Hexacyclic Chelated Lithium Stable Solvates for Highly Reversible Cycling of High-Voltage Lithium Metal Battery

Regulating Amine Substitution in Fluorosulfonyl-Based Flame-Retardant Electrolytes for Energy-Dense Lithium Metal Batteries

Sulfone-based electrolytes offer unusually high anodic and thermal stability that in principle makes them promising candidates for fabricating energy-dense lithium metal batteries (LMBs). Their uses in practical batteries are currently limited by their inability to sustain long-term Li metal plating/stripping processes due to their high reactivity toward the Li metal. Here, we report on the design and synthesis of a unique family of fluorosulfonyl group-based (FSO2-) molecules, modified with ethyl (FSE)/N,N-dimethyl (FSNDM)/N,N-diethyl (FSNDE)/N-pyrrolidine (FSNP) end groups to create exceptionally stable single-salt single-solvent electrolytes. The flammability, solvation structure, ion transport, Li metal deposition kinetics, and high-voltage stability of the electrolytes are systematically studied. It is shown that the electrolytes are nonflammable, possess weak solvation characteristics, yet manifest high room-temperature ionic conductivities (1.6-6.1 mS cm-1) and low solution viscosities. In comparison to FSE, the FSNDM-, FSNDE-, and FSNP-based electrolytes exhibit an exceptionally reversible Coulombic efficiency for Li metal plating/stripping (>99.71% over 800 cycles) and exhibit typical oxidative stability at voltages exceeding 4.6 V. Deployed as electrolytes in Li metal batteries (20 μm Li anode and 3 g A h-1 electrolyte) with high-loading (18.5 mg cm-2) LiNi0.8Co0.1Mn0.1O2 cathodes, 329 cycles have been achieved before 80% capacity retention. Six Ah Li metal pouch cells based on the designed electrolytes also exhibit high stability and high energy density (496 W h kg-1) for over 150 cycles with at most 2.7% volume expansion. Our findings demonstrate that through an intentional molecular design, sulfone electrolytes provide a robust route toward nonflammable Li metal compatible electrolytes with practical high-voltage cathodes.

Topological Design of Highly Conductive Weakly Solvating Electrolytes for Ultrastable Sodium Metal Batteries Operating at -60 °C and Below

Weakly solvating electrolytes (WSE) can favor reversible Na batteries at -40 °C for some extreme applications because of the low desolvation energy. However, it is challenging to enable reversible Na batteries at lower temperatures. Herein, we uncover that the low ionic conductivity of WSE reduces reaction kinetics at -60 °C. Accordingly, a highly conductive weakly solvating electrolyte (HCWSE) is designed by introducing additives of strongly solvating solvents and the dilution of NaPF6. The additive can dominate the solvation sheath, increase the dissociation of NaPF6 and the fluidity of the electrolyte, and thus greatly improve the ionic conductivity. Furthermore, the binding energy between Na+ and solvents is proposed as a descriptor to determine the solvating power of solvents, based on which a series of ultralow-temperature HCWSEs have been topologically designed by facilely introducing strong-solvation ether additives into the weak-solvation solvents. As a demonstration, the HCWSE showcases the long cycling of Na||Na cell at -60 °C with an overpotential of 42 mV under 1 mA cm-2 for 1200 h. The Na||NNFM (Na0.75Ni0.25Fe0.25Mn0.5O2) cell exhibits a reversible capacity of 79.2 mAh g-1 after 160 cycles. The cells also achieve impressive performances at -70 °C.

Designing High Donor Number Anion Additive for Stable Lithium Metal Batteries

The electrolytes in energy-dense lithium metal batteries (LMBs) face the challenge of being compatible with both the lithium anode and the high voltage cathodes. Adjusting the solvation structures of the electrolytes by regulating the interaction between ions and solvents is an effective strategy to improve the stability of LMBs. Herein, lithium trifluoroacetate (LiTFA) endowed with high donor number is introduced into ether-based electrolytes as an additive to regulate the solvation structure and further stabilize the interphase as well as accelerate the interfacial kinetic of LMBs. Due to the strong interaction between TFA- with Li+, the anion-rich solvation structure with reduced solvent coordination capability is constructed, contributing to the formation of inorganic-rich interphase layers and facilitate charge transfer reaction. Consequently, the designed electrolyte improves the reversibility of Li plating/stripping with high Coulombic efficiency of 99.24% and enables long-term cycling of Li||LiNi0.8Co0.1Mn0.1O2 (NCM811) over 100 cycles with a capacity retention of 84.37% under the condition of lean electrolyte, limited Li source and conventional Li-salt concentration. This work provides an effective and low-cost strategy to adjust the solvation structure and improve the stability of LMBs without largely sacrificing the intrinsic physicochemical property (viscosity, wettability, ionic conductivity etc.) of electrolytes.

Unveiling the Role of Fluorination in Hexacyclic Coordinated Ether Electrolytes for High-Voltage Lithium Metal Batteries

Fluorinated ethers have become promising electrolyte solvent candidates for lithium metal batteries (LMBs) because they are endowed with high oxidative stability and high Coulombic efficiencies of lithium metal stripping/plating. Up to now, most reported fluorinated ether electrolytes are -CF3-based, and the influence of ion solvation in modifying degree of fluorination has not been well-elucidated. In this work, we synthesize a hexacyclic coordinated ether (1-methoxy-3-ethoxypropane, EMP) and its fluorinated ether counterparts with -CH2F (F1EMP), -CHF2 (F2EMP), or -CF3 (F3EMP) as terminal group. With lithium bis(fluorosulfonyl)imide as single salt, the solvation structure, Li-ion transport behavior, lithium deposition kinetics, and high-voltage stability of the electrolytes were systematically studied. Theoretical calculations and spectra reveal the gradually reduced solvating power from nonfluorinated EMP to fully fluorinated F3EMP, which leads to decreased ionic conductivity. In contrast, the weakly solvating fluorinated ethers possess higher Li+ transference number and exchange current density. Overall, partially fluorinated -CHF2 is demonstrated as the desired group. Further full cell testing using high-voltage (4.4 V) and high-loading (3.885 mAh cm-2) LiNi0.8Co0.1Mn0.1O2 cathode demonstrates that F2EMP electrolyte enables 80% capacity retention after 168 cycles under limited Li (50 μm) and lean electrolyte (5 mL Ah-1) conditions and 129 cycles under extremely lean electrolyte (1.8 mL Ah-1) and the anode-free conditions. This work deepens the fundamental understanding on the ion transport and interphase dynamics under various degrees of fluorination and provides a feasible approach toward the design of fluorinated ether electrolytes for practical high-voltage LMBs.