18650 battery 3.7v 2000mah

　　Interface modification technology helps high-performance 18650 battery 3.7v 2000mah

　　With the application of high-nickel materials and silicon-carbon composite materials, the current energy density of lithium-ion batteries has reached more than 300Wh/kg, which is very close to the limit energy density of conventional lithium-ion batteries of 350Wh/kg. There is not much room for further improvement. . The theoretical specific capacity of metallic lithium can reach 3860mAh/g, and it also has a low voltage platform (-3.04V vs standard hydrogen electrode). It is an ideal anode material for the next generation of high specific energy lithium-ion batteries.

　　However, when metallic lithium is used as the negative electrode of a lithium-ion battery, it will cause problems such as dendrite growth during repeated charging and discharging, causing the lithium metal negative electrode to pulverize and expand, and even cause a short circuit between the positive and negative electrodes. Interface modification is an effective method to inhibit the growth of lithium dendrites and is widely used in the development of 18650 battery 3.7v 2000mah. Recently, Yuliang Gao (first author), Keyu Xie (corresponding author) and Bingqing Wei (corresponding author) of Northwestern Polytechnical University performed silanization treatment on the surface of metallic lithium anode, which significantly inhibited the growth of Li dendrites and reduced By eliminating interface side reactions, the capacity retention rate of a 1Ah soft-pack battery can reach more than 96% after 160 cycles at a rate of 1C, and the gas production of the battery is significantly suppressed.

　　Although a lot of progress has been made in the research on metallic lithium anodes, most of these studies are based on button batteries, and most of us use soft-pack batteries in actual use, so these results are often difficult to apply in actual batteries. application. In order to simulate as much as possible the problems faced in actual use, YuliangGao directly used soft-pack batteries in his research. The positive electrode material of the soft-pack battery prepared in the experiment is NCM523, with a surface capacity density of 3.38mAh/cm2. The surface of the metallic lithium negative electrode has been silanized, and the amount of electrolyte added is 2.7g/Ah.

　　In the experiment, the author used tetraethoxysilane (TEOS) to treat the surface of metallic lithium. The reaction between tetraethoxysilane and metallic lithium is mainly achieved by reacting with the hydrogen and oxygen functional groups on its surface (as shown in the following formula) . As you can see from the picture below, before treatment, the surface of metallic Li shows a metallic luster. The SEM picture also shows that the surface of metallic Li is very rough at this time, with a large number of dots on the surface. After treatment, the surface of metallic Li forms a very Dense surface layer.

　　In order to further analyze the composition of the surface layer of metallic lithium, the author used Raman spectroscopy to analyze the surface composition of metallic lithium. From the figure h below, you can see that the Si-O-Si in LixSiOy has the characteristic peaks at 800 and 2170/cm. Increasing trend in intensity with processing time. At the same time, the intensity of the characteristic peaks of metallic lithium at 516 and 1840/cm is also gradually decreasing, indicating that after treatment, a dense and continuously covering surface layer is formed on the surface of metallic lithium, the main component of which is LixSiOy.

　　According to the space charge theory, the Li+ concentration gradient in the interface layer is the main reason for the formation of Li dendrites. Therefore, forming a uniform layer of Li+ diffusion channels on the surface of the metallic lithium anode can effectively inhibit the growth of Li dendrites. Therefore, the author used the AC impedance method to measure the ion conductivity of Li+ in the surface layer (MSI) formed above. The results showed that the ion conductivity of Li+ in the surface layer reached 9.8×10-5S/cm. At the same time, the The Li+ migration number in the surface layer is 0.77, which can provide a fast channel for the diffusion of Li+ and reduce the Li+ concentration gradient on the surface of metallic Li, thus achieving the purpose of inhibiting the growth of Li dendrites.

　　The author also used the COMSOL tool to simulate the Li concentration gradient on the metallic Li surface. The analysis showed that after constant current charging for 3600 seconds, the metallic Li surface layer can well reduce the concentration gradient on the metallic Li surface (as shown in Figure c below).

　　At the same time, the author analyzed the mechanical strength of the surface layer formed after surface treatment of metallic Li. The author selected a total of 7 points for measurement. After measuring the Young's modulus of 7 points of the MSI-Li anode after surface treatment It is 6.25-9.45GPa, which is higher than the 6GPa required to suppress Li dendrites. The Young's modulus of untreated P-Li is only 0.21-0.32GPa. Therefore, the good mechanical strength of the surface layer produced by TEOS treatment on the metal Li surface can well inhibit the growth of metal Li dendrites.

　　The stability of the SEI film is particularly important for metal Li anodes. Li2CO3, LiOH and Li2O in conventional SEI films can dissolve in the electrolyte, thereby forming a porous SEI film, which in turn causes an increase in side reactions. Therefore, the author used a symmetrical structure Li-Li battery to examine the stability of the interface film. Before starting the test, the charge exchange impedance of the ordinary metal lithium anode P-Li was 52Ω, and the surface-treated metal lithium anode MSI-Li was 58Ω. are basically the same, but after 96 hours of charge and discharge, the impedance of the ordinary metal lithium anode increased to 236Ω, while the impedance of the surface-treated lithium metal anode only increased to 66Ω, which shows that the surface protective layer MSI can well stabilize the metal Li anode. /electrolyte interface. After 20 cycles at a current density of 1mA/cm2, the author used a scanning electron microscope to examine the morphology of the surface of the metallic lithium anode. From the picture below, you can see that the SEI film on the surface of the ordinary metallic lithium anode has become rough and cracked (bottom) Figure h), while the surface of the surface-treated lithium metal anode still maintains a smooth surface (Figure i below), which also means that the MSI protective layer on the surface of the lithium metal anode can well stabilize the electrode interface.

　　In order to further verify the effect of the MSI protective layer in practice, the author assembled a 1Ah soft-pack battery with an NCM523 cathode (area capacity density 3.38mAh/cm2), a negative electrode/positive electrode redundancy ratio of 2.96, and a liquid injection volume of 2.7g/Ah. The author used a 1C rate to perform a cycle test on the battery. As can be seen from Figure b below, after 80 cycles, the battery using ordinary metal lithium anode began to decline rapidly, and the capacity had declined to 0.756 at the 89th cycle. Ah (capacity retention rate 84%), and the battery with surface treatment can still produce 0.872Ah (capacity retention rate 96%) after 160 cycles, which shows that the MSI protective layer can stabilize the metal well. The interface of the lithium anode improves the cycle performance of the battery.

　　EIS is an effective method to analyze the internal interface changes of lithium-ion batteries. From the test results of e and f below, it can be found that the charge exchange impedance of ordinary metal lithium anode batteries increased significantly from 34mΩ to 155mΩ after 85 cycles, while the MSI protected The metallic lithium anode only increased from 12mΩ to 33mΩ, which shows that the MSI layer can well reduce the side reactions between the metallic lithium anode and the electrolyte, reduce the thickness of the dead lithium layer on the surface of the metallic lithium anode, and promote the diffusion of Li+.

　　By dissecting the lithium metal battery after cycling, it can be found that the MSI-protected lithium metal anode still maintains a smooth and complete surface after 20 cycles, but the surface of the ordinary metal lithium anode shows a rough and discontinuous surface. After 85 cycles, the ordinary metal lithium anode was severely damaged, the surface was loose and porous, and the thickness of the surface layer also increased significantly to 51um, while the MSI-protected metal lithium anode showed a more dense and complete appearance, with the surface The thickness of the layer has only increased to 25um. This is mainly because on the one hand, the MSI layer can isolate metallic lithium from the electrolyte, thereby reducing electrode side reactions. At the same time, the MSI layer also has better mechanical strength and can effectively inhibit the growth of Li dendrites. At the same time, MSI has better ionic conductivity. The rate can also make Li+ diffusion more uniform and inhibit the growth of dendrites.

　　Gas production is also a common problem in soft-pack lithium-ion batteries. Gas production will cause the battery to bulge, causing the distance between the positive and negative electrodes inside the battery cell to increase, causing the battery impedance to increase, thereby affecting the battery's cycle life. Therefore, the author studied the impact of the MSI protective layer on battery gas production. As can be seen from the figure below, the battery began to produce obvious gas after 40 cycles at 1C rate, but the MSI-protected metal lithium battery produced more obvious gas. Less than ordinary metal lithium secondary batteries, MSI-protected metal lithium batteries also produce less gas than ordinary metal lithium secondary batteries after storage. This is mainly because the MSI protective layer can prevent the contact between metallic lithium and the electrolyte, thereby reducing the occurrence of side reactions. At the same time, the good mechanical strength of the MSI protective layer can also withstand the damage of metallic lithium dendrites and volume expansion, which also plays a role. The role of protecting metallic lithium anode.

　　After analysis, the main components of the gas generated by metal lithium batteries during circulation and storage are CH4 (50.71%), CO (42.6%), CO2 (2.84%), O2 (1.69%), C2H6 (1.41%), H2 (0.65%) ), C2H4 (0.03%), C3H6 (0.03%), C3H8 (0.01%) and C4H10 (0.01%), among which CH4 and CO are the main components of the gas generated by the battery.

　　YuliangGao uses tetraethoxysilane (TEOS) to treat the surface of metallic lithium, forming a LixSiOy protective layer on the surface of metallic lithium. This protective layer not only has good mechanical strength, but can effectively inhibit the growth of Li dendrites. It also has good ionic conductivity, which can effectively reduce the Li+ concentration gradient on the surface of the metallic lithium anode and reduce the growth of dendrites, thus greatly improving the cycle life of 18650 battery 3.7v 2000mah.

　　This article mainly refers to the following literature. The article is only used for the introduction and review of relevant scientific works, as well as classroom teaching and scientific research, and shall not be used for commercial purposes. If you have any copyright issues, please feel free to contact us.

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