CN111751741B - A kind of non-destructive testing method for the threshold voltage of lithium evolution of lithium ion battery - Google Patents

Abstract

The invention discloses a non-destructive testing method for lithium-evolution threshold voltage of lithium ion batteries, comprising the steps of: Step 1: In the first step, the battery is charged with a constant current with a normal charging current I of a preset size, and the battery is charged at a fixed time interval or every time At intervals of a fixed voltage value, it immediately enters a stop charging sleep phase until the charging reaches the set cut-off voltage; the second step is to draw the first resistance-voltage curve; the third step is to use a preset small current I', carry out constant current charging to the same type of battery, draw and obtain the second resistance-voltage curve; the fourth step, compare the second resistance-voltage curve with the first resistance-voltage curve as a reference curve; read the battery at The threshold voltage at which lithium deposition begins to occur when constant current charging is performed at current I. The invention determines the threshold voltage at which lithium precipitation occurs during the battery charging process by intermittently measuring the DC resistance of the battery during the battery charging process, and analyzing the resistance-voltage curve.

Description

A kind of non-destructive testing method of lithium-ion battery lithium evolution threshold voltage

technical field

The invention relates to the technical field of batteries, in particular to a non-destructive testing method for lithium-evolution threshold voltage of lithium-ion batteries.

Background technique

At present, lithium-ion batteries have the advantages of high specific energy, many cycles, and long storage time. They are not only widely used in portable electronic devices such as mobile phones, digital cameras and laptop computers, but also widely used in electric vehicles and electric bicycles. As well as large and medium-sized electric equipment such as power tools, the performance requirements for lithium-ion batteries are getting higher and higher.

In current commercial lithium-ion batteries, graphite is still a commonly used anode material for lithium-ion batteries. Because its lithium intercalation potential is close to that of metal lithium, it is easy to occur under certain conditions of use, such as high-current charging or low-temperature charging. Lithium precipitation phenomenon, which seriously affects the cycle performance of lithium-ion batteries, and may even cause safety problems such as short circuit in the battery due to the formation of lithium dendrites.

At present, the commonly used detection method for lithium precipitation is generally by disassembling the battery and manually observing whether there is metallic lithium on the surface of the negative electrode sheet based on experience. Although this method is relatively straightforward, it is an after-the-fact judgment and cannot detect the threshold voltage parameter when the battery begins to precipitate lithium.

Among them, it should be noted that the threshold means the limit, so the threshold is also called the critical value, which refers to the lowest or highest value that an effect can produce. In this patent, the threshold voltage when the battery starts to precipitate lithium means: the lowest voltage corresponding to the time when the battery starts to precipitate lithium during the charging process, that is, when the voltage exceeds this voltage, the battery starts and will continue to precipitate lithium, so it is called a battery The threshold voltage at which lithium precipitation occurs, that is, the threshold voltage at which lithium precipitation occurs in the battery.

Through the detection of the lithium deposition threshold voltage of the battery at different temperatures and different charging currents, it can be determined when the lithium deposition occurs in the battery under these conditions, so as to limit the charging cut-off voltage under the corresponding charging current to be below the threshold voltage. , to ensure good performance of the battery and safe use. Especially in the current market demand for the improvement of battery fast charging capability, when developing the fast charging system, the R&D personnel try to shorten the charging time as much as possible, and at the same time refer to parameters such as the lithium deposition threshold voltage to effectively avoid the battery during use. Lithium precipitation occurs. Because once lithium precipitation occurs in the battery, due to the high activity of the precipitation lithium, it is easy to have side reactions with the electrolyte, and the generated side reaction products are deposited on the surface of the negative electrode, which will increase the battery impedance and affect the performance of the battery. When the battery increases, lithium continues to grow on the surface of the negative electrode, and the formed lithium dendrites easily pierce the separator, resulting in an internal short circuit in the battery, and even safety accidents such as thermal runaway of the battery.

As the battery is used, its performance will decline, and the original charging system may no longer be applicable. Therefore, in the following specific technical solutions of the present invention, the present invention proposes a detection method for the threshold voltage of lithium evolution, which can be applied to the detection in the whole life cycle of the battery. The charging system will help prolong the life of the battery and ensure safety.

SUMMARY OF THE INVENTION

The purpose of the present invention is to provide a non-destructive testing method for the lithium evolution threshold voltage of a lithium ion battery in view of the technical defects existing in the prior art.

To this end, the present invention provides a non-destructive testing method for the lithium evolution threshold voltage of a lithium ion battery, comprising the following steps:

The first step is to charge a battery with a constant current with a normal charging current I of a preset size, and during the constant current charging process, every fixed period of time or every fixed voltage value, that is, immediately enter a stop Charging and dormancy stage, until the battery is charged to the set cut-off voltage;

Among them, in each stop charging and dormancy stage, the starting voltage V _s of the battery at the beginning of the stop charging is collected in real time, and after a preset sleep duration, the end voltage V _r of the battery at the end of the sleep is collected in real time, and then according to the preset Design the formula to calculate and obtain the initial DC resistance R s when the battery is charged with the normal charging current I constant current, and the initial DC resistance R _s when the charging is stopped at the beginning of each stop charging dormancy stage; that is, the DC resistance of the battery is obtained by intermittent measurement;

In the second step, take the initial voltage V _s of the battery in each stop charging and dormancy stage as the abscissa, and take the corresponding measured initial DC resistance R _s of the battery in each stop charging and dormancy stage as the ordinate, and draw to obtain The resistance-voltage curve in the process of constant current charging of the battery with the normal charging current I is defined as the first resistance-voltage curve;

The third step is to perform constant current charging on the same type of battery with a preset small current I', and in the process of constant current charging, at every fixed time interval or at every fixed voltage value, immediately enter a Stop the charging and dormancy stage until the battery is charged to the set cut-off voltage; then, take the starting voltage V _s ' of the battery in each stop charging and dormancy stage as the abscissa, to correspond to the measured battery in each stop charging and dormancy stage. The initial DC resistance R _s ' is the ordinate, and the resistance-voltage curve during the constant current charging process of the battery with a small current I' is drawn and obtained, which is defined as the second resistance-voltage curve;

Among them, in each stop charging and dormancy stage, the starting voltage V _s ' of the battery at the beginning of the stop charging is collected in real time, and after a preset sleep period, the end voltage V _r ' of the battery at the end of the sleep is collected in real time, and then According to the preset calculation formula, calculate and obtain the initial DC resistance R s ' when the battery is charged at a constant current with a small current I', and the initial DC resistance R _s ' at the beginning of each stop charging dormancy stage; that is, the DC resistance of the battery is obtained by intermittent measurement;

In the fourth step, the second resistance-voltage curve is used as the reference curve, and the first resistance-voltage curve is compared with the first resistance-voltage curve; when the first resistance-voltage curve appears in the process of increasing with the voltage, the trend is different from the reference curve. When the inflection point of the first DC resistance value decreases, it means that the battery begins to precipitate lithium, and reading the corresponding battery voltage V _s,L in the first resistance-voltage curve at this time means that the battery is charging at the normal charging current I. During constant current charging, the threshold voltage at which lithium deposition begins to occur.

Among them, in the first step, the preset calculation formula is as follows:

When the battery is charged at a constant current with a normal charging current I, the initial DC resistance R _s =(V _s -V _r )/I at the beginning of each stop charging sleep phase at the beginning of the stop charging;

In the third step, the preset calculation formula is as follows:

When the battery is charged at a constant current with a small current I', the initial DC resistance R _s '=(V _s '-V _r ')/I' at the beginning of each stop charging sleep phase.

Therefore, in the first and third steps, every fixed period of time immediately enters a stop charging dormancy stage, in which:

For each fixed time interval, the calculation formula of the time length T of the fixed time is as follows:

Time length T=Q/I ₀ *3600*A%, in milliseconds;

Among them, Q is the battery capacity, I ₀ is the size of the charging current; the value range of A% is 0.02% to 5%;

In the first step, the charging current magnitude I ₀ is equal to the normal charging current I; in the third step, the charging current magnitude I ₀ is equal to the small current I'.

Among them, in the first and third steps, every time a fixed voltage value is interval, it immediately enters a stop charging sleep stage, wherein:

Fixed voltage value, the set value range is 1mV ~ 100mV;

In the first step and the third step, the sleep preset time period is specifically 0.01-50s.

Among them, the voltage value of fixed size, the set value range is 5mV-50mV;

In the first and third steps, the preset sleep duration is 0.1s-10s.

Among them, the value range of the small current I' is 0.01C ~ 0.5C.

Among them, the value range of the small current I' is 0.05C-0.3C.

It can be seen from the above technical solutions provided by the present invention that, compared with the prior art, the present invention provides a non-destructive testing method for the lithium evolution threshold voltage of a lithium ion battery. It is of great practical significance to determine the threshold voltage of lithium precipitation during the charging process of the battery by analyzing the resistance-voltage curve.

Description of drawings

Fig. 1 is the flow chart of the nondestructive detection method of a kind of lithium ion battery lithium evolution threshold voltage improved by the present invention;

2 is a non-destructive testing method for a lithium-ion battery lithium-evolution threshold voltage improved by the present invention, and a schematic diagram of a DC impedance test in Example 1;

Fig. 3 is a kind of non-destructive testing method of lithium-ion battery lithium evolution threshold voltage improved by the present invention, in Example 1, the resistance-voltage curve schematic diagram of the battery in the process of charging with 1C current constant current;

Fig. 4 is a kind of non-destructive testing method of lithium ion battery lithium evolution threshold voltage improved by the present invention, in Example 1, the resistance-voltage reference curve schematic diagram of the battery in the process of charging with 0.2C current constant current;

Fig. 5 is a kind of non-destructive testing method of the lithium-ion battery lithium evolution threshold voltage improved by the present invention, in embodiment 2, the resistance-voltage curve schematic diagram of the battery in the process of charging with 0.7C current constant current;

6 is a non-destructive testing method for a lithium-ion battery lithium evolution threshold voltage improved by the present invention, in Example 2, a schematic diagram of a resistance-voltage reference curve during constant current charging of the battery with a 0.2C current.

Detailed ways

In order to make those skilled in the art better understand the solution of the present invention, the present invention is further described in detail below with reference to the accompanying drawings and embodiments.

Referring to FIGS. 1 to 6 , the present invention provides a non-destructive testing method for the threshold voltage of lithium ion battery evolution, which specifically includes the following steps:

The first step is to charge a battery with a constant current with a normal charging current I of a preset size, and during the constant current charging process, every fixed period of time or every fixed voltage value, that is, immediately enter a stop Charging and dormancy stage, until the battery is charged to the set cut-off voltage;

Among them, in each stop charging and dormancy stage, the initial voltage V _s of the battery at the beginning of the stop charging (the voltage when the battery is charged with the preset current I constant current for a certain period of time or voltage value) is collected in real time, and the battery is preset in the sleep state. After the duration, the end voltage V _r of the battery at the end of the dormancy (that is, the battery voltage after the pre-set dormancy duration) is collected in real time, and then according to the preset calculation formula, it is calculated to obtain when the battery is charged with the normal charging current I constant current, at each The initial DC resistance R _s when the charging is stopped at the beginning of the dormant phase of charging; that is, the DC resistance of the battery is obtained by intermittent measurement;

In the second step, take the initial voltage V _s of the battery in each stop charging and dormancy stage as the abscissa, and take the corresponding measured initial DC resistance R _s of the battery in each stop charging and dormancy stage as the ordinate, and draw to obtain The resistance-voltage curve in the process of constant current charging of the battery with the normal charging current I is defined as the first resistance-voltage curve;

The third step is to use a preset small current I' to test the same type of batteries (for example, batteries of the same product model in the same batch, that is, batteries with the same specifications, as described below, for example, all 21700 cylindrical lithium-ion batteries) battery) for constant current charging, and in the process of constant current charging, every fixed time interval or every interval of a fixed voltage value, that is, immediately enter a stop charging dormancy stage, until the battery is charged to the set cut-off voltage; then , take the initial voltage V _s ' of the battery in each stop charging and dormancy stage as the abscissa, and take the measured initial DC resistance R _s ' of the battery in each stop charging and dormancy stage as the ordinate, draw the battery to obtain The resistance-voltage curve in the process of constant current charging with small current I' is defined as the second resistance-voltage curve;

Among them, in each stop charging and dormancy stage, the initial voltage V _s ' (the voltage when the battery is charged with the preset current I constant current for a certain period of time or voltage value) is collected in real time when the battery starts to stop charging in real time, and in the dormancy pre-set After setting the time length, collect the end voltage V _r ' of the battery at the end of the dormancy in real time (that is, the battery voltage after the dormancy preset time length), and then calculate and obtain according to the preset calculation formula when the battery is charged with a small current I ' constant current, in The initial DC resistance R _s ' when the charging is stopped at the beginning of each stop-charging dormancy stage; that is, the DC resistance of the battery is obtained by intermittent measurement;

In the fourth step, the second resistance-voltage curve is used as the reference curve, and the first resistance-voltage curve is compared with the first resistance-voltage curve; when the first resistance-voltage curve appears in the process of increasing with the voltage, the trend is different from the reference curve. When the first inflection point of the DC resistance value decreases (that is, the inflection point of the DC resistance reduction), it means that the battery begins to precipitate lithium. Read the corresponding battery voltage V _sL in the first resistance-voltage curve at this time, that is, the battery is in the When constant current charging is performed with normal charging current I, the threshold voltage at which lithium precipitation begins to occur, which can also be referred to as the maximum threshold voltage at which lithium precipitation does not occur when the battery is charged with normal charging current I.

Wherein, the trend of the first resistance-voltage curve is different from that of the reference curve in the process of increasing with the voltage, for example, the first resistance-voltage curve may be an upward trend (the DC resistance increases), and the reference curve is a downward trend; Or the first resistance-voltage curve has a downward trend (DC resistance decreases), while the reference curve has an upward trend

In the first and third steps, the time, voltage, current and capacity data during the charging process of the battery are collected in real time.

In the first step, in terms of specific implementation, the preset calculation formula is as follows:

When the battery is charged at a constant current with a normal charging current I, the initial DC resistance R _s =(V _s -V _r )/I at the beginning of the charging stop at the beginning of each stop charging sleep period.

In the third step, in terms of specific implementation, the preset calculation formula is as follows:

When the battery is charged at a constant current with a small current I', the initial DC resistance R _s '=(V _s '-V _r ')/I' at the beginning of each stop charging sleep phase.

For the present invention, in terms of specific implementation, the test environment (that is, the working environment) in the first step is the same as the test environment (that is, the working environment) in which the constant current charging with a small current I' is used in the third step.

In the third step, in terms of specific implementation, the preset small current I' is preferably the charging current at which the battery does not precipitate lithium under the evaluation conditions, for example, 0.01C to 0.5C at normal temperature (for example, 5 to 25 degrees Celsius). , perform an intermittent DC resistance test with this small current, and use the drawn resistance-voltage curve shape as a reference curve.

It should be noted that, for the present invention, the method for detecting the threshold voltage of lithium deposition provided by the present invention is non-destructive testing, and does not require additional processing of the battery or high-precision testing equipment, so it is suitable for all types of batteries in various The detection of the threshold voltage of lithium evolution in the working environment, and is suitable for the detection of the threshold voltage of lithium evolution in all stages of the battery life cycle.

In the first and third steps, in terms of specific implementation, every fixed period of time, it immediately enters a stage of stop charging and dormancy, in which:

For every fixed time interval, the time length T of the fixed time (ie, the interval time) can be based on the charging required by the battery per charging a preset percentage A% of the battery capacity Q (for example, the charging amount of SOC of 0.02% to 5%). time is calculated; the calculation formula of time length T is as follows:

Time length T=Q/I ₀ *3600*A%, in milliseconds;

Among them, Q is the battery capacity, I ₀ is the charging current; the value range of A% is preferably 0.02% to 5%;

In the first step, the charging current magnitude I ₀ is equal to the normal charging current I; in the third step, the charging current magnitude I ₀ is equal to the small current I'.

For example, when charging with 1C current, the interval time is 36s (that is, equal to this charging time), that is, the battery is charged with a constant current of 1C, and sleeps once every 36s for intermittent testing of DC impedance.

In the present invention, in the first step and the third step, every time there is a fixed voltage value, it immediately enters a stop charging dormancy stage, wherein:

The voltage value of a fixed size (ie, the interval voltage value) can be set in the range of 1mV-100mV, preferably 5mV-50mV.

In the first and third steps, in terms of specific implementation, the preset sleep duration is specifically 0.01-50s, preferably 0.1s-10s.

In the present invention, in the fourth step, the method uses the first resistance-voltage curve obtained when the normal charging current I is charged with constant current, and the reference curve obtained when the small current does not precipitate lithium (that is, the first resistance-voltage curve). The shape of the two resistance-voltage curves) was compared and analyzed to obtain the threshold voltage of lithium deposition of the battery.

In the third step, in terms of specific implementation, the value range of the small current I' depends on different test environments, and is generally 0.01C to 0.5C, preferably 0.05C to 0.3C.

In the fourth step, in terms of specific implementation, the method uses the first resistance-voltage curve obtained when the normal charging current I is charged with constant current, and the reference curve obtained when the small current does not precipitate lithium (that is, the first resistance-voltage curve obtained under the condition of no lithium precipitation). Two resistance-voltage curves) shapes are compared, when the first resistance-voltage curve measured in the charging process with the normal charging current I, the first inflection point that is different from the reference curve appears in the process of increasing the voltage. When , it means that the battery starts to precipitate lithium. Read the corresponding battery voltage Vs,L in the first resistance-voltage curve at this time, which is the threshold voltage at which the battery starts to precipitate lithium when it is charged with a normal charging current I constant current, It can also be referred to as the maximum threshold voltage at which the battery does not precipitate lithium when it is charged with a normal charging current I.

Based on the above technical solutions, the present invention provides a non-destructive testing method for the lithium evolution threshold voltage of a lithium ion battery. By intermittently measuring its DC resistance during the charging process of the battery, and analyzing the resistance-voltage curve, the battery can be determined. The threshold voltage at which lithium evolution occurs during charging. The specific steps are: charge the battery with a constant current with a set current (that is, a normal charging current I with a preset size), sleep for a certain period of time at intervals of a certain time or voltage, and use it to measure the DC resistance under the voltage until the battery is charged. to the set cut-off voltage; take the battery charging voltage as the abscissa and the measured DC resistance as the ordinate, draw the first resistance-voltage curve; and the reference resistance-voltage curve in the process of charging with a preset small current In contrast, when the measured first resistance-voltage curve has a first falling inflection point that is different from the reference curve, it means that the battery begins to precipitate lithium, and read the corresponding value in the first resistance-voltage curve at this time. The battery voltage is the threshold voltage at which lithium precipitation occurs when the battery is charged at this current.

Therefore, the detection method of the present invention does not require additional processing of the battery, belongs to the non-destructive testing of the battery, and does not require the use of high-precision and expensive equipment, so it is suitable for the detection of the lithium-evolution threshold voltage of all types of batteries under various working environments. And it is suitable for the detection of lithium deposition threshold voltage at each stage of the battery life cycle.

In order to understand the technical solutions of the present invention more clearly, the following description is made with reference to specific embodiments.

The present invention will be described in detail below by taking the test of a commercial cylindrical lithium ion battery as an example, in conjunction with the accompanying drawings, so as to further illustrate the substantial features and significant progress of the present invention.

Example 1.

In this embodiment, the test sample is a 21700 cylindrical lithium-ion experimental battery with a 1C capacity of 4.62Ah. The threshold voltage at which lithium deposition occurs in the battery.

The battery testing equipment is a conventional charging and discharging instrument. In this embodiment, the equipment used is the Arbin BT2000 charging and discharging testing system. Specifically, the following test steps are included:

The first step: the normal charging current I of the preset size is 4.62A (ie 1C), and the battery is charged with constant current, and the cut-off voltage is 4.2V. Considering that lithium precipitation should not occur when the battery is charged at a low state of charge, when setting the test interval of DC resistance, the interval before 40% SOC is longer, and it is set to sleep for 3s every 180s of charging, starting from 40% SOC, It sleeps for 3s every 36s of charging, and is used to intermittently measure the DC resistance of the battery. During charging, time, voltage, current, and capacity data are collected during battery charging.

The calculation method of intermittent DC resistance is: as shown in Figure 2, the voltage of the battery when the battery is charged with a constant current of 4.62A for a certain period of time is recorded as V _s = 3.588V, and the battery voltage after sleep for 3s is V _r = 3.468V , then the DC resistance when the battery is charged to V _s =3.588V is R _s =(V _s -V _r )/I=(3.588-3.468)/4.62=0.0260Ω=26.0mΩ.

Step 2: During constant current charging with normal charging current I, take the charging voltage V _s of the battery in each charging stage as the abscissa, and the measured corresponding DC resistance Rs as the ordinate, and draw the battery charging process with 1C current The first resistance-voltage curve in , as shown in accompanying drawing 3;

The third step: the second resistance-voltage curve measured with a small current I' of 0.2C during the charging process, as shown in Figure 4, as a reference curve, will be measured with a normal charging current I of 1C during the charging process. Comparing the obtained first resistance-voltage curve with this reference curve, it can be found that the first resistance-voltage curve measured during 1C charging has a falling inflection point at about 4.11V, and the battery begins to precipitate lithium. That is, the threshold voltage at which lithium precipitation begins to occur when the battery is charged at 1C current is 4.11V, which is also the maximum threshold voltage at which lithium precipitation does not occur when the battery is charged at 1C.

It should be noted that the method of obtaining the second resistance-voltage curve measured during the charging process with a small current I' of 0.2C is the same as that in the charging process with a 1C current. Specifically: charge the same type of battery with a constant current of 0.92A, sleep for 3s every 180s of charging, start from 40% SOC, sleep for 3s every 36s of charging, to measure the DC resistance of the battery intermittently. During charging, time, voltage, current, and capacity data are collected during battery charging.

According to R _s '=(V _s '-V _r ')/I', the intermittent DC resistance R _s ' of the battery under the charging voltage V _s ' at each stage can be calculated when a small current I' of 0.2C is charged. , and then take the charging voltage V _s ' as the abscissa, and the measured corresponding DC resistance Rs' as the ordinate, draw the second resistance-voltage curve (ie the reference curve) during the charging process of the battery with 0.2C current, As shown in Figure 4. It can be found that in the process of 0.2C current charging, the DC resistance of the battery first decreases and then tends to be flat with the increase of the battery voltage, and then slightly increases at the end of the charging period.

It should be noted that, for the detection method of the present invention, the principle is: when the battery is charged with lithium precipitation, a lithium deposition layer will be formed on the surface of the negative electrode, forming a parallel circuit with the original negative electrode sheet, causing the current to be shunted during battery charging. part, so the battery resistance will decrease. It is shown in the resistance-voltage curve, that is, when the battery undergoes lithium precipitation, the curve of its DC impedance with the charging voltage will deviate from the original trend and the inflection point of impedance reduction will appear. This inflection point means that the battery begins to undergo lithium precipitation. Threshold voltage for lithium evolution of the battery.

Example 2.

In this embodiment, the test sample is a 21700 cylindrical experimental battery that has been cycled 1000 times, and the battery capacity has decayed to 4.08Ah. Lithium precipitation, and test the threshold voltage of lithium precipitation when the battery is charged at 0.7C.

The battery testing equipment is a conventional charge-discharge instrument, and the equipment used in this embodiment is the Arbin BT2000 charge-discharge test system. Specifically, the following test steps are included:

The first step: the normal charging current I of the preset size is 0.7C=2.856A, and the battery is charged with constant current, and the cut-off voltage is 4.2V. Dormancy for 3s per 260s of charging, starting from 40% SOC, and dormancy for 3s per 52s of charging, for intermittent determination of the DC resistance of the battery. During charging, time, voltage, current, and capacity data are collected during battery charging.

The calculation method of intermittent DC resistance is shown in Figure 2. The voltage of the battery when the battery is charged with a constant current of 2.856A for a certain period of time is recorded as V _s , and the battery voltage after sleep for 3s is V _r , then when the battery is charged to V _s The DC resistance is R _s =(V _s -V _r )/I.

Step 2: When performing constant current charging with a normal charging current I of 0.7C, take the charging voltage Vs of the battery in each charging stage as the abscissa, and the measured corresponding DC resistance R _s as the ordinate, draw the battery as 0.7 The resistance-voltage curve during the C current charging process is shown in Figure 5;

The third step: the second resistance-voltage curve measured during the charging process with a small current I' of 0.2C, as shown in Figure 6, as a reference curve, will be measured with a normal charging current I of 0.7C during the charging process. Comparing the first resistance-voltage curve with this reference curve, it can be found that the resistance-voltage curve measured during the 0.7C charging process has the first falling inflection point at around 3.88V, at which time the battery begins to precipitate lithium, that is, When the battery is charged with a current of 0.7C, the threshold voltage at which lithium precipitation begins to occur is 3.88V.

It should be noted that the method of obtaining the second resistance-voltage curve measured during the charging process with a small current I' of 0.2C is the same as the charging process with a 0.7C current. Specifically: charge the battery with a constant current of 0.816A, sleep for 3s every 180s of charging, start from 40% SOC, sleep for 3s every 36s of charging, and measure the DC resistance of the battery intermittently. During charging, time, voltage, current, and capacity data are collected during battery charging.

According to R _s '=(V _s '-V _r ')/I', when calculating the small current I' of 0.2C for charging, the intermittent DC resistance R _s ' of the battery under the charging voltage V _s ' in each stage, Then take the charging voltage V _s ' as the abscissa, and the measured corresponding DC resistance Rs' as the ordinate, and draw the second resistance-voltage curve (ie the reference curve) during the charging process of the battery with 0.2C current, such as Figure 6. It can be found that during the charging process with a small current I' of 0.2C, the DC resistance of the battery after cycle decay decreases first and then tends to be flat with the increase of the battery voltage, and increases significantly at the end of charging, especially compared to For an uncycled battery, the DC resistance at the end of charging showed a large increase.

Therefore, based on the above technical solutions, it can be seen that the non-destructive testing method for the threshold voltage of lithium ion batteries provided by the present invention does not require additional processing of the battery, but only needs to perform intermittent DC resistance testing on the battery. The trend analysis of the curve can obtain the threshold voltage of lithium evolution of the battery, so it is suitable for the detection of the threshold voltage of lithium evolution of all types of batteries under various working environments, and is suitable for the detection of the threshold voltage of lithium evolution in all stages of the battery life cycle. Therefore, it can provide detection methods and formulation basis for the determination of battery use boundary conditions and the optimization of charging system, and has a wide range of application prospects in battery health state management.

Of course, those skilled in the art can also calculate other lithium evolution threshold parameters according to other parameters collected during charging, such as the state of charge corresponding to lithium evolution, etc., which should all fall within the protection scope of the present invention.

To sum up, compared with the prior art, the present invention provides a non-destructive testing method for the threshold voltage of lithium-ion battery lithium evolution. The method measures the DC resistance of the battery intermittently during the charging process of the battery, and measures the resistance of the battery. -Analyzing the voltage curve to determine the threshold voltage of lithium precipitation during the charging process of the battery has great practical significance.

The above are only the preferred embodiments of the present invention. It should be pointed out that for those skilled in the art, without departing from the principles of the present invention, several improvements and modifications can be made. It should be regarded as the protection scope of the present invention.

Claims (7)

1. a non-destructive testing method for lithium ion battery threshold voltage of lithium evolution, is characterized in that, comprises the following steps: The first step is to charge a battery with a constant current with a normal charging current I of a preset size, and during the constant current charging process, every fixed period of time or every fixed voltage value, that is, immediately enter a stop Charging and dormancy stage, until the battery is charged to the set cut-off voltage; Among them, in each stop charging and dormancy stage, the starting voltage V _s of the battery at the beginning of the stop charging is collected in real time, and after a preset sleep duration, the end voltage V _r of the battery at the end of the sleep is collected in real time, and then according to the preset Design the formula to calculate and obtain the initial DC resistance R s when the battery is charged with the normal charging current I constant current, and the initial DC resistance R _s when the charging is stopped at the beginning of each stop charging dormancy stage; that is, the DC resistance of the battery is obtained by intermittent measurement; In the second step, take the initial voltage V _s of the battery in each stop charging and dormancy stage as the abscissa, and take the measured initial DC resistance R _s of the battery in each stop charging and dormancy stage as the ordinate, draw the battery to obtain the battery The resistance-voltage curve in the process of constant current charging with the normal charging current I is defined as the first resistance-voltage curve; The third step is to perform constant current charging on the same type of battery with a preset small current I', and in the process of constant current charging, at every fixed time interval or at every fixed voltage value, immediately enter a Stop the charging and dormancy stage until the battery is charged to the set cut-off voltage; then, take the starting voltage V _s ' of the battery in each stop charging and dormancy stage as the abscissa, to correspond to the measured battery in each stop charging and dormancy stage. The initial DC resistance R _s ' is the ordinate, and the resistance-voltage curve during the constant current charging process of the battery with a small current I' is drawn and obtained, which is defined as the second resistance-voltage curve; Among them, in each stop charging and dormancy stage, the starting voltage V _s ' of the battery at the beginning of the stop charging is collected in real time, and after a preset sleep period, the end voltage V _r ' of the battery at the end of the sleep is collected in real time, and then According to the preset calculation formula, calculate and obtain the initial DC resistance R s ' when the battery is charged at a constant current with a small current I', and the initial DC resistance R _s ' at the beginning of each stop charging dormancy stage; that is, the DC resistance of the battery is obtained by intermittent measurement; In the fourth step, the second resistance-voltage curve is used as the reference curve, and the first resistance-voltage curve is compared with the first resistance-voltage curve; when the first resistance-voltage curve appears in the process of increasing with the voltage, the trend is different from the reference curve. When the inflection point of the first DC resistance value decreases, it means that the battery begins to precipitate lithium, and reading the corresponding battery voltage V _s,L in the first resistance-voltage curve at this time means that the battery is charging at the normal charging current I. During constant current charging, the threshold voltage at which lithium deposition begins to occur. 2. nondestructive testing method as claimed in claim 1, is characterized in that, in the first step, preset calculation formula is as follows: When the battery is charged at a constant current with a normal charging current I, the initial DC resistance R _s =(V _s -V _r )/I at the beginning of each stop charging sleep phase at the beginning of the stop charging; In the third step, the preset calculation formula is as follows: When the battery is charged at a constant current with a small current I', the initial DC resistance R _s '=(V _s '-V _r ')/I' at the beginning of each stop charging sleep phase. 3. nondestructive testing method as claimed in claim 1, is characterized in that, in the first step and the 3rd step, every interval of a fixed time, namely immediately enters a stop charging dormancy stage, wherein: For each fixed time interval, the calculation formula of the time length T of the fixed time is as follows: Time length T=Q/I ₀ *3600*A%, in milliseconds; Among them, Q is the battery capacity, I ₀ is the size of the charging current; the value range of A% is 0.02% to 5%; In the first step, the charging current magnitude I ₀ is equal to the normal charging current I; in the third step, the charging current magnitude I ₀ is equal to the small current I'. 4. nondestructive testing method as claimed in claim 1 is characterized in that, in the first step and the 3rd step, every interval of a voltage value of a fixed size, namely immediately enters a stop charging dormancy stage, wherein: Fixed voltage value, the set value range is 1mV ~ 100mV; In the first step and the third step, the sleep preset time period is specifically 0.01-50s. 5. nondestructive testing method as claimed in claim 4 is characterized in that, the voltage value of fixed size, the set value range is 5mV-50mV; In the first and third steps, the preset sleep duration is 0.1s-10s. 6. The nondestructive testing method according to any one of claims 1 to 5, wherein the value range of the small current I' is 0.01C to 0.5C. 7. The nondestructive testing method according to claim 6, wherein the value range of the small current I' is 0.05C-0.3C.

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