SEI Formation on Sodium Metal Electrodes in Superconcentrated Ionic Liquid Electrolytes and the Effect of Additive Water

Abstract

We have previously reported that water addition (∼1000 ppm) to an N-methyl-N-propylpyrrolidinium bis(fluorosulfonyl)imide (C 3 mpyrFSI) superconcentrated ionic liquid electrolyte (50 mol % NaFSI) promoted the formation of a favorable solid electrolyte interphase (SEI) and resulted in enhanced cycling stability. This study reports the characterization of Na−metal anode surfaces cycled with these electrolytes containing different water concentrations (up to 5000 ppm). Morphological and spectroscopic characterization showed that water addition greatly influences the formation of the SEI and that ∼1000 ppm of water promoted the formation of an active and more uniform deposit, with larger quantities of SEI species (S, O, F, and N) present. Water addition to the electrolyte system is also proposed to promote the formation of a new complex between the FSI anions, water molecules, and sodium cations as components of the SEI. For both dry and wet (∼1000 ppm) electrolytes, the SEIs were mainly composed of NaF, metal oxide (i.e., Na 2 O), and the complex, suggested to be Na 2 [SO 3 -N-SO 2 F]•nH 2 O (n = 0−2). Postcycling SEM analysis of the Na−metal electrodes after extensive cycling (500 cycles, 1.0 mA•cm −2 , 1.0 mA•.cm −2 ) was used to estimate the minimal average cycling efficiency (ACE), which was enhanced by water addition: up to ∼99% for the 1000 ppm cell compared to ∼98% for the dry cell. Two distinct deposit morphologies, a microporous and a compact layer deposit, were evident after extended cycling in the wet and dry electrolytes. The presence of both the microporous and compact layer deposits on Na−metal surfaces cycled with the wet electrolyte, along with the distinct chemistry and morphology of the SEI, all contributed to a more stable symmetric cell voltage profile and lower cell polarization. In contrast, a higher fraction of microporous deposits and the absence of compact layer formation in the dry electrolyte were associated with higher cell polarization potentials and the occurrence of dendrites.

Tuning the Formation and Structure of the Silicon Electrode/Ionic Liquid Electrolyte Interphase in Superconcentrated Ionic Liquids

“…On all spectra, this FSI signal contains one extremely sharp and one broader component (Figure S7) that can be attributed to mobile, liquid-like nondegraded FSI − anions and non-or less mobile confined FSI containing species, such as LiFSI. Ferdousi et al 61 recently reported the formation of complexes suggested to be Na 2 [SO 3 -N-SO 2 F]•nH 2 O (n = 0− 2) in the Na system, resulting from the partial decomposition of FSI. In these complexes, the local environment of fluorine is expected to be close to that in FSI anion and the corresponding 19 F NMR shift close to that of the nondegraded FSI signal.…”

Stable and Efficient Lithium Metal Anode Cycling through Understanding the Effects of Electrolyte Composition and Electrode Preconditioning

“…2−4 Recently, high salt concentration (or superconcentrated) organic solvents and their nonflammable alternatives, such as ionic liquid (IL) electrolytes, were found to be successful in achieving high Coulombic efficiency for metal deposition with a uniform morphology, likely resulting from the formation of a favorable solid−electrolyte interphase (SEI). 5−8 Other attempts have been made to enhance the transport properties and electrochemical behavior of superconcentrated electrolytes in the applicable temperature range by introducing additional low molecular weight components, such as water 9 and/or organic solvents. 10−12 Moreover, the optimal cycling protocols have been also designed to deliver long-term stable cycling.…”

Asymmetric Sodiophilic Host Based on a Ag-Modified Carbon Fiber Framework for Dendrite-Free Sodium Metal Anodes

“…Unfortunately, the Na metal anode still faces severe problems in its practical applications. , During the cycling process, uneven Na plating/stripping, unstable solid electrolyte interphase (SEI), and large volume expansion can induce Na dendrite growth and active Na loss, eventually leading to low Coulombic efficiency (CE), short cycling life, and safety hazards. , To address the abovementioned issues, various strategies have been employed, mainly including the optimization of liquid electrolytes, , the introduction of artificial SEI films on the Na metal surface, , the use of solid-state electrolytes, and the construction of three-dimensional (3D) conductive hosts. − Among them, three-dimensional (3D) hosts have the merits of good conductivity and large surface area, which can not only significantly reduce the local current density but also alleviate the Na volume change during cycling. Thus, Na dendrite growth can be effectively suppressed and the CE of the Na metal anode can be largely improved .…”