CN112684342A - Method for evaluating heat generation rate in charging/discharging process of sodium ion battery - Google Patents
Method for evaluating heat generation rate in charging/discharging process of sodium ion battery Download PDFInfo
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- 229910001415 sodium ion Inorganic materials 0.000 title claims abstract description 127
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- 238000000034 method Methods 0.000 title claims abstract description 98
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- 238000005516 engineering process Methods 0.000 claims abstract description 12
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- DGAQECJNVWCQMB-PUAWFVPOSA-M Ilexoside XXIX Chemical compound C[C@@H]1CC[C@@]2(CC[C@@]3(C(=CC[C@H]4[C@]3(CC[C@@H]5[C@@]4(CC[C@@H](C5(C)C)OS(=O)(=O)[O-])C)C)[C@@H]2[C@]1(C)O)C)C(=O)O[C@H]6[C@@H]([C@H]([C@@H]([C@H](O6)CO)O)O)O.[Na+] DGAQECJNVWCQMB-PUAWFVPOSA-M 0.000 description 5
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- Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
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- Y02E60/10—Energy storage using batteries
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Abstract
The invention relates to a method for evaluating heat generation rate in a sodium ion battery charging/discharging process, and belongs to the technical field of sodium ion batteries. The method takes the characteristics of stable open-circuit voltage and less side reaction of the organic electrolyte of the sodium-ion battery into consideration, measures the voltage and temperature change coefficient of the sodium-ion battery from charging and discharging to different charge states by adopting a potential method, calculates the entropy change by utilizing the voltage and temperature change coefficient, obtains the reversible heat production rate according to the entropy change, and further improves the accuracy of the reversible heat production rateDegree; meanwhile, consider a sodium ion battery Na+The overpotential caused by the internal resistance of the battery is tested by adopting a constant current intermittent titration technology in the diffusion of the organic electrolyte and the solid particles, and the measured overpotential value is more accurate, so that the accurate irreversible heat production rate can be obtained. The method can analyze the heat production rule of the battery and the contribution of heat production rate sources of all parts to the total heat production rate, and provides guarantee for optimizing the battery design and optimizing a battery thermal management system.
Description
Technical Field
The invention relates to a method for evaluating heat generation rate in a sodium ion battery charging/discharging process, and belongs to the technical field of sodium ion batteries.
Background
In recent years, the fossil energy crisis and the environmental protection have made people's gaze shift to new sustainable secondary energy, and as an energy storage battery which has been commercially applied and has a mature application technology, a lithium ion battery has almost occupied the "4C" market (the "4C" market is four application markets of consumer electronics, computers, networks and communications) because of its advantages of long cycle life, high working voltage, high capacity, small self-discharge, stable discharge performance, wide working temperature range, no memory effect, long cycle life, and the like. At present, lithium ion batteries are frequently applied to electric automobiles as energy storage power batteries, and the economic and environment-friendly characteristics of the power batteries are mainly considered. However, the power battery is not mature and perfect at present, and the application technology of the power battery has great disadvantages. One is the shortage and uneven distribution of lithium resources; secondly, the lithium ion battery has great potential safety hazard when being applied to the electric automobile, such as thermal safety accidents of battery pack combustion, explosion and the like.
The room temperature sodium ion battery, which is the most potential "switching person" of the lithium ion battery, is one of the subjects of the most intense research in the energy field at present. Na (Na)+Standard electrode potential (-2.71V vs SHE) vs. Li for the/Na couple+the/Li (-3.04V vs SHE) is higher by about 0.3V, so the electrolyte with lower decomposition voltage is suitable for the sodium-ion battery, namely, the electrolyte of the room-temperature sodium-ion battery has better safety performance. However, sodium ion electricityThe security problem of pools still does not come in small amounts. Firstly, the relatively mature cathode material of the conventional room-temperature sodium ion battery is a hard carbon cathode, but the standard electrochemical equilibrium potential of the hard carbon cathode is 0.1V, a certain sodium precipitation phenomenon still exists under certain conditions, and sodium dendrite is possibly generated on the cathode side, so that potential safety risk exists; secondly, the room temperature sodium ion battery is used as a large energy storage battery and a power battery, and the absolute value of the current passing through the room temperature sodium ion battery is larger, so that the heat generation rate per unit volume is relatively larger, and the thermal safety is particularly important; more importantly, although the room-temperature sodium ion battery can adopt electrolyte with lower decomposition voltage, the electrolyte of the sodium ion battery with better ion conductivity and more application is still organic electrolyte, and the organic electrolyte contains larger chemical energy, so that the danger caused by the release of the chemical energy is huge when the battery is out of control thermally. In addition, the data show that the current safety accidents of the electric automobile are mainly thermal runaway accidents.
At present, the application of the sodium ion battery still stays in an experimental stage, and the problem to be solved firstly is to clearly understand the thermodynamic and kinetic characteristics of the sodium ion battery in the charging and discharging processes, so that the safe application of the sodium ion battery can be ensured.
The practical problem of the sodium ion battery, namely searching for a suitable electrode material of the sodium ion battery, is considered secondly. It is well known that the electrode material is critical to the energy density of the battery. At present, the most promising negative electrode material for sodium ion batteries and the most mature technology development is a hard carbon negative electrode. Hard carbon has a larger interlayer distance relative to graphite, can store a large amount of sodium ions, and thus has a larger energy. However, the positive electrode material of the sodium ion battery is still in the exploration stage, and the main positive electrode materials of the sodium ion battery are roughly divided into three types: respectively a layered transition metal oxide positive electrode material, a Prussian blue analogue positive electrode material and a polyanionic positive electrode material. The layered transition metal oxide anode material is easy to generate phase change in the charge and discharge processes to cause structure attenuation and capacity reduction. The Prussian blue analogue anode material contains crystal water in the synthesis process, and the structure is unstable in the charge-discharge process; polyanion cathode materials have three-dimensionally stable sodium ion diffusion channels, but the materials have poor electronic conductivity, so the polyanion cathode materials are coated with a conductive agent when in use.
Entropy change is a measure of the ordering or disordering of sodium ions in the electrode material lattice, and the reversible heat generation rate during charging and/or discharging of a sodium-ion battery can be calculated from the entropy change, a set test temperature, and the charging and/or discharging current. Therefore, analyzing the thermodynamic characteristics of the sodium-ion battery during charging and/or discharging through the entropy change test helps to capture thermodynamic information of the sodium-ion battery during charging and/or discharging.
As is well known, in electrochemistry, an overpotential is a difference between an electrode potential at which an electrode reaction deviates from equilibrium and an equilibrium potential of the electrode reaction, and its physical meaning is to cause irreversible heat of a battery, including ohmic heat, heat of polarization reaction, and heat generated by diffusion of internal resistance. Overpotential is the embodiment of a kinetic process and can greatly influence the progress of electrode reaction. The irreversible heat production rate is therefore a matter of information on the dynamics of the battery. And the total heat production rate of the battery is the sum of the reversible heat production rate and the irreversible heat production rate, so that the thermodynamics and kinetics information of the sodium-ion battery can be clearly understood by analyzing the rule of the total heat production rate in the charging and/or discharging processes of the sodium-ion battery, the heat production rule of the battery can be analyzed, and the heat management system of the battery is optimized.
At present, basic theories such as a positive and negative sodium storage mechanism of a sodium ion battery, structural evolution in a charging and discharging process, an interface reaction between an electrode and an electrolyte and the like are deeply explored, and at present, a research on a heat generation rate in the charging and discharging process of the sodium ion battery at room temperature is mainly calorimetry, namely, the temperature change of the sodium ion battery is directly tested in the charging and discharging process, and the heat generation rate is determined according to the temperature change condition.
Disclosure of Invention
The invention aims to provide an evaluation method of heat generation rate in the charging/discharging process of a sodium-ion battery, which provides a basis for the optimization of structural design and the optimization of a thermal management system of the sodium-ion battery.
The invention provides a method for evaluating the heat generation rate in the charging/discharging process of a sodium-ion battery to solve the technical problems, which comprises the following steps:
1) performing constant-current charge and discharge test on the sodium ion battery to be tested to ensure that the electrochemical performance of the sodium ion battery to be tested tends to be stable;
2) when the electrochemical performance of the sodium ion battery to be tested tends to be stable, carrying out potentiometric test on the sodium ion battery to be tested, and determining entropy change of the sodium ion battery in the charging and/or discharging process;
the entropy change determining process in the charging and/or discharging process of the sodium-ion battery cell is as follows: charging and/or discharging the sodium-ion battery to different charge states according to a specific multiplying power, acquiring open-circuit voltage corresponding to each set temperature in each charge state, fitting the change relation between the open-circuit voltage and the temperature to obtain the voltage temperature change coefficient in each charge state, calculating the entropy change in different charge states according to the voltage temperature change coefficient, and calculating the reversible heat production rate of the sodium-ion battery in the charging and/or discharging process according to the entropy change, the charging and/or discharging current and the set temperature;
3) testing overpotential caused by the internal resistance of the battery when the sodium-ion battery is charged and/or discharged to different charge states at constant current under various set temperatures by using a constant current intermittent titration technology; calculating the irreversible heat generation rate of the sodium ion battery in the charging and/or discharging process according to the obtained overpotential when the sodium ion battery is charged and/or discharged to different charge states and the battery charging and/or discharging current;
4) calculating the total heat yield in the charging and/or discharging process of the sodium-ion battery according to the reversible heat yield obtained in the step 2) and the irreversible heat yield obtained in the step 3).
The method considers the characteristics of stable open-circuit voltage, high sodium coulombic efficiency and less side reaction of the organic electrolyte of the sodium-ion battery, adopts a potential method to measure the voltage temperature change coefficient of the sodium-ion battery from charging and discharging to different charge states, utilizes the voltage temperature change coefficient to calculate the entropy change, and calculates the entropy change according to the entropyThe reversible heat production rate is obtained, and the accuracy of the reversible heat production rate is further improved; meanwhile, consider a sodium ion battery Na+The overpotential caused by the internal resistance of the battery is tested by adopting a constant current intermittent titration technology in the diffusion of the organic electrolyte and the solid particles, and the measured overpotential value is more accurate, so that the accurate irreversible heat production rate can be obtained. The method can analyze the heat production rule of the battery, the contribution of heat production rate sources of all parts to the total heat production rate and the influence of exploration temperature and multiplying power on the total heat production rate, and provides guarantee for optimizing the battery design and optimizing a battery thermal management system.
Further, in order to accurately obtain the entropy change of the sodium ion battery, the calculation formula of the sodium ion entropy change in the step 2) is as follows:
wherein E is the open circuit voltage of the sodium ion battery at different states of charge; Δ S is the entropy change at different states of charge; n is the charge transference number; f is the Faraday constant; t is the temperature; p is pressure;
represents the derivative of the open circuit voltage E with respect to the temperature T, i.e. the voltage temperature coefficient.
Further, in order to accurately determine the reversible heat generation rate of the sodium-ion battery in the charging and discharging processes, the calculation formula of the reversible heat generation rate in the charging and/or discharging processes of the sodium-ion battery in the step 2) is as follows:
wherein q isrevIs the reversible heat generation rate per unit volume in the charging and/or discharging process of the sodium ion battery; i is the current charged and/or discharged; t is the set battery temperature; n is the charge transference number; f is the Faraday constant; v is the volume of the cell.
Further, in order to accurately determine the irreversible heat generation rate of the sodium-ion battery in the charging and discharging processes, the calculation formula of the irreversible heat generation rate in the charging and/or discharging processes of the sodium-ion battery in the step 3) is as follows:
wherein q isirrIs the irreversible heat generation rate per unit volume in the charging and/or discharging process of the sodium ion battery; i is the current charged and/or discharged; v is the volume of the single battery; etaIRIs an overpotential caused by ohmic internal resistance and polarization internal resistance; etaDIs an overpotential caused by the internal resistance to sodium ion diffusion.
Further, the calculation formula of the total heat production rate during the charging and/or discharging process of the sodium-ion battery in the step 4) is as follows:
wherein q istotalThe total heat yield per unit volume in the charging and/or discharging process of the sodium ion battery; q. q.srevIs the reversible heat generation rate per unit volume in the charging and/or discharging process of the sodium ion battery; q. q.sirrIs the irreversible heat generation rate per unit volume during the charging and/or discharging process of the sodium ion battery.
Drawings
FIG. 1 is a flow chart of a method for evaluating the heat generation rate during the charge/discharge of a sodium-ion battery according to the present invention;
FIG. 2 shows Na at different temperatures at 20% SOC in example 1 of the present invention3V2(PO4)3Graph of open circuit voltage versus time for a @ C/Na cell;
FIG. 3 is Na at 20% SOC in example 1 of the present invention3V2(PO4)3The curve graph of the open-circuit voltage of the @ C/Na battery along with the temperature;
FIG. 4 shows Na at 20 ℃ in example 1 of the present invention3V2(PO4)3Entropy change of @ C/Na cellA change curve graph of the SOC;
FIG. 5 is a schematic diagram showing the GITT test conducted during the charging of a sodium-ion battery in example 1 of the present invention;
FIG. 6 is a schematic diagram of the GITT test conducted during discharge of a sodium-ion battery in example 2 of the present invention;
FIG. 7 shows Na at 20 ℃ in 1C charging in example 1 of the present invention3V2(PO4)3The curve chart of the irreversible heat production rate, the reversible heat production rate and the total heat production rate of the @ C/Na battery along with the change of the SOC.
Detailed Description
The following further describes embodiments of the present invention with reference to the drawings.
The invention leads the electrochemical performance of the sodium ion battery to be tested to tend to be stable by carrying out constant current charging and/or discharging test on the sodium ion battery to be tested; when the electrochemical performance of the sodium-ion battery to be tested tends to be stable, carrying out potentiometric test on the sodium-ion battery to be tested, determining the entropy change of the sodium-ion battery in the charging and/or discharging process, further calculating the reversible heat production rate of the sodium-ion battery in the charging and/or discharging process, and realizing thermodynamic evaluation; and measuring overpotential in the charging and/or discharging process of the sodium-ion battery by using a constant current intermittent titration technology, further calculating the irreversible heat generation rate in the charging and/or discharging process of the sodium-ion battery, and realizing evaluation on the dynamic performance in the charging and/or discharging process of the sodium-ion battery. The implementation flow of the method is shown in fig. 1, and the specific implementation process is described in detail in embodiment 1 and embodiment 2.
The evaluation method for evaluating the thermodynamic (entropy change and reversible heat generation rate) and kinetic (overpotential and irreversible heat generation rate) characteristics of the sodium-ion battery in the charging and discharging processes is universal in the application of different types of sodium-ion batteries. Of course, except in extreme cases (e.g., where the method is not applicable when assessing thermodynamic and kinetic information of the cell under thermal runaway conditions). The invention can effectively evaluate the thermodynamic and kinetic properties of a half battery or a full battery which is composed of a transition metal oxide positive electrode, a polyanionic compound positive electrode, a Prussian blue analogue positive electrode and a negative electrode material and has stable performance.
Therefore, the following examples are only for further illustrating the present invention and do not limit the scope of application of the present invention. The basic parameters of the specific setting are changed according to the performance characteristics and actual needs of different electrode materials.
The batteries to be tested selected in the embodiment 1 and the embodiment 2 are both Na3V2(PO4)3The @ C/Na sodium ion battery is a typical sodium ion battery formed by polyanion compound cathode material and sodium metal cathode. The model is CR2032 button cell battery, with diameter of 7.9mm, thickness of 3.6mm and volume of 1.765 × 10-4And L. The surface of the battery to be tested is covered with heat insulation cotton for heat insulation, and electrochemical test is carried out in a sealed environment. In example 1, the performance of charging to different SOCs during charging is shown, and the performance of discharging to different SODs during discharging is shown in example 2, but the performance of charging to different SOCs and discharging to different SODs may be evaluated as other embodiments.
Example 1
1. And carrying out constant-current charge and discharge test on the battery to be tested, so that the electrochemical performance of the battery to be tested tends to be stable.
Before the constant-current charging and discharging test is carried out, the running state of the equipment is checked, and if the running state is good, the preparation work before the test is carried out: fixing a PT100 thermocouple on the central part of the surface of the battery by using heat conducting glue, wrapping the battery by using heat insulation cotton after connecting a charging and discharging battery tester clamp, putting the battery into a programming temperature control incubator, setting a charging and discharging test program and a temperature control program, setting the temperature in the incubator to be 20 ℃, and closing the incubator door. After the temperature in the box is stable, the battery is allowed to stand in the box for 2 hours to ensure that the thermodynamics of the battery and the environment reach an equilibrium state, and Na is added3V2(PO4)3The @ C/Na battery is connected to the positive and negative clamps of the MACRRO battery tester, a program is set, and the test is started when the battery temperature and the indoor temperature reach thermodynamic equilibrium.
Setting a charging and discharging voltage interval to be 2.5-4.0V at room temperature, then performing constant-current charging and discharging circulation for 2 circles at a rate of 0.1C, and standing for 2 hours; circulating for 2 circles at 0.5C multiplying power, and standing for 2 hours; then circulating for 2 circles at 1C multiplying power and standing for 2 hours; finally, circulating for 10 circles at 0.1C multiplying power and standing for 20 hours; at this time, the electrochemical performance of the battery tends to be stable.
2. And (4) indirectly testing entropy change of the battery charged to different SOC by using a potential method, and further calculating the reversible heat production rate.
The first is the preparation before testing: fixing a PT100 thermocouple on the central part of the surface of the battery by using heat conducting glue, wrapping the battery by using heat insulation cotton after connecting a charging and discharging battery tester clamp, putting the battery into a programming temperature control incubator, setting a charging and discharging test program and a temperature control program, setting the temperature in the incubator to be 20 ℃, and closing the incubator door. After the temperature in the box is stable, the battery is allowed to stand in the box for 2 hours to ensure that the thermodynamics of the battery and the environment reach an equilibrium state, and then the voltage temperature change coefficient dE/dT test is started. The specific execution steps are as follows:
(1) pre-circulation: cycling for 10 times at 20 ℃, 0.1C multiplying power and a voltage interval of 2.5-4.0V, and acquiring the capacity information of the battery after the charge-discharge cycle of the battery is stable;
(2) adjusting the SOC: after the charge state of the battery is stable, the battery is charged to a specified SOC state at a constant current of 0.1C, and in order to avoid experimental errors caused by self-discharge of a half battery under high voltage, the battery is completely discharged at 0.1C and then charged to the specified SOC state in one step when the SOC is adjusted.
(3) And (3) testing open circuit voltage: firstly, the battery is balanced for 20 hours after reaching the target SOC at 20 ℃, then the open-circuit voltage is recorded, then the battery is respectively kept at the temperatures of 10 ℃, 20 ℃, 30 ℃ and 40 ℃ for 2 hours, the open-circuit voltage value is collected, one data point is recorded every 10 seconds, and finally the data values recorded in the last half hour are averaged, and the result is shown in figure 2.
Through the steps (2) and (3), the open-circuit voltage of the battery when the battery is charged to different SOC under different temperatures can be obtained, wherein Na is obtained under different temperatures under 20% SOC3V2(PO4)3The open-circuit voltage of the @ C/Na cell varies with time as shown in FIG. 2Wherein Na at 20% SOC3V2(PO4)3The open-circuit voltage curve of the @ C/Na cell with temperature is shown in FIG. 3.
(4) Calculation and plotting: according to EocvAnd performing data fitting on the temperature to obtain a voltage temperature change relation dE/dT under different SOCs, then calculating an entropy change delta S according to the following calculation formula, and finally performing data fitting on the delta S and different SOCs of the battery. Wherein, the calculation formula of Δ S is as follows:
wherein EocvIs the open circuit voltage of the battery at different SOC; Δ S is the entropy change at different SOCs; Δ H is the enthalpy change at different SOC/SOD; n is the charge transference number; f is the Faraday constant; t is the temperature, here
For the convenience of recording, dE/dT is used as a unified expression in the subsequent steps, and represents a voltage temperature coefficient, and E is another expression of open-circuit voltage.
Through the process, the delta S under different SOC in the charging process can be determined, and corresponding delta S-SOC curves can be fitted, as shown in FIG. 4, the curve in the graph is an entropy change curve under 20 ℃, and entropy change curves under other temperatures can also be obtained according to requirements.
And calculating the reversible heat generation rate of the sodium-ion battery in the charging process according to the obtained entropy change delta S, the charging current and the set temperature, wherein the specific calculation formula is as follows:
wherein q isrevIs the unit volume of the sodium ion battery in the charging processReversible heat production rate; i is the current charged; t is the set battery temperature; n is the charge transference number; f is the Faraday constant; v is the volume of the cell.
3. And (3) directly testing overpotential caused by the internal resistance of the battery during constant-current charging of the battery by adopting a constant-current intermittent titration technology, and calculating the irreversible heat generation rate.
Constant current intermittent titration technique test (GITT): the essence of the technology is that a complete constant current charging/discharging test is divided into a plurality of sections, a standing time is added after each section of constant current charging/discharging is finished, the purpose is to enable the charge state of the battery to reach a stable state, and in short, GITT is formed by a series of processes of pulse, constant current and relaxation. As shown in FIG. 5, the transient voltage at the end of constant current charging and discharging of the battery and the transient voltage at the beginning of relaxation have a sudden change, and the part of the overpotential is caused by the ohmic internal resistance and polarization internal resistance of the battery and is marked as etaIR(ii) a After the charging is finished, the current does not pass through the battery, but Na is generated due to the concentration difference of sodium ions between the electrode and the electrolyte+The diffusion continues under the action of the concentration difference, the voltage of the battery changes, and the relaxation process is to stabilize the charge state of the battery and make the voltage reach a stable value, namely the open-circuit voltage under the SOC. Therefore, the potential difference at the beginning and end of the relaxation process is generally understood to be represented by Na+Overpotential due to diffusion, denoted as ηD。
For this embodiment, the battery with a stable state of charge is placed in a thermostat, the temperature of the thermostat is set to 20 ℃, after the temperature in the thermostat is stable, the battery is allowed to stand in the thermostat for 2 hours to allow the battery to reach thermal equilibrium with the environment, and then the start program starts the constant current intermittent titration technical test. The state of charge of the battery is adjusted by adopting 1C multiplying power, if the change of one period is recorded every 10% of SOC, 10 periods exist in the whole GITT test curve, and therefore the whole charging process is divided into 10 stages. To ensure the state of charge balance after each stage of battery charging, the relaxation time per stage was set to 2 hours. The specific steps can be decomposed into:
(1) adjusting the charge state of the sodium-ion battery to the next stage according to the 1C multiplying power (the charge state adjustment of the battery at the stage is different from the charge state adjustment of the open-circuit voltage testing stage);
(2) standing for 2 hours;
(3) repeating the two operations for 10 times until the battery voltage rises to 4.0V;
(4) processing data, calculating eta according to GITT test curveIR、ηD。
Calculating the irreversible heat generation rate in the charging process of the sodium ion battery according to the obtained overpotential for constant current charging of the sodium ion battery and the battery charging current, wherein the calculation formula of the irreversible heat generation rate is as follows:
wherein q isirrIs the irreversible heat generation rate per unit volume in the charging process of the sodium ion battery; i is the charging current; v is the volume of the single battery; etaIRIs an overpotential caused by ohmic internal resistance and polarization internal resistance; etaDIs an overpotential caused by the internal resistance to sodium ion diffusion.
4. The total heat yield during sodium charging was calculated.
The heat production rate comprises a reversible heat production rate and an irreversible heat production rate, wherein the reversible heat production rate reflects the thermodynamic characteristics of the battery, the irreversible heat production rate reflects the kinetic characteristics of the battery, the total heat production rate in the charging process of the battery is equal to the sum of the reversible heat production rate and the irreversible heat production rate, and the specific formula is as follows:
wherein q istotalIs the total heat production rate; q. q.sirrIs the irreversible heat production rate; q. q.srevIs the reversible heat production rate; r is the internal resistance of the battery (comprising ohmic internal resistance, polarization internal resistance and diffusion internal resistance) in the step 3; i is a charge-discharge current; t is the temperature; Δ S is the entropy change at different SOCs; n is the charge transference number; f is a Faraday constant; etaIRIs an overpotential caused by ohmic internal resistance and polarization internal resistance; etaDIs an overpotential caused by the internal resistance of sodium ion diffusion; v is the volume of the cell.
Calculating the reversible heat generation rate q in the open circuit voltage test stagerevAnd the irreversible heat production rate q calculated in the constant current intermittent titration test stageirrAnd (5) substituting the equation into the equation (5) to calculate the total heat production rate of the unit volume of the battery, drawing by taking the SOC as an independent variable, and fitting data to obtain a battery electrode reaction heat production rule. For the present example, Na at 20 ℃ in 1C charging3V2(PO4)3The irreversible heat generation rate, reversible heat generation rate, total heat generation rate of @ C/Na battery are shown in FIG. 7.
To verify the present invention, Na determined in the present example was used3V2(PO4)3The relation between total heat production rate and heat production per unit volume of @ C/Na battery and SOC variation and Na recording by PT100 thermocouple and HY1000 digital temperature display instrument3V2(PO4)3The temperature variation relation along with the charging time in the charging process of the @ C/Na battery is compared, and the variation trend of the temperature variation relation and the charging time variation relation is the same, so that the heat generation rate obtained by the method is accurate, and the precision can reach +/-5%.
Example 2
This example is for Na3V2(PO4)3The heat production rate in the discharging process of the @ C/Na battery is used for analyzing the thermodynamics and the kinetics in the discharging process of the sodium-ion battery, the process is similar to the charging process in the embodiment 1, and the specific process is as follows.
1. And carrying out constant current charging and/or discharging test on the battery to be tested, so that the electrochemical performance of the battery to be tested tends to be stable.
The equipment was checked for good performance before testing to ensure that the temperature of the cell was in thermal equilibrium with the ambient temperature (procedure as in example 1). Mixing Na3V2(PO4)3The @ C/Na battery is connected to the positive and negative clamps of the MACRRO battery tester, and a program is set. The test was started when the cell temperature and the room temperature reached thermodynamic equilibrium. At room temperature, settingThe charging and discharging voltage interval is 2.5-4.0V. Then performing constant current charge-discharge circulation for 2 circles at a rate of 0.1C, and standing for 2 hours; circulating for 2 circles at 0.5C multiplying power, and standing for 2 hours; then circulating for 2 circles at 1C multiplying power and standing for 2 hours; finally, circulating for 10 circles at 0.1C multiplying power and standing for 20 hours; at the moment, the electrochemical performance of the battery tends to be stable, after the battery is fully charged with 0.1C multiplying power, the constant voltage is kept at 4.0V for 2 hours, or the current is reduced to 0.05C, so that the internal charge state of the battery is balanced.
2. And (3) indirectly testing entropy changes under different SOD by using a potential method, and calculating corresponding reversible heat production rate.
Fixing a PT100 thermocouple on the central part of the surface of the battery by using heat conducting glue, wrapping the battery by using heat insulation cotton after connecting a charging and discharging battery tester clamp, putting the battery into a programming temperature control incubator, setting a charging and discharging test program and a temperature control program, setting the temperature in the incubator to be 20 ℃, and closing the incubator door. After the temperature in the box is stable, the battery is allowed to stand in the box for 2 hours to ensure that the thermodynamics of the battery and the environment reach an equilibrium state, and then the voltage-temperature change coefficient dE/dT test is started. The specific execution steps are as follows:
(1) pre-circulation: cycling for 10 times at 20 ℃, 0.1C multiplying power and a voltage interval of 2.0-4.0V, and acquiring the capacity information of the battery after the charge-discharge cycle of the battery is stable;
(2) and (3) full charging: charging to 4.0V at constant current with 0.1C multiplying power, and setting the charge state at the moment as 0% SOD;
(3) adjusting SOD: after full charge, 0.1C constant current discharge to the specified SOD state. In order to avoid experimental error caused by self-discharge of the battery under high voltage, the SOD is fully charged by 0.1C and then discharged to a specified SOD state in one step each time the SOD is adjusted.
(4) And (3) testing open circuit voltage: firstly, the battery is balanced for 20 hours after reaching the target SOD at the temperature of 20 ℃, then the open-circuit voltage is recorded, then the open-circuit voltage value is collected at each temperature of five temperatures of 0 ℃, 10 ℃, 20 ℃, 30 ℃ and 20 ℃ for 2 hours, a data point is recorded every 10 seconds, and finally the average value of the recorded data values is taken.
(5) Calculation and plotting: and (3) performing data fitting on the temperature according to the OCV value to obtain a voltage temperature change coefficient dE/dT under different SODs, then calculating the magnitude of delta S according to a formula (2), and finally performing data fitting on the delta S and different SODs of the battery, or obtaining an entropy change curve under other temperatures according to needs.
Substituting the obtained delta S, the discharge current and the set temperature value into a formula (3) to calculate Na3V2(PO4)3The reversible heat generation rate of the @ C/Na cell under different SOD conditions.
3. And (3) directly testing overpotential caused by the internal resistance of the battery when the battery is charged and discharged at constant current by adopting a constant current intermittent titration technology, and calculating the irreversible heat generation rate.
For this embodiment, first, a battery at a constant current full point is placed in a thermostat, the temperature of the thermostat is set to 20 ℃, after the temperature in the thermostat is stable, the battery is allowed to stand in the thermostat for 2 hours to make the battery and the environment reach thermal equilibrium, a program is started to start a constant current intermittent titration technology test, and a schematic diagram of GITT of a discharging process is shown in fig. 6; the charge state of the battery is adjusted by adopting 1C multiplying power to the battery, if the change of one period is recorded every 10% SOD, 10 periods exist in the whole GITT test curve, and the whole discharging process needs to be divided into 10 stages. The specific steps can be decomposed into:
(1)1C multiplying power is used for adjusting the battery charge state to the next stage (the battery charge state adjustment in the stage is different from the charge state adjustment in the open-circuit voltage testing stage);
(2) standing for 2 hours;
(3) repeating the two operations for 10 times until the voltage of the battery is reduced to 2.5V;
(4) processing data, calculating eta according to GITT test curveIR、ηD。
Substituting the obtained overpotential for constant current discharge of the sodium-ion battery and the battery discharge current into a formula (4) to calculate Na3V2(PO4)3Irreversible heat generation rate q of @ C/Na battery under different SOD conditionsirr。
4. And calculating the total heat production rate of the battery during the discharge process.
For the present embodiment, the reversible yield of the open circuit voltage test phase is calculatedHeat rate qrevAnd the irreversible heat production rate q calculated in the constant current intermittent titration test stageirrSubstituting the formula (5) to calculate the total heat production rate of the unit volume of the battery, drawing by using SOD as an independent variable, performing data fitting, determining the heat production rule in the discharge process of the battery, and realizing the research on the thermodynamic property and the kinetic property in the discharge process of the battery.
In order to verify the accuracy of the evaluation of the heat generation rate in the discharge process of the invention, Na determined in the embodiment can be used3V2(PO4)3The relationship between total heat production rate per unit volume of @ C/Na battery and variation of SOD with heat production and Na recording by PT100 thermocouple and HY1000 digital temperature display instrument3V2(PO4)3The temperature variation relation along with the discharge time in the discharging process of the @ C/Na battery is compared, the temperature variation relation and the discharge time variation relation are found to keep the same variation trend, the accuracy of the invention is shown, and the accuracy can reach +/-5%.
Claims (5)
1. A method for evaluating a heat generation rate during a charge/discharge process of a sodium ion battery, the method comprising the steps of:
1) performing constant-current charge and discharge test on the sodium ion battery to be tested to ensure that the electrochemical performance of the sodium ion battery to be tested tends to be stable;
2) when the electrochemical performance of the sodium-ion battery to be tested tends to be stable, carrying out potentiometric test on the sodium-ion battery to be tested, determining the entropy change of the sodium-ion battery in the charging and/or discharging process, and calculating the reversible heat production rate of the sodium-ion battery in the charging and/or discharging process according to the entropy change, the charging and/or discharging current and the set temperature;
the entropy change determining process in the charging and/or discharging process of the sodium-ion battery cell is as follows: charging and/or discharging the sodium-ion battery to different charge states according to a specific multiplying power, acquiring open-circuit voltage corresponding to each set temperature in each charge state, fitting the change relationship between the open-circuit voltage and the temperature to obtain the voltage temperature change coefficient in each charge state, and calculating entropy change in different charge states according to the voltage temperature change coefficient;
3) testing overpotential caused by the internal resistance of the battery when the sodium-ion battery is charged and/or discharged to different charge states at constant current under various set temperatures by using a constant current intermittent titration technology; calculating the irreversible heat generation rate of the sodium ion battery in the charging and/or discharging process according to the obtained overpotential when the sodium ion battery is charged and/or discharged to different charge states and the battery charging and/or discharging current;
4) calculating the total heat yield in the charging and/or discharging process of the sodium-ion battery according to the reversible heat yield obtained in the step 2) and the irreversible heat yield obtained in the step 3).
2. The method for estimating the heat generation rate during the charge/discharge of the sodium-ion battery according to claim 1, wherein the formula for calculating the entropy change of the sodium ions in the step 2) is as follows:
wherein E is the open circuit voltage of the sodium ion battery at different states of charge; Δ S is the entropy change at different states of charge; n is the charge transference number; f is the Faraday constant; t is the temperature; p is pressure;
represents the derivative of the open circuit voltage E with respect to the temperature T, i.e. the voltage temperature coefficient.
3. The method for evaluating the heat generation rate during the charge/discharge of a sodium-ion battery according to claim 1, wherein the calculation formula of the reversible heat generation rate during the charge and/or discharge of the sodium-ion battery in the step 2) is as follows:
wherein q isrevIs the reversible heat generation rate per unit volume in the charging and/or discharging process of the sodium ion battery; i is charging and/or the current discharged; t is the set battery temperature; n is the charge transference number; f is the Faraday constant; v is the volume of the cell.
4. The method for evaluating the heat generation rate during the charge/discharge of a sodium-ion battery according to claim 1, wherein the calculation formula of the irreversible heat generation rate during the charge and/or discharge of the sodium-ion battery in the step 3) is as follows:
wherein q isirrIs the irreversible heat generation rate per unit volume in the charging and/or discharging process of the sodium ion battery; i is the current charged and/or discharged; v is the volume of the single battery; etaIRIs an overpotential caused by ohmic internal resistance and polarization internal resistance; etaDIs an overpotential caused by the internal resistance to sodium ion diffusion.
5. The method for evaluating the heat generation rate during the charge/discharge of a sodium-ion battery according to claim 3 or 4, wherein the total heat generation rate during the charge and/or discharge of the sodium-ion battery in the step 4) is calculated by the following formula:
wherein q istotalThe total heat yield per unit volume in the charging and/or discharging process of the sodium ion battery; q. q.srevIs the reversible heat generation rate per unit volume in the charging and/or discharging process of the sodium ion battery; q. q.sirrIs the irreversible heat generation rate per unit volume during the charging and/or discharging process of the sodium ion battery.
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