Stepped current charging method for preventing lithium precipitation of lithium ion battery cathode
Technical Field
The invention belongs to the field of optimized charging of lithium ion batteries.
Background
Lithium ion battery negative pole lithium deposition is induced by low temperature, high state of charge (SOC) and large rate current. The lithium ion battery is faced with the problems of difficult charging, easy lithium precipitation during charging and the like at low temperature, seriously damages the driving range and safety of the electric automobile, and forms one of limiting factors for restricting the popularization of the electric automobile. How to charge electric energy into a battery quickly and safely under an extremely cold condition is one of key technologies for promoting popularization and application of an electric vehicle in a cold region, in order to avoid lithium precipitation of a battery cathode, the charging speed is extremely slow at a low temperature due to extremely small charging current which can be accepted by the battery because the internal resistance of the battery is sharply increased at the low temperature, and a slightly large harmonic current may cause lithium precipitation of the battery cathode, so that the service life of the battery is accelerated and attenuated, and even a safety problem is caused.
At present, charging methods at normal temperature are quite multiple, and the charging methods mainly comprise conventional constant current and constant voltage (CC-CV) charging, multi-stage constant current charging, pulse charging, sinusoidal alternating current charging, high-power pre-charging, constant power charging, optimized charging considering multiple targets and the like. However, most of these methods set the charging current multiplying power empirically, the phenomenon of low charging efficiency or high SOC lithium evolution occurs, and the normal temperature charging method is difficult to be directly applied to low temperature charging, such as CV charging method, and during constant voltage charging at low temperature, the negative electrode potential is likely to be lower than the lithium evolution potential, which may cause lithium evolution at the negative electrode. In addition, the dynamics of the battery is delayed at low temperature, the maximum charging current acceptable by the battery is very small, and when the battery is charged by directly using a normal-temperature charging method at low temperature, the charging time is very long, so that the practical requirement of quick charging is difficult to meet.
Disclosure of Invention
Aiming at the defects in the prior art, the invention is based on a three-electrode battery manufactured by using a commercial battery, a high-sensitivity model parameter in a model is identified through elaborate experimental design, a lithium ion battery electrochemical model with accurate physical meaning of the model parameter and higher model precision is established, the maximum acceptable boundary current for preventing lithium precipitation of a negative electrode of the lithium ion battery in any initial charge state can be calculated in a wider temperature interval, and finally, the stepped current charging method for preventing lithium precipitation of the negative electrode of the lithium ion battery is provided.
In order to achieve the above purposes, the technical scheme adopted by the invention is as follows:
in the patent, relevant experiments of three-electrode battery design are tested aiming at positive and negative solid-phase diffusion coefficients, positive and negative impedance parameters and positive and negative exchange current densities, the model is ensured to have higher precision, the model parameters have accurate physical meanings, a lithium ion battery electrochemical model suitable for a wider temperature region is constructed, according to lithium analysis criteria, initial current is input through the model to research the maximum acceptable charging current of the lithium ion battery without lithium analysis, the input initial SOC of the model is changed, the maximum acceptable charging current of the lithium ion battery in different initial states can be obtained, safety margins are further considered, a stepped current charging method for preventing lithium ion battery negative electrode lithium analysis is provided, the charging efficiency is highest on the premise that no lithium analysis occurs, and the service life of the battery is prolonged to the maximum extent.
In order to achieve the purpose, the invention adopts the following technical scheme:
a stepped current charging method for preventing lithium precipitation of a negative electrode of a lithium ion battery comprises the following steps:
s1, adding a reference electrode on the commercial battery, manufacturing a three-electrode battery, and carrying out electrochemical impedance spectrum and capacity increment curve tests on the three-electrode battery and the commercial battery to verify the effectiveness of the reference electrode;
s2, placing the three-electrode battery in an incubator environment at 25 ℃ for standing for 8 hours to ensure that the battery reaches a thermal equilibrium state, and carrying out a rated capacity test: charging at a constant current and a constant voltage of 4.2V at 1C until the current is reduced to 0.05C, standing for 1 hour, discharging at 1C to a cut-off voltage of 2.5V, standing for 1 hour, and performing charge-discharge circulation for three times, wherein the average value of the three discharge capacities is used as a rated capacity C0;
s3, carrying out a constant current intermittent titration technique and an electrochemical impedance spectrum experiment on the three-electrode battery at different environmental temperatures, and respectively determining high-sensitivity model parameters, namely positive and negative solid-phase diffusion coefficients, positive and negative impedance parameters and positive and negative exchange current densities so as to ensure that the model has higher precision and the model parameters have accurate physical meanings;
s4, establishing a high-precision electrochemical model according to the high-sensitivity model parameters obtained in the step S3 and the model parameters obtained by manufacturers and literatures, and determining the lithium analysis criterion formula as phis,NS-φ l,NS0, wherein phis,NSRepresents the solid phase potential of the negative electrode,. phil,NSThe liquid phase potential at the interface of the negative electrode-diaphragm is shown, so the criterion of lithium analysis is that the solid phase potential of the negative electrode is equal to the liquid phase potential at the interface of the negative electrode-diaphragm;
s5, changing the input conditions of the electrochemical model in the step S4, namely changing the temperature, the initial SOC and the current multiplying power, and determining the maximum acceptable current meeting the lithium analysis criterion;
and S6, setting a safety margin of 10% according to the maximum acceptable current obtained in the step S5, and taking 90% of the maximum acceptable current as the boundary current of the charging control.
The specific steps of step S3 are:
s31, placing the three-electrode battery in a to-be-tested environment temperature and standing for 8 hours to ensure that the battery reaches a thermal equilibrium state, emptying the three-electrode battery by using 1C rate current, and standing for 1 hour;
s32, setting the frequency range of electrochemical impedance spectrum testing to be 10 millihertz-10 kilohertz, setting the amplitude of sinusoidal alternating voltage to be 5mV, standing for 40min, and measuring impedance parameters and exchange current density of different SOC points through the step, wherein the impedance parameters comprise membrane impedance, charge transfer impedance and the like;
s33, the constant current intermittent titration technique is that a constant current is applied to a measuring system under a certain specific environment and is cut off after a period of time, the change of the system potential along with the time in the applied current period and the voltage reaching the balance after relaxation are observed, the relaxation information of the overpotential in the electrode process can be obtained by analyzing the change of the potential along with the time, and then the reaction kinetic information is conjectured and calculated. When the constant current is intermittently dripped, setting to charge for 6min by using current with 0.05C multiplying power, standing for 1 hour, and measuring the solid phase diffusion coefficients of the positive electrode and the negative electrode at different SOC points;
s34, filling the mixture into a container (5% C0-0.05C x 0.1h) with the rate of 0.2C, and standing for 1 hour;
s35, repeating the steps S32-S34 to obtain anode and cathode impedance parameters, anode and cathode exchange current densities and anode and cathode solid phase diffusion coefficients of the three-electrode battery at different SOC points in the charging direction under a certain specific environment;
and S36, changing the test environment temperature in the S31, and obtaining anode and cathode impedance parameters, anode and cathode exchange current densities and anode and cathode solid phase diffusion coefficients of the three-electrode battery at different temperatures at different SOC points in the charging direction.
In step S5, the maximum acceptable current of the battery in different initial states can be obtained in a wider temperature range by changing the temperature, the initial SOC state and the current multiplying power input by the electrochemical model.
In step S5, in consideration of the calculation error of the maximum charging current determined based on the electrochemical model and the calculation error of the maximum current at the untested temperature point, the calculated acceptable maximum charging current may be greater than or less than the true acceptable maximum charging current, and in order to prevent the battery from lithium precipitation under various operating conditions, a safety margin of 10% is set, that is, a current of 90% of the maximum acceptable current calculated by the model is used as the boundary current of the charging control.
The three-electrode battery manufactured by the commercial battery is used for carrying out a constant current intermittent titration technology and an electrochemical impedance spectrum experiment, so that the anode and cathode solid-phase diffusion coefficients and anode and cathode impedance parameters of the three-electrode battery are obtained, and the established electrochemical model is more in line with the performance characteristics of the commercial battery.
The invention provides a stepped current charging method for preventing lithium separation of a negative electrode of a lithium ion battery.
The invention has the beneficial effects that: the invention provides a platform for researching the charging strategy of the lithium ion battery. The established lithium ion battery model can simulate the maximum lithium non-analysis boundary current under different working conditions, and a stepped current charging method for preventing lithium analysis of the lithium ion battery cathode can be formulated based on the model basis. The stepped charging method is based on a lithium ion battery lithium separation theory, breaks through traditional experience selection, can effectively prevent lithium separation of a negative electrode, reduces the safety risk of the lithium ion battery, improves the charging efficiency, and provides an important reference value for the field of optimized charging of the lithium ion battery.
Drawings
The invention has the following drawings:
FIG. 1 is a schematic diagram of a three electrode cell made from a commercial battery;
FIG. 2 is a graphical representation of the maximum charge current acceptable to a battery at various temperatures when charging is initiated from 0% SOC;
FIG. 3 is a schematic diagram of a stepped charging current curve from the 0% SOC point, taking into account safety margin;
FIG. 40 is a schematic view of a step charging current curve at different SOC initial charging times.
Detailed Description
The present invention will be described in further detail with reference to the accompanying drawings.
S1, fig. 1 shows a schematic diagram of a three-electrode battery made of a commercial battery. Electrochemical impedance spectroscopy and capacity increment curve tests were performed on three-electrode batteries and commercial batteries: the electrochemical impedance spectrum testing frequency range is 10 milli-hertz to 10 kilohertz, and the amplitude of the sine alternating voltage is set to be 5 mV; the capacity increment curve test sets that 0.05C is charged to 4.2V, the battery is kept stand for 1 hour, 0.05C is discharged to 2.5V, the effectiveness of a reference electrode is verified, and an electrochemical model established by using a three-electrode battery to perform the test is more in line with the performance characteristics of a commercial battery;
s2, testing the rated capacity of the three-electrode battery at room temperature of 25 ℃, placing the three-electrode battery in a 25 ℃ incubator environment for standing for 8 hours to ensure that the battery reaches a thermal balance state, charging the battery to 0.05 ℃ by using a 1C constant current and constant voltage of 4.2V, standing for 1 hour, discharging the battery to a cut-off voltage of 2.5V by using 1C, standing for 1 hour, and carrying out charge-discharge circulation for three times, wherein the average value of three discharge capacities is used as the rated capacity C0;
s3, carrying out a constant current intermittent titration technique and an electrochemical impedance spectrum experiment on the three-electrode battery at different environmental temperatures, and respectively determining a positive and negative solid-phase diffusion coefficient, a positive and negative impedance parameter and a positive and negative exchange current density so as to ensure that the model has higher precision and the model parameters have accurate physical meanings;
the specific steps of step S3 are:
s31, placing the three-electrode battery in a to-be-tested environment temperature and standing for 8 hours to ensure that the battery reaches a thermal equilibrium state, emptying the three-electrode battery by using 1C rate current, and standing for 1 hour;
s32, setting the frequency range of electrochemical impedance spectrum testing to be 10 milli-hertz-10 kilohertz, setting the amplitude of sine alternating voltage to be 5mV, standing for 40min, and measuring anode and cathode impedance parameters and anode and cathode exchange current densities of different SOC points by the step, wherein the impedance parameters comprise membrane impedance, charge transfer impedance and the like;
s33, the constant current intermittent titration technique is that a constant current is applied to a measuring system under a certain specific environment and is cut off after a period of time, the change of the system potential along with the time in the applied current period and the voltage reaching the balance after relaxation are observed, the relaxation information of the overpotential in the electrode process can be obtained by analyzing the change of the potential along with the time, and then the reaction kinetic information is conjectured and calculated. The testing technology is set to charge for 6min by using current with 0.05C multiplying power, and the static state is set for 1 hour, so that the solid phase diffusion coefficients of the positive electrode and the negative electrode at different SOC points can be measured;
s34, filling the mixture into a container (5% C0-0.05C x 0.1h) with the rate of 0.2C, and standing for 1 hour;
s35, repeating the steps S32-S34 to obtain anode and cathode impedance parameters, anode and cathode exchange current densities and anode and cathode solid phase diffusion coefficients of the three-electrode battery at different SOC points in the charging direction under a certain specific environment;
s36, changing the test environment temperature in the S31 to obtain anode and cathode impedance parameters, anode and cathode exchange current densities and anode and cathode solid phase diffusion coefficients of the three-electrode battery at different SOC points in the charging direction at different temperatures;
s4, establishing a high-precision electrochemical model based on the high-sensitivity model parameters obtained in the step S3 and the model parameters obtained by manufacturers and literatures, and determining the lithium analysis criterion formula as phis,NS-φ l,NS0, wherein phis,NSRepresents the solid phase potential of the negative electrode,. phil,NSThe liquid phase potential at the interface of the negative electrode and the diaphragm is shown, namely the lithium analysis criterion is that the solid phase potential of the negative electrode is equal to the liquid phase potential at the interface of the negative electrode and the diaphragm;
s5, changing the input conditions of the electrochemical model in the step S4, namely changing the temperature, the initial SOC and the current multiplying power, and determining the maximum acceptable current meeting the lithium analysis criterion.
And S6, setting a safety margin of 10% according to the maximum acceptable current obtained in the step S5, and taking 90% of the maximum acceptable current as the boundary current of the charging control.
FIG. 2 shows the maximum charging current acceptable for a battery at different temperatures during initial charging from 0% SOC, with the model inputting different initial SOC points, different initial current rates, according to the lithium analysis criterion formula φs,NS-φl,NSObtaining the maximum acceptable current multiplying power of different SOC points as 0, and obtaining an SOC-acceptable maximum charging current curve as shown in fig. 2 by linear interpolation of the maximum current of the SOC points, wherein the acceptable maximum charging current of the lithium ion battery is reduced along with the increase of the SOC.
Fig. 3 shows a stepwise charging current from the 0% SOC point taking into account a safety margin, and considering a calculation error of the maximum charging current determined based on the electrochemical model and a calculation error of the maximum current at the untested temperature point, the calculated acceptable maximum charging current may be greater or less than the true acceptable maximum charging current, and in order to prevent the battery from precipitating lithium under various operating conditions, a safety margin of 10% is set, that is, a current of 90% of the maximum acceptable current calculated by the model is used as a boundary current for the charging control.
Considering that the lithium ion battery is not charged from an empty state in the actual use process, fig. 4 shows the step charging current when different SOC start charging is performed at 0 ℃, different initial SOC states are set by the lithium ion battery electrochemical model, and the initial SOC states are set with phis,NS-φl,NSAnd (3) taking 0 as a criterion for lithium analysis to obtain the maximum acceptable charging current in different initial states, wherein obviously, the lower the initial SOC is, the larger the acceptable charging current of the lithium ion battery in the initial charging stage is.
It should be understood that the present invention is only illustrative and not restrictive for clear description of the embodiments of the present invention, and that various changes and modifications may be made by those skilled in the art based on the above description.
The above embodiments are merely illustrative, and not restrictive, and those skilled in the relevant art can make various changes and modifications without departing from the spirit and scope of the invention, and therefore all equivalent technical solutions also belong to the scope of the invention.
Those not described in detail in this specification are within the skill of the art.