Anomalous Voltage Change in Charge and Discharge Curves for Electrode Materials Showing Plateau-like Behavior

Recently T. Sasaki et al.¹ reported on a phenomenon of an anomalous voltage change that occurs when electrode materials showing a plateau-like behavior (during charge/discharge) are partially charged and/or discharged. They studied the anomalous voltage change in lithium iron phosphate (LiFePO₄) as well as in lithium titanate (Li₄Ti₅O₁₂). The anomalous voltage change was referred to as ‘voltage bump’ which appears already after only one cycle of partial charge and discharge. The voltage bump was explained in connection to a particle-by-particle charge/discharge model. The reported electrochemical tests were performed at room temperature in two-electrode coin-type or three-electrode cells with Li-metal as reference and counter electrode (half cell).

In this work, partial charge/discharge experiments (Fig. 1) were used to study the effect of temperature on the anomalous voltage change. The investigations were carried out on commercially available large-format cells based on LiFePO₄/graphite system (full cell). Increasing the number of partial cycles leads to an interference of single voltage bumps, but the voltage rise was observed to saturate at a given voltage level (Fig. 2). An increased voltage bump was observed at smaller temperature, while an opposite effect was obtained at higher temperature (Fig. 2). Resulting voltage bumps in the range of up to 20 mV were observed, depending on temperature. The observed behavior is discussed in view of the particle-by-particle model and kinetic effects. Such a slight voltage change can lead to substantial miscalculations in estimating the state of charge of cells and batteries.

¹Tsuyoshi Sasaki, Yoshio Ukyo, Petr Novák, Nature Materials, published online: 14 April 2013, DOI: 10.1038/NMAT3623.