Hybrid materials, which combine inorganic and molecular components, often exhibit structural flexibility that enables unusual functional responses. Among them, Prussian blue analogs (PBAs) are a promising class for post-lithium battery technologies. Here, we show that nonequilibrium transformation processes govern the charge-storage mechanism of a PBA electrode, K2Mn[Fe(CN)6]. Ostensibly, this behavior mirrors that observed in high-rate cycling of conventional cathodes such as LiFePO4 yet arises here for fundamentally different reasons-namely, low elastic moduli and cooperative distortions inherent to the hybrid framework. Using operando x-ray absorption spectroscopy with Metropolis matrix factorization and x-ray diffraction, we show that framework flexibility limits transport kinetics and promotes collective, metastable pathways. Our results not only highlight various directions for PBA cathode optimization but also suggest a broader relevance of nonequilibrium mechanisms for mass transport in hybrid materials beyond PBAs alone.

Lithium argyrodites based on the composition Li 6 PS 5 X (X = Cl, Br, I) are among the most intensely researched solid electrolyte materials. To enable application in commercial all-solid-state lithium metal batteries, separators must be fabricated with large areas and a thin (<50 µm) form factor to maximize the cell-level energy density and provide the mechanical flexibility required for roll-to-roll processing. Unfortunately, the fabrication techniques commonly used for lithium-ion electrode films such as slurry casting and calendering create large-scale porosity, rendering the separators susceptible to dendrite penetration at current densities well below the requirements of fast charging [1]. To increase the dendrite resistance, the pore size must be reduced. Significant attention has been directed at densification processes which could achieve this, including sintering and warm isostatic pressing (WIP). However, avoiding the thermal decomposition of Li 6 PS 5 Cl is challenging. We have investigated the use of radio frequency (RF) magnetron sputtering to produce pore-free Li 6 PS 5 Cl films without the need for subsequent densification. The success of this approach has been demonstrated previously for LiPON – the preeminent thin-film electrolyte with a critical current density for dendrite propagation exceeding 10 mA cm -2 [2]. In our investigation, we used a desktop RF magnetron sputtering system housed within an argon-filled glovebox to deposit films from unconsolidated powder mixtures of Li 6 PS 5 Cl and Li 2 S. A low power density (~1 W cm -2 ) and high chamber pressure (2*10 -2 mbar) were required to avoid excessive heating and consequent decomposition of the target. Cross-sectional scanning electron micrographs revealed the absence of porosity and other microstructural flaws in films between 1 and 10 µm thick, while energy dispersive X-ray spectroscopy (EDX) and X-ray photoelectron spectroscopy (XPS) measurements confirmed that the chemical compositions were close to the targeted stoichiometry. X-ray diffraction patterns showed that the films were nanocrystalline and consisted principally of the Li 6 PS 5 Cl argyrodite phase, with a small volume fraction of impurities including Li 3 PS 4 . Post-deposition annealing on an Li 6 PS 5 Cl powder bed was used to increase the grain size and thus the Li + conductivity. Rapid grain growth occurred at temperatures as low as 450 °C, which enabled thermal decomposition of the argyrodite to be avoided. Interestingly, like LiPON, the films showed a high resistance to fracture during nanoindentation experiments, suggesting that the dendrite resistance of these films will be similarly high. Sputter-deposited films such as these will be useful for fundamental studies on commercially relevant lithium argyrodite separators, since the properties of dense thin films are likely to be somewhat different from those of the porous separators studied to date. [1] Kim, S., Chart, Y. A., Narayanan, S., & Pasta, M. (2022). Thin Solid Electrolyte Separators for Solid-State Lithium-Sulfur Batteries. Nano Letters , 22 (24), 10176–10183. https://doi.org/10.1021/acs.nanolett.2c04216 [2] Neudecker, B. J., Dudney, N. J., & Bates, J. B. (2000). “Lithium-Free” Thin-Film Battery with In Situ Plated Li Anode. Journal of The Electrochemical Society , 147 (2), 517. https://doi.org/10.1149/1.1393226

Understanding the structure-property relationships of solid-electrolyte interphases (SEIs) is critical for the rational design of electrolytes for beyond-Li-ion battery chemistries. However, the nanometre-scale dimensions, chemical heterogeneity and beam sensitivity of SEIs make their structural characterisation exceptionally challenging. Here, we present a data acquisition and analysis workflow for four-dimensional scanning transmission electron microscopy (4D-STEM) applied to beam-sensitive, heterogeneous lithium-metal SEI samples. By combining data dimensionality reduction with unsupervised machine-learning approaches, we perform real-space clustering to enhance signal-to-noise ratios across large datasets. Automated Bragg reflection analysis was then used to identify distinct crystallographic phases within the SEI. This approach enables nanoscale phase mapping of dominant species across large fields of view with nanometre-level lateral resolution. We identified lithium oxide and lithium hydroxide as the predominant crystalline phases in the lithium-metal SEI.

Electrochemical impedance spectroscopy (EIS) is widely used to probe the solid electrolyte interphase (SEI) under realistic conditions, without causing damage to its structure. However, the models and experimental conditions often raise concerns about the reliability of the results. In this work, we present an extensive EIS study of lithium metal in the model electrolyte lithium bis-(fluorosulfonyl)-imide in tetraglyme, analyzing the system at equilibrium as a function of time, temperature, and salt concentration using a setup designed to minimize artifacts. We apply information theory to determine the number of independent degrees of freedom and constrain the number of Voigt elements used in fitting. Our analysis reveals strong correlations among processes, warranting caution when assigning physical meaning. X-ray photoelectron spectroscopy and 4D-scanning transmission electron microscopy measurements are used to support the interpretation and provide complementary insights into the chemical nature of the interphase. The unique and extensive dataset we have collected, comprising over 12000 highly reproducible impedance spectra, will serve as a valuable resource to the community for further analysis and for supporting additional modeling and experimental efforts.

Lithium phosphorus oxynitride (LiPON) is one of the few solid electrolytes that form a truly passivating solid electrolyte interphase (SEI) when in contact with metallic lithium. Investigations into the origin of this stability may provide the insights needed to replicate it in the SEIs of alternative solid electrolyte materials. In this study, we used in situ lithium plating X-ray photoelectron spectroscopy (XPS) to investigate the formation and evolution of the Li-LiPON SEI. We show that the SEI is chemically and structurally inhomogeneous, with the fully reduced compounds identified in previous studies (Li2O, Li3N, and Li3P) concentrated near the lithium metal side and partially lithiated species, including Li x P, predominant closer to the LiPON side. Li3P and Li x P have recently been suggested as enablers of continuous SEI growth in thiophosphate solid electrolytes. We suggest that the stability of the Li-LiPON SEI is derived from a combination of the LiPON reduction potential (0.68 V vs Li+/Li), which is below the oxidation potentials of the fully reduced SEI compounds, and the graded structure of the SEI, which ensures that the most reduced species are not in physical or electrical contact with the LiPON layer.

Lithium phosphorus oxynitride (LiPON) is one of the few solid electrolytes that forms a truly passivating solid electrolyte interphase (SEI) when in contact with metallic lithium. Investigations into the origin of this stability may provide the insights needed to replicate it in the SEIs of alternative solid electrolyte materials. In this study we used in situ lithium plating X-ray photoelectron spectroscopy (XPS) to investigate the formation and evolution of the Li-LiPON SEI for the first time. We show that the SEI is chemically and structurally inhomogeneous, with the fully reduced compounds identified in previous studies (Li2O, Li3N and Li3P) concentrated near the lithium metal side and partially lithiated species, including Lix P, predominant closer to the LiPON side. Li3P and Lix P have recently been suggested as enablers of continuous SEI growth in thiophosphate solid electrolytes. We suggest that the stability of the Li-LiPON SEI is derived from a combination of the LiPON reduction potential (0.68 V vs. Li+/Li), which is below the oxidation potentials of the fully reduced SEI compounds, and the graded structure of the SEI, which ensures that the most reduced species are not in physical or electrical contact with the LiPON layer.

The solid electrolyte interphase that forms on Li6PS5Cl argyrodite solid electrolytes has been reported to continually grow through a diffusion-controlled process, yet this process is not fully understood. Here, we use a combination of electrochemical and X-ray photoelectron spectroscopy techniques to elucidate the role of phosphorus in this growth mechanism. We uncover how Li6PS5Cl can decompose at potentials well above the full reduction to Li3P, forming partially lithiated phosphorus species, LixP. We provide evidence of a gradient of LixP species throughout the solid electrolyte interphase and propose a growth mechanism in which the rate-determining step is the diffusion of lithium through LixP. We predict continuous solid electrolyte interphase growth as long as metallic lithium is present and a LixP percolation pathway exists, highlighting the importance of understanding and engineering solid electrolyte interphase composition and nanostructure in solid-state batteries. We believe that this growth mechanism would apply to any solid electrolyte interphase that can contain partially lithiated phosphorus, or potentially any lithium alloy.

Solid-state lithium metal batteries show substantial promise for overcoming theoretical limitations of Li-ion batteries to enable gravimetric and volumetric energy densities upwards of 500 Wh kg−1 and 1,000 Wh l−1, respectively. While zero-lithium-excess configurations are particularly attractive, inhomogeneous lithium plating on charge results in active lithium loss and a subsequent coulombic efficiency penalty. Excess lithium is therefore currently needed; however, this negatively impacts energy density and thus limiting its thickness is essential. Here we discuss the viability of various technologies for realizing thin lithium films that can be scaled up to the volumes required for gigafactory production. We identify thermal evaporation as a potentially cost-effective route to address these challenges and provide a techno-economic assessment of the projected costs associated with the fabrication of thin, dense lithium metal foils using this process. Finally, we estimate solid-state pack costs made using thermally evaporated lithium foils. Preparing suitable lithium anodes is crucial for high-performance solid-state batteries. This study evaluates methods for producing thin lithium films, emphasizing thermal evaporation as a cost-effective approach while estimating associated pack costs.

Solid state lithium-metal batteries show substantial promise for overcoming theoretical limitations of state-of-the-art Li-ion batteries to enable gravimetric and volumetric energy densities upwards of 500 Wh kg−1 and1000WhL−1, respectively. Whilezerolithiumexcessconfigurationsareparticularlyattractivefrom both an energy density and a manufacturing standpoint, inhomogeneous lithium plating on charge results in active lithium loss and a subsequent coulombic efficiency penalty accrued from Li anode irreversibility, which severely limits cycle life. Excess lithium in the form of a lithium foil is therefore currently needed to extend cycle life to practical values. On the other hand, excess lithium negatively impacts energy density and thus limiting its thickness is essential to derive any practical benefit from adoption of a lithium metal-based battery architecture. Currently, costs associated with the production of these foils at such thicknesses and scale are a major impediment to adoption by industry. Here we discuss the viability of various technologies in the realisation of thin lithium films that can be scaled up to surface areas required for gigafactory production. We identify thermal evaporation as a potentially cost-effective route to address these challenges and provide a technoeconomic assessment of the projected costs associated with the fabrication of thin, dense lithium metal foils using this process. Finally we estimate solid state pack costs made using thermally evaporated Li foils.

Transport and thermodynamic properties are integral parameters to understand, model, and optimize state-of-the-art and next-generation battery electrolytes. The accurate measurement of these properties is experimentally challenging as well as time- and resource-intensive, and consequently, reports are scarce. Their dependence on temperature is explored even less and is commonly limited to a few temperature points. Recently, we introduced an operando Raman gradient analysis (ORGA) tool to extract transport and thermodynamic properties. Here, we expand the capabilities of ORGA by incorporating a temperature-sensitive external reference into the design. With this enhancement, we are able to visualize the local concentration of any Raman-active species in the electrolyte and detect lithium filament nucleation. We demonstrate and validate this new functionality of ORGA via an examination of lithium bis(fluorosulfonyl)imide (LiFSI) in tetraethylene glycol dimethyl ether (G4) as a function of temperature. All transport properties and activation energies are reported, and the effect of temperature is discussed.

Potassium-ion batteries (KIBs) are a promising alternative to conventional lithium-ion batteries with reduced critical mineral dependency but accurate three-electrode characterization is hindered by the lack of a suitable reference electrode. Potassium metal is frequently used as a reference electrode out of necessity, but its high reactivity and unstable potential limit its reliability. Here we investigate the K-In and K-Bi alloy systems, synthesize two-phase In-In4K and Bi-Bi2K alloys, and identify Bi-Bi2K as a promising material owing to its stable potential of 1.07 V vs K+/K. We prove the use of Bi-Bi2K as a reference electrode by cycling graphite in three-electrode cells and demonstrate that it results in significantly less electrolyte reduction than potassium metal, facilitating the accurate electrochemical characterization necessary to accelerate KIB development.

Solid-state lithium-based batteries offer higher energy density than their Li-ion counterparts. Yet they are limited in terms of negative electrode discharge performance and require high stack pressure during operation. To circumvent these issues, we propose the use of lithium-rich magnesium alloys as suitable negative electrodes in combination with Li6PS5Cl solid-state electrolyte. We synthesise and characterise lithium-rich magnesium alloys, quantifying the changes in mechanical properties, transport, and surface chemistry that impact electrochemical performance. Increases in hardness, stiffness, adhesion, and resistance to creep are quantified by nanoindentation as a function of magnesium content. A decrease in diffusivity is quantified with 6Li pulsed field gradient nuclear magnetic resonance, and only a small increase in interfacial impedance due to the presence of magnesium is identified by electrochemical impedance spectroscopy which is correlated with x-ray photoelectron spectroscopy. The addition of magnesium aids contact retention on discharge, but this must be balanced against a decrease in lithium diffusivity. We demonstrate via electrochemical testing of symmetric cells at 2.5 MPa and 30∘C that 1% magnesium content in the alloy increases the stripping capacity compared to both pure lithium and higher magnesium content alloys by balancing these effects.

In situ X-ray photoelectron spectroscopy (XPS) techniques have proven to be powerful tools for the characterisation of the solid electrolyte interphase (SEI) formed between the anode and solid electrolyte (SE) in solid-state batteries. XPS offers access to time and operational condition-resolved information on the SEI’s chemical composition in the absence of destructive sample preparation. Here we present a Virtual Electrode Plating XPS (VEP-XPS) investigation of the composition and stability of the SEI formed between lithium metal and two different solid electrolytes: Li10GeP2S12 (LGPS)andLi1.5Al0.5Ge1.5(PO4)3 (LAGP). LAGP shows slower SEI formation kinetics, as proven by the emergence of a metallic lithium signal, while LGPS exhibits rapid SEI growth that prevents metallic lithium from plating. We attribute these observations to the SEI composition, distribution and physical properties of secondary de- composition products and in particular to the mixed ion-electron conductive Li3P which can be observed in LGPS and not in LAGP

Understanding and characterizing the transport and thermodynamic properties of electrolytes are critical for optimizing battery performance. In this study, we employ operando Raman gradient analysis (ORGA) to characterize the concentration-dependent diffusion coefficient, transference number, ionic conductivity, and thermodynamic factor of potassium bis(fluorosulfonyl)imide (KFSI) in triethyl phosphate (TEP), an ideal model system and one of the most promising K-ion battery electrolytes. ORGA demonstrates results consistent with conventional state-of-the-art methods while proving to be significantly more electrolyte- and time-efficient. Additionally, we probe, for the first time, the concentration-dependent transport and thermodynamic properties of KFSI-TEP, providing key parameters for K-ion battery modeling.

Abstract Potassium-ion batteries (KIBs) are emerging as a promising alternative technology to lithium-ion batteries (LIBs) due to their significantly reduced dependency on critical minerals. KIBs may also present an opportunity for superior fast-charging compared to LIBs, with significantly faster K-ion electrolyte transport properties already demonstrated. In the absence of a viable K-ion electrolyte, a full-cell KIB rate model in commercial cell formats is required to determine the fast-charging potential for KIBs. However, a thorough and accurate characterisation of the critical electrode material properties determining rate performance—the solid state diffusivity and exchange current density—has not yet been conducted for the leading KIB electrode materials. Here, we accurately characterise the effective solid state diffusivities and exchange current densities of the graphite negative electrode and potassium manganese hexacyanoferrate $${{{{\rm{K}}}}}_{2}{{{\rm{Mn}}}}[{{{\rm{Fe}}}}{({{{\rm{CN}}}})}_{6}]$$ K 2 Mn [ Fe ( CN ) 6 ] (KMF) positive electrode, through a combination of optimised material design and state-of-the-art analysis. Finally, we present a Doyle-Fuller-Newman model of a KIB full cell with realistic geometry and loadings, identifying the critical materials properties that limit their rate capability.

In situ X-ray photoelectron spectroscopy (XPS) techniques have proven to be powerful tools for the characterisation of the solid electrolyte interphase (SEI) formed between the anode and solid electrolyte (SE) in solid-state batteries. XPS offers access to time and operational condition-resolved information on the SEI's chemical composition in the absence of destructive sample preparation. Here we present a Virtual Electrode Plating XPS (VEP-XPS) investigation of the composition and stability of the SEI formed between lithium metal and two different solid electrolytes: Li10GeP2S12 (LGPS) and Li1.5Al0.5Ge1.5(PO4)3 (LAGP). LAGP shows slower SEI formation kinetics, as proven by the emergence of a metallic lithium signal, while LGPS exhibits rapid SEI growth that prevents metallic lithium from plating. We attribute these observations to the SEI composition, distribution and physical properties of secondary decomposition products and in particular to the mixed ion-electron conductive Li3P which can be observed in LGPS and not in LAGP.

The lithium price has increased more than sevenfold since the start of 2021 (as of May 2022), reaching unprecedented price levels and demonstrating significant challenges for the security of supply of lithium for lithium-ion batteries (LIBs) 1 . With forecasts showing a potential significant lithium supply deficit by 2030 1 , the case for alternative chemistries based on abundant minerals which can fulfil some LIB functions has never been stronger. Sodium-ion batteries (NIBs) and potassium-ion batteries (KIBs) are emerging as promising complementary technologies to lithium-ion batteries (LIBs) due to the availability and low cost of sodium and potassium, and the minerals comprising their leading electrodes 2 , 3 . KIBs have a significant advantage over NIBs as K + can reversibly intercalate into the graphite electrodes used in LIBs, thus one of the primary components of KIBs is already available at commercial scale, unlike for NIBs 2 . The lower charge density of K + compared to Li + has also been suggested to result in superior ion transport in the electrolyte with KIBs potentially able to deliver superior rate capability and low-temperature performance. However, a comprehensive characterisation of the ionic transport and thermodynamic properties of nonaqueous K-ion electrolytes, critical to the development of KIBs, has not yet been reported. Here, for the first time, we fully characterise the ionic transport and thermodynamic properties of a nonaqueous K-ion electrolyte 4 , potassium bis(fluorosulfonyl)imide (KFSI) in 1,2-dimethoxyethane (DME) and compare it with its Li-ion equivalent (LiFSI in DME) over the concentration range 0.25–2 m. This was realised by developing a K metal preparation protocol enabling sufficient K metal stability for electrolyte characterisation. Our results demonstrate that the K-ion electrolyte indeed displays significantly higher salt diffusion coefficients and transference numbers than the Li-ion electrolyte, evidencing the potential for high-power applications of KIBs. IEA. Global EV Outlook 2022. (IEA, 2022). Dhir, S., Wheeler, S., Capone, I. & Pasta, M. Outlook on K-Ion Batteries. Chem vol. 6 2442–2460 (2020). doi: 10.1016/j.chempr.2020.08.012 Hosaka, T., Kubota, K., Hameed, A. S. & Komaba, S. Research Development on K-Ion Batteries. Chem. Rev. acs.chemrev.9b00463 (2020) doi:10.1021/acs.chemrev.9b00463. Dhir, S., Jagger, B., Maguire, A. & Pasta, M. Characterising the Ionic Transport and Thermodynamic Properties of Potassium-ion Electrolytes. Res. Sq. (2022) doi:10.21203/RS.3.RS-2310020/V1. Figure 1

Non-aqueous potassium-ion batteries (KIBs) represent a promising complementary technology to lithium-ion batteries due to the availability and low cost of potassium. Moreover, the lower charge density of K+ compared to Li+ favours the ion-transport properties in liquid electrolyte solutions, thus, making KIBs potentially capable of improved rate capability and low-temperature performance. However, a comprehensive study of the ionic transport and thermodynamic properties of non-aqueous K-ion electrolyte solutions is not available. Here we report the full characterisation of the ionic transport and thermodynamic properties of a model non-aqueous K-ion electrolyte solution system comprising potassium bis(fluorosulfonyl)imide (KFSI) salt and 1,2-dimethoxyethane (DME) solvent and compare it with its Li-ion equivalent (i.e., LiFSI:DME), over the concentration range 0.25-2 molal. Using tailored K metal electrodes, we demonstrate that KFSI:DME electrolyte solutions show higher salt diffusion coefficients and cation transference numbers than LiFSI:DME solutions. Finally, via Doyle-Fuller-Newman (DFN) simulations, we investigate the K-ion and Li-ion storage properties for K∣∣graphite and Li∣∣graphite cells.

Lithium alloys have the potential to overcome anode-side challenges in solid state batteries. In this work we synthesize and characterize lithium-rich magnesium alloys, quantifying the changes in mechanical properties, transport, and surface chemistry that impact electrochemical performance. Increases in hardness, stiffness,adhesion, and creep are quantified by nanoindentation as a function of magnesium content. A decrease in diffusivity is quantified with chronopotentiometry and6Li PFG-NMR, and an increase in interfacial impedance due to the presence of magnesium is identified with electrochemical impedance spectroscopy which is correlated with XPS data. Throughout, changes in properties are linked to electrochemical performance. This work provides a framework to investigate other lithium alloy systems.