In the previous article, "Talking about the Improvement of Li + Migration and Rapid Charge in Lithium-ion Batteries", we mentioned that the transfer number of Li + is of great significance for improving the quick charge capability of Li-ion batteries. In this article, we will continue Take everyone to learn how to effectively improve the Li-ion battery electrolyte Li + migration number.
From the introduction of the last article, we can also see that it is self-evident that the development of electrolytes with higher migration numbers of Li + for fast-charging batteries is actually ongoing. For example, The Li + conductor ceramic material is a single ion conductor and therefore has a migration number of 1. The solid polymer electrolyte in Figure b, by immobilizing anions on the polymer framework, avoids the migration of anions and enhances the migration of Li + In order to improve the performance of the copolymer electrolyte, one also added small molecules of solvent and nanoparticles, as shown in Figure d.Another way is to fix the liquid electrolyte of anions, such as the use of tool anions, High concentration of electrolyte and other methods to improve the liquid electrolyte Li + migration number.
The above methods to increase the number of electrolyte Li + migration also have their own advantages and limitations, then we will work with you and the following analysis.
Ceramic based single ion conductor
Solid oxide electrolytes have been very hot in recent years, a ceramic-based single ion conductor, the current study is relatively hot mainly garnet phase metal oxide materials, such as Li 7La 3Zr 2O12, And lithium thiosulfate, Li 2S-P 2S5Etc. These materials have very high ionic conductivities, which can reach 25mS / cm at room temperature and only Li + in these electrolytes can migrate, so it is a very ideal electrolyte candidate.But the ceramic electrolyte should be real Application of lithium-ion batteries is also facing many challenges, first of all due to the brittleness of the ceramic material is very strong, so the thickness of less than 100um, or even 20um electrolyte membrane process is very difficult to achieve.Meanwhile, the traditional solid-phase synthesis methods In the ceramic electrolyte to form a large number of grain boundaries and pores, thereby greatly reducing the electrical conductivity of ceramic electrolyte (ceramic electrolytes actually because of the existence of grain boundary resistance, lithium ion conductivity is much lower than the theoretical value).
Monomer electrolyte
In this electrolyte, the anions are immobilized on a long chain of polymers and therefore do not move under the action of an electric field. Therefore, the diffusion of Li + can only take place in a vacancy form, which also results in the electrolyte The diffusion rate of Li + is significantly lower than that of the liquid electrolyte, rendering it very low at room temperature.The following figure summarizes that since the first discovery of single ion conductor polymer electrolytes in 1985, the conductivity of different polymers has been at 30 and 90 ° C Conductivity increased (according to published papers in different years finishing.) Can be seen from the figure in the past 20 years single ion conductor polymer electrolyte room temperature conductivity has been maintained at 10-5S / cm or so, little increase.
Additive Modified Polymer Electrolyte
In view of the difficulty of improving the performance of a single ion conductor polymer electrolyte, attempts have been made to improve the conductivity by adding small amounts of small molecule solvents and nanoparticles to it.The main role of small molecule solvents is to increase the solubility of Li + The concentration of the sub-polymer electrolyte to add a certain number of nanoparticles can also effectively improve the conductivity of the electrolyte, such as phase PEO / LiCiO 4Add part of TiO 2Can significantly improve the conductivity of the electrolyte.Recent studies have shown that the conductivity can be increased to 10-4 S / cm with a Li + mobility of more than 0.9 by adding some additives to the polymer electrolyte. In the future, with respect to polymer electrolytes Research should focus on a deeper understanding of the reaction mechanism between 'additive-salt-polymer'.
Liquid electrolyte
Liquid electrolyte is the most popular choice of electrolyte for lithium-ion batteries at present, so how to improve the number of Li + migration in liquid electrolyte is our most concern.The conductivity of liquid electrolyte can reach 10mS / cm, which is much higher than other types of However, due to the limitation of Li + solvation shell, the transfer number of Li + is generally less than 0.5, which greatly limits the ability of lithium-ion battery with liquid electrolyte to rapidly charge.To solve this problem, we can proceed from two aspects , We can start with the limitation of anion migration. For example, Archer et al. Proposed the idea of immobilizing anions on nanoparticles in 2013. Another idea is to use lithium salts with high concentration, such as high concentration of LTFSI electrolyte Li + migration number can reach 0.7, the study suggests that this may be unique solution structure makes the anion mobility greatly reduced, while the cation is less affected.
Overall, we are currently working to improve the liquid electrolyte is still relatively small, so there is a very bright future in this regard.Kyle M. Diederichsen at the University of California, Berkeley that the soluble polymer anion lithium salt is to achieve high Li + The effective way to migrate several liquid electrolytes deserves our further study in this area.
