Energy Storage Devices Research Articles

Constructing stable solid electrolyte interphases to enhance the calendar life of anode-free lithium metal batteries

Advancement of anode-free lithium metal batteries (AFLMBs) has been appreciated due to their exceptional volumetric energy density and manufacturing simplicity. However, AFLMBs suffer from poor cycle performance due to the absence of excess lithium and current research lacks an understanding of lithium corrosion and calendar aging in AFLMBs. Here, we investigate the practical effects of chemical and galvanic corrosion on battery performance (calendar life) through a modified full-cell cycling experiment. The impact of galvanic corrosion is negligible, whereas chemical corrosion during a rest period reduces the capacity retention (CR) from 47% to 32% over 100 cycles. Consequently, a corrosion control strategy is implemented, substituting 1,2-dimethoxyethane with 1,2-dimethoxypropane (DMP), which has weaker lithium ion solvation properties due to its inductive effect. This approach results in the formation of a Li2O-rich solid-electrolyte interphase (SEI) layer, which effectively prevents direct contact of lithium with the electrolyte and the formation of additional SEI layers. Consequently, the decrease in CR due to chemical corrosion is considerably mitigated in DMP-based electrolyte, with only a slight decrease from 51% to 49%. This study provides insights into lithium corrosion in practical AFLMBs and offers guidelines for designing less-corrosive electrolytes to improve AFLMB performance for diverse industrial applications as energy-storage devices.

Capacity planning of storage batteries for remote island microgrids with physical energy storage with CO2 phase changes

Energy storage devices based on the CO2 thermal cycle (CTC) lose competitiveness in cold regions because of the decreased energy volume density and charge–discharge efficiency at low ambient temperatures. The CO2 hybrid thermal cycle (CHS) combines the CTC with the CO2 hydrate thermal cycle (CHT) to maintain a high energy volume density and charge–discharge efficiency regardless of the ambient temperature. The CHS-based energy storage system has been shown to have an energy volume density of 80–140 kWh/m3 and charge–discharge efficiency of 60 %–106 %. However, the economic feasibility of this system has not been considered. In this study, a numerical analysis was performed on the practical application and economic feasibility of CHS-based energy storage for the 100 % renewable energy microgrid of a pair of remote islands. In a case analysis of CHS installation in a microgrid for a remote island with 100 % renewable energy, the construction cost of CHS was 2/3 of that of pumped storage; the equipment cost reduction of CHS strongly depends on the type and amount of renewable energy to be interconnected, and therefore, the overall optimization of the power system is necessary.

Cellulose-based water-in-salt ZnBr2 hydrogels with multiple functions for energy storage devices

A new simple preparation method for water-in-salt-type cellulose hydrogels filled with ZnBr2 is reported. The ZnBr2 hydrogel electrolyte can be applied to zinc-ion hybrid supercapacitors and Zn-Br interfacial batteries. The water-in-salt ZnBr2 hydrogel serves as a stable electrolyte for good electrochemical performance in zinc-ion hybrid supercapacitors without redox reactions and can participate in the charging/discharging process of the Zn‒Br interfacial battery through the redox reaction of halogen anions. The assembled zinc-ion hybrid supercapacitor exhibits a specific capacitance of 241.8 F g-1 at 0.5 A g-1. Furthermore, after a 10000-cycle-long charging/discharging test at 5 A g-1, the coulombic efficiency is found at nearly 100%. The interfacial battery reaches the highest energy efficiency of 66% at a current density of 3 mA cm-2 at 0–1.8 V and displays a good cycle life with an 88% average coulombic efficiency at 3 mA cm-2. This work expands the application of hydrogels and provides new possibilities for optimizing the high performance of electrolytes.

Application and research of three-dimensional composite fiber interlayer in lithium-sulfur battery

Lithium-sulfur (Li-S) batteries are regarded to be the most probable‌ electrochemical energy storage devices for large-scale applications because of their high energy and green characteristics. Nevertheless, the polysulfides generated during the cycle of Li-S batteries can cause the "shuttle effect", reducing the utilization rate of sulfur (S) and electrochemical performance. In this study, a novel three-dimensional composite fiber (TDCF) interlayer was prepared and placed between the S cathode and the PP separator to adsorb polysulfide and restrain the "shuttle effect". The results show that the capacity retention rate of Li-S battery is still higher 3:50 ratio, 200ml of anhydrous ethanol was added to moisten, than 70% after 200 charge-discharge cycles at 1C.

Zinc cobalt sulfide with carbon (graphene oxide and CNT) based nanocomposite as positive electrode for asymmetric coin cell supercapacitors

Abstract The world dependence on portable electronic devices has increased the demand for high-performance energy storage devices. The use of transition metal sulfides as faradaic electrode materials for electrochemical energy storage is rapidly increasing due to their high energy density. Herein Zinc Cobalt Sulfide (ZCS) with graphene oxide (GO) and carbon nanotubes (CNT) were used to create an interconnected ZCS composite network using a solvothermal technique. The materials were characterized by utilizing XRD, FT-Raman, TGA, FESEM/EDX, XPS, and BET. The electrochemical performance of the materials was examined using cyclic voltammetry (CV), galvanostatic charge–discharge (GCD), and electrochemical impedance spectroscopy (EIS). The prepared electrodes exhibited both pseudocapacitor behavior and double-layer capacitor behavior, indicating the hybrid nature. Furthermore, All the electrode ZCS, ZCS/GO, ZCS/CNT, and ZCS/GO/CNT electrodes demonstrated higher capacitance behavior, with values of 420, 551, 585 and 811 F g−1 at 1 A/g. Among these ZCS/GO/CNT electrode exhibits outstanding electrochemical properties, with a notable retention of 81.08% at 10 Ag−1 because Combining ZCS nanoparticles with interconnected GO and CNT provides excellent electronic conductivity and stability. The assembled ZCS/GO/CNT//graphene oxide asymmetric coin cell (ACC) supercapacitor showed a high energy density of 33.3 Wh kg–1 at a power density of 624 W kg–1. The 3D nanostructure of ZCS/GO/CNT/Graphene oxide has great potential for developing foldable energy storage devices.

Functional metal-organic frameworks derived electrode materials for electrochemical energy storage: a review.

Pristine metal-organic frameworks (MOFs) are built through self-assembly of electron rich organic linkers and electron deficient metal nodes via coordinate bond. Due to the unique properties of MOFs like highly tunable frameworks, huge specific surface areas, flexible chemical composition, flexible structures and a large volume of pores, they are being used to design the electrode materials for electrochemical energy storage devices. As per the literature, MOFs (including manganese, nickel, copper, and cobalt-based zeolitic imidazolate frameworks (ZIFs), University of Oslo (UiO) MOFs, Hong Kong University of Science and Technology (HKUST) MOFs and isoreticular MOFs (IRMOFs)) have attracted much attention in the field of supercapacitors (SCs)/batteries. According to their dimensionality such as 1D, 2D and 3D, pristine MOFs are mainly used as SC materials. Highly porous materials and their composites are capable for intercalation of metal ions (Na+/Li+). Moreover, the supramolecular features (π⋯π, C-H⋯π, hydrogen bond interactions) of redox stable MOFs provide better insight for electrochemical stability. So, this review provides an in-depth analysis of pure MOFs and MOF derived composites (MOF composites and MOF derived porous carbon) as electrode materials and also discusses their metal ion charge storage mechanism. Finally, we provide our perspectives on the current issues and future opportunities for supercapacitor materials.

Carbon nanostructures for high-frequency line-filtering supercapacitors

Supercapacitors (SCs) are considered one of the front-runner energy storage devices for future electronic and automobile device applications. Even though their high-power densities, fast charge/discharge, and long cycling stabilities make them promising for future applications, their charge-discharge takes place below 1 Hz, a major issue for using them as capacitors for line filtering applications. Therefore, developing ultrafast electrochemical supercapacitors with alternating current (AC) line filtering functions has gained research attention to replace conventional aluminum electrolytic capacitors (AEC). Most available SCs possess resistive features rather than capacitive at 120 Hz because of the electrode geometry and configurations, which is a bottleneck in the research of line-filtering SCs. Addressing this challenge could be possible by developing novel electrode materials using hybrid nanostructures to meet the critical requirements for line filtering functions. This mini-review focuses on the advancement and challenges of carbon nanostructure-based electrode materials for AC line filtering applications and the future directions of this growing research area.

Hydrothermal synthesis of rGO and MnCoS composite for enhanced supercapacitor application

Nanostructured materials incorporating transition metal sulfides have demonstrated considerable potential across various applications, particularly in the realms of energy production and storage. Sulfide-based material preparation is a challenging and costly procedure that requires a high temperature and reducing atmosphere. This work reports that manganese cobalt sulfide (MCS) and reduced graphene oxide composite manganese cobalt sulfide (rMCS) were successfully prepared through a hydrothermal method. Various characterization techniques were employed to analyze the prepared materials, including X-ray diffraction, field emission scanning electron microscopy, transmission electron microscopy, Brunauer-Emmett-Teller analysis, and X-ray photoelectron spectroscopy. In a three-electrode system, MCS and rMCS electrodes exhibit an excellent specific capacitance of 1695 and 1925 F g−1 at 1 A g−1 current density respectively. MCS delivers the capacitance retention of 99% and rMCS exhibits the capacitance retention of 100% capacitance retention over 5000 consecutive cycles. The constructed asymmetric supercapacitor electrode (rMCS//rGO) exhibits the energy and power density of 64 Wh kg−1 at 799 W kg−1, respectively with outstanding cyclic stability of 97.4% even after 10,000 cycles. The exceptional electrochemical properties of MCS with rGO composite electrode indicate that they would make an outstanding electrode material for cutting-edge energy storage devices.

Customizing Biochar Formulations: Enabling Sustainable and High-Performance Electrochemical Energy Storage Devices

Customizing Biochar Formulations: Enabling Sustainable and High-Performance Electrochemical Energy Storage Devices

Fabrication and High Dielectric Properties of Sandwich-Structured Ba0.6Sr0.4TiO3/Polyvinylidene Fluoride Layered Composites with Multiscale Parallel Interfaces.

Ba0.6Sr0.4TiO3/polyvinylidene fluoride (BST/PVDF) dielectric functional composites have been widely used in flexible wearable devices, capacitors, and energy storage devices. In addition to the ceramic phase type, polymer matrix type, composition, and interfacial connectivity of BST/PVDF composite materials, their morphology also significantly influences their electrical characteristics. Therefore, herein, sandwich-structured BST/PVDF layered composites were designed and prepared via tape-casting processing using different types of BST fillers [i.e., formless zero-dimensional (0D)-BST, rod-like one-dimensional (1D)-BST, and plate-like two-dimensional (2D)-BST]. The microstructures and electrical characteristics of sandwich-structured BST/PVDF composites were studied in relation to the BST morphology. The effects of the internal mechanisms of different interfacial models on the breakdown strength of BST/PVDF composites were discussed. According to our findings, unlike the 0D-BST and 1D-BST powders, 2D-BST powders form multiscale parallel interfaces in sandwich-structured composites due to their unique lamella-like morphology, which enhances the breakdown strength of sandwich-structured composites. Sandwich-structured BST/PVDF composites containing 2D-BST powders exhibit good electrical characteristics with an energy storage density of 19.71 J/cm3, an energy storage efficiency of 85.3%, a dielectric constant of 30.4 (1 kHz), a dielectric loss of 0.036 (1 kHz), and a dielectric tunability of 93.2%. This study provides a method for preparing functional composites with high dielectric tunability and high energy storage characteristics.

Multi-scale self-attention feature decoupling transfer network-based cross-domain capacity prediction of lithium-ion batteries

Lithium-ion battery capacity prediction is paramount for improving the safety and reliability of energy storage devices. However, accurately predicting capacity continues to pose a challenge, stemming from the diverse range of operational conditions and the lack of capacity labels under unpredictable scenarios. To tackle this problem, a multi-scale self-attention feature decoupling transfer network is proposed for predicting the capacity of lithium-ion batteries across diverse operational conditions. More specifically, multi-scale multi-head self-attentive encoders are designed to adaptively extract multi-scale domain-invariant features by using a series of loss function constraints. Then, transfer learning integrated with domain-invariant features is applied to migrate the knowledge of the source domain to the target domain for cross-domain capacity prediction. Finally, the performance of our methods is authenticated through utilization of two battery dataset, each including three different charging and discharging scenarios. Experimental results indicate that the proposed method effectively extracts domain-invariant features from charging data characterized by varying distributions, thereby mitigating the effects of distributional differences. Compared with the state-of-the-art methods, the proposed method shows excellent accuracy and bustling ability in two datasets.

Grid Integration of Offshore Wind Energy: A Review on Fault Ride Through Techniques for MMC-HVDC Systems

Over the past few decades, wind energy has expanded to become a widespread, clean, and sustainable energy source. However, integrating offshore wind energy with the onshore AC grids presents many stability and control challenges that hinder the reliability and resilience of AC grids, particularly during faults. To address this issue, current grid codes require offshore wind farms (OWFs) to remain connected during and after faults. This requirement is challenging because, depending on the fault location and power flow direction, DC link over- or under-voltage can occur, potentially leading to the shutdown of converter stations. Therefore, this necessitates the proper understanding of key technical concepts associated with the integration of OWFs. To help fill the gap, this article performs an in-depth investigation of existing alternating current fault ride through (ACFRT) techniques of modular multilevel converter-based high-voltage direct current (MMC-HVDC) for OWFs. These techniques include the use of AC/DC choppers, flywheel energy storage devices (FESDs), power reduction strategies for OWFs, and energy optimization of the MMC. This article covers both scenarios of onshore and offshore AC faults. Given the importance of wind turbines (WTs) in transforming wind energy into mechanical energy, this article also presents an overview of four WT topologies. In addition, this article explores the advanced converter topologies employed in HVDC systems to transform three-phase AC voltages to DC voltages and vice versa at each terminal of the DC link. Finally, this article explores the key stability and control concepts, such as small signal stability and large disturbance stability, followed by future research trends in the development of converter topologies for HVDC transmission such as hybrid HVDC systems, which combine current source converters (CSCs) and voltage source converters (VSCs) and diode rectifier-based HVDC (DR-HVDC) systems.

Risk assessment for Na-Zn liquid metal batteries

Background Na-Zn liquid metal batteries, which operate at 600 °C, have recently been proposed as inexpensive stationary energy storage devices. As with any other electrochemical cell, their fabrication and operation involves certain risks, which need to be well understood in order to be minimised. Methods A risk assessment according to ISO 12100 is performed at the cell level for operating Na-Zn cells in the laboratory environment. Hazard identification and risk evaluation are systematically addressed, including a thorough literature review, theoretical calculations and selected experiments. Results Cell overpressure is found to be one of the main risks – and might be caused either by mistakes in battery production (humidity) or operation (over-charge/discharge). In terms of cell housing, the weakest component is clearly the feedthrough. Its failure might lead to the release of hazardous aerosols to the environment. In this context, the candidate electrolyte components LiCl and BaCl2 are especially dangerous, and should therefore be reduced or avoided if possible. Conclusions Overall, Na-Zn cells are expected to reach a very high safety level, similar to state-of-the-art ZEBRA technology, as they are not prone to thermal runaway. However, considering the still low TRL level and open questions concerning the durability of certain parts of their housing, the batteries should preferably be operated under a fume hood.

3D Binder-Free Mo@CoO Electrodes Directly Manufactured in One Step via Electric Discharge Machining for In-Plane Microsupercapacitor Application

Cobalt oxide-based in-plane microsupercapacitors (IPMSCs) stand out as a favorable choice for various applications in energy sources for the Internet of Things (IoT) and other microelectronic devices due to their abundant natural resources and high theoretical specific capacitance. However, the low electronic conductivity of cobalt oxide greatly hinders its further application in energy storage devices. Herein, a new manufacturing method of electric discharging machining (EDM), which is simple, safe, efficient, and environment-friendly, has been developed for synthesizing Mo-doped and oxygen-vacancy-enriched Co-CoO (Mo@Co-CoO) integrated microelectrodes for efficiently constructing Mo@Co-CoO IPMSCs with customized structures in a single step for the first time. The Mo@Co-CoO IPMSCs with three loops (IPMSCs3) exhibited a maximum areal capacitance of 30.4 mF cm−2 at 2 mV s−1. Moreover, the Mo@Co-CoO IPMSCs3 showed good capacitive behavior at a super-high scanning rate of 100 V s−1, which is around 500–1000 times higher than most reported CoO-based electrodes. It is important to note that the IPMSCs were fabricated using a one-step EDM process without any assistance of other material processing techniques, toxic chemicals, low conductivity binders, exceptional current collectors, and conductive fillers. This novel fabrication method developed in this research opens a new avenue to simplify material synthesis, providing a novel way for realizing intelligent, digital, and green manufacturing of various metal oxide materials, microelectrodes, and microdevices.

Ternary g-C3N4/Co3O4/CeO2 nanostructured composites for electrochemical energy storage supercapacitors

Extensive use of fossil fuels causes heavy discharge of carbon dioxide, depleting energy resources and this requires environmentally friendly and effective energy storage materials. Hybrid supercapacitors (HSCs) are recently developed as effective energy storage materials enabling high capacitance retention rate and quick charging. Herein, synthesis of two-dimensional g-C3N4 nanosheets supported onto three-dimensional flower-like Co3O4/CeO2 (CoCe) ternary synergistic heterostructures are developed as effective electrodes for hybrid supercapacitor applications. Addition of g-C3N4 produces substantial surface active sites, enabling its synergistic effect with CoCe to enhance electrochemical performance having exceptional conductivity. The CoCe/g-C3N4 ternary composite electrode exhibits a higher specific capacitance of 1088.3 F g−1 at 1 A g-1 with 96 % of recycling stability over 5000 cycles, which is ∼5.5 and ∼5 folds higher specific capacitance than the pristine g-C3N4 and CoCe electrodes. EIS analysis revealed that CoCe/g-C3N4 electrode offered reduced charge transfer resistance compared to pristine electrodes. The fabricated two-electrode HSC device displays outstanding retention after 10,000 cycles with an ultra-high specific capacitance of 119.8 F g−1, excellent energy density 37.4 Wh kg−1 and power density of 749.9 W kg−1. This research showcases the perspectives of CoCe/g-C3N4 ternary electrodes in hybrid supercapacitors and other renewable energy storage devices.

A low-impedance cation exchange membrane for extending the voltage window of a BiFeO3-based hybrid supercapacitor through the pH-decoupling strategy

Aqueous hybrid supercapacitors usually suffer from narrow voltage window and poor energy density limited by the thermodynamic decomposition of water. Based on necessary ion exchange membranes, the pH-decoupling strategy can be employed to extend the voltage windows of hybrid supercapacitors. We propose a novel low-impedance cation exchange membrane (CEM) with the interpenetrating network structure for the pH-decoupling strategy, which can be served as the separator for a AC//BiFeO3 supercapacitor to enhance its electrochemical performances. Two CEMs (CEM-1 and CEM-2) with different contents of the sulfonic acid (-SO3H) groups, are fabricated through the copolymerization of various monomers according to the predetermined stoichiometric ratios. FTIR, Raman and XPS spectra reflect that the CEM-2 has the higher content of the -SO3H groups than the CEM-1. The membrane parameter measurements confirm that the CEM-2 has the higher water content, lower membrane resistance and smaller membrane thickness than the CEM-1. The AC//BiFeO3 supercapacitor with the CEM-2 separator provides the higher gravimetric specific capacitance of 61 F g−1 at 1 A g−1, better coulombic efficiencies (97 % ∼ 103 %) and lower Rs value (39.74 Ω) than the AC//BiFeO3 supercapacitors with the CEM-1 separator. Moreover, the AC//BiFeO3 supercapacitor with the CEM-2 separator delivers the maximum energy density of 41.0 Wh kg−1 at the power density of 1.1 kW kg−1. The pH-decoupling strategy with this low-impedance CEM, opens up a new avenue for developing high-performance aqueous electrochemical energy storage devices.

Enhancing the Filler Utilization of Composite Gel Electrolytes via In Situ Solution-Processable Method for Sustainable Sodium-Ion Batteries.

The composite gel electrolyte (CGE), which combines the advantages of inorganic solid-state electrolytes and solid polymer electrolytes, is regarded as the ultimate candidate for constructing batteries with high safety and superior electrode-electrolyte interface contact. However, the ubiquitous agglomeration of nanofillers results in low filler utilization, which seriously reduces structural uniformity and ion transport efficiency, thus restricting the development of consistent and durable batteries. Herein, a solution-processable method to in situ construct CGE with high filler utilization is introduced. The homogeneous metal-organic framework fillers contribute to uniform ionic and electronic filed distribution, realizing a stable electrode-electrolyte interface. Consequently, the CGE with high filler utilization achieves an ultra-long lifespan of 10000 cycles with a capacity retention of 80.2%. This work provides guidance for constructing high-performance CGEs in electrochemical energy-storage devices.

Crystal structure of nickel orthovanadate (Ni3V2O8) at 299 (3) K and 1323 (8) K: an X-ray diffraction study.

Nickel orthovanadate is a promising material with potential applications in energy storage and photocatalytic devices. The crystal structure of Ni3V2O8 at 299 (3) K and 1323 (8) K was studied using X-ray powder diffraction. The sample was a single-phase orthorhombic kagome-staircase-Ni3(VO4)2-type structure (space group Cmca) at both temperatures. The phase purity and morphology was studied using energy-dispersive X-ray spectroscopy and scanning electron microscopy. The refined unit-cell parameters at 299 (3) K are a = 5.93384 (4) Å, b = 11.38318 (7) Å and c = 8.23818 (5) Å, and at 1323 (8) K are a= 6.02077 (7) Å, b = 11.48838 (7) Å and c = 8.32611 (9) Å. The obtained results indicate thermal expansion anisotropy, with a largest expansivity along a. Variations in Ni-O and V-O bonds with temperature are observed. The variation in the Ni-O bond is about one order higher in magnitude than that of the V-O bond, signifying the high rigidity of V-O bonds. The unit-cell size variations with rising effective ionic volume of the divalent A ion in the A3B2O8 family [A = Ni, Mg, Zn, Co, Mn (experimental data) and also A = Cu, Cd (theoretical data), B = V or As] are analyzed. Based on experimental and theoretical data, trends within the family are observed and the unit-cell size for reported solid solution of nickel (87%) and copper (13%) mixture in (Ni1-xCux)3V2O8 are predicted. Predictions are also provided for some hypothetical A3B2O8 ternary compound and solid solutions.

Investigation of Onboard Power Generation Method Using Waste Energy at Altitude Conditions

Abstract Demand for increased onboard power generation on small aircraft is ever growing due to increased electrical power demand. The objective of this study is to investigate onboard power generation using waste energy at various altitude and ambient conditions. Experiments were performed on a 2-liter turbocharged direct-injection intermittent combustion engine fueled with jet fuel (F-24). The turbocharger has an integrated motor-generator system that is designed to generate electricity using the enthalpy in the engine exhaust, or to use as a motor to rotate the turbocharger using the energy stored in onboard energy storage device. An electrically actuated wastegate was used to control useful exhaust flowrate that is used to generate electricity. An external power supply system was used to emulate the energy storage. The engine was installed in the US DEVCOM Army Research Laboratory?s altitude chamber in which absolute outside air pressure was controlled up to 37.6 kPa to simulate the altitude conditions up to 7,620 m (25,000 ft). Three different ambient conditions were selected to represent International Standard Atmosphere (ISA), polar, and tropical conditions. Based on the experimental results, a response surface model was developed to predict the power generation of the motor-generator integrated turbocharger using waste energy as a function of altitude, engine power, and the wastegate position. The result from this study provides an insight into onboard power generation method using waste energy at altitude conditions.

Transition Metal (Co, Ni, Fe) Selenides by Selenization of Gallic Acid based MOFs used as Na‐Ion Battery Anodes

AbstractSodium‐ion batteries (NIBs) are gaining momentum, thanks to the increasing demand for energy storage devices and the abundant reserves and low sodium cost. Transition metals are well‐established materials due to their high conductivity and electrochemical activity. In this work, metal selenides (MSex) (M: Ni, Co, Fe) are obtained by facile selenization in a single step of transition gallic acid based metal organic frameworks (MOFs) under Ar flow at 600 °C. As the powders undergo selenization, the resulting MSex particles are encapsulated within the amorphous carbon network formed by the decomposition of the gallate ligand. The microstructures are examined by HR‐TEM analyses and the characteristic interplanar spacing of each transition metal selenide is measured and found to coincide with the XRD pattern. Meanwhile, the specific surface areas were measured as 121, 152, and 155 m2/g for CoSe2, NiSe and FeSe, respectively. The resulting NiSe/C, CoSe2/C and FeSe/C nanomaterials are tested as NIB negative electrodes and are shown to have a capacity of 315, 312, and 363 mAh/g, respectively, after 100 cycles at a current density of 100 mA/g while Na‐ion diffusion coefficients (DNa+) are calculated in the range of 10−10–10−7 cm2/s by galvanostatic intermittent titration (GITT) technique.