Sodium separation detection method of sodium ion battery

Abstract

The invention provides a sodium separation detection method of a sodium ion battery, the sodium ion battery comprises hard carbon as a negative electrode and a sodium ion electrode material as a positive electrode, and the method comprises the following steps: applying a certain pressure to the sodium ion battery and installing a pressure sensor; charging the sodium ion battery, recording the pressure detected by the pressure sensor in real time along with the change of the charging capacity in the charging process, and obtaining the pressure change / electric quantity change corresponding to different charging capacities, namely a dP / dQ curve; and selecting the maximum value in the dP / dQ curve below the actual capacity and setting the maximum value as 1, carrying out normalization processing on other numerical values in proportion, when dP / dQ is less than 1, determining that the negative electrode does not generate sodium precipitation reaction, and when dP / dQ is greater than 1, determining that the negative electrode has sodium precipitation reaction. According to the invention, the sodium precipitation detection can be carried out at low cost without disassembling the battery, and the in-situ detection problem of sodium precipitation of the hard carbon negative electrode can be solved.

Description

Technical Field

[0001] The present invention belongs to the technical field of electrochemical energy storage batteries, and in particular relates to a sodium precipitation detection method for a sodium ion battery. Background Art

[0002] At present, due to environmental pollution and energy crisis, people have begun to turn their attention to clean and renewable energy sources such as wind energy, solar energy, and tidal energy. However, most of these new energy sources are intermittent, so it is necessary to develop energy storage technology to better utilize them. Electrochemical energy storage represented by lithium-ion batteries has attracted widespread attention due to its advantages such as mature technology, fast response, and no influence from terrain. However, the reserves of lithium in the earth's crust are low, and the distribution of lithium ore is uneven, resulting in high prices for lithium salts, which in turn increases the cost of lithium-ion batteries and limits their application in large-scale power storage systems. Sodium and lithium are in the same main group and have similar physical and chemical properties. Sodium reserves are abundant and the cost is low. Therefore, sodium-ion batteries have good application prospects and development potential in large-scale energy storage systems.

[0003] Hard carbon is a commonly used anode in sodium-ion batteries. Because the potential for sodium ion intercalation and deintercalation in hard carbon is close to the potential for sodium metal deposition and stripping, sodium easily precipitates on the anode surface during high-rate cycling or low-temperature operation. The precipitated sodium grows into sodium dendrites, which can pierce the separator after prolonged cycling, causing a battery short circuit and posing a safety hazard. Therefore, detecting sodium precipitation at the anode is of great significance.

[0004] Battery disassembly can yield intuitive conclusions, but due to the active chemical properties of sodium metal, the disassembly process poses certain safety risks. Furthermore, the battery cannot be used after disassembly, necessitating the development of in-situ detection methods. Microcalorimetry can determine whether sodium is precipitated by measuring heat changes during the battery reaction; in-situ infrared fiber loss spectroscopy can detect the negative electrode in situ. However, these methods require expensive specialized equipment and can be used in laboratories, but are not suitable for industrial production and practical applications.

[0005] In order to solve the problem of in-situ detection of sodium precipitation on hard carbon negative electrodes, a low-cost sodium precipitation detection method that does not require disassembly of the battery, that is, non-destructive testing, is urgently needed to provide more possibilities for the practical development of sodium-ion battery systems. Summary of the Invention

[0006] Problems to be solved by the invention: In view of the above problems, the object of the present invention is to provide a method for detecting sodium precipitation in a sodium ion battery, which can detect sodium precipitation of a hard carbon negative electrode without disassembling the battery.

[0007] Technical means to solve the problem: The present invention provides a method for detecting sodium precipitation in a sodium ion battery. The sodium ion battery includes hard carbon as a negative electrode and a sodium ion electrode material as a positive electrode. The method is characterized by comprising: applying a certain pressure to the sodium ion battery and installing a pressure sensor; charging the sodium ion battery, and during the charging process, recording the pressure detected by the pressure sensor in real time as the charging capacity changes, to obtain a pressure change / electricity change corresponding to different charging capacities, i.e., a dP / dQ curve; selecting a maximum value in the dP / dQ curve below the actual capacity and setting it to 1, and performing proportional normalization on the remaining values; when dP / dQ is less than 1, it means that no sodium precipitation reaction has occurred at the negative electrode, and when dP / dQ is greater than 1, it means that the sodium precipitation reaction has occurred at the negative electrode.

[0008] According to the present invention, sodium precipitation detection can be performed at low cost without disassembling the battery, which can solve the problem of in-situ detection of sodium precipitation on hard carbon negative electrodes. Specifically, the embedding of sodium ions in the hard carbon negative electrode and the deposition of sodium ions on the negative electrode surface will produce a certain volume change. If a certain preload is applied to the battery, then when the volume changes, the battery will also produce a certain pressure change. The volume expansion caused by the embedding of sodium ions into hard carbon and the deposition of sodium ions with the same charge is different, and thus the pressure change is also different. Obviously, the pressure change caused by sodium ion deposition at the same charge is greater than the pressure change caused by sodium ion embedding into hard carbon. Therefore, the pressure change / charge change (dP / dQ) can be used to determine whether the reaction at the negative electrode is sodium precipitation or normal sodium ion embedding into hard carbon. This method does not require disassembly of the battery, greatly improving the safety of the hard carbon negative electrode. This provides a possibility for the practical development of sodium ion battery systems.

[0009] Alternatively, in the present invention, during the charging process, the current density is 0.01C to 5C.

[0010] Alternatively, in the present invention, a pressure between 10 and 100 psi is applied to the sodium ion battery.

[0011] Alternatively, in the present invention, the sodium ion battery is a soft-pack battery with an air bag.

[0012] Alternatively, in the present invention, when measuring the actual capacity of the backup sodium ion battery, the method includes: preparing a hard carbon half-cell with the hard carbon as the positive electrode and the sodium as the negative electrode; charging the hard carbon half-cell at a low current density until the sodium ions are completely precipitated from the hard carbon, and then discharging the hard carbon half-cell at the same low current density, while recording the discharge curve until an inflection point appears in the discharge curve; and taking the capacity corresponding to the inflection point in the discharge curve as the actual capacity.

[0013] In the present invention, the battery may be charged to 1 to 4 V in order to completely precipitate sodium ions from the hard carbon.

[0014] Alternatively, in the present invention, the voltage is controlled below 0 V during the discharge process until an inflection point appears on the discharge curve.

[0015] Alternatively, in the present invention, the inflection point is a point in the discharge curve where voltage change / charge change, ie, dV / dQ=0.

[0016] Effects of the invention: The present invention provides a method for detecting sodium deposition in sodium-ion batteries. By applying a certain pressure to a soft-pack battery and monitoring the pressure changes during charge and discharge in real time, this method allows for in-situ monitoring of whether hard carbon deposition is occurring. The reliability of this method is verified by disassembling batteries under different charge and discharge conditions. The proposed hard carbon detection method is simple, requires no battery disassembly, and is safe and reliable. It allows for quick and convenient monitoring of hard carbon anodes, effectively improving the safety of sodium-ion batteries using hard carbon anodes in practical applications. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1 is a schematic diagram of the detection of sodium precipitation on hard carbon according to the present invention; Figure 2 is the discharge curve of the hard carbon half-cell of Example 1. DETAILED DESCRIPTION

[0018] The present invention will be further described below with reference to the following embodiments. It should be understood that the following embodiments are only intended to illustrate the present invention, not to limit the present invention. The components of the embodiments of the present invention generally described and shown in the figures herein may be arranged and designed in various different configurations.

[0019] In actual use, the actual capacity of hard carbon materials varies depending on their source, so it is necessary to measure their actual capacity. However, if the actual capacity of a specific hard carbon material is known, this step can be omitted. Furthermore, the actual capacity of the hard carbon material is the charge capacity of the sodium-ion battery.

[0020] For the sodium-ion battery to be tested, a hard carbon material made from the same material as the hard carbon negative electrode in the sodium-ion battery is selected. A hard carbon half-cell is prepared with the hard carbon material as the positive electrode and the sodium sheet as the negative electrode. The hard carbon half-cell is then charged and discharged at a low current density to determine the actual capacity of the hard carbon material. Specifically, the battery is charged at a low current density at room temperature until sodium ions are completely released from the hard carbon. Lower current densities result in a capacity closer to the theoretical capacity. For example, in this embodiment, a current density of 0.01C to 0.1C can be used, but this is not limited to this. Furthermore, controlling the voltage too high during charging can lead to electrolyte decomposition, while controlling the voltage too low can prevent complete sodium ion release and thus hinder the full capacity. For example, in this embodiment, the voltage can be charged to 1 to 4V, preferably 1.5 to 2.5V, but this is not limited to this. The hard carbon half-cell is then discharged at the same low current density, with the voltage kept below 0 V during discharge to ensure complete sodium ion embedding into the hard carbon. For example, in this embodiment, the voltage can be kept below -0.1 V. The charge-discharge curves during this process are recorded. A voltage inflection point appears below 0 V. The inflection point in the discharge curve is the point where the voltage change / charge change, or dV / dQ, equals 0. It is known that sodium ion embedding occurs before this inflection point, and sodium ion precipitation occurs after this inflection point. Therefore, the capacity corresponding to the inflection point is taken as the actual capacity of the hard carbon material. This determines the actual capacity, Q, of the hard carbon material before sodium precipitation.

[0021] like Figure 1 As shown, a sodium ion battery is prepared using the same hard carbon material as the negative electrode. A certain pressure is applied to the sodium ion battery and a pressure sensor is installed. Afterwards, the battery is charged at a low current density, and the specific operation is similar to the above. To ensure that the battery is fully charged without sodium precipitation on the hard carbon, the charging capacity is set to the actual capacity (Q) measured above. For example, in this embodiment, the battery is charged to the actual capacity Q at a low current density of 0.1C. During the charging process, the pressure (P) detected by the pressure sensor is recorded in real time as the charging capacity changes, thereby obtaining the pressure change / charge change corresponding to different charging capacities, that is, the dP / dQ curve.

[0022] The point where dP / dQ is the maximum value in the dP / dQ curve calculated by measurement is set to 1, and the remaining values ​​are normalized proportionally. Since the above measurement process has always used a very small current density, and the measurement process ensures that sodium precipitation will not occur (the maximum capacity is the actual capacity), a standard sodium precipitation critical value (i.e., the dP / dQ maximum value) is obtained. It can be seen from this that in practical applications, as long as dP / dQ exceeds the maximum value, it can be directly identified as sodium precipitation. That is, dP / dQ=1 is regarded as the critical point of sodium precipitation. When dP / dQ<1, it is considered that no sodium precipitation reaction has occurred at the negative electrode, and when dP / dQ>1, it is considered that the sodium precipitation reaction at the negative electrode has occurred.

[0023] Therefore, based on the above, the present invention provides a sodium precipitation detection method for a sodium ion battery, which is as follows: a certain pressure is applied to the sodium ion battery to be detected and a pressure sensor is installed, and then the sodium ion battery to be detected is charged. At this time, the battery can be charged and discharged under different working conditions according to actual conditions, for example, at different current densities and temperatures, the current density can be 0.01C~5C, and the temperature can be -30℃~50℃, but not limited thereto. Then, dP / dQ is measured and calculated as described above. When dP / dQ < 1, it is judged that no sodium precipitation reaction has occurred at the negative electrode. When dP / dQ > 1, it is judged that the sodium precipitation reaction at the negative electrode has occurred, thereby completing the sodium precipitation detection of the sodium ion battery. In addition, although sodium precipitation only occurs during the charging process, in actual use, dP / dQ can be measured without distinction throughout the entire process.

[0024] In addition, in this embodiment, the sodium ion battery can be provided with an air bag to eliminate the influence of gas on the battery pressure. The minimum current density can be selected between 0.01 and 0.1C to ensure accurate measurement of the hard carbon capacity. The pre-applied pressure can be selected between 10 and 100 psi. Specifically, in this embodiment, a known mold can be used to apply pressure to the sodium ion battery, such as Figure 1 As shown, it includes a fixture and a pressure sensor, wherein the fixture can be a metal plate and / or a hard plastic plate, etc., but is not limited thereto. Using a metal plate to compress the sodium-ion soft-pack battery is equivalent to applying a certain preload, and a pressure sensor is placed between the metal plate and the sodium-ion soft-pack battery. During the battery charge and discharge cycle, the insertion and extraction of sodium ions in the negative electrode will produce a certain volume change, which in turn produces a certain pressure change. The pressure change dP can be obtained by the pressure sensor.

[0025] In addition, the reliability of the detection method of the present invention can be verified by disassembling the battery. Specifically, the sodium ion battery under different dP / dQ in the above test process can be disassembled, and the disassembled hard carbon negative electrode can be immersed in water respectively, and the pH value of the water can be tested after standing for a period of time. When sodium precipitation occurs in the battery, if the hard carbon negative electrode is immersed in water, the precipitated metallic sodium will dissolve in the water and significantly increase the pH of the water. Therefore, if the pH value exceeds 7, it is considered that sodium precipitation occurs at the negative electrode, and the result is compared with the detection result of the detection method of the present invention to verify the reliability. Among them, the immersion time of the hard carbon negative electrode in water can be between 1 and 10 hours, and the pH value can be verified by a pH meter, phenolphthalein solution, purple litmus solution, pH test paper, etc. to determine whether sodium precipitation has occurred.

[0026] As can be seen from the above, the present invention utilizes the fact that the pressure change caused by sodium ion deposition is greater than the pressure change caused by sodium ion embedding into hard carbon at the same charge, and uses this pressure change to determine whether sodium deposition is occurring at the negative electrode. Specifically, electrode materials undergo volume changes during cycling. With the exception of a very small number of zero-strain materials, sodium ions generally expand when embedded into the material. Due to the mechanical constraints of the battery case, the pressure applied to the battery case changes. Because the volume change of the hard carbon negative electrode exceeds that of the positive electrode, the pressure change of the entire battery is essentially controlled by the hard carbon. Therefore, the pressure change of the battery can be used to reveal the electrochemical processes of the hard carbon. When the charge capacity is constant, the number of sodium ions deposited or embedded into the hard carbon at the negative electrode is the same. Clearly, the volume / pressure increase caused by sodium ion deposition is much greater than the volume / pressure change caused by sodium ion embedding into the hard carbon. Therefore, the presence of sodium deposition in the hard carbon can be described by the ratio dP / dQ. The maximum dP / dQ value during sodium ion embedding into the hard carbon within the actual capacity range is used as the critical value. When dP / dQ exceeds this value, sodium deposition is considered to have occurred at the negative electrode of the sodium ion battery.

[0027] Compared with the prior art, the hard carbon sodium precipitation detection method according to the present invention is simple to operate, requiring only applying pressure to the battery and detecting the pressure changes during the battery charging and discharging process; low cost, requiring no expensive instruments and equipment; and highly safe, requiring no disassembly of the sodium ion battery, thereby eliminating potential safety hazards.

[0028] The following examples are further given to illustrate the present invention in detail. It should also be understood that the following examples are only used to further illustrate the present invention and cannot be understood as limiting the scope of protection of the present invention. Some non-essential improvements and adjustments made by those skilled in the art based on the above content of the present invention all fall within the scope of protection of the present invention. The specific process parameters and the like in the following examples are only examples within a suitable range, that is, those skilled in the art can make selections within a suitable range through the description herein, and are not limited to the specific numerical values ​​exemplified below. Example

[0029] The hard carbon half-cell produced by a certain manufacturer on the market was assembled with sodium as the negative electrode for testing. The charge and discharge curves are shown in the figure below. Figure 2 As shown in FIG, the capacity corresponding to the discharge voltage inflection point is 295 mAh / g, and this capacity is taken as the actual specific capacity of the hard carbon negative electrode.

[0030] A sodium ion transfer battery was assembled using the hard carbon as the negative electrode and sodium iron pyrophosphate as the positive electrode. An initial pressure of 30 psi was applied to the soft-pack battery, and the battery was charged and discharged at a current density of 0.01C with a capacity of 295 mAh / g. The dP / dQ curve was monitored in real time, and it was found that the maximum dP / dQ value during the process was 3.4 psi / mAh, which was normalized to 1.

[0031] Seven sodium-ion pouch cells identical to the above-described battery were assembled and tested. An initial pressure of 30 psi was applied to the pouch cells. The assembled cells were charged at a current density of 1C at 25°C to charge capacities of 0, 75, 150, 225, 295, 375, and 450 mAh / g, respectively. The dP / dQ values ​​for each of the seven cells were measured and calculated, as shown in Table 1 below.

[0032] Table 1: dP / dQ of the soft pack battery of Example 1 at different SOC

[0033] According to Table 1 above, it can be found that when the battery capacity is less than the actual capacity (295 mAh / g) (as mentioned above, no sodium precipitation occurs within this range), the dP / dQ is the maximum value when the capacity is 75 mAh / g. This value is normalized to 1, and the remaining values ​​are normalized proportionally. At this time, when the capacity is greater than 295 mAh / g, dP / dQ is greater than 1, indicating that the hard carbon begins to precipitate sodium.

[0034] Next, the above results were verified. Specifically, after charging was completed, seven sodium-ion soft-pack batteries were disassembled and the hard carbon negative electrode was immersed in 200 mL of water. After standing for 1 hour, the pH of the aqueous solution was measured using a pH meter, as shown in Table 2 below.

[0035] Table 2: pH value of the hard carbon negative electrode of Example 1 after immersion in water

[0036] According to Table 2, when the capacity is greater than 295 mAh / g, the pH of the aqueous solution in which the hard carbon anode is immersed becomes strongly alkaline, indicating that sodium precipitation has occurred. This is consistent with the detection results of the present invention, further verifying the reliability of the detection method described in the present invention.

[0037] The above specific embodiments further describe the purpose, technical solutions and beneficial effects of the present invention in detail. It should be understood that the above is only a specific embodiment of the present invention and is not limited to the scope of protection of the present invention. Without departing from the purpose of the basic characteristics of the present invention, the present invention can be embodied in various forms. Therefore, the embodiments of the present invention are used for illustration rather than limitation. Since the scope of the present invention is defined by the claims rather than the specification, and all changes that fall within the scope defined by the claims or the equivalent range of the scope defined by the claims should be understood to be included in the claims. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A method for detecting sodium precipitation in a sodium ion battery, wherein the sodium ion battery comprises hard carbon as a negative electrode and a sodium ion battery material as a positive electrode, characterized in that: include: Applying a certain pressure to the sodium ion battery and installing a pressure sensor; Charging the sodium ion battery, and during the charging process, recording the pressure detected by the pressure sensor in real time as the charging capacity changes, to obtain a pressure change / electricity change corresponding to different charging capacities, i.e., a dP / dQ curve; Below the actual capacity, the maximum value in the dP / dQ curve is selected and set to 1, and the remaining values ​​are normalized proportionally. When dP / dQ is less than 1, sodium precipitation reaction does not occur at the negative electrode, and when dP / dQ is greater than 1, sodium precipitation reaction has occurred at the negative electrode.

2. The sodium analysis detection method for a sodium ion battery according to claim 1, wherein During the charging process, the current density is 0.01C~5C.

3. The sodium precipitation detection method for sodium ion battery according to claim 1, wherein A certain pressure between 10 and 100 psi is applied to the sodium ion battery.

4. The sodium precipitation detection method for sodium ion battery according to claim 1, wherein The sodium ion battery is a soft-pack battery with an air bag.

5. The sodium precipitation detection method for sodium ion battery according to claim 1, wherein When determining the actual capacity of the backup sodium ion battery, it includes: preparing a hard carbon half-cell using the hard carbon as a positive electrode and the sodium as a negative electrode; charging the hard carbon half-cell at a low current density until sodium ions are completely precipitated from the hard carbon, and then discharging the hard carbon half-cell at the same low current density, while recording the discharge curve until an inflection point appears on the discharge curve; The capacity corresponding to the inflection point in the discharge curve is taken as the actual capacity.

6. The sodium precipitation detection method for a sodium ion battery according to claim 5, wherein In order to completely precipitate sodium ions from the hard carbon, charge to 1~4V.

7. The sodium precipitation detection method for a sodium ion battery according to claim 5, wherein During the discharge process, the voltage is controlled below 0 V until an inflection point appears on the discharge curve.

8. The sodium precipitation detection method for a sodium ion battery according to claim 5, wherein: The inflection point is the point in the discharge curve where the voltage change / charge change, ie, dV / dQ=0.

Images