Conventional Liquid Electrolytes Research Articles

Enhancing Molecular Electrocatalysis of CO2 Reduction with Pressure-Tunable CO2 -Expanded Electrolytes.

Electrochemical studies of CO2 conversion by molecular catalysts are typically carried out in a narrow range of near-ambient CO2 pressures wherein low CO2 solubilities in the liquid phase can limit the rate of CO2 reduction. In this study, five-fold rate enhancements are enabled by pairing CO2 -expanded electrolytes (CXEs), a class of media that accommodate multimolar concentrations of CO2 in organic solvents at modest pressures, with a homogeneous molecular electrocatalyst, [Re(CO)3 (bpy)Cl] (1, bpy=2,2'-bipyridyl). Analysis of cyclic voltammetry data reveals pressure-tunable rate behavior, with first-order kinetics at moderate CO2 pressures giving way to zero-order kinetics at higher pressures. The significant enhancement in the space-time yield of CO demonstrates that CXEs offer a simple yet powerful strategy for unlocking the intrinsic potential of molecular catalysts by mitigating CO2 solubility limitations commonly encountered in conventional liquid electrolytes. Moreover, our findings reveal that 1, a workhorse molecular catalyst, performs with intrinsic kinetic behavior, which is competitive with fast enzymes under optimal conditions in CXEs.

Indigo As Single Additive in Liquid Electrolyte for Lithium Metal Batteries

Rechargeable lithium ion batteries (LIBs) have been successfully developed and widely used to power today’s portable electronic devices. The long term success in electric vehicles and energy storage system relies on rising the energy density, low temperature efficiency, safety and increased cycle life.In parallel, lithium metal batteries (LMBs), described as a system with Li0 as anode and metal oxide as cathode (NMC, LFP, LMO) are arising as aim of research for a plethora of groups. Lithium metal anode is considered as ideal anode due to high theoretical capacity (3860 mAhg-1), lower negative electrochemical potential and lower density (0.534 g.cm-3).1 Researchers used complex liquid electrolyte (ionic liquid, etc.) systems in order to avoid lithium dendrite and degradation, conventional liquid electrolyte (LiPF6, carbonate solvants) degraded faster than expected, and consequently, generated cells fading. Therefore conventional liquid electrolyte in such cell was not used.Use organic additive in electrolyte and/or electrodes is considered as one of the most economical and effective’s approaches for solved many problems as cited previously. The additive can interact with electrolyte or anode to prevent degradation or enhanced cell performances.This presentation will outline commercial organic compounds: Indigo and his derivatives use as single additive in NMC/Li batteries with conventional liquid electrolyte, which considerably increased the cycle life of Li metal batteries. Dyes such Indigoïd contain electron density donor (-NH-) and acceptor (-C=O) groups linked by conjugated bonds, which participate to their versatile electrochemical properties.Used as additive in cathode materials or electrolyte, indigo modified the electrode structure and changed the composition of the Li-ions battery electrolyte and solid electrolyte interface (SEI) on lithium metal. 1.Zhang, H., Eshetu, G.G.,Judez, X., Li, C., Armand, M., Angew. Chem. Int. Ed. 2018, 57, 15002-15027 Figure 1

In-Situ Cycling in Liquid Electrochemical TEM of FIB Prepared Semi All Solid State 2-D Micro-Battery

All solid state Li-ion micro-battery is a promising candidate to power miniaturized sensors for Internet of things (IOT) and other electronic devices. The LiNi0.5Mn1.5O4 spinel (LNMO) is a potential positive electrode material for Li-ion micro-batteries offering a theoretical capacity of 147 mAh/g (65 µAh/cm2/µm for a bulk density of 4.47 g/cm3) and operates up to now at the highest potential (around 4.7 V vs. Li/Li+) [1]. Fabricated micro-battery has a stacking of 4 layers sandwiched on each other [from bottom to top-Si (0.385mm)/ Al2O3 (100 nm)/ Pt (30 nm) / LNMO (400 nm)] where Al2O3 and Pt are deposited by ALD and LNMO is deposited by PVD (Fig1a). 100% theoretical capacity was achieved on the first discharge (till 4.4V vs. Li+/Li) and 80% capacity still retained even after 300 cycles. Further thin film is characterized by Raman and XRD to confirm the phase of LNMO.Charging and discharging of Li-ion batteries can result in significant transformation of the cathode structure. It is crucial to know which structural changes occur at the atomic scale to understand the evolution of the electrochemical performance and degradation routes of the different cathode materials. The in situ cycling in liquid TEM has given us an opportunity to enter into the battery so as to spot the slightest textural, chemical or structural modifications of the electrode materials resulting in important advances in knowledge on electrochemical energy storage [2-3].Here, our approach is based on the cycling a semi all-solid-state battery Si/Al2O3/Pt/LNMO inside the TEM using liquid-electrochemical cell (Fig1b) with conventional liquid electrolyte (LiClO4, EC: DMC). The Pt current collector of semi all solid state micro-battery is connected to the Pt part of reference electrode on the e-chips used for TEM study (Fig 1c). First, using FIB preparation technique, we sliced a full 2-D “micro-battery” making it as thin to observe/analyse by TEM techniques. Then, we modified the micro-battery FIB lamellar design using FIB-SEM tool to get good electrical contact and reduced polarisation. Several technological problems have to be overcome in the process. For the instance, it is mandatory to obtain a good electrical contact between the Pt reference electrode of e-chip and Pt of FIB lamellar, which is achieved by making a Pt bridge between them. 4-Probe electrical conductivity performed locally confirms the good electrical conductivity between the different parts of FIB lamellar and Pt reference electrode of e-chip.The final modified version of FIB lamellar of semi all solid state 2D micro-battery [from bottom to top-Si (0.385mm)/ Al2O3 (100nm)/ Pt (630nm) / LNMO (400nm)/ Pt (2µm)] is then cycled inside the liquid electrochemical TEM holder in potential window of 4.1 V-4.8 V Vs Li+/Li. The flow of the electrolyte (LiClO4 EC:DMC 1:1) inside the TEM holder was further controlled by microfluidic controller with the flow rate of 2 µL/min. CV was recorded at a sweep rate of 0.1mV/s and two plateaus at 4.4 V and 4.6 V was observed corresponding to Ni2+/3+ and Ni3+/4+ oxidation respectively (Fig 1d).The semi all solid 2-D micro-battery was further studied using ASTAR (Automatic crystal orientation and phase mapping) device [5] to perform the phase mapping of two possible crystalline structures i.e. ordered phase and disordered phase of LiMn1.5Ni0.5O4 spinel material along with some Ni rich impurities LixNi1-xO [4].The orientation mapping was also performed to gain some information regarding grain boundaries (Fig 1e-insert electron diffraction pattern), grain size distribution (maximum grains of less than 10 nm in size) and disorientation between the grains ( disorientation of 55o between the two selected grains) etc. A region of interest of 1.2 µm2 was chosen for the scanning area with a step size of 5 nm. Several TEM analyses were also made such as HRTEM, EELS, EDX and STEM-HAADF at high resolution on pristine sample as well as post-mortem sample (after cycling). Thanks to ASTAR technique to provide us information regarding the epitaxial effect between the stacking of different layers (observed till few nm of thickness), porosity , morphological and structural changes between pristine and after cycling.

Using Experiment and First-Principles to Assess Electrochemical Windows of Common Solid Electrolytes for Their Application in All Solid-State Lithium Batteries.

During the last decades, lithium batteries have been developed to power a growing number of portable applications and to meet the needs of an increasingly mobile society. Their industrial and commercial application has always occurred in two successive steps of equal importance: the discovery of new electrode materials and/or electrolytes, followed by their extensive optimisation. A new generation of lithium batteries has been recently developed to meet high expectations in terms of safety, stability and capacity: All-Solid-State Lithium Batteries (ASSLB), where the conventional liquid electrolyte (LiPF6 + EC/DEC) is replaced by a safer and more stable ceramic, polymer or glass solid electrolyte (SE). ASSLB are partly developed with the prospect of using high potential materials as positive electrode and lithium metal as negative electrode. This is only possible through SE stated large electrochemical windows. Nevertheless, values for these electrochemical windows are very divergent in the literature published through the last decades. Recently, several studies have come to specifically decry the frequent overestimation of SE electrochemical stabilities 1,2. Establishing a robust procedure to determine SE real electrochemical windows has become detrimental. Our work is focused on developing a combined theoretical and experimental approach to better assess the electrochemical stability of widely used SE such as Li1.3Al0.3Ti1.7(PO4)3, Li1.5Al0.5Ti1.5(PO4)3 and LiLaTi2O6. In this presentation, we shed light on the importance of selecting the right experimental setup and explore the link between experimental and interpreted thermodynamic results.1) Y. Tian, T. Shi, W. Richards, J. Li, J. Kim, S-H. Bo, G. Ceder. Energy Environ. Sci., 2017, 10, 11502)Z. Zhang, Y. Shao, B. Lotsch, Y-S. Hu, H. Li, J. Janek, L. Nazar, C-W. Nan, J. Maier, M. Armand, L. Chen. Energy Environ. Sci. 2018,11, 1945-1976.

Operando Electrochemical Pressiometry Using Three-Electrode Cells Probing (Electro)Chemo-Mechanics for All-Solid-State Lithium-Ion Batteries

Due to the nonflammability of inorganic solid electrolytes (SEs) and the wide operation temperature ranges, bulk-type all-solid-state lithium or lithium-ion batteries (ASLBs) have been emerged as a promising power source. In particular ASLBs using sulfide SEs, such as Li10GeP2S12 (LGPS, 1.2 × 10-2 S cm-1) and Li3P7S11 (1.7 × 10-2 S cm-1), are attractive because of the extremely high ionic conductivities of sulfide SE materials, which are comparable to those of conventional organic liquid electrolytes used for lithium-ion batteries.For the success of practical ASLBs, however, a current level of understanding and development on the electrode microstructure and interfacial (electro)chemistry is insufficient. While ASLBs consist of all the solid-state components (i.e., electrode active materials, SEs, conducting additives such as carbon, binders), volumetric strains of electrode active materials upon repeated charge-discharge cycles cause a serious deterioration of the performances. Thus, (electro)chemo-mechanics have been emerging as the critical issue. However, the relevant reports are scarce to date. Moreover, complex interfacial evolutions which could also contribute to the (electro)chemo-mechanical behaviors have been overlooked.In this work, we first report an operando pressiometry using three-electrode cells to enable deconvolution of voltages and volumetric strains of positive and negative electrodes in-situ. Figure 1 shows the result of Li[Ni,Co,Mn]O2/Li6PS5Cl/graphite all-solid-state full cells cycled at 30 °C. The pressiometry profiles reveal signatures of phase transitions of electrode active materials, providing valuable information in terms of the assessment of the electrochemical performances and/or the management of battery system. More details will be discussed in the presentation. Figure 1

Thermally and Oxidatively Stable Polymer Electrolyte for Lithium Batteries Enabled By Phthalate Plasticization

Solid-state lithium batteries are desirable alternatives to lithium ion batteries given their high theoretical energy densities associated with Li metal anodes and the potentially superior safety from the elimination of flammable liquid electrolytes. Compelling ceramic lithium conductors with ionic conductivities surpassing those of conventional liquid electrolytes have been developed, but they suffer from high fabrication and processing costs. Moreover, achieving intimate solid-solid contact at the electrolyte-electrode interface is challenging. Polymer electrolytes are alternative materials that offer processing ease and the ability to fully infiltrate and wet composite electrodes. Poly(ethylene oxide) (PEO) mixed with lithium salt is the archetypical polymer electrolyte and solid-state cells using LiFePO4 cathode and lithium metal anode have been successfully commercialized. However, a significant limitation of PEO is its low oxidative stability, which restricts its application to lower voltage cathode (e.g. LiFePO4). Thus, polymer electrolytes with higher oxidative stabilities are desperately needed. In previous work, our group investigated hydrogenated nitrile butadiene rubber (HNBR) as a candidate for high-voltage application because of its superior oxidative stability of 5.3 V vs. Li/Li+. This polymer readily dissolves lithium bis(trifluoromethane)sulfonimide (LiTFSI) salt and has a high lithium transference number (0.59 compared to 0.37 for PEO). However, HNBR-LiTFSI suffers from low ionic conductivity (3.6×10-8 S·cm-1 at 20°C), which severely limits transport in full solid-state cells. In this work, phthalates were employed as the plasticizer to increase the conductivity of the HNBR-based electrolyte. FTIR shows that phthalates not only enhance the segmental mobility of the polymer chains but also coordinate with Li+ and participate in the solvation of lithium salt. Phthalates have a broad liquid-phase window preventing potential phase separation in the polymer electrolyte and evaporation at elevated temperatures. Their low HOMOs (e.g. -10.7 eV for di-ethyl-phthalate) should translate to higher oxidative resistance. A series of di-alkyl-phthalate homologues differing in the length of the alkyl branches were investigated in this report. Plasticized polymer electrolytes were prepared by blending HNBR-LiTFSI and a given amount of phthalate in acetone solution, and then casting into a mold and evaporating the volatile acetone. The plasticized electrolyte can withstand 159°C without significant evaporation; the decomposition of HNBR occurs at over 400°C even in a pure O2 environment. The conductivity of HNBR-LiTFSI with di-ethyl-phthalate (up to 20 wt.%) was 7.3×10-7 S·cm-1 at 20°C and 4.4×10-5 S·cm-1 at 70°C. The electrochemical oxidative stability of both HNBR-LiTFSI and plasticized-HNBR-LiTFSI was assessed by linear sweep voltammetry. The oxidation potential was reduced from 5.3 V to 5.2 V vs. Li/Li+ after introducing phthalates. Electrochemical impedance analysis showed that a resistive interface developed between HNBR-based electrolytes and lithium, indicating a chemical incompatibility between HNBR and lithium metal. However, this problem was readily addressed through a laminated electrolyte structure where a layer of PEO/LiTFSI physically separated plasticized HNBR/LiTFSI from direct contact with lithium metal. The effectiveness of this structure was confirmed by over 2000 hours of reversible galvanostatic cycling of symmetric lithium metal electrodes at 70°C.

Interface Engineering of Sulfide-Based All-Solid-State Lithium Batteries

All-solid-state lithium batteries (ASSLBs) have gained intensive attention worldwide because of their intrinsic safety and potential high energy density.1 As a critical component in ASSLBs, various solid-state electrolytes have been extensively studied over the past decade. So far, the ionic conductivity of solid-state electrolytes, particularly sulfide electrolytes (SEs, i.e. Li10GeP2S12) is close to that of conventional liquid electrolytes. However, the electrochemical performance of SE-based ASSLBs is hindered by the large interfacial resistance between electrodes and sulfide electrolytes. The underlying reasons are detrimental interfacial reactions and insufficient solid-solid contact between electrodes and SEs in ASSLBs.2 In our research, taking the advantages of atomic/molecular layer deposition techniques, an interfacial layer with designed functionalities can be conformably interposed between the electrodes and solid-state sulfide electrolytes, aiming at overcoming the interfacial resistance. At the anode interface, a artifical solid electrolyte interphase (SEI) has been engineered to suppress the interfacial rections and lithium dendrite growth, sucessfully enabling the use of Li metal in SE-based ASSLBs.3 At the cathode interface, a dual-shell interfacial nanostructure was rationally designed, in which the inner shell LiNbO3 suppressing the interfacial reactions, while the outer shell Li10GeP2S12 providing an intimate electrode-electrolyte contact.4 As a result, the dual-shell strucutured LGPS@LNO@LCO cathode exhibits a high initial specific capacity of 125.8 mAh.g-1 (1.35 mAh.cm-2) with an initial Coulombic efficiency of 90.4% at 0.1 C and 87.7 mAh.g-1 even at 1C. Furthermore, a plastic crystal electrolyte has been developed, which can simultaneously suppress the interfacial reactions and lithium dendrite growth in ASSLBs.5 The above work not only demonstrates various strategies to enable high-energy-density ASSLBs but also provides new insights into the interfacial challenges of ASSLBs.

(Invited) Lithium Batteries Enabled By Thin Film Composite Solid Electrolyte Separators

Over the past decade, solid-state lithium ion conductors with conductivities on par and even exceeding the conductivities of conventional liquid electrolytes have been developed. However, practical solid-state lithium metal batteries have not yet been realized due to challenges associated with fabricating and integrating solid electrolyte separators into electrochemical cells. To achieve the power metrics and effective energy densities of existing lithium ion cells, solid electrolytes must be fabricated with comparable area specific resistances and thicknesses on the order of 10 μm, without a significant increase in cost. Thin film composite separators, where a dense solid electrolyte film is coated onto microporous membranes (separators), are one possible solution. The solid thin film electrolyte enables reversible cycling of Li metal by isolating it from the catholyte and inhibiting growth of dendritic structures, while the porous scaffold mechanically supports the thin film and hosts the catholyte within its porosity for efficient ion transport.In this work, lithium phosphate oxynitride (Lipon) thin films with thicknesses on the order of 50 - 100 nm were deposited onto microporous polypropylene separators. These composite separators were robust and flexible; the Lipon film didn’t fracture as the separator was handled, enabling the integration into standard lithium cells, such as coin cells. Due to its thinness, the area specific resistance (ASR) of the solid electrolyte layer was 5 - 10 Ω-cm2, and a relatively low interfacial resistance of 2.5 Ω-cm2 between the Lipon and liquid electrolyte was observed. The Lipon layer was shown to be chemically and physically stable in several common liquid electrolyte formulations. Negligible evolution of interfacial resistance and reaction products at the Lipon/liquid electrolyte were observed after extended exposure to these electrolytes. In Li cycling tests, the Lipon layer promoted smooth, dense Li morphologies in electrodeposits with areal capacities exceeding 15 mAh cm-2. Without the dense Lipon layer, the morphology of the Li electrodeposits was rough – consistent with “mossy” lithium morphology typically observed in liquid electrolytes. In operando impedance spectroscopy showed that a stable resistance of the Lipon bulk was maintained during Li plating, suggesting that the Lipon layer remains intact. The solid electrolyte membrane was also shown to inhibit the polysulfide shuttle in Li-S cells, preventing the reaction of the polysulfide with Li metal. The Li-S cell using the composite separators showed high coulombic efficiency and stable cycling performance. In LiMn2O4-Li cells, the thin film composite separators protected the Li metal from reactions with Mn ions dissolved in liquid electrolyte. The development of these composite solid electrolyte separator offers a promising path to energy dense lithium metal batteries. This work was supported by the ARPA-E IONICS program, U.S. Department of Energy, award DE-AR0000775.

(Invited) Hexagonal Boron Nitride Ionogel Electrolytes for Solid-State Batteries

Ionogel electrolytes based on ionic liquids (ILs) and a gelling solid matrix are an attractive alternative to conventional liquid electrolytes for energy storage applications. In particular, ILs offer several advantages as an electrolyte including nonflammability, nonvolatility, high electrochemical stability, and high ionic conductivity. Moreover, combining ILs with a gelling solid matrix leads to a solid-state electrolyte that is sufficiently mechanically robust to replace both the liquid electrolyte and separator in a single component, thus allowing easier packaging, streamlined manufacturing, and minimal risk of leakage. For ionogel electrolytes, hexagonal boron nitride (hBN) is an especially desirable solid matrix due to its excellent chemical and thermal stability in addition to its mechanical robustness [1]. This talk will address recent efforts to realize high-performance hBN/IL ionogel electrolytes using hBN that has been solution-exfoliated into two-dimensional nanosheets using ethyl cellulose (EC) stabilizing polymers [2]. In addition to the intrinsic advantages of hBN, EC-exfoliated hBN nanosheets significantly enhance both ionic conductivity (> 1 mS/cm) and mechanical strength (storage modulus > 1 MPa) compared to conventional hBN microparticles, providing additional competitive advantages for energy storage applications [3]. The high thermal stability of hBN/IL ionogel electrolytes also enables safe, high-rate operation (up to 10C charge/discharge rate) at high temperatures up to 175 °C, which represents the highest operating temperature to date for solid-state lithium-ion batteries.[1] K.-S. Chen, et al., ACS Energy Letters, 2, 2026 (2017).[2] A. C. M. de Moraes, et al., Advanced Functional Materials, 29, 1902245 (2019).[3] W. J. Hyun, et al., ACS Nano, 13, 9664 (2019).

Drifts (Diffuse Reflectance Infrared Fourier Transform Spectroscopy) - a Way to Assess the Reactivity of Solid Electrolytes with Ambient Air

All-Solid-State-Batteries (ASSBs) are envisioned to be the next-generation lithium-ion batteries (LIBs). Replacing the inflammable organic electrolyte in conventional liquid electrolyte LIBs by a non-inflammable inorganic solid electrolyte (SE) is one of the concepts to increase battery safety. Moreover, ASSBs may offer higher gravimetric and volumetric energy density compared to liquid electrolyte based technology.[1,2] However, their high reactivity with ambient air is a major obstacle which prevents ASSBs from industrial scale processing, as production under inert atmosphere is not possible on this level.[3] Thus, industrial ASSB production would have to take place in dry-rooms, leading to the question whether SE handling in dry-room atmospheres containing O2 and CO2 and ppm-levels of moisture is possible.Therefore, we will examine the reactivity of the commercially available solid electrolyte Li10SnP2S12 (LSPS) with O2 and CO2 as well as with the typical H2O concentration in ambient air, followed by reactivity studies with a mixture of CO2 and H2O vapor. For this purpose, we used Diffuse Reflectance Infrared Fourier-Transform Spectroscopy (DRIFTS), a highly surface sensitive infrared technique.The hermetically sealed DRIFTS cell containing the SE powder was set up such that it could be purged with dry or humidified streams of inert gas (Ar), O2, and CO2, or mixtures thereof while monitoring in situ the chemical changes of the LSPS solid electrolyte and the formed gaseous species by DRIFTS. Figure 1 shows stacked IR spectra of LSPS after exposure to different gasses, stating no reactivity with O2 or CO2 but noticeable changes in the IR spectrum for exposure to moisture and a combination of CO2 and moisture, respectively, indicating decomposition reactions.The spectral analysis of the decomposition process is supported by ex situ X-Ray Diffractometry (XRD) analysis of the LSPS powder after exposure to the different gas streams. Concluding, the impact of exposing LSPS to the above mentioned gas mixtures on the ionic conductivity of the solid electrolyte will be demonstrated. Acknowledgements: This work was carried out as part of the research project ASSB coordinated by ZAE Bayern. The project is funded by the Bavarian Ministry of Economic Affairs, Regional Development and Energy.

PY13FSI-Infiltrated SBA-15 as Nonflammable and High Ion-Conductive Ionogel Electrolytes for Quasi-Solid-State Sodium-Ion Batteries.

Exploring electrolytes of high safety is essential to pave the practical route for sodium-ion batteries (SIBs) toward their important applications in large-scale energy storage and power supplies. In this regard, ionogel electrolytes (IEs) have been highlighted owing to their high ionic conductivity, prominent electrochemical and thermal stability, and, more crucially, high interfacial wettability. However, present studies lack an understanding of the interaction of IEs, which determines the ion desolvation and migration. In this article, IEs comprising an SBA-15 host, an ionic liquid, sodium salt, and poly(vinylidene fluoride)-hexafluoro propylene (PVDF-HFP) have been proposed by mechanical ball milling and roller pressing. The component ratio has been optimized based on the balance between ionic conductivity and self-supporting capability of IEs. The optimal IEs showed sufficiently high ionic conductivity (2.48 × 10-3 S cm-1 at 30 °C), wide electrochemical window (up to 4.8 V vs Na+/Na), and high Na+ transference number (0.37). Due to the presence of SBA-15 and an ionic liquid, the IEs exhibited much improved thermal resistance than that of the conventional organic liquid electrolytes (OLEs). Furthermore, Fourier transform infrared (FT-IR) spectroscopy revealed the hydrogen bonding interaction between silanols and the dissolved salts, not only anchoring anions for immobilization but also promoting the dissociation of sodium salts. After being matched with the Na3V2(PO4)3 (NVP) cathode and metallic Na anode, the SIBs presented a specific discharge capacity of up to 110.7 mA h g-1 initially at room temperature with 92% capacity retention after 300 cycles. The improved safety and electrochemical performance provided insights into rationally regulating IEs and their interactions with the prospect of strengthening their practical applications in SIBs.

Recent Progress in Solid Electrolytes for Energy Storage Devices

AbstractWith the rapid advances in safe, flexible, and even stretchable electronic products, it is important to develop matching energy storage devices to more effectively power them. However, the use of conventional liquid electrolytes produces volatilization and leakage that are dangerous and requires strict packaging layers that are typically rigid. To this end, solid electrolytes that can overcome these problems have attracted increasing attention in recent decades. In this review article, three main types of solid electrolytes (i.e., inorganic, polymer, and composite electrolytes) are first described and compared in terms of their structures and properties. The advantages of solid electrolytes to make safe, flexible, stretchable, wearable, and self‐healing energy storage devices, including supercapacitors and batteries, are then discussed. The remaining challenges and possible directions are finally summarized to highlight future development in this field.

Poly(ethylene disulfide)/graphene oxide nanocomposites: Dynamic-mechanical and electrochemical properties

In this work, dynamic-mechanical and electrochemical properties of polyethylene disulfide and polyethylene disulfide/graphene oxide (GO) nanocomposites are investigated to explore their possible application in rechargeable batteries. The crude polyethylene disulfide, as well as GO and sodium dodecylbenzenesulfonate (SDBS) modified-GO loaded nanocomposites are synthesized through the in situ interfacial polymerization. The GO loaded nanocomposite presents a glass transition temperature of 4.5 °C and a high storage modulus of 115 MPa at 25 °C, which is 17% and 155% higher than that for the crude polyethylene disulfide and the SDBS-modified-GO loaded nanocomposite, respectively. Although the electrical conductivity of the GO loaded nanocomposite is slightly higher than other two materials (due to the slightly higher electrical conductivity of GO nanosheets), the electrical conductivity of all polysulfide materials is very close and in the range of 10−6 and 10−4 S/m at low (10 Hz) and high frequencies (106 Hz), respectively. Notably, the polyethylene disulfide/GO nanocomposite presents a Coulombic efficiency of 97% in a lithium cell with a conventional liquid-electrolyte.

Electrochemical studies on symmetric solid-state Na-ion full cell using Na3V2(PO4)3 electrodes and polymer composite electrolyte

Ubiquitous low-cost sodium ore is abundant and increasingly of interest for sodium-based battery research for grid-scale energy storage systems. The conventional organic liquid electrolytes in currently used sodium-based batteries endure inflammability, thermal instability, and risk of safety issues. The recent trend in developing solid-state-Li batteries also thrives on exploring the all-solid-state Na battery. Herein, we report a solid-state Na-ion full cell working at room temperature using a polyvinylidene fluoride (PVDF)-based composite polymer electrolyte and NaZr2(PO4)3-structured Na3V2(PO4)3 (NVP) electrode. PVDF-based composite polymer electrolyte (CPE) with NaCF3SO3 salt and SiO2 filler displays high conductivity with electrochemical stability. The symmetric solid-state full cell delivers a specific capacity of 76 mA h g−1 at 0.5C rate, and a specific energy of 126 W h kg−1 based on the total mass of the NVP cathode only. The solid-state Na-ion full cell can retain 70% of its initial specific capacity after 100 continuous charge-discharge cycles.

Flexible, stable, fast-ion-conducting composite electrolyte composed of nanostructured Na-super-ion-conductor framework and continuous Poly(ethylene oxide) for all-solid-state Na battery

Solid-state electrolytes have attracted increasing attentions, as they can solve the safety issues related to the use of flammable liquid electrolytes. However, it is still challenging to achieve high ionic conductivity of solid-state electrolytes and to create low interface impedance between solid electrolyte and solid electrode at room temperature. Herein, we report the development of high-performance NASICON/Poly(ethylene oxide) composite electrolytes (CEs) for all-solid-state Na batteries that can be used at room temperature (23.8 °C). With the novel combination of nanostructured NASICON framework and continuous poly(ethylene oxide) filler, an outstanding room-temperature ionic conductivity of 1.4×10−4 S cm−1 is obtained. The fast Na-ion diffusion pathways at the 3D inter-connected NASICON-Poly(ethylene oxide) interfaces are visually observed for the first time via scanning probe microscopy, contributing to the superior ionic conductivity in CEs. The NASICON framework further strengthens the mechanical structure of CEs and effectively suppresses Na dendrites. Intimate and stable contact is achieved between the solid-state CEs and solid ceramic electrodes at room temperature, along with low electrolyte-electrode interface impedance and good durability, due to the high flexibility of CEs. Prototypes of all-solid-state Na batteries using the CEs are developed, demonstrating excellent electrochemical performance, which verifies that the present solid-state CEs are promising alternatives to the conventional liquid electrolytes.

A collagen-based electrolyte-locked separator enables capacitor to have high safety and ionic conductivity

The conventional liquid electrolytes (LEs) have a high level of ionic conductivity; however, they often suffer from the poor processability and safety risks of potential leakage. Although solid-state electrolytes (SSEs) can solve these inherent problems of LEs, the ionic conductivity of most SSEs is several magnitudes lower than these of LEs. Herein, we report a novel strategy by building liquid ion-transport channels in a solid framework and prepared an electrolyte-locked separator (ELS) using a collagen fiber membrane (CFm). The liquid electrolyte was primarily infiltrated in the smaller voids of CFm, and its ionic conductivity could attain to 9.0 × 10−3 S cm−1 when the electrolyte absorption (EA) reached up to 112.0%. After centrifuging treatment, the electrolyte retentions (ER) and ionic conductivities of ELS were 108.93% and 8.37 × 10−3 S cm−1, respectively, which were much higher than those of commercial cellulose separator (CS), exerting excellent liquid-locking performances. In particular, the electrical double-layer capacitors (EDLC) assembled by ELS or CS were characterized and exhibited similar electrochemical performance, demonstrating the satisfactory ability and applicability of ELS for commercial use. In addition, the ELS-based EDLC exhibited favorable flexibility with relative lower loss of capacitance under different angles of bending.

A Flexible Solid Electrolyte with Multilayer Structure for Sodium Metal Batteries

AbstractSolid electrolytes (SEs) can potentially address the inherent safety problems of conventional organic liquid electrolytes. However, their low ionic conductivity and large interfacial resistance limit the practical applications of SEs. Here, a flexible solid electrolyte with a multilayer structure is fabricated by the UV curing of an interpenetrating network of poly(ether‐acrylate) (ipn‐PEA) in the Na3Zr2Si2PO12/poly(vinylidene fluoride‐hexafluoropropylene) porous skeleton (NZSP/PVDF‐HFP), exhibiting a high Na+ transference number of 0.63 and a suitable ionic conductivity of above 10−4 S cm−1 at 60 °C. In addition, due to the unique structure of the internal rigidity and external flexibility, the composite solid electrolyte can effectively mitigate interfacial ion transfer issues while guaranteeing a certain mechanical strength, and largely inhibiting the formation of dendrite and dead sodium. The solid sodium metal batteries using Na3V2(PO4)3 (NVP) as a cathode possess a discharge capacity of 85 mA h g−1 after 100 cycles at 0.5 C, and achieve above 90% of capacity retention rate during 100 cycles at 0.1 C for Na2/3Ni1/3Mn1/3Ti1/3O2 (NTMO) at 60 °C. The flexible solid electrolyte with multilayer structure shows a great advantage for managing the ionic conductivity and interface resistance problem, suggesting a promise as a practical sodium metal battery.

A Single-Ion Conducting Borate Network Polymer as a Viable Quasi-Solid Electrolyte for Lithium Metal Batteries.

Lithium-ion batteries have remained a state-of-the-art electrochemical energy storage technology for decades now, but their energy densities are limited by electrode materials and conventional liquid electrolytes can pose significant safety concerns. Lithium metal batteries featuring Li metal anodes, solid polymer electrolytes, and high-voltage cathodes represent promising candidates for next-generation devices exhibiting improved power and safety, but such solid polymer electrolytes generally do not exhibit the required excellent electrochemical properties and thermal stability in tandem. Here, an interpenetrating network polymer with weakly coordinating anion nodes that functions as a high-performing single-ion conducting electrolyte in the presence of minimal plasticizer, with a wide electrochemical stability window, a high room-temperature conductivity of 1.5 × 10-4 S cm-1 , and exceptional selectivity for Li-ion conduction (tLi+ = 0.95) is reported. Importantly, this material is also flame retardant and highly stable in contact with lithium metal. Significantly, a lithium metal battery prototype containing this quasi-solid electrolyte is shown to outperform a conventional battery featuring a polymer electrolyte.

Initiated Chemical Vapor Deposition of Ultrathin Polymer Coatings at Graphite Electrodes for Enhanced Performance in Li-Ion Batteries

The solid-electrolyte interphases (SEIs) that form in situ at Li-ion battery electrodes play a critical role in determining cell- and system-level characteristics, but may not contribute optimized performance. Power, cycle life, and safety are improved when the electrodes are deliberately modified with nanoscale, Li+-conducting coatings. Herein, we use initiated chemical vapor deposition (iCVD) to apply ultrathin (<40 nm) polymer coatings, based on the monomer, 1,3,5-trivinyl-1,3,5-trimethylcyclotrisiloxane (V3D3), at the surfaces of powder-composite graphite electrodes. The conformal nature of the polymer deposition is verified by scanning electron microscopy (SEM), while X-ray photoelectron spectroscopy (XPS) confirms the expected chemical structure and nanoscale nature of the polymer. Poly(V3D3)-coated graphite electrodes are electrochemically evaluated in coin cells that include conventional liquid Li-ion electrolytes and LiCoO2-based cathodes. Galvanostatic tests on these cells demonstrate significant improvements in specific power, coulombic efficiency, cycle life, and tolerance to overcharge conditions vs equivalent cells containing unmodified graphite anodes, while impedance analysis shows reduced charge-transfer resistance and more consistent cell-to-cell behavior with polymer-coated graphite anodes. Finally, we use XPS with depth profiling to demonstrate that some conventional SEI products do form within/beneath the poly(V3D3) coating after extensive cycling, but in speciations such as LiF, that are favorable for electrochemical performance.

Poly(N-methylpyrrole) barrier coating and SiO2 fillers based gel electrolyte for safe and reversible Li–S batteries

Lithium-sulfur (Li–S) battery with a modified cathode, and a gel polymeric electrolyte is implemented. A poly(N-methylpyrrole) (PNMPy) coating is applied over a S@reduced graphene oxide (S@RGO) composite and a gel polymeric electrolyte composed of lithium imide, poly(methyl methacrylate), and fumed silica nanoparticles (Li+/PMMA/SiO2) substitutes the conventional liquid electrolyte in a Li–S@RGO-PNMPy cell. The PNMPy overlayer at the S@RGO composite, allows the unobstructed passage of Li-ions between the cathode and the electrolyte during charge-discharge by the virtue of a high Li-ion diffusion coefficient of ∼10−6 cm2 s−1, while it simultaneously functions as a physical barrier and efficiently restricts the polysulfide dissolution and shuttle, thus increasing the capacity retention from 13 to 40%, after 500 cycles. The Li+/PMMA/SiO2 gel is characterized by a liquid-like ionic conductivity that varies from 2.6 to 86 mS cm−1, over a temperature window of 5–75 °C and a wide electrochemical voltage stability window of ∼1–3 V versus Li/Li+, thereby enabling its use in all Li-chalcogenide batteries. Notwithstanding the higher magnitude of stable capacity at the end of 500 charge-discharge cycles of 476 mAh gSulfur−1 for the liquid electrolyte based Li–S@RGO-PNMPy cell, the analogous Li+/PMMA/SiO2 gel based cell demonstrates a superior capacity retention, a better reversibility or Coulombic efficiency response over the cycle life and is safer than its’ liquid counterpart. The simplicity of the two novel approaches relying on the abilities of the PNMPy overlayer and the Li+/PMMA/SiO2 gel in bringing about improvements in the performance of the Li–S cell with technological ramifications is a pivotal step towards developing commercial Li–S batteries.