Introduction

Since their commercialization in the 1990s, Li-ion batteries (LIBs) have been widely used in consumer electronics and have recently found large-scale energy storage applications in electric vehicles and smart grids1,2,3. However, the currently used LIBs contain flammable liquid organic electrolytes and therefore pose safety risks4. To mitigate these risks, one can replace organic liquid electrolytes with inorganic solid electrolytes, which feature higher thermal stability and are not susceptible to leakage5. This replacement affords high-energy-density all-solid-state batteries (ASSBs), which have attracted much attention, as exemplified by attempts to use solid electrolytes in combination with high-voltage cathodes6,7, high-capacity sulfur electrodes8,9, and Li metal anodes10,11,12,13.

Among the various types of solid (e.g., sulfide-, oxide-, hydride-, and halide-based) electrolytes developed to date5,14,15,16, the sulfide-based ones feature high conductivities and interface formability, and are therefore particularly well suited for ASSBs. In particular, sulfide electrolytes with Li10GeP2S12-17,18, argyrodite-19,20, and Li7P3S11-type21,22 crystal structures have high conductivities (>10−3 S cm−1) comparable to those of liquid electrolytes. Sulfide electrolytes are easily deformed by pressing at room temperature23, allowing one to form favorable electrode/electrolyte interfaces with high contact areas, and ensure sufficient ion conduction. However, the volume changes of the electrode active materials during charge/discharge induce local contact losses at ASSB electrode/electrolyte interfaces24,25. Typical sulfide electrolytes contain Li+ ions and isolated anions such as PS43−. Previously, sulfide polymers with (–P–S–S–)n units were predicted to electrochemically form at cathode/electrolyte interfaces during charging (delithiation)26,27. In contrast, thermodynamic calculations and structural analyses of the related interfaces indicate that such polymers undergo disproportionation into P2S74− and elemental sulfur28,29,30. To date, no agreement has been reached, and the related discussions are still ongoing.

The improvement of existing processing technologies is vital for ASSB commercialization, with wet slurry coating recognized as a practical and scalable way of fabricating sheet-type ASSBs31. In this process, binders help to adhere the composites (containing the electrode active materials, solid electrolytes, and carbon-based conductive additives) to the current collector, make the composite sheets flexible, and allow roll-to-roll manufacturing. Both conventional LIBs and ASSBs use organic polymers, such as polyvinylidenefluoride32, styrene-butadiene-styrene copolymers33, and acrylonitrile-butadiene copolymers34,35 as binders. However, the insulating nature of these binders hinders ionic and electronic conduction in ASSBs. To mitigate this problem, one can use volatile binders that can be thermally removed to afford binder-less ASSBs36. Another strategy is to use Li+-conductive polymer binders based on organic polymer electrolytes or hybrids of ionic liquid and organic polymer electrolytes37,38,39. Sulfide polymers will be a promising Li+-conductive binder, and thus reduce hetero interfacial resistances in ASSBs.

Herein, we polymerized isolated PS43− anions by reacting them with I2 (Fig. 1). The reaction of two PS43− anions with I2 afforded a dimer comprising two PS4 tetrahedra bridged with a disulfide (S–S) bond, whereas higher I2 loadings afforded a potentially flexible chain polymer with a (–P–S–S–)n structure. We demonstrated the mechanochemical and liquid-phase syntheses of the sulfide polymer. This polymer was compared with previously reported structures like P2S74− to provide useful suggestions for the design of cathode/electrolyte interfaces in ASSBs and was used as a binder in sheet-type ASSBs. LiI, a by-product of the above reaction, is also a Li-ion conductor40 and can enhance the ionic conductivity of sulfide electrolytes41,42 which suggests that sulfide polymer-LiI composites hold great promise as solid electrolytes.

Fig. 1: Schematics of the I2-induced polymerization of Li3PS4.
figure 1

The reaction of Li3PS4 with I2 afforded a chain polymer with a (–P–S–S–)n structure.

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Results

Mechanochemical synthesis of sulfide polymers and their characterization

Li3PS4 was prepared by ball milling a mixture of Li2S and P2S5 and was further ball-milled with I2 to afford sulfide polymers. We synthesized three kinds of sulfide polymers with different molar ratios; Li3PS4-I2 (2:1), Li3PS4-I2 (4:3), and Li3PS4-I2 (1:1). Powder X-ray diffraction (PXRD) patterns acquired after the reaction with I2 revealed the presence of LiI (PDF#01-075-5397) (Fig. 2(a)), and thus indicated the occurrence of Li3PS4 polymerization. The Raman spectrum of the amorphous polymerization product featured two main peaks at 477 and 390–420 cm−1 (Fig. 2(b)). The former peak, ascribed to S–S bond stretching, gained intensity with increasing I2 loading, which reflected the concomitant increase in the extent of S–S cross-linking. Although elemental S exhibits a similar S–S stretching peak at 473 cm−1 43, the peak at 477 cm−1 had a different origin, as it did not lose intensity after the polymer was washed with toluene to dissolve elemental S (Supplementary Fig. 1). The peak at 390–420 cm−1 was assigned to the stretching of P–S bonds and shifted to lower wavenumbers with increasing I2 loading (cf. the P–S peak of pristine Li3PS4 at 421 cm−1), which was ascribed to the progressive polymerization of Li3PS4 through S–S bridging. Supplementary Table 1 lists the Raman shifts of selected lithium thiophosphate anions44,45,46,47, viz. P2S74− (pyro-thiodiphosphate, two corner-sharing PS4 tetrahedra), P2S64− (hypo-thiodiphosphate with P–P bond), P2S62− (meta-thiodiphosphate, two edge-sharing PS4 tetrahedra), and PS3− (meta-thiophosphate, chain of corner-sharing PS4 tetrahedra). The Raman shift of Li3PS4-I2 (2:1), which featured two S–S bond-linked PS4 tetrahedra (P2S84−) was similar to that of P2S74−, and that of Li3PS4-I2 (1:1), which featured a chain structure (PS4−), was similar to that of PS3−. The structural difference between Li3PS4-I2 polymers and the known anions of P2S74− or PS3− is that in the former case, the PS4 tetrahedra are linked through S–S bonds, whereas in the latter case, they share the corner S atoms. Because this difference is small, the corresponding Raman shifts were similar. Similar behavior has been reported for other thiophosphates such as Li3PS4+n and P4S10+m48,49.

Fig. 2: Structural data for Li3PS4-I2 polymers prepared by ball milling.
figure 2

a PXRD patterns and (b) Raman spectra. The diffraction peaks of cubic LiI are indicated with black circles.

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Li3PS4-I2 (1:1) featured a particle size of 1–10 µm (Fig. 3(a)) and had good deformability (as did other sulfide electrolytes), affording a highly dense compact without clear original particle shapes after pressing at room temperature (Fig. 3(b)). The conductivities of Li3PS4-I2 (2:1), (4:3), and (1:1) were 2.9 × 10−4, 1.7 × 10−4, and 3.0 × 10−5 S cm−1, respectively, i.e., these values decreased with progressive polymerization (Fig. 3(c) and Supplementary Fig. 2).

Fig. 3: Morphologies and ion-conduction behaviors of Li3PS4-I2 polymers.
figure 3

SEM images of (a) the Li3PS4-I2 (1:1) powder prepared by ball milling and (b) the fracture cross-section of the pellet prepared by cold-pressing the powders at 333 MPa. c Room temperature conductivities of Li3PS4-I2 polymers. The error bar show standard deviation of Li3PS4-I2 (2:1) for n = 3, Li3PS4-I2 (4:3) for n = 3, and Li3PS4-I2 (1:1) for n = 5, respectively.

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Anisole, an aprotic solvent with a comparatively low donor number, is used as the binder solvent of sulfide electrolyte-based sheet-type ASSBs, as it does not attack these electrolytes36. Figure 4(a) shows that the solubility of Li3PS4-I2 polymers in anisole increased with increasing I2 loading and was highest for Li3PS4-I2 (1:1). On the other hand, Li2S-P2S5 solid electrolytes (ortho-, pyro-, and meta-thiophosphates) exhibited low solubility in anisole (Supplementary Fig. 3). Upon drying, the supernatant of the Li3PS4-I2 (1:1) solution in anisole afforded a sticky gel, in line with the formation of long chains (Fig. 4(b)). The Raman spectrum of ball milling-prepared Li3PS4-I2 (1:1) was compared with that of the dried supernatant of the corresponding anisole solution (Fig. 4(c)). Both the spectra featured similar S–S and P–S peaks around 477 and 390 cm−1, respectively, indicating that these components were similar. However, the P–S peak of the ball milling-prepared Li3PS4-I2 (1:1) was broader than that of the polymer recovered from the anisole solution, indicating that a greater variety of structures was formed in the former case. Figure 4(d) presents the 31P NMR spectra of the two kinds of Li3PS4-I2 (1:1) polymers, revealing the presence of three main peaks at 86–89, 100, and 120 ppm. The main peak at 86–89 ppm was tentatively assigned to the chain of disulfide-bond-linked PS4 tetrahedra, whereas the other peaks could not be assigned. Supplementary Table 1 lists the 31P NMR data for the known anions of lithium thiophosphates, revealing that a subpeak at ~120 ppm has been observed in the spectrum of PS3− 45. The above unknown peaks may reflect the presence of branched structures or variable-size rings in addition to the regular chains of PS4 tetrahedra, although further structural analysis is needed to reveal the details.

Fig. 4: Solubilities of ball milling-prepared Li3PS4-I2 polymers in anisole.
figure 4

a Photographs of solutions of 10 wt% Li3PS4-I2 (2:1), 20 wt% Li3PS4-I2 (4:3), and 30 wt% Li3PS4-I2 (1:1) in anisole after centrifugation at 2599 × g. b Photograph of the supernatant of the Li3PS4-I2 (1:1) polymer solution during drying at 60 °C. c Raman and (d) 31P NMR spectra of the Li3PS4-I2 (1:1) polymer prepared by ball milling and the dried supernatant of its anisole solution.

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Liquid-phase synthesis of sulfide polymers

Li3PS4-I2 polymers were also prepared by liquid-phase synthesis (LS), i.e., by stirring Li3PS4 and I2 in anisole at 60 °C for one day. In the case of Li3PS4-I2 (1:1), the product was a clear brown solution at room temperature, while for Li3PS4-I2 (2:1) and (4:3) (Fig. 5(a)), we observed precipitates that were identified as unreacted Li3PS4 based on their Raman spectra (Fig. 5(b)). Since I2 is soluble in anisole, the polymerization of Li3PS4 progressed from the surface of the insoluble Li3PS4. As in the case of ball milling-prepared Li3PS4-I2 (1:1), evaporation of the Li3PS4-I2 (1:1) solution obtained after LS afforded a sticky gel, which contained hexagonal LiI (PDF#01-075-5395) (Supplementary Fig. 4). Notably, hexagonal LiI was also observed in the dried supernatant of the anisole solution of ball milling-prepared Li3PS4-I2 (1:1). This hexagonal phase was reported to be metastable and undergo a slow structural change to the cubic phase even at room temperature50. However, the fact that hexagonal LiI mainly precipitated upon the drying (at 60 °C) of Li3PS4-I2 (1:1) polymers isolated from anisole solutions (the solution produced during LS and the supernatant obtained by dispersing ball milling–produced polymer) suggested that the sulfide polymer matrix prohibited the hexagonal-to-cubic transformation. The NMR spectrum of Li3PS4-I2 (1:1) prepared by LS was similar to that of the dried supernatant of the anisole solution of the ball milling-prepared polymer, i.e., the Li3PS4-I2 polymer with a (–P–S–S–)n chain structure could be also synthesized by LS. The conductivity of Li3PS4-I2 (1:1) prepared by LS was 2.3 × 10−6 S cm−1 after drying at 200 °C (Supplementary Fig. 5), which was lower than the conductivity of ball milling-produced polymer.

Fig. 5: Liquid-phase synthesis of Li3PS4-I2 polymers.
figure 5

a Schematics of the synthetic process. Photographs of Li3PS4-I2 polymer solutions obtained after stirring Li3PS4 and I2 at 60 °C for one day. The Li3PS4-I2 (1:1) polymer solution afforded a sticky gel upon drying. b Raman and (c) 31P NMR spectra of Li3PS4-I2 polymers prepared by liquid-phase synthesis.

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Binder applications of sulfide polymers

The Li3PS4-I2 polymer was used as a binder for ASSBs. Sheets containing Li3PS4 as a solid electrolyte were prepared by coating a slurry comprising this thiophosphate and ball milling-prepared Li3PS4-I2 (1:1) (Fig. 6(a–c) and Supplementary Movie 1) in anisole. The obtained sheets with a thickness of ~80 µm could be bent around a 16-mm-diameter cylinder, although Li3PS4 powder was detached from the sheet in the absence of the binder. The conductivity of the Li3PS4 sheet with the sulfide polymer binder (2.3 × 10−4 S cm−1, Supplementary Fig. 6) was close to that of the sheet without the binder (3.2 × 10−4 S cm−1).

Fig. 6: Sheets of Li3PS4 solid electrolytes prepared by wet slurry coating.
figure 6

a Appearance of the sheet prepared using the Li3PS4-I2 (1:1) polymer as a binder. The results of bending around a 16-mm-diameter cylinder are displayed for sheets prepared (b) with and (c) without the binder.

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We also prepared composite electrode sheets (thickness: ~40 µm) comprising LiNi1/3Mn1/3Co1/3O2 (NMC), an argyrodite-type solid electrolyte (Li6PS5Cl with a conductivity of 2.2 × 10−3 S cm−1), and Li3PS4-I2 (1:1) as a binder (Supplementary Fig. 7) and evaluated the performance of the corresponding all-solid-state cell. We adopted the argyrodite-type solid electrolyte in the composite electrode due to its high ionic conductivity in the 10−3 S cm−1 range, ease of synthesis, and low costs of raw materials, which are advantageous for mass production51,52. As a result, typical charge-discharge curves of the NMC active material without obvious side reactions at 2.6–4.3 V (vs. Li/Li+) were observed (Fig. 7(a)). The small cell resistance after the first charge indicated that the Li+-conductive binder did not act as a resistive component (Fig. 7(b)). In addition, the above cell featured high cycling performance (Fig. 7(c)), retaining 93.8% of its capacity between cycles 5 and 200. These results suggest that the prepared Li3PS4-I2 polymer can be used as an ion-conductive binder for sheet-type ASSBs.

Fig. 7: Electrochemical performance of an all-solid-state cell containing Li3PS4-I2 (1:1) as a polymer binder.
figure 7

a First charge-discharge curve, b Nyquist plot after first charge, and (c) cycling performance of an all-solid-state Li-In/Li3PS4/NMC cell containing Li3PS4-I2 (1:1) as a polymer binder. The inset displays the enlarged view at the beginning and end of ten cycles.

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Discussion

Herein, solid electrolytes prepared by I2-induced polymerization of Li3PS4 were shown to contain S–S bridges, as indicated by the Raman peak at 477 cm−1. This peak was retained after the polymer was washed with toluene to remove elemental sulfur and thus truly originated from the S–S linkages in the polymer. The solubility of the sulfide polymer in anisole increased with increasing I2 loading, which was in contrast with the low solubilities of ortho-, pyro-, and meta-thiophosphate Li2S-P2S5 electrolytes. The formation of a sticky gel upon the drying of the Li3PS4-I2 (1:1) solution was in line with the formation of chain structures. Notably, the sulfide polymer did not undergo disproportionation into the known thiophosphates and elemental S and was assumed to contain PS4 tetrahedra cross-linked with S–S bonds. On the other hand, the sulfide polymer was similar to the known thiophosphates with corner-sharing PS4 tetrahedra in terms of the Raman shifts of P–S bonds and 31P NMR shifts, which indicated the similarity between the chemical environments of P atoms. The same behavior has been observed in reports on the structural analysis of cathode/electrolyte interfaces29,53. Further structural analyses of the sulfide polymer are expected to provide critical insights into the related cathode/electrolyte interfaces to facilitate their design in ASSBs.

Liquid-phase synthesis is well suited for mass production54. The sulfide polymer could be prepared by both liquid-phase and mechanochemical syntheses, although the sulfide polymers prepared by liquid-phase possessed lower conductivities. The polymer structure and the composite state with LiI would influence the conductivities of sulfide polymers. LiI is a poor ionic conductor40, however, doping it can improve the conductivity of sulfide electrolytes41,42. Although LiI crystal was observed in the XRD patterns, some amount of LiI were considered to be doped in the amorphous sulfide polymers and enhance the conductivities. The ball milling process was assumed to increase doped-LiI in the sulfide polymer, and made the higher conductivity. Removing redundant LiI crystal with poor ionic conductivity and increasing doped-LiI will improve the conductivities of sulfide polymers. The sulfide polymer was used as a binder to prepare sheets comprising Li3PS4 and NMC composites as solid electrolytes. These sheets were bendable, and all-solid-state cells containing NMC composite sheets exhibited low resistance and high capacity retention. The NMC loading (~2 mg) was low compared to the practical value, and it was difficult to prepare thick electrode sheets with >100 µm thickness without cracking. Further works such as designing a molecular weight and a structure of the sulfide polymer will improve the adhesiveness of the binder. Thus, the flexible and ionically conductive sulfide polymer prepared herein is expected to facilitate the development of more robust electrode/electrolyte interfaces in ASSBs.

Methods

Material synthesis

Solid electrolytes were prepared by ball milling appropriate amounts of Li2S (Furuuchi Chemical Corp.) and P2S5 (Sigma-Aldrich) at 500 rpm for 20 h using ZrO2 pots (volume: 45 mL) and balls (diameter: 5 mm). The thus obtained Li3PS4 was polymerized by ball milling with I2 (Kojundo Chemical Lab. Co.) at 500 rpm for 20 h. Dehydrated heptane (Fujifilm Wako Pure Chemical Corp.) was added in the pots with the starting materials as a medium for ball milling. The heptane medium was removed after ball milling by drying the samples at room temperature. The extent of S–S bridging was determined by I2 loading. The I2-induced polymerization of Li3PS4 and I2 was also carried out by one-day stirring in anisole at 60 °C and a total solid content of 25 wt%. The resulting Li3PS4-I2 polymer solution was dried in a vacuum at 60 °C. All the samples were synthesized in a dry Ar atmosphere.

Material characterization

PXRD patterns (SmartLab, Rigaku; Cu Kα radiation, 2θ = 10−60°, step = 0.02°) were recorded in sealed vessels under dry Ar. Raman spectra (LabRAM HR Evolution, HORIBA) were recorded in the wavenumber range 100–800 cm−1 using an exposure of 1 s 30 times and an excitation laser with a wavelength of 532 nm. For Raman measurements, the samples were sealed in glass tubes filled with dry Ar. Solid-state magic-angle-spinning 31P NMR (ECZ400R, JEOL) measurements were performed at a spinning speed of 12 kHz. The powdered samples were sealed in spinners in a dry Ar-filled glove box. A 90° 3.8 μs pulse with a recycle delay of 300 s was used, and the observed frequency was 161.944 MHz. Powder morphologies and pellet fracture cross-sections were examined by scanning electron microscopy (SEM; JSM-7800F, JEOL). Sample pellets were prepared by pressing the powders at 333 MPa and room temperature. All SEM samples were prepared in a dry Ar-filled glove box and transferred in sealed vessels.

Preparation of solid electrolyte and electrode composite sheets

The Li3PS4-I2 (1:1) polymer synthesized by ball milling was used as a binder. Solid electrolyte sheets were prepared by mixing Li3PS4 powders with the Li3PS4-I2 (1:1) polymer in anisole in a weight ratio of 95:5 followed by coating on Al current collector sheets. Electrode composite sheets were also prepared by coating slurries of LiNbO3-coated LiNi1/3Co1/3Mn1/3O2 (NMC; active material)55, argyrodite-type Li6PS5Cl (solid electrolyte)56,57, acetylene black, and the Li3PS4-I2 (1:1) polymer in anisole (70:30:5:5, w/w/w/w) on Al current collector sheets. The sheets were dried at 60 °C in a dry Ar atmosphere overnight and further dried in a vacuum at 160 °C for 3 h.

Conductivity measurements

Powders or sheets were put into a polyimide tube with a diameter of 10 mm and sandwiched between two stainless steel rods as an ion-blocking electrode. The cells were pressed at 333 MPa, and conductivities were measured at least three times at room temperature using an AC impedance analyzer (cell test system 1470E, Solartron Analytical; lead wires connected to stainless steel rods) in the frequency range from 10 Hz to 1 MHz. All procedures were conducted under dry Ar.

All-solid-state cell assembly and electrochemical measurements

NMC composite sheets (NMC loading = ~2 mg) punched into circles with a diameter of 9.5 mm and a Li3PS4 separator layer (80 mg) were put into a polyimide tube (10 mm in diameter) and pressed together at 296 MPa. A Li-In alloy foil (Li foil: thickness 0.2 mm, diameter 4 mm, and In foil: thickness 0.1 mm, diameter 8 mm) was placed on the surface of the separator side of the bilayer pellet as a counter electrode. The three-layered pellet was sandwiched between two stainless steel rods as a current collector, and a pressure of 111 MPa was applied to assemble the cell. All processes were performed in a dry Ar-filled glove box (DBO-2NKP, Miwa Manufacturing Co., Ltd.). Charge-discharge tests were carried out at 0.071 mA cm−2 in the first five cycles and at 0.14 mA cm−2 in the subsequent cycles at 30 °C under dry Ar using an electrochemical measurement device (BTS-2004, Nagano Co.). Cell resistance was measured after the first charge using an AC impedance analyzer (cell test system 1470E, Solartron Analytical). The cells were uniaxially pressed under 75 MPa during the electrochemical measurements.