Ab Initio Modeling of Electrolyte Molecule Ethylene Carbonate Decomposition Reaction on Li(Ni,Mn,Co)O2 Cathode Surface

Universal Neural Network Potential-Driven Molecular Dynamics Study of CO2/O2 Evolution at the Ethylene Carbonate/Charged-Electrode Interface

Long-term durability and safety are required to develop Li-ion batteries that can operate at high voltages. However, side reactions, including the release of O2 from the electrode and CO2 from the organic electrolyte, occur at the positive-electrode/electrolyte interface during charging at high voltages. In this study, universal neural network potential (UNNP)-driven molecular dynamics (MD) calculations are used to investigate the mechanism of the reaction between LixCoO2 (0 ≤ x ≤ 1) or LixNiO2 (0 ≤ x ≤ 1), as the positive-electrode material, and an ethylene-carbonate-based electrolyte, with a solid-liquid interface composed of ∼1700 atoms. Molecular CO2 and O2 evolve from the partially or fully Li-deintercalated LixNiO2, while no gas-evolution reactions are observed for LixCoO2. Hence, compared LixNiO2, the LiCoO2 electrode is more stable toward the decomposition of ethylene carbonate in the charged state. The decomposition reactions at the solid-liquid interface during charging are also analyzed using a NN force field. This study provides a robust approach involving MD simulations using UNNP to better understand the side reactions in electrochemical devices, which can guide manufacturers in selecting appropriate materials.

Adsorptive Shield Derived Cathode Electrolyte Interphase Formation with Impregnation on LiNi0.8Mn0.1Co0.1O2 Cathode: A Mechanism-Guiding-Experiment Study

Lithium difluoro(oxalate) borate (LiDFOB) contributes actively to cathode-electrolyte interface (CEI) formation, particularly safeguarding high-voltage cathode materials. However, LiNixCozMnyO2-based batteries benefit from the LiDFOB and its derived CEI only with appropriate electrolyte design while a comprehensive understanding of the underlying interfacial mechanisms remains limited, which makes the rational design challenging. By performing ab initio calculations, the CEI evolution on the LiNi0.8Co0.1Mn0.1O2 has been investigated. The findings demonstrate that LiDFOB readily adheres to the cathode via semidissociative configuration, which elevates the Li deintercalation voltage and remains stable in solvent. Electrochemical processes are responsible for the subsequent cleavage of B-F and B-O bonds, while the B-F bond cleavage leading to LiF formation is dominant in the presence of adequate Li+ with a substantial Li intercalation energy. Thus, impregnation is established as an effective method to regulate the conversion channel for efficient CEI formation, which not only safeguards the cathode's structure but also counters electrolyte decomposition. Consequently, in comparison to utilizing LiDFOB as an electrolyte additive, employing LiDFOB impregnation in the NCM811/Li cell yields significantly improved cycling stability for over 2000 h.

Inert Filler Selection Strategies in Li-Ion Gel Polymer Electrolytes

The main role of inert fillers in polymer electrolytes is to enhance ionic conductivity. However, lithium ions in gel polymer electrolytes (GPEs) conduct in liquid solvent rather than along the polymer chains. So far, the main role of inert fillers in improving the electrochemical performance of GPEs is still unclear. Here, various low-cost and common inert fillers (Al2O3, SiO2, TiO2, ZrO2) are introduced into GPEs to study their effects on Li-ion polymer batteries. It is found that the addition of inert fillers has different effects on ionic conductivity, mechanical strength, thermal stability, and, dominantly, interfacial properties. Compared with other gel electrolytes containing SiO2, TiO2, or ZrO2 fillers, those with Al2O3 fillers exhibit the most favorable performance. The high performance is ascribed to the interaction between the surface functional groups of Al2O3 and LiNi0.8Co0.1Mn0.1O2, which alleviates the decomposition of the organic solvent by the cathode, resulting in the formation of a high-quality Li+ conductor interfacial layer. This study provides an important reference for the selection of fillers in GPEs, surface modification of separators, and cathode surface coating.

Solvent-mediated oxide hydrogenation in layered cathodes

Self-discharge and chemically induced mechanical effects degrade calendar and cycle life in intercalation-based electrochromic and electrochemical energy storage devices. In rechargeable lithium-ion batteries, self-discharge in cathodes causes voltage and capacity loss over time. The prevailing self-discharge model centers on the diffusion of lithium ions from the electrolyte into the cathode. We demonstrate an alternative pathway, where hydrogenation of layered transition metal oxide cathodes induces self-discharge through hydrogen transfer from carbonate solvents to delithiated oxides. In self-discharged cathodes, we further observe opposing proton and lithium ion concentration gradients, which contribute to chemical and structural heterogeneities within delithiated cathodes, accelerating degradation. Hydrogenation occurring in delithiated cathodes may affect the chemo-mechanical coupling of layered cathodes as well as the calendar life of lithium-ion batteries.

Insights from Ab Initio Molecular Dynamics on the Interface Reaction between Electrolyte and Li2MnO3 Cathode during the Charging Process

The complex interface reactions are crucial to the performance of the Li2MnO3 cathode material. Here, the interface reactions between the liquid electrolyte and the typical surfaces of Li2MnO3 during the charging process are systematically investigated by ab initio molecular dynamics (AIMD) simulation and first-principles calculation. The results indicate that these interface reactions lead to the formation of hydroxide radicals, oxygen, carbon dioxide, carbonate radicals, and other products, which are consistent with the experimental findings. These processes primarily result from the conversion of the stable closed-shell O2- into reactive oxygen ions by electron loss. All surfaces exhibit some degree of layered- and spinel-like phase transitions during the AIMD simulations, consistent with the experiment. This is mainly attributed to the decrease in the Mn-O bond strength and the increase in the Li/O ion vacancy concentration. This study offers valuable theoretical insights into the interface reaction between lithium-rich cathode materials and liquid electrolytes.

Exploring the Molecular-Scale Structures at Solid/Liquid Interfaces of Li-Ion Battery Materials: A Force Spectroscopy Analysis with Sparse Modeling

In this study, we clarify the liquid structure formed at the interface between LiCoO2 (LCO), the cathode material of Li-ion batteries, and propylene carbonate (PC), which is used as a solvent in the electrolyte, on a molecular scale. We apply sparse modeling-based modal analysis to force spectroscopy data measured by frequency modulation atomic force microscopy (FM-AFM) and show that each component in the FM-AFM force curve, such as oscillatory solvation force, background, and noise, can be automatically decomposed. Moreover, by combining detailed force curve analysis with solid/liquid interface simulations based on first-principles calculation, we have identified that there are distinct damped vibrational modes in the force curves at the LCO/PC interface with a period of about 0.57 nm and those with shorter periods, which likely correspond to the solvation forces associated with bulk-state PC molecules and those with PC molecules in "lying down" orientations.

Hybrid Density Functional Theory Comparison of Oxygen Release and Solvent Decomposition Kinetics on LixNiO2 Surfaces

High-nickel-content layered oxides are among the most promising electric vehicle battery cathode materials. However, their interfacial reactivity with electrolytes and tendency toward oxygen release (possibly yielding reactive 1O2) remain degradation concerns. Elucidating the most relevant (i.e., fastest) interfacial degradation mechanism will facilitate future mitigation strategies. We apply screened hybrid density functional (HSE06) calculations to compare the reaction kinetics of LixNiO2 surfaces with ethylene carbonate (EC) with those of O2 release. On both the (001) and (104) facets, EC oxidative decomposition exhibits lower activation energies than O2 release. Our calculations, coupled with previously computed liquid-phase reaction rates of 1O2 with EC, strongly question the role of "reactive 1O2" species in electrolyte oxidative degradation. The possible role of other oxygen species is discussed. To deal with the challenges of modeling LixNiO2 surface reactivity, we emphasize a "local structure" approach instead of pursuing the global energy minimum.

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.

Thermal Stability and Outgassing Behaviors of High-nickel Cathodes in Lithium-ion Batteries

LiNiO2 -based high-nickel layered oxide cathodes are regarded as promising cathode materials for high-energy-density automotive lithium batteries. Most of the attention thus far has been paid towards addressing their surface and structural instability issues brought by the increase of Ni content (>90 %) with an aim to enhance the cycle stability. However, the poor safety performance remains an intractable problem for their commercialization in the market, yet it has not received appropriate attention. In this review, we focus on the gas generation and thermal degradation behaviors of high-Ni cathodes, which are critical factors in determining their overall safety performance. A comprehensive overview of the mechanisms of outgassing and thermal runaway reactions is presented and analyzed from a chemistry perspective. Finally, we discuss the challenges and the insights into developing robust, safe high-Ni cathodes.

A fluorinated cation introduces new interphasial chemistries to enable high-voltage lithium metal batteries

Fluorides have been identified as a key ingredient in interphases supporting aggressive battery chemistries. While the precursor for these fluorides must be pre-stored in electrolyte components and only delivered at extreme potentials, the chemical source of fluorine so far has been confined to either negatively-charge anions or fluorinated molecules, whose presence in the inner-Helmholtz layer of electrodes, and consequently their contribution to the interphasial chemistry, is restricted. To pre-store fluorine source on positive-charged species, here we show a cation that carries fluorine in its structure is synthesized and its contribution to interphasial chemistry is explored for the very first time. An electrolyte carrying fluorine in both cation and anion brings unprecedented interphasial chemistries that translate into superior battery performance of a lithium-metal battery, including high Coulombic efficiency of up to 99.98%, and Li⁰-dendrite prevention for 900 hours. The significance of this fluorinated cation undoubtedly extends to other advanced battery systems beyond lithium, all of which universally require kinetic protection of highly fluorinated interphases.

Hyphenated DEMS and ATR-SEIRAS techniques for in situ multidimensional analysis of lithium-ion batteries and beyond

A wide spectrum of state-of-the-art characterization techniques have been devised to monitor the electrode-electrolyte interface that dictates the performance of electrochemical devices. However, coupling multiple characterization techniques to realize in situ multidimensional analysis of electrochemical interfaces remains a challenge. Herein, we presented a hyphenated differential electrochemical mass spectrometry and attenuated total reflection surface enhanced infrared absorption spectroscopy analytical method via a specially designed electrochemical cell that enables a simultaneous detection of deposited and volatile interface species under electrochemical reaction conditions, especially suitable for non-aqueous, electrolyte-based energy devices. As a proof of concept, we demonstrated the capability of the homemade setup and obtained the valuable reaction mechanisms, by taking the tantalizing reactions in non-aqueous lithium-ion batteries (i.e., oxidation and reduction processes of carbonate-based electrolytes on Li1+xNi0.8Mn0.1Co0.1O2 and graphite surfaces) and lithium-oxygen batteries (i.e., reversibility of the oxygen reaction) as model reactions. Overall, we believe that the coupled and complementary techniques reported here will provide important insights into the interfacial electrochemistry of energy storage materials (i.e., in situ, multi-dimensional information in one single experiment) and generate much interest in the electrochemistry community and beyond.

Unraveling the Dynamic Correlations between Transition Metal Migration and the Oxygen Dimer Formation in the Highly Delithiated LixCoO2 Cathode

The voltage-window expansion can increase the practical capacity of Li_xCoO₂ cathodes, but it would lead to serious structural degradations and oxygen release induced by transition metal (TM) migration. Therefore, it is crucial to understand the dynamic correlations between the TM migration and the oxygen dimer formation. Here, machine-learning-potential-assisted molecular dynamics simulations combined with enhanced sampling techniques are performed to resolve the above question using a representative CoO₂ model. Our results show that the occurrence of the Co migration exhibits local characteristics. The formation of the Co vacancy cluster is necessary for the oxygen dimer generation. The introduction of the Ti dopant can significantly increase the kinetic barrier of the Co ion migration and thus effectively suppress the formation of the Co vacancy cluster. Our work reveals atomic-scale dynamic correlations between the TM migration and the oxygen sublattice's instability and provides insights about the dopant's promotion of the structural stability.

Hollow-core optical fibre sensors for operando Raman spectroscopy investigation of Li-ion battery liquid electrolytes

Improved analytical tools are urgently required to identify degradation and failure mechanisms in Li-ion batteries. However, understanding and ultimately avoiding these detrimental mechanisms requires continuous tracking of complex electrochemical processes in different battery components. Here, we report an operando spectroscopy method that enables monitoring the chemistry of a carbonate-based liquid electrolyte during electrochemical cycling in Li-ion batteries with a graphite anode and a LiNi_0.8Mn_0.1Co_0.1O₂ cathode. By embedding a hollow-core optical fibre probe inside a lab-scale pouch cell, we demonstrate the effective evolution of the liquid electrolyte species by background-free Raman spectroscopy. The analysis of the spectroscopy measurements reveals changes in the ratio of carbonate solvents and electrolyte additives as a function of the cell voltage and show the potential to track the lithium-ion solvation dynamics. The proposed operando methodology contributes to understanding better the current Li-ion battery limitations and paves the way for studies of the degradation mechanisms in different electrochemical energy storage systems.

High-voltage liquid electrolytes for Li batteries: progress and perspectives

Since the advent of the Li ion batteries (LIBs), the energy density has been tripled, mainly attributed to the increase of the electrode capacities. Now, the capacity of transition metal oxide cathodes is approaching the limit due to the stability limitation of the electrolytes. To further promote the energy density of LIBs, the most promising strategies are to enhance the cut-off voltage of the prevailing cathodes or explore novel high-capacity and high-voltage cathode materials, and also replacing the graphite anode with Si/Si-C or Li metal. However, the commercial ethylene carbonate (EC)-based electrolytes with relatively low anodic stability of ∼4.3 V vs. Li⁺/Li cannot sustain high-voltage cathodes. The bottleneck restricting the electrochemical performance in Li batteries has veered towards new electrolyte compositions catering for aggressive next-generation cathodes and Si/Si-C or Li metal anodes, since the oxidation-resistance of the electrolytes and the in situ formed cathode electrolyte interphase (CEI) layers at the high-voltage cathodes and solid electrolyte interphase (SEI) layers on anodes critically control the electrochemical performance of these high-voltage Li batteries. In this review, we present a comprehensive and in-depth overview on the recent advances, fundamental mechanisms, scientific challenges, and design strategies for the novel high-voltage electrolyte systems, especially focused on stability issues of the electrolytes, the compatibility and interactions between the electrolytes and the electrodes, and reaction mechanisms. Finally, novel insights, promising directions and potential solutions for high voltage electrolytes associated with effective SEI/CEI layers are proposed to motivate revolutionary next-generation high-voltage Li battery chemistries.

Suppression Mechanisms of the Solid-Electrolyte Interface Formation at the Triple-Phase Interfaces in Thin-Film Li-Ion Batteries

Side reactions of the charge/discharge in Li-ion batteries (LIBs) generate a solid-electrolyte interface (SEI) onto an electrode surface, resulting in the degradation of the lifetime of a cell. The suppression of SEI formations has attracted much attention for achieving longer cyclable LIBs. Our research group has previously reported that few SEI were observed at triple-phase interfaces (TPIs) consisting of BaTiO₃, LiCoO₂, and electrolyte interfaces in LIBs with excellent cyclability and ultrahigh-speed chargeability. An investigation on the suppression mechanisms of SEI formations at TPIs should yield important information on understanding the undesirable side reactions. Therefore, we have explored the suppression mechanisms of SEI formations by preparing epitaxial thin films and evaluating the surface of the samples after the electrochemical treatment. The results of X-ray photoelectron spectroscopy and scanning electron microscopy with energy-dispersive X-ray analysis measurements suggested that the decomposition of LiPF₆ was suppressed at TPIs, implying that the generation of PF₅ via the decomposition of LiPF₆ contributed to SEI formation.

In-Situ Intermolecular Interaction in Composite Polymer Electrolyte for Ultralong Life Quasi-Solid-State Lithium Metal Batteries

Solid-state lithium metal batteries built with composite polymer electrolytes using cubic garnets as active fillers are particularly attractive owing to their high energy density, easy manufacturing and inherent safety. However, the uncontrollable formation of intractable contaminant on garnet surface usually aggravates poor interfacial contact with polymer matrix and deteriorates Li⁺ pathways. Here we report a rational designed intermolecular interaction in composite electrolytes that utilizing contaminants as reaction initiator to generate Li⁺ conducting ether oligomers, which further emerge as molecular cross-linkers between inorganic fillers and polymer matrix, creating dense and homogeneous interfacial Li⁺ immigration channels in the composite electrolytes. The delicate design results in a remarkable ionic conductivity of 1.43×10^-3  S cm^-1 and an unprecedented 1000 cycles with 90 % capacity retention at room temperature is achieved for the assembled solid-state batteries.

Role of Surface Acidity in the Surface Stabilization of the High-Voltage Cathode LiNi0.6Mn0.2Co0.2O2

Metal oxide coatings have been reported to be an effective approach for stabilizing cathode interfaces, but the associated chemistry is unclear. In this work, thin films of TiO₂, ZnO, and Cr₂O₃, which have different surface acidities/basicities, were used to modify the surface chemistry of LiNi_0.6Mn_0.2Co_0.2O₂ and study the acidity's role in the cathode/electrolyte interphase composition and impedance under high-voltage cycling (4.5 V vs Li/Li⁺). Cathodes with more acidic surfaces provided higher initial specific capacity and capacity retention with cycling. More basic surfaces had higher initial impedance and greater impedance growth with cycling. These differences appeared to depend on the degree of LiPF₆ salt decomposition at the interface, which was related to acidity, with more neutral surfaces having a LiF/Li _x PO _y F _z ratio close to unity, but basic surfaces had substantially more LiF. This chemistry was more significant than the cathode electrolyte interphase (CEI) thickness as the more acidic surfaces formed a thicker CEI than the basic surface, resulting in better capacity retention. These results suggest that the Brønsted acidity of cathodes directly influences electrolyte degradation, ion transport, and thus, cell lifetime.

Solvent oligomerization pathways facilitated by electrolyte additives during solid-electrolyte interphase formation

The solid-electrolyte interphase (SEI) layer formation is known to play an important role in determining the lifetime of lithium-ion batteries. A thin, stable SEI layer is linked to overall improved battery performance and longevity, however, the factors and mechanisms that lead to optimal SEI morphology and composition are not well understood. Inclusion of electrolyte additives (fluoroethylene carbonate, FEC; and vinylene carbonate, VC) is often necessary for improving SEI characteristics. To understand how these electrolyte additives impact SEI formation, molecular dynamics (MD) and density functional theory (DFT) simulations were employed to study the reaction networks and oligomerization pathways, respectively, for three systems containing ethylene carbonate (EC), a lithium ion, and FEC or VC. MD simulations suggest radical oligomerization pathways analogous to traditional oligomerization with nucleophilic alkoxide species via SN1 reaction mechanisms. Both SN1 and SN2 mechanisms were studied for all three systems using DFT. Oligomerization reactions were studied with both a standard alkoxide species and a ring-opened EC radical as the nucleophiles and EC, FEC, and VC as the electrophiles. For all cases, FEC and VC exhibited lower free energy barriers and more stable adducts when compared with EC. We conclude that one of the role of additives is to modify the oligomerization process of EC by introducing branching points (FEC) or termination points (VC).

Fading Mechanisms and Voltage Hysteresis in FeF2 -NiF2 Solid Solution Cathodes for Lithium and Lithium-Ion Batteries

The rapid development of ultrahigh-capacity alloying or conversion-type anodes in rechargeable lithium (Li)-ion batteries calls for matching cathodes for next-generation energy storage devices. The high volumetric and gravimetric capacities, low cost, and abundance of iron (Fe) make conversion-type iron fluoride (FeF₂ and FeF₃ )-based cathodes extremely promising candidates for high specific energy cells. Here, the substantial boost in the capacity of FeF₂ achieved with the addition of NiF₂ is reported. A systematic study of a series of FeF₂ -NiF₂ solid solution cathodes with precisely controlled morphology and composition reveals that the presence of Ni may undesirably accelerate capacity fading. Using a powerful combination of state-of-the-art analytical techniques in combination with the density functional theory calculations, fundamental mechanisms responsible for such a behavior are uncovered. The unique insights reported in this study highlight the importance of careful selection of metals and electrolytes for optimizing electrochemical properties of metal fluoride cathodes.

The interaction of ethylammonium tetrafluoroborate [EtNH3+][BF4−] ionic liquid on the Li(001) surface: towards understanding early SEI formation on Li metal

Dissociation of an ionic liquid is not necessarily a requirement for the formation of an SEI layer.

Accelerated Evolution of Surface Chemistry Determined by Temperature and Cycling History in Nickel-Rich Layered Cathode Materials

Nickel-rich layered cathode materials have the potential to enable cheaper and higher energy lithium ion batteries. However, these materials face major challenges (e.g., surface reconstruction, microcracking, potential oxygen evolution) that can hinder the safety and cycle life of lithium ion batteries. Many studies of nickel-rich materials have focused on ways to improve performance. Understanding the effects of temperature and cycling on the chemical and structural transformations is essential to assess the performance and suitability of these materials for practical battery applications. The present study is focused on the spectroscopic analysis of surface changes within a strong performing LiNi_0.8Mn_0.1Co_0.1O₂ (NMC811) cathode material. We found that surface chemical and structural transformations (e.g., gradient metal reduction, oxygen loss, reconstruction, dissolution) occurred quicker and deeper than expected at higher temperatures. Even at lower temperatures, the degradation occurred rapidly and eventually matched the degradation at high temperatures. Despite these transformations, our performance results showed that a better performing nickel-rich NMC is possible. Establishing relationships between the atomic, structural, chemical, and physical properties of cathode materials and their behavior during cycling, as we have done here for NMC811, opens the possibility of developing lithium ion batteries with higher performance and longer life. Finally, our study also suggests that a separate, systematic, and elaborate study of surface chemistry is necessary for each NMC composition and electrolyte environment.

Kinetics-Controlled Degradation Reactions at Crystalline LiPON/Lix CoO2 and Crystalline LiPON/Li-Metal Interfaces

Detailed understanding of solid-solid interface structure-function relationships is critical for the improvement and wide deployment of all-solid-state batteries. The interfaces between lithium phosphorous oxynitride (LiPON) solid electrolyte material and lithium metal anode, and between LiPON and Li_x CoO₂ cathode, have been reported to generate solid-electrolyte interphase (SEI)-like products and/or disordered regions. Using electronic structure calculations and crystalline LiPON models, we predict that LiPON models with purely P-N-P backbones are kinetically inert towards lithium at room temperature. In contrast, transfer of oxygen atoms from low-energy Li_x CoO₂ (104) surfaces to LiPON is much faster under ambient conditions. The mechanisms of the primary reaction steps, LiPON structural motifs that readily reacts with lithium metal, experimental results on amorphous LiPON to partially corroborate these predictions, and possible mitigation strategies to reduce degradations are discussed. LiPON interfaces are found to be useful case studies for highlighting the importance of kinetics-controlled processes during battery assembly at moderate processing temperatures.

Molecular Surface Modification of NCM622 Cathode Material Using Organophosphates for Improved Li-Ion Battery Full-Cells

Surface coating is a viable strategy for improving the cyclability of Li_{1+ x}(Ni_{1- y- z}Co _yMn _z)_{1- x}O₂ (NCM) cathode active materials for lithium-ion battery cells. However, both gaining synthetic control over thickness and accurate characterization of the surface shell, which is typically only a few nm thick, are considerably challenging. Here, we report on a new molecular surface modification route for NCM622 (60% Ni) using organophosphates, specifically tris(4-nitrophenyl) phosphate (TNPP) and tris(trimethylsilyl) phosphate. The functionalized NCM622 was thoroughly characterized by state-of-the-art surface and bulk techniques, such as attenuated total reflection infrared spectroscopy, X-ray photoelectron spectroscopy, and time-of-flight secondary ion mass spectrometry (ToF-SIMS), to name a few. The comprehensive ToF-SIMS-based study comprised surface imaging, depth profiling, and three-dimensional visualization. In particular, tomography is a powerful tool to analyze the nature and morphology of thin coatings and is applied, to our knowledge, for the first time, to a practical cathode active material. It provides valuable information about relatively large areas (over several secondary particles) at high lateral and mass resolution. The electrochemical performance of the different NCM622 materials was evaluated in long-term cycling experiments of full-cells with a graphite anode. The effect of surface modification on the transition-metal leaching was studied ex situ via inductively coupled plasma optical emission spectroscopy. TNPP@NCM622 showed reduced transition-metal dissolution and much improved cycling performance. Taken together, with this study, we contribute to optimization of an industrially relevant cathode active material for application in high-energy-density lithium-ion batteries.

Degradation Mechanism of Dimethyl Carbonate (DMC) Dissociation on the LiCoO2 Cathode Surface: A First-Principles Study

The degradation mechanism of dimethyl carbonate electrolyte dissociation on the (010) surfaces of LiCoO₂ and delithiated Li_1/3CoO₂ were investigated by periodic density functional theory. The high-throughput Madelung matrix calculation was employed to screen possible Li_1/3CoO₂ supercells for models of the charged state at 4.5 V. The result shows that the Li_1/3CoO₂(010) surface presents much stronger attraction toward dimethyl carbonate molecule with the adsorption energy of -1.98 eV than the LiCoO₂(010) surface does. The C-H bond scission is the most possible dissociation mechanism of dimethyl carbonate on both surfaces, whereas the C-O bond scission of carboxyl is unlikely to occur. The energy barrier for the C-H bond scission is slightly lower on Li_1/3CoO₂(010) surface. The kinetic analysis further shows that the reaction rate of the C-H bond scission is much higher than that of the C-O bond scission of methoxyl by a factor of about 10³ on both surfaces in the temperature range of 283-333 K, indicating that the C-H bond scission is the exclusive dimethyl carbonate dissociation mechanism on the cycled LiCoO₂(010) surface. This study provides the basis to understand and develop novel cathodes or electrolytes for improving the cathode-electrolyte interface.

Chemical Reactivity Descriptor for the Oxide-Electrolyte Interface in Li-Ion Batteries

Understanding electrochemical and chemical reactions at the electrode-electrolyte interface is of fundamental importance for the safety and cycle life of Li-ion batteries. Positive electrode materials such as layered transition metal oxides exhibit different degrees of chemical reactivity with commonly used carbonate-based electrolytes. Here we employed density functional theory methods to compare the energetics of four different chemical reactions between ethylene carbonate (EC) and layered (Li_xMO₂) and rocksalt (MO) oxide surfaces. EC dissociation on layered oxides was found energetically more favorable than nucleophilic attack, electrophilic attack, and EC dissociation with oxygen extraction from the oxide surface. In addition, EC dissociation became energetically more favorable on the oxide surfaces with transition metal ions from left to right on the periodic table or by increasing transition metal valence in the oxides, where higher degree of EC dissociation was found as the Fermi level was lowered into the oxide O 2p band.