Difluorobenzene‐Based Locally Concentrated Ionic Liquid Electrolyte Enabling Stable Cycling of Lithium Metal Batteries with Nickel‐Rich Cathode

Abstract

the high reactivity of both the lithium metal anode (LMA) and Ni-rich NMC usually leads to poor interfacial compatibility with the conventional electrolytes and consequently limited cyclability. [4][5][6][7] Without a sufficiently protective solidelectrolyte interphase (SEI) on LMAs, side reactions between lithium and electrolytes cause lithium dendritic growth and low stripping/plating coulombic efficiency (CE). [8][9][10] At the cathode/electrolyte interface, parasitic electrolyte degradation occurs due to the highly reactive Ni 4+ species generated upon delithiation. This is accelerated by increasing Ni content in the cathode, resulting in limited reversibility of the Ni-rich NMC cathode and thickening of the cathode/electrolyte interphase (CEI) upon cycling. [11,12] Among the strategies proposed to enhance the cyclability of LMBs, electrolyte engineering appears to be one of the most effective and feasible approaches, as the electrolyte plays a keyrole in the CEI and SEI formation. [13][14][15][16][17][18][19] Ionic liquid electrolytes (ILEs), with high electrochemical stability, are valuable options for Li/Ni-rich NMC cells in this context. [20][21][22][23] For instance, Wu et al. [24] have recently reported highly stable cycling of Li/LiNi 0.88 Co 0.09 Mn 0.03 O 2 cells up to 300 cycles with a capacity retention of 88% in [LiTFSI] 0.2 [Pyr 14 FSI] 0.8 (LiTFSI = lithium bis(trifluoromethanesulfonyl)imide, Pyr 14 FSI = N-butyl-Nmethyl pyrrolidinium bis(fluorosulfonyl)imide). Unfortunately, the excellent performance has been only achieved with low cathode mass loading (<5 mg cm −2 ), or/and low current density Lithium metal batteries (LMBs) with nickel-rich cathodes are promising candidates for next-generation, high-energy batteries. However, the highly reactive electrodes usually exhibit poor interfacial compatibility with conventional electrolytes, leading to limited cyclability. Herein, a locally concentrated ionic liquid electrolyte (LCILE) consisting of lithium bis(fluorosulfonyl)imide (LiFSI), 1-ethyl-3-methylimidazolium bis(fluorosulfonyl)imide (EmimFSI), and 1,2-difluorobenzene (dFBn) is designed to overcome this challenge. As a cosolvent, dFBn not only promotes the Li + transport with respect to the electrolyte based on the ionic liquid only, but also has beneficial effects on the electrode/electrolyte interphases (EEIs) on lithium metal anodes (LMAs) and LiNi 0.8 Mn 0.1 Co 0.1 O 2 (NMC811) cathodes. As a result, the developed LCILE enables dendritefree cycling of LMAs with a coulombic efficiency (CE) up to 99.57% at 0.5 mA cm −2 and highly stable cycling of Li/NMC811 cells (4.4 V) at C/3 charge and 1 C discharge (1 C = 2 mA cm −2 ) for 500 cycles with a capacity retention of 93%. In contrast, the dFBn-free electrolyte achieves lithium stripping/plating CE, and the Li/NMC811 cells' capacity retention of only 98.22% and 16%, respectively under the same conditions. The insight into the coordination structure, promoted Li + transport, and EEI characteristics gives fundamental information essential for fu...

Insights into the Effect of Lithium Doping on the Deep Eutectic Solvent Choline Chloride:Urea

“… values between 0.44 and 0.61 at 30 °C were also reported by Watanabe and coworkers in solvated IL systems consisting of equimolar mixtures of a lithium salt and a glyme [ 12 , 26 ]. However, values found here for ChCl:U-LiCl are in line or even higher than transference numbers found for traditional mixtures of ILs and lithium salts (i.e., in the range 0.02–0.15 are common values for alkyl-imidazolium and alkyl-pyrrolidinium ILs doped with lithium salts [ 27 , 28 , 29 ]). Therefore, even bearing in mind that apparent transference numbers are only an approximate concept that does not take into account correlated motion of ions, the values found here for the prototypical ChCl:U-LiCl system are probably too low for battery applications at present.…”

Locally Concentrated Ionic Liquid Electrolyte with Partially Solvating Diluent for Lithium/Sulfurized Polyacrylonitrile Batteries

“…The self‐diffusion coefficient of Li + in FEmF is 3.5 × 10 −11 m 2 s −1 , which is 2.5 times of that in FE. [ 31 ] The apparent Li + transference number of FEmF was calculated to be 0.118, which is slightly lower than that of FE (0.135). Considering the ionic conductivities and the Li + transference numbers, one can infer that Li + transport in FEmF is faster than in FE, which is identical to the higher Li + self‐diffusion coefficient in FEmF.…”

“…Even more important, the remarkable CE of 99.72% is the highest value ever recorded for LCILEs. [ 12,23–25,31 ] Therefore, the presence of partially solvating mFBn does not cause negative effects on the compatibility of the electrolyte toward LMAs.…”

“…To date, a high lithium stripping/plating CE of 99.56% has been recorded with rationally designed LCILEs. [ 31 ] With the remarkable compatibility toward LMAs, LCILEs might be promising candidates for Li/SPAN cells. Nonetheless, the electrochemical behavior of SPAN cathodes in ILEs or LCILEs has not been investigated yet.…”

Tailoring Electrolyte Solvation for LiF-Rich Solid Electrolyte Interphase toward a Stable Li Anode

“…Among all reported dilutants, the fluorinated aromatic compounds have gained our interest due to their nonsolvating property and high fluorine-donating capacity, which benefits deriving an SEI layer with rich LiF. − To single out a suitable crowding dilutant, the LUMO energy levels of a series of fluorinated aromatic compoundsbenzene (Ben), methoxybenzene (mBen), fluorobenzene (fBen), 1,2-difluorobenzene (1,2-dfBen), and (trifluoromethoxy)­benzene (tfmBen)were calculated as they represent the stability of solvents . As shown in Figure a, 1,2-dfBen shows the lowest LUMO energy level of −0.94 eV, suggesting that it will decompose preferentially on the Li anode and facilitate the formation of a LiF-rich SEI.…”