KR102591610B1 - Method of Measuring IR Drop for Zinc-Bromine Cell - Google Patents

Abstract

The present invention relates to a method for measuring IR Drop for zinc-bromine batteries, and more specifically, to a method for measuring voltage drop in the High SOC (State of Charge) region.

Description

{Method of Measuring IR Drop for Zinc-Bromine Cell}

The present invention relates to a method of measuring the voltage drop (IR Drop) for a zinc-bromine battery, and more specifically, to a method of measuring the voltage drop in the High SOC (State of Charge) region.

Recently, hybrid cars, plug-in hybrid cars, and electric cars have been in the spotlight due to environmental pollution problems. In particular, the development of technology for batteries, which are essential for these vehicles, is considered very important.

However, these batteries generally have a lifespan. In other words, as the car is used, the internal resistance naturally increases and the output decreases, and on the other hand, the overall charging capacity decreases. If such performance deterioration occurs, it can lead to a decrease in fuel efficiency and performance of hybrid vehicles or plug-in hybrid vehicles, so measuring the performance of these batteries is important.

Among various batteries, redox flow batteries, an electrochemical method, are being widely studied, and zinc-bromine batteries in particular are attracting attention due to their advantages such as price competitiveness and high discharge voltage.

In order to determine the degree of performance deterioration of the battery and accurately identify the cause of the deterioration, it is important to measure the internal resistance of the battery. Representative methods for measuring internal resistance include the current blocking method and the alternating current impedance method.

The AC impedance method measures the resistance value of a battery at a specific frequency to determine its characteristics. However, in order to measure all the resistance components of each unit cell of a battery pack using the impedance measurement method, all cells must be individually tested sequentially or the cells to be measured must be individually tested. As equipment is needed to simultaneously input a number of alternating currents, equipment configuration becomes complicated and costs increase.

Compared to the AC impedance method, the current blocking method has a relatively simple analysis device and can quickly measure resistance within a few seconds by observing the voltage change that appears instantaneously when the current is cut off. Korean Patent Publication No. 10-2023-0023943 relates to a method of measuring battery internal resistance by the current interruption method, and discloses a method of calculating internal resistance, including ohmic resistance and polarization resistance, by performing current interruption twice. In addition, Korean Patent Publication No. 10-2015-0028410 relates to a method of measuring battery internal resistance, using a direct current internal resistance measurement method that measures the internal resistance of the battery by applying current to the battery, and applying charge and discharge current to the battery. A method of measuring internal resistance is disclosed, characterized in that the current pattern is adjusted according to the equivalent parameters of the battery equivalent impedance model.

However, due to the characteristics of zinc-bromine batteries, in Low SOC (State of Charge), where EDL (Electric Double Layers) reaction and pseudo capacitor reaction mainly occur, the voltage drop value is not clearly distinguished and various overvoltages are mixed. Therefore, there is a problem of low reliability of the measured values.

Accordingly, the present inventors measured the voltage drop in the High SOC region where Zn deposition occurs, rather than the Low SOC (State of Charge) region where EDL reaction and capital capacitor reaction mainly occur in the existing constant current charging and discharging method. It was confirmed that only the voltage drop value can be easily measured without various overvoltages mixed together. In addition, the present invention was completed after confirming that the internal resistance can be calculated through the measured voltage drop and that the voltage drop can be easily estimated even when the applied current value is changed.

The purpose of the present invention is to provide a method for measuring the voltage drop (V _{IR Drop} ) caused by the materials (current collector, electrode, lead tab, etc.) that make up the zinc-bromine battery.

Another object of the present invention is to provide a method for easily estimating the voltage drop even when the applied current value changes.

In order to achieve the above object, the present invention provides a voltage drop measurement method for zinc-bromine batteries in the High SOC (State of Charge) region that satisfies the following conditions:

a) Constant current charge state;

b) potential region above 1.5 V; and

c) State of charge greater than 0.3 mAh/cm ² .

The present invention also provides a method for estimating the voltage drop (V _{IR Drop-estimate} ) according to a change in the applied current value, comprising the following steps:

a) Calculating the internal resistance (R _{IR Drop} ) according to Equation 2 below using the voltage drop (V _{IR Drop} ) measured by the above method,

[Equation 2]

[R _{IR Drop} ]=[V _{IR Drop} ]/[I _Input ]

Here, I _Input is the applied current value; and

b) calculating the voltage drop (V _{IR Drop-estimate} ) using the calculated internal resistance (R _{IR Drop} ) by Equation 3 below,

[Equation 3]

[V _{IR Drop-estimate} ] = [I _spec. ] * [R _{IR Drop} ]

Here, I _spec. is the changed current value.

In the case of the existing constant current charging and discharging method, it was difficult to measure only the voltage drop value because various overvoltages were often mixed together. However, the voltage drop measurement method for zinc-bromine batteries according to the present invention measures the voltage drop in the High SOC section where Zn deposition mainly occurs, and the measurement is performed within a short time to determine the overvoltage of only the voltage drop value that reflects less other overvoltage factors. can be measured. In addition, the voltage drop measurement method for zinc-bromine batteries is economical because it can be confirmed simultaneously while conducting the existing constant current charging and discharging evaluation, so there is no need to introduce additional procedures for evaluation.

Figure 1 shows a voltage current curve according to an embodiment of the present invention.
Figure 2 shows a voltage curve over time of a zinc-bromine battery in a high SOC region according to an embodiment of the present invention.

Unless otherwise defined, all technical and scientific terms used in this specification have the same meaning as commonly understood by a person skilled in the art to which the present invention pertains. In general, the nomenclature used herein is well known and commonly used in the art.

The term 'IR Drop' used in the present invention has the same meaning as Voltage Drop and literally refers to a phenomenon in which the parasitic impedance of the power supply network increases and this impedance combines with the current to drop the voltage.

Due to the nature of zinc-bromine batteries, in Low SOC (State of Charge), where EDL reaction and pseudo capacitor reaction mainly occur, the voltage drop value is not clearly distinguished and various overvoltages are mixed, making it difficult to use the existing constant current charging and discharging method. In this case, there was a problem with low reliability of the measured values. In the present invention, in order to measure the voltage drop (V _{IR Drop} ) caused by the materials (current collector, electrode, lead tab, etc.) that make up the zinc-bromine battery, the voltage drop is measured in the High SOC section where Zn deposition mainly occurs. It was confirmed that it was possible to measure the overvoltage of only the voltage drop value that reflected a small amount of other overvoltage factors by performing the measurement within a short period of time.

Therefore, from one perspective, the present invention relates to a method of measuring voltage drop in a high SOC (state of charge) region that satisfies the following conditions as a voltage drop measurement method for zinc-bromine batteries.

a) Constant current charge state;

b) potential region above 1.5 V; and

c) State of charge greater than 0.3 mAh/cm ² .

In the present invention, the charging capacity can be calculated as (applied current * time / area), and in the zinc bromine battery, the electrode refers to the anode or cathode.

In the present invention, condition b) is to have a potential range of 1.5V or more, more preferably 1.5V to 3V, and most preferably 1.5V to 2.5V. If the potential range is less than 1.5, it is undesirable because it is a Low SOC region and EDL reaction and pseudocapacitor reaction mainly occur. If the potential range is more than 1.5V, zinc deposition process (Zn deposition) is the main reaction, so the voltage change is relatively small. If the battery voltage rises above 3V, the overvoltage is large, so the percentage of unwanted concentration overvoltage included in the measured value may increase. Therefore, 1.5V to 3V is the preferred measurement range because the ratio of concentration overvoltage is low, and 1.5V to 2.5V is the most desirable measurement range because the ratio of concentration overvoltage is even lower. Here, a large overvoltage generally means that a high charging current is applied, thereby increasing the effect of the concentration overvoltage.

In the present invention, condition c) is a state of charge of 0.3 mAh/cm ² or more when the battery electrode thickness is 10 to 300 um; When the electrode thickness of the battery is 300 to 600 um, a state of charge of 0.6 mAh/cm ² or more; Alternatively, when the electrode thickness of the battery is 600 to 1000 um, it is preferable that the battery be charged at 1.0 mAh/cm ² or more. As the electrode thickness increases, the reaction area where the EDL reaction can occur increases. Therefore, in order to maintain an optimal measurement state in which EDL reaction occurs less (voltage fluctuations are not large), an appropriate state of charge is required according to the electrode thickness.

In the present invention, the voltage drop measurement of the zinc-bromine battery is performed in the High SOC region that satisfies the above conditions, measuring the voltage (V _Charge ) in the charging state by applying the current (I _Input ) and the current ( It is desirable to include the step of measuring the voltage (V _Rest ) when the I _Input ) is blocked, and to measure the voltage drop (V _{IR Drop} ) according to the formula below.

[V _{IR Drop} ] = [V _Charge ] - [V _Rest ]

Additionally, in the present invention, the current blocking time is preferably 0.1 to 5 seconds, and more preferably 1 to 3 seconds. Here, the current blocking time refers to the time to block the current in the High SOC area that satisfies the above-mentioned conditions, and the voltage difference that occurs at this time is considered a voltage drop.

In the present invention, it is preferable to measure the voltage drop by charging for T1 time in the Low SOC area that satisfies the following conditions, then charging for T2 time in the High SOC area, and then cutting off the current.

a) Constant current charge state; and

b) potential region below 1.5 V,

Here, T2 > T1.

In the present invention, a section may occur that does not satisfy both the Low SOC area and the High SOC area that satisfy the above-mentioned conditions, and this can be regarded as a Middle SOC area.

Additionally, in the present invention, the state of charge can be calculated through (applied current * charging time)/electrode area value.

If the time to reach the High SOC section has not been long, the EDL reaction may still be included. In order to rule out this, it is recommended to charge in the High SOC area by more than the capacity charged in the Low SOC area and then measure the voltage drop. desirable.

In the present invention, the voltage drop includes the Ohmic resistance generated at the interface between the electrode material and the current collector, the current collector, the lead tab, the conductor, etc., and the resistance value generated by charge transfer. It is characterized by

As the battery is used, the internal resistance naturally increases and the output decreases. At this time, it is important to measure the internal resistance of the battery in order to determine the degree of performance deterioration of the battery and accurately identify the cause of the deterioration. In the present invention, it was confirmed that the internal resistance (R _{IR Drop} ) can be calculated using the voltage drop (V _{IR Drop} ) measured in the High SOC region where Zn deposition mainly occurs. In addition, it was confirmed that the voltage drop can be calculated using the internal resistance value according to the present invention for the data required for the design of low-power batteries where the voltage drop value is difficult to observe or high-power batteries where the drop in concentration difference is large.

From another perspective, the present invention relates to a method of estimating the voltage drop (V _{IR Drop-estimate} ) according to a change in the applied current value, including the following steps.

a) Calculating the internal resistance (R _{IR Drop} ) according to Equation 2 below using the voltage drop (V _{IR Drop} ) measured by the above method,

[Equation 2]

[R _{IR Drop} ]=[V _{IR Drop} ]/[I _Input ]

Here, I _Input is the applied current value; and

b) calculating the voltage drop (V _{IR Drop-estimate} ) using the calculated internal resistance (R _{IR Drop} ) by Equation 3 below,

[Equation 3]

[V _{IR Drop-estimate} ] = [I _spec. ] * [R _{IR Drop} ]

Here, I _spec. is the changed current value.

Because the voltage drop measured in the High SOC area is used, rather than the Low SOC (State of Charge) area where EDL reaction and pseudo capacitor reaction mainly occur, the accurate internal resistance value can be easily obtained.

Hereinafter, the present invention will be described in more detail through examples. These examples are only for illustrating the present invention, and it will be apparent to those skilled in the art that the scope of the present invention is not limited by these examples.

Example 1 - Voltage Drop (V _{IR Drop} ) measurement

In this example, the voltage drop of the zinc-bromine battery was measured.

The zinc-bromine cell used in this example is a zinc-bromine cell with an electrode thickness of 80 um and an area of 4*6 cm ^2. After stabilizing the zinc-bromine cell, Charging and discharging were performed by applying a current of mA.

In a constant current charging state, the voltage drop was measured in each of the Low SOC and High SOC areas that satisfied the above-mentioned conditions, and the current cut-off time was 1 second and the current was 120 mA.

The voltage drop (V _{IR Drop} ) measures the voltage (V _Charge ) in a charging state when current (I _Input ) is applied, and then measures the voltage (V _Rest ) when the current (I _Input ) is blocked, using the formula below: The voltage drop value was calculated according to .

[V _{IR Drop} ] = [V _Charge ] - [V _Rest ]

In the case of Low SOC, the 'charging time' and 'charging capacity' from the start of charging to reaching 1.5V are measured, and in the case of High SOC, from the point of reaching more than 0.3 mAh/cm ² in the potential area of 1.5V or more until the Rest state. This shows the results of measuring the 'charging time' and 'charging capacity'. Here, the point at which the charging capacity divided by the area of the electrode becomes 0.3 mAh/cm2 is the starting point of the High SOC region.

As can be seen in Table 1 below, in the Low SOC region, the deviation of the voltage drop measured for each cycle is quite large (standard deviation 11.59). This is because EDL reaction and pseudo capacitor reaction mainly occur in the Low SOC area, so it can be seen that various overvoltages are mixed and the voltage drop values are not clearly distinguished. However, it can be seen that in the High SOC area, the standard deviation (2.17) is significantly lower than the voltage drop deviation measured in the Low SOC area.

As can be seen in Table 1, the charging time and charging capacity in the Low SOC area and High SOC area when measuring the 2nd cycle are as follows.

- Charging time in Low SOC area: 26 seconds

- Charging capacity in Low SOC area: 0.87 mAh

- Charging time in High SOC area: 1585 seconds

- Charging capacity in High SOC area: 52.8 mAh (=2.2 mAh/cm ² )

Additionally, in Figure 2, the voltage drop (V _{IR Drop} ) can be confirmed when measuring the 2nd cycle in the High SOC region of Table 1 above.

Example 2 - Voltage drop (V _{IR Drop} ) value is used to determine the internal resistance (R _{IR Drop} ) calculate

In this example, the internal resistance was calculated using the voltage drop value obtained in Example 1.

In this example, the internal resistance (R _{IR Drop} ) was calculated using the voltage drop (V _{IR Drop} ) value obtained in Example 1 and the current density value applied to the battery (see the formula below).

[R _{IR Drop} ]=[V _{IR Drop} ]/[I _Input ]

Example 3 - Calculating expected voltage drop at different current densities

In this example, the expected voltage drop was calculated when the current density was different using the internal resistance value obtained in Example 2.

In this example, the expected voltage drop (V _{IR Drop-estimate} ) was calculated using the changed specific current density (I _spec. ) and the internal resistance (R _{IR Drop} ) value obtained in Example 2 (see equation below).

[V _{IR Drop-estimate} ] = [I _spec. ] * [R _{IR Drop} ]

The internal resistance value measured at a current density of 120 mA applied in Examples 1 and 2 (average of 2 to 5 cycles) was 0.940 Ω (High SOC region), and it can be confirmed that the expected voltage drop value using the above equation is 56.375 mV.

Table 3 shows the results of measuring the voltage drop by applying a specific current density (60 mA). The expected voltage drop value calculated in this example (56.375 mV) and the value measured in the High SOC area (58 mV) I was able to confirm something similar to this.

Therefore, it was confirmed that the voltage drop can be calculated using the internal resistance value according to the present invention for the data required for designing low-output batteries where the voltage drop value is difficult to observe or high-output batteries where the drop in concentration difference is large.

Claims (7)

This is a voltage drop (V _{IR Drop} ) measurement method for zinc-bromine batteries. Charge the battery for T1 hours in the Low SOC (State of Charge) area, then charge it for T2 hours in the High SOC area, cut off the current, and measure the voltage drop. ,
The Low SOC area is a) constant current (Constant) charging state; and b) satisfies the condition of being a potential region of less than 1.5 V,
The High SOC area is a) constant current (Constant) charging state; b) potential region above 1.5 V; and c) satisfies the condition of being charged at 0.3 mAh/cm ² or more,
A method characterized in that T2 > T1.
According to paragraph 1,
Measuring the voltage (V _Charge ) in a charging state while applying a current (I _Input ), and
Including measuring the voltage (V _Rest ) when the current (I _Input ) is blocked,
How to measure voltage drop (V _{IR Drop} ) using Equation 1 below.
Equation 1: [V _{IR Drop} ] = [V _Charge ] - [V _Rest ]
The method of claim 2, wherein the current blocking time is 0.1 to 5 seconds.
delete The method of claim 1, wherein the voltage drop is a resistance generated by Ohmic resistance and charge transfer occurring at the interface between the electrode material and the current collector, the current collector, the lead tab, and/or the conductor. A method comprising:
The method of claim 1, wherein condition c) is
When the electrode thickness of the battery is 10 to 300 um, a state of charge of 0.3 mAh/cm ² or more;
When the electrode thickness of the battery is 300 to 600 um, a state of charge of 0.6 mAh/cm ² or more; or
When the electrode thickness of the battery is 600 to 1000 um, the state of charge is 1.0 mAh/cm ² or more.
A method characterized in that.
A method of estimating the voltage drop (V _{IR Drop-estimate} ) according to a change in the applied current value, including the following steps:
a) Calculate the internal resistance (R _{IR Drop} ) according to Equation 2 below using the voltage drop (V _IR Drop) measured by any one of paragraphs 1 to 3, 5, and 6. step,
[Equation 2]
[R _{IR Drop} ]=[V _{IR Drop} ]/[I _Input ]
Here, I _Input is the applied current value; and
b) calculating the voltage drop (V _{IR Drop-estimate} ) using the calculated internal resistance (R _{IR Drop} ) by Equation 3 below,
[Equation 3]
[V _{IR Drop-estimate} ] = [I _spec. ] * [R _{IR Drop} ]
Here, I _spec. is the changed current value.