Solid Electrolyte Interphase Growth Research Articles

Binary Cation Matrix Electrolyte and Its Effect on Solid Electrolyte Interphase Suppression and Evolution of Si Anode.

An unstable solid electrolyte interphase (SEI) has been recognized as one of the biggest challenges to commercializing silicon (Si) anodes for high-energy-density batteries. This work thoroughly investigates a binary cation matrix of Mg2++Li+ electrolyte and its role in SEI development, suppression, and evolution of a Si anode. Findings demonstrate that introducing Mg ions dramatically reduces the SEI growth before lithiation occurs, primarily due to the suppression of solvent reduction, particularly ethylene carbonate (EC) reduction. The Mg2+ alters the Li+ cation solvation environment as EC preferably participates in the oxophyllic Mg2+ solvation sheath, thereby altering the solvent reduction process, resulting in a distinct SEI formation mechanism. The initial SEI formation before lithiation is reduced by 70% in the electrolyte with the presence of Mg2+ cations. While the SEI continues to develop in the postlithiation, the inclusion of Mg ions results in an approximately 80% reduction in the postlithiation SEI growth. Continuous electrochemical cycling reveals that Mg2+ plays a crucial role in stabilizing the deep-lithiated Si phases, which effectively mitigates side reactions, resulting in controlled SEI growth and stable interphase while eliminating complex LixSiy formation. Mg ions promote the development of a notably more rigid and homogeneous SEI, characterized by a reduced dissipation (ΔD) in the Mg2++Li+ ion matrix compared to the solely Li+ system. This report reveals how the Mg2++Li+ ion matrix affects the SEI evolution, viscoelastic properties, and electrochemical behavior at the Si interface in real time, laying the groundwork for devising strategies to enhance the performance and longevity of Si-based next-generation battery systems.

Elucidating the Transport of Electrons and Molecules in a Solid Electrolyte Interphase Close to Battery Operation Potentials Using a Four-Electrode-Based Generator-Collector Setup.

In lithium-ion batteries, the solid electrolyte interphase (SEI) passivates the anode against reductive decomposition of the electrolyte but allows for electron transfer reactions between anode and redox shuttle molecules, which are added to the electrolyte as an internal overcharge protection. In order to elucidate the origin of these poorly understood passivation properties of the SEI with regard to different molecules, we used a four-electrode-based generator-collector setup to distinguish between electrolyte reduction current and the redox molecule (ferrocenium ion Fc+) reduction current at an SEI-covered glassy carbon electrode. The experiments were carried out in situ during potentiostatic SEI formation close to battery operation potentials. The measured generator and collector currents were used to calculate passivation factors of the SEI with regard to electrolyte reduction and with regard to Fc+ reduction. These passivation factors show huge differences in their absolute values and in their temporal evolution. By making simple assumptions about molecule transport, electron transport, and charge transfer reaction rates in the SEI, distinct passivation mechanisms are identified, strong indication is found for a transition during SEI growth from redox molecule reduction at the electrode | SEI interface to reduction at the SEI | electrolyte interface, and good estimates for the transport coefficients of both electrons and redox molecules are derived. The approach presented here is applicable to any type of electrochemical interphase and should thus also be of interest for interphase characterization in the fields of electrocatalysis and corrosion.

Two-Step Growth Mechanism of the Solid Electrolyte Interphase in Argyrodyte/Li-Metal Contacts.

The structure and growth of the solid electrolyte interphase (SEI) region between an electrolyte and an electrode is one of the most fundamental yet less well-understood phenomena in solid-state batteries. We present an atomistic simulation of the SEI growth for one of the currently promising solid electrolytes (Li6PS5Cl), based on ab initio-trained machine learning interatomic potentials, for over 30,000 atoms during 10 ns, well beyond the capabilities of conventional molecular dynamics. This unveils a two-step growth mechanism: a Li-argyrodite chemical reaction leading to the formation of an amorphous phase, followed by a kinetically slower crystallization of the reaction products into a 5Li2S·Li3P·LiCl solid solution. The simulation results support the recent, experimentally founded hypothesis of an indirect pathway of electrolyte reduction. These findings shed light on the intricate processes governing SEI evolution, providing a valuable foundation for the design and optimization of next-generation solid-state batteries.

Physics-Informed Design of Hybrid Pulse Power Characterization Tests for Rechargeable Batteries

Industry-standard diagnostic methods for rechargeable batteries, such as hybrid pulse power characterization (HPPC) tests for hybrid electric vehicles, provide some indications of state of health (SoH), but lack a physical basis to guide protocol design and identify degradation mechanisms. We develop a physics-based theoretical framework for HPPC tests, which are able to accurately determine specific mechanisms for battery degradation in porous electrode simulations. We show that voltage pulses are generally preferable to current pulses, since voltage-resolved linearization more rapidly quantifies degradation without sacrificing accuracy or allowing significant state changes during the measurement. In addition, asymmetric amounts of information gain between charge /discharge pulses are found from differences in electrode kinetic scales. We demonstrate our approach of physics-informed HPPC on simulated Li-ion batteries with nickel-rich cathodes and graphite anodes. Multivariable optimization by physics-informed HPPC rapidly determines kinetic parameters that correlate with degradation phenomena at the anode, such as solid-electrolyte interphase (SEI) growth and lithium plating, as well as at the cathode, such as oxidation-induced cation disorder. If validated experimentally, standardized voltage protocols for HPPC tests could play a pivotal role in expediting battery SoH assessment and accelerating materials design by providing new electrochemical features for interpretable machine learning of battery degradation.

Long‐Life High‐Voltage Sodium‐Ion Batteries Enabled by Electrolytes with Cooperative Na+‐Solvation

AbstractStabilizing the electrode interphases is urgently required to enhance the lifetime of high‐voltage sodium‐ion batteries (SIBs). However, the continuous anode solid–electrolyte interphase (SEI) growth associated with electron leakage and the fragile cathode–electrolyte interphase (CEI) lead to capacity fade at high voltage; and yet the solvation‐interphase‐performance relationship is inadequately addressed. Herein, a cooperative Na+‐solvation strategy is reported to stabilize the interphases by a holistic design of electrolytes combining soft and moderate co‐solvents. The rationally regulated Na+‐solvation leads to CEI/SEI with the desired thickness and component stability. As such, remarkable cycling stability is achieved for 4.3‐V Na3V2O2(PO4)2F (NVOPF) cathodes with 83.3% capacity retention over 3000 cycles at 1 C, significantly outperforming the carbonate counterpart (41.6% capacity retention). Meanwhile, the restrained SEI growth via reducing the formation of electron‐leaking Na2CO3 stabilizes the long‐term cycling of the hard carbon (HC) anode. The assembled NVOPF||HC full cells achieve superior rate capability (up to 15 C) and stable cycling stability over 500 cycles. The demonstrated engineering of electrolyte chemistry, Na+‐solvation, and interphase structure/component contributes toward the rational establishment of design rules for high‐voltage SIBs and possibly other similar chemistries.

Flexible bidirectional pulse charging regulation achieving long-life lithium-ion batteries

Typical application scenarios, such as vehicle to grid (V2G) and frequency regulation, have imposed significant long-life demands on lithium-ion batteries. Herein, we propose an advanced battery life-extension method employing bidirectional pulse charging (BPC) strategy. Unlike traditional constant current charging methods, BPC strategy not only achieves comparable charging speeds but also facilitates V2G frequency regulation simultaneously. It significantly enhances battery cycle ampere-hour throughput and demonstrates remarkable life extension capabilities. For this interesting conclusion, adopting model identification and postmortem characterization to reveal the life regulation mechanism of BPC: it mitigates battery capacity loss attributed to loss of lithium-ion inventory (LLI) in graphite anodes by intermittently regulating the overall battery voltage and anode potential using a negative charging current. Then, from the perspective of internal side reaction, the life extension mechanism is further revealed as inhibition of solid electrolyte interphase (SEI) and lithium dendrite growth by regulating voltage with a bidirectional pulse current, and a semi-empirical life degradation model combining SEI and lithium dendrite growth is developed for BPC scenarios health management, the model parameters are identified by genetic algorithm with the life simulation exhibiting an accuracy exceeding 99%. This finding indicates that under typical rate conditions, adaptable BPC strategies can extend the service life of LFP battery by approximately 123%. Consequently, the developed advanced BPC strategy offers innovative perspectives and insights for the development of long-life battery applications in the future.

Lignin‐derived hard carbon anode with a robust solid electrolyte interphase for boosted sodium storage performance

AbstractHard carbon is regarded as a promising anode candidate for sodium‐ion batteries due to its low cost, relatively low working voltage, and satisfactory specific capacity. However, it still remains a challenge to obtain a high‐performance hard carbon anode from cost‐effective carbon sources. In addition, the solid electrolyte interphase (SEI) is subjected to continuous rupture during battery cycling, leading to fast capacity decay. Herein, a lignin‐based hard carbon with robust SEI is developed to address these issues, effectively killing two birds with one stone. An innovative gas‐phase removal‐assisted aqueous washing strategy is developed to remove excessive sodium in the precursor to upcycle industrial lignin into high‐value hard carbon, which demonstrated an ultrahigh sodium storage capacity of 359 mAh g−1. It is found that the residual sodium components from lignin on hard carbon act as active sites that controllably regulate the composition and morphology of SEI and guide homogeneous SEI growth by a near‐shore aggregation mechanism to form thin, dense, and organic‐rich SEI. Benefiting from these merits, the as‐developed SEI shows fast Na+ transfer at the interphases and enhanced structural stability, thus preventing SEI rupture and reformation, and ultimately leading to a comprehensive improvement in sodium storage performance.

Anionic and cationic co-driving strategy for enhanced lithium storage and migration on Si-based anodes

Alloy-based anodes are considered to be a promising choice for next-generation high-energy density devices; nevertheless, lithiation-induced anisotropic swelling and ongoing solid electrolyte interphase growth and cracking severely limit practical applicability. Herein, an anionic and cationic co-driving strategy is proposed for the alloy-based (Si) anode that aims to improve structural stability with controlled ion migration pathways and enhanced reaction kinetics, eventually leading to better capacity, high-rate performance, and cycle performance for lithium storage. Ex-situ tests and density functional theory simulations show that Co-HHTP has both anionic and cationic co-storage capabilities. Furthermore, the strong interactions between Co-HHTP and anionic species may impede anion transport towards the silicon surface, hence mitigating the recurrent degradation of the solid electrolyte interphase. As a proof of concept, the Si-based anode, fitted with Co-HHTP, delivers a high initial Coulombic efficiency of 80.4 %, a large reversible capacity (1648.0 mAh g−1 at 0.2 A g−1), and an ultralow attenuation rate of 0.034 % per cycle over 1000 cycles. The proposed approach provides a new strategy for a high-performance anode through functional coating structural construction coupled with anionic and cationic co-storage that confine anion diffusion and facilitate lithium storage and migration.

Interface engineering of Si-based anodes with fluorinated binder enabling lean-additive lithium-ion batteries

Silicon (Si) is a promising alternative to graphite anode for increasing the energy density of litihum-ion batteries owing to its high theoretical capacity. However, the severe volume change of Si during cycling leads to unstable solid electrolyte interphase (SEI) growth, resulting in rapid capacity fading, particularly under lean-additive electrolyte conditions. In this study, we introduce an innovative approach to interface engineering of Si anodes using a fluorinated binder, a method that forms a stable and uniform lithium fluoride (LiF)-based SEI layer under lean-additive electrolyte conditions without the need for common additives. The LiF layer not only improves the structural stability of Si electrodes but also enhances the electrolyte stability, thereby preventing undesired SEI growth by electrolyte decomposition. Benefiting from this proposed binder, the Si-based anode retains a high reversible capacity (93.7 %) after 400 cycles at 0.9 A g−1 and the full cell, pairing with LiNi0.8Co0.1Mn0.1O2 cathode, delivers a high areal capacity of 4.23 mAh cm−2 (93.4 % retention) after 200 cycles under a lean-additive electrolyte (only 3 wt% additive). The interface stabilization by fluorinated binder provides insights into the rational design of a binder for the application of Si anodes in high-energy-density lithium-ion batteries.

Consistently Tuned Battery Lifetime Predictive Model of Capacity Loss, Resistance Increase, and Irreversible Thickness Growth

Predicting changes in cell resistance and thickness as the battery ages can be as important as capacity fade in informing vehicle resale value, pack replacement schedules, and viability for repurposing before eventual recycling. Three well-known degradation mechanisms, namely SEI growth, lithium plating, and electrode particle fracture due to reversible expansion and contraction, are revisited and tuned using capacity fade data from cells experiencing plating and having significant loss of anode active material as the dominant degradation mechanism. Therefore, lifetime prediction achieved in this work cannot rely on SEI degradation that can be tuned mostly with calendar aging data. More importantly, our model uses capacity-predictive irreversible SEI growth, net growth in plating, and accumulation of particle fracture as the cells cycle to predict the irreversible increases in cell thickness through a single set of tuned parameters. Last but not least, the capacity-predictive degradation can also predict the increased resistance through another single set of parameters. These two sets of scaling parameters achieve for the first time a consistently tuned model of degradation mechanisms for lifetime prediction of changes in battery capacity, resistance, and thickness as the battery ages. The model prediction has been validated in conditions not used for parameter tuning.

Current Imbalance in Dissimilar Parallel-Connected Batteries and the Fate of Degradation Convergence

Abstract This paper proposes an analytical framework describing how initial capacity and resistance variability in parallel-connected battery cells may inflict additional variability or reduce variability while the cells age. We derive closed-form equations for current and SOC imbalance dynamics within a charge or discharge cycle. These dynamics are represented by a first-order equivalent circuit model and validated against experimental data. To demonstrate how current and state of charge (SOC) imbalance leads to cell degradation, we developed a successive update scheme in which the intercycle imbalance dynamics update the intracycle degradation dynamics, and vice versa. Using this framework, we demonstrate that current imbalance can cause convergent degradation trajectories, consistent with previous reports. However, we also demonstrate that different degradation assumptions, such as those associated with SOC imbalance, may cause divergent degradation. We finally highlight the role of different cell chemistries, including different OCV function nonlinearities, on system behavior, and derive analytical bounds on the SOC imbalance using Lyapunov analysis.

Delineating the Impact of Transition-Metal Crossover on Solid-Electrolyte Interphase Formation with Ion Mass Spectrometry.

Lithium-metal batteries (LMB) employing cobalt-free layered-oxide cathodes are a sustainable path forward to achieving high energy densities, but these cathodes exhibit substantial transition-metal dissolution during high-voltage cycling. While transition-metal crossover is recognized to disrupt solid-electrolyte interphase (SEI) formation on graphite anodes, experimental evidence is necessary to demonstrate this for lithium-metal anodes. In this work, advanced high-resolution 3D chemical analysis is conducted with time-of-flight secondary-ion mass spectrometry (TOF-SIMS) to establish spatial correlations between the transition metals and electrolyte decomposition products found on cycled lithium-metal anodes. Insights into the localization of various chemistries linked to crucial processes that define LMB performance, such as lithium deposition, SEI growth, and transition-metal deposition are deduced from a precise elemental and spatial analysis of the SEI. Heterogenous transition-metal deposition is found to perpetuate both heterogeneous SEI growth and lithium deposition on lithium-metal anodes. These correlations are confirmed across various lithium-metal anodes that are cycled with different cobalt-free cathodes and electrolytes. An advanced electrolyte that is stable to higher voltages is shown to minimize transition-metal crossover and its effects on lithium-metal anodes. Overall, these results highlight the importance of maintaining uniform SEI coverage on lithium-metal anodes, which is disrupted by transition-metal crossover during operation at high voltages.

Unraveling the Interplay between Degradation Mechanisms during Battery Cycling Conditions: Influence on SEI Growth

Although the solid electrolyte interphase (SEI) is integral for the operation and stability of Li-ion batteries, its continuous growth over the battery life leads to in loss of cyclable material and overall degradation which results in capacity face. Therefore, a proper understanding of the mechanisms and factors involved in the SEI evolution during battery operation is integral for both reliable ageing models, allowing proper prognosis of the state of health (SoH), and improvement of battery lifetime.Generally, studies of SEI growth have focused on calendar ageing experiments, in which the battery is left undisturbed for long periods of time and periodic check-up cycles are performed to estimate the capacity loss over time. Despite significant advances in the understanding of SEI formation with this kind of experiments, the specific limiting mechanism for SEI growth is still under discussion, with electron or Li-interstitial diffusion through the SEI being the main candidates in recent studies. Calendar ageing provides a suitable framework to study SEI growth in almost isolation from other degradation mechanisms but neglects the synergistic effects that can occur between them in real operation conditions, in which the SEI growth will be influenced by other degradation processes. For example, it has been reported that lithium plating can favor SEI formation and that new SEI will growth in exposed active material through cracks in the particles or in the existing SEI. Although initial efforts towards accounting for these synergies are already present in the literature, they have been applied to sparse sets of data and particularized for single chemistries.In this work we focus on the study SEI growth during cell operation and its interplay with other degradation mechanisms in the cell in different regimes to partially isolate degradation processes (e.g., varying cut-off voltages to favor lithium plating). To this end, we carry out electrochemical cycling tests with LNO/Gr and NMC811/Gr full coin cells and also assemble 3-electrode cells to differentiate between positive and negative electrode contributions. Moreover, the individual contributions from the different degradation mechanisms are assessed by combining EIS measurements, incremental capacity analysis and morphological characterization.The results of the characterization are then used to parametrize the implementation of degradation models (SEI, particle cracking, lithium plating, loss of active material...), with both semi-empirical and physics-based approaches, within a Doyle-Fuller-Newman pseudo-2D framework. The results of the model are then compared with the full coin cells experiments and used to evaluate the degradation synergies and provide best-practice approaches to balance battery performance and lifetime. Moreover, the flexibility of the model with degradation is tested using larger publicly available and literature datasets for different chemistries to assess its suitability in cases for which the complete model parameterization is not available (as it is often the case in commercial cells).

Battery Venting Caused By Fast Charging

Lithium-ion batteries (LIBs) experience high risks of failure during unreasonable fast charging. Various degradation mechanisms inside the LIBs such as lithium (Li) plating, solid electrolyte interphase growth and electrolyte dry-out can undermine the electrochemical performance and induce massive gas generation alongside the operation. As a typical failure behaviour, venting occurs when the internal pressure of battery cell exceeds the safety threshold, releasing the gaseous products as well as depressurizing the cell. Recent studies1–3 of thermal runaway (TR) tests after fast charging have found that accumulated plated Li can react with electrolyte at elevated external temperatures, decreasing both self-heating and onset temperatures of TR. Yet, how fast-charging degradation will affect battery venting behaviours remains unclear.To answer this question, in this study, a series of fast charging experiments under controlled cooling as well as adiabatic TR tests were conducted with 51 Ah LiNi0.6Mn0.2Co0.2O2/graphite prismatic LIBs. The results showed that the prismatic battery underwent severe Li plating with over 30% capacity fade after 10 cycles of 4C charging. In the adiabatic TR tests, the aged batteries after 4C charging exhibited lower self-heating temperature and venting temperature as compared to the fresh batteries, by 51 ℃ and 34.4 ℃ respectively. This can be explained by the additional gas generation from the reaction between plated Li and electrolyte in the fast-charged batteries. Gas analyses of 120 ℃ hot-box tests for the degraded anodes with the electrolyte showed that the vented gas at the early venting stage primarily consisted of carbon dioxide, carbon monoxide and ethylene. The same charging protocols were then conducted in a natural convection thermal chamber with poor heat dissipation, where the temperature rise was much higher than those under forced convection as expected. Excessive cycling operations with off-limit charge rates (5C and 5.6C) were observed to directly cause battery swelling and venting without TR, and the temperature decreasing rates during rest after each charging process tend to slow down and even remain stable during a certain period of time. This indicates that the exothermic reaction between plated Li and electrolyte can be directly triggered by fast charging if heat removal is not effective enough. The findings provide useful insights into battery venting induced by fast charging, which can shed light on battery safety evaluation for fast charging techniques.Acknowledgements:This work is supported by the Faraday Institution BESAFE project (FIRG038), the Imperial College President’s PhD Scholarship Scheme (funded by EPSRC), the National Natural Science Foundation of China (Grant No. 52207241), China National Postdoctoral Program for Innovative Talents (grant no. BX20220171), China Postdoctoral Science Foundation (grant no. 2022M711760), and Shuimu Tsinghua Scholar Program (grant no. 2021SM130).

Unexplored Reaction Pathways of Ethylene Carbonate during Solid Electrolyte Interphase Formation

The solid electrolyte interphase (SEI) keeps eluding our grasp. Fundamental understanding of Li-ion battery performance will not be complete until the major underlying reaction pathways of SEI formation and growth have been described.1 Although the SEI has been observed to be a complex and highly diverse matrix of in-/organic crystalline and amorphous aggregates, likely only few compounds are functional, i.e. passivate the electrode while at the same time sustaining the Li+ transport. The aim of the presentation is to provide experimental evidence for hitherto largely unexplored reaction pathways of several commonly observed SEI compounds (such as lithium ethyl dicarbonate, abbr. LEDC) and prove how the formation of the same can be altered by the choice of electrolyte additives. For instance, Electrochemical Quartz Crystal Microbalance with Dissipation monitoring complemented by Online Electrochemical Mass Spectrometry is demonstrated to separate electrochemical from chemical reactions in a multistep SEI formation process.2-3 Based on our experimental findings, the role and implication of each compound (be it LiOH, Li2O, LEDC, etc.) in the SEI formation process will be discussed.4

(Battery Division Early Career Award Sponsored by Neware Technology Limited) Understanding Interfacial Mechanisms that Govern Performance in Solid-State Batteries

Solid-state batteries offer the promise of improved energy density and safety compared to lithium-ion batteries. Here, I will present our emerging understanding of the key differences between how high-capacity anode materials behave in solid-state batteries compared to in conventional liquid-electrolyte batteries. The electro-chemo-mechanical evolution of materials at solid-solid electrochemical interfaces is different than at solid/liquid interfaces, and contact evolution in particular plays a critical role in determining the behavior of solid-state batteries. I will focus on mechanisms governing lithium metal anodes (including lithium in the “anode-free” configuration) and alloy anodes. Lithium metal anodes in solid-state batteries are intrinsically limited by void formation during stripping and dendrite growth during plating. Anode-free solid-state batteries, in which there is no initial lithium metal at the anode interface, offer extremely high energy density, but there is a lack of understanding of how their behavior differs from excess-lithium electrodes. Using X-ray tomography, cryo-FIB, and finite-element modeling, we show that anode-free solid-state batteries are intrinsically limited by current concentrations at the end of stripping due to localized lithium depletion. This causes accelerated short circuiting compared to lithium-excess cells. Based on these results, the beneficial influence of metal alloy interfacial layers on controlling lithium evolution and mitigating contact loss from localized lithium depletion, including at low stack pressures, will be discussed. X-ray tomography is further shown to be particularly useful in observing the dynamic evolution of lithium metal, including void formation and filament growth. The second part of the talk focuses on alloy anodes. Alloy anodes typically exhibit fast capacity decay in lithium-ion batteries because of excessive solid-electrolyte interphase growth. We show that alloy anodes in solid-state batteries can exhibit improved interfacial stability and enhanced cyclability. Furthermore, in situ measurement of stack pressure evolution during cycling shows that the volume changes of alloy anodes can lead to large pressure swings within the cell, giving insight into electrode composite evolution. Based on these insights, we present a new design for dense foil alloy anodes. This design offers a paradigm that does away with slurry coating, potentially reducing manufacturing costs. Taken together, these findings show the importance of controlling chemo-mechanics and interfaces in solid-state batteries for improved energy storage capabilities.

Unveiling the Dynamics of Solid Electrolyte Interphase Evolution in Li-Ion Batteries Via Operando X-Ray Reflectivity: Experimental Insights for Advanced Energy Storage

The Solid-Electrolyte-Interphase (SEI) is a crucial component of Li-ion batteries (LIBs), as it passivates the highly reactive electrode from the electrolyte and helps to stabilize the electrode/electrolyte interface. However, the SEI formation and evolution can also limit the cycling performance and the capacity retention of the battery. Accurate electrochemical models must therefore account for the SEI and properly simulate it. In order to investigate experimentally the SEI, various techniques have been deployed, including electron microscopy1, X-ray photoelectron spectroscopy (XPS)2,3 and Nuclear magnetic resonance (NMR)4.Providing reliable information on the SEI evolution during cycling, and thus benchmarking models, is challenging with these ex-situ or destructive techniques, as they hardly allow in situ observation. On the other hand, synchrotron X-ray reflectivity (XRR) can be used for the operando investigation of interfacial phenomena. It provides information on the thickness, density, and roughness of the SEI, as well as their changes upon (de)lithiation or in out-of-equilibrium conditions. Moreover, operando XRR can be performed under controlled battery operating conditions, providing relevant information on the SEI. Recently, several works reported that operando XRR experiment can provide in situ structural information of the formation and the growth of SEI with sub-nanometer resolution during the cycling5–7.Herein, we report the results of operando XRR experiments performed at the beamline BM32 of the European Synchrotron Radiation Facility (ESRF) to investigate the SEI on Si anode using novel, sustainable fluorine-free electrolytes. Indeed, in the quest for environmentally friendly and safe LIBs, removing the fluorinated electrolytes that are toxic and release corrosive compounds is a necessary step. One way to enhance the safety and sustainability of LIBs is to replace the conventional fluorinated electrolytes by fluorine-free alternatives. A F-free composition based on the lithium bis(oxalato) borate (LiBOB) salt recently showed promising performance in commercially relevant cell configurations8. As with conventional electrolytes, the F-free electrolyte reacts at the surface of the electrodes during the first charge, thus forming the SEI. Characterizing the F-free SEI growth and stability is thus key to understand its behavior and to improve the cell performance for this promising green electrolyte.In this work, operando XRR experiments were performed in electrochemical cells with different electrolyte compositions: 1 M LiPF6 in ethylene carbonate (EC):ethyl methyl carbonate (EMC) = 3:7 vol % (LP57), 0.7 M LiBOB in EC:EMC = 3:7 vol % (LiBOB), and 0.7 M LiBOB in EC:EMC = 3:7 vol % with 2 wt% vinylene carbonate (LiBOB-VC) during at least three cycles. XRR data show that the formations of SEI layers and their evolutions during the cycling for these two types of electrolytes are different. Furthermore, a layer with low electron density (ED) is still present after total delithiation in the LiBOB-cells, while the ED evolutions are generally reversible for LP57 cell. This suggests that the F-free SEI has different properties during cycling than the SEI formed with conventional salt LiPF6.In summary, operando XRR is a powerful technique for investigating the impact of new electrolyte chemistry and formulation on the SEI properties, which significally influence batteries performances. Moreover, the in situ and in real-time experimental insights with sub-nanometer resolution from this technique provides valuable information for the development of more accurate and predictive models for the SEI evolution. Its unique capabilities make it an essential tool for advancing the understanding of SEI behavior and designing more efficient and stable energy storage systems.Figure 1: (a) Principle of operando X-ray reflectometry. (b) XRR data and fit-derived EDP of LiBOB cell upon lithiation. Figure 1

Amorphization of Inorganic Solid Electrolyte Interphase Components and Its Impact on Enhancing Their Transport and Mechanical Properties.

The safety and cycle life of lithium-ion batteries (LIBs) are largely determined by the solid electrolyte interphase (SEI) formed on the surface of the anode. However, there is still a lack of understanding regarding the structure and properties of the individual SEI components. Among others, lithium oxide (Li2O), lithium carbonate (Li2CO3), and lithium fluoride (LiF) are known to be the main components of the inorganic SEI layer in conventional LIBs, but their intrinsic protective roles remain controversial. Herein, we present the transformational effects of their amorphous phase on the mechanical and transport characteristics, based on first-principles calculations. Our studies clearly demonstrate that their amorphous phases exhibit significantly improved Li-ion conductivity when compared to the crystalline structures. Additionally, among them, amorphous LiF emerges as a frontrunner for fast Li+ ion transportation, reversing the conventionally understood hierarchy. Under ambient conditions, the amorphous phases of LiF, Li2O, and Li2CO3 are thermodynamically unstable and tend to undergo recrystallization. However, this work highlights that exceptionally ductile and resilient amorphous phases can form if SEI formation and growth would involve some admixing of lithiophilic impurities like nitrogen (N) within the host matrices.

Fundamental Understanding and Quantification of Capacity Losses Involving the Negative Electrode in Sodium-Ion Batteries.

Knowledge about capacity losses related to the solid electrolyte interphase (SEI) in sodium-ion batteries (SIBs) is still limited. One major challenge in SIBs is that the solubility of SEI species in liquid electrolytes is comparatively higher than the corresponding species formed in Li-ion batteries. This study sheds new light on the associated capacity losses due to initial SEI formation, SEI dissolution and subsequent SEI reformation, charge leakage via SEI and subsequent SEI growth, and diffusion-controlled sodium trapping in electrode particles. By using a variety of electrochemical cycling protocols, synchrotron-based X-ray photoelectron spectroscopy (XPS), gas chromatography coupled with mass spectrometry (GC-MS), and proton nuclear magnetic resonance (1 H-NMR) spectroscopy, capacity losses due to changes in the SEI layer during different open circuit pause times are investigated in nine different electrolyte solutions. It is shown that the amount of capacity lost depends on the interplay between the electrolyte chemistry and the thickness and stability of the SEI layer. The highest capacity loss is measured in NaPF6 in ethylene carboante mixed with diethylene carbonate electrolyte(i.e., 5 µAhh-1/2 pause or 2.78 mAhg·h-1/2 pause ) while the lowest value is found in NaTFSI in ethylene carbonate mixed with dimethoxyethance electrolyte (i.e., 1.3 µAhh-1/2 pause or 0.72 mAhg·h-1/2 pause ).

Characterization of Cycle-Aged Commercial NMC and NCA Lithium-ion Cells: I. Temperature-Dependent Degradation

Lithium-ion batteries are widely used in applications from consumer electronic devices to stationary energy storage. Appropriate management of batteries is challenging due to limited data on their performance and materials degradation. Previous studies have focused on characterization of single cells under specific operating conditions. In the present work, commercial 18650 lithium-ion cells with LiNixMnyCo1-x-yO2 (NMC) and LiNixCoyAl1-x-yO2 (NCA) positive electrodes were characterized by a wide range of electrochemical and materials techniques after cycling at 15, 25, or 35 °C to ∼80% capacity. The NCA cells exhibit weak temperature dependence in their cycle aging and materials degradation. The NMC cells exhibited increased capacity fade and materials degradation as ambient temperature decreased. All cells exhibited loss of lithium inventory as their primary degradation mode. However, the NCA cells only showed evidence of solid electrolyte interphase (SEI) growth whereas the NMC cells showed signs of Li plating at 15 °C, transitioning to SEI growth at 35 °C. The NMC cells displayed signs of loss of active material at the positive electrode at lower temperatures, suggesting that Li plating is correlated to additional processes that increase the rate of degradation. These results highlight the importance of avoiding broad generalizations about Li-ion battery temperature dependence.