A ZIF-67-derived–sulfur sandwich structure for high performance Li–S batteries

With the explosive development of portable electronic devices, electric vehicles (EVs), and smart grids, the demand for high energy density storage devices has grown over the past decades.^1–3 Nevertheless, traditional lithium ion batteries’ (LIBs’) electrode materials such as LiFeO₄, LiCoO₂ are no longer satisfactory to the surging demand due to their limited energy density.^4–6 Therefore, it is urgent to develop high-energy-density battery system to meet the increasing requirement. Lithium sulfur batteries (LSBs) are commonly considered as one of the most promising candidates for the next generation battery system because of its high theoretical capacity (1675 mA h g⁻¹) and high energy density (2600 W h kg⁻¹), which is about 3–5 times higher than that of the commercial LIBs. More than that, low cost and toxicity are also as advantages of LSBs for practical application.^2,7–9 However, despite these promising advantages, there still exist several issues for LSBs in large-scale application. Low electrical conductivity of S and final product, Li₂S, shuttling effect of polysulfides (PSs), and volume change during cycling greatly reduce its capacity and cycle life. Especially for the shuttling effect, soluble polysulfides would diffuse from the cathode to the anode and then react with lithium, causing low Coulombic efficiency (CE) and rapid capacity decay.^3,8,10

In the past decades, many considerable methods have been proposed to address these problems. Nazar and co-workers first used porous carbon, CMK-3, to trap the PSs within its pores and limit the shuttling effect.¹¹ Ever since, many carbonaceous materials, such as carbon cloth,^12,13 carbon nanotubes,^14,15 and carbon fibers,^16,17 have been investigated for LSBs to enhance the electrical conductivity as well as prohibit the diffusion of polysulfides. Nevertheless, the physical barrier effect between carbonaceous materials (nonpolar) and PSs (polar) makes it hard to efficiently prevent the shuttling due to the weak interaction in between. Some polar materials (metal oxides, sulfides, and phosphides) were then reported to effectively enhance the cycling performance of LSBs through the strong chemical interaction between the polar materials and PSs.^12,13,18,19

In this consideration, we designed a sandwich electrode to build a fully confined structure with sulfur immobilized in the middle of the ZIF-67-derived phosphide (PZ67) coatings, named PZ67/S/PZ67, which combines the effects of chemical interaction, physical barrier, and high conductivity for high performance LSBs. The obtained hybrid electrode exhibits an improved capacity and cycling stability. The initial capacity of the PZ67/S/PZ67-based cell can reach 1160 mA h g⁻¹ at the current density of 0.5 C (1 C = 1672 mA h g⁻¹) and still achieve about 715 mA h g⁻¹ after 300 cycles. In the higher current density of 1 C, PZ67/S/PZ67 electrode also exhibits a considerable initial capacity of 1040.3 mA h g⁻¹ and guarantees 576 mA h g⁻¹ after 300 cycles.

Typically, the shuttling effect is the biggest gap for LSBs’ practical application.²⁰ During cycling, the soluble PSs diffuse from the cathode area to the anode area and then repeat react with lithium metal, which would form solid Li₂S₂ and Li₂S on the Li metal [Fig. 1(a)]. This process will cause irreversible capacity loss, consume lots of electrolytes, and fresh Li metal. Thus, keeping PSs within the cathode area is an effective method to enhance the battery performance, especially for cycling stability. With this consideration in mind, a sandwich-structured cathode was designed to form a fully confined electrode structure to prohibit random diffusion of soluble PSs [Fig. 1(d)]. The polar component, CoP, in situ formed in the PZ67 coatings, can chemically interact with PSs, and the physical barrier is devoted to reduce the PSs transport. The sandwich structure can also restrain volume expansion of the electrode during cycling. Except that, the highly conductive PZ67 materials can provide superior electric conductivity for improving the reaction kinetics of sulfur. The synergistic effect of the above-mentioned advantages will create a fully confined structure to achieve a high performance LSBs. In comparison, other two partially confined structures, PZ67/S and S/PZ67, were also examined in this paper to better understand the effect of the electrode design [Figs. 1(b) and 1(c)].

The XRD pattern of the as-synthesized ZIF-67 particles is shown in Fig. 2(a), the diffraction peaks of ZIF-67 correspond well with the simulated one. After phosphate treatment, the diffraction peaks of the product, PZ67, are observed at 31.6°, 36.3°, 45.1°, and 48.4°, which can be assigned to the (0 1 1), (1 1 1), (2 1 0), (2 0 2) plane of the CoP phase (JPCDS No. 29-0497), respectively. The morphologies of ZIF-67 and PZ67 were revealed by scanning electron microscopy (SEM) in Figs. 2(c) and 2(d). Both of the synthesized ZIF-67 and PZ67 show particle size ranging from 100 to 200 nm, indicating that there is no obvious size change after phosphate treatment. However, the surface of PZ67 particles becomes very rough. SEM element mapping of PZ67 in Fig. 2(e) verified the uniform distribution of Co and P elements.

The porous feature of the PZ67 particle was investigated by adsorption/desorption measurement (Fig. S1). The isotherm of PZ67 shows a typical IV curves with a Brunauer-Emmet-Teller (BET) surface area of 115 m² g⁻¹, and the Barrett-Joyner-Halenda (BJH) pore size distribution data [Fig. S1(b)] disclose the presence of mesopores with an average pore size of ∼4.1 nm. Such mesopores are benefit for enhancing Li⁺ transfer and confining the PSs diffusion during cycling, as well as inhibiting the volume variations. Unexpectedly, PZ67 also exhibits a strong capability of adsorbing Li₂S₄ (as a representative of polysulfides, Fig. S2). The brown solution turns colorless when added PZ67 after 1 h, which indicates that PZ67 is highly PS-philic.

The electrochemical performance of these four different structure electrodes were investigated by assembling 2032 coin cells to perform discharge-charge cycles within the voltage range of 1.8–2.8 V. The mass loading of sulfur is ∼2 mg cm⁻² for each cell. As shown in Fig. S3, the cell based on fully confined cathode, PZ67/S/PZ67, exhibits a high initial capacity of 1368.5 mA h g⁻¹ at the current density of 0.2 C. However, for another two partially confined cells, PZ67/S and S/PZ67, the initial capacity is about 945 and 1200 mA h g⁻¹, respectively. As for the nonconfined cell, bare S, it shows an initial capacity of 1027.7 mA h g⁻¹ and it suddenly drops to 881.3 mA h g⁻¹ in the second cycle. After 100 cycles, the PZ67/S/PZ67 cell retained higher capacity of over 900 mA h g⁻¹, compared to 381, 622, 651 mA h g⁻¹ of S, PZ67/S, and S/PZ67 cells.

When the current density was increased to 0.5 C, the PZ67/S/PZ67 cell also achieved a high capacity of 1158.1 mA h g⁻¹ in the first cycle. The capacity maintained about 715.1 mA h g⁻¹ even after 300 cycles (Fig. 3). In contrast, the S cell delivered about 900 mA h g⁻¹ in the first cycle and lost 63.5% of its initial capacity after 300 cycles (330.5 mA h g⁻¹). As for another two cells, the PZ67/S cell achieved 807.1 mA h g⁻¹ and 489.2 mA h g⁻¹ at the first cycle and after 300 cycles, respectively. Similarly, the value of the S/PZ67 cell was 818.3 mA h g⁻¹ and 481.6 mA h g⁻¹, respectively. The PZ67/S/PZ67 cell even achieved a high capacity of 916.7 mA h g⁻¹ at 1 C. These results illustrated that the synergistic effect of the polar component (CoP), the high surface area, as well as the high conductivity of PZ67/S/PZ67 can effectively decrease the shuttling effect; hence, the battery performance can be dramatically improved.

Galvanostatic intermittent titration techniques (GITTs) and electrochemical impedance spectroscopy (EIS) were performed to investigate the effect of PZ67 for high electrochemical reaction kinetics of LSBs. GITT was first developed by Weppner and Huggins to determine the chemical diffusion coefficient,²¹ and we can get the information of charge/discharge overpotential and equilibrium potential of batteries from this technique. During GITT measurements, every cell was discharged and charged using a current density of 0.2 C with a duration of 10 min and then rested for 10 min to reach cell’s equilibrium potential at open circuit. The discharge/charge overpotential is mainly attributed to the ion diffusion, and the sudden change of cell’s potential is depended on the charge transfer and Ohm resistance.²² As shown in Fig. 4(a), the overpotential of PZ67/S, S/PZ67, and PZ67/S/PZ67 cells is much smaller than that of the S cell. The quasiequilibrium potential of the PZ67/S/PZ67 cell is more closely to the value of theoretical potential than other cells, especially for charge equilibrium potential [Fig. 4(b)]. The impedance of the cells was measured in the frequency range from 100 kHz to 0.1 Hz at the open circuit potential [Fig. 4(c)] to obtain more information about the electrode kinetics. Similar to the GITT result, the PZ67/S/PZ67 cell exhibited a small charge transfer resistance (R_ct) of 32 Ω, much smaller than the S cell of 178 Ω and other two cells (both about 58 Ω). This phenomenon implies that the introduction of the PZ67/S/PZ67 structure into LSBs greatly enhanced the ion transport, electronic conductivity, and reaction kinetics, which enables high capacity release.

In conclusion, we designed a sandwich-structured electrode to prohibit the shuttling effect and enhance the reaction kinetics in LSBs. Benefiting from the polar, porous, and highly conductive components derived from ZIF-67, PZ67 exhibits a synergistic effect that can effectively enhance the battery performance. As a result, the PZ67/S/PZ67 structured cathode shows a high initial capacity of 1158.1 mA h g⁻¹ at the current density of 0.5 C and maintains more than 710 mA h g⁻¹ after 300 cycles, which is about 2.2 times of that of a Li–S cell without PZ67. This result indicates that such a unique electrode structure with porous and polar PZ67 materials provide a practical way for designing a high performance sulfur cathode in the application of lithium sulfur batteries.