Operando Measurements of Electrolyte Lithium Ion Concentration during Fast-Charging

Abstract

The ability to charge a Li-ion battery at high charging rates is critical for widespread electric vehicle adoption, but fast-charging rates can cause battery degradation and performance issues. A primary challenge for fast charging is finding electrolytes with sufficient lithium ion transport to operating under high current densities. However, there are few options for measuring ion transport in operating batteries. A novel diagnostic using Fourier transform infrared spectroscopy (FTIR) with attenuated total reflection (ATR) is reported for operando measurements of lithium ion concentration in a liquid electrolyte during battery operation. This diagnostic uses solvation shifting of solvent molecule vibrational modes to quantify lithium concentration. The diagnostic was calibrated using EC/EMC/LiPF6 electrolytes with known lithium concentrations from 0.03 mol/liter to 1.83 mol/liter. The concentration of PF6 - was also determined with a similar procedure. The resulting diagnostic can measure lithium concentrations from 0.001 to over 2 mol/liter with a minimum detection limit of 0.0011 moles/liter. The diagnostic is demonstrated using a lithium-ion half cell with a graphite anode and EC/EMC/LiPF6 electrolyte assembled in an argon-filled glove box. Measurements on the current collector side of the anode showed that as charging rates increased (i.e. during lithiation), lithium ion concentrations in the adjacent electrolyte decreased. The reverse trend was observed upon delithiation. These results demonstrate the utility of this diagnostic in quantifying ion depletion in electrolytes under fast charge conditions.