Sodium-lithium hybrid battery and SOC calculation method thereof

Abstract

The invention discloses a sodium-lithium hybrid battery and an SOC calculation method thereof, the sodium-lithium hybrid battery is formed by connecting a plurality of lithium battery modules in series, and each battery module comprises a plurality of battery cells; wherein one battery module or a plurality of battery modules comprise at least one sodium battery cell. The hybrid battery has the advantages that the sodium battery is added into the lithium iron phosphate battery, so that the hybrid battery is realized, the SOC value of the hybrid battery can be calculated with high accuracy, and the SOC estimation error caused by a gentle voltage platform of the pure lithium iron phosphate battery is avoided.

Description

Technical Field

[0001] The present invention relates to the field of battery production and manufacturing and battery SOC estimation, and particularly relates to a sodium-lithium hybrid battery for improving the calculation accuracy of battery pack SOC and its SOC calculation method. Specifically, it relates to a method for calculating the SOC of a lithium iron phosphate battery based on the characteristics of a sodium battery, which suppresses the spread of thermal runaway through the high thermal stability of the sodium battery and improves the accuracy of lithium battery SOC estimation by using the slope characteristics of its charge-discharge curve. Background Art

[0002] For the SOC of a lithium iron phosphate battery, the ampere-hour integration method is usually used to calculate the SOC, and the formula is as follows

[0003]

[0004] -SOC 0 : Initial state of charge (e.g., 100% when fully charged);

[0005] -Q n : Rated capacity of the battery (unit: Ah);

[0006] -I(τ): Real-time charge-discharge current (positive for charging, negative for discharging, unit: A);

[0007] -τ: Time variable (unit: hour);

[0008] -η: Coulomb efficiency.

[0009] The traditional ampere-hour integration method depends on the accuracy of the current sensor, but there are limitations in actual applications. Sensor noise and zero drift will cause the accumulation of current integration errors and lead to a gradual increase in SOC errors. Therefore, it is necessary to correct the SOC through the battery voltage. The common methods are OCV correction and the equivalent battery model method, that is, establishing the relationship between voltage and SOC (such as a first-order model, a second-order model) to calculate the SOC. However, whether it is OCV or the voltage during charge and discharge, when the mapping relationship with SOC is in the 10%-90% SOC interval, it will show a significant flattening characteristic (i.e., "voltage plateau"), making it difficult to accurately calibrate through the voltage method, as shown in the mapping between SOC and discharge voltage Figure 1 shown.

[0010] The SOC and voltage mapping characteristics of the sodium battery are better, and it is easier to calculate the accurate SOC through the voltage, as shown in Figure 2 shown. The charge-discharge curve of the sodium battery has significant slope characteristics (such as when discharging, the voltage drops from 3.75V to 2.5V, and the slope dV / dQ≈0.05V / Ah). Its voltage change rate is much higher than the gentle voltage plateau of the lithium iron phosphate battery (in the 3.2 - 3.4V interval, ΔV / ΔSOC≈0.01V / 1%). This characteristic makes it easier to model the relationship between the SOC and voltage of the sodium battery.

[0011] Since sodium batteries have good SOC calculation characteristics and lithium iron phosphate batteries have relatively stable and high energy density, if the two are combined, it is possible to ensure a certain energy density on the basis of accurate SOC calculation. How to mix the two and how to accurately calculate the SOC after mixing are the issues that need to be considered. Summary of the Invention

[0012] The purpose of the present invention is to overcome the deficiencies of the prior art and provide a sodium-lithium hybrid battery and its SOC calculation method. By adding a sodium battery to a lithium iron phosphate battery, a hybrid battery is realized, and a high-accuracy SOC value calculation of the hybrid battery is achieved.

[0013] To achieve the above object, the technical solution adopted by the present invention is: a sodium-lithium hybrid battery, the sodium-lithium hybrid battery is formed by connecting a plurality of lithium battery modules in series, and each battery module includes a plurality of battery cells; at least one sodium battery cell is included in one battery module or a plurality of battery modules.

[0014] The sodium battery cell is connected in series with the remaining battery cells in its battery module.

[0015] The initial capacity of the sodium battery cell connected in series in the battery module is greater than (Q L *(100 + SOH L ) * CYC L ) / ((100 + SOH N ) * CYC N );

[0016] Where the initial capacity of the lithium battery is Q L , the cut-off SOH is SOH L , the number of cycles is CYC L , the cut-off SOH of the sodium battery is SOH N , the number of cycles is CYC N .

[0017] The initial capacity of the sodium battery cell is (Q L *(100 + SOH L ) * CYC L ) / ((100 + SOH N ) * CYC N ) + M;

[0018] Where M is the redundant capacity added to the sodium battery, and its value is in Q L *a% to Q L *b%, where a and b are constants.

[0019] The value of M is in Q L *5% to QL *20%.

[0020] A method for calculating the SOC of a sodium-lithium hybrid battery, which includes calculating the SOC value of the sodium battery cell in real time after the sodium battery cells and lithium battery cells are mixed to form a sodium-lithium hybrid battery, and correcting and calculating the SOC value of the hybrid battery based on the SOC value of the sodium battery cell.

[0021] The method for calculating the SOC of the hybrid battery includes:

[0022] Step 1: Test the charge-discharge rate curves of the sodium battery cells. The rate curves are used to dynamically calculate the SOC of the sodium battery in real time;

[0023] Step 2: Charge the hybrid battery system to full charge, then collect the voltage of the sodium battery cell, compare it with the charge curve of the corresponding charge rate, and obtain the SOC of the sodium battery when it is full, denoted as SOC NF ;

[0024] Step 3: Discharge the hybrid battery system until it is empty, collect the voltage of the sodium battery, compare it with the discharge curve of the corresponding rate, and obtain the SOC of the sodium battery when it is empty, denoted as SOC NE ;

[0025] Step 4: When the sodium-lithium hybrid battery is in the working state, the calculated value of the SOC of the hybrid battery is calculated using the following formula:

[0026] Hybrid battery SOC = (SOC N - SOC NE ) / (SOC NF - SOC NE ) * 100%

[0027] where SOC N is the SOC of the sodium battery cell.

[0028] The SOC of the sodium battery cell is calculated by combining the rate curve and Ah integration.

[0029] When any one lithium battery cell in the hybrid battery reaches the full charge cut-off voltage, it is determined that the hybrid battery is fully charged. When any one lithium battery cell reaches the discharge cut-off voltage, it is determined that the hybrid battery is discharged.

[0030] The advantages of the present invention are as follows: By adding a sodium battery to a lithium iron phosphate battery, a hybrid battery is realized, and a highly accurate calculation of the SOC value of the hybrid battery is achieved, avoiding the SOC estimation error caused by the flat voltage platform of a pure lithium iron phosphate battery. Design the capacity of the sodium battery in the hybrid battery to improve the discharge performance of the entire hybrid battery. After adding the sodium battery, it will not affect the discharge of the entire battery system and can achieve the method of correcting the lithium battery SOC with the sodium battery. Brief Description of the Drawings

[0031] The following briefly describes the content expressed in each drawing of the specification of the present invention and the marks in the drawings:

[0032] Figure 1 It is a schematic diagram of the voltage change curve of a lithium iron phosphate battery as the battery SOC changes in the prior art;

[0033] Figure 2 It is a schematic diagram of the voltage change curve of a sodium battery as the remaining Ah of the battery changes in the prior art;

[0034] Figure 3 It is a schematic diagram of the combination of the sodium-lithium hybrid battery of the present invention. Detailed Description of the Preferred Embodiment

[0035] The following further describes in detail the specific embodiments of the present invention by describing the optimal embodiments with reference to the drawings.

[0036] As Figure 3 shown, taking the lithium iron phosphate battery system as an example, in order to improve the accuracy of the SOC calculation of the lithium iron phosphate battery system, a sodium battery cell is connected in series into the lithium iron phosphate battery to form a sodium-lithium hybrid battery system, and then the accurate calculation of the SOC of the entire hybrid battery system is realized based on the SOC of the sodium battery.

[0037] As Figure 3 shown in the sodium-lithium hybrid battery, the sodium-lithium hybrid battery system is formed by connecting a plurality of lithium battery modules in series. Each battery module includes a plurality of battery cells, and the battery cells inside are all connected in series to form a battery module; at least one sodium battery cell is included in one battery module or a plurality of battery modules. Taking Figure 3 as an example, four battery modules are connected in series to form a battery pack. One of the battery cells in one of the battery modules is a sodium battery cell, and the rest of the battery cells are lithium iron phosphate battery cells. The battery cells in each battery module are connected in series to form a module, and the battery modules are connected in series with other battery modules to finally form a hybrid battery system. In Figure 3 , the sodium battery cell is in module 3, and the sodium battery cell in module 3 is connected in series with the remaining lithium iron phosphate battery cells.

[0038] The initial capacity of the sodium battery cell connected in series in the battery module should be greater than (Q L *(100 + SOH L ) * CYC L ) / ((100 + SOH N ) * CYC N );

[0039] where the initial capacity of the lithium battery is Q L , and the cut-off SOH is SOHL The number of cycles is CYC L , and the cut-off SOH of the sodium battery is SOH N The number of cycles is CYC N .

[0040] In a preferred embodiment, the initial capacity of the sodium battery cell is (Q L *(100 + SOH L ) * CYC L ) / ((100 + SOH N ) * CYC N ) + M;

[0041] where M is the redundant capacity added to the sodium battery, and its value ranges from Q L *a% to Q L *b%, and a and b are constants. Preferably, the value of M ranges from Q L *5% to Q L *20%.

[0042] Principle description of the sodium-lithium hybrid battery system:

[0043] 1. Thermal runaway suppression architecture

[0044] Among the power batteries currently installed in vehicles, most are lithium iron phosphate batteries, and their SOC is relatively difficult to calculate. However, due to the excellent voltage characteristics of sodium batteries, it is convenient to calculate the SOC of the battery pack. Therefore, sodium battery cells can be connected in series in the lithium iron phosphate battery system and used as the benchmark for the SOC of the lithium iron phosphate battery. Its architecture is as Figure 3 shown. Figure 3 In the lithium battery system composed of four modules in, the gray represents the lithium battery and the black represents the sodium battery. Among them, a sodium battery cell is connected in series at the middle position of the entire system. Its advantage is that the sodium battery cell has better safety. When the lithium battery system experiences thermal runaway, the sodium battery can utilize its high heat capacity and stability characteristics to delay the combustion of the lithium battery system.

[0045] 2. Calculation of sodium battery capacity

[0046] The cycle life of lithium iron phosphate batteries is usually 5000 - 12000 times, while the cycle life of sodium batteries is usually 3000 - 6000 times. The number of cycles here is the capacity of the battery that decays from SOH = 100% to the cut-off SOH. Usually, the cut-off SOH of both lithium batteries and sodium batteries is between 70% and 80%.

[0047] Since the attenuation rates of lithium batteries are different, if the rated capacities of sodium batteries and lithium batteries are the same, the capacity of sodium batteries will attenuate faster, resulting in the capacity of sodium batteries being much smaller than that of lithium batteries after running for a period of time. Since they are in series, the maximum amount of electricity that the entire energy storage system can discharge is the capacity of sodium batteries. Therefore, factors such as the attenuation rate of sodium batteries and the cut-off SOH need to be considered when configuring the capacity of sodium batteries, so as to achieve the method of using sodium batteries to correct the SOC of lithium batteries without affecting the discharge of the entire battery system.

[0048] Let the initial capacity of the lithium battery be Q L and the cut-off SOH be SOH L and the number of cycles be CYC L , let the cut-off SOH of the sodium battery be SOH N and the number of cycles be CYC N . All these data can be obtained from the cell manufacturer. Based on these, the configurable capacity of the sodium battery is calculated, that is, the capacity of the configured sodium battery needs to meet the capacity requirements of the lithium battery throughout its life cycle. Since the cycle life of sodium batteries is much shorter than that of lithium batteries, the configured capacity of sodium batteries must be larger. The capacity that can be discharged during the entire life cycle of the sodium battery is:

[0049] Accumulated capacity of sodium battery = Q N *(100 + SOH N ) * CYC N

[0050] The capacity that can be discharged during the entire life cycle of the lithium battery is:

[0051] Accumulated capacity of lithium battery = Q L *(100 + SOH L ) * CYC L

[0052] The accumulated capacity of the sodium battery needs to be > the accumulated capacity of the lithium battery, that is, the following formula

[0053] Q N *(100 + SOH N ) * CYC N > Q L *(100 + SOH L ) * CYC L

[0054] After simplification, the capacity of the sodium battery is not less than:

[0055] Q N > (Q L *(100 + SOH L ) * CYC L ) / ((100 + SOH N ) * CYC N)

[0056] The goal in engineering practice is to use sodium batteries to correct the SOC of lithium batteries. To prevent sodium batteries from becoming the weak link in the entire battery system, factors such as the imbalance and inconsistency of conventional lithium batteries need to be considered. Therefore, some redundant capacity needs to be added based on the minimum capacity as the initial capacity for the final sodium battery setting, which is:

[0057] Q N =(Q L *(100 + SOH L )*CYC L ) / ((100 + SOH N )*CYC N ) + M

[0058] where M is the redundant capacity added to the sodium battery, and its range is from Q L *5% to Q L *20%

[0059] In the above embodiment, after the connection structure of the sodium-lithium hybrid battery and the selection of the initial capacity of the sodium battery, a sodium-lithium hybrid battery system is formed according to the series structure and capacity. This battery system has the advantages of lithium iron phosphate batteries, and at the same time, due to the addition of sodium battery cells, the accurate calculation of the SOC of the entire hybrid system can be based on the sodium battery cells.

[0060] A method for calculating the SOC of a sodium-lithium hybrid battery includes, after the sodium and lithium battery cells are mixed to form a sodium-lithium hybrid battery, calculating the SOC value of the sodium battery cells in real time, and correcting and calculating the SOC value of the hybrid battery based on the SOC value of the sodium battery cells.

[0061] The process of correcting the SOC of lithium batteries based on sodium batteries: After the lithium-sodium batteries are mixed, the sodium batteries are used to calculate the SOC of the lithium batteries.

[0062] Step 1, test the charge and discharge curves of the sodium battery at different charge and discharge rates, and the SOC of the sodium battery can be calculated in real time and dynamically using these rate curves;

[0063] Step 2, fully charge the entire system, that is, any lithium battery cell reaches the full charge cut-off voltage. When the system is fully charged, collect the voltage of the sodium battery, compare it with the corresponding charge curve, and obtain the SOC of the sodium battery when it is fully charged, denoted as SOC NF

[0064] Step 3, discharge the entire hybrid battery system until any lithium battery cell reaches the discharge cut-off voltage. When the system is discharged, collect the voltage of the sodium battery, compare it with the corresponding discharge curve, and obtain the SOC of the sodium battery when it is discharged, denoted as SOC NE

[0065] Step 4, after the pre-correction steps of Steps 2 and 3, when the hybrid battery is applied to engineering practice, the lithium battery is fully charged, that is, its SOC = 100% corresponding to the SOC of the sodium battery NF , the lithium battery is discharged, that is, its SOC = 0% corresponding to the SOC of the sodium battery NE , since the sodium and lithium battery cells are in series, the SOCs of the two are linearly corresponding, and the SOC calculation formula for the lithium battery system is:

[0066] SOC = (SOC N - SOC NE ) / (SOC NF - SOC NE ) * 100%

[0067] where SOC N is the SOC of the sodium battery and can be calculated by combining the most common rate curve and Ah integration method.

[0068] Based on the above method, accurate estimation of the SOC of series and parallel battery modules containing sodium battery cells can also be carried out.

[0069] Obviously, the specific implementation of the present invention is not limited by the above methods. As long as various non-substantive improvements are made using the method concept and technical solution of the present invention, they are all within the protection scope of the present invention.

Claims

1. A sodium-lithium hybrid battery, characterized in that: The sodium-lithium hybrid battery comprises a plurality of lithium battery modules connected in series, each battery module comprises a plurality of battery cells; one battery module or a plurality of battery modules comprises at least one sodium battery cell.

2. A sodium-lithium hybrid battery as claimed in claim 1, characterized in that: The sodium battery cell is connected in series with the remaining cells in the battery module in which it is located.

3. A sodium-lithium hybrid battery according to claim 1 or 2, characterized in that: The initial capacity of the sodium battery cells connected in series in the battery module is greater than (Q L *(100+SOH L )*CYC L ) / ((100+SOH N )*CYC N ); The initial capacity of the lithium battery is Q L 、SOH cut-off is SOH L , the number of cycles is CYC L , the sodium battery cut-off SOH is SOH N , the number of cycles is CYC N .

4. A sodium-lithium hybrid battery as claimed in claim 3, characterized in that: The initial capacity of the sodium battery cell is (Q L *(100+SOH L )*CYC L ) / ((100+SOH N )*CYC N )+M; in M is the redundant capacity of the sodium battery, and its value is in Q L *a%~Q L *b%, a and b are constants.

5. A sodium-lithium hybrid battery as claimed in claim 4, characterized in that: The value of M is in Q L *5%~Q L *20%.

6. A method for calculating the SOC of a sodium-lithium hybrid battery according to any one of claims 1 to 5, characterized in that: After the sodium-lithium battery cells are mixed to form a sodium-lithium hybrid battery, the SOC value of the sodium battery cell is calculated in real time, and the SOC value of the hybrid battery is obtained by correcting the calculation based on the SOC value of the sodium battery cell.

7. The method for calculating the SOC of a sodium-lithium hybrid battery according to claim 6, characterized in that: The SOC calculation method of the hybrid battery includes: Step 1: Test different charge and discharge rate curves of the sodium battery cell. The rate curve is used to dynamically calculate the SOC of the sodium battery in real time. Step 2: Charge the hybrid battery system to full power, then collect the voltage of the sodium battery cell, compare the charging curve of the response charging rate, and obtain the SOC of the sodium battery when fully charged, recorded as SOC NF ; Step 3: Discharge the hybrid battery system until it is empty, collect the voltage of the sodium battery, compare the discharge curves at the corresponding rates, and obtain the SOC of the sodium battery when it is empty, recorded as SOC NE ; Step 4: When the sodium-lithium hybrid battery is in working state, the SOC of the hybrid battery is calculated using the following formula: Hybrid battery SOC = (SOC N -SOC NE ) / (SOC NF -SOC NE )*100% Among them, SOC N It is the SOC of the sodium battery cell.

8. The method for calculating the SOC of a sodium-lithium hybrid battery according to claim 7, characterized in that: The SOC of the sodium battery cell is calculated by combining the rate curve and Ah integration.

9. The method for calculating the SOC of a sodium-lithium hybrid battery according to claim 6, characterized in that: When any one of the lithium battery cells in the hybrid battery reaches the full charge cut-off voltage, the hybrid battery is judged to be fully charged, and when any one of the lithium battery cells reaches the discharge empty cut-off voltage, the hybrid battery is judged to be discharged empty.

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