Tuning ion conducting pathways using holographic polymerization

Abuse-Tolerant Electrolytes for Lithium-Ion Batteries

Safety issues currently limit the development of advanced lithium-ion batteries (LIBs) and this is exacerbated when they are misused or abused. The addition of small amounts of fillers or additives into common liquid electrolytes can greatly improve resistance to abuse without impairing electrochemical performance. This review discusses the recent progress in such abuse-tolerant electrolytes. It covers electrolytes with shear thickening properties for tolerating mechanical abuse, electrolytes with redox shuttle additives for suppressing electrochemical abuse, and electrolytes with flame-retardant additives for resisting thermal abuse. It aims to provide insights into the functioning of such electrolytes and the understanding of electrolyte composition-property relationship. Future perspectives, challenges, and opportunities towards practical applications are also presented.

Nanophase-Separated, Elastic Epoxy Composite Thin Film as an Electrolyte for Stable Lithium Metal Batteries

The design of solid polymer electrolytes (SPE) with high ionic conductivity and excellent mechanical properties is challenging because these two properties are often conflicting. To achieve both, a reaction-controlled strategy is proposed based on the nanophase separation of an ionic transport pathway and a supporting matrix to balance ionic mobility and mechanical properties. Specifically, an elastic epoxy polymer electrolyte (eEPE), synthesized via two-step polymerization, combines outstanding mechanical strength (toughness of 3.4 MJ m^-3) and high ionic conductivity (3.5 × 10^-4 S cm^-1 at 25 °C). The nanostructured eEPE is both tough and flexible, therefore promotes uniform deposition of Li even under a high current density (2 mA cm^-2 and 2 mAh cm^-2). Importantly, eEPE composite films greatly improve the safety performance of the LiFePO₄/Li pouch cells: safe operations are achieved under several abusive conditions. This work highlights an alternative route for high-safety solid-state lithium metal batteries of the next generation.

Designing Comb-Chain Crosslinker-Based Solid Polymer Electrolytes for Additive-Free All-Solid-State Lithium Metal Batteries

Developing solid polymer electrolytes (SPEs) is a promising approach to realize practical dendrite-free lithium metal batteries (LMBs). Tuning the nanoscale polymer network chemsitry is of critical importance for SPE design. In this work, we took lessons from the rubber chemistry and developed a series of comb-chain crosslinker-based SPEs (ConSPEs) using a preformed polymer as the multifunctional crosslinker. The high-functionality crosslinker increased the connectivity of nanosized cross-linked domains, which led to a robust network with dramatically improved toughness and superior lithium dendrite resistance even at a current density of 2 mA cm^-2. The uniform and flexile network also dramatically improved the anodic stability to over 5.3 V versus Li/Li⁺. Additive-free, all-solid-state LMBs with the ConSPE showed high discharge capacity and stable cycling up to 10 C rate, and could be stably cycled at 25 °C. Our results demonstrated that ConSPEs are promising for high-performance and dendrite-free LMBs.

Monochromatic "Photoinitibitor"-Mediated Holographic Photopolymer Electrolytes for Lithium-Ion Batteries

A new polymer electrolyte based on holographic photopolymer is designed and fabricated. Ethylene carbonate (EC) and propylene carbonate (PC) are introduced as the photoinert substances. Upon laser-interference-pattern illumination, photopolymerization occurs within the constructive regions which subsequently results in a phase separation between the photogenerated polymer and unreacted EC-PC, affording holographic photopolymer electrolytes (HPEs) with a pitch of ≈740 nm. Interestingly, both diffraction efficiency and ionic conductivity increase with an augmentation of the EC-PC content. With 50 wt% of EC-PC, the diffraction efficiency and ionic conductivity are ≈60% and 2.13 × 10-4 S cm-1 at 30 °C, respectively, which are 60 times and 5 orders of magnitude larger than the electrolyte without EC-PC. Notably, the HPEs afford better anisotropy and more stable electrochemical properties when incorporating N,N-dimethylacrylamide. The HPEs exhibit good toughness under bending, excellent optical transparency under ambient conditions, and astonishing capabilities of reconstructing colored images. The HPEs here open a door to design flexible and transparent electronics with good mechanical, electrical, and optical functions.

Aligned Ionogel Electrolytes for High-Temperature Supercapacitors

Ionogels are a new class of promising materials for use in all-solid-state energy storage devices in which they can function as an integrated separator and electrolyte. However, their performance is limited by the presence of a crosslinking polymer, which is needed to improve the mechanical properties, but compromises their ionic conductivity. Here, directional freezing is used followed by a solvent replacement method to prepare aligned nanocomposite ionogels which exhibit enhanced ionic conductivity, good mechanical strength, and thermal stability simultaneously. The aligned ionogel based supercapacitor achieves a 29% higher specific capacitance (176 F g-1 at 25 °C and 1 A g-1) than an equivalent nonaligned form. Notably, this thermally stable aligned ionogel has a high ionic conductivity of 22.1 mS cm-1 and achieves a high specific capacitance of 167 F g-1 at 10 A g-1 and 200 °C. Furthermore, the diffusion simulations conducted on 3D reconstructed tomography images are employed to explain the improved conductivity in the relevant direction of the aligned structure compared to the nonaligned. This work demonstrates the synthesis, analysis, and use of aligned ionogels as supercapacitor separators and electrolytes, representing a promising direction for the development of wearable electronics coupled with image based process and simulations.

2D MXene-containing polymer electrolytes for all-solid-state lithium metal batteries

Nanocomposite polymer electrolytes (CPEs) are promising materials for all-solid-state lithium metal batteries (LMBs) due to their enhanced ionic conductivities and stability to the lithium anode. MXenes are a new two-dimensional, 2D, family of early transition metal carbides and nitrides, which have a high aspect ratio and a hydrophilic surface. Herein, using a green, facile aqueous solution blending method, we uniformly dispersed small amounts of Ti₃C₂T _x into a poly(ethylene oxide)/LiTFSI complex (PEO₂₀-LiTFSI) to fabricate MXene-based CPEs (MCPEs). The addition of the 2D flakes to PEO simultaneously retards PEO crystallization and enhances its segmental motion. Compared to the 0D and 1D nanofillers, MXenes show higher efficiency in ionic conductivity enhancement and improvement in the performance of LMBs. The CPE with 3.6 wt% MXene shows the highest ionic conductivity at room temperature (2.2 × 10^-5 S m^-1 at 28 °C). An LMB using MCPE with only 1.5 wt% MXene shows rate capability and stability comparable with that of the state-of-the-art CPELMBs. We attribute the excellent performance to the 2D geometry of the filler, the good dispersion of the flakes in the polymer matrix, and the functional group-rich surface.

Anisotropic charge transport in ion-conductive photoresponsive polyethylene oxide-based mesomorphic materials

The mechanism of charge motion in conductive and photosensitive mesogenic block copolymers containing polyethylene oxide (PEO) segments is investigated over a wide frequency and temperature range with the broadband dielectric spectroscopy technique. It is found that the ultraviolet (UV) irradiation, the UV intensity, and the anchoring conditions of mesogenic unit in the cells produce changes in conductivity properties and in the molecular arrangement. The anisotropic nature of the conductivity is established.

Photoinitiation and Inhibition under Monochromatic Green Light for Storage of Colored 3D Images in Holographic Polymer-Dispersed Liquid Crystals

Holographic photopolymer composites have garnered a great deal of interest in recent decades, not only because of their advantageous light sensitivity but also due to their attractive capabilities of realizing high capacity three-dimensional (3D) data storage that is long-term stable within two-dimensional (2D) thin films. For achieving high performance holographic photopolymer composites, it is of critical importance to implement precisely spatiotemporal control over the photopolymerization kinetics and gelation during holographic recording. Though a monochromatic blue light photoinitibitor has been demonstrated to be useful for improving the holographic performance, it is impractical to be employed for constructing holograms under green light due to the severe restriction of the First Law of Photochemistry, while holography under green light is highly desirable considering the relatively low cost of laser source and high tolerance to ambient vibration for image reconstruction. Herein, we disclose the concurrent photoinitiation and inhibition functions of the rose bengal (RB)/N-phenylglycine (NPG) system upon green light illumination, which result in significant enhancement of the diffraction efficiency of holographic polymer-dispersed liquid crystal (HPDLC) gratings from zero up to 87.6 ± 1.3%, with an augmentation of the RB concentration from 0.06 × 10^-3 to 9.41 × 10^-3 mol L^-1. Interestingly, no detectable variation of the ϕ^1/2k_p/k_t^1/2, which reflects the initiation efficiency and kinetic constants, is given when increasing the RB concentration. The radical inhibition by RBH^• is believed to account for the greatly improved phase separation and enhanced diffraction efficiency, through shortening the weight-average polymer chain length and subsequently delaying the photopolymerization gelation. The reconstructed colored 3D images that are easily identifiable to the naked eye under white light demonstrate great potential to be applied for advanced anticounterfeiting.

Mechanism for Conducting Filament Growth in Self-Assembled Polymer Thin Films for Redox-Based Atomic Switches

The switching mechanisms of atomic switches based on poly(ethylene oxide) (PEO) are systematically investigated. By using self-assembled PEO and Ag-PEO thin films, stack-structured devices exhibit stable bipolar switching behavior over 10(3) cycles. Direct observation of filament growth behavior in planar-structured devices reveals the effects of the polymer thin-film morphology, and the presence of electrochemically active electrodes, on the switching characteristics.

Hybrid electrolytes with controlled network structures for lithium metal batteries

Solid polymer electrolytes (SPEs) with tunable network structures are prepared by a facile one-pot reaction of polyhedral oligomeric silsesquioxane and poly(ethylene glycol). These SPEs, with high conductivity and high modulus, exhibit superior resistance to lithium dendrite growth even at high current densities. Measurements of lithium metal batteries with a LiFePO4 cathode show excellent cycling stability and rate capability.

Ionic conductivity enhancement of polymer electrolytes with ceramic nanowire fillers

Solid-state electrolytes provide substantial improvements to safety and electrochemical stability in lithium-ion batteries when compared with conventional liquid electrolytes, which makes them a promising alternative technology for next-generation high-energy batteries. Currently, the low mobility of lithium ions in solid electrolytes limits their practical application. The ongoing research over the past few decades on dispersing of ceramic nanoparticles into polymer matrix has been proved effective to enhance ionic conductivity although it is challenging to form the efficiency networks of ionic conduction with nanoparticles. In this work, we first report that ceramic nanowire fillers can facilitate formation of such ionic conduction networks in polymer-based solid electrolyte to enhance its ionic conductivity by three orders of magnitude. Polyacrylonitrile-LiClO4 incorporated with 15 wt % Li0.33La0.557TiO3 nanowire composite electrolyte exhibits an unprecedented ionic conductivity of 2.4 × 10(-4) S cm(-1) at room temperature, which is attributed to the fast ion transport on the surfaces of ceramic nanowires acting as conductive network in the polymer matrix. In addition, the ceramic-nanowire filled composite polymer electrolyte shows an enlarged electrochemical stability window in comparison to the one without fillers. The discovery in the present work paves the way for the design of solid ion electrolytes with superior performance.

Anisotropic ion transport in nanostructured solid polymer electrolytes

We discuss recent progresses on anisotropic ion transport in solid polymer electrolytes.