Overcoming the Obstacle of Polymer–Polymer Resistances in Double Layer Solid Polymer Electrolytes

Double-layer solid polymer electrolytes (DLSPEs) comprising one layer that is stable toward lithium metal and one which is stable against a high-voltage cathode are commonly suggested as a promising strategy to achieve high-energy-density lithium batteries. Through in-depth EIS analysis, it is here concluded that the polymer–polymer interface is the primary contributor to electrolyte resistance in such DLSPEs consisting of polyether-, polyester-, or polycarbonate-bad SPEs. In comparison to the bulk ionic resistance, the polymer–polymer interface resistance is approximately 10-fold higher. Nevertheless, the interfacial resistance was successfully lowered by doubling the salt concentration from 25 to 50 wt % LiTFSI owing to improved miscibility at the interface of the two polymer layers.


Synthesis of PTMC and PCL
High molecular weight poly(trimethylene carbonate) (PTMC) and poly(ε-caprolactone) (PCL) were synthesized using a method described elsewhere. 1 Described briefly, 200 mmol of TMC or CL monomer were added together with 40 μL of 1M Sn(Oct) 2 catalyst in toluene solution in a stainless steel reactor. All constituents, including the reactor, were dried prior to the synthesis in order to remove traces of water. The TMC and CL monomers were dried by distillation over CaH 2 under reduced pressure. The reactor was then placed in an oven set at 130 °C for 72 h. The contents were thoroughly mixed by shaking the reactor every 30 min during the first 90 min in the oven. After the polymerization, the reactor was allowed to cool and the final product could be divided into small pieces for later use. In the case of PTMC, the obtained product was transparent and rubbery, and according to previous gel permeation chromatography (GPC) measurements, the M w was approximately 185 000 -334 000 g mol −1 . In contrast, synthesized PCL was opaque, milky white in color, solid and had a M w of 740 000 g mol −1 . Despite differences in M w , PEO, PCL and PTMC can still be compared, since ionic conductivity and transference numbers are expected to remain constant above 100 000 g mol −1 . 2,3 All steps were carried out in argon atmosphere.

Polymer electrolyte preparation and assembly of the DLSPEs
The single-layer solid polymer electrolyte films were prepared by first dissolving polymer and LiTFSI salt by heating and stirring overnight. PEO was dissolved in THF, while PCL and PTMC were dissolved in ACN. The ratio of polymer to solvent was 0.02 g mL −1 for PEO and 0.0267 g mL −1 for PCL and PTMC. The quantity of salt is expressed relative to the total weight of the SPE i.e. polymer and salt. The solutions were subsequently cast in PTFE molds 30 mm in diameter (dried at 60 °C for 12 hours prior to use) and placed in an oven connected to an external pump. The solvent was evaporated using a method described elsewhere. 4 In short, the pressure in the oven was lowered to 200 mbar and kept at 30 °C for 20 hours after which the pressure was lowered to approximately 1.5 mbar and the temperature was increased to 60 °C and held for an additional 40 h. Next, the resulting films were placed between two PTFE sheets and heated to 80 °C for 30 min in an MTI 6T hydraulic lamination hot press. After 30 minutes of heating, the films were hot pressed at 25 MPa for an additional 30 min at the same temperature. The pressure was retained until a temperature of 40 °C was reached after the heater had been turned off. Similarly, the double-layer solid polymer electrolytes were assembled by placing two single layer films (after hot pressing) between two PTFE sheets and heating at 50 °C for 20 min followed by pressing at 1 MPa for 25 mins. Once again, the pressure was maintained until the temperature was 40 °C after the heater had been turned off. Using this method, it was possible to manufacture DLSPEs with a well-defined interface with no gaps, see Figure S6. Next, 13 mm in diameter films were punched out and their thickness was determined using a Mitituyo digital indicator micrometer. Finally, the films were hermetically sealed in 2025 coin cells (Hohsen, dried at 60 °C for 60 hours prior) along with a PTFE spacer ring. All steps were carried out in argon atmosphere.

Electrochemical impedance spectroscopy
The impedance-frequency response of the polymer electrolyte films was studied using electrochemical impedance spectroscopy (EIS). Prior to measurement, the polymer films were annealed at an elevated temperature to ensure good contact between the electrolyte and the stainless-steel blocking electrodes. The impedance-frequency response was measured between 10 7 and 1 Hz with an amplitude of 50 mV at temperatures ranging from 25 to 90 °C using a Schlumberger impedance/Gain-Phase analyzer SI 1260. The bulk resistance of the single layer polymer electrolyte films was determined by simulating impedance response using a Debye equivalent circuit in ZView v.3.2b (Scribner Associates). 4 S3

Scanning electron microscopy
Morphological characterization of the polymer-polymer interface was done using a Carl Zeiss Merlin scanning electron microscope (SEM) with a beam current of 100 pA and an acceleration voltage of 3 kV. Cross-sections of the DLSPEs were prepared by cutting with a scalpel. All steps were done under argon atmosphere.

Differential scanning calorimetry
The glass transition temperatures and melting points of the polymer electrolyte films was determined using differential scanning calorimetry (DSC) on a TA instrument DSC Q2000. Polymer electrolyte samples were hermitically sealed in aluminum pans and then cooled to −80 °C at 5 °C min −1 followed by thermal equilibration. Next, samples were heated to 100 °C at 10 °C min −1 and once again allowed to thermally equilibrate, after which the thermal cycle was repeated. Finally, glass transition temperatures and melting points were determined from the second heating cycle using TA instruments Universal Analysis 2000 v. 4.5A.