Many battery applications target fast charging to achieve an 80 % rise in state of charge (SOC) in < 15 min. However, in the case of all-solid-state batteries (SSBs), they typically take several hours to reach 80 % SOC while retaining a high specific energy of 400 W h kg(cell)(-1). We specify design strategies for fast-charging SSB cathodes with long cycle life and investigate the fast-charging capability of a sulfide-based single crystal Li-Ni-Mn-Co oxide composite cathode. At 30 degrees C and charging at 15 mA cm(-2), a specific capacity of 150 mA h g(-1) was achieved in similar to 8 min, with 81 % capacity retention after 3000 cycles. Critically, a 3-electrode arrangement was used to avoid the common problem of overcharging at high current densities. By following the design strategy and optimized manufacturing, a 210 mu m thick cathode was able to be charged at an extraordinary current density of 50 mA cm(-2) to reach an areal capacity of 8 mA h cm(-2) in only 10 min, suggesting practical cathodes for SSBs with 400 W h kg(cell)(-1) may be within reach.

Prussian blue analogues (PBAs) are a highly tunable family of materials with properties suitable for a wide variety of applications. Although their straightforward aqueous synthesis allows for the facile preparation of a diverse set of compositions, the use of water as the solvent has hindered the preparation of specific compositions with highly sought-after properties. A typical example is Cr[Cr(CN)6]: its predicted strong magnetic interactions have motivated many attempts at its synthesis but with limited success. The lack of control over vacancies, crystallinity, and the oxidation state has prevented the experimental validation of its theoretical magnetic properties. Here, we report the nonaqueous synthesis of vacancy-suppressed, nanocrystalline chromium hexacyanochromate. The control over vacancies and the oxidation state leads to stronger magnetic interactions with a markedly increased absolute Weiss temperature (Θ = -836(6) K) and magnetic ordering temperature of (240 ± 10) K. Our results challenge the notion of the solvent as merely reaction medium and introduce a pathway for exploring moisture- and air-sensitive PBA compositions.

Lithium-ion batteries (LIBs) are playing an increasingly important role in enabling the transition to a low-carbon global economy, in electric vehicle and grid storage applications. Despite increasing LIB ubiquity, there remains gaps in our understanding of how to optimize further LIB electrode design and structure, and how to achieve such designs in practice and at scale without excessive trial-and-error experimentation. For example, thicker electrodes are desirable for improved volumetric capacity but particularly during rapid charging and discharging cycles, a significant through-electrode thickness Li ion concentration gradient in the electrolyte builds up. This leads to a spatially varying concentration overpotential that in turns leads to uneven utilization of the active material, resulting in diminished capacity and accelerated degradation. Although models of electrode dynamics can help qualitatively to understand concentration polarization effects as a function of electrode design, there are few practical experimental tools to visualize or resolve Li-ion concentration gradients in practice. For example, the widely-used energy dispersive X-ray spectroscopy (EDS) in a scanning electron microscopy cannot resolve elements with very low atomic number such as Li. In this presentation we describe the development of a methodology to visualize Li-ion concentration gradients across a range of electrodes based on secondary ion mass spectroscopy (SIMS). We combine SIMS with EDS to collect elemental maps of all principle LIB electrode elements in a single workflow, which is also extended to 3D by further combining with a plasma field ion microscope (P-FIB) capability. We apply the new methodology for LIB cathodes based on LiFePO4 (LFP) and LiMn2O4 (LMO) that are at different state of overall charge, achieved at a range charging rates. The electrodes also span a range of thicknesses from 100 to 700 µm. We show how the approach can operate over cross-sections large enough to encompass all the electrode thickness while maintaining sufficient spatial resolution to capture key features of the local Li concentration. While EDS elemental maps, with appropriate calibration, can be correlated to local element concentration with good accuracy, SIMS spectra and specifically the intensity of the 7Li+ peaks cannot be readily related to local Li concentration since the yield of 7Li+ ion relates to additional factors, such as the local atomic environment, surface topology and more. We describe how we inter-relate the EDS and SIMS maps for non-Li elements to account for some of these features, and show how these reveals the underlying Li distributions. We consider how these measurements of the Li concentration in the solid particles of the electrode relate to the local state of charge, and the conditions in the electrolyte when charge/discharge was halted and the sample was retrieved from the cell for examination. Fine-scale variations in local 7Li+ ion intensity are also revealed and explained in terms of local microstructural features such as particle size, any particle cracking, the growth of secondary electrolyte interphase, etc.

Sluggish electrolyte transport properties result in a tradeoff between energy density and rate capability in lithium-ion batteries. To increase energy density, electrodes are made thicker and less porous. However, once thick enough, lithium transport in the electrolyte becomes the rate limiting process, and capacities at elevated C rates are reduced as a result of underutilisation of active material near the current collector. Strategies have been proposed to overcome these limitations, including pore engineering to reduce through-plane tortuosity, to varying degrees of success. We introduce bilayer cathodes that aim to improve the rate performance of thick electrodes by controlling the through-thickness charging rate. The bilayer cathode structure is comprised of two discrete sublayers containing the active materials lithium iron phosphate (LFP) and lithium nickel manganese cobalt oxide (NMC). Due to LFP and NMC having open-circuit voltage (OCV) profiles in different voltage windows, the through-thickness charging rate is dependent on the location of the two active materials. The bilayer electrodes are manufactured by multi-pass doctor blade coating and, in principle, could also be produced using twin-slot dies which would require only a minor modification to the current commercial manufacturing methods. The electrochemical performance of the bilayer cathodes are compared with a blended electrode (single layer containing intimately mixed LFP and NMC) and a uniform NMC electrode, all at constant areal capacity (4.5 mAh cm-2) and porosity (30%). We report significant differences in voltage profiles when charging from 0% state of charge as well as intermediate (e.g. 50%) states of charge in electrodes containing the same mass fraction of LFP and NMC, demonstrating that the location of the active material through the electrode thickness impacts behaviour. Moreover, the best performing bilayer electrode structure (LFP layer adjacent to the current collector, NMC layer on top of the LFP layer) outperforms the uniform NMC electrode in fast charge tests. At 3C, the best performing bilayer cathode maintains 84% of its capacity whilst the uniform NMC electrode maintains only 53%. In discharge, the same electrodes both maintain approximately 52% capacity at 3C, demonstrating the anisotropic charging/discharge performance introduced by the bilayer structure. An understanding of the through-thickness charging rate, and distribution of state of charge, throughout charging is required to explain why the bilayer cathode outperforms a conventional uniform cathode. In uniform electrodes, with the same active material through the thickness of the electrode, a gradient in state of charge is formed due to a gradient in resistance to charge through the electrode thickness. This is due to much higher ionic resistance within the electrolyte than the electronic resistance within the composite electrode. In thick electrodes this effect is pronounced enough so that at the end of charge there is far greater underutilisation of active material in the part of the electrode nearest the current collector. In the bilayer cathode structure, by placing active material with a lower OCV near the current collector, we can, somewhat counterintuitively, achieve much more even through-thickness charging compared to uniform electrodes, minimising underutilisation of active material near the current collector and increasing rate performance.

Growth in the Li-ion battery market continues to accelerate, driven primarily by the increasing need for economic energy storage for electric vehicles. Electrode manufacture by slurry casting is the first main step in cell production but much of the manufacturing optimisation is based on trial and error, know-how and individual expertise. Advancing manufacturing science that underpins Li-ion battery electrode production is critical to adding to the electrode manufacturing value chain. Overcoming the current barriers in electrode manufacturing requires advances in materials, manufacturing technology, in-line process metrology and data analytics, and can enable improvements in cell performance, quality, safety and process sustainability. In this roadmap we explore the research opportunities to improve each stage of the electrode manufacturing process, from materials synthesis through to electrode calendering. We highlight the role of new process technology, such as dry processing, and advanced electrode design supported through electrode level, physics-based modelling. Progress in data driven models of electrode manufacturing processes is also considered. We conclude there is a growing need for innovations in process metrology to aid fundamental understanding and to enable feedback control, an opportunity for electrode design to reduce trial and error, and an urgent imperative to improve the sustainability of manufacture.

Potassium-ion (K-ion) batteries are a promising complementary technology to lithium-ion in which less scarce and less expensive elements can be used; potassium instead of lithium, aluminum instead of copper, and cobalt-free cathodes. They have the advantage over sodium-ion batteries in that graphite, the standard commercial anode for lithium-ion, can be used[1] and potassium has a lower reduction potential (K+/K) than that of sodium and even lithium. Currently, the research focus is on developing energy-dense, high-voltage cathode materials. Prussian blue analogues, with their open-framework structure, are well-suited to reversibly insert the relatively large potassium ion.[2] Potassium manganese hexacyanoferrate (KMF) is of particular interest due to its high theoretical capacity (155 mAh g-1) and high voltage (around 4 V vs. K+/K) making it one of the most promising K-ion candidate cathode materials.[3] However, significant challenges remain before KMF K-ion batteries are suitable for commercialization. Current collector corrosion at high voltages and material instability lead to low coulombic efficiency on cycling and severe capacity fade.[4] In this work we report a significant improvement in KMF electrode performance by utilizing a highly stable ionic liquid electrolyte. This electrolyte suppresses corrosion of the current collector and deleterious reactions on the cathode. Performance of the KMF material is significantly controlled by its composition and particle size, which can be controlled during synthesis. We employ high-throughput techniques to systematically study a wide synthesis parameter space in order to minimize hexacyanoferrate vacancy and water content whilst controlling for particle size. We found that changing particle size considerably impacted specific capacity and cyclability, and a compromise must be made. Our optimized KMF material had a specific capacity of 119 mA h g-1, a coulombic efficiency of 99.3%, and relatively good cyclability. Crucially, graphite also displayed excellent electrochemical performance in the same ionic liquid electrolyte (maintaining 99% of initial capacity after 400 cycles and a coulombic efficiency of more than 99.9%) and exhibited a highly reversible electrode reaction studied by in operando XRD. Optimized KMF | ionic liquid electrolyte | graphite full-cell chemistry reported here proves a viable strategy for highly efficient high-voltage potassium-ion batteries. [1] Komaba et al., Potassium intercalation into graphite to realize high-voltage/high-power potassium-ion batteries and potassium-ion capacitors, Electrochemistry Communications (2015), 60, 172-175 [2] Hurlbutt et al., Prussian Blue Analogs as Battery Materials, Joule (2018), 2, 10, 1950-1960 [3] Bie et al., A novel K-ion battery: hexacyanoferrate(II)/graphite cell, J. Mater. Chem. A (2017), 5, 9, 4325-4330 [4] Hosaka et al. Highly concentrated electrolyte solutions for 4 V class potassium-ion batteries, Chem. Commun. (2018), 54, 8387-8390

Potassium-ion batteries (KIB) are a promising complementary technology to lithium-ion batteries because of the comparative abundance and affordability of potassium. Currently, the most promising KIB chemistry consists of a potassium manganese hexacyanoferrate (KMF) cathode, a Prussian blue analog, and a graphite anode (723Whl−1 and 359Whkg−1 at 3.6V). No electrolyte has yet been formulated that is concurrently stable at the high operating potential of KMF (4.02V vs K+/K) and compatible with K+ intercalation into graphite, currently the most critical hurdle to adoption. Here we combine a KMF cathode and a graphite anode with a KFSI in Pyr1,3FSI ionic liquid electrolyte for the first time and show unprecedented performance. We use high-throughput techniques to optimize the KMF morphology for operation in this electrolyte system, achieving 119 mA h g−1 at 4 V vs K+/K and a coulombic efficiency >99.3%. In the same ionic liquid electrolyte graphite shows excellent electrochemical performance and we demonstrate reversible cycling by operando XRD. These results are a significant and essential step forward towards viable potassium-ion batteries.

The application of transition metal fluorides as energy-dense cathode materials for lithium ion batteries has been hindered by inadequate understanding of their electrochemical capabilities and limitations. Here, we present an ideal system for mechanistic study through the colloidal synthesis of single-crystalline, monodisperse iron(ii) fluoride nanorods. Near theoretical capacity (570 mA h g−1) and extraordinary cycling stability (>90% capacity retention after 50 cycles at C/20) is achieved solely through the use of an ionic liquid electrolyte (1 m LiFSI/Pyr1,3FSI), which forms a stable solid electrolyte interphase and prevents the fusing of particles. This stability extends over 200 cycles at much higher rates (C/2) and temperatures (50 °C). High-resolution analytical transmission electron microscopy reveals intricate morphological features, lattice orientation relationships and oxidation state changes that comprehensively describe the conversion mechanism. Phase evolution, diffusion kinetics and cell failure are critically influenced by surface-specific reactions. The reversibility of the conversion reaction is governed by topotactic cation diffusion through an invariant lattice of fluoride anions and the nucleation of metallic particles on semicoherent interfaces. This new understanding is used to showcase the inherently high discharge rate capability of FeF2. The application of metal fluorides as cathodes for lithium ion batteries has been hindered by inadequate understanding of their electrochemical capabilities. Reversible conversion reaction in iron fluoride nanocrystals is shown to be due to topotactic cation diffusion and nucleation of metallic particles.

Mechanochemical grinding of polycrystalline powders of the Prussian blue analogue (PBA) Mn[Co(CN)$_{\textbf6}$]$_{\textbf{2/3}}\boldsymbol\Box_{\textbf{1/3}}\cdot\boldmath x$H$_{\textbf 2}$O and K$_{\textbf 3}$Co(CN)$_{\textbf 6}$ consumes the latter and chemically modifies the former. A combination of inductively-coupled plasma and X-ray powder diffraction measurements suggests the hexacyanometallate vacancy fraction in this modified PBA is reduced by approximately one third under the specific conditions we explore. We infer the mechanochemically-driven incorporation of [Co(CN)$_{\textbf 6}$]$^{\textbf 3-}$ ions onto the initially-vacant sites, coupled with intercalation of charge-balancing K$^+$ ions within the PBA framework cavities. Our results offer a new methodology for the synthesis of low vacancy PBAs, unlocking novel, high capacity PBA battery materials.

Due to the rapid battery market expansion, and the limited and geographically concentrated lithium and cobalt resources, there is significant concern regarding the short-term supply and long-term sustainability of lithium-ion batteries (LIBs). Potassium-ion batteries (KIBs) are emerging as a promising complementary technology to LIBs due to the relative abundance of potassium. KIBs can also use graphite anodes providing a critical advantage over sodium-ion batteries (NIBs). In this perspective, we provide an overview of the most promising cathodes, anodes, and electrolytes to date for KIBs. We also present a concise techno-economic model to critically compare the most promising KIB chemistries and evaluate if they can compete with a leading NIB and LIBs. Finally, we identify five critical research challenges that need to be addressed for KIBs to become a viable technology.

Prussian blue analogues (PBAs) have recently shown outstanding electrochemical properties ascribable to their unique open-framework crystal structure that allows the reversible insertion of alkali ions with negligible perturbation to the framework itself. Many hexacyanoferrate materials have shown excellent properties and are some of the most promising sodium- and potassium-ion cathode materials in both aqueous and organic electrolytes. However, there is a distinct lack of candidate PBA materials that operate at low potentials, as their characteristic crystalline framework shows instability. In this article, we characterize the structure and electrochemical behavior of manganese hexacyanochromate, which exhibits reversible sodium insertion at -0.86 V vs standard hydrogen electrode (1.84 V vs Na+/Na) while maintaining the characteristic PBA cubic structure. This is the lowest redox potential of reported PBA materials and shows fast kinetics in a high-voltage water-in-salt electrolyte. Further reduction in potential in an organic electrolyte shows decomposition of the crystalline structure.

Prussian blue analogues (PBAs) have recently shown outstanding electrochemical properties ascribable to their unique open-framework crystal structure that allows the reversible insertion of alkali ions with negligible perturbation to the framework itself. Many hexacyanoferrate materials have shown excellent properties and are some of the most promising sodium- and potassium-ion cathode materials in both aqueous and organic electrolytes. However, there is a distinct lack of candidate PBA materials that operate at low potentials as their characteristic crystalline framework shows instability. In this article we characterise the structure and electrochemical behavior of manganese hexacyanochromate which exhibits reversible sodium insertion at - 0.86 V vs. SHE (1.84 V vs. Na+/Na), whilst maintaining the characteristic PBA cubic structure. This is the lowest redox potential of reported PBA materials and shows fast kinetics in a high voltage water-in-salt electrolyte. Further reduction in potential in an organic electrolyte shows decomposition of the crystalline structure.

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.2018.07.017 Song, 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

Sodium batteries and solid-state electrolytes are two research directions in the effort to develop electrochemical energy storage that goes beyond the lithium ion. In this issue of Chem, Goodenough and colleagues combine a sodium-metal anode, a NASICON solid electrolyte, and a Prussian blue analog cathode to create an energy-dense, long-lived battery.

Prussian blue analogs have significant promise as active materials for the next generation of battery electrodes with improved cycle life and rate capability. Their useful electrochemical properties include two independent redox centers per unit cell; a nanoporous, open framework for rapid ion conduction; high stability during ion (de) insertion; and structural and electrochemical tunability for diverse applications. Here we share insights into how control of the five main crystallographic features (two transition-metal ions, the inserting ion, defects, and water) imparts control over the ion-insertion reaction. We then identify five key opportunities to expand our understanding of these materials, including the role of water in their ion conduction, modeling, synthesis methods, use as anode materials, and technoeconomics. Further research in these areas will accelerate the development of new, high-performing battery electrodes.