High-Performance Lithium Metal Batteries Enabled by a Fluorinated Cyclic Ether with a Low Reduction Potential

Tuning the End Alkyl Chain of the Ether Solvent to Stabilize the Electrode/Electrolyte Interfaces in the NCM-Li Battery

Lithium metal batteries (LMBs) combined with a high-voltage nickel-rich cathode show great potential in meeting the growing need for high energy density. The lack of advanced electrolytes has been a major obstacle in the commercialization of high-voltage lithium metal batteries (LMBs), as these electrolytes need to effectively support both a stable lithium metal anode (LMA) and a high-voltage cathode (>4 V vs Li+/Li). In this work, by extending the two terminal methyl groups in DIGDME and TEGDME to n-butyl groups, we design a new weakly solvating electrolyte (2 M LIFSI+TEGDBE) that enables the stable cycling of NMC83 (LiNi0.83Co0.12Mn0.05O2) cathodes. The NMC83 cell exhibits a high and stable Coulombic efficiency (CE) of over 99%, as well as capacity retention of approximately 99.8% after 100 cycles at 0.3 C. X-ray photoelectron spectroscopy analysis (XPS) and high-resolution transmission electron microscope (HRTEM) images revealed that the anion species decomposed first, resulting in the formation of a cathode-electrolyte interface (CEI) film predominantly consisting of decomposition products from the anions on the positive electrode surface. This work links the functional group of solvents with the solvation structure and electrochemical performance of ether-based electrolytes, providing a distinctive sight to design advanced electrolytes for high-energy-density LMBs.

Reduction-Tolerance Electrolyte Design for High-Energy Lithium Batteries

Lithium batteries employing Li or silicon (Si) anodes hold promise for the next-generation energy storage systems. However, their cycling behavior encounters rapid capacity degradation due to the vulnerability of solid electrolyte interphases (SEIs). Though anion-derived SEIs mitigate this degradation, the unavoidable reduction of solvents introduces heterogeneity to SEIs, leading to fractures during cycling. Here, we elucidate how the reductive stability of solvents, dominated by the electrophilicity (EPT) and coordination ability (CDA), delineates the SEI formed on Li or Si anodes. Solvents exhibiting lower EPT and CDA demonstrate enhanced tolerance to reduction, resulting in inorganic-rich SEIs with homogeneity. Guided by these criteria, we synthesized three promising solvents tailored for Li or Si anodes. The decomposition of these solvents is dictated by their EPTs under similar solvation structures, imparting distinct characteristics to SEIs and impacting battery performance. The optimized electrolyte, 1 M lithium bis(fluorosulfonyl)imide (LiFSI) in N-Pyrrolidine-trifluoromethanesulfonamide (TFSPY), achieves 600 cycles of Si anodes with a capacity retention of 81 % (1910 mAh g-1). In anode-free Cu||LiNi0.5Co0.2Mn0.3O2 (NCM523) pouch cells, this electrolyte sustains over 100 cycles with an 82 % capacity retention. These findings illustrate that reducing solvent decomposition benefits SEI formation, offering valuable insights for the designing electrolytes in high-energy lithium batteries.

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.

Regulating Li Nucleation and Growth Heterogeneities via Near-Surface Lithium-Ion Irrigation for Stable Anode-Less Lithium Metal Batteries

The inhomogeneous nucleation and growth of Li dendrite combined with the spontaneous side reactions with the electrolytes dramatically challenge the stability and safety of Li metal anode (LMA). Despite tremendous endeavors, current success relies on the use of significant excess of Li to compensate the loss of active Li during cycling. Herein, a near-surface Li+ irrigation strategy is developed to regulate the inhomogeneous Li deposition behavior and suppress the consequent side reactions under limited Li excess condition. The conformal polypyrrole (PPy) coating layer on Cu surface via oxidative chemical vapor deposition technique can induce the migration of Li+ to the interregional space between PPy and Cu, creating a near-surface Li+-rich region to smooth diffusion of ion flux and uniform the deposition. Moreover, as evidenced by multiscale characterizations including synchrotron high-energy X-ray diffraction scanning, a robust N-rich solid-electrolyte interface (SEI) is formed on the PPy skeleton to effectively suppress the undesired SEI formation/dissolution process. Strikingly, stable Li metal cycling performance under a high areal capacity of 10 mAh cm-2 at 2.0 mA cm-2 with merely 0.5 × Li excess is achieved. The findings not only resolve the long-standing poor LMA stability/safety issues, but also deepen the mechanism understanding of Li deposition process.

Reinforcing the Electrode/Electrolyte Interphases of Lithium Metal Batteries Employing Locally Concentrated Ionic Liquid Electrolytes

Lithium metal batteries (LMBs) with nickel-rich cathodes are promising candidates for next-generation high-energy-density batteries, but the lack of sufficiently protective electrode/electrolyte interphases (EEIs) limits their cyclability. Herein, trifluoromethoxybenzene is proposed as a cosolvent for locally concentrated ionic liquid electrolytes (LCILEs) to reinforce the EEIs. With a comparative study of a neat ionic liquid electrolyte (ILE) and three LCILEs employing fluorobenzene, trifluoromethylbenzene, or trifluoromethoxybenzene as cosolvents, it is revealed that the fluorinated groups tethered to the benzene ring of the cosolvents not only affect the electrolytes' ionic conductivity and fluidity, but also the EEIs' composition via adjusting the contribution of the 1-ethyl-3-methylimidazolium cation (Emim+ ) and bis(fluorosulfonyl)imide anion. Trifluoromethoxybenzene, as the optimal cosolvent, leads to a stable cycling of LMBs employing 5 mAh cm-2 lithium metal anodes (LMAs), 21 mg cm-2 LiNi0.8 Co0.15 Al0.05 (NCA) cathodes, and 4.2 µL mAh-1 electrolytes for 150 cycles with a remarkable capacity retention of 71%, thanks to a solid electrolyte interphase rich in inorganic species on LMAs and, particularly, a uniform cathode/electrolyte interphase rich in Emim+ -derived species on NCA cathodes. By contrast, the capacity retention under the same condition is only 16%, 46%, and 18% for the neat ILE and the LCILEs based on fluorobenzene and benzotrifluoride, respectively.

Spatially Confined Silver Nanoparticles in Mercapto Metal-Organic Frameworks to Compartmentalize Li Deposition Toward Anode-Free Lithium Metal Batteries

As the promising next-generation energy storage solution, lithium metal battery (LMB) has gained great attention but still suffers from troubles associated with the highly active metallic lithium. Herein, it is aimed to develop an anode-free LMB engaging no Li disk or foil by modifying the Cu current collector with mercapto metal-organic frameworks (MOFs) impregnating Ag nanoparticles (NPs). While the polar mercapto groups facilitate and guide Li+ transport, the highly lithiophilic Ag NPs help to enhance the electric conductivity and lower the energy barrier of Li nucleation. Furthermore, the MOF pores allow compartmentalizing bulk Li into a 3D matrix Li storage so that not only the local current density is reduced, but also is the plating/stripping reversibility greatly enhanced. As a result, full cells pairing the prelithiated Ag@Zr-DMBD/Cu anodes with LiFePO4 cathodes demonstrate a high initial specific capacity of 159.8 mAh g-1 , first-cycle Coulombic efficiency of 96.6%, and long-term cycling stability over 1000 cycles with 99.3% capacity retention at 1 C. This study underlines the multi-aspect functionalization of MOFs to impart lithiophilicity, polarity, and porosity to achieve reversible Li plating/stripping and paves the way for realizing high-performance anode-free LMBs through exquisite modification of the Cu current collector.

Tuning the Li+ Solvation Structure by a "Bulky Coordinating" Strategy Enables Nonflammable Electrolyte for Ultrahigh Voltage Lithium Metal Batteries

In battery electrolyte design principles, tuning Li⁺ solvation structure is an effective way to connect electrolyte chemistry with interfacial chemistry. Although recent proposed solvation tuning strategies are able to improve battery cyclability, a comprehensive strategy for electrolyte design remains imperative. Here, we report a solvation tuning strategy by utilizing molecular steric effect to create a "bulky coordinating" structure. Based on this strategy, the designed electrolyte generates an inorganic-rich solid electrolyte interphase (SEI) and cathode-electrolyte interphase (CEI), leading to excellent compatibility with both Li metal anodes and high-voltage cathodes. Under an ultrahigh voltage of 4.6 V, Li/NMC811 full-cells (N/P = 2.0) hold an 84.1% capacity retention over 150 cycles and industrial Li/NMC811 pouch cells realize an energy density of 495 Wh kg^-1. This study provides innovative insights into Li⁺ solvation tuning for electrolyte engineering and offers a promising path toward developing high-energy Li metal batteries.