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

Abstract

Stable metal anode cycling for high energy density batteries can be realized through modification of electrolyte composition and optimization of formation protocols, i.e., electrode interphase preconditioning conditions. However, the relationship between these and the electrochemical performance is still unclear due to a lack of molecular level understanding of electric double layer (EDL) changes with modification of these two parameters. Herein, we examine the impact of ionic liquid (IL) electrolyte composition (salt concentration and cosolvent) and preconditioning cycling conditions on Li anode performance through EDL changes affecting both the solid–electrolyte interphase (SEI) and deposition morphology. Each electrolyte composition results in a particular interfacial Li-ion solvation environment, which controls the reductive stability, Li deposition potential, and ultimately the composition of properties of the SEI. The latter is dependent on the EDL composition such as the IL cation/Li-anion ratio or the presence of other surface active additives. It is found that in a superconcentrated electrolyte, a high current density (≥10.0 mA cm–2/1.0 mAh cm–2) is beneficial during the metal anode preconditioning step, compared with the case of low Li salt-containing IL. This correlates with a predominance of Li x (anion) y (x > y) at a highly negatively charged interface, which is present when higher current densities are used for preconditioning, as suggested by molecular dynamics simulations. In contrast, for the lower viscosity superconcentrated electrolyte containing 20 wt % of ether cosolvent, a more moderate preconditioning step current density (6.0 mA cm–2/1.0 mAh cm–2) leads to an optimized deposition morphology and improved cycling performance. This is a consequence of the competing processes of ion transport at the interface, which controls the Li+ ion flux and the intrinsic reduction kinetics occurring at the more negative electrode.

Applying Classical, Ab Initio, and Machine-Learning Molecular Dynamics Simulations to the Liquid Electrolyte for Rechargeable Batteries

“…Both TFSI – and trifluoromethanesulfonate (OTF – ) are anions, but the former absorbed and the latter desorbed as the positive electrode polarization increased. Compared with anions, Li + cations show a broader distribution, as they can locate between layers of anions and solvents. ,, The attraction from Li + is likely to induce anions accumulating at negative potentials, especially in HCEs, which favors the formation of a uniform SEI and stable cycling of alkali metal anodes. ,, These all demonstrate the effects of the surface charge on the interfacial structures as well as the selective partitioning of different species on the surface. , Many experimental results, some of which are in contradiction to expectations from bulk solvation structures, could also be reasonably interpreted by the dependence of interfacial structures on surface properties and electrolyte compositions. , For instance, EC and DMC molecules coordinate with Li + nearly in an equal ratio according to MD simulations on bulk electrolytes . However, experiments reveal that the decomposition products of EC prevail in forming an SEI initially, ,, which disagrees with bulk structures but can be rationalized by the above-mentioned interfacial structure in which EC molecules replace DMC on the charged electrode .…”

Polar Organic Cations at Electrified Metal with Superconcentrated Ionic Liquid Electrolyte and Implications for Sodium Metal Batteries

“…The interfacial concentration of both [C3mpyr] + and [C2O1mpyr] + cations noticeably decreases from −3.8 μC cm 2 to −15.2 μC cm 2 charge conditions, which is due to an increase of positively charged Na x FSI y (x > y) aggregates at the interface. 3 Meanwhile, the presence of [C2mpyr] + near Au (111) slightly increases and remains very high under the same charging conditions. This phenomenon correlates with the polarity of organic cations where [C2mpyr] + is the least polar ion compared to its counterparts; therefore, with its more localized charge of +1.0, [C2mpyr] + strongly interacts with the electrode surface, allowing a strong competition with nonpolar [Na] + to screen the negative surface charge of Au (111) in comparison to the other organic cations.…”

“…Besides, we also showed that sodium/lithium anode preconditioning cycling at high current density with superconcentrated ILs electrolytes (1:1 salt:IL) leads to low resistive interphase with a uniform deposition morphology and/or inorganic rich chemistry. 3,13 However, regarding the importance of the IL cation, there is a lack of understanding of how this affects the interfacial nanostructuring and the resultant mechanism of charge transfer at metal anodes, especially for superconcentrated IL electrolytes. On the other hand, such an effect has been widely investigated in relation to ion transport in the bulk phase and the cycling performance of metal anodes with a wide range of metal salt concentrations for alkali metal battery applications.…”

“…The DFT analysis of the bulk phase for these electrolytes shows that [P1222] + with highly localized +1.0 charge has a weaker ion− ion association compared to that of [C3mpyr] + . 3 This is a result of their different geometry ("spherical" vs "planar" shape), which affects the ionic interactions between a positively charged phosphorus atom or the nitrogen of the pyrrolidinium ring with other Li x FSI y /[FSI] − species. The effect of the nature of the organic cation on reductive/ oxidative stability of 50 mol % NaFSI containing electrolyte was examined using density functional theory (DFT) calculations for their bulk phase with an explicit solvent model, as shown in Figure 3d,e.…”

“…Electrolyte design and fundamental understandings of the charge transfer processes are essential for the rational development of energy-storage devices. Ionic and/or molecular structuring at the electrode/electrolyte interface, i.e., within the so-called electric-double layer (EDL), has shown a direct influence on battery performance due to the impact on the solid electrolyte interphase (SEI) formation and mechanism of charge transfer. − Successful electrolytes for high-voltage alkali metal-ion batteries should be electrochemically stable within ≥5.0 V; ionic liquids (ILs) are promising electrolytes having a wider electrochemical window than conventional neutral organic solvents, i.e., carbonates, ethers etc . The ILs are defined as molten salts with a liquid phase <100 °C which consists of an organic cation and an organic/inorganic anion .…”