Sulfolane-based Electrolyte Research Articles

Sulfolane-Based Flame-Retardant Electrolyte for High-Voltage Sodium-Ion Batteries

HighlightsNaTFSI/SUL:OTE:FEC facilitates the formation of S, N-rich, dense and robust cathode–electrolyte interphase on NaNMF cathode, which improves the cycling stability under high voltage.By utilizing NaTFSI/SUL:OTE:FEC, the Na||NaNMF batteries achieved an impressive retention of 81.15% after 400 cycles at 2 C with the cutoff voltage of 4.2 V.The study offers a reference for the utilization of sulfolane-based electrolytes in sodium-ion batteries (SIBs), while the nonflammability of the NaTFSI/SUL:OTE:FEC enhances the safety of SIBs.

Tetra-Peg Gels with Highly Concentrated Sulfolane-Based Electrolytes Exhibiting High Li-Ion Transference Numbers

To achieve high safety of lithium-ion batteries (LIBs), non-flammable electrolytes and prevention of liquid electrolytes are desirable. Sulfolane (SL) is a thermally stable and non-flammable solvent, and can dissolve high concentration Li salts. Recently, highly concentrated electrolytes (HCEs) containing Li salts over 3 mol dm−3 have attracted much attention for their attractive properties such as high thermal stability and wide electrochemical windows. We previously reported that SL-based HCEs exhibit Liion hopping conduction mechanism.1,2 In SL-based HCEs, a unique solvation structure is formed in which different Li+ ions are cross-linked by the SL and anions. In the cross-linked structure, Li+ ions dynamically exchange the ligands (SL and anion) and diffuse/migrate faster than SL solvent and anion, leading to high transference numbers of Li+ ion (>0.5). The high transference number of Li+ is effective in suppressing the concentration polarization during high-rate charging and discharging of a LIB.Gelation of the HCEs can prevent the leakage of liquids and further improve the safety of LIBs. In the case of polymer gel electrolytes, liquid electrolytes are incorporated in the polymer network. Ionic conductivity of a gel electrolyte is generally lower than that of its parent liquid electrolyte. However, to prepare self-standing and mechanically robust gel electrolytes high polymer concentrations (typically over 20 wt%) are required. To achieve both high ionic conductivity and mechanical toughness of a gel electrolyte, the use of tetra-arm poly(ethylene glycol) (TPEG) has been proposed.3 TPEG gels obtained by the end-coupling reaction of two symmetrical TPEGs with different terminals exhibit excellent mechanical properties.4 TPEG forms a homogeneous polymer network, allowing the resulting gels to uniformly disperse external stresses. Consequently, TPEG gels possess excellent mechanical toughness even at low polymer concentrations (<10 wt%). In this work, we prepared self-standing TPEG gel membranes containing a high concentration LiN(SO2CF3)2/SL electrolyte.5 The mechanical, ion transport, and electrochemical properties of the gels were characterized. TPEG gels exhibited high transference numbers of Li+. The low polymer concentration and homogeneous polymer network in the gel electrolyte were useful in achieving both mechanical reliability and high Li+ transport ability. Li/LiCoO2 cell with a TPEG gel electrolyte membrane could discharge at a current density of 2.9 mA cm−2 despite the low ionic conductivity (0.26 mS cm−1) of the gel electrolyte. Acknowledgements : This study was partially supported by the JSPS KAKENHI (Grant No. JP19H05813, JP22H00340, and JP23K17370). References K. Dokko, et al, J. Phys. Chem. B, 2018, 122, 10736–10745.A. Nakanishi, et al., J. Phys. Chem. C, 2019, 123, 14229–14238.K. Fujii, et al., Soft Matter, 2012, 8, 1756–1759T. Sakai, et al., Macromolecules, 2008, 41, 5379–5384.N. Tasaki, et al., Phys. Chem. Chem. Phys., 2023, 25, 17793–17797

Tracking Degradation of Sulfolane-Based Electrolytes in 5 V LNMO Cells

Lithium-ion batteries (LIBs) are today the most used energy storage for portable electronics and electric vehicles – but are laden with concerns of materials scarcity and availability, alongside performance and cost. The development of high-voltage cobalt-free cathode active materials, such as LiNi0.5Mn1.5O4 (LNMO), represents one promising route for next-generation LIBs, due to a combination of high capacity and low cost. Combining LNMO with conventional LIB electrolytes, based on carbonate solvents, however, typically leads to severe capacity fading due to a range of different degradation mechanisms1, not the least due to the high electrochemical potential reached during charge, up to 5 V vs. Li+/Li°.Here we use a range of sulfolane-based electrolytes to improve the stability2 and track the degradation by intermittent current interruption (ICI), which is a rather new protocol to determine the internal resistance of battery cells during cycling3. The ICI data is presented in the form of heat maps, in order to better visualize the variation of internal resistance as function of the cell voltage. Furthermore, Raman spectroscopy is used to identify the decomposition products resulting from the degradation and the data is correlated with the ICI data. Overall this allows us to shine further light on the prospects of sulfolane-based electrolytes for 5 V cells as compared to conventional LIB electrolytes such as e.g. LP40.References Guo, K. et al. High‐Voltage Electrolyte Chemistry for Lithium Batteries. Small Science 2, 2100107 (2022).Xu, J. Critical Review on cathode–electrolyte Interphase Toward High-Voltage Cathodes for Li-Ion Batteries. Nano-Micro Letters vol. 14 (2022).Yin, L. et al. Implementing intermittent current interruption into Li-ion cell modelling for improved battery diagnostics. Electrochim Acta 427, (2022).

Effect of the anionic composition of sulfolane based electrolytes on the performances of lithium-sulfur batteries

In lithium-sulfur batteries, cell design, specifically electrolyte design, has a key impact on the battery performance. The effect of lithium salt anion donor number (DN) (DN[PF6]− = 2.5, DN[N(SO2CF3)2]− = 5.4, DN[ClO4]− = 8.4, DN[SO3CF3]− = 16.9, and DN[NO3]− = 21.1) on the patterns of lithium-sulfur batteries and lithium metal electrode performances with sulfolane-based electrolytes is investigated. An increase in DN of lithium salt anions leads to an increase in the depth and rate of electrochemical reduction of sulfur and long-chain lithium polysulfides and to a decrease in those for medium- and short-chain lithium polysulfides. DN of lithium salt anions has weak effect on the discharge capacity of lithium-sulfur batteries and the Coulomb efficiency during cycling, with the exception of LiSO3CF3 and LiNO3. An increase in DN of lithium salt anions leads to an increase in the cycling duration of lithium metal anodes and to a decrease in the presence of lithium polysulfides. In sulfolane solutions of LiNO3 and LiSO3CF3, lithium polysulfides do not affect the cycling duration of lithium metal anodes.

Simulation of Lithium-Sulfur Batteries and Optimization of Cathode Structure

Recently, the spread of the mobile devices and clean energy, the demand for high-power and high-capacity secondary batteries has been growing. However, the energy density of lithium-ion batteries (LiB) that is widely used nowadays is reaching its design limit. Instead of LiB, lithium-sulfur batteries (LiSB) are expected as one of the next-generation secondary batteries. Compared to LiB, LiSB has the advantages of higher energy density, lower cost, and easier weight saving. In LiB, the driving force is the insertion and desorption reaction of lithium ions between the cathode and anode, while in LiSB, is the complex sulfur dissolution and precipitation reaction. In LiSB, the electrolyte determines the dissolution and precipitation of sulfur, which is thought to have a significant impact on battery characteristics. Therefore, new electrolyte systems are being developed daily and techniques to elucidate the reaction mechanisms are needed. Furthermore, the effect of carbon black, a conductive material, on battery characteristics has also been considered. In this study, we investigated the optimal initial lithium ion concentration and calculation focused on the precipitation form of each carbon species for the design of optimal electrode structures.Kumaresan's one-dimensional model [1] based on porous electrode theory was used. Sulfur was used as the cathode active material and lithium metal as the anode active material. Electron and ionic potentials were calculated from the electro-neutrality condition, concentration distribution in the electrolyte was calculated from the non-stationary mass balance equation, and electrochemical reactions at the interface between active material and electrolyte were calculated using the Butler-Volmer equation. Furthermore, stepwise reactions were considered by incorporating the electrochemical reaction of sulfur and the dissolution and precipitation reactions into the concentration distribution and the interfacial reactions. The temperature in the electrode layer was assumed to be constant, and no heat loss or side reactions due to overvoltage were assumed to occur. In LiSB, sulfur dissolved in the electrolyte on the cathode surface is sequentially reduced as polysulfide ions as the discharge progresses. It then combines with lithium ions transferred from the anode and expands to about 1.8 times. Therefore, based on our previous research [2], the effect of the expansion and contraction of the electrode particles was reflected by dynamic changes in the structural properties of the electrode layer.Ionic conductivity and transport number are important parameters that affect the properties of the electrolyte. The higher the lithium ion concentration, the higher the transport number, but the lower the ionic conductivity [3]. In this study, we focused on sulfolane-based electrolyte and investigated the effect of initial lithium ion concentration on discharge capacity by incorporating the relationship between ionic conductivity and transport number into a model. As a result, the discharge capacity reached its maximum when the initial lithium ion concentration was 3.0 M. In the static state, the ionic conductivity was highest around 1.6 M, but considering the operating conditions during discharge, the concentration distribution became be more remarkable due to the low transport number, which may have resulted in excessive Li2S precipitation near the separator. As an extension to the microstructure of the electrode layer, we focused on carbon black (CB). Two types of CB (KB, MCND) were targeted. When MCND and KB were used as CB under the same conditions in the experiment, it was confirmed that the discharge characteristics of MCND were significantly better than those of KB. In LiSB, Li2S precipitation has a significant effect on discharge characteristics. Therefore, we focused on the pores of both CBs. We reconstructed the binarized structure from X-ray CT images of both CBs, hypothetically precipitated Li2S in the pores, and calculated the relative conductivity. The relative conductivity represents the transport property and is highly dependent on the structure of the electrode layer. As a calculation result, the relative conductivity of KB became significantly smaller as the porosity decreased. From the above, the transport properties in both CBs were confirmed by calculation.As described above, it was suggested that an optimum lithium ion concentration exists, and the effect of different CB structures on transport properties was qualitatively evaluated. In the future, it is necessary to confirm the validity of the optimal concentration through experiments and to quantitatively evaluate the effect of different CB.AcknowledgmentThis study was supported by JST ALCA-SPRING Grant JPMJAL1301, Japan.

Potassium Bis(fluorosulfonyl)imide Is an Effective Additive for Improving Anode Cyclability in Sulfolane-Based Electrolytes for Li–S Batteries

Potassium Bis(fluorosulfonyl)imide Is an Effective Additive for Improving Anode Cyclability in Sulfolane-Based Electrolytes for Li–S Batteries

Performance Stabilization of Lithium-Sulfur Batteries Containing Sulfolane-based Electrolyte and Microporous Cathode by Controlling Working Voltage Range

For lithium-sulfur (Li-S) batteries, high-concentration electrolyte that inhibits the dissolution of Li polysulfide has been widely studied; one such electrolyte contains sulfolane. This study investigates the conditions under which a microporous activated carbon cathode, derived from azurmic acid, operates stably in a sulfolane-based electrolyte. We expected this cathode to maintain a stable capacity in a sulfolane-based electrolyte because its micropores stabilize the S species. However, Li-S batteries containing this cathode and electrolyte exhibit significant capacity decay during cycling. The cutoff voltage during charge-discharge cycling is varied to suppress the capacity decay. At a discharge voltage of 1.4 V or lower, the cycle life of the Li-S batteries is significantly reduced. Conversely, increasing the cutoff voltage during discharge suppresses the capacity decay of Li-S batteries. On the other hand, increasing the upper voltage limit during charging increases the reversible capacity. Thus, the operating voltage range is optimized. This study indicates that the voltage range of Li-S batteries should be carefully determined depending on the type of cathode material and electrolyte.

Tailored interface composition improves the integrity of electrode/electrolyte interphases for high-voltage Ni-rich lithium metal batteries in a sulfolane-based electrolyte

High-voltage lithium metal batteries (LMBs) matched with Ni-rich cathodes can deliver high energy density to satisfy the rapid development of power battery systems. However, conventional solid electrolyte interphase (SEI) or cathode electrolyte interphase (CEI) composed of Li2CO3, Li2O, LiF and alkyl lithium carbonate cannot meet the requirements for high-voltage LMBs in terms of stability and mechanical performance. Herein, a sulfolane-based electrolyte assisted with co-solvent and dual-salt strategies is developed to realize tailored LiF-boride-sulfide-containing interphases on both electrodes, in which the decomposition product RSOx can combine various inorganic and organic components. The improved integrity of interphases can mitigate the continuous decomposition of the electrolyte, thereby promoting uniform and dense lithium deposition and protecting the structure of cathode materials. Thus, the prepared sulfolane-based electrolyte results in an average Coulombic efficiency of 99.1% for Li||Cu cells and enables 4.7 V Li||LiNi0.8Co0.1Mn0.1O2 cells to maintain 80.0% capacity retention after 200 cycles. Besides, Cu||LiNi0.8Co0.1Mn0.1O2 anode-free cells (3.5 mAh cm−2, capacity retention of 70.5% after 50 cycles) further confirm the practicality of the prepared electrolyte. This work offers a direct and efficient method to enhance the integrity and stability of interphases by optimizing the electrolyte composition.

Distinct Differences in Li-Deposition/Dissolution Reversibility in Sulfolane-Based Electrolytes Depending on Li-Salt Species and Their Solvation Structures

Distinct Differences in Li-Deposition/Dissolution Reversibility in Sulfolane-Based Electrolytes Depending on Li-Salt Species and Their Solvation Structures

Low Concentration Sulfolane-Based Electrolyte for High Voltage Lithium Metal Batteries.

Electrolyte engineering is crucial for the commercialization of lithium metal batteries. Here, lithium metal is stabilized in the highly reactive sulfolane-based electrolyte under low concentration (0.25 M) for the first time. Inorganic-polymer hybrid solid electrolyte interphase (SEI) with high ionic conductivity, low bonding with lithium and high flexibility enables dense chunky lithium deposition and high plating/stripping efficiency. Low concentration electrolyte (LCE) also enables excellent cycling stability of LiNi0.5 Co0.2 Mn0.3 O2 (NCM523)/Li cells at 1 C (90.7 % retention after 500 cycles) and 0.3 C (83.3 % retention after 1000 cycles). With a low N/P ratio (≈2), the capacity retention for NCM523/Li cells can achieve 94.3 % after 100 cycles at 0.3 C. Exploring the LCE is of paramount significance because it provides more possibilities of the lithium salt selections, especially reviving some lithium salts that are excluded before due to their low solubility. More importantly, LCE has the significant advantage of commercialization due to its cost-effectiveness.

Tetra-arm poly(ethylene glycol) gels with highly concentrated sulfolane-based electrolytes exhibiting high Li-ion transference numbers.

We demonstrate that tetra-arm poly(ethylene glycol) gels containing highly concentrated sulfolane-based electrolytes exhibit high Li+ transference numbers. The low polymer concentration and homogeneous polymer network in the gel electrolyte are useful in achieving both mechanical reliability and high Li+ transport ability.

A Sulfolane-Based Electrolyte Optimized for Microporous Activated Carbon-Sulfur Composites for Lithium Sulfur Batteries

The dissolution of lithium polysulfide into electrolytes is a major limitation of Li-S batteries that hinders their practical applications. Lithium polysulfide dissolution is suppressed by using sulfur-complexed microporous-carbon cathodes in lithium–sulfur batteries. This study reports the use of a localized high-concentration electrolyte (LHCE), in which a sulfolane (SL) high-concentration electrolyte is diluted with hydrofluoroether (HFE), for the strong suppression of lithium polysulfide elution. A previously reported sulfolane electrolyte in the solvated state exhibits a short cycle life of eight cycles, whereas the sulfur-containing microporous-carbon cathode with a sulfolane-based electrolyte prepared in the ratio of lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) : SL : HFE = (0.572–0.953) : 2.00 : 1.26 (mol) exhibits a discharge capacity of more than 550 mAh g−1, even after 50 charge–discharge cycles. Increasing the LiTFSI ratio decreases the response current during discharge, as indicated by cyclic voltammetry. A sulfur cathode with microporous carbon (AZC) shows equivalent performance on decreasing the amount of solvated LiTFSI. This study could guide the design and development of electrolytes for sulfur-containing microporous-carbon cathodes.

Towards stable sodium metal battery with high voltage output through dual electrolyte design

The application of sodium metal batteries (SMBs) is hindered by the notable challenges of side reactions between electrolyte and Na as well as the growth of Na dendrites. Herein, we report that a unique design of dual electrolytes within double separators, namely lean diglyme-based electrolyte hosted by the polypropylene separator together with sulfolane-based electrolyte hosted by the glass fiber separator (denoted as D-e@PP/S-e@GF), can effectively prevent Na dendrite growth and suppress side reactions. D-e@PP/S-e@GF ingeniously combines the advantages of D-e and S-e by using their wettability difference on separators. Finite element method (FEM) simulations and in-situ optical microscopy reveal that the D-e@PP layer promotes uniform deposition and significantly reduces side reactions. The S-e@GF layer can be paired with high voltage cathodes. D-e@PP/S-e@GF enables a high CE of Na plating/stripping as high as 97.22% over 560 cycles at 0.5 mAh cm−2 and ultra-long cycle life of more than 1900 h at 1 mAh cm−2 in Na||Na symmetric cell. Furthermore, this design also demonstrates excellent cycling stability of full cells using Na3V2(PO4) (NVP) and Prussian Blue (PB) as cathodes. The unique design of dual electrolytes within double separators provides a new and promising avenue to develop high-performance SMBs.

Development of a Polarizable Force Field for Molecular Dynamics Simulations of Lithium-Ion Battery Electrolytes: Sulfone-Based Solvents and Lithium Salts.

A many-body polarizable force field (PFF) was developed for molecular dynamics (MD) simulations of sulfone-based solvents and lithium salts. Development of the polarizable force field included parameterization of atomic polarizabilities, electrostatic interactions, and van der Waals interactions of electrolyte components. 1λ6-thiolane-1,1-dione or sulfolane (SLF) compound was selected as one of the most appropriate solvents for high-voltage battery electrolytes. Atomic polarizabilities for the sulfolane solvent and lithium salts were obtained by means of a combination of quantum mechanics (QM) and molecular mechanics (MM) approaches using the isotropic atomic dipole polarizable (IADP) model. High-quality atomic polarizabilities were refined for 10 atomic types. Intermolecular interactions of Li+ ions with SLF were parameterized to reproduce the binding energies at the MP2/aug-cc-pvDZ level of theory in the gas phase. Intermolecular interactions of Li+ ions with polyatomic anions, such as nitrate [NO3]-, tetrafluoroborate [BF4]-, perchlorate [ClO4]-, hexafluorophosphate [PF6]-, bis(fluorosulfonyl)imide [FSI]-, and bis(trifluoromethylsulfonyl)imide [TFSI]-, were parameterized employing a similar methodology. A series of molecular dynamics simulations was performed for sulfolane-based electrolytes at several different lithium salt concentrations. Thermodynamic, structural, and transport properties were evaluated to validate the force field parameters against available simulation and experimental data. Transport properties of sulfolane were significantly improved as compared with those obtained from MD simulations using a nonpolarizable force field (NFF). A newly developed polarizable potential was shown to reproduce Li+ ion dynamics as a function of salt concentration. Faster diffusion of Li+ ions, among other electrolyte components, was obtained for high salt concentration electrolytes.

An overlooked issue for high-voltage Li-ion batteries: Suppressing the intercalation of anions into conductive carbon

Summary High-voltage Li-ion batteries have been extensively studied to increase energy density of batteries. However, their cycling stability has remained poor, despite various strategies being proposed to overcome the issues of high-potential cathodes, such as electrolyte oxidation and transition metal dissolution. Herein, we report anion intercalation into the cathode conductive carbon as an overlooked yet critical issue. We propose a concentrated sulfolane (SL)-based electrolyte that prevents anion intercalation via two mechanisms: (1) by offering a high activation barrier to intercalation via its strong anion-Li+ interaction and (2) by forming a sulfur-containing, anion-blocking SL-derived interphase. This electrolyte, used with graphitized acetylene black, which is oxidatively stable but usually susceptible to anion intercalation, enables the stable operation of a Li2CoPO4F/graphite full cell (cut-off voltage = 5.2 V), with 93% capacity retention after 1,000 cycles and an average Coulombic efficiency of ≥99.9%. This is a pivotal strategy to enhance the reversibility of >5 V Li-ion batteries on a commercial level.

Characterization of Solid Electrolyte Interphase Formed in Li[N(CF3SO2)2]-Sulfolane-Based Electrolytes

A highly concentrated electrolyte composed of sulfolane (SL) and lithium bis(trifluoromethylsulfonyl)amide (LiTFSA) attracts attention as an electrolyte for the lithium-sulfur battery because of the low solubility of lithium polysulfides. 1,1,2,2-Tetrafluoroethyl 2,2,3,3-tetrafluoropropyl ether (HFE) is often added to the electrolyte in order to decrease the viscosity[1]. It is important to improve the cyclability of the Li anode to develop the Li-S battery having a high energy density. The deposition and dissolution of Li have been known to be affected by the formation and growth of a solid electrolyte interphase on the anode surface. In the present study, formation of the SEI on Cu and Li electrodes in LiTFSA-SL-based electrolytes was investigated by electrochemical impedance spectroscopy (EIS), X-ray photoelectron spectroscopy (XPS), and transmission electron microscopy (TEM).The mixtures of LiTFSA, SL and HFE at various ratios were used as the electrolytes. The SEI was formed on a Cu substrate by holding its potential at 0 V vs. Li|Li(I) in the electrolytes. The impedance measurements were conducted at the open circuit potential using a coin-type or three-electrode cell using a Li foil as a reference and counter electrode. The samples for TEM and XPS analysis were prepared by immersing a Cu foil or grid in the electrolytes in contact with Li. After washing the sample with monoglyme, TEM observation and XPS measurement were conducted without exposure to air. The SEI was formed on Li deposited galvanostatically on a Cu grid placed in a coin-type cell.Formation of the SEI was not confirmed on a Cu electrode after keeping the potential at 0 V for 24 hours in LiTFSA-SL (1 : 2) by EIS, suggesting sluggish formation and/or poor adhesion of the reduced products. On the other hand, a semicircle attributable to the SEI appeared in LiTFSA-SL-HFE (1 : 2 : 2), indicated the decomposition of HFE played an important role in the SEI formation.The XPS spectra of a Cu electrode after the SEI formation at 0 V in LiTFSA-SL-HFE indicated the existence of TFSA– and/or its decomposition products in the SEI. On the other hand, LiF was found to be absent in the SEI, suggesting LiF was not formed by the cathodic decomposition of the electrolyte on a Cu electrode.The SEI with a thickness of 10-100 nm was observed by TEM on a Cu grid after the SEI formation in LiTFSA-SL-HFE. The shrinkage of the film was observed during the observation, suggesting that the SEI was composed of organic materials. No spot was observed in the electron diffraction diagrams of the SEI. On the other hand, the ring asignable to Li2O was observed in the electron diffraction diagram of the SEI formed on Li deposits. Thus, the composition of the SEI formed on Li was considered different from that on Cu.AcknowledgmentThis study was supported by Advanced Low Carbon Technology Research and Development of Program (ALCA) of the Japan Science and Technology Agency (JST).Reference[1] A. Nakanishi et al., J. Phys. Chem. C, 123, 14229 (2019).

Effects of Lithium Bis(fluorosulfonyl)imide Concentration on Performances of Lithium-ion Batteries Containing Sulfolane-based Electrolytes

Carbonate-free electrolytes based on lithium bis(fluorosulfonyl)imide (LiFSI) and sulfolane (SL) (LiFSI/SL) are promising alternatives to conventional electrolytes based on lithium hexafluorophosphate and ethylene carbonate. In this study, we show the effects of LiFSI concentration on performances of LiCoO2/graphite full cells with LiFSI/SL (1–2.25 mol dm−3). The LiFSI/SL electrolytes behave as ideal Walden electrolytes. Cells containing LiFSI/SL with various concentrations exhibit excellent cycle stabilities regardless of their concentrations though they have free-state solvent molecules. Furthermore, the cells containing LiFSI/SL with higher LiFSI concentrations show superior high-rate performances with high discharge capacities and discharge voltages due to high lithium dissociation and lithium ion transference number. The cell containing 1.5 mol dm−3 LiFSI/SL specifically shows excellent low-temperature performance with high discharge capacity and discharge voltage. The low-temperature performance is ascribed from non-crystallinity of the electrolyte as well as the high lithium dissociation.

Long-Term Stable Lithium Metal Anode in Highly Concentrated Sulfolane-Based Electrolytes with Ultrafine Porous Polyimide Separator.

Highly concentrated solutions composed of lithium bis(fluorosulfonyl)imide (LiFSI) and sulfolane (SL) are promising liquid electrolytes for lithium metal batteries because of their high anodic stability, low flammability, and high compatibility with lithium metal anodes. However, it is still challenging to obtain the stable lithium metal anodes in the concentrated electrolytes due to their poor wettability to the conventional polyolefin separators. Here, we report that the highly concentrated 1:2.5 LiFSI/SL electrolyte coupled with a three-dimensionally ordered macroporous polyimide (3DOM PI) separator enables the stable lithium plating/stripping cycling with an average Coulombic efficiency of ca. 98% for over 400 cycles at 1.0 mA cm-2. The 3DOM PI separator shows good electrolyte wettability and large electrolyte uptake due to its high porosity and polar constituent of the imide structure, allowing superior cycling performance in the highly concentrated solution, compared with the polyolefin separators. Electrochemical and spectroscopic analyses reveal that the superior cycling stability in the concentrated electrolyte is attributed to the formation of highly stable and Li+ ion conductive solid electrolyte interphase (SEI) layer derived from FSI- anions, which reduces the side reactions of SL with lithium metal, prevents the growth of lithium dendrites, and suppresses the increase in cell impedance over long-term cycling. Our findings demonstrate that polar and porous separators could effectively improve the affinity to the concentrated electrolytes and allow the formation of the anion-derived SEI layer by increasing the salt concentration of the electrolytes, achieving the long-term stable lithium metal anode.

Effect of Salt Concentration in Sulfolane-Based Electrolyte on Long-Term Li Plating/Stripping Behavior

Lithium metal is considered as the ultimate anode material for rechargeable batteries because it has the highest theoretical specific capacity (3,860 mAh g−1) and the lowest electrochemical potential (−3.04 V versus standard hydrogen electrode) of all possible candidates. However, dendritic Li growth and low Li Coulombic efficiency (CE) during Li plating/stripping hinder the practical applications of rechargeable lithium-metal batteries. In recent years, highly stable Li plating/stripping cycling has been achieved in the concentrated electrolytes of lithium bis(fluorosulfonyl) imide (LiFSI) in various solvents.1 , 2 Among the reported electrolytes, sulfolane (SL) is a promising solvent for high energy lithium metal batteries, since it has high anodic stability and low flammability. The concentrated electrolyte composed of LiFSI and SL shows high CE of ca. 98% and stable cycling of Li metal anode, however, the effect of LiFSI concentration on the long-term cyclability is unclear. Concentrated LiFSI/SL electrolytes have high viscosity and thus poor wettability toward conventional polyolefin separators, preventing stable Li plating/stripping cycling. We have previously reported that three dimensionally ordered macroporous polyimide (3DOM PI) membrane has high electrolyte wettability owing to its high porosity of ca. 70% and the relatively polar constituent of imide structure.3 Furthermore, uniform distribution of pores in a hexagonal close-packed arrangement of 3DOM PI membrane provides even current distribution during Li plating/stripping processes, preventing the dendritic Li growth. Therefore, 3DOM PI separator is expected to stabilize Li plating–stripping cycling. Here we studied the effect of LiFSI concentration in SL-based electrolyte on long-term Li plating/stripping behavior by using 3DOM PI separator. 3DOM PI separator exhibits good wettability toward concentrated LiFSI/SL electrolyte and highly stable Li plating/stripping than surfactant-coated polypropylene separator, enabling the evaluation of the long-term cyclability. Higher concentrated LiFSI/SL electrolyte realizes superior cyclability with low overpotential and high CEs during cycling. The ratio of F and N is higher, while that of C is much lower in the SEI on the Li metal cycled in the higher concentrated electrolyte. These results suggest that the FSI-derived SEI in the highly concentrated electrolyte suppresses decomposition of SL and subsequent increase in interfacial resistance, leading to long-term stable Li plating–stripping cycling. Acknowledgement This work is supported by The Furukawa Battery Co., Ltd. and 3DOM Inc. References X. Fan, L. Chen, X. Ji, T. Deng, S. Hou, J. Chen, J. Zheng, F. Wang, J. Jiang, K. Xu and C. Wang, Chem, 4, 174 (2018). X. Ren, S. Chen, H. Lee, D. Mei, M. H. Engelhard, S. D. Burton, W. Zhao, J. Zheng, Q. Li, M. S. Ding, M. Schroeder, J. Alvarado, K. Xu, Y. S. Meng, J. Liu, J.-G. Zhang and W. Xu, Chem, 4, 1877 (2018).Y. Maeyoshi, S. Miyamoto, H. Munakata and K. Kanamuraamura, J. Power Sources, 350, 103 (2017).

Electrochemical deposition of silicon from a sulfolane-based electrolyte: Effect of applied potential

As a low-cost, non-volatile, highly polar and chemically stable solvent, sulfolane has a high implementation potential for electroplating in organic media. This is the first report of the electrodeposition of silicon from sulfolane-based electrolytes. Voltammetric and chronoamperometric techniques, coupled with a quartz crystal microbalance, have been used to perform and characterize the process. Resonance frequency, f, damping, w, and apparent molar mass, Mapp are used as sensitive parameters for the evaluation of the silicon layer formation. Si electrodeposition displays a strong dependence on the applied potential. Close to theoretical values for Mapp and minimal w are observed at low deposition overpotentials, which allow an in situ quantitative mass evaluation. At higher overpotentials the process efficiency decreases due to simultaneous electrolyte decomposition. The electrodeposition of elemental Si (approx. 60% of the entire Si content) is evidenced by X-ray photoelectron spectroscopy. Additionally, the formation of a binary metal compound with the Cu substrate might be a key factor in the very good adhesion and mechanical stability of the layer.