Rethinking the Design of Ionic Conductors Using Meyer?Neldel?Conductivity Plot
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Gao, Yirong↗
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Anisotropic ionic transportation performances of (100) and (112) planes in MAPbI3 single crystal
Highlights: • Anisotropic ion transportations of MAPbI{sub 3} are studied. • Photon excited ion transportation is investigated. • Linear increase trends with incident light power of ion transportation are observed. • Enhanced electron and ion transportations have been observed on (112) plane of MAPbI{sub 3}. • (112) plane of MAPbI{sub 3} shows more active transportation properties than (100). - Abstract: Organic–Inorganic hybrid perovskites attract intensive attentions due to significant optoelectronic performances. However, abnormal phenomena such as photocurrent hysteresis and slow photoconductivity response caused by ion migration made ionic transportation properties to be a hot topic in this area. Thinking of serious influences such as efficiency decay of excited carrier separation and poor reliability are caused by ion migration, systematical research on this topic is still highly demanded. Here, anisotropic ionic transportation performances of (100) and (112) planes in MAPbI{sub 3} single crystal have been investigated with various photon excitation conditions. Enhanced ionic conductance had been extracted on (112) plane both with and without illuminations. Comparing to exponential saturation revealed by the electrons, the ionic conductance exhibits a linear increase trends with incident light power in MAPbI{sub 3}.
36 MATERIALS SCIENCE↗
Transport and mechanical behavior in PEO-LLZO composite electrolytes
Abstract Composite solid electrolytes (CEs), wherein ion-conducting polymer and ceramic/glass is mixed, are promising candidates for all-solid-state batteries due to their promise of acceptable ionic conductivity and mechanical properties compared to their individual constituents. While numerous studies have focused on improving the performance of CEs, it is still unclear what the material targets are that can result in improved macroscopic performance especially in light of the coupled needs for high transport and high mechanical strength in these materials. In this study, a two-dimensional (2D) mathematical model is developed to investigate electrochemical and mechanical characteristics of CEs. The model is compared to CEs consisting of poly-ethylene-oxide (PEO) polymer and lithium lanthanum zirconium oxide (LLZO) ceramic material with examination of the impact of varying LLZO volume fractions. The potential drop at the PEO-LLZO interface is evaluated using the junction potential theory. Using experimental data from the literature, the model estimates the ionic conductivity, effective transference number, and mechanical stiffness of the CEs. While the mechanical stiffness improves with increasing volume fraction of LLZO, the impact on conductivity and transference number depends on interfacial resistance at the interface. Finally, the study reports CE’s potential to enhance Li-ion transport and mechanical properties to inhibit lithium (Li) dendrite growth.
25 ENERGY STORAGE↗
Enhanced Ion Transport and Molecular Packing Stability in Asymmetric 2D Nanostructured π‐Conjugated Thieno[3,2‐b]Thiophene‐Based Liquid Crystal
Organic semiconductors based on liquid crystal (LC) molecules have attracted increasing interest. Here, in this work, two linear LCs based on 2,5‐bis(thien‐2‐yl)thieno[3,2‐b]thiophene (BTTT) mesogen are designed and synthesized, including BTTT/dEO3 with two symmetrically attached tri(ethylene oxide) groups and BTTT/mEO6 with one asymmetrically attached hexa(ethylene oxide) group. These two molecules have comparable functional‐group compositions but different molecular geometries, leading to their moderately different material performances. Both LCs show smectic mesophases with relatively low transition temperatures as confirmed by differential scanning calorimetry and polarized optical microscopy. A combination of experimental grazing incidence wide‐angle X‐ray scattering and molecular dynamics (MD) simulations reveals a herringbone packing motif of BTTT segments in both LCs while a smaller molecular tilt angle in BTTT/mEO6. Ionic conductivities are measured by doping LCs with different amounts of ionic dopants, lithium bis(trifluoromethanesulfonyl)imide (LiTFSI). BTTT/mEO6 shows better smectic phase stability to higher LiTFSI doping ratios. Both LCs exhibit similar ionic conductivities in the smectic phases, but BTTT/mEO6 outperforms BTTT/dEO3 by a factor of three in the amorphous phase at higher temperatures. MD simulations, performed to examine the ion solvation environment, reveal that BTTT/mEO6 is more efficient in coordinating Li‐ions and screening their interactions with TFSI‐ions which further promote ionic transport.
grazing incidence wide-angle X-ray scattering↗
Critical role of acceptor dopants in designing highly stable and compatible proton-conducting electrolytes for reversible solid oxide cells
Proton-conducting electrolytes are receiving increasing attention due to their high ionic conductivity at intermediate temperatures, enabling the operation of solid oxide cells with high energy efficiency at low cost. However, the effect of B-site dopants on the properties of doped barium hafnate-cerate electrolyte materials, especially in single cells under operating conditions, has not been systematically studied. Here we report our findings in the development of a series of proton-conducting electrolytes with a general formula of BaHf 0.1 Ce 0.7 R 0.2 O 3–δ (BHCR172, R = Yb, Er, Y, Gd, Sm). Here, the results reveal that electrical conductivity, ionic transference number, chemical stability against steam and CO 2 , and compatibility with NiO during sintering are all closely correlated with the dopant size. In particular, the reaction with NiO is found to strongly affect the properties of the electrolytes and hence cell performance. Among all tested compositions, BaHf 0.1 Ce 0.7 Yb 0.2 O 3–δ (BHCYb172) shows excellent chemical stability and minimal reactivity towards NiO, as predicted from density functional theory (DFT)-based calculations and confirmed by experimental results. In addition, proton-conducting reversible solid oxide cells (P-ReSOCs) based on the optimized electrolyte composition, BHCYb172, demonstrate exceptional performance and stability, achieving a remarkable peak power density of 1.74 W cm –2 (O 2 as the oxidant) at 600 °C in the fuel cell mode and a high current density of 2.0 A cm –2 at 1.3 V and 600 °C in the steam electrolysis mode while maintaining excellent durability for over 1000 h.
25 ENERGY STORAGE↗
Physically Based Modeling of PEMFC Cathode Catalyst Layers: Effective Microstructure and Ionomer Structure–Property Relationship Impacts
Abstract This work presents a pseudo-two-dimensional proton exchange membrane fuel cell (PEMFC) model incorporating Nafion ionomer structure–property relationships in the cathode catalyst layer (CL) to capture and explain losses at low Pt loading. Structural data from neutron reflectometry and thin film Nafion conductivity measurements predict variations in the oxygen diffusion coefficient and ionic conductivity with changing CL ionomer thickness and Pt loading. By including these structure–property relationships, predicted polarization curves agree closely with previously published experimental data from cells with Pt loadings between 0.025 and 0.2 mg/cm2. Results demonstrate that structure–property relationships based on physically measurable ionomer and CL properties provide a feasible interpretation of PEMFC CL phenomena for a range of Pt loadings and help explain previously unaccounted-for losses at low Pt. Results also show that simulations must account for surface species coverage variations in order to properly capture the kinetic losses. Finally, results suggest that an increase in ionomer thickness surrounding the C/Pt surfaces may lead to improved cell performance due to improved ionic conductivity.
Electrochemistry↗
Transition from Vehicular to Structural Ionic Transport in Electrified Alkali Aqueous Solutions
A molecular understanding of the solvation and dynamics of ions under static electric fields is crucial for modelling a wide range of natural and technological processes. Yet, traditional simulation methods suffer from a trade-off that has to be made between accuracy and statistical convergence. To bridge this gap, herein we extend our recently introduced Perturbed Neural Network Potential Molecular Dynamics (PNNP MD) approach to investigate the solvation structures and ionic transport mechanisms of electrified alkali cationic solutions. We obtain ionic conductivities for Li+, Na+ and Cs+ from the field dependence of the ionic current density in good agreement with experiment. Surprisingly, the migration mechanism is found to be strikingly different for the three ions despite their similar ionic conductivities. While Li+ conducts predominantly through vehicular migration of a stable 4-fold coordinated ion at all field strengths, Cs+ conducts strictly through a structural diffusion mechanism, where 9-12 transient first shell water coordination bonds are broken and renewed. Notably, aqueous Na+ emerges as a “Goldilocks” ion: its ion-water interactions are strong enough to maintain distinct 5-6 fold coordination shells at zero field (unlike Cs+) yet labile enough to be strongly perturbed by electric fields (unlike Li+). As a consequence, we observe an electric field-induced transition from vehicular to structural ionic transport for Na+ that is accompanied by a marked increase in the ionic current density. Our results imply that the conductance mechanism of ions with moderate ion-solvent interactions can be effectively tuned by external electric fields.
Joll, Kit↗
Poly(vinylferrocene) as an Ionomer and Sulfur-Confining Additive for Lithium–Sulfur Batteries
Lithium-sulfur (Li-S) batteries are promising owing to their high energy density, environmental benignity, and low cost. Most of the commonly used binders in Li-S battery cathodes are inert and have no significant effect on the challenges of sulfur as a cathode material, such as the polysulfide shuttle effect, low ionic/electronic conductivity, and a sluggish redox reaction. In this work, we demonstrate the use of poly(vinylferrocene) (PvFc) as a binder additive that can effectively increase the ionic conductivity of the cathode and act as a sulfide confining agent. Electrochemical tests performed with PvFc as part of the binder mixture used to cast the cathodes demonstrate an increase in rate capability and cycle life when compared to the baseline samples. Ionic conductivity measurements and X-ray photoelectron spectroscopy suggest that the π-cation molecular interaction between the cyclopentadienyl rings from ferrocene and Li + act like an ion couple with ferrocene acting as a static, covalently bound acceptor of Li + ions that enhances their mobility through the cathode. This coupled with the affinity between the ferrocenyl cations and the Li salt anions, which provides more distribution of counterions for Li + movement and improves accessibility to the cathode S reservoir, make PvFc a promising ionomer for Li-S batteries. In addition, the π-cation bonds between cyclopentadienyl and lithium polysulfides produce a sulfide confining effect that mitigates capacity fade through polysulfide dissolution. Furthermore, this work demonstrates an expansion in the utility of PvFc as a component in Li-ion batteries, which so far is mostly limited to use as an active material in organometallic batteries.
25 ENERGY STORAGE↗
Dynamic Monkey Bar Mechanism of Superionic Li‐ion Transport in LiTaCl 6
Abstract The LiTaCl 6 solid electrolyte has the lowest activation energy of ionic conduction at ambient conditions (0.165 eV), with a record high ionic conductivity for a ternary compound (11 mS cm −1 ). However, the mechanism has been unclear. We train machine‐learning force fields (MLFF) on ab initio molecular dynamics (AIMD) data on‐the‐fly and perform MLFF MD simulations of AIMD quality up to the nanosecond scale at the experimental temperatures, which allows us to predict accurate activation energy for Li‐ion diffusion (at 0.164 eV). Detailed analyses of trajectories and vibrational density of states show that the large‐amplitude vibrations of Cl − ions in TaCl 6 − enable the fast Li‐ion transport by allowing dynamic breaking and reforming of Li−Cl bonds across the space in between the TaCl 6 − octahedra. We term this process the dynamic‐monkey‐bar mechanism of superionic Li + transport which could aid the development of new solid electrolytes for all‐solid‐state lithium batteries.
Lei, Ming↗
Dynamic Monkey Bar Mechanism of Superionic Li–ion Transport in LiTaCl 6
Here, the LiTaCl 6 solid electrolyte has the lowest activation energy of ionic conduction at ambient conditions (0.165 eV), with a record high ionic conductivity for a ternary compound (11 mS cm –1 ). However, the mechanism has been unclear. We train machine-learning force fields (MLFF) on ab initio molecular dynamics (AIMD) data on-the-fly and perform MLFF MD simulations of AIMD quality up to the nanosecond scale at the experimental temperatures, which allows us to predict accurate activation energy for Li-ion diffusion (at 0.164 eV). Detailed analyses of trajectories and vibrational density of states show that the large-amplitude vibrations of Cl – ions in TaCl 6 – enable the fast Li-ion transport by allowing dynamic breaking and reforming of Li–Cl bonds across the space in between the TaCl 6 – octahedra. We term this process the dynamic-monkey-bar mechanism of superionic Li + transport which could aid the development of new solid electrolytes for all-solid-state lithium batteries.
25 ENERGY STORAGE↗
Correlating Transport and Structural Properties in Li 1+ x Al x Ge 2– x (PO 4 ) 3 (LAGP) Prepared from Aqueous Solution
Li 1+ x Al x Ge 2– x (PO 4 ) 3 (LAGP) is a solid lithium-ion conductor belonging to the NASICON family, representing the solid solution of LiGe 2 (PO 4 ) 3 and AlPO 4 . The typical syntheses of LAGP either involve high-temperature melt-quenching, which is complicated and expensive, or a sol–gel process requiring costly organic germanium precursors. Herein, we report a simple method based on aqueous solutions without the need of ethoxide precursors. Using synchrotron and neutron diffraction, the crystal structure, the occupancies for Al and Ge, and the distribution of lithium were determined. Substitution of germanium by aluminum allows for an increased Li + incorporation in the material and the actual Li + content in the sample increases with the nominal Li + content and a solubility limit is observed for higher aluminum content. By means of impedance spectroscopy, an increase in the ionic conductivity with increasing lithium content is observed. Whereas the lithium ionic conductivity improves, due to the increasing carrier density, the bulk activation energy increases. This correlation suggests that changes in the transport mechanism and correlated motion may be at play in the Li 1+ x Al x Ge 2– x (PO 4 ) 3 solid solution.
37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
Structure–Transport Properties Governing the Interplay in Humidity-Dependent Mixed Ionic and Electronic Conduction of Conjugated Polyelectrolytes
Polymeric mixed ionic-electronic conductors (MIECs) are of broad interest in the field of energy storage and conversion, opto-electronics, and bioelectronics. A class of polymeric MIECs are conjugated polyelectrolytes (CPEs), which possess a p-conjugated backbone imparting electronic transport characteristics along with side chains comprised of a pendant ionic group to allow for ionic transport. Here, our study focuses on the humidity-dependent structure-transport properties of poly[3-(potassium-n-alkanoate) thiophene-2,5-diyl] (P3KnT) CPEs with varied side-chain lengths of n = 4, 5, 6, and 7. UV-Vis spectroscopy along with electronic paramagnetic resonance (EPR) spectroscopy reveal the infiltration of water leads to a hydrated, self-doped state that allows for electronic transport. The resulting humidity-dependent ionic conductivity (σ i ) of the thin films shows a monotonic increase with relative humidity (RH) while electronic conductivity (σ e ) follows a nonmonotonic profile. The values of σ e continue to rise with increasing RH reaching a local maximum after which σ e begins to decrease. P3KnTs with higher n values demonstrate greater resiliency to increasing RH before suffering decrease in σ e . This drop in σ e is attributed to two factors. First, disruption of the locally-ordered π-stacked domains observed through in situ humidity-dependent grazing incidence wide angle X-ray scattering (GIWAXS) experiments can account for some of the decrease in σ e . A second and more dominant factor is attributed to the swelling of the amorphous domains where electronic transport pathways connecting ordered domains are impeded. P3K7T is most resilient to swelling (based on ellipsometry and water uptake measurements) where sufficient hydration allows for high σ i (1.0 × 10 -1 S/cm at 95% RH) while not substantially disrupting σ e (1.7 × 10 -2 S/cm at 85% RH and 8.0 × 10 -3 S/cm at 95% RH). Overall, our study highlights the complexity of balancing electronic and ionic transport in hydrated CPEs.
37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
Fuel cell components and systems having carbon-containing electrically-conductive hollow fibers
According to one embodiment, a system includes a structure having an ionically-conductive, electrically-resistive electrolyte/separator layer covering an inner or outer surface of a carbon-containing electrically-conductive hollow fiber and a catalyst coupled to the hollow fiber, an anode extending along at least part of a length of the structure, and a cathode extending along at least part of the length of the structure, the cathode being on an opposite side of the hollow fiber as the anode. In another embodiment, a method includes acquiring a structure having an ionically-conductive, electrically-resistive electrolyte/separator layer covering an inner or outer surface of a carbon-containing electrically-conductive hollow fiber and a catalyst along one side thereof, adding an anode that extends along at least part of a length of the structure, and adding a cathode that extends along at least part of the length of the structure on an opposite side as the anode.
25 ENERGY STORAGE↗
Local charge-screening and polarization switching in a ferroelectric ionic-superconductor
For ferroelectric ionic-conductors, polarization switching is complicated by the interplay between ion mobility and charge screening effects. When the ionic charge carriers also play a key role in the domain reversal, such as in Rb-doped KTiOPO 4 (RKTP), a higher level of complexity is introduced. RKTP provides an ideal platform for investigating the relationship between ionic conductivity and polarization reversal because its highly anisotropic crystal properties allow selective modification of material characteristics through diffusive cation doping. Here, we use indiffused Ba/K doping to create a significant increase in the ionic conductivity. Time-of-flight secondary ion mass spectrometry is employed to map Ba/K doping within the RKTP crystal and correlate it to changes in ionic mobility and polarization switching characteristics under an external field applied to the nonpolar face. Using band-excitation piezoresponse force microscopy, we demonstrate a selective switching-inhibition mechanism driven by the enhanced charge screening.
36 MATERIALS SCIENCE↗
Direct Measurement of Crossover and Interfacial Resistance of Ion-Exchange Membranes in All-Vanadium Redox Flow Batteries
Among various components commonly used in redox flow batteries (RFBs), the separator plays a significant role, influencing resistance to current as well as capacity decay via unintended crossover. It is well-established that the ohmic overpotential is dominated by the membrane and interfacial resistance in most aqueous RFBs. The ultimate goal of engineering membranes is to improve the ionic conductivity while keeping crossover at a minimum. One of the major issues yet to be addressed is the contribution of interfacial phenomena in the influence of ionic and water transport through the membrane. In this work, we have utilized a novel experimental system capable of measuring the ionic crossover in real-time to quantify the permeability of ionic species. Specifically, we have focused on quantifying the contributions from the interfacial resistance to ionic crossover. The trade-off between the mass and ionic transport impedance caused by the interface of the membranes has been addressed. The MacMullin number has been quantified for a series of electrolyte configurations and a correlation between the ionic conductivity of the contacting electrolyte and the Nafion® membrane has been established. The performance of individual ion-exchange membranes along with a stack of various separators have been explored. We have found that utilizing a stack of membranes is significantly beneficial in reducing the electroactive species crossover in redox flow batteries compared to a single membrane of the same fold thickness. For example, we have demonstrated that the utilization of five layers of Nafion ® 211 membrane reduces the crossover by 37% while only increasing the area-specific resistance (ASR) by 15% compared to a single layer Nafion ® 115 membrane. Therefore, the influence of interfacial impedance in reducing the vanadium ion crossover is substantially higher compared to a corresponding increase in ASR, indicating that mass and ohmic interfacial resistances are dissimilar. We have expanded our analysis to a combination of commercially available ion-exchange membranes and provided a design chart for membrane selection based on the application of interest (short duration/high-performance vs. long-term durability). The results of this study provide a deeper insight into the optimization of all-vanadium redox flow batteries (VRFBs).
UV/Vis spectroscopy↗
Study of segmental dynamics and ion transport in polymer–ceramic composite electrolytes by quasi-elastic neutron scattering
Composite electrolytes composed of a polymer electrolyte and an ion-conducting ceramic electrolyte are promising in fulfilling the requirements for a stable lithium metal anode. In this work, we identify the effects of the surface of a lithium-ion-conducting ceramic, the Ohara LICGC™ ceramic, on the segmental dynamics and ionic conductivity of polymer electrolyte consisting of poly(ethylene oxide) and lithium bis(trifluoromethanesulfonyl)imide (LiTFSI). Using quasi-elastic neutron scattering, we study the segmental motion of PEO chains under the confinement of LiTFSI salt and Ohara ceramic, in their melt state (363 K). We compare the relaxation time, τ, and the monomeric friction coefficient, ζ, of four samples: neat PEO, PEO + Ohara ceramic, PEO + LiTFSI and PEO + LiTFSI + Ohara ceramic. In the absence of LiTFSI, Ohara ceramic posed negligible change in the segmental dynamics of PEO. In contrast, with the presence of LiTFSI, Ohara ceramic slowed down the segmental motion of PEO chains by ~60% compared to neat PEO + LiTFSI. The intrinsic ionic conductivity of the polymer phase in the composite decreased by ~30% compared to the neat polymer electrolyte. The underpinnings of these results may be that polymer chains in the vicinity of the ceramic surface are less mobile due to coordination with surface bound lithium ions.
36 MATERIALS SCIENCE↗
Synthesis, crystal structure and properties of alluaudite-like triple molybdate Na{sub 25}Cs{sub 8}Fe{sub 5}(MoO{sub 4}){sub 24}
A new triple molybdate Na{sub 25}Cs{sub 8}Fe{sub 5}(MoO{sub 4}){sub 24} was synthesized using solid state reactions and studied with X-ray powder diffraction, second harmonic generation (SHG) technique, differential scanning calorimetry, Mössbauer and dielectric impedance spectroscopy. Single crystals of Na{sub 25}Cs{sub 8}Fe{sub 5}(MoO{sub 4}){sub 24} were obtained and its structure was solved (the space group P1{sup ¯}, a=12.5814(5), b=13.8989(5), c=28.4386(9) Å, α=90.108(2), β=90.064(2), γ=90.020(2)°, V=4973.0(3) Å{sup 3}, Z=2, R=0.0440). Characteristic features of the structure are polyhedral layers composed of pairs of edge-shared FeO{sub 6} and (Fe, Na)O{sub 6} octahedra, which are connected by bridging MoO{sub 4} tetrahedra. The layers share common vertices with bridging MoO{sub 4} tetrahedra to form an open 3D framework with the cavities occupied by the Cs{sup +} and Na{sup +} cations. The compound undergoes first-order phase transformation at 642 K and above this phase transition, electrical conductivity reaches 10{sup −3}–10{sup −2} S cm{sup −1}. Thus, Na{sub 25}Cs{sub 8}Fe{sub 5}(MoO{sub 4}){sub 24} may be considered as a promising compound for developing new materials with high ionic conductivity. - Graphical abstract: A new triple molybdate Na{sub 25}Cs{sub 8}Fe{sub 5}(MoO{sub 4}){sub 24} was synthesized and structurally characterized, its physicochemical properties were studied. - Highlights: • New compound Na{sub 25}Cs{sub 8}Fe{sub 5}(MoO{sub 4}){sub 24} was synthesized. • Physicochemical properties of the compound were studied. • The first-order phase transformation is observed. • Electrical conductivity above 642 K is (10{sup −2}–10{sup −3}) S cm{sup −1}. • New compound may be considered as promising object with high ionic conductivity.
37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
Sol–gel synthesis and lithium ion conduction properties of garnet-type Li{sub 6}BaLa{sub 2}Ta{sub 2}O{sub 12}
Graphical abstract: Li{sub 6}BaLa{sub 2}Ta{sub 2}O{sub 12}, prepared by sol–gel synthesis method, shows a total Li-ion conductivity of 1.69 × 10{sup −5} S/cm at 298 K and an activation energy is 0.40 eV. Highlights: ► Garnet-type fast ion conductor LLBTO was synthesized by modified sol–gel Pechini method. ► The synthesis temperature is significantly lower (200 K) than conventional solid state synthesis methods. ► The ionic conductivity of sol–gel synthesized LLBTO is 1.69 × 10{sup −5} S/cm. -- Abstract: High lithium ion conductive garnet-type barium lanthanum lithium tantalate, Li{sub 6}BaLa{sub 2}Ta{sub 2}O{sub 12} (LLBTO), was prepared by a modified sol–gel Pechini method from the appropriate mixtures of lithium carbonate, lanthanum oxide, barium carbonate and tantalum ethoxide. The thermal decomposition of the precursor powder was investigated by TG/DTA analysis. The LLBTO precursor powders were annealed at various temperatures between 923 and 1123 K for 6 h in air. The transformation process from precursor powder to crystalline garnet-like phase was analyzed by X-ray powder diffraction (XRPD). The morphology of the powders annealed at various temperatures was investigated by scanning electron microscopy (SEM). The resultant pelletized Li{sub 6}BaLa{sub 2}Ta{sub 2}O{sub 12}, prepared by sol–gel synthesis method, shows a total Li-ion conductivity of 1.69 × 10{sup −5} S/cm at 298 K and an activation energy is 0.40 eV. The ionic conductivities reported in this study are slightly higher than those reported for LLBTO sample synthesized by conventional solid state synthesis route.
36 MATERIALS SCIENCE↗