An ion/electron-conductive cathode for all-solid-state Li-S batteries

All-solid-state Li-S batteries suffer from the insulating nature of S/Li2S and severe cathodic volume changes during cycling. Here we reported a Li2S-based cathode with high mixed-conductivity and structural stability, showing 87% theoretical discharge capacity and 80% retention rate for 100 cycles.
Published in Chemistry and Materials
An ion/electron-conductive cathode for all-solid-state Li-S batteries
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To meet the energy supply needs of future devices such as electric vehicles, the development of high-capacity and long-cycle secondary batteries that meet sustainable development needs is required. 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 sulfur1. Moreover, the shuttle effect caused by the dissolution and diffusion of lithium polysulfide intermediates (Li2Sx, 4 < x < 8) in liquid electrolytes is radically suppressed in solid-state redox reactions2.

 However, three issues prevent ASSLSBs from efficiently utilizing active materials (AMs), 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 the AM and the SE. Therefore, the development of a solid-state cathode composite that can simultaneously overcome these three drawbacks is vital.

 According to previous research, the following strategies are key to realizing an optimal cathode composite: (1) reducing the particle size of the cathode SE (<1 μm)3, (2) enhancing the mixed (ion/electron) conductivity4, and (3) constructing a stable cathode framework and 3D conductive pathways5. 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 framework, must be established.

 In our recent study published in Communications Materials, we reported a Li2S-based cathode composite featuring (i) a hybrid (AM–CR10) of the AM and a carbon replica with ~10-nm-sized pores (CR10), (ii) a Li6PS5Br SE (SE-liq) obtained by liquid-phase synthesis, and (iii) vapour-grown carbon fibres (VGCFs), exhibiting outstanding mixed conductivity and a stable cathode structure, prepared using liquid-phase methods (Fig. 1(a)). This AM–CR10/SE-liq/VGCF composite is hereafter termed ACSV. The AM–CR10 hybrid is yielded by dissolving Li2S–LiI (the AM) in EtOH in a mass ratio of 3:1 and then precipitating it in the ~10-nm-sized pores of CR10. Notably, modifying Li2S with LiI enhances its ionic conductivity and the redox kinetics of Li2S/S. After the mild liquid-phase procedure, the porous CR10 is sufficiently filled with the AM without undergoing collapse. The highly conductive SE-liq (2.22 mS cm−1 at 25 °C) possessing a small particle-size distribution (<1 μm) is successfully synthesized using the THF–EtOH solvent system. The ACSV cathode composite with a 3D ion/electron-conductive structure can then be prepared by mixing AM–CR10, high-conductivity SE-liq, and rod-shaped VGCFs (Fig. 1(b)).

 In the charge/discharge profile of ACSV at a current density of 0.05 C at room temperature, 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)).

 Consequently, our 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 in the future.

To learn more about this cathode design technology developed by researchers from Tokyo Institute of Technology, please refer to our recent publication in Communications Materials: A composite cathode with a three-dimensional ion/electron-conducting structure for all-solid-state lithium–sulfur batteries. (https://doi.org/10.1038/s43246-024-00537-w)

 References:

  1. Liu, Y., He, P. & Zhou, H. Rechargeable solid‐state Li–Air and Li–S batteries: materials, construction, and challenges. Energy Mater. 8, 1701602 (2018).
  2. Kinoshita, S., Okuda, K., Machida, N., Naito, M. & Sigematsu, T. All-solid-state lithium battery with sulfur/carbon composites as positive electrode materials. Solid State Ionics 256, 97–102 (2014).
  3. Peng, L. et al. Tuning solid interfaces via varying electrolyte distributions enables high‐performance solid‐state batteries. Energy Environ. Mater. 6, e12308 (2023).
  4. Hakari, T., Hayashi, A. & Tatsumisago, M. Li2S‐based solid solutions as positive electrodes with full utilization and superlong cycle life in all‐solid‐state Li/S batteries. Sustain. Syst. 1, 1700017 (2017).
  5. Suzuki, K. et al. High cycle capability of all-solid-state lithium–sulfur batteries using composite electrodes by liquid-phase and mechanical mixing. ACS Appl. Energy Mater. 1, 2373–2377 (2018).

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