Impedance Diagnostic for Overcharged Lithium-Ion Batteries

We use electrochemical impedance spectroscopy (EIS) to monitor the resistance increases associated with Li-ion batteries during and after overcharge. EIS is commonly used to measure the resistance within a Li-ion cell and can show changes in resistance behavior originating from chemical and electrochemical reactions occurring within bulk electrodes, electrolyte, and along electrode/electrolyte interfaces [1]. The physical processes in electrochemical cells have different time constants or effective capacitances, which result in different frequency responses [2]. We report the impedance characteristics for overcharged batteries are markedly different from those of the healthy batteries. At 500 Hz, the results are independent of state of charge for healthy batteries, and grossly different for overcharged batteries [3]. The changes at 500 Hz are not coincidental, as this frequency responsive to the cell passivation layers that form upon overcharge. From the results, we recommend a singlepoint impedance-based diagnostic tool for monitoring battery health. Commercial Li-ion prismatic cells (Full River 10 to 300 mAh) are used for these experiments. EIS measurements were collected using a Solartron SI 1260 impedance gain-phase analyzer driven by an EG&G PAR263A potentiostat. Impedance data were collected under open circuit conditions using a ±10 mV amplitude perturbation between 50 kHz and 10 mHz at various voltages during charge, overcharge and discharge. The upper limit for the charge voltage ranged from 4.2 V to 5.0 V, while the discharge voltage cutoff was held constant at 2.8 V throughout all experiments. EIS data were collected and analyzed by ZPlot and ZView software packages (Scribner Associates Inc.). The batteries were charged and discharged at constant 1C rates (30 mA) at approximately 23°C. Repeated charge/discharge (2.8–4.2 V) and overcharge/discharge (2.8 – 4.4, 4.6, 4.8, 5.0 V) data were measured using a Maccor Series 4300 battery tester. Overcharged LiCoO2|C cells have drastically different impedance spectra as compared to properly operated ones (charged between 2.8 – 4.2 V). A soft overcharge to 4.4 V results in small changes in the impedance spectrum compared to the recommended 4.2 V upper charging limit. When overcharged above 4.4 V, the impedance characteristics change dramatically. The shapes of the impedance spectra irreversibly change upon discharge after a severe 5.0 V overcharge and no longer resemble the impedance spectra measured for the battery charged to 4.2 V (see reference 3). This irreversibility in the impedance response for overcharged Li-ion cells is most pronounced at 500 Hz (Fig 1). A single overcharge to 4.6 V causes the battery impedance at 500 Hz to drop significantly, reflecting a change in the structure of the electrode passivation layers. With further overcharge cycles, the battery impedance at 500 Hz increases, reflecting an increase in the cell resistance, and the first steps toward an overheating battery. These trends are independent of battery sizes from 10 to 300 mAh, but more work is needed to determine these trends across different chemistries (i.e. LiFePO4 cathodes), battery packs, and for cells operating at different temperatures. This work points to a simple prognostic and diagnostic for the safe operation of Li-ion batteries, and also sheds some information on the causes of overcharge failure in batteries.