A Liquid-Phase-Synthesized Cathode Composite with a Three-Dimensional Ion/Electron-Conducting Structure for All-Solid-State Lithium–Sulfur Batteries
Authors: Jiang Peilu , Zhou Huangkai , Song Subin , Suzuki Kota , Watanabe Kenta , Yamaguchi Yumi , Matsui Naoki , Hori Satoshi , Kanno Ryoji , Hirayama Masaaki 
Published: December 2024

All-solid-state lithium–sulfur batteries (ASSLSBs) with solid electrolytes (SEs) are considered promising next-generation energy storage systems owing to their high theoretical specific capacity (S: 1675 mAh g−1, Li2S: 1166 mAh g−1), high energy density (2500 Wh kg−1), non-flammability, and the natural abundance and low toxicity of sulfur.1 Moreover, the shuttle effect caused by the dissolution and diffusion of lithium polysulfide intermediates (Li2S
x
, 4 < x < 8) in liquid electrolytes is radically suppressed in solid-state redox reactions.2 However, three issues prevent ASSLSBs from efficiently utilizing active materials, resulting in low electrochemical performances: (1) the insulating property of Li2S/S towards both electrons and ions, (2) the severe stress and strain caused by the volumetric change (80%) of the sulfur cathode during lithiation/delithiation, and (3) the poor solid/solid interfacial contact between active material and SE. Therefore, the development of a solid-state cathode composite that can simultaneously overcome these three drawbacks is vital.
Typically, a cathode composite of an ASSLSB comprises an SE, active material (S or Li2S), and conductive additives. The following strategies based on previous studies are key to realizing an optimal Li2S-based cathode composite: (1) reducing the particle size of the cathode SE (<1 μm),3 (2) enhancing the mixed (ion/electron) conductivity,4 and (3) constructing a stable cathode framework and 3D conductive pathways.5 However, approaches that can concurrently achieve all three requirements have rarely been reported. Additionally, alternatives to the commonly used mechanical mixing method, which can lead to poor interfacial contact and disintegration of the cathode structure, must be established.
Herein, we report a Li2S-based cathode composite (AM–CR10/SE-liq/VGCF, or ACSV) with high mixed-conductivity and stability, fabricated by infiltrating a Li2S–LiI active material (AM) solution to a mesoporous carbon replica with ~10-nm-sized pores (CR10), followed by mixing the AM–CR10 composite with a liquid-phase-synthesized Li6PS5Br solid electrolyte (SE-liq) and vapor-grown carbon fibers (VGCFs) (Fig. 1(a)).
The Rietveld analysis of the X-ray diffraction (XRD) patterns of liquid-phase-synthesized xLi2S-LiI (x = 1, 3, 5) indicated the possibility that part of the I− anions were incorporated into the Li2S lattice. First-principles density functional theory (DFT) calculation and Arrhenius plots measurement for 3Li2S-LiI (AM) revealed that the I− incorporation not only enhanced the ionic/electronic conductivities of Li2S but also promoted the Li2S/S conversion redox kinetics. The results of N2 adsorption/desorption testing for AM-CR10 based on Brunauer-Emmett-Teller (BET) and Barret-Joyner-Halenda (BJH) analysis ensured that the porous CR10 with mechanical reinforcement was filled with AM sufficiently without destruction, which can largely alleviate the volume expansion. Additionally, the XRD patterns and Raman spectra of the Li6PS5Br solid electrolyte (SE-liq) indicated that argyrodite-type Li6PS5Br was successfully obtained. The ionic conductivity of SE-liq determined by alternating current electrochemical impedance spectroscopy (EIS) was 2.22 mS·cm-1, which is comparable to that of Li6PS5Br (SE-bm) synthesized by the conventional ball milling method (2.26 mS·cm-1). Moreover, the field emission scanning electron microscope (FE-SEM) images and particle size analysis clarified that the particle size distribution of SE-liq was 0.1-1 μm and much smaller than that of SE-bm (1-30 μm). The high-resolution SEM images of the cathode composite ACSV suggested the presence of three-dimensional conductive pathways (Fig. 1(b)), while the conductivity measurements ensured that ACSV possessed a mixed ionic/electronic conductive behavior. In the charge/discharge profile of ACSV at a current density of 0.05 C at room temperature, the initial discharge capacity was 841.0 mAh·g−1. The discharge capacity increased to 1009.2 mAh·g−1 at the 20th cycle after activation, reaching 86.6% of the theoretical capacity of the Li2S material (1166 mAh·g-1) (Fig. 1(d)). Furthermore, the discharge capacity retention rate up to 100 cycles at a current density of 0.1C was 82.8%, while the Coulombic efficiency remained approximately 100% (Fig. 1(c)).
In conclusion, the ASSLSB with this ACSV composite cathode exhibited exceptional capacity and reversibility, resulting from its mechanically sturdy configuration, high lithium storage capability, and three-dimensional mixed conductive structure. This study affirmed the potency of designing a Li2S-based composite cathode using liquid-phase methods to mitigate the insulating property of Li2S, inhibit the volumetric change within the cathode, and improve the solid/solid interfacial contact between Li2S and SE, thereby helping achieve high-performance ASSLSBs.

References:

1. Liu Y., et al., Energy Mater.
8, 1701602 (2018).
2. Lin Z. and Liang C., Mater. Chem. A
3, 936–958 (2015).
3. Peng L., et al., Energy Environ. Mater.
6, e12308 (2023).
4. Han F., et al., Nano Lett.
16, 4521–4527 (2016).
5. Suzuki K., et al., ACS Appl. Energy Mater.
1, 2373–2377 (2018).

Figure 1