Synthesis, Crystal Structure, and Conductivity of a Weakly Coordinating Anion/Cation Salt for Electrolyte Application in Next-Generation Batteries

Conspectus Research at historically black colleges and universities (HBCUs) started with humble beginnings by G. W. Carver at Tuskegee Institute AL, the nation’s first HBCU. He is now remembered as the man who transformed one crop, peanuts to more than 300 useful products such as food, beverages, medicines, cosmetics, and chemicals. However, research was not the focus of most of the newly founded HBCUs to provide, primarily, liberal arts education and training in agriculture for the black minority. HBCUs remained segregated, lacking facilities such as libraries and scientific/research equipment comparable to those at traditionally white institutions. While the Civil Rights Act of 1964 heralded the dawn of “equal opportunity” and progressive desegregation in the South, many public HBCUs had to close or merge with white institutions due to loss of funding and/or students. In order to remain competitive in enrollment and financial support of the best talents, HBCUs have been expanding their research and federal contracts by working in collaboration with research-intensive institutions and/or minority-serving institutions (MSIs). Albany State University (ASU), an HBCU with a great tradition of in-house and extramural undergraduate research, has partnered with the laboratory of Dr. John Miller at Brookhaven National Laboratory (BNL) to offer the best training and mentorship to our undergraduates. Students synthesized and performed conductivity measurements on a new generation of ion-pair salts. One of these constitutes, potentially, a nonaqueous electrolyte for the next generation of high-energy-density batteries owing to its electrochemical properties. The quest for rechargeable batteries with greater energy density and capable of shorter recharge time at the “pump” for electrical vehicles (EVs) is leading the development of electrolytes with higher ionic mobility and greater limiting conductivity. In order to achieve high energy density, it is vital for an electrolyte to be electrochemically stable while operating at high voltages. The development of a weakly coordinating anion/cation electrolyte for energy storage applications offers a challenge of technological significance. This class of electrolytes is advantageous for the investigation of electrode processes in low-polarity solvents. The improvement arises from the optimization of both ionic conductivity and solubility of the ion pair formed between a substituted tetra-arylphosphonium (TAPR) cation and tetrakis-fluoroarylborate (TFAB), a weakly coordinating anion. The chemical “push–pull” between cation and anion affords a highly conducting ion pair in low-polarity solvents such as tetrahydrofuran (THF) and tert-butyl methyl ether (TBME). The limiting conductivity value of the salt, namely, tetra-p-methoxy-phenylphosphonium-tetrakis(pentafluorophenyl)borate or TAPR/TFAB (R = p-OCH3), is in the range of lithium hexafluorophosphate (LiPF6) used in lithium-ion batteries (LIBs). This TAPR/TFAB salt can improve the efficiency and stability of batteries over those of existing and commonly used electrolytes by optimizing the conductivity tailored to the redox-active molecules. LiPF6 dissolved in carbonate solvents is unstable with high-voltage electrodes that are required to achieve greater energy density. In contrast, the TAPOMe/TFAB salt is stable and has a good solubility profile in low-polarity solvents given its relatively great size. And it constitutes a low-cost supporting electrolyte capable of bringing nonaqueous energy storage devices to compete with existing technologies.


X-ray Crystallography of ZW1 (GM-B157 in CCDC)
Experimental: The material (ZW1) was used as supplied. The data for ZW1 (GM-B157) including single crystal X-ray ( Figure S1, Table S1) and crystal packing ( Figure S2) were collected from a shock-cooled single crystal at 100(2) K on a Bruker D8 VENTURE dual wavelength Mo/Cu four-circle diffractometer with a microfocus sealed X-ray tube using a mirror optics as monochromator and a Bruker PHOTON II detector. The diffractometer was equipped with an Oxford Cryostream 800 low temperature device and used Mo radiation (λ = 0.71073 Å).
All data were integrated with SAINT and a none absorption correction using SADABS was applied. [1,2] The structure was solved by dual methods using SHELXS-97 and refined by fullmatrix least-squares methods against F 2 by SHELXL-2014. [3,4] All non-hydrogen atoms were refined with anisotropic displacement parameters. The hydrogen atoms were refined freely with anisotropic displacement parameters. Crystallographic data for the structures reported in this paper have been deposited with the Cambridge Crystallographic Data Centre (CCDC). [5] These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/structures. This report and the CIF file were generated using FinalCif. [6] Figure S1. X-ray single crystal structure of ZW1 (GM-B157 in CCDC)   (Table S2). These conductivity results show that the highest limiting conductivity of the synthesized salts belongs to TAP R -TFAB (R=pOMe) at 78.6 S/m which is closest to TBA-PF6, the leading industrial supporting electrolyte.
We performed electrochemical experiments to probe the effect of both the solvent and the synthesized electrolyte TAP R -TFAB ion-pair (R= pOCH3) as well as TBA-TFAB and TBA-PF6 on conductivity in low-polar media (tetrahydrofuran THF and tert-butyl methyl ether TBME). Data for TA OMe P + TFABin THF correlate well with preliminary data: the values for Λ0 and KA were 90.1 Scm 2 mol -1 and 4.37 x 10 3 . In TBME, the value for Λ0 was lower than predicted by the Walden product (113 Scm 2 mol -1 ).

TBA + TFAB -TA OMe PTFAB TBA + PF6and TA 3,4-diOMe PTFAB
There's a reduction in the values of association constant KA from TBA + TFABelectrolyte to the proposed electrolyte TA OMe P + TFABin THF ( Figure S3). This result is promising as it suggests that the novel TA OMe P + TFABsalt is formed by a anion and a cation with a much weaker ion pairing ability than the established electrolyte TBA + TFAB -. A decrease in the KA values from TBA + TFABto our synthetic TA OMe P + TFABis also observed in the TBME conductivity tests shown below ( Figure S4) further confirming TA OMe P + TFABas a more weakly coordinating electrolyte than TBA + TFAB -.
The strategy of increasing the size and bulkiness of the cation of the ion-pair seems to correlate well with a decrease in ion-pairing and an increase in limiting conductivity of the ion-pair in these low-polarity solvents.

NMR Spectroscopy and Mass Spectrometry Data
All NMR data were initially acquired at BNL, Upton NY and at UCONN, Storrs CT in 2018-2019. Soon after all restrictions related to the Covid-19 pandemic were lifted, new NMR data were recorded at USF-Tampa FL NMR facility using a 600 MHz Agilent spectrometer. High resolution mass spectrometry were acquired at the University of South Carolina (USC) Mass Spectrometry facility in Columbia SC. Single crystal X-ray crystallography data were acquired at Emory University, Atlanta GA.

Tetrakis(p-methoxy-phenyl) phosphonium bromide 2 (P1)
To an oven-dried 50-mL round-bottomed flask containing a magnetic stir bar, a reflux condenser fitted with a CaCl2 drying tube, p-bromoanisole (1 mmol -0.19g) was dissolved in o-xylene (20 mL), followed by the addition of tris(p-methoxyphenyl) phosphine (1.1 mmol -0.39g) and of tris-palladium dibenziledene (5% mol -0.050g). The reaction mixture was heated to reflux using a heating mantle for three hours, and then cooled briefly by removal from the heating mantle. Additional tris-palladium dibenziledene catalyst (5% mol -0.050g) were added and the reaction mixture was refluxed for another 3 hr. The reaction mixture was cooled to room temperature and vacuum filtered using hexane to wash the residue (3x). The dry crude product which crystallized in the mother-liquor was further purified by recrystallization in hot ethanol yielding 0.52g (90%) of the phosphonium bromide 2. Figure S8.

Tetrakis(p-methoxy-phenyl) phosphonium-tetrakis(pentafluorophenyl)borate salt (TAP R /TFAB, with R= p-OMe) 3 (ZW1).
To an oven dried 50-mL round-bottomed flask, containing a magnetic stir bar, a reflux condenser fitted with a CaCl2 drying tube, phosphonium bromide 2 (1 mmol -0.54g) was dissolved in methanol (20 mL), and lithium tetrakis(pentafluorophenyl)borate LiTFAB was added (1 mmol -0.69g). The reaction mixture was gently heated to 70 ℃ using a heating mantle for 3 hr. The reaction mixture was cooled to room temperature and the precipitate was vacuum filtered using ice-cold methanol to wash the crystals (3x). The dry product was collected in the filter paper without further purification. Yield: ~1.0g (88%) of crude product, recrystallized in hot ethanol to yield of needle-like white crystals (0.25g) as first batch of high purity salt 3.