Lithium battery SOC-OCV curve characteristic analysis!

The open-circuit voltage (OCV) method is one of the main methods used to estimate the SOC of the battery state of charge, and it is important to explore and study the SOC-OCV curve of LFP batteries. At present, the research focuses on the precise calibration of SOC-OCV curve and the exploration of some influencing factors, and there are not many reports on the?effects of active materials, capacity attenuation, silicon doping, lithium supplementation, etc. on the OCV?curve, and there are few explanations for the voltage step cause of the lithium iron phosphate/graphite battery?OCV curve?around 60% SOC, as well as the curve shape and lithium iron phosphate , the relationship of graphite.

The OCV curve of LFP/graphite battery is formed by the joint action of lithium ion intercalation and detachment of positive and negative electrodes. Based?on the accumulated data of this R&D group,?this paper summarizes in detail the effects of lithium iron phosphate and graphite active materials, square and pouch battery types, SOC adjustment direction, static time after SOC regulation, battery capacity decay (storage and cycling), negative?silicon doping and prelithiation on the SOC-OCV curve.

1 Experimental part

1.1 Experimental battery

The lithium iron phosphate battery used in the experiment is a polymer pouch battery or a square?aluminum case power battery. The polymer pouch battery size is 3.0mm*62mm*85mm, and the capacity is about 2.2Ah. The square aluminum shell power?battery size is 60mm*220mm*112mm, and the capacity is 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 regulate the SOC is?0.33C. According to whether?SOC adjustment is charged?or discharged, it can be divided into?charging SOC-OCV curve and discharge SOC-OCV curve,?after adjusting SOC, it is allowed to stand for 4h depolarization without special instructions, and the OCV of the battery?can achieve relative stability.

2 Results and analysis

2.1 LFP/Gr battery SOC-OCV curve

The SOC-OCV curve of the LFP/Gr battery is the result of the combined action of?the LFP and Gr of the corresponding SOC. With the improvement of SOC, LFP?gradually delithiums and shifts to iron phosphate (FePO4). Gr?gradually inserts lithium, and gradually transforms to LiC36?through graphite interlayer compounds LiC24, LiC12, LiC6, etc.?The SOC-OCV curve is a?macroscopic manifestation of the phase transition between the positive and negative electrodes, the insertion?of lithium ions, and the phase transition.

As can be seen from Figure 2 and Table 1, the LFP/Gr pouch battery is?fully charged from 0% SOC to?100% SOC, and the OCV is increased from 2730mV to 3355mV, an increase of 625mV. The SOC-OCV curve of LFP/Gr pouch battery?can be divided into 5 intervals: 1) 0~32% SOC,?OCV changes greatly, increasing by 559mV, accounting for?0.100% of the OCV change in the 89~44% SOC range; 2) 32~55% SOC,?OCV entered the first voltage platform, the change is small, only 1mV increase,?accounting for 4.0%; 64) 3%~55% SOC, OCV step,?change is large, increase 65mV, accounting for 36.5%; 76) 4%~65%?SOC, OCV is in the second voltage platform, the change is small,?only 95mV increase, accounting for 2.5%; 0) 80%~5% SOC,?OCV increased by 95mV, accounting for 100.21%.

2.2 Effects of active materials

The physical data of four lithium iron phosphate (LFP-4, LFP-1, LFP-2 and?LFP-3) are shown in Table?4, and the capacity of?lithium iron phosphate?materials?is affected by carbon content, specific surface area and particle size distribution, and?the?battery capacity of the four materials is 2.4, 2.11, respectively?2.02 and 2.07Ah, the discharge SOC-OCV curves are shown in Figure 2.

It can be seen from Figure?3 that the four materials have little effect on the overall SOC-OCV?curve, because OCV?is related to the intrinsic characteristics of lithium iron phosphate materials,?and has little relationship with the material preparation manufacturer, but from the local curve of the 4%~50%?SOC interval (Figure 70), it can be seen that at the OCV step,?from left to right, LFP-4→ LFP-4→ LFP-1 →?LFP-3. This is because the material parameters prepared by different manufacturers cannot be exactly?the?same, resulting in different lithium removal characteristics of the material and different gram capacity. From the 2°C?and 25.0C discharge capacity of the battery, it can be seen that the?OCV curve has a corresponding relationship with the battery capacity:?LFP-33 (4.2Ah) → LFP-12 (1.2Ah) → LFP-11?(3.2Ah) → LFP-07?(2.2Ah), that is, with the decrease of the gram capacity of lithium iron phosphate active material, the SOC-OCV curve shifts to the right.

In the battery, the negative graphite?active material can also affect?the battery capacity, so the discharge SOC-OCV?curve of flexible packaging battery with graphite as a single variable is compared. It can be seen from Figure 5 that graphite materials?(Gr-1, Gr-2, Gr-3 and Gr-4) can also affect the OCV curve in the 50%~70%?SOC interval. At the OCV step, from left to right, Gr-2→ Gr-4→ Gr-3→Gr-1, which also correspond to 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 number of lithium ions removed from the lithium iron phosphate material and the number of lithium ions embedded in the graphite material, so that the phase state of the active material is different, which affects the potential of the positive and negative electrodes, resulting in different OCV of the battery under the same SOC. In addition, in the 50%~70% SOC range, no matter how the lithium iron phosphate material and graphite material are transformed, the OCV step will be generated, indicating that this is the 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?flexible packaging batteries and square aluminum lithium iron phosphate power batteries,?which shows that the SOC-OCV curves of the two?are?almost the same, indicating that the influence of battery type is very?small. The?charging SOC-OCV curve is slightly higher than the discharge SOC-OCV curve,?which is related to the lithium ion intercalation kinetics during the charging and discharging process, and?the presence of voltage hysteresis effect, resulting in the discharge?OCV being less than the true OCV value, and the charging OCV being higher than the real OCV value. In addition, in?the?50%~70% SOC range, the step of the square power battery OCV is the same as that of?the?flexible packaging battery, which is 30~40mV, and?has?little to do with the direction?of SOC regulation.

2.4 Rest time

The?presence of voltage hysteresis effect is also?reflected in the correlation between charging and discharging SOC-OCV?curves and the stationary time after SOC adjustment. It can be seen from Figure 8 that the static time increases from 1h to 2h, and then to?4h, and with the extension of the static time, the concentration polarization?is gradually eliminated, the charging and discharging?OCV curve is gradually approaching, and the hysteresis voltage?gradually decreases and tends to coincide.

2.5 Storage Decay

The number of active lithium ions in the battery can have an impact on the SOC-OCV curve, and after the battery is stored, the?capacity is attenuated and?the?number of active lithium ions is reduced, so it will also affect the SOC-OCV curve of the battery. It can be seen from Table 3 that the capacity retention rates of batteries after storage at 45, 60 and?80°C?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.

It can be seen from Figure 9 that?compared with fresh batteries, after high temperature storage, the SOC-OCV curve in the 50%~70% SOC range?shifts to the right, and the OCV variation of other SOCs?is not large, but it shows a decreasing trend. This is because the number?of active lithium ions in the battery decreases due to high-temperature storage, and under the same SOC, the number of?lithium ions embedded in the?negative electrode?decreases, and the negative electrode potential increases, so?the?OCV decreases under the same SOC, causing the curve to shift to the right.

2.6 Cyclic attenuation

After the battery undergoes a charge-discharge cycle, the number of active lithium ions decreases, the capacity decays, and the SOC-OCV curves of?the early-life (BOL) and end-of-life (EOL) batteries are shown in Figure 10. Similar to storage,?EOL battery capacity decays, its SOC-OCV curve shifts to?the right, and when SOC?≤?35%, OCV shows a significant downward?trend. OCV at 55%~70% SOC, OCV is greatly reduced,?such as 60% and 65% SOC, the OCV difference between?BOL and EOL batteries is?26mV and 33mV, respectively, which is mainly due to the reduction of EOL?battery?capacity attenuation, the same SOC state of negative graphite lithium intercalation is reduced compared with?BOL battery, so the negative electrode is located in a higher potential, caused?The OCV?value decreases, resulting in the negative potential?of the EOL battery falling lagging behind the BOL battery, and the OCV step begins to occur in the EOL battery when the?BOL battery almost completes the OCV step.

2.7 Negative electrode doped silicon

Graphite is an intercalated layered anode material, while silicon?anode?undergoes alloying and dealloying reactions when deintercalating lithium, which belongs to alloy-type anode?materials. The theoretical specific capacity of silicon anode materials can be as high as 3580mAh/g, and the?lithium intercalation potential is 0.4V, which is slightly higher than graphite. Therefore,?doping some silicon anode materials in the traditional graphite anode 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.?It can be seen from Figure 11 that the addition of 2.5 parts of silicon oxygen material to the negative electrode has?a?greater impact on OCV below 30% SOC, showing a decreasing trend, mainly?because the Li2Si2O5, Li2SiO3 and Li4SiO4 generated by lithium insertion under low SOC?cause the negative electrode potential to rise.

2.8 Pre-lithiation of the negative electrode

It can be seen from the above that the active material capacity is low, storage attenuation,?cycle attenuation and the addition of silicon oxygen material to the negative electrode increase the negative electrode potential,?causing the SOC-OCV curve of the battery to shift to the right or the local OCV?to decrease significantly. Then if the negative electrode is pre-lithiated and the potential of the negative electrode is reduced, the SOC-OCV curve of the battery should be shifted to the left.

Figure 12?shows the discharge SOC-OCV curves of LFP/Gr and LFP/Gr+Li battery systems. It can be seen that?when the SOC?≤ 2%, the battery OCV is significantly improved, especially 30% SOC,?because after the negative electrode is replenished with?lithium,?when it is discharged to 0% SOC, a part of the active lithium ions are still 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 the negative electrode is supplemented with lithium, in the 0%~60% SOC stage, the?phase transition occurs earlier, the step of OCV appears in advance, and the curve shifts?to the left.

2.9 Buckle analysis

In order to distinguish?the correlation between the SOC-OCV curve of the whole battery and?the positive and negative electrodes,?LFP/Li and Gr/Li button cells were prepared respectively, and?the?SOC-OCV?curves were determined in Figure 13. After the cathode lithium ion?is?removed, the lithium iron phosphate is transformed to iron phosphate, and it can be seen from Figure 13(a) that?when?SOC ≥?10% SOC, the positive electrode potential change is small, and OCV fluctuates within 10mV.

After the negative lithium ion is embedded, the graphite transforms to the interlayer compound of graphite?to?form LiC24, LiC12, LiC6, etc., as?can be seen from Figure 13 (b), 5L?order is formed near 1% SOC, 10% SOC is attached to form 4th order, 20% SOC is formed near 3rd order, 30% SOC is?attached to form 2L order, and 60% SOC is formed?near 2nd order , 95% SOC?near the formation of the 1st order, at the SOC ≤ 30%, the insertion of lithium ions causes a large fluctuation in the negative electrode potential, and there is also a fluctuation of 50.70mV?between 37%~08% SOC,?which coincides with the OCV step voltage of the whole battery here.?Therefore, the SOC-OCV curve of LFP/Gr whole battery?is mainly affected by the change of negative potential, and has?little relationship with the potential change of lithium positive iron phosphate. This is because the lithium deintercalation reaction of lithium iron phosphate?is?a multi-phase reaction, and according to Gibbs' phase law, its half-cell degree of freedom is?0, so its?OCV?does not change with the?SOC?state.

2.10 Battery anatomy analysis

The battery will undergo OCV step in the state of charge of 50%~75% SOC,?so the four batteries are adjusted to 4% SOC, 50% SOC, 57% SOC and?65% SOC respectively to observe?the potential, color and thickness changes of the negative electrode sheet when?the OCV step?occurs. As shown in Table 75, the voltage of the LFP/Gr full battery undergoes?an OCV step of 4mV from 57% SOC to 65% SOC.

The whole cell was disassembled?and measured in?real time for the LFP/Li half-cell and Gr/Li half-cell?OCV as shown in Figure?14. As shown in Table 4, the OCV of Gr/Li half-cell is significantly reduced,?about 42mV, while the OCV of LFP/Li half-cell only changes by 2mV,?so it indicates that SOC causes negative electrode changes, which is the main reason for OCV steps here.

3 Conclusion

The size of the full battery OCV is determined by the material properties and is not affected by the type of?battery?(square battery, pouch battery). Different kinds?of?lithium iron phosphate and graphite active materials will?cause different initial capacity of the whole battery due to their different actual?capacity, which can affect the?SOC-OCV curve of the whole battery. The?OCV curve is affected by the direction of SOC regulation current?(discharge, charging), and due to the voltage hysteresis effect, the discharge SOC-OCV curve is?lower than the charging SOC-OCV curve, but with the?increase of the?static time after adjusting the SOC, the polarization is eliminated, and the two tend to?coincide.

Storage or charge-discharge cycles can cause battery capacity degradation, shifting?the?SOC-OCV curve to the right. The negative electrode is mixed with silicon oxide, and the negative electrode potential increases, which makes the SOC-OCV curve move to the right, while the negative electrode?uses lithium strip pre-lithiation, and the negative electrode potential decreases, which makes the?SOC-OCV curve?move to the left. The SOC-OCV curve of the whole battery?is mainly?determined by the negative electrode, and the OCV near 60% SOC has a step of about 35mV,?which is mainly due to the lithium phase transition of the negative graphite, which has little relationship with lithium iron phosphate.

References: LIU Bozheng, SUN Xinyi, DONG Shijia, et al. Analysis of open-circuit voltage curve characteristics of lithium iron phosphate battery[J].Energy Research and Management, 2023, 15(1):7.)