Interface characteristics at high temperatures of high power charging lithium battery

Interface characteristics at high temperatures of high power charging lithium battery

In recent years, with the increasing popularity of plug-in hybrid and 48V mild hybrid systems, the demand for high-power lithium-ion batteries is increasing. In 2002, the United States listed the research on high-power lithium-ion batteries as part of the United States’ “Advanced Technology Development Program” to optimize the chemical system of the battery and optimize the design of the battery. High-power lithium-ion batteries have a large discharge current during use, so more heat is generated during the discharge process.

At high temperatures, the decomposition of the electrolyte at the positive and negative interfaces increases significantly, resulting in increased electrode interface impedance, which affects the performance of the battery. In order to analyze the interface properties of high-power lithium-ion batteries at high temperature, AM Andersson (first author, corresponding author) and J. Liu (corresponding author) of Argonne National Laboratory in the United States investigated high-power 18650 batteries at different temperatures The storage characteristics under different SoCs were studied, and the characteristics of the positive and negative interfaces of the battery after storage were analyzed.

The battery used by the author in the experiment is a 18650 battery with a capacity of 1Ah, the positive electrode material is LiNi0.8Co0.2O2, the formula is 84% ​​active material, 4% SFG-6, 4% acetylene black, 8% pVDF , the thickness of Al foil is 20um, the negative electrode is graphite system, the formula is 75% MCMB-6 mesocarbon microspheres, 17% SFG-6, 8% pVDF, and the thickness of copper foil is 12um.

The separator used in the battery is a 37um three-layer composite separator from Celgard, the electrolyte solvent is EC:DEC (1:1), and the lithium salt is LipF6 (1M). The following picture is the SEM picture of the positive electrode material. From the picture, it can be seen that the morphology of LiNi0.

8Co0.2O2 is similar to that of common ternary materials. They are all secondary particle balls formed by agglomeration of a large number of primary particles, with a diameter of 5 -10um or so.

The following picture shows the SEM image of the positive electrode after storage at 40°C. From the picture, it can be seen that many small particles (the author thinks it is LiF) appeared on the surface of the positive electrode after storage, but the same phenomenon was not found on the negative electrode, so the author believes that This may be due to the oxidative decomposition of lithium salts on the surface of the positive electrode. The figure below shows the XpS analysis results of the LiNi0.

8Co0.2O2 cathode powder. There are two reaction peaks at 285 and 290eV in the C1s diagram, of which the characteristic peak at 285eV corresponds to hydrocarbon impurities, while the characteristic peak at 290eV mainly comes from the surface of the cathode.

of carbonate impurities. The following figure shows the XpS analysis results of the surface of the positive electrode just after coating. The characteristic peaks in the C1s figure mainly come from graphite, acetylene black and pVDF.

The characteristic peaks of Li2CO3 overlap with those of pVDF. The characteristic peaks in the F1s figure There are two important peaks, one is the characteristic peak near 687.6eV, which is mainly from the pVDF binder, and the other is the characteristic peak near 684.

7eV, which is mainly from LiF. Regarding the source of LiF on the surface of the positive electrode, the author believes that it may be in the homogenate. In the process, the pVDF binder reacts to generate HF under the catalysis of alkali, and HF reacts with Li2CO3 on the surface of the positive electrode to generate LiF.

The figure below shows the XpS image of the surface of the LiNi0.8Co0.2O2 positive electrode after the first charge-discharge cycle.

From the C1s image, it can be seen that in addition to pVDF, Li2CO3, and graphite, there is a polymerization near 285.1eV on the positive electrode surface. phase.

Two characteristic peaks can be observed in the F1s diagram, the characteristic peak at 687.6 eV is from pVDF binder, and the characteristic peak near 684.9 eV is from LiF.

The figure below shows the XpS analysis results of the positive electrode after storage at 40°C and 60% SoC. From the C1s, F1s and O1s figures, it can be seen that the surface composition of the battery after storage is basically the same as that after the first charge and discharge, but in the p2p figure The original single characteristic peak is split into two characteristic peaks, of which the higher energy peak is composed of two peaks of 136.7eV and 137.

5eV, which is basically consistent with the surface characteristics of the positive electrode after formation, and the lower energy peak is located at 134.2eV and 135eV , the author thinks that this characteristic peak is mainly from the covalent bond formed by p atoms and atoms with lower electronegativity (such as O), of which the peak at 135eV comes from p2O5, and the peak at 134.2eV may be from Li2pFO3.

The element content of the positive electrode surface after storage at different temperatures and SoCs is shown in the figure below. It can be seen from the figure below that the content of C and O elements on the positive electrode surface increases with the increase of storage temperature, while the element F is just the opposite. As the storage temperature increases, the content of F element on the surface of the positive electrode gradually decreases, which is mainly due to the coverage of the decomposition product on the surface of the positive electrode, which makes the F element signal in the positive electrode pVDF binder significantly decrease.

The figure below shows the XpS analysis results of the positive electrode surface before and after DEC cleaning after storage at 40°C, 60% SoC and 70°C, 60% SoC respectively. It can be seen from the figure below that the positive electrode after storage at 40°C is cleaned by DEC. The LiF content on the surface of the negative electrode was significantly reduced, but the LiF content of the positive electrode after storage at 70 °C did not decrease significantly after cleaning, which indicated that the LiF generated at the lower storage temperature was loosely bound on the surface of the positive electrode, while at the lower storage temperature.

LiF generated at higher temperature will be strongly attached to the surface of the positive electrode. Since the XpS tool can only measure the surface material with a limited thickness, in order to analyze the distribution of elements at different depths, the author used Ar+ sputtering to process the surface of the electrode. With the extension of the processing time, the material on the surface of the electrode was Erosion, exposing the underlying material, so as to realize the detection of materials at different depths.

The figure below shows the relationship between the content of different elements on the electrode surface and the sputtering time (figure a is the control sample, and figure b below is the sample after storage at 60 °C and 40% SoC). The content of C element is much higher than other types of elements, which is mainly due to the huge surface area of ​​carbon black conductive agents, and with the extension of sputtering time, more carbon elements are exposed, and the content of carbon elements also increases. showing an increasing trend.

The content of O element comes from two parts, one part comes from impurities in the electrode surface layer, and the other part comes from LiNi0.8Co0.2O2 in the bottom layer, so we see that the content of O element shows a decreasing trend at the beginning with the increase of sputtering time , and then began to show an upward trend.

The figure below shows the XpS analysis results of the negative electrode surface, where the figure a below is the negative electrode without contact with the electrolyte, the figure b below is the negative electrode surface after the formation cycle, and the figure c below is after storage at 70°C and 60% SoC. On the surface of the negative electrode, it can be seen from the figure a below that the characteristic peaks on the negative electrode surface that are not in contact with the electrolyte mainly come from graphite and pVDF, and there are no extra characteristic peaks. From Figure b below, it can be seen that the characteristic peak of graphite near 284.

5eV does not appear after the negative electrode is formed, which indicates that the surface of the graphite negative electrode has been covered by the SEI film, and the broad characteristic peak appears in the range of 289.5-291eV, It shows that a layer of carbonate appears on the surface of the negative electrode, such as lithium alkyl carbonate and Li2CO3 commonly found on the surface of the negative electrode, and this characteristic peak shifts to around 290.4eV after high temperature storage, indicating that the Li2CO3 component becomes dominant at this time, which indicates that Lithium alkyl carbonates decompose into more stable Li2CO3 components at high temperatures.

The main peaks near 687.6eV in the F1s figure are from pVDF and LipF6/LixpFy, and the peak at 685.5eV is mainly from LiF.

From Figure c below, it can be seen that the characteristic peaks around 685eV on the surface of the negative electrode after storage are significantly enhanced, indicating that The LiF content on the anode surface increased significantly after high temperature storage. A.M.

Andersson’s research shows that there is Li2CO3 impurity in the LiNi0.8Co0.2O2 positive electrode, and during the electrode preparation process, the HF and LiNi0.

8Co0.2O2 positive electrode that appear due to the alkali-catalyzed decomposition of pVDF react, and LiF products will be generated on the positive electrode surface. The impurities on the surface of the positive electrode after formation are mainly LiF formed by the decomposition of LipF6, while the surface of the negative electrode is mainly composed of various carbonate and polymer impurities.

After high temperature storage, the impurities on the surface of the positive electrode include polycarbonate, LiF, LixpFy and LixpFyOz, while on the negative electrode surface, alkyl lithium carbonate and polycarbonate were transformed into more stable Li2CO3 after high temperature storage, and impurities such as LiF, LixpFy and LixpFyOz were also observed on the negative electrode surface. This article mainly refers to the following documents. The article is only used for the introduction and review of related scientific works, as well as classroom teaching and scientific research, and shall not be used for commercial purposes.

Please feel free to contact us with any copyright issues. SurfaceCharacterizationofElectrodesfromHighpowerLithium-IonBatteries,JournalofTheElectrochemicalSociety,149(10)A1358-A13692002,A.M.

Andersson,D.p.Abraham,R.

Haasch,S.MacLaren,J.LiuandK.

Aminea.

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