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

Abstract

The latest advances in the stabilization of Li/Na metal battery and Li-ion battery cycling has highlighted the importance of electrode/electrolyte interface (Solid Electrolyte Interphase -SEI) and its direct link to cycling behaviour. In order to understand the structure and properties of the SEI, we used combined experimental and computational studies to unveil how the ionic liquid (IL) cation nature and salt concentration impact the silicon/IL electrolyte interfacial structure and the formed SEI. The nature of IL cation is found to be important to control the electrolyte reductive decomposition that influences the SEI composition and properties, and the reversibility of the Li-Si alloying process. Also, increasing the Li salt concentration changes the interface structure for a favorable and less resistive SEI. The most promising interface for the Si-based battery was found to be in P 1222 FSI with 3.2 m LiFSI which leads to an optimal SEI after 100 cycles in which LiF and trapped LiFSI are the only distinguishable lithiated and fluorinated products detected. This study shows a clear link between the nano-structure of the IL electrolyte near the electrode surface, the resulting SEI, and the Si negative electrode cycling performance. More importantly, this work will aid rational design of Si-based Li-ion batteries using IL electrolytes in an area that has so far been neglected, reinforcing the benefits of superconcentrated electrolyte systems.

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

“…It is important to note that not only the dipole moment of the organic cation alone but also its geometry and chemistry should be considered as a factor influencing the EDL composition of superconcentrated ILs. For example, for 50 mol % LiFSI containing P1222FSI and C3mpyrFSI electrolytes near Au (111) at −14.4 μC cm 2 , the phosphonium system has ∼30% more Li x FSI y ( x > y ) aggregates and almost no IL cation compared to that of pyrrolidinium system. , The [C3mpyr] + also demonstrated higher affinity toward a negatively charged silicon surface than [P1222] + in the same electrolyte which was observed through both MD simulation and differential capacitance measurements . This is even though the dipole moment of [P1222] + is only 0.9424 D which is more than twice smaller compared to that of [C3mpyr] + .…”

“…3,30 The [C3mpyr] + also demonstrated higher affinity toward a negatively charged silicon surface than [P1222] + in the same electrolyte which was observed through both MD simulation and differential capacitance measurements. 24 This is even though the dipole moment of [P1222] + is only 0.9424 D which is more than twice smaller compared to that of [C3mpyr] + . 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] + .…”

“…14−17 Of the existing literature investigating the IL cation effect on the interface, most studies focus on neat ILs and the EDL capacitor applications, 18−22 and some focus on the lowsalt-containing systems (≤20 mol % of metal salt) for battery applications. 23,24 Recently, we compared the interfacial behavior of two salt-concentrated ILs containing 50 mol % of lithium bis(fluorosulfonyl)imide (LiFSI) and cations of similar size at negatively charged gold and silica interfaces. The calculated effective cation volumes at MP2/6-311++G(d,p) level of theory for triethyl(methyl)phosphonium ([P1222] + ) and N-methyl-N-propylpyrrolidinium ([C3mpyr] + ) are 120.78 and 111.30 cm 3 mol −1 , respectively.…”

“…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. , 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. − Of the existing literature investigating the IL cation effect on the interface, most studies focus on neat ILs and the EDL capacitor applications, − and some focus on the low-salt-containing systems (≤20 mol % of metal salt) for battery applications. , Recently, we compared the interfacial behavior of two salt-concentrated ILs containing 50 mol % of lithium bis­(fluorosulfonyl)­imide (LiFSI) and cations of similar size at negatively charged gold and silica interfaces. The calculated effective cation volumes at MP2/6-311++G­(d,p) level of theory for triethyl­(methyl)­phosphonium ([P1222] + ) and N -methyl- N -propylpyrrolidinium ([C3mpyr] + ) are 120.78 and 111.30 cm 3 mol –1 , respectively.…”

“…It was found that the “spherical” [P1222] + cation interacts weakly with the anode surface compared to the relatively “planar” [C3mpyr] + cation. Therefore, more Li x FSI y complexes are found at the interface of [P1222] + , which correlated with more stable lithium electrochemistry for the silicon anode. ,, Although this previous literature introduces some understanding of the role of IL cation on the battery electrode interfaces, it is still unclear how to rationalize the chemistry of organic cations in superconcentrated ILs and an optimum current density for stable metal anode cycling for different IL chemistries. Overall, there is a lack of reports connecting the polarity of organic cations with their interfacial nanostructure in superconcentrated ILs and relevant electrochemical behavior during metal anode cycling.…”