Ion Migration Mechanism Research Articles

Stability of Perovskite Thin Films under Working Condition: Bias‐Dependent Degradation and Grain Boundary Effects

AbstractWitnessed by the rapid increase of power conversion efficiency to 25.5%, organic–inorganic hybrid perovskite solar cells (PSCs) are becoming promising candidates of next‐generation photovoltaics. However, PSCs can be unstable under the influence of light and bias. Especially, grain boundaries (GBs) are vulnerable to attack by light and bias in perovskite films, leading to degradation of photovoltaic properties of PSCs. Herein, photocurrent atomic force microscopy and Kelvin probe force microscopy are employed to systematically investigate the bias‐dependent charge transport behaviors and stability of (FAPbI3)0.85(MAPbBr3)0.15 perovskite under working condition. Bias‐dependent morphology and photocurrent images show irreversible decomposition of the perovskite at a bias of 0.1 V or below, which is accelerated by light illumination, leading to formation of an interfacial layer that restricts carrier transport. Meanwhile, GBs appear to enhance carrier transport at larger bias, but serve as breakthrough sites for perovskite decomposition at smaller bias. Introducing excess methylammonium iodide promotes decomposition, while potassium iodide passivation greatly relieves the decomposition. These results support the ion migration mechanism of decomposition through interfaces and GBs. This work provides a deeper understanding of bias‐induced degradation of PSCs as well as bias‐dependent double‐edged roles of GBs, and forms valuable guidance for appropriate operation of PSCs.

Unraveling the mechanism of ion and electron migration in composite solid-state electrolyte using conductive atomic force microscopy

Composite solid electrolytes (CSEs) which formed a flexible interface with electrodes are considered as promising electrolytes candidates for all-solid-state batteries (ASSBs). However, the role of inorganic particles and migration of Li-ions still need to be investigated. Herein, LLZO-PEO composite electrolyte is prepared with different weight ratios (0, 50, 75 wt. %) of LLZO and in-situ conductive atomic force microscopy (c-AFM) is employed to observe the changes in electrolyte topography and mechanical properties at different temperatures. Furthermore, c-AFM provides novel insights into the inhomogeneous migration of ions and electrons within the electrolyte. At high temperatures, Li-ions migrate along the amorphous PEO interphase of 0 and 50 wt. % LLZO-PEO electrolytes. With the increase of LLZO content (75 wt. %), LLZO particles form a continuous ionic network and Li-ions migrate through the LLZO particles. At low temperatures, Li-ions can only migrate along the amorphous PEO. Also, the addition of LLZO inhibits the migration of electrons and improves the insulative properties of the composite electrolyte. The current work provides novel insights into the design and development of composite electrolytes for next-generation ASSBs.

Temperature‐ and Bias‐Dependent Degradation and Regeneration of Perovskite Solar Cells with Organic and Inorganic Hole Transport Layers

Hybrid halide perovskite solar cells have drawn widespread attention with the achievement of high power conversion efficiencies. However, poor stability remains the greatest barrier preventing their commercialization. Performance degradation and recovery have a complicated dependence on the environment and a dependence on the applied bias, which affects ion migration. Herein, solar cells with an organic hole transport layer and cells with an inorganic hole transport layer are compared. A type of degradation of the organic transport layer is examined, which is reversible by applying a forward bias soak, and how the degradation arises from ion migration mechanisms is explained. Experimental current–voltage and capacitance transient measurements are conducted as a function of temperature. The resulting S‐kink and positive capacitance decay are explained in terms of the modeled effects of a changing ion density at the hole transport layer. An irreversible degradation is found upon heating to more than 100 °C. On the contrary, the inorganic hole transport layer is found to eliminate the observable effects of ion migration, even at elevated temperatures, so long as air exposure is avoided.

First-principles studies of oxygen ion migration behavior for different valence B-site ion doped SrFeO3-δ ceramic membranes.

Density functional theory calculations were performed to investigate the structural, electronic, and oxygen ion migration properties of B-site ion doped SrFeO3-δ perovskite (B = Al, Zr, Nb, and W) materials, which were used as oxygen transport membranes (OTMs) for pure oxygen output and catalytic reactions. The results of our calculations indicate that the Fe-O bond length increased and the M-O bond length decreased with the doping of Zr, Nb, and W. And the doping of Al caused the valence state of Fe ions to increase. The states near the Fermi level were mainly contributed by Fe atoms and O atoms. The strength of the Fe-O bond gradually weakened with the increase in the valence of the doped ions. Through studying the oxygen vacancy defect and the mechanism of oxygen ion migration, it was found that the doping of Al promoted the migration of oxygen ions, while the doping of Zr, Nb, and W limited the migration of oxygen ions. This study provides important insights into the behavior of oxygen ion migration in doped SrFeO3-δ perovskite materials.

Studies on the improved electro-chemical performance and the sodium ion migration mechanism of Na0·44MnO2-CNT electrodes for aqueous sodium batteries

Here, A kind of carbon nanotubes (CNT) wrapping Na0·44MnO2 has been synthesized via a high-temperature solid state method and showed better electro-chemical performance in 1 M Na2SO4 aqueous solution as cathode material for aqueous sodium ion batteries. This rod-like nano-composite exhibited a first charge capacity of 70.1 mA h·g−1 with the reversible capacity retention of about 63.4% after 300 cycles at a current density of 50 mA g−1. It can also remain reversible specific capacity of 53.1, 44.7, 34.8, and 19.7 mA h·g−1 at the current density of 1 C, 2 C, 5 C and 10 C, respectively. The Na+ migration mechanism and kinetics of Na0·44MnO2 are fully investigated by molecular dynamics (MD) and the density functional theory (DFT). A SQS-8 quasi-random structure is used to model the structure of the material. The results show that Na+ is highly vigorous and propagates 1D along the c-axis. A minimum of 0.081 eV and 0.51 eV is required for Na+ to travel through the large S-shaped tunnel and the small tunnel, respectively.

Improved Adsorption and Migration of Divalent Ions Over C4N Nanosheets: Potential Anode for Divalent Batteries

Divalent ion batteries are potential substitutes to existing rechargeable batteries because of the high-energy density, safety, and low cost. However, the applications of divalent batteries are strongly dependent on the availability of efficient electrode materials. We herein report the adsorption and migration mechanism of divalent ions like Mg+2 and Ca+2, on two-dimensional carbon nitride monolayer (C4N). The adsorption of both Mg and Ca ions are much stronegr on C4N saturated with functional groups like hydrogen (-H), hydroxyl (-OH), and carboxylic (-COOH) as compared to the pristine monolayers, which implies the improvement in metal storage caused by the functional groups. On functionalized C4N, the first Mg binds within the binding energy range of 1.5-2.2 eV having migration barrier, at the saturated sites, of around 0.5 eV, which indicates desirable binding and robust diffuse. However, the functional groups tend to act as the trapping sites for Ca ions and the diffusion might get hindered as compared to Mg ions. In addition to binding and diffusion mechanism, charge transfer, electronic structures and open circuit volatges have also been calculated for both Mg and Ca on pristine and functionalized C4N monolayers. We find that -H, -OH, and -COOH play important role for the cyclic performance of C4N as the prospective anode material for divalent ion batteries.

A resistive device with electrolyte as active electrode

Due to the outstanding performance of resistance random access memory (RRAM) in the memory field, the study of resistive switching (RS) phenomena has become extremely noticeable in the recent years. The mechanism of metal conductive filamentary RRAM is already clear, but the conditions of the RS are still unclear. Therefore, this paper aims to explore the conditions for the occurrence of resistive, using a new RS structure called Electrolyte-Oxide-Semiconductor (EOS). This structure is based on the formation of metal conductive filament and exhibits the unipolar switching characteristics. Due to the formation or rupture of the conductive filaments, this device exhibits different resistance states. A series model of electrolyte and conductive filaments is used to explain the IV curve of this device. Compared with the device using a metal active electrode, the active electrode of this device is originally ionized. Therefore, it would be a better tool to explore the mechanism of ion migration and the formation of conductive filaments. Materials screening of metal in RRAM would also be more efficient.

Redox MXene Artificial Synapse with Bidirectional Plasticity and Hypersensitive Responsibility

AbstractArtificial synapses are key elements for the nervous system which is an emulation of sensory and motor neuron signal transmission. Here, the design and fabrication of redox‐behavior the metal carbide nanosheets, termed MXene artificial synapse, which uses a highly‐conductive MXene electrode, are reported. Benefiting from the special working mechanism of ion migration with adsorption and insertion, the device achieves world‐record power consumption (460 fW) of two‐terminal synaptic devices, and so far, the bidirectionally functioned synaptic device could effectively respond to ultra‐small stimuli at an amplitude of ±80 mV, even exceeding that of a biological synapse. Potential applications have also been demonstrated, such as dendritic integration and memory enhancement. The special strategy and superior electrical characteristics of the bidirectionally functioned electronic device pave the way to high‐power‐efficiency brain‐inspired electronics and artificial peripheral systems.

Dynamics of Hydroxyl Anions Promotes Lithium Ion Conduction in Antiperovskite Li2OHCl.

Li2OHCl is an exemplar of the antiperovskite family of ionic conductors, for which high ionic conductivities have been reported, but in which the atomic-level mechanism of ion migration is unclear. The stable phase is both crystallographically defective and disordered, having ∼1/3 of the Li sites vacant, while the presence of the OH- anion introduces the possibility of rotational disorder that may be coupled to cation migration. Here, complementary experimental and computational methods are applied to understand the relationship between the crystal chemistry and ionic conductivity in Li2OHCl, which undergoes an orthorhombic to cubic phase transition near 311 K (≈38 °C) and coincides with the more than a factor of 10 change in ionic conductivity (from 1.2 × 10-5mS/cm at 37 °C to 1.4 × 10-3 mS/cm at 39 °C). X-ray and neutron experiments conducted over the temperature range 20-200 °C, including diffraction, quasi-elastic neutron scattering (QENS), the maximum entropy method (MEM) analysis, and ab initio molecular dynamics (AIMD) simulations, together show conclusively that the high lithium ion conductivity of cubic Li2OHCl is correlated to "paddlewheel" rotation of the dynamic OH- anion. The present results suggest that in antiperovskites and derivative structures a high cation vacancy concentration combined with the presence of disordered molecular anions can lead to high cation mobility.

Transparent characterization of spatial-temporal evolution of gangue solid wastes' void structures during compression based on CT scanning

In the solid backfill mining, gangue solid wastes (GSW) are used as the main filling materials. The void ratio and void structure of GSW filling materials under the loaded compression in the goaf are the key factors that affect the heavy metal ion migration with the mine water. In this paper, the transparency of the internal structure of GSW filling materials was realized by CT scanning experiment and 3D reconstruction technology, and then the spatial-temporal evolution characteristics of void ratio and void structure of GSW filling materials were studied under the loaded compression. The results show that: (a) With the progress of loading process, the connectivity between adjacent voids is getting worse, and closed voids are gradually formed; (b) The void's equivalent diameter is mainly concentrated in the range of 15 mm - 25 mm; (c) The change in the overall void ratio of GSW is mainly caused by the change of large voids, and the change of small voids has slight contribution to the change of the overall void ratio. However, the sharp increase in the number of small voids can reflect the poor connectivity and permeability between the voids. This research is of great significance to further understand the migration mechanism of heavy metal ions in the filled goaf, and protect the groundwater environment.

Insights on the ion migration throughout the nano-channel of ettringite under an external electric field: Structure, dynamics, and mechanisms

In this research, molecular dynamics simulations were employed to unravel the transmission mechanisms of chloride ions and sodium ions throughout ettringite nanopores as exposed to an external electric field 0–0.04 V/Å. The current was generated by the directional migration of cations and anions, proportional to the intensity of electric field, among which chlorides account for 80% while sodium contributes little. The migration of sodium ions was limited as a result of the attraction from the oxygen atoms in the sulfate ions on the ettringite surface. This effect weakened due to the increase in the electric field intensity. Furthermore, the relocation rate of ions may obey in the following ranking: Cl− > Ow > Na+. The development of this work can reveal the mechanism of ion migration and diffusion inside the pores of concrete during the application of the electrochemical desalination method, which has a guiding significance for the durability design of concrete structures.

Analysis of intracellular copper ions content and distribution in algae by gold nanorods method

A novel approach was introduced for detection of copper ions presented in microalgae cells using gold nanorods (GNRs) and for determination of factors affecting the migration of copper ions between water and microalgae cells. It was feasible to detect the copper ions in algae cells (in both intracellular and extracellular fluids) by gold nanorods and investigate the migration mechanism of copper ions in algae cells. That pH of water environment had influenced the migration process. The copper ion concentration had a significant impact on the transition process. When the pH equals 7.1, the copper ion concentration reached its highest value in the microalgae cells. The combination of copper ions with microalgae cells and the binding mechanism were investigated that copper ions and microalgae cells were both related to organic functional groups appearing around the surface of microalgae cells. The binding sites of copper ions and algal cells are amino, hydroxyl and carboxyl groups that had significant impact on the metal migration.

Modeling Film Conductivity for Ion Migration Analysis in Perovskite Solar Cells

Ion migration under stress is a major degradation challenge for commercialization of perovskite solar cells. This paper presents three different approaches to model the conductivity variation in perovskite films at elevated temperature, prolonged irradiation levels and strain induced in the lattice. The conductivity variation under any of these stressing conditions can represent ion migration across the perovskite layer because the conductivity is related to mobility and carrier densities. Ion migration can elevate the density of electrons/holes across the film if the stress condition reduce/lift the activation energy at the interface of the film and smoothens the ion passivation to the perovskite layer. In addition, the conductivity of the film is related to device metrics (e.g. fill factor) and also to materials properties (e.g. mobility and bandgap) and, therefore, it can be a significant measure of stress impact on device stability. For example, the conductivity falls under stress which represents the ion migration enhancement. In our modeling, we have shown that the conductivity is inversely related to temperature and strain (σ ∝ 1/kT, σ ∝ 1/ε) but is directly related to irradiation level (σ ∝ ξ1/2). We have fitted our modeling to experimental data reported in the literature and extracted the activation energy of ion migration mechanism under every stress condition. The band diagrams of the mechanism are presented and it is shown that stress can foster the ion migration by reducing the activation energy at the interfaces of the perovskite layer. However, the impact of heating is worse on film conductivity, whereas irradiation and strain have a moderate or slight effect on it, respectively. Our findings may provide a practical solution to obtain a measure of ion migration in perovskite solar cells for aging analysis of the cell stability and recovery rates under different stressing conditions.

Improving electrical properties of ZnO–Bi2O3–Sb2O3–MnO2 varistors by doping with pre-synthesized Bi–Si–O phase

Abstract The electric properties and microstructure of ZnO -Bi2O3– Sb2O3– MnO2 (ZBSM) varistor ceramics as a function of pre-synthesized mixture of 6Bi2O3 SiO2 (Bi–Si–O) were investigated. The introduction of Bi–Si–O increases liquid phase content during the sintering process, accelerating the solid solubility of MnO2, Sb2O3 and possibly partial Si ions towards ZnO grains. As results, the interface state density significantly increases to 16.43 × 1016/m2 in the 4 wt% Bi–Si–O ZBSM ceramic, which leads to the highest value of barrier height of 5.87 eV and nonlinear coefficient of 54.56. At the same time, the electric breakdown voltage and leakage current density decreases to 613.82 V/mm and 0.001 μA mm−2, respectively. Besides, grain boundary resistance gradually increases with increasing Bi–Si–O dopant (x ≤ 4 wt%) and then decreases sharply to a very low value when x = 6 wt%, which consists with the variation of leakage current density. Furthermore, only one relaxation occurred in ZBSM-xBi-Si-O varistors which can be confirmed by the Cole-Cole plot analysis. Oxygen absorption and ion migration mechanisms are used to explain the enhancement of nonlinear electrical properties.

Molecular dynamics study on applying layered graphene oxide membranes for separating cadmium ions from water

Because cadmium can seriously damage human organs, we explored the prospect of using layered graphene oxide membranes to separate cadmium ions from water by molecular dynamics. The effects of three geometric parameters on membrane performance were investigated. The results indicate that the effects of the interlayer spacing and offset value on the salt rejection rate are much greater than the corresponding effects on water permeability. By contrast, for the gap size, comparable effects on the salt rejection rate and water permeability were observed. Then, the mechanism of water transport and ion migration were investigated. The number-density map revealed why the gap size has a more significant effect on water permeability. The radial distribution function and coordinate number indicate that Cd2+ is surrounded by two hydration shells. The second shell is more flexible so that it allows the loss of several water molecules to enable Cd2+ to pass through the membrane. Potential of mean force indicates the energy barriers for the migration of Cd2+ and Cl- on the surface of nanosheets are much larger than that for water molecules. The water permeability can reach up to 458 L/(m2⋅h⋅bar), which is much higher than the value for commercial membranes.

Defect proliferation in CsPbBr3 crystal induced by ion migration

Ion migration in halide perovskite materials usually brings an intractable problem in the working stability of solar cells and photoelectrical detectors. The mechanism of ion migration and its impact on physical properties are still open questions. In this work, the ion migration behavior in solution-grown CsPbBr3 crystals was observed by the hysteresis in current–voltage curves and the temperature dependent reversed current–time measurements. Defect proliferation phenomena (new defects of [VCs]− and [PbBr]2+) originating from ion migration were verified by thermally stimulated current spectroscopy. Our results also give evidence that Cs+ ions also participate in the process of ion migration except the widely considered Br− ions. Furthermore, the photoelectric properties of the CsPbBr3 device were found to be seriously deteriorated after the ion migration. Our work demonstrates the strong correlation between the ion migration and physical properties in halide perovskites.

Progress and Perspective of Ceramic/Polymer Composite Solid Electrolytes for Lithium Batteries.

Solid composite electrolytes (SCEs) that combine the advantages of solid polymer electrolytes (SPEs) and inorganic ceramic electrolytes (ICEs) present acceptable ionic conductivity, high mechanical strength, and favorable interfacial contact with electrodes, which greatly improve the electrochemical performance of all‐solid‐state batteries compared to single SPEs and ICEs. However, there are many challenges to overcome before the practical application of SCEs, including the low ionic conductivity less than 10−3 S cm−1 at ambient temperature, poor interfacial stability, and high interfacial resistance, which greatly restrict the room temperature performance. Herein, the advances of SCEs applied in all‐solid‐state lithium batteries are presented, including the Li ion migration mechanism of SCEs, the strategies to enhance the ionic conductivity of SCEs by various morphologies of ICEs, and construction methods of the low resistance and stable interfaces of SCEs with both cathode and anode. Finally, some typical applications of SCEs in lithium batteries are summarized and future development directions are prospected. This work presents how it is quite significant to further enhance the ionic conductivity of SCEs by developing the novel SPEs with the special morphology of ICEs for advanced all‐solid‐state lithium batteries.

Electrical properties and oxide ion conducting mechanism in Na-doped LaPO4

In this paper, series of Na-doped La1-xNaxPO4-x (0 ≤ x ≤ 0.2) materials are synthesized by solid state reaction method, and single phase has been obtained for 0 ≤ x ≤ 0.1. The doped materials show significantly improved conductivity under dry environment by about four orders of magnitude, but dominated by sodium ion conduction, with ~ 10% oxide ion conduction. The oxide ion conducting mechanism in LaPO4-based materials is studied for the first time by molecular dynamic simulations based on interatomic potential approach. This work therefore provides a comprehensive view of the underlying oxide ion migration mechanism for LaPO4-based materials.

The physics of ion migration in perovskite solar cells: Insights into hysteresis, device performance, and characterization

AbstractPerovskite solar cells promise to reach the highest efficiencies at the lowest costs, but the mobile ions create challenges in both cell measurements and performance improvement. Being able to understand the ion migration mechanism and its impacts would help to solve relevant issues and may open up opportunities for further development. Although some efforts have been made here, explanations are often too exotic resulting in not only some controversies but the neglect of some important aspects addressable by familiar concepts. This paper uses familiar knowledge in solar cell device physics to explain the ion migration mechanism and its impacts under conditions of interests. It is shown that, when depletion layers contract such as in a forward current‐voltage scan, tardy ionic charges can distort the perovskite bands hence reduce the net currents before they move to new equilibriums, causing the hysteresis effect. As ion migration impacts the electron and hole distributions, it can affect the Shockley‐Read‐Hall and Auger processes differently, hence may either reduce or increase the ratio of radiative to nonradiative recombination. This previously unaddressed mechanism for variation in the electron‐hole recombination can explain photoluminescence quenching and enhancement as well as performance degradation and improvement during ion migration.

Light-Enhanced Ion Migration in Two-Dimensional Perovskite Single Crystals Revealed in Carbon Nanotubes/Two-Dimensional Perovskite Heterostructure and Its Photomemory Application.

Two-dimensional (2D) hybrid perovskite sandwiched between two long-chain organic layers is an emerging class of low-cost semiconductor materials with unique optical properties and improved moisture stability. Unlike conventional semiconductors, ion migration in perovskite is a unique phenomenon possibly responsible for long carrier lifetime, current–voltage hysteresis, and low-frequency giant dielectric response. While there are many studies of ion migration in bulk hybrid perovskite, not much is known for its 2D counterparts, especially for ion migration induced by light excitation. Here, we construct an exfoliated 2D perovskite/carbon nanotube (CNT) heterostructure field effect transistor (FET), not only to demonstrate its potential in photomemory applications, but also to study the light induced ion migration mechanisms. We show that the FET I–V characteristic curve can be regulated by light and shows two opposite trends under different CNT oxygen doping conditions. Our temperature-dependent study indicates that the change in the I–V curve is probably caused by ion redistribution in the 2D hybrid perovskite. The first principle calculation shows the reduction of the migration barrier of I vacancy under light excitation. The device simulation shows that the increase of 2D hybrid perovskite dielectric constant (enabled by the increased ion migration) can change the I–V curve in the trends observed experimentally. Finally, the so synthesized FET shows the multilevel photomemory function. Our work shows that not only we could understand the unique ion migration behavior in 2D hybrid perovskite, it might also be used for many future memory function related applications not realizable in traditional semiconductors.