Efficient Lithium Metal Cycling at a Wide Range of Pressures from an Anion-Derived Solid-Electrolyte Interphase Framework

Abstract

Advanced electrolytes were developed to improve the cyclability of lithium (Li) metal anodes, yet their working mechanisms remain unclear. Here, we study the Li cycling performance under different pressures in a 1 M Li bis(fluorosulfonyl)imide/fluorinated 1,4-dimethoxybutane electrolyte. A consistently long cycle life is achieved over a wide range of pressures (30−600 psi). This is due to a completely different Li plating mode with more favorable deposition morphologies compared to that in a conventional carbonate electrolyte, which exhibits increasing cycle stability with increased pressure. We show that this is enabled by the properties of an anion-derived residual solid-electrolyte interphase (rSEI) framework on the electrode surface, an undercharacterized structure with profound implications for Li metal cycling. This anion-derived rSEI chemistry is likely the key to a prolonged cycle life of Li metal batteries and should be vigorously addressed in future electrolyte designs. With a low electrode potential (−3.04 V) and a high specific capacity (3860 mAh/g), lithium (Li) metal anodes have great potential for the realization of next-generation batteries with high specific energies. However, commercialization of Li metal anodes is plagued by their poor cyclability, rooted in their high chemical reactivity, dendritic growth pattern, and large change in volume during Li metal deposition. A series of approaches, such as the design of “host” structures and artificial solidelectrolyte interphases (SEIs), have been utilized to address these issues. Still, the cycling performance of Li metal anodes is far from what would be required for practical applications. Recently, electrolyte engineering has arisen as one of the most promising strategies. A series of advanced electrolyte systems, including high-concentration electrolytes (HCEs), localized high-concentration electrolytes (LHCEs), dual-salt electrolytes, all-fluorinated electrolytes, liquefied gas electrolytes, and a single-salt, single-solvent electrolyte, altered the deposition morphology of Li metal, leading to much improved Coulombic efficiencies (CEs). The improved CEs in these advanced electrolytes were usually attributed to a much more passivating compact SEI (cSEI, the SEI film in intimate contact with the Li metal surface). However, this theory cannot explain why the CEs of many advanced electrolytes are lower during initial cycles than those of baseline carbonate electrolytes (Figure S1), and then gradually increase during continuous cycling. Furthermore, a recent study (unpublished results) shows that Li metal corrodes at similar or even faster rates in some of the advanced electrolytes than in baseline carbonate electrolytes during storage, suggesting that the compact SEIs in these advanced electrolytes are not as passivating as expected. Therefore, a thorough understanding of the working mechanisms of these high-performing electrolytes remains lacking. Compared to that of Li-ion batteries, operation of Li metal batteries leads to more pronounced pressure fluctuation due to the aggravated electrode volume change. However, pressure is rarely regulated or optimized for the testing of Li metal batteries with advanced electrolytes. Experimental and simulation studies indicated that increased pressure was generally beneficial for Li cycling but also dependent on the electrolyte. Because the recently reported advanced electrolytes enabled Li deposition in favorable yet slightly varied morphologies, the dependencies of their performance on pressure are thus an interesting and critical Received: December 5, 2020 Accepted: February 1, 2021 Leter http://pubs.acs.org/journal/aelccp © XXXX American Chemical Society 816 https://dx.doi.org/10.1021/acsenergylett.0c02533 ACS Energy Lett. 2021, 6, 816−825 D ow nl oa de d vi a ST A N FO R D U N IV o n Fe br ua ry 1 0, 2 02 1 at 0 1: 38 :0 9 (U T C ). Se e ht tp s: //p ub s. ac s. or g/ sh ar in gg ui de lin es f or o pt io ns o n ho w to le gi tim at el y sh ar e pu bl is he d ar tic le s.