The Stability of Polymers in Liquid Li-S Battery

PVDF (Kynar HSV 900), PVDF-HFP (Kynar Flex 2801) and PEO particles (MW = 5,000,000, Sigma-Aldrich) were respectivelyadded to the dioxolane (DOL) and 1,2-dimethoxyethane(DME) (1:1, v/v)) under stirring for 3 h.

Polymer membranes were prepared by tape casting method. PVDF and PVDF-HFP particles were added to N-methyl pyrrolidinone (NMP) respectively under stirring for 4 h. The resulting slurry was cast on a PTFE plate, and the solvents evaporated in an air-circulating oven at 50°C for 5 h. Then the membranes were peeled off and dried under vacuum at 50°C for 10 h. PEO particles were dispersed in acetonitrile to form a slurry. Then the slurry was prepared to membrane using the same method as PVDF and PVDF-HFP membranes do.

DOL and DME(1:1, v/v) was added dropwise into PVDF, PVDF-HFP and PEO membranes respectively until the membranes were immersed.

Cells were fabricated using coin type cells (CR2016) by stacking cathode, anode and separator in a glove box filled with argon (99.9995%). Galvanostatically discharge/charge test was operated in the range of 1.5–2.8V at a current density of 500 mA/g. PVDF membrane, PVDF-HFP membrane and celgard 2325 were used as separators respectively. C/S composite and lithium metal were respectively used for cathode and anode of Li/S cell. 1 M LiTFSI + 0.2 MLiNO₃/DOL and DME (1:1, v/v) was used as electrolyte.

Aluminum foil and lithium metal were respectively used for cathode and anode of contrast cell (1), marked as Al-Li cell. C/S composite and stainless steel sheet were respectively used for cathode and anode of contrast cell (2), marked as S-Steel cell. PVDF-HFP membrane was stacked between celgard 2325 separator and C/S composite cathode of Li/S cell to prepare contrast cell (3), marked as PVDF-HFP(+). PVDF-HFP membrane was stacked between celgard 2325 separator and lithium anode of Li/S cell to prepare contrast cell (4), marked as PVDF-HFP(−).

The PVDF and PVDF-HFP membranes which had been immersed in lithium polysulfide solution for 10 days were marked as PVDF (Li₂S_x) and PVDF-HFP(Li₂S_x), respectively.

Surface morphology of the polymer membranes were studied by a field-emission scanning electron microscope (SEM, Hitachi S-4800). X-ray diffraction (XRD) pattern was recorded with a Bruker D/8 X-ray diffractometer using Cu Karadiation (k = 1.5418 Å) to identify the crystal structure of polymer membranes. Fourier transform infrared (FT-IR) spectra were used to confirm the structure and composition of PVDF and PVDF-HFP membranes.

PEO, PVDF and PVDF-HFP particles are all dissolved in electrolyte after stirring, as shown in Figure 1. After immersed by ether electrolyte, PVDF and PVDF-HFP membranes remain stable but PEO membrane dissolved, as shown in Figure 2. The surface morphology changes of PVDF and PVDF-HFP membranes after immersed by electrolyte were shown in Figure 3. After immersed by electrolyte, the pores on the surface of PVDF membrane disappeared but the cracking and shedding turned up,similarly, the pores on the surface of PVDF-HFP membrane minified. It can be seen, although PVDF and PVDF-HFP membranes immersed electrolyte remains stable, they have swollen in electrolyte.

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Figure 4a shows the initial discharge/charge curves of celgard 2325, PVDF and PVDF-HFP separators at the current of 500 mA/g between 1.5 and 2.8 V. All of the curves exhibit two discharging plateaus but at different voltage ranges. For celgard 2325 separator, the discharge plateaus at around 2.35 V and 2.1V could be attributed to lithiation of S₈ molecules and the charge plateau at around 2.3V could be attributed to delithiation of S₈ molecules. However, PVDF-HFP separator discharge plateaus fall to around 2.1V and 1.8V. Furthermore, PVDF separator discharge plateaus fall to around 1.8V and 1.5V. Severe polarization phenomenon of PVDF separator is attributed to high surface tightness (Figure 3d), membrane thickness(32μm) and crystallinity,²⁰ which also led to high mechanical strength of membrane and long cycle life of cell, shown as Figure 4b. Besides, the polarization phenomenon of PVDF-HFP separator worse than celgard 2325 separator but lighter than PVDF separator attributed to high surface tightness (Figure 3b) but lower membrane thickness(21μm) and lighter crystallinity,²¹ which also led to lighter polarization maybe with constantly dissolution of membrane as the cycles go on and internal short circuit of cell after 48 cycles, shown as Figure 4b.

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The colorless and transparent PVDF and PVDF-HFP membranes turned black more or less after cycles, shown as Figure 5a and Figure 5b, which revealed some physical or chemical reactions of PVDF and PVDF-HFP membranes in Li/S cell or during cycles. After cycles, celgard 2325 turned a little yellow but still was transparent, shown as Figure 5c. In consideration that PVDF-HFP membrane turn black thoroughly, take PVDF-HFP membrane as the representative to study the cause of blackening.

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The most intuitive reason of blackening may be the carbon on the cathode is immersed with the electrolyte into the membrane. To analyze the components of black material on PVDF-HFP membrane after cycles, SEM, EDS mapping analysis and XRD were used. Figures 6–7 are SEM images and elemental maps of PVDF-HFP and PVDF-HFP cross section after cycles. Figure 8 is the XRD spectra for PVDF-HFP membrane. Figure 6 shows the surface of PVDF-HFP membrane was covered by nanoparticles and the elements C, F, S were visuallyobserved on the maps of PVDF-HFP membrane. It is obvious that the fluorine (Figure 7c) and sulfur (Figure 7d) are homogenously dispersed on the surface, but the carbon is not dispersed uniformly and failed to match with nanoparticles. Meanwhile, the maps of the cross section show the homogenous distribution of fluorine and sulfur at the longitudinal direction, but carbon is more dispersed on the surface of membrane. In view that PVDF-HFP membrane turn black uniformly all through, the black materials should be homogenous dispersed in the membrane. So it may be sulfur instead of carbon that makes the membrane black. Meanwhile, the simple carbon and sulfur peaks cannot be observed in the XRD spectrum (Figure 8), which means the sulfur and carbon in PVDF-HFP membrane is not single substances. So, the carbon element on the surface of membrane should come from the electrolyte instead of conductive agent and the sulfur element should come from polysulfide instead of sulfur. Causation of blackening may be the cathode, anode, electrolyte or all of them. To further confirm the reason, some contrast cells were prepared and shelved for 10 days. Figure 9 shows the membranes in different contrast cells after shelving for 10 days. PVDF-HFP membrane in Li-S cell after shelving for 10 days turn black (Figure 9a), that means cycle is not the neccesary factor of blackening. PVDF-HFP membranes in Al-Li and S+Steel cells have not turn black (Figure 9b and Figure 9c), that means the causation of blackening could not be the cathode, anode and electrolyte but all of them. PVDF-HFP(+) turns black totally (Figure 9d) and PVDF-HFP(−) turns black lightly (Figure 9e), that means the causation should be something generating from cathode and capable of diffusing to anode side. So it is inferred that the causation should be the intermediate product Li₂S_x, which is confirmed by Figure 9f, as shows that PVDF-HFP immersed by Li₂S_x solution turn black.

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XRD and FI-IR were used for PVDF-HFP membranes treated in different conditions to study the structural transformation of PVDF-HFP membranes. Figure 10a shows the XRD patterns for pure PVDF-HFP membrane, PVDF-HFP(+), PVDF-HFP(−) and PVDF-HFP(Li₂S_x). Pure PVDF-HFP membrane is crystalline and shows itspeaks at 2-Theta = 18.2, 20, 26.6 and 38, correspond to the (1 0 0), (0 2 0), (1 1 0) and (0 2 1) crystalline peaks of PVDF. Peaks found at 18.2, 26.6 and 38 were completely destroyed and the peak found at 20 disappeared gradually in PVDF-HFP(−), PVDF-HFP(+) and PVDF-HFP(Li₂S_x), that meanscystallinity of the PVDF-HFP membrane was disrupted by the addition of Li₂S_x. The XRD patterns for PVDF-HFP(Li₂S_x) and PVDF-HFP after cycles (Figure 8) show the humps, which is a typical characteristic of amorphous materials. The segmental mobility of polymer chains is much higher in amorphousregions than thecrystalline. The amorphous nature producesgreater ionic diffusivity in accordance with the high ionic conductivity but low mechanical strength, which is quite in accord with the electrochemical performance (Figure 4).

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Figure 10b shows FT-IR spectra for pure PVDF-HFP membrane, PVDF-HFP (+), PVDF-HFP(−) and PVDF-HFP(Li₂S_x). The peaks near 488, 532, 615, 764, 795, 974 and 1149 cm⁻¹ are for the non polar α-phase of PVDF and its co-polymer PVDF-HFP, while those near 431, 776, 812, 833 and 1233 cm⁻¹are exclusive for the γ-phase. The peaks near 761,794 and 974 cm⁻¹ are observed in pure PVDF-HFP membrane FT-IR spectrum but all of them disappear and the peak near 834 cm⁻¹ turns up in PVDF-HFP(+), PVDF-HFP(−) and PVDF-HFP(Li₂S_x) FT-IR spectrum, which means the non polar α- phase PVDF-HFP membrane transform to γ-phase after reacting with Li₂S_x. The peaks at 1143 and 1350 cm⁻¹ are observed in PVDF-HFP (Li₂S_x), which are ascribed to the sulfone formation. The oxygen element in sulfone is presumably derived from the electrolyte.

Figure 11 shows XRD and FT-IR spectra for pure PVDF membrane before and after being immersed by Li₂S_x solution. The appearance of hump shows the amorphous structure of PVDF (Li₂S_x). The peaks near 761,794 and 974 cm⁻¹ diminish and the peak near 1231cm⁻¹ turn up, which means the non polar α-phase PVDF membrane transform to γ-phase after reacting with Li₂S_x. The transformations of crystalline phase and cystallinity are according with the PVDF-HFP membrane, but the peaks at 1143 and 1350 cm⁻¹ have not been observed, which means the action of PVDF in Li-S cell is not as intensive as PVDF-HFP. The same result is illustrated by photo of PVDF membrane after cycles (Figure 5a).

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In summary, PVDF-HFP can react with Li₂S_x in liquid Li-S cell to form sulfone, which induce the crystalline phase transformation and cystallinity decrease of PVDF-HFP membrane. PVDF can also react with Li₂S_x in liquid Li-S cell, but the action is less intensive than PVDF-HFP.