Impedance Informed, High Frequency Pulse Charging As Method to Enable Stable Low Temperature Li-Ion Battery Operation
Authors: Klein Emily J , Carter Rachel E. , Love Corey T 
Published: February 2020

Low temperature cycling of Li-ion batteries can have two major complications: incomplete intercalation in the anode and lithium metal deposition.1-2 These complications lead to a loss in capacity and cell instability caused by lithium deposits inside the cell. Instability within the cell can cause the battery to begin an exothermic reaction and go into thermal runaway, resulting in a potentially hazardous reaction. Herein we use a pulse charging technique to overcome the limitations associated with low temperature cycling. Commercial 18650 cells were cycled 50 times at 25°C (ambient), 0°C, and at 0°C using a pulse charging technique. The parameters of pulse charge were determined using a single point impedance technique.3 Electrochemical impedance spectroscopy (EIS) was acquired at 10% increments of state of charge (SOC). The frequency least dependent on state of charge indicates a time constant of ion movement where the solid electrolyte interface (SEI) at the anode is least inhibiting to ion flow. Therefore, this frequency was imitated with a square-wave, current pulse to ameliorate the sluggish SEI kinetics often blamed for onset of lithium deposition in low temperature charging. The magnitude of current was prescribed by the cell manufacturer and square pulses were applied until the voltage cutoff was reached. The charge was then topped off with a CV step and constant current discharged in the same manner as the conventionally charged 25°C and 0°C controls. The cells cycled at 25°C remained stable across 50 cycles: staying around 2.6A with 98% capacity retention. The pulsed cell and the cold cell begin with a capacity deficit due to enhanced cell impedance at the cold conditions. The capacity of the cold cell starts to fade, and at the 20th cycle a large drop in capacity is apparent. The drop off in capacity is due to plating of lithium within the cell, which is illustrated in the differential capacity assessment and computed tomography renderings of the cells after charging. The pulses cell does not exhibit fade like the cold cell, maintains a constant capacity, and has an impressive 99% capacity retention at the 50th cycle, which is comparable to the ambient cell.
Figure 1. Charge/discharge behavior over 50 cycles for (a) constant current at 25°C (ambient), (b) pulsed charging at 0°C, and (c) constant current charging at 0°C. (d) Capacity retention over 50 cycles for the three charging and temperature conditions.

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