Sodium–Sulfur Flow Battery for Low‐Cost Electrical Storage

A new sodium–sulfur (Na–S) flow battery utilizing molten sodium metal and flowable sulfur‐based suspension as electrodes is demonstrated and analyzed for the first time. Unlike the conventional flow battery and the high‐temperature Na–S battery, the proposed flow battery system decouples the energy and power thermal management by operating at different temperatures for the storage tank (near room temperature) and the power stack (100–150 °C). The new Na–S flow battery offers several advantages such as easy preparation and integration of the electrode, low energy efficiency loss due to temperature maintenance, great tolerance of the volume change of the metal anode, and efficient utilization of sulfur. The Na–S flow battery has an estimated system cost in the range of $50–100 kWh^?1 which is very competitive for grid‐scale energy storage applications.     

On the Reversibility and Fragility of Sodium Metal Electrodes

Metallic sodium is receiving renewed interest as a battery anode material because the metal is earth‐abundant, inexpensive, and offers a high specific storage capacity (1166 mAh g^?1 at ?2.71 V vs the standard hydrogen potential). Unlike metallic lithium, the case for Na as the anode in rechargeable batteries has already been demonstrated on a commercial scale in high‐temperature Na||S and Na||NiCl₂ secondary batteries, which increases interest. The reversibility of room temperature sodium anodes is investigated in galvanostatic plating/stripping reactions using in situ optical visualization and galvanostatic polarization measurements. It is discovered that electronic disconnection of mossy metallic Na deposits (“orphaning”) is a dominant source of anode irreversibility in liquid electrolytes. The disconnection is shown by means of direct visualization studies to be triggered by a root‐breakage process during the stripping cycle. As a further step toward electrode designs that are able to accommodate the fragile Na deposits, electrodeposition of Na is demonstrated in nonplanar electrode architectures, which provide continuous and morphology agnostic access to the metal at all stages of electrochemical cycling. On this basis, nonplanar Na electrodes are reported, which exhibit exceptionally high levels of reversibility (Coulombic efficiency >99.6% for 1 mAh cm^?2 Na throughput) in room‐temperature, liquid electrolytes.     

An Advanced Na–FeCl2 ZEBRA Battery for Stationary Energy Storage Application

Sodium‐metal chloride batteries, ZEBRA, are considered one of the most important electrochemical devices for stationary energy storage applications because of its advantages of good cycle life, safety, and reliability. However, sodium–nickel chloride (Na–NiCl₂) batteries, the most promising redox chemistry in ZEBRA batteries, still face great challenges for the practical application due to its inevitable feature of using Ni cathode (high materials cost). Here, a novel intermediate‐temperature sodium–iron chloride (Na–FeCl₂) battery using a molten sodium anode and Fe cathode is proposed and demonstrated. The first use of unique sulfur‐based additives in Fe cathode enables Na–FeCl₂ batteries can be assembled in the discharged state and operated at intermediate temperature (<200 °C). The results presented demonstrate that intermediate‐temperature Na–FeCl₂ battery technology could be a propitious solution for ZEBRA battery technologies by replacing the traditional Na–NiCl₂ chemistry.     

Layered Oxide Cathodes for Sodium‐Ion Batteries: Phase Transition,Air Stability,and Performance

The increasing demand for replacing conventional fossil fuels with clean energy or economical and sustainable energy storage drives better battery research today. Sodium‐ion batteries (SIBs) are considered as a promising alternative for grid‐scale storage applications due to their similar “rocking‐chair” sodium storage mechanism to lithium‐ion batteries, the natural abundance, and the low cost of Na resources. Searching for appropriate electrode materials with acceptable electrochemical performance is the key point for development of SIBs. Layered transition metal oxides represent one of the most fascinating electrode materials owing to their superior specific capacity, environmental benignity, and facile synthesis. However, three major challenges (irreversible phase transition, storage instability, and insufficient battery performance) are known for cathodes in SIBs. Herein, a comprehensive review on the latest advances and progresses in the exploration of layered oxides for SIBs is presented, and a detailed and deep understanding of the relationship of phase transition, air stability, and electrochemical performance in layered oxide cathodes is provided in terms of refining the structure–function–property relationship to design improved battery materials. Layered oxides will be a competitive and attractive choice as cathodes for SIBs in next‐generation energy storage devices.     

Recent Progress in Electrode Materials for Sodium‐Ion Batteries

Grid‐scale energy storage systems (ESSs) that can connect to sustainable energy resources have received great attention in an effort to satisfy ever‐growing energy demands. Although recent advances in Li‐ion battery (LIB) technology have increased the energy density to a level applicable to grid‐scale ESSs, the high cost of Li and transition metals have led to a search for lower‐cost battery system alternatives. Based on the abundance and accessibility of Na and its similar electrochemistry to the well‐established LIB technology, Na‐ion batteries (NIBs) have attracted significant attention as an ideal candidate for grid‐scale ESSs. Since research on NIB chemistry resurged in 2010, various positive and negative electrode materials have been synthesized and evaluated for NIBs. Nonetheless, studies on NIB chemistry are still in their infancy compared with LIB technology, and further improvements are required in terms of energy, power density, and electrochemical stability for commercialization. Most recent progress on electrode materials for NIBs, including the discovery of new electrode materials and their Na storage mechanisms, is briefly reviewed. In addition, efforts to enhance the electrochemical properties of NIB electrode materials as well as the challenges and perspectives involving these materials are discussed.     

Na‐Ion Batteries for Large Scale Applications: A Review on Anode Materials and Solid Electrolyte Interphase Formation

The urgent need for optimizing the available energy through smart grids and efficient large‐scale energy storage systems is pushing the construction and deployment of Li‐ion batteries in the MW range which, in the long term, are expected to hit the GW dimension while demanding over 1000 ton of positive active material per system. This amount of Li‐based material is equivalent to almost 1% of current Li consumption and can strongly influence the evolution of the lithium supply and cost. Given this uncertainty, it becomes mandatory to develop an energy storage technology that depends on almost infinite and widespread resources: Na‐ion batteries are the best technology for large‐scale applications. With small working cells in the market that cannot compete in cost ($/W h) with commercial Li‐ion batteries, the consolidation of Na‐ion batteries mainly depends on increasing their energy density and stability, the negative electrodes being at the heart of these two requirements. Promising Na‐based negative electrodes for large‐scale battery applications are reviewed, along with the study of the solid electrolyte interphase formed in the anode surface, which is at the origin of most of the stability problems.     

Universal Soldering of Lithium and Sodium Alloys on Various Substrates for Batteries

The high theoretical specific capacity of lithium (Li) metal and the nonflammability of solid‐state electrolytes (SSEs) make the solid‐state Li metal battery a promising option to develop safe batteries with high energy density. To make the switch from liquid to solid‐state electrolyte, the high interfacial resistance resulting from the poor solid–solid contacts between Li metal and SSEs needs to be addressed. Herein, a one‐step soldering technique to quickly coat molten Li onto different substrates including metals, ceramics, and polymers is presented. It is deduced that the surface energy and viscosity of the molten Li can be tuned by adding alloy elements, which improves the wettability against various substrates. When soldered onto the surface of garnet‐based SSEs, the Li alloys exhibit significantly improved contact, which leads to an interface resistance as low as ≈7 Ω cm². While cycling under high loads, the newly plated Li still maintains tight contact with the garnet surface and demonstrates excellent electrochemical stability. Several Li binary alloys as well as sodium (Na) binary alloys are successfully tested on various substrates to demonstrate the versatility of this soldering technique for potential battery applications.     

Frogspawn‐Coral‐Like Hollow Sodium Sulfide Nanostructured Cathode for High‐Rate Performance Sodium–Sulfur Batteries

Room‐temperature (RT) sodium–sulfur (Na–S) batteries are attractive cost‐effective platforms as the next‐generation energy storage systems by using all earth‐abundant resources as electrode materials. However, the slow kinetics of Na–S chemistry makes it hard to achieve high‐rate performance. Herein, a facile and scalable approach has been developed to synthesize hollow sodium sulfide (Na₂S) nanospheres embedded in a highly hierarchical and spongy conductive carbon matrix, forming an intriguing architecture similar to the morphology of frogspawn coral, which has shown great potential as a cathode for high‐rate performance RT Na–S batteries. The shortened Na‐ion diffusion pathway benefits from the hollow structures together with the fast electron transfer from the carbon matrix contributes to high electrochemical reactivity, leading to superior electrochemical performance at various current rates. At high current densities of 1.4 and 2.1 A g^?1, high initial discharge capacities of 980 and 790 mAh g^?1_sulfur can be achieved, respectively, with reversible capacities stabilized at 600 and 400 mAh g^?1_sulfur after 100 cycles. As a proof of concept, a Na‐metal‐free Na–S battery is demonstrated by pairing the hollow Na₂S cathode with tin‐based anode. This work provides guidance on rational materials design towards the success of RT high‐rate Na–S batteries.     

Enhanced Cyclability of Lithium–Oxygen Batteries with Electrodes Protected by Surface Films Induced via In Situ Electrochemical Process

Although the rechargeable lithium–oxygen (Li–O₂) batteries have extremely high theoretical specific energy, the practical application of these batteries is still limited by the instability of their carbon‐based air‐electrode, Li metal anode, and electrodes, toward reduced oxygen species. Here a simple one‐step in situ electrochemical precharging strategy is demonstrated to generate thin protective films on both carbon nanotubes (CNTs), air‐electrodes and Li metal anodes simultaneously under an inert atmosphere. Li–O₂ cells after such pretreatment demonstrate significantly extended cycle life of 110 and 180 cycles under the capacity‐limited protocol of 1000 mA h g^?1 and 500 mA h g^?1, respectively, which is far more than those without pretreatment. The thin‐films formed from decomposition of electrolyte during in situ electrochemical precharging processes in an inert environment, can protect both CNTs air‐electrode and Li metal anode prior to conventional Li–O₂ discharge/charge cycling, where reactive reduced oxygen species are formed. This work provides a new approach for protection of carbon‐based air‐electrodes and Li metal anodes in practical Li–O₂ batteries, and may also be applied to other battery systems.     

Progress in High‐Voltage Cathode Materials for Rechargeable Sodium‐Ion Batteries

Room‐temperature rechargeable sodium‐ion batteries are considered as a promising alternative technology for grid and other storage applications due to their competitive cost benefit and sustainable resource supply, triumphing other battery systems on the market. To facilitate the practical realization of the sodium‐ion technology, the energy density of sodium‐ion batteries needs to be boosted to the level of current commercial Li‐ion batteries. An effective approach would be to elevate the operating voltage of the battery, which requires the use of electrochemically stable cathode materials with high voltage versus Na⁺/Na. This review summarizes the recent progress with the emerging high‐voltage cathode materials for room‐temperature sodium‐ion batteries, which include layered transitional‐metal oxides, Na‐rich materials, and polyanion compounds. The key challenges and corresponding strategies for these materials are also discussed, with an emphasis placed on the intrinsic structural properties, Na storage electrochemistry, and the voltage variation tendency with respect to the redox reactions. The insights presented in this article can serve as a guide for improving the energy densities of room‐temperature Na‐ion batteries.     

Scaling Printable Zn–Ag2O Batteries for Integrated Electronics

Printed batteries are an emerging solution for integrated energy storage using low‐cost, high accuracy fabrication techniques. While several printed batteries have been previously shown, few have designed a battery that can be incorporated into an integrated device. Specifically, a fully printed battery with a small active electrode area (<1 cm²) achieving high areal capacities (>10 mAh cm^?2) at high current densities (1–10 mA cm^?2) has not been demonstrated, which represents the minimum form‐factor and performance requirements for many low‐power device applications. This work addresses these challenges by investigating the scaling limits of a fully printed Zn–Ag₂O battery and determining the electrochemical limitations for a mm²‐scale battery. Processed entirely in air, Zn–Ag₂O batteries are well suited for integration in typical semiconductor packaging flows compared to lithium‐based chemistries. Printed cells with electrodes as small as 1 mm² maintain steady operating voltages above (>1.4 V) at high current densities (1–12 mA cm^?2) and achieve the highest reported areal capacity for a fully printed battery at 11 mAh cm^?2. The findings represent the first demonstration of a small, packaged, fully printed Zn–Ag₂O battery with high areal capacities at high current densities, a crucial step toward realizing chip‐scale energy storage for integrated electronic systems.     

Recent Progress of the Solid‐State Electrolytes for High‐Energy Metal‐Based Batteries

Secondary batteries based on metal anodes (e.g., Li, Na, Mg, Zn, and Al) are among the most sought‐after candidates for next‐generation mobile and stationary storage systems because they are able to store a larger amount of energy per unit mass or volume. However, unstable electrodeposition and uncontrolled interfacial reactions occuring in liquid electrolytes cause unsatisfying cell performance and potential safety concerns for the commercial application of these metal anodes. Solid‐state electrolytes (SSEs) having a higher modulus are considered capable of inhibiting difficulties associated with the anodes and may enable building of safe all‐solid‐state metal batteries, yet several challenges, such as insufficient room‐temperature ionic conductivity and poor interfacial stability between the electrode and the electrolyte, hinder the large‐scale development of such batteries. Here, research and development of SSEs including inorganic ceramics, organic solid polymers, and organic–inorganic hybrid/composite materials for metal‐based batteries are reviewed. The comparison of different types of electrolytes is discussed in detail, in the context of electrochemical energy storage applications. Then, the focus of this study is on recent advances in a range of attractive and innovative battery chemistries and technologies that are enabled by SSEs. Finally, the challenges and future perspectives are outlined to foresee the development of SSEs.     

Mixed Metal Sulfides for Electrochemical Energy Storage and Conversion

Mixed metal sulfides (MMSs) have attracted increased attention as promising electrode materials for electrochemical energy storage and conversion systems including lithium‐ion batteries (LIBs), sodium‐ion batteries (SIBs), hybrid supercapacitors (HSCs), metal–air batteries (MABs), and water splitting. Compared with monometal sulfides, MMSs exhibit greatly enhanced electrochemical performance, which is largely originated from their higher electronic conductivity and richer redox reactions. In this review, recent progresses in the rational design and synthesis of diverse MMS‐based micro/nanostructures with controlled morphologies, sizes, and compositions for LIBs, SIBs, HSCs, MABs, and water splitting are summarized. In particular, nanostructuring, synthesis of nanocomposites with carbonaceous materials and fabrication of 3D MMS‐based electrodes are demonstrated to be three effective approaches for improving the electrochemical performance of MMS‐based electrode materials. Furthermore, some potential challenges as well as prospects are discussed to further advance the development of MMS‐based electrode materials for next‐generation electrochemical energy storage and conversion systems.     

Electrolytes and Interphases in Sodium‐Based Rechargeable Batteries: Recent Advances and Perspectives

For sodium (Na)‐rechargeable batteries to compete, and go beyond the currently prevailing Li‐ion technologies, mastering the chemistry and accompanying phenomena is of supreme importance. Among the crucial components of the battery system, the electrolyte, which bridges the highly polarized positive and negative electrode materials, is arguably the most critical and indispensable of all. The electrolyte dictates the interfacial chemistry of the battery and the overall performance, having an influence over the practical capacity, rate capability (power), chemical/thermal stress (safety), and lifetime. In‐depth knowledge of electrolyte properties provides invaluable information to improve the design, assembly, and operation of the battery. Thus, the full‐scale appraisal of both tailored electrolytes and the concomitant interphases generated at the electrodes need to be prioritized. The deployment of large‐format Na‐based rechargeable batteries also necessitates systematic evaluation and detailed appraisal of the safety‐related hazards of Na‐based batteries. Hence, this review presents a comprehensive account of the progress, status, and prospect of various Na⁺‐ion electrolytes, including solvents, salts and additives, their interphases and potential hazards.     

Yolk–Shell NiS2 Nanoparticle‐Embedded Carbon Fibers for Flexible Fiber‐Shaped Sodium Battery

Fiber‐shaped rechargeable batteries hold promise as the next‐generation energy storage devices for wearable electronics. However, their application is severely hindered by the difficulty in fabrication of robust fiber‐like electrodes with promising electrochemical performance. Herein, yolk–shell NiS₂ nanoparticles embedded in porous carbon fibers (NiS₂?PCF) are successfully fabricated and developed as high‐performance fiber electrodes for sodium storage. Benefiting from the robust embedded structure, 3D porous and conductive carbon network, and yolk–shell NiS₂ nanoparticles, the as‐prepared NiS₂?PCF fiber electrode achieves a high reversible capacity of about 679 mA h g^?1 at 0.1 C, outstanding rate capability (245 mA h g^?1 at 10 C), and ultrastable cycle performance with 76% capacity retention over 5000 cycles at 5 C. Notably, a flexible fiber‐shaped sodium battery is assembled, and high reversible capacity is kept at different bending states. This work offers a new electrode‐design paradigm toward novel carbon fiber electrodes embedded with transition metal oxides/sulfides/phosphides for application in flexible energy storage devices.     

Fe‐Based Tunnel‐Type Na0.61[Mn0.27Fe0.34Ti0.39]O2 Designed by a New Strategy as a Cathode Material for Sodium‐Ion Batteries

Sodium‐ion batteries are promising for grid‐scale storage applications due to the natural abundance and low cost of sodium. However, few electrodes that can meet the requirements for practical applications are available today due to the limited routes to exploring new materials. Here, a new strategy is proposed through partially/fully substituting the redox couple of existing negative electrodes in their reduced forms to design the corresponding new positive electrode materials. The power of this strategy is demonstrated through the successful design of new tunnel‐type positive electrode materials of Na_0.61[Mn_0.61‐xFe_xTi_0.39]O₂, composed of non‐toxic and abundant elements: Na, Mn, Fe, Ti. In particular, the designed air‐stable Na_0.61[Mn_0.27Fe_0.34Ti_0.39]O₂ shows a usable capacity of ≈90 mAh g^?1, registering the highest value among the tunnel‐type oxides, and a high storage voltage of 3.56 V, corresponding to the Fe³⁺/Fe⁴⁺ redox couple realized for the first time in non‐layered oxides, which was confirmed by X‐ray absorption spectroscopy and Mössbauer spectroscopy. This new strategy would open an exciting route to explore electrode materials for rechargeable batteries.     

A Novel High Capacity Positive Electrode Material with Tunnel‐Type Structure for Aqueous Sodium‐Ion Batteries

Aqueous sodium‐ion batteries have shown desired properties of high safety characteristics and low‐cost for large‐scale energy storage applications such as smart grid, because of the abundant sodium resources as well as the inherently safer aqueous electrolytes. Among various Na insertion electrode materials, tunnel‐type Na_0.44MnO₂ has been widely investigated as a positive electrode for aqueous sodium‐ion batteries. However, the low achievable capacity hinders its practical applications. Here, a novel sodium rich tunnel‐type positive material with a nominal composition of Na_0.66[Mn_0.66Ti_0.34]O₂ is reported. The tunnel‐type structure of Na_0.44MnO₂ obtained for this compound is confirmed by X‐ray diffraction and atomic‐scale spherical aberration‐corrected scanning transmission electron microscopy/electron energy‐loss spectrum. When cycled as positive electrode in full cells using NaTi₂(PO₄)₃/C as negative electrode in 1 m Na₂SO₄ aqueous electrolyte, this material shows the highest capacity of 76 mAh g^?1 among the Na insertion oxides with an average operating voltage of 1.2 V at a current rate of 2 C. These results demonstrate that Na_0.66[Mn_0.66Ti_0.34]O₂ is a promising positive electrode material for rechargeable aqueous sodium‐ion batteries.     

High‐Abundance and Low‐Cost Metal‐Based Cathode Materials for Sodium‐Ion Batteries: Problems,Progress, and Key Technologies

Recently, room‐temperature stationary sodium‐ion batteries (SIBs) have received extensive investigations for large‐scale energy storage systems (EESs) and smart grids due to the huge natural abundance and low cost of sodium. The SIBs share a similar “rocking‐chair” sodium storage mechanism with lithium‐ion batteries; thus, selecting appropriate electrodes with a low cost, satisfactory electrochemical performance, and high reliability is the key point for the development for SIBs. On the other hand, the carefully chosen elements in the electrodes also largely determine the cost of SIBs. Therefore, earth‐abundant‐metal‐based compounds are ideal candidates for reducing the cost of electrodes. Among all the high‐abundance and low‐cost metal elements, cathodes containing iron and/or manganese are the most representative ones that have attracted numerous studies up till now. Herein, recent advances on both iron‐ and manganese‐based cathodes of various types, such as polyanionic, layered oxide, MXene, and spinel, are highlighted. The structure–function property for the iron‐ and manganese‐based compounds is summarized and analyzed in detail. With the participation of iron and manganese in sodium‐based cathode materials, real applications of room‐temperature SIBs in large‐scale EESs will be greatly promoted and accelerated in the near future.     

Transition Metal Oxide Anodes for Electrochemical Energy Storage in Lithium‐ and Sodium‐Ion Batteries

Lithium‐ion batteries (LIBs) with outstanding energy and power density have been extensively investigated in recent years, rendering them the most suitable energy storage technology for application in emerging markets such as electric vehicles and stationary storage. More recently, sodium, one of the most abundant elements on earth, exhibiting similar physicochemical properties as lithium, has been gaining increasing attention for the development of sodium‐ion batteries (SIBs) in order to address the concern about Li availability and cost—especially with regard to stationary applications for which size and volume of the battery are of less importance. Compared with traditional intercalation reactions, conversion reaction‐based transition metal oxides (TMOs) are prospective anode materials for rechargeable batteries thanks to their low cost and high gravimetric specific capacities. In this review, the recent progress and remaining challenges of conversion reactions for LIBs and SIBs are discussed, covering an overview about the different synthesis methods, morphological characteristics, as well as their electrochemical performance. Potential future research directions and a perspective toward the practical application of TMOs for electrochemical energy storage are also provided.     

MoS2 Nanosheets Vertically Aligned on Carbon Paper: A Freestanding Electrode for Highly Reversible Sodium‐Ion Batteries

The development of sodium‐ion batteries for large‐scale applications requires the synthesis of electrode materials with high capacity, high initial Coulombic efficiency (ICE), high rate performance, long cycle life, and low cost. A rational design of freestanding anode materials is reported for sodium‐ion batteries, consisting of molybdenum disulfide (MoS₂) nanosheets aligned vertically on carbon paper derived from paper towel. The hierarchical structure enables sufficient electrode/electrolyte interaction and fast electron transportation. Meanwhile, the unique architecture can minimize the excessive interface between carbon and electrolyte, enabling high ICE. The as‐prepared MoS₂@carbon paper composites as freestanding electrodes for sodium‐ion batteries can liberate the traditional electrode manufacturing procedure, thereby reducing the cost of sodium‐ion batteries. The freestanding MoS₂@carbon paper electrode exhibits a high reversible capacity, high ICE, good cycling performance, and excellent rate capability. By exploiting in situ Raman spectroscopy, the reversibility of the phase transition from 2H‐MoS₂ to 1T‐MoS₂ is observed during the sodium‐ion intercalation/deintercalation process. This work is expected to inspire the development of advanced electrode materials for high‐performance sodium‐ion batteries.