Modeling and Testing Considerations for Electrolytes that Enable Extreme Fast Charging of Lithium-ion Cells

The ability to achieve fast charge rates has become a pragmatic issue for timely implementation of advanced batteries in numerous applications. Recent guidance from the U.S. DOE has put forth a 10-minute charge (denoted 6C) as the target for Li-ion cell chemistries being designed for extreme fast charging (XFC) of electric vehicles [1]. The choice of electrolyte is a central consideration to achieve XFC goals. The design of new electrolytes for XFC chemistries may involve moving to solvents that are less viscous, more diffusive and yet that might be more volatile, calling for balanced considerations in applications. Additionally, the choice of stabilizing additives is a key metric because of more extensive redox behavior at the surface of electrodes under XFC [2].<br/>The primary target for this study is development of electrolytes for XFC Li-ion cells based on an effective combination of modeling approach using the Idaho National Laboratory Advanced Electrolyte Model (INL-AEM) and parallel laboratory techniques. First, AEM provides rapid early screening of multi-solvent electrolyte candidates for XFC based on an assortment of cell-relevant properties, such as transport properties (e.g., conductivity and diffusivity), lithium desolvation energies, and electrode pore wetting rates. Multi-solvent systems provide a balanced set of properties, wherein lower molecular-weight solvents offer reduced viscosity, increased species diffusivity, and mitigation of concentration polarization at high charge rates. Then, performance of selected electrolytes is tested using coin and pouch cells having NMC532 cathodes with graphite electrodes. Lab testing results coincides with property predictions from the AEM and a macro-scale cell model. Results indicate combinations of low-molecular weight solvents are key for fast-charge electrolytes as they extend the useful conductivity range to both low and higher salt concentrations, and possess higher self-diffusivities compared to conventional solvents. One of selected electrolytes, called B26 (EC:DMC:DEC:EP:PN (20:40:10:15:15, wt.) w/ 3% VC and FEC) + LiPF₆), outperformed other tested electrolytes in conditions including capacity cycle life and fast charging ability. Under fast charging conditions, B26 shows increased charge acceptance, decreased lithium metal plating at the anode, decreased concentration polarization, and decreased aging.<br/> <br/><b>Reference</b><br/>[1] Michelbacher et al., J. Power Sources, 367 (2017)<br/>[2] Ning et al. Energy Storage Materials, 44, 296-312 (2022)