Chapter II. Synthesis and application of TFSI resembled salt additives for Li battery
2.3. Results and discussion
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Research background II: Self-healing electrostatic shield
One theory of Li dendrite formation is the higher electric field at tips of the metal tending to attract more Li-ions. Therefore, some group hypothesized that if the tip was replaced by other host metal, lithium accumulation might occur at near the tip, so it can alter dendrite formation. Zhang’s group found out cations which had 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 protuberances, which forced further deposition of lithium to adjacent regions of the anode and mitigated the dendrite formation.4a,12 Sun’s group further applied this theory to solid-state lithium batteries using polymer electrolyte (PEO-Cs+)11
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Scheme 2.1. Trials and errors: 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 h; (b) two step reaction:
methanesulfonamide (1.0 mmol), KOtBu (1equiv), MeOH (1 M) at rt, 1 h; trifluoromethanesulfonyl chloride (1 equiv), MeCN (1 M) under reflux, 48 h; LiOH·H2O (1 equiv), H2O (1 M). (c) (b) two step reaction: trifluoromethanesulfonamide (1.0 mmol), LiOtBu (1equiv), MeOH (1 M) at rt, 1 h;
methanesulfonyl chloride (1 equiv), MeCN (1 M) under reflux, 48 h; LiOH·H2O (1 equiv), H2O (1 M).
Research topic II: Exploration of synthetic route to cation exchanged FSI salt
Since the metal-metal exchange was very common in organic and inorganic synthesis, we just assumed that lithium and silver exchange would be possible. Therefore, we planned to synthesize AgFSI derivatives from LiFSI (lithium bis(fluorosulfonyl)imide) and diverse Ag salt (AgNO3, Ag2CO3, AgF, etc.) in different reaction temperature. We tested a lot of different reaction conditions such as 150 ◦C very harsh condition with 48 h long time, but it was not effective because of quite strong bond between lithium cation and FSI anion caused by small atomic size of lithium. To solve this problem, we tested other FSI salt which is KFSI (potassium bis(fluorosulfonyl)imide), and fortunately the reaction showed great yield and selectivity in mild condition with very short reaction time. We could not explain why this phenomenon happened, but as a synthesis of electrolyte additive which could be applied to a self- healing electrostatic shield agent, this reaction could be utilized effectively. (Scheme 2.2.)
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Scheme 2.2. Trials and errors: Synthesis of AgFSI (Bis(fluoro)sulfonamide silver(I) salt) (a) Use of LiFSI instead of KFSI: LiFSI (1.0 mmol), AgNO3 (1 equiv), MeCN (1 M) at rt, 30 min; (b) Use of other silver(I) sources such as Ag2CO3 and AgF: KFSI (1.0 mmol), Ag(I) source (1 equiv), MeCN (1 M) at rt, 30 min.
Synthesis of lithium (methanesulfonyl)(trifluoromethanesulfonyl)imide [LiMTFSI] salt
To a solution of trifluoromethansulfonamide (1 mmol) in MeOH (1 mL), KOtBu (1 equiv) was added.
The reaction was kept stirring for 1 h at rt. Then, MeOH was fully evaporated, and remaining solid was diluted with MeCN (1 mL). Methanesulfonyl chloride (1 equiv) was added dropwisely to the solution at 0 °C, and the reaction was kept stirring for 48 h at 80 °C. (Figure 2.2.a) After cooling to rt, the precipitates of reaction mixture were filtered, and the filtrate was evaporated. (Figure 2.2.b) Oily phased mixture was diluted with water (1 mL), and LiOH·H2O (1 equiv) was added. After removal of water by vacuum evaporator, excess amount of MeCN was added and filtered one more time. The desired product was obtained by evaporation of the filtrate. (white powder, 130 mg, 56%) (Figure 2.2.c);
1H NMR (400 MHz, acetonitrile-d3): 2.96 (s); 13C NMR (100 MHz, acetonitrile-d3): 121.4 (q), 43.7 (s);
19F NMR (377 MHz, acetonitrile-d3): -79.39; 7Li NMR (156 MHz, acetonitrile-d3): -2.21. (Scheme 2.3.a)
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Figure 2.2. Pictures of each stage of synthesis of LiMTFSI salt additive (a) After reaction of potassium trifluoromethanesulfonamide salt with methanesulfonyl chloride in MeCN for 48 h at 80 °C: White precipitates formation. (b) After filtration of reaction (a): Oily phase. (c) After completion of reaction procedure: white powder was obtained (130 mg, 56%)
Synthesis of silver bis(fluoromethanesulfonyl)imide [AgFSI·2MeCN] salt
To a solution of KFSI (potassium bis(fluorosulfonyl)imide) (2 mmol, 438 mg), AgNO3 (1 equiv, 2 mmol, 340 mg) was added. The reaction was kept stirred for 30 min at room temperature. Then, the white solid precipitate (KNO3) was filtered, and combined filtrate was evaporated under vacuo. Excess amount of dichloromethane was poured for extra washing and filtration, and the remaining white solid was filtered again. Evaporation of combined filtrate gained the yellowish oily product (740 mg, 100%).
We could characterize AgFSI·2MeCN salt simply by comparing nmr spectra with KFSI. From 1H nmr spectrum, we confirmed that acetonitrile existed after drying over vacuo 24 h. Also, we could check the reaction was over by 19F nmr spectrum. Compared to the spectrum of KFSI, the peak was slightly changed (-0.1). We utilized trifluorotoluene as an internal standard, so we could double-check the product; 1H NMR (400 MHz, methanol-d3): 2.17 (s); 19F NMR (377 MHz, methanol-d3): 50.78.
(Scheme 2.3.b)
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Scheme 2.3. Synthesis of LiMTFSI and AgFSI·2MeCN. (a) Two-step synthesis of LiMTFSI:
trifluoromethanesulfonamide (1.0 mmol), KOtBu (1 equiv), MeOH (1 M) at rt, 1 h; methanesulfonyl chloride (1.0 equiv), MeCN (1 M) under reflux condition, 48 h; Addition of LiOH·H2O (1 equiv), water (1 M). (b) Synthesis of AgFSI·2MeCN: KFSI (Bis(fluoro)sulfonamide potassium salt) (1.0 mmol), AgNO3 (1 equiv), MeCN (1 M) at rt, 30 min.
Cell performance
From cell test Nam-Soon Choi’s group, we revealed preliminary cell data of addition of LiMTFSI in common liquid electrolyte. We utilized 2.5 M LiFSI DME + 1% LiDFBP + 3% LiNO3 as a reference electrolyte condition to double-check effects of the additive as a lithium salt. 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 might be weakly bonded to carbon due to electro-withdrawing effect of trifluoromethane functional group. In this reason, hydrogen was detached during charge and discharge, and it caused electrode or electrolyte oxidation by generating the toxic HF. From lots of researches, we all understood HF corrosion at the surface of electrode, so we needed to substitute methanesulfonyl chloride to other sulfonyl chloride without hydrogen such as nonafluorobutanesulfonyl chloride. (Figure 2.3. & Table 2.1.)
To evaluate AgFSI·2MeCN salt as an electrolyte additive, we also chose 2 M LiFSI DME + 1%
LiDFBP + 3% LiNO3 as a partner condition due to its handness. We added few amount (0.05%) of various Ag salt into the electrolyte, and fortunately result was not bad. Though there was extreme
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increase in capacity, coulombic efficiency and lifetime, but data showed a hopeful result. Using various silver salt additives in a very small portion increased the capacity in amount of around 4 mAh g-1. At the same time, the lifetime was increased in amount of about 30 cycle. From this result, we could confirm that silver might be effective to stabilize the surface of electrode and electrolyte. For the detailed evaluation, we needed to test the additive in more various conditions. (Figure 2.4. &
Table 2.2.)
To test further, we progressed to check overpotential factor. From the data, electro-deposition overpotential was slightly decreased meaning that silver salt could generate better cyclability. Because there was no data about cell mechanism of silver salt, we guessed that silver(I) might act as a self- healing electrostatic shield agent like cesium cation. Also, FSI anion was the most effective because of compatibility with LiFSI. (Figure 2.4.)