Lithium Ion Accelerated Charge Procedure Developed from Half Cell Characterization
Authors: Yourey William , Fu Yanbao , Li Ning , Battaglia Vince , Tong Wei 
Published: February 2020

With the continued implementation of electric vehicles (EV) into the transportation market, consumer convenience is a major hurdle to overcome, namely, an EV recharge time similar to that of refilling a gasoline vehicle tank. To accomplish this, fast charging lithium-ion cells are needed. The majority of commercial cells are manufactured with varied positive electrode active materials, coupled with a graphite negative electrode. Along with the reversibility, a significant benefit of graphite is the fact that the lithiated graphite has a voltage close to that of Li/Li+, resulting in a large overall cell voltage. The downside of this low voltage is that as attempts are made to fast charge or quickly lithiate the graphite, a small polarization results in a voltage below that of Li/Li+, which is favorable to lithium metal plating on the negative graphite electrodes1. To overcome this issue, material development and modification are being perused to reduce lithium ion diffusion length and resistance with the goal to limit negative electrode polarization during charge 2.

Meanwhile, modified charging procedures for commercial lithium-ion cells with conventional graphite negative electrodes such as multistage constant current charging and boost charging, are being implemented, where charging currents are initially high and reduced as cell voltage limits are reached 3,4. These processes largely rely on overall cell voltage to drive the charging current selection and appear independent of negative electrode loading, composition, porosity, and cell matching ratio. The presented work provides an outline for the development of an accelerated charge procedure through half-cell characterization of the limiting graphite negative electrode, which may be applied to any graphite electrode formulation and full cell design. This work provides an example of an accelerated charging procedure based off negative electrode performance and percentage of negative electrode utilization. The outlined procedure reduces charging current when percentages of negative electrode lithiation are reached, where the current and lithiation percentage result in a negative electrode voltage of ~10 mV vs. Li/Li+. Also demonstrated is a measure of charging efficiency, where an ideal fast charging process would involve a negative electrode held at a voltage slightly above Li/Li+ (10 mV), and an overall cell voltage below the cell charging voltage limit, maximizing current and minimizing cell charge time. The described work was performed on cells with a capacity loading of ~2.4 mAh/cm2, representative a commercial cell loading, where recharge times to 80% state of charge occurred in approximately 34 minutes. While demonstrating similar capacity fade to cells cycled using conventional CCCV techniques.

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