Since Sony introduced the first commercial lithium-ion battery, graphite materials have been the mainstream of negative electrode materials.Graphite negative electrode rapidly decreases its potential after lithium intercalation, which is close to the potential of the negative electrode of Li metal. This feature is good on the one hand, The voltage of the ion battery increases the energy density of the lithium-ion battery. On the one hand, the low potential leads to the decomposition of the carbonate-based organic electrolyte to form a passivation layer. The solid electrolyte membrane, that is, Usually we say that SEI is the key to protect the negative electrode and electrolyte. SEI membrane can avoid direct contact between electrolyte and negative electrode to reduce the decomposition of electrolyte. At the same time, it is also possible to avoid the intercalating of Li + and solvent molecules, causing graphite Therefore, the majority of electrolyte manufacturers in the process of electrolyte design should first consider how to form a more stable SEI film, so a variety of additives to help improve the quality of SEI film forming additives An endless stream, such as our common VC, FEC, are commonly used SEI film forming additives, but recently from King Abdullah Studies (this is a super-wealthy universities interested can find out) of JunMing and others have found, SEI film lithium-ion batteries in the role did not we think important.
Most of the solvents used in the commercial lithium-ion battery electrolyte will suffer from the problem of Li + -solvent co-embedding. In particular, the PC solvent will cause serious damage to the graphite structure during the co-embedding, leading to the exfoliation of the graphite layer, In general, we think that the negative SEI film plays a key role in preventing co-embedding of solvents, but a study by Jun Ming et al. Found that the lithium salt of electrolyte (LiTFSI) Concentration and addition of inorganic additives (LiNO3) play a greater role in preventing solvent co-embedding.
Why do we say that we first take a look at the experiment of Jun Ming et al. Firstly, Jun Ming assembled a graphite / metal Li half-cell and then injected a commercial electrolyte solution (LiPF6, 1mol / L, EC: DMC = 1: 1) During the first discharge, with the insertion of Li, the potential of the negative electrode decreased, and the decomposition of the electrolyte and the formation of the SEI film took place. In the subsequent cycles, the cycle curve of the battery became very smooth due to the formation of the SEI film, Indicating that the SEI membrane can play a protective role, but when Jun Ming removed the SEI membrane-forming negative electrode as described above and reinserted into the half-cell, a new etchant (1.0 M / 0.4 M electrolyte, ie LITFSI, 1M , LiNO3, 0.4M, DOL / DME = 1: 1, a commonly used electrolyte in Li-S batteries), it was found that a large irreversible capacity was still formed during the first discharge / charge, However, if we increase the lithium salt concentration of the above electrolyte to 2.5 M (2.5 M / 0.4 M electrolyte LITFSI, 2.5 M, LiNO3, 0.4 M, DOL / DME = 1: 1), can significantly inhibit the decomposition of the electrolyte (as shown in Figure c below.) Then Jun Ming In addition, the negative electrode of the SEI film is formed in the ethereal 2.5M / 0.4M electrolyte, and the electrolyte is again put into the 1.0M / 0.4M ether solution of low concentration, and the electrolyte begins to decompose obviously on the negative electrode surface d), indicating that 1) the SEI film formed in the carbonate electrolyte can not effectively prevent the delamination and spalling of the graphite; 2) increasing the lithium salt concentration can effectively inhibit the failure of the graphite negative electrode.
So what is the reason why high concentrations of LiTFSI electrolyte can significantly improve the stability of the graphite negative electrode? To this end, Jun Ming were configured with different LiTFSI concentrations and formulations of the electrolyte were tested, the results shown below. As can be seen in Figure d, the stability of the electrolyte has been significantly improved as the LiTFSI concentration increases (from 1M to 10M) (the drop in electrolyte concentration at 10M is mainly due to the excessively high concentration, leading to increased viscosity and ion mobility Difficult), and the addition of LiNO3 in the electrolyte can further enhance the stability of the electrolyte.Using XRD studies shows that graphite can reversibly insert and desorb Li + in 2.5M / 0.4M electrolyte with higher concentration, but at higher concentration Low 1.0M / 0.4M electrolyte, due to graphite in the role of co-solvent under the action of exfoliation, resulting in Li + irreversible.
In order to study the mechanism of the above phenomena, Jun Ming used Raman spectroscopy to study the solution structure of the electrolyte. The results show that the TFSI- ions in the electrolyte exist in several different forms depending on the intensity of the Li + interaction: Ion FI, loose ion pair LIP, compact ion pair IIP and polymeric ion pair AIP. The experimental data show that: 1) At lower concentration, TFSI- mainly exists in the form of FI or LIP. When lithium salt concentration is increased, TFSI - will shift to LIP and IIP; 2) DME will dissolve lithium salt better than DOL such that TFSI- is more present as FI; 3) NO3- can significantly enhance Li + -TFSI- Interaction.
In general, we believe that Li + dissolves to form a solvated shell that contains both anionic and solvent molecules. Jun Ming mimics molecular dynamics using the density function theory to arrive at the structure of the solvated shell As shown in the above figure, Li + has a strong interaction with O in TFSI-, and NO3- can replace TFSI- in the solvated shell (Figure e) or replace the solvent molecule Figure f), so as to achieve the effect of weakening the interaction between Li + -solvents.At the same time, the above study also shows that the structure of the solvation shell does not necessarily need to be electrically neutral, for example, Can further interact with Li + to form a larger structure, further weakening the force of Li + -solvent.
HOP data of heterogeneous degree of order also proved the above calculation results. In general, the larger the degree of ordinal parameter is, the stronger the interaction between Li + and solvent. Jun Ming found that 1.0M / 0.4M solution of When the lithium salt concentration is increased to 2.5M / 0.4M, the degree of order parameter is reduced to 1.8, indicating that increasing lithium salt concentration can effectively reduce the Li + -solvent interaction without adding LiNO3 2.5M / The order parameter of 0M electrolyte is 4.7, which is much higher than 1M / 0.4M electrolyte with LiNO3 addition, indicating that NO3- can effectively reduce the Li + -solvent interaction.
Jun Ming's research has opened up a whole new idea in electrolyte development - reducing the Li + -solvent interaction, reducing the total intercalation of solvents, and improving the cycle performance of graphite negative electrodes.Lower Li + -solvent interaction can be achieved by increasing the lithium salt Concentration, the use of weakly soluble DOL solvent system, the electrolyte to the addition of inorganic salts (such as LiNO3 or NaNO3, etc.) means to significantly reduce the phenomenon of solvent co-embedding, which for future electrolyte design, especially S The design of positive battery electrolyte provides a new idea, which is of great significance to the application of S positive battery.
