Complex Hydride-Based Gel Polymer Electrolytes for Rechargeable Ca-Metal Batteries.

Rechargeable Ca batteries offer the advantages of high energy density, low cost, and earth-abundant constituents, presenting a viable alternative to lithium-ion batteries. However, using polymer electrolytes in practical Ca batteries is not often reported, despite its potential to prevent leakage and preserve battery flexibility. Herein, a Ca(BH4)2-based gel-polymer electrolyte (GPE) is prepared from Ca(BH4)2 and poly(tetrahydrofuran) (pTHF) and tested its performance in Ca batteries. The electrolyte demonstrates excellent stability against Ca-metal anodes and high ionic conductivity. The results of infrared spectroscopy and 1H and 11B NMR indicate that the terminal ─OH groups of pTHF reacted with BH4 - anions to form B─H─(pTHF)3 moieties, achieving cross-linking and solidification. Cyclic voltammetry measurements indicate the occurrence of reversible Ca plating/stripping. To improve the performance at high current densities, the GPE is supplemented with LiBH4 to achieve a lower overpotential in the Ca plating/stripping process. An all-solid-state Ca-metal battery with a dual-cation (Ca2+ and Li+) GPE, a Ca-metal anode, and a Li4Ti5O12 cathode sustained >200 cycles, confirming their feasibility. The results pave the way for further developing lithium salt-free Ca batteries by developing electrolyte salts with high oxidation stability and optimal electrochemical properties.

Understanding Ion Dynamics in Closoborate-Type Lithium-Ion Conductors on Different Time-Scales.

The lithium-ion transport mechanism in 0.7Li(CB9H10)-0.3Li(CB11H12) complex hydride solid electrolyte was studied over a wide time-scale (ns-ms) by choosing appropriate techniques for assessing ionic motion on the desired time-scale using nuclear magnetic resonance (NMR) relaxation, AC impedance, and pulsed field gradient-NMR (PFG-NMR) measurements. The 7Li NMR line width decreased with increasing temperature, and the spin-lattice relaxation time T1 for the cation and anions showed a minimum near 303 K, indicating that the lithium ions and the anions were highly mobile. The activation energy estimated from the analysis of the NMR relaxation time matched well with the values estimated from the AC impedance and PFG-NMR. This confirms that the lithium-ion motion in 0.7Li(CB9H10)-0.3Li(CB11H12) is the same over a wide time-scale, suggesting steady Li-ion motion over a wide transport range. This understanding offers insights into strategies for designing complex hydride lithium superionic conductors.

Stereolithography 3D Printed Carbon Microlattices with Hierarchical Porosity for Structural and Functional Applications.

Hierarchically porous carbon microlattices (HPCMLs) fabricated by using a composite photoresin and stereolithography (SLA) 3D printing is reported. Containing magnesium oxide nanoparticles (MgO NPs) as porogens and multilayer graphene nanosheets as UV-scattering inhibitors, the composite photoresin is formed to simple cubic microlattices with digitally designed porosity of 50%. After carbonization in vacuum at 1000 °C and chemical removal of MgO NPs, it is realized that carbon microlattices possessing hierarchical porosity are composed of the lattice architecture (≈100 µm), macropores (≈5 µm), mesopores (≈50 nm), and micropores (≈1 nm). The linear shrinkage after pyrolysis is as small as 33%. Compressive strength of 7.45 to 10.45 MPa and Young's modulus of 375 to 736 MPa are achieved, proving HPCMLs a robust mechanical component among reported carbon materials with a random pore structure. Having a few millimeters in thickness, the HPCMLs can serve as thick supercapacitor electrodes that demonstrate gravimetric capacitances 105 and 13.8 F g-1 in aqueous and organic electrolyte, reaching footprint areal capacitances beyond 10 and 1 F cm-2 , respectively. The results present that the composite photoresin for SLA can yield carbon microarchitectures that integrate structural and functional properties for structural energy storages .

Calcium Metal Batteries with Long Cycle Life Using a Hydride-Based Electrolyte and Copper Sulfide Electrode.

As potential alternatives to Li-ion batteries, rechargeable Ca metal batteries offer advantageous features such as high energy density, cost-effectiveness, and natural elemental abundance. However, challenges, such as Ca metal passivation by electrolytes and a lack of cathode materials with efficient Ca2+ storage capabilities, impede the development of practical Ca metal batteries. To overcome these limitations, the applicability of a CuS cathode in Ca metal batteries and its electrochemical properties are verified herein. Ex situ spectroscopy and electron microscopy results show that a CuS cathode comprising nanoparticles that are well dispersed in a high-surface-area carbon matrix can serve as an effective cathode for Ca2+ storage via the conversion reaction. This optimally functioning cathode is coupled with a tailored, weakly coordinating monocarborane-anion electrolyte, namely, Ca(CB11 H12 )2 in 1,2-dimethoxyethane/tetrahydrofuran, which enables reversible Ca plating/stripping at room temperature. The combination affords a Ca metal battery with a long cycle life of over 500 cycles and capacity retention of 92% based on the capacity of the 10th cycle. This study confirms the feasibility of the long-term operation of Ca metal anodes and can expedite the development of Ca metal batteries.

Topological Data analysis of Ion Migration Mechanism.

Topological data analysis based on persistent homology has been applied to the molecular dynamics simulation for the fast ion-conducting phase (α-phase) of AgI to show its effectiveness on the ion migration mechanism analysis. Time-averaged persistence diagrams of α-AgI, which quantitatively record the shape and size of the ring structures in the given atomic configurations, clearly showed the emergence of the four-membered rings formed by two Ag and two I ions at high temperatures. They were identified as common structures during the Ag ion migration. The averaged potential energy change due to the deformation of the four-membered ring during Ag migration agrees well with the activation energy calculated from the conductivity Arrhenius plot. The concerted motion of two Ag ions via the four-membered ring was also successfully extracted from molecular dynamics simulations by our approach, providing new insight into the specific mechanism of the concerted motion.

Increasing the ionic conductivity and lithium-ion transport of photo-cross-linked polymer with hexagonal arranged porous film hybrids.

High ionic conductivity, suitable mechanical strength, and electrochemical stability are the main requirements for high-performance poly(ethylene oxide)-based electrolytes. However, the low ionic conductivity owing to the crystallinity of the ethylene oxide chain that limits the discharge rate and low-temperature performance has restricted the development and commercialization of these electrolytes. Lithium electrolytes that combine high ionic conductivity with a high lithium transference number are rare and are essential for high-power batteries. Here, we report hexagonal arranged porous scaffolds for holding prototype polyethylene glycol-based composite electrolytes containing solvate ionic liquid. The appealing electrochemical and thermal properties indicate their potential as electrolytes for safer rechargeable lithium-ion batteries. The porous scaffolds in the composite electrolytes ensure better electrochemical performance towing to their shortened pores (sizes of 3-14 µm), interconnected pathways, and improved lithium mobility. We demonstrate that both molecular design and porous microstructures are essential for improving performance in polymer electrolytes.

Monocarborane cluster as a stable fluorine-free calcium battery electrolyte.

High-energy-density and low-cost calcium (Ca) batteries have been proposed as 'beyond-Li-ion' electrochemical energy storage devices. However, they have seen limited progress due to challenges associated with developing electrolytes showing reductive/oxidative stabilities and high ionic conductivities. This paper describes a calcium monocarborane cluster salt in a mixed solvent as a Ca-battery electrolyte with high anodic stability (up to 4 V vs. Ca2+/Ca), high ionic conductivity (4 mS cm-1), and high Coulombic efficiency for Ca plating/stripping at room temperature. The developed electrolyte is a promising candidate for use in room-temperature rechargeable Ca batteries.

Unveiling Pseudocapacitive Charge Storage Behavior in FeWO4 Electrode Material by Operando X-Ray Absorption Spectroscopy.

In nanosized FeWO4 electrode material, both Fe and W metal cations are suspected to be involved in the fast and reversible Faradaic surface reactions giving rise to its pseudocapacitive signature. In order to fully understand the charge storage mechanism, a deeper insight into the involvement of the electroactive cations still has to be provided. The present paper illustrates how operando X-ray absorption spectroscopy is successfully used to collect data of unprecedented quality allowing to elucidate the complex electrochemical behavior of this multicationic pseudocapacitive material. Moreover, these in-depth experiments are obtained in real time upon cycling the electrode, which allows investigating the reactions occurring in the material within a realistic timescale, which is compatible with electrochemical capacitors practical operation. Both Fe K-edge and W L3 -edge measurements point out the involvement of the Fe3+ /Fe2+ redox couple in the charge storage while W6+ acts as a spectator cation. The result of this study enables to unambiguously discriminate between the Faradaic and capacitive behavior of FeWO4 . Beside these valuable insights toward the full description of the charge storage mechanism in FeWO4 , this paper demonstrates the potential of operando X-ray absorption spectroscopy to enable a better material engineering for new high capacitance pseudocapacitive materials.

Enhanced Electrochemical Performance of Ultracentrifugation-Derived nc-Li3VO4/MWCNT Composites for Hybrid Supercapacitors.

Nanocrystalline Li3VO4 dispersed within multiwalled carbon nanotubes (MWCNTs) was prepared using an ultracentrifugation (uc) process and electrochemically characterized in Li-containing electrolyte. When charged and discharged down to 0.1 V vs Li, the material reached 330 mAh g(-1) (per composite) at an average voltage of about 1.0 V vs Li, with more than 50% capacity retention at a high current density of 20 A g(-1). This current corresponds to a nearly 500C rate (7.2 s) for a porous carbon electrode normally used in electric double-layer capacitor devices (1C = 40 mA g(-1) per activated carbon). The irreversible structure transformation during the first lithiation, assimilated as an activation process, was elucidated by careful investigation of in operando X-ray diffraction and X-ray absorption fine structure measurements. The activation process switches the reaction mechanism from a slow "two-phase" to a fast "solid-solution" in a limited voltage range (2.5-0.76 V vs Li), still keeping the capacity as high as 115 mAh g(-1) (per composite). The uc-Li3VO4 composite operated in this potential range after the activation process allows fast Li(+) intercalation/deintercalation with a small voltage hysteresis, leading to higher energy efficiency. It offers a promising alternative to replace high-rate Li4Ti5O12 electrodes in hybrid supercapacitor applications.