Chapter VI: Design Rules for Passivating Self-Assembled Monolayers to a
6.2 Introduction
Interfacial stability is essential to enhance cycling performance and longevity of energy storage materials, which is crucial for large-scale energy grid systems and electrical vehicles [175, 197, 122, 54, 182, 178, 105, 53, 59]. As is well known, lithium metal anode is one of strong candidates for next-generation advanced battery
materials thanks to the highest theoretical capacity (3,860 mA h g−1and 2,061 mA h cm−1) and the lowest electrochemical potential (-3.04 V vs. SHE) [175, 8, 182, 29, 178, 105]. However, its practical applications have been delayed due to the limited interfacial stability of lithium metal, including dendritic lithium deposition, due to its high reactivity, raising serious safety issues that lead to battery failure.
At electrochemical interfaces, typical liquid electrolytes (e.g. a mixture of cyclic and linear carbonates) immediately and spontaneously degrade via various competitive electrochemical reactions [197, 122, 137], resulting in the in situ formation of a structurally and chemically heterogeneous thin film called a solid electrolyte interphase (SEI) [132, 29, 5, 142, 184, 24]. A functionally useful SEI passivates the electrode, conducting ions yet providing electronic insulation, by mitigating detrimental electrolyte degradation that leads to battery capacity loss, aging, and failure. The composition and structure of the SEI do depend on the choice of solvent chemistry, and sacrificial additives [162]. For instance, fluoroethylene carbonate (FEC) was found successful in enhancing SEI stability at battery cycling by enriching the SEI with inorganic lithium fluoride (LiF) component [162, 68].
The other way to introduce a passivating SEI to suppress the lithium dendrites is ex situ fabrication [63, 83, 68, 207]. One of the advantages of the artificial SEI is the ability to control its composition and structure in a systematic way [177]. For example, Li-C composite microparticles, coated with a self-assembled monolayer (SAM), exhibit high specific capacity and high Coulombic efficiency [83]. The SAM film of octadecylphosphonic acid (OPA) was coated on a lithium electrode due to the reactive phosphonic acid group. Further, the saturated hydrocarbon chain drives the densely packed SAM layer that blocks moisture in air or trace amounts of water in solvents. The slurry-coated Li anode was also found to retain 82.5%
capacity after 250 cycles at 1C when paired with a commercial LiFePO4cathode.
Another successful implementation of an artificial SEI is hard-shell protection for a metal cathode that enables reversible room-temperature fluoride-ion shuttle, paired with liquid ether solvents [38]. Fluoride-ion battery (FIB) is one of advanced high- energy-density batteries, which enables multivalent fluoride conversion reactions in addition to the electronegativity and low mass of fluorine for higher energy density than lithium-ion one [7, 62, 38, 200]. However, the fluoride-ion battery operates only at elevated temperatures. Related challenges for the fluoride-ion shuttle in liquid electrolytes include (i) low solubility of metal fluoride electrolyte salts and (ii) low chemical stability due to formation of HF−2 ( 0.7 V) or irreversible F−complexation.
A recent study reported room-temperature fluoride-ion shuttle that ethereal liquid solvents readily transport fluoride ions of dry tetraalkylammonium fluoride salts, including bis(2,2,2-trifluoroethyl) ether (BTFE) and glyme [38]. Alpha-hydrogens in the ether solvents are responsible for solvating fluoride ions, maintaining high chemical stability against hydrogen abstraction reactions.
A novel cathode, consisting of a copper core and a lanthanum trifluoride shell, enables the room-temperature cycling, paired with the liquid ether solvents. An artificial hard-shell SEI provides high electrochemical stability by mitigating chal- lenges associated with cathode metal dissolution. The hard-shell protection permits a reversible fluoride ion shuttle that is related to the conversion of Cu to CuF2. Nevertheless, anode stabilization is required for safe operation at complete cycles.
This chapter presents simulation-aided design recipes for SAMs to passivate a metal anode for fluoride-ion batteries. The primary goal is to provide molecular insights into fluoride-ion solvation structure and dynamics at the functional SAM-decorated metal interface. A functional SAM layer should primarily satisfy the following: (i) high film formation ability, (ii) high fluoride-ion solubility, (iii) poor solvent pene- tration, and (iv) fast (de-)fluorination. Herein, inspired by the ether solvents found to enable room temperature fluoride-ion shuttle, four different chemical structures of the SAM molecule are investigated, combining carbon monofluoride (CFx) and ether moieties [38].
We conduct all-atom simulations with polarizable metal electrodes coated with a SAM layer to investigate fluoride-ion solvation structure and dynamics. Four SAM layers are studied; they have a different spacer part of different chemistry, yet the same ethyl anchor and end group. The fluoride-ion SAM solvation structure sug- gests that a functional SAM molecule needs to exhibit both fluorinated and ether moieties in order to enable facile F−shuttle between the bulk electrolyte and a metal electrode. Thanks to the polarizable metal electrode included in our model, the kinetics of fluoride-ion SAM solvation are estimated using non-equilibrium simu- lation trajectories with a reversed bias potential. The fluoride-ion SAM solvation dynamics is found consistent with the associated structure. In particular, a sizable free-energy barrier for F− penetration at a SAM/electrolyte boundary is found for all SAM molecules studied, suggesting that optimizing the end group of the SAM molecule could facilitate the F−transfer across the SAM/electrolyte boundary.
Simulations also propose a scenario for the previous experiment to successfully protect the Ce or Ca anode surface with FOTS (1H,1H,2H,2H-Perfluorooctyl-
SAM𝑎 Spacer feature Fluorinated? 𝛼-CH2moiety?
E-(CH2)2-(CF2CH2OCH2CF2)2F BTFE-like Yes Yes E-(CH2)2-(CF2CH2)3-CF3 PVDF-like Yes No𝑏 E-(CH2)2-(CH2OCH2)3-CF3 Glyme-like No Yes
E-(CH2)2-(CF2)7-CF3 FOTS-like Yes No
Table 6.1: Four SAM molecules considered in this study. 𝑎E represents an electrode surface. 𝑏CH2moiety is present next to CF2without an oxygen atom.
trichlorosilane) additives, confirmed by cyclic voltammetry and electrochemical impedance spectroscopy studies [38]. Results for a FOTS-like SAM layer suggest that the passivation is enabled mainly by steric repulsion with unfavorable interac- tions of the FOTS-like SAM with the BTFE electrolyte and the ions.