Atomic/molecular layer deposition for energy storage and conversion.

Energy storage and conversion systems, including batteries, supercapacitors, fuel cells, solar cells, and photoelectrochemical water splitting, have played vital roles in the reduction of fossil fuel usage, addressing environmental issues and the development of electric vehicles. The fabrication and surface/interface engineering of electrode materials with refined structures are indispensable for achieving optimal performances for the different energy-related devices. Atomic layer deposition (ALD) and molecular layer deposition (MLD) techniques, the gas-phase thin film deposition processes with self-limiting and saturated surface reactions, have emerged as powerful techniques for surface and interface engineering in energy-related devices due to their exceptional capability of precise thickness control, excellent uniformity and conformity, tunable composition and relatively low deposition temperature. In the past few decades, ALD and MLD have been intensively studied for energy storage and conversion applications with remarkable progress. In this review, we give a comprehensive summary of the development and achievements of ALD and MLD and their applications for energy storage and conversion, including batteries, supercapacitors, fuel cells, solar cells, and photoelectrochemical water splitting. Moreover, the fundamental understanding of the mechanisms involved in different devices will be deeply reviewed. Furthermore, the large-scale potential of ALD and MLD techniques is discussed and predicted. Finally, we will provide insightful perspectives on future directions for new material design by ALD and MLD and untapped opportunities in energy storage and conversion.

Oxygen-Based Anion Redox for Lithium Batteries.

ConspectusThe importance of current Li-ion batteries (LIBs) in modern society cannot be overstated. While the energy demands of devices increase, the corresponding enhancements in energy density of battery technologies are highly sought after. Currently, many different battery concepts, such as Li-S and metal-air among many others, have been investigated. However, their practical implementation has mostly been restricted to the prototyping stage. In fact, most of these technologies require rework of existing Li-ion battery manufacturing facilities and will naturally incur resistance to change from industry. For this reason, one specifically attractive technology, anionic redox in transition metal oxides, has gained much attention in the recent years. Its ability to be directly used in already established processes and higher energy density with similar electrolyte formulation make it a key materials research direction for next generation Li-ion batteries. In regular LIBs, the redox active centers are the transition metal cation. In anion redox, both the anion (typically O) and the transition metal cation are utilized as redox centers with enormous implications for increasing energy density. This new material can be highly competitive for replacing the current LIB technologies. However, much is still unknown about its cycling mechanism. Upon activating the O redox couples, most cationic and anionic redox active materials will either evolve O2 or undergo irreversible structural degradation with associated severe decreases in electrochemical performance. By understanding the transition from full anion redox to partial cationic and anionic redox, we hope readers can gain a deeper understanding of the topic.This Account will focus mainly on the work that was conducted by our group at Argonne National Laboratory. The phenomenon of cationic and anionic redox in a lithium-ion battery cathode will first be discussed. Our work in resonant inelastic X-ray scattering to investigate the spectroscopic features of O after delithiation has found potential "fingerprint" signals that could likely be used to identify and confirm reversible O redox if corroborated with other techniques. To follow, we will examine our work on Li-O2 batteries. While our group and the research community have had many significant contributions and improvements to the field of Li-O2 (such as decreasing overpotential and achieving cyclability in air environment), its practical application is still far from realization. Perhaps our most important contribution to this area is the discovery that Ir deposited on reduced graphene oxide can be used to halt the reduction of O2 at the LiO2 oxidation state. This not only significantly decreases the charge overpotential but also presents the important concept of oxidation-state controlled discharge. Subsequently, we will focus on our oxidation state-controlled redox-based charging of oxygen in a pure oxygen redox Li-ion battery. Future implications of this technology will be emphasized.

Cationic and anionic redox in lithium-ion based batteries.

Lithium-ion batteries have proven themselves to be indispensable among modern day society. Demands stemming from consumer electronics and renewable energy systems have pushed researchers to strive for new electrochemical technologies. To this end, the advent of anionic redox, that is, the sequential or simultaneous redox of the cation and anion in a transition metal oxide based cathode for a Li-ion battery, has garnered much attention due to the enhanced specific capacities. Unfortunately, the higher energy densities are plagued with problems associated with the irreversibility of anionic redox. Much effort has been placed on finding a suitable composition of transition metal oxide, with some groups identifying the underlying features and relationship for anion redox and cationic redox to occur reversibly. Accordingly, it is timely to review anionic redox in terms of what anionic redox is with emphasis on the mechanism and the evidence underlying its discovery and validation. To follow will be a section defining the nature of the transition metal and oxygen bond accompanied by three subsequent sections bridging the redox spectrum from pure anionic, to a mix of cationic and anionic and pure cationic.

From Sodium-Oxygen to Sodium-Air Battery: Enabled by Sodium Peroxide Dihydrate.

Metal-air batteries have attracted extensive research interests due to their high theoretical energy density. However, most of the previous studies were limited by applying pure oxygen in the cathode, sacrificing the gravimetric and volumetric energy density. Here, we develop a real sodium-"air" battery, in which the rechargeability of the battery relies on the reversible reaction of the formation of sodium peroxide dihydrate (Na2O2·2H2O). After an oxygen evolution reaction catalyst is applied, the charge overpotential is largely reduced to achieve a high energy efficiency. The sodium-air batteries deliver high areal capacity of 4.2 mAh·cm-2 and have a decent cycle life of 100 cycles. The oxygen crossover effect is largely suppressed by replacing the oxygen with air, whereas the dense solid electrolyte interphase formed on the sodium anode further prolongs the cycle life.

Cation Additive Enabled Rechargeable LiOH-Based Lithium-Oxygen Batteries.

Lithium-oxygen (Li-O2 ) batteries have attracted extensive research interest due to their high energy density. Other than Li2 O2 (a typical discharge product in Li-O2 batteries), LiOH has proved to be electrochemically active as an alternative product. Here we report a simple strategy to achieve a reversible LiOH-based Li-O2 battery by using a cation additive, sodium ions, to the lithium electrolyte. Without redox mediators in the cell, LiOH is detected as the sole discharge product and it charges at a low charge potential of 3.4 V. A solution-based reaction route is proposed, showing that the competing solvation environment of the catalyst and Li+ leads to LiOH precipitation at the cathode. It is critical to tune the cell chemistry of Li-O2 batteries by designing a simple system to promote LiOH formation/decomposition.

Tuning Li2O2 Formation Routes by Facet Engineering of MnO2 Cathode Catalysts.

In lithium-oxygen batteries, the solubility of LiO2 intermediates in the electrolyte regulates the formation routes of the Li2O2 discharge product. High-donor-number electrolytes with a high solubility of LiO2 tend to promote the formation of Li2O2 large particles following the solution route, which eventually benefits the cell capacity and cycle life. Here, we propose that facet engineering of cathode catalysts could be another direction in tuning the formation routes of Li2O2. In this work, ß-MnO2 crystals with high occupancies of {111} or {100} facets were adopted as cathode catalysts in Li-O2 batteries with a tetra(ethylene)glycol dimethyl ether electrolyte. The {111}-dominated ß-MnO2 catalyzed the formation of the Li2O2 discharge product into large toroids following the solution routes, while {100}-dominated ß-MnO2 facilitated the formation of Li2O2 thin films through the surface routes. Further computational studies indicate that the different formation routes of Li2O2 could be related to different adsorption energies of LiO2 on the two facets of ß-MnO2. Our results demonstrate that facet engineering of cathode catalysts could be a new way to tune the formation route of Li2O2 in a low-donor-number electrolyte. We anticipate that this new finding would offer more choices for the design of lithium-oxygen batteries with high capacities and ultimately a long cycle life.

Atomic Layer Co3 O4 Nanosheets: The Key to Knittable Zn-Air Batteries.

Flexible, wearable, and portable energy storage devices with high-energy density are crucial for next-generation electronics. However, the current battery technologies such as lithium ion batteries have limited theoretical energy density. Additionally, battery materials with small scale and high flexibility which could endure the large surface stress are highly required. In this study, a yarn-based 1D Zn-air battery is designed, which employs atomic layer thin Co3 O4 nanosheets as the oxygen reduction reaction/oxygen evolution reaction catalyst. The ultrathin nanosheets are synthesized by a high-yield and facile chemical method and show a thickness of only 1.6 nm, corresponding to few atomic layers. The 1D Zn-air battery shows high cycling stability and high rate capability. The battery is successfully knitted into clothes and it shows high stability during the large deformation and knotting conditions.

Toward Highly Efficient Electrocatalyst for Li-O2 Batteries Using Biphasic N-Doping Cobalt@Graphene Multiple-Capsule Heterostructures.

For the promotion of lithium-oxygen batteries available for practical applications, the development of advanced cathode catalysts with low-cost, high activity, and stable structural properties is demanded. Such development is rooted on certain intelligent catalyst-electrode design that fundamentally facilitates electronic and ionic transport and improves oxygen diffusivity in a porous environment. Here we design a biphasic nitrogen-doped cobalt@graphene multiple-capsule heterostructure, combined with a flexible, stable porous electrode architecture, and apply it as promising cathodes for lithium-oxygen cells. The biphasic nitrogen-doping feature improves the electric conductivity and catalytic activity; the multiple-nanocapsule configuration makes high/uniform electroactive zones possible; furthermore, the colander-like porous electrode facilitates the oxygen diffusion, catalytic reaction, and stable deposition of discharge products. As a result, the electrode exhibits much improved electrocatalytic properties associated with unique morphologies of electrochemically grown lithium peroxides.

Understanding the Role of Lithium Iodide in Lithium-Oxygen Batteries.

Lithium-oxygen (Li-O2 ) batteries possess a high theoretical energy density, which means they could become a potential alternative to lithium-ion batteries. Nevertheless, the charging process of Li-O2 batteries requires much higher energy, due to the insulating nature of the discharge product. It has been revealed that the anion additive, lithium iodide (LiI), can tune the cell chemistry to form lithium hydroxide (LiOH) as the product and facilitate the kinetics during the charging process. Although numerous studies have been reported, the role of this additive is still under investigation. Herein, the recent advances focusing on the use of LiI in Li-O2 batteries are reviewed, its catalytic behavior on discharge and charge is discussed, and its synergistic effect with water is understood. The ambiguity existing among the studies are also revealed, and solutions to the current issues are introduced.

Hydrous Nickel-Iron Turnbull's Blue as a High-Rate and Low-Temperature Proton Electrode.

Proton batteries are emerging as a promising solution for energy storage; however, their development has been hindered by the lack of suitable cathode materials. Herein, a hydrous Turnbull's blue analogue (TBA) of Ni[Fe(CN)6]2/3·4H2O has been investigated as a viable proton cathode. Particularly, it shows an extremely high rate performance up to 6000 C (390 A g-1) at room temperature and delivers good capacity values at a low temperature of -40 °C in an aqueous electrolyte. The excellent rate capability is also amenable to high mass loadings of 10 mg cm-2. Such fast and low-temperature rate behavior likely stems from the fast proton conduction that is afforded by the Grotthuss mechanism inside the TBA structure. Furthermore, advanced characterization, including in operando synchrotron X-ray diffraction (XRD), and X-ray absorption near-edge structure (XANES) were employed to understand the changes of crystal structures and the oxidation-states of metal elements of the electrodes.

Toward Highly Selective Electrochemical CO2 Reduction using Metal-Free Heteroatom-Doped Carbon.

There are growing interests in metal-free heteroatom-doped carbons for electrochemical CO2 reduction. Previous studies extensively focus on the effect of N-doping, and their products severely suffer from low current density (mostly <2 mA cm-2) and limited selectivity (<90%). Here, it is reported that heteroatom codoping offers a promising solution to the above challenge. As a proof of concept, N,P-codoped mesoporous carbon is prepared by annealing phytic-acid-functionalized ZIF-8 in NH3. In CO2-saturated 0.5 m NaHCO3, the catalyst enables CO2 reduction to CO with great selectivity close to 100% and large CO partial current density (≈8 mA cm-2), which are, to the best of knowledge, superior to all other relevant competitors. Theoretical simulations show that the improved activity and selectivity are stemmed from the enhanced surface adsorption of *COOH and *CO intermediates as a result of the synergy of N and P codoping.

Lithiation-Induced Non-Noble Metal Nanoparticles for Li-O2 Batteries.

Low-cost and highly active electrocatalysts are attractive for Li-O2 applications. Herein, a 3D interconnected plate architecture consisting of ultrasmall Co-Ni grains embedded in lithium hydroxide nanoplates (Co2Ni@LiOH) is designed and prepared by a lithiation strategy at room temperature. This catalyst exhibits a remarkably reduced charge potential of ∼3.4 V at 50 µA cm-2, which leads to the high roundtrip efficiency of ∼79%, among the best levels reported and a cycle life of up to 40 cycles. The well-aligned network facilitates the oxygen diffusion and the electrolyte penetration into the electrode. The enhanced electrical conductivity network improves the charge transport kinetics and more active sites are exposed, which facilitate the adsorption and dissociation of oxygen during the oxygen reduction reaction and the oxygen evolution reaction. This new catalyst design inspires the development of an effective non-noble metal catalyst for Li-O2 batteries.

Bamboo-Like Nitrogen-Doped Carbon Nanotube Forests as Durable Metal-Free Catalysts for Self-Powered Flexible Li-CO2 Batteries.

The Li-CO2 battery is a promising energy storage device for wearable electronics due to its long discharge plateau, high energy density, and environmental friendliness. However, its utilization is largely hindered by poor cyclability and mechanical rigidity due to the lack of a flexible and durable catalyst electrode. Herein, flexible fiber-shaped Li-CO2 batteries with ultralong cycle-life, high rate capability, and large specific capacity are fabricated, employing bamboo-like N-doped carbon nanotube fiber (B-NCNT) as flexible, durable metal-free catalysts for both CO2 reduction and evolution reactions. Benefiting from high N-doping with abundant pyridinic groups, rich defects, and active sites of the periodic bamboo-like nodes, the fabricated Li-CO2 battery shows outstanding electrochemical performance with high full-discharge capacity of 23 328 mAh g-1 , high rate capability with a low potential gap up to 1.96 V at a current density of 1000 mA g-1 , stability over 360 cycles, and good flexibility. Meanwhile, the bifunctional B-NCNT is used as the counter electrode for a fiber-shaped dye-sensitized solar cell to fabricate a self-powered fiber-shaped Li-CO2 battery with overall photochemical-electric energy conversion efficiency of up to 4.6%. Along with a stable voltage output, this design demonstrates great adaptability and application potentiality in wearable electronics with a breath monitor as an example.

In situ quantification of interphasial chemistry in Li-ion battery.

The solid-electrolyte interphase (SEI) is probably the least understood component in Li-ion batteries. Considerable effort has been put into understanding its formation and electrochemistry under realistic battery conditions, but mechanistic insights have mostly been inferred indirectly. Here we show the formation of the SEI between a graphite anode and a carbonate electrolyte through combined atomic-scale microscopy and in situ and operando techniques. In particular, we weigh the graphitic anode during its initial lithiation process with an electrochemical quartz crystal microbalance, which unequivocally identifies lithium fluoride and lithium alkylcarbonates as the main chemical components at different potentials. In situ gas analysis confirms the preferential reduction of cyclic over acyclic carbonate molecules, making its reduction product the major component in the SEI. We find that SEI formation starts at graphite edge sites with dimerization of solvated Li+ intercalation between graphite layers. We also show that this lithium salt, at least in its nascent form, can be re-oxidized, despite the general belief that an SEI is electrochemically inert and its formation irreversible.

High-Rate and Long-Term Cycle Stability of Li-S Batteries Enabled by Li2S/TiO2-Impregnated Hollow Carbon Nanofiber Cathodes.

The high theoretical energy density of lithium-sulfur (Li-S) batteries makes them an alternative battery technology to lithium ion batteries. However, Li-S batteries suffer from low sulfur loading, poor charge transport, and dissolution of lithium polysulfide. In our study, we use the lithiated S, Li2S, as the cathode material, coupled with electrospun TiO2-impregnated hollow carbon nanofibers (TiO2-HCFs), which serve as the conductive agent and protective barrier for Li2S in Li-S batteries. TiO2-HCFs provide much improved electron/ionic conductivity and serve as a physical barrier, which prevents the dissolution of lithium polysulfides. The Li2S/TiO2-HCF composite delivers a discharge capacity of 851 mA h gLi2S-1 at 0.1C and the bilayer TiO2-HCFs/Li2S/TiO2-HCF composite delivers a high specific capacity of 400 mA h gLi2S-1 at 5C.

Kinetics Tuning the Electrochemistry of Lithium Dendrites Formation in Lithium Batteries through Electrolytes.

Lithium batteries are one of the most advance energy storage devices in the world and have attracted extensive research interests. However, lithium dendrite growth was a safety issue which handicapped the application of pure lithium metal in the negative electrode. In this investigation, two solvents, propylene carbonate (PC) and 2-methyl-tetrahydrofuran (2MeTHF), and four Li+ salts, LiPF6, LiAsF6, LiBF4 and LiClO4 were investigated in terms of their effects on the kinetics of lithium dendrite formation in eight electrolyte solutions. The kinetic parameters of charge transfer step (exchange current density, j0, transfer coefficient, α) of Li+/Li redox system, the mass transfer parameters of Li+ (transfer number of Li+, tLi+, diffusion coefficient of Li+, DLi+), and the conductivity (κ) of each electrolyte were studied separately. The results demonstrate that the solvents play a critical role in the measured j0, tLi+, DLi+, and κ of the electrolyte, while the choice of Li+ salts only slightly affect the measured parameters. The understanding of the kinetics will gain insight into the mechanism of lithium dendrite formation and provide guidelines to the future application of lithium metal.

Ultralong Cycle Life Achieved by a Natural Plant: Miscanthus × giganteus for Lithium Oxygen Batteries.

Large energy-storage systems and electric vehicles require energy devices with high power and high energy density. Lithium oxygen (Li-O2) batteries could achieve high energy density, but they are still facing problems such as low practical capacity and poor cyclability. Here, we prepare activated carbons (MGACs) based on the natural plant Miscanthus × giganteus (MG) through slow pyrolysis. It possesses a large surface area, plenty of active sites, and high porosity, which are beneficial to the utilization of oxygen electrode in Li-O2 batteries. The MGACs-based oxygen electrode delivers a high specific capacity of 9400 mAh/g at 0.02 mA/cm2, and long cycle life of 601 cycles (with a cutoff capacity of 500 mAh/g) and 295 cycles (with a cutoff capacity of 1000 mAh/g) at 0.2 mA/cm2, respectively. Additionally, the material exhibits high rate capability and high reversibility, which is a promising candidate for the application in Li-O2 batteries.

Red Phosphorus-Embedded Cross-Link-Structural Carbon Films as Flexible Anodes for Highly Reversible Li-Ion Storage.

Red phosphorus (P) is considered to be one of the most attractive anodic materials for lithium-ion batteries (LIBs) due to its high theoretical capacity of 2596 mAh g-1. However, intrinsic characteristics such as the poor electronic conductivity and large volume expansion at lithiation impede the development of red P. Here, we design a new strategy to embed red P particles into a cross-link-structural carbon film (P-C film), in order to improve the electronic conductivity and accommodate the volume expansion. The red P/carbon film is synthesized via vapor phase polymerization (VPP) followed by the pyrolysis process, working as a flexible binder-free anode for LIBs. High cycle stability and good rate capability are achieved by the P-C film anode. With 21% P content in the film, it displays a capacity of 903 mAh g-1 after 640 cycles at a current density of 100 mA g-1 and a capacity of 460 mAh g-1 after 1000 cycles at 2.0 A g-1. Additionally, the Coulombic efficiency reaches almost 100% for each cycle. The superior properties of the P-C films together with their facile fabrication make this material attractive for further flexible and high energy density LIB applications.

New Insights into the Instability of Discharge Products in Na-O2 Batteries.

Sodium-oxygen batteries currently stimulate extensive research due to their high theoretical energy density and improved operational stability when compared to lithium-oxygen batteries. Cell stability, however, needs to be demonstrated also under resting conditions before future implementation of these batteries. In this work we analyze the effect of resting periods on the stability of the sodium superoxide (NaO2) discharge product. The instability of NaO2 in the cell environment is demonstrated leading to the evolution of oxygen during the resting period and the decrease of the cell efficiency. In addition, migration of the superoxide anion (O2(-)) in the electrolyte is observed and demonstrated to be an important factor affecting Coulombic efficiency.

Mesocarbon Microbead Carbon-Supported Magnesium Hydroxide Nanoparticles: Turning Spent Li-ion Battery Anode into a Highly Efficient Phosphate Adsorbent for Wastewater Treatment.

Phosphorus in water eutrophication has become a serious problem threatening the environment. However, the development of efficient adsorbents for phosphate removal from water is lagging. In this work, we recovered the waste material, graphitized carbon, from spent lithium ion batteries and modified it with nanostructured Mg(OH)2 on the surface to treat excess phosphate. This phosphate adsorbent shows one of the highest phosphate adsorption capacities to date, 588.4 mg/g (1 order of magnitude higher than previously reported carbon-based adsorbents), and exhibits decent stability. A heterogeneous multilayer adsorption mechanism was proposed on the basis of multiple adsorption results. This highly efficient adsorbent from spent Li-ion batteries displays great potential to be utilized in industry, and the mechanism study paved a way for further design of the adsorbent for phosphate adsorption.