The limited resources and uneven distribution of lithium stimulate strong motivation to develop new rechargeable batteries that use alternative charge carriers. Potassium-ion batteries (PIBs) are at the top of the list of alternatives because of the abundant raw materials and relatively high energy density, fast ion transport kinetics in the electrolyte, and low cost. However, several challenges still hinder the development of PIBs, such as low reversible capacity, poor rate performance, and inferior cycling stability. Research on the cathode is currently focused on developing materials with high energy density and cycling stability, mainly including layered transition metal oxides, polyanion compounds, organic compounds, etc. Anodes based on intercalation reactions, conversion reactions, and alloying with potassium are currently under development, and promising results have been published. This review comprehensively summarizes the research effort to date on the electrode material optimization (e.g., crystals, morphology, reaction mechanisms, and interface control), the synthesis methods, and the full cell fabrication for PIBs to enhance the electrochemical potassium storage and provide a platform for further development in this battery system.

Potassium-ion batteries: outlook on present and future technologies

Sodium-based dual-ion batteries (Na-DIBs) show a promising potential for large-scale energy storage applications due to the merits of environmental friendliness and low cost. However, Na-DIBs are generally subject to poor rate capability and cycling stability for the lack of suitable anodes to accommodate large Na+ ions. Herein, we propose a molecular grafting strategy to in situ synthesize tin pyrophosphate nanodots implanted in N-doped carbon matrix (SnP2O7@N-C), which exhibits a high fraction of active SnP2O7 up to 95.6 wt% and a low content of N-doped carbon (4.4 wt%) as the conductive framework. As a result, this anode delivers a high specific capacity ∼400 mAh g-1 at 0.1 A g-1, excellent rate capability up to 5.0 A g-1 and excellent cycling stability with a capacity retention of 92% after 1200 cycles under a current density of 1.5 A g-1. Further, pairing this anode with an environmentally friendly KS6 graphite cathode yields a SnP2O7@N-C||KS6 Na-DIB, exhibiting an excellent rate capability up to 30 C, good fast-charge/slow-discharge performance and long-term cycling life with a capacity retention of ∼96% after 1000 cycles at 20 C. This study provides a feasible strategy to develop high-performance anodes with high-fraction active materials for Na-based energy storage applications.

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Molecular grafting towards high-fraction active nanodots implanted in N-doped carbon for sodium dual-ion batteries.

Recently, research on novel and low cost batteries has been widely conducted to realize large-scale energy storage systems. However, few of the battery systems have delivered performance equal to that of Li-ion batteries. Herein, we propose non-aqueous K-ion batteries by developing hexacyanoferrate(II) compounds (so-called Prussian blue analogues), K1.75Mn[FeII(CN)6]0.93·0.16H2O and K1.64Fe[FeII(CN)6]0.89·0.15H2O, as affordable positive electrode materials. In particular, K1.75Mn[FeII(CN)6]0.93·0.16H2O prepared by a simple precipitation process delivers a high capacity of 141 mA h g−1 at 3.8 V as the average operating potential, resulting in a comparable energy density to that of LiCoO2, with excellent cyclability and rate performance in K half-cells. Operando X-ray diffraction measurements reveal that the excellent electrochemical performance of this material is attributed to its open and flexible framework, which can realize fully reversible K+ extraction/insertion and a structural change from monoclinic to tetragonal via cubic phases. For the first time, we demonstrate an inexpensive high-voltage K-ion full cell with a K1.75Mn[Fe(CN)6]0.93·0.16H2O/graphite configuration to prove its feasibility as a new promising battery system for an environmentally friendly future.

A Novel K-Ion Battery; Hexacyanoferrate(II)/Graphite Cell

Potassium-ion batteries are a compelling technology for large scale energy storage due to their low-cost and good rate performance. However, the development of potassium-ion batteries remains in its infancy, mainly hindered by the lack of suitable cathode materials. Here we show that a previously known frustrated magnet, KFeC2O4F, could serve as a stable cathode for potassium ion storage, delivering a discharge capacity of ~112 mAh g−1 at 0.2 A g−1 and 94% capacity retention after 2000 cycles. The unprecedented cycling stability is attributed to the rigid framework and the presence of three channels that allow for minimized volume fluctuation when Fe2+/Fe3+ redox reaction occurs. Further, pairing this KFeC2O4F cathode with a soft carbon anode yields a potassium-ion full cell with an energy density of ~235 Wh kg−1, impressive rate performance and negligible capacity decay within 200 cycles. This work sheds light on the development of low-cost and high-performance K-based energy storage devices. The abundance and low cost of potassium makes potassium batteries a promising technology for large scale energy storage. Here the authors apply a previously known frustrated magnet, KFeC2O4F, as the cathode in which the unique structure and Fe2+/Fe3+ redox enable excellent cycling stability.

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A fluoroxalate cathode material for potassium-ion batteries with ultra-long cyclability.

Fast Rate and Long Life Potassium-Ion Based Dual-Ion Battery through 3D Porous Organic Negative Electrode

Sodium-ion batteries (NIBs) are the most promising alternatives to lithium-ion batteries in the development of renewable energy sources. The advancement of NIBs depends on the exploration of new electrode materials and fundamental understanding of working mechanisms. Herein, via experimental and simulation methods, we develop a mixed polyanionic compound, Na2 Fe(C2 O4 )SO4 ⋅H2 O, as a cathode for NIBs. Thanks to its rigid three dimensional framework and the combined inductive effects from oxalate and sulfate, it delivered reversible Na insertion/desertion at average discharging voltages of 3.5 and 3.1 V for 500 cycles with Coulombic efficiencies of ca. 99 %. In situ synchrotron X-ray measurements and DFT calculations demonstrate the Fe2+ /Fe3+ redox reactions contribute to electron compensation during Na+ desertion/insertion. The study suggests mixed polyanionic frameworks may provide promising materials for Na ion storage with the merits of low cost and environmental friendliness.

A Low-Cost and Environmentally Friendly Mixed Polyanionic Cathode for Sodium-Ion Storage

Low-cost electrochemical energy storage systems (EESSs) are urgently needed to promote the application of renewable energy sources such as wind and solar energy. In analogy to lithium-ion batteries, the cost of EESSs depends mainly on charge-carrier ions and redox centers in electrodes, and their performance is limited by positive electrodes. In this context, this Minireview evaluates several EESS candidates and summarizes the known mixed polyanionic compounds (MPCs)-a family with robust frameworks and large channels for ion storage and migration. After comprehensive analysis, it is pointed out that a deeper exploration of MPCs may generate numerous novel crystallographically interesting compounds and excellent cathode materials for low-cost energy storage applications.

Mixed Polyanionic Compounds as Positive Electrodes for Low-Cost Electrochemical Energy Storage

The intrinsic physical and chemical properties of materials are largely governed by the bonding and electronic structures of their fundamental building units. The majority of cathode materials contain octahedral TMO6 (TM = transition metal), which dominates the redox chemistry during electrochemical operation. As a less symmetric form of TMO6 , the trigonal prismatic geometry is not a traditionally favored coordination configuration as it tends to lose the crystal-field stabilization energy and thus generate large ligand repulsion. Herein, a K-ion battery cathode design, K2 Fe(C2 O4 )2 , is shown​, where the TMO6 trigonal prism (TP) is not only electrochemically active but stable enough to allow for excellent cycling stability. Detailed synchrotron X-ray absorption spectroscopy measurements reveal the evolution of localized fine structure, evidencing the electrochemical activity, reversibility, and stability of the TP motif. The findings are expected to expand the toolbox for the rational design of electrode materials by taking advantage of TP as a structural gene.

K-Ion Battery Cathode Design Utilizing Trigonal Prismatic Ligand Field.

Potassium storage systems are attracting more and more attention for large scale energy storage owing to the great abundance of potassium in the earth. The development of potassium-based devices has so far been hindered by the lack of appropriate potassium-host materials since the large radius of potassium has put forward the requirement of more rigid frameworks and larger channels for K ion migration, in comparison with lithium and sodium-based devices. In this study, we report an iron-based oxalate as a positive electrode for K-ion storage. KLi3Fe(C2O4)3@C delivered a stable capacity of 140 mA h g−1 at 120 mA g−1, and showed a 91% capacity retention after 200 cycles. By combining cyclic voltammetry analysis and synchrotron X-ray absorption spectroscopy, the K storage mechanism is demonstrated to be a combined Fe2+/Fe3+ redox reaction and pseudocapacitive process. Remarkably, it displays a stable capacity of ∼130 mA h g−1 and little attenuation over 5000 cycles at 240 mA g−1 in the lifespan test. A K-based full cell was successfully constructed via pairing this cathode with a soft carbon anode and exhibited good rate capability together with a capacity retention of ∼90% for 200 cycles, indicating its promising prospect as an environment friendly and stable K-storage cathode.

An iron-based polyanionic cathode for potassium storage with high capacity and excellent cycling stability

Renewable energies, such as solar and wind, have been explored and widely applied for alleviating problems associated with the depletion of fossil fuel resources and environmental pollution. The intermittent and fluctuating features of these renewable energies require development of efficient energy storage and conversion systems. Sodium-ion batteries (SIBs) are considered one of the most promising candidates for large-scale energy storage due to the low cost and earth abundance of sodium resources. A major challenge for the practical application of SIBs is the development of appropriate cathodes with high energy densities and cycling stabilities. Layered oxide cathodes have received significant attention because of their relatively simple synthetic routes and high capacities stemming from their layered structures. However, they often suffer from moisture sensitivity and structural degradation upon repeated Na+ insertion/extraction, leading to severe performance fading. This review summarizes and discusses the degradation mechanisms of these layered oxide cathodes and modulation strategies for addressing the stability issues. Understanding the mechanisms behind structural instability would provide better insight for improving SIBs' cathode materials, which has critical implications for the designs and applications of SIBs as renewable energy systems. 

https://onlinelibrary.wiley.com/doi/pdfdirect/10.1002/cnl2.7

Tianyi Song

Papers

A Low-Cost and Environmentally Friendly Mixed Polyanionic Cathode for Sodium-Ion Storage

Mixed Polyanionic Compounds as Positive Electrodes for Low-Cost Electrochemical Energy Storage

K-Ion Battery Cathode Design Utilizing Trigonal Prismatic Ligand Field.

An iron-based polyanionic cathode for potassium storage with high capacity and excellent cycling stability

Structural degradation mechanisms and modulation technologies of layered oxide cathodes for sodium‐ion batteries