(Keynote) Polyanionic Compounds for K-Ion Batteries

Prussian blue analogues (PBAs) are a framework material that are of great interest for use as cathode materials in Na- and K-ion batteries. Na-ion PBA batteries have been commercialized for use in stationary storage and are now in development for use in electric vehicles. The high operating potential of K-ion PBAs and their compatibility with pre-existing graphite anodes offer the exciting prospect of use in electric vehicles where high specific energy density is necessary [1].The leading cathode material is the Manganese/Iron PBA – K x Mn[Fe(CN)6] y (0<x<2, 0.67<y<1.00). Attempts to maximise the specific capacity of PBA cathodes have been made by eliminating [Fe(CN)6] vacancies (y=1) which incorporates a full x = 2 formula units of K+ ions in the structure through electronic balancing [2]. Whilst making these low-vacancy materials has been made possible by altering the synthesis, optimising the electrochemical performance has proved more challenging [3]. Accessing the full capacity with high retention on cycling is one problem, and achieving the same excellent rate capability of the previously studied high-vacancy PBAs is also non-trivial [3,4] .High-vacancy PBAs have an apparent simple cubic structure, and cycle electrochemically in a solid-solution [4]. Conversely, the low-vacancy PBAs with high K-ion content have coherent structural distortions and undergo phase transitions on cycling [3,5]. The phase transitions are believed to be responsible for poor capacity retention and rate capability [2]. There is also the theory that vacancies provide additional diffusion pathways for K+ ions [Fig. 1] [2]. For the development of this material as a cathode for K-ion batteries to be successful, it is vital that the diffusion kinetics and structural changes on electrochemical cycling are well-understood from a fundamental perspective.For the first time we report diffusion coefficients by using Potentiostatic Intermittent Titration across multiple states of charge of a PBA cathode [6]. Linking the ionic diffusion in the material with the structural phase(s) present from operando X-ray diffraction gives us a mechanistic insight into the cathode’s charge/discharge behaviour. This informs materials design for tuning the structure of the pristine material to optimise its electrochemical performance in a K-ion battery.

Alkali-ion intercalation compounds are widely used as cathode materials for rechargeable batteries, including Li-, Na-, and K-ion batteries. For many years, researchers have developed layered oxide cathode materials for Na- and K-ion batteries given that the layered oxides show high reversible capacity and high working voltage in Li-ion technology. However, our works have demonstrated that the layered oxide compounds may not be good cathode candidates for Na and K systems. There are two main issues:[1-4] (i) Na and K transition metal oxide compounds form non-stoichiometric composition (x<1.0 in A x MO2, A= Na and K). It leads practical difficulty of realizing Na- and K-ion batteries because all the Na and K ions should come from the cathode in rocking-chair batteries. (ii) As the insertion ion size increases, the voltage curve becomes much sloped. The sloped voltage curves result in low specific capacity and average voltage. Both the problems are attributable to much stronger Na+-Na+ and K+-K+ interaction than Li+-Li+ in the layered oxide structure. In this respect, polyanion compounds that have 3 dimensional arrangements of Na and K ions, resulting in longer Na+-Na+ and K+-K+ distance, will be better candidates. We proposed KVPO4F[5] as an example of polyanion compounds and it has stoichiometric composition and provides high average voltage of ~4.3 V (vs. K/K+) with a reversible capacity of ~105 mAh/g.We further investigated the effect of intercalation ion species in K x VPO4F (x~0).[6] Our work demonstrates the voltage for Na intercalation is even higher than that for Li insertion unlike the common belief that Li insertion voltage is always higher than Na insertion. The lower Li intercalation voltage is likely attributed to unstable Li site in large cavity, making less stable discharged product upon Li insertion vs. Na insertion. In addition, we found that Li intercalation is more sluggish than Na and K intercalation in K x VPO4F (x~0). Since Li ion is too small compared to cavity in the cathode, Li ions are undercoordinated in the transition state, in which a Li ion is coordinated by two anions only. Therefore, Li ion migration barrier is much higher than Na and K migration. This finding teaches us that large cavity size (or channel size) is not always good for fast alkali ion migration and we need to finely tune the cavity size suitable for each intercalating ion species.

K-ion batteries need efficient positive electrode materials with open structural frameworks to accommodate the large ionic radius of K+. In that direction, polyanionic compounds are of great interest. Among them, KVPO4F is the most studied one. Its electrochemical redox processes still remain unclear, especially at high voltage. To tackle this issue, X-ray photoelectron spectroscopy was used to reveal the electrochemical redox processes of KxVPO4F. First, a carbon coating was performed and allowed increasing the overall electrochemical performance while mitigating electrolyte degradation at high voltage. Then, XPS analysis showed a high reversibility of the redox processes although the K+ extraction (and insertion) from x = 0.5 to 0 was hindered, possibly by structural constraints while electrolyte degradation occurred mostly above 4.5 V.

Synthesis of truncated octahedral zinc-doped manganese hexacyanoferrates and low-temperature calcination activation for lithium-ion battery

In recent years Prussian blue analogue materials have received much attention as battery materials. Two distinct transition metal sites connected through cyanide ligands create an open-framework structure containing interstitial sites capable of containing a range of alkali metal cations.1 Compositional control, as well as defect and water content, allow tunability of many properties. They show excellent electrochemical properties and are some of the most promising candidate cathode materials for sodium- and potassium-ion batteries.2 Operating in both aqueous and organic electrolytes they have shown high rate capability and extremely long cycle lives attributed to negligible change in lattice constant on cycling and fast ionic mobility.3 A majority of current literature focuses on hexacyanoferrate's as cathode materials, operating at around 0.8 V vs. SHE. However, to utilise the excellent properties in a full cell you need an anode with comparable cycle life and kinetics. Through exchanging Fe with Mn, manganese hexacyanomanganate had a redox couple at 0 V vs. SHE. This, in combination with copper hexacyanoferrate produced a full cell with an average discharge voltage of 1 V and long cycle life.4 Efforts to produce lower potential PBA materials have often resulted in loss of the cubic structure and reversible capacity ascribable to a conversion-type mechanism.5 In this work we investigate manganese hexacyanochromate as a low potential PBA material. Through substitution of Cr into the structure we have lowered the reduction potential to -0.86 V vs. SHE, whilst operating in a high voltage aqueous electrolyte. This is the lowest reduction potential reported with the confirmation that the open-framework structure is maintained, and reversible capacity arises from sodium insertion into the material. Two types of water in the structure are characterised through high resolution XRD, TGA and FTIR, and the crucial role water plays in electrochemical activity and structural integrity is highlighted. It was found that when operating in a non-aqueous electrolyte the material is dehydrated and during electrochemical reduction the crystalline structure is lost. This study isolates the important role water plays within the structure and that it is vital to consider when investigating low potential PBA materials for alkali-ion batteries. Hurlbutt, K., Wheeler, S., Capone, I. & Pasta, M. Prussian Blue Analogs as Battery Materials. Joule (2018). doi:10.1016/J.JOULE.2018.07.017Song, J. et al. Removal of interstitial H2O in hexacyanometallates for a superior cathode of a sodium-ion battery. J. Am. Chem. Soc. 137, 2658–2664 (2015).Wessells, C. D., Huggins, R. a & Cui, Y. Copper hexacyanoferrate battery electrodes with long cycle life and high power. Nat. Commun. 2, 550 (2011).Pasta, M. et al. Full open-framework batteries for stationary energy storage. Nat. Commun. 5, 3007 (2014).Deng, L. et al. Investigation of the Prussian Blue Analog Co3[Co(CN)6]2 as an Anode Material for Nonaqueous Potassium-Ion Batteries. Adv. Mater. 1802510 (2018). doi:10.1002/adma.201802510 Figure 1

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.

HighlightsMn-based electrode materials, including oxides, Prussian blue analogues and polyanion compounds, are introduced systematically for aqueous sodium-ion batteries.The composition, crystal structure, morphology and electrochemical performance of Mn-based electrode materials are reviewed.The improvement methods of electrochemical performance, such as electrolyte optimization, element doping or substitution, morphology optimization and carbon modification, are discussed.

In-situ-grown Co-doped Ni-hexacyanoferrate/Ni-foam composites as battery-type electrode materials for aqueous hybrid supercapacitors

Li-ion batteries (LIBs) have succeeded in powering small portable electric devices and are currently expanding to large scale applications (i. e. electric vehicles and grid-level energy storage systems) but it is debatable whether the lithium reserve can meet the increasing demands on the emerging large scale applications. Thus, K-ion batteries (KIBs) and Na-ion batteries (NIBs) are considered alternative energy storage systems due to the natural abundance of K and Na reserve. KIB technology is particularly interesting because K has a lower standard redox potential than Na in non-aqueous carbonate-based electrolytes that are commonly used for NIBs.1-3 It indicates that KIBs can potentially have a higher cell voltage than NIBs when the cathode materials provide the same working voltage as their analogues in Na system. More importantly, graphite, which is a standard anode for LIBs, cannot reversibly intercalate Na ions, thus NIBs require expensive hard carbon anodes. On the other hand, graphite can store and release K ions reversibly, which in fact sparks research interest in KIBs.3,4 The discovery of novel positive electrodes is a critical step toward realizing KIBs. In this work, we develop new cathode materials with layered-structure (i. e. KxTMO2, TM = Co and Mn)5, 6 for KIBs and investigate K-storage properties and mechanism in them by in-situ diffraction and electrochemical characterization combined with theoretical first-principles calculations. Here, we also found that the alkali ion species significantly affect electrochemical properties in layered transition metal oxides. When the size of alkali ion increases (Li+ < Na+ < K+), the voltage curves become more sloped. This is because the formation energy of layered compounds changes more significantly according to K concentrations in K x TMO2 due to the stronger interaction between K+-K+ than Na and Li systems. It can deteriorate the specific capacity and operating voltage, and thus energy density. Therefore, it is important to design novel electrode materials that can minimize the interaction between K ions. This presentation will also provide a good guideline how to design electrode materials for KIBs. References Eftekhari, A. et al. Potassium Secondary Batteries. ACS Appl. Mater. Sci., 9, 4404. (2016).Marcus, Y. Thermodynamic functions of transfer of single ions from water to non-aqueous and mixed solvents: Part 3 - Standard potentials of selected electrodes. Pure and Applied Chemistry 57, 1129. (1985).Komaba, S. et al. Potassium intercalation into graphite to realize high-voltage/high-power potassium-ion batteries and potassium-ion capacitors. Electrochem. Commun. 60, 172. (2015).Jian, Z., et al. Carbon Electrodes for K-Ion Batteries. J. Am. Chem. Soc. 137, 11566. (2015).Kim, H. et al. K-Ion Batteries Based on a P2-Type K0.6CoO2 Cathode. Adv. Energy Mater. 7, 1700098. (2017)Kim, H. et al. Investigation of potassium storage in layered P3-type K0.5MnO2 cathode. Adv. Mater. 29, 1702480 (2017)

Medium- and high-entropy materials as positive electrodes for sodium-ion batteries: Quo Vadis?

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

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

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

Research status and prospect of rechargeable magnesium ion batteries cathode materials

Prussian blue analogs (PBAs) are promising cathode materials for potassium-ion batteries (KIBs) owing to their large open framework structure. As the K+ migration rate and storage sites rely highly on the periodic lattice arrangement, it is rather important to guarantee the high crystallinity of PBAs. Herein, highly crystalline K2 Fe[Fe(CN)6 ] (KFeHCF-E) is synthesized by coprecipitation, adopting the ethylenediaminetetraacetic acid dipotassium salt as a chelating agent. As a result, an excellent rate capability and ultra-long lifespan (5000 cycles at 100mA g-1 with 61.3% capacity maintenance) are achieved when tested in KIBs. The highest K+ migration rate of 10-9 cm2 s-1 in the bulk phase is determined by the galvanostatic intermittent titration technique. Remarkably, the robust lattice structure and reversible solid-phase K+ storage mechanism of KFeHCF-E are proved by in situ XRD. This work offers a simple crystallinity optimization method for developing high-performance PBAs cathode materials in advanced KIBs.