1.The Charging and Discharging Modes of Button Cells
The charging and discharging tests of button lithium batteries typically use constant current charging (CC), constant current-constant voltage charging (CC-CV), constant voltage charging (CV), and constant current discharging (DC) to test and analyze the battery's charging and discharging behavior. By analyzing the data changes during this process, various electrochemical performance parameters of the battery or material, such as capacity, coulombic efficiency, charging and discharging plateau, and internal parameter variations, can be characterized.
2.The charging and discharging test methods of button lithium batteries.
The stepwise charging and discharging mode is mainly used for testing the DC internal resistance, polarization, and diffusion impedance performance. Considering the content of active materials and the effect of electrode size on the test current, constant current charging is often expressed in terms of current density, such as mA/g (current per unit mass of active material) or mA/cm² (current per unit area of the electrode).
The magnitude of the charging and discharging current is often represented by the charge/discharge rate, which is calculated as:
Charge/discharge rate (C) = Charge/discharge current (mA) / Rated capacity (mA·h).
For example, if a battery with a rated capacity of 1000 mA·h is charged or discharged at 500 mA, the charge/discharge rate is 0.5 C. According to the industry standard QCT/743-2006 for lithium-ion batteries used in electric vehicles, the general charging and discharging current for lithium-ion batteries is C/3. Therefore, tests with C/3 charge/discharge behavior are commonly seen in laboratory testing of lithium-ion batteries.
Rate performance testing can take three forms:
1.Constant current-constant voltage charging at the same rate, followed by testing at different discharge rates to characterize and evaluate the battery's performance at various discharge rates.
2.Constant current discharge at the same rate, followed by testing at different charging rates to characterize the battery's charging performance at different rates.
3.Both charge and discharge are tested at the same rate.
When testing the cycle performance of a battery, the main task is to determine the charge/discharge mode and perform periodic cycles until the battery's capacity drops to a specified value (usually 80% of its rated capacity). The number of charge/discharge cycles the battery undergoes is recorded, or the remaining capacity after a certain number of cycles is compared to characterize the battery's cycle performance. Additionally, the testing environment of the battery can have a certain effect on its charge/discharge performance.
3.Basic Data Analysis
Voltage Analysis
The open-circuit voltage of an assembled lithium-ion battery refers to the potential difference between the positive and negative electrodes when no current flows through the external circuit. It can be measured directly using a multimeter (with an accuracy of no less than 0.1 mV; it is recommended to use a high-impedance voltmeter to prevent self-discharge) or directly read from the battery testing system after connection.
This value is only the initial open-circuit voltage after the battery assembly. The open-circuit voltage at full SOC (State of Charge) should be measured using the Galvanostatic Intermittent Titration Technique (GITT), which will be introduced in later articles. The working voltage refers to the instantaneous potential difference between the positive and negative electrodes when current flows through the external circuit, which can be directly reflected in the battery testing system data.
The working voltage is given by the equation:
U=E0±IRiU = E_0 \pm I R_iU=E0±IRi
where E0E_0E0 is the thermodynamic equilibrium voltage, RiR_iRi is the internal resistance or contact resistance present in the button cells, such as the ohmic resistance of a structural component, charge transfer impedance, or diffusion impedance, and III is the test current. The working voltage is related to the magnitude of the current.
Discharge Average Voltage Analysis
The discharge average voltage requires processing the curve mathematically, as follows:
Qmax=∫0EI dVQ_{\text{max}} = \int_{0}^{E} I \, dVQmax=∫0EIdV
where QmaxQ_{\text{max}}Qmax is the discharge capacity in the curve, and EEE is the voltage on the discharge curve's vertical axis.
Capacity Analysis
Battery capacity is an important performance indicator of lithium-ion batteries. It represents the amount of charge stored in a lithium-ion battery under specific conditions, typically measured in Ah (ampere-hours) or mAh (milliampere-hours) (1 Ah = 1000 mAh). The main method to determine the lithium-ion battery capacity is to integrate the current over time when the battery discharges from 100% SOC to 0% SOC (i.e., within the test voltage range). The formula is:
Q=∫0tI dtQ = \int_0^t I \, dtQ=∫0tIdt
where QQQ is the battery capacity (Ah), III is the current (A), and ttt is the test time (h). 1 mAh is equivalent to 3.6 Coulombs. Typically, the capacity data can be directly read from the testing system software.
For the tested battery material, capacity analysis generally requires determining three key data points:
a. First charge capacity is the charging capacity when the lithium-ion battery completes its first charge.
b. First discharge capacity is the discharge capacity when the lithium-ion battery completes its first discharge.
c. Reversible capacity refers to the capacity value after the battery has stabilized through several cycles (the test value at room temperature is also called rated capacity). Typically, the discharge capacity of the 3rd to 5th cycle is selected, and sometimes the discharge capacity after 10 weeks is used.
In practical applications, the analysis of the specific capacity, areal capacity, and volumetric capacity of the tested materials or electrodes is more valuable for reference. For instance, specific capacity refers to the discharge capacity per unit mass of active material, C=QmC = \frac{Q}{m}C=mQ; areal capacity refers to the discharge capacity per unit area of the test electrode, C=QSC = \frac{Q}{S}C=SQ; and volumetric capacity refers to the discharge capacity per unit volume of the electrode, C=QVC = \frac{Q}{V}C=VQ.
In these equations, CCC is the specific discharge capacity (mA·h/g for mass, mA·h/cm² for area, or mA·h/cm³ for volume), QQQ is the discharge capacity (mA·h), mmm is the mass of the active material (g), SSS is the area of the test electrode (cm²), and VVV is the volume of the test electrode (cm³). The specific capacity parameter is useful for directly comparing the performance of tested materials, while areal and volumetric capacities are more useful for the practical application of the materials, especially when matching the capacities of the positive and negative electrodes. It is recommended to provide all three specific capacities when publishing research.
Button cell data can also be used to evaluate the energy density of the positive electrode active material (W), which refers to the energy that can be stored and released by unit mass of positive electrode material. It is calculated as W=EQmW = \frac{E Q}{m}W=mEQ, where EEE is the average discharge voltage, and QQQ is the specific capacity. The common units for energy density are W·h/kg (specific energy), and it can also be expressed in volumetric energy density (W·h/L).
Typically, the mass ratio of the positive electrode active material in a battery cell is 30%–50%, depending on the compaction and true density of the material. Therefore, based on the energy density of the positive electrode material, a rough estimate of the corresponding full-cell energy density can be made. This is useful for evaluating positive materials and predicting cell energy density when it is not feasible to develop a complete battery.
Discharge/Charge Curve Analysis
The charge/discharge curve reflects the charging and discharging behavior of battery materials. Analyzing the charge/discharge curve of button cells is crucial for understanding the material's performance and electrochemical behavior. This is especially important for analyzing half-cell charge/discharge curves, as it allows for a targeted analysis of the characteristics and behavior of a specific material. Charge/discharge curves can appear in different forms, such as the more common "cross-type" curve (Figure 1) and "cyclic-type" curve (Figure 2).
The "cross-type" charge/discharge curves of half-cells assembled with different materials.
The "cycling-type" charge/discharge curves of half-cells assembled with different materials.
A large amount of data can be extracted from the charge/discharge curves of button cells. Below is a brief introduction to the reading and analysis
of some of the data.
The charge/discharge curve of a graphite/metal lithium button half-cell
The de-intercalation and intercalation of lithium ions in the positive and negative electrode materials correspond to the plateau or sloping regions on the charge/discharge curve (as well as the oxidation/reduction peaks in cyclic voltammetry and differential capacitance curves). By analyzing the changes in each plateau region, the electrochemical reaction behavior of the material can be studied.
Typically, the number of charging and discharging voltage plateaus or slopes is the same. If the total charge and discharge capacity is identical, but the capacity of each corresponding plateau/slope differs, this indicates significant differences in the thermodynamic reaction pathways or kinetic characteristics of lithium intercalation and de-intercalation in the material. Figure 3 shows the typical charge/discharge curve for graphite anode material.
The charge/discharge curve shows that during the charging and discharging of the graphite/metal lithium half-cell, the graphite electrode exhibits three distinct charge/discharge plateaus at 0.08/0.1 V, 0.11/0.14 V, and 0.2/0.22 V, respectively. These correspond to the two-phase transition processes of three lithium-graphite intercalation compounds. The starting point of the plateau corresponds to the initiation of phase transition, and the endpoint of the plateau corresponds to the end of the phase transition. Plateau behavior implies that the electrochemical potential of the main material is independent of the ion occupancy in the material.
Sloping regions in the charge/discharge curve generally correspond to solid solution reactions or capacitive behavior. Sloping behavior indicates a direct relationship between the electrochemical potential of the main material and the ion occupancy in the material. Therefore, by analyzing the charge/discharge curve, one can preliminarily determine the number of phase transition reactions occurring during the process, whether it involves two-phase transitions, solid solution reactions, or capacitive behaviors related to adsorption/desorption. This can guide structural studies, such as X-ray diffraction (XRD).
Polarization Analysis
Charge and Discharge Curve of Lithium-Rich Cathode Material (Li₁.₂Ni₀.₁₃Co₀.₁₃Mn₀.₅₄O₂)
During the charge and discharge process of lithium-ion batteries, polarization is inevitable, especially during high-rate charging and discharging. It is essential to study the capacity variation caused by polarization and analyze the polarization situation based on the charge and discharge curves. Compared to analyzing the electrode process kinetics through GITT (Galvanostatic Intermittent Titration Technique), PITT (Potential Intermittent Titration Technique), or Electrochemical Impedance Spectroscopy (EIS), the kinetic information obtained from the charge and discharge curves is more intuitive.
Typically, at lower charge and discharge rates (such as 0.05C, 0.02C, 0.01C, or lower, depending on the material), the capacity measured can largely ignore the capacity variation caused by polarization. The difference in capacity values measured at a given rate compared to the low-rate capacity values can be considered as the capacity change due to polarization.
In the constant current-constant voltage (CC-CV) charging and constant current discharging curve, the polarization can be characterized by the ratio of the constant current charging capacity to the total charging capacity and the ratio of the constant voltage charging capacity to the total charging capacity. The lower the ratio of constant current charging capacity to total charging capacity or the higher the ratio of constant voltage charging capacity to total charging capacity, the greater the polarization.
Differential Curve Analysis
In the process of analyzing the battery charge-discharge curve, to facilitate the study of the curve, the differential processing of the curve is performed, transforming the platform regions into peak curves. Common methods used are the incremental capacity curve (dQ/dV vs. V) and the differential voltage curve (dV/dQ vs. Q) to analyze the charge-discharge curve.
Differential capacity curves of several cathode material half-cells.
The differential capacity curve, abbreviated as the IC curve (Figure 5), is widely used, but data processing must be done with caution due to the presence of voltage platforms (i.e., dV = 0). The oxidation and reduction peaks in the curve correspond to the charging and discharging platforms in the charge-discharge curve and are also related to the oxidation and reduction peaks in the cyclic voltammetry (CV) curve. Based on the peak positions in this curve, the redox reactions can be confirmed and judged, as referenced in the literature.
Additionally, the movement and attenuation of the peak positions have certain comparative value. For example, if the peak position shifts, it indicates a shift in the charging and discharging platform potential near that voltage, which is related to the difficulty of lithium embedding and de-embedding caused by structural changes in the material. Changes in the intensity of a specific peak can represent variations in the length of the corresponding charge-discharge platform.
The differential voltage curve of the discharge process of silicon-carbon composite material/metal lithium sheet half-cell.
The differential voltage curve, abbreviated as dV curve, can be used to assign peak positions based on literature or experimental comparison, and the capacity performance of different materials or platforms can be preliminarily judged according to the x-coordinate of the peak positions. This curve is relatively easy to process and is commonly used in the analysis of composite material electrode sheets.
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