Conventional Liquid Electrolytes Research Articles

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.

Ni‐Rich Layered Cathode Materials with Electrochemo‐Mechanically Compliant Microstructures for All‐Solid‐State Li Batteries

AbstractWhile Ni‐rich cathode materials combined with highly conductive and mechanically sinterable sulfide solid electrolytes are imperative for practical all‐solid‐state Li batteries (ASLBs), they suffer from poor performance. Moreover, the prevailing wisdom regarding the use of Li[Ni,Co,Mn]O2 in conventional liquid electrolyte cells, that is, increased capacity upon increased Ni content, at the expense of degraded cycling stability, has not been applied in ASLBs. In this work, the effect of overlooked but dominant electrochemo‐mechanical on the performance of Ni‐rich cathodes in ASLBs are elucidated by complementary analysis. While conventional Li[Ni0.80Co0.16Al0.04]O2 (NCA80) with randomly oriented grains is prone to severe particle disintegration even at the initial cycle, the radially oriented rod‐shaped grains in full‐concentration gradient Li[Ni0.75Co0.10Mn0.15]O2 (FCG75) accommodate volume changes, maintaining mechanical integrity. This accounts for their different performance in terms of reversible capacity (156 vs 196 mA h g−1), initial Coulombic efficiency (71.2 vs 84.9%), and capacity retention (46.9 vs 79.1% after 200 cycles) at 30 °C. The superior interfacial stability for FCG75/Li6PS5Cl to for NCA80/Li6PS5Cl is also probed. Finally, the reversible operation of FCG75/Li ASLBs is demonstrated. The excellent performance of FCG75 ranks at the highest level in the ASLB field.

Understanding interface stability in solid-state batteries

Solid-state batteries (SSBs) using a solid electrolyte show potential for providing improved safety as well as higher energy and power density compared with conventional Li-ion batteries. However, two critical bottlenecks remain: the development of solid electrolytes with ionic conductivities comparable to or higher than those of conventional liquid electrolytes and the creation of stable interfaces between SSB components, including the active material, solid electrolyte and conductive additives. Although the first goal has been achieved in several solid ionic conductors, the high impedance at various solid/solid interfaces remains a challenge. Recently, computational models based on ab initio calculations have successfully predicted the stability of solid electrolytes in various systems. In addition, a large amount of experimental data has been accumulated for different interfaces in SSBs. In this Review, we summarize the experimental findings for various classes of solid electrolytes and relate them to computational predictions, with the aim of providing a deeper understanding of the interfacial reactions and insight for the future design and engineering of interfaces in SSBs. We find that, in general, the electrochemical stability and interfacial reaction products can be captured with a small set of chemical and physical principles. The reliable operation of solid-state batteries requires stable or passivating interfaces between solid components. In this Review, we discuss models for interfacial reactions and relate the predictions to experimental findings, aiming to provide a deeper understanding of interface stability.

Fluorinated Ether Based Electrolyte Enabling Sodium-Metal Batteries with Exceptional Cycling Stability.

Sodium-metal batteries with conventional organic liquid electrolytes have disadvantages including dendrite deposition and safety concern. In this work, we report a low-flammable electrolyte (NaPF6-FRE) consisting of 1 M NaPF6 in 1,2-dimethoxyethane (DME), fluoroethylene carbonate (FEC), and 1,1,1,3,3,3-hexafluoroisopropylmethyl ether (HFPM) (2:1:2, in volume ratio). The symmetric Na and Na||Cu cells with a 1 M NaPF6-DME electrolyte absorbed in a porous separator, such as the porous glass-fiber, show very poor cycling performance. In addition, the cell with a Na3V2(PO4)3 (NVP) cathode and 1 M NaPF6-DME electrolyte shows low Coulombic efficiency. FEC was added into the NaPF6-DME-based electrolyte to reduce the irreversible capacity of the NVP cathode and improve the Coulombic efficiency of the cell. However, the high reactivity of FEC with the Na electrode leads to formation of an unstable solid electrolyte interphase (SEI) and large interfacial resistance, and HFPM was further added to stabilize the Na electrode surface by forming a new fluorine-containing organic layer. The new prepared low-flammable electrolyte (NaPF6-FRE) with 1 M NaPF6 in DME, FEC, and HFPM (2:1:2, in volume ratio) shows a wide electrochemical window of 5.2 V. The Na symmetric cells with this low-flammable electrolyte show superior cycling performance for 800 h with a stable voltage profile at 0.5 mA cm-2, 0.5 mA h cm-2 and 1 mA cm-2, 1 mA h cm-2, respectively. The NVP||Na cells show an excellent capacity retention of 94% after 2000 cycles and superior Coulombic efficiency of 99.9% on average at 5 C.

Nanofiber-reinforced polymer electrolytes toward room temperature solid-state lithium batteries

Safe and efficient utilization of electrochemical energy is of prime importance for e-mobility and sustainable development of the current society. Solid state batteries (SSBs) have emerged as one of the most promising solutions to address aforementioned challenges due to the replacement of conventional liquid electrolytes with inherently safer solid electrolytes. Polymer electrolyte (PE)-based SSBs have better processability and flexibility than inorganic electrolyte-based ones; however, the room temperature (RT) operation of the PE-based SSBs remains as one of the most critical issues. Herein, a nanofiber-reinforced polymer electrolyte (NRPE) comprising of poly(vinylidene fluoride) fibers along with a high molecular weight though flowable polymer matrix is proposed as an innovative electrolyte for SSBs. These NRPEs are self-standing, highly conductive, and stable against Li metal (Li°) electrode, endowing the Li° || LiFePO4 cells with good performances at operational temperatures down to RT. The outstanding physicochemical and electrochemical properties of NRPEs make them as appealing candidates for attaining high-performance SSBs.

Li0.35La0.55TiO3 Nanofibers Enhanced Poly(vinylidene fluoride)-Based Composite Polymer Electrolytes for All-Solid-State Batteries.

Using polymer electrolytes with relatively high mechanical strength, enhanced safety, and excellent flexibility to replace the conventional liquid electrolytes is an effective strategy to curb the Li-dendrite growth in Li-metal batteries (LMBs). However, low ionic conductivity, unsatisfactory thermal stability, and narrow electrochemical window still hinder their applications. Here, we fabricate Li0.35La0.55TiO3 (LLTO) nanofiber-enabled poly(vinylidene fluoride) (PVDF)-based composite polymer electrolytes (CPEs) with enhanced mechanical property and wide electrochemical window. The results show that 15 wt % of LLTO nanofibers synergize with PVDF, giving a flexible electrolyte membrane with significantly improved performance, such as high ionic conductivity (5.3 × 10-4 S cm-1), wide electrochemical window (5.1 V), high mechanical strength (stress 9.5 MPa, strain 341%), and good thermal stability (thermal degradation 410 °C). In addition, an all-solid-state Li-metal battery of sandwich-type LiFePO4/PVDF-CPE (15 wt % of LLTO)/Li delivers satisfactory cycling stability and outstanding rate performance. A reversible capacity of 121 mA h g-1 is delivered at 1 C after 100 cycles. This work exemplifies that the introduction of LLTO nanofibers can improve the electrochemical performances of PVDF-based CPEs used as electrolytes for all-solid-state LMBs.

Interfacial Reactions and Performance of Li7La3Zr2O12-Stabilized Li-Sulfur Hybrid Cell.

Herein, we report on the characterization of a Li-S hybrid cell containing a garnet solid electrolyte (Li7La3Zr2O12, LLZO) and conventional liquid electrolyte. While the liquid electrolyte provided ionically conductive pathways throughout the porous cathode, the LLZO acted as a physical barrier to protect the Li metal anode and prevent polysulfide shuttling during battery operation. This hybrid cell exhibited an initial capacity of 1000 mAh/g(S) and high Coulombic efficiency (>99%). The interface between the liquid electrolyte and LLZO was studied using electrochemical impedance spectroscopy and X-ray photoelectron spectroscopy (XPS). These results indicate that a spontaneous interfacial reaction layer formed between the LLZO and liquid electrolyte. XPS depth profiling experiments indicate that this layer consisted of Li-enriched phases near the surface (e.g., Li2CO3) and intermediate Li-La-Zr oxides in subsurface regions. The reaction layer extended well beyond the LLZO surface, and bulk pristine LLZO was not observed even at the deepest sputtering depths used in this study (∼90 nm). Overall, these results highlight that developing stable electrode/electrolyte interfaces is critical for solid-state batteries and their hybrids.

(Invited) Direct Fabrication of Cathode Layer Onto Oxide-Based Solid Electrolyte with Good Interface

All-solid-state lithium batteries with oxide-based solid electrolytes are expected as one of the next generation batteries with larger energy density and higher safety than conventional lithium-ion batteries. Among the oxide-based solid electrolytes, Li7La3Zr2O12 (LLZO) is one of the leading candidates for fabrication of the all-solid-state lithium batteries, because of high ionic conductivity of ~10−4 S cm−1 and high stability against lithium metal.[1] However, it is difficult to form an interface between the oxide-based solid electrolytes and cathode particles. In order to apply the all-solid-state lithium batteries, simple techniques for the interface formation are strongly required. We focus on two approaches. One of them is aerosol deposition (AD) method.[2] Cathode layer can be directly deposited onto the oxide-based solid electrolytes by AD method. We deposited a LiCoO2-Li3BO3 composite particles onto an Al-doped LLZO (Li6.25Al0.25La3Zr2O12, Al-LLZO) pellet. Li3BO3 is plastic material and it plays a role of binder for formation of layer. We succeeded the formation of LiCoO2-Li3BO3 layer on Al-LLZO. A test cell with LiCoO2-Li3BO3 / Al-LLZO / Li construction worked as a secondary battery. In addition, interestingly, better battery performances was obtained by annealing of the cell consisting of Al-LLZO pellet and LiCoO2-Li3BO3 cathode layer. Charge and discharge curves are shown in Figure 1a, which was measured at 0.1 C at 60 °C. The initial cycle showed great charge and discharge performance. Although the capacity gradually decreased during 100 cycles. Another technique is a quasi-solid cathode. It was prepared by mixing a conventional cathode slurry and a lithium ion conductive liquid. The slurry was directly painted onto Al-LLZO pellet. After drying, an interfacial resistance was significantly decreased. Namely, the interface between Al-LLZO and cathode layer was formed by addition of the lithium ion conductive liquid. Since the amount of lithium ion conductive liquid was small relative to the amount of cathode particles, the quasi-solid cathode did not have fluidity. We tested several kinds of lithium ion conductive liquids, i.e. conventional liquid electrolytes, solvated ionic liquids and ionic liquids with lithium salts. As a result, ionic liquid was the best additive for the quasi-solid cathode. Furthermore, we assembled a test cell consisting of LiCoO2-based quasi-solid cathode / Al-LLZO / Li. The cell worked for 100 cycles at 0.1 C under 60 °C with good cycling performance, as shown in Figure 1b. Development of simple techniques for the interface formation is very important to fabricate all solid state batteries. Both methods proposed in this study are not so difficult. It can be said that they are promising techniques to establish the practical application of all-solid-state lithium batteries. [1] R. Murugan, V. Thangadurai, W. Weppner, Angew. Chem. Int. Ed., 2007, 46, 7778–7781. [2] S. Iwasaki, T. Hamanaka, T. Yamakawa, W. C. West, K. Yamamoto, M. Motoyama, T. Hirayama, Y. Iriyama, J. Power Sources, 2014, 272, 1086–1090. Figure 1

(Invited) Keeping It Simple: Anode Free Lithium Metal Cells with Liquid Electrolytes As a Path to Higher Energy Density

In the pursuit of higher energy density, both academia and industry are intensely researching many alternative rechargeable battery concepts beyond conventional lithium-ion cells. The most promising alternatives will be those that can achieve cell level energy density larger than that of state-of-the-art lithium ion batteries at a comparable or lower cost. Based on these criteria, anode free lithium metal cells emerge as potentially useful lithium-ion alternatives as their simple cell construction can enable both high energy density and low cost assembly. In anode free cells, the negative electrode begins as a bare copper current collector and the lithium metal anode is plated in-situ from the lithium inventory in the cathode. These cells can achieve energy density 40-50% greater than lithium-ion at the cell level and have a simpler cell construction that eliminates the need to handle lithium metal foils or coat active materials on the negative current collector. However, the cycling stability of these cells in conventional carbonate liquid electrolytes is often terrible due to the low CE of lithium metal and the lack of an excess lithium reservoir. In this way, anode free cells exaggerate the technical challenges of lithium metal which makes them excellent test vehicles for optimizing lithium metal cycling. In this presentation we will highlight our progress in improving lithium metal cycling efficiency in liquid electrolytes toward the development of safe, practical, high energy density anode free cells. Much of this work focuses on the development and characterization of anode free LiNi0.5Mn0.3Co0.2O2 (NMC 532)//Cu wound pouch cells. We demonstrate that the cycling stability and safety of these cells is directly correlated to the morphology of the lithium plated during charge. High surface area mossy lithium deposits in conventional liquid electrolytes lead to fast capacity fade and high anode reactivity. We will also show how many different factors including, cycling rate, charge protocol, applied pressure, temperature, and electrolyte composition affect lithium morphology. Our latest results will be highlighted which show that with the appropriate electrolyte chemistry and cycling conditions, smooth lithium morphology with densely packed columns 50 um in diameter can be achieved after cycling in liquid electrolytes. This excellent lithium morphology significantly improves the capacity retention (Figure 1) of anode free cells and limits the reactivity of the deposited lithium. Cell failure mechanisms will also be addressed with insight as to how these failures can be overcome to further improve cycle life. These results demonstrates that stable cycling of dendrite free lithium metal is possible in simple anode free cells with liquid electrolytes which represents a promising and practical avenue for the design of high energy density batteries. Figure 1

Highly Safe and High Energy Density Lithium-Ion Battery Using Less Volatile Electrolyte

Lithium-ion batteries (LIBs) are now used in the variety of fields including vehicles, and industrial and portable uses. Since the current LIBs use volatile and flammable electrolyte liquid, they have a risk on safety, e.g. explosion owing to an unexpected short. For ensuring the LIB safety, the cooling and safety mechanisms need to be installed in battery packs used for large scale applications, i.e. electric vehicles (EVs), and they compromise battery energy density and cost competitiveness. To overcome the issue related to the LIB safety, we recently developed a novel electrolyte having the lower vapor pressure than the conventional organic liquid electrolyte, and incorporated it into LIB. High energy density of over 300 Wh/kg and high output power of over 1,600 W/kg were successfully demonstrated. In the presention, the concept of our less volatile electrolyte, and evaluated results on performance of our highly safe, high energy density and high power LIB will be reported. This work was supported by Innovative Science and Technology Initiative for Security, ATLA, Japan.

Plastic Crystal Polymer Electrolytes with Anion-Trapping Boron Moieties for Room Temperature All-Solid-State Sodium Batteries

Sodium-ion batteries (SIBs) have been considered as promising alternatives to lithium-ion batteries (LIBs) particularly in large scale electric energy storage applications for renewable energy and grid due to low cost and the huge abundance of sodium resources. However, increasing safety issues caused by the use of conventional organic liquid electrolytes (LEs) greatly hinder the commercial application of SIBs in electric vehicles or hybrid electric vehicles [1]. Replacing LEs with solid electrolytes (SEs) can inherently address such safety problems. However, it is a compelling challenge to develop SEs with superior room-temperature ionic conductivity, high ion transference number, considerable mechanical property, and low electrolyte/electrode interfacial resistance [2]. Recently, we developed a new type of nonwoven supported plastic crystal polymer electrolyte containing anion-trapping boron moieties (B-PCPE), which prepared by in situ growth method using UV-curing technique. In addition, the plastic crystal electrolyte modified electrodes were also designed to reduce interfacial resistance. The B-PCPE shows excellent room temperature ionic conductivity (0.36 mS/cm) and greatly enhanced sodium transference number (0.62) due to the presence of anion-trapping boron moieties. All-solid-state SIB assembled using the B-PCPE with anion acceptor exhibits excellent cycle performance and rate capability at room temperature. The cell delivers first discharge capacity of 104.8 mAh/g with Coulombic efficiency of 94.1% and the capacity retention is about 80.1% after 120 cycles at 0.1 C. We believe this B-PCPE is a promising candidate for practical application and our work provides a new perspective to design high-performance solid electrolyte for ambient-temperature all-solid-state batteries with exceptional rate capability and cycle stability.

(Invited) Progress in Solid State Ion Conductors- Have You Made Any Significant Impact?

Currently, solid state (ceramic) ion conductors are being considered to replace conventional liquid and gel electrolytes in numerous electrochemical energy technologies such as batteries due to their high voltage and chemical-thermal stabilities. There are several family of inorganic crystalline and amorphous compounds discovered as fast ion conductors since 1950. However, their application in modern devices are limited today. Only a few solid ion conductors have been used in commercial electrochemical technologies. This talk will report milestones in solid state ionics, including Professors Huggins’s key contributions.

Enhanced Performance and Safety in Intermediate Temperature Lithium-Sulfur Batteries

Due to the high theoretical capacity and energy density, lithium-sulfur is considered one of the most promising battery chemistries. Difficulties with passivation and polysulfide shuttle have traditionally plagued the lithium anode and sulfur cathode, respectively, in conventional liquid electrolyte batteries. Solid-state electrolytes, such as lithium garnets, can largely address these issues as the dense ceramic layer helps block polysulfide shuttle and are stable to metallic lithium. However, poor electron transfer kinetics resulting from high electronic resistance and large volumetric changes of sulfur limit the development of solid-state Li-S batteries. At intermediate temperatures, the diffusivity of sulfur is thermally activated, facilitating electron transfer. Here we mimic encapsulated sulfur through infiltration of sulfur, carbon, and a series of binders into one side of a porous-dense-porous garnet structure. The garnet provides ionic conduction through the cell while helping to entrap sulfur with aid of binder. Cell cycling was performed to determine the effect of binder on specific capacity and capacity fade. Electrochemical impedance spectroscopy was used to elucidate the changes occurring within the cathode. While the pore size is larger than what is used for most encapsulated sulfur cathodes, thermal activation allows certain binder systems enhanced performance. In addition to conductive benefits, this large-scale enclosure of sulfur helps avoid corrosion issues possible with sulfur at elevated temperature.

Remarkable Conductivity of a Self-Healing Single-Ion Conducting Polymer Electrolyte, Poly(ethylene-co-acrylic lithium (fluoro sulfonyl)imide), for All-Solid-State Li-Ion Batteries.

Single-ion conducting polymer electrolyte (SICPE) is a safer alternative to the conventional high-performance liquid electrolyte for Li-ion batteries. The performance of SICPEs-based Li-ion batteries is limited due to the low Li+ conductivities of SICPEs at room temperature. Herein, we demonstrated the synthesis of a novel SICPE, poly(ethylene-co-acrylic lithium (fluoro sulfonyl)imide) (PEALiFSI), with acrylic (fluoro sulfonyl)imide anion (AFSI). The solvent- and plasticizer-free PEALiFSI electrolyte, which was assembled at 90 °C under pressure, exhibited self-healing properties with remarkably high Li+ conductivity (5.84 × 10-4 S cm-1 at 25 °C). This is mainly due to the self-healing behavior of this electrolyte, which induced to increase the proportion of the amorphous phase. Additionally, the weak interaction of Li+ with the resonance-stabilized AFSI anion is also responsible for high Li+ conductivity. This self-healed SICPE showed high Li+ transference number (ca. 0.91), flame and heat retardancy, and good thermal stability, which concurrently delivered ca. 88.25% (150 mAh g-1 at 0.1C) of the theoretical capacitance of LiFePO4 cathode material at 25 °C with the full-cell configuration of LiFePO4/PEALiFSI/graphite. Furthermore, the self-healed PEALiFSI-based all-solid-state Li battery showed high electrochemical cycling stability with the capacity retention of 95% after 500 charge-discharge cycles.

High-Voltage Sulfolane Plasticized UV-Curable Gel Polymer Electrolyte.

A high-voltage electrolyte can match high-voltage positive electrode material to fully exert its capacity. In this research, a sulfolane plasticized polymer electrolyte was prepared by in situ photocuring. First, the effect of the sulfolane content on the ionic conductivity of the gel polymer electrolyte was investigated. Results showed that the ionic conductivity variation trend was in good agreement with the exponential function model for curve fitting. Second, the activation energy was calculated from the results of the variable temperature conductivity tests. The activation energy was inversely proportional to the sulfolane content. For the sulfolane content of 80 wt. % in gel polymer electrolyte (GPE)-80 (19.5 kJ/mol), the activation energy was close to conventional liquid electrolyte (9.5 kJ/mol), and the conductivity and electrochemical window were 0.64 mS/cm and 5.86 V, respectively. The battery cycle performance test showed that the initial specific discharge capacities of GPE-80 and liquid electrolyte were 176.8 and 148.3 mAh/g, respectively. After 80 cycles, the discharge capacities of GPE-80 and liquid electrolyte were 115.8 and 41.1 mAh/g, and the capacity retention rates were 65.5% and 27.7%, respectively; indicating that GPE-80 has a better specific discharge capacity and cycling performance than the liquid electrolyte. SEM images indicated that GPE-80 can suppress the growth of lithium dendrites. The EDS test showed that GPE-80 can inhibit the dissolution of metal ions in the cathode material.

Characterizing the Li-Solid-Electrolyte Interface Dynamics as a Function of Stack Pressure and Current Density

Summary The replacement of conventional liquid electrolytes with solid electrolytes has the potential to safely enable energy-dense Li-metal anodes. Because of the challenges surrounding solid-solid interfaces, it is crucial to better understand the Li-metal-solid-electrolyte interface. This work utilizes stack pressure to correlate mechanics with the electrochemical behavior of Li-electrolyte cells during galvanostatic cycling. Symmetric cells are constructed using Li7La3Zr2O12 and tested using AC and DC techniques under dynamic stack pressure conditions. It is demonstrated that significant polarization occurs during galvanostatic cycling at a current-dependent “critical stack pressure.” Using reference electrodes, this effect is isolated to the Li stripping electrode. This suggests that at low pressures, the Li stripping rate exceeds the rate at which mechanical deformation replenishes the interface, inducing the formation of voids and ultimately increasing resistance. This analysis not only motivates the need for further understanding of the Li-metal-solid-electrolyte interface but also provides guidelines for the future design of all-solid-state batteries.

Mo3Nb2O14: A high-rate intercalation electrode material for Li-ion batteries with liquid and garnet based hybrid solid electrolytes

Herein we report Mo3Nb2O14 possessing a layered open framework structure as potential candidate for Li-ion batteries with conventional liquid electrolyte as well as with a hybrid solid electrolyte (garnet oxide-ionic liquid). The material delivers a discharge capacity of 329 mA h g−1 at 1C rate in a liquid electrolyte-based cell, corresponding to insertion of 8.5 Li. Moreover, it exhibits excellent rate capability (up to 50C rate) and good capacity retention. Considering the high capacity and high rate capabilities of the material we further investigate its performance in presence of garnet based quasi (hybrid) solid electrolyte owing to the attractive features of solid-state batteries. When hybrid solid electrolyte is used, the electrode material delivers an initial discharge capacity of 206 mA h g−1 at high current rate of 1C and exhibits excellent capacity retention of 165 mA h g−1 (80%) at the end of 100 charge-discharge cycles. The ultra-fast Li+ insertion/extraction can be attributed to the three-dimensional channel structure of the Mo3Nb2O14 which allows facile lithium ion transport. Hence, this work supports the idea of revisiting layered type metal oxides as electrodes for developing high energy density and safe lithium ion batteries.

Bicontinuously crosslinked polymer electrolyte membranes with high ion conductivity and mechanical strength

Recently, safety has become an important issue for energy storage devices due to explosion and leakage of conventional liquid electrolytes. Traditional polymer electrolytes also have a strong dependence on the tradeoff relationship and do not exhibit sufficient ionic conductivity as a solid; thus, they are often used as a gel. We report the preparation of a highly ion-conductive, mechanically strong solid electrolyte membranes based on bicontinuously crosslinked polymer electrolytes for solid-state supercapacitors. The solid electrolyte membranes are synthesized in situ via facile ring-opening polymerization in the presence of a conductive ionic liquid without any solvents. The crosslinking structure is based on the reaction of the amines of the polyethylenimine oligomer with the epoxy groups of hydrophilic poly(ethylene glycol) diglycidyl ether and hydrophobic bisphenol A diglycidyl ether at 50 °C for 3 h. This unique structure endows the solid electrolyte membranes with high tensile strength as well as high ion conductivity (2.41 × 10−3 S cm−1 at 25 °C). All the membranes have good thermal stability and an amorphous phase in which the ionic species are uniformly distributed in the matrix. Upon application to solid-state supercapacitors based on carbon electrodes, a capacitance of 19.8 F g−1 at 2 mV s−1 is obtained, with an excellent capacitance retention and a coulombic efficiency close to 100% for 10,000 cycles. The solid electrolyte membrane has potential applications in structural supercapacitors and batteries, which require high mechanical strength.

A Novel Material Class of Solid-State Nano-Composite Electrolytes with High Li-Ion Conductivity

All-solid-state Li-ion cells are expected to become the next major cell technology. Besides the fact that they are intrinsically safer compared to today’s cells with a flammable liquid organic electrolyte, a solid-state electrolyte also enables the introduction of a metallic lithium anode, resulting in cell energy densities of >1000 Wh/L. This is significantly higher compared to the practical limit of 800 Wh/L of the liquid Li-ion cell technology. Applying metallic lithium electrodes with a liquid electrolyte is challenging due to the likelihood of dendrite formation during cell charging (i.e. during lithium plating) [1]. It is expected that a solid-state electrolyte is a better candidate to deal with the lithium dendrite formation problem, as it forms a physical barrier for the dendrites to grow [2]. For many years, it was considered practically impossible to engineer solid-state materials having ionic conductivities that are equally high and surpassing those of the liquid electrolytes, as ion diffusion in solids is typically much slower compared to diffusion in liquids. A high Li-ion conductivity through the electrolyte is essential for the Li-ion cells to be charged and discharged at high rate. We here present a novel class of nano-composite electrolytes (nano-SCEs) for which ion conductivity values are obtained higher than those seen in conventional liquid electrolytes. These nano-SCEs constitute a nano-porous SiO2 monolith that encloses an adsorbed ionic liquid electrolyte. Different from to so-called ionogel electrolytes – which follow a similar approach in that they confine an ILE in a porous oxide matrix [3] – our electrolytes feature the unique property that the ionic conductivity of the composite is higher than to that of the ionic liquid electrolyte [4]. The so-called enhancement factor reaches values of > 200 %, as shown in the Figure 1. The high ion conductivity and enhancement factor were established by engineering the surface chemistry in the nano-SCE [4]. Along the pore walls of the oxide matrix, the ionic liquid electrolyte molecules are organizing themselves such that “highway” conduction paths are created for fast Li-ion transport. The exact nature of these highway conduction paths strongly depends on the chemistry of the ionic liquid electrolyte. Figure 1 show the influence of the ILE chemistry on the nano-SCE enhancement factor. Interestingly, the enhancement factor increases with increasing ILE viscosity. In conclusion, a new material class of composite electrolytes is presented having the unique feature of enhanced ion conductivity. The conduction promotion arises from the chemical interaction between the ILE and the nano-porous SiO2 matrix. Furthermore, the enhancement factor can be controlled by engineering the ILE viscosity.

Mutifunctional Epoxy-Based Solid Polymer Electrolytes for Energy Storage Systems

For the development of next-generation safe energy storage (batteries or supercapacitors), energy conversion (solar cells), and electromechanical transduction devices (ionic actuators), solid polymer electrolytes (SPEs) having good electrochemical stability are widely considered as promising materials to substitute for conventional organic liquid electrolytes that have flammability and explosion issues. The SPEs could also offer a perfect solution to the enhancement of energy or power density, due to the possibility to use lithium metal anode that has a high theoretical specific capacity, and would even enable their use in multifunctional wearable or structural energy storage systems for portable electronics or transport applications. However, the key challenge facing the development of the SPEs for energy storage applications is to achieve high mechanical performance without sacrificing the requisite ionic conductivity: increasing conductivity typically leads to a reduction of the modulus. We prepare cross-linkable epoxy-based networked SPEs including Li salts (LiTFSI) with either plastic crystals (PCs), tetraglymes (G4s) or ionic liquids (ILs). The epoxy resins are particularly attractive as polymer matrices for solid polymer electrolytes due to their high mechanical performance combined with good adhesive properties and corrosion resistance. The selected electrolyte components (PC, G4 or IL) are allowed to boost ionic conductivity owing to solvating the lithium cation and plasticizing the epoxy matrix. As a result, the thermal curing of a homogeneous mixture of epoxy and electrolyte components can generate a two-phase system in which the epoxy phase is selected to provide mechanical strength and the electrolyte phase is selected to maximize ionic conductivity, via polymerization-induced phase separation, allowing for a bicontinuous microphase separation morphology. To further improve the physical properties of the SPEs, inorganic nanoparticles such TiO2, SiO2, or Al2O3 are incorporated in the nanocomposite SPEs, allowing for the combination of the advantages of both inorganic materials and organic polymers. This can be the strategy to achieve the simultaneous improvement in both mechanical strength and ion conduction of nanocomposite SPEs. In the current investigation, we conduct an investigation of the effect of electrolyte and inorganic types and their concentration on the ion conduction and dielectric and viscoelastic response of epoxy-based networked SPEs to investigate their ion and polymer dynamics. We measure storage and loss moduli, viscosity, ionic conductivity, dielectric constant, Young’s modulus of a series of epoxy-based SPEs, where the concentration of epoxy resin, electrolyte, and inorganic filler is varied, using oscillatory and steady shear, dielectric relaxation spectroscopy, and dynamic mechanical analyzer, respectively. These results are complemented by morphological studies from FESEM, microstructural and physical interaction studies from Fourier transform infrared (FTIR) spectroscopy, intermolecular interaction from Density functional theory (DFT) calculations, and thermal properties from DSC. Our study leads to insight regarding optimal design of multifunctional solid polymer electrolytes for energy storage devices.