Polarization Study and AC-Impedance Measurement in the Horizontal Plane of a Lithium-ion Battery Using Eight Reference Electrodes

Abstract

Eight reference electrodes (Li/Li⁺) were embedded around a square (30 mm × 30 mm) lithium-ion battery (LIB) cell (Type A or Type B), and the electrical potential changes of the positive and negative electrodes against the reference electrodes (Rx; x-1–8) during charge and discharge were studied. Type A has a jutting positive-side (JP-side) and a jutting negative-side (JN-side). During the charge and discharge of Type A, significant potential shifts (>1.3 V) in the horizontal plane of the cell were observed near the JP-side and JN-side, but which were relieved after several hours. The potential shifts in the horizontal plane of the cell (the difference between the maximum and the minimum of P-Rx (x = 1–8) or N-Rx (x = 1–8)) were caused by the relaxation current between P and JP-side or between N and JN-side. AC impedance measurements also were conducted with the cells several hours after they reached a specific SOC level. Linear relations were obtained at wide frequencies between the ratio of P-Rx (x = 1–8) and Z′ (Real) or Z′′ (Imaginary) resistance. The sharing rate of P-Rx (x = 1–8) in Z′ (Real) or Z′′ (Imaginary) resistance was mainly determined by the position of the reference electrode which was close to the edge of the positive electrode or that of the negative one. It was found that when a reference electrode is installed outside of the cell, it is difficult to divide the impedance into both electrodes by the AC impedance measurements with the reference electrode.