Accurate Internal Monitoring of Batteries by Embedded Piezoelectric Sensors.

Advancements in operando techniques have unraveled the complexities of the Electrode Electrolyte Interface (EEI) in electrochemical energy storage devices. However, each technique has inherent limitations, often necessitating adjustments to experimental conditions, which may compromise accuracy. To address this challenge, a novel battery cell design is introduced, integrating piezoelectric sensors with electrochemical analysis for surface-sensitive operando measurements. This innovative approach aims to overcome conventional limitations by accommodating commercial-grade battery electrodes within a single body, alongside a piezoelectric sensor. This enables operando electrogravimetric measurements to be realized, and the electrochemistry of a battery to be more faithfully reproduced at the sensor level. A proof of concept is carried out on both Li-ion (LiFePO4//Graphite) and Na-ion (Na3V2(PO4)2F3//Hard carbon) systems, utilizing commercially available powder electrodes. In both cases, the results reveal rational mass variations at the sensor level during the cycling of commercial electrodes with mass loadings several orders of magnitude higher, while performing Galvanostatic Charge Discharge (GCD) tests across various C-rates. This innovative design opens up possibilities for a broader application of operando electrogravimetry within the battery community, to enhance the understanding of EEI behavior and facilitate the development of more efficient energy storage solutions.

Disclosing the Interfacial Electrolyte Structure of Na-Insertion Electrode Materials: Origins of the Desolvation Phenomenon.

Among a variety of promising cathode materials for Na-ion batteries, polyanionic Na-insertion compounds are among the preferred choices due to known fast sodium transfer through the ion channels along their framework structures. The most interesting representatives are Na3V2(PO4)3 (NVP) and Na3V2(PO4)2F3 (NVPF), which display large Na+ diffusion coefficients (up to 10-9 m2 s-1 in NVP) and high voltage plateaux (up to 4.2 V for NVPF). While the diffusion in the solid material is well-known to be the rate-limiting step during charging, already being thoroughly discussed in the literature, interfacial transport of sodium ions from the liquid electrolyte toward the electrode was recently shown to be important due to complex ion desolvation effects at the surface. In order to fill the blanks in the description of the electrode/electrolyte interface in Na-ion batteries, we performed a molecular dynamics study of the local nanostructure of a series of carbonate-based sodium electrolytes at the NVP and the NVPF interfaces along with careful examination of the desolvation phenomenon. We show that the tightness of solvent packing at the electrode surface is a major factor determining the height of the free energy barrier associated with desolvation, which explains the differences between the NVP and the NVPF structures. To rationalize and emphasize the remarkable properties of this family of cathode materials, a complementary comparative analysis of the same electrolyte system at the carbon electrode interface was also performed.

Targeting the Right Metrics for an Efficient Solvent-Free Formulation of PEO:LiTFSI:Li6PS5Cl Hybrid Solid Electrolyte.

Hybrid solid electrolytes (HSEs) aim to combine the superior ionic conductivity of inorganic fillers with the scalable process of polymer electrolytes in a unique material for solid-state batteries. Pursuing the goal of optimizing the key metrics (σion ≥ 10-4 S·cm-1 at 25 °C and self-standing property), we successfully developed an HSE based on a modified poly(ethylene oxide):LiTFSI organic matrix, which binds together a high loading (75 wt %) of Li6PS5Cl particles, following a solvent-free route. A rational study of available formulation parameters has enabled us to understand the role of each component in conductivity, mixing, and mechanical cohesion. Especially, the type of activation mechanism (Arrhenius or Vogel-Fulcher-Tammann (VFT)) and its associated energy are proposed as a new metric to unravel the ionic pathway inside the HSE. We showed that a polymer-in-ceramic approach is mandatory to obtain enhanced conduction through the HSE ceramic network, as well as superior mechanical properties, revealed by the tensile test. Probing the compatibility of phases, using electrochemical impedance spectroscopy (EIS) alongside 7Li nuclear magnetic resonance (NMR), reveals the formation of an interphase, the quantity and resistivity of which grow with time and temperature. Finally, electrochemical performances are evaluated by assembling an HSE-based battery, which displays comparable stability as pure ceramic ones but still suffers from higher polarization and thus lower capacity. Altogether, we hope these findings provide valuable knowledge to develop a successful HSE, by placing the optimization of the right metrics at the core of the formulation.

Detangling electrolyte chemical dynamics in lithium sulfur batteries by operando monitoring with optical resonance combs.

Challenges in enabling next-generation rechargeable batteries with lower cost, higher energy density, and longer cycling life stem not only from combining appropriate materials, but from optimally using cell components. One-size-fits-all approaches to operational cycling and monitoring are limited in improving sustainability if they cannot utilize and capture essential chemical dynamics and states of electrodes and electrolytes. Herein we describe and show how the use of tilted fiber Bragg grating (TFBG) sensors to track, via the monitoring of both temperature and refractive index metrics, electrolyte-electrode coupled changes that fundamentally control lithium sulfur batteries. Through quantitative sensing of the sulfur concentration in the electrolyte, we demonstrate that the nucleation pathway and crystallization of Li2S and sulfur govern the cycling performance. With this technique, a critical milestone is achieved, not only towards developing chemistry-wise cells (in terms of smart battery sensing leading to improved safety and health diagnostics), but further towards demonstrating that the coupling of sensing and cycling can revitalize known cell chemistries and break open new directions for their development.

Decoupling the roles of Ni and Co in anionic redox activity of Li-rich NMC cathodes.

Li[LixNiyMnzCo1-x-y-z]O2 (lithium-rich NMCs) are benchmark cathode materials receiving considerable attention due to the abnormally high capacities resulting from their anionic redox chemistry. Although their anionic redox mechanisms have been much investigated, the roles of cationic redox processes remain underexplored, hindering further performance improvement. Here we decoupled the effects of nickel and cobalt in lithium-rich NMCs via a comprehensive study of two typical compounds, Li1.2Ni0.2Mn0.6O2 and Li1.2Co0.4Mn0.4O2. We discovered that both Ni3+/4+ and Co4+, generated during cationic redox processes, are actually intermediate species for triggering oxygen redox through a ligand-to-metal charge-transfer process. However, cobalt is better than nickel in mediating the kinetics of ligand-to-metal charge transfer by favouring more transition metal migration, leading to less cationic redox but more oxygen redox, more O2 release, poorer cycling performance and more severe voltage decay. Our work highlights a compositional optimization pathway for lithium-rich NMCs by deviating from using cobalt to using nickel, providing valuable guidelines for future high-capacity cathode design.

Three-dimensional operando optical imaging of particle and electrolyte heterogeneities inside Li-ion batteries.

Understanding (de)lithiation heterogeneities in battery materials is key to ensure optimal electrochemical performance. However, this remains challenging due to the three-dimensional morphology of electrode particles, the involvement of both solid- and liquid-phase reactants and a range of relevant timescales (seconds to hours). Here we overcome this problem and demonstrate the use of confocal microscopy for the simultaneous three-dimensional operando measurement of lithium-ion dynamics in individual agglomerate particles, and the electrolyte in batteries. We examine two technologically important cathode materials: LixCoO2 and LixNi0.8Mn0.1Co0.1O2. The surface-to-core transport velocity of Li-phase fronts and volume changes are captured as a function of cycling rate. Additionally, we visualize heterogeneities in the bulk and at agglomerate surfaces during cycling, and image microscopic liquid electrolyte concentration gradients. We discover that surface-limited reactions and intra-agglomerate competing rates control (de)lithiation and structural heterogeneities in agglomerate-based electrodes. Importantly, the conditions under which optical imaging can be performed inside the complex environments of battery electrodes are outlined.

Visualizing Water Reduction with Diazonium Grafting on a Glassy Carbon Electrode Surface in a Water-in-Salt Electrolyte.

Aqueous batteries are regaining interest, thanks to the extended working stability voltage window in a highly concentrated electrolyte, namely the water-in-salt electrolyte. A solid-electrolyte interphase (SEI) forms on the negative electrode to prevent water access to the electrode surface. However, we further reported that the formed SEI layer was not uniform on the surface of the glassy carbon electrode. The SEI after passivation will also show degradation during the remaining time of open-circuit voltage (OCV); hence, it calls for a more stable passivation layer to cover the electrode surface. Here, a surface modification was successfully achieved via artificial diazonium grafting using monomers, such as poly(ethylene glycol), α-methoxy, ω-allyloxy (PEG), and allyl glycidyl cyclocarbonate (AGC), on glassy carbon. Physical and electrochemical measurements indicated that the hydrophobic layer composed of PEG or AGC species was well grafted on the electrode surface. The grafted hydrophobic coatings could protect the electrode surface from the water molecules in the bulk electrolyte and then suppress the free water decomposition (from LSV) but still migrating lithium ions. Furthermore, multiple cycles of CV with one-hour resting OCV identified the good stability of the hydrophobic grafting layer, which is a highlight compared with our precious work. These findings relying on the diazonium grafting design may offer a new strategy to construct a stable artificial SEI layer that can well protect the electrode surface from the free water molecule.

Electrochromic Corundum-like Compound Based on the Reversible (De)insertion of Lithium: Li2Ni2W2O9.

The search for efficient anodic electrochromic materials is essential to the development of electrochromic devices, such as smart windows. Magnetron-sputtered lithium-nickel-tungsten mixed oxides are good candidates to tackle this issue; however, they display a complicated microstructure, making it difficult to pinpoint the origin of their electro-optical properties. Herein, by exploring the Li2O-NiO-WO3 phase diagram, we obtained a new phase, Li2Ni2W2O9, that crystallizes in the orthorhombic Pbcn space group. This material can reversibly uptake/release 0.75 Li+ (31 mA h·g-1) when cycled between 2.5 and 5.0 V versus Li+/Li. Moreover, through operando optical microscopy, we show that this new phase is electrochromic, and crucial information can be accessed about the diffusion-limited insertion of lithium at the single-particle scale. This study sets the ground for future syntheses of electrochemically active materials crystallizing in the ramsayite structure and details how the electrochromic properties of battery materials can be used to shed some light on their electrochemical mechanisms.

Decoupling Parasitic Reactions at the Positive Electrode Interfaces in Argyrodite-Based Systems.

Li-ion batteries are the key stones of electric vehicles, but with the emergence of solid-state Li batteries for improving autonomy and fast charging, the need for mastering the solid electrolyte (SE)/electrode material interfaces is crucial. All-solid-state-batteries (ASSBs) suffer from long-term capacity fading with enhanced decomposition reactions. So far, these reactions have not been extensively studied in Li6PS5Cl-based systems because of the complexity of overlapping degradation mechanisms. Herein, those reactions are studied in depth. We investigated their effects under various operating conditions (temperature, C-rate, voltage window), types of active materials, and with or without carbon additives. From combined resistance monitoring and impedance spectroscopy measurements, we could decouple two reactions (NMC/SE and VGCF/SE) with an inflection dependent on the cutoff potential (3.6 or 3.9 V vs Li-In/In are studied) on charge and elucidate their distinct repercussions on cycling performances. The pernicious effect of carbon additives on both the first cycle and power performances is disclosed, so as its long-term effect on capacity retention. As a mean to resolve these issues, we scrutinized the benefits of a coating layer around NMC particles to prevent high potential interactions, minimize the drastic loss of capacity observed with bare NMC, and simply propose to get rid of carbon additives. Altogether, we hope these findings provide insights and novel methodologies for designing innovative performing solid-state batteries.

Correlation between Electrolyte Chemistry and Solid Electrolyte Interphase for Reversible Ca Metal Anodes.

The development of rechargeable Ca metal batteries (RCMBs) is hindered by the Ca2+ passivating solid electrolyte interphases (SEIs). The cation solvation structure dictated by electrolyte chemistry plays a critical role in the SEIs properties. While a relatively weak cation-solvent binding is preferred in Li metal anodes to promote anion-derived SEIs, we demonstrate an enhanced Ca deposition/stripping reversibility under a strong cation-solvent interaction, which is materialized in strongly-solvating solvent and highly-dissociated salt combinations. Such electrolyte formulations benefit the formation of solvent-occupied solvation structure and minimize the anion reduction, resulting in organic-rich/CaF2 -poor SEIs for reversible Ca metal anodes. Furthermore, RCMBs paired with an organic cathode using the optimized electrolytes are demonstrated as a proof-of-concept. Our work reveals the paradigm shift in SEIs design for Ca metal anodes, opening up new opportunities for emerging RCMBs.

Deciphering the Double-Layer Structure and Dynamics on a Model LixMoO3 Interface by Advanced Electrogravimetric Analysis.

A major feature of the electrolyte/electrode interface (EEI) that affects charge storage in lithium-ion batteries is the electrical double layer (EDL), but most of the available experimental approaches for probing its structuration have limitations due to electrical field and redox reaction disturbances, hence explaining why it is frequently overlooked. Herein we show that this is no longer true by using an advanced electrochemical quartz crystal microbalance (EQCM)-based method in the form of ac-electrogravimetry. For proof of concept, we studied the effect of various solvent/salt combinations, differing in their dipole moment and size/weight, respectively, on the structure of the EDL forming at the EEI of LixMoO3. We show that a significant amount of solvated lithium ions and anions contribute to charge compensation at the interface, and by varying the nature of the solvents (cyclic vs noncyclic), we provide a solid experimental proof of the direct relationship between the ions' solvation and solvent polarity. Moreover, we demonstrated a disappearance of the anionic motion in the less polar solvent (DMC) most likely due to plausible formation of contact ion pairs and agglomerates at the EDL level. Altogether, ac-electrogravimetry, when combined with classical EQCM, stands as an elegant and powerful method to experimentally assess the chemical structure and dynamics of the electrical double layer. We hope that the community will start to adopt it to better engineer interfaces of electrochemical energy storage devices.

Fluorinated Materials as Positive Electrodes for Li- and Na-Ion Batteries.

Fluorine is known to be a key element for various components of batteries since current electrolytes rely on Li-ion salts having fluorinated ions and electrode binders are mainly based on fluorinated polymers. Metal fluorides or mixed anion metal fluorides (mainly oxyfluorides) have also gained a substantial interest as active materials for the electrode redox reactions. In this review, metal fluorides for cathodes are considered; they are listed according to the dimensionality of the metal fluoride subnetwork. The synthesis conditions and the crystal structures are described; the electrochemical properties are briefly indicated, and the nature of the electron transport agent is noted. We stress the crucial importance of the elaboration processes to induce the presence of cation disorders, of anion substitutions (mainly F-/O2- or F-/OH-) or vacancies. Finally, we show that an accurate structural characterization is a key step to enable enhanced material performances to overcome several lasting roadblocks, namely the large irreversible capacity and poor energy efficiency that are frequently encountered.

Capturing dynamic ligand-to-metal charge transfer with a long-lived cationic intermediate for anionic redox.

Reversible anionic redox reactions represent a transformational change for creating advanced high-energy-density positive-electrode materials for lithium-ion batteries. The activation mechanism of these reactions is frequently linked to ligand-to-metal charge transfer (LMCT) processes, which have not been fully validated experimentally due to the lack of suitable model materials. Here we show that the activation of anionic redox in cation-disordered rock-salt Li1.17Ti0.58Ni0.25O2 involves a long-lived intermediate Ni3+/4+ species, which can fully evolve to Ni2+ during relaxation. Combining electrochemical analysis and spectroscopic techniques, we quantitatively identified that the reduction of this Ni3+/4+ species goes through a dynamic LMCT process (Ni3+/4+-O2- → Ni2+-On-). Our findings provide experimental validation of previous theoretical hypotheses and help to rationalize several peculiarities associated with anionic redox, such as cationic-anionic redox inversion and voltage hysteresis. This work also provides additional guidance for designing high-capacity electrodes by screening appropriate cationic species for mediating LMCT.

Probing the Electrode-Electrolyte Interface of a Model K-Ion Battery Electrode─The Origin of Rate Capability Discrepancy between Aqueous and Non-Aqueous Electrolytes.

Li-ion batteries are the electrochemical energy storage technology of choice of today's electrical vehicles and grid applications with a growing interest for Na-ion and K-ion systems based on either aqueous or non-aqueous electrolyte for power, cost, and sustainable reasons. The rate capability of alkali-metal-ion batteries is influenced by ion transport properties in the bulk of the electrolyte, as well as by diverse effects occurring at the vicinity of the electrode and electrolyte interface. Therefore, identification of the predominant factor affecting the rate capability of electrodes still remains a challenge and requires suitable experimental and computational methods. Herein, we investigate the mechanistic of the K+ insertion process in the Prussian blue phase, Fe4III[FeII(CN)6]3 in both aqueous and non-aqueous electrolytes, which reveals drastic differences. Through combined electrochemical characterizations, electrochemical-quartz-crystal-microbalance and ac-electrogravimetric analyses, we provide evidences that what matters the most for fast ion transport is the positioning of the partially solvated cations adsorbed at the material surface in aqueous as opposed to non-aqueous electrolytes. We rationalized such findings by molecular dynamics simulations that establish the K+ repartition profile within the electrochemical double layer. A similar trend was earlier reported by our group for the aqueous versus non-aqueous insertion of Li+ into LiFePO4. Such a study unveils the critical but overlooked role of the electrode-electrolyte interface in ruling ion transport and insertion processes. Tailoring this interface structuring via the proper salt-solvent interaction is the key to enabling the best power performances in alkali-metal-ion batteries.

Optical sensors for operando stress monitoring in lithium-based batteries containing solid-state or liquid electrolytes.

The study of chemo-mechanical stress taking place in the electrodes of a battery during cycling is of paramount importance to extend the lifetime of the device. This aspect is particularly relevant for all-solid-state batteries where the stress can be transmitted across the device due to the stiff nature of the solid electrolyte. However, stress monitoring generally relies on sensors located outside of the battery, therefore providing information only at device level and failing to detect local changes. Here, we report a method to investigate the chemo-mechanical stress occurring at both positive and negative electrodes and at the electrode/electrolyte interface during battery operation. To such effect, optical fiber Bragg grating sensors were embedded inside coin and Swagelok cells containing either liquid or solid-state electrolyte. The optical signal was monitored during battery cycling, further translated into stress and correlated with the voltage profile. This work proposes an operando technique for stress monitoring with potential use in cell diagnosis and battery design.

Activation of anionic redox in d0 transition metal chalcogenides by anion doping.

Expanding the chemical space for designing novel anionic redox materials from oxides to sulfides has enabled to better apprehend fundamental aspects dealing with cationic-anionic relative band positioning. Pursuing with chalcogenides, but deviating from cationic substitution, we here present another twist to our band positioning strategy that relies on mixed ligands with the synthesis of the Li2TiS3-xSex solid solution series. Through the series the electrochemical activity displays a bell shape variation that peaks at 260 mAh/g for the composition x = 0.6 with barely no capacity for the x = 0 and x = 3 end members. We show that this capacity results from cumulated anionic (Se2-/Sen-) and (S2-/Sn-) and cationic Ti3+/Ti4+ redox processes and provide evidence for a metal-ligand charge transfer by temperature-driven electron localization. Moreover, DFT calculations reveal that an anionic redox process cannot take place without the dynamic involvement of the transition metal electronic states. These insights can guide the rational synthesis of other Li-rich chalcogenides that are of interest for the development of solid-state batteries.

Correlating ligand-to-metal charge transfer with voltage hysteresis in a Li-rich rock-salt compound exhibiting anionic redox.

Anionic redox is a double-edged sword for Li-ion cathodes because it offers a transformational increase in energy density that is also negated by several detrimental drawbacks to its practical implementation. Among them, voltage hysteresis is the most troublesome because its origin is still unclear and under debate. Herein, we tackle this issue by designing a prototypical Li-rich cation-disordered rock-salt compound Li1.17Ti0.33Fe0.5O2 that shows anionic redox activity and exceptionally large voltage hysteresis while exhibiting a partially reversible Fe migration between octahedral and tetrahedral sites. Through combined in situ and ex situ spectroscopic techniques, we demonstrate the existence of a non-equilibrium (adiabatic) redox pathway enlisting Fe3+/Fe4+ and O redox as opposed to the equilibrium (non-adiabatic) redox pathway involving sole O redox. We further show that the charge transfer from O(2p) lone pair states to Fe(3d) states involving sluggish structural distortion is responsible for voltage hysteresis. This study provides a general understanding of various voltage hysteresis signatures in the large family of Li-rich rock-salt compounds.

Extending insertion electrochemistry to soluble layered halides with superconcentrated electrolytes.

Insertion compounds provide the fundamental basis of today's commercialized Li-ion batteries. Throughout history, intense research has focused on the design of stellar electrodes mainly relying on layered oxides or sulfides, and leaving aside the corresponding halides because of solubility issues. This is no longer true. In this work, we show the feasibility of reversibly intercalating Li+ electrochemically into VX3 compounds (X = Cl, Br, I) via the use of superconcentrated electrolytes (5 M LiFSI in dimethyl carbonate), hence opening access to a family of LixVX3 phases. Moreover, through an electrolyte engineering approach, we unambiguously prove that the positive attribute of superconcentrated electrolytes against the solubility of inorganic compounds is rooted in a thermodynamic rather than a kinetic effect. The mechanism and corresponding impact of our findings enrich the fundamental understanding of superconcentrated electrolytes and constitute a crucial step in the design of novel insertion compounds with tunable properties for a wide range of applications including Li-ion batteries and beyond.