Electrolyte Development and Its Application for Fast Charging of Li-Ion Batteries

Abstract

Energy density, cost and safety are, more than ever, the most significant barriers to overcome in order to increase the wide acceptance of Li-ion batteries (LIBs) in electric vehicles (EVs).1 The U.S. Department of Energy (DOE) has set ultimate goals for battery electric vehicles (BEVs), which include reducing the production cost of the battery pack to $150/kWh, increasing the electrical range of the battery to 300 miles, and decreasing the charging time to 15 minutes or less.2 Increasing electrode thickness, and hence increasing active material loading, is an effective way to achieve these energy density and cost targets.3 The caveat, however, is that thicker electrodes fail during fast charging. One of the solutions for enabling extreme fast charging while retaining most of the battery energy can be achieved through the significant enhancement of the Li-ion mass-transport in electrolytes such that enough Li ions are available for intercalation in graphite. The mass transport of Li-ions can be evaluated by two macroscopic characteristic values: 1) the Li ions ionic conductivity that is related to the total flux of charge carriers, 2) the Li ion transference number that is related to the fraction of the total current that is carried by Li ions. An electrolyte with both higher Li ions conductivity and transference numbers is ideal for higher Li ions transport, and hence would be a step toward realizing cells with higher charging rates. The fast charging performance of high-energy density (NMC811/graphite) Li-ion cells iss studied when different lithium salts were used in the electrolyte. The new electrolyte shows both higher ionic conductivity and Li-ion transference number compared to LiPF6 electrolyte. During a 12-minute fast charging step, cells with LiPF6 electrolyte reached the cut-off voltage in 4.2 minutes, which is much earlier compared to 7.4 minutes for the new electrolyte. The new electrolyte showed 13% capacity improvement in the first cycle of the 12-minute charge. The capacity retention was also significantly higher at 87.7% after 500 cycles with less lithium plating observed. (1) Needell, Z. A.; et al. Potential for Widespread Electrification of Personal Vehicle Travel in the United States. Nat. Energy 2016, 1 (9). (2) Howell, D.; et al. Enabling Fast Charging: Enabling Fast Charging: A Technology Gap Assessment. 2017, No. October. (3) Du, Z.; et al. Understanding Limiting Factors in Thick Electrode Performance as Applied to High Energy Density Li-Ion Batteries. J. Appl. Electrochem. 2017, 47 (3), 405–415. Figure 1