Thermodynamic effects of solid electrolyte interphase formation from solvation and ionic association in water-in-salt electrolytes

Abstract

Water-in-Salt-Electrolytes (WiSEs) are a promising class of next-generation electrolytes. Unlike classical dilute electrolytes or more conventional battery electrolytes, WiSEs are characterised by their super-concentrated salt concentration with only a small amount of water, which gives rise to their expanded electrochemical stability window (ESW). The expansion of the ESW is, in part, due to the formation of an inorganic solid electrolyte interphase (SEI) that passivates the anode; this principle is also important in graphite and Li-metal anodes, and beyond Li-ion technologies. The solvation and ionic associations are key descriptors in understanding the expansion of the ESW. Specifically, as reactions which lead to the SEI (or cathode electrolyte interphase, CEI) must occur at the electrode-electrolyte interface, the distribution of reactants and their various solvation environments are critical. This distribution near the interface is referred to as the electrical double layer (EDL), in the absence of reactions. Here we further develop and analyse a recently proposed thermodynamic theory of hydration and ionic associations in the EDL of WiSEs. We parameterize this theory from bulk molecular dynamics simulations and benchmark it against EDL simulations, finding good qualitative agreement. Using this thermodynamic theory, we rationalise changes in the ESW through: changes in the activity in the bulk electrolyte through the Nernst equation, which directly changes the stability of the electrolytes; and thermodynamic changes to the kinetics of these reactions, from the Butler-Volmer equation and coupled ion electron transfer kinetics, through the concentration of reactant species in the Helmholtz layer.

Figures (7)

• Figure 1: EDL of 21m WiSEs at negative electrode ($\sigma$ = -0.2 C/m$^2$) as a function from the interface, in dimensionless units, where $\kappa$ is the inverse Debye length, from simulations (a-d) and theory (e-h). In the former, the grey region indicates the minimum distance from the electrode at which a species was never found. a,e) Volume fraction of each species ($\bar{\phi}_i$). b,f) Volume fractions of hydrated cations (Simulation $\bar{\phi}_{10x}$ & Theory $\bar{\phi}_{104}$), free anions ($\bar{\phi}_{010}$), free water ($\bar{\phi}_{001}$), and aggregates and gel, if present ($\bar{\phi}_{Agg}$). c,g) Association probabilities ($\bar{p}_{ij}$). d,h) Product of the ionic association probabilities, $\bar{p}_{+-}\bar{p}_{-+}$, where the dashed line indicates the critical line for gelation and the red shaded region is the standard deviation.
• Figure 2: Cluster distribution of 21m water-in-LiTFSI near the negative electrode ($\sigma$ = -0.2 C/m$^2$) at different distances from the interface, from theory (a-c) and simulations (d-f).
• Figure 3: EDL of 21m WiSEs at positive electrode ($\sigma$ = 0.2 C/m$^2$) as a function from the interface, in dimensionless units, where $\kappa$ is the inverse Debye length, from simulations (a-d) and theory (e-h). In the former, the grey region indicates the minimum distance from the electrode at which a species was never found. a,e) Volume fraction of each species ($\bar{\phi}_i$). b,f) Volume fractions of hydrated cations (Simulation $\bar{\phi}_{10x}$ & Theory $\bar{\phi}_{104}$), free anions ($\bar{\phi}_{010}$), free water ($\bar{\phi}_{001}$), and aggregates and gel, if present ($\bar{\phi}_{Agg}$). c,g) Association probabilities ($\bar{p}_{ij}$). d,h) Product of the ionic association probabilities, $\bar{p}_{+-}\bar{p}_{-+}$, where the dashed line indicates the critical line for gelation, with the shaded region denoting its standard deviation.
• Figure 4: Cluster distribution of 21m water-in-LiTFSI near the positive electrode ($\sigma$ = 0.2 C/m$^2$) at different distances from the interface, from theory (a-c) and simulations (d-f).
• Figure 5: Activity of ions, water and hydrated cations in the bulk as a function of molality, computed with the sticky cation approximation for LiTFSI.