What is galvanostatic method

The galvanostatic method is to obtain the performance characteristics of the Lithium Iron Phosphate Battery by testing the voltage change with time under the condition of constant current. The electrochemical properties achievable by this method include capacity, reversibility, resistance, and diffusion rate. The galvanostatic method can be divided into two categories according to the different end conditions.

  1. Voltage cut-off control method
    The continuous charge-discharge experiment is to measure the voltage change with time in a given voltage range and constant current condition. It is an electrochemical analysis method that measures the amount of electricity under continuous charge and discharge and voltage changes over time.

Figure 1 shows the voltage change over time with the lower and upper limits set to 0V and 3V, respectively. From the figure, we can see that the voltage changes slightly after each cycle, which indicates that the electrode material undergoes a reversible reaction involving lithium ions. Figure 2 lists the charge-discharge capacity for each charge-discharge cycle, from which the coulombic efficiency of each charge-discharge step can be calculated.

The charge-discharge capacity of the charge-discharge cycle

The differential capacitance curve is a plot of dQ/dV versus voltage based on time and voltage values ​​obtained from galvanostatic testing. The units of the differential capacitance curve can also be expressed as dt/dV using the equation below.

Figure 3 shows the differential capacitance curve (dQ/dV) of a battery composed of graphite and lithium metal from the voltage cut-off controlled constant current test. The data is from the first cycle in Figure 1. In this way, we can accurately measure the voltage value of the electrochemical reaction.

Differential Capacitance Curves (dQdV) for Graphite and Lithium Metal Composition

The differential capacitance curve is similar to the cyclic voltammetry curve, except that there is a constant overpotential. In cyclic voltammetry, in order to differentiate the electrochemical reaction types, the scan rate must be reduced. The voltage of the electrochemical reaction can be read more easily from the differential capacitance curve.

Constant capacity cut-off control method
Constant capacitance cut-off control is a constant current method that tests the characteristics of the negative electrode by controlling the number of charges. Unlike the positive electrode material, when the negative electrode material is charged to a voltage close to that of metallic lithium, the intercalation of lithium and the precipitation of metallic lithium may occur simultaneously. Given the relatively flatness of the voltage curve, the voltage cutoff control method should not be used, as small changes in voltage can cause significant changes in charge. To solve this problem, the battery can be charged to the required capacity and then voltage-controlled discharged.

Constant pressure method
The constant voltage method is a simpler method than the constant current method, which can make the redox reaction of the battery reach the electrochemical equilibrium.

  1. Constant voltage charging
    There is an ion concentration gradient on the surface and inside of the material involved in deintercalation, the magnitude of which depends on the applied current. Unlike the electrode surface, the charging inside the electrode is not complete. If it is only charged to the rated capacity by the constant current method, the electrode material may be damaged when the surface potential exceeds the rated voltage. To avoid this problem, charge at constant current and then charge at constant voltage within the rated voltage range, which maximizes the energy storage capacity of the battery.
  2. Potential step test
    Potential step test is a method based on constant voltage control and by gradually increasing or decreasing the battery voltage, it is necessary to set the charge cut-off current and time for each step. Using this method we can obtain open circuit voltage and voltage transient signals, which can be used to derive differential capacitance curves and diffusion rates.

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