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Hard carbons are extensively studied for application as anode materials in sodium-ion batteries, but only recently a great interest has been focused toward the understanding of the sodium storage mechanism and the comprehension of the structure–function correlation. Although several interesting mechanisms have been proposed, a general mechanism explaining the observed electrochemical processes is still missing, which is essentially originating from the remaining uncertainty on the complex hard carbons structure. The achievement of an in-depth understanding of the processes occurring upon sodiation, however, is of great importance for a rational design of optimized anode materials. In this review, we aim at providing a comprehensive overview of the up-to-date known structural models of hard carbons and their correlation with the proposed models for the sodium-ion storage mechanisms. In this regard, a particular focus is set on the most powerful analytical tools to study the structure of hard carbons (upon de-/sodiation) and a critical discussion on how to interpret and perform such analysis. Targeting the eventual commercialization of hard carbon anodes for sodium-ion batteries – after having established a fundamental understanding – we close this review with a careful evaluation of potential strategies to ensure a high degree of sustainability, since this is undoubtedly a crucial parameter to take into account for the future large-scale production of hard carbons.

Summary Potassium-ion batteries using non-aqueous electrolyte currently face fast growing interest since they hold the promise of combining the advantageous properties of lithium- and sodium-ion batteries without having the individual drawbacks. In this review, recent studies are analyzed and the main material classes as well as novel research strategies are discussed.

The recently introduced seawater battery concept is an eco-friendly energy storage system that offers appealing electrochemical performance. Its radically innovative design, compared to conventional lithium-ion batteries, makes use of seawater as an almost infinite sodium reservoir for the positive electrode and, thereby, avoids the use of expensive, scarce, and toxic elements like nickel and cobalt. So far, the problems identified mostly originate from the available negative electrode active materials. In this study, a starch-derived hard carbon was used to optimize the system. Due to its improved disordered structure compared with commercial hard carbon, the starch hard carbon exhibits an increased reversible capacity, current-rate capability, and cycling ability. The material, in fact, depicts a high maximum power density of 700 W kg−1 (based on hard carbon weight) upon discharge at 900 mA g−1, while still being active at 2700 mA g−1. These results represent an important step toward practical application of the sodium-based seawater battery technology.

The recently launched concept of Semi-Solid Flow Batteries (SSFBs) shows a strong potential for flexible energy storage, but the liquid-dispersed state of the electrode materials introduces several aspects of which a scientific understanding is lacking. We studied the effect of electrochemical cycling on the rheological and electrical properties of a SSFB anolyte containing Li 4 Ti 5 O 12 (LTO) and Ketjen Black (KB) particles in EC:DMC solvent with 1 M LiPF 6 , using an adapted rheometer that allows in situ electrochemical cycling and electrical impedance spectroscopy. Charging (lithiation) caused a reduction in the electronic conductivity, yield stress and high shear viscosity of the fluid electrode. For mildly reducing voltages (1.4 V), these changes were partially reversed on discharging. For more reducing voltages these changes were stronger and persistent. The finding of comparable trends for a fluid electrode without the LTO, lends support to a simplistic interpretation, in which all trends are ascribed to the formation of a surface layer around the conductive KB nanoparticles. This Solid Electrolyte Interphase (SEI) insulates particles and reduces the van der Waals attractions between them. SEI layers formed at less reducing voltages, partially dissolve during the subsequent discharge. Those formed at more reducing voltages, are thicker and permanent. As these layers increase the electronic resistance of the fluid electrode by (more than) an order of magnitude, our findings highlight significant challenges due to SEI formation that still need to be overcome to realize SSFBs.

Sodium-ion batteries are regarded as a complementary drop-in technology to lithium-ion batteries because they promise lower cost and a higher degree of environmental friendliness. Among other reasons, these benefits come from the use of manganese-based materials, whose stabilization via cation substitution is intensively studied to improve the electrochemical performance. Although multiple elements have been considered as substituent, surprisingly, boron has not been reported for layered sodium-ion cathode materials up to date. Our investigation of layered Na2/3B0.11Mn0.89O2 reveals an unexpectedly good electrochemical performance, with charge and discharge capacities of more than 175 mAh g−1 at 10 mA g−1 and 135 mAh g−1 at 500 mA g−1. The measured capacities are among the highest ever reported for sodium-based layered oxides in the potential range of 4.0–2.0 V vs. Na/Na+.

Sodium-ion batteries are an appealing alternative to lithium-ion batteries because they use raw materials that are less expensive, more abundant and less toxic. The background leading to such promises is carefully assessed in terms of cell and battery production, as well as raw material supply risks, for sodium-ion and modern lithium-ion batteries.

Herein, we present the synthesis and characterization of negative (a hard carbon/carbon black composite) and positive (K0.3MnO2) active materials for K-ion batteries as well as their combination in a non-aqueous K-ion cell. The hard carbon/carbon black composite can deliver up to 200 mAh g−1 while the layered birnessite K0.3MnO2 delivers up to 136 mAh g−1. The K-ion cell exhibits an interesting and encouraging cycling performance for 100 cycles. These exciting new insights demonstrate the potential of K-ion batteries, which are worth to be further investigated in greater detail.

Modern industrial agriculture is strongly influenced by product norms and standards, resulting in massive amounts of fresh fruit that is left in the field or wasted in spite of their good nutritional value. Herein, we present the synthesis of hard carbon from natural apple biowaste, and its use of biomass is a suitable strategy for the development of cheap and powerful carbon-based active materials for Na-ion batteries. The hard carbon exhibits a good rate capability [112 mAh g−1 at 5 C (1 A g−1)], excellent long-term cycling stability (1000 cycles at 5 C), and high specific capacity (245 mAh g−1 at 0.1 C) with full retention after 80 cycles. The full capacity (250 mAh g−1) of the hard carbon is also obtained in Na-ion cells by using the layered P2-type NaxNi0.22Co0.11Mn0.66O2 cathode. The good electrochemical performance as well as the low cost and environmental friendliness of the apple-biowaste-derived hard carbon proves its suitability for future Na-ion batteries.

P-type layered oxides are promising cathode materials for sodium-ion batteries and a wide variety of compounds have been investigated so far. Nevertheless, detailed studies on how to link synthesis temperature, structure and electrochemistry are still rare. Herein, we present a study on P-type NaxNi0.22Co0.11Mn0.66O2 materials, investigating the influence of synthesis temperature on their structure and electrochemical performance. The change of annealing temperature leads to various materials of different morphologies and either P3-type (700 °C), P3/P2-type (750 °C) or P2-type (800–900 °C) structure. Galvanostatic cycling of P3-type materials revealed high initial capacities but also a high capacity fade per cycle leading to a poor long-term cycling performance. In contrast, pure P2-type NaxNi0.22Co0.11Mn0.66O2, synthesized at 800 °C, exhibits lower initial capacities but a stable cycling performance, underlined by a good rate capability, high coulombic efficiencies and high average discharge capacity (117 mA h g−1) and discharge voltage (3.30 V vs. Na/Na+) for 200 cycles.

We report the first proof of concept for a non-aqueous semi-solid flow battery (SSFB) based on Na-ion chemistry using P2-type NaxNi0.22Co0.11Mn0.66O2 and NaTi2(PO4)3 as positive and negative electrodes, respectively. This concept opens the door for developing a new low-cost type of non-aqueous semi-solid flow batteries based on the rich chemistry of Na-ion intercalating compounds.

Sodium-based layered oxides undergo several structural changes upon the (de-)sodiation process and reveal a strong tendency towards water intercalation at lower sodium contents. However, valuable information about their handling during the investigation are still rare. Herein, we report an investigation of the water sensitivity of layered NaxNi0.22Co0.11Mn0.66O2 with mixed P2/P3 structure via X-ray diffraction. At lower sodium contents, i.e. below x ≈ 0.33 or above 3.6 V vs. Na/Na+, a strong tendency for the uptake of water is observed, supported by the appearance of new diffraction peaks confirming a dramatically increased interlayer distance. However, at high sodium content (x > 0.33) the material can be processed in open air without major changes.

This paper reports the development and characterization of high-performance sodium-ion cells based on a starch-derived hard carbon anode and a mixed layered oxide cathode with the overall composition of P2/P3/O2-Na0.76Mn0.5Ni0.3Fe0.1Mg0.1O2. At first, both materials are tested in sodium-metal half cells to demonstrate the cycling performance. On the basis of this electrochemical characterization, the systematic development of sodium-ion cells is shown. Special attention is drawn towards activation, balancing, voltage range, as well as charging procedure. Important parameters such as energy and voltage efficiency, the influence of the constant voltage step at the end of the charge, as well as the potential and voltage profiles are discussed in greater detail. The developed sodium-ion cells show high specific energies (200–240 W h per kg of cathode and anode active materials), high average discharge voltages (3.3 V), high energy efficiencies, and extraordinary long-term cycling stability (e.g. 80 % capacity retention after 700 cycles). This performance underlines the beneficial combination of hard carbon and layered oxides and demonstrates sodium-ion batteries to be a promising energy-storage technology.

Recent concerns about the availability of raw materials for lithium-ion batteries (LIBs) have led to an increased interest in alternative secondary battery technologies. Recently, from about 2010 on, researchers’ attention shifted back to sodium-ion batteries (NIBs) as they offer advantages like the replacement of the copper current collector of the anode by aluminum and low cost and abundant Na-reserves instead of relatively high cost and poorly distributed Li-reserves. These advantages are considerable, since NIBs show a comparable electrochemical performance to LIBs.1 A significant advantage of potassium-ion batteries (KIBs), if compared with NIBs, is the lower potential of K/K+ related to the standard hydrogen electrode (SHE). The standard potentials are reported as −2.71 V for Na/Na+, −2.94 V for K/K+ and −3.04 V for Li/Li+.2 Therefore, in theory, KIBs should deliver cell voltages similar to LIBs and higher than NIBs. Nevertheless, it needs to be considered that the higher radius and increased mass of K-ions result in decreased gravimetric and volumetric energy densities (76 pm and 6.941 g mol-1 for Li+; 100 pm and 22.990 g mol-1 for Na+; 140 pm and 39.098 g mol-1 for K+).2,3 Taking these aspects into account, KIBs could potentially show a combination of the advantages of LIBs and NIBs without suffering of their drawbacks. With this work we demonstrate as positive and negative K-ion materials, including a K-ion full cell, may show interesting electrochemical performances which are worth to be investigated in greater detail by a wider scientific community. References 1. N. Yabuuchi, K. Kubota, M. Dahbi, and S. Komaba, Chemical Reviews, 114, 11636–11682 (2014). 2. S. Komaba, T. Hasegawa, M. Dahbi, and K. Kubota, Electrochemistry Communications, 60, 172–175 (2015). 3. M. E. Wieser et al., Pure and Applied Chemistry, 85, 2013 (2013).

Sodium-Ion Batteries (SIBs) have established themselves as possible low-cost alternative to the common lithium-ion battery (LIB). The cost advantage is underlined by a powerful electrochemical performance, superior to any other secondary battery except for LIBs. Consequently, this has led to a rapidly increasing scientific interest within the last five years. In accordance with the economical concept of diversification SIBs are discussed to be promising alternative for applications, in which cost-efficient energy storage (in terms of $/kWh) is the most important criterion. In order to boost SIBs into such applications (e.g. for the decentralized and fragmented power grid of the future) new, powerful but also rather cheap materials composed of abundant and environmentally friendly elements, need to be used in order to cover the lower electrochemical performance. In fact, research community and research efforts are, currently, focusing on the material screening and development. [1-3] With respect to layered materials strong progress in terms of their understanding, characterization and their performance has been achieved. In this presentation we will focus on the design and development of layered cathode materials for powerful next generation SIBs. In the past, we have synthesized and characterized layered P2- NaxNi0.22Co0.11Mn0.66O2material. [4-6] In this presentation we will show how the composition of this material can be modified and reveal the effect on the electrochemical performance.[7-9] Final intention of the presentation is to: -illustrate the influence of the composition on the electrochemical performance, -illustrate how layered cathode materials can be designed, -demonstrate the feasibility of these principals via a new layered Na- based oxide. The final material delivers a stable capacity of about 110 mAh g-1 (3.4 V average discharge potential) at a high current rate of 1C (180 mA g-1) between 4.3-2.5 V (vs. Na/Na+) in conventional organic carbonate based electrolyte. This performance is among the best reported for layered Na-based oxides so far and certainly is promising for the application in future Na-based electrochemical energy storage devices. [1] S.-W. Kim et al. Adv. Energy Mater. 2 (2012), 710. [2] J.M. Tarascon Nature Chemistry 2 (2010), 510. [3] N. Yabuuchi et al. Chem. Rev. 114 (2014), 11636. [4] D. Buchholz et al. Chem. Mater. 25 (2013), 142. [5] D. Buchholz et al. J. Mater. Chem. A 2 (2014), 13415. [6] L.G. Chagas et al. J. Mater. Chem. A 2 (2014) 20263. [7] I. Hasa et al. Adv. Energy Mater. 4 (2014), 140083. [8] I. Hasa et al. ACS Appl. Mater. Interfaces 7 (2015), 5206. [9] D. Buchholz et al. J. Power Sources 282 (2015), 581.

Currently, sodium-ion batteries face a strong increase of the scientific and commercial interest as they represent a relatively young and unexploited research field and a possible low cost alternative towards the lithium-ion technology as well. Sodium based raw materials are abundant and easily available, thus resulting in cheaper mining abilities. In addition, the low cost advantage is not only due to the lower price of sodium but rather more interconnected with the feasible use of aluminium as anode current collector since sodium is not alloying with it. The avoidance of copper thus results not only in a cheaper but also in a lighter battery with increased energy density. [1-3] Concerning positive electrode materials, polyanionic frameworks and layered oxides have been largely investigated and now it is commonly accepted that they are suitable cathode materials for sodium-ion batteries. Generally, superior electrochemical performances compared to other secondary battery systems, except for the lithium-ion technology, have been demonstrated for sodium based systems.[1,4-9] One major advantage compared to polyanionic compounds surely is the lower molecular weight of layered oxides, finally resulting in improved specific capacities. On the other hand, layered materials often suffer of large volume changes upon the whole (de-)sodiation process leading to a fading electrochemical performance. Consequently, the composition of the transition metal layer in these oxides is crucial to ensure a good electrochemical performance and a stable long term cycling behavior. Within the class of layered transition metal based oxides, materials with an O- and P-type structure are known, whereas the latter ones exhibit a smaller sodium reservoir but reveal an improved reversibility of the electrochemical redox process at higher potentials, finally leading to an improved electrochemical performance. In the past we have developed the synthesis of layered NaxMO2(M= transition metal) materials by firing a mixture of a transition metal hydroxide precursor, obtained via co-precipitation method, and sodium hydroxide in a simple two step solid state annealing process in air, followed by a water treatment. [7, 8, 9] Herein, we report the synthesis and structural characterization of different novel layered oxides of the type NaxMO2 with P-type structure, synthesized at various annealing temperatures. Furthermore, we demonstrate the influence of structure and the type and ratio of the employed transition metals on the electrochemical performance of these materials. References [1] S.-W. Kim et al. Adv. Energy Mater. 2 (2012) 710 [2] J.M. Tarascon Nature Chemistry 2 (2010) 510 [3] H. Baker, H. Okamoto, ASM Handbook, Volume 3, ASM International, USA, 1992 [4] M. Sathiya et al. Chem. Mater. 24 (2012) 1846 [5] N. Yabuuchi et al. Nature Mater. 11 (2012) 512 [6] D. Kim et al. Electrochem. Commun. 18 (2012) 66 [7] D. Buchholz et al. Chem. Mater. 25 (2013) 142 [8] D. Buchholz et al. Electrochim. Acta (2013), doi.org/10.1016/j.electacta.2013.02.109 [9] D. Buchholz et al. Tailoring the Electrochemical Performance of P2- Na0.45Ni0.22Co0.11Mn0.66O2 as High Voltage or High Capacity Cathode Material for Sodium-Ion Batteries, 224th ECS Meeting San Francisco, USA, October 2013