Technical field

The invention relates to the field of lithium ion batteries, and in particular to a method for diagnosing lithium plating in lithium ion batteries by using electrochemical impedance spectroscopy.

Background art

Due to the unique advantages of lithium ion batteries (LIB), such as high energy and power density, low memory effect, and environmental friendliness, they have been widely used as energy sources for electric vehicles and hybrid electric vehicles, thereby replacing conventional fossil fuels. However, lithium ion batteries still face some key technical challenges, such as lithium metal plating on the anode. Lithium plating is kinetically favorable because the working potential of graphite is very close to the potential of metallic lithium deposition. The lithium plating on the anode in a lithium ion battery is closely related to charging conditions, such as low temperature, high charging rate and overcharging. These conditions result in high anode polarization and force anode potential to reach the threshold of metallic lithium plating, thereby leading to anode lithium plating. Lithium metal is usually deposited in a dendritic or mossy shape, which is one of the main causes of aging and safety accidents, such as short circuit caused by lithium accumulation. Therefore, the study of lithium plating is very important for the operation of lithium ion battery under different working conditions. In order to determine whether lithium plating occurs, many methods are used as general techniques.

Visual observation techniques, such as naked eye, optical microscope, SEM or TEM, can be used to observe the plating and morphology of lithium metal on the graphite anode. Different observation techniques determine whether the lithium ion battery needs to be disassembled or at least needs to be specially designed. Disassembly will cause irreversible damage to the battery. When lithium plating occurs, many measures can be taken to eliminate it, and the battery can be further recycled in the remaining life.

Electrochemical impedance spectroscopy (EIS) is a widely used tool for characterizing lithium ion batteries. The result is usually presented in the form of a Nyquist plot. The Nyquist plot contains two "semicircles" . The low-frequency semicircle is attributed to the charge transfer at the electrode/electrolyte interface, and the high-frequency semicircle is attributed to the interface between electrode particles and metal current collectors. In a very simple model, Ret is expected to follow the Arrhenius formula:

1/Rct =Ae ^{(-Ea/(kB T} », wherein Ea represents an activation energy associated with the site where lithium ion transitions through the solid electrolyte interface in the material, kB represents a Boltzmann constant, T represents temperature, and A represents a proportionality constant.

Conventional electrochemical impedance spectroscopy analysis usually requires attention to meet the environmental factors of electrochemical system, such as temperature control, so that the only corresponding causal relationship between measurement signal and disturbance signal is guaranteed during the analysis process, thereby eliminating any other interference signals. Therefore, the electrochemical impedance spectroscopy analysis for lithium ion batteries in the prior art is performed at the same temperature.

For example, CN106680726A discloses a method for testing the cycle performance of a lithium ion battery. The method includes performing a preset testing operation in real time after the lithium ion battery is subjected to a preset charge-discharge cycle test operation of different cycles. The testing operation includes real-time detection of the battery state of charge of the lithium ion battery. When the battery state of charge of the lithium ion battery reaches the preset battery state of charge value, the electrochemical AC impedance test is performed on the lithium ion battery to obtain preset AC impedance test parameters. However, the AC impedance test in this patent application document was not carried out at different temperatures.

CN106199451 A discloses a method for testing the optimal compaction density of lithium iron phosphate positive plates of lithium ion batteries, wherein the test steps involve performing two electrochemical tests by using electrochemical workstation IVIUM-n-STAT. AC impedance spectroscopy test is carried out first, followed by linear sweep test. The specific AC impedance spectroscopy test is performed at starting voltage of 3.42V-3.43V, scanning frequency of 100000-0.01 Hz, and current range of 100mA. After open circuit voltage is stable, linear sweep is performed with voltage amplitude of 50mV, voltage interval of 1mV, scan rate of 1mV/s, and current range of 1mA. Finally, the relevant performance of the lithium iron phosphate positive plate was obtained by the result analysis. The AC impedance test in this patent application document was not carried out at different temperatures, either.

Summary of the invention

There is a need in the art for a method for quickly analyzing and/or determining whether lithium plating occurs on the anode in a battery without disassembling the battery.

The inventor has found that electrochemical impedance spectroscopy method can be used to determine whether lithium plating occurs in a battery without disassembling the battery. The inventors has found that since electrochemical impedance spectroscopy has temperature dependence, the results show differences when measured at different temperatures. Therefore, when electrochemical impedance spectroscopy measurement is performed in lithium ion batteries with and without lithium plating, the results of electrochemical impedance spectroscopy will exhibit different trends. Specifically, for lithium ion batteries without lithium plating, the curve in the Nyquist plot of electrochemical impedance spectroscopy shows a tendency to decrease in the real part (horizontal axis direction) as the temperature increases; on the contrary, in the case of a lithium ion battery with lithium plating, the curve in the Nyquist plot of electrochemical impedance spectroscopy shows a tendency to increase in the real part (horizontal axis direction) as the temperature increases. These two opposite results therefore provide a standard for quickly evaluating whether lithium plating occurs in a lithium ion battery.

Therefore, the present invention relates to a method for determining whether lithium plating occurs on the anode of a lithium ion battery using electrochemical impedance spectroscopy at different temperatures. This method is very effective in diagnosing lithium deposition.

In one aspect, the present invention relates to a method for analyzing the occurrence of lithium plating on the anode of a lithium ion battery using electrochemical impedance spectroscopy. The method is characterized by performing electrochemical impedance spectroscopy analysis on complete/undisassembled lithium-ion batteries under different temperature conditions, and determining the occurrence of lithium plating on the anode of the lithium ion battery according to the arrangement of the curves attributable to different temperatures on the obtained Nyquist plot.

In another aspect, the present invention relates to a method for analyzing the occurrence of lithium plating on the anode of a lithium ion battery using electrochemical impedance spectroscopy, characterized by determining the occurrence of lithium plating according to the sequence of the arrangment of electrochemical impedance spectroscopy curves obtained under different temperature conditions.

In still another aspect, the present invention relates to a method for analyzing the occurrence of lithium plating on the anode of a lithium ion battery using electrochemical impedance spectroscopy, characterized in that in the Nyquist plot of electrochemical impedance spectroscopy obtained at different temperatures, one or more curves obtained from low to high temperature are arranged from high frequency region to low frequency region, indicating the occurrence of lithium precipitation, or one or more curves obtained from low to high temperature are arranged from left to right in the real part, indicating the occurrence of lithium precipitation.

In yet another aspect, the present invention relates to a method for analyzing the occurrence of lithium plating on the anode of a lithium ion battery using electrochemical impedance spectroscopy, characterized in that the temperature interval between different temperature conditions for performing electrochemical impedance spectroscopy analysis is 5 ~20°C, preferably 5-15°C, more preferably 5-10°C.

In still another aspect, the present invention relates to a method for analyzing the occurrence of lithium plating on the anode of a lithium ion battery using electrochemical impedance spectroscopy, characterized in that the temperature change between different temperature conditions for performing electrochemical impedance spectroscopy analysis is continuous.

In still another aspect, the present invention relates to a method for analyzing the occurrence of lithium plating on the anode of a lithium ion battery using electrochemical impedance spectroscopy, characterized in that multiple electrochemical impedance spectroscopy measurements are performed at each test temperature, preferably 1-10 measurements, more preferably 2-8 measurements, most preferably 3-5, 3 or 4 measurements.

Embodiments

Unless otherwise defined, all technical and scientific terms used herein have the same meanings as commonly understood by those skilled in the art. Although any methods and materials similar or equivalent to those described herein can be used in the practice or testing of the present invention, preferred methods and materials are now described.

In the present application, the lithium ion battery is the smallest basic functional unit of the lithium ion battery pack. In the sense of the invention, the lithium ion battery pack represents not only the rechargeable battery pack (secondary battery pack) but also the non -rechargeable battery pack (primary battery pack). Rechargeable lithium-ion batteries are synonymous with lithium-ion secondary batteries. The two terms also include lithium battery packs and lithium ion accumulator. The battery pack is composed of at least two connected batteries. Typically, in a lithium ion battery pack, two or more lithium-ion batteries are connected in series or in parallel. Lithium-ion batteries here include two opposite electrodes, a negative anode and a positive cathode. The two electrodes are electrically and physically separated from each other by a separator arranged between the electrodes. Lithium ion batteries are usually filled with electrolyte. The separator can be penetrated by lithium ions, so ions can be exchanged between the anode and the cathode during charging or discharging.

As used herein, "active material" means the part of the electrode that stores lithium ions. In the case of the cathode, the active material may be a lithium-containing compound such as a lithium metal oxide complex. In the case of the opposite anode electrode, the active material may be silicon or lithiated silicon.

As used herein, the term "anode" refers to an electrode capable of donating electrons when the battery is in operation, which is also referred to as a negative electrode in the nomenclature.

As the active material of the anode, all materials known in the related technical field can be used. Regarding the anode in the sense of the present invention, there are no limitations. In particular, it is also possible to use mixtures of different active anode materials.

The anode material can be selected from lithium-metal oxides such as lithium titanium oxide, metal oxides (e.g. Fe2Os, ZnO, ZnFe2O4), carbon- containing materials such as graphite (synthetic graphite, natural graphite), graphene, mesophase carbon, doped carbon, hard carbon, soft carbon, fullerene, mixtures of silicon and carbon, silicon, lithium alloy, lithium metal and mixtures thereof. As anode materials, niobium pentoxide, tin alloy, titanium dioxide, tin dioxide and silicon can also be used.

The anode material may also be a material that can be alloyed with lithium. It may be a lithium alloy or an unlithiated or partially lithiated precursor, and a lithium alloy is generated from the precursor during formation. Preferred materials that can be alloyed with lithium are lithium alloys selected from the group consisting of silicon-based alloys, tin-based alloys, and antimony-based alloys.

As the active material of the cathode, all materials known in the related technical field can be used. Regarding the cathode in the sense of the present invention, there are no limitations. In particular, it is also possible to use mixtures of different active cathode materials.

The electrochemical workstation used for the electrochemical impedance spectroscopy measurement of the method of the present invention is not particularly limited. It can be a conventional electrochemical workstation in the field, including a single-channel electrochemical workstation, a multi-channel electrochemical workstation, an integrated electrochemical workstation, etc., such as various types of electrochemical workstations provided by manufacturers or trademarks Zahner, Gamry, Vertex, etc. It is well known for those skilled in the art to plot the results obtained by the electrochemical workstation into corresponding Nyquist plot and to analysis it.

Considering in conjunction with the accompanying drawings, other objects, advantages and new features of the present invention will become apparent from the following detailed description of one or more preferred embodiments. The present disclosure is written for those skilled in the art. Although the present disclosure uses terms that may be unfamiliar to laymen, those skilled in the art should be familiar with the terms used herein.

Description of the drawings Figure 1 is the curve obtained by electrochemical impedance spectroscopy measurement of a sample at different temperatures; due to continuous change of temperature, 3-4 measurements are performed at each test temperature.

It can be seen from the figure that there are multiple curves in each temperature group, and as the temperature increases at intervals of about 5-10 °C, each group of curves belonging to the same temperature in the electrochemical impedance spectroscopy measured in this case moves to the right along the transverse axis in the Nyquist plot, that is, these curves groups are arranged from low to high in the real part as the temperature increases.

Fig. 2 is an SEM photograph of a graphite electrode sheet on which lithium metal is plated.

Figure 3 is an SEM photograph of a fresh (no lithium plated) graphite electrode sheet.

Figure 4 is a photograph of a fresh (no lithium plated) graphite electrode sheet.

Fig. 5 is a photograph of a graphite electrode sheet on which lithium metal is plated.

Examples

Two commercial soft-pack batteries with graphite anodes with good consistency were selected in parallel and cycled at 0°C and 0.3C rate to cause lithium plating. After 80 cycles, the capacity retention rate of the battery was about 90%.

Example 1 (disassembly inspection for comparison)

One of the batteries was disassembled to check the appearance of lithium plating. Visual observation showed that the two had different states. It was found through observation that the gray area due to the plating of lithium metal can be visually observed on the anode (see the comparison between Fig. 4 and Fig. 5). After taking pictures of their surfaces using SEM method, changes were observed in their microscopic morphology.

Example 2 (Electrochemical Impedance Spectroscopy of the Invention) As a comparison, instead of disassembling the other battery, the electrochemical impedance spectroscopy measurement was performed on the Autolab PGSTAT302N electrochemical workstation of Eco Chemie in the potentiostatic mode at different temperatures (30°C, 35°C, 45°C, 56°C) (the test parameters were: 5 mV / 10' ¹ ~10 ⁵ Hz) to check the lithium plating. The results were summarized in Figure 1, from which the trend of lithium plating in the battery can be clearly seen. As the temperature increased, the curve (group) continued to move to the right on the Nyquist plot. For impedance, the real part increased gradually. This change was not consistent with the temperature reversal in the case of lithium ion battery without lithium plating, that is, the real part decreased with the increase of measurement temperature. Therefore, the reason for this phenomenon can be attributed to lithium plating: with the increase of temperature, the plated lithium metal will be re-embedded in graphite. This reduced electronic conductivity and increased internal resistance, and the arrangement of curves in the electrochemical impedance spectroscopy increased with increasing temperature in the real part.