After more than two decades of development, lithium-ion batteries have made remarkable progress in materials and design. The specific energy of 260Wh/kg or more, which is increased from the initial 80Wh/kg, has continued to increase. Nickel ternary material/silicon-carbon material is the main direction for the development of high-specific-energy batteries. With the gradual maturation of positive and negative materials and supporting binders, conductive agents, and electrolytes, high specific energy of 300 Wh/kg will be realized by 2020. The goal is basically not too difficult. Although silicon-carbon materials can temporarily meet the design requirements for high-specific-energy batteries, the next-generation 400Wh/kg of new-generation high-specification power battery silicon-carbon materials cannot do anything.
From the current level of technological development, Li-S, Li-air, and all-solid-state Li metal batteries are the most promising next-generation high-specific-energy battery solutions. All of these batteries will be applied to metal Li anodes without exception. The theoretical capacity of the negative electrode reaches 3800mAh/g, and it has excellent electronic conductivity. It is a very ideal negative electrode material, but the negative electrode of metal Li has to face a serious problem when used in a secondary battery. Crystals. The appearance of metallic Li dendrites not only causes Li loss, but also causes internal short circuits in extreme cases, leading to serious safety problems. Therefore, a large number of scholars have invested a lot of energy in the development of Li dendrites can be inhibited Technology, such as our previous report “Tsinghua University: Inducing Directional Growth of Li Dendrites and Resolving the Safety Problems of Metallic Lithium Negatives!” reported that Peichao Zou et al. of Tsinghua University avoided Li by inducing the growth direction of Li dendrites. The dendrite penetrates the diaphragm to achieve the purpose of avoiding the internal short circuit. In addition, we are still in the article “Opportunities and Challenges of Metallic Lithium Electrodes” in the article on the current suppression of metal Li dendrites. A comprehensive review of the means of growth, you can click the link to view the original text.
Dendrites are a relatively common phenomenon in the metallurgical industry. For example, dendrite problems may occur in the production of electrolytic Cu and Zn. In particular, the research on fiery hot ambient temperature ionic liquid electrolysis Al in recent years has also been dendrite. Problems are bothered. The root cause of dendrite formation is local polarization, resulting in uneven current distribution, and the same is true for the generation of Li dendrites in secondary batteries. Therefore, the key to suppressing the growth of Li dendrites is how to reduce local polarization. For example, it has been reported that the addition of a small amount of alkali metal elements such as Cs+ and Rb+, which have a slightly lower reduction potential than Li+, in the electrolytic solution can significantly inhibit the growth of Li dendrite, as shown in the figure below. When the crystals are generated, the local current density increases, attracting nearby Cs+ and Rb+. However, since the reduction potentials of the two metal ions are relatively low, deposition does not occur, and the cations accumulated on the surface of Li dendrite will affect Li+. Repelling effect, thereby inhibiting the growth of Li dendrites.
Recently, Hanqing Jiang of Shenzhen State University, Shenzhen University and Hunan University discovered that mechanical stress has an important influence on the growth of metallic Li dendrites. Lithium is deposited on a flexible substrate by depositing Li in the process of deposition. The release of stress effectively inhibited the growth of Li dendrites.
The flexible substrate designed by Hanqing Jiang is shown in the figure below. It is mainly composed of thin copper foil and a flexible substrate (polydimethylsiloxane (PDMS)). When Li is deposited on the above substrate, the stress generated will lead to copper. Foil wrinkles, so as to achieve the purpose of stress relief (as shown in Figure a and b below), and if the rigid base, due to stress can not be released, resulting in Li dendrite formation (shown in Figure c).
The figure below shows the folds caused by Li deposition on flexible substrates with different thicknesses (200, 400, and 800 nm) during the charging process by Hanqing Jiang using the button cell. From the figure we can see that the copper foil substrate first appeared 1D. The phenomenon of wrinkling, as the Li deposition time increases, the copper foil exhibits 2D wrinkles. This phenomenon also verifies the assumption that Li will generate stress during the deposition process. At the same time, we also notice the wavelengths and metals of these folds appearing on the flexible substrate. The deposition of Li is not related, but is closely related to the thickness of the copper foil. The wavelengths of the folds for the 200nm, 400nm and 800nm copper foils are 25um, 50um and 100um, respectively.
The following figure shows the deposition process of Li on the hard substrate and the flexible substrate. It can be seen that after 5 minutes of deposition, more protrusions have appeared on the hard substrate (below a), and Li deposition is very uneven. The metal Li deposited on the flexible substrate is relatively uniform and has no sharp bulges. After deposition for 1 h, a large number of sharp Li dendrites with different diameters (see figure c) have appeared on the hard substrate, and the flexibility The metal Li layer on the substrate is very uniform, with no observed Li dendrites (Fig. d). After 100 cycles, the hard substrate is already covered with Li dendrites of the metal, whereas Li on the flexible substrate is still relatively This shows that the stress release mechanism of the flexible substrate can well inhibit the growth of Li dendrite.
Hanqing Jiang believes that Li dendrite growth is to release the stress generated during Li deposition, but this theory still lacks the support of relevant data, so Hanqing Jiang established a model to analyze the growth process of Li dendrite. There are several in the model. A key factor affects the growth process of Li dendrites. The first is the stress that Li generates during the deposition process. This is mainly because the surface Li is embedded in the Li crystal boundary at a non-equilibrium state, resulting in the generation of stress (approximately 100MPa). Secondly, the SEI film formed on the surface of Li inhibits the release of stress through the surface creep of the metal Li. The third is the presence of planar defects in the metal Li, which promotes the growth of the metal Li dendrites.
In the above model, Li dendrite grows because the stress generated at the Li grain boundary alters the chemical potential of Li here, resulting in the deposition rate of Li that is consistently higher than the average Li deposition rate (as shown in Figure c above). The calculations show that the growth rate of Li dendrites on hard substrates can reach 8.4-9.8 nm/s, much higher than the growth rate of Li coatings, and the growth rate of Li dendrites on flexible substrates is only 0.3 nm/s. This is even slower than the growth rate of the Li coating, which naturally does not produce Li dendrites, indicating that the flexible substrate can well suppress the growth of Li dendrites through the release of stress.
In order to further improve the performance of the flexible substrate, Hanqing Jiang prepared a flexible current collector with a 3D structure (as shown in the figure below). The current collector of the 3D structure can effectively reduce the current density at the electrode surface and reduce the thickness of the metal Li on the electrode surface. Therefore, the growth of Li dendrite can be suppressed better and the cycling performance of the battery can be improved.
Hanqing Jiang compares the electrochemical performance of 3D flexible current collectors, copper foils and foamed copper foils (shown below). The following figures b, c and d show the three current collectors at 1mA/cm2, 2mA/cm2 and 3mA respectively. Charging for 1 h at a current density of /cm2 and then discharging to a 1 V cycle performance curve, the 3D flexible current collector is significantly improved in cycle performance. At a current density of 1 mA/cm2, the 3D flexible current collector in the first 200 cycles The Coulomb efficiency is above 98%, while the Foam copper foil and copper foil in the first 90 times have a Coulomb efficiency of only 90% and 95%, and then they begin to become very unstable.
In order to verify the practicality of 3D flexible current collectors, Hanqing Jiang fabricated a full cell with LiFePO4 as a positive electrode with a 3D flexible current collector pre-inserted with lithium (2mAh/cm2) as a negative electrode, and tested the battery. The electrochemical performance (shown in the figure below) is 100 cycles at a rate of 1C. The capacity retention rate of the 3D flexible current collector can reach 85.6%, while the capacity retention rate of the battery using the copper foil as the negative electrode current collector is only 55.3%. The capacity retention rate of the battery using the foamed copper foil as the negative current collector is only 34.4%.
Hanqing Jiang et al.'s work made us realize that the stress produced by Li in the deposition process is a key factor that leads to the generation and growth of Li dendrites. The flexible substrate is used as a current collector to liberate metallic Li during the deposition process through the collector folds. The stress generated in this process can effectively inhibit the growth of Li dendrites and improve the cycling performance of metallic Li cells. This is very important for the development of lithium metal batteries. At present, the battery needs further improvement in cycle performance and energy density. Raise to improve the usability of the battery.
