Competitive Solvation Enhanced Stability of Lithium Metal Anode in Dual-Salt Electrolyte.

The development of lithium metal batteries is hindered by the low Coulombic efficiency and poor cycling stability of the metallic lithium. The introduction of consumptive LiNO3 as an additive can improve the cycling stability, but its low solubility in the carbonate electrolytes makes this strategy impractical for long-term cycling. Herein we propose LiNO3 as a cosalt in the LiPF6-LiNO3 dual-salt electrolyte to enhance the cycling stability of lithium plating/stripping. Competitions among the components and the resultant substitution of NO3- for PF6- in the solvation shell facilitate the formation of a Li3N-rich solid electrolyte interphase (SEI) film and suppress the LiPF6 decomposition. The highly Li+ conductive and stable SEI film effectively tailors the lithium nucleation, suppresses the formation of lithium dendrites, and improves the cycling performance. The competitive solvation has profound importance for the design of a complex electrolyte to meet the multiple requirements of secondary lithium batteries.

Additive-Assisted Novel Dual-Salt Electrolyte Addresses Wide Temperature Operation of Lithium-Metal Batteries.

In this study, self-synthesized lithium trifluoro(perfluoro-tert-butyloxyl)borate (LiTFPFB) is combined with lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) to formulate a novel 1 m dual-salt electrolyte, which contains lithium difluorophosphate (LiPO2 F2 ) additive and dominant carbonate solvents with low melting point and high boiling point. The addition of LiPO2 F2 into this novel dual-salt electrolyte dramatically improves cycleability and rate capability of a LiNi0.5 Mn0.3 Co0.2 O2 /Li (NMC/Li) battery, ranging from -40 to 90 °C. The NMC/Li batteries adopt a Li-metal anode with low thickness of 100 µm (even 50 µm) and a moderately high cathode mass loading level of 10 mg cm-2 . For the first time, this paper provides valuable perspectives for developing practical lithium-metal batteries over a wide temperature range.

Dual-Salt Mixed Electrolyte for High Performance Aqueous Aluminum Batteries.

A dual-salt electrolyte with 5 M Al(OTF)3 and 0.5 M LiOTF is proposed for aqueous aluminum batteries, which can effectively prevent the corrosion caused by the hydrogen evolution reaction. With the addition of LiOTF in the electrolyte, the solvation phenomenon has changed with the coordination mode of Al3+ conversion from an all octahedral structure to a mixed octahedral and tetrahedral structure. This change can reduce the hydrogen bond between water molecules, which will minimize the occurrence of hydrogen evolution reactions. Moreover, the new electrolyte improves the cycle life of the battery. With MnO as the cathode, 2.1 V high charging platform and 1.5 V high discharge platform can be obtained. The electrochemical stability window (ESW) has been improved to 3.8 V. The first cycle capacity is up to 437 mAh g-1, which can be maintained at 103 mAh g-1 after 100 cycles. This work provides solutions for the future development of electrolyte for aqueous aluminum batteries.

Synergistic Dual-Salt Electrolyte for Safe and High-Voltage LiNi0.8Co0.1Mn0.1O2//Graphite Pouch Cells.

Concerns about thermal safety and unresolved high-voltage stability have impeded the commercialization of high-energy lithium-ion batteries bearing LiNi0.8Co0.1Mn0.1O2 (NCM811) cathodes. Enhancing the cathode structure and optimizing the electrolyte formula have demonstrated significant potential in improving the high-voltage properties of batteries while simultaneously minimizing thermal hazards. The current study reports the development of a high-voltage lithium-ion battery that is both safe and reliable, using single-crystal NCM811 and a dual-salt electrolyte (DSE). After 200 cycles at high voltage (up to 4.5 V), the capacity retention of the battery with DSE was 98.80%, while that for the battery with a traditional electrolyte was merely 86.14%. Additionally, in comparison to the traditional electrolyte, the DSE could raise the tipping temperature of a battery's thermal runaway (TR) by 31.1 °C and lower the maximum failure temperature by 76.1 °C. Moreover, the DSE could effectively reduce the battery's TR heat release rate (by 23.08%) as well as eliminate concerns relating to fire hazards (no fire during TR). Based on material characterization, the LiDFOB and LiBF4 salts were found to facilitate the in situ formation of an F- and B-rich cathode-electrolyte interphase, which aids in inhibiting oxygen and interfacial side reactions, thereby reducing the intensity of redox reactions within the battery. Therefore, the findings indicate that DSE is promising as a safe and high-voltage lithium-ion battery material.

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.

LiFSI and LiDFBOP Dual-Salt Electrolyte Reinforces the Solid Electrolyte Interphase on a Lithium Metal Anode.

Metallic lithium (Li) has great potential as an anode material for high-energy-density batteries due to its high specific capacity. However, the uncontrollable dendritic lithium growth on the metallic lithium surface limits its practical application owing to the instability of the solid electrolyte interphase (SEI). A tailored SEI composition/structure can mitigate or inhibit the lithium dendrites' growth, thereby enhancing the cyclability of the Li-metal anode. In this work, excellent cycling stability of lithium metal anodes was achieved by utilizing a novel dual-salt electrolyte based on lithium bis(fluorosulfonyl) imide (LiFSI) and lithium difluorobis(oxalato) phosphate (LiDFBOP) in carbonate solvents. By combining surface/microstructural characterization and computations, we reveal that the preferential reduction of LiDFBOP occurs prior to LiFSI and carbonate solvents and its reduction products (Li2C2O4 and P-O species) bind to LiF, resulting in a favorable compact and protective SEI on the Li electrodes. It was found that the improved oxidative stability was accompanied by reduced corrosion of the current collector. A Li/Li symmetrical cell with a designed dual-salt electrolyte system exhibits stable polarization voltage over 1000 h of cycle time. In addition, the LiFSI-LiDFBOP advantage of this dual-salt electrolyte system enables the Li/LiFePO4 cells with significantly enhanced cycling stability. This work demonstrates that constructing a tailored SEI using a dual-salt electrolyte system is vital for improving the interfacial stability of lithium metal batteries.

Dioxolanone-Anchored Poly(allyl ether)-Based Cross-Linked Dual-Salt Polymer Electrolytes for High-Voltage Lithium Metal Batteries.

Novel cross-linked polymer electrolytes (XPEs) are synthesized by free-radical copolymerization induced by ultraviolet (UV)-light irradiation of a reactive solution, which is composed of a difunctional poly(ethylene glycol) diallyl ether oligomer (PEGDAE), a monofunctional reactive diluent 4-vinyl-1,3-dioxolan-2-one (VEC), and a stock solution containing lithium salt (lithium bis(trifluoromethanesulfonyl)imide, LiTFSI) in a carbonate-free nonvolatile plasticizer, poly(ethylene glycol) dimethyl ether (PEGDME). The resulting polymer matrix can be represented as a linear polyethylene chain functionalized with cyclic carbonate (dioxolanone) moieties and cross-linked by ethylene oxide units. A series of XPEs are prepared by varying the [O]/[Li] ratio (24 to 3) of the stock solution and thoroughly characterized using physicochemical (thermogravimetric analysis-mass spectrometry, differential scanning calorimetry, NMR, etc.) and electrochemical techniques. In addition, quantum chemical calculations are performed to elucidate the correlation between the electrochemical oxidation potential and the lithium ion-ethylene oxide coordination in the stock solution. Later, lithium bis(fluorosulfonyl)imide (LiFSI) salt is incorporated into the electrolyte system to produce a dual-salt XPE that exhibits improved electrochemical performance, a stable interface against lithium metal, and enhanced physical and chemical characteristics to be employed against high-voltage cathodes. The XPE membranes demonstrated excellent resistance against lithium dendrite growth even after reversibly plating and stripping lithium ions for more than 1000 h with a total capacity of 0.5 mAh cm-2. Finally, the XPE films are assembled in a lab-scale lithium metal battery configuration by using carbon-coated LiFePO4 (LFP) or LiNi0.8Co0.15Al0.05O2 (NCA) as a cathode and galvanostatically cycled at 20, 40, and 60 °C. Remarkably, at 20 °C, the NCA-based lithium metal cells displayed excellent cycling stability and good capacity retention (>50%) even after 1000 cycles.

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

We present a synergistic strategy to boost the cycling performance of Li-metal batteries. The strategy is based on the combined use of a micropattern (MP) on the surface of the Li-metal electrode and an advanced dual-salt electrolyte (DSE) system to more efficiently control undesired Li-metal deposition at higher current density (∼3 mA cm-2). The MP-Li electrode induces a spatially uniform current distribution to achieve dendrite-free Li-metal deposition beneath the surface layer formed by the DSE. The MP-Li/DSE combination exhibited excellent synergistic rate capability improvements that were neither observed with the MP-Li system nor for the bare Li/DSE system. The combination also resulted in the Li||LiMn2O4 battery attaining over 1 000 cycles, which is twice as long at the same capacity retention (80%) compared with the control cells (MP-Li without DSE). We further demonstrated extremely fast charging at a rate of 15 C (19.5 mA cm-2).

Stacking of Tailored Chalcogenide Nanosheets around MoO2-C Conductive Stakes Modulated by a Hybrid POM⊂MOF Precursor Template: Composite Conversion-Insertion Cathodes for Rechargeable Mg-Li Dual-Salt Batteries.

Mg anode has pronounced advantages in terms of high volumetric capacity, resource abundance, and dendrite-free electrochemical plating, which make rechargeable Mg-based batteries stand out as a representative next-generation energy storage system utilized in the field of large-scale stationary electric grid. However, sluggish Mg2+ diffusion in cathode lattices and facile passivation on the Mg anode hinder the commercialization of Mg batteries. Exploring a highly electroactive cathode prototype with hierarchical nanostructure and compatible electrolyte system with the capability of activating both an anode and a cathode is still a challenge. Here, we propose a POM⊂MOF (NENU-5) core-shell architecture as a hybrid precursor template to achieve the stacking of tailored chalcogenide nanosheets around MoO2-C conductive stakes, which can be employed as conversion-insertion cathodes (Cu1.96S-MoS2-MoO2 and Cu2Se-MoO2) for Mg-Li dual-salt batteries. Li-salt modulation further activates the capacity and rate performance at the cathode side by preferential Li-driven displacement reaction in Cu+ extrusible lattices. The heterogeneous conductive network and conformal dual-doped carbon coating enable a reversible capacity as high as 200 mAh/g with a coulombic efficiency close to 100%. The composite cathode can endure a long-term cycling up to 400 cycles and a high current density up to 2 A/g. The diversity of MOF-based materials infused by functional molecules or clusters would enrich the nanoengineering of electrodes to meet the performance demand for future multivalent batteries.

Effects of Imide-Orthoborate Dual-Salt Mixtures in Organic Carbonate Electrolytes on the Stability of Lithium Metal Batteries.

The effects of lithium imide and lithium orthoborate dual-salt electrolytes of different salt chemistries in carbonate solvents on the cycling stability of lithium (Li) metal batteries are systematically and comparatively investigated. Two imide salts (LiTFSI and LiFSI) and two orthoborate salts (LiBOB and LiDFOB) are chosen for this study and compared with the conventional LiPF6 salt. Density functional theory calculations indicate that the chemical and electrochemical stabilities rank in the following order: LiTFSI-LiBOB > LiTFSI-LiDFOB > LiFSI-LiDFOB > LiFSI-LiBOB. The experimental cycling stability of the Li metal batteries with the electrolytes ranks in the following order: LiTFSI-LiBOB > LiTFSI-LiDFOB > LiFSI-LiDFOB > LiPF6 > LiFSI-LiBOB, which is in well accordance with the calculation results. The LiTFSI-LiBOB can effectively protect the Al substrate and form a more robust surface film on Li metal anode, while the LiFSI-LiBOB results in serious corrosion to the stainless steel cell case and a thicker and looser surface film on Li anode. The key findings of this work emphasize that the salt chemistry is critically important for enhancing the interfacial stability of Li metal anode and should be carefully manipulated in the development of high-performance Li metal batteries.

High-Capacity Mg-Organic Batteries Based on Nanostructured Rhodizonate Salts Activated by Mg-Li Dual-Salt Electrolyte.

A magnesium battery is a promising candidate for large-scale transportation and stationary energy storage due to the security, low cost, abundance, and high volumetric energy density of a Mg anode. But there are still some obstacles retarding the wide application of Mg batteries, including poor kinetics of Mg-ion transport in lattices and low theoretical capacity in inorganic frameworks. A Mg-Li dual-salt electrolyte enables kinetic activation by dominant intercalation of Li-ions instead of Mg-ions in cathode lattices without the compromise of a stable Mg anode process. Here we propose a Mg-organic battery based on a renewable rhodizonate salt ( e. g., Na2C6O6) activated by a Mg-Li dual-salt electrolyte. The nanostructured organic system can achieve a high reversible capacity of 350-400 mAh/g due to the existence of high-density carbonyl groups (C═O) as redox sites. Nanocrystalline Na2C6O6 wired by reduced graphene oxide enables a high-rate performance of 200 and 175 mAh/g at 2.5 (5 C) and 5 A/g (10 C), respectively, which also benefits from a high intrinsic diffusion coefficient (10-12-10-11 cm2/s) and pesudocapacitance contribution (>60%) of Na2C6O6 for Li-Mg co-intercalation. The suppressed exfoliation of C6O6 layers by a firmer non-Li pinning via Na-O-C or Mg-O-C and a dendrite-resistive Mg anode lead to a long-term cycling for at least 600 cycles. Such an extraordinary capacity/rate performance endows the Mg-Na2C6O6 system with high energy and power densities up to 525 Wh/kg and 4490 W/kg (based on active cathode material), respectively, exceeding the level of high-voltage insertion cathodes with typical inorganic structures.