Localized High-concentration Electrolyte Research Articles

(Invited) Long-Life Sodium-Sulfur Batteries Enabled By a Localized High Concentration Electrolyte

The high abundance of both sulfur and sodium on earth makes room-temperature sodium-sulfur (Na-S) batteries a low-cost, environmentally benign, sustainable energy storage technology.1 High reactivity and instability of the Na-metal anode and the formation and migration of sodium polysulfides (NaPSs) at the sulfur cathode need to be mitigated to make this technology viable. The root cause of these issues can be the traced to the electrolytes that are conventionally used.To simultaneously solve the issues mentioned above, we utilize a localized high concentration electrolyte (LHCE).2 LHCE is prepared by diluting a concentrated salt solution with an inert solvent. The resulting unique solvation structure of the electrolyte favors the decomposition of the sodium salt to yield a thin, robust, ion-conducting SEI on both the anode and the cathode. This enables highly efficient and reversible stripping and plating at the Na-metal anode. At the sulfur cathode, this SEI changes the sulfur redox pathway by preventing the formation of NaPSs and steers it towards a quasi-solid-state conversion reaction. These benefits significantly prolong the life of the battery and bring the Na-S technology a step closer to viability. References A. Manthiram and X. Yu, Small, 11, 2108–2114 (2015).J. He, A. Bhargav, W. Shin, and A. Manthiram, J. Am. Chem. Soc., 143, 20241–20248 (2021). Figure 1

Advanced flame-retardant electrolyte for highly stabilized K-ion storage in graphite anode

Although graphite anodes operated with representative de/intercalation patterns at low potentials are considered highly desirable for K-ion batteries, the severe capacity fading caused by consecutive reduction reactions on the aggressively reactive surface is inevitable given the scarcity of effective protecting layers. Herein, by introducing a flame-retardant localized high-concentration electrolyte with retentive solvation configuration and relatively weakened anion-coordination and non-solvating fluorinated ether, the rational solid electrolyte interphase characterized by well-balanced inorganic/organic components is tailored in situ. This effectively prevented solvents from excessively decomposing and simultaneously improved the resistance against K-ion transport. Consequently, the graphite anode retained a prolonged cycling capability of up to 1400cycles (245 mA h g−1, remaining above 12mon) with an excellent capacity retention of as high as 92.4%. This is superior to those of conventional and high-concentration electrolytes. Thus, the optimized electrolyte with moderate salt concentration is perfectly compatible with graphite, providing a potential application prospect for K-storage evolution.

Protecting Li Metal Anode While Suppressing “Shuttle Effect” of Li-S Battery Through Localized High-Concentration Electrolyte

Protecting Li Metal Anode While Suppressing “Shuttle Effect” of Li-S Battery Through Localized High-Concentration Electrolyte

Stable Sodium-Based Batteries with Advanced Electrolytes and Layered-Oxide Cathodes.

Sodium-ion batteries offer a promising alternative to the more expensive, resource-limited lithium-ion batteries, in particular to accommodate the growing demand for large-scale energy storage. One of the main challenges for sodium-ion batteries, however, is the poor electrolyte stability, which leads to rapid capacity fade during cycling. Recent advances in the lithium-ion-battery field have expanded our understanding of electrolyte compositions and stability, paving the way for better sodium-ion-battery electrolytes. Two of the most promising new classes of electrolytes are evaluated herein with a sodium layered-oxide cathode, for the first time: a localized high-concentration electrolyte (LHCE) composed of sodium bis(fluorosulfonyl)imide, dimethyl ether, and tetrafluoropropyl ether and a "highly fluorinated" electrolyte (HFE) composed of 20% fluoroethylene carbonate with a lithium difluorophosphate additive. With a combination of electrochemical and post-mortem characterization techniques, the stability of each electrolyte is assessed with the O3-type Na(Ni0.3Fe0.4Mn0.3)O2 cathode and sodium metal anode. Both electrolytes significantly improve the surface and bulk stability of the cathode, but only the LHCE has a meaningful improvement on sodium metal stability. For the purpose of developing a long-lasting, sodium-ion full cell, both classes of electrolyte show great promise.

Regulating Solvation Structure in Nonflammable Amide‐Based Electrolytes for Long‐Cycling and Safe Lithium Metal Batteries

AbstractThe cycling stability of lithium metal batteries is steadily improving. The safety issues, which mainly result from the employment of flammable solvents, should be strongly considered for practical Li metal batteries. Nonflammable solvents can mitigate fire hazards; however, their employment irreversibly deteriorates the cycling stability of working batteries owing to intrinsic high reactivity against Li metal. Herein, regulating solvation structure in a dimethylacetamide (DMAC)‐based electrolyte is proposed to achieve compatibility between cycling stability and nonflammability of electrolytes. DMAC, a nonflammable solvent, is employed to construct a nonflammable localized high‐concentration electrolyte (LHCE). In the DMAC‐based LHCE, there are abundant aggregate clusters resulting in the formation of anion‐derived solid electrolyte interphase to circumvent parasitic reactions between DMAC solvents and Li metal and to improve the uniformity of Li deposition, which ensures the compatibility between cycling stability under practical conditions and nonflammability of electrolytes. This work opens an emerging avenue to construct long‐cycling and safe Li metal batteries by manipulating solvation structure in nonflammable electrolytes.

Anion-Containing Solvation Structure Reconfiguration Enables Wide-Temperature Electrolyte for High-Energy-Density Lithium-Metal Batteries.

The demand for high-energy-density lithium batteries (LBs) that work under a wide temperature range (-40 to 60 °C) has been increasing recently. However, the conventional lithium hexafluorophosphate (LiPF6)-based ester electrolyte with a solvent-based solvation structure has limited the practical application of LBs under extreme temperature conditions. In this work, a novel localized high-concentration electrolyte (LHCE) system is designed to achieve the anion-containing solvation structure with less free solvent molecules using lithium difluorophosphate (LiPO2F2) as a lithium salt, which enables wide-temperature electrolyte for LBs. The optimized solvation structure contributes to the cathode-electrolyte interface (CEI) with abundant LiF and P-O components on the surface of the LiNi0.5Co0.2Mn0.3O2 (NCM523) cathode, effectively inhibiting the decomposition of electrolyte and the dissolution of transition-metal ions (TMIs). Moreover, the weakened Li+-dipole interaction is also beneficial to the desolvation process. Therefore, the 4.3 V Li||NCM523 cell using the modified electrolyte maintains a high capacity retention of 81.0% after 200 cycles under 60 °C. Meanwhile, a considerable capacity of 70.9 mAh g-1 (42.0% of that at room temperature) can be released at an extremely low temperature of -60 °C. This modified electrolyte dramatically enhances the electrochemical stability of NCM523 cells by regulating the solvation structure, providing guidelines for designing a multifunctional electrolyte that works under a wide temperature range.

Revisiting the corrosion mechanism of LiFSI based electrolytes in lithium metal batteries

Lithium bis(fluorosulfonyl)imide (LiFSI), regarded as one of the most promising alternative of lithium hexafluorophosphate (LiPF6), seriously weakens the electrochemical performance of lithium metal batteries at high voltages, due to its extreme corrosion in nonaqueous electrolyte towards some components of the batteries. Though studies have shown that the aluminum (Al) collector will be corroded in LiFSI electrolytes, few attentions have been paid to the corrosion of steel components of lithium metal batteries, the corrosion intensity of which is much more severe than that of Al. Herein, by comparing the electrochemical corrosion behaviors of Al foil and stainless steel (SS) in LiFSI electrolytes, the corrosion products were characterized and analyzed, and the corrosion mechanism of SS was further proposed. Based on the corrosion mechanism, a strategy to inhibit the corrosion by using high concentration electrolyte (HCE) was also proposed. Moreover, introduction of 1,1,2,2-tetrafluoroethyl 2,2,2-trifluoroethyl ether into HCE, forming a localized high concentration electrolyte (LHCE), which not only addresses the viscosity and wettability issues of HCE but also inhibits the corrosion in conventional concentration electrolyte. As a result, Li||LiCoO2 coin cells using HCE and LHCE show excellent cycling stability with capacity fading rates of 0.53% and 0.26% per cycle, respectively, at 1 C (1 C = 180 mAh g − 1) between 3 – 4.45 V. The same corrosion inhibition regular also has been proved in pouch cells.

Facile Dual-Protection Layer and Advanced Electrolyte Enhancing Performances of Cobalt-free/Nickel-rich Cathodes in Lithium-Ion Batteries.

Despite cobalt (Co)-free/nickel (Ni)-rich layered oxides being considered as one of the promising cathode materials due to their high specific capacity, their highly reactive surface still hinders practical application. Herein, a polyimide/polyvinylpyrrolidone (PI/PVP, denoted as PP) coating layer is demonstrated as dual protection for the LiNi0.96Mg0.02Ti0.02O2 (NMT) cathode material to suppress surface contamination against moist air and to prevent unwanted interfacial side reactions during cycling. The PP-coated NMT (PP@NMT) preserves a relatively clean surface with the bare generation of lithium residues, structural degradation, and gas evolution even after exposure to air with ∼30% humidity for 2 weeks compared to the bare NMT. In addition, the exposed PP@NMT significantly enhances the electrochemical performance of graphite||NMT cells by preventing byproducts and structural distortion. Moreover, the exposed PP@NMT achieves a high capacity retention of 86.7% after 500 cycles using an advanced localized high-concentration electrolyte. This work demonstrates promising protection of Co-free/Ni-rich layered cathodes for their practical usage even after exposure to moist air.

Stable Lithium Metal Plating/Stripping in a Localized High-Concentration Cyclic Carbonate-Based Electrolyte

Li metal is the ultimate anode material for rechargeable Li batteries because of its high capacity and low electrochemical potential. However, Li metal anodes suffer from low Coulombic efficiency and poor cycling stability owing to the growth of Li dendrites. In this study, we report that a localized high-concentration electrolyte comprising lithium bis(fluorosulfonyl)imide (LiFSI), ethylene carbonate (EC), propylene carbonate (PC), and 1,1,2,2-tetrafluoroethyl 2,2,3,3-tetrafluoropropyl ether (HFE) achieves stable Li plating/stripping cycling with a Coulombic efficiency of >98 %. In contrast to LiFSI/EC : PC electrolytes, this electrolyte shows good wettability on a polypropylene separator. Li metal deposited in this electrolyte displays a large, granular, and dense morphology. Spectroscopic analyses confirm strong FSI−–Li+ coordination in this electrolyte, leading to the formation of a solid electrolyte interphase (SEI) layer enriched with LiF and sulfurous compounds derived from FSI−. These results indicate that the SEI layer facilitates the deposition of compact Li and effectively prevents Li loss owing to electrolyte decomposition and dead Li formation, resulting in highly reversible Li plating/stripping cycling. This electrolyte design can be an effective strategy for developing high-energy-density Li metal batteries.

Isoxazole-Based Electrolytes for Lithium Metal Protection and Lithium-Sulfurized Polyacrylonitrile (SPAN) Battery Operating at Low Temperature

A new electrolyte system using isoxazole as the salt dissolving solvent has been developed and studied for lithium metal batteries. By using fluoroethylene carbonate (FEC) as an additive and 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether (TTE) as a diluent for localized high concentration electrolyte (LHCE), isoxazole-based electrolytes were successfully implemented in lithium metal batteries, demonstrating excellent lithium metal protection capability. Utilizing several advanced characterization techniques (including synchrotron-based X-ray absorption spectroscopy and photoelectron spectroscopy), the solid electrolyte interphase (SEI) formed on the Li-metal anode after employing these electrolytes was thoroughly investigated. The high ionic conductivity of isoxazole at low temperature and the low impedance of SEI formed in LHCE significantly improved the low-temperature performance of Li-sulfurized polyacrylonitrile (SPAN) batteries, delivering 273.8 mAh g−1 capacity at −30 °C with 99.85% capacity retention after 50 cycles.

Influence of electrolyte structural evolution on battery applications: Cationic aggregation from dilute to high concentration

AbstractIn the context of the rapid expansion of the electric vehicle market, the request for high‐energy‐density lithium‐ion batteries (LIBs) has steadily increased, hence the demand for highly stable electrolytes. Over the past years, efforts have been devoted to the improvement of electrolyte materials. As a benchmarking breakthrough, the achievement of high concentration electrolyte (HCE) can be attributed to the altered cationic aggregation (i.e., cation‐solvent and cation‐anion coordination environment), which offers technical superiority over the widely applied conventional dilute electrolytes. More recently, based on the understanding of the dilute electrolyte and HCE, the concept of localized HCE (LHCE) has been proposed and extensively investigated. All these findings reveal a roadmap of electrolyte optimization for high‐performance LIBs via coordination structure regulation. Through elucidating the correlation of structure evolution therein and its critical effects on battery performance, this review aims to establish the design principle of electrolytes based on their structures other than component studies and thereby accelerate the development of high‐performance electrolytes.

Stable lithium metal batteries enabled by localized high-concentration electrolytes with sevoflurane as a diluent

A localized high concentration electrolyte, prepared using sevoflurane as a diluent in carbonate, promotes the formation of highly robust LiF-rich solid-electrolyte interphases on both Li-metal anodes and NMC811 cathodes, leading to long-life high-voltage lithium batteries.

In situ formation of circular and branched oligomers in a localized high concentration electrolyte at the lithium-metal solid electrolyte interphase: a hybrid ab initio and reactive molecular dynamics study

Developing advanced electrolytes has been considered as a promising approach to stabilize the lithium metal anode via the formation of a stable solid electrolyte interphase (SEI) that can protect the Li anode to enable long-term cycling stability.

A localized high concentration electrolyte for 4 V-class potassium metal batteries

A new localized high concentration electrolyte (LHCE) formulation enables improved oxidation stability (4.3 V), K anode passivation and Al anti-corrosion at the same time.

New nonflammable tributyl phosphate based localized high concentration electrolytes for lithium metal batteries

A new tributyl phosphate based nonflammable localized high concentration electrolyte exhibits good electrochemical performance.

Molecular Insight into Nano-Heterogeneity of Localized High-Concentration Electrolyte: Correlation with Lithium Dynamics and Solid-Electrolyte Interphase Formation

Molecular Insight into Nano-Heterogeneity of Localized High-Concentration Electrolyte: Correlation with Lithium Dynamics and Solid-Electrolyte Interphase Formation

A nonflammable phosphate-based localized high-concentration electrolyte for safe and high-voltage lithium metal batteries

The well-designed electrolyte enables high-voltage Li‖Ni6Mn2Co2O2 batteries with both enhanced safety and superior reversibility.

DFT–ReaxFF hybrid molecular dynamics investigation of the decomposition effects of localized high-concentration electrolyte in lithium metal batteries: LiFSI/DME/TFEO

Due to its low electrochemical potential and high theoretical specific energy, lithium-metal batteries (LMBs) have been considered as a promising advanced energy storage system for portable applications such as electric vehicles (EVs). However, the uncontrolled growth of lithium dendrites during cycling has remained a challenge. By utilizing an inert solvent to "dilute" the high concentration electrolytes, the concept of localized high-concentration electrolytes (LHCEs) has recently been demostrated as an effective solution to enable the dendrite-free cycling of LMBs. In this work, we investigated the reactions of 2 M lithium bis(fluorosulfonyl)imide (LiFSI) in a mixture of dimethoxyethane (DME)/tris(2,2,2-trifluoroethyl) orthoformate (TFEO) electrolyte at a Li metal anode. The SEI formation mechanism is investigated using a hybrid ab initio and reactive force field (HAIR) method. The 1n reactive HAIR trajectory reveals the important initial reduction reactions of LiFSI, TFEO, and DME. Particularly, both FSI anions and TFEO decompose quickly to release a considerable amount of F-, which leads to a LiF-rich SEI inorganic inner layer (IIL). Furthermore, TFEO produces a significant amount of unsaturated carbon products, such as thiophene, which can potentially increase the conductivity of SEI to increase the battery performance. Meanwhile, XPS analysis is utilized to further investigate the evolution of the atomic environment in SEI. Future designs of better electrolytes can be greatly aided by these results.

Stable Dendrite-Free Sodium-Sulfur Batteries Enabled by a Localized High-Concentration Electrolyte.

Ambient-temperature sodium-sulfur batteries are an appealing, sustainable, and low-cost alternative to lithium-ion batteries due to their high material abundance and specific energy of 1274 W h kg-1. However, their viability is hampered by Na polysulfide (NaPS) shuttling, Na loss due to side reactions with the electrolyte, and dendrite formation. Here, we demonstrate that a solid-electrolyte interphase rich in inorganic components can be realized at both the sulfur cathode and the Na anode by tweaking the solvation structure of the electrolyte. This transforms the sulfur redox process from conventional dissolution-precipitation chemistry into a quasi-solid-state reaction, which eliminates NaPS shuttling and facilitates dendrite-free Na-metal plating and stripping. With the solvated ionic liquid electrolyte structure, a high initial capacity of 922 mA h g-1 with a capacity fade of as low as 0.10% per cycle over 300 cycles was achieved. The scalability of this approach to pouch cells with practically necessary parameters demonstrates its potential for practical viability.

Decomposition Reactivities of Carbonate Electrolyte Vs. Localized High Concentration Electrolytes on NaNiO2 Cathode Surface

Cathode electrolyte interphase (CEI) layers formed by the electrolyte decomposition on the positive electrode have been thought to be critical for the electrochemical performance of sodium-ion batteries. The conventional carbonate electrolyte decomposition often leads to unstable CEI layers, which are one of the known causes of capacity loss and structural degradation over repeated charging cycles. To solve this obstacle, the conception of localized high concentration electrolyte (LHCE) was proposed to maintain localized solvation structure of active electrolyte salts by using hydrofluoroethers as diluents. However, the cathode decomposition mechanisms of LHCE electrolytes have been rarely studied and the formation of CEI layers at cathode-LHCE interfaces are still unclear. Here, density functional theory is applied to investigate the oxidative decompositions of carbonate electrolyte and LHCE electrolyte on layered NaNiO2 surface. We compared the oxidation potentials of electrolyte molecules, reactions mechanisms of bulk electrolytes and decomposition reactions on (0 0 2) and (1 0 -2) surfaces of NaNiO2. Our results indicated that H-transfer reactions were prone to occur during carbonate electrolyte decomposition, while solvent(diluent) dissociation reactions were the primary pathways for LHCE decomposition. The proton transfer reactions with cathode surface oxygen atoms contribute to the formation of hydroxyl group (-OH). We predicted that the charge reduction of surface nickel atoms bonded with -OH groups is the main reason for the capacity fading and structural transformation of NaNiO2 cathode.