Ternary Sulfur/Polyacrylonitrile/SiO2 Composite Cathodes for High-Performance Sulfur/Lithium Ion Full Batteries

Abstract

In the present study, a novel sulfur/lithium-ion full battery was assembled while using ternary sulfur/polyacrylonitrile/SiO₂ (S/PAN/SiO₂) composite as the cathode and prelithiated graphite as the anode. For anode, Stabilized Lithium Metal Powder (SLMP) was successfully transformed into lithiated graphite anode. For cathode, scanning electron microscopy (SEM) and transmission electron microscopy (TEM) revealed that SiO₂ was uniformly distributed on S/PAN composites, where SiO₂ served as an effective additive due to its ultra high absorb ability and enhanced ability in trapping soluble polysulfide. The tested half-cell based on S/PAN/SiO₂ composite revealed high discharge capacity of 1106 mAh g⁻¹ after 100 cycles at 0.2 C. The full cell based on prelithiated graphite//S/PAN/SiO₂ composite system delivered a specific capacity of 810 mAh g⁻¹ over 100 cycles.

1. Introduction

Battery storage systems have been widely used in various applications, such as wind and solar energy storage, emergency back-up power, peak shaving, load-leveling, and transportation electrification [1,2]. Lithium-ion secondary batteries are widely used in mobile communications, notebook computers [3,4], digital cameras, and other small electronic devices due to several interesting features [5], including high energy storage density, long service life, and elevated rated voltage [6]. However, the energy densities of lithium-ion batteries do not currently meet the demand, hence the development of new energy storage systems becomes important [7,8,9,10,11,12]. Lithium/sulfur (Li/S) batteries have high theoretical energy densities reaching up 2600 Wh kg⁻¹, making them the most promising energy chemical power systems. In regard to this, elemental sulfur as cathode has low cost when combined with its environmental friendliness and high theoretical specific capacity (1672 mAh g⁻¹) [13,14,15,16,17,18]. However, despite these criteria, the development of Li/S batteries still faces numerous challenges. Elemental sulfur is electrically insulating and the polysulfide generated during charging and discharging processes (Li+S₈→Li₂S_x (4 ≤ x ≤ 8)) is highly soluble in electrolytes [19,20].

The above issues could be solved by adding various types of conductive carbon materials [21,22,23]. Also, some polymer and oxide additives might enhance the structural stability of composites and improve the conductivity of matrix materials. On the other hand, it is important to suppress the “shuttle effect” of polysulfides [24].

Among composites, sulfur/polyacrylonitrile (S/PAN) showed high sulfur utilization with elevated initial capacity [21,25]. However, the poor electrical conductivities of S/PAN binary composites render the cycling and rate performances hindered. To this end, the Li/S battery exhibits enhanced electrochemical performance due to the different types of additives in the sulfur cathode, which results in a change in morphology and/or absorbing properties. The additive has the advantages of small dose and remarkable effect. Therefore, the use of additives in Li/S cells to change the morphology of S composite is one of the effective methods to improve the battery performance [26,27].

A method of containment at the cathode is to provide additives in the cathode matrix that can attract and hold polysulfides so that it does not diffuse to the negative electrode. To employ this concept [28], our previous study showed that the morphology of S/PAN composite greatly changed by adding small amounts of additives. Also, the composition morphology has been transferred from smooth to rough, effectively improving the electrochemical reaction at the electrode. In this work, we successfully synthesized S/PAN/SiO₂ composites as cathode materials by wet ball milling, followed by heat treatment. The addition of small amounts of SiO₂ nanoparticles was found to be beneficial for optimizing the surface morphology and favoring the homogeneous distribution of individual components. The above studies were based on traditional Li/S battery system, employing lithium metal foil as anode characterized by safety hazards, possible dendrite formation, short-circuiting, and cell thermal runway [29].

To improve the safety concerns of large-scale production of lithium/sulfur batteries, one promising strategy is to develop pre-lithiated commercial graphite anodes while using stabilized lithium metal powders (SLMP). In previous studies, some studies investigated the potential use of SLMP for overcoming the irreversible capacities of various anode systems [30,31]. Herein, we developed a novel sulfur/lithium-ion battery with pre-lithiated graphite anode, and the performances of the resulting pre-lithiated graphite//S/PAN/SiO₂ composite battery were discussed.

2. Materials and Methods

The ternary cathode materials were prepared by first mixing 8 g sulfur (Shanghai Huzheng Nano Technology Co., Ltd., Shanghai, China), 2 g polyacrylonitrile (PAN) (Sigma-Aldrich, Shanghai, China), and 0.5 g SiO₂. Anhydrous ethanol (Aladdin, Shanghai, China) was then added to the mixture for wet ball milling at 600 rpm and 2 h. Next, the obtained ball-milled product was dried and heat-treated under protective N₂ atmosphere at 450 °C for 6 h. For comparison, S/PAN binary composite was also prepared while using the same experimental conditions.

The samples were characterized by X-ray diffraction (XRD, Bruker D8, Bruker, Karlsruhe, Germany), scanning electron microscopy (SEM, ZEISS, SUPRA, Jena, Germany), X-ray photoelectron spectroscopy (XPS, ESCALAB, 250Xi, Thermo Fisher, Waltham, MA, USA), transmission electron microscopy (TEM, JEOL, Tokyo, Japan), and Fourier transform infrared (FTIR, Bruker, Ettlingen, Germany) spectroscopy. The surface area was calculated using the Brunauer-Emmett-Teller (BET, ASAP 2020, Micromeritics, Atlanta, GA, USA).

The S/PAN/SiO₂ and S/PAN electrodes were prepared by evenly grinding 80 wt % S/PAN/SiO₂ (S/PAN) composites, 10 wt % Super P, and 10 wt % PVDF (50 mm diameter, Shanghai Xingya, Shanghai, China). The mixture was then dropwise added to NMP to yield a slurry, which was coated on nickel foam and dried in 60 °C oven for 12 h. the obtained platforms were then cut into circular electrodes. The graphite anode was comprised of graphite, Super P, and PVDF at the mass ratio of 90:3:7. A surface application technique was employed to apply SLMP suspended in xylene slurry onto the prefabricated graphite anodes. An SLMP: graphite molar ratio of 11:60 was utilized to compensate for the irreversible capacity and lithiating the graphite. Upon solvent evaporation, the anode sheets were calendered using manual rolling mill at 3 MPa. The cathode loading of each cell was 2.5 mg cm⁻² and the graphite active material loading was about 3 mg cm⁻².

The assembled 2025 button cell was completed in an argon-filled glove box. In half cells, a lithium chip was used as anode. In full cells, lithium was substituted by the lithiated graphite anode. The electrolyte was composed of 1M LiPF₆ (Li zhiyuan, Shanghai, China) solution dissolved in ethyl carbonate (EC), dimethylcarbonate (DMC), and diethyl carbonate (DEC) at volume ratio of 1:1:1. Coin cells were assembled and pre-conditioned for 24 h at room temperature. The galvanostatic charge/discharge tests were conducted at different current densities (1 C = 1672 mA g⁻¹) and voltage of 1.0–3.0 V Li⁺/Li.

3. Results and Discussion

Figure 1a represents the XRD patterns of S/PAN/SiO₂ composite. The characteristic Fddd orthorhombic crystal structure peaks of elemental sulfur vanished from the XRD patterns of S/PAN composite. This could be due to trapping of S in the internal structure of the composite to form highly dispersed state, with crystalline sulfur and PAN most likely being converted to amorphous sulfurized PAN by heat-treatment [10,20]. Comparison of the characteristic bands between S/PAN/SiO₂ and S/PAN composites revealed a broad peak at 23°, which can be indexed to mixed peaks of S/PAN and amorphous SiO₂ [32,33]. The latter was probably induced by the dispersion of SiO₂ on the S/PAN surface and a slight shift in characteristic peaks of the ternary composites to the left. It also shows that SiO₂ and other components did not react during ball milling and heat treatment. Chemical analysis has shown that the sulfur content in the S/PAN/SiO₂ ternary composite was 45 wt %.

Figure 1. X-ray diffraction (XRD) and Fourier transform infrared (FTIR) images of S/PAN (S/PAN/SiO2) ternary composite.

In order to determine the various functional groups in the S/PAN/SiO₂ composite, the FTIR analysis of the sample presents in Figure 1b. The characteristic peaks at 805, 1252, 1365 and 1502 cm⁻¹ indicated the presence of C=C and C=N bonds in the composite [30]. The peaks at 882 cm⁻¹ can be attributable to the S-S bonds, and those at 1046 and 1092 cm⁻¹ can be assigned to the C-S stretching. The presence of the three peaks suggested that sulfur particles and PAN formed sulfurized-polyacrylonitrile [34]. The peak at 1116 cm⁻¹ was associated with the asymmetric stretching vibration of Si-O-Si in SiO₂ [33].

The microscopic morphologies of S/PAN/SiO₂ composites were revealed by SEM. The surface morphology of S/PAN binary composite underwent remarkable changes in the presence of small amounts of SiO₂ nanoparticles. The typical S/PAN nanostructure depicted in Figure 2a was composed numerous agglomerated particles. The S/PAN composite showed a bulk structure with very compact particles and smooth surface (Figure 2b). By comparison, the S/PAN/SiO₂ ternary composite consisted of nanosized primary particles (Figure 2c,d), resulting in a rough surface of the ternary composite. The BET specific surface area of S/PAN/SiO₂ composite was estimated to 33.75 m² g⁻¹, which was significantly higher than those of S/PAN binary composites that were published previously [20,31]. Therefore, the surface area was increased significantly by adding the additive with a nano structure. This should facilitate the contact between the electrolyte and electrodes. Moreover, the ternary composite contained many nano-sized particles, creating a three-dimensional (3D) porous nanostructure, it is beneficial to ion diffusion in Li/S battery and the SiO₂ suppresses the separation and agglomeration of active materials in the composite. The TEM image of S/PAN/SiO₂ composite were shown in Figure 2e,f, the “dark dots” were the amorphous SiO₂ particles well-dispersed in the composite bulk, which agrees well with the wide peaks in the XRD patterns. The as-prepared ternary composite was enabled to maintain the homogeneous distribution of its components and unchanged morphology during discharge-charge cycling, and retain the reactive sites in its nanosized structure [35].

Figure 2. SEM images (a,b) of S/PAN composite, SEM (c,d) and TEM images (e,f) of S/PAN/SiO2 ternary composites.

To further determine the chemical bonds in the S/PAN/SiO₂ composite, the XPS analysis of the sample is shown in Figure 3a–d. The C 1s, S 2p, and Si 2p peaks were all detected in the spectra. Figure 3b revealed the presence of a high-resolution peak of C 1s, as well as two peaks corresponding to C-C (284.6 eV) and C-N/C-S. The XPS C 1s spectra of S/PAN/SiO₂ composite confirmed the presence of a distinct peak at 285.8 eV, corresponding to the C-S/C-N bonds. This indicated the existence of certain chemical bonding between S and PAN in the S/PAN/SiO₂ composite. Figure 3c reveals the high-resolution S 2p peak. The S 2p_3/2 peak located at 161.4 and 163.3 eV were associated with C-S bond, attributed to single C-S bond and C-S bond in short-chain sulfide. The S 2p_1/2 peak at 164.6 eV was assigned to the S-S bond [21]. In XPS spectrum of Si 2p (Figure 3d), the peak at 104.8 eV was also present in the characteristic peak of SiO₂ [36]. These data were in accordance with the XRD patterns.

Figure 3. (a) Survey X-ray photoelectron spectroscopy (XPS) spectrum of S/PAN/SiO2 composite, (b) C 1s, (c) S 2p and (d) Si 2p spectra of S/PAN/SiO2 composite.

To figure out the electrochemical performances, the Li/S half cells were assembled while using lithium metal as anode, S/PAN/SiO₂ composites as cathode, and 1M LiPF₆ as electrolyte. Figure 4a shows the discharge/charge curves of S/PAN/SiO₂ half batteries. A method of containment at the cathode is to provide additives in the cathode matrix that can attract and hold polysulfides so that they do not diffuse to the negative electrode. We utilized SiO₂ as an additive to S/PAN electrode. The main interaction that the polysulfides have with the additive is through surface sorption, and therefore the surface area is increased significantly by synthesizing the additive with a nano structure. the SiO₂ additive was able to sorb polysulfides during the intermediate discharge and release them near the end of discharge so that they could be further reduced in the S/PAN matrix with most of the sulfur being reversibly sorbed in the SiO₂. The kinetics improvement and the polarization decrease achieved in the system by the addition of SiO₂ could be very beneficial for the utilization of the low-conductive sulfur active material in the composite cathode, and consequently improved the energy and power density of the battery [20,30]. The initial discharge curve consisted of a rapid drop curve around 2.4 V and flat discharge curve around 1.6 V. The initial charge-discharge process was typical of lithium-sulfur battery reactions [10]. During the following two cycles, the discharge curve was mainly composed of two lines with different inclinations. The first part of the curve looked relatively flat (from 2.2 V to 1.6 V) and the second was steep (from 1.6 V to 1.0 V). The two curves indicated the available capacity of about 1266 mAh g⁻¹. Figure 4b confirmed a coulombic efficiency of binary and ternary composite electrode half cells close to 100%. However, the S/PAN/SiO₂ half-cell showed a higher specific discharge capacity, which can be attributed to added nanosized SiO₂. Moreover, the S/PAN/SiO₂ electrode maintained a discharge capacity of 1106 mAh g⁻¹ at 0.2 C after 100 cycles. The S/PAN/SiO₂ half-cell exhibited specific discharge capacities of 1268, 969, 806, and 622 mAh g⁻¹ at current densities of 0.2, 0.5, 1 and 2 C, respectively (Figure 4c). By comparison, the S/PAN half-cell was also tested under the same conditions and the data are gathered in Figure 4d. Obviously, the cell with the S/PAN/SiO₂ composite cathode showed an enhanced rate capability. This is, again, due to the significant improvement of the charge transfer properties of the composite cathode and its stability by the SiO₂ additive that was observed in this work [37].

Figure 4. (a) Discharge/charge performance of S/PAN/SiO2 half-cell at 0.2 C between 1 V and 3 V. (b) Cycling performance and coulombic efficiency of S/PAN/SiO2 half-cell at 0.2 C. (c) Rate performance of S/PAN/SiO2 cell at various current densities. (d) Rate performance of S/PAN/SiO2 and S/PAN half-cell at various current densities.

As shown in Figure 5, the prelithiated graphite//S/PAN/SiO₂ composite system exhibited an initial capacity of 804 mAh g⁻¹ and specific capacity of 810 mAh g⁻¹ after 100th cycle. Hence, extremely low capacity decay rate was registered. Figure 5a depicts the coulomb efficiency of the full cell, which was close to 100%. The cycling curve of the full cell indicated a slight decrease from the initial cycle up to the 10th cycle then slowly increased up to 100th cycle. Although the amorphous S/PAN can provide good mechanical support for SiO₂ spheres, the structural stability cannot be guaranteed due to repeated volume changes during cycling. This led to decrease in capacity. Meanwhile, the added amorphous nano-sized SiO₂ absorbed more electrolyte and polysulfide on the surface of rough ternary composite. The discharge capacity also gradually increased in subsequent cycles, which is may due to that the pre-lithiated graphite was not fully lithiated, meaning that the electrode underwent slow activation. When the electrode was in contact with the electrolyte, the SLMP in the anode released lithium ions, making it slow to activate. As the battery cycles increases, the anode side of the graphite was completely lithiated, and the battery exhibited a gradual increase in discharge specific capacity [28,29,38].

Figure 5. (a) Discharge/charge performance of S/PAN/SiO2 full-cell at 0.2 C between 0.1 V and 3 V. (b) Cycling performance and coulombic efficiency of S/PAN/SiO2 full-cell at 0.2 C.

When compared to other related work (Table 1), our S/PAN/SiO₂ composite exhibited superior electrochemical performance.

[ Table omitted. See PDF. ]

4. Conclusions

Sulfur/lithium-ion full batteries were assembled while using ternary S/PAN/SiO₂ composite as cathode and a prelithiated graphite as anode. SLMP was successfully applied to lithiation of graphite anode. Nano-sized SiO₂ was found to be uniformly distributed on S/PAN composites, which served as an effective additive due to its ultra high absorbtion ability and enhanced trapping soluble polysulfide. The S/PAN/SiO₂ composite cathode half-cell showed a high discharge capacity of 1106 mAh g⁻¹ after 100 cycles at 0.2 C. The pre-lithiated graphite//S/PAN/SiO₂ composite full cell system delivered a specific capacity of 810 mAh g⁻¹ over 100 cycles. These findings look promising for future use in energy conversion and storage devices.

Author Contributions

Formal analysis, Y.H. and T.T.; Investigation, Y.H. and Z.S.; Project administration, Y.Z.; Supervision, Y.Z. and Z.C.; Writing-original draft, Y.H. and Z.S.; Writing-review & editing, Y.Z. and Z.C.

Funding

This work was supported by the Program for the Outstanding Young Talents of Hebei Province; Guangdong Provincial Science and Technology Project (2017A050506009).

Conflicts of Interest

The authors declare no conflict of interest.

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