Abstract
AbstractEffective utilization of Li‐metal electrodes is vital for maximizing the specific energy of lithium–oxygen (Li–O2) batteries. Many conventional electrolytes that support Li–O2 cathode processes (e.g., dimethyl sulfoxide, DMSO) are incompatible with Li‐metal. Here, a wide range of ternary solutions based on solvent, salt, and ionic liquid (IL) are explored to understand how formulations may be tailored to enhance stability and performance of DMSO at Li‐metal electrodes. The optimized formulations therein facilitate stable Li plating/stripping performances, Columbic efficiencies >94%, and improved performance in Li–O2 full cells. Characterization of Li surfaces reveals the suppression of dendritic deposition and corrosion and the modulation of decomposition reactions at the interface within optimized formulations. These observations are correlated with spectroscopic characterization and simulation of local solvation environments, indicating the persistent importance of DMSO–Li+‐cation interactions. Therein, stabilization remains dependent on important molar ratios in solution and the 4:1 solvent‐salt ratio, corresponding to ideal coordination spheres in these systems, is revealed as critical for these ternary formulations. Importantly, introducing this stable, non‐volatile IL has negligible disrupting effects on the critical stabilizing interactions between Li+ and DMSO and, thus, may be carefully introduced to tailor other key electrolyte properties for Li–O2 cells.
Highlights
Introduction mechanisms in LiO2 cells, and the design of new materials to enable rechargeability at the cathode, is critical to the realization of a practically viable device
While the measured efficiency of this process remains insufficient for any practical consideration,[32] the stability enhancement achieved through formulation design is considerable for a dimethyl sulfoxide (DMSO)-based system for Li–O2 application and presently the largest stability demonstrated for an additive-free DMSO electrolyte
The introduction of a stable cyclic alkylammonium-[TFSI] Ionic liquids (ILs) and the tailoring of the important molar ratios between the three components resulted in significantly enhanced stability of Li-metal plating/stripping cycling, achieving >900 h cycling with no increase in overpotential
Summary
The reactivity, or instability, of DMSO in conventional electrolyte formulations toward Li metal (in the absence of strong SEI forming additives) is first demonstrated using symmetrical Li|electrolyte|Li coin cells under galvanostatic control (Figure 1 (i)). The poor Columbic efficiency of low-to-moderately concentrated DMSO/Li[TFSI] electrolytes is expected and has been reported previously (1 mol dm−3 Li[TFSI] in DMSO, CE ≈ 25–50%).[15,16c,34] the stability enhancements gained from more highly concentrated DMSO/Li[TFSI] have been reported in symmetrical Li|Li cell cycling and by observation of Li-metal surfaces by SEM,[16a] as well as a Columbic efficiency of ≈85% in 3 mol dm−3 Li[TFSI] in DMSO.[16c] the apparent stabilization realized in the IL-containing solutions, 0.2xLi in C(8:2) and 0.15xLi in E, are found to be larger These values even exceed that achieved by introduction of two well-established SEI forming components, vinylene carbonate and Li[NO3],[15] and is comparable to highly concentrated DMSO formulations containing 4 mol dm−3 Li[NO3]/Li[FSI].[34] The [FSI]− anion has been demonstrated previously to contribute to improved performance and the formation of stable and reliable SEI at Li-metal interfaces,[35] in HCEs,[10a,c] but, as discussed, is considered incompatible toward superoxide generation at the cathode of Li–O2 cells.[12] While the measured efficiency of this process remains insufficient for any practical consideration (wherein values exceeding 99.97% would be required),[32] the stability enhancement achieved through formulation design is considerable for a DMSO-based system for Li–O2 application and presently the largest stability demonstrated for an additive-free DMSO electrolyte. The effect of crossover of dissolved O2 could have more pronounced effects on the interphase characteristics of an actively cycling electrode (due to possible exposure to fresh Li surfaces on dendrites or via fractures in the SEI during plating/stripping), but Figure 3 indicates this is minimal in the studied formulations and testing regime
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