Aluminum Sulfur Research Articles

Electrocatalysis for Continuous Multi-Step Reactions in Quasi-Solid-State Electrolytes Towards High-Energy and Long-Life Aluminum-Sulfur Batteries.

Aluminum-sulfur (Al-S) batteries of ultrahigh energy-to-price ratios are a promising energy storage technology, while they suffer from a large voltage gap and short lifespan. Herein, we propose an electrocatalyst-boosting quasi-solid-state Al-S battery, which involves a sulfur-anchored cobalt/nitrogen co-doped graphene (S@CoNG) positive electrode and an ionic-liquid-impregnated metal-organic framework (IL@MOF) electrolyte. The Co-N4 sites in CoNG continuously catalyze the breaking of Al-Cl and S-S bonds and accelerate the sulfur conversion, endowing the Al-S battery with a shortened voltage gap of 0.43 V and a high discharge voltage plateau of 0.9 V. In the quasi-solid-state IL@MOF electrolytes, the shuttle effect of polysulfides has been inhibited, which stabilizes the reversible sulfur reaction, enabling the Al-S battery to deliver 820 mAh g-1 specific capacity and 78 % capacity retention after 300 cycles. This finding offers novel insights to design Al-S batteries for stable energy storage.

Electrocatalysis for Continuous Multi‐Step Reactions in Quasi‐Solid‐State Electrolytes Towards High‐Energy and Long‐Life Aluminum–Sulfur Batteries

AbstractAluminum–sulfur (Al−S) batteries of ultrahigh energy‐to‐price ratios are a promising energy storage technology, while they suffer from a large voltage gap and short lifespan. Herein, we propose an electrocatalyst‐boosting quasi‐solid‐state Al−S battery, which involves a sulfur‐anchored cobalt/nitrogen co‐doped graphene (S@CoNG) positive electrode and an ionic‐liquid‐impregnated metal–organic framework (IL@MOF) electrolyte. The Co−N4 sites in CoNG continuously catalyze the breaking of Al−Cl and S−S bonds and accelerate the sulfur conversion, endowing the Al−S battery with a shortened voltage gap of 0.43 V and a high discharge voltage plateau of 0.9 V. In the quasi‐solid‐state IL@MOF electrolytes, the shuttle effect of polysulfides has been inhibited, which stabilizes the reversible sulfur reaction, enabling the Al−S battery to deliver 820 mAh g−1 specific capacity and 78 % capacity retention after 300 cycles. This finding offers novel insights to design Al−S batteries for stable energy storage.

Bifunctional TiN@N-doped-graphene catalyst based high sulfur content cathode for reversible Aluminum-Sulfur batteries

Aluminum-sulfur (Al-S) batteries are drawing extensive attentions for the development of economical battery systems owing to the high theoretical capacity of 1672 mAh g−1 and low-cost naturally abundant electrode materials, Al and S. However, Al-S batteries are beset by poor reversibility and low actual capacity, only few reversible Al-S cells have been reported until now. In this work, factors affecting Al–S batteries performance are explored and explained, such as the inactive discharge product(Al2S3), polysulfides diffusion and the self-discharge reaction on the surface of SEI-layer-free aluminum anode. And we demonstrate an improved Al–S battery by effectively adjust the voltage window and develop TiN@N-doped-graphene catalyst materials to modify the sulfur cathode. This catalyst is for the first time applied in Al–S batteries and explored its positive effects during the reversible Al-S reactions. Comprehensive electrochemical testing and ex-situ characterization certificates that, in the discharging process, the shuttle effect of polysulfides is suppressed based on the strong adsorption of catalyst; in the charging process, sulfide redox kinetics are promoted and the decomposition reaction barrier is reduced by the catalyst. The Al-S cell delivers an initial capacity of ∼993 mAh g−1 and maintains a capacity of ∼500 mAh g−1 after 200 cycles.

Towards understanding aluminum sulfur batteries with imidazolium-based electrolytes: A phenomenological model

Aluminum sulfur batteries with ionic liquid electrolytes are promising next-generation energy storage devices due to the high abundance of both aluminium and sulfur. However, very little understanding of the discharge mechanism is currently available, which hampers their development. Herein, a mathematical model that considers the complex electrochemical reduction of the sulfide species as well as the formation of the various polysulfides is developed to describe the discharge performance and the reversibility of Al–S cells at different current densities. The model is validated with experimental data obtained from Swagelok cells composed of Al metal anode, S@CNT cathode, and EMIMCl-AlCl3 ionic liquid electrolyte. The cells exhibited different discharge mechanisms and Al2S3 precipitation routes at the different current densities. The contact resistance between the composite electrode and the current collector was the main factor limiting the discharge performance at high current densities. The reversibility of the Al–S cells based on the formation of Al2S3 precipitates strongly depends on the operating current density. The developed model will serve as a tool for other researchers to enhance the electrochemical performance of aluminum sulfur batteries.

Binder-free S@Ti3C2Tx sandwich structure film as a high-capacity cathode for a stable aluminum-sulfur battery

The rechargeable aluminum-sulfur (Al-S) battery is a promising alternative-energy storage device with high energy density and made of cheap raw materials. However, Al-S batteries face several obstacles, especially the shuttle effect. Herein, a binder-free S@Ti3C2Tx sandwich structure film with uniform sulfur dispersion was designed. The two-dimensional (2D) layered material Ti3C2Tx not only has the function of binder and conductive agent but also is a promising host for sulfur anchoring. As a result, S@Ti3C2Tx film showed an initial capacity of 489 mA h g−1 at 300 mA g−1 and retained the value at 415 mA h g−1 after 280 stable cycles, with an average Coulombic efficiency of ∼95%. The film displayed higher capacity and stability than the S + Ti3C2Tx cathode prepared by the slurry-coating method (the initial capacity was 317 mA h g−1 and then decayed to 222 mA h g−1 after 160 cycles). The main capacity of S@Ti3C2Tx film in the Al-S battery came from the reversible redox reaction of S2− and S. This new 2D material combined with a controllable electrode structure design paves the way for the development of Al-S batteries.

Low-voltage-hysteresis aluminum-sulfur battery with covalently functionalized mesoporous graphene.

A pyridyl-functionalized mesoporous graphene is developed to accommodate sulfur for Al-S batteries, which can significantly reduce the voltage hysteresis to ∼0.43 V. The reaction kinetics of the Al-S battery are accelerated by the catalyst-free carbon host, ascribed to both the mesoporous graphene structure and the covalently functionalized pyridyl groups.

Cover Feature: Understanding the Oxidation and Reduction Reactions of Sulfur in Rechargeable Aluminum‐Sulfur Batteries with Deep Eutectic Solvent and Ionic Liquid Electrolytes (ChemSusChem 1/2022)

The Cover Feature shows that aluminum–sulfur (Al–S) battery energy storage containers could store electrical energy from photovoltaic and wind turbine systems. The Al–S battery was made of earth-abundant elements such as Al, S, Cl, N, C, and H. In addition, it shows high safety because of the use of an AlCl3-based non-flammable deep eutectic solvent electrolyte. More information can be found in the Full Paper by Y. Bian et al.

Rechargeable Aluminum-chalcogen Batteries: Molecular-Level Mechanistic Insights & Scientific Perspectives

Rechargeable aluminum metal batteries have recently garnered significant interest due to its low cost, earth abundance, inherent safety, and high volumetric and gravimetric capacities. Commercialization of rechargeable aluminum battery systems has been limited due to the small number of (i) electrolytes that enable reversible electrodeposition of Al metal and (ii) positive electrode materials that are (electro)chemically within those electrolytes while providing high capacity and cycle life. Metal-chalcogen (e.g., sulfur, selenium) batteries are among the most promising candidates for low-cost, high-energy-density electrochemical energy storage systems; however, to date, rechargeable aluminum-chalcogen batteries have seldom been explored scientifically or technologically.Here, rechargeable aluminum-sulfur (Al-S) and aluminum-selenium (Al-Se) cells were prepared with a chloroaluminate ionic liquid electrolyte (AlCl3:[EMIm][Cl], molar ratio of 1.5:1) and their electrochemical reaction mechanisms and products were studied at a molecular-level with solid-state nuclear magnetic resonance (NMR) spectroscopy. Galvanostatic cycling of both Al-S and Al-Se cells showed similar initial capacities and overpotentials to previously reported systems. For Al-S systems, the higher specific capacity upon charge, compared to discharge, points towards either an unwanted side reaction or deleterious polysulfide-shuttle-like behavior. This phenomenon is heavily prevalent at low current densities and affects reversibility. For Al-Se systems, two different reaction mechanisms that have been previously reported were found to be dependent on the applied current density and the crystallinity of the selenium. We were able to consolidate both reaction mechanisms during a single charge/discharge step, significantly increasing specific capacity and energy density. For both Al-S and Al-Se systems, significant capacity fade was also observed over multiple galvanostatic cycles. Solid-state 27Al single-pulse magic-angle-spinning (MAS) NMR spectra acquired on sulfur electrodes cycled to different states-of-charge reveal the presence of both solid and liquid discharge products. Solid-state 2D 27Al{27Al} multiple-quantum (MQ-)MAS measurements reveal the amorphous nature of the solid discharge product, while 2D 27Al{27Al} exchange spectroscopy (EXSY) NMR spectra reveal chemical exchange between electrolyte-soluble sulfur species coordinating with AlCl4 - and Al2Cl7 - after discharge. For Al-Se systems, solid-state 27Al and 77Se single-pulse MAS NMR measurements also reveal liquid-state reaction products in the electrolyte, leading to loss of active material from the cathode, while revealing compositional changes in the selenium electrode. Scientific perspectives on these aluminum-chalcogen battery chemistries will be discussed. This work also highlights the importance that molecular-level analytical tools, such as solid-state NMR spectroscopy, can have in clarifying electrochemical mechanisms in emerging electrochemical systems.

A New Strategy to Improve the Performance of Aluminum-Sulfur Battery

Both aluminum and sulfur are promising materials for future-generation electrochemical energy storage technology owing to their natural abundance, low cost, and high capacity. However, a rechargeable aluminum–sulfur (Al–S) battery with high energy density suffers from poor reversibility and very short cycle life. This paper demonstrates an Al–S battery which comprised of a MWCN/S cathode, aluminum anode and deep eutectic solvent of AlCl3/acetamide. The Al–S battery delivers an initial capacity of ∼453 mAh g−1 and maintains a capacity of up to ∼358 mAh g−1 after 200 cycles.

A cobalt-based metal-organic framework and its derived material as sulfur hosts for aluminum-sulfur batteries with the chemical anchoring effect.

With the urgent need to explore high-performance electrochemical energy storage systems, rechargeable Al-ion batteries (AIBs) have attracted attention from researchers and engineers due to their traits, such as abundance and safety. Among all the issues waiting to be solved, the development of a reliable positive electrode material with high specific capacity is an absolute priority for the commercialization of AIBs. Sulfur has a natural advantage when used as the active material, and its theoretical specific capacity is as high as 1675 mA h g-1. MOFs and MOF-derived materials have been proved to be promising hosts for Li-S batteries. Herein, we report a novel Al-S battery system employing MOF (ZIF-67) and MOF-derived materials as sulfur host materials. After being chemically combined with sulfur, the composite still maintains its unique well-defined polyhedron morphology. The voltage hysteresis phenomenon is effectively alleviated with the aid of the host matrix. DFT calculations confirm that ZIF-67 and carbonized ZIF-67-700 polyhedrons can act as an anchor point towards sulfur (S8) and polysulfides (Al2S3, Al2S6, Al2S12, and Al2S18), preventing the detrimental dissolution and shuttle effect. These findings can enlighten future researchers regarding Al-S batteries and broaden the application of MOFs in the field of electrochemical energy storage systems.

Recent advances in cathode engineering to enable reversible room-temperature aluminium-sulfur batteries.

The rigorous requirements, such as high abundance, cost-effectiveness, and increased storage capacities, pose severe challenges to the existing Li-ion batteries' long-term sustainability. Room-temperature aluminum–sulfur (Al–S) chemistry, in particular, is gaining importance due to its high theoretical energy density (1700 W h kg−1). Al–S battery technology is one of the emerging metal–sulfur candidates that can surpass current Li-ion chemistries. When coupled with sulfur, aluminum metal brings a cheap and energy-rich option to existing battery technologies. Owing to the unique virtues of the Al–S battery, it has garnered increasing interest among scientific communities. Al–S chemistry has been investigated for quite some time, yet the cell performance remained in its infancy, which poses a challenge to this technology's viability. Besides stabilizing the Al metal anode, the most important challenge in the practical development of Al–S batteries is the development of a suitable sulfur cathode material. Owing to the complexity of this multivalent system, numerous factors have been taken into account, but the best sulfur cathode is yet to be identified. A detailed exploration of sulfur cathodes and their implications on the battery performance are discussed in this mini-review article. We present a detailed picture of cathode materials that may serve as the reference guide for developing more practical cathode materials. Also, fundamental principles and challenges encountered in the development of the sulfur cathodes are highlighted. Through the knowledge disseminated in this mini-review, the development in the multivalent post-Li-ion battery can be accelerated. A glimpse of the future outlook on the Al–S battery system with different potential solutions is also discussed.

Understanding Electrochemical Reaction Mechanisms and Properties of Rechargeable Aluminum-Sulfur and Aluminum-Selenium Batteries

The continual rise in global energy demands coupled with recent incremental advances in the cost and energy density of lithium-ion batteries have given researchers an impetus to investigate high-energy-density batteries such as those that use multivalent-ions. Rechargeable Al metal batteries have recently garnered significant interest due to its low cost, earth abundance, inherent safety, and high volumetric (8040 mAh cm -3) and gravimetric (2980 mAh g -1) capacities due to the trivalent nature of Al3+ ions (3-electron redox). Commercialization of rechargeable aluminum battery systems has been limited, however, due to the small number of (i) electrolytes that enable reversible electrodeposition of Al metal at room temperature and (ii) positive electrode materials that are compatible with the electrolytes while providing high capacity and cycle life. Sulfur (S) and selenium (Se) are promising positive electrode materials for rechargeable battery systems as they exhibit very high specific capacities during electrochemical conversion reactions.Sulfur is a very low-cost material due to its earth abundance and generation as a byproduct of the petroleum industry, while it can provide a very high specific capacity up to 1675 mAh g-1. Commercialization of battery systems with sulfur electrodes has been hindered due to its low electrical conductivity, high volume expansion during galvanostatic cycling, and deleterious polysulfide shuttle reactions during charge and discharge1-3. Selenium, like sulfur, is also a chalcogen and is expected to exhibit similar electrochemical reactions. Although Se is a heavier element and consequently exhibits lower specific capacity (675 mAh g-1), it possesses significantly higher electronic conductivity compared to sulfur4 that would be beneficial in mitigating key challenges that faces sulfur electrodes, such as low active material utilization and high charge transfer overpotentials during galvanostatic cycling. To date, there are few reports of Al-S2,3, and Al-Se4,5 systems. There is no clear consensus on the electrochemical reaction mechanism during the charge/discharge process, while capacity fade plagues both systems during galvanostatic cycling.Here, aluminum-sulfur (Al-S) and aluminum-selenium (Al-Se) cells were investigated with a chloroaluminate ionic liquid electrolyte, AlCl3:[EMIm][Cl] (molar ratio of 1.5:1), whose electrochemical reaction mechanisms and ensuing reaction products were analyzed via a combination of bulk (XRD, NMR) and surface (XPS) analyzation techniques. Electrochemical experiments for Al-S batteries showed high initial capacities close to 1200 mAh g-1, which exhibited a inverse correlation with galvanostatic cycling rate. For Al-S batteries a higher capacity was observed upon charge, compared to discharge, pointing towards either an unwanted side reaction or polysulfide-shuttle-like behavior. This behavior occurred predominately at low current densities and affected the reversibility of the system. For the Al-Se batteries, the two different reaction mechanisms that have been previously reported were found to be dependent on (i) the applied current density and (ii) the crystallinity of the active material, as shown by XRD. We demonstrate that, with appropriate cycling conditions, both reaction mechanisms can occur during a single charge/discharge step, thus significantly increasing the capacity and the electrochemical window of the Al-Se cell. A significant capacity fade was also observed over a course of multiple cycles showing similar behavior compared to Al-S cells.To understand the reaction mechanisms of Al-S and Al-Se batteries at a molecular level, solid-state and liquid-state 27Al NMR and 77Se NMR measurements (on Al-Se systems) were acquired on the electrodes and electrolyte at different states-of-charge, respectively. XPS was also performed on cycled chalcogen electrodes to understand surface compositions. The combination of electrochemical, spectroscopic, and X-ray diffraction data yield insights into the reaction mechanisms and products, including the aluminum coordination environments and relative populations in these environments. This work establishes the potential of conversion-type aluminum-chalcogen batteries as high-energy-density energy storage systems and highlights the importance that molecular-level analytical tools like NMR spectroscopy can have in clarifying electrochemical mechanisms in emerging electrochemical systems.Reference L. Ellis, K. T. Lee, and L. F. Nazar, Chemistry of Materials, 22, 691 (2010).Gao et al., Angew. Chemie - Int. Ed., 55, 9898–9901 (2016).. X. Yu, M. J. Boyer, G. S. Hwang, and A. Manthiram, Chem, 4, 586–598 (2018).Liu et al., Nano Energy, 66, 104159 (2019)Li, J. Liu, X. Huo, J. Li, and F. Kang, ACS Appl. Mater. Interfaces, 11, 45709–45716 (2019).

A Critical Evaluation of the Effect of Electrode Thickness and Side Reactions on Electrolytes for Aluminum-Sulfur Batteries.

The high abundance and low cost of aluminum and sulfur make the Al–S battery an attractive combination. However, significant improvements in performance are required, and increasing the thickness and sulfur content of the sulfur electrodes is critical for the development of batteries with competitive specific energies. This work concerns the development of sulfur electrodes with the highest sulfur content (60 wt %) reported to date for an Al–S battery system and a systematic study of the effect of the sulfur electrode thickness on battery performance. If low‐cost electrolytes made from acetamide or urea are used, slow mass transport of the electrolyte species is identified as the main cause of the poor sulfur utilization when the electrode thickness is decreased, whereas complete sulfur utilization is achieved with a less viscous ionic liquid. In addition, the analysis of very thin electrodes reveals the occurrence of degradation reactions in the low‐cost electrolytes. The new analysis method is ideal for evaluating the stability and mass transport limitations of novel electrolytes for Al–S batteries.

Theoretical Insights into the Charge and Discharge Processes in Aluminum–Sulfur Batteries

With the limited number of studies available (mostly experimental), the field of aluminum–sulfur (Al–S) batteries is in urgent need of understanding their complex electrochemical reactions. Herein, the ab initio molecular dynamics (AIMD) simulations are used to obtain a detailed understanding of the involved charging and discharging processes in Al–S batteries by analysis of interfacial systems S8(001)/[EMIM]AlCl4 and Al2S3(001)/[EMIM]AlCl4–electrolyte, respectively. We observe that during the discharging process, the reduction of S8 follows a layer-by-layer mechanism and involves the formation of various cationic and anionic intermediate species, which drives the formation of Al polysulfides during the course of the discharging process. The evolution of the discharge voltage profile studied over a limited timescale shows two voltage domains: the first voltage domain (1.87–2.10 V) corresponds to the interface effects of surface–electrolyte and second the voltage domain of 1.38–1.50 V starts coinciding with the experimental value of 1.30 V and involves the reduction of S8 to higher-order polysulfides. We also observe the diffusion of these higher-order Al polysulfides to the electrolyte, which is in accordance with the experimentally observed solvation of higher-order Al poylsulfides into the electrolyte. The evolution of the atomistic structure and reaction voltage during charging shows that the top atomic sublayer structural distortions are mainly limited to the layer where Al atoms are removed and not the other inner atomic layers, which could be the reason for the poor electrochemical reversibility and an increased overall charging voltage (1.75 V) in experimentally studied Al–S batteries. We believe that the new atomistic insights obtained about the formation of various intermediate species, competition of different reaction mechanisms, and importance of local Al concentration could help improve the understanding of the complex electrochemical processes observed in Al–S batteries.

A rechargeable aqueous aluminum-sulfur battery through acid activation in water-in-salt electrolyte.

We demonstrated a rechargeable aqueous Al-S battery based on a water-in-salt electrolyte with the configuration Al‖Al(OTF)3 + LiTFSI + HCl‖S/C. The superconcentrated LiTFSI trapped water molecules to inhibit the hydrolysis of aluminum polysulfides in the cathode, and the HCl additive provided a mild acidic environment to enable repeatable oxidation-reduction reactions in the anode.

Design of a composite cathode and a graphene coated separator for a stable room-temperature aluminum–sulfur battery

A S/host composite cathode and a graphene coated separator are designed for a room-temperature Al–S battery with stable and high capacity.

A low-cost deep eutectic solvent electrolyte for rechargeable aluminum-sulfur battery

Aluminum-sulfur (Al-S) battery is a promising candidate of next generation rechargeable batteries owing to its high theoretical energy density, high safety and low cost, but currently greatly impeded by the shortage of high-performance and cost-effective electrolytes. In this work, a low-cost deep eutectic solvent, i.e., AlCl3/acetamide, as the electrolyte for reversible room-temperature Al-S battery has been reported. The Al-S battery delivers an initial capacity above 1500 mA h g−1 and good rate performance, which is most probably caused by the presence of AlCl4−, Al2Cl7− and [AlCl2·(acetamide)2]+ ions as indicated by spectroscopic analysis. Furthermore, X-ray photoelectron spectroscopy on the discharged sulfur cathode suggests the presence of aluminum sulfides including Al2S3 as electrochemical conversion products. Density functional theory calculations on the energy barriers provide insights into the electrochemical reaction pathways. This AlCl3/acetamide based Al-S battery presents a promising prospect for high-performance and low-cost energy storage system in future.

Carbonized‐MOF as a Sulfur Host for Aluminum–Sulfur Batteries with Enhanced Capacity and Cycling Life

AbstractThe rechargeable aluminum–sulfur (Al–S) battery is a promising next generation electrochemical energy storage system owing to its high theoretical capacity of 1672 mAh g−1 and in combining low‐cost and naturally abundant elements, Al and S. However, to date, its poor reversibility and low lifespan have limited its practical application. In this paper, a composite cathode is reported for Al–S batteries based on S anchored on a carbonized HKUST‐1 matrix (S@HKUST‐1‐C). The S@HKUST‐1‐C composite maintains a reversible capacity of 600 mAh g−1 at the 75th cycle and a reversible capacity of 460 mAh g−1 at the 500th cycle under a current density of 1 A g−1, with a Coulombic efficiency of around 95%. X‐ray diffraction and Auger spectrum results reveal that the Cu in HKUST‐1 forms S–Cu ionic clusters. This serves to facilitate the electrochemical reaction and improve the reversibility of S during charge/discharge. Additionally, Cu increases the electron conductivity at the carbon matrix/S interface to significantly decrease the kinetic barrier for the conversion of sulfur species during battery operation.

Room-Temperature Aluminum-Sulfur Batteries with a Lithium-Ion-Mediated Ionic Liquid Electrolyte

Summary Aluminum-sulfur (Al-S) chemistry is attractive for the development of future-generation electrochemical energy storage technologies. However, to date, only limited reversible Al-S chemistry has been demonstrated. This paper demonstrates a highly reversible room-temperature Al-S battery with a lithium-ion (Li + -ion)-mediated ionic liquid electrolyte. Mechanistic studies with electrochemical and spectroscopic methodologies revealed that the enhancement in reversibility by Li + -ion mediation is attributed to the chemical reactivation of aluminum polysulfides and/or sulfide by Li + during electrochemical cycling. The results obtained with X-ray photoelectron spectroscopy and density functional theory calculations suggest the presence of a Li 3 AlS 3 -like product with a mixture of Li 2 S- and Al 2 S 3 -like phases in the discharged sulfur cathode. With Li + -ion mediation, the cycle life of room-temperature Al-S batteries is greatly improved. The cell delivers an initial capacity of ∼1,000 mA hr g −1 and maintains a capacity of up to 600 mA hr g −1 after 50 cycles.

An Aluminum–Sulfur Battery with a Fast Kinetic Response

AbstractThe electrochemical performance of the aluminum‐sulfur (Al‐S) battery has very poor reversibility and a low charge/discharge current density owing to slow kinetic processes determined by an inevitable dissociation reaction from Al2Cl7− to free Al3+. Al2Cl6Br− was used instead of Al2Cl7− as the dissociation reaction reagent. A 15‐fold faster reaction rate of Al2Cl6Br− dissociation than that of Al2Cl7− was confirmed by density function theory calculations and the Arrhenius equation. This accelerated dissociation reaction was experimentally verified by the increase of exchange current density during Al electro‐deposition. Using Al2Cl6Br− instead of Al2Cl7−, a kinetically accelerated Al‐S battery has a sulfur utilization of more than 80 %, with at least four times the sulfur content and five times the current density than that of previous work.