Understanding the Effect of Interlayer on the Performance of All-Solid-State Li2s Batteries
Authors: Shin Minjeong , Gewirth Andrew A. 
Published: February 2020

Li−S batteries are considered a promising technology for next generation batteries, due to their high energy density and low cost of sulfur. The traditional liquid Li−S batteries, however, suffer from severe capacity fading and low Coulombic efficiency owing to processes occurring at the sulfur cathode. The electrochemical conversion reaction between S8 and Li2S involves a phase change where soluble lithium polysulfide intermediates are produced, causing so-called polysulfide shuttle phenomena. In order to mitigate the issues related to polysulfides, various approaches such as physical confinement of polysulfides or fabricating protective layer on Li anode were employed. However, the shuttle reaction still remains, affecting the cycling performance of the battery.

One promising approach to proceed the Li−S battery chemistry without producing polysulfide intermediates is to utilize non-solvating electrolyte such as inorganic solid electrolytes (SE). The use of solid electrolyte eliminates the possibility of polysulfide dissolution, improving the energy efficiency of Li−S batteries. In addition, sulfide solid electrolyte such as Li7P­3S­11­ exhibits an ionic conductivity of ~10-3 S cm-1 comparable to that of the conventional 1 M ethereal electrolytes. Despite these advantages, the performance of all-solid-state Li−S battery (ASSLSB) incorporating inorganic solid electrolyte is much worse than Li−S cells with liquid electrolytes in terms of active material utilization and rate capability. The biggest challenge lies in constructing sufficient Li+ ion transport channel within the composite cathode, since SEs do not infiltrate into cathodes as liquid electrolytes.

There have been considerable research efforts to improve the ionic conductivity of the cathode, such as ball-milling S8 (or Li2S), conductive carbon, and SE to prepare the composite cathode. However, only limited improvement was achieved on increasing active material utilization by simply ball-milling the three components. This is likely due to the poor physical contact between SE and cathode active material, leaving the active material electrochemically inactive (Figure 1a).

In an effort to improve the interfacial contact and increase the capacity, we report a strategy of using an interlayer between the electrode/electrolyte interfaces (Figure 1b). Our solid-state cell with interlayers exhibits much higher capacity compared to that of bare sample without interlayers, achieving ~80% active material utilization at cycle 60 with dramatically reduced impedance relative to the bare cell.

The interlayer in ASSLSB infiltrates into void space in composite cathode creating contact between the active material and SE resulting in high active material utilization. This increased contact leads to smaller cell impedance due to the decreased interfacial resistance. Additionally, the interlayer provides buffer space for volume contraction and expansion during electrochemical cycling, thereby preventing the physical delamination of cathode components.

Figure 1