectra of different Li anodes before or after five cycles at 0.1 mA cm-2 with 10 h steps. (A) C1s, (B) O 1s, (C) Li 1s, (D) F 1s.
Performance in LFP|TSPE|Li cells
LFP|TSPE|Li cells were assembled to assess the effect of the art-SEI in a battery cell. In this case, ordinary porous LFP electrodes (LFP, super C65, PVDF with a weight ratio of 8:1:1 and an active material mass loading of 2.8 mg cm-2) were used. As can be seen from Figure 8 A and B, both cells deliver a similar initial discharge capacity (159 and 163 mAh g-1 for the bare and art-SEI cells, respectively). Then, the bare Li cells experience a significantly increased ohmic polarization and fast capacity fading with a low capacity retention of only 60% after 62 cycles, while the coated Li cells exhibit a relatively stable ohmic polarization and significantly improved cycling stability with a capacity retention of 85% after 350 cycles. A similar improvement was also observed with a higher mass loading of LFP of ca. 5.6 mg cm-2 and cycling at 0.1 C. As shown in Supplementary Figure 7, coated Li cells exhibit excellent cyclability with a capacity retention of 80% after 43 cycles while bare Li cells show fast capacity decay to 56% of the initial discharge capacity. The relatively low performance of these cells and initial capacity variation likely originates from the poor contact between the polymer electrolyte and the porous cathode as suggested by the increase of capacity after the first cycle for both cells. In fact, previous reports used dense LFP electrodes including a TSPE of similar composition (i.e., PEO, PY14TFSI, and LiTFSI)[14-16].
Figure 8. Cycling performance of LFP|TSPE|Li cells cycling at 0.2 C. (A) Voltage profiles; (B) Evolution of the specific capacity over cycling.
Both cells use the TSPE, so the difference is due to the interface engineering. Here, on the one hand, the art-SEI is not, a priori, a full single-ion conductor, since the liquid fraction of the TSPE can penetrate at least the outer layer of the art-SEI. Nevertheless, the anionic function grafted onto the polymers used to prepare the art-SEI should, in principle, prevent Li depletion in the outer layer of the art-SEI to levels below the Li content of the polymer. On the other hand, even though the outer art-SEI layer is swollen by some of the IL, we can still expect that the deposited layer, after reacting with the electrolyte, still constitutes a physical barrier, as compared with the very flexible TSPE used (especially at 80 °C).
CONCLUSION
Homogeneous and transparent crosslinked ternary OPEO/LiTFSI/PYR14TFSI (10:1:1, mol) electrolyte membranes were prepared via UV curing. The conductivity of this electrolyte reaches 3.0 × 10-4 S cm-1 at
DECLARATIONS
Authors’ contributions
Made substantial contributions to conception and design of the study and performed data analysis and interpretation: Wan J, Paillard E
Performed data acquisition: Wan J, Hou X, Wan M, Briatico Vangosa F
Provided technical and material support: Briatico Vangosa F, Bresser D, Li J
Wrote the first version of the manuscript: Wan J
All authors revised the manuscript.
Availability of data and materials
The data are available upon request from the authors.
Financial support and sponsorship
The China Scholarship Council (CSC) provided financial support for Wan J (2020063100).
Conflicts of interest
Paillard E is the associate editor of the journal Energy Materials, while the other authors have declared that they have no conflicts of interest.
Ethical approval and consent to participate
Not applicable.
Consent for publication
Not applicable.
Copyright
© The Author(s) 2024.
Supplementary Materials
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Wan, J.; Wan M.; Hou X.; Vangosa F. B.; Bresser D.; Li J.; Paillard E. Combining ternary, ionic liquid-based, polymer electrolytes with a single-ion conducting polymer-based interlayer for lithium metal batteries. Energy Mater. 2024, 4, 400074. http://dx.doi.org/10.20517/energymater.2024.50
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