Validation of a Combinatorial Approach Toward the Discovery of Electrolyte Formulations for Lithium-Ion Batteries

Combinatorial methods have opened exciting new prospects for screening materials for application in a growing number of technological areas ranging from drug discovery to semiconductors for photovoltaics^1,2. These efforts have been paralleled more recently by the use of computers to explore the properties of yet to be synthesized materials incorporating just about every stable element in the periodic table. However, application of these combinatorial techniques to electrolytes for lithium ion batteries have been rather scarce³where optimization of solvent formulations has apparently been performed by coarse trial and error. This contribution describes a unique instrument capable of producing large numbers of electrolyte mixtures involving solvents either currently in use or being considered in the L-ion industry.

EXPERIMENTAL

All organic solvent formulations were prepared with a programmable non-contact liquid handling instrument (Digilab MicroSYS) using the synQUAD dispensing technique placed inside an Ar (Airgas High Purity) filled high quality glovebox (MBraun UNIlab) to avoid contamination with atmospheric components and reduce safety hazards.  During operation, the pressure within the glove box varied between 2.0 and 6.0 mbar.  The solvents involved in these studies were all provided by BASF (formerly Novolyte, Independence, OH), and included diethyl carbonate, DEC, dimethyl carbonate, DMC, propylene carbonate, PC ethyl methyl carbonate, EMC and a solution of room temperature solid, ethylene carbonate, EC, in EMC.  The composition of all organic solvent mixtures prepared by the combinatorial instrument was assayed using a Hewlett Packard GC/MS (controlled by Xcaliber) equipped with an Rtx-200MS GC column (30 m in length, 0.25 mm I.D. and film thickness equal to 1 μm, RESTEK Corporation). For all the measurements including the standards, 1 μL diluted solvent (1 μL of the solvent mixtures of interest in 1 mL acetonitrile (HPLC purity, Sigma Aldrich) is injected into GC-MS for each run with a 5 μL syringe (Fisher Scientific). Standard curves relating integrated peak area and volume or weight of the solvent of interest in acetonitrile was obtained by running the solvent at four different concentrations. RESULTS AND DISCUSSION

Shown in Figure 1 is a typical chromatogram of a solvent mixture, where each of the peaks is associated with an individual solvent as indicated. Figure 2 shows a comparison between the GC/MS analysis and the intended combinatorial ratios for eight different samples. AS evidenced from these data the agreement between the two sets of data .is indeed excellent yielding for a selected specimen average error of only 2.1%. Values for other specimens ranged between 1.2 and 2.1 % providing evidence that the methodology developed displays high degree of accuracy and as such can assist the discovery of new solvent formulations for application in batteries, supercapacitors and redox flow cells. An assessment of the electrochemical  performance of mixtures such as those featured in this report is in progress and will be reported in due course.

Acknowledgements

This work was supported by a grant from BASF (formerly Novolyte, Independence, OH) through a subcontract for the Ohio Third frontier Program.