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Next‐generation Li‐ion batteries are expected to exhibit superior energy and power density, along with extended cycle life. Ni‐rich high‐capacity layered nickel manganese cobalt oxide electrode materials (NMC) hold promise in achieving these objectives, despite facing challenges such as capacity fade due to various degradation modes. Crack formation within NMC‐based cathode secondary particles, leading to parasitic reactions and the formation of inactive crystal structures, is a critical degradation mechanism. Mechanical and chemical degradation further deteriorate capacity and lifetime. To mitigate these issues, an artificial cathode electrolyte interphase can be applied to the active material before battery cycling. While atomic layer deposition (ALD) has been extensively explored for active material coatings, molecular layer deposition (MLD) offers a complementary approach. When combined with ALD, MLD enables the deposition of flexible hybrid coatings that can accommodate electrode material volume changes during battery operation. This study focuses on depositing TiO2‐titanium terephthalate thin films on a LiNi0.8Mn0.1Co0.1O2 electrode via ALD‐MLD. The electrochemical evaluation demonstrates favorable lithium‐ion kinetics and reduced electrolyte decomposition. Overall, the films deposited through ALD‐MLD exhibit promising features as flexible and protective coatings for high‐energy lithium‐ion battery electrodes, offering potential contributions to the enhancement of advanced battery technologies and supporting the growth of the EV and stationary battery industries. [ABSTRACT FROM AUTHOR]

Alleviating the decomposition of the electrolyte is of great significance to improving the cycle stability of cathodes, especially for LiCoO2 (LCO), its volumetric energy density can be effectively promoted by increasing the charge cutoff voltage to 4.6 V, thereby supporting the large‐scale application of clean energy. However, the rapid decomposition of the electrolyte under 4.6 V conditions not only loses the transport carrier for lithium ion, but also produces HF and insulators that destroy the interface of LCO and increase impedance. In this work, the decomposition of electrolyte is effectively suppressed by changing the adsorption force between LCO interface and EC. Density functional theory illustrates the LCO coated with lower electronegativity elements has a weaker adsorption force with the electrolyte, the adsorption energy between LCO@Mg and EC (0.49 eV) is weaker than that of LCO@Ti (0.73 eV). Meanwhile, based on the results of time of flight secondary ion mass spectrometry, conductivity‐atomic force microscopy, in situ differential electrochemical mass spectrometry, soft X‐ray absorption spectroscopy, and nuclear magnetic resonance, as the adsorption force increases, the electrolyte decomposes more seriously. This work provides a new perspective on the interaction between electrolyte and the interface of cathode and further improves the understanding of electrolyte decomposition. [ABSTRACT FROM AUTHOR]

Abstract Alleviating the decomposition of the electrolyte is of great significance to improving the cycle stability of cathodes, especially for LiCoO2 (LCO), its volumetric energy density can be effectively promoted by increasing the charge cutoff voltage to 4.6 V, thereby supporting the large‐scale application of clean energy. However, the rapid decomposition of the electrolyte under 4.6 V conditions not only loses the transport carrier for lithium ion, but also produces HF and insulators that destroy the interface of LCO and increase impedance. In this work, the decomposition of electrolyte is effectively suppressed by changing the adsorption force between LCO interface and EC. Density functional theory illustrates the LCO coated with lower electronegativity elements has a weaker adsorption force with the electrolyte, the adsorption energy between LCO@Mg and EC (0.49 eV) is weaker than that of LCO@Ti (0.73 eV). Meanwhile, based on the results of time of flight secondary ion mass spectrometry, conductivity‐atomic force microscopy, in situ differential electrochemical mass spectrometry, soft X‐ray absorption spectroscopy, and nuclear magnetic resonance, as the adsorption force increases, the electrolyte decomposes more seriously. This work provides a new perspective on the interaction between electrolyte and the interface of cathode and further improves the understanding of electrolyte decomposition.

This study explores how fluoroethylene carbonate (FEC) influences the solid electrolyte interface (SEI) layer formation during battery cycling process. FEC improves SEI properties, producing a uniform, chemically stable layer enriched with lithium fluoride. This enhances mechanical resilience and electrochemical stability. FEC also suppresses electrolyte deformation and decomposition, maintaining its initial state. The findings highlight the significance and comprehension of electrolyte additives, offering an electrolyte research pathway for improving Li-ion battery performance and durability.

Non-aqueous electrolyte liquids such as carbonate solvents have been widely employed in the commercial lithium-ion batteries and in the development of next-generation rechargeable batteries. The decomposition products of the organic electrolyte and additive molecules contribute to the formation of solid electrolyte interphases (SEIs) on the electrode surface, which have key impacts on battery's electrochemical performance. The rational engineering of electrolyte systems demands precise understanding of the electrochemical reaction pathways as well as the decomposition products of the electrolytes. Mass spectrometry (MS) is a well-established molecular analytical approach that can provide critical information for unambiguous structure assignment based on its mass-resolving power. In recent years, the application of MS for battery research has been expanding rapidly, providing valuable insights about the chemical species generated during battery operation and electrolyte evolution. This review aims to summarize the recent advances of MS technique-based analysis of electrolyte decomposition products as well as SEIs, and thus to demonstrate the high utility of MS methods for characterization of battery systems.

[Display omitted] • FEC builds SEI layer with Li 2 CO 3 and LiF on electrode, whether graphite or silicon. • The durable SEI layer enhances capacity and effectively reduces volume expansion. • FEC prevents the chemical conversion and reductive decomposition of electrolyte. • FEC inhibits dicarboxylate formation, thereby preserving electrolyte performance. This study explores how fluoroethylene carbonate (FEC) influences the solid electrolyte interface (SEI) layer formation during battery cycling process. FEC improves SEI properties, producing a uniform, chemically stable layer enriched with lithium fluoride. This enhances mechanical resilience and electrochemical stability. FEC also suppresses electrolyte deformation and decomposition, maintaining its initial state. The findings highlight the significance and comprehension of electrolyte additives, offering an electrolyte research pathway for improving Li-ion battery performance and durability. [ABSTRACT FROM AUTHOR]

Analysis of the performance evolution and failure mechanisms of commercial Li-ion batteries is crucial for improving testing methods, accurately modeling battery performance, and ensuring safe battery operation. Here, we present the results of a 2-year aging study conducted on commercial large-format LiNiMnCoO 2 -graphite pouch cells. Loss of lithium inventory (LLI) and loss of positive electrode active material (LAM PE) are shown to dominate capacity fade, as quantified by differential voltage-capacity analysis; only a small amount of LAM PE was measured in extracted electrode material, indicating that LAM PE in full cells was due to electrode dry-out. Resistance evolution, observed by direct current pulses and electrochemical impedance spectroscopy analyzed using the distribution of relaxation times, shows complex trends. Particle cracking and electrode expansion is theorized to cause most changes to resistance. Post-mortem measurements reveal a 10% increase in electrode stack thickness and substantial gas generation, with lithium plating observed in extreme cycling conditions, causing large resistance increases. Circumstantial evidence for self-discharge via redox shuttle, which decomposes the electrolyte, is shown. Overall, electrolyte stability was determined to be the limiting factor for cell lifetime. The impacts of many degradation mechanisms on diagnostic signals substantially overlap, making it challenging to monitor cell health and safety. [Display omitted] • 2-year calendar and cycle aging of commercial large-format lithium-ion pouch cells. • Measurement and analysis of voltage-capacity curves, d.c. pulse, and a.c. impedance. • Aged cells show microstructural damage, thickness increase, and gas generation. • Performance loss driven by SEI growth, failure driven by electrolyte decomposition. • Observed failures include burst pouches and lithium plating induced short-circuit. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • In operando EPR spectroscopy is utilized to monitor redox processes in LCO. • In the pristine battery, Co2+ due to electrolyte decomposition was found. • A pure oxygen redox mechanism is found for standard battery cycling. • Cobalt remains redox inactive during charging and discharging. Abstract Electron paramagnetic resonance (EPR) spectroscopy was utilized in operando to analyze redox mechanisms in lithium cobalt oxide (LCO) cathode material during battery cycling. In the pristine battery, a considerable amount of paramagnetic Co2+ was detected that was formed during initial contact of the electrolyte with the LCO. The Co2+ got oxidized during the formation cycle. The absence of an EPR signal during subsequent battery cycles, irrespective of the state-of-charge, indicates that no EPR active Co4+ is formed. This suggests the presence of an anionic oxygen redox mechanism in the active material. [ABSTRACT FROM AUTHOR]

In lithium-ion batteries with carbonate electrolytes, the formation of lithium alkoxides at the anode impairs the electrochemical performance and the cycle life of the cells through destabilisation of the cathode–electrolyte interface. To fully understand the effect of electrolyte composition on the stability of the cathode–electrolyte interface, and therefore to minimise alkoxide formation and improve cycling stability, we study different carbonate solvents and mixtures thereof. Electrolytes that promote the formation of ethoxide are found to be more detrimental to the cell performance than those forming methoxide. The presence of cyclic carbonates in the electrolyte-solvent mixture alleviates the detrimental effects of ethoxide-forming solvents on the electrochemical performance of Li 1.05 (Ni 0.33 Co 0.33 Mn 0.33 ) 0.95 O 2 by reducing the solubility of the ethoxide. [ABSTRACT FROM AUTHOR]

• Crosstalk reactions greatly influence capacity fading of lithium-ion batteries. • A four-electrode cell with Al 2 O 3 electrodes was built to study crosstalk reactions. • Constant-current electrolysis and cyclic voltammetry were used in this regard. • A kinetic model was setup to quantitatively elucidate the changes in electricity. • The model provided adequate fits to the crosstalk-reaction-induced electricity. Crosstalk reactions in Li-ion batteries play an important role in the capacity fading caused by imbalance of the states-of-charge between positive and negative electrodes. A quantitative understanding of these reactions can enable estimation of battery life. Because crosstalk reactions are complex phenomena in which chemical species produced by a side reaction at one electrode react at the other electrode, a generator–collector-type electrochemical cell is required to investigate these reactions. Therefore, a new four-electrode cell was constructed in this study, in which oxidative decomposition of an electrolyte was achieved by constant-current electrolysis at one Al 2 O 3 electrode, and the reduction of the generated oxidants was detected by cyclic voltammetry (CV) at the other Al 2 O 3 electrode. CV indicated that the cathodic current due to the crosstalk reaction increased during electrolysis, and that two cathodic peaks appeared at 1.3 and 1.8 V. The cathodic current eventually stabilized and decreased to zero after electrolysis. A kinetic equation that expressed the crosstalk-reaction-induced cathodic current provided a decent fit to the observed current via kinetic parameters. This kinetic equation is remarkably useful for quantitatively understanding the electrochemical nature of crosstalk reactions and can help predict the side-reaction currents in actual batteries through the derived kinetic parameters, which can determine battery life. [ABSTRACT FROM AUTHOR]

Abstract: Four different electrolytes are prepared by dissolving a Li salt in three different room-temperature ionic liquids (RTILs) and also in a conventional organic solvent. The cathodic (electrochemical reduction) stability of these electrolytes is compared at both ambient and elevated temperature by potential cycling on a TiO2-B electrode. At room temperature, the stability of pyrrolidinium- and piperidinium-based RTILs is comparable with that of the carbonate-based organic solvent, which is in contrast to the severely decomposed imidazolium-based RTIL. At elevated temperature (120°C), the imidazolium-based RTIL undergoes even more significant cathodic decomposition that results in the deposition of a resistive surface film and leads to eventual cell degradation. By contrast, the cathodic decomposition and concomitant film deposition are not serious with pyrrolidinium- and piperidinium-based RTILs even at this high-temperature, so that the TiO2-B/Li cell operates with reasonably good cycle performance. The latter two RTILs appear to be promising solvents for lithium-ion batteries that are durable against occasional exposure to high-temperature. [Copyright &y& Elsevier]

Abstract: We present the results of novel type additives to improve the cathode cycleability performance. Benzene derivatives (biphenyl and o-terphenyl) and heterocyclic compounds (furan, thiophene, N-methylpyrrole and 3,4-ethylenedioxythiophene), which have lower oxidation potentials than those of electrolyte solvents are picked up. We use MO calculations in the selection of additives and prove that the calculated HOMO values agree well with the measured oxidation potentials. To clarify the additive performance, electrochemical properties and cycleability of the additive are investigated. The additives are found to be decomposed on cathode to form very thin film. We have named this resulting novel-type thin surface film as electro-conducting membrane (ECM) since it is different from solid electrolyte interphase (SEI) by the point of its electro-conductivity. The nature and the component of ECM are studied with X-ray photoelectron spectroscopy (XPS) and auger electron spectroscopy (AES) in details. It is concluded that these additives, which were formerly known as overcharge protecting proofs, contribute to improve cathode cycleability by forming very thin cathode surface layer in the case of slight amount of addition. [Copyright &y& Elsevier]

The thermal decomposition of LiPF6 in the solid state and as solutions in dialkylcarbonates has been investigated. The thermal decomposition of LiPF6 was investigated with differential scanning calorimatry (DSC) suggesting decomposition to LiF and PF5. In solution, PF5 reacts with dialkylcarbonates to form a variety of decomposition products including carbon dioxide (CO2), ethers (R2O), alkylfluorides (RF), phosphorus oxyfluoride (OPF3), and fluorophosphates (OPF2OR, OPF(OR)2). The structure of the decomposition products are supported by nuclear magnetic resonance (NMR) spectroscopy and gas chromatography with mass selective detection (GC–MS). [Copyright &y& Elsevier]

The degradation of lithium-ion batteries resulted from electrolyte decomposition may be indicated by the change in diffusion coefficient, D e , and concentration, C e. Although D e and C e are included in the liquid-phase diffusion process of a pseudo two-dimensional (P2D) model, these two important parameters are always inundated or contaminated when the complete P2D model is reduced. To obtain D e and C e reliably, a new parameter-lumped model of concentration overpotential, based on the reduced liquid-phase diffusion partial differential equation (PDE)in P2D model by Padé approximation, is developed with two main contributions. First, as the basis of Padé-based approximation, new analytic solutions of liquid-phase diffusion PDE are derived under the improved boundary conditions with the mass conservation principle. Second, in the proposed parameter-lumped model, the target parameters of D e and C e are grouped with other unknown but fixed parameters, respectively, which guarantees that even if the specific values of target parameters cannot be obtained, their changing trajectories can also be independently and effectively observed by lumped parameters. Finally, the simulation accuracies of reduced dynamic electrolyte concentration expressions are compared with that from other simplified methods, and the identification accuracy of the lumped parameters is verified under different working conditions. • Target parameters for electrolyte decomposition are lumped respectively. • The changes in target parameters can be identified unambiguously. • Padé approximation is applied to simplify liquid-phase diffusion process. • Analytical expression for diffusion is proposed under improved boundary conditions. • Accuracy of Padé approximation is compared with that of other simplified methods. [ABSTRACT FROM AUTHOR]