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The initial formation cycles are critical to the performance of a lithium-ion battery (LIB), particularly in the case of silicon anodes, where the high surface area and extreme volume expansion during cycling make silicon susceptible to detrimental side reactions with the electrolyte. The solid electrolyte interface (SEI) that is formed during these initial cycles serves to protect the surface of the anode from a continued reaction with the electrolyte, and its composition reflects the composition of the electrolyte. In this work, ReaxFF reactive force field simulations were used to investigate the interactions between ether-based electrolytes with high LiTFSI salt concentrations (up to 4 mol/L) and a silicon oxide surface. The simulation investigations were verified with galvanostatic testing and post-mortem X-ray photoelectron spectroscopy, revealing that highly concentrated electrolytes resulted in the faster formation and SEIs containing more inorganic and silicon species. This study emphasizes the importance of understanding the link between electrolyte composition and SEI formation. This ReaxFF approach demonstrates an accessible way to tune electrolyte compositions for optimized performance without costly, time-consuming experimentation.

Electrochemical reduction of CO2 in traditional aqueous electrolytes suffers from low faradaic efficiency towards desired products which can be traced back to low CO2 solubility and strong competition from the hydrogen evolution reaction. The use of non‐conventional electrolytes aims to mitigate these issues. This review will give a focused overview summarizing some of the most recent contributions on the electrochemical conversion of CO2 in organic solvents, ionic liquids, solid electrolytes, and brines. We summarize the findings in terms of activity, selectivity, and durability for each of the systems. In addition, it provides an outlook about the role of water, cations, and anions in the reaction. We also highlight the challenges of the electrochemical reduction of CO2 in each of the electrolytes. All the studies referred to in this review contribute meaningfully to reaching the technical targets for CO2 electrolyzers in non‐conventional electrolytes. [ABSTRACT FROM AUTHOR]

Li-metal batteries employing Li-metal anodes with the highest capacity density have attracted considerable attention. However, the Li-metal deposition is often nonuniform, which reduces the cycle performance of Li-metal batteries. The three-dimensionally ordered macroporous polyimide (3DOM PI) separator has provided much better Li deposition/dissolution cycle performance owing to its unique and ordered pore structure, but its low mechanical strength is an obstacle to practical use. In this study, we added polybenzimidazole (PBI) to increase the mechanical strength of the 3DOM PI separator. Both the 3DOM PI and 3DOM PI+PBI separators with pore sizes of 300 and 100 nm were prepared to investigate the effect of pore size on the mechanical strength of the separator. The 3DOM PI+PBI separators exhibited higher tensile strength, higher thermal stability, better Li deposition/dissolution cycle performance, and higher charge–discharge performance of the Li-metal battery than the 3DOM PI separators. Here, it was suggested that the increase in the mechanical strength of the separator prevented the compression of the pores in the separator by external pressure and provided a more uniform Li+ ion flux inside the separator.

The initial formation cycles are critical to the performance of a lithium-ion battery (LIB), particularly in the case of silicon anodes, where the high surface area and extreme volume expansion during cycling make silicon susceptible to detrimental side reactions with the electrolyte. The solid electrolyte interface (SEI) that is formed during these initial cycles serves to protect the surface of the anode from a continued reaction with the electrolyte, and its composition reflects the composition of the electrolyte. In this work, ReaxFF reactive force field simulations were used to investigate the interactions between ether-based electrolytes with high LiTFSI salt concentrations (up to 4 mol/L) and a silicon oxide surface. The simulation investigations were verified with galvanostatic testing and post-mortem X-ray photoelectron spectroscopy, revealing that highly concentrated electrolytes resulted in the faster formation and SEIs containing more inorganic and silicon species. This study emphasizes the importance of understanding the link between electrolyte composition and SEI formation. This ReaxFF approach demonstrates an accessible way to tune electrolyte compositions for optimized performance without costly, time-consuming experimentation. [ABSTRACT FROM AUTHOR]

MXene-based aqueous symmetric supercapacitors (SSCs) are attractive due to their good rate performances and green nature. However, it remains a challenge to reach voltages much over 1.2 V, which significantly diminishes their energy density. Herein, we report on Mo1.33CTz MXene-based SSCs possessing high voltages in a 19.5 M LiCl electrolyte. Benefiting from the vacancy-rich structure and high stable potential window of Mo1.33CTz, the obtained SSCs deliver a maximum energy density of >38.2 mWh cm−3 at a power density of 196.6 mW cm−3 under an operating voltage of 1.4 V, along with excellent rate performance and impressive cycling stability. This highly concentrated LiCl electrolyte is also applicable to Ti3C2Tz, the most widely studied MXene, achieving a maximum energy density of >41.3 mWh cm−3 at a power density of 165.2 mW cm−3 with an operating voltage of 1.8 V. The drop in energy density with increasing power in the Ti3C2Tz cells was steeper than for the Mo-based cells. This work provides a roadmap to develop superior SSCs with high voltages and high energy densities.

Abstract Organic electrode materials have become a hot research field in lithium‐ion batteries. However, the dissolution issue of organic materials (especially small molecules) in traditional electrolytes has become one of the important reasons to limit their application. The usage of highly concentrated electrolyte (HCE, >3 M) has been demonstrated to solve this problem, where the electrochemical performance of Pillar[5]quinone (P5Q) in 4.2 M LiTFSA/AN electrolyte was investigated. The HCE can avoid the reaction between acetonitrile molecules and lithium metal anode, reduce the dissolution of organic materials, and display excellent battery performance. At a current density of 0.2 C, a high specific capacity of 310 mAh g−1 (Ctheo = 446 mAh g−1) was maintained after 900 cycles, and the reversible capacity is higher than 113 mAh g−1 even at 10 C, indicating a good rate capability. This research would expand the new application of acetonitrile‐based electrolyte in organic secondary battery.

ABSTRACT: In recent research, the importance of electrolytes with high Li+ transference number (tLi) and ionic conductivity (σion) has been emphasized to realize rapid charge for Li secondary batteries. Simultaneously fulfilling high tLi and σion is still unsolved in liquid electrolytes; however, highly concentrated electrolytes (HCEs) of weakly coordinating solvents and Li salts will be promising for addressing this challenge. This idea is inspired by a recent study by Angell et al. on superprotonic ionic liquids comprising a weak Brønsted base and a superacid; highly labile and exchangeable H+ can be formed between significantly weak proton accepting sites. Here, we studied weakly coordinating fluoroethylene carbonate (FEC)-based electrolytes with lithium bis(fluorosulfonyl)amide (Li[FSA]) and compared with ethylene carbonate (EC)-based electrolytes. Experimental and computational studies indicated that solvent and ion exchange is more pronounced in the FEC-based HCE, resulting in higher tLiPP (0.73) and ionic conductivity (1.02 mS cm−1) compared to those of the EC-based HCE (tLiPP= 0.53 and σion= 0.84 mS cm−1). However, the FEC-based HCE exhibited lower electrochemical stability due to the intrinsically lower reductive stability of FEC and the oxidative decomposition of the liberated solvent in the HCE. Despite the superior transport properties, the Li/LiCoO2 cell with the FEC-based electrolyte showed lower discharge capacities and lower Coulombic efficiencies at higher current densities due to side reactions of the electrolyte. This study demonstrates that weak Li-solvent interactions can simultaneously enhance tLi and σion of HCEs, but they have the potential to sacrifice the electrochemical stability.

Dual‐ion batteries (DIBs) have attracted much attention owing to their low cost, high voltage, and environmental friendliness. As the source of active ions during the charging/discharging process, the electrolyte plays a critical role in the performance of DIBs, including capacity, energy density, and cycling life. However, most used electrolyte systems based on the LiPF6 salt demonstrate unsatisfactory performance in DIBs. We have successfully developed a 7.5 mol kg−1 lithium bis(fluorosulfonyl)imide (LiFSI) in a carbonate electrolyte system. Compared with diluted electrolytes, this highly concentrated electrolyte exhibits several advantages: 1) enhanced intercalation capacity and cycling stability of the graphite cathode, 2) optimized structural stability of the Al anode, and 3) significantly increased battery energy density. A proof‐of‐concept DIB based on this concentrated electrolyte exhibits a discharge capacity of 94.0 mAh g−1 at 200 mA g−1 and 96.8 % capacity retention after 500 cycles. By counting both the electrode materials and electrolyte, the energy density of this DIB reaches up to ≈180 Wh kg−1, which is among the best performances of DIBs reported to date. [ABSTRACT FROM AUTHOR]

This paper compares and combines two common methods to improve the cycle performance of lithium metal (Li) electrodes. One technique is to establish a micro-structured current collector by chemical separation of a copper/zinc alloy. Furthermore, the use of a highly concentrated ether-based electrolyte is applied as a second approach for improving the cycling behavior. The influence of the two measures compared with a planar current collector and a 1 M concentrated carbonate-based electrolyte, as well as the combination of the methods, are investigated in test cells both with Li and lithiumnickel cobaltmanganese oxide (NCM) as counter electrodes. In all cases Li is in-situ plated onto the micro-structured current collectors respectively a planar copper foil without presence of any excess Li before first deposition. In experiments with Li counter electrodes, the effect of a structured current collector is not visible whereas the influence of the electrolyte can be observed. With NCM counter electrodes and carbonate-based electrolyte structured current collectors can improve Coulombic efficiency. The confirmation of this outcome in experiments with highly concentrated ether-based electrolyte is challenging due to high deviations. However, these results indicate, that improvements in Coulombic efficiency achieved by structuring the current collector's surface and using ether-based electrolyte do not necessarily add up, if both methods are combined in one cell. [ABSTRACT FROM AUTHOR]

Dual‐carbon batteries (DCBs) are a promising candidate for smart grid applications, owing to their low cost, high power capability, and environmentally friendly benefits. As an essential component of DCBs, electrolytes not only act as a medium for ion migration during the running of the battery, but also provide active ions to be intercalated into carbon electrodes, exerting significant impacts on the electrochemical performance and safety of the DCB. In this Review, we discuss recent progress in electrolyte research for DCBs, including conventional liquid electrolytes, highly concentrated electrolytes, and gel polymer electrolytes, with a focus on their stability and compatibility with carbon electrodes. Finally, we present perspectives on the current limitations and future research directions of electrolytes for DCBs. Some recommendations for battery evaluation are also offered. [ABSTRACT FROM AUTHOR]

Highly concentrated LiTFSI - acetonitrile electrolytes have recently been shown to stabilize graphite electrodes in lithium-ion batteries (LIBs) much better than comparable more dilute systems. Here we revisit this system in order to optimise the salt concentration vs. both graphite and lithium metal electrodes with respect to electrochemical stability. However, we observe an instability regardless of concentration, making lithium metal unsuitable as a counter electrode, and this also affects evaluation of e.g. graphite electrodes. While the highly concentrated electrolytes have much improved electrochemical stabilities, their reductive decomposition below ca. 1.2 V vs. Li + /Li° still makes them less practical vs. graphite electrodes, and the oxidative reaction with Al at ca. 4.1 V vs. Li + /Li° makes them problematic for high voltage LIB cells. The former originates in an insufficiently stable solid electrolyte interphase (SEI) dissolving and continuously reforming – causing self-discharge, as observed by paused galvanostatic cycling, while the latter is likely caused by aluminium current collector corrosion. Yet, we show that medium voltage LiFePO 4 positive electrodes can successfully be used as counter and reference electrodes. [ABSTRACT FROM AUTHOR]

This paper compares and combines two common methods to improve the cycle performance of lithium metal (Li) electrodes. One technique is to establish a micro-structured current collector by chemical separation of a copper/zinc alloy. Furthermore, the use of a highly concentrated ether-based electrolyte is applied as a second approach for improving the cycling behavior. The influence of the two measures compared with a planar current collector and a 1 M concentrated carbonate-based electrolyte, as well as the combination of the methods, are investigated in test cells both with Li and lithium nickel cobalt manganese oxide (NCM) as counter electrodes. In all cases Li is in-situ plated onto the micro-structured current collectors respectively a planar copper foil without presence of any excess Li before first deposition. In experiments with Li counter electrodes, the effect of a structured current collector is not visible whereas the influence of the electrolyte can be observed. With NCM counter electrodes and carbonate-based electrolyte structured current collectors can improve Coulombic efficiency. The confirmation of this outcome in experiments with highly concentrated ether-based electrolyte is challenging due to high deviations. However, these results indicate, that improvements in Coulombic efficiency achieved by structuring the current collector’s surface and using ether-based electrolyte do not necessarily add up, if both methods are combined in one cell.

Lithium-sulfur (Li-S) batteries can theoretically deliver high energy densities exceeding 2500 Wh kg -1 . However, high sulfur loading and lean electrolyte conditions are two major requirements to enhance the actual energy density of the Li-S batteries. Herein, the use of carbon-dispersed highly concentrated electrolyte (HCE) gels with sparingly solvating characteristics as sulfur hosts in Li-S batteries is proposed as a unique approach to construct continuous electron-transport and ion-conduction paths in sulfur cathodes as well as achieve high energy density under lean-electrolyte conditions. The sol-gel behavior of carbon-dispersed sulfolane-based HCEs was investigated using phase diagrams. The sol-to-gel transition was mainly dependent on the amount of the carbonaceous material and the Li salt content. The gelation was caused by the carbonaceous-material-induced formation of an integrated network. Density functional theory (DFT) calculations revealed that the strong cation-π interactions between Li + and the induced dipole of graphitic carbon were responsible for facilitating the dispersion of the carbonaceous material into the HCEs, thereby permitting gel formation at high Li-salt concentrations. The as-prepared carbon-dispersed sulfolane-based composite gels were employed as efficient sulfur hosts in Li-S batteries. The use of gel-type sulfur hosts eliminates the requirement for excess electrolytes and thus facilitates the practical realization of Li-S batteries under lean-electrolyte conditions. A Li-S pouch cell that achieved a high cell-energy density (up to 253 Wh kg -1 ) at a high sulfur loading (4.1 mg cm -2 ) and low electrolyte/sulfur ratio (4.2 μL mg -1 ) was developed. Furthermore, a Li-S polymer battery was fabricated by combining the composite gel cathode and a polymer gel electrolyte.

It has been researched that highly concentrated electrolytes (HCEs) can solve the problem of the excessive decomposition of dilute electrolytes at a high voltage, but the mechanism is not clear. In this work, the antioxidation mechanism of HCE at a high voltage was investigated by in situ electrochemical tests and theoretical calculations from the perspective of the solvation structure and physicochemical property. The results indicate that compared with the dilute electrolyte, the change of solvation structures in HCE makes more PF 6 - anions easier to be oxidized prior to the dimethyl carbonate solvents, resulting in a more stable cathode-electrolyte interphase (CEI) film. First, the lower oxidation potential of the solvation structure with more PF 6 - anions inhibits the side effects of the electrolyte effectively. Second, the CEI film, consisted of LiF and Li x PO y F z generated from the oxidation of PF 6 - and Li 3 PO 4 generated from the hydrolysis of LiPF 6 via the soluble PO 2 F 2 - intermediate, can reduce the interface impedance and improve the conductivity. Intriguingly, the high viscosity of HCEs and the hydrolysis of LiPF 6 are proven to play a positive role in enhancing the interfacial stability of the electrolyte/electrode at a high voltage. This study builds a deep understanding of the bulk and interface properties of HCEs and provides theoretical support for their large-scale application in high-voltage battery materials.

The use of water-in-salt electrolytes (WiSEs) in electrochemical capacitors is an interesting alternative to state-of-the-art electrolytes based on organic solvents. However, making use of their wide electrochemical stability in electrochemical capacitors has been challenging, as shown in battery materials. In this work, we used three concentrations of LiTf 2 N in water (7, 15 and 21 mol kg−1) with activated carbon, and the determination of the maximum operating voltage showed stability up to 2.2, 2.4 and 2.6 V, respectively. This determination suggested positive/negative-electrode mass balances of 2.1, 1.5 and 1.2, but the experiments showed that the cycle life of devices using WiSEs of 15 and 21 mol kg−1 can be improved. Thus, we find that increasing the amount of active material in the positive electrode gave higher capacitance retention over cycling. With the optimization for a longer cycle life, the observed capacitance retentions for the devices with WiSEs of 7, 15 and 21 mol kg−1 were 97%, 70% and near 100%, respectively, after 10000 cycles. The higher amount of active material in the positive electrode penalized the energy-power performance, but the higher electrochemical stability compensated for it, and WiSEs of 7, 15 and 21 mol kg−1 stored 15, 16 and 21 W h kg−1 when they operated at 0.1 A g−1. Image, graphical abstract [ABSTRACT FROM AUTHOR]

In this work we demonstrate the application of a highly concentrated aqueous electrolyte to a hybrid supercapacitor cell. We combine an 8 m Sodium bis(trifluoromethanesulfonyl)imide (NaTFSI) aqueous electrolyte with a nanostructured VO 2 -cathode to enhance the voltage widow up to 2.4 V in a full cell. With the enhanced potential window, we are able to exploit the full contribution of the VO 2 material, where a part is outside the stability window of standard alkaline aqueous electrolytes. We show that the VO 2 material in the highly concentrated electrolyte provides a faradaic contribution even at the highest current density (25 A/g) and in this way increases the energy content also in high power conditions. The full cell shows a good efficiency but also a capacity fade over 500 cycles (39%) which is most likely related to dissolution of VO 2. [ABSTRACT FROM AUTHOR]

Nowadays, alkali metal-oxygen batteries such as Li-, Na-, and K-O 2 batteries have been investigated extensively because of their ultrahigh energy density. However, the oxygen crossover of oxygen batteries and the intrinsic drawbacks of the metal anodes (i.e., large volume changes and dendrite issues) have still been unsolved key problems. Here, we demonstrate a novel design of the K-ion oxygen battery using a graphite intercalation composite as the anode in a highly concentrated ether-based electrolyte. Instead of the metal K anode, the potassium graphite intercalation compound as the anode is depotassiated/potassiated in a binary form below 0.3 V (vs. K + /K); correspondingly, the discharged product KO 2 is formed/decomposed at the carbon nanotube cathode, and an all-carbon full cell exhibits impressive cycling stability with a working voltage of 2.0 V. Furthermore, the utilization of graphite intercalation chemistry has been demonstrated to be applicable in Li-O 2 batteries as well. Therefore, this study may provide a new strategy to resolve the key problems of the alkali metal-oxygen batteries.

To elucidate what properties control and practically limit ion transport in highly concentrated electrolytes (HCEs), the viscosity, ionic conductivity, ionicity, and transport numbers were studied for nine model electrolytes and connected to the rate capability in Li-ion battery (LIB) cells. The electrolytes employed the LiTFSI salt in three molar ratio concentrations; 1 : 2, 1 : 4, and 1 : 16 (LiTFSI:X) vs. solvents (X) with different permittivities; tert-butyl methyl ether (MTBE), tetrahydrofuran (THF) and propylene carbonate (PC). While the low polarity MTBE creates liquid electrolytes, ion-pairing limits the ionic conductivity despite extremely low viscosities. For the less concentrated 1 : 16 LiTFSI:MTBE and 1 : 16 LiTFSI:THF electrolytes the ionic diffusivities decrease with increased temperature, a sign of aggregation, but still their ionic conductivities and LIB performance increase. In general, the low ionic conductivity and high viscosity both limit the use of HCEs in LIBs, and no compensating mechanism seems to be present., (© 2020 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Developing a long-term stable electrolyte is one of the most enormous challenges for Li-O 2 batteries. Equally, the high flammability of frequently used solvents seriously weakens the electrolyte safety in Li-O 2 batteries, which inevitably restricts their commercial applications. Here, a binary mixture of highly concentrated tetraglyme electrolyte (HCG4) and 1,1,2,2-tetrafluoroethyl 2,2,3,3-tetrafluoropropyl ether (TTE) was used for a novel electrolyte (HCG4/TTE) in Li-O 2 batteries, which exhibit good wettability, enhanced ionic conductivity, considerable nonflammability, and high electrochemical stability. Being a co-solvent, TTE can contribute to increasing ionic conductivity and to improving flame retardance of the as-prepared electrolyte. The cell with this novel electrolyte displays an enhanced cycling stability, resulting from the high electrochemical stability during cycling and the formation of electrochemically stable interfaces prevents parasitic reactions occurring on the Li anode. These results presented here demonstrate a novel electrolyte with a high electrochemical stability and considerable safety for Li-O 2 batteries.