Ionic-electronic Conductors Research Articles

Li Dynamics in Mixed Ionic-Electronic Conducting Interlayer of All-Solid-State Li-metal Batteries.

Lithium-metal (Li0) anodes potentially enable all-solid-state batteries with high energy density. However, it shows incompatibility with sulfide solid-state electrolytes (SEs). One strategy is introducing an interlayer, generally made of a mixed ionic-electronic conductor (MIEC). Yet, how Li behaves within MIEC remains unknown. Herein, we investigated the Li dynamics in a graphite interlayer, a typical MIEC, by using operando neutron imaging and Raman spectroscopy. This study revealed that intercalation-extrusion-dominated mechanochemical reactions during cell assembly transform the graphite into a Li-graphite interlayer consisting of SE, Li0, and graphite-intercalation compounds. During charging, Li+ preferentially deposited at the Li-graphite|SE interface. Upon further plating, Li0-dendrites formed, inducing short circuits and the reverse migration of Li0. Modeling indicates the interface has the lowest nucleation barrier, governing lithium transport paths. Our study elucidates intricate mechano-chemo-electrochemical processes in mixed conducting interlayers. The behavior of Li+ and Li0 in the interlayer is governed by multiple competing factors.

Programmable Ferroelectricity in Hf0.5Zr0.5O2 Enabled by Oxygen Defect Engineering.

Ferroelectricity, especially the Si-compatible type recently observed in hafnia-based materials, is technologically useful for modern memory and logic applications, but it is challenging to differentiate intrinsic ferroelectric polarization from the polar phase and oxygen vacancy. Here, we report electrically controllable ferroelectricity in a Hf0.5Zr0.5O2-based heterostructure with Sr-doped LaMnO3, a mixed ionic-electronic conductor, as an electrode. Electrically reversible extraction and insertion of an oxygen vacancy into Hf0.5Zr0.5O2 are macroscopically characterized and atomically imaged in situ. Utilizing this reversible process, we achieved multilevel polarization states modulated by the electric field. Our study demonstrates the usefulness of the mixed conductor to repair, create, manipulate, and utilize advanced ferroelectric functionality. Furthermore, the programmed ferroelectric heterostructures with Si-compatible doped hafnia are desirable for the development of future ferroelectric electronics.

Charge Carrier Induced Structural Ordering And Disordering in Organic Mixed Ionic Electronic Conductors.

Operational stability underpins the successful application of organic mixed ionic-electronic conductors (OMIECs) in a wide range of fields, including biosensing, neuromorphic computing, and wearable electronics. In this work, both the operation and stability of a p-type OMIEC material of various molecular weights are investigated. Electrochemical transistor measurements reveal that device operation is very stable for at least 300 charging/discharging cycles independent of molecular weight, provided the charge density is kept below the threshold where strong charge-charge interactions become likely. When electrochemically charged to higher charge densities, an increase in device hysteresis and a decrease in conductivity due to a drop in the hole mobility arising from long-range microstructural disruptions are observed. By employing operando X-ray scattering techniques, two regimes of polaron-induced structural changes are found: 1) polaron-induced structural ordering at low carrier densities, and 2) irreversible structural disordering that disrupts charge transport at high carrier densities, where charge-charge interactions are significant. These operando measurements also reveal that the transfer curve hysteresis at high carrier densities is accompanied by an analogous structural hysteresis, providing a microstructural basis for such instabilities. This work provides a mechanistic understanding of the structural dynamics and material instabilities of OMIEC materials during device operation.

On the Influence of Li3InCl6−PEDOT:PSS Hybrids in Solid‐State Batteries Prepared via an Aqueous One‐Pot Approach

AbstractSolid‐state batteries (SSBs) utilizing halide solid electrolytes (SE) have garnered attention due to their enhanced stability when paired with oxide‐based cathode active materials. However, the dynamic interparticle contact during cycling in SSBs poses challenges to their stability and performance. To mitigate this problem, in this study, we present a one‐pot, aqueous synthesis of composites that integrate ion conductivity, electron conductivity, and mechanical stability into a single material. The developed composites consist of a halide SE, lithium indium chloride (Li3InCl6), and a conductive polymer (CP), poly(3,4‐ethylendioxythiophene)/poly(styrene sulfonate) (PEDOT:PSS). The successful synthesis was verified using spectroscopic, thermal, scattering, and microscopy methods, with Kelvin Probe Force Microscopy (KPFM) demonstrating the distribution of PEDOT:PSS at the grain boundaries between Li3InCl6 particles. Upon incorporating our composite material with lithium nickel manganese cobalt oxide (NMC) cathode active material (CAM) as catholyte, an increase in the partial electronic transport was observed with increasing CP content. A direct correlation between the partial electronic transport of the catholytes and the initial discharge capacities was demonstrated. This study lays the groundwork for the preparation of multi‐functional catholytes under more sustainable conditions, without the need for organic solvents, extremely high temperatures, or special environments.

Highly sintering-resistant iron oxide with a hetero-oxide shell for chemical looping water splitting

Chemical looping water splitting is a promising technology that couples carbonaceous fuel oxidation to produce H2 with inherent CO2 separation. Redox catalyst with long-term performance is the key issue but is hampered in traditional Fe2O3-based approaches due to Fe sintering. This study reports a one-step sol-gel synthetic method to construct Fe2O3@Hetero-oxide redox catalysts with core-shell structure, leading to outstanding anti-sintering property in TGA analysis (>250 cycles, 5000 min). Fixed-bed reactor tests showed that hydrogen yield approached theoretical value (16.7 mmol/g-Fe2O3), with ∼100 % H2 purity. This result is significantly better than dispersing a mixed ionic-electronic conductor (MIEC) in Fe2O3 matrix using high energy ball milling. Although the latter can enhance the durability due to facilitated O2− and electron transportation, Fe sintering and redox catalysts deactivation is inevitable. This study offers a generalized structure design and synthetic approach for anti-sintering redox catalysts within chemical looping concepts.

Ta/Nb doping motivating cubic phase stability and CO2 tolerance of Sr2Co1.6Fe0.4O6-δ double perovskite for high-performing SOFC cathode

Developing cathodes that are both phase-stable and tolerant to carbon dioxide (CO2) is a crucial and challenging task for solid-oxide fuel cells (SOFCs) operating at reduced temperatures. The activity of oxygen reduction reaction (ORR) in SOFC cathodes is typically influenced by factors such as geometric features and oxygen vacancies. In this study, new and robust cathodes for SOFCs, Sr2Co1.5Fe0.4Ta0.1O5+δ (SCFT) and Sr2Co1.5Fe0.4Nb0.1O5+δ (SCFN) are developed by doping a small amount of niobium (Nb) and tantalum (Ta) into Sr2Co1.6Fe0.4O5+δ (SCF) using a simple solid-state reaction technique. The effects of Nb/Ta doping on the crystal structure, oxygen vacancies, electrical conductivity, and electrochemical properties of SCFT and SCFN are investigated. The results reveal that these materials possess a fully cubic structure in the Fm-3m space group, with improved oxygen vacancy and electrical conductivity. They also exhibit good chemical compatibility with electrolytes at 1100 °C for 8 h and remain thermally stable when exposed to air and CO2 at 700 °C, without any additional phase formation. The area-specific resistance (ASR) of the SCFT cathode is found to be only 0.06 Ω cm2, whereas the SCFN cathode has an ASR of 0.10 Ω cm2. The superior performance of SCFT can be attributed to the higher oxygen vacancy, which enhances oxygen surface exchange kinetics and diffusivity compared to SCFN. When tested in a single cell using humidified H2 as the fuel and ambient air as the oxidant, the SCFT cathode achieves a power density of 656 mW cm−2 at 700 °C, while the SCFN cathode reaches 559 mW cm−2, indicating the potential of SCFT as a promising cathode material for SOFCs. The excellent performance of these materials can be attributed to their mixed electronic-ionic conduction properties, unique structural features, and high observed oxygen vacancy, which contribute to their remarkable electronic conductivity and significant ionic mobility.

Nanocrystalline cellulose-based mixed ionic–electronic conductor for bioelectronics

A mixed ionic–electronic conductor (MIEC) comprising sulfated cellulose nanocrystals (S-CNCs) as nanotemplate is investigated in organic electrochemical transistors (OECTs) with remarkable channel current and high transconductance values.

Contrasting interchain order and mixed ionic–electronic conduction in conjugated polymers: an isoindigo case study

Using isoindigo-based polymers, this study investigates mixed ionic–electronic conduction in highly ordered conjugated polymers by systematically incorporating polar side chains, which promote ion insertion while preserving interchain stacking.

Adjusting molecular weight optimizes electronic transport of extrinsically N-type doped conjugated polymer incorporating glycolated side chains

Adjusting the molecular weight of n-type conjugated polymers incorporating glycolated side chains optimizes the power factor of organic thermoelectrics and mixed electronic–ionic conduction of organic electrochemical transistors.

The mixed ionic-electronic conductors studied by advanced transmission electron microscopy

The mixed ionic-electronic conductors studied by advanced transmission electron microscopy

Side chain engineering in indacenodithiophene-co-benzothiadiazole and its impact on mixed ionic–electronic transport properties

This well-studied polymer system does not conform to existing design criteria for imparting mixed conduction. The reasons for this anomalous behaviour are unravelled and used to outline more robust design criteria for new organic semiconductors.

Polyethylene glycol-decorated n-type conducting polymers with improved ion accessibility for high-performance organic electrochemical transistors.

High-performance n-type organic mixed ionic-electronic conductors (OMIECs) are essential for advancing complementary circuits based on organic electrochemical transistors (OECTs). Despite significant progress, current n-type OMIECs often exhibit lower transconductance and slower response times compared to their p-type counterparts, limiting the development of OECT-based complementary circuits. Optimizing the conjugated backbone and side chain structures of OMIECs is critical for enhancing both ion and electron transport efficiencies while maintaining a delicate balance between the two. In this study, hydrophilic polyethylene glycol (PEG) side chains were incorporated into the highly conductive n-type polymer poly(3,7-dihydrobenzo[1,2-b:4,5-b']difuran-2,6-dione) (PBFDO) backbone to achieve this goal. The incorporation of PEG chains improved ion accessibility, and by adjusting the PEG content, the electronic and ionic transport properties were fine-tuned, ultimately enhancing the performance of OECTs and related p-n complementary circuits. The n-type OECTs based on PBFDO-PEG50wt% demonstrated exceptional transfer characteristics, including a transient response time (τON) as low as 72 μs, a high geometry-normalized transconductance exceeding 400 S cm-1, and an impressive μC* value surpassing 720 F cm-1 V-1 s-1. Notably, the use of PBFDO-PEG50wt% in a complementary inverter resulted in a voltage gain of 20 V/V, more than five times higher than that achieved with unmodified PBFDO (<4 V/V). These findings highlight the importance of balancing electron and ion transport characteristics in OMIECs to achieve high performance in OECTs and their associated circuits, and they validate PEG decoration as an effective approach.

Disorder-to-order transition of regiorandom P3HT upon electrochemical doping

Upon repeated electrochemical doping and thermal annealing, regiorandom P3HT undergoes a disorder-to-order structural change.

Electrochemical modulation of mechanical properties of glycolated polythiophenes.

Electrochemical doping of organic mixed ionic-electronic conductors is key for modulating their conductivity, charge storage and volume enabling high performing bioelectronic devices such as recording and stimulating electrodes, transistors-based sensors and actuators. However, electrochemical doping has not been explored to the same extent for modulating the mechanical properties of OMIECs on demand. Here, we report a qualitative and quantitative study on how the mechanical properties of a glycolated polythiophene, p(g3T2), change in situ during electrochemical doping and de-doping. The Young's modulus of p(g3T2) changes from 69 MPa in the dry state to less than 10 MPa in the hydrated state and then further decreases down to 0.4 MPa when electrochemically doped. With electrochemical doping-dedoping the Young's modulus of p(g3T2) changes by more than one order of magnitude reversibly, representing the largest modulation reported for an OMIEC. Furthermore, we show that the electrolyte concentration affects the magnitude of the change, demonstrating that in less concentrated electrolytes more water is driven into the film due to osmosis and therefore the film becomes softer. Finally, we find that the oligo ethylene glycol side chain functionality, specifically the length and asymmetry, affects the extent of modulation. Our findings show that glycolated polythiophenes are promising materials for mechanical actuators with a tunable modulus similar to the range of biological tissues, thus opening a pathway for new mechanostimulation devices.

PEDOT-based stretchable optoelectronic materials and devices for bioelectronic interfaces.

The rapid development of wearable and implantable electronics has enabled the real-time transmission of electrophysiological signals in situ, thus allowing the precise monitoring and regulation of biological functions. Devices based on organic materials tend to have low moduli and intrinsic stretchability, making them ideal choices for the construction of seamless bioelectronic interfaces. In this case, as an organic ionic-electronic conductor, poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) has low impedance to offer a high signal-to-noise ratio for monitoring bioelectrical signals, which has become one of the most promising conductive polymers. However, the initial conductivity and stretchability of pristine PEDOT:PSS are insufficient to meet the application requirements, and there is a trade-off between their improvement. In addition, PEDOT:PSS has poor stability in aqueous environments due to the hygroscopicity of the PSS chains, which severely limits its long-term applications in water-rich bioelectronic interfaces. Considering the growing demands of multi-function integration, the high-resolution fabrication of electronic devices is urgent. It is a great challenge to maintain both electrical and mechanical performance after miniaturization, particularly at feature sizes below 100 μm. In this review, we focus on the combined improvement in the conductivity and stretchability of PEDOT:PSS, as well as the corresponding mechanisms in detail. Also, we summarize the effective strategies to improve the stability of PEDOT:PSS in aqueous environments, which plays a vital role in long-term applications. Finally, we introduce the reliable micropatterning technologies and PEDOT:PSS-based stretchable optoelectronic devices applied at bio-interfaces.

Mixed ionic-electronic conduction in Ruddlesden–Popper and Dion–Jacobson layered hybrid perovskites with aromatic organic spacers

Mixed ionic-electronic conduction in Ruddlesden–Popper and Dion–Jacobson layered hybrid perovskites with aromatic organic spacers

Resistive switching memories with enhanced durability enabled by mixed-dimensional perfluoroarene perovskite heterostructures.

Hybrid halide perovskites are attractive candidates for resistive switching memories in neuromorphic computing applications due to their mixed ionic-electronic conductivity. Moreover, their exceptional optoelectronic characteristics make them effective as semiconductors in photovoltaics, opening perspectives for self-powered memory elements. These devices, however, remain unexploited, which is related to the variability in their switching characteristics, weak endurance, and retention, which limit their performance and practical use. To address this challenge, we applied low-dimensional perovskite capping layers onto 3D mixed halide perovskites using two perfluoroarene organic cations, namely (perfluorobenzyl)ammonium and (perfluoro-1,4-phenylene)dimethylammonium iodide, forming Ruddlesden-Popper and Dion-Jacobson 2D perovskite phases, respectively. The corresponding mixed-dimensional perovskite heterostructures were used to fabricate resistive switching memories based on perovskite solar cell architectures, showing that the devices based on perfluoroarene heterostructures exhibited enhanced performance and stability in inert and ambient air atmosphere. This opens perspectives for multidimensional perovskite materials in durable self-powered memory elements in the future.

2D MXene electrochemical transistors.

The solid-state field-effect transistor, FET, and its theories were paramount in the discovery and studies of graphene. In the past two decades another transistor based on conducting polymers, called organic electrochemical transistor (ECT), has been developed and largely studied. The main difference between organic ECTs and FETs is the mode and extent of channel doping; while in FETs the channel only has surface doping through dipoles, the mixed ionic-electronic conductivity of the channel material in organic ECTs enables bulk electrochemical doping. As a result, organic ECTs maximize conductance modulation at the expense of speed. To date ECTs have been based on conducting polymers, but here we show that MXenes, a class of 2D materials beyond graphene, enable the realization of electrochemical transistors (ECTs). We show that the formulas for organic ECTs can be applied to these 2D ECTs and used to extract parameters like mobility. These MXene ECTs have high transconductance values but low on-off ratios. We further show that conductance switching data measured using ECT, in combination with other in situ-ex situ electrochemical measurements, is a powerful tool for correlating the change in conductance to that of the redox state, to our knowledge, this is the first report of this important correlation for MXene films. 2D ECTs can draw great inspiration and theoretical tools from the field of organic ECTs and have the potential to considerably extend the capabilities of transistors beyond those of conducting polymer ECTs, with added properties such as extreme heat resistance, tolerance for solvents, and higher conductivity for both electrons and ions than conducting polymers.

Resistive switching in benzylammonium-based Ruddlesden-Popper layered hybrid perovskites for non-volatile memory and neuromorphic computing.

Artificial synapses based on resistive switching have emerged as a promising avenue for brain-inspired computing. Hybrid metal halide perovskites have provided the opportunity to simplify resistive switching device architectures due to their mixed electronic-ionic conduction, yet the instabilities under operating conditions compromise their reliability. We demonstrate reliable resistive switching and synaptic behaviour in layered benzylammonium (BzA) based halide perovskites of (BzA)2PbX4 composition (X = Br, I), showing a transformation of the resistive switching from digital to analog with the change of the halide anion. While (BzA)2PbI4 devices demonstrate gradual set and reset processes with reduced power consumption, the (BzA)2PbBr4 system features a more abrupt switching behaviour. Moreover, the iodide-based system displays excellent retention and endurance, whereas bromide-based devices achieve a superior on/off ratio. The underlying mechanism is attributed to the migration of halide ions and the formation of halide vacancy conductive filaments. As a result, the corresponding devices emulate synaptic characteristics, demonstrating the potential for neuromorphic computing. Such resistive switching and synaptic behaviour highlight (BzA)2PbX4 perovskites as promising candidates for non-volatile memory and neuromorphic computing.

Azonia-Naphthalene: A Cationic Hydrophilic Building Block for Stable N-Type Organic Mixed Ionic-Electronic Conductors.

Conjugated polymers that can efficiently transport both ionic and electronic charges have broad applications in next-generation optoelectronic, bioelectronic, and energy storage devices. To date, almost all the conjugated polymers have hydrophobic backbones, which impedes efficient ion diffusion/transport in aqueous media. Here, we design and synthesize a novel hydrophilic polymer building block, 4a-azonia-naphthalene (AN), drawing inspiration from biological systems. Because of the strong electron-withdrawing ability of AN, the AN-based polymers show typical n-type charge transport behaviors. We find that cationic aromatics exhibit strong cation-π interactions, leading to smaller π-π stacking distance, interesting ion diffusion behavior, and good morphology stability. Additionally, AN enhances the hydrophilicity and ionic-electronic coupling of the polymer, which can help to improve ion diffusion/injection speed, and operational stability of organic electrochemical transistors (OECTs). The integration of cationic building blocks will undoubtedly enrich the material library for high-performance n-type conjugated polymers.