High-Rate Cycling of Lithium-Metal Batteries Enabled by Dual-Salt Electrolyte-Assisted Micropatterned Interfaces

3-(2,2,2-Trifluoroethoxy)propionitrile-based electrolytes for high energy density lithium metal batteries

In this study, 3-(2,2,2-trifluoroethoxy)-propionitrile (FEON), a fluorinated nitrile compound with high oxidative stability, low volatility and non-flammability, is introduced as an electrolyte solvent for high-energy density Li|NCM batteries. After optimization of the electrolyte as (0.8 M LiTFSI + 0.2 M LiODFB)/FEC : FEON (1 : 3, by vol., abbreviated as FF13), the FEON-based electrolyte exhibits better cycling performance for both the lithium metal anode and 4.4 V high-voltage NCM cathode, compared with those of a commercial carbonate electrolyte of 1 M LiPF₆/EC : EMC : DMC (1 : 1 : 1, by vol.). As for the FF13 electrolyte, the maximum coordination number of 3 for FEON molecules in the solvation structure is disclosed through molecular dynamics simulation combined with Fourier transform infrared spectroscopy and nuclear magnetic resonance spectroscopy measurements. Furthermore, the solid electrolyte interphase on the lithium metal anode is enriched with organic components and LiF, which is proposed from FEON decomposition based on density functional theory calculations and X-ray photoelectron spectroscopy analysis. All the above results demonstrate that fluorinated nitrile electrolytes constitute a promising platform for high energy density Li|NCM batteries.

Dendrite Suppression by Lithium-Ion Redistribution and Lithium Wetting of Lithium Zeolite Li2(Al2Si4O12) in Liquid Electrolytes

Lithium metal is considered a next-generation anode material for high-voltage, high-energy-density batteries; however, its commercialization is limited because of dendrite formation during charging, which leads to short-circuiting and fire. Li metal is coated with a lithium zeolite Li2(Al2Si4O12) (bikitaite - BKT) for dendrite suppression. The BKT-coated Li metal anode exhibits enhanced cycle performance for both Li/LMO (over 982 cycles) and Li/Li cells (over 2000 h at 0.52.0 mAh cm-2 and 693 h at 2.0 mAh cm-2). Moreover, the voltage profile of the Li/Li cells deviates from the conventional Li plating behavior. We hypothesize that this is due to the Li wetting of the BKT particles during plating, which leads to the formation of an interconnected three-dimensional (3D) Li network. Furthermore, BKT, a Li conductor, promotes even Li+-ion distribution during plating, resulting in the uniform deposition of Li and, consequently, suppressed dendrite formation. This work provides evidence that BKT can be potentially used in Li metal batteries.

Synergistic Effect of a Dual-Salt Liquid Electrolyte with a LiNO3 Functional Additive toward Stabilizing Thin-Film Li Metal Electrodes for Li Secondary Batteries

Li metal thickness has been considered a key factor in determining the electrochemical performance of Li metal anodes. The use of thin Li metal anodes is a prerequisite for increasing the energy density of Li secondary batteries intended for emerging large-scale electrical applications, such as electric vehicles and energy storage systems. To utilize thin (20 μm thick) Li metal anodes in Li metal secondary batteries, we investigated the synergistic effect of a functional additive (Li nitrate, LiNO₃) and a dual-salt electrolyte (DSE) system composed of Li bis(fluorosulfonyl)imide (LiTFSI) and Li bis(oxalate)borate (LiBOB). By controlling the amount of LiNO₃ in DSE, we found that DSE containing 0.05 M LiNO₃ (DSE-0.05 M LiNO₃) significantly improved the electrochemical performance of Li metal anodes. DSE-0.05 M LiNO₃ increased the cycling performance by 146.3% [under the conditions of a 1C rate (2.0 mA cm^-2), DSE alone maintained 80% of the initial discharge capacity up to the 205th cycle, whereas DSE-0.05 M LiNO₃ maintained 80% up to the 300th cycle] and increased the rate capability by 128.2% compared with DSE alone [the rate capability of DSE-0.05 M LiNO₃ = 50.4 mAh g^-1, and DSE = 39.3 mAh g^-1 under 7C rate conditions (14.0 mA cm^-2)]. After analyzing the Li metal surface using scanning electron microscopy and X-ray photoelectron spectroscopy, we were able to infer that the stabilized solid electrolyte interphase layer formed by the combination of LiNO₃ and the dual salt resulted in a uniform Li deposition during repeated Li plating/stripping processes.

Simultaneous Stabilization of the Solid/Cathode Electrolyte Interface in Lithium Metal Batteries by a New Weakly Solvating Electrolyte

So far, the practical application of Li metal batteries has been hindered by the undesirable formation of Li dendrites and low Coulombic efficiencies (CEs). Herein, 1,2-diethoxyethane (DEE) is proposed as a new electrolytic solvent for lithium metal batteries (LMBs), and the performances of 1.0 m LiFSI in DEE are evaluated. Because of the low dielectric constant and dipole moment of DEE, the majority of the FSI^- exists in associated states like contact ion pairs and aggregates, which is similar to the highly concentrated electrolytes. These associated complexes are involved in the reduction reaction on the Li metal anode, forming sound solid electrolyte interphase layers. Furthermore, free FSI^- ions in DEE are observed to participate in the formation of cathode electrolyte interphase layers. These passivation layers not only suppress dendrite growth on the Li anode but also prevent unwanted side-reactions on the LiFePO₄ cathode. The average CE of the Li||Cu cells in LiFSI-DEE is observed to be 98.0%. Moreover, LiFSI-DEE also plays an important role in enhancing the cycling stability of the Li||LiFP cell with a capacity retention of 93.5% after 200 cycles. These results demonstrate the benefits of LiFSI-DEE, which creates new possibilities for high-energy-density rechargeable LMBs.

Highly Stable Porous Polyimide Sponge as a Separator for Lithium-metal Secondary Batteries

To inhibit Li-dendrite growth on lithium (Li)-metal electrodes, which causes capacity deterioration and safety issues in Li-ion batteries, we prepared a porous polyimide (PI) sponge using a solution-processable high internal-phase emulsion technique with a water-soluble PI precursor solution; the process is not only simple but also environmentally friendly. The prepared PI sponge was processed into porous PI separators and used for Li-metal electrodes. The physical properties (e.g., thermal stability, liquid electrolyte uptake, and ionic conductivity) of the porous PI separators and their effect on the Li-metal anodes (e.g., self-discharge and open-circuit voltage properties after storage, cycle performance, rate capability, and morphological changes) were investigated. Owing to the thermally stable properties of the PI polymer, the porous PI separators demonstrated no dimensional changes up to 180 °C. In comparison with commercialized polyethylene (PE) separators, the porous PI separators exhibited improved wetting ability for liquid electrolytes; thus, the latter improved not only the physical properties (e.g., improved the electrolyte uptake and ionic conductivity) but also the electrochemical properties of Li-metal electrodes (e.g., maintained stable self-discharge capacity and open-circuit voltage features after storage and improved the cycle performance and rate capability) in comparison with PE separators.