Correction for ‘The role of an elastic interphase in suppressing gas evolution and promoting uniform electroplating in sodium metal anodes’ by Chen Gong et al., Energy Environ. Sci., 2023, 16, 535–545, https://doi.org/10.1039/D2EE02606F.

The role of ether electrolyte solvent in facilitating efficient Na metal anode cycling performance is identified to be preventing adverse gas formation and favouring a flexible conformal SEI.

Red phosphorus (RP) is a promising anode material for alkali-ion batteries due to its ability to alloy at low potentials with Li, Na and K to form the phases Li3P, Na3P and KP, delivering a high theoretical capacity of 2596, 2596 and 865 mAh g-1, respectively. However, its main disadvantage is its very large volumetric expansion during the alloying reaction (168% with Li, 331% with Na and 161% with K) that can cause, loss of electrical contact, low Coulombic efficiency, and ultimately poor cycle life. Other alloying materials such as silicon have shown to suffer from the fracturing of the particles during the reaction with lithium. In this work, we discuss if this is the case for phosphorus too [1]. Through comprehensive electrochemo-mechanical characterisation and modelling of the cycling stresses, we show that RP can be cycled at high current densities without fracture. A model that can simulate the volume expansion of RP during the reaction with the alkali metals Li, Na and K, as well as the generation and evolution of stresses on the particles is developed. Its use required the implementation of the elastic, plastic and fracture properties of RP that are measured by novel application of in-situ nanoindentation and powder compression. The validity of the model is also tested through in-situ TEM observation with extreme conditions (anisotropic ion diffusion and high current density) where no catastrophic failure was observed. The mechanical and the electrochemical characterisation, together with the model developed in this work allows for predictions to be made for the application of RP in alkali-ion batteries. REFERENCE [1] Capone et al., Matter 2020 3 (6), 2012-2028

To match the high capacity of metallic anodes, all-solid-state batteries (ASSBs) re- quire high energy density, long-lasting composite cathodes such as Ni-Mn-Co (NMC)- based lithium oxides mixed with a solid-state electrolyte (SSE). However in practice, cathode capacity typically fades due to NMC cracking and increasing NMC/SSE in- terface debonding because of NMC pulverization, which is only partially mitigated by the application of a high cell pressure during cycling. Using smart processing proto- cols we report a single crystal particulate LiNi0.83Mn0.06Co0.11O2 and Li6PS5Cl SSE composite cathode with outstanding discharge capacity of 210 mAh g−1 at 30 °C. A first cycle coulombic efficiency of >85%, and >99% thereafter, was achieved despite a 5.5% volume change during cycling. A near-practical discharge capacity at a high areal capacity of 8.7 mAh cm−2 was obtained using a novel asymmetric anode/cathode cycling pressure of only 2.5 MPa/0.2 MPa.

Red phosphorus (RP) is a promising anode material for potassium-ion batteries because of its theoretical capacity of 865mAhg–1 delivered at an average potential of 0.5V vs K+/K. However, its alloy reactionto form KP entails a volume expansion of 162% resulting in severe stresses that lead to SEI and electrode fracture, loss of electric contact, and ultimately reduced cycle life. Moreover, its low electronic conductivity (10-14 Scm–1) limits rate capability. Here, we report a RP-graphite composite prepared by a two step ball milling procedure to control particle size and optimize carbon coating. Electrodes prepared with the composites achieve high capacity (723mAhg–1) at C/20 and retaining 75% at 5C. It also shows very good cycling stability, retaining more than 96% of the capacity after 100 cycles at 1C.

The solid electrolyte interphase (SEI), a complex layer that forms over the surface of electrodes exposed to battery electrolyte, has a central influence on the structural evolution of the electrode during battery operation. For lithium metallic anodes, tailoring this SEI is regarded as one of the most effective avenues for ensuring consistent cycling behavior, and thus practical efficiencies. While fluoride-rich interphases in particular seem beneficial, how they alter the structural dynamics of lithium plating and stripping to promote efficiency remains only partly understood. Here, operando liquid-cell transmission electron microscopy is used to investigate the nanoscale structural evolution of lithium electrodeposition and dissolution at the electrode surface across fluoride-poor and fluoride-rich interphases. The in situ imaging of lithium cycling reveals that a fluoride-rich SEI yields a denser Li structure that is particularly amenable to uniform stripping, thus suppressing lithium detachment and isolation. By combination with quantitative composition analysis via mass spectrometry, it is identified that the fluoride-rich SEI suppresses overall lithium loss through drastically reducing the quantity of dead Li formation and preventing electrolyte decomposition. These findings highlight the importance of appropriately tailoring the SEI for facilitating consistent and uniform lithium dissolution, and its potent role in governing the plated lithium's structure.

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

Red phosphorus (RP) is a promising anode material for potassium-ion batteries because of its theoretical capacity of 865 mAh/g delivered at an average potential of 0.5 V vs. K+/K. However, its alloy reaction to form KP entails a volume expansion of 162% resulting in severe stresses that lead to SEI and electrode fracture, loss of electric contact, and ultimately reduced cycle life. Moreover, its low electronic conductivity (10(-14) S/cm) limits rate capability. Here, we report an RP-graphite composite prepared by a twostep ball-milling procedure to control particle size and optimize carbon coating. Raman operando on graphite in the composite suggests that the carbon coating reversibly expands and contracts due to the volume expansion of RP particles. Electrodes prepared with the composite achieve high capacity (723 mAh/gP) at C/20 and retaining 75% at 5C. It also shows very good cycling stability, retaining more than 96% of the capacity after 100 cycles at 1C. (C) 2021 Elsevier Ltd. All rights reserved.

Red phosphorus (RP) is a promising anode material for alkali-ion batteries due to a high theoretical capacity at low potentials when alloying with lithium, sodium, and potassium. Most alloy anode materials display large volume changes during cycling, which can lead to particle fracturing, low Coulombic efficiency, loss of electrical contact, and ultimately poor cycle life. In this paper we outline, through comprehensive electrochemo-mechanical characterization and modeling of the cycling stresses, why RP can be cycled at high current densities without fracture. Application of in situ nanoindentation and powder compression allows for measurement of the elastic, plastic, and fracture properties of RP. In situ transmission electron microscopy observation with extreme conditions (anisotropic ion diffusion and high current density) was used to validate the model, observing no catastrophic failure of RP particles. Electrochemo-mechanical characterization with geometry and stress modeling allows for predictions to be made for application of RP in alkali-ion batteries.

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.

In this study we address the Li-ion de-insertion/insertion mechanisms from/into the lattice of the mixed olivine LiCo1/3Fe1/3Mn1/3PO4(LCFMP). This mechanism is driven by a subtle interplay of structural, electronic and thermodynamic features. We aim at dissecting this complex landscape that is tightly connected to the long-term electrochemical performance of this material as a positive electrode in lithium-ion cells. To this end, we report advanced structural characterization, based onex situsynchrotronradiation diffraction on samples at different lithium contents. We couple this analysis with first-principles simulations, for a directvis-a-viscomparison. Our results show that (1) the mixing of the three transition-metal (TM) cations in the olivine lattice leads to a solid solution, providing the olivine lattice with the necessary flexibility to retain its single-phase structure during cell operation; (2) the electronic features of the three TMs are responsible for the observed electrochemical performance; (3) the de-lithiation of the olivine lattice is a thermodynamically driven process. Last but not least, our integrated experimental and theoretical results reveal the subtle features behind the formation of antisite defects that selectively involve Li-Co couples. In conclusion, our study provides the necessary scientific foundations to understand the structure-property-function relationships in LCFMP olivines, paving the way for further development and optimization of this material for application in Li-ion batteries.

Transition metal fluorides display the combination of high capacity and high electrode potential that are essential to boosting the energy density of lithium ion batteries. However, their application as cathode materials has been hindered by an incomplete understanding of their electrochemical capabilities and limitations. Herein, we present a system of single crystalline, monodisperse iron (II) fluoride nanorods and detail how their morphology and uniformity make them ideal for mechanistic study. High-resolution analytical transmission electron microscopy reveals intricate morphological features, lattice orientation relationships, and oxidation state changes that redefine the conversion reaction with unprecedented spatial resolution. We first introduce the presence of surface specific reactions and examine how they critically influence phase evolution, diffusion kinetics and cell failure. Next, we examine how 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 semi-coherent interfaces. We will focus on how each mechanistic feature establishes new principles for improving reaction hysteresis, kinetics and reversibility and how this new holistic understanding can inform the more effective application of metal fluorides. To ensure a pertinent result, ex-situ data was extracted from coin cells that deliver near theoretical capacity (570 mAh g-1) and extraordinary cycling stability (>90 % capacity retention after 100 cycles). This exceptional performance was enabled solely through the use of an ionic liquid electrolyte. Using a combination of impedance spectroscopy, TEM, and X-ray photoelectron spectroscopy, we explain how the solid electrolyte interphase forming ability of this electrolyte precludes the major failure mechanisms associated with transition metal fluorides. We conclude this talk by examining the complementary nature of this cathode electrolyte pairing, particularly with regards to high-temperature cycling. Figure 1

Red phosphorus is a promising anode material thanks to its ability to form alloys with lithium, sodium and potassium, thus achieving high theoretical capacity (2596 mAh g -1 with Li and Na and 865 mAh g -1 with K). However, it suffers from its low electronic conductivity and a huge volume expansion that leads to the formation of stress with consequent fracture and pulverization of the particle. In this work, red phosphorus particles have been analyzed with in-situ TEM techniques in order to study the mechanism of the crack formation during the volume expansion with lithium, sodium and potassium. This experiment is coupled with computational modelling of the particles and the simulation of the diffusion of the ions through the particles to understand the development of stresses that can lead to the nucleation of the crack and the failure of the particle.

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.

In this study we address the Li-ion de-insertion/insertion mechanisms from/into the lattice of the mixed olivine LiCo1/3Fe1/3Mn1/3PO4 (LCFMP).

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.

Sodium-ion batteries will have an important role as a complement to lithium-ion in a future where lithium or cobalt, two critical elements for lithium-ion batteries, become scarce or prohibitively expensive. Red phosphorus (RP) is a promising candidate as an anode for sodium-ion batteries because of its low potential and high specific capacity. Its main disadvantage is its 490% volumetric expansion during sodiation. This leads to particle pulverization and substantial reduction of the cycle life. Furthermore, RP has an extremely low electronic conductivity of 10-14 S cm-1. Both issues have been previously addressed by ball milling RP with a carbon matrix. This decreases the RP particle size and also forms a more electronically conductive composite. However, it is challenging to determine the RP particle size independent of the size of the composite particles. Consequently, little is known about how much the RP particle size must be reduced to improve anode performance. Here we quantify the relationship between the RP particle-size distribution and its cycle life for the first time by separating the ball milling process into two steps. An initial wet ball milling is used to control the RP particle-size distribution, which is measured via dynamic light scattering. This is followed by a dry milling step to produce RP-graphite composites. We found that wet milling breaks apart the largest RP particles in the range of 2 to 10 µm decreases the Dv90 from 1.85 to 1.26 µm and significantly increases the cycle life of the RP. Furthermore, we determined that the length of time of the second milling step affects the uniformity of the carbon distribution in the composite. Photoelectron spectroscopy and transmission electron microscopy confirms the successful formation of a carbon coating, thus improving the performance of the resulting material. The RP with a Dv90 of 0.79 µm mixed with graphite for 48h delivered 1,354 mA h g-1 with high coulombic efficiency (>99%) and cyclability (88% capacity retention after 100 cycles). These results are an important step in the development of cyclable, high-capacity anodes for sodium-ion batteries.

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.