Characteristic analysis of lithium battery SOC-OCV curve!
Aug,12,24
The open circuit voltage (OCV) method is one of the main methods used to estimate the state of charge (SOC) of batteries,
and exploring and studying the SOC-OCV curve of LFP batteries is very important.
At present, research focuses on the precise calibration of SOC-OCV curves and the exploration of some influencing factors.
There are few reports on the effects of active materials, capacity decay, silicon doping, lithium supplementation, etc. on OCV curves,
and there is little explanation for the voltage step of OCV curves near 60% SOC in lithium iron phosphate/graphite batteries,
as well as the relationship between curve shape and lithium iron phosphate and graphite.
The OCV curve of LFP/graphite batteries is formed by the combined action of positive and negative lithium ion insertion and extraction.
Based on the data accumulated by our research and development team,
this article provides a detailed summary of the effects of lithium iron phosphate and graphite active materials, square and soft pack battery types,
SOC adjustment direction, SOC adjustment settling time, battery capacity degradation (storage and cycling),
negative electrode silicon doping, and pre lithiation on the SOC-OCV curve.
1 Experimental section
1.1 Experimental Battery
The lithium iron phosphate battery used in the experiment is a polymer soft pack battery or a square aluminum shell power battery.
The size of the polymer soft pack battery is 3.0mm * 62mm * 85mm, with a capacity of approximately 2.2Ah.
The size of the square aluminum shell power battery is 60mm * 220mm * 112mm, with a capacity of 172Ah.
1.2 Performance Testing
The SOC-OCV curve measurement method for LFP/Gr batteries is shown in Figure 1, and the current used to adjust SOC is 0.33C.
According to whether SOC adjustment is based on charging or discharging, it can be divided into charging SOC-OCV curve and discharging SOC-OCV curve.
After adjusting SOC, if there are no special instructions, let it stand for 4 hours to depolarize, and the OCV of the battery can reach relative stability.
2 Results and Analysis
2.1 SOC-OCV curve of LFP/Gr battery
The SOC-OCV curve of LFP/Gr battery is the result of the combined action of LFP and Gr corresponding to SOC.
With the increase of SOC, LFP gradually delithiates and transforms into iron phosphate (FePO4) phase;
And Gr gradually intercalates lithium, and through the interlayer compounds LiC36, LiC24, LiC12, etc., it gradually transforms into LiC6 phase.
The SOC-OCV curve is a macroscopic manifestation of the phase transition between the positive and negative electrodes during the extraction and insertion of lithium ions.
From Figure 2 and Table 1, it can be seen that the OCV of LFP/Gr soft pack batteries increases from 2730mV to 3355mV,
an increase of 625mV, from 0% SOC empty charge to 100% SOC full charge.
The SOC-OCV curve of LFP/Gr soft pack batteries can be divided into five intervals: 1) 0-32% SOC,
with a large change in OCV, increasing by 559mV, accounting for 89.44% of the OCV change in the 0-100% SOC interval; 2) 32-55% SOC,
OCV enters the first voltage platform with small changes, only increasing by 4mV, accounting for 0.64%; 3) 55% to 65% SOC,
OCV undergoes a step change, with a significant increase of 36mV, accounting for 5.76%; 4) 65% to 95% SOC, OCV is in the second voltage platform,
with small changes, only increasing by 5mV, accounting for 0.80%; 5) 95% to 100% SOC, OCV increased by 21mV, accounting for 3.36%.
2.2 Effects of Active Materials
The physical property data of four types of lithium iron phosphate (LFP-1, LFP-2, LFP3, and LFP-4) are shown in Table 2.
The capacity of lithium iron phosphate materials is affected by carbon content, specific surface area, and particle size distribution.
The battery capacities of the four materials are 2.11, 2.02, 2.07, and 2.12 Ah, respectively. The discharge SOC-OCV curves are shown in Figure 3.
From Figure 3, it can be seen that the four materials have little effect on the overall SOC-OCV curve.
This is because OCV is related to the intrinsic properties of lithium iron phosphate materials
and has little relationship with the material preparation manufacturer.
However, from the local curve in the 50% to 70% SOC range (Figure 4),
it can be seen that at the OCV step, from left to right, the order is LFP-4 → LFP-1 → LFP3 → LFP-2.
This is because the material parameters prepared by different manufacturers cannot be exactly the same,
resulting in different lithium insertion and extraction characteristics of the materials,
and differences in the performance of gram capacity.
From the discharge capacity of the battery at 25 ℃ and 0.33C,
it can be seen that there is a corresponding relationship between the OCV curve and the battery capacity:
LFP-4 (2.12Ah) → LFP-1 (2.11Ah) → LFP3 (2.07Ah) → LFP-2 (2.02Ah),
that is, as the capacity of the lithium iron phosphate active material decreases, the SOC-OCV curve shifts to the right.
In batteries, negative graphite active materials can also affect battery capacity,
so the discharge SOC-OCV curves of flexible packaging batteries with graphite as a single variable were compared.
As shown in Figure 5, graphite materials (Gr-1, Gr-2, Gr-3, and Gr-4) can also affect the OCV curve in the 50% to 70% SOC range.
At the OCV step, from left to right, the sequence is Gr-2 → Gr-4 → Gr-3 → Gr-1,
which also corresponds to the battery capacity (Figure 6): Gr-2 (2.21Ah) → Gr-4 (2.20Ah) → Gr-3 (2.19Ah) → Gr-1 (2.11Ah).
The size of the battery capacity reflects the amount of lithium ions released from lithium iron phosphate material
and the amount of lithium ions embedded in graphite material,
resulting in differences in the phase state of the active material and affecting the potential of the positive and negative electrodes,
leading to different OCVs of the battery at the same SOC.
In addition, within the 50% to 70% SOC range, regardless of how the lithium iron phosphate material or graphite material is transformed,
a step change in OCV will occur, indicating that this is an intrinsic characteristic of the LFP/Gr battery system.
2.3 Battery type and SOC adjustment direction
Figure 7 shows the charging and discharging SOC-OCV curves of the flexible packaging battery and the square aluminum shell lithium iron phosphate power battery.
It can be seen that the SOC-OCV curves of the two are almost the same, indicating that the influence of battery type is minimal.
The charging SOC-OCV curve is slightly higher than the discharging SOC-OCV curve,
which is related to the lithium-ion deintercalation kinetics during the charging and discharging process.
The presence of voltage hysteresis effect leads to a discharge OCV lower than the true OCV value and a charging OCV higher than the true OCV value.
In addition, within the range of 50% to 70% SOC, the step of OCV for square power batteries is consistent with that of flexible packaging batteries,
both ranging from 30 to 40mV, and is not significantly related to the direction of SOC regulation.
Figure 7 Charging and discharging SOC-OCV curves
2.4 Rest time
The existence of voltage hysteresis effect is also reflected in the correlation between charging
and discharging SOC-OCV curves and the settling time after adjusting SOC.
From Figure 8, it can be seen that the settling time increases from 1 hour to 2 hours, and then to 4 hours.
As the settling time prolongs, the concentration polarization gradually disappears, the charge discharge OCV curve gradually approaches,
the hysteresis voltage gradually decreases, and tends to coincide.
2.5 Storage attenuation
The amount of active lithium ions in the battery can have an impact on the SOC-OCV curve.
After storage, the capacity of the battery decreases and the amount of active lithium ions decreases, which also affects the SOC-OCV curve of the battery.
From Table 3, it can be seen that the capacity retention rates of the batteries stored at 45, 60, and 80 ℃ are 98.9%, 96.4%, and 91.7%, respectively,
corresponding to the OCV size of 60% SOC. That is, the higher the capacity retention rate, the larger the OCV of 60% SOC.
From Figure 9, it can be seen that compared to fresh batteries, after high-temperature storage,
the SOC-OCV curve in the 50% to 70% SOC range shifts to the right,
while the OCV of other SOC changes little but shows a decreasing trend.
This is because high-temperature storage causes a decrease in the number of active lithium ions in the battery.
At the same SOC, the number of lithium ions embedded in the negative electrode decreases, and the negative electrode potential increases.
Therefore, at the same SOC, the OCV decreases, causing the curve to shift to the right.
2.6 Cyclic attenuation
After the battery undergoes charge and discharge cycles, the number of active lithium ions decreases and the capacity decays.
The SOC-OCV curves of the battery at the beginning of life (BOL) and end of life (EOL) are shown in Figure 10.
Similar to storage, the capacity of EOL batteries decreases, and their SOC-OCV curve shifts to the right.
When SOC ≤ 35%, OCV shows a significant decreasing trend. When the OCV is between 55% and 70% SOC, there is a significant decrease in OCV.
For example, at 60% and 65% SOC, the OCV of BOL and EOL batteries differ by 26mV and 33mV, respectively.
This is mainly due to the reduced capacity decay of EOL batteries.
Compared to BOL batteries, the negative electrode graphite lithium insertion is reduced at the same SOC state,
resulting in a higher potential at the negative electrode and a decrease in OCV value.
As a result, the decrease in negative electrode potential of EOL batteries lags behind that of BOL batteries.
When BOL batteries almost complete the OCV step, EOL batteries begin to undergo OCV step.
2.7 Silicon doped negative electrode
Graphite is an intercalated layered negative electrode material,
while silicon negative electrodes undergo alloying and dealloying reactions during lithium deintercalation,
belonging to the alloy type negative electrode material;
The theoretical specific capacity of silicon negative electrode material can reach up to 3580mAh/g,
with a lithium extraction potential of 0.4V, slightly higher than graphite.
Therefore, doping some silicon negative electrode materials into traditional graphite negative electrodes may have an impact on the SOC-OCV curve of the battery.
Figure 11 shows the discharge SOC-OCV curves of LFP/Gr and LFP/Gr+SiO2 battery systems.
From Figure 11, it can be seen that the addition of 2.5 parts of silicon oxide material to the negative electrode has a significant impact on
the OCV below 30% SOC, showing a decreasing trend.
This is mainly due to the increase in negative electrode potential caused by the lithium insertion of Li2Si2O5, Li2SiO3, and Li4SiO4 at low SOC.
2.8 Negative electrode pre lithiation
From the above, it can be seen that the low capacity of the active material, storage attenuation, cycling attenuation,
and the addition of silicon oxide materials to the negative electrode cause an increase in the negative electrode potential,
resulting in a rightward shift in the SOC-OCV curve of the battery or a significant decrease in local OCV.
So if the negative electrode is pre lithiated and the potential of the negative electrode is reduced, the SOC-OCV curve of the battery should shift to the left.
Figure 12 shows the discharge SOC-OCV curves of two battery systems, LFP/Gr and LFP/Gr+Li.
It can be seen that when SOC ≤ 30%, the OCV of the battery is significantly improved, especially at 0% SOC.
This is because after the negative electrode is recharged with lithium,
when discharged to 0% SOC, there are still some active lithium ions stored in the negative electrode,
and the potential of the negative electrode is relatively low, so the OCV of the battery is higher.
In addition, after lithium supplementation to the negative electrode, phase transition occurs earlier in the 60% to 75% SOC stage,
and the step of OCV appears earlier, causing the curve to shift to the left.
2.9 Power outage analysis
In order to distinguish the correlation between the SOC-OCV curve of the entire battery and the positive and negative electrodes,
LFP/Li and Gr/Li button batteries were prepared separately, and their SOC-OCV curves were measured as shown in Figure 13.
After the release of positive lithium ions, lithium iron phosphate transforms into iron phosphate.
As shown in Figure 13 (a), when SOC ≥ 10% SOC, the positive electrode potential changes little, and the OCV fluctuates within 10mV.
After the insertion of negative lithium ions, graphite transforms into graphite interlayer compounds, forming LiC24, LiC12, LiC6, etc.
As shown in Figure 13 (b), a 1L order is formed near 5% SOC, a 4th order is formed near 10% SOC, a 3rd order is formed near 20% SOC,
a 2L order is formed near 30% SOC, a 2nd order is formed near 60% SOC, and a 1st order is formed near 95% SOC.
When SOC ≤ 30%, the insertion of lithium ions causes a significant fluctuation in the negative electrode potential,
with a fluctuation of 37.08 mV between 50% and 70% SOC, which is consistent with the OCV step voltage of the entire battery at this point.
Therefore, the SOC-OCV curve of LFP/Gr full battery is mainly affected by the change in negative electrode potential,
and has a relatively small relationship with the potential change of positive electrode lithium iron phosphate.
This is because the lithium deintercalation reaction of lithium iron phosphate is a multiphase reaction, and according to the Gibbs phase law,
its half cell degree of freedom is 0, so its OCV does not change with the SOC state.
2.10 Analysis of battery anatomy
When the battery is charged at 50% to 75% SOC, an OCV step will occur.
Therefore, four batteries were adjusted to 50% SOC, 57% SOC, 65% SOC, and 75% SOC respectively to observe the changes in the potential, color,
and thickness of the negative electrode during the OCV step.
As shown in Table 4, the voltage of the LFP/Gr full battery undergoes a 30mV OCV step from 57% SOC to 65% SOC.
Disassemble the entire battery, as shown in Figure 14, and measure the OCV of LFP/Li half cell and Gr/Li half cell in real-time.
As shown in Table 4, the OCV of the Gr/Li half cell significantly decreased by about 42mV,
while the OCV of the LFP/Li half cell only changed by 2mV,
indicating that SOC caused a negative electrode change and was the main reason for the step change in OCV at this location.
Figure 15 shows the thickness and color changes of 50% to 75% SOC negative electrode sheets.
As shown in Figure 15, with the increase of SOC, the color of the polarizer changes from black purple to dark purple,
then dark yellow to golden yellow, and the thickness of the polarizer slightly increases.
After the OCV step change of the entire battery from 57% SOC to 65% SOC, except for the edge of the negative electrode,
most of the electrode colors changed from dark purple to dark yellow, with a thickness change of only 2um,
indicating that the transformation of graphite interlayer compounds has little effect on the graphite interlayer spacing at this time.
3 Conclusion
The size of the full battery OCV is determined by the material properties and is not affected by the battery type (square battery, soft pack battery).
Different types of lithium iron phosphate and graphite active materials, due to their varying actual capacity,
can cause differences in the initial capacity of the entire battery,
which can affect the SOC-OCV curve of the entire battery. The OCV curve is affected by the direction of SOC regulation current (discharge, charge).
Due to voltage hysteresis effect, the discharge SOC-OCV curve is lower than the charge SOC-OCV curve.
However, as the settling time increases after SOC regulation, polarization is eliminated and the two tend to overlap.
Storage or charge discharge cycles can cause a decrease in battery capacity, leading to a rightward shift in the SOC-OCV curve.
Mixing silicon oxide material into the negative electrode increases the negative electrode potential, causing the SOC-OCV curve to shift to the right,
while pre lithiation with lithium strips reduces the negative electrode potential, causing the SOC-OCV curve to shift to the left.
The SOC-OCV curve of the entire battery is mainly determined by the negative electrode, with a step of about 35mV occurring in the OCV near 60% SOC,
mainly due to the phase transition of graphite intercalation in the negative electrode, and has a relatively small relationship with lithium iron phosphate.
