The synthesis and application of bifunctional flame retardants and TFSI were similar to salt additives for Li. The thesis/dissertation consists of two chapters focusing on the synthesis and application of bifunctional flame retardants and TFSI-like salt additives for Li battery. We combined common flame retardants such as phosphate and an SEI-forming agent such as vinyl or vinyl carbonate into a single molecule using simple synthesis and purification.
The list of TFSI resembles salt additives for Li battery. a) Possible flame retardant mechanism of DMMP. Photographs of each stage of the additive synthesis of LiMTFSI salt (a) After reaction of potassium trifluoromethanesulfonamide salt with methanesulfonyl chloride in MeCN for 48 h at 80 °C: Formation of white precipitates. Performance table of NK series cells in 5 criteria: charge capacity, discharge capacity, coulombic efficiency and lifetime of each electrolytic system.
Table of performance of LiMTFSI salt cells by 5 criteria: charge capacity, discharge capacity, coulombic efficiency and lifetime of each electrolyte system. Table of performance of AgFSI·2MeCN salt cells when charging 5 criteria: capacity, discharge capacity, coulombic efficiency and lifetime of each electrolyte system. a) General electrolyte additives used in lithium battery. This work: bifunctional flame retardant additives. a) Synthesis of vinyl flame retardant containing C-bonded phosphorus.
Trial and error: synthesis of LiMTFSI (Methane(trifluoromethane)sulfonamide lithium salt) (a) one-step reaction: methanesulfonamide (1.0 mmol), trifluoromethanesulfonyl chloride (1 equiv), LiOH·H2O (1 equiv), MeCN (1 M) under reflux, 24 hour; (b) two-step reaction:.
Synthesis and application of bifunctional flame retardants for Li battery
- Abstract
- Introduction
- Results and discussion
- Conclusion
- References
- Supporting information
- NMR Spectra
To overcome this problem, flame retardant additives based on organic halogenated compounds or organic phosphorus compounds have been used. Then remaining hydrogen and hydroxyl radical can be captured by radical chain reaction.3a,3b,3c (Figure 1.1.). a) Possible flame retardant mechanism of DMMP. They showed improved cell performance compared to former additives, but this does not mean that they improved the electrochemical performance better than the usual flame retardant free lithium ion battery.
These flame retardant additives were effective in eliminating heat generation, accumulation and explosion, but it was inevitable to increase cell resistance and viscosity.5a,5b,5c. They used LiFSI:triethylphosphate=1:2 solution with 5% (volume) FEC additive in a lithium ion battery, and it showed ~99.8% coulombic efficiency and 91% cyclic stability comparable to cell performance in carbonate electrolytes. They showed that each interlayer consisted of a polymeric form and that cell performance was dramatically improved, even more than that of the cell that used the same amount of FEC additive instead7 (Figure 1.2).
We focus on examining and comparing the nonflammability and electrochemical performance of several multifunctional additives consisting of common electrolyte additives (VC, FEC, etc.) and a phosphate known as a flame retardant. Therefore, the problem of cell performance and safety can be solved at the same time with a small amount of an additive. The most important point in this work is the diversification of additives with the introduction of binders such as oxygen, nitrogen between the usual SEI forming additives and flame retardant.
Performance table of NK series cells according to 5 criteria: charge capacity, discharge capacity, coulombic efficiency and lifetime of each electrolyte system. From the preliminary data of the fundamental test with Choi's group, we assumed that the vinyl functional groups had no positive effects on the cell function. From the data, we summarized that the 5% electrolyte system NKOP-1 was the best in all factors, which meant the strongest targeted flame retardant and the optimal concentration, which in small amounts did not affect the cell's performance.
From this result, we can conclude that O-bonded phosphorus containing vinyl flame retardant gives more PO radical under certain heating conditions. Although two flame retardant additives, unfortunately, could not satisfy the cell performance test, but there was little possibility and advantage as a means of retarding the flame, because the synthesis was very common and cheap. Also, the in-vivo flame retardant additive showed much cheaper and easier ways to handle the lithium battery compared to other gloves, such as an outer protective film.
In other words, the flame retardancy could be solved by a phosphate backbone and at the same time the cell performance could be increased by a nitrogen-bonded vinyl group. To solve the common problems of using flame retardants in liquid electrolyte, we have designed and synthesized new bifunctional flame retardants containing both phosphate and vinyl in one molecule.
Synthesis and application of TFSI resembled salt additives for Li battery
- Abstract
- Introduction
- Results and discussion
- Conclusion
- References
- Supporting information
- NMR Spectra
However, the Li-metal anode has many problems: Li-dendrite formation-SEI breakdown due to the alloy-type charge-discharge process, low coulombic efficiency due to the highly reactive Li-metal anode, and corrosion of the aluminum current collector. The common lithium salt LiPF6 can be thermally decomposed to form pentafluorophosphorane (PF5), and the HF produced by the explosive reaction with water greatly promotes the decomposition of the electrolyte.5 It also creates a needle-like dendrite on the surface of the metal lithium anode, so that a large research for the use of other lithium salts such as LiTFSI and LiFSI. Among them, Chen's group developed a new class of "Solvent-in-Salt" electrolytes with an ultrahigh salt concentration (7 M LiTFSI-DME:DOL = 1:1).
It provided a high lithium-ion transfer number (0.73) and improved the cycling and safety performance of lithium rechargeable batteries by suppressing Li dendrite growth and shape change on the Li metal anode surface. These excellent performances can be attributed to the increased coordination of the solvents and the increased availability of the lithium ion concentration in the electrolyte. In their system, excellent results were obtained even with high current densities (10 mA cm-2) and large cycles (6,000) in a lithium half-cell. Also, the highly concentrated electrolytic system showed higher thermal stability, lower flammability and reduced aluminum corrosion due to the small amount of free solvent.7 However, these two systems require significantly higher costs than electrolytes. overall due to 2 ~ 7 larger amounts of Li salts.
Armand's group fully investigated various asymmetric lithium bis(sulfonyl)imide salt anions for solid polymer electrolytes (SPEs) and recently reported new hydrogen-containing anions that achieved higher Li-ion conductivities than commercialized TFSI anions. Although the overall conductivity of asymmetric lithium salt with PEO electrolyte decreased, there was no report using asymmetric Li salt in non-aqueous Li metal battery, so it is meaningful to expand the choice of different asymmetric lithium bis(sulfonyl)imide salt.11. Zhang's group found that cations which had an effective reduction potential below the standard reduction potential of lithium ion, such as Cs+ and Rb+, could form a positively charged electrostatic shield around the initial growth tip of the protrusions, forcing further deposition of lithium to adjacent areas of anode and decreased dendrite formation.4a,12 Sun's group further applied this theory to solid-state lithium batteries using polymer electrolyte (PEO-Cs+)11.
We tested many different reaction conditions, such as 150 ◦C very harsh conditions with a 48-hour time, but it was not effective due to a fairly strong bond between lithium cation and FSI anion caused by small atomic size of lithium. From the cell test Nam-Soon Choi's group, we revealed preliminary cell data from addition of LiMTFSI in common liquid electrolyte. Compared to the reference condition, addition of LiMTFSI showed slightly decreased cell performance in a point of view of cyclability and coulombic efficiency.
We guessed that hydrogen of LiMTFSI may be weakly bonded to carbon due to electrowithdrawing effect of trifluoromethane functional group. We also tested the additives with newly developed electrolyte system revealed by Prof. Nam-soon Choi's group. There was no dramatic effect in our research, but luckily the possibility existed to include in-vivo TFSI like salt electrolyte system.
In the future, if we change our research in more detail, then we expect the next generation of lithium metal batteries. Cell performance table for AgFSI·2MeCN salt in 5 criteria charge capacity, discharge capacity, coulombic efficiency and lifetime for each electrolyte system.
