Nonlinear aging characteristics of lithium-ion cells under different operational conditions

• Turning point from linear to nonlinear aging characteristics observed.
• Aggravated by high charging currents, high depths of discharge, low temperatures.
• Appearance of aging induced lithium plating.
• Localized areas of the anode covered with thick surface films.
• Aging behavior upon prolonged cycling crucial for 2nd-life-applications.

The potential reuse of lithium-ion batteries exhausted upon electric vehicle operation is a broadly discussed topic. However, a profound understanding of battery aging behavior is a prerequisite to assess overall system cost and economic benefit of battery reuse: Whereas the capacity fade under load is commonly reported to show a linear dependency on charge throughput, a turning point to nonlinear aging characteristics is observed at residual capacities of about 80% under different operational conditions. Based on a widespread aging matrix, results indicate an earlier appearance of nonlinear characteristics at high charging rates and operational voltage windows but low temperatures. Scanning electron microscopy images reveal areas of thick surface films at the anode while the cathode stays unmodified. Therefore, a thickening of the solid electrolyte interphase, catalyzed by oxidation products migrating to the anode, is believed to be the dominating aging mechanism before the turning point. Afterwards, lithium plating might even occur at moderate temperatures and charging rates due to deteriorated ionic kinetics and graphite active material loss. As all these mechanisms are dependent on the cell potential, an optimized operational strategy may avoid or retard nonlinear aging characteristics and extend the life of lithium-ion cells.