Chemical Origin of in Situ Carbon Dioxide Outgassing from a Cation-Disordered Rock Salt Cathode.

In situ carbon dioxide (CO2) outgassing is a common phenomenon in lithium-ion batteries (LiBs), primarily due to parasitic side reactions at the cathode-electrolyte interface. However, little is known about the chemical origins of the in situ CO2 released from emerging Li-excess cation-disordered rock salt (DRX) cathodes. In this study, we selectively labeled various carbon sources with 13C in cathodes containing a representative DRX material, Li1.2Mn0.4Ti0.4O2 (LMTO), and performed differential electrochemical mass spectrometry (DEMS) during galvanostatic cycling in a carbonate-based electrolyte. When charging LMTO cathodes, electrolyte solvent (EC) decomposition is the dominant source of the CO2 outgassing. The amount of EC-originated CO2 is strongly correlated with the total surface area of carbon black in the electrode, revealing the critical role of electron-conducting carbon additives in the electrolyte degradation mechanisms. In addition, unusual bimodal CO2 evolution during the first cycle is found to originate from carbon black oxidation. Overall, the underlying chemical origin of in situ CO2 release during battery cycling is highly voltage- and cycle-dependent. This work further provides insights into improving the stability of DRX cathodes in LiBs and is envisioned to help guide future relevant material design to mitigate parasitic reactions in DRX-based batteries.

Lithium-Ion Transport and Exchange between Phases in a Concentrated Liquid Electrolyte Containing Lithium-Ion-Conducting Inorganic Particles.

Understanding Li+ transport in organic-inorganic hybrid electrolytes, where Li+ has to lose its organic solvation shell to enter and transport through the inorganic phase, is crucial to the design of high-performance batteries. As a model system, we investigate a range of Li+-conducting particles suspended in a concentrated electrolyte. We show that large Li1.3Al0.3Ti1.7P3O12 and Li6PS5Cl particles can enhance the overall conductivity of the electrolyte. When studying impedance using a cell with a large cell constant, the Nyquist plot shows two semicircles: a high-frequency semicircle related to ion transport in the bulk of both phases and a medium-frequency semicircle attributed to Li+ transporting through the particle/liquid interfaces. Contrary to the high-frequency resistance, the medium-frequency resistance increases with particle content and shows a higher activation energy. Furthermore, we show that small particles, requiring Li+ to overcome particle/liquid interfaces more frequently, are less effective in facilitating Li+ transport. Overall, this study provides a straightforward approach to study the Li+ transport behavior in hybrid electrolytes.

An Experimental Approach to Assess Fluorine Incorporation into Disordered Rock Salt Oxide Cathodes.

Disordered rock salt oxides (DRX) have shown great promise as high-energy-density and sustainable Li-ion cathodes. While partial substitution of oxygen for fluorine in the rock salt framework has been related to increased capacity, lower charge-discharge hysteresis, and longer cycle life, fluorination is poorly characterized and controlled. This work presents a multistep method aimed at assessing fluorine incorporation into DRX cathodes, a challenging task due to the difficulty in distinguishing oxygen from fluorine using X-ray and neutron-based techniques and the presence of partially amorphous impurities in all DRX samples. This method is applied to "Li1.25Mn0.25Ti0.5O1.75F0.25" prepared by solid-state synthesis and reveals that the presence of LiF impurities in the sample and F content in the DRX phase is well below the target. Those results are used for compositional optimization, and a synthesis product with drastically reduced LiF content and a DRX stoichiometry close to the new target composition (Li1.25Mn0.225Ti0.525O1.85F0.15) is obtained, demonstrating the effectiveness of the strategy. The analytical method is also applied to "Li1.33Mn0.33Ti0.33O1.33F0.66" obtained via mechanochemical synthesis, and the results confirm that much higher fluorination levels can be achieved via ball-milling. Finally, a simple and rapid water washing procedure is developed to reduce the impurity content in as-prepared DRX samples: this procedure results in a ca. 10% increase in initial discharge capacity and a ca. 11% increase in capacity retention after 25 cycles for Li1.25Mn0.25Ti0.50O1.75F0.25. Overall, this work establishes new analytical and material processing methods that enable the development of more robust design rules for high-energy-density DRX cathodes.

Ion Transport in (Localized) High Concentration Electrolytes for Li-Based Batteries.

High concentration electrolytes (HCEs) and localized high concentration electrolytes (LHCEs) have emerged as promising candidates to enable higher energy density Li-ion batteries due to their advantageous interfacial properties that result from their unique solvent structures. Using electrophoretic NMR and electrochemical techniques, we characterize and report full transport properties, including the lithium transference numbers (t+) for electrolytes ranging from the conventional ∼1 M to HCE regimes as well as for LHCE systems. We find that compared to conventional electrolytes, t+ increases for HCEs; however the addition of diluents to LHCEs significantly decreases t+. Viscosity effects alone cannot explain this behavior. Using Onsager transport coefficients calculated from our experiments, we demonstrate that there is more positively correlated cation-cation motion in HCEs as well as fast cation-anion ligand exchange consistent with a concerted ion-hopping mechanism. The addition of diluents to LHCEs results in more anticorrelated motion indicating a disruption of concerted cation-hopping leading to low t+ in LHCEs.

A Critical Analysis of Chemical and Electrochemical Oxidation Mechanisms in Li-Ion Batteries.

Electrolyte decomposition limits the lifetime of commercial lithium-ion batteries (LIBs) and slows the adoption of next-generation energy storage technologies. A fundamental understanding of electrolyte degradation is critical to rationally design stable and energy-dense LIBs. To date, most explanations for electrolyte decomposition at LIB positive electrodes have relied on ethylene carbonate (EC) being chemically oxidized by evolved singlet oxygen (1O2) or electrochemically oxidized. In this work, we apply density functional theory to assess the feasibility of these mechanisms. We find that electrochemical oxidation is unfavorable at any potential reached during normal LIB operation, and we predict that previously reported reactions between the EC and 1O2 are kinetically limited at room temperature. Our calculations suggest an alternative mechanism in which EC reacts with superoxide (O2-) and/or peroxide (O22-) anions. This work provides a new perspective on LIB electrolyte decomposition and motivates further studies to understand the reactivity at positive electrodes.

Nonintrusive thermal-wave sensor for operando quantification of degradation in commercial batteries.

Monitoring real-world battery degradation is crucial for the widespread application of batteries in different scenarios. However, acquiring quantitative degradation information in operating commercial cells is challenging due to the complex, embedded, and/or qualitative nature of most existing sensing techniques. This process is essentially limited by the type of signals used for detection. Here, we report the use of effective battery thermal conductivity (keff) as a quantitative indicator of battery degradation by leveraging the strong dependence of keff on battery-structure changes. A measurement scheme based on attachable thermal-wave sensors is developed for non-embedded detection and quantitative assessment. A proof-of-concept study of battery degradation during fast charging demonstrates that the amount of lithium plating and electrolyte consumption associated with the side reactions on the graphite anode and deposited lithium can be quantitatively distinguished using our method. Therefore, this work opens the door to the quantitative evaluation of battery degradation using simple non-embedded thermal-wave sensors.

Enhanced Electrochemical Performance of Disordered Rocksalt Cathodes Enabled by a Graphite Conductive Additive.

Cobalt-free cation-disordered rocksalt (DRX) cathodes are a promising class of materials for next-generation Li-ion batteries. Although they have high theoretical specific capacities (>300 mA h/g) and moderate operating voltages (∼3.5 V vs Li/Li+), DRX cathodes typically require a high carbon content (up to 30 wt %) to fully utilize the active material which has a detrimental impact on cell-level energy density. To assess pathways to reduce the electrode's carbon content, the present study investigates how the carbon's microstructure and loading (10-20 wt %) influence the performance of DRX cathodes with the nominal composition Li1.2Mn0.5Ti0.3O1.9F0.1. While electrodes prepared with conventional disordered carbon additives (C65 and ketjenblack) exhibit rapid capacity fade due to an unstable cathode/electrolyte interface, DRX cathodes containing 10 wt % graphite show superior cycling performance (e.g., reversible capacities ∼260 mA h/g with 85% capacity retention after 50 cycles) and rate capability (∼135 mA h/g at 1000 mA/g). A suite of characterization tools was employed to evaluate the performance differences among these composite electrodes. Overall, these results indicate that the superior performance of the graphite-based cathodes is largely attributed to the: (i) formation of a uniform graphitic coating on DRX particles which protects the surface from parasitic reactions at high states of charge and (ii) homogeneous dispersion of the active material and carbon throughout the composite cathode which provides a robust electronically conductive network that can withstand repeated charge-discharge cycles. Overall, this study provides key scientific insights on how the carbon microstructure and electrode processing influence the performance of DRX cathodes. Based on these results, exploration of alternative routes to apply graphitic coatings is recommended to further optimize the material performance.

Ion correlation and negative lithium transference in polyelectrolyte solutions.

Polyelectrolyte solutions (PESs) recently have been proposed as high conductivity, high lithium transference number (t+) electrolytes where the majority of the ionic current is carried by the electrochemically active Li-ion. While PESs are intuitively appealing because anchoring the anion to a polymer backbone selectively slows down anionic motion and therefore increases t+, increasing the anion charge will act as a competing effect, decreasing t+. In this work we directly measure ion mobilities in a model non-aqueous polyelectrolyte solution using electrophoretic Nuclear Magnetic Resonance Spectroscopy (eNMR) to probe these competing effects. While previous studies that rely on ideal assumptions predict that PESs will have higher t+ than monomeric solutions, we demonstrate that below the entanglement limit, both conductivity and t+ decrease with increasing degree of polymerization. For polyanions of 10 or more repeat units, at 0.5 m Li+ we directly observe Li+ move in the "wrong direction" in an electric field, evidence of a negative transference number due to correlated motion through ion clustering. This is the first experimental observation of negative transference in a non-aqueous polyelectrolyte solution. We also demonstrate that t+ increases with increasing Li+ concentration. Using Onsager transport coefficients calculated from experimental data, and insights from previously published molecular dynamics studies we demonstrate that despite selectively slowing anion motion using polyanions, distinct anion-anion correlation through the polymer backbone and cation-anion correlation through ion aggregates reduce the t+ in non-entangled PESs. This leads us to conclude that short-chained polyelectrolyte solutions are not viable high transference number electrolytes. These results emphasize the importance of understanding the effects of ion-correlations when designing new concentrated electrolytes for improved battery performance.

Tuning Bulk Redox and Altering Interfacial Reactivity in Highly Fluorinated Cation-Disordered Rocksalt Cathodes.

Lithium-excess, cation-disordered rocksalt (DRX) materials have been subject to intense scrutiny and development in recent years as potential cathode materials for Li-ion batteries. Despite their compositional flexibility and high initial capacity, they suffer from poorly understood parasitic degradation reactions at the cathode-electrolyte interface. These interfacial degradation reactions deteriorate both the DRX material and electrolyte, ultimately leading to capacity fade and voltage hysteresis during cycling. In this work, differential electrochemical mass spectrometry (DEMS) and titration mass spectrometry are combined to quantify the extent of bulk redox and surface degradation reactions for a set of Mn2+/4+-based DRX oxyfluorides during initial cycling with a high-voltage charging cutoff (4.8 V vs Li/Li+). Increasing the fluorine content from 7.5 to 33.75% is shown to diminish oxygen redox and suppresses high-voltage O2 evolution from the DRX surface. Additionally, electrolyte degradation processes resulting in the formation of both gaseous species and electrolyte-soluble protic species are observed. Subsequently, DEMS is paired with a fluoride-scavenging additive to demonstrate that increasing fluorine content leads to increased dissolution of fluorine from the DRX material into the electrolyte. Finally, a suite of ex situ spectroscopy techniques (X-ray photoelectron spectroscopy, inductively coupled plasma optical emission spectroscopy, and solid-state nuclear magnetic resonance spectroscopy) are employed to study the change in DRX composition during charging, revealing the dissolution of manganese and fluorine from the DRX material at high voltages. This work provides insight into the degradation processes occurring at the DRX-electrolyte interface and points toward potential routes of interfacial stabilization.

Particle Surface Cracking Is Correlated with Gas Evolution in High-Ni Li-Ion Cathode Materials.

High-Ni layered oxide cathode materials (LiNixTM(1-x)O2, where x > 0.8) are of great interest because they offer increased capacity compared to current commercial materials within a narrow voltage range. However, recent studies have shown that these materials in their current form suffer from capacity fading when an upper cutoff voltage above 4.3 V vs Li/Li+ is used. While many studies have focused on the H2 → H3 transition as the primary cause of capacity fading, gas evolution studies show that degradation processes cannot be attributed to the H2 → H3 transition alone. In this work, differential electrochemical mass spectrometry (DEMS) is combined with titration mass spectrometry (TiMS) to measure gases evolved in a lithium half-cell during cycling as well as surface species which evolve gas upon addition of strong acid to an extracted cathode. Along with qualitative observations of particle cracking by scanning electron microscopy (SEM), these results reveal correlations between particle cracking, electrolyte reactivity, and carbonate oxidation and deposition on the cathode surface during the first charge of high-Ni cathode materials.

Impact of Platinum Primary Particle Loading on Fuel Cell Performance: Insights from Catalyst/Ionomer Ink Interactions.

A variety of electrochemical energy conversion technologies, including fuel cells, rely on solution-processing techniques (via inks) to form their catalyst layers (CLs). The CLs are heterogeneous structures, often with uneven ion-conducting polymer (ionomer) coverage and underutilized catalysts. Various platinum-supported-on-carbon colloidal catalyst particles are used, but little is known about how or why changing the primary particle loading (PPL, or the weight fraction of platinum of the carbon-platinum catalyst particles) impacts performance. By investigating the CL gas-transport resistance and zeta (ζ)-potentials of the corresponding inks as a function of PPL, a direct correlation between the CL high current density performance and ink ζ-potential is observed. This correlation stems from likely changes in ionomer distributions and catalyst-particle agglomeration as a function of PPL, as revealed by pH, ζ-potential, and impedance measurements. These findings are critical to unraveling the ionomer distribution heterogeneity in ink-based CLs and enabling enhanced Pt utilization and improved device performance for fuel cells and related electrochemical devices.

Single-Crystal LiNix Mny Co1- x - y O2 Cathodes for Extreme Fast Charging.

Ni-rich layered LiNix Mny Co1- x - y O2 (NMCs, x ≥ 0.8) are poised to be the dominating cathode materials for lithium-ion batteries for the foreseeable future. Conventional polycrystalline NMCs, however, suffer from severe cracking along the grain boundaries of primary particles and capacity loss under high charge and/or discharge rates, hindering their implementation in fast-charging electric vehicular (EV) batteries. Single-crystal (SC) NMCs are attractive alternatives as they eliminate intergranular cracking and allow for grain-level surface optimization for fast Li transport. In the present study, the authors report synthetic approaches to produce SC LiNi0.8 Co0.1 Mn0.1 O2 (NMC811) samples with different morphologies: Oct-SC811 with predominating (012)-family surface and Poly-SC811 with predominating (104)-family surface. Poly-SC811, representing the first experimentally synthesized NMC811 single crystals with (104) surface, delivers superior performance even at the ultra-high rate of 6 C. Through detailed X-ray analysis and electron microscopy characterization, it is shown that the enhanced performance originates from better chemical and structural stabilities, faster Li+ diffusion kinetics, suppressed side reactions with electrolyte, and excellent cracking resistance. These insights provide important design guidelines in the future development of fast-charging NMC-type cathode materials.

Interplay between Cation and Anion Redox in Ni-Based Disordered Rocksalt Cathodes.

The reversibility of the redox processes plays a crucial role in the electrochemical performance of lithium-excess cation-disordered rocksalt (DRX) cathodes. Here, we report a comprehensive analysis of the redox reactions in a representative Ni-based DRX cathode. The aim of this work is to elucidate the roles of multiple cations and anions in the charge compensation mechanism that is ultimately linked to the electrochemical performance of Ni-based DRX cathode. The low-voltage reduction reaction results in the low energy efficiency and strong voltage hysteresis. Our data reveal that the Mo migration between octahedral and tetrahedral sites enhances the O reduction potential, thus offering a potential strategy to improve energy efficiency. This work highlights the important role that the high-valence transition metal plays in the redox chemistry and provides useful insights into the potential pathway to further address the challenges in Ni-based DRX systems.

Layered-rocksalt intergrown cathode for high-capacity zero-strain battery operation.

The dependence on lithium-ion batteries leads to a pressing demand for advanced cathode materials. We demonstrate a new concept of layered-rocksalt intergrown structure that harnesses the combined figures of merit from each phase, including high capacity of layered and rocksalt phases, good kinetics of layered oxide and structural advantage of rocksalt. Based on this concept, lithium nickel ruthenium oxide of a main layered structure (R[Formula: see text]m) with intergrown rocksalt (Fm[Formula: see text]m) is developed, which delivers a high capacity with good rate performance. The interwoven rocksalt structure successfully prevents the anisotropic structural change that is typical for layered oxide, enabling a nearly zero-strain operation upon high-capacity cycling. Furthermore, a design principle is successfully extrapolated and experimentally verified in a series of compositions. Here, we show the success of such layered-rocksalt intergrown structure exemplifies a new battery electrode design concept and opens up a vast space of compositions to develop high-performance intergrown cathode materials.

Cation-disordered rocksalt-type high-entropy cathodes for Li-ion batteries.

High-entropy (HE) ceramics, by analogy with HE metallic alloys, are an emerging class of solid solutions composed of a large number of species. These materials offer the benefit of large compositional flexibility and can be used in a wide variety of applications, including thermoelectrics, catalysts, superionic conductors and battery electrodes. We show here that the HE concept can lead to very substantial improvements in performance in battery cathodes. Among lithium-ion cathodes, cation-disordered rocksalt (DRX)-type materials are an ideal platform within which to design HE materials because of their demonstrated chemical flexibility. By comparing a group of DRX cathodes containing two, four or six transition metal (TM) species, we show that short-range order systematically decreases, whereas energy density and rate capability systematically increase, as more TM cation species are mixed together, despite the total metal content remaining fixed. A DRX cathode with six TM species achieves 307 mAh g-1 (955 Wh kg-1) at a low rate (20 mA g-1), and retains more than 170 mAh g-1 when cycling at a high rate of 2,000 mA g-1. To facilitate further design in this HE DRX space, we also present a compatibility analysis of 23 different TM ions, and successfully synthesize a phase-pure HE DRX compound containing 12 TM species as a proof of concept.

Reduction of carbon dioxide at a plasmonically active copper-silver cathode.

Electrochemically deposited copper nanostructures were coated with silver to create a plasmonically active cathode for carbon dioxide (CO2) reduction. Illumination with 365 nm light, close to the peak plasmon resonance of silver, selectively enhanced 5 of the 14 typically observed copper CO2 reduction products while simultaneously suppressing hydrogen evolution. At low overpotentials, carbon monoxide was promoted in the light and at high overpotentials ethylene, methane, formate, and allyl alcohol were enhanced upon illumination; generally C1 products and C2/C3 products containing a double carbon bond were selectively promoted under illumination. Temperature-dependent product analysis in the dark showed that local heating is not the cause of these selectivity changes. While the exact plasmonic mechanism is still unknown, these results demonstrate the potential for enhancing CO2 reduction selectivity at copper electrodes using plasmonics.

Role of Redox-Inactive Transition-Metals in the Behavior of Cation-Disordered Rocksalt Cathodes.

Owing to the capacity boost from oxygen redox activities, Li-rich cation-disordered rocksalts (LRCDRS) represent a new class of promising high-energy Li-ion battery cathode materials. Redox-inactive transition-metal (TM) cations, typically d0 TM, are essential in the formation of rocksalt phases, however, their role in electrochemical performance and cathode stability is largely unknown. In the present study, the effect of two d0 TM (Nb5+ and Ti4+ ) is systematically compared on the redox chemistry of Mn-based model LRCDRS cathodes, namely Li1.3 Nb0.3 Mn0.4 O2 (LNMO), Li1.25 Nb0.15 Ti0.2 Mn0.4 O2 (LNTMO), and Li1.2 Ti0.4 Mn0.4 O2 (LTMO). Although electrochemically inactive, d0 TM serves as a modulator for oxygen redox, with Nb5+ significantly enhancing initial charge storage contribution from oxygen redox. Further studies using differential electrochemical mass spectroscopy and resonant inelastic X-ray scattering reveal that Ti4+ is better in stabilizing the oxidized oxygen anions (On - , 0 < n < 2), leading to a more reversible O redox process with less oxygen gas release. As a result, much improved chemical, structural and cycling stabilities are achieved on LTMO. Detailed evaluation on the effect of d0 TM on degradation mechanism further suggests that proper design of redox-inactive TM cations provides an important avenue to balanced capacity and stability in this newer class of cathode materials.

In Situ ATR-SEIRAS of Carbon Dioxide Reduction at a Plasmonic Silver Cathode.

Illumination of a voltage-biased plasmonic Ag cathode during CO2 reduction results in a suppression of the H2 evolution reaction while enhancing CO2 reduction. This effect has been shown to be photonic rather than thermal, but the exact plasmonic mechanism is unknown. Here, we conduct an in situ ATR-SEIRAS (attenuated total reflectance-surface-enhanced infrared absorption spectroscopy) study of a sputtered thin film Ag cathode on a Ge ATR crystal in CO2-saturated 0.1 M KHCO3 over a range of potentials under both dark and illuminated (365 nm, 125 mW cm-2) conditions to elucidate the nature of this plasmonic enhancement. We find that the onset potential of CO2 reduction to adsorbed CO on the Ag surface is -0.25 VRHE and is identical in the light and the dark. As the production of gaseous CO is detected in the light near this onset potential but is not observed in the dark until -0.5 VRHE, we conclude that the light must be assisting the desorption of CO from the surface. Furthermore, the HCO3- wavenumber and peak area increase immediately upon illumination, precluding a thermal effect. We propose that the enhanced local electric field that results from the localized surface plasmon resonance (LSPR) is strengthening the HCO3- bond, further increasing the local pH. This would account for the decrease in H2 formation and increase the CO2 reduction products in the light.

Definition of Redox Centers in Reactions of Lithium Intercalation in Li3RuO4 Polymorphs.

Cathodes based on layered LiMO2 are the limiting components in the path toward Li-ion batteries with energy densities suitable for electric vehicles. Introducing an overstoichiometry of Li increases storage capacity beyond a conventional mechanism of formal transition metal redox. However, the role and fate of the oxide ligands in such intriguing additional capacity remain unclear. This reactivity was predicted in Li3RuO4, making it a valuable model system. A comprehensive analysis of the redox activity of both Ru and O under different electrochemical conditions was carried out, and the effect of Li/Ru ordering was evaluated. Li3RuO4 displays highly reversible Li intercalation to Li4RuO4 below 2.5 V vs Li+/Li0, with conventional reactivity through the formal Ru5+-Ru4+ couple. In turn, it can also undergo anodic Li extraction at 3.9 V, which involves O states to a much greater extent than Ru. This reaction competes with side processes such as electrolyte decomposition and, to a much lesser extent, oxygen loss. Although the associated capacity is reversible, reintercalation unlocks a different, conventional pathway also involving the formal Ru5+-Ru4+ couple despite operating above 2.5 V, leading to chemical hysteresis. This new pathway is both chemically and electrochemically reversible in subsequent cycles. This work exemplifies both the challenge of stabilizing highly depleted O states, even with 4d metals, and the ability of solids to access the same redox couple at two very different potential windows depending on the underlying structural changes. It highlights the importance of properly defining the covalency of oxides when defining charge compensation in view of the design of materials with high capacity for Li storage.