KI-Assisted Formation of Spindle-like Prussian White Nanoparticles for High-Performance Potassium-Ion Battery Cathodes.

Prussian white (PW) is considered as a promising cathode material for potassium-ion batteries (KIBs) due to its low cost and high theoretical capacity. However, the high water content and structural defects and the strict synthesis conditions of PW lead to its unsatisfactory cycling performance and low specific capacity, hindering its practical applications. Herein, a template-engaged reduction method is proposed, using MIL-88B(Fe) as a self-template and KI as the reducing agent to prepare K-rich PW with low defects and water content. Furthermore, the hierarchical porous spindle-like morphology can be inherited from the precursor, furnishing sufficient active sites and reducing the ion diffusion path. Consequently, when applied as a KIB cathode material, spindle-like PW (K1.72Fe[Fe(CN)6]0.96·0.342H2O) manifested remarkable potassium storage properties. Notably, a full cell assembled by the spindle-like PW cathode and graphite anode exhibited a large energy density of ∼216.7 Wh kg-1, demonstrating its huge potential for energy storage systems.

Electrolyte Interphases in Aqueous Batteries.

The narrow electrochemical stability window of water poses a challenge to the development of aqueous electrolytes. In contrast to non-aqueous electrolytes, the products of water electrolysis do not contribute to the formation of a passivation layer on electrodes. As a result, aqueous electrolytes require the reactions of additional components, such as additives and co-solvents, to facilitate the formation of the desired solid electrolyte interphase (SEI) on the anode and cathode electrolyte interphase (CEI) on the cathode. This review highlights the fundamental principles and recent advancements in generating electrolyte interphases in aqueous batteries.

The Influence of Ions on the Electrochemical Stability of Aqueous Electrolytes.

The electrochemical stability window of water is known to vary with the type and concentration of dissolved salts. However, the underlying influence of ions on the thermodynamic stability of aqueous solutions has not been fully understood. Here, we investigated the electrolytic behaviors of aqueous electrolytes as a function of different ions. Our findings indicate that ions with high ionic potentials, i.e., charge density, promote the formation of their respective hydration structures, enhancing electrolytic reactions via an inductive effect, particularly for small cations. Conversely, ions with lower ionic potentials increase the proportion of free water molecules-those not engaged in hydration shells or hydrogen-bonding networks-leading to greater electrolytic stability. Furthermore, we observe that the chemical environment created by bulky ions with lower ionic potentials impedes electrolytic reactions by frustrating the solvation of protons and hydroxide ions, the products of oxygen evolution reaction (OER) and hydrogen evolution reaction (HER), respectively. We found that the solvation of protons plays a more substantial role than that of hydroxide, which explains a greater shift for OER than for HER, a puzzle that cannot be rationalized by the notion of varying O-H bond strengths of water. These insights will help the design of aqueous systems.

Anticatalytic Strategies to Suppress Water Electrolysis in Aqueous Batteries.

Aqueous electrolytes are the leading candidate to meet the surging demand for safe and low-cost storage batteries. Aqueous electrolytes facilitate more sustainable battery technologies due to the attributes of being nonflammable, environmentally benign, and cost effective. Yet, water's narrow electrochemical stability window remains the primary bottleneck for the development of high-energy aqueous batteries with long cycle life and infallible safety. Water's electrolysis leads to either hydrogen evolution reaction (HER) or oxygen evolution reaction (OER), which causes a series of dire consequences, including poor Coulombic efficiency, short device longevity, and safety issues. These are often showstoppers of a new aqueous battery technology besides the low energy density. Prolific progress has been made in the understanding of HER and OER from both catalysis and battery fields. Unfortunately, a systematic review on these advances from a battery chemistry standpoint is lacking. This review provides in-depth discussions on the mechanisms of water electrolysis on electrodes, where we summarize the critical influencing factors applicable for a broad spectrum of aqueous battery systems. Recent progress and existing challenges on suppressing water electrolysis are discussed, and our perspectives on the future development of this field are provided.

Strengthening Aqueous Electrolytes without Strengthening Water.

Aqueous electrolytes typically suffer from poor electrochemical stability; however, eutectic aqueous solutions-25 wt.% LiCl and 62 wt.% H3 PO4 -cooled to -78 °C exhibit a significantly widened stability window. Integrated experimental and simulation results reveal that, upon cooling, Li+ ions become less hydrated and pair up with Cl- , ice-like water clusters form, and H⋅⋅⋅Cl- bonding strengthens. Surprisingly, this low-temperature solvation structure does not strengthen water molecules' O-H bond, bucking the conventional wisdom that increasing water's stability requires stiffening the O-H covalent bond. We propose a more general mechanism for water's low temperature inertness in the electrolyte: less favorable solvation of OH- and H+ , the byproducts of hydrogen and oxygen evolution reactions. To showcase this stability, we demonstrate an aqueous Li-ion battery using LiMn2 O4 cathode and CuSe anode with a high energy density of 109 Wh/kg. These results highlight the potential of aqueous batteries for polar and extraterrestrial missions.

From Copper to Basic Copper Carbonate: A Reversible Conversion Cathode in Aqueous Anion Batteries.

Dual-ion batteries that use anions and cations as charge carriers represent a promising energy-storage technology. However, an uncharted area is to explore transition metals as electrodes to host carbonate in conversion reactions. Here we report the reversible conversion reaction from copper to Cu2 CO3 (OH)2 , where the copper electrode comprising K2 CO3 and KOH solid is self-sufficient with anion-charge carriers. This electrode dissociates and associates K+ ions during battery charge and discharge. The copper active mass and the anion-bearing cathode exhibit a reversible capacity of 664 mAh g-1 and 299 mAh g-1 , respectively, and relatively stable cycling in a saturated mixture electrolyte of K2 CO3 and KOH. The results open an avenue to use carbonate as a charge carrier for batteries to serve for the consumption and storage of CO2 .

Reversible Copper Cathode for Nonaqueous Dual-Ion Batteries.

Most reported cathodes of nonaqueous dual-ion batteries (DIBs) host anions via insertion reactions. It is necessary to explore new cathode chemistry to increase the battery energy density. To date, transition metals have yet to be investigated for nonaqueous DIBs, albeit they may offer high capacity in anodic conversion reactions. Here, we report that bulk copper powder exhibits a high reversible capacity of 762 mAh g-1 at 3.2 V vs. Li+ /Li and relatively stable cycling in common organic electrolytes. The operation of the copper electrode is coupled with the transfer of anion charge carriers. An anion exchange membrane separator is employed to prevent Cu2+ from crossing from the catholyte to the anode side. We designed an unbalanced electrolyte with a more concentrated anolyte than a catholyte. This addresses the concentration overpotential ensued during charge and facilitates the high specific capacity and enhanced reversibility. This finding provides a promising direction for high-energy DIBs.

Strategies towards Low-Cost Dual-Ion Batteries with High Performance.

Rocking-chair based lithium-ion batteries (LIBs) have extensively applied to consumer electronics and electric vehicles (EVs) for solving the present worldwide issues of fossil fuel exhaustion and environmental pollution. However, due to the growing unprecedented demand of LIBs for commercialization in EVs and grid-scale energy storage stations, and a shortage of lithium and cobalt, the increasing cost gives impetus to exploit low-cost rechargeable battery systems. Dual-ion batteries (DIBs), in which both cations and anions are involved in the electrochemical redox reaction, are one of the most promising candidates to meet the low-cost requirements of commercial applications, because of their high working voltage, excellent safety, and environmental friendliness compared to conventional rocking-chair based LIBs. However, DIB technologies are only at the stage of fundamental research and considerable effort is required to improve the energy density and cycle life further. We review the development history and current situation, and discuss the reaction kinetics involved in DIBs, including various anionic intercalation mechanism of cathodes, and the reactions at the anodes including intercalation and alloying to explore promising strategies towards low-cost DIBs with high performance.

A Non-aqueous H3 PO4 Electrolyte Enables Stable Cycling of Proton Electrodes.

A non-aqueous proton electrolyte is devised by dissolving H3 PO4 into acetonitrile. The electrolyte exhibits unique vibrational signatures from stimulated Raman spectroscopy. Such an electrolyte exhibits unique characteristics compared to aqueous acidic electrolytes: 1) higher (de)protonation potential for a lower desolvation energy of protons, 2) better cycling stability by dissolution suppression, and 3) higher Coulombic efficiency owing to the lack of oxygen evolution reaction. Two non-aqueous proton full cells exhibit better cycling stability, higher Coulombic efficiency, and less self-discharge compared to the aqueous counterpart.

Reversible Insertion of Mg-Cl Superhalides in Graphite as a Cathode for Aqueous Dual-Ion Batteries.

Oxidative anion insertion into graphite in an aqueous environment represents a significant challenge in the construction of aqueous dual-ion batteries. In dilute aqueous electrolytes, the oxygen evolution reaction (OER) dominates the anodic current before anions can be inserted into the graphite gallery. Herein, we report that the reversible insertion of Mg-Cl superhalides in graphite delivers a record-high reversible capacity of 150 mAh g-1 from an aqueous deep eutectic solvent comprising magnesium chloride and choline chloride. The insertion of Mg-Cl superhalides in graphite does not form staged graphite intercalation compounds; instead, the insertion of Mg-Cl superhalides makes the graphite partially turbostratic.

Reverse Dual-Ion Battery via a ZnCl2 Water-in-Salt Electrolyte.

Dual-ion batteries are known for anion storage in the cathode coupled to cation incorporation in the anode. We flip the sequence of the anion/cation-storage chemistries of the anode and the cathode in dual-ion batteries (DIBs) by allowing the anode to take in anions and a cation-deficient cathode to host cations, thus operating as a reverse dual-ion battery (RDIB). The anion-insertion anode is a nanocomposite having ferrocene encapsulated inside a microporous carbon, and the cathode is a Zn-insertion Prussian blue, Zn3[Fe(CN)6]2. This unique battery configuration benefits from the usage of a 30 m ZnCl2 "water-in-salt" electrolyte. This electrolyte minimizes the dissolution of ferrocene; it raises the cation-insertion potential in the cathode, and it depresses the anion-insertion potential in the anode, thus widening the full cell's voltage by 0.35 V compared with a dilute ZnCl2 electrolyte. RDIBs provide a configuration-based solution to exploit the practicality of cation-deficient cathode materials in aqueous electrolytes.

A Four-Electron Sulfur Electrode Hosting a Cu2+ /Cu+ Redox Charge Carrier.

The elemental sulfur electrode with Cu2+ as the charge carrier gives a four-electron sulfur electrode reaction through the sequential conversion of S↔CuS↔Cu2 S. The Cu-S redox-ion electrode delivers a high specific capacity of 3044 mAh g-1 based on the sulfur mass or 609 mAh g-1 based on the mass of Cu2 S, the completely discharged product, and displays an unprecedently high potential of sulfur/metal sulfide reduction at 0.5 V vs. SHE. The Cu-S electrode also exhibits an extremely low extent of polarization of 0.05 V and an outstanding cycle number of 1200 cycles retaining 72 % of the initial capacity at 12.5 A g-1 . The remarkable utility of this Cu-S cathode is further demonstrated in a hybrid cell that employs an Zn metal anode and an anion-exchange membrane as the separator, which yields an average cell discharge voltage of 1.15 V, the half-cell specific energy of 547 Wh kg-1 based on the mass of the Cu2 S/carbon composite cathode, and stable cycling over 110 cycles.

A Dual Plating Battery with the Iodine/[ZnIx (OH2 )4-x ]2-x Cathode.

Plating battery electrodes typically deliver higher specific capacity values than insertion or conversion electrodes because the ion charge carriers represent the sole electrode active mass, and a host electrode is unnecessary. However, reversible plating electrodes are rare for electronically insulating nonmetals. Now, a highly reversible iodine plating cathode is presented that operates on the redox couples of I2 /[ZnIx (OH2 )4-x ]2-x in a water-in-salt electrolyte. The iodine plating cathode with the theoretical capacity of 211 mAh g-1 plates on carbon fiber paper as the current collector, delivering a large areal capacity of 4 mAh cm-2 . Tunable femtosecond stimulated Raman spectroscopy coupled with DFT calculations elucidate a series of [ZnIx (OH2 )4-x ]2-x superhalide ions serving as iodide vehicles in the electrolyte, which eliminates most free iodide ions, thus preventing the consequent dissolution of the cathode-plated iodine as triiodides.

An Aqueous Dual-Ion Battery Cathode of Mn3 O4 via Reversible Insertion of Nitrate.

We report reversible electrochemical insertion of NO3 - into manganese(II, III) oxide (Mn3 O4 ) as a cathode for aqueous dual-ion batteries. Characterization by TGA, FTIR, EDX, XANES, EXAFS, and EQCM collectively provides unequivocal evidence that reversible oxidative NO3 - insertion takes place inside Mn3 O4 . Ex situ HRTEM and corresponding EDX mapping results suggest that NO3 - insertion de-crystallizes the structure of Mn3 O4 . Kinetic studies reveal fast migration of NO3 - in the Mn3 O4 structure. This finding may open a new direction for novel low-cost aqueous dual-ion batteries.

Insights on the Proton Insertion Mechanism in the Electrode of Hexagonal Tungsten Oxide Hydrate.

This study reveals the transport behavior of lattice water during proton (de)insertion in the structure of the hexagonal WO3·0.6H2O electrode. By monitoring the mass evolution of this electrode material via electrochemical quartz crystal microbalance, we discovered (1) WO3·0.6H2O incorporates additional lattice water when immersing in the electrolyte at open circuit voltage and during initial cycling; (2) The reductive proton insertion in the WO3 hydrate is a three-tier process, where in the first stage 0.25 H+ is inserted per formula unit of WO3 while simultaneously 0.25 lattice water is expelled; then in the second stage 0.30 naked H+ is inserted, followed by the third stage with 0.17 H3O+ inserted per formula unit. Ex situ XRD reveals that protonation of the WO3 hydrate causes consecutive anisotropic structural changes: it first contracts along the c-axis but later expands along the ab planes. Furthermore, WO3·0.6H2O exhibits impressive cycle life over 20 000 cycles, together with appreciable capacity and promising rate performance.

Electrolytes, SEI Formation, and Binders: A Review of Nonelectrode Factors for Sodium-Ion Battery Anodes.

Through intense effort in recent years, knowledge of Na-ion batteries has been advanced significantly, pertaining to electrodes. Often, such progress has been accompanied by using a convenient choice of electrolyte or binder. Nevertheless, it has been witnessed that "external" factors to electrodes, such as electrolytes, solid electrolyte interphase, and binders, affect the functions of electrodes profoundly. And generally, certain types of electrodes favor some electrolytes or binders. With a rapidly increasing number of publications in the area, trends in terms of electrolytes and binders are possibly exploitable. Unfortunately, the field has yet to see a review article that devotes itself to these nonelectrode aspects of Na-ion batteries. Here, the gap is filled by conducting a comprehensive review of these nonelectrode external factors, especially by looking into their correlation with electrochemical properties, such as cycle life, and first cycle coulombic efficiency. Not only are the representative reports reviewed, but also quantitative analyses on the database that are constructed are provided. With such analyses, some new data-driven perspectives are postulated, which are of great value to the community.

Pillared graphite anodes for reversible sodiation.

There has been a major effort recently to develop new rechargeable sodium-ion electrodes. In lithium ion batteries, LiC6 forms from graphite and desolvated Li cations during the first charge. With sodium ions, graphite only shows a significant capacity when Na+ intercalates as a solvated complex, resulting in ternary graphite intercalation compounds (GICs). Although this chemistry has been shown to be highly reversible and to support high rates in small test cells, these GICs can require >250% volume expansion and contraction during cycling. Here we demonstrate the first example of GICs that reversibly sodiate/desodiate without any significant volume change. These pillared GICs are obtained by electrochemical reduction of graphite in an ether/amine co-solvent electrolyte. The initial gallery expansion, 0.36 nm, is less than half of that in diglyme-based systems, and shows a similar capacity. Thermal analyses suggest the pillaring phenomenon arises from stronger co-intercalate interactions in the GIC galleries.

Fundamentally Addressing Bromine Storage through Reversible Solid-State Confinement in Porous Carbon Electrodes: Design of a High-Performance Dual-Redox Electrochemical Capacitor.

Research in electric double-layer capacitors (EDLCs) and rechargeable batteries is converging to target systems that have battery-level energy density and capacitor-level cycling stability and power density. This research direction has been facilitated by the use of redox-active electrolytes that add faradaic charge storage to increase energy density of the EDLCs. Aqueous redox-enhanced electrochemical capacitors (redox ECs) have, however, performed poorly due to cross-diffusion of soluble redox couples, reduced cycle life, and low operating voltages. In this manuscript, we propose that these challenges can be simultaneously met by mechanistically designing a liquid-to-solid phase transition of oxidized catholyte (or reduced anolyte) with confinement in the pores of electrodes. Here we demonstrate the realization of this approach with the use of bromide catholyte and tetrabutylammonium cation that induces reversible solid-state complexation of Br2/Br3-. This mechanism solves the inherent cross-diffusion issue of redox ECs and has the added benefit of greatly stabilizing the reactive bromine generated during charging. Based on this new mechanistic insight on the utilization of solid-state bromine storage in redox ECs, we developed a dual-redox EC consisting of a bromide catholyte and an ethyl viologen anolyte with the addition of tetrabutylammonium bromide. In comparison to aqueous and organic electric double-layer capacitors, this system enhances energy by factors of ca. 11 and 3.5, respectively, with a specific energy of ∼64 W·h/kg at 1 A/g, a maximum power density >3 kW/kg, and cycling stability over 7000 cycles.

Mg-Ion Battery Electrode: An Organic Solid's Herringbone Structure Squeezed upon Mg-Ion Insertion.

We report that crystalline 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA), an organic solid, is highly amenable to host divalent metal ions, i.e., Mg2+ and Ca2+, in aqueous electrolytes, where the van der Waals structure is intrinsically superior in hosting charge-dense ions. We observe that the divalent nature of Mg2+ causes unique squeezing deformation of the electrode structure, where it contracts and expands in different crystallographic directions when hosting the inserted Mg-ions. This phenomenon is revealed experimentally by ex situ X-ray diffraction and transmission electron microscopy, and is investigated theoretically by first-principles calculations. Interestingly, hosting one Mg2+ ion requires the coordination from three PTCDA molecules in adjacent columns of stacked molecules, which rotates the columns, thus reducing the (011) spacing but increasing the (021) spacing. We demonstrate that a PTCDA Mg-ion electrode delivers a reversible capacity of 125 mA h g-1, which may include a minor contribution of hydronium storage, a good rate capability by retaining 75 mA h g-1 at 500 mA g-1 (or 3.7 C), and a stable cycle life. We also report Ca2+ storage in PTCDA, where a reversible capacity of over 80 mA h g-1 is delivered.

Na-Ion Battery Anodes: Materials and Electrochemistry.

The intermittent nature of renewable energy sources, such as solar and wind, calls for sustainable electrical energy storage (EES) technologies for stationary applications. Li will be simply too rare for Li-ion batteries (LIBs) to be used for large-scale storage purposes. In contrast, Na-ion batteries (NIBs) are highly promising to meet the demand of grid-level storage because Na is truly earth abundant and ubiquitous around the globe. Furthermore, NIBs share a similar rocking-chair operation mechanism with LIBs, which potentially provides high reversibility and long cycling life. It would be most efficient to transfer knowledge learned on LIBs during the last three decades to the development of NIBs. Following this logic, rapid progress has been made in NIB cathode materials, where layered metal oxides and polyanionic compounds exhibit encouraging results. On the anode side, pure graphite as the standard anode for LIBs can only form NaC64 in NIBs if solvent co-intercalation does not occur due to the unfavorable thermodynamics. In fact, it was the utilization of a carbon anode in LIBs that enabled the commercial successes. Anodes of metal-ion batteries determine key characteristics, such as safety and cycling life; thus, it is indispensable to identify suitable anode materials for NIBs. In this Account, we review recent development on anode materials for NIBs. Due to the limited space, we will mainly discuss carbon-based and alloy-based anodes and highlight progress made in our groups in this field. We first present what is known about the failure mechanism of graphite anode in NIBs. We then go on to discuss studies on hard carbon anodes, alloy-type anodes, and organic anodes. Especially, the multiple functions of natural cellulose that is used as a low-cost carbon precursor for mass production and as a soft substrate for tin anodes are highlighted. The strategies of minimizing the surface area of carbon anodes for improving the first-cycle Coulombic efficiency are also outlined, where graphene oxide was employed as dehydration agent and 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) was used to unzip wood fiber. Furthermore, surface modification by atomic layer deposition technology is introduced, where we discover that a thin layer of Al2O3 can function to encapsulate Sn nanoparticles, leading to a much enhanced cycling performance. We also highlight recent work about the phosphorene/graphene anode, which outperformed other anodes in terms of capacity. The aromatic organic anode is also studied as anode with very high initial sodiation capacity. Furthermore, electrochemical intercalation of Na ions into reduced graphene oxide is applied for fabricating transparent conductors, demonstrating the great feasibility of Na ion intercalation for optical applications.