Achieving fast and stable Li⁺ transport in lithium-sulfur battery via a high ionic conduction and high adhesion solid polymer electrolyte

Abstract

Solid-state lithium (Li)-sulfur (S) batteries are promising secondary batteries because of their high energy density and high safety, but their practical application is severely hindered by poor Li-ions (Li⁺) transport in batteries due to low ionic conduction of the electrolyte and unstable electrode/electrolyte interface. Here, we address the issue by using a polyurethane (PU)-based electrolyte. The polar urethane/urea groups of PU reduce the hopping energy barrier of Li⁺, which results in high ionic conductivity of 1.8 × 10^-4 S cm^-1 (25 °C), high ion transference number of 0.54, and low activation energy of 0.39 eV. In addition, the polar urethane/urea groups endow the electrolyte with high adhesion, which allows the electrode/electrolyte interfaces to self-heal timely after being damaged during cycling. Benefiting from these merits, a symmetric Li||Li cell using the polyolefin-PU-bis(trifluoromethane)sulfonimide lithium salt electrolyte can cycle for approximately 800 h with a stable overpotential (approximately 40 mV), and a solid-state Li-S battery using the electrolyte delivers a specific capacity of approximately 610 mAh g^-1 after testing for 125 cycles at a S loading of about 4 mg cm^-2. Self-healing of the electrode/electrolyte interfaces during cycling was observed in situ by a laser confocal microscope. This study demonstrates the importance of polar groups in electrolytes in maintaining a fast and stable Li⁺ transport, which can be applied to other solid-state batteries.

Keywords

Solid-state Li-S battery / urethane/urea groups / low energy barrier / self-healing electrode/electrolyte interface / in-situ observation

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