Concentrated LiFSI–Ethylene Carbonate Electrolytes and Their Compatibility with High-Capacity and High-Voltage Electrodes

Abstract

The unusual physical and chemical properties of electrolytes with excessive salt contents have resulted in rising interest in highly concentrated electrolytes, especially for their application in batteries. Here, we report strikingly good electrochemical performance in terms of conductivity and stability for a binary electrolyte system, consisting of lithium bis(fluorosulfonyl)imide (LiFSI) salt and ethylene carbonate (EC) solvent. The electrolyte is explored for different cell configurations spanning both high-capacity and high-voltage electrodes, which are well known for incompatibilities with conventional electrolyte systems: Li metal, Si/graphite composites, LiNi 0.33 Mn 0.33 Co 0.33 O 2 (NMC111), and LiNi 0.5 Mn 1.5 O 4 (LNMO). As compared to a LiTFSI counterpart as well as a common LP40 electrolyte, it is seen that the LiFSI:EC electrolyte system is superior in Li-metal–Si/graphite cells. Moreover, in the absence of Li metal, it is possible to use highly concentrated electrolytes (e.g., 1:2 salt:solvent molar ratio), and a considerable improvement on the electrochemical performance of NMC111–Si/graphite cells was achieved with the LiFSI:EC 1:2 electrolyte both at the room temperature and elevated temperature (55 °C). Surface characterization with scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS) showed the presence of thicker surface film formation with the LiFSI-based electrolyte as compared to the reference electrolyte (LP40) for both positive and negative electrodes, indicating better passivation ability of such surface films during extended cycling. Despite displaying good stability with the NMC111 positive electrode, the LiFSI-based electrolyte showed less compatibility with the high-voltage spinel LNMO electrode (∼4.7 V vs Li + /Li).

Unveiling the Mystery of Lithium Bis(fluorosulfonyl)imide as a Single Salt in Low-to-Moderate Concentration Electrolytes of Lithium Metal and Lithium-Ion Batteries

“…Thus, many previous controversies about the corrosion of Al in LiFSI electrolytes can be reasonably explained by the influence of electrolyte solvents on the "threshold potential." This also reasonably explains the success of the HCE strategy [18][19][20][21][22][23] since high salt concentration reduces the molar fraction of free solvents in the solution, which as a result enhances the stability of solvents against oxidation and reduction at extreme potentials.…”

“…10 Owing to the concern with Al corrosion, LiFSI is at present mainly employed as an additive [11][12][13] or a co-salt [14][15][16][17] to improve the performances of Li and Li-ion batteries. In efforts to use LiFSI as a single electrolyte solute in the Li metal and Li-ion batteries, several strategies have shown success in suppressing Al corrosion, for example, high concentration electrolytes (HCEs), [18][19][20][21][22][23] fluorinated solvents 24,25 and particularly using fluorinated co-solvents to form localized high concentration electrolytes (LHCEs), [26][27][28] and ionic liquids (ILs). [29][30][31] From the viewpoint of practical applications, however, all these strategies are not acceptable because on one hand the high concentration and fluorinated solvents increase the cost and on the other hand the fluorinated solvents introduce detrimental impacts on the environment.…”

An electrochemical evaluation of state-of-the-art non-flammable liquid electrolytes for high-voltage lithium-ion batteries

“…The lattice oxygen atoms of NMC in particular are known to participate in the oxidation of solvent molecules, lowering the effective electrochemical stability limit considerably [48]. For instance, even though EC-based electrolytes often show little oxidative current against inert electrodes [49,50], when electrodes containing active materials are used, EC is the component that limits the oxidative stability of traditional electrolytes [51].…”

Electrolyte Additive-Controlled Interfacial Models Enabling Stable Antimony Anodes for Lithium-Ion Batteries

“…have been being widely dedicated to stabilizing the alloying anode (e.g., Sb, , Sn, , Bi, , Si , ), the complex synthetic procedures and also the boundaries’ impedance between nanostructures may raise the cost and safety issues in batteries. Alternatively, the electrolyte design including introducing film-forming additives (e.g., FEC, , VC, , LiDFOB, , LiPO 2 F 2 , ), increasing salt concentrations to form a high-concentration electrolyte, , adding a dilute solvent (e.g., HFE, BTFE, OTE) to form a localized high-concentration electrolyte, and changing the kind of solvent (e.g., THF/2-MeTHF, , DME, − SN) have also been widely developed to stabilize the alloying anode. Among them, introducing film-forming additives in the electrolyte is one of the most economic and effective methods to improve the alloying anode performance.…”