In Situ NMR Analysis of Lithium Metal Surface Deposits from Electrolytes with Varying Salt Concentrations

Abstract

Lithium metal constitutes a promising anode material, based on its high theoretical specific capacity (3 860 mAh g−1), and low electrochemical potential (−3.04 V vs. SHE). Non-homogeneous lithium deposition, however, may lead to the formation of reactive high surface area lithium (HSAL) upon cycling, eventually yielding losses of active material, safety risks, and insufficient cycling efficiency. HSAL with e.g. needle-like structure does not only cause higher capacity losses based on side reactions and solid electrolyte interface (SEI) formation, but also could result in internal short circuits by penetration of separator layers. In contrast, low surface area lithium (LSAL) such as nodule-like deposits[1] may reduce unwanted side reactions as well as safety risks. The actual microstructure of lithium deposits is governed by extrinsic factors including cell pressure, temperature, current densities, as well as surface properties of lithium metal electrodes but also the explicit choice of electrolyte constituents, and is therefore crucial for the overall cell performance.[2] In this work, highly concentrated electrolytes are exploited to achieve improved cycling stability and reduced HSAL formation, where the formation of a highly robust SEI, more uniform lithium deposits, fewer side reactions on the anode surface, as well as higher viscosity of the electrolytes were identified as key factors.[1, 3] Electrochemical in situ investigations, scanning electron microscopy (SEM) [4] and in situ 7Li nuclear magnetic resonance (NMR) spectroscopy [5, 6] are applied to in detail elucidate lithium deposition phenomena in symmetrical Li/Li cells. In addition to SEM data, in situ 7Li NMR constitutes an alternative, highly viable spectroscopic method that affords (semi-) quantitative and qualitative monitoring of lithium microstructure growth upon cycling, thereby unraveling critical processes during electrodeposition while delivering counterstrategies for unrestrained HSAL growth and options for achieving significantly improved electrochemical performance and life time of the considered cells. [1] J. Qian, et al., Nature Communications, 6 (2015) 6362. [2] X.-B. Cheng, et al., Chemical Reviews, 117 (2017) 10403-10473. [3] L. Suo, et al., Nature Communications, 4 (2013) 1481. [4] G. Bieker, et al., Physical Chemistry Chemical Physics, 17 (2015) 8670-8679. [5] H.J. Chang, et al., The Journal of Physical Chemistry C, 119 (2015) 16443-16451. [6] R. Bhattacharyya, et al., Nature Materials, 9 (2010) 504-510.