Stabilizing Batteries with Salts

Designing Better Electrolyte Components for Lithium-Ion Batteries

Researchers at North Carolina State University are cooking up next-generation electrolytes for lithium-ion batteries, and their recipe calls for more salt. The Ionic Liquids and Electrolytes for Energy Technologies (ILEET) Laboratory, run by Professor Wesley Henderson, creates and characterizes new lithium salts and ionic liquids to formulate electrolytes with a wider range of operating temperatures and voltages. Electrolytes in current Li-ion batteries are limited by poor low temperature performance (< -10°C) and decompose at elevated temperatures (>55°C), potentially causing a fire hazard. To circumvent these issues, Henderson is developing new electrolyte components based on organoborates related to bis(oxalato)borate (BOB^–) and cyanocarbanions. These anions are first synthesized as lithium salts, and then scaled up to form ionic liquids, which are salts that remain liquid even at room temperature or below and have low volatility.

Henderson_Fig1 — Figure 1. The chemical structure of LiBOB (left) and its crystal structure (right). The elemental color key is: Li (purple), O (red), B (tan), and C (grey). Ref: Zavalij, et al. Acta Crystallogr. 2003, B59, 753-759.

To determine whether these new salts improve upon state-of-the-art electrolytes, the ILEET Lab uses a suite of characterization tools. Thermal gravimetric analysis (TGA) reveals a salt’s thermal stability while differential scanning calorimetry (DSC) provides insight into the phases of these salts at various temperatures and compositions in electrolyte solvents, allowing researchers to construct phase diagrams. Powder x-ray diffraction (XRD) and Raman spectroscopy can then be used to determine solvate structures in the solid and liquid phases, respectively. Once solvate structures are known, they can be correlated to properties such as ionic conductivity and solubility. One of Henderson’s main goals is to use structural information and phase diagrams to predict the properties of new salts and determine their optimum compositions in electrolytes.

Derivatives of LiBOB are being synthesized to investigate the effect of different functional groups on physical and electrochemical properties. LiBOB has low solubility in carbonate solvents, which may be due to its high degree of ionic association (each Li⁺is coordinated by five oxygen atoms, Figure 1) and corresponding formation of aggregated solvate structures. Since higher solubility is desirable for increasing the availability of electroactive lithium in the battery, LiBOB is being tweaked according to the design principle of weaker ionic association between the anion and lithium cations. One such modification involves replacing one of the oxalate ligands with two fluorines to form lithium difluoro(oxalato)borate (LiDFOB), which has been shown to have higher solubility than LiBOB in carbonate solvents.

The improved solubility of LiDFOB can be explained by examining its solvate structure in a commonly used electrolyte solvent, dimethyl carbonate (DMC). The crystal structure of a (DMC)_3/2:LiDFOB solvate reveals that each Li⁺is coordinated by two anions and two DMC molecules. One DMC is ordered and coordinates a single Li⁺ while the other DMC is disordered over two positions and coordinates two Li⁺(Figure 2). Each DFOB^–coordinates only two Li⁺, whereas crystalline solvates of LiBOB in DMC indicate that BOB^– coordinates three or more Li⁺. Therefore, fluorine substitution weakens the ionic association within the lithium salt, thereby facilitating dissolution by solvents.

Henderson_Fig2 — Figure 2. Chemical structure of LiDFOB (left) and its crystal structure as a (DMC)3/2:LiDFOB solvate (right). The elemental color key is: Li (purple), O (red), B (tan), F (green), and C (grey).

Besides organoborate-based anions, the ILEET Lab is investigating cyanocarbanions. One such candidate, dicyanotriazolate (DCTA), has been found to strongly coordinate with Li⁺, forming an aggregate in solvents. Computational calculations suggest that improved Li⁺dissociation can be achieved by increasing the number of cyano-groups in the anion. Subsequently, cyanocarbanions with extensive resonance across the structures will be prepared to determine how the lithium salts with these anions compare with LiDCTA.

All newly synthesized lithium salts will be characterized in an array of standard carbonate- and sulfone-based electrolytes. Once the parameter space for each binary system is mapped onto a phase diagram, thermal and electrochemical stability tests can be performed on promising compositions. Further evaluation of performance in battery cells will be done in collaboration with the U.S. Army Research Laboratory. These systematic studies are paving the path toward customized lithium salts and ionic liquids for specific battery applications, such as operation under high-voltage or extreme temperature conditions.