Molecular Design of Mono-Fluorinated Ether-Based Electrolyte for All-Climate Lithium-ion Batteries and Lithium-Metal Batteries.

Fluorinated-ethers are promising electrolyte solvents in lithium metal batteries, for their high antioxidant and excellent reductive stability on Li anode. However, fluorinated-ethers with high fluorination degree suffer from low ionic conductivity and narrow temperature adaptibility. Herein, we synthesize a mono-fluorinated linear ether of bis(2-fluoroethoxy) methane (BFME) with enhanced solvated ability. The -OCH2O- structure and fluoride substitution on the ß-C position endows the BFME electrolyte with moderate affinity to Li+, thereby improving the ionic conductivity and decreasing the Li+-desolvation energy barrier at a wide temperature range of -60 ̶ 60 oC. Additionally, the electrolyte with anion-participated solvation structure demonstrates high film-forming ability by forming LiF-rich interfacial film on the electrode surfaces, rendering the graphite anode with an initial Coulombic efficiency (CE) of 94.9% and a Li plating/stripping CE of 99.8% by Aurbach method. Consequently, the Graphite||LiFePO4 pouch cells delivered 83.2%, 92.5% and 81.2% capacity retention after 1250, 200 and 300 cycles at 25, -20 oC and 60 oC, respectively. Moreover, the Li||LFP pouch cell with 3 Ah capacity can operate for 65 cycles with 99% capacity retention, verifying the effectiveness of the BFME electrolyte in stabilizing the interfaces and broadening the temperature adaptibility of lithium-ion and lithium metal batteries.

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

Electrolyte engineering is crucial for developing high-performance lithium metal batteries (LMB). Here, we synthesized two cosolvents methyl bis(fluorosulfonyl)imide (MFSI) and 3,3,4,4-tetrafluorotetrahydrofuran (TFF) with significantly different reduction potentials and add them into LiFSI-DME electrolytes. The LiFSI/TFF-DME electrolyte gave an average Li Coulombic efficiency (CE) of 99.41 % over 200 cycles, while the average Li CEs for MFSI-based electrolyte is only 98.62 %. Additionally, the TFF-based electrolytes exhibited a more reversible performance than the state-of-the-art fluorinated 1,4-dimethoxylbutane electrolyte in both Li||Cu half-cell and anode-free Cu||LiNi0.8 Mn0.1 Co0.1 O2 full cell. More importantly, the decomposition product from bis(fluorosulfonyl)imide anion could react with ether solvent, which destroyed the SEI, thus decreasing cell performance. These key discoveries provide new insights into the rational design of electrolyte solvents and cosolvents for LMB.

A Safe and Sustainable Lithium-Ion-Oxygen Battery based on a Low-Cost Dual-Carbon Electrodes Architecture.

People anticipate high-energy-density battery technology with better security, stability, and sustainability. By tuning the advantage of specific capacity, the lithium-metal anode is replaced with a graphite intercalation compound and a conceptual prototype lithium-ion-oxygen battery based on a low-cost dual-carbon electrodes architecture is proposed. The lithium-ion involves a (de)intercalation process into the graphite anode and an O2 /Li2 O2 redox conversion on the carbon-nanotube cathode. After a thorough examination as to the electrode compatibility with current electrolytes, a nonflammable fluorinated ether electrolyte is proposed to render a highly coordinated solvation sheath and low lithium salt concentration (1 mol). Herein, the compatibility with graphite anode is investigated, which maintains high capacity retention (88.1%) after long-term lifespan (over 1 year). In view of the ultrahigh reversibility (average Coulombic efficiency over 99.93%) of the graphite anode, a lithium-ion-oxygen coin cell with high depth-of-discharge of 80% and 60% deliver a satisfactory life over 150 and 300 cycles, respectively. Moreover, systematic spectroscopy characterizations demonstrate a reversible and efficient 2e- O2 /Li2 O2 redox reaction without relying on noble-metal catalysts. Lastly, in the engineering aspect, a high-energy-density pouch cell (302.52 W h kg-1 based on the weight of the entire pouch) with cost-effective and environmentally friendly carbon-composed cell components is successfully fabricated.

In Situ Interfacial Tuning To Obtain High-Performance Nickel-Rich Cathodes in Lithium Metal Batteries.

Nickel-rich layered oxides are currently considered the most practical candidates for realizing high-energy-density lithium metal batteries (LMBs) because of their relatively high capacities. However, undesired nickel-rich cathode-electrolyte interactions hinder their applicability. Here, we report a satisfactory combination of an antioxidant fluorinated ether solvent and an ionic additive that can form a stable, robust interfacial structure on the nickel-rich cathode in ether-based electrolytes. The fluorinated ether 1,1,2,2-tetrafluoroethyl-1H,1H,5H-octafluoropentyl ether (TFOFE) introduced as a cosolvent into ether-based electrolytes stabilizes the electrolytes against oxidation at the LiNi0.8Mn0.1Co0.1O2 (NCM811) cathode while simultaneously preserving the electrochemical performance of the Li metal anode. Lithium difluoro(bisoxalato)phosphate (LiDFBP) forms a uniform cathode-electrolyte interphase that limits the generation of microcracks inside secondary particles and undesired dissolution of transition metal ions such as nickel, cobalt, and manganese from the cathode into the electrolyte. Using TFOFE and LiDFBP in ether-based electrolytes provides an excellent capacity retention of 94.5% in a Li|NCM811 cell after 100 cycles and enables the delivery of significantly increased capacity at high charge and discharge rates by manipulating the interfaces of both electrodes. This research provides insights into advancing electrolyte technologies to resolve the interfacial instability of nickel-rich cathodes in LMBs.

A Micelle Electrolyte Enabled by Fluorinated Ether Additives for Polysulfide Suppression and Li Metal Stabilization in Li-S Battery.

The Li-S battery is a promising next-generation technology due to its high theoretical energy density (2600 Wh kg-1) and low active material cost. However, poor cycling stability and coulombic efficiency caused by polysulfide dissolution have proven to be major obstacles for a practical Li-S battery implementation. In this work, we develop a novel strategy to suppress polysulfide dissolution using hydrofluoroethers (HFEs) with bi-functional, amphiphlic surfactant-like design: a polar lithiophilic "head" attached to a fluorinated lithiophobic "tail." A unique solvation mechanism is proposed for these solvents whereby dissociated lithium ions are readily coordinated with lithiophilic "head" to induce self-assembly into micelle-like complex structures. Complex formation is verified experimentally by changing the additive structure and concentration using small angle X-ray scattering (SAXS). These HFE-based electrolytes are found to prevent polysulfide dissolution and to have excellent chemical compatibility with lithium metal: Li||Cu stripping/plating tests reveal high coulombic efficiency (>99.5%), modest polarization, and smooth surface morphology of the uniformly deposited lithium. Li-S cells are demonstrated with 1395 mAh g-1 initial capacity and 71.9% retention over 100 cycles at >99.5% efficiency-evidence that the micelle structure of the amphiphilic additives in HFEs can prohibit polysulfide dissolution while enabling facile Li+ transport and anode passivation.

Improving Cell Resistance and Cycle Life with Solvate-Coated Thiophosphate Solid Electrolytes in Lithium Batteries.

Solid electrolytes (SEs) have become a practical option for lithium ion and lithium metal batteries because of their improved safety over commercially available ionic liquids. The most promising of the SEs are the thiophosphates, whose excellent ionic conductivities at room temperature are comparable to those of commercially utilized ionic liquids. Hybrid solid-liquid electrolytes exhibit higher ionic conductivities than their bare SE counterparts because of decreased grain boundary resistance, enhanced interfacial contact with electrodes, and decreased degradation at the interface. In this study, we add lithium bis(trifluoromethane sulfonyl)imide and a highly fluorinated ether solvate electrolyte to the surface of Li7P3S11 (LPS) and Li10GeP2S12 (LGPS) pellets and evaluate their overall cell resistance in Li-Li symmetric cells relative to their bare Li/SE/Li counterparts. Time-resolved electrochemical impedance spectroscopy shows an order of magnitude lower cell resistance for LGPS-solvate than for bare LGPS. In contrast, the LPS-solvate system exhibits a higher cell resistance than bare LPS. Scanning electron microscopy and energy dispersive X-ray spectroscopy show that LGPS allows for the total permeation of the solvate into the bulk SE. Although LPS has smaller grain sizes and higher porosity, it has a higher solubility in 1,1,2,2-tetrafluoroethyl 2,2,3,3-tetrafluoropropyl ether (TTE), which results in an LPS-TTE interlayer on the surface of the pellet, thereby increasing overall cell resistance. Cyclic voltammetry of the bare and hybrid SE cells shows an order of magnitude higher current density for the LGPS-solvate cell over the bare LGPS. Bare LPS shorts after two cycles, whereas the LPS-solvate cell does not short within the timeframe of the experiment (100 cycles). This study suggests that solvates can be used to improve the cell resistance and current density of SEs by altering the grain boundary structures and the interphase between electrode and electrolyte.

Synergistic Effect of Partially Fluorinated Ether and Fluoroethylene Carbonate for High-Voltage Lithium-Ion Batteries with Rapid Chargeability and Dischargeability.

The roles of a partially fluorinated ether (PFE) based on a mixture of 1,1,1,2,2,3,3,4,4-nonafluoro-4-methoxybutane and 2-(difluoro(methoxy)methyl)-1,1,1,2,3,3,3-heptafluoropropane on the oxidative durability of an electrolyte under high-voltage conditions, the rate capability of the graphite and 5 V-class LiNi0.4Mn1.6O4 (LNMO) electrodes, and the cycling performance of graphite/LNMO full cells are examined. Our findings indicate that the use of PFE as a cosolvent in the electrolyte yields thermally stable electrolytes with self-extinguishing ability. Electrochemical tests confirm that the PFE combined with fluoroethylene carbonate (FEC) effectively alleviates the oxidative decomposition of the electrolyte at the high-voltage LNMO cathode and enables reversible electrochemical reactions of the graphite anodes and LNMO cathodes at high rates. Moreover, the combination of PFE, which mitigates electrolyte decomposition at high voltages, and FEC, which stabilizes the anode-electrolyte interface, enables the reversible cycling of high-voltage full cells (graphite/LNMO) with a capacity retention of 70.3% and a high Coulombic efficiency of 99.7% after 100 cycles at 1C rate at 30 °C.