Formation of Linear Oligomers in Solid Electrolyte Interphase via Two‐Electron Reduction of Ethylene Carbonate

Abstract

Solid electrolyte interphase (SEI) plays a significant role in enhancing the stability and durability of lithium metal batteries (LMBs) by separating highly reactive lithium metal anode (LMA) from the electrolyte to avoid continuous degradation. However, the underlying reaction mechanism is still far from clear. Herein, a hybrid ab initio and reactive force field (HAIR) method is employed to extend the ab initio molecular dynamics (AIMD) to 1 ns, which provides crystal information about the reaction mechanism of elementary reactions that can explain the components and morphology evolution of SEI formation. Specifically, HAIR simulation confirms the two‐electron (2e–) reduction of ethylene carbonate (EC) by releasing CO and CO2, agreeing with phenomenal experiment observation. As the unsaturated intermediates accumulate, polymerization reactions occur, producing linear polyethylene oxide (PEO), Li2OCO2CH2CH2, Li2OCO2(CH2)4, etc., which regulate the formation of outer organic layer (OOL) that consists of linear polyethylene oxide (PEO), Li2OCO2CH2CH2, Li2OCO2(CH2)4, etc., and the inner inorganic layer (IIL) mainly consists of LiF and Li2O. Simulations at low concentration (LC, 1M) and high concentration (HC, 5M) reveal significantly different reaction pathways when HC electrolyte can significantly promote the formation of homogenous LiF that has been regarded as an important component to facilitate robust SEI.

Optimization of Lithium Metal Anode Performance: Investigating the Interfacial Dynamics and Reductive Mechanism of Asymmetric Sulfonylimide Salts

“…At 2.51 ps, in a Li + -rich environment, EC undergoes a one-electron reduction, breaking its C-O bond and transforming from a ring to a chain structure, forming OC2H4OCO − . Subsequently, OC2H4OCO − follows two pathways: It accepts another electron to form OC2H4O2 − , releasing one CO at 4.00 ps (Figure 3a), and releases CO2 while forming the free radical anion •C2H4O − , consistent with previous DFT calculations [52]. At 61.21 ps, •C2H4O − is further reduced by Li 0 and forms Li2O along with one electron transfer (Figure 3b).…”

Optimization of Lithium Metal Anode Performance: Investigating the Interfacial Dynamics and Reductive Mechanism of Asymmetric Sulfonylimide Salts

“…At 2.51 ps, in a Li +rich environment, EC undergoes a one-electron reduction, breaking its C-O bond and transforming from a ring to a chain structure, forming OC2H4OCO -. Subsequently, OC2H4OCO -follows two pathways: it accepts another electron to form OC2H4O2 -, releasing one CO at 4.00 ps (Figure 3a), and releases CO2 while forming the free radical anion •C2H4O -, consistenting with previous DFT [45]. At 61.21 ps, •C2H4O -is further reduced by Li 0 and forms Li2O along with one electron transfer (Figure 3b).…”

Temperature-dependent interphase formation and Li+ transport in lithium metal batteries

“…7c ) 41 – 47 . For example, EC is subjected to ring-opening and C − O breaking to generate RC = OLi, then recombining or decomposition to Li 2 CO 3 /ROCO 2 Li and ROLi 42 , while the latter reaction is partly hindered at the low temperature. Consequently, Li 2 CO 3 /ROCO 2 Li and ROLi are dominated in the SEI formed at 25 and 0 °C but much reduced at −20 °C replaced by its intermediate products RC = OLi as evidenced by the XPS (Fig.…”

“…Decreasing the temperature alters the thermodynamic reaction of electrolyte decomposition, resulting in different reaction pathways and products (Fig. 7c) [41][42][43][44][45][46][47] . For example, EC is subjected to ring-opening and C − O breaking to generate RC = OLi, then recombining or decomposition to Li 2 CO 3 /ROCO 2 Li and ROLi 42 , while the latter reaction is partly hindered at the low temperature.…”