Lithium Battery Staircase Charging Method and Cycle Degradation Mechanism
Currently, the primary charging system used for applications and testing of lithium-ion batteries is the constant current-constant voltage (CC-CV) charging method.
This charging method is simple and convenient to operate.
However, as the demand for fast charging of lithium-ion batteries continues to rise, the limitations of this method become more evident. Especially in the case of high current constant current-constant voltage charging, it can directly impact the battery’s lifespan.
Even after a certain period of use, the potential risks associated with high current constant current-constant voltage charging increase.
There are other representative charging systems such as the Multi-Segment Constant Current (MSCC) and Pulse Charging (PC) methods.
Multi-Segment Charging can be understood as breaking the charging process into several segments of CC-CV charging, and the choice of segments depends on the fundamental charging characteristics of the battery.
Pulse Charging, on the other hand, involves periodic variations in charging current in terms of magnitude and direction. This charging method is relatively complex to operate and demands high precision in equipment response.
The charging process of lithium-ion batteries involves complex phase transformations of positive and negative electrode materials, interfacial electrochemical reactions, polarization effects, and irreversible reactions.
From the battery’s CC-CV charging voltage-capacity curve, it can also be observed that during the constant current charging phase, the battery capacity does not linearly increase with the charging voltage but shows significant variations at different states of charge (SOC). This is determined by the positive and negative electrode materials and battery design.
This article, based on the charging characteristics of lithium-ion batteries and taking into account the characteristics of material phase transitions, establishes a multi-segment charging system to improve the battery’s charging efficiency while ensuring battery cycle life.
1. Experiment
1.1 Experimental Content
A square cell (NCM811/graphite system, designed capacity of 64.0Ah, voltage range 2.8-4.2V) was used to determine the multi-step charging regime, validate the multi-step charging cycle, and analyze the degradation mechanism. With the aim of ensuring the cycle life of the cell, a fast charging strategy was configured for the square cell, allowing for a 30-minute charge to reach 80% SOC.
1.2 Analysis and Testing
1.2.1 Rate Charging Performance
The cell’s rate charging performance was tested at different rates (0.2C, 0.8C, 1.0C, 1.2C, 1.6C, 2C). Rate charging performance testing was conducted using the BT-2000 equipment (5V, 200A) from Arbin Inc. in the United States.
1.2.2 Multi-Step Fast Charging Cycle
The cell underwent performance testing in a multi-step fast charging regime. Fast charging cycle testing was conducted using the BT-2000 equipment (5V, 200A) from Arbin Inc. in the United States. Capacity calibration at 0.2C was performed after every 100 fast charging cycles.
1.2.3 Charging DC Internal Resistance
Charging DC Internal Resistance (DCIR) testing was performed on the cell before, during, and after the fast charging cycles.
The charging DCIR testing was carried out from 0% to 75% SOC, with data points taken at every 5% SOC. The DCIR testing for lithium-ion batteries was completed using the BT-2000 equipment (5V, 200A) from Arbin Inc. in the United States.
2. Results and Discussion
2.1 Multi-Step Fast Charging Regime
2.1.1 Charging Rate Performance
The charging rate performance data for the cell are presented in Table 1.
Charging the cell at 2C resulted in a constant current capacity ratio of 80.92%.
Charging at 1.6C achieved a constant current capacity ratio of 82.98%. This indicates excellent charging rate performance for the cell, and using the 1.6C constant current constant voltage charging regime can meet the goal of reaching 80% SOC in 30 minutes. The instantaneous voltage for the 80% SOC at 1.6C charging is 4.168V, which is very close to the cell’s charging cut-off voltage.
The constant current capacity ratios for the cell at 1.2C, 1.0C, and 0.8C were 85.55%, 87.47%, and 90.73%, respectively.
Table 1 Lithium-Ion Battery Charging Rate Performance
Charging Rate | Total Charging Time/min | Total Charging Capacity/Ah | Constant Current Time/min | Constant Voltage Time/min | Charging to 80% SOC Time/min | Charging to 80% SOC Voltage/V | Charging to 80% SOC Current/A | Constant Current Capacity/Ah | Constant Voltage Capacity/Ah | Constant Current Capacity Ratio/% |
2.0 C | 47.11 | 64.68 | 26.17 | 20.94 | 26.00 | 4.194 | 119.99 | 52.34 | 12.34 | 80.92 |
1.6 C | 54.60 | 64.69 | 33.56 | 21.05 | 32.50 | 4.168 | 95.99 | 53.69 | 11.01 | 82.98 |
1.2 C | 67.32 | 64.55 | 46.03 | 21.29 | 43.17 | 4.166 | 71.99 | 55.23 | 9.33 | 85.55 |
1.0 C | 77.83 | 64.88 | 56.76 | 21.07 | 50.00 | 4.116 | 59.99 | 56.76 | 8.13 | 87.47 |
0.8 C | 91.93 | 64.84 | 73.53 | 18.40 | 65.00 | 4.090 | 47.99 | 58.83 | 6.01 | 90.73 |
2.1.2 Determination of Staircase Charging Scheme
The dV/dQ curves for different charging rates of the cell are shown in Figure 1. The characteristic peaks in the dV/dQ curves mainly reflect the phase transitions of the positive and negative electrode active materials during lithium extraction and insertion processes.
Taking the dV/dQ curve for 0.2C charging as an example:
- Characteristic Peak 1 (5% SOC) primarily reflects the initial phase transition of the overall positive and negative electrode materials.
- Characteristic Peak 2 (15% SOC) corresponds to the phase transition of the negative electrode material.
- Characteristic Peak 3 (20% SOC) corresponds to the phase transition of the positive electrode material.
- Characteristic Peak 4 (55% SOC) is mainly composed of phase transitions in both the positive and negative electrode materials, with a predominant contribution from the negative electrode material.
- Characteristic Peak 5 (80% SOC) mainly reflects the phase transition of the positive electrode material.
- Characteristic Peak 6 (98% SOC) is determined by both the positive and negative electrode materials.
As the charging rate increases, the phase transitions of the positive and negative electrode materials occur earlier, resulting in the coexistence of multiple phases.
This is evident in the leftward shift or even disappearance of some phase transition peaks.
Figure 1: Battery Charging dV/dQ vs. SOC Curve at Different Charging Rates
When the charging rate reaches 1.6C：
Compared to 1.2C, there is no change in Feature Peak 1 (5% SOC). Feature Peak 2 (15% SOC), which reflects the phase transition of the negative electrode material, disappears, as does Feature Peak 4. Although Feature Peak 5 still exists, it is very close to Feature Peak 6, which shifts significantly to the left (82% SOC) at 1.6C charging rate.
This indicates that at a charging rate of 1.6C, the phase transition reactions of the positive and negative electrode materials at low SOC (≤55%) cannot be distinguished, and the absolute value of dV/dQ in this range is lower than at high SOC (>55%). Phase transition reactions of the positive and negative electrode materials at high SOC (>55%) cannot be distinguished either, and there is a significant change in the phase transition reactions as SOC approaches 100%.
When the charging rate reaches 2.0C：
Compared to 1.6C, Feature Peak 5 disappears, and Feature Peak 6 (98% SOC) shifts significantly to the left (81% SOC). This indicates that at a charging rate of 2.0C, the phase transition reactions of the positive and negative electrode materials cannot be distinguished below 81% SOC.
In summary, to achieve the fast charging goal of 30 minutes for 80% SOC while avoiding potential lithium plating risks, the charging steps are initially determined based on the SOC at which clear phase transition peaks occur, such as 5% SOC, 55% SOC, and 80% SOC.
Charging rates are selected based on the absolute values of dV/dQ, with higher charging rates chosen for intervals with smaller absolute values of dV/dQ, and lower charging rates for intervals with larger absolute values of dV/dQ.
For example, when SOC is less than 55%, a charging rate of 2C can be selected, and when SOC is greater than 55%, the charging rate should not exceed 1.6C.
The charging DCIR (Direct Current Internal Resistance) curve of the cells is shown in Figure 2. The cells (7# and 8#) exhibit relatively high charging DCIR when at low SOC, especially at SOC 0, where the charging DCIR is 2.68mΩ.
In the range of 0% to 5% SOC, the charging DCIR is ≥1.77mΩ.
As the SOC of the cells increases, their charging DCIR rapidly decreases.
From the perspective of charging DCIR, employing lower charging rates during the 0-5% SOC interval can effectively reduce energy losses.
Figure 2: Charging DCIR Curve of the Cells
Based on the combined Charging dV/dQ-SOC curve and Charging DCIR curve, the cell’s step charging regimen is determined, as shown in Figure 3.
This step charging regimen consists of 7 steps. The first five steps are combined for a total of 30 minutes, accumulating 80% of the charging capacity. The remaining 20% of the charging capacity is charged at 0.5C to the cut-off voltage.
Figure 3: Cell Step Charging Regimen
Figure 4 shows the practical application results of this step-charging regimen. This step-charging regimen can achieve the fast charging goal of 80% SOC in 30 minutes. The time required to fully charge the cell is 61.73 minutes. The average charging rate is approximately 1.4C. The voltage when the cell reaches 80% SOC is 4.1V, and the instantaneous current is 0.93C, which is significantly lower than 1.6C.
Figure 4: SOC Curves of Battery Cell Voltage-to-Rate and SOC Curves of Charging Time for Step Charging
2.2 Step Charging Cycle Verification and Degradation Analysis
2.2.1 Step Charging Cycle Curves
The battery cell underwent cycling performance tests using step fast-charging and 1C constant current-constant voltage charging, with a discharge rate of 1C.
Figure 5 displays the cycle curves for step charging. Both methods completed 100% depth of discharge (DOD) from 2.8V to 4.2V.
Under the step charging scheme, the cell completed 800 cycles with a capacity retention rate of ≥91.99% at 1C discharge.
In comparison, the cell completed 800 cycles at 1C constant current/constant voltage charging, with a capacity retention rate of ≥94.06%.
The difference in capacity retention between the two methods at 1C discharge is 2.07%.
Figure 5: Battery Cell Step Charging Cycle Curves
2.2.2 Step Charging Cycle Degradation Analysis
The step charging curves for the battery cell at different cycle numbers are shown in Figure 6.
- In the 5th cycle of step charging, the fast charging goal of 30 minutes to reach 80% SOC is achieved.
- After 200 cycles of step charging, the fast charging capacity is reduced by 1.0%, reaching 79.0% SOC during a 30-minute charge. This loss is mainly attributed to the high-rate charging in the second step.
- After 400 cycles, the fast charging capacity drops to 78.7% SOC during a 30-minute charge, showing a 1.3% loss, with no significant difference in the step charging curve at cycle 400.
- After 800 cycles, the fast charging capacity experiences a 3.03% loss, reaching 76.97% SOC during a 30-minute charge.
In summary, the capacity loss of the battery cell occurs primarily during the 2C charging in the second step of the step charging cycle. Subsequent steps with lower charging rates compensate for the charging capacity.
Figure 6: Step Charging Curves at Different Cycle Numbers for the Battery Cell
The dV/dQ-SOC curves for 0.2C charging after different cycling modes are shown in Figure 7. After step charging cycles and 1.0C cycles, the dV/dQ-SOC curves are essentially the same. Moreover, the characteristic peak positions of the phase transitions of the positive and negative active reaction materials during lithium extraction and insertion in the dV/dQ curve have not significantly changed.
This indicates that there has been no significant structural change in the positive and negative electrode materials under the two cycling modes, and there has been no capacity degradation caused by material failure.
Figure 7: dV/dQ Charging Curves of the Cell after Different Cycling Modes
With an increase in the number of cycles, the relative absolute values of the dV/dQ curve slightly increase, mainly due to the rising internal resistance of the cell during DC charging.
The increase in direct current internal resistance is primarily a result of the thickening of the solid electrolyte interface (SEI) layers on the surfaces of both positive and negative electrode materials.
The thickening of the SEI and CEI layers is primarily attributed to the cumulative side reactions during charge and discharge processes, leading to a loss of active lithium ions in the system, which is reflected as a decrease in the cell’s discharge capacity. This capacity decay occurs within a normal range.
3. Conclusion
Based on the charging characteristics of a 60Ah lithium-ion battery with a ternary/graphite system, a stepped charging regimen was developed.
Under the stepped charging regimen, the battery achieved the fast-charging goal of 80% SOC in 30 minutes and completed 800 cycles with a capacity retention rate of ≥91.99%.
Further analysis revealed that the capacity decay during the stepped charging cycles was primarily due to the loss of active lithium ions, with no significant abnormalities observed in the positive and negative electrode materials.
The method for establishing this stepped charging regimen is simple, fast, and accurate. It ensures the cyclic performance of lithium-ion batteries while selectively improving the battery’s charging efficiency. This approach holds significant practical value for fast-charging applications of lithium-ion batteries.