A full-life-cycle lithium-free fast charging method based on a polarization model

Abstract

The present invention proposes a full-life-cycle lithium-free fast charging method based on a polarization model. The present invention analyzes the internal mechanism of lithium plating in the battery. By implanting a reference electrode into the battery, accurate positive and negative electrode potentials of the battery are obtained. The polarization model is calibrated using online parameter identification. Subsequently, only the current input of the battery needs to be obtained to estimate the positive and negative electrode potentials and the terminal voltage at the next moment. Subsequently, the current magnitude is controlled by a current controller to ensure that the negative electrode potential is not less than the threshold value, so that lithium plating does not occur in the battery under any charging conditions.

Description

Technical Field

[0001] The present invention relates to the technical field of lithium-ion battery charging, and particularly to a full-life-cycle lithium-free fast charging method based on a polarization model. Background Art

[0002] With the oil crisis and people's increasing emphasis on environmental protection, new energy vehicles have been vigorously promoted by the government and favored by consumers. As the core of current new energy vehicles, lithium-ion batteries are particularly excellent in terms of energy, efficiency, lifespan, environmental friendliness, etc. However, lithium-ion batteries have relatively strict requirements for their own operating conditions. At too high temperatures, the battery is prone to thermal runaway and accelerated aging. At too low charging temperatures and too large charging current rates, lithium plating is likely to occur. At too small charging current rates, the battery charging time is extremely long. How to ensure that lithium plating does not occur during the charging of lithium-ion batteries under any conditions is a difficult problem in the industry. Currently, the main methods for battery charging at home and abroad are constant current and constant voltage, pulse, intermittent, step-down current, etc. However, these existing charging methods are difficult to ensure that lithium plating does not occur completely during the charging process. Summary of the Invention

[0003] The object of the present invention is to propose a full-life-cycle lithium-free fast charging method based on a polarization model to overcome the defect of easy lithium plating existing in existing fast charging technologies.

[0004] To achieve the above object, the present invention proposes a full-life-cycle lithium-free fast charging method based on a polarization model, which ensures the range of the negative electrode potential during the battery charging process to avoid lithium plating through a polarization model and a current controller, including the following steps:

[0005] S1: implant a reference electrode into a fresh battery and reference batteries with different aging degrees, and identify the relevant parameters of the positive and negative electrodes of the fresh battery and reference batteries based on an online identification algorithm;

[0006] S2: according to the results of the online identification algorithm for identifying the ohmic internal resistances of the positive and negative electrodes of the fresh and reference batteries, combined with the battery aging mechanism, proportionally allocate the internal resistances of the reference batteries with different aging degrees to obtain the relative change relationship between the overall ohmic internal resistance of the batteries with different aging degrees and the ratio of the ohmic internal resistances of the positive and negative electrodes, and build a complete full-life-cycle lithium-free fast charging polarization circuit model of the battery;

[0007] S3: for a test battery to be optimized for charging, determine the aging degree and relevant parameters of the test battery through the online identification algorithm and input them into the polarization circuit model, and the polarization circuit model outputs the negative electrode potential V of the test battery in real time; an ;

[0008] S4: The current controller receives the negative electrode potential output by the polarization circuit model and performs fuzzy control. an >V threshold When the negative electrode potential of the test battery is less than the negative electrode potential threshold, that is, V an <V threshold When the current controller reduces the current by a preset rate as the change amount and monitors the change of the negative electrode potential in real time to achieve the negative electrode potential maintained at the V threshold Nearby target.

[0009] Furthermore, the SOH of the fresh battery is 100%; the SOH of the reference battery at different aging degrees is any value between 70% and 100%.

[0010] Furthermore, in step S2, the relative change relationship between the overall ohmic internal resistance of the battery at different aging degrees and the ratio of the positive and negative electrode ohmic internal resistance is: as the aging degree of the lithium-ion battery increases, the overall ohmic internal resistance of the battery increases proportionally; the increased ohmic internal resistance after aging is distributed between the positive and negative electrodes according to a preset ratio.

[0011] Furthermore, the preset ratio of the positive and negative electrodes is any value between 0 and 1.

[0012] Furthermore, in step S2 and step S3, the online identification method calculates the open circuit voltage, ohmic internal resistance, polarization internal resistance and time constant according to the battery current and voltage.

[0013] Furthermore, the battery parameters include: positive electrode open circuit voltage, positive electrode ohmic internal resistance, positive electrode polarization internal resistance, positive electrode time constant and negative electrode open circuit voltage, negative electrode ohmic internal resistance, negative electrode polarization internal resistance and negative electrode time constant.

[0014] Further, in step S3, the polarization model is composed of two first-order RC models of positive and negative electrodes, wherein the full battery terminal voltage is expressed as:

[0015] U t =V ca -V an

[0016] Where: U t is the full battery terminal voltage, V ca is the positive output potential, V an is the negative electrode output potential;

[0017] The output potential of the positive and negative electrodes can be expressed by the open circuit potential and the ohmic overpotential and polarization overpotential (V p ), where the output potential of the positive electrode is expressed as:

[0018]

[0019] Wherein: is the open - circuit voltage of the battery positive electrode, and I bat is the real - time current of the battery, is the ohmic internal resistance of the positive electrode, is the polarization over - potential of the positive electrode;

[0020] Among them, the expression formula for the output potential of the negative electrode is:

[0021]

[0022] Wherein: is the open - circuit voltage of the battery negative electrode, and I bat is the real - time current of the battery, is the ohmic internal resistance of the negative electrode, is the polarization over - potential of the negative electrode;

[0023] The relationship between the transient polarization over - potential on the RC link and current and time can be expressed as follows. Among them, the expression formula for the polarization over - potential of the positive electrode is:

[0024]

[0025] Wherein: Δt is the time step, and τ ca is the time constant of the positive electrode, the polarization internal resistance of the positive electrode;

[0026] The expression formula for the polarization over - potential of the negative electrode is:

[0027]

[0028] Wherein: Δt is the time step, and τ an is the time constant of the negative electrode, is the polarization internal resistance of the negative electrode;

[0029] The actual model parameters are obtained through HPPC and dynamic working condition experiments on the battery and using the online identification method.

[0030] Furthermore, the polarization model can automatically select corresponding parameters according to the current ambient temperature and the battery aging state.

[0031] Furthermore, in step S4, the negative electrode potential threshold is any value between 0mv and 100mv.

[0032] Compared with the prior art, the advantages of the present invention are:

[0033] 1. Based on the internal mechanism of lithium deposition in lithium-ion batteries, the present invention obtains accurate positive and negative electrode potentials through the implantation of a reference electrode. Ensuring that the negative electrode potential is not less than 0 can guarantee that no lithium deposition occurs during charging. The present invention performs parameter identification on lithium-ion batteries in different aging states based on online identification to obtain a polarization model parameter table for the entire life cycle. Then, through the calibrated polarization model, the positive and negative electrode potentials and the battery terminal voltage during the battery charging process are estimated. Finally, the current is reasonably regulated through a PID controller to ensure that the negative electrode potential fluctuates within a reasonable range, realizing that no lithium deposition reaction occurs during the charging process of lithium-ion batteries.

[0034] 2. The full-life-cycle lithium-deposition-free fast charging method based on the polarization model of the present invention can accurately estimate the true positive and negative electrode potentials of the battery. For batteries in different aging states and at different temperatures, the model parameters can be automatically updated through the polarization model to more accurately estimate the battery terminal voltage and the negative electrode potential. By controlling with a current controller, it can ensure that the current during the battery charging process is near the set negative electrode potential threshold without lithium deposition. BRIEF DESCRIPTION OF THE DRAWINGS

[0035] Figure 1 It is a flowchart of the full-life-cycle lithium-deposition-free fast charging method based on the polarization model in an embodiment of the present invention;

[0036] Figure 2 It is a first-order RC equivalent circuit diagram in an embodiment of the present invention;

[0037] Figure 3 It is a schematic diagram of the polarization model in an embodiment of the present invention;

[0038] Figure 4 It is a schematic diagram of the change of the positive and negative electrode ohmic internal resistance with the overall ohmic internal resistance of the battery in an embodiment of the present invention;

[0039] Figure 5 It is a schematic diagram of the negative electrode potential during a single lithium-deposition-free fast charge in an embodiment of the present invention;

[0040] Figure 6 It is a schematic diagram of the current during a single lithium-deposition-free fast charge in an embodiment of the present invention. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0041] To make the objectives, technical solutions, and advantages of the present invention clearer, the technical solutions of the present invention will be further described below.

[0042] As Figure 1 shown, the present invention estimates the negative electrode potential of the battery through the polarization model, and then uses a current controller to control the current to reasonably increase or decrease the current to achieve the purpose of fast charging without lithium deposition. The specific implementation process includes steps S1, S2, S3, and S4.

[0043] S1 As Figure 2, Figure 3 , implant a reference electrode into the fresh battery and the reference battery at different aging degrees, and identify the relevant parameters of the positive and negative electrodes of the fresh battery and the reference battery based on the online identification algorithm;

[0044] S11 Implant a reference electrode into the test lithium-ion battery to obtain the positive and negative electrode potentials during the use of this type of battery.

[0045] S12 Conduct experimental tests on the test battery with the implanted reference, and obtain the open-circuit voltage, ohmic internal resistance, time constant, and polarization internal resistance corresponding to the positive and negative electrodes of the test battery through the online identification method. The online identification method is based on the first-order RC equivalent circuit model, and the least squares method with a forgetting factor is used to optimize the parameter identification scheme.

[0046] S2 As Figure 4 shown, according to the results of the online identification algorithm for the ohmic internal resistance of the positive and negative electrodes of the fresh and reference batteries, combined with the battery aging mechanism, proportionally allocate the internal resistance of the reference battery at different aging degrees to obtain the relative change relationship between the overall ohmic internal resistance of the battery at different aging degrees and the proportion of the ohmic internal resistance of the positive and negative electrodes, and build a complete non-lithium-depositing and fast full-polarization circuit model of the battery under the full life cycle;

[0047] The specific steps of the above-mentioned step S2 are as follows:

[0048] S21 Conduct online parameter identification on the preset aging battery to obtain the ohmic internal resistance of the positive and negative electrodes;

[0049] S22 Compare the overall ohmic internal resistance of the fresh test battery (100%) and multiple aging test batteries (x%), and calculate the corresponding positive electrode ohmic internal resistance and negative electrode ohmic internal resistance ratio:

[0050] Positive and negative electrode ratio of fresh battery (100% SOH)

[0051] Positive electrode ohmic internal resistance Negative electrode ohmic internal resistance

[0052] Positive electrode ohmic internal resistance after aging Negative electrode ohmic internal resistance after aging

[0053] Positive and negative electrode ratio of aging battery (x% SOH):

[0054] According to multiple ratios and the overall ohmic internal resistance value of the battery, the proportional relationship between the change in the overall ohmic internal resistance of the battery and the ohmic internal resistances of the positive and negative electrodes is corresponding, and further, the proportional relationship between the ohmic internal resistances of the positive and negative electrodes corresponding to different overall ohmic internal resistances in the whole life cycle is obtained.

[0055] S3 is as Figure 5 shown. For the test battery to be optimized for charging, the aging degree and related parameters of the test battery are determined by an online identification algorithm and input into the polarization circuit model, and the negative electrode potential V of the test battery is output by the model in real time an .

[0056] In the step S3, the polarization model is composed of two first-order RC models for the positive and negative electrodes. Among them, the full-cell terminal voltage can be expressed as:

[0057] U t = V ca - V an

[0058] In the formula: U t is the full-cell terminal voltage, V ca is the output potential of the positive electrode, V an is the output potential of the negative electrode.

[0059] The output potentials of the positive and negative electrodes can be calculated using the open-circuit potential and the additional voltage drop caused by the ohmic overpotential and the polarization overpotential (V p ). Among them, the expression for the output potential of the positive electrode is:

[0060]

[0061] In the formula: is the open-circuit voltage of the positive electrode of the battery, I bat is the real-time current of the battery, is the ohmic internal resistance of the positive electrode, is the polarization overpotential of the positive electrode.

[0062] Among them, the expression formula for the output potential of the negative electrode is:

[0063]

[0064] In the formula: is the open-circuit voltage of the negative electrode of the battery, I bat is the real-time current of the battery, is the ohmic internal resistance of the negative electrode, is the polarization overpotential of the negative electrode.

[0065] The relationship between the transient polarization overpotential on the RC link and the current and time can be expressed as follows. Among them, the expression formula for the polarization overpotential of the positive electrode is:

[0066]

[0067] where: Δt is the time step, τ ca is the positive electrode time constant, and the positive electrode polarization internal resistance.

[0068] The expression formula for the negative electrode polarization overpotential is:

[0069]

[0070] where: Δt is the time step, τ an is the negative electrode time constant, and is the negative electrode polarization internal resistance.

[0071] The actual model parameters need to be obtained through HPPC and dynamic working condition experiments on the battery and using the online identification method.

[0072] S4 is as Figure 6 , the current controller receives the negative electrode potential output by the polarization circuit model and performs fuzzy control. When the negative electrode potential of the test battery is greater than the negative electrode potential threshold (V an > V threshold ), the current controller increases the current with a preset magnification as the variation; when the negative electrode potential of the test battery is less than the negative electrode potential threshold (V an < V threshold ), the current controller decreases the current with a preset magnification as the variation and monitors the change of the negative electrode potential in real time to achieve the goal of maintaining the negative electrode potential near the V threshold .

[0073] The specific steps of the step S4 are as follows:

[0074] S41 uses the negative electrode potential predicted by the polarization model and the current at the previous moment as the input of the current controller. When the negative electrode potential (V an ) is greater than the set threshold (V threshold ), the current controller outputs a current approaching the maximum charge magnification acceptable to the battery; after the negative electrode potential (V an ) is less than or equal to the set threshold (V threshold ), the current controller outputs a current that decreases by a percentage of the input current to ensure that the negative electrode potential fluctuates near the set threshold and is not less than 0 during the charging process until the charging is completed to obtain a fast charge curve without lithium deposition;

[0075] S42 changes the temperature and aging state, and the polarization model automatically selects the corresponding parameters, and continues to obtain fast charge curves under multiple states to achieve fast charging without lithium deposition throughout the life cycle;

[0076] In the embodiments of the present invention, the polarization model is based on a first-order RC equivalent circuit model. In practice, the specific use is not limited to the first-order RC equivalent circuit model and is applicable to an n (n≥1)-order RC equivalent circuit model.

[0077] The above are only the preferred embodiments of the present invention and do not impose any limitation on the present invention. Any person skilled in the art, within the scope of the technical solution of the present invention, makes any form of equivalent replacement or modification and other changes to the technical solution and technical content disclosed by the present invention, which are all within the content of the technical solution of the present invention and still fall within the protection scope of the present invention.

Claims

1. A full-life-cycle lithium-free fast charging method based on a polarization model, characterized in that Ensure the range of the negative electrode potential during the battery charging process through a polarization model and a current controller to avoid lithium plating, including the following steps: S1: Implant a reference electrode into a fresh battery and reference batteries with different aging degrees, and identify the relevant parameters of the positive and negative electrodes of the fresh battery and reference batteries based on an online identification algorithm; S2: According to the results of the online identification algorithm for identifying the ohmic internal resistances of the positive and negative electrodes of the fresh and reference batteries, combined with the battery aging mechanism, proportionally allocate the internal resistances of the reference batteries with different aging degrees, obtain the relative change relationship between the overall ohmic internal resistance of the batteries with different aging degrees and the proportion of the ohmic internal resistances of the positive and negative electrodes, and build a complete non-lithium-plating and fast polarization circuit model of the battery under the full life cycle; S3: For the test battery to be optimized for charging, determine the aging degree and related parameters of the test battery through the online identification algorithm and input them into the polarization circuit model, and the polarization circuit model outputs the negative electrode potential V of the test battery in real time an ; S4: The current controller receives the negative electrode potential output by the polarization circuit model and performs fuzzy control; when the negative electrode potential of the test battery is greater than the negative electrode potential threshold, i.e., V an > V threshold , the current controller increases the current with a preset magnification as the change amount; when the negative electrode potential of the test battery is less than the negative electrode potential threshold, i.e., V an < V threshold , the current controller decreases the current with a preset magnification as the change amount and monitors the change of the negative electrode potential in real time to achieve the goal of maintaining the negative electrode potential near the V threshold .

2. The all-life-cycle lithium-free fast charging method based on the polarization model according to claim 1, wherein The SOH of the fresh battery is 100%; the SOH of the reference batteries with different aging degrees is any value between 70% and 100%.

3. The full-life-cycle lithium-free fast charging method based on the polarization model according to claim 1, wherein In step S2, the relative change relationship between the overall ohmic internal resistance of the batteries with different aging degrees and the proportion of the ohmic internal resistances of the positive and negative electrodes is as follows: as the aging degree of the lithium-ion battery increases, the overall ohmic internal resistance of the battery increases proportionally; for the increased ohmic internal resistance after aging, the positive and negative electrodes are allocated according to a preset ratio.

4. The full-life-cycle lithium-free fast charging method based on the polarization model according to claim 3, wherein The preset ratio of the positive and negative electrodes is any value between 0 and 1.

5. The all-life-cycle lithium-free fast charging method based on the polarization model according to claim 1, characterized in that In steps S2 and S3, the online identification algorithm calculates the open-circuit voltage, ohmic internal resistance, polarization internal resistance, and time constant based on the battery current and voltage.

6. The full-life-cycle lithium-free fast charging method based on the polarization model according to claim 5, wherein The parameters of the battery include: positive open-circuit voltage, positive ohmic internal resistance, positive polarization internal resistance, positive time constant, and negative open-circuit voltage, negative ohmic internal resistance, negative polarization internal resistance, and negative time constant.

7. The full-life-cycle lithium-free fast charging method based on the polarization model according to claim 1, characterized in that In step S3, the polarization model consists of two first-order RC models for the positive and negative electrodes, where the full-cell terminal voltage is expressed as: U t = V ca -V an Where: U t is the full cell terminal voltage, V ca is the positive electrode output potential, V an is the negative electrode output potential; The output potentials of the positive and negative electrodes can be calculated using the open-circuit potential and the additional voltage drops caused by the ohmic overpotential and the polarization overpotential (V p ). The expression for the output potential of the positive electrode is as follows: Where: is the open-circuit voltage of the battery positive electrode, I bat is the real-time current of the battery, is the ohmic internal resistance of the positive electrode, is the polarization overpotential of the positive electrode; where the expression formula for the negative electrode output potential is: Wherein: is the open-circuit voltage of the battery negative electrode, I bat is the real-time current of the battery, is the ohmic internal resistance of the negative electrode, is the polarization overpotential of the negative electrode; The relationship between the transient polarization overpotential on the RC link with current and time can be expressed as follows, where the expression formula for the positive electrode polarization overpotential is: where: Δt is the time step, τ ca is the positive electrode time constant, the positive electrode polarization internal resistance; The expression formula for the negative electrode polarization overpotential is: where: Δt is the time step, and τ an is the negative electrode time constant, and is the negative electrode polarization internal resistance; The actual model parameters are obtained through HPPC and dynamic operating condition experiments on the battery using the online identification algorithm.

8. The full-life-cycle lithium-free fast charging method based on the polarization model according to claim 7, characterized in that The polarization model can automatically select the corresponding parameters according to the current ambient temperature and the battery aging state.

9. The full-life-cycle lithium-free fast charging method based on the polarization model according to claim 7, characterized in that, In step S4, the negative electrode potential threshold is any value between 0 mV and 100 mV.

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