Ferdousi Shammi A, O'Dell Luke A, Hilder Matthias, Barlow Anders J, Armand Michel, Forsyth Maria, Howlett Patrick C
Institute for Frontier Materials (IFM), Deakin University, Burwood, Victoria 3125, Australia.
Centre for Materials and Surface Science (CMSS), Department of Chemistry and Physics, La Trobe University, Melbourne, Victoria 3086, Australia.
ACS Appl Mater Interfaces. 2021 Feb 3;13(4):5706-5720. doi: 10.1021/acsami.0c18119. Epub 2021 Jan 26.
We have previously reported that water addition (∼1000 ppm) to an -methyl--propylpyrrolidinium bis(fluorosulfonyl)imide (CmpyrFSI) 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., NaO), and the complex, suggested to be Na[SO-N-SOF]·HO ( = 0-2). Postcycling SEM analysis of the Na-metal electrodes after extensive cycling (500 cycles, 1.0 mA·cm, 1.0 mA·.cm) 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.
我们之前报道过，向N-甲基-N-丙基吡咯烷双（氟磺酰）亚胺（CmpyrFSI）超浓离子液体电解质（50 mol% NaFSI）中添加水（约1000 ppm）可促进形成有利的固体电解质界面（SEI），并提高循环稳定性。本研究报道了使用这些含有不同水浓度（高达5000 ppm）的电解质循环后的钠金属阳极表面的表征。形态学和光谱表征表明，添加水对SEI的形成有很大影响，约1000 ppm的水促进了活性且更均匀沉积物的形成，其中存在大量的SEI物种（S、O、F和N）。还提出向电解质体系中添加水可促进作为SEI组分的FSI阴离子、水分子和钠阳离子之间形成新的络合物。对于干燥和潮湿（约1000 ppm）的电解质，SEI主要由NaF、金属氧化物（即NaO）和该络合物组成，推测为Na[SO₂-N-SO₂F]·H₂O（x = 0 - 2）。在进行大量循环（500次循环，1.0 mA·cm⁻²，1.0 mAh·cm⁻²）后，对钠金属电极进行循环后扫描电子显微镜分析，以估计最小平均循环效率（ACE），添加水可提高该效率：对于1000 ppm电池，高达约99%，而干燥电池约为98%。在潮湿和干燥电解质中进行长时间循环后，明显出现了两种不同的沉积物形态，即微孔和致密层沉积物。在使用潮湿电解质循环的钠金属表面上同时存在微孔和致密层沉积物，以及SEI独特的化学性质和形态，都有助于形成更稳定的对称电池电压曲线和更低的电池极化。相比之下，干燥电解质中较高比例的微孔沉积物和致密层的缺失与更高的电池极化电位和枝晶的出现有关。
