Concentration Polarization of Salt‐Containing Liquid Electrolytes

Using the example of Li‐battery electrolytes, the importance of the concept of conservative ensembles for polarization behavior and transference measurements of salt‐containing liquid electrolytes is stressed. The conventional evaluation of the stationary values fails if the ion pair is mobile and can act as a vehicle of a single ion such as Li⁺. The necessary generalization is considered. While the analytical form of the time dependence of voltage or current is not affected provided the ion pairing is sufficiently fast, the diffusion coefficient contains nontrivial extra contributions. Finally, soggy‐sand electrolytes are inspected, in which polarization occurs in the space‐charge zones.     

3D Ion-Conducting,Scalable, and Mechanically Reinforced Ceramic Film for High Voltage Solid-State Batteries

Concerning the safety aspects of Li⁺ ion batteries, an epoxy-reinforced thin ceramic film (ERTCF) is prepared by firing and sintering a slurry-casted composite powder film. The ERTCF is composed of Li⁺ ion conduction channels and is made of high amounts of sintered ceramic Li_1+xTi_2-xAl_x(PO₄)₃ (LATP) and epoxy polymer with enhanced mechanical properties for solid-state batteries. The 2D and 3D characterizations are conducted not only for showing continuous Li⁺ ion channels thorough LATP ceramic channels with over 10⁻⁴ S cm⁻¹ of ionic conductivity but also to investigate small amounts of epoxy polymer with enhanced mechanical properties. Solid-state Li⁺ ion cells are fabricated using the ERTCF and they show initial charge–discharge capacities of 139/133 mAh g⁻¹. Furthermore, the scope of the ERTCF is expanded to high-voltage (>8 V) solid-state Li⁺ ion batteries through a bipolar stacked cell design. Hence, it is expected that the present investigation will significantly contribute in the preparation of the next generation reinforced thin ceramic film electrolytes for high-voltage solid-state batteries.     

Unprecedented Improvement of Single Li‐Ion Conductive Solid Polymer Electrolyte Through Salt Additive

Solid‐state lithium metal (Li°) batteries (SSLMBs) are believed to be the most promising technologies to tackle the safety concerns and the insufficient energy density encountered in conventional Li‐ion batteries. Solid polymer electrolytes (SPEs) inherently own good processability and flexibility, enabling large‐scale preparation of SSLMBs. To minimize the growth of Li° dendrites and cell polarization in SPE‐based SSLMBs, an additive‐containing single Li‐ion conductive SPE is reported. The characterization results show that a small dose of electrolyte additive (2 wt%) substantially increases the ionic conductivity of single Li‐ion conductive SPEs as well as the interfacial compatibility between electrode and SPE, allowing the cycling of SPE‐based cells with good electrochemical performance. This work may provide a paradigm shift on the design of highly cationic conductive electrolytes, which are essential for developing safe and high‐performance rechargeable batteries.     

Decomposition of Trace Li2CO3 During Charging Leads to Cathode Interface Degradation with the Solid Electrolyte LLZO

A major challenge for lithium-containing electrochemical systems is the formation of lithium carbonates. Solid-state electrolytes circumvent the use of organic liquids that can generate these species, but they are still susceptible to Li₂CO₃ formation from exposure to water vapor and carbon dioxide. It is reported here that trace quantities of Li₂CO₃, which are re-formed following standard mitigation and handling procedures, can decompose at high charging potentials and degrade the electrolyte–cathode interface. Operando electrochemical mass spectrometry (EC–MS) is employed to monitor the outgassing of solid-state batteries containing the garnet electrolyte Li₇La₃Zr₂O₁₂ (LLZO) and using appropriate controls CO₂ and O₂ are identified to emanate from the electrolyte–cathode interface at charging potentials > 3.8 V (vs Li/Li⁺). The gas evolution is correlated with a large increase in cathode interfacial resistance observed by potential-resolved impedance spectroscopy. This is the first evidence of electrochemical decomposition of interfacial Li₂CO₃ in garnet cells and suggests a need to report “time-to-assembly” for cell preparation methods.     

Amine‐Functionalized Boron Nitride Nanosheets: A New Functional Additive for Robust,Flexible Ion Gel Electrolyte with High Lithium‐Ion Transference Number

Ion gel electrolytes show great potential in solid‐state batteries attributed to their outstanding characteristics. However, because of the strong ionic nature of ionic liquids, ion gel electrolytes generally exhibit low lithium‐ion transference number, limiting its practical application. Amine‐functionalized boron nitride (BN) nanosheets (AFBNNSs) are used as an additive into ion gel electrolytes for improving their ion transport properties. The AFBNNSs‐ion gel shows much improved mechanical strength and thermal stability. The lithium‐ion transference number is increased from 0.12 to 0.23 due to AFBNNS addition. More importantly, for the first time, nuclear magnetic resonance analysis reveals that the amine groups on the BN nanosheets have strong interaction with the bis(trifluoromethanesulfonyl)imide anions, which significantly reduces the anion mobility and consequently increases lithium‐ion mobility. Battery cells using the optimized AFBNNSs‐ion gel electrolyte exhibit stable lithium deposition and excellent electrochemical performance. A LiFePO₄|Li cell retains 92.2% of its initial specific capacity after the 60th cycle while the cell without AFBNNSs‐gel electrolyte only retains 53.5%. The results not only demonstrate a new strategy to improve lithium‐ion transference number in ionic liquid electrolytes, but also open up a potential avenue to achieve solid‐state lithium metal batteries with improved performance.     

High Energy Density All‐Solid‐State Batteries: A Challenging Concept Towards 3D Integration

Rechargeable all‐solid‐state batteries will play a key role in many autonomous devices. Planar solid‐state thin film batteries are rapidly emerging but reveal several drawbacks, such as a relatively low energy density and the use of highly reactive metallic lithium. In order to overcome these limitations a new 3D‐integrated all‐solid‐state battery concept with significantly increased surface area is presented. By depositing the active battery materials into high‐aspect ratio structures etched in, for example silicon, 3D‐integrated all‐solid‐state batteries are calculated to reach a much higher energy density. Additionally, by adopting novel high‐energy dense Li‐intercalation materials the use of metallic Lithium can be avoided. Sputtered Ta, TaN and TiN films have been investigated as potential Li‐diffusion barrier materials. TiN combines a very low response towards ionic Lithium and a high electronic conductivity. Additionally, thin film poly‐Si anodes have been electrochemically characterized with respect to their thermodynamic and kinetic Li‐intercalation properties and cycle life. The Butler‐Vollmer relationship was successfully applied, indicating favorable electrochemical charge transfer kinetics and solid‐state diffusion. Advantageously, these new Li‐intercalation anode materials were found to combine an extremely high energy density with fast rate capability, enabling future 3D‐integrated all‐solid‐state batteries.     

Liquid‐Crystalline Electrolytes for Lithium‐Ion Batteries: Ordered Assemblies of a Mesogen‐Containing Carbonate and a Lithium Salt

Thermotropic liquid‐crystalline (LC) electrolytes for lithium‐ion batteries are developed for the first time. A rod‐like LC molecule having a cyclic carbonate moiety is used to form self‐assembled two‐dimensional ion‐conductive pathways with lithium salts. Electrochemical and thermal stability, and efficient ionic conduction is achieved for the liquid crystal. The mixture of the carbonate derivative and lithium bis(trifluoromethylsulfonyl)imide is successfully applied as an electrolyte in lithium‐ion batteries. Reversible charge–discharge for both positive and negative electrodes is observed for the lithium‐ion batteries composed of the LC electrolyte.     

Flexible Stable Solid‐State Al‐Ion Batteries

Rechargeable aluminum‐ion batteries (AIBs) are regarded as promising candidates for post‐lithium energy storage systems (ESSs). For addressing the critical issues in the current liquid AIB systems, here a flexible solid‐state AIB is established using a gel‐polymer electrolyte for achieving robust electrode–electrolyte interfaces. Different from utilization of solid‐state systems for alleviating the safety issues and enhancing energy density in lithium‐ion batteries, employment of polymeric electrolytes mainly focuses on addressing the essential problems in the liquid AIBs, including unstable internal interfaces induced by mechanical deformation and production of gases as well as unfavorable separators. Particularly, such gel electrolyte enables the solid‐state AIBs to present an ultra‐fast charge capability within 10 s at current density of 600 mA g^?1. Meanwhile, an impressive specific capacity ≈120 mA h g^?1 is obtained at current density of 60 mA g^?1, approaching the theoretical limit of graphite‐based AIBs. In addition to the well‐retained electrochemical performance below the ice point, the solid‐state AIBs also hold great stability and safety under various critical conditions. The results suggest that such new prototype of solid‐state AIBs with robust electrode–electrolyte interfaces promises a novel strategy for fabricating stable and safe flexible ESSs.     

Solid‐State Plastic Crystal Electrolytes: Effective Protection Interlayers for Sulfide‐Based All‐Solid‐State Lithium Metal Batteries

All‐solid‐state lithium metal batteries (ASSLMBs) have attracted significant attention due to their superior safety and high energy density. However, little success has been made in adopting Li metal anodes in sulfide electrolyte (SE)‐based ASSLMBs. The main challenges are the remarkable interfacial reactions and Li dendrite formation between Li metal and SEs. In this work, a solid‐state plastic crystal electrolyte (PCE) is engineered as an interlayer in SE‐based ASSLMBs. It is demonstrated that the PCE interlayer can prevent the interfacial reactions and lithium dendrite formation between SEs and Li metal. As a result, ASSLMBs with LiFePO₄ exhibit a high initial capacity of 148 mAh g^?1 at 0.1 C and 131 mAh g^?1 at 0.5 C (1 C = 170 mA g^?1), which remains at 122 mAh g^?1 after 120 cycles at 0.5 C. All‐solid‐state Li‐S batteries based on the polyacrylonitrile‐sulfur composite are also demonstrated, showing an initial capacity of 1682 mAh g^?1. The second discharge capacity of 890 mAh g^?1 keeps at 775 mAh g^?1 after 100 cycles. This work provides a new avenue to address the interfacial challenges between Li metal and SEs, enabling the successful adoption of Li metal in SE‐based ASSLMBs with high energy density.     

Isolation of Solid Solution Phases in Size‐Controlled LixFePO4 at Room Temperature

State‐of‐the‐art LiFePO₄ technology has now opened the door for lithium ion batteries to take their place in large‐scale applications such as plug‐in hybrid vehicles. A high level of safety, significant cost reduction, and huge power generation are on the verge of being guaranteed for the most advanced energy storage system. The room‐temperature phase diagram is essential to understand the facile electrode reaction of Li_xFePO₄ (0 < x < 1), but it has not been fully understood. Here, intermediate solid solution phases close to x = 0 and x = 1 have been isolated at room temperature. Size‐dependent modification of the phase diagram, as well as the systematic variation of lattice parameters inside the solid‐solution compositional domain closely related to the electrochemical redox potential, are demonstrated. These experimental results reveal that the excess capacity that has been observed above and below the two‐phase equilibrium potential is largely due to the bulk solid solution, and thus support the size‐dependent miscibility gap model.     

Materials Research Advances towards High-Capacity Battery/Fuel Cell Devices

The world has entered an era featured with fast transportations, instant communications, and prompt technological revolutions, the further advancement of which all relies fundamentally, yet, on the development of cost-effective energy resources allowing for durable and high-rate energy supply. Current battery and fuel cell systems are challenged by a few issues characterized either by insufficient energy capacity or by operation instability and, thus, are not ideal for such highly-demanded applications as electrical vehicles and portable electronic devices. In this mini-review, we present, from materials perspectives, a few selected important breakthroughs in energy resources employed in these applications. Prospectives are then given to look towards future research activities for seeking viable materials solutions for addressing the capacity, durability, and cost shortcomings associated with current battery/fuel cell devices.     

Engineering Solid Electrolyte Interphase for Pseudocapacitive Anatase TiO2 Anodes in Sodium‐Ion Batteries

Anatase TiO₂ is considered as one of the promising anodes for sodium‐ion batteries because of its large sodium storage capacities with potentially low cost. However, the precise reaction mechanisms and the interplay between surface properties and electrochemical performance are still not elucidated. Using multimethod analyses, it is herein demonstrated that the TiO₂ electrode undergoes amorphization during the first sodiation and the amorphous phase exhibits pseudocapacitive sodium storage behaviors in subsequent cycles. It is also shown that the pseudocapacitive sodium storage performance is sensitive to the nature of solid electrolyte interphase (SEI) layers. For the first time, it is found that ether‐based electrolytes enable the formation of thin (≈2.5 nm) and robust SEI layers, in contrast to the thick (≈10 nm) and growing SEI from conventional carbonate‐based electrolytes. First principle calculations suggest that the higher lowest unoccupied molecular orbital energies of ether solvents/ion complexes are responsible for the difference. TiO₂ electrodes in ether‐based electrolyte present an impressive capacity of 192 mAh g^?1 at 0.1 A g^?1 after 500 cycles, much higher than that in carbonate‐based electrolyte. This work offers the clarified picture of electrochemical sodiation mechanisms of anatase TiO₂ and guides on strategies about interfacial control for high performance anodes.     

Intercalated Electrolyte with High Transference Number for Dendrite‐Free Solid‐State Lithium Batteries

Solid‐state lithium (Li) batteries using solid electrolytes and Li anodes are highly desirable because of their high energy densities and intrinsic safety. However, low ambient‐temperature conductivity and poor interface compatibility of solid electrolytes as well as Li dendrite formation cause large polarization and poor cycling stability. Herein, a high transference number intercalated composite solid electrolyte (CSE) is prepared by the combination of a solution‐casting and hot‐pressing method using layered lithium montmorillonite, poly(ethylene carbonate), lithium bis(fluorosulfonyl)imide, high‐voltage fluoroethylene carbonate additive, and poly(tetrafluoroethylene) binder. The electrolyte presents high ionic conductivity (3.5 × 10^?4 S cm^?1), a wide electrochemical window (4.6 V vs Li⁺/Li), and high ionic transference number (0.83) at 25 °C. In addition, a 3D Li anode is also fabricated via a facile thermal infusion strategy. The synergistic effect of high transference number intercalated electrolyte and 3D Li anode is more favorable to suppress Li dendrites in a working battery. The solid‐state batteries based on LiFePO₄ (Al₂O₃ @ LiNi_0.5Co_0.2Mn_0.3O₂), CSE, and 3D Li deliver admirable cycling stability with discharge capacity 145.9 mAh g^?1 (150.7 mAh g^?1) and capacity retention 91.9% after 200 cycles at 0.5 C (92.0% after 100 cycles at 0.2 C) at 25 °C. This work affords a splendid strategy for high‐performance solid‐state battery.     

Reversible Lithium Storage with High Mobility at Structural Defects in Amorphous Molybdenum Dioxide Electrode

This work demonstrates that structural defects in amorphous metal oxide electrodes can serve as a reversible Li⁺ storage site for lithium secondary batteries. For instance, molybdenum dioxide electrode in amorphous form (a‐MoO₂) exhibits an unexpectedly high Li⁺ storage capacity (up to four Li per MoO₂ unit), which is larger by a factor of four than that for the crystalline counterpart. The conversion‐type lithiation is discarded for this electrode from the absence of Mo metal and lithium oxide (Li₂O) in the lithiated a‐MoO₂ electrode and the retention of local structural framework. The sloping voltage profile in a wide potential range suggests that Li⁺ ions are inserted into the structural defects that are electrochemically nonequivalent. This electrode also shows an excellent cycle stability and rate capability. The latter feature is seemingly due to a rather opened Li⁺ diffusion pathway provided by the structural defects. A high Li⁺ mobility is confirmed from nuclear magnetic resonance study.     

Janus‐Faced,Dual‐Conductive/Chemically Active Battery Separator Membranes

Battery separators are supposed to be electrical insulators to prevent internal short‐circuit failure between electrodes as well as having porous channels to allow ion transport. Here, as a multifunctional membrane strategy to dispel this stereotypical belief about battery separators, a new class of Janus‐faced, dual (ion/electron)‐conductive/chemically active battery separators (denoted as “Janus separators”) based on a heterolayered nanofiber mat architecture is demonstrated. The Janus separator, which is fabricated through in‐series, concurrent electrospraying/electrospinning processes, consists of an ion‐conductive/metal ion‐chelating support layer (a mat of densely packed, thiol‐functionalized silica particles spatially besieged by polyvinylpyrrolidone/polyacrylonitrile nanofibers) and a dual‐conductive top layer (a thin mat of polyetherimide nanofibers wrapped with multi‐walled carbon nanotubes). The support layer acts as a chemical trap that can capture heavy metal ions dissolved in liquid electrolytes and the top layer serves as an upper current collector for cathodes to boost the redox reaction kinetics. Notably, the unusual porous microstructure of the top layer is theoretically elucidated using molecular dynamics simulation. Benefiting from such material/structural uniqueness, the Janus separator enables significant improvements in fast‐rate charge/discharge reactions (even for high‐mass loading cathodes) and in the high‐temperature cycling performance, which lie far beyond those achievable with conventional polyethylene separators.     

Reversible Conversion Reactions and Small First Cycle Irreversible Capacity Loss in Metal Sulfide‐Based Electrodes Enabled by Solid Electrolytes

Solid‐state batteries can potentially enable new classes of electrode materials which are unstable against liquid electrolytes. Here, SnS nanocrystals, synthesized by a wet chemical method, are used to fabricate a Li‐ion electrode, and the electrochemical properties of this electrode are examined in both solid and liquid electrolyte designs. The SnS‐based solid‐state cell delivers a capacity of 629 mAh g^?1 after 100 cycles and exhibits an unprecedentedly small irreversible capacity in the first cycle (8.2%), while the SnS‐based liquid cell shows a rapid capacity decay and large first cycle irreversible capacity (44.6%). Cyclic voltammetry (CV) experiments show significant solid electrolyte interphase (SEI) formation in the liquid cell during the first discharge while SEI formation by electrolyte reduction in the solid‐state cell appears negligible. Along with CV, X‐ray photoelectron spectroscopy and energy dispersive spectroscopy are used to investigate the differences between the solid‐state and liquid cells. The reaction chemistry of SnS in solid‐state cells is also studied in detail by ex situ X‐ray diffraction and X‐ray absorption spectroscopy. The overarching findings are that use of a solid electrolyte suppresses materials degradation and electrolyte reduction which leads to a small first cycle irreversible capacity and stable cycling.     

Remarkably Improved Electrode Performance of Bulk MnS by Forming a Solid Solution with FeS – Understanding the Li Storage Mechanism

Transition metal compounds based on conversion reactions are promising electrode materials for lithium‐ion batteries due to their higher lithium storage capacity compared with currently available commercial battery electrodes. Most of the studies on these materials in the literature focus on transition metal oxides and fluorides, and not much work on transition metal sulphides has been reported, partially due to their relatively poor electrochemical performance. Here, synthesis and characterization of a series of solid solution Fe_xMn_1‐xS (x = 0.2, 0.5, 0.8) monosulphide compounds is reported. Interestingly, hexagonal FeS and cubic MnS can form a solid solution of Fe_xMn_1‐xS (x < 0.57). It is demonstrated that the lithium storage voltage can be tuned by changing the Fe concentration in the Fe_xMn_1‐xS matrix; meanwhile, the discharge‐charge coulombic efficiency and cycle stability of Fe_xMn_1‐xS are greatly enhanced in comparison with that of pure MnS. A half cell using Fe_0.5Mn_0.5S as electrode material achieves a high first cycle coulombic efficiency of 78.0% and a high reversible capacity of ca. 477 mAh g^?1 after 35 cycles, while for pure MnS the first cycle coulombic efficiency is only 45.9% and the capacity rapidly fades to ≈200 mAh g^?1 after 15 cycles. Although the solid solution state of Fe_0.5Mn_0.5S cannot be retained during conversion reaction as indicated by X‐ray diffraction (XRD), X‐ray absorption spectroscopy (XAS), and transmission electron microscopy (TEM), the initial discharge “polarization”, which has been considered as one of the major hurdles for conversion reaction, can be significantly reduced by this type of material design. In addition, the size and distribution of the nucleated nanophases might also be altered by the initial solid solution state of Fe_0.5Mn_0.5S, contributing to the improved electrochemical performance reported here.