Potassium-Ion Batteries: Sustainable Strategies (KIBSS)

DFG form 3.06 – 03/25 page 1 of 16 Deutsche Forschungsgemeinschaft Kennedyallee 40 ∙ 53175 Bonn, Germany ∙ Postal address: 53170 Bonn, Germany Tel.: + 49 228 885-1 ∙ Fax: + 49 228 885-2777 ∙ postma[email protected] ∙ www.dfg.de DFG Potassium-Ion Batteries: Sustainable trate Strategies (KIBSS) Final Report under the DFG Cooperation Project for joint German-Russian Project Proposals Disclaimer: The English translation of this document is provided for informational purposes. In the event of a discrepancy between the English and the German versions, the German text takes precedence. DFG form 3.06 – 03/25 page 2 of 16 Deutsche Forschungsgemeinschaft Kennedyallee 40 ∙ 53175 Bonn, Germany ∙ Postal address: 53170 Bonn, Germany Tel.: + 49 228 885-1 ∙ Fax: + 49 228 885-2777 ∙ postma[email protected] ∙ www.dfg.de DFG FINAL REPORT 1 General Information DFG reference number: MI 945/6-1 (DM) & JE 980/1-1 (FJ) Project number: 448719339 Project title: Nachhaltige Strategien für K-Ionen Batterien Name(s) of the applicant(s): Dr. Daria Mikhailova, Dr. Fabian Jeschull Official address(es): DM: Leibniz Institute for Solid State and Materials Research (IFW) Dresden e.V., Institute for Complex Materials, Helmholtzstr. 20, D-01069, Dresden, Germany FJ: Karlsruhe Institute of Technology (KIT), Institute for Applied Materials – Energy Storage Systems (IAM-ESS), Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany Name(s) of the co-applicants: Name(s) of the cooperation partners: Reporting period (entire funding period): 01.04.2021 – 21.03.2025 (36 months + 12 month cost-neutral prolongation) DFG form 3.06 – 03/25 page 3 of 16 Deutsche Forschungsgemeinschaft Kennedyallee 40 ∙ 53175 Bonn, Germany ∙ Postal address: 53170 Bonn, Germany Tel.: + 49 228 885-1 ∙ Fax: + 49 228 885-2777 ∙ postma[email protected] ∙ www.dfg.de DFG 2 Summary In course of the energy transition towards more sustainable power generation, e-mobility and portable electronics, cost-effective and sustainable energy storage solutions are key. K-ion batteries (KIBs) ranks high amongst the potential battery chemistries, in terms of energy density and prospective costs and abundance of raw materials. In this respect KIBs are expected to compete with graphite/LiFePO4 chemistries and outperform Na-ion batteries. KIBs show a broad technological overlap with other monovalent cell chemistries and could even leverage existing value chains of Li-based battery technologies (e.g. for graphite electrodes). Research results over the past 10 years clearly demonstrated that KIBs won’t be a drop-in solution and rather pose element-specific challenges related to the high reactivity of potassium and its intercalation compounds towards electrolyte components, poor passivation properties of the solid electrolyte interphase (SEI) and (solid-state) diffusion limitations in the active materials. Along the three major cell components, negative and positive electrodes and electrolyte, “Potassium-Ion Batteries: Sustainable Strategies” (KIBSS) set out to address these challenges through novel material developments, and gaining deeper understanding of the degradation pathways and failure mechanisms in KIBs (including dual-ion cell concepts). For this reason, investigating hard carbon from renewable feedstocks and Fe-based Prussian white electrodes played a central role from a sustainability perspective. The teams at IFW Dresden and KIT combined surface-analytical, electrochemical and bulk-analytical techniques, also at synchrotron facilities, as a complementary approach for a better holistic understanding of the cell chemistry. A major part of the work focused on the SEI formation on carbonaceous electrodes (graphite & hard carbons) in different electrolyte environments with the aim to increase cycle life and capacity loss through thinner, yet more protective passivation layers. It was observed repeatedly that experimental results from half-cell setups, i.e. electrochemical tests against Kmetal, lead to strong interferences in the analysis of electrode interfaces and electrolyte compositions, owing to the evolution of parasitic compounds that a root cause for self-discharge (in presence of potassium). In contrast, full cell configurations based on graphite negative electrodes and Prussian white (K2Fe[Fe(CN)6]) or polyanionic KVPO4F displayed fundamentally different aging behaviour, highlighting the need of revisiting established testing procedures in the KIB field (or post-Li field in general) that were carried over from Li-ion technologies. Overall, the results suggest that a more dedicated electrolyte and surface/interface design strategies are still needed for the targeted cycle life and energy, with concentrated electrolyte salt mixtures and sulfur-based electrolyte additives pointing into a promising direction. DFG form 3.06 – 03/25 page 4 of 16 Deutsche Forschungsgemeinschaft Kennedyallee 40 ∙ 53175 Bonn, Germany ∙ Postal address: 53170 Bonn, Germany Tel.: + 49 228 885-1 ∙ Fax: + 49 228 885-2777 ∙ postma[email protected] ∙ www.dfg.de DFG Zusammenfassung Für Technologies zur nachhaltigeren Stromerzeugung, E-Mobilität und tragbarer Elektronik sind günstige und nachhaltige Energiespeichersysteme ein Schlüsselelement. K-Ionen Batterien (KIBs) zeichnen sich unter den potenziellen Zellchemien durch eine gute Energiedichte auf einer günstigen und gut verfügbaren Rohstoffbasis aus. Bspw. wird erwartet, dass KIBs mit Graphit/LiFePO4-Batterien konkurrieren könnten und dabei selbst Na-Ionen Batterien übertreffen. Die Entwicklung von KIBs profitiert von den chemo-physikalischen Gemeinsamkeiten anderer monovalenter Zellchemien und perspektivisch auch durch bereits bestehende Wertschöpfungsketten im Li-Ionen Bereich (z.B. für Graphitelektroden). Nähere Untersuchungen in den letzten 10 Jahre zeigen jedoch auch, dass KIBs keineswegs eine „Drop-in“-Lösung darstellen, sondern lediglich andere, element-spezifische Herausforderungen mit sich bringen. Dies umfasst z.B. die hohe Reaktivität von Kalium und dessen Interkalationsverbindungen gegenüber Elektrolytkomponenten, schlechten Passivierungseigenschaften der Solid Electrolyte Interphase (SEI), sowie Beschränkungen der Festkörperdiffusion in Aktivmaterialien. Entlang der drei Hauptzellkomponenten (negativen und positiven Elektroden sowie Elektrolyt), zielt das Projekt "Nachhaltige Strategien für K-Ionen Batterien" (KIBSS) auf langlebigere KIBs ab, durch neuartige Materialentwicklungen und dem Aufbau eines tiefergreifenden Verständnisses der Degradationswege und Materialermüdung (einschließlich Dual-Ionen-Zellkonzepten). Dies umfasst insbesondere hard carbon aus erneuerbaren Rohstoffen und Fe-basierte Berliner Blau Derivate (K2Fe[Fe(CN)6]) als Aktivmaterialien. Die Teams am IFW Dresden und KIT folgten hierbei einen komplementären Ansatz durch die Kombination von oberflächenanalytische, elektrochemischen und bulk-analytischen Methoden, u.a. an Synchrotroneinrichtungen. Die Arbeit konzentrierte sich hierbei auf die SEI-Bildung an kohlenstoffbasierten Elektroden (Graphit und hard carbon) in unterschiedlichen Elektrolytumgebungen mit dem Ziel Kapazitätsverluste durch dünnere, jedoch stärker schützende Passivierungsschichten einzugrenzen und dadurch die Lebensdauer zu steigern. Experimente in Halbzellenformaten, d.h. elektrochem. Messungen gegen K-Metall, zeigten hierbei, dass Nebenreaktionen sich signifikant auf die Proben und deshalb auf Messergebnisse auswirkten. Parasitäre Zersetzungsprodukte, die an der Kaliumelektrode entstehen, sind u.a. Ursache für Selbstentladungsprozesse von positiven Elektroden. Im Gegensatz dazu zeigten Vollzellkonfigurationen basierend auf negativen Graphitelektroden und K2Fe[Fe(CN)6] oder KVPO4F ein grundlegend anderes Alterungsverhalten. Dies stellt etablierte Testverfahren für die Untersuchung von KIBs (bzw. post-Li Zellen im Allgemeineren) in Frage. Abschließend lässt sich feststellen, dass mit Blick auf die angestrebte Lebensdauer und Energiedichten weitreichendere Strategien zum Elektrolytund Grenzflächendesign erforderlich sein werden, wobei konzentrierte Elektrolytsalzmischungen und schwefelbasierte Elektrolytadditive in eine vielversprechende Richtung weisen. DFG form 3.06 – 03/25 page 5 of 16 Deutsche Forschungsgemeinschaft Kennedyallee 40 ∙ 53175 Bonn, Germany ∙ Postal address: 53170 Bonn, Germany Tel.: + 49 228 885-1 ∙ Fax: + 49 228 885-2777 ∙ postma[email protected] ∙ www.dfg.de DFG 3 Progress Report Background and Objectives. The overarching goal of KIBSS was to develop novel electrode materials with low material fatigue and high reversibility and to gain a detailed understanding of the failure mechanisms in K-ion battery cell components. In addition, dual-ion cell concepts were explored. Major challenges related to the reversible K-ion insertion/intercalation of positive and negative electrode materials (1), electrolyte formulations (liquid or solid) with high anodic and cathodic stability and/or improved passivation properties (2). The underlying mechanisms of phase transformations in active materials and processes at the electrode-electrolyte interface were studied by a combination of both in-house and synchrotron-based bulk and surface sensitive techniques. A main goal was to assemble a lab-scale prototype cell at the end of the funding period. The work programme was aligned along the three main cell components, i.e. negative electrode (“anode”; lead IFW Dresden (DE)), electrolyte (lead KIT (DE)) and positive electrode (“cathode”; lead Skoltech (RU)). The objectives were defined more specifically for the individual research directions: (i) Carbonaceous materials for K-ion and K-based dual-ion cell concepts, including the synthesis of functionalized hard carbons, electrochemical testing in suitable electrolytes and ex-situ & in-situ characterization by spectroscopic methods. (ii) Investigations on the degradation pathways and SEI formation of carbonate-based electrolyte formulations, including electrolyte additives in half & full cell configurations, as well as development of polyether-based solid polymer electrolytes. (iii) Material tailoring of polyanionic compounds & Prussian white derivatives and their structural and electrochemical characterization, supported by DFT calculations. Deviations from original concept. In the original proposal, the project partners split the workload by cell component. The development of novel positive electrode materials was assigned to the Russian partners at Skoltech. Because of the political tension between Russia and Europe, this collaboration was discontinued after ca. 1 year after begin of the project. A research exchange in April 2021 (Anna Khudyshkina (KIT), PhD student to Skoltech) and November/December 2021 (Paulina Morozova (Skoltech), PhD student, to KIT), enabled a knowledge transfer for a positive electrode reference material and also resulted in a joint publication (ref. A8, section 4.1). The proposed joint KIB protype (WP4) was thus not possible at the end of the project. The activities of both German partners focused in the following on negative electrodes and electrode-electrolyte interphases as major roadblocks for long cycle life in potassium-ion batteries. The positive reference material was used, applied in full cells (against graphite) in electrochem. and surface analytical study (ref. 9, 10, section 4.1). DFG form 3.06 – 03/25 page 6 of 16 Deutsche Forschungsgemeinschaft Kennedyallee 40 ∙ 53175 Bonn, Germany ∙ Postal address: 53170 Bonn, Germany Tel.: + 49 228 885-1 ∙ Fax: + 49 228 885-2777 ∙ postma[email protected] ∙ www.dfg.de DFG Description of the project-specific results and findings. In the following sections the contributions by IFW Dresden and KIT to the work packages within the framework of KIBSS are summarized. IFW Dresden. Commercially available carbons were systematically tested in various potassium electrolytes described in the literature, including in-house synthesised Nand S-doped hard carbons. Operando synchrotron experiments allowed to evaluate the impact of temperature on the bulk behaviour of graphite during anionic insertion. Chemical synthesis of KCx compounds was done to establish the reference XPS spectra for battery research. Alternative electrolytes and anode materials for KIBs were prepared and characterised accordingly. Carbonaceous Anodes in Commonly Used Electrolytes. On the first step, a comparison of cyclic behaviour of commercially available graphite KS6L (Imerys, France) and hard carbon (Carbotron P, Japan) was tested in standard 0.8 M KPF6/EC:DEC (1:1, v.) and super-concentrated 3M KFSI/DME electrolytes. Targeting the “green agenda”, a focus was made on using water-based binders: sodium carboxymethyl cellulose (CMC), sodium alginate and polyacrylic acid (PAA). Both graphite and hard carbon exhibited significantly better long-term performance when water-based binders were applied, compared to polyvinylidene fluoride (PVDF), which also required the use of toxic N-methyl pyrrolidone (NMP) in the electrode preparation procedure. The best long-term performance was achieved using the Na alginate binder, in agreement with literature data, reporting high mechanical stability of such electrode composites [1]. Concerning the electrolytes, the best results were achieved for the anodes cycled in 3M KFSI/DME. We decided to avoid using higher concentrations due to economic reasons. It was possible to reproduce the operando XRD experiment [2], reporting the mixed mechanism of pure K-ions insertion and solvent co-intercalation. The process is sequential and highly reversible. Thus, stable long-term cycling of commercial carbons in the specified electrolyte was achieved. The same was done with in-house synthesised hard carbons, doped with heteroatoms N or S. Although this part of work does not contain any scientific novelty, it was essential to conduct it, targeting the good scientific practice. A summary of this part of the work is available online in the form of a free-accessible dataset [ZENODO]. In parallel, alternative materials were evaluated as KIB anodes. As the result, a bismuth-containing (37.5 mass %, ICP-OES) carbon nanocomposite was designed (Fig.1a). It exhibited remarkable capacity retention: a value of around 300 mAh∙g-1 was kept for 500 cycles at 100 mA∙g-1 current density. Additionally, it demonstrated a possibility to be used in capacitors as well as in batteries [A4]. Another direction was represented by Ti3C2Tx MXenes, which were systematically studied within a frame of a master thesis of S. Kordian. Although the electrochemical performance was moderate, due to their chemical flexibility, these materials DFG form 3.06 – 03/25 page 7 of 16 Deutsche Forschungsgemeinschaft Kennedyallee 40 ∙ 53175 Bonn, Germany ∙ Postal address: 53170 Bonn, Germany Tel.: + 49 228 885-1 ∙ Fax: + 49 228 885-2777 ∙ postma[email protected] ∙ www.dfg.de DFG represent an enormous playground for optimisation of various parameters, targeting the battery-related characteristics [3]. The M.Sc. thesis of S. Kordian is accessible free of charge on the website of SLUB (Saxon State Library – State and University Library Dresden). Model Experiments on Potassium Intercalation into Graphite. Using highly-oriented pyrolytic graphite (HOPG) and chemical intercalation of potassium from vapour, we were able to establish model XPS spectra of K-graphite intercalation compounds (GIC), which could be used by battery researchers [A1, A2]. The main advantage of our chemical approach is that everything except pure insertion of K-ions into graphite is excluded from the experiment, allowing to consider the obtained data as a reference to distinguish between pure cationic intercalation and solvent co-intercalation. Temperature-dependent Anionic Intercalation into Graphite. Due to amphoteric properties of carbon and a rather unique crystal structure of graphite, not only cationic, but also anionic intercalation into this material is possible, providing an opportunity of building dualgraphite (dual-ion) batteries (DIB). Insertion of anions into graphite as well as cationic intercalation, represents a sequential process, known as staging. Its mechanism at room temperature is well-characterised [4], however, the effects of temperature elevation are lacking systematic studies, especially in the case of KIBs. Another issue is related to complications in electrolyte development. Besides challenges with an exceptionally high reactivity of potassium, one should also consider very high anionic intercalation potentials, often exceeding 5.0 vs. K+/K. For purely fundamental studies, we revisited sulfones, proposed in the work as solvents stable at such high voltages [4]. Although 2m KPF6/EMS (ethyl methyl sulfone)-based dual-ion system degrades rather rapidly, it was applicable for operando synchrotron XRD studies (Fig.1b and 1c). We found that irrelative to the alkali metal, bulk behaviour of graphite upon anionic intercalation at 298 K was the same. However, after temperature elevation to 333 K, the mechanism changed completely. Instead of sequential formation of corresponding GICs, multiple phases of uncertain origin formed. We excluded the formation of crystallo-solvates and assumed either co-intercalation of solvent or simultaneous formation of multiple intercalated phases, including the one with maximal intercalant concentration (MIC). The results were reported at the Electrochemical Society meeting in 2023 [B2]. Moreover, we performed a similar experiment with series of super-concentrated Li-based electrolytes and were able to observe that the described phenomenon was anion-specific. The latter was reported at a conference [B3] as well as in a peer-reviewed publication [A3]. Overall, the outcomes opened additional questions about suitable conditions for the DIBs operation. Attempts of implementing hard carbons as DIB cathodes were unsuccessful, allowing to assume the ordered structure as a necessary requirement for anionic intercalation. A supplementary operando Raman spectroscopic study (unpublished) showed that tG/tD bands ratio in graphite increases, when anions are DFG form 3.06 – 03/25 page 8 of 16 Deutsche Forschungsgemeinschaft Kennedyallee 40 ∙ 53175 Bonn, Germany ∙ Postal address: 53170 Bonn, Germany Tel.: + 49 228 885-1 ∙ Fax: + 49 228 885-2777 ∙ postma[email protected] ∙ www.dfg.de DFG inserted. Also, it confirmed that the origin of relatively low coulombic efficiency of DIB does not lay in the surface of graphite electrodes: the initial tG/tD ratio is retained after charge-discharge as well as the bands positions. KIT. A complementary analytical approach was chosen to understand degradation mechanisms at electrode-electrolyte interfaces of KIBs. This comprised surface-sensitive X-ray photoelectron spectroscopy (XPS) for solid surface deposits, nuclear magnetic resonance (NMR) spectroscopy and gas chromatography mass spectrometry (GCMS) to study liquid, soluble decomposition products for correlation to electrochemical results in various cell setups. SEI Formation and Electrolyte Degradation. Only few solvents are compatible with graphite negative electrodes, owing to co-intercalation of solvent molecules and subsequent decomposition that disintegrates the graphite structure. Unless a protective surface layer is formed prior to the intercalation process (e.g. high concentrations of KFSI, see above) or deliberate co-intercalation processes at around 1 V are performed, it will be carbonate-based electrolytes that are to this date most compatible with graphite. In the framework of KIBSS, KIT compared the SEI layer formation between a lithiumand a potassium-based half-cell configuration, using the same graphite electrodes from the same batch that can intercalate either Lior K-ions [A8]. The only two components that were different was the cation in the electrolyte and the alkali metal counter electrode. XPS on cycled graphite electrodes shows that the surface layer grows significantly thicker, which could be a major origin of increased cell resistance in half cell configurations. The elemental distribution further suggested that the SEI layer exhibits a considerably larger organic fraction than SEI layers formed in a Li-containing electrolyte, rendering the layer more prone to recurrent dissolution and thus SEI reformation reactions. Moreover, XPS also showed that merely by storing graphite in a cell against potassium metal leads to a significant deposition of potassium salts, in stark contrast to the Li-setup. In fact, complementary NMR and gas chromatography (GCMS) measurements on electrolyte samples exposed to metallic potassium, illustrate the rapid concentration build-up of parasitic 333 K Figure 1. (a) Rate capability tests of the Bi@RPC carbon composite anode in a K//Bi@RPC cell. (b) Operando studies of PF6intercalation into graphite in a potassium-based electrolyte at 298K; (b) – the same at 333 K. a b c DFG form 3.06 – 03/25 page 9 of 16 Deutsche Forschungsgemeinschaft Kennedyallee 40 ∙ 53175 Bonn, Germany ∙ Postal address: 53170 Bonn, Germany Tel.: + 49 228 885-1 ∙ Fax: + 49 228 885-2777 ∙ postma[email protected] ∙ www.dfg.de DFG bis(alkyl carbonates) in K-based electrolytes that may diffuse across the cell during cycling and storage and deposit (or even react) there (crosstalk)[A10]. The reactivity towards potassium is lower for diethyl carbonate (DEC), which might explain its popular (though probably unconscious) choice for anode-half-cell experiments. A more detailed study was performed with graphite electrodes from both half and full cell setups using hard X-ray photoelectron spectroscopy (HAXPES) study for higher probing depths[A11]. Interestingly, the SEI layers between the two setups shows fundamental differences in composition and thickness, thus raising the question how representative XPS measurements on samples derived from half-cell configurations really are in the case of KIB systems? At the very least, it is a complication in the analysis of XPS results, as the surface layer composition is significantly biased by the counter electrode, especially if potassium metal is used, and results on impeding factors to cycle life and capacity loss are difficult to translate or not transferable at all to the actual K-ion full cells case. Crosstalk Phenomena in Half Cells. Bis(alkyl carbonates) formed at the potassium metal interface are soluble species that can interfere with electrochemical processes. Using the aforementioned K2Fe[Fe(CN)6] (KFF) positive electrode, it could be shown that parasitic side reactions occur that appear as an additional step-like voltage feature in voltage profiles of KFF/potassium half cells. The feature can also be observed in many publications on these materials, usually without further comment to the unexpected additional voltage step. For a thorough electrochemical investigation, a potassium-free 3-electrode cell setup (Fig. 2) was developed based on a (traditional) Ag/AgCl chemistry but for a thin layer cell setup (usually potassium metal is used, however, its side reactions make it unsuitable for this purpose)[A13]. The setup was leveraged in various half and full cell setups to demonstrate that bis(alkyl carbonates), and thus by proxy potassium, are the origin of the additional electrode process. The detrimental crosstalk also induced a selfdischarge mechanism in KFF, which is a critical disadvantage. Ethylene Sulfate Additive. According to literature side reactions at the KFF electrode can be suppressed through the electrolyte additive 1,3,2-dioxathiolande-2,2-dioxide (DTD, or ethylene sulfate). The 3-electrode study showed that this is indeed the case in half cells, i.e. in the presence of K-metal, where it seems to suppress the bis(alkyl carbonate) formation[5]. DTD showed improved capacity retention in a collaborative work with IFW Dresden on KVPO4F Figure 2. Illustration of 3-electrode setup and potassium-induced side reactions (ref11). DFG form 3.06 – 03/25 page 16 of 16 Deutsche Forschungsgemeinschaft Kennedyallee 40 ∙ 53175 Bonn, Germany ∙ Postal address: 53170 Bonn, Germany Tel.: + 49 228 885-1 ∙ Fax: + 49 228 885-2777 ∙ postma[email protected] ∙ www.dfg.de DFG - Ulf-Christian Rauska – “Towards Mechanically Stable PEO-based Electrolytes for Potassium Batteries” (student project/”Vertieferarbeit”, M.Sc., Chemistry, KIT) - Celine Röder – “Electrolyte Components for High-Voltage Positive Electrodes in Potassium-Ion Batteries” (thesis, M.Sc., Chemistry, KIT) - Selina Kordian – „Untersuchungen an schichtartigen Karbiden als Elektrodenmaterialien für K-Ionen-Batterien“ (Masterarbeit, IFW Dresden, BTU) 2024 - Ekaterina Serebrennikova – “Sulfur-containing electrolyte additives for potassium-ionbatteries” (student project/”Vertieferarbeit”, M.Sc., Chemistry, KIT) - Benjamin Brecht – “Fluorine-free Electrolyte Salts for Solid Polymer Electrolyte Applications in Post-Li Batteries” (thesis, M.Sc., Chemistry, KIT) Iurii Panasenko – “Strategies to Mitigate Electrolyte Degradation Processes in Potassium Batteries” (PhD thesis, KIT) - Congcong Liu – “Carbon-based anode materials for sodiumand potassium-ion-batteries” (PhD thesis, IFW Dresden, TU Dresden) 5.1 Doctoral researchers involved: Doctoral researchers Gender (m/f/d) Doctoral status (ongoing, finished, discontinued) Start and (where applicable) finish of doctoral studies: MM/YYYY – MM/YYYY Funding within the framework of the project MM/YYYY – MM/YYYY Panasenko, Iurii m finished 09/2021 – 12/2025 09/2021 – 03/2025 Liu, Congcong f not finished yet 09/2021 -12/2025