Comparing the Physicochemical, Electrochemical, and Structural Properties of Boronium versus Pyrrolidinium Cation-Based Ionic Liquids and Their Performance as Li-Ion Battery Electrolytes

Abstract

Ionic liquid-type electrolytes (ILELs) based on boronium cations, (trimethylamine)(dimethylethylamine)dihydroborate [N 111 N 112 BH 2 ] + , are revisited as they have barely been studied for Li battery applications as alternatives to the ubiquitous [C x mpyr] + and [R 4 P] + cations. We demonstrate the potential of binary and ternary ILELs with bis-(trifluoromethanesulfonyl)imide [TFSI] − and bis(fluorosulfonyl)imide [FSI] − anions in comparison with N-butyl-N-methylpyrrolidinium [C 4 mpyr][TFSI] as a reference. The conductivity of neat ILs and their 0.5 mol kg −1 Li-salt-containing mixtures (σ 25°C = 0.5− 1.68 mS cm −1 ), differential scanning calorimetry (T g = −83 to −77 °C, T m = 14 to 28 °C), solid−solid-state transitions for [N 111 N 112 BH 2 ][FSI], cyclic voltammetry (electrochemical stability window 5.6 V), Li|LFP and Li|Li cells, and a crystal structure of the Li environment have been investigated. The binary mixture of [N 111 N 112 BH 2 ][TFSI] + 0.8 mol kg −1 of Li[TFSI] yielded a crystalline material for which the X-ray structure showed a fourcoordinate square-planar [Li(TFSI) 2 ] − environment with O ∩ O chelation, while two-coordinate environments were reported for Li + centers in [C x pyr][Li 2 (TFSI) 3 ] with higher anion numbers, both tetrahedral monodentate coordination by four TFSI anions and five-coordinate, square-based pyramidal coordination by three TFSI anions. Charge−discharge cycling studies were conducted on lithium metal electrodes in a symmetrical Li|Li coin cell configuration for 1500 charge−discharge cycles at 50 °C and a current density of 0.2 mA cm −2 . When used as an electrolyte in Li|LFP half cells, several hundred charge−discharge cycles with high initial discharge capacity (∼155 mA h g −1 at 50 °C) and good retention (0.03 to 0.05% capacity loss per cycle) are possible at cycle rates of 0.1−0.25 C (0.07−0.2 mA cm −2 ). A good discharge capacity (∼135 mA h g −1 ) was also achieved at 20 °C/0.1 C with the ternary electrolyte system.

Physical and Electrochemical Analysis of N-Alkylpyrrolidinium-Substituted Boronium Ionic Liquids

“…Each BIL undergoes a single glass transition at low temperatures (less than −60 °C) consistent with many previously studied IL systems, including past BILs. , Of the BILs prepared, 1 exhibits a significantly lower T g (−91.07 °C) than those containing N -alkylpyrrolidinium moieties. The T g of BIL 1 (containing two dimethylethylamine ligands) is slightly lower than that of the previously studied analogue [(N 111 )­(N 112 )­BH 2 ]­[TFSI] ( T g reported as −79.2 and −83.0 °C, respectively). Many factors are at play when considering the glass-forming behavior of ILs, including charge distribution across the ions as well as cation–anion interactions.…”

“…In addition to suppressing the secondary oxidation and reduction signals seen in the neat IL, Li­[TFSI] extends the stable electrochemical window of the electrolyte at platinum by nearly 1.4 V vs Ag/Ag + . Similar extensions of the reducing limiting potential in lithium-IL electrolyte systems are well known and have been attributed to the breakdown of anionic species into stable solid-electrolyte interface (SEI) layers boosted by the presence of lithium ions. ,,, The presence of new reductive ( E red < −3.6 V vs Ag/Ag + ) and oxidative ( E ox = −2.8 and −2.4 V) peaks, associated with the addition of Li + to the electrochemical system, is characteristic of electroplating/stripping behavior of past studied ionic liquid electrolytes. , Electrochemical reversibility for faradaic processes can be determined by analyzing the corresponding charges for both the reduction and oxidation processes, with a ratio of Q red / Q ox approaching unity denoting electrochemical reversibility . Integration of the oxidative and reductive current reveals that the lithium electrodeposition stripping in BIL 2 does not reach reversibility ( Q red / Q ox = 1.56), favoring reductive processes over oxidations.…”

“…To date N , N -dialkylpyrrolidiniums (i.e., [14pyrr]­[TFSI]) have been the primary class of ILs capable of exhibiting electrochemical stabilities comparable to the BIL electrolytes presented in this study (c.a. 5.6 V). , Yet, pyrrolidinium ILs experience limitations in their synthetic derivatization, restricted to altering the two alkyl side chains, compared to the [L 1 L 2 BH 2 ] template. When factoring in the limiting oxidation potential for each respective BIL, electrochemical solvent windows for BILs 2 and 3 reach an impressive 6.1 and 6.3 V, respectively (Figure ).…”

“…Alternatively, the reductive current contributing to the tailing seen ca. −4 V (vs Ag/Ag + ) is likely a combination of electroplating and the irreversible breakdown of electrolytes obscuring the true reversibility of Li + within BIL systems . Regardless, the results above demonstrate how BIL electrolytes can sustain the large negative potential needed to support lithium ion electrochemistry, among other high voltage processes needed for electrochemical energy storage.…”

Ionic Liquids and Electrolytes with Flexible Aromatic Anions

“…[9][10][11] Ionic liquid (IL) based electrolytes is one route pursued, which uses the fact that ILs offer wide liquidus ranges, high thermal stabilities, negligible volatilities, and are non-flammable -hence promising with respect to safety. [12,13] The most commonly studied IL-based electrolytes for LIBs are made by dissolving either lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) [14,15] or lithium bis(fluorosulfonyl)imide (LiFSI) [16] in an IL of the same anions and well-known cations such as phosphonium, [17] pyrrolidinium, [18] imidazolium, [19] morpholinium [20] or piperidinium. [21] The use of these fluorinated anions is, however, less desirable due to their sensitivity to moisture, corrosion of current collectors, and potential health and environmental risks during both production and recycling.…”