Capacity Fading Mechanism Research Articles

Cathode-Electrolyte Interphase in Lithium Batteries Revealed by Cryogenic Electron Microscopy

Summary Cathode electrolyte interphase (CEI), the intimate coating layer formed on the positive electrode, has been thought to be critical. However, many aspects of CEI remain unclear. This originates from the lack of effective tools to characterize structural and chemical properties of these sensitive interphases at nanoscale. Here, we develop a protocol to preserve the native state and directly visualize the interface on the positive electrode using cryogenic electron microscopy. We find that under normal operation conditions, there does not exist an intimate coating layer at the single-particle level in carbonate-based electrolyte. However, upon brief external electrical shorting, a solid-electrolyte interphase, which usually forms on anodes, could form on cathodes and be electrochemically converted into a stable, conformal CEI in situ. The conformal CEI helps improve Coulombic efficiency and overall capacity retention of the battery. This generates a different perspective of CEI in commercial carbonate-based electrolytes than previously understood.

LiNi0.8Co0.15Ti0.05O2: synthesis by solid state reaction and investigation of structural and electrochemical properties with enhanced battery performance

Solid state synthesis is an essential technique for large-scale production of electrode active materials in battery industry. However, solid state synthesis of LiNi0.8Co0.15Al0.05O2 (NCA), which is a well-known commercial cathode material for Li-ion batteries, provides electrochemically inactive compound. Here, we report the solid state synthesis of LixNi0.8Co0.15Ti0.05O2 where x = 1.03, 1.06, and 1.09, which is a modified version of conventional NCA. Our thorough studies consist of characterization of compounds by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), and magnetization measurements. The results point out the significant effects of Li content on structural and magnetic properties of the samples. Battery performance tests show that Li1.06Ni0.8Co0.15Ti0.05O2 exhibits better cycling properties than conventional NCA. X-ray absorption spectroscopy (XAS) technique is utilized to determine structural modifications upon cycling of this compound via ex-situ analysis. We conclude that substitution of Ti ions in Li1.06Ni0.8Co0.15Ti0.05O2 improves the cycling capability of the cells by reducing the formation of NiO insulating layer which hinders the redox reactions. The capacity value of x = 1.06 sample increases up to 150 mAh g−1 at C/3 rate during cycling and the capacity fade is negative for the first 10 cycles. Possible mechanism for the negative capacity fade is also discussed.

Roles of Mn and Ni in Li-rich Mn-Ni-Fe oxide cathodes

Cobalt free, lithium-rich manganese, nickel, and iron composite oxides (nanoscale mixture of monoclinic, Li2MnO3, and rhombohedral, LiMO2, phases) are promising cathodes as they show attractive electrochemical performance in initial cycles and can reduce the cost and toxicity of lithium ion battery. However, capacity and voltage fading on cycling are the downsides. To better understand the electrochemical performance and degradation mechanism, a series of Li1.2Mn0.40+xNi0.30-xFe0.10O2 materials with x = 0, 0.05, 0.10, and 0.15 was synthesized and systematically studied. The results of this study suggest that two phase composition is beneficial for superior electrochemical performance. The series showed first discharge capacity at 0.1C of 267, 204, 226, and 277 mAhg−1 for x = 0, 0.05, 0.10, and 0.15 respectively, with capacity retention after 100 cycles at 0.3C of 57 %, 57 %, 94 % and 67 %. For the first time we report EXAFS data analysis at all three K-edges: Mn, Ni and Fe before and after extended cycling. Ni reduction to Ni2+ after cycling and voltage fading increased with Ni fraction in the series. There is no trend of Mn reduction (Li2MnO3 activation) on cycling with Mn content indicating composition selection is crucially important for improved performance. The information on changes in electronic states and atomic distances, which allows proposing comprehensive electrochemical (role of Li2MnO3 phase) and degradation (Li+/Ni2+ exchange, voltage fading, and capacity fading) mechanisms.

Sodium‐Ion Batteries: Understanding the Capacity Fading Mechanisms of O3‐Type Na[Ni0.5Mn0.5]O2 Cathode for Sodium‐Ion Batteries (Adv. Energy Mater. 37/2020)

Sodium‐Ion Batteries: Understanding the Capacity Fading Mechanisms of O3‐Type Na[Ni_0.5Mn_0.5]O₂ Cathode for Sodium‐Ion Batteries (Adv. Energy Mater. 37/2020)

Capacity fading mechanism of Li2O loaded NiFe2O4/SiO2 aerogel anode for lithium-ion battery: Ex-situ XPS analysis

The aerogel form of 0–5 wt% of Li2O loaded NiFe2O4/SiO2 composite aerogel were successfully synthesized using the sol-gel process followed by the supercritical ethanol drying. The addition of small Li2O wt.% enhances the electrical conductivity of NiFe2O4/SiO2 composite aerogels. Based on the AC impedance analysis, the high conducting 5% Li2O + 95% [NiFe2O4/SiO2] composite aerogel with conductivity 2.1 × 10−11 S cm−1 was taken for the detailed analysis. We observed the maximum initial reversible discharge capacity of 930 mA h g−1, and it gradually fades to 370 mA h g−1 at 50th discharge. To elucidate the capacity-fading mechanism, the ex-situ X-ray Photoelectron Spectroscopy (XPS) technique is used to identify the chemical composition at different states of charge-discharge cycling. The capacity fading arises mainly from irreversible LixSiOy accumulation, which causes the loss of active lithium. Meanwhile, the metal to metal oxide conversion is partially reversible between Ni0↔ Ni2+, Fe0↔Fe3+ during the subsequent cycles. To further support, the SEM images were taken for the respective electrodes, and the cell resistance and lithium diffusion coefficient (DLi+) was calculated from the electrochemical impedance spectroscopy (EIS) at the respective samples taken for ex-situ XPS analysis. The DLi+ gradually decreases from 1.8 × 10−13 cm2 s−1 to 9.6 × 10−14 cm2 s−1, and the cell resistance increases during the 1st to 10th discharge cycle. We conclude that the prolonged accumulation of irreversible LixSiOy phase will hinder the DLi+ in subsequent cycles, which leads to capacity-fading. The detailed results and electrochemical performance will be discussed.

Radical Charge Population and Energy: Critical Role in Redox Potential and Cycling Life of Piperidine Nitroxyl Radical Cathodes in Aqueous Zinc Hybrid Flow Batteries.

Redox-active 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) derivatives have recently been investigated to expand the choice of catholyte for aqueous flow batteries (AFBs). However, the effects of substituent R in 4-position on redox potential and corresponding capacity fading mechanism are still unclear. Here, we conduct comparative studies of four R-TEMPO with R = -OH, -NH2, -COOH, and -NHCOCH3 in zinc hybrid AFBs. Experimental and theoretical analyses reveal that low-radical head charge population sum and radical energy, depending on R in 4-position, play a critical role in enhancing redox potential and cycling life of R-TEMPO. The electronic effect brought along by N-acetyl could redistribute the charge and lower systematic energy, making the ring-opening joint sturdy and therefore suppress the side reactions. Accordingly, the 4-NHCOCH3-TEMPO/Zn battery achieves a high capacity retention of >99.65%/day and an open-circuit voltage of 1.71 V. Our findings on the effects of substituent are greatly anticipated to boost the high-energy density, long-life, and eco-friendly TEMPO-based AFBs.

Capacity fading mechanism of conversion-type FeF3 electrode: Investigation by electrochemical operando nuclear magnetic resonance spectroscopy

FeF3 has attracted considerable attention as a positive electrode material for next-generation rechargeable lithium ion batteries, because of its low cost, low risk, and high energy density, which facilitate a conversion-type lithiation/delithiation reaction. However, the conversion reaction of the FeF3 electrode is known to suffer from capacity fading during repeated discharge–charge cycles. Herein, we find an interesting correlation between capacity fading behavior and spectral evolutions in electrochemical operando nuclear magnetic resonance (NMR) measurements. The operando 7Li NMR spectra demonstrate the reversible formation of metallic Fe by the conversion process during the early discharge–charge cycles. However, it is gradually suppressed after repeated cycles. Moreover, LiF is augmented at the fully charged states, indicating that FeF3 is no longer recovered after repeated cycles. The active material can converge into FeF2 and LiF in the degraded electrode. Another factor associated with capacity degradation is the electrolyte decomposition occurring at high voltages, which results in a resistive film coating the electrode surface. We therefore conclude that the film accumulation on repeated discharging inhibits the conversion reaction to metallic Fe and LiF, leading to a characteristic capacity fading behavior of FeF3.

Understanding the Capacity Fading Mechanisms of O3‐Type Na[Ni0.5Mn0.5]O2 Cathode for Sodium‐Ion Batteries

AbstractA spherical O3‐type Na[Ni0.5Mn0.5]O2 cathode, composed of compactly‐packed nanosized primary particles, is synthesized by the coprecipitation method to examine its capacity fading mechanism. The electrochemical performance cycled at different upper cut‐off voltages demonstrate that the P3′ to O3′ phase transition above 3.6 V is primarily responsible for the loss of the structural stability of the O3‐type Na[Ni0.5Mn0.5]O2 cathode. The capacity retention is greatly improved by avoiding the P3′ to O3′ phase transition, and 94.2% and 90.7% of the initial capacities (108.9 mAh g−1 at 3.35 V and 125.4 mAh g−1 at 3.58 V) are retained after 100 cycles. During cycling at 4.0 V, rapid capacity fading (75.5% of 147.5 mAh g−1 after 100 cycles) is observed. The poor Na+ ion intercalation stability is directly attributed to the extent of microcracks caused by the abrupt change in the lattice structure. Microcracks traversing the entire secondary particle compromise the mechanical integrity of the cathode and accelerate electrolyte infiltration into the particle interior, causing the subsequent degradation of the exposed internal surfaces. Thus, suppressing microcracks in secondary particles is one of the key challenges for improving the cycling stability of hierarchical structured O3‐type Na[Ni0.5Mn0.5]O2 cathodes.

Effects of ester-based electrolyte composition and salt concentration on the Na-storage stability of hard carbon anodes

Hard carbon (HC) is the most feasible anode for Na-ion batteries (NIBs). However, HC electrodes suffer from low initial coulombic efficiency (ICE), poor rate capability and cyclic instability. Here, a systematic investigation was carried out to understand the effect of Na salt, esters-based solvent and concentration on the electrochemical performance of hard carbon spheres (HCSs) electrodes. Based on the experimental results, 2 M NaTFSI/DC:DMC electrolyte delivers the best initial reversible capacities of 332 mAh·g−1, high ICE of 83.5% and excellent electrochemical stability of 90% after 100 cycles at 0.1C and 80% after 300 cycles at 1C. In addition, the drastic capacity fading phenomenon of HCSs electrode occurs in ultrahigh concentration electrolyte (5 M NaTFSI/EC:DMC) after cycles, and this drastic capacity fading mechanism is revealed by the ex-situ strategies of the Raman, SEM and capacity-recovery experiment. The proposed moderate concentrated electrolyte and failure mechanism of ultrahigh concentration electrolyte for HC electrode provide further insights in developing preferable electrolyte for NIBs.

Cagelike CoSe2@N-Doped Carbon Aerogels with Pseudocapacitive Properties as Advanced Materials for Sodium-Ion Batteries with Excellent Rate Performance and Cyclic Stability.

Electrochemical conversion reaction based electrodes offer a high sodium storage capacity in rechargeable batteries by utilizing the variable valence states of transition metals. Thus, transition metal chalcogenides (TMCs) as such materials have been intensively investigated in recent years to explore the possibilities of practical application in rechargeable sodium-ion batteries; however, it is hindered by poor rate performance and a high-cost preparation method. In addition, some issues in regards to conversion reactions remain poorly understood, including incomplete reversible reaction processes, polarization, and hysteresis. Herein, a novel cagelike CoSe2@N-doped carbon aerogels hybrid composite was designed and prepared by a facile and high-efficiency sol-gel technology. Benefiting from the surface engineering optimization, high charge transfer, and low-energy diffusion barrier, the CoSe2@N-doped carbon aerogels exhibit a high pseudocapacitive property. Most importantly, the CoSe2 anode has been carefully investigated at different discharge/charge states by X-ray absorption near edge spectroscopy technologies and density functional theory (DFT) simulations, which deeply reveal the capacity fading mechanism and phase transition behavior.

Understanding capacity fading mechanism of thick electrodes for lithium-ion rechargeable batteries

The use of thick electrodes with high-loading density of active material is one of the most practical strategies to increase the volumetric/specific energy density of lithium-ion battery, while taking advantage of the current electrode chemistry. However, their use is accompanied by serious deterioration of electrochemical performance, especially exhibiting poor capacity retention with low power capability. Here, the degradation behavior of the LiNi0·6Co0·2Mn0·2O2, one of widely adopted cathodes, is comparatively investigated under high loading levels as high as 28 mg cm−2 over the extended cycling. It is revealed that the charge transport limitation is cumulatively dominated by the lithium ionic diffusion rather than the electronic conduction in the thick electrode. More importantly, as the cycle proceeds, the thick electrode gets exposed to a serious reaction inhomogeneity because of the negative feedback between the accumulated ion transport limitation and locally increasing resistance. It leads to the generation of current hot spot in the electrode and the corresponding local material degradation, which further inhibit the charge transport, resulting in unavoidable capacity fading. This finding proposes that rational electrode architecture detouring the hot spot generation needs to be considered with respect to the ion transport and the electrode material degradation toward the high-loading electrodes.

In-Situ Monitoring of Electrode Degradation in Vanadium Redox-Flow Batteries By Distribution of Relaxation Times Analysis

Recent studies[1,2] suggest that ion crossover through the membrane of a vanadium redox-flow battery (VRFB) can not be considered the only source of irreversible capacity loss. Instead, degradation of the macroporous carbon felt electrodes has to be taken into account as well. Especially in the negative half-cell, any degradation-induced alteration of the fibres’ surface composition will significantly impede the kinetics of the V2+/V3+ redox-system. Since the resulting increase in kinetic overvoltage will limit the accessible capacity, a reasonable lifetime prediction of a VRFB is only possible if all aspects of capacity fade mechanisms, including electrode degradation are well understood. This raises the demand for a measurement technique, which allows for in-operando state-of-health-assessment of the electrodes in a non-invasive fashion. Employing electrochemical impedance spectroscopy (EIS), the characteristics of any electrochemical system can be easily probed by applying non-harmful, small-amplitude sinusoidal perturbations. Interpretation of EIS usually relies on fitting suitable equivalent circuit models to experimental data. However, validation of such models can be a cumbersome task due to the inherent ambiguity of EIS, meaning that different circuits may approximate the same data set equally well. Furthermore, a full cell spectrum will always yield a superposition of the respective half-cell impedances. Discriminating the contributions of the individual half-cell electrodes to the impedance of a VRFB has therefore been attempted by implementing reference electrodes. However, these are known to be a source of artefacts and distortion in the high frequency range.In the present study we apply the distribution of relaxation times (DRT) analysis[3] to EIS data of a single cell VRFB. This innovative approach allows us to resolve up to three different time constants per frequency decade. It therefore enables processes that overlap in the classical Bode- and Nyquist representation to be displayed as distinct peaks. Consequently, the individual half-cells can be studied from full cell spectra, circumventing the necessity of finding appropriate equivalent circuit models and rendering the implementation of reference electrodes obsolete.We outline our experimental strategy for unravelling the physical nature of the processes related to the individual signals in the DRT spectrum and finally demonstrate how these findings enable an online monitoring of electrode degradation in VRFB. Keywords: electrochemical impedance spectroscopy, distribution of relaxation times, electrode degradation, vanadium redox-flow battery

Microstructural analyses of all-solid-state Li–S batteries using LiBH4-based solid electrolyte for prolonged cycle performance

Abstract Complex hydride materials have been widely investigated as potential solid electrolytes because they have good compatibility with the lithium metal anodes used in all-solid-state batteries. However, the development of all-solid-state batteries utilizing complex hydrides has been difficult as these cells tend to have short cycle lives. This study investigated the capacity fading mechanism of all-solid-state lithium–sulfur (Li–S) batteries using Li4(BH4)3I solid electrolytes by analyzing the cathode microstructure. Cross-sectional scanning electron microscopy observations after 100 discharge–charge cycles revealed crack formation in the Li4(BH4)3I electrolyte and an increased cathode thickness. Raman spectroscopy indicated that decomposition of the Li4(BH4)3I solid electrolyte occurred at a constant rate during the cycling tests. To combat these effects, the cycle life of Li–S batteries was improved by increasing the amount of solid electrolyte in the cathode.

Self-discharge tests to measure side-reaction currents of a Li[Li1/3Ti5/3]O4 electrode

The lifetime performance of lithium-ion batteries is a critical issue for automobile and stationary applications. The difference in the side-reaction current (ISR) of electrodes causes deviations of the state of charge (SOC) of the electrodes leading to the capacity fading of lithium-ion batteries. Establishment of a method to measure the ISR is important for understanding the capacity fading mechanism. We report herein that a novel and simple method to determine the ISR in lithium-ion batteries, the self-discharge test, was developed and applied to lithium-ion cells with lithium titanium oxide (Li[Li1/3Ti5/3]O4, LTO), lithium aluminum manganese oxide (Li[Li0.1Al0.1Mn1.8]O4, LAMO), and lithium nickel manganese oxide (Li[Ni1/2Mn3/2]O4, LiNiMO) as electrodes. According to the self-discharge test results, the ISR of LTO is affected by another electrode of LAMO or LiNiMO. The ISR of LTO in LTO/LiNiMO cells larger than that in LTO/LAMO cells is explained by the additional-ISR of LTO, which results from side reactions such as the reduction of oxidized products generated at the positive electrode. The side reactions at the positive electrode are accelerated with increasing electrode potential, meaning that the higher potential of the positive electrode resulted in the larger additional-ISR of LTO. The real side-reaction current of the LTO electrode in lithium-ion cells is the sum of the intrinsic and the additional current (real-ISR = intrinsic-ISR + additional-ISR).

Interfacial Bonding of Metal‐Sulfides with Double Carbon for Improving Reversibility of Advanced Alkali‐Ion Batteries

AbstractEngineering interfacial properties of metal‐sulfides toward excellent electrochemical capability is imperative for advanced energy‐storage materials. However, they still suffer from an unclear mechanism of capacity fading, along with ineffective physical–chemical evolution. Herein, a highly‐effective Sb2S3 with double carbon is designed with interfacial SbC bonds and double carbon, which boosts promoting of ion transferring and alleviates the separation of both active phases (Sb, S). Through “voltage‐cutting” manners, the key elements of capacity improvement about phase transitions are further determined. As expected, even at 5.0 A g−1, the lithium‐storage capacity remains about 674 mAh g−1. Utilized as sodium ion battery (SIB) anode, the rate capacity still reaches up to 366 mAh g−1 at 3.0 A g−1, much larger than that of Sb2S3. Obtaining the full cell of Ni–Fe Prussian blue analog versus M‐Sb2S3@DC, the reversible capacity is 330 mAh g−1 at 0.5 A g−1. Supported by kinetic analysis, the excellent rate properties are determined by the surface‐controlling behaviors, mainly resulting from the decreased capacitive resistance and improved ion moving. Furthermore, the reassembling evolution of active phases is revealed in detail by ex situ techniques. This work is expected to offer significant insights into interfacial evolutions toward advanced energy‐storage systems.

Toward quantifying capacity losses due to solid electrolyte interphase evolution in silicon thin film batteries.

To understand the origins of failure and limited cycle life in lithium-ion batteries (LIBs), it is imperative to quantitatively link capacity-fading mechanisms to electrochemical and chemical processes. This is extremely challenging in real systems where capacity is lost during each cycle to both active material loss and solid electrolyte interphase (SEI) evolution, two indistinguishable contributions in traditional electrochemical measurements. Here, we have used a model system in combination with (1) precision measurements of the overall Coulombic efficiency via electrochemical experiments and (2) x-ray reflectivity measurements of the active material losses. The model system consisted of a 515 Å thick amorphous silicon (a-Si) thin film on silicon carbide in half-cell geometry using a carbonate electrolyte with LiPF6 salt. This approach allowed us to quantify the capacity lost during each cycle due to SEI evolution. Combined with electrochemical analysis, we identify SEI growth as the major contribution to capacity fading. Specifically, the continued SEI growth results in increasing overpotentials due to increased SEI resistance, and this leads to lower extent of lithiation when the cutoff voltage is reached during lithiation. Our results suggest that SEI grows more with increased time spent at low voltages where electrolyte decomposition is favored. Finally, we extracted a proportionality constant for SEI growth following a parabolic growth law. Our methodology allows for the quantitative determination of lithium-ion loss mechanisms in LIBs by separately tracking lithium ions within the active materials and the SEI and offers a powerful method of quantitatively understanding LIB loss mechanisms.

Capacity fading of a LiFePO4/graphite cell during shallow cycling due to planar inhomogeneity in LiFePO4

The capacity fading mechanism of a LiFePO4/graphite cell (nominal capacity 3 Ah) was investigated. We report that capacity fading resulted from the centimeter-scale planar inhomogeneous reaction on the LiFePO4 cathode after operation at a shallow state of charge (SOC), operated between SOC 40% and 60%. It was confirmed by (i) wide variations of electrode capacities of the disassembled electrode and by (ii) the two-dimensional X-ray diffraction (XRD) analysis. Li-ion content could be estimated by (i) comparing the residual capacities of several electrode regions of the LiFePO4 sheet after disassembly of the cell and (ii) the intensity ratio variations of the typical XRD peaks of LiFePO4 and FePO4 at 5 mm × 5 mm grids. The inhomogeneity was induced by using only the two-phase region in the LiFePO4. Electrode reversible capacities are not a dominant factor for capacity fading, and we found the inhomogeneity has a possibility for restoration. Therefore, we should consider a suitable operating procedure of the storage system for electricity grid use with long-term service life.

Does Polarization Increase Lead to Capacity Fade?

“Polarization increase reduces capacity” is frequently used to explain capacity fading in rechargeable batteries. To verify this empirical law, failure mode and effect analysis (FMEA) was used to identify capacity fade mechanism and derive the contribution of each failure mode in graphite–LiCoO2 cells cycled between 3.00 V and 4.35 V. The thermodynamic and kinetic attributes to the capacity fade at the material, electrode, and cell levels were quantified respectively. Loss of Li inventory dominates in the capacity fade, followed by the loss of active materials in the electrodes. The capacity loss due to the impedance increase in the cell was relatively insignificant, contrary to what often conceived. This work emphasizes the importance of using quantitative FMEA to assess cell degradation and conduct failure analysis so the contributions from material, electrode, to the cell level can be distinctly identified. The polarization increase does not affect the charge retention significantly.

Insights into P2-Type Layered Positive Electrodes for Sodium Batteries: From Long- to Short-Range Order.

P2-type Fe- and Mn-based layered sodium transition metal oxides are promising positive electrode materials for sodium batteries due to their high energy density and low costs of the constituting transition metals. However, poor structural reversibility and fast capacity decay have prevented their breakthrough so far. Herein, the real-time dynamic phase transitions and capacity fading mechanism of the P2 Na0.67Fe0.5Mn0.5O2 positive electrode are revealed by operando X-ray diffraction, operando/ex situ X-ray absorption spectroscopy, neutron powder diffraction, and neutron pair distribution functions. Upon the desodiation process, a layered OP4 phase with long-range order is found as an intermediate state. With further deep desodiation, the formation of a Na-depleted ramsdellite phase with a short coherent length of 30 Å is observed for the first time. However, the transition from OP4 to ramsdellite is considered to be irreversible due to the breakdown of the layered structural characteristics, resulting in poor cycling performance in a variety of Fe-based layered sodium transition metal oxides. This work suggests that stabilizing the crystal structure by substitution or chemical modification can be a favorable strategy to avoid the degradation of positive electrodes and thus to improve the cycling performance.

Design of Thin-Film Interlayer between Silicon Electrode and Current Collector Using a Chemo-Mechanical Degradation Model

To enhance delamination limitations in silicon electrode, a thin-film interlayer between silicon electrode and copper current collector is designed using a chemo-mechanical degradation model. The chemo-mechanical degradation model considers the formation of the solid electrolyte interphase on the surface and within the cracks of the silicon electrode, the physical isolation of active materials and the resistance due to loss of contact between the silicon composite electrode and the copper foil as the main capacity fading mechanisms. The model is validated with experimental data collected from coin cells made of silicon electrode with a bare and an adhesive thin film laminated copper foil. The reduction in the delamination limitations depends on the interplay of the adhesion strength, conductivity, coverage and thickness of adhesive thin film on the surface of the copper foil.