Long-Term Stable Lithium Metal Anode in Highly Concentrated Sulfolane-Based Electrolytes with Ultrafine Porous Polyimide Separator

Abstract

Highly concentrated solutions composed of lithium bis­(fluorosulfonyl)­imide (LiFSI) and sulfolane (SL) are promising liquid electrolytes for lithium metal batteries because of their high anodic stability, low flammability, and high compatibility with lithium metal anodes. However, it is still challenging to obtain the stable lithium metal anodes in the concentrated electrolytes due to their poor wettability to the conventional polyolefin separators. Here, we report that the highly concentrated 1:2.5 LiFSI/SL electrolyte coupled with a three-dimensionally ordered macroporous polyimide (3DOM PI) separator enables the stable lithium plating/stripping cycling with an average Coulombic efficiency of ca. 98% for over 400 cycles at 1.0 mA cm–2. The 3DOM PI separator shows good electrolyte wettability and large electrolyte uptake due to its high porosity and polar constituent of the imide structure, allowing superior cycling performance in the highly concentrated solution, compared with the polyolefin separators. Electrochemical and spectroscopic analyses reveal that the superior cycling stability in the concentrated electrolyte is attributed to the formation of highly stable and Li+ ion conductive solid electrolyte interphase (SEI) layer derived from FSI– anions, which reduces the side reactions of SL with lithium metal, prevents the growth of lithium dendrites, and suppresses the increase in cell impedance over long-term cycling. Our findings demonstrate that polar and porous separators could effectively improve the affinity to the concentrated electrolytes and allow the formation of the anion-derived SEI layer by increasing the salt concentration of the electrolytes, achieving the long-term stable lithium metal anode.

Lithium metal anodes: Present and future

“…For guiding uniform deposition of Li crystal seeds in the early plating process, magnesium (Mg) nanoparticles were deposited on single side of separator, resulting in dendrite-free lithium metal anode over 400 cycles [133] . Maeyoshi et al developed a porous and polar separator that effectively enhanced the uptake of the concentrated electrolytes and then induced the formation of the anion-derived SEI layer for high-performance lithium metal anode [134] . Wang et al prepared HAPs/PVA separator with an outstanding Young's modulus and high mechanical flexibility, which efficiently restrained the growth of Li dendrite and induced a homogeneous stable SEI [135] .…”

Polyimide separators for rechargeable batteries

“…In their report, monodispersed silica particles were used as the template to create a 3D-ordered microporous (3DOM) structure. The authors later used this 3DOM PI separator in highly concentrated sulfolane-based electrolytes, hoping to resolve the poor wettability issue in PP separators [43]. The polarity and high porosity of the PI membrane effectively improved the affinity of the separator toward the concentrated electrolytes and allowed for the formation of an anion-derived SEI layer, achieving a long-term stable Li metal anode.…”

“…The Kanamura group developed 3DOM PI separators using a colloidal crystal templating method [43,44,[109][110][111][112]. Fig.…”

“…Fig. 11(e) shows the fabrication process of such a separator [43]. In their procedure, silica particles with submicron diameters were dispersed in dimethylacetamide together with PAA.…”

“…Recently, a new concept of solvent-in-salt or concentrated electrolytes has been proposed. [43] Such electrolytes contain a high concentration of Li salts which change the solvation structure of the electrolyte, significantly improving the electrochemical stability toward the Li metal. However, the concentrated electrolytes usually wet poorly in the PP separators.…”