Formation Of Cathode Electrolyte Interphase Research Articles

Anion-mediated interphase construction enabling high-voltage solid-state lithium metal batteries

Enhancing the interphase stability of polymer electrolytes with high-voltage cathodes is crucial for the development of high-energy-density lithium metal batteries (LMBs), especially in wearable devices area. Herein, lithium difluorophosphate (LiDFP) is incorporated into the solid polymer electrolyte of poly(ethylene oxide) (PEO) to enhance the formation of a robust cathode-electrolyte interphase (CEI) in high-voltage LiCoO2 LMBs. Through combining electrochemical measurements, spectroscopic techniques and theoretical calculations, the underlying modification mechanism is revealed. Due to its stronger anionic coordination with both polymer electrolyte chain and proton, and more oxidation-active resultant anion-coordinated polymer compared with lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), LiDFP suppresses the formation of the strong acid HTFSI deriving from the deprotonated process of the terminal O-H group thus avoiding the further oxidative attack to the polymeric framework. Furthermore, under the synergetic decomposition of HDFP intermediate, the polymer-derived radical helps to reconstruct a chemo-mechanically stable and Li-conductive CEI layer, which consists of abundant LiF/Li3PO4/LixPOyFz inorganic species distributed in lithiated organic macromolecules. The in-situ constructed CEI effectively passivates catalytic sites on LiCoO2 directly against PEO, resulting in well-maintained layered structure during high-voltage cycling. The proposed interphasial chemistry that regulates CEI formation provides directive knowledge of electrolyte optimization for high-voltage solid-state batteries.

Reconstruction of solvation structure at the cathode-electrolyte interface via partial de-solvation in a metal-organic framework

The formation of cathode-electrolyte interphase (CEI) is closely related to the solvation structure in direct contact with the cathode. The de-solvated solvent molecules are subject to the attack of highly catalytic transition metal ions in the cathode materials (such as Ni-rich LiNi0.8Co0.1Mn0.1O2, Ni-rich NCM), especially at high voltage or high temperature, leading to the poor CEI structure that accelerates the deterioration of rechargeable batteries. Considerable strategies have been proposed to solve this issue. Herein, we have constructed a porous Mg-MOF buffer layer on the surface of Ni-rich NCM, which not only greatly reduces the contact between solvent molecules and cathodes to alleviate the extensive interface reactions, but also promote the de-solvation of Li+ solvation structure through strong affinity between solvents and Mg2+. The partially de-solvated structure induces the anion to participate in the solvation structure and contributes to the formation of a uniform LiF-rich CEI with superior mechanical integrity and chemical stability. Therefore, the NCM with Mg-MOF buffer layer exhibits improved cycling stability at both elevated temperature of 45 °C and high voltage of 4.5 V with lithium as anode. Moreover, the modified NCM/graphite full cells also deliver ultra-stable rechargeable capability with a remarkable capacity retention of 87.3% over 1000 cycles as compared to the only 21.7% of pristine NCM based cells.

Investigating Cathode Electrolyte Interphase Formation in NMC 811 Primary Particles through Advanced 4D-STEM ACOM Analysis

Investigating Cathode Electrolyte Interphase Formation in NMC 811 Primary Particles through Advanced 4D-STEM ACOM Analysis

Additive-rejuvenated anions (De)intercalation into graphite cathode enables optimum dual-ion battery

Cathode-electrolyte interphase (CEI) has particular function in maintaining electrolyte and cathode stability. However, the CEI will also cause some negative effects on ion transport. It is crucial for optimizing the structure of CEI through additives in the electrolyte to improve the electrode reaction. In this study, we proposed using C3H3FO3 (FEC) as an additive to optimize an aqueous LiTFSI (bistrifluoromethanesulfonimide lithium salt) electrolyte. The incorporation of FEC modified the solvation structure of TFSI–, inhibit the formation of CEI with thicker size, thereby facilitating the intercalation of TFSI– into the selected graphite cathode. Consequently, the graphite's discharge capacity was doubled, reaching 47 mAh g−1. The influence of TFSI– solvation structure on CEI formation was further studied by the emerging correlative SEM, Raman imaging and TOF-SIMS techniques and theoretical calculation techniques. Using this optimization strategy, we successfully constructed an aqueous dual-ion battery using C24H10N2O4 and graphite as the anode and cathode with an impressive potential window of 2.55 V, which delivered the energy density of 66 Wh kg−1 at the power density of 128 W kg–1.

Non‐Sacrificial Additive Enables a Non‐Passivating Cathode Interface for 4.6 V Li||LiCoO2 Batteries

AbstractVarious electrolyte additives are developed to construct a cathode electrolyte interphase (CEI) layer for high‐voltage LiCoO2 since the cathode suffers severe interfacial degradation when increasing the cut‐off voltage over 4.55 V. However, the CEI derived from the additive sacrificial reaction faces the risk of rupture due to the corrosion reaction and the volumetric variation of the cathode. Herein, a non‐passivating cathode interface is realized for 4.6 V LiCoO2 with a non‐sacrificial electrolyte additive (TBAClO4) by regulating the solvent environment at the interface rather than the preferential decomposition for CEI formation. Owing to the novel protection mechanism, the cell performance shows little dependence on the CEI‐formation process. Therefore, an ultra‐high initial coulombic efficiency (96.63%) and excellent cycling stability (81% capacity retention after 300 cycles) are achieved in Li||LiCoO2 batteries. Moreover, even with the electrolyte containing 1000 ppm H2O, the remarkable water capture ability of the additive together with its interfacial regulation enables the 4.6 V Li||LiCoO2 battery to retain 80% capacity after 200 cycles. This non‐sacrificial strategy provides new insights into high‐voltage electrolyte additive design for high‐energy‐density lithium metal batteries.

Interfacial Electrochemical Lithiation and Dissolution Mechanisms at a Sulfurized Polyacrylonitrile Cathode Surface.

Advances in sulfurized-polyacrylonitrile (SPAN)-based cathode materials promise safer and more efficient lithium-sulfur (Li-S) battery performance. To elucidate electrolyte-cathode interfacial electrochemistry and polysulfide (PS) dissolution, we emulate discharge SPAN reactions via ab initio molecular dynamics (AIMD) simulations. Plausible structures and their lithiation profiles are cross-validated via Raman/IR spectroscopy and density functional theory (DFT). Lithium bis(fluorosulfonyl)imide (LiFSI) plays versatile roles in the Li-SPAN cell electrochemistry, primarily as the major source in forming the cathode-electrolyte interphase (CEI), further verified via X-ray photoelectron spectroscopy and AIMD. Besides being a charge carrier and CEI composer, LiFSI mediates the PS generation processes in SPAN electrochemical lithiation. Analysis of AIMD trajectories during progressive lithiation reveals that, compared to carbonates, ether solvents enable stronger solvation and chemical stabilization for both salt and SPAN structures. Differentiated CEI formation and electrochemical lithiation decomposition pathways and products are profoundly associated with the intrinsic nature of lithium bonding with oxygen and sulfur.

A Facile Synthesis of Al2O3-Coated LiNi1/3Co1/3Mn1/3O2 with Improved Cycle Life Prepared By a Wet-Chemical Method

Aging of lithium-ion batteries (LIBs) and its prevention has been one of the focal points of LIB research in the recent years. The stability of the well-established anode material graphite far surpasses the stability of emerging high energy cathodes (NCM Ni83, Li- and Mn-rich NCM)1,2. Additionally, the degradation of the solid electrolyte interphase (SEI) on the anode side mainly originates from dissolved TM species of the cathode material3. Thus, to bring the high energy LIBs to market, the main focus has been on the mitigation of cathode degradation. Application of protective coatings is an excellent method for stability improvement and is often preferred over doping and structural modifications4. Nevertheless, a pressing need remains for a more sustainable, cost-effective, and easily scalable method to synthesize coatings on cathode materials.In this work, we study LiNi1/3Co1/3Mn1/3O2 (NCM 111) as a model material and coat it with Al2O3 via a wet-chemical process. Wet-chemical processes have in the past often used toxic precursors or solvents, rendering the sustainability of these methods questionable5,6. Thus, here a wet-chemical coating process is developed based on a green solvent (ethanol) and a non-toxic raw material (aluminium isopropoxide). The coating is based on hydrogen bond formation between a hydrolyzed precursor (Al(OH)3) and the oxygen groups on the surface of the commercially sourced active material (NCM111) to form a uniform coating of Al(OH)3 which is converted to Al2O3 after a sintering step in air at 500°C. In addition to characterizing the effects of coating on the cathode active material, we also investigated how the coating procedure alone, without adding the aluminium precursor, affects the active material. This is done to separate the effects of solution washing and sintering from those of the coating formation on the active material. The pristine, coated, and washed and sintered NCM powders were investigated by SEM, EDS, XPS, XRD, and TEM methods, and electrochemical tests in half cells.The capacity of the uncoated NCM decreases to 80% of initial capacity during the first 200 cycles at 1 C rate. A significant improvement in both capacity retention and rate capability is observed for both the washed and coated samples. Although coated samples display further cycle life improvements, most of the improvements can be attributed to the ethanol washing away residual lithium compounds (RLCs) such as LiCO3, LiOH and LiO, which are left on the particle surface after the synthesis or formed during the storage in ambient atmosphere. RLCs can either accelerate electrolyte and electrode decomposition via several reaction pathways7 or participate in cathode-electrolyte interphase formation, resulting in increased impedance and decreased cycle life. Further improvements in the Al2O3-coated samples due to the coating serving as a protective layer that helps to prevent unwanted reactions between the cathode and the electrolyte.This study demonstrates Al2O3-coated NCM 111 with improved electrochemical performance. Al2O3 is deposited via a facile and environmentally friendly wet-chemical coating process. The study also highlights the importance of washing the active material to remove residual lithium compounds, which can have a significant impact on cycle life and performance.Authors acknowledge the support of M-Era.net project "Inert Coatings for Prevention of Ageing of NMC Cathode for Lithium-Ion Batteries” (InCoatBat).

Synergistic Effect of Lithium (difluoromethanesulfonyl)(trifluoromethanesulfonyl)Imide (LiDFTFSI) and Vinylene Carbonate (VC) on High Performance of NMC811║Graphite Cells

Lithium-ion batteries (LIBs) have gained increasing importance in energy storage systems, driven by the growing demands of grid storage, automotive, and portable consumer applications. To meet the need for high energy density batteries, one promising approach involves the utilization of high capacity layered transition metal oxide cathodes, such as nickel-rich LiNi0.8Mn0.1Co0.1O2 (NMC811), which can deliver a high reversible specific capacity of over 180 mAh·g-1 [1,2]. However, due to the structural and interfacial instability[3], nickel-rich NMC cathode still faces challenges in long-term galvanostatic cycling. For these reasons, design of novel electrolyte formulations, which enable formation of an effective cathode electrolyte interphase (CEI), is highly desirable. Recent studies have highlighted the cross-talk between the cathode and anode, indicating that the evolution of the solid electrolyte interphase (SEI) can impact the formation of the CEI[4]. Thus, establishing an effective SEI/CEI pair is essential for achieving long-term cycling of nickel-rich NMC cathode-based cells. Electrolyte optimization plays a crucial role in facilitating the formation of a desirable SEI/CEI pair, leading to an improved cell performance and longevity. Lithium (difluoromethanesulfonyl)(trifluoro-methanesulfonyl)imide (LiDFTFSI) has proven to be promising in solid-polymer-electrolyte batteries due to the good SEI/CEI formation ability and suppressed Al-dissolution[5]. Additionally, LiDFTFSI exhibits also good compatibility with Li-metal batteries[6], heralding promising applications in Li-ion batteries. However, there is lack of systematic research investigating the potential impact of LiDFTFSI on the cathode as well as on resulting CEI formation and dynamics.In this work, we demonstrated enhanced galvanostatic cycling performance of NMC811||graphite cells achieved by utilizing LiDFTFSI and lithium hexafluorophosphate (LiPF6) in a blended salt organic carbonate-based electrolyte formulation. Comprehensive electrochemical and post mortem analysis revealed that the LiDFTFSI alone can effectively mitigate the structural changes in the NMC811 electrode by facilitating the formation of modified CEI. However, the continued growth of an inhomogeneous CEI, caused by the cross-talk effect between electrodes, adversely affected long-term cycling stability. To address this, vinylene carbonate (VC) was introduced to the electrolyte. Synergistic effect with LiDFTFSI leads to the formation of effective and uniform SEI and CEI. As a result, 720 charge/discharge cycles were achieved in NMC811||graphite cells with LiDFTFSI and VC containing electrolytes at 1C while maintaining 80% state-of-health (SOH80%).

An Electrolyte with Less Space-Occupying Diluent at Cathode Inner Helmholtz Plane for Stable 4.6 V Lithium-Ion Batteries.

Reasonably elevating the working voltage (≥4.4 V vs. Li/Li+ ) of the cathode is one of the efficient approaches to maximize the energy density of lithium-ion batteries (LIBs). As a preferred partner for high-voltage LIB systems, localized high-concentration electrolyte (LHCE), characterized by a stronger Li solvation structure, less free solvent, and robust electrode/electrolyte interphase has attracted much attention in academic circles. Herein, we systematically studied the role of the diluent in LHCE on the formation of the cathode electrolyte interphase (CEI) and elucidated that the existing anion-diluent pairing in the inner Helmholtz plane (IHP) results in an uneven CEI and subsequent battery degradation under high voltage. A m-fluorotoluene (mFT) diluent was further employed in the LHCE containing lithium difluoro(oxalato)borate (LiDFOB) to facilitate a uniform and rich-anion-derived CEI, since the weaker interaction of HmFT -BDFOB - , as compared to the HHhydrofluoroether -BDFOB - , reduces the influence of mFT in IHP or initial CEI formation. Consequently, the mFT-dominated LHCE propels the high-voltage performance of LIBs one step forward, endowing a 4.6 V-class 1.2-Ah graphite||LiNi0.8 Co0.1 Mn0.1 O2 pouch cells a 90.4 % capacity retention after 130 cycles. Our study thus describes a new index affecting the CEI formation and proposes novel strategies to deeply optimize the high-voltage LIBs.

An Electrolyte with Less Space‐Occupying Diluent at Cathode Inner Helmholtz Plane for Stable 4.6 V Lithium‐Ion Batteries

AbstractReasonably elevating the working voltage (≥4.4 V vs. Li/Li+) of the cathode is one of the efficient approaches to maximize the energy density of lithium‐ion batteries (LIBs). As a preferred partner for high‐voltage LIB systems, localized high‐concentration electrolyte (LHCE), characterized by a stronger Li solvation structure, less free solvent, and robust electrode/electrolyte interphase has attracted much attention in academic circles. Herein, we systematically studied the role of the diluent in LHCE on the formation of the cathode electrolyte interphase (CEI) and elucidated that the existing anion‐diluent pairing in the inner Helmholtz plane (IHP) results in an uneven CEI and subsequent battery degradation under high voltage. A m‐fluorotoluene (mFT) diluent was further employed in the LHCE containing lithium difluoro(oxalato)borate (LiDFOB) to facilitate a uniform and rich‐anion‐derived CEI, since the weaker interaction of HmFT−BDFOB−, as compared to the HHhydrofluoroether−BDFOB−, reduces the influence of mFT in IHP or initial CEI formation. Consequently, the mFT‐dominated LHCE propels the high‐voltage performance of LIBs one step forward, endowing a 4.6 V‐class 1.2‐Ah graphite||LiNi0.8Co0.1Mn0.1O2 pouch cells a 90.4 % capacity retention after 130 cycles. Our study thus describes a new index affecting the CEI formation and proposes novel strategies to deeply optimize the high‐voltage LIBs.

Surface-interspersed nanoparticles induced cathode-electrolyte interphase enabling stable cycling of high-voltage LiCoO2

The development of high-voltage LiCoO2 (LCO) is crucial for achieving lithium-ion batteries with a high volumetric energy density. However, LCO experiences accelerated degradation at high voltages due to severe interface and structure instability. A uniform and robust cathode-electrolyte interphase (CEI) serves as a vital barrier for protecting the interface. However, the smooth surface of single-crystalline LCO, lacking grain boundaries, poses challenges in generating an effective CEI. Herein, ZrO2 nano-rivets are constructed on the surface of LCO to provide phase boundaries for preferential film-forming sites, inducing the formation of a uniform and robust CEI. The high-quality CEI derived from ZrO2 nano rivets stabilizes the fragile surface of high-voltage LCO and facilitates lithium-ion diffusion. In addition, a Zr diffusion layer is simultaneously built in the LCO bulk. Zr diffusion into the LCO lattice not only effectively suppresses unfavorable phase transitions to stabilize the bulk structure, but also mitigates oxygen charge deficiencies at the highly delithiated state, therefore stabilizing lattice oxygen. Consequently, the modified LCO exhibits excellent capacity retention of 80% after 700 cycles at 4.6 V. This work emphasizes the significance of material surface properties in CEI formation and provides new insights for the design of CEI.

High-Voltage Degradation of Ni-Rich Cathode Materials in All-Solid-State Batteries

Lithium-ion battery (LIB) has been widely used as electrochemical energy storage devices in a variety of fields in our daily lives. In particular, recently there are strong market demands in the energy storage systems and electric vehicles, mainly due to its high energy and power densities. However, the customers’ strong needs for the safer and longer-lasting battery than commercial ones have sparked special interest in all-solid-state lithium battery (ASSB), which has a great possibility of replacing the conventional LIB that uses liquid electrolyte solution. Though much effort has been made for its practical applications, there are still a number of critical issues involved in the electrode/cell design, fabrication, scale-up, efficient operating conditions, etc. Upper cut-off voltage is one of the most important parameters, having a determinative effect on the energy density and cycling stability of the ASSB esp. when one uses Ni-rich layered oxide cathodes. Actually, the upper cut-off voltage is treated with high priority also in the LIB systems. In the case of the LIB, the secondary particle cracking is followed by gap-filling with electrolyte during the repetitive high voltage charging, giving additional electrochemical active area. Whereas, the particle cracking in the ASSB usually leads to the isolation of the primary particles. In addition, it is reported that the interface between active materials and solid electrolyte is degraded in a complex manner due to simultaneous chemical side reactions (e.g. cathode electrolyte interphase (CEI) formation) and mechanical breakdown (e.g. contact loss between electrolyte and particle), different from that of the LIB. Therefore, an analysis of cathode degradation in the ASSB needs to be dealt with in a different way from that in the LIB.In this work, the degradation of ASSB, consisting of sulfide-based solid electrolyte and Ni-rich cathode, undergoing the high voltage charge cycling is systematically studied using the electrochemical techniques. For this purpose, first, the deterioration tendency of reversible capacity and cell resistance was analyzed in constant current-constant voltage (CC-CV) charging mode at different upper cut-off voltages (CC charging) and the following charging times (CV charging). In addition, the effect of charging rate on the degradation trend was investigated in terms of the duration time that the cathode potential stays higher than a specific potential value in CC charging mode. It was confirmed that there are particular degradation tendencies of capacity and resistance for upper cut-off voltage and duration time, and the empirical relations were accordingly derived. Notably, the degradation trends between capacity and resistance were observed differently, strongly indicative of different governing factor for each degradation process. In the experiments at various charging rates, the lower charging rate led to larger degradation of the cathode. It is suggested that the application of a driving force for irreversible phase transition (i.e., from H2 to H3) for longer time at lower charging rate is the most probable reason.In order to more clearly understand the governing factors affecting high voltage cathode degradation, the degradation trends of capacity and resistance at the aforementioned operating conditions were thoroughly investigated using incremental capacity-differential voltage (IC-DV) analysis and electrochemical impedance spectroscopy (EIS), respectively. From the IC-DV analysis, it proved that the loss of active materials (LAM) is accelerated at higher cut-off voltage, longer duration time, and lower charging rate, indicating that the LAM might be one of the governing factors for capacity degradation. It is notable, by the way, that the interfacial resistance of the cathode was considerably increased at high voltage charging and implies the loss of lithium inventory (LLI) due to the CEI formation could be another major degradation mode. Accordingly, we tried to estimate the contribution of LLI to total degradation from the comparison of half-cell and full-cell analyses. On the other hand, from the EIS analysis, it was found that the increasing rate of the interfacial resistance was far higher than the decreasing rate of capacity. This tells us that the degradation of the cathode|electrolyte interface is mainly responsible for the resistance or power degradation at high voltage charging. In this presentation, the deterioration modes of ASSB at various high voltage charging conditions are explained in terms of the interface and bulk of the cathode. Moreover, the strategy to minimize the degradation at high voltage charging will be discussed.

In situ formation of partially carbonized polyvinylpyrrolidone as a green binder for high-voltage Li-ion battery cathode with improved electrochemical performance

We propose the in situ formation of partially carbonized polyvinylpyrrolidone (PC-PVP) as a green binder for high-voltage LiNi0.5Mn1.5O4 (LNMO) cathodes. A mixture of water and ethanol with an equal volume ratio is shown to enable a more uniform dispersion of LNMO particles in the active material matrix with fewer crevices rather than pure water or pure ethanol. Conformal coverage of LNMO with PC-PVP can be achieved through a stepwise thermal process, which not only enhances the electrical conductance of the cathode material but also protects the electrode surface from overreaction with the electrolyte at high voltages. This can be attributed to the coral reef-like structure of the PC-PVP coating layer, which offers abundant electrical contact without sacrificing ion transport pathways. In addition, the specific nitrogen-containing groups of PC-PVP promote ion transport within the electrode. Furthermore, a carbon paper (CP) substrate is introduced as a current collector to support the PC-PVP/LNMO material layer and to establish a good interconnection. Owing to these features, the PC-PVP/LNMO/CP electrode outperforms the PVDF/LNMO/Al counterpart in terms of high-voltage cycling stability and rate capability. The improved electrode performance is supported by the reduced cathode–electrolyte interphase (CEI) resistance, which corresponds to mitigated electrolyte decomposition and stable CEI formation.

Specific Adsorption-Oxidation Strategy in Cathode Inner Helmholtz Plane Enabling 4.6 V Practical Lithium-Ion Full Cells.

Increasing the cutoff voltage effectively maximizes the available capacity of the state-of-art layered-oxide cathodes (LiTMO2). However, the spontaneous dehydrogenation-oxidation of carbonates in the cathode inner Helmholtz plane (C-IHP) under high voltage/temperature leads to side effects, including weak cathode electrolyte interphase (CEI) and cathode structural collapse. Here, we report a specific adsorption-oxidation (Ad-O) mechanism that dominates the later CEI formation through molecular regulation in C-IHP. The two tailored additives with specific electron-rich groups will enter the C-IHP and mask the active sites of cathodes, thereby reducing the weak CEI generation from conventional carbonates. As-formed hierarchical CEI with inner LiF and outer B-F/-CN rich organic structure will further protect the aggressive cathode from harmful electrolyte corrosion under harsh conditions of high voltages (4.6 V) and elevated temperatures (60 °C). This synergistic strategy guided by the specific Ad-O mechanism enables 3.5 Ah LiNi0.8Co0.1Mn0.1O2/Graphite pouch cells, which remarkably achieve 270 Wh/kg with 450 cycles.

Regulating Cathode‐Electrolyte Interphase by Confining Functional Aluminum Compound within Ni‐Rich Cathodes

AbstractThe oxidative capability of Ni4+ and high operation voltage of nickel‐rich LiNi1−x−yCoxMnyO2 (Ni‐rich NCM) cause its continuous and deleterious side reactions with electrolyte and irreversible phase transition, which hinder its industrial application. To mitigate these issues, Al (CF3SO3)3 is proposed as a solid electrolyte additive that can be readily oxidized to regulate the cathode‐electrolyte interphase (CEI) due to the highest occupied molecular orbital‐level of CF3SO3−, meanwhile being confined within the single‐crystalline NCM811. CF3SO3− prior to the electrolyte is oxidized upon increasing voltage to produce sulfur components and involve CEI formation. Concurrently, the released Al3+ ions are combined with reactive oxygen from NCM811 particles and HF from the electrolyte to form Al2O3 and AlF3, respectively. A robust sandwich CEI film containing sulfur and aluminum species is formed, which cannot only prevent decomposition of the electrolyte, but also alleviate the formation of inactive rock‐salt phase on NCM811 surface. Consequently, such CEI leads to high‐performance batteries with a high‐capacity retention of 91.5% after 200 cycles under 0.5 C compared to 72.4% of pristine NCM811. This facile and environmentally benign method provides a new avenue to develop high‐capacity and durable cathodes for lithium‐ion batteries.

Deciphering the critical effect of cathode-electrolyte interphase by revealing its dynamic evolution

Cathode electrolyte interphase (CEI) layer plays a crucial role in determining the electrochemical performance of lithium-ion batteries. Limited by the sensitive nature of CEI and the lack of characterization techniques, its dynamic evolution during cycling, its formation mechanism, and its specific impact on battery performance are not yet fully understood. Herein, we systematically investigate the dynamic evolution of CEI layer and its critical effect on the cycling performance of LiCoO2 cathode by diverse characterization techniques. We find that cycling voltage plays a key role in affecting CEI formation and evolution, and a critical potential (4.05 V vs. Li) is identified, which acts as the switching potential between CEI deposition and decomposition. We show that CEI starts deposition in the discharge process when the potential is below 4.05 V, and CEI decomposition occurs when the potential is higher than 4.05 V. When the battery is cycled below such a critical potential, a stable CEI layer is developed, which leads to superior cycling stability. When the battery is cycled above such a critical potential, a CEI-free cathode interface is observed, which also demonstrates good cycle stability. However, when the critical potential falls in the cycling voltage range, CEI deposition and decomposition are repeatedly switched on during cycling, leading to the dynamically unstable CEI layer. The unstable CEI layer causes continuous interfacial reaction and degradation, resulting in battery performance decay. Our work deepens the understanding of the CEI formation and evolution mechanisms, and clarifies the critical effect of CEI layer on cycling performance, which provides new insights into stabilizing the electrode–electrolyte interface for high-performance rechargeable batteries.

Voltage-dependent formation of cathode–electrolyte interphase with independent metallic layer in LiNi0.8Mn0.1Co0.1O2 cathode for high-energy density lithium-ion batteries

Layered LiNi0.8Mn0.1Co0.1O2 (NMC811) has emerged as the promising cathode material for next generation lithium-ion batteries owing to its high capacity. The double redox reaction between Ni2+/Ni3+ and Ni3+/Ni4+ is the major source of the large capacity, but it often leads to the formation of an unstable cathode–electrolyte interphase (CEI) layer. Herein, the correct nature of the CEI layer for the NMC811 battery has been studied with a particular attention to the voltage dependency (4.3 V vs. 4.7 V). A CEI metallic layer consisting of transition metal (TM) composites (i.e. 54.9MnOx, 54.9MnF3, 58.7NiO, 58.7NiF3, 58.9CoOx, and 58.9CoF3) has been long considered as a part of inorganic layer. However, the present study found that the metallic layer can be present as an independent CEI layer depending on the cut-off voltages. To further understand the nature of the CEI formation with the cathode material, a density-functional theory was conducted. The Fermi level with respect to the redox couples of the TMs/oxygen orbitals and their covalency not only affect the cathode stability but also modulate the CEI property. It is also proposed that the covalency of TM(3d)-O(2p) is the key factor that determines the localization of the TM metallic layer.

Understanding the Cathode–Electrolyte Interphase in Lithium‐Ion Batteries

The electrode–electrolyte interface is one of the major components enabling Li‐ion batteries (LIBs) to function reversibly. Often, the solid–electrolyte interphase (SEI) at the anode is regarded as the key interface that determines the cycle life, capacity fade, and overall safety of batteries. There are a plethora of SEI literatures that exist; however, the cathode–electrolyte interphase (CEI) remains relatively unexplored. Unlike in the case of SEI, a detailed understanding of CEI formation and its association with battery performance is not present. This review gives insight into the recent progress in understanding the CEI in LIBs. Though there is a relative dearth of literature, the CEI is generally considered as a heterogeneous multicomponent film formed due to the decomposition of electrolyte at the cathode surface. Besides understanding the thermodynamic properties and relevant kinetic reactions, one of the main challenges lies in developing and stabilizing the CEI layer due to its complex structural composition. Extensive research efforts to engineer a stable CEI are reviewed, including the use of electrolyte additives, artificial engineering, and heteroatom doping of cathode. Furthermore, promising characterization techniques and future outlook in forming a robust CEI for both existing LIB and post‐LIB systems are highlighted.

Depth-Dependent Understanding of Cathode Electrolyte Interphase (CEI) on the Layered Li-Ion Cathodes Operated at Extreme High Temperature

The high-temperature operation of Li-ion batteries is highly dependent on the stability of the cathode electrolyte interphase (CEI) formed during lithiation–delithiation reactions. However, knowledge on the nature of the CEI is limited and its stability under extreme temperatures is not well understood. Therefore, herein, we investigate a proof-of-concept study on stabilizing CEI on model LiNi0.33Mn0.33Co0.33O2 (NMC333) at an extreme operation condition of 100 °C using the thermally stable pyrrolidinium-based ionic liquid electrolyte. The electrochemical lithiation–delithiation reactions at 100 °C and the CEI evolution upon different cycling conditions are investigated. Further, the depth-dependent CEI chemistry was investigated using energy-tunable synchrotron-based hard X-ray photoelectron spectroscopy (HAXPES). The results reveal that the high-temperature operation accelerated the CEI formation compared to room temperature, and the surface of the interphase layer is rich in boron-based inorganic moieties than the deeper surface. Further, bulk-sensitive X-ray absorption spectroscopy (XAS) was used to investigate the transition-metal redox contributors during high-temperature electrochemical reactions; similar to room temperature, the Ni2+/4+ redox couple is the only charge-compensating redox couple during high-temperature operation. Finally, the physical nature of the conformal CEI on the cathode particles was visualized with high-resolution transmission electron microscopy, which confirms that the significant degradation of cathode particles without conformal CEI is due to the transformation of a layer-to-spinel formation at extreme temperature. In this study, understanding this high-temperature interfacial chemistry of NMC cathodes through advanced spectroscopy and microscopy will shed light on transforming the ambient-temperature Li-ion chemistry into high-temperature applications.

Design principles for the fabrication of high-performance cathode-electrolyte interphase for Li-ion batteries

Cathode-electrolyte interphase (CEI) is vital to the safety and lifespan of high-voltage cathode materials. However, the CEI formation mechanism in Li-ion batteries remains unclear. In a recent Matter article, Zhang et al. report on a real-time formation mechanism. Here, we discuss challenges and propose design principles in the fabrication of high performance CEI. Cathode-electrolyte interphase (CEI) is vital to the safety and lifespan of high-voltage cathode materials. However, the CEI formation mechanism in Li-ion batteries remains unclear. In a recent Matter article, Zhang et al. report on a real-time formation mechanism. Here, we discuss challenges and propose design principles in the fabrication of high performance CEI. In situ TEM visualization of LiF nanosheet formation on the cathode-electrolyte interphase (CEI) in liquid-electrolyte lithium-ion batteriesZhang et al.MatterFebruary 7, 2022In BriefThe formation behaviors of LiF nanocrystals have been studied on anodes in Li-ion batteries; there is no research work reporting LiF crystal growth on cathodes. Here, we report a real-time LiF crystal formation on positively charged titanium (Ti) electrodes in an electrochemical liquid cell with 150-nm-thick Li hexafluorophosphate (LiPF6)/propylene carbonate (PC) liquid electrolyte loaded. The LiF nanocrystals show two-dimensional (2D) morphologies on the electrode surface, which can serve as a cathode electrolyte interface (CEI). Full-Text PDF