MacMullin value, interface impedance and ion conductivity

MacMullin value

The ratio between the resistivity of the diaphragm and the resistivity of the electrolyte is called the MacMullin (NM) value. The MacMullin value can characterize the relative value of the diaphragm in the battery resistance, thereby reflecting the influence of the diaphragm on the electrochemical performance. Generally speaking, the NM value of lithium-ion batteries is close to 8. The smaller the value, the better. In fact, the NM value can better characterize the ion permeability of the diaphragm than the ion conductivity, because it eliminates the influence of the electrolyte on the result. The calculation formula for the NM value is

MacMullin value, interface impedance and ion conductivity
(1-1)

In the formula: ρ-Resistivity of the diaphragm (Ω); ρe-Resistivity of the electrolyte (Ω).

Interface impedance

The specific test steps of the interface impedance are: in a glove box filled with argon, sandwich the diaphragm between the electrodes, and add an appropriate amount of electrolyte to assemble the button cell. Use the AC impedance test function in electrochemical work to set test parameters. Figure 1 depicts a simplified AC impedance spectrum, the Nyquist plot, which can be used to study the electrochemical interface behavior of the battery. The semicircles (or overlapping semicircles) represent different physical or electrochemical processes, and the semicircles in the high frequency region represent the impedance of the solid electrolyte interface layer (SEI film); the semicircle in the mid-low frequency region represents the charge transfer resistance between the electrode and the electrolyte; the straight line in the ultra-low frequency region represents the impedance caused by the diffusion of lithium ions. The overall interface impedance (Ri or RSEI) can be estimated by subtracting the bulk impedance (Rb). In addition to the SEI film, the compatibility between the diaphragm and the electrode can also change the impedance value of the battery.

MacMullin value, interface impedance and ion conductivity
17Figure 1 – Simplified AC impedance spectrum

Ion conductivity

Ionic conductivity mainly reflects the transmission capacity of ions in the electrolyte, and is also a comprehensive characterization of the thickness of the diaphragm, porosity, pore size, pore tortuosity, and electrolyte infiltration degree. It is an important parameter of the diaphragm material. The ionic conductivity of the non-aqueous liquid electrolyte of lithium-ion batteries is generally in the range of 10-3~10-2S/cm, and the ionic conductivity of the diaphragm/electrolyte system required for use in lithium-ion batteries at room temperature is on the order of 10-3S/cm. Although the separator can prevent a short circuit between the positive and negative electrodes, its existence causes the effective ion conductivity in the electrolyte to decrease, and the battery impedance increases. Some separators can even cause the ion conductivity to drop by 1 to 2 orders of magnitude.

Conductivity is the ability of an object to conduct electrons or ions, and it is the reciprocal of resistivity. Among them, the ionic conductivity of the diaphragm after immersing in the electrolyte can be calculated by the AC impedance method. The specific test steps are: in a glove box filled with argon, place the diaphragm between two stainless steel plates, and add an appropriate amount of electrolyte to assemble a button cell. Use the AC impedance test function in the electrochemical workstation to set parameters for testing. The intersection of the oblique line and the real axis of the high-frequency region obtained in the experimental results can be approximated as the resistance value of the diaphragm. The calculation formula is

MacMullin value, interface impedance and ion conductivity
(1-2)

In the formula: Rb-the resistance of the diaphragm (Ω); A-the area of the diaphragm (cm2); L-the thickness of the diaphragm (μm).