KR20160056423A - Improved Organic-Inorganic Composite Electrolyte to Enhance Performance of Lithium-Sulfur Batteries - Google Patents

Abstract

Proposed is a newly designed organic-inorganic composite electrolyte which improves the cycle lifespan of a lithium-sulfur battery. Silica nano-particles having negative electric charges are distributed into a polymer electrolyte to restrict polysulfide to be dissolved while a lithium-sulfur battery is charged or discharged. Specifically, by concentrating the silica nano-particles near a positive electrode surface, the dissolving may be greatly reduced due to effective charge-to-charge repulsion. As the result of developing the composite gel polymer electrolyte (gCPE) having a special inner structure, long-time stability which is a long cherished object is achieved. Thus, a new lithium-sulfa battery which has a high discharge capability of 1140 mAh/g at the first cycle and 970 mAh/g at the 100th cycle and improved durability may be developed.

Description

TECHNICAL FIELD [0001] The present invention relates to a high-performance lithium-sulfur composite electrolyte,

More particularly, the present invention relates to an electrolyte for a lithium-sulfur secondary battery, a method for manufacturing the same, and a secondary battery using the same.

Sulfur is one of the most popular cathode active materials for large capacity energy storage media due to its remarkable high theoretical capacity (1675 mAh / g) and energy density (2567 Wh / kg). Huang is also environmentally friendly, affordable, and resource-rich.

Despite these positive aspects, lithium-sulfur cells undergo severe capacity reduction during the cycle, mainly due to the dissolution of lithium polysulfide intermediates in the liquid electrolyte. The dissolved polysulfide adsorbs like a solid film that does not dissolve on the surface of the electrode during sustained charge / discharge, resulting in increased resistance of the cell, reduced Coulomb efficiency, and reduced active material.

Numerous studies have been conducted to solve the problem of polysulfide dissolution. Nanostructured carbon materials are applied as the anode skeleton, and the method of physically injecting sulfur is proceeding most significantly. This approach could significantly improve the cycle life of the lithium-sulfur battery over several hundred cycles. Another promising means for preventing polysulfide dissolution is the development of new electrolytes. In particular, the polymer electrolyte can act as a physical barrier to dissolved polysulfide intermediates moving from the anode to the cathode. Polymer electrolytes have the additional advantage that they are mechanically stable, easy to process, and form flexible. However, these electrolytes have received relatively less attention than the cathode materials.

The most widely studied polymer electrolytes are solid polymer electrolytes (SPEs) Gel polymer electrolytes (GPEs) . Polymers based on polyethylene oxide (PEO) have long been considered promising candidates for SPEs. However, especially at low temperatures, low ionic conductivity (<10 ^-5 S / cm) inhibits practical use of batteries. In order to replace SPEs, polymer electrolytes (GPEs), in which liquid electrolytes of high ionic conductivity are embedded in a mechanically rigid polymeric frame, are in the spotlight. Nonetheless, the application of GPEs to lithium-sulfur batteries has resulted in very poor long-term stable results over hundreds of cycles and unbelievable results. A poor long term characteristic is due to the dissolution of lithium polysulfides into GPEs through liquid components.

There are three advantages of CPEs beyond the general GPEs in lithium-sulfur battery systems. First, it can have improved mechanical properties without reducing the specific ionic conductivity. Secondly, the movement of the lithium polysulfide intermediate between the electrodes can be controlled, since it acts as a physical barrier when the polysulfide reaches the liquid electrolyte. Third, the introduction of inorganic particles with large surface area and high dielectric constant can improve lithium salt separation. Although the application of CPEs to lithium-sulfur cells has shown tremendous potential, the successful development of lithium-sulfur batteries using CPEs, which surpass the performance of conventional liquid electrolyte based lithium-sulfur batteries, remains at an early stage.

Accordingly, a problem to be solved by the present invention is to provide new CPEs capable of increasing the cycle life and energy density of a lithium-sulfur battery.

The CPEs according to the present invention provide a crosslinked complex polymer electrolyte comprising silica nanoparticles (SNPs), polyethylene glycol diacrylate (PEGDA), and lithium salt-doped tetraethylene glycol dimethyl ether (TEGDME). The electrolyte may be in a free standing form.

In one aspect, the present invention relates to a method for preparing CPEs having a thickness of 80 mm by mixing silica nanoparticles (SNPs), polyethylene glycol diacrylate (PEGDA) and lithium salt-doped tetraethylene glycol dimethyl ether (TEGDME) RTI ID = 0.0 &gt; UV &lt; / RTI &gt;

SNPs have a diameter of about 220 nm and a high negative charge of -47 mV, which can delay the dissolution of the polysulfide in the electrolyte with electrostatic repulsion. The completion of the UV crosslinking was confirmed by Fourier transform infrared spectroscopy (FT-IR).

According to the present invention, a new high performance lithium-sulfur battery, an electrolyte used therefor, and a method for producing the same are provided.

1 schematically shows the outline of the manufacturing method according to the present invention
Fig. 1 (a) Cross-sectional SEM pictures of hCPE and gCPE. Illustrated images show cyclic voltammetry results of Li / hCPE / S and Li / gCPE / S cells in three regions (b) enlarged in the cross direction of gCPE.
Fig. 2 (a) Constant current charge / discharge capacity values of Li / gCPE / S, Li / hCPE / S, and Li / GPE / S at room temperature, 0.5C and 1.7-2.8V. (b) charge / discharge voltage profiles of the three lithium-sulfur batteries obtained in the first cycle. (c) Positive surface structure of gCPE, hCPE, and GPE removed from lithium-sulfur cell after five cycles
Fig. (B) Li / gCPE / SC constant current charge / discharge capacity at a room temperature, 0.2C, 1.9-2.7V, a) Li / gCPE / S- Voltage curve

Figure 1 (Scheme 1) depicts the process for preparing a composite polymer electrolyte. (HCPE) in which the SNPs are uniformly distributed in the crosslinked PEO matrix, when the SNPs are subjected to a density gradient in the vertical direction of the electrolyte by thermal pretreatment at 80 ° C. (gCPE, due to the low surface tension of the SNPs, Most dense form) was produced by a simple UV curing process. The photographs shown in Figure 1 (Scheme 1) show the flexible and transparent nature of the synthesized composite polymer electrolyte.

The cross - sectional shapes of hCPE and gCPE were confirmed by scanning electron microscopy (SEM) and the distribution of SNPs was different. Figure 2 (a) shows that the density of the SNPs is position-dependent and increases from the bottom outward in the three vertically enlarged regions of the gCPE (outside, center, and substrate sides).

Interestingly, the ionic conductivity of gCPE is about 10% higher than that of hCPE, and the distribution of SNPs in the electrolyte has a positive effect on the migration of lithium ions (hCPE is 20% higher than GPE without SNPs Thus, the negatively charged surface of closely packed SNPs provides a more efficient pathway for lithium ions.

The gCPE and gCPE electrolyte, the lithium metal cathode, and the coin cell consisting of the sulfur / carbon black anode were assembled and cyclic voltammetry was performed (gCPE had a dense SNPsurface in the direction of the sulfur electrode). Representative results of Li / hCPE / S and Li / gCPE / S at mV s ^-1 scanning speed are shown in Fig. 1b. Two reduction peaks at 1.9 V and 2.4 V (vs. Li / Li ⁺ ) and an oxidation peak at 2.4 V were clearly identified. In the Li / hCPE / S system, relatively small changes were observed in the Li / gCPE / S system, while the position of the reduction peaks and the decrease in intensity occurred during the continuous cycle process. In the case of the gCPE system, not only the active material exists but also the formation of the solid electrolyte membrane on the anode surface is reduced.

The constant current discharging / charging capacity of Li / gCPE / S and Li / hCPE / S cells can be observed at room temperature, 0.5 C, 1.7-2.8 V in FIG. As a comparative experiment, the results of Li / GPE / S can also be seen. The Li / gCPE / S system showed an initial discharge capacity of 830 mAh / g (based on sulfur weight) and decreased to 500 mAh / g with 80% Coulomb efficiency after 80 cycles, It is a figure. This clearly contrasts the low discharge capacity and poor capacity conservation values for Li / hCPE / S (240 mAh / g, 28%) and Li / GPE / S (133 mAh / g, 20%) under the same cycle conditions It accomplishes. In Fig. 3b (Fig. 2b), the first charging / discharging curve of the three lithium-sulfur batteries can be seen, with 2.0 V and 2.4 V (vs. Li / Li ⁺ ) at discharge, can confirm.

For Li / hCPE / S cells, most capacity decreases occur rapidly during the first few cycles, which means that the active material is significantly reduced in the electrolyte. These results demonstrate that the extension of the dissolution of the polysulfide is dependent on the structure of the electrolyte. The surface states of the gCPE, hCPE and GPE in the anode direction were investigated by scanning electron microscopy (SEM) and energy dispersive X-ray (EDX). After five charge / discharge cycles, . As shown in Fig. 3c (Fig. 2c), the surface of gCPE showed a smooth state with a small amount of sulfur bundles when compared to the observation of giant sulfur crystals in hCPE and GPE. This difference can be seen in the visuals of the illustrations of FIG. 3c (FIG. 2c). EDX analysis of the amount of sulfur on the gCPE, hCPE, and GPE surfaces resulted in 10, 16, and 27 wt%, respectively, indicating that the position of negatively charged SNPs near the anode surface did not pass through the sulfide intermediates most effectively do. This is responsible for the improved battery performance of the Li / gCPE / S system. Figure 3 (a) (Fig. 2a).

The use of a sulfur anode optimized with gCPE can demonstrate improved performance of lithium-sulfur cells. (S-GDL) containing two kinds of sulfur anodes, Li ₂ S ₈ anion solution in the gas diffusion layer and an electrode (SC) containing Li ₂ S in the MCMB were prepared. / Discharge cycle characteristics can be seen in FIGS. 4A and 4B (FIGS. 3A and 3B, respectively). The discharge capacity of the Li / gCPE / S-GDL system was stabilized at around 700 mAh / g after 30 cycles and after 150 cycles it was led to a reversible discharge capacity of 725 mAh / g, corresponding to 78% capacity retention. Improved cycling is also proven in the Li / gCPE / SC system, maintaining capacity retention of 85% and reversible discharge capacity of 970 mAh / g after 100 cycles. Typical charge / discharge voltage curves of Li / gCPE / SC cells at room temperature, 1.9 - 2.7V are shown in Fig. 3c (Fig. 3c).

In summary, a new design of CPEs has been proposed to achieve improved cycle life of lithium-sulfur batteries. The use of gCPE enabled a stable discharge capacity of 970 mAh / g after 100 cycles. Improved long-term cycle life can be believed to be the result of SNPs with negative charge located near the anode surface, in order to reduce the dissolution of the polysulfide into a polymer electrolyte during the cycle. The internal structure of the CPEs plays an important role in determining the performance of the lithium-sulfur battery.

Claims (2)

Cross - linked polyelectrolytes containing silica nanoparticles (SNPs), polyethylene glycol diacrylate (PEGDA), and lithium salt - doped tetraethylene glycol dimethyl ether (TEGDME). (SNPs), polyethylene glycol diacrylate (PEGDA), and lithium salt-doped tetraethylene glycol dimethyl ether (TEGDME) are mixed and crosslinked to form a composite polymer electrolyte.

Images