Introduction

Solid polymer electrolytes have been received attention as electrolyte materials for all solid secondary lithium batteries because of advantages in better safety, flexibility, light weight, and processability in cell design in comparison with liquid electrolyte. Polyether-based electrolytes such as poly(ethylene oxide) (PEO) has been widely studied as electrolyte materials for all solid secondary lithium polymer batteries [1,2,3,4,5,6] and organic electronics such as electric double layer transistor (EDL) [7, 8]. Unfortunately, their ionic conductivities do not reach to 10−3 S/cm at room temperature though some methods, such as network formation, usage of random and block copolymers, and addition of plasticizers such as carbonate- and ether-type solvents, oligomeric PEO, hyperbranched polymers, and inorganic fillers, and carbon dioxide treatment, have been adopted to improve the ionic conductivity of the solid polymer electrolytes [9,10,11,12,13].

Recently, polycarbonate-based electrolytes as potential novel polymer electrolytes have been attracting attention, and their electrochemical and mechanical properties and their application to polymer batteries have been investigated actively. For example, poly(ethylene carbonate) and poly(2-alkoxymethylethylene carbonate), obtained by alternating copolymerization of carbon dioxide with alkyloxy glycidyl ethers [14,15,16,17], poly(trimethylene carbonate) and poly(2-alkoxymethyl-2-ethyltrimethylene carbonate) [18,19,20,21,22,23,24], poly(ethylene carbonates) having both a rigid benzene ring and different ethylene oxide (EO) side chain lengths [25], and alternating copolymers of vinylene carbonate with vinyl ethers with various EO side chain lengths [26], were reported. Moreover, as an interesting fact related to polycarbonate-based electrolytes, Tominaga et al. pointed out that polycarbonate-based electrolytes showed the high lithium transference number (t +) value, larger than 0.5, in the high lithium salt concentrations [15, 16]. We also investigated the polymer electrolytes composed of poly(2,2-dimethoxypropylene carbonate) with lithium salts, such as LiTFSI and LiFSI, and found that polymer electrolytes composed of poly(2,2-dimethoxypropylene carbonate) with LiFSI showed relatively high ionic conductivity values and high transference numbers of lithium ion larger than 0.5 [27].

In addition to development of novel polymer electrolytes, there are demands on high energy density, high energy, and safety on lithium-ion batteries. High voltage is one of factors leading to high energy density for lithium-ion batteries [28, 29]. In general, the voltage of the cell is limited by electrochemical window of the organic solvent in the electrolyte system. Therefore, attempts to explore electrolytes with large electrochemical windows (> 5 V), especially solvents, to match the high-voltage cathode materials have been made. It is reported that sulfite- and sulfone-based electrolytes in solvents such as dimethyl sulfite, diethyl sulfite, ethyl methyl sulfone, and tetramethylene sulfone are promising electrolytes for high-voltage lithium-ion batteries [30,31,32]. To date, few studies have been dedicated to the polymer electrolyte with sulfite structure and sulfone one [33]. Here, we focused to a sulfite structure because of expectation of high-voltage stability and similarity to a carbonate structure. And also, electrochemical and thermal properties of polysulfite-based polymer electrolytes are interesting in comparison with polycarbonate-based ones. In this work, we prepared poly(butylene sulfite) (poly-1) as a challenge to explore the possibility of polymer electrolytes and investigated electrochemical and thermal properties of the poly-1-based electrolytes with lithium salts such as lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) and lithium bis(fluorosulfonyl)imide (LiFSI).

Experimental section

Materials

1,4-Butanediol, thionyl chloride, trifluoromethanesulfonic acid (TfOH) (all reagents, TCI Chemicals) were used without further purification. Acetonitrile (Sigma, H2O < 0.005%) was used as received. Lithium bis(trifluoromethanesulfonyl)imide (LiTFSI, LiN(SO2CF3)2, Kishida Chemical Co.) and lithium bis(fluorosulfonyl)imide (LiFSI, LiN(SO2F)2, Kishida Chemical Co.) were dried under vacuum at room temperature prior to use and kept inside an argon-filled glove box.

Preparation of polymer

Butylene sulfite (1) and its polymer (poly-1) were synthesized according to the procedure reported previously [34] (Scheme 1).

Scheme 1
scheme 1

Syntheses of butylene sulfite (1) and poly(butylene sulfite) (poly-1)

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Butylene sulfite (1)

1,4-Butanediol (36.9 mL, 554 mmol) was dissolved in dichloromethane (127 mL) and then cooled to 0 °C in ice bath. Into the resulting solution, a solution of thionyl chloride (40.8 mL, 745.4 mmol) in dichloromethane (77 mL) was added dropwisely over 5 h. The mixture solution was stirred for 1 h at room temperature and then refluxed for 1 h with stirring. The reaction solution was washed with distilled water, saturated sodium bicarbonate aqueous solution, and dried over anhydrous magnesium sulfate. The filtrate was placed under reduced pressure, and then the residue was distilled to give 1 (14.6 g, 38.7% yield) as colorless liquid: bp 104 °C (20 mmHg); IR (KBr, cm−1): νCH 2958, νS = O 1102; 1H NMR (CDCl3, ppm): δ 4.46–4.73 (m, 2H), 3.94–4.02 (m, 2H), 1.82–1.90 (m, 4H); 13C NMR (CDCl3, ppm): δ 64.0 (OCH2), 28.3 (CH2).

Polymerization of 1

Butylene sulfite (1) (2.0 g, 14.7 mmol) and trifluoromethanesulfonic acid (13 μL, 0.15 mmol) were placed in an ampoule and then purged with nitrogen gas. The ampoule was placed at 25 °C for 1.5 h for polymerization. Pyridine (40 μL, 0.50 mmol) was added into the ampule to stop the polymerization and then poured it into excess amount of diethyl ether. The polymer was dissolved in a small amount of dichloromethane, and the resulting solution was poured into an excess amount of diethyl ether to deposit the polymer. The polymer was dried under reduced pressure to give poly(butylene sulfite) (poly-1) (1.52 g, 76.0%) a pale brown viscous liquid: IR (KBr, cm−1): νCH 2960, 2891, νS = O 1199, 924, 703; 1H NMR (CDCl3, ppm): δ 4.10 (m, 2H), 3.97 (m, 2H), 1.80 (s, 4H); 13C NMR (CDCl3, ppm): δ 62 (OCH2), 26 (CH2); M n = 21,000 (M w/M n = 1.84) .

Preparation of polymer electrolyte

All preparation procedures were carried out inside a dry argon-filled glove box kept at dew point of − 85 °C to avoid moisture contamination. Given amounts of poly-1 and lithium salt (LiTFSI or LiFSI) were dissolved in acetonitrile and vigorously stirred for 1 h. The resulting solution was poured onto a Teflon Petri dish, and acetonitrile was evaporated very slowly at room temperature for 24 h, and then, the polymer electrolytes were dried at 80 °C under reduced pressure for 24 h using electric furnace equipped in the glove box and cooled, and then stored inside the glove box.

Measurements

IR spectra were recorded on a JASCO FT/IR-4100 spectrometer. 1H and 13C NMR spectra were taken on a JEOL JNM-A500 (500 MHz for 1H) spectrometer using tetramethylsilane as an internal standard. The number-average (M n) and weight-average (M w) molecular weights of poly-1 were estimated by gel permeation chromatography (GPC) on a JASCO PU-1580 equipped with a JASCO RI-930 refractive index detector and two TOSOH TSKgel Multipore HXL-M columns using tetrahydrofuran (THF) as an eluent at a flow rate of 1.0 mL min−1 and polystyrene standards for calibration at room temperature.

The ionic conductivities of the polymer electrolytes were measured by the electrochemical impedance spectroscopy. The samples were fixed inside a Teflon O-ring spacer with known thickness and sandwiched between two stainless steel (SS) electrode discs acting as ion-blocking electrodes and set in a thermostat oven chamber. The measurements were carried out using Solartron 1260 frequency response analyzer over a frequency range of 1 Hz to 1 MHz and in a temperature range of 0 to 80 °C with an amplitude of 10 mV. All samples were first kept at 80 °C for at least 12 h and then measured by cooling cycle. The measurements were carried out after keeping the samples for 1 h at each temperature to attain thermal equilibration. Nyquist plots (semi-circles) were computed from the raw experimental data. The resistance of the polymer electrolyte was taken at the maximum impedance value. The conductivity values (σ) were derived from the equation of σ = (1/R b)(L/A), where R b is the bulk resistance determined from the Nyquist plot, L is the thickness of the sample, and A is the area of the sample.

The electrochemical stability (oxidation stability) of the polymer electrolytes was evaluated by running a linear sweep voltammetry (10 mV/s) in two electrode cell as Li/polymer electrolyte/SS cell, where Li and SS were used as a counter electrode and a blocking working electrode, respectively. A Solartron 1287 electrochemical interface was used for the voltammetry measurement at 30, 60, or 80 °C. Lithium transference number (t +) was determined at 30 °C by the two-impedance polarization coupling technique developed by Bruce et al., where cells with symmetrical non-blocking lithium metal electrode were used [35, 36].

Thermal stability of polymer electrolytes was investigated with thermogravimetry-differential thermal analysis instrument TG-DTA 6200 (Seiko Instruments Inc.) at a heating rate of 5 °C/min in nitrogen. The glass transition temperature (T g) of the polymer electrolytes were determined by differential scanning calorimetry (DSC) using EXSTER6000 thermal analysis instrument DSC 6200 (Seiko Instruments Inc.) under a nitrogen gas flow. About 10-mg amount of sample was weighted, loaded in an aluminum pan, and then sealed. The measurement was carried out in a temperature range of − 100 to 110 °C at a heating rate of 10 °C/min.

Results and discussion

Thermal property of polymer electrolytes

Thermal property such as glass transition temperatures (T g) and decomposition temperatures at 5 wt% weight loss (T d5) of the poly-1 and the polymer electrolytes composed of poly-1 with lithium salts such as LiTFSI and LiFSI at various lithium salt concentrations were investigated by the DSC and TGA measurements. The DSC traces of the polymer electrolytes are shown in Fig. 1a for the poly-1/LiTFSI system and Fig. 1b for the poly-1/LiFSI system, respectively. The thermal degradation behavior of the poly-1 and the polymer electrolytes measured by TGA is shown in Fig. 2a for the poly-1/LiTFSI system and Fig. 2b for the poly-1/LiFSI system, respectively. The results of thermal properties of the poly-1 and polymer electrolytes are summarized in Table 1, together with the thermal property of LiTFSI and LiFSI salts.

Fig. 1
figure 1

DSC traces of the polymer electrolytes of a the poly-1/LiTFSI and b the poly-1/LiFSI systems, respectively

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Fig. 2
figure 2

TG curves of the poly-1 and the polymer electrolytes of a the poly-1/LiTFSI and b the poly-1/LiFSI systems, respectively

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Table 1 Thermal properties of the poly-1 and the polymer electrolytes composed of poly-1 with lithium salts (LiTFSI and LiFSI) at various lithium salt concentrations
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Poly-1 and all of polymer electrolytes do not show melting peaks and crystallization exotherm, indicating that they are noncrystalline, that is, amorphous. The T g of poly-1 is to be − 74.1 °C, and the T g’s of polymer electrolytes are to be − 56.3 to −19.6 °C for the poly-1/LiTFSI system and − 48.3 to − 44.0 °C for the poly-1/LiFSI system, respectively, indicating that the complexation of the poly-1 with lithium salts such as LiTFSI and LiFSI raises T g in both systems, that is, interaction formation between dissociated lithium cations and the sulfite groups in the poly-1 chains. As shown in Table 1, the T g values of the polymer electrolytes for the poly-1/LiTFSI system increase significantly with increasing the lithium salt concentrations, the behavior of which is similar to that of polyether-based polymer electrolytes due to the strong coordination of cations with the dipoles ether chains [4,5,6]. On the other hand, in contrast to the poly-1/LiTFSI system, the T g values for the poly-1/LiFSI system are almost constant or decrease with increasing the lithium salt concentrations, which is similar to the polycarbonate-based polymer electrolytes [15, 19, 20].

Polymer electrolytes of the poly-1/LiTFSI system and the poly-1/LiFSI system had T d5 of 178–192 and 145–183 °C, respectively, indicating that the polymer electrolytes for the poly-1/LiTFSI system were thermally more stable than those for the poly-1/LiFSI system. And also, the T d5 values increase for the poly-1/LiTFSI system and decrease for the poly-1/LiFSI system with increasing lithium salt concentrations. This behavior is different from the poly(2,2-dimethoxypropylene carbonate)-based polymer electrolytes with LiTFSI and LiFSI, where the T d5 values decreased with increasing lithium salt concentrations for both lithium salt systems [27]. It is not clear at present why different thermal behavior is observed between LiTFSI and LiFSI toward poly-1. It seems that the difference in chemical structures, that is, carbonate group and sulfite one, might affect the coordination state of lithium cation toward polymer chains.

Ionic conductivity of polymer electrolytes

The ionic conductivities of the polymer electrolytes composed of poly-1 with LiTFSI and LiFSI were measured as a function of temperature and lithium salt concentration. The temperature dependence of ionic conductivity for these polymer electrolytes at five [LiTFSI]/[repeating unit (RU)] ratios of 1/15, 1/10, 1/5, 1/2, and 1/1 and at four [LiFSI]/[repeating unit (RU)] ratios of 1/10, 1/5, 1/2, and 1/1 in a temperature range of 0 to 80 °C were shown in Fig. 3a, b, respectively.

Fig. 3
figure 3

Temperature dependence of the ionic conductivities for the polymer electrolytes of a the poly-1/LiTFSI system at five [LiTFSI]/[repeating unit (RU)] ratios of 1/15, 1/10, 1/5, 1/2, and 1/1 and b the poly-1/LiFSI system at four [LiFSI]/[repeating unit (RU)] ratios of 1/10, 1/5, 1/2, and 1/1

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Ionic conductivities in both polymer electrolyte systems increase with increased temperature as shown in Fig. 3a, b. Both polymer electrolyte systems show Vogel-Tammann-Fülcher (VTF) behavior like as typical polymer electrolytes, where the transportation of the lithium ions is coupled to the segmental motion of polymer chains [6, 37]. And also, the relationships of ionic conductivity for the poly-1/LiTFSI system and the poly-1/LiFSI system at selected temperatures such as 20, 50, and 80 °C with T g were shown in Fig. 4a, b, respectively.

Fig. 4
figure 4

The relationship of ionic conductivity at 20, 50, and 80° C of the polymer electrolytes for a the poly-1/LiTFSI system and b the poly-1/LiFSI system with T g

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From Fig. 4a, ionic conductivities of the polymer electrolytes for the poly-1/LiTFSI system increased with increasing lithium salt concentrations, reached a maximum value at the [LiTFSI]/[repeating unit (RU)] ratio of 1/10, and then decreased in further increased salt concentrations. Similar behavior was observed for the polymer electrolytes for the poly(trimethylene carbonate)/LiTFSI system [18]. At the [LiTFSI]/[repeating unit (RU)] ratio of 1/10, the highest ionic conductivity values are 2.36 × 10−4 S/cm at 80 °C and 1.01 × 10−5 S/cm at 20 °C. On the other hand, glass transition temperatures (T g) increased significantly with an increase in lithium salt concentrations, due to a physical crosslinking formation by the strong coordination of lithium cation with the polymer chains. Observation of the maximum ionic conductivity value for the poly-1/LiTFSI system is explained well with the increase in the carrier number and the rise in T g, which is similar to the typical ether-type polymer electrolytes [4,5,6]. On the other hand, as shown in Fig. 4b, ionic conductivities of the polymer electrolytes for the poly-1/LiFSI system increase with increasing lithium salt concentrations, and glass transition temperatures (T g) are almost constant or decrease with increasing the lithium salt concentrations as mentioned above. At the [LiFSI]/[repeating unit (RU)] ratio of 1/1, the highest ionic conductivity values are 1.25 × 10−3 S/cm at 80 °C and 5.93 × 10−5 S/cm at 20 °C. This behavior in ionic conductivity for the poly-1/LiFSI system is similar to that for the polymer electrolyte for the poly(2,2-dimethoxypropylene carbonate)/LiFSI system reported previously, but different from the polymer electrolyte for the poly(2,2-dimethoxypropylene carbonate)/LiTFSI system [27], ascribing to the difference in chemical structure between carbonate group and sulfite one.

Here, in order to obtain some information related to the chemical structure between carbonate group and sulfite one, we calculated the partial charges on the oxygen atoms in diethyl sulfite and diethyl carbonate with the Extended Hückel method [38]. The results are shown in Fig. 5.

Fig. 5
figure 5

Partial charges on the oxygen atoms for diethyl sulfite and diethyl carbonate calculated with the Extended Hückel method

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The partial charges on oxygen atoms in diethyl sulfite are larger than those in diethyl carbonate, suggesting that the oxygen atoms in diethyl sulfite may coordinate more strongly to lithium ion compared with those in diethyl carbonate. It is, therefore, considered that the different coordinative ability between sulfite group and carbonate one might cause the difference in the ionic conductivity behavior between the poly-1/LiFSI system and poly-1/LiTFSI system.

Electrochemical properties of polymer electrolytes

The electrochemical stability of the polymer electrolytes for the poly-1/LiTFSI and poly-1/LiFSI systems at four different [Li salt]/[repeating unit] ratios of 1/10, 1/5, 1/2, and 1/1 was measured by linear sweep voltammetry at 60 and 80 °C for the poly-1/LiTFSI system and at 30 °C for the poly-1/LiFSI system, respectively. The measurements at 60 and 80 °C were performed to obtain the large electric current flow. Linear sweep voltammograms of the polymer electrolytes are shown in Fig. 6 for the poly-1/LiTFSI system at the [LiTFSI]/[RU] ratio of 1/1 at 80 °C and the poly-1/LiFSI system at the [LiFSI]/[RU] ratio of 1/1 at 30 °C, respectively. The results of breakdown voltages are summarized in Table 2.

Fig. 6
figure 6

Linear sweep voltammograms of the polymer electrolytes for the poly-1/LiTFSI system at the [LiTFSI]/[RU] ratio of 1/1 at 80° C and the poly-1/LiFSI system at the [LiFSI]/[RU] ratio of 1/1 at 30° C, respectively, at the scanning rate of 10 mV/s

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Table 2 Breakdown voltages (V) and lithium transference numbers (t +) for the polymer electrolytes composed of the poly-1 with LiTFSI and LiFSI at various lithium salt concentrations
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The polymer electrolytes for the poly-1/LiTFSI and the poly-1/LiFSI systems have the breakdown voltages of 4.1–4.3 V at 60 and 80 °C and 4.3–4.4 V at 30 °C, respectively, which are lower breakdown voltages than 5 V expected for the sulfite structure. It is reported that the poly(ethylene carbonate)-LiFSI 80 wt% electrolyte and the poly(trimechylene carbonate)-LiTFSI electrolyte at the [LiTFSI]/[RU] ratio of 1/8 have electrochemical stabilities of about 5.3 V vs Li/Li+ [17] and up to 5 V vs Li/Li+ [20], respectively. The poly(butylene sulfite)-based electrolytes have definitely lower electrochemical stability than the poly(carbonate)-based ones, though they are stable until 4 V. From Fig. 5, as lithium ion is considered to coordinate strongly a sulfite group in comparison with a carbonate one, the S(=O)–O bond energy might be reduced greatly by lithium ion coordination, resulting in the lower electrochemical stability of poly(sulfite)-based electrolytes. Lithium transference numbers (t +) at 30 °C for the poly-1/LiTFSI and the poly-1/LiFSI systems were measured at the [lithium salt]/[repeating unit (RU)] ratios of 1/10 and 1/1, and the results are summarized in Table 2. Unfortunately, t + at 30 °C in the [lithium salt]/[repeating unit (RU)] ratio of 1/1 for the poly-1/LiTFSI system could not be determined because ionic conductivity of the poly-1/LiTFSI system was too low to measure. The t + values at the [lithium salt]/[repeating unit (RU)] ratio of 1/10 are 0.57 for the poly-1/LiTFSI system and 0.56 for the poly-1/LiFSI system, respectively, and also the t + value of the poly-1/LiFSI system at the [LiFSI]/[repeating unit (RU)] ratio of 1/1 is 0.61. In the case of poly-1/LiFSI system, it is considered that an increase in the t + value at higher LiFSI salt concentration is due to an increase in carrier number. Such high t + values, larger than 0.5, of these polymer electrolytes are reported for the polymer electrolytes of poly(ethylene carbonates) and derivatives with lithium salts in high salt concentrations [14,15,16,17]. The t + values for both systems are significantly higher than those (0.1–0.3) of conventional PEO-based polymer electrolytes, where the movement of ions is coupled with segmental motion of the polymer chains [4,5,6].

Conclusions

Poly(butylene sulfite) (poly-1) was prepared, and ionic conductivities, thermal, and electrochemical properties of the polymer electrolytes composed of poly-1 with LiTFSI and LiFSI were investigated. Polymer electrolytes for the poly-1/LiTFSI system showed the ionic conductivity behavior similar to the typical ether-type polymer electrolytes and the poly(trimethylene carbonate)/LiTFSI system. The highest ionic conductivity values at the [LiTFSI]/[repeating unit] ratio of 1/10 were 2.36 × 10−4 S/cm at 80 °C and 1.01 × 10−5 S/cm at 20 °C. On the other hand, ionic conductivities of the polymer electrolytes for the poly-1/LiFSI system increased with an increase in lithium salt concentrations, and ionic conductivity values at the [LiFSI]/[repeating unit] ratio of 1/1 were 1.25 × 10−3 S/cm at 80 °C and 5.93 × 10−5 S/cm at 20 °C. Glass transition temperature (T g) increased with lithium salt concentrations for the poly-1/LiTFSI system, but T g for the poly-1/LiFSI system was almost constant regardless of lithium salt concentrations. Polymer electrolytes showed high t + to be 0.57 for the poly-1/LiTFSI system and 0.56 for the poly-1/LiFSI system, respectively. TG measurement indicated that the polymer electrolytes for the poly-1/LiTFSI system (T d5 of 178–192 °C) were thermally more stable than those for the poly-1/LiFSI system (T d5 of 145–183 °C). Polysulfite-based electrolytes have electrochemical stabilities up to 4 V.