Chapter II. Synthesis and application of TFSI resembled salt additives for Li battery
2.2. Introduction
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materials show the highest theoretical energy density (~650 Wh kg-1 and ~950 Wh kg-1) and lowest cost among all of batteries.3
However, Li metal anode has many problems: Li dendrite formation-SEI decomposition because of alloy type charge-discharge process, low coulombic efficiency due to highly reactive lithium metal anode, and aluminum current collector corrosion. To solve these problems, many researchers have struggled to apply various chemical and physical methods that include electrolyte design, artificial SEI formation, self-healing electrostatic shield, and so on.4a,4b
Novel electrolyte design: highly concentrated electrolyte system
Common lithium salt LiPF6 can be thermally decomposed to produce pentafluorophosphorane (PF5) and HF caused by explosive reaction with water promotes the decomposition of electrolyte terribly.5 Also, it generates needle-like shaped dendrite at the surface of lithium metal anode, so there were lots of researches to use other lithium salts such as LiTFSI and LiFSI. Among them, Chen's group developed a new class of 'Solvent-in-Salt’ electrolyte with ultrahigh salt concentration (7 M LiTFSI-DME:DOL = 1:1). It gave high lithium-ion transference number (0.73) and enhanced cyclic and safety performance of rechargeable Li batteries by suppression of Li dendrite growth and shape change at the surface of Li metal anode. While less concentrate electrolyte exhibited an apparent coulombic efficiency above 100%
meaning a sign of 'polysulphide shuttle effect’, ultrahigh concentrated system revealed 74% after 100 cycles.6 Zhang’s group presented highly concentrated electrolyte system (4 M LiFSI-DME) gave a high- rate cycling of a Li metal anode up to 99.1% coulombic efficiency without dendrite formation. These excellent performances can be attributed to the increased solvent coordination and increased availability of lithium ion concentration in the electrolyte. In their system, great result was delivered even at high current density (10 mA cm-2) and large cycles (6,000) in a lithium half-cell.Also, highly concentrated electrolyte system showed higher thermal stability, lower flammability, and decreased aluminum corrosion due to little amount of free solvent.7 However, these two systems need really higher costs than general electrolyte because of 2 ~ 7 larger amounts of Li salts. Also, LiTFSI and LiFSI are 2 ~ 161 times more expensive than LiPF6 in common chemical company TCI, so above researches cannot satisfy a necessary condition related to commercial market. (Figure 2.1.)
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Figure 2.1. Each price per lithium salt. Expensiveness in this order: LiFSI > LiTFSI > LiPF6
Artificial SEI formation: surface coating before cycling
During first cycle of Li-ion battery, inorganic and organic SEI, of which the LiF rich SEI can successfully protect the electrode surface while lithium ion can penetrate into the layer were formed by interaction between electrode and electrolyte, but because lithium metal itself is too reactive and unstable SEI can be generated, it is insufficient to prevent the surface with only spontaneously generated SEI.8a,8b,8c,8d,8e Therefore, several researches have focused on artificial SEI formation before first cycle.
Archer's group utilized strong Lewis acid Aluminum iodide to initiate polymerization of dioxolane at the Li metal surface and to localize halide salts in the formed polymeric SEI thin film.9 Yang's group developed an artificial SEI mainly composed of amorphous titanium oxide and lithium n-butoxide. The generated SEI exhibited not only efficient mechanical strength but also facilitated Li-ion transport ability.10 These all researches were a lot meaningful in that they solve representative problems of Li metal battery such as Li metal dendrite formation and low coulombic efficiency. However, it is still insufficient to commercialize Li metal battery into electric transportation industry like EV vehicle and aircraft because of their high costs, safety concerns, and increased weight of battery.
Research background I: TFSI resembled asymmetric Li salt
Armand’s group fully examined various asymmetric anions of lithium bis(sulfonyl)imide salt into solid polymer electrolytes (SPEs), and recently reported novel hydrogen-containing anions achieved higher Li ion conductivities than commercialized TFSI anions. Also, they successfully tested the novel SPEs derived from asymmetric Li-salts in several battery systems such as Li-LiFePO4, Li-S and Li-O2
batteries. Though the total conductivity of lithium asymmetric salt with PEO electrolyte decreased, there was no report that used asymmetric Li-salt in nonaqueous Li-metal battery, so it is meaningful to extend choice of various asymmetric lithium bis(sulfonyl)imide salt.11
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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