Phase Transfer‐Mediated Degradation of Ether‐Based Localized High‐Concentration Electrolytes in Alkali Metal Batteries

Abstract Localized high‐concentration electrolytes (LHCEs) have attracted interest in alkali metal batteries due to the advantages of forming stable solid‐electrolyte interphases (SEIs) on anodes and good chemical/electrochemical stability. Herein, a new degradation mechanism is revealed for ether‐based LHCEs that questions their compatibility with alkali metal anodes (Li, Na, and K). Specifically, the ether solvent reacts with alkali metals to generate solvated electrons (es −) that attack hydrofluoroether co‐solvents to form a series of byproducts. The ether solvent essentially acts as a phase‐transfer reagent that continuously transfers electrons from solid‐phase metals into the solution phase, thus inhibiting the formation of stable SEI and leading to continuous alkali metal corrosion. Switching to an ester‐based solvating solvent or intercalation anodes such as graphite or molybdenum disulfide has been shown to avoid such a degradation mechanism due to the absence of es −.

The stability tests for Li, Na, and K metals were performed under three different conditions: (1) pure TFTFE solution (denoted as #1_Li/Na/K); (2) mixed solution of TFTFE and DME (denoted as #2_Li/Na/K); and (3) localized high-concentration electrolyte (LHCE) of TFTFE and DME with LiFSI, NaFSI or KFSI as salt (denoted as #3_Li/Na/K). The detailed recipes of ether-based solutions are listed in Table S2. For the control samples of ester-based solutions, 260 µL of TFTFE and 200 µL of PC or DEC were mixed. The fresh metal disks were firstly placed in the airtight vials with septa. An extra electrical tape was used to further seal the vial. The prepared solutions were then injected into the sealed vials with syringes. After the total reaction, the gas, liquid, and solid products were characterized. The glovebox antechamber was used to further remove the residual solvents from the reacted solid products. A filter was used when necessary.

TFTFE stability evaluation in the presence of solvated electrons and potassium anions
A fresh K plate was placed in pure TEGDME solvent (200 µL) for 1 minute, and the obtained dark blue solution was quickly transferred into a vial containing pure TFTFE solution (200 µL) with a pipette. 600 µL D2O (Sigma-Aldrich, 99.9%) was then added into the above mixture solution for NMR tests.

Potassiated graphite (KC8) and potassiated molybdenum disulfide stability tests in LHCE components
KC8 electrode preparation: KC8 electrodes were prepared based on a previous report with a subtle modification. [1] Specifically, the synthesis processes were conducted in a homemade Swagelok battery setup containing two bulk stainless-steel pieces and a polyethylene hold. [2] Two O-rings are applied as the isolating layer to prevent the outside atmosphere from penetrating the cell. Two-electrode half cells were assembled using a K foil as the reference/counter electrode, and graphite (~1 mg cm -2 ) coated on a copper foil as the working electrode. After dropping ~250 µL high-concentration electrolyte (HCE, 6.91 m) of KFSI/DME on the K foil, a piece of graphite electrode was directly put on the K surface without any separator. After resting for 24 hours, the golden KC8 electrodes were obtained, followed by DME wash for future use.

MoS2 electrode preparation:
The commercial MoS2 powder (Sigma-Aldrich) was mixed with Super P carbon black powder (Timcal) and poly(vinylidene fluoride) (PVDF, Sigma-Aldrich) with the mass ratio of 7:2:1 wt %. After adding the specific amount of NMP drops, the formed slurry was ground for 30 minutes and then doctor-bladed on a Cu foil, followed by drying treatment under vacuum at 100 o C for 1 day. The MoS2 electrodes were punched with the diameter of 12 mm and the typical active material loading is about 2 mg cm -2 .
Potassiated MoS2 (K0.4MoS2) preparation: Half-cells with the metallic K anodes and the MoS2 counter electrodes were assembled in an argon-filled glovebox. A piece of a glass fiber (GF/A, Whatman) and a Celgard membrane were used as the separator. A carbonate-based electrolyte (80 μL, 1 mol L -1 KFSI in ethylene carbonate/diethyl carbonate (1:1 by volume)) was added on the separator to wet the electrodes. The coin cells were tested in the galvanostatic mode at the current density of 20 mA g -1 MoS2 on the Neware battery analyzer (BTS3000). The normal cutoff voltage window is 0.5~3.0 V (vs. K/K + ). The potassiated MoS2 was obtained after the second discharge step and rinsed with DME solvent for further use.
For the stability tests, the freshly prepared KC8 or K0.4MoS2 electrodes were soaked in the mixed solution of 260 µL TFTFE and 220 µL DME or the 1.42 m KFSI-based LHCE as the same recipe in Table S2. The reacted liquids were collected for NMR tests with the solvent of D2O.

Characterization of reaction products
The side reactions between the alkali metals and the solutions may produce gas and solid products (see Table S2). A gastight syringe (SGE 250 µL, gastight with a needle, Sigma-Aldrich) was used for headspace sampling, and the evolved gaseous products (250 μL) were directly injected into a gas chromatograph (GC, Agilent Technologies 7820A) for components analyses. A flame ionization detector (FID) and a thermal conductivity detector (TCD) were equipped in the GC. Nitrogen was used as the carrier gas, and a porapak Q stainless steel packed column was employed to separate the injected gases. The reacted solid products were directly dissolved in D2O for 1 H and 19 F nuclear magnetic resonance (NMR) tests with a 400 MHz NMR spectrometer (Bruker, AVIII 400). A filter with a pore size of 0.45 µm was used to filtrate the insoluble solids when necessary. The insoluble solid products of #2_K were washed with DME and H2O before the Raman test. A Renishaw inVia microscope equipped with a 633 nm excitation laser (laser power 6 mW) was used to collect the Raman signal for the #2_K black solid sample. X-ray diffraction (XRD) characterizations were tested on a Rigaku D/MAX 2500/PC with a Cu Kα source. The self-adhesive polymer film (3M) was used to protect the air-sensitive samples from exposure during XRD tests. X-ray photoelectron spectroscopy (XPS) spectra were recorded on a Kratos Axis XPS spectrometer with an Al Kα radiation. The obtained spectra were calibrated with reference to the C-C peak (284.8 eV). The deconvolution treatment was finished using the CasaXPS software with a combined Gaussian-Lorentzian profile after the background subtraction. The Fourier Transform Infrared Spectroscopy (FTIR) spectrum of #2_K solid product was recorded using the Perkin-Elmer® Spectrum™ FT-IR/NIR spectrometer (Perkin Elmer Inc., Tres Cantos, Madrid) in the mid-IR mode, equipped with a Universal attenuated total reflectance (ATR) sampling device involving diamond crystal. A pressure of approximate 76 ± 1 N was applied to squeeze the solid sample powder towards the diamond.  [3] C5H4F8O (TTE) 1.5M-LiFSI (DME/TTE = 3:5 by volume) Li//Cu LiF, C-F, C=O, C-O, S=O XPS [4] LiFSI-1.0DME-3TTE Li//LiCoO2 LiF, Li2S, SOx, Sn 2-XPS [5] LiFSI-1.0DME-3TTE Li//NMC811 LiF, Li2O, C-F, C=O XPS [6] LiFSI-1.2DME-3TTE Li//NMC811  The cross-section optical image of the cycled Na anodes (inset of Figure S1b). Note that the corresponding FSI salts were used for all the cases.
The Na-Cu cell shows good stability at the current of 0.5 mA for 500 cycles. The black species are found at the edge area (inset of Figure 1c) after disassembling the cycled battery. However, they are absent inside the bulk Na anode, as evidenced by the shining metal cluster from its cross-section image (inset of Figure S1b). From the 1 H and 19 F NMR spectra of the cycled Na anode (Figure 1g, 1h), the CF3CH2Oanion peaks are easily distinguished, further verifying the TFTFE decomposition during Na plating/stripping processes.
It is demonstrated that the Li-Cu cells deliver good cyclic stability with a high average coulombic efficiency of 98.5% at the current of 0.5 mA for 240 cycles. But the anode darken phenomenon can be found from the black species in the inset of Figure 1d. After dissolving the cycled Li anode into D2O, the CF3CH2Oanion peaks are detected in the 1 H and 19 F NMR spectra (Figure 1g, 1h). This verifies that the TFTFE decomposition was triggered by the abundant electrons flow during cell cycling. For the pure TFE, there is only one quartet from 3.9 to 4.1 ppm with a coupling constant of 9.2 Hz ( Figure  S2c). The hydroxy should be deuterated by the D2O solvent. The -CF3 peak could be found at -76.81 ppm in the 19 F NMR spectrum ( Figure S2d).    After 2 days resting, the surface of aged K or Na metal was scratched and dissolved into D2O for the NMR tests. The reacted liquid was directly mixed with D2O for NMR tests. There is no sign of TFTFE decomposition in both the reacted solid and liquid samples. Figure S5. (a, b) Gas chromatography (GC) tests of the series of K reaction gas products; (c) The normalized solid increments of #2_M and #3_M samples based on #1_M (M = Li, Na, or K) or #4_K; The side reaction between bare K and DME solvent is denoted as #4_K. (d) The GC data of the #4_K reaction gas products. The data source is from Table S2. The insets are the zoom-in data of the dashed area. As for the normalization process, the relative data of #2_M and #3_M are calculated by dividing their absolute values to #1_M or #4_M.
After normalized the solid increments to #1_M or #4_K, all the Li reactions show negligible change. In comparison, over 500 times of solid increment was generated when DME participated in the reaction of K and TFTFE, which is consistent with the observation of the severe degradation to K. The solid increment of #3_K is much larger than that of #2_K, which is mainly due to the unreacted solid KFSI salt residue. Figure S6. (a) The optical images of #2_K reacted solid product before (left) and after D2O (middle) or DME (right) wash; (b) XRD patterns of #2_K and #3_K solid products after DME wash; (c) The Raman and (d) FTIR spectrum of the insoluble #2_K solid product after DME and water wash. The inset of S6d is the zoomed-in fingerprint region in the range of 750-1800 cm -1 .
The black solid product was found to be insoluble in both water and DME. The good light adsorption property of the unsaturated electronic structure of carbon atoms in this black substance is consistent with the partial transformation from sp 3 to sp 2 bond species in the Raman spectrum. [15] In addition, the FTIR technique was used to verify the existence of the sp 2 (C=C) carbon as a corrosion product. After washing treatment with DME and water several times, the final black solids were subject to FTIR test and the fluorine-containing functional groups can be distinguished from the fingerprint region ( Figure S6d). The broad peak at 1580 cm -1 is related to C=C stretching vibration from the sp 2 hybridized carbon. [16,17] The bands at 1273 cm -1 and 1064 cm -1 can be ascribed to the C-O stretching and C-C vibration. [17,18] The existence of C-F vibration can be deduced from the peaks at 960 cm -1 and 1161 cm -1 . [17] Figure S7. XPS spectra of the (a-c) #2_K and (d-h) #3_K solid products after DME wash.
The sp 2 carbon (C=C) species can be easily identified at 284.1 eV, apart from the sp 3 carbon (C-C) species at 284.8 eV in the #2_K and #3_K reaction solids. [19] (https://sites.cardiff.ac.uk/xpsaccess/reference/carbon) The KF can be identified in the F 1s spectrum at the binding energy of 682.9eV and the K 2p spectrum from two peaks ranging from 292 to 296 eV. The signals of the C-F bond are located at the binding energies of 688.05 eV and 288.5 eV. The formation of C-O species at 286.1 eV and 531.5 eV can be ascribed to the TFTFE and/or DME decomposition products. The M-O (metal-O) signals are also observed at 530.06 eV. There are also apparent signals of SOx (165.9 eV, S 2p3/2; 167.08 eV, S 2p1/2) and N-SOx (398.06 eV, N 1s) species from salt anion decomposition. [9,20] Most importantly, the polymer species in O 1s at the binding energy of 533.1 eV, [21] combining with the C=C, C-C, C-F, and C-O signals, suggested the existence of fluoropolymer species, which could further verify the proposed mechanism.  19 F NMR spectra of the #2_K gas products in C6D6.
The DME/TFTFE mixture solution was injected into a sealed vial with a K disk inside. After 20 minutes, the formed gases were collected and injected into the C6D6 solution (Cambridge Isotope Laboratories, Inc.) with a syringe. After resting overnight, the liquid sample was subjected to NMR characterization.
The apparent peak at 5.25 ppm in the 1 H spectrum can be ascribed to the singlet of C2H4. Based on the previous report, no signal of C2F4 could be found due to the absence of a peak at -131 ppm in the 19 F spectrum. [22] Note that a tiny signal at -81.76 ppm in the 19 F spectrum can be identified and further assigned to CH2CF2. Nevertheless, the corresponding doublet of CH2CF2 from 4.47 to 4.52 ppm is invisible from the 1 H spectrum, probably due to the trace amount generation or trace dissolution amount in C6D6. Figure S9. The possible formation mechanism of CH2CF2 induced by the nucleophilic attack from solvated electrons and potassium anions.
As proposed here, the TFTFE molecular may exhibit the C-O bond cleavage after nucleophilic attack from solvated electrons and potassium anions, which contributes to CH2CF2 formation. A piece of K disk was first soaked in KFSI_LHCE solution for 2 minutes and then taken out to stop the reaction. After removing the solvent, the surface layer of the K disk was scratched and dissolved into D2O for the NMR test. Figure S11. Aging the partially corroded K metal with a black byproduct surface layer in (a) TFTFE/DME mixture for 5 minutes, and (b) pure TFTFE or DME solution for 2 days. The inset of (a) is the byproduct residue after aging, and the insets of (b) are the corresponding cross-section pictures of the partially corroded K after 2 days aging. (c) The 19 F NMR spectra of the aged solutions by resting the partially corroded K in TFTFE/DME mixture, pure TFTFE, or pure DME. D2O was used for NMR tests. The peaks labeled as a, b, and c are ascribed to the pristine TFTFE molecule. 20 µL of KFSI_LHCE solution was firstly dropped on K metal discs and rested for 20 minutes to form a partially corroded surface. Then, the partially corroded K metals with KFSI_LHCE-derived SEI layer were further rested in TFTFE/DME mixture, pure TFTFE or pure DME solution, respectively. Figure S12. The optical images of KC8 after aging in the (a) TFTFE/DME and KFSI-based LHCE for a specific time; (b) 19 F NMR spectrum of the reacted liquid after aging KC8 in TFTFE/DME mixture, D2O was used for NMR tests. The peaks labeled as a, b, and c are ascribed to the pristine TFTFE molecule; (c) The XRD patterns of the pristine MoS2 powder and potassiated MoS2 (K0.4MoS2) electrode; (d) The electrochemical performances of the K-MoS2 half-cell based on 1 mol L -1 KFSI/EC-DEC (1:1 by vol.) at the voltage range of 0.5~3.0 V (vs. K + /K). The K0.4MoS2 was prepared by stopping the cell at the 2 nd discharge process.
The MoS2 electrode was firstly prepared with commercial MoS2 powder and the corresponding XRD pattern in Figure S12c matches the standard database pattern (JCPDF #37-1492). After the intercalation of K + into MoS2 interlayer by constructing a K-MoS2 half-cell ( Figure S12d), the (002) peak at 14.4° disappears and two new peaks at 10.7° and 38.9° are clearly observed, which agrees well with the intercalated compound of hexagonal K0.4MoS2. [23] Figure S13. (a-b) The GC tests of the Na series reaction gas products; The (c) 1 H and (d) 19 F NMR spectra of the Na series reaction solid products in D2O. Figure S14. (a-b) The GC tests of the Li series reaction gas products; The (c) 1 H and (d) 19 F NMR spectra of the Li series reaction solid products in D2O.