Manipulated Fluoro-Ether Derived Nucleophilic Decomposition Products for Mitigating Polarization-Induced Capacity Loss in Li-Rich Layered Cathode.

Electrolyte engineering is a fascinating choice to improve the performance of Li-rich layered oxide cathodes (LRLO) for high-energy lithium-ion batteries. However, many existing electrolyte designs and adjustment principles tend to overlook the unique challenges posed by LRLO, particularly the nucleophilic attack. Here, we introduce an electrolyte modification by locally replacing carbonate solvents in traditional electrolytes with a fluoro-ether. By benefit of the decomposition of fluoro-ether under nucleophilic O-related attacks, which delivers an excellent passivation layer with LiF and polymers, possessing rigidity and flexibility on the LRLO surface. More importantly, the fluoro-ether acts as "sutures", ensuring the integrity and stability of both interfacial and bulk structures, which contributed to suppressing severe polarization and enhancing the cycling capacity retention from 39 % to 78 % after 300 cycles for the 4.8 V-class LRLO. This key electrolyte strategy with comprehensive analysis, provides new insights into addressing nucleophilic challenge for high-energy anionic redox related cathode systems.

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

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

Clarification of Decomposition Pathways in a State-of-the-Art Lithium Ion Battery Electrolyte through 13 C-Labeling of Electrolyte Components.

The decomposition of state-of-the-art lithium ion battery (LIB) electrolytes leads to a highly complex mixture during battery cell operation. Furthermore, thermal strain by e.g., fast charging can initiate the degradation and generate various compounds. The correlation of electrolyte decomposition products and LIB performance fading over life-time is mainly unknown. The thermal and electrochemical degradation in electrolytes comprising 1 m LiPF6 dissolved in 13 C3 -labeled ethylene carbonate (EC) and unlabeled diethyl carbonate is investigated and the corresponding reaction pathways are postulated. Furthermore, a fragmentation mechanism assumption for oligomeric compounds is depicted. Soluble decomposition products classes are examined and evaluated with liquid chromatography-high resolution mass spectrometry. This study proposes a formation scheme for oligo phosphates as well as contradictory findings regarding phosphate-carbonates, disproving monoglycolate methyl/ethyl carbonate as the central reactive species.

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

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

New Insights into the Operating Voltage of Aqueous Supercapacitors.

The main limitation of aqueous supercapacitors (SCs) lies in their narrow operating voltages, especially when compared with organic SCs. Fundamental understanding of factors relevant to the operating voltage helps providing guidance for the assembly of high-voltage aqueous SCs. In this regard, this concept analyzes the deciding factors for the operating voltage of aqueous SCs. Strategies applied to expand the operating voltage are summarized and discussed from the aspects of electrolyte, electrode, and asymmetric structure. Dynamic factors associated with water electrolysis and maximally using the available potential ranges of electrodes are particularly emphasized. Finally, other promising approaches that have not been explored and their challenges are also elaborated, hoping to provide more insights for the design of high-voltage aqueous SCs.

Unveiling a Novel Decomposition Pathway in Propylene Carbonate-Based Supercapacitors: Insights from a Jelly Roll Configuration Study.

This research elucidates novel insights into the electrochemical properties and degradation phenomena of propylene carbonate (PC)-based supercapacitors at a large-scale 18650 cylindrical jelly-roll cell level. Central to our findings is the identification of 2-ethyl-4-methyl-1,3-dioxolane (EMD) as a hitherto undocumented decomposition by-product, highlighting the nuanced complexity of PC electrolyte stability. We further demonstrate that elevated operational voltages precipitate accelerated electrolyte degradation, underscoring the criticality of defining the operational voltage window for maximizing device longevity. Employing advanced analytical techniques, including gas chromatography-mass spectrometry (GC-MS), this study meticulously analyzes electrolyte decomposition mechanisms. The outcomes offer pivotal insights into the operational constraints and chemical resilience of PC-based supercapacitors, contributing significantly to the optimization of supercapacitor design and application. By delineating a specific decomposition pathway, this investigation enriches the understanding of electrochemical dynamics in supercapacitor systems, providing a foundation for future research and technological advancement in energy storage devices.

Suppressed Electrolyte Decomposition Behavior to Improve Cycling Performance of LiCoO2 under 4.6 V through the Regulation of Interfacial Adsorption Forces.

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

Dual-salt strategy tuning the solvation structure to achieve high cycling stability for FeS2 cathodes.

Pyrite FeS2, as a promising conversion-type cathode material, faces rapid capacity degradation due to challenges such as polysulfide shuttle and massive volume changes. Herein, a localized high-concentration electrolyte (LHCE) based on dual-salt lithium bis(fluorosulfonyl)imide (LiFSI) and lithium bis(trifluoromethanesulphonyl)imide (LiTFSI) is designed to address the challenges. By the dual-salt strategy, we tailor a more desirable solvation structure than that in the single-salt system. Specifically, the solvation structure involving FSI- and TFSI- enables milder electrolyte decomposition, which reduces initial capacity loss. Meanwhile, it facilitates the formation of a stable and flexible cathode/electrolyte interphase (CEI), effectively mitigating side effects and accommodating volume changes. Consequently, the micro-sized FeS2 realizes a capacity of 641 mAh g-1 after 600 cycles with a retention rate of 90%, significantly improving the cycling stability of the FeS2 cathode. This work underscores the pivotal role of solvation structure in modulating electrochemical performances and provides a simple and effective electrolyte design concept for conversion-type cathodes.

Unraveling Propylene Oxide Formation in Alkali Metal Batteries.

The increasing need for electrochemical energy storage drives the development of post-lithium battery systems. Among the most promising new battery types are sodium-based battery systems. However, like its lithium predecessor, sodium batteries suffer from various issues like parasitic side reactions, which lead to a loss of active sodium inventory, thus reducing the capacity over time. Some problems in sodium batteries arise from an unstable solid electrolyte interphase (SEI) reducing its protective power e. g., due to increased solubility of SEI components in sodium battery systems. While it is known that the electrolyte affects the SEI structure, the exact formation mechanism of the SEI is not yet fully understood. In this study, we follow the initial SEI formation on a piece of sodium metal submerged in propylene carbonate with and without the electrolyte salt sodium perchlorate. We combine X-ray photoelectron spectroscopy, gas chromatography, and density functional theory to unravel the sudden emergence of propylene oxide after adding sodium perchlorate to the electrolyte solvent. We identify the formation of a sodium chloride layer as a crucial step in forming propylene oxide by enabling precursors formed from propylene carbonate on the sodium metal surface to undergo a ring-closing reaction. Based on our combined theoretical and experimental approach, we identify changes in the electrolyte decomposition process, propose a reaction mechanism to form propylene oxide and discuss alternatives based on known synthesis routes.

Improved Electrochemical Performance of Spinel LiNi0.5Mn1.5O4 Cathode Materials with a Dual Structure Triggered by LiF at Low Calcination Temperature.

High-voltage spinel LiNi0.5Mn1.5O4 (LNMO), which has the advantages of high energy density, low cost, environmental friendliness, and being cobalt-free, is considered one of the most promising cathode materials for the next generation of power lithium-ion batteries. However, the side reaction at the interface between the LNMO cathode material and electrolyte usually causes a low specific capacity, poor rate, and poor cycling performance. In this work, we propose a facilitated method to build a well-tuned dual structure of LiF coating and F- doping LNMO cathode material via simple calcination of LNMO with LiF at low temperatures. The experimental results and DFT analysis demonstrated that the powerful interface protection due to the LiF coating and the higher lithium diffusion coefficient caused by F- doping effectively improved the electrochemical performance of LNMO. The optimized LNMO-1.3LiF cathode material presents a high discharge capacity of 140.3 mA h g-1 at 1 C and 118.7 mA h g-1 at 10 C. Furthermore, the capacity is retained at 75.4% after the 1000th cycle at 1 C. Our research provides a concrete guidance on how to effectively boost the electrochemical performance of LNMO cathode materials.

Sulfide-Based All-Solid-State Lithium-Sulfur Batteries: Challenges and Perspectives.

Lithium-sulfur batteries with liquid electrolytes have been obstructed by severe shuttle effects and intrinsic safety concerns. Introducing inorganic solid-state electrolytes into lithium-sulfur systems is believed as an effective approach to eliminate these issues without sacrificing the high-energy density, which determines sulfide-based all-solid-state lithium-sulfur batteries. However, the lack of design principles for high-performance composite sulfur cathodes limits their further application. The sulfur cathode regulation should take several factors including the intrinsic insulation of sulfur, well-designed conductive networks, integrated sulfur-electrolyte interfaces, and porous structure for volume expansion, and the correlation between these factors into account. Here, we summarize the challenges of regulating composite sulfur cathodes with respect to ionic/electronic diffusions and put forward the corresponding solutions for obtaining stable positive electrodes. In the last section, we also outlook the future research pathways of architecture sulfur cathode to guide the develop high-performance all-solid-state lithium-sulfur batteries.

Quantitative Analysis of Origin of Lithium Inventory Loss and Interface Evolution over Extended Fast Charge Aging in Li Ion Batteries.

During the extreme fast charging (XFC) of lithium-ion batteries, lithium inventory loss (LLI) and reaction mechanisms at the anode/electrolyte interface are crucial factors in performance and safety. Determining the causes of LLI and quantifying them remain an essential challenge. We present mechanistic research on the evolution and interactions of aging mechanisms at the anode/electrolyte interface. We used NMC532/graphite pouch cells charged at rates of 1, 6, and 9 C up to 1000 cycles for our investigation. The cell components were characterized after cycling using electrochemical measurements, inductively coupled plasma optical emission spectroscopy, 7Li solid-state nuclear magnetic resonance spectroscopy, and high-performance liquid chromatography/mass spectrometry. The results indicate that cells charged at 1 C exhibit no Li plating, and the increase of SEI thickness is the dominant source of the Li loss. In contrast, Li loss in cells charged at 9 C is related to the formation of the metallic plating layers (42%) the SEI layer (38.1%) and irreversible intercalation into the bulk graphite (19%). XPS analysis suggests that the charging rate has little influence on the evolution of SEI composition. The interactions between competing aging mechanisms were evaluated by a correlation analysis. The quantitative method established in this work provides a comprehensive analytical framework for understanding the synergistic coupling of anodic degradation mechanisms, forecasting SEI failure scenarios, and assessing the XFC lithium-ion battery capacity fade.

Enabling Long Cycle Life and High Rate Iron Difluoride Based Lithium Batteries by In Situ Cathode Surface Modification.

Metals fluorides (MFs) are potential conversion cathodes to replace commercial intercalation cathodes. However, the application of MFs is impeded by their poor electronic/ionic conductivity and severe decomposition of electrolyte. Here, a composite cathode of FeF2 and polymer-derived carbon (FeF2 @PDC) with excellent cycling performance is reported. The composite cathode is composed of nanorod-shaped FeF2 embedded in PDC matrix with excellent mechanical strength and electronic/ionic conductivity. The FeF2 @PDC enables a reversible capacity of 500 mAh g-1 with a record long cycle lifetime of 1900 cycles. Remarkably, the FeF2 @PDC can be cycled at a record rate of 60 C with a reversible capacity of 107 mAh g-1 after 500 cycles. Advanced electron microscopy reveals that the in situ formation of stable Fe3 O4 layers on the surface of FeF2 prevents the electrolyte decomposition and leaching of iron (Fe), thus enhancing the cyclability. The results provide a new understanding to FeF2 electrochemistry, and a strategy to radically improve the electrochemical performance of FeF2 cathode for lithium-ion battery applications.

Stable Li Metal-Electrolyte Interface Enabled by SEI Improvement and Cation Shield Functionality of the Azamacrocyclic Ligand in Carbonate Electrolytes.

To promote the reversible cycleability of Li metal negative electrodes, a Li-chelating azamacrocyclic ligand molecule is introduced into a carbonate-based electrolyte intended for lithium metal batteries. Reversible Li plating and stripping on the Cu electrode are found to be the outcomes of the bifunctional effects of adding the lithium nitrate-chelating azamacrocyclic ligand. The negatively shifted redox potential of the Li-chelating macrocyclic ligand, relative to that of the free Li-ion, acted as a cationic shielding molecule for smooth Li deposition, and the Li3N-based solid electrolyte interphase (SEI) film derived from the nitrate anion strengthened the interphasial characteristics of the Li metal negative electrode. Cationic shielding and Li3N-based SEI composition could help enhance the cycleability of the Li metal in a cascading manner. Consequently, the physicochemical characteristics of the lithium nitrate-chelated 1,4,8,11-tetramethyl-1,4,8,11-tetraazacylcotetradecane molecule exhibit stable Li/LiNi0.8Co0.1Mn0.1O2 cycleability.

Online sample pretreatment for analysis of decomposition products in lithium ion battery by liquid chromatography hyphenated with ion trap-time of flight-mass spectrometry or inductively coupled plasma-sector field-mass spectrometry.

Lithium ion batteries are essential power sources for mobile electronic devices like cell phones, tablets and increasingly used in the field of electromobility and energy transition. The commonly applied liquid electrolytes in commercial cells contain a conducting salt at relatively high concentration (LiPF6, ≥1 mol/L). For analytical battery electrolyte investigations, it is necessary to protect the column and mass spectrometer from salt precipitation and clogging. Thus, dilution of the sample is necessary which results in higher limits of detection and limits of quantification. In this study, a comprehensive online sample preparation approach for reversed phase liquid chromatography with an online-solid phase extraction was developed, which allows higher injections volumes and lower dilution factors. For the method development of the online-solid phase extraction, pristine electrolytes were used with trimethyl phosphate and triethyl phosphate as model substances for organo(fluoro)phosphates with weak and strong retention on the extraction column. Organo(fluoro)phosphates are potential hazardous decomposition products, due to their structural similarity to chemical warfare agents like sarin, and therefore their quantification is beneficial for toxicological assessment. The optimization of chromatographic parameters was performed using electrochemically aged electrolytes. For substance independent quantification with a plasma-based technique, an isocratic separation method was implemented. Using optimized conditions, LiPF6 could be removed quantitatively and the injection volume was increased up to a factor of 50, while the dilution factor could be decreased up to a factor of ten. Eleven different organo(fluoro)phosphates with an overall concentration of 133 mg/kg were found. Therefore, limit of detection and limit of quantification were improved significantly (LOQ: ≤100 µg kg-1 phosphorus content, LOD: ≤35 µg kg-1 phosphorus content). In summary, a fast online sample preparation for liquid chromatographic investigations of lithium ion battery electrolytes was implemented, validated on electrochemically aged lithium ion battery electrolyte.

Fast sample preparation for organo(fluoro)phosphate quantification approaches in lithium ion battery electrolytes by means of gas chromatographic techniques.

Lithium ion batteries are essential power sources in portable electronics, electric vehicles and as energy storage devices for renewable energies. During harsh battery cell operation as well as at elevated temperatures, the electrolyte decomposes and inter alia organo(fluoro)phosphates are formed due to hydrolysis of the conducting salt lithium hexafluorophosphate (LiPF6). Since these phosphorus-containing decomposition products possess a potential toxicity based on structural similarities compared to chemical warfare agents, quantification is of high interest regarding safety estimates. In this study, two comprehensive approaches for the precipitation of highly concentrated PF6¯ were investigated, allowing the separation from target analytes (organo(fluoro)phosphates) and improving mass spectrometry-based quantification techniques. Trimethyl phosphate was used as a polar, non-acidic organophosphate reference substance for method development via liquid chromatography-mass spectrometry. Six solvents were examined regarding precipitation reaction and selectivity. Thermally degraded electrolytes were analyzed after precipitation by means of gas chromatography-flame ionization detector, demonstrating the applicability of the developed sample preparations. The optimized method was applied successfully without influencing any volatile and non-acidic decomposition products. Using optimized conditions, a precipitation rate of 98% PF6¯ was achieved. Consequently, a fast and easy sample preparation for gas chromatographic investigations on lithium ion battery electrolytes was implemented, applicable for routine analysis.

Porous Materials Applied in Nonaqueous Li-O2 Batteries: Status and Perspectives.

Porous materials possessing high surface area, large pore volume, tunable pore structure, superior tailorability, and dimensional effect have been widely applied as components of lithium-oxygen (Li-O2 ) batteries. Herein, the theoretical foundation of the porous materials applied in Li-O2 batteries is provided, based on the present understanding of the battery mechanism and the challenges and advantageous qualities of porous materials. Furthermore, recent progress in porous materials applied as the cathode, anode, separator, and electrolyte in Li-O2 batteries is summarized, together with corresponding approaches to address the critical issues that remain at present. Particular emphasis is placed on the importance of the correlation between the function-orientated design of porous materials and key challenges of Li-O2 batteries in accelerating oxygen reduction reaction (ORR)/oxygen evolution reaction (OER) kinetics, improving the electrode stability, controlling lithium deposition, suppressing the shuttle effect of the dissolved redox mediators, and alleviating electrolyte decomposition. Finally, the rational design and innovative directions of porous materials are provided for their development and application in Li-O2 battery systems.

A method for quantitative analysis of gases evolving during formation applied on LiNi0.6Mn0.2Co0.2O2 ∣∣ natural graphite lithium ion battery cells using gas chromatography - barrier discharge ionization detector.

To understand the overall processes behind the decomposition of state-of-the-art organic liquid electrolytes in lithium ion batteries (LIBs), it is necessary to investigate and quantify the permanent gases and light hydrocarbons evolving during electrolyte decomposition. In this work a convenient way of sampling gas from pouch cells without any previous preparation of the cell as well as a comprehensive gas chromatographic (GC) investigation of the gas phase is shown. A barrier discharge ionization detector (BID) was utilized for gas quantification and a multi component gas standard in combination with a gas mixing device was implemented to prepare calibration standards for validation. Therefore, sensitivity, linearity and reproducibility as well as the limits of detection (LOD) and limits of quantification (LOQ) were determined. Gas samples from pouch cells using LiNi0.6Mn0.2Co0.2O2 as cathode material and natural graphite (NMC622 ∣∣ NG) as anode material were analysed after formation. Gas volume and gas composition are key factors for a sufficient formation of LIBs and of interest for research with respect to the development of new materials and additives.

Gassing in Sn-Anode Sodium-Ion Batteries and Its Remedy by Metallurgically Prealloying Na.

Despite the high theoretical energy densities of tin as the anode of sodium-ion batteries, its electrochemical performance has been plagued by the inadequate initial Coulombic efficiency (ICE) and poor cycle life. While it is generally believed that mechanical degradation, namely, pulverization and subsequent loss of electrical contact, is the underlying cause, here, we show that gassing is an essential problem in sodium-ion batteries with Sn anode. Since the gas generation appears at a certain voltage, reducing the voltage by metallurgically prealloying Sn with Na could be a solution, with an additional benefit of compensating for "live" Na loss in future cycles. When the metallurgically alloyed foil is used as the anode in Na3V2(PO4)2F3 (NVPF)//alloy full cells, the ICE is significantly improved from 24.68 to 75%, and from the 2nd cycle to the 100th cycle, the average Coulombic efficiency can be maintained up to 99.44%. The full cell can run for 100 cycles, with acceptable capacity decay to 81.4 mAh/g(NVPF) from the initial 112.5 mAh/g(NVPF).

Preparative hydrophilic interaction liquid chromatography of acidic organofluorophosphates formed in lithium ion battery electrolytes.

The expansion of lithium ion battery (LIB) application is accompanied by the growth of battery pack sizes. This progression emphasizes the consideration of electrolyte safety as well as environmental aspects in case of abuse, accident, or recycling. Hexafluorophosphate is one of the most commonly used conducting salt anions in electrolytes. It has great potential to degrade to various acidic and non-acidic organo(fluoro)phosphates with presence of water and during battery cell operation. Consequently, toxicological investigation on these organo(fluoro)phosphates has emerged because they either have structural similarities as chemical warfare agents or play a widespread physiological role as phosphates in the human body. This circumstance underlines the need of isolated examination of these compounds for safety assessment. In this work, we used hydrophilic interaction liquid chromatography for the extraction of acidic organofluorophosphates from thermally aged LIB electrolytes. The developed two-step fractionation method provided high separation selectivity towards acidic head groups, which allowed the separation of undesired matrix and target compounds. These findings facilitate isolated toxicological investigations on organofluorophosphates that are beneficial for environmental and safety research, the battery cell industry, and human safety surveillance in regard to aged LIB electrolytes.