Recent Progress in Cathode Materials for Ca-Ion Batteries.

Fundamentals and key components of sodium-ion batteries: Challenges and future perspectives

The World's energy consumption has increased drastically over the past several decades, particularly driven by increasing population and economic activities as well as rising living standards with technological advancements. Since the industrial revolution, fossil fuels have been the major source of energy productions, which is today shown as the primary cause of the ongoing climate change and associated environmental challenges in threatening the existence of many living species and human life and comfort. According to the statistics provided by International Energy Agency (IEA),1 the global energy supply was 14 282 Mtoe in 2018 that is 115% higher than that of the value for 1973. Fossil fuels, such as natural gas, coal and oil are still playing a critical role and covering more than 80% of global energy demands. The concerns associated with global warming and depletion of energy sources have been directing the attention to renewable and environmentally-benign energy solutions. However, efficient, affordable, and nature-friendly energy storage systems are the key to using renewable energy sources effectively. In this study, a comprehensive literature search is conducted to study and bibliografically evalute Canada's energy storage research and development activities over the last five decades. The present study intends to present the cumulative picture on energy storage reseach and development efforts and assess how universities, research institutes and funding agencies have contributed to this particular field. The literature search compiles a total of fifty-year data on energy storage-related research and development activities conducted in Canada. The number of conducted studies associated with energy storage regarding research articles, books, research projects, dissertations, and patents is presented in the period from 1971 to 2021. Furthermore, the Canadian institutes are ranked based on their scholarly ouputs and publications to indicate the most productive ones in the field of energy storage based on the data presented in the literature sources. Energy storage is described as the capture of harvested energy for later use to maintain the balance fluctuations between energy demand and supply. As the world moves towards a low-carbon economy, energy storage systems are exepcted to even play a more critical role in extracting more energy from renewable resources.10 The intermittent characteristic of renewable resources, specifically solar and wind, is one of the major challenges ahead of a reliable and steady renewable energy supply. This can be avoided by storing excess energy to re-use when the renewable resources are not sufficient or unavailable. Energy storage systems can be categorized in different ways in regards to the form of converted and stored energy. Figure 1 presents an overview of energy storage system categories which are considered in this study. It is important to note that chemical energy storage allows to storage and transportation of large amounts of energy for long distances with minimal self-discharge rates, which is crucial for the distribution and effective use of renewable energy resources. Figure 2 presents the energy density of various chemical fuels and batteries. Despite possessing high energy density, environmental concerns associated with the production and consumption of fossil-based chemical fuels, e.g. diesel, gasoline or natural gas, should be adrressed. In regards to nature-friendly chemical energy storage, particularly power-to-gas chemical storage, hydrogen is attracting favorable attention nowadays. In this regard, hydrogen appears to be the lightest and most abundant chemical element in the universe, but is not available freely (by itself). However, it occurs naturally within a compound such as water (H2O); therefore, it is an intermediate energy form. Excess electricity from renewables can be utilized in hydrogen production processes, i.e. electrolysis of water, to store energy in chemical form. Power-to-hydrogen chemical energy storage is a unique and clean method, and it can be compatible with electrochemical storage since hydrogen possesses much greater energy content. One of the major challenges with hydrogen is the low calorific value per unit of volume (MJ/m3) due to its low density. This can be avoided by deploying alternative hydrogen carriers, such as ammonia. Liquefaction at atmospheric conditions, high hydrogen atom content and high energy density characteristics make ammonia a favorable medium for storage and transportation of energy. It is further to note that batteries and capacitors are devices allowing electrochemical storage of electricity. The potential energy is stored in chemical form in batteries, while capacitors are expected to store their potential energy in the form of an electric field.14 Batteries are thus most commonly used and matured options that propose high energy densities and voltages. Lithium-ion (Li-ion), sodium-sulphur (NaS), nickel-cadmium (NiCd), lead acid (Pb-acid), lead-carbon batteries, zebra batteries (Na-NiCl2) and flow batteries are the existing battery types used in various applications. Due to their high specific energy (per unit weight) and high specific energy (per unit volume), lithium batteries attract considerable attention in electrical energy storage. Capacitors can be classified as electrostatic capacitors, electrolytic capacitors, and electrochemical capacitors, among which electrochemical, so-called supercapacitors, promote the highest capacitance per unit of volume owing to a porous structure 15. As a mechanical energy storage method, pumped hydro accounts for more than 95% of the global installed energy storage capacity.15 The main advantage of mechanical energy storage is to readily deliver energy. The method is easy to adapt for conversion and storage of energy from various sourcessuch as water current, wave, or tidal. Pumped hydro, compressed air, and flywheel are the commonly practiced mechanical energy storage methods. Pumped hydro and compressed air are bound to specific geographical characteristics, which is the main restriction for the implementation of these energy storage methods. A flywheel stores the energy obtained from a rotor mass spinning at very high speeds as kinetic energy. The stored kinetic energy can be later used to produce electricity by altering the angular velocity of the flywheel through a decelerating torque and using an electric motor in reverse 16. Despite of being non-suitable for long-term energy storage, flywheels offer favorable advantages, that is, long-life spin, high energy densities and low maintenance costs, and fast charging.17 Thermal energy storage (TES) systems are effective systems to store heat in a reservoir for a period of time (defined or undefined) for later use, that is, space heating and cooling, electricity production, hot water. If the reservoir temperature is kept under the atmospheric temperature, the storage process is called cold thermal energy storage.18 Operating temperature range, capacity requirements, charging and discharging requirements are some of important parameters when choosing an appropriate TES system for a desired application. Since the major proportion of produced and consumed energy is heat around the world, TES systems carries a significant weight regarding the development of effective energy systems . Furthermore, energy losses from the systems commonly occur in the form of heat. Decreasing losses via further utilizing from waste energy is one of many way to mitigate environmental impacts, which can be accomplished by implementing TES systems in a cost effective manner. In the current study, an overview of Canada's energy storage related studies from 1971 to 2021 is presented. The scope of this study is to present the contributions of Canadian universities, research institutes and funders to energy storage research and development efforts over the last five decades. The keywords and methodology that are used for the literature search are depicted in Figure 3. The collected and compiled data for the corresponding fields are presented in figures, and results are discussed in the following sections. In this study, an online literature search is conducted on Canada's energy storage related studies over the last five decades. The current section presents collected and compiled data in charts. The findings of the conducted study are discussed in detail. The most productive institutes of Canada regarding conducting or sponsoring energy storage-related studies are presented. The documents associated with energy storage in all subject areas are presented in Figure 4. In this regard, the highest number of energy storage-related studies were conducted in the subject area of engineering in Canada from 1971 to 2021. The fields of energy and chemistry appear as the other two major subject areas in which high number of energy storage research was conducted. While compiling data, attention has been paid to any potential field conflicts concerning the multi attribution of interdisciplinary publications. Therefore, it should be noticed that chemistry may have an impact on the subject areas of biochemistry, genetics, and molecular biology. The same impact of engineering could be seen on the subject area of energy. The data collected and analysed for comparative evaluation reveals that the Canadian institutes have produced 37 386 energy storage-related academic research publications over the last 50-year period. In this regard, Figure 5A presents the breakdown of energy storage-related publications affiliated to Canada in the corresponding material types in accordance with the publication years. Figure 5A shows that studies in this field consistently increases every year. Consequently, the year 2020 appears as the most productive one in this regard as of April 2021. In the light of this trendline, predicting that the year 2021 will be more productive than 2020 would not be wrong. Figure 5B has been prepared as a heat map to show Canada's provinces where energy storage-related studies were conducted over the last 50 years. In this regard, Ontario appears as the most productive province of Canada in terms of scholarly publications. However, one has to keep in mind that Ontario has the highest number of contributing institutes/universities compared to the rest of the provinces in Canada. Figure 6 presents the breakdown of the number of energy storage-related books, book chapters, and editorials in accordance with publication years in Canada. The year 2018 was the most productive year of Canada by producing 200 documents in the corresponding classification. The other important point that should be pinned in the figure is the year 2005. Compared to the previous year, almost five times more energy storage-related books were published in 2005. Compared to 2019, the number of published books decreased almost by half in 2020. The impact of COVID-19 is recognized as one of numerous reasons behind this result. The number of energy storage-related dissertations completed in Canada over the last five decades is primarily searched through and collected from Library and Archives Canada. The search results are, in this regard, presented in Figure 7. It can be noted that the data presented in Figure 7 is based on the number of energy storage-related theses available in the library; and thus, the results are subject to change in accordance with instantaneous new data entry to the library. As of April 2021, there are 626 energy storage-related MSc and PhD theses/dissertations entered in the online system of Library and Archives Canada. The highest number of theses/dissertations was recorded in the year of 2013 with a value of 62. Based on data collected from the website of Canadian Intellectual Property Office, the number of energy storage-related patents of Canada is 11 719 for the last 50 years. The corresponding results are presented Figure 8. According to this, the year of 2018 appeared to be the Canada's most productive year in terms of patents registered under the title of energy storage. Canada is blessed with institutions that have been supporting energy storage-related research and developments by providing funds. Over the evaluated period of 50 years, the Natural Sciences and Engineering Research Council of Canada (NSERC) appears as the major funding agency for Canada's energy storage related research, development, partnership, strategic innovation and commercialization efforts and has supported more than 8500 projects in the field of energy storage. A total number of projects that have been funded by Canadian institutes have become 18021 from 1971 to 2021. It is most likely to observe a major increase in the number of projects by the impact of the recent hydrogen strategic plan of Canada. Being an excellent energy storage medium is one of many advantages of hydrogen. Therefore, there is a significant correlation in between hydrogen and energy storage research efforts. The subject matter report is already named as call-to-action for green hydrogen transition for the energy sector. In the report, the importance of dedicating more domestic resources to green hydrogen efforts to achieve the targets by 2050 is emphasized clearly (Figure 9). The development and implementation efforts of effective, affordable, and nature-friendly energy storage systems are recognized as a key requirement in achieving a green energy transition. In this regard, Canada intends to be the country leading and inspiring many others. Figure 10 shows the a number of Canadian institutes/universities that have produced more than 100 scholarly publications on energy storage under the subject area of energy. According to this, the total number of such scholarly publications has been 9125 over the past five decades. The results clearly show that there are two universities, such as University of Waterloo with 809 publications and Ontario Tech. University (formerly University of Ontario Institute of Technology) with 697 publications, leading in this particular area. University of Waterloo appears as the most productive Canadian university with a total number of 809 publications while Ontario Tech. University, which is one of the Canada's youngest universities, follows with a total number of 697 publications. The specific affiliations in the publications show that the researchers working in the productive environment of Clean Energy Research Laboratory (CERL) at Ontario Tech. have played a major role in this achievement. The data that support the findings of this study are available in Scopus, Library and Archives Canada and Canadian Intellectual Property Office with the reference number of [17], [18], and [19], respectively. These data were derived from the following resources available in the public domain: https://www.scopus.com/search/form.uri?display=basic&zone=header&origin, https://www.bac-lac.gc.ca/eng/services/theses/Pages/search.aspx, https://www.ic.gc.ca/opic-cipo/cpd/eng/search/basic.html?wt_src=cipo-patent-main.

Energy storage materials derived from Prussian blue analogues

Cathode materials of metal-ion batteries for low-temperature applications

<p indent="0mm">The pursuit of next-generation energy storage systems has intensified research on lithium-sulfur (Li-S) batteries, which offer high energy density and low cost. However, the practical application of Li-S batteries is hindered by issues such as the shuttle effect, low electrical conductivity of sulfur, and large volume changes during cycling. Organic electrode materials possess advantages such as safety, environmental friendliness, sustainability, design flexibility, and low cost, making them promising cathode materials for high-capacity lithium batteries. Among them, organosulfur has received increasing attention with the development of lithium-sulfur batteries. Similar to sulfur cathodes, organosulfur electrodes can provide a relatively high theoretical capacity based on the reversible breaking and formation of S–S bonds. However, inherent problems such as poor conductivity, sluggish redox kinetics, unsatisfactory cycling stability, and the solubility of cycling products seriously hinder the development of organosulfur cathode materials. Doping heteroatoms into organosulfur molecules to design and synthesize new organosulfur molecules is an effective strategy to improve the performance of organic sulfur cathodes. Heteroatom doping offers a versatile method to modify the electronic structure and chemical reactivity of organosulfur cathodes. By introducing elements such as oxygen (O), nitrogen (N), selenium (Se), tellurium (Te) and phosphorus (P), researchers aim to tailor the material’s properties to improve conductivity, accelerate redox reactions, stabilize electrode interfaces, and mitigate dissolution issues of reaction intermediates. Hybridized organosulfur cathode materials have received more and more attention due to their unique design strategy and flexible electrochemical modulation. Therefore, the research progress in heteroatom-containing organosulfur cathode materials is reviewed comprehensively in this paper, summarizing their applications in lithium battery systems. It primarily discusses heteroatom-containing small molecule organic sulfur and polymer organic sulfur materials, and concludes with the challenges and prospects for organic sulfur cathode materials. It covers synthetic strategies to incorporate heteroatoms, characterization techniques to elucidate their structural and electrochemical properties, and their performance as cathode materials in various battery configurations. The discussion encompasses mechanistic insights into how heteroatoms influence charge storage mechanisms, ion diffusion kinetics, and overall battery performance metrics. Despite the promising advances, challenges remain, including the precise control of heteroatom doping levels, maintaining structural integrity during cycling, and optimizing synthesis methods for scalability and cost-effectiveness. Furthermore, the environmental impact and recyclability of these materials are critical considerations for their sustainable application in energy storage technologies. Looking forward, the development of heteroatom-containing organic sulfur cathode materials holds significant potential to address current limitations and unlock new opportunities for next-generation rechargeable batteries. Future research directions focus on refining synthetic techniques, enhancing material stability and cycling performance, exploring novel heteroatom combinations, and integrating these materials into practical battery systems that meet the increasing demands for high-energy-density and sustainable energy storage solutions. In conclusion, heteroatom-doped organic sulfur cathode materials represent a significant advancement in the field of Li-S batteries. The synergistic effect of organic-inorganic hybridization, combined with the tunability of heteroatom doping, offers a promising avenue for the development of high-performance energy storage systems. Further research is needed to fully exploit the potential of these materials and to address the remaining challenges for their practical application.

The research to discover a suitable cathode material for calcium-ion batteries (CIBs) remains challenging, despite their potential advantages of high energy density and cost-effectiveness. Herein, we report K0.5V2O5 as a cathode material for CIBs. During the initial charging of the electrode, potassium ions were replaced with 0.12 Ca. The calcium-exchanged electrode exhibited a reversible discharge capacity of ∼100 mAh g–1 at 0.1 C rate at room temperature with an average voltage of ∼3.0 V (vs Ca/Ca2+). The reversible capacity is higher than any reported cathode materials without crystal water, demonstrating the feasibility of further development of high-capacity crystal-water-free CIB cathode materials.

This study investigates the electrochemical properties of ball-milled copper hexacyanoferrate (CuHCF), a Prussian blue analogue, as a cathode material in aqueous calcium-ion batteries (CIBs). X-ray diffraction analysis confirmed that the ball milling process did not destroy the crystal structure of the CuHCF active material. The general grain size and crystal surface of the synthesized CuHCF active materials were confirmed from the scanning electron microscopy (SEM) images. The electrochemical test results revealed that prolonged ball milling improved the charge/discharge capacity in the initial cycle. After 200 cycles, structural collapse of the CuHCF electrode occurred, as observed by SEM.

The ever-growing market of electrochemical energy storage impels the advances on cost-effective and environmentally friendly battery chemistries. Lithium-ion batteries (LIBs) are currently the most critical energy storage devices for a variety of applications, while sodium-ion batteries (SIBs) are expected to complement LIBs in large-scale applications. In respect to their constituent components, the cathode part is the most significant sector regarding weight fraction and cost. Therefore, the development of cathode materials based on Earth’s abundant elements (Fe and Mn) largely determines the prospects of the batteries. Herein, we offer a comprehensive review of the up-to-date advances on Fe- and Mn-based cathode materials for LIBs and SIBs, highlighting some promising candidates, such as Li- and Mn-rich layered oxides, LiNi0.5Mn1.5O4, LiFe1-xMnxPO4, NaxFeyMn1-yO2, Na4MnFe2(PO4)(P2O7), and Prussian blue analogs. Also, challenges and prospects are discussed to direct the possible development of cost-effective and high-performance cathode materials for future rechargeable batteries.

Calcium-ion batteries (CIBs) represent a promising alternative to the current lithium-ion batteries (LIBs). Calcium ions can transfer two electrons per ion. Thus, in principle, the capacity of a host material can double due to the divalency of calcium, provided the host material can release and accept the transferred electrons. The final battery product is expected to be cost-effective, owing to the earth-abundance of calcium reserves. The redox potential of calcium is close to that of lithium, enabling a high cell voltage. The larger ionic radius of calcium (1.0 Å) compared to those of other divalent ions (0.60–0.74 Å) could be a disadvantage because a narrow diffusion pathway sufficient for smaller ions will not allow the passage of larger Ca ions. However, the larger ionic radius ensures a lower effective intercalant-ion charge density, which can be rather advantageous for the diffusion in the host materials as well as in the electrolytes.Recent discoveries of reversible plating or alloying of calcium provoked considerable interest in calcium-based rechargeable batteries. Theoretical calculations also predict that the energy barriers for Ca diffusion are lower than other multivalent ions in some structures, and the development of positive materials are anticipated. However, only a few cathode materials have been reported so far, only to exhibit low energy-storage capability and poor cyclability. In this talk, the synthesis, structural and electrochemical properties of new cathode materials for nonaqueous CIBs will be presented, which were recently developed in our group. Some examples include NASICON-type NaV2(PO4)3, FeV3O9∙1.2H2O, Ca0.13MoO3·(H2O)0.41. These stimulating discoveries will lead to the development of new strategies for obtaining high energy density CIBs.

The rapid worldwide energy consumption is necessitating more energy generation from environmentally benign sources like solar, wind, or tide. However, the lack of efficient energy storage devices is the major limitation for the practical harvest of these clean energies. Rechargeable batteries are the promising solution for the efficient energy storage. We have witnessed the great success of secondary lithium-ion batteries (LIBs) in portable electronic devices in the last two decades. However, the cost and safety barriers make traditional LIBs not desirable to meet the requirements for large power applications, such as the electric or plug-in hybrid vehicles. In contrast, the multivalent ion rechargeable batteries such as Mg1-3 and Ca4 , 5 ion batteries are more promising as larger and safer energy storage devices due to their advantages in safety, cost and capacity. For instance, compared to lithium, magnesium metal is non-toxic, more air resistant and free from the dangerous dendrite formation. The higher natural abundance of magnesium makes it more than 20 times cheaper than lithium in cost. In addition, the divalent characteristic of magnesium ion (Mg2+) enables magnesium to have a nearly doubled volumetric capacity as an anode than the lithium (3832 mA cm-3 vs 2062 mA cm-3). All these merits have brought significant attention on magnesium-ion batteries as the new generation rechargeable energy storage device. Inspired by the pioneer work by Aurbach group for the development of the famous dichloro-complex magnesium electrolyte (2EtAlCl2-MgBu2),6 a lot of efforts have been devoted to the development of high voltage electrolytes for rechargeable magnesium-ion batteries. In contrast to the rapid development of effective magnesium electrolytes capable of reversibly plating/stripping magnesium with wide electrochemical windows, the design of high potential cathode material for magnesium ion batteries is far behind. To date, the Chevrel phase Mo6S8 is still the only cathode material with excellent cycling reversibility and reasonable rate capability in rechargeable magnesium-ion batteries. However, Mo6S8 could only operate at a relatively low voltage in magnesium-ion batteries (< 1.3 V) with a typical below 100 mAh g-1 capacity. Thus, development of high operating voltage cathode material deserves great attention in nowadays. Herein, we would like to present our recent progress on the development of high voltage and high performance rechargeable magnesium-ion batteries using redox active organic molecules as cathode material. As shown in Figure 1(a), when the 0.3M Mg(HMDS)2-4MgCl2 in THF solution was used as the electrolyte,7 excellent reversible redox processes have been observed in the cyclic voltammetry measurement when the redox active organic molecule was used as the cathode material. The redox potential was detected at around 1.7 V (vs Mg/Mg2+), which is much higher than the well-known Chevral phase Mo6S8. The compatibility and reversibility of the organic cathode material with the electrolyte and magnesium metal anode was further demonstrated in the batteries upon cycling. As shown in Figure 1(b), excellent cycling performance has been observed with around 120 mAh g-1 initial specific discharge capacity at the current rate of 0.5C, and still about 102 mAh g-1specific discharge capacity was obtained in 100 cycles (around 84% capacity retention). The results further suggest the great success of using redox active organic molecules as cathode material for secondary multivalent-ion batteries. (1) Muldoon, J.; Bucur, C. B.; Gregory, T. Chem Rev 2014, 114, 11683. (2) Saha, P.; Datta, M. K.; Velikokhatnyi, O. I.; Manivannan, A.; Alman, D.; Kumta, P. N. Prog Mater Sci 2014, 66, 1. (3) Yoo, H. D.; Shterenberg, I.; Gofer, Y.; Gershinsky, G.; Pour, N.; Aurbach, D. Energ Environ Sci 2013, 6, 2265. (4) Padigi, P.; Goncher, G.; Evans, D.; Solanki, R. J Power Sources 2015, 273, 460. (5) Lipson, A. L.; Pan, B.; Lapidus, S. H.; Liao, C.; Vaughey, J. T.; Ingram, B. J. Chem Mater 2015, ASAP. (6) Aurbach, D.; Lu, Z.; Schechter, A.; Gofer, Y.; Gizbar, H.; Turgeman, R.; Cohen, Y.; Moshkovich, M.; Levi, E. Nature 2000, 407, 724. (7) Liao, C.; Sa, N.; Key, B.; Burrell, A. K.; Cheng, L.; Curtiss, L. A.; Vaughey, J. T.; Woo, J. J.; Hu, L. B.; Pan, B. F.; Zhang, Z. C. Journal of Materials Chemistry A 2015, 3, 6082. Figure 1

The lack of suitable cathode materials has hampered the further development of calcium-ion batteries (CIBs). A novel composite cathode material, namely BaV6O16·3H2O@GO, was fabricated, which delivers a high specific capacity of 285.72 mA h g-1 at 50 mA g-1 after 50 cycles and a long cycle life, benefiting from a large layer spacing and robust structure. This study provides guidance for the development of vanadium-based cathode materials for CIBs.

Recent progress of Prussian blue analogues as cathode materials for nonaqueous sodium-ion batteries

High-entropy cathode materials for sodium-ion batteries: Correlating synthesis, crystal structure and electrochemical properties

Increasing awareness of environmental pollution and requirement of reliable energy are driving to develop various energy storage systems. Among them, rechargeable batteries play a key role in our modern society. "Building better batteries" remains an ongoing process to cater diverse energy demands starting from small-scale consumer electronics to large-scale automobiles and grid storage. Constantly promoting the development of battery technologies towards better, cheaper and safer properties is the persistent mission of scientific researchers. Due to the similar working principle to Li-ion batteries, abundant resource advantages, and mature fabrication technologies, Na-ion batteries (NIBs) have become a rising star and aroused a great deal of interest particularly for the application in the grid electrical energy storage. To realize the goal, the scientific community is making unremitting endeavors in the search for more efficient, reliable and economical materials and technologies, where the new designing and discovering, the endless improvements and innovations, as well as the proposed solutions and strategies are the best proof. This special issue presents a body of work discussing the important topics in NIB research: i) the synthetic condition- and composition-dependent layered oxides, ii) the multielectron reactions of polyanionic materials, iii) the application of Prussian blue analogs in organic or aqueous electrolytes, iv) the synthesis of anode materials and understanding of storage mechanism, and v) the effect of salt concentration on electrolyte and electrode property. This issue begins with a research news of sodium-oxygen batteries to review the cell chemistry and reaction mechanism, and consists of three communications, five reviews and nine research papers to introduce the above-mentioned topics, discuss them in detail, and summarize recent advances. As one of the main components of NIBs, cathode materials play a key role in overall performance, and consequently capture considerable interest. Among them, layered metal oxides, polyanionic compounds and Prussian blue analogs have been intensively examined due to their respective advantages. Four papers cover sodium layered oxides (investigating the synthetic condition, element doping and structure design for the material modification), six papers involve polyanionic cathodes (with regard to the versatile skeleton, multielectron reaction and tunable redox potential to realize high power or high energy density batteries), and two papers concern Prussian blue analogs (possessing rigid open framework with large interstitial sites and capable of applying in both organic and aqueous electrolytes). Compared with cathode materials, the selection of anode materials suffer from limited options. Two reviews summarize the state-of-the-art achievements of carbonaceous anodes: one aims to help readers comprehensively learn how to fabricate one-element doped and several-elements co-doped carbons to tune the anode performance, and the other intends to demonstrate the sodium storage ability of graphitic carbons with strategies of solvent co-intercalation, graphite expanding and active material compositing. One research paper on the cobalt/chalcogenides in B, N co-doped graphene for sodium storage is a live example combining the main content of the two reviews. Besides, another research paper reports evidence for reversible silicon redox activity by investigating the sodium storage mechanism in silicon oxycarbide. Electrolytes are also crucial to influence the whole performance of battery system. Organic electrolytes containing solvents and salts are still the mostly used type, where the salt-solvent coordination greatly impacts the ionic conductivity, viscosity, electrochemical stability window etc. One research paper systematically investigates the salt concentration on the properties of organic carbonate-based electrolyte and finds that viscosity plays a major role in influencing ionic conductivity rather than ions and molecules' coordination. The best electrolyte should make a good compromise between physicochemical and electrochemical properties. This special issue in Small Methods attempts to offer highlights on the new developments of electrode material design, electrolyte composition optimization, sodium storage mechanism interpretation, and electrochemical performance improvement for NIBs in a broad range. Even there are still many challenges to be resolved, given the facts that important achievements have been realized and over 10 companies in the world start to commercialize this technology, in particular, HiNa Battery Company has successfully installed a Na-ion battery system with a capacity of 30 kW/100 kWh for the energy storage in Liyang city of China (Figure 1). We are sure that NIBs will have promising applications in the future. It has been our pleasure to edit this special issue, and we would like to thank Small Methods editors, particularly, Dr. Guangchen Xu, for proposing the special issue and handling all the papers. We would also express our earnest thanks to all the authors for their commitment and great work, and appreciate all the reviewers who have made excellent contributions to guarantee this high-quality issue for the community.

Layered oxide materials are widely used in the field of energy storage and conversion due to their high specific energy, high efficiency, long cycle life, and high safety. Herein, We summarize the latest research progress in the field of layered metal oxide cathode materials from three aspects: challenges faced, failure mechanisms, and modification methods. We also compare the characteristics of lithium‐based layered oxides and sodium‐based layered oxides, and predict future development directions. The layered oxide cathode materials for sodium‐ion batteries and lithium‐ion batteries exhibit overall structural and operational similarities. There are also some differences, such as lattice parameters and application extent. Sodium‐ion battery cathode materials need to explore new materials and address structural instability issues, while lithium‐ion batteries require finding alternative materials and improving production efficiency. Future challenges for both types of materials include enhancing capacity and cycle performance, elucidating deep mechanisms, reducing costs, and improving resource sustainability. Future development should focus on balancing cycle stability and charge cut‐off voltage to meet the growing demand for battery applications.