- Open circuit voltage
The open circuit voltage is the voltage measurement when the battery is in equilibrium and there is no potential difference between the two electrodes. In other words, the voltage of the electrodes is stabilized when no current is flowing. This value reflects the Gibbs free energy under thermodynamic equilibrium conditions. The closed circuit voltage (CCV) test is the voltage value when the electrode is connected to the external circuit and there is current passing through it. To accurately describe the thermodynamic state of an electrode, it is necessary to correlate the open circuit voltage with the chemical composition.
The open circuit voltage provides information on the electrode material voltage, the occurrence of internal short circuits and the initial interfacial chemistry. By testing the electrode open circuit voltage during the charge-discharge cycle, we can investigate the reversibility of charge-discharge. The variation of open circuit voltage with time provides information for us to study electrochemical reactions, such as the charge transfer and self-discharge phenomena of electrode materials.
Figure 1 shows the open circuit voltage of a cell composed of single-walled carbon nanotubes (SWCNTs) and lithium metal as electrodes and 1M LiPF6/EC:DEC (1:1 by volume) as electrolyte. The voltage test range is 0~3V. During the test, a constant current of 50 mA/g is applied for 1h, and then no current is applied for 1h, alternately. The closed-circuit voltage measured within 1h of constant current is represented by a thick dashed line, while the open-circuit voltage value at zero current is represented by a solid line.
- Linear Sweep Voltammetry
Linear sweep voltammetry is an electrochemical analysis method that sweeps the voltage of a battery over a given voltage range at a rate v (V/s), and the results are expressed in the form of a current-voltage curve. Significant changes in current can be observed when the battery undergoes oxidation or reduction reactions over the test voltage range. We can predict and analyze the reactions taking place in the battery by testing the current and voltage values corresponding to these points. Based on these characteristics, linear sweep voltammetry is widely used to evaluate the electrochemical stability of electrolytes.
Figure 2 is an example of linear sweep voltammetry showing the change in current with applied voltage at a constant voltage sweep rate. The battery consists of lithium metal and electrodes, with 1M LiPF6/DMC as the electrolyte. It can be seen that the current only changes slightly when the voltage is below 4.5V, and the current changes significantly in the range of 4.6~5.2V, and rises rapidly from 5.2V. This indicates that the redox reaction within the cell occurs at voltages above 4.6 V, and the electrolyte shows stable electrochemical performance below 4.6 V.
- Cyclic Voltammetry
Cyclic voltammetry is an electrochemical analysis method that scans the voltage of a cell at a constant voltage scan rate over a given voltage range. Similar to linear sweep voltammetry, cyclic voltammetry observes changes in current by applying a voltage at a constant sweep rate. But each cycle of cyclic voltammetry repeats the same experiment, and the current-voltage curve obtained by cyclic voltammetry is different from the linear curve obtained by linear sweep voltammetry. Cyclic voltammetry provides information on redox reactions taking place in a battery including voltage, charge, reversibility, and persistence (the persistence of a reversible electrochemical reaction). The size of the scan rate depends on the purpose of the experiment, and the lowest possible scan rate is recommended for detailed analysis of electrochemical reactions.
Figure 3 is a typical cyclic voltammogram showing current as a function of applied voltage. When the scanning direction is (+), the anodic current initiates the oxidation reaction, while the (-) direction is the reduction reaction. Figure 4 presents the cyclic voltammetry results of the natural graphite anode in the range of 0–3 V. The battery consisted of “natural graphite/[1M LiPF6/(PC:EC:DEC)]/lithium”. The cathodic current represents the electrochemical reaction induced by lithium intercalation, and the anodic current corresponds to the extraction of lithium.
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