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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.

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.

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.