BF3 Complexing Phosphate/Phosphonate Ionic Liquids: Synthesis, Characterization and Thermophysical Properties

A new group of BF3 complexing phosphate/phosphonate ionic liquids (ILs) [Emim][X(BF3)2] (X=dimethyl phosphate, diethyl phosphate, methyl phosphonate, and ethyl phosphonate) were synthesized and characterized. Key thermophysical properties of the new complex ionic liquids, including density, viscosity, conductivity, surface tension, solid-liquid phase transition, and thermal stability were determined and compared with those of [Emim][X]. Some other important thermophysical properties such as isobaric thermal expansion coefficient, molecular volume, standard molar entropy, and lattice potential energy were obtained from measured density data, and the free volume was estimated by a linear equation presented in this article, while critical temperature, normal boiling temperature, and enthalpy of vaporization were estimated from measured surface tension and density data. Furthermore, Fragility study shows that [Emim][X(BF3)2] should be considered as fragile liquids, while [Emim][X] could be considered as extremely fragile liquids. The ionicity of [Emim][X(BF3)2] was predicted by Walden rule, and the result shows that these ILs fit well with Walden law. The key features of these complex ILs are their extremely low glass transition (-95.33~-98.46 °C) without melting, considerably low viscosities (33.876~58.117 mPa ⋅ s), and high values of free volume fraction (comparable to [Omim][BF4], [Emim][NTf2], and [Emim][TCB]).

Azophosphines: Synthesis, Structure and Coordination Chemistry

The conceptual replacement of nitrogen with phosphorus in common organic functional groups unlocks new properties and reactivity. The phosphorus-containing analogues of triazenes are underexplored but offer great potential as flexible and small bite-angle ligands. This manuscript explores the synthesis and characterisation of a family of air-stable azophosphine-borane complexes, and their subsequent deprotection to the free azophosphines. These compounds are structurally characterised, both experimentally and computationally, and highlight the availability of the phosphorus lone pair for coordination. This is confirmed by demonstrating that neutral azophosphines can act as ligands in Ru complexes, and can coordinate as monodentate or bidentate ligands in a controlled manner, in contrast to their nitrogen analogues.

Role of Salt Concentration in Stabilizing Charged Ni-Rich Cathode Interfaces in Li-Ion Batteries

The cathode-electrolyte interphase (CEI) in Li-ion batteries plays a key role in suppressing undesired side reactions while facilitating Li-ion transport. Ni-rich layered cathode materials offer improved energy densities, but their high interfacial reactivities can negatively impact the cycle life and rate performance. Here we investigate the role of electrolyte salt concentration, specifically LiPF6 (0.5-5 m), in altering the interfacial reactivity of charged LiN0.8Mn0.1Co0.1O2 (NMC811) cathodes in standard carbonate-based electrolytes (EC/EMC vol %/vol % 3:7). Extended potential holds of NMC811/Li4Ti5O12 (LTO) cells reveal that the parasitic electrolyte oxidation currents observed are strongly dependent on the electrolyte salt concentration. X-ray photoelectron and absorption spectroscopy (XPS/XAS) reveal that a thicker LixPOyFz-/LiF-rich CEI is formed in the higher concentration electrolytes. This suppresses reactions with solvent molecules resulting in a thinner, or less-dense, reduced surface layer (RSL) with lower charge transfer resistance and lower oxidation currents at high potentials. The thicker CEI also limits access of acidic species to the RSL suppressing transition-metal dissolution into the electrolyte, as confirmed by nuclear magnetic resonance (NMR) spectroscopy and inductively coupled plasma optical emission spectroscopy (ICP-OES). This provides insight into the main degradation processes occurring at Ni-rich cathode interfaces in contact with carbonate-based electrolytes and how electrolyte formulation can help to mitigate these.

Reinforcement of Positive Electrode-Electrolyte Interface without Using Electrolyte Additives Through Thermoelectrochemical Oxidation of LiPF6 for Lithium Secondary Batteries

Owing to the limited electrochemical stability window of carbonate electrolytes, the initial formation of a solid electrolyte interphase and surface film on the negative and positive electrode surfaces by the decomposition of the electrolyte component is inevitable for the operation of lithium secondary batteries. The deposited film on the surface of the active material is vital for reducing further electrochemical side reactions at the surface; hence, the manipulation of this formation process is necessary for the appropriate operation of the assembled battery system. In this study, the thermal decomposition of LiPF6 salt is used as a surface passivation agent, which is autocatalytically formed during high-temperature storage. The thermally formed difluorophosphoric acid is subsequently oxidized on the partially charged high-Ni positive electrode surface, which improves the cycleability of lithium metal cells via phosphorus- and fluorine-based surface film formation. Moreover, the improvement in the high-temperature cycleability is demonstrated by controlling the formation process in the lithium-ion pouch cell with a short period of high-temperature storage before battery usage.

H2O/HF Scavenging Mechanism in Cellulose-Based Separators for Lithium-Ion Batteries with Enhanced Cycle Life

Lithium-ion batteries (LIBs) are increasingly being integrated into the transportation industry due to their high energy density, durability, and low cost. With the growing demand for transportation and other emerging applications, there is a concurrent rise in concern over LIB material sourcing and recycling. This urges the development of LIBs with extended cycle lifespans. One mechanism of capacity fading in LIBs is the dissolution of transition metals into the electrolyte after the cathode is etched with hydrofluoric acid (HF). HF is readily generated by the hydrolysis of the LIB electrolyte salt, lithium hexafluorophosphate (LiPF6), which makes minimizing moisture in the electrolyte a priority in manufacturing. In this study, a nonwoven, cellulose-based separator (CBS) is introduced as an alternative separator for battery technologies to scavenge residual water and HF from the electrolyte. The CBS is shown to be capable of scavenging varying amounts of water from the electrolyte based on different drying processes of the CBS, and a mechanism for this water scavenging is identified based on the materials present in the CBS. In addition, the chemical and electrochemical performance of the CBS in real battery conditions is investigated. Results suggest an effective H2O/HF scavenging capability in the CBS that allows LIB coin cells to have over 17% higher capacity retention than those with conventional separators. Furthermore, studies of the industrially manufactured, commercially relevant cylindrical and pouch cells show remarkable 761 and 103% improvements in the 60% capacity lifetime, respectively. The environmental friendliness, low cost, and easy application empowered by the cycle life improvements shown in this work make this nonwoven CBS a promising candidate for improving industry-level LIB performance.

Mechanism of Alkene Hydrofunctionalization by Oxidative Cobalt(salen) Catalyzed Hydrogen Atom Transfer

Oxidative MHAT hydrofunctionalization of alkenes provides a mild cobalt-catalyzed route to forming C-N and C-O bonds. Here, we characterize relevant salen-supported cobalt complexes and their reactions with alkenes, silanes, oxidant, and solvent. These stoichiometric investigations are complemented by kinetic studies of the catalytic reaction and catalyst speciation. We describe the solution characterization of an elusive cobalt(III) fluoride complex, which surprisingly is not the species that reacts with silane under catalytic conditions; rather, a cobalt(III) aquo complex is more active. Accordingly, the addition of water (0.15 M) speeds the catalytic reaction, and kinetic studies show that water addition enables catalytic product formation in 2 h at -50 °C in acetone. Under these conditions, cobalt(III) resting states can be observed by UV-vis spectrophotometry, including a cobalt(III)-alkyl complex. It comes from a transient cobalt(III) hydride complex that is formed in the turnover-limiting step of the catalytic cycle. This hydride readily degrades but not to H2; it releases H+ through a bimetallic pathway that explains the [Co]2 dependence of the off-cycle reaction. In contrast, the rate of the catalytic reaction follows the power law kobs[Co]1[silane]1. Because of the different [Co] dependence of the catalytic reaction and the degradation reaction, lower catalyst loading improves the yield of the catalytic reaction by reducing the relative rate of unproductive silane/oxidant consumption. These studies illuminate mechanistic details of oxidative MHAT hydrofunctionalization of alkenes and lay the groundwork for understanding other catalytic reactions mediated by cobalt hydride and cobalt alkyl complexes.

High Response and Selectivity of the SnO2 Nanobox Gas Sensor for Ethyl Methyl Carbonate Leakage Detection in a Lithium-Ion battery

It is well-known that metal-oxide semiconductors (MOS) have significant gas sensing activity and are widely used in harmful gas monitoring in various environments. With the rapid development of new energy vehicles, the monitoring of the gas composition and concentration in LIB has become an effective way to avoid safety problems. However, the study of typical electrolyte solvent detection, such as EMC and DMC detection by the MOS sensor, is still in its infancy. Here, the SnO2 nanoboxes are synthesized by coordination dissolution using cubic Cu2O as the template, and its sensor shows high sensitivity (0.27 to 10 ppb EMC), excellent response (32.46 to 20 ppm EMC), and superior selectivity. Additionally, the sensor possesses fast and clear response to lithium-ion battery (LIB) leakage simulation tests, suggesting that it should be a promising candidate for LIB safety monitors. These sensing performances are attributed to large specific surface area, small grain size, and high size/thickness ratio of nanoboxes. More importantly, DFT calculations confirm the adsorption of EMC on the surface of the SnO2 nanoboxes, and the EMC decomposition processes catalyzed by SnO2 are deduced by in situ FTIR and GC-MS.

Unraveling the Hydrolysis Mechanism of LiPF6 in Electrolyte of Lithium Ion Batteries

Lithium hexafluorophosphate (LiPF6) has been the dominant conducting salt in lithium-ion battery (LIB) electrolytes for decades; however, it is extremely unstable in even trace water (ppm level). Interestingly, in pure water, PF6- does not undergo hydrolysis. Hereby, we present a fresh understanding of the mechanism involved in PF6- hydrolysis through theoretical and experimental explorations. In water, PF6- is found to be solvated by water, and this solvation greatly improved its hydrolytic stability; while in the electrolyte, it is forced to "float" due to the dissociation of its counterbalance ions. Its hydrolytic susceptibility arises from insufficient solvation-induced charge accumulation and high activity in electrophilic reactions with acidic species. Tuning the solvation environment, even by counterintuitively adding more water, could suppress PF6- hydrolysis. The undesired solvation of PF6- anions was attributed to the perennial LIB electrolyte system, and our findings are expected to inspire new thoughts regarding its design.

Synthesis, Characterization, and Physicochemical Properties of New [Emim][BF3X] Complex Anion Ionic Liquids

A new series of complex anion ionic liquids (ILs) [Emim][BF3X] (X = CH3SO3, EtSO4, HSO4, Tosylate) were synthesized and characterized by nuclear magnetic resonance, elemental analysis, differential scanning calorimetry analysis, and thermogravimetry. The physicochemical properties of these ILs, such as density, viscosity, conductivity, and surface tension, were measured and correlated with thermodynamic and empirical equations in the temperature range of 293.15-358.15 K under ambient conditions, and the thermal expansion coefficient, standard molar entropy, lattice potential energy, viscosity activation energy, surface enthalpy, and surface entropy were further calculated from experimental values. According to the temperature-dependent viscosity and conductivity, [Emim][BF3X] ILs follow the Walden rule, and they are classified as "good (or super) ionic liquids".

Modulating Ionic Transport and Interface Chemistry via Surface-Modified Silica Carrier in Nano Colloid Electrolyte for Stable Cycling of Li-Metal Batteries

Tailoring the Li+ microenvironment is crucial for achieving fast ionic transfer and a mechanically reinforced solid-electrolyte interphase (SEI), which administers the stable cycling of Li-metal batteries (LMBs). Apart from traditional salt/solvent compositional tuning, this study presents the simultaneous modulation of Li+ transport and SEI chemistry using a citric acid (CA)-modified silica-based colloidal electrolyte (C-SCE). CA-tethered silica (CA-SiO2 ) can render more active sites for attracting complex anions, leading to further dissociation of Li+ from the anions, resulting in a high Li+ transference number (≈0.75). Intermolecular hydrogen bonds between solvent molecules and CA-SiO2 and their migration also act as nano-carrier for delivering additives and anions toward the Li surface, reinforcing the SEI via the co-implantation of SiO2 and fluorinated components. Notably, C-SCE demonstrated Li dendrite suppression and improved cycling stability of LMBs compared with the CA-free SiO2 colloidal electrolyte, hinting that the surface properties of the nanoparticles have a huge impact on the dendrite-inhibiting role of nano colloidal electrolytes.

Quantifying Dissolved Transition Metals in Battery Electrolyte Solutions with NMR Paramagnetic Relaxation Enhancement

Transition metal dissolution is an important contributor to capacity fade in lithium-ion cells. NMR relaxation rates are proportional to the concentration of paramagnetic species, making them suitable to quantify dissolved transition metals in battery electrolytes. In this work, ⁷Li, ³¹P, ¹⁹F, and ¹H longitudinal and transverse relaxation rates were measured to study LiPF₆ electrolyte solutions containing Ni²⁺, Mn²⁺, Co²⁺, or Cu²⁺ salts and Mn dissolved from LiMn₂O₄. Sensitivities were found to vary by nuclide and by transition metal. ¹⁹F (PF₆^-) and ¹H (solvent) measurements were more sensitive than ⁷Li and ³¹P measurements due to the higher likelihood that the observed species are in closer proximity to the metal center. Mn²⁺ induced the greatest relaxation enhancement, yielding a limit of detection of ∼0.005 mM for ¹⁹F and ¹H measurements. Relaxometric analysis of a sample containing Mn dissolved from LiMn₂O₄ at ∼20 °C showed good sensitivity and accuracy (suggesting dissolution of Mn²⁺), but analysis of a sample stored at 60 °C showed that the relaxometric quantification is less accurate for heat-degraded LiPF₆ electrolytes. This is attributed to degradation processes causing changes to the metal solvation shell (changing the fractions of PF₆^-, EC, and EMC coordinated to Mn²⁺), such that calibration measurements performed with pristine electrolyte solutions are not applicable to degraded solutions-a potential complication for efforts to quantify metal dissolution during operando NMR studies of batteries employing widely-used LiPF₆ electrolytes. Ex situ nondestructive quantification of transition metals in lithium-ion battery electrolytes is shown to be possible by NMR relaxometry; further, the method's sensitivity to the metal solvation shell also suggests potential use in assessing the coordination spheres of dissolved transition metals.

Accessing the Primary Solid-Electrolyte Interphase on Lithium Metal: A Method for Low-Concentration Compound Analysis

Despite large research efforts in the fields of lithium ion and lithium metal batteries, there are still unanswered questions. One of them is the formation of the solid-electrolyte interphase (SEI) in lithium-metal-anode-based battery systems. Until now, a compound profile analysis of the SEI on lithium metal was challenging as the amounts of many compounds after simple contact of lithium metal and the electrolyte were too low for detection with analytical methods. This study presents a novel approach on unravelling the SEI compound profile through accumulation in the gas, liquid electrolyte, and solid phase. The method uses the intrinsic behavior of lithium metal to spontaneously react with the liquid electrolyte. In combination with complementary, state-of-the-art analytical instrumentation and methods, this approach provides qualitative and quantitative results on all three phases revealing the vast variety of compounds formed in carbonate-based electrolytes.

Solution NMR of Battery Electrolytes: Assessing and Mitigating Spectral Broadening Caused by Transition Metal Dissolution

NMR spectroscopy is a powerful tool that is commonly used to assess the degradation of lithium-ion battery electrolyte solutions. However, dissolution of paramagnetic Ni²⁺ and Mn²⁺ ions from cathode materials may affect the NMR spectra of the electrolyte solution, with the unpaired electron spins in these paramagnetic solutes inducing rapid nuclear relaxation and spectral broadening (and often peak shifts). This work establishes how dissolved Ni²⁺ and Mn²⁺ in LiPF₆ electrolyte solutions affect ¹H, ¹⁹F, and ³¹P NMR spectra of pristine and degraded electrolyte solutions, including whether the peaks from degradation species are at risk of being lost and whether the spectral broadening can be mitigated. Mn²⁺ is shown to cause far greater peak broadening than Ni²⁺, with the effect of Mn²⁺ observable at just 10 μM. Generally, ¹⁹F peaks from PF₆ ^- degradation species are most affected by the presence of the paramagnetic metals, followed by ³¹P and ¹H peaks. Surprisingly, when NMR solvents are added to acquire the spectra, the degree of broadening is heavily solvent-dependent, following the trend of solvent donor number (increased broadening with lower solvent donicity). Severe spectral broadening is shown to occur whether Mn is introduced via the salt Mn(TFSI)₂ or is dissolved from LiMn₂O₄. We show that the weak ¹⁹F and ³¹P peaks in spectra of electrolyte samples containing micromolar levels of dissolved Mn²⁺ are broadened to an extent that they are no longer visible, but this broadening can be minimized by diluting electrolyte samples with a suitably coordinating NMR solvent. Li₃PO₄ addition to the sample is also shown to return ¹⁹F and ³¹P spectral resolution by precipitating Mn²⁺ out of electrolyte samples, although this method consumes any HF in the electrolyte solution.

Transition Metal Dissolution Mechanisms and Impacts on Electronic Conductivity in Composite LiNi0.5Mn1.5O4 Cathode Films

The high-voltage LiNi0.5Mn1.5O4 (LNMO) spinel cathode material offers high energy density storage capabilities without the use of costly Co that is prevalent in other Li-ion battery chemistries (e.g., LiNixMnyCozO2 (NMC)). Unfortunately, LNMO-containing batteries suffer from poor cycling performance because of the intrinsically coupled processes of electrolyte oxidation and transition metal dissolution that occurs at high voltage. In this work, we use operando electron paramagnetic resonance (EPR) and nuclear magnetic resonance (NMR) spectroscopies to demonstrate that transition metal dissolution in LNMO is tightly coupled to HF formation (and thus, electrolyte oxidation reactions as detected with operando and in situ solution NMR), indicative of an acid-driven disproportionation reaction that occurs during delithiation (i.e., battery charging). Leveraging the temporal resolution (s-min) of magnetic resonance, we find that the LNMO particles accelerate the rate of LiPF6 decomposition and subsequent Mn2+ dissolution, possibly due to the acidic nature of terminal Mn-OH groups. X-ray photoemission electron microscopy (XPEEM) provides surface-sensitive and localized X-ray absorption spectroscopy (XAS) measurements, in addition to X-ray photoelectron spectroscopy (XPS), that indicate disproportionation is enabled by surface reconstruction upon charging, which leads to surface Mn3+ sites on the LNMO particle surface that can disproportionate into Mn2+(dissolved) and Mn4+(s). During discharge of the battery, we observe high quantities of metal fluorides (in particular, MnF2) in the cathode electrolyte interphase (CEI) on LNMO as well as the conductive carbon additives in the composite. Electronic conductivity measurements indicate that the MnF2 decreases film conductivity by threefold compared to LiF, suggesting that this CEI component may impede both the ionic and electronic properties of the cathode. Ultimately, to prevent transition metal dissolution and the associated side reactions in spinel-type cathodes (particularly those that operate at high voltages like LNMO), the use of electrolytes that offer improved anodic stability and prevent acid byproducts will likely be necessary.

Protocol for quantitative nuclear magnetic resonance for deciphering electrolyte decomposition reactions in anode-free batteries

In this protocol, we describe the quantification of electrolytes using nuclear magnetic resonance. We detail the steps involved for battery cycling, sample preparation, instrument operation, and data analysis. The protocol can be used to quantify electrolyte decomposition reactions and the apparent electron transfer numbers of different electrolyte components. The protocol is optimized for lithium-based anode-free batteries but can also be applied to other rechargeable batteries. For complete details on the use and execution of this protocol, please refer to Zhou et al. (2022).¹.

Onset Potential for Electrolyte Oxidation and Ni-Rich Cathode Degradation in Lithium-Ion Batteries

High-capacity Ni-rich layered metal oxide cathodes are highly desirable to increase the energy density of lithium-ion batteries. However, these materials suffer from poor cycling performance, which is exacerbated by increased cell voltage. We demonstrate here the detrimental effect of ethylene carbonate (EC), a core component in conventional electrolytes, when NMC811 (LiNi_0.8Mn_0.1Co_0.1O₂) is charged above 4.4 V vs Li/Li⁺-the onset potential for lattice oxygen release. Oxygen loss is enhanced by EC-containing electrolytes-compared to EC-free-and correlates with more electrolyte oxidation/breakdown and cathode surface degradation, which increase concurrently above 4.4 V. In contrast, NMC111 (LiNi_0.33Mn_0.33Co_0.33O₂), which does not release oxygen up to 4.6 V, shows a similar extent of degradation irrespective of the electrolyte. This work highlights the incompatibility between conventional EC-based electrolytes and Ni-rich cathodes (more generally, cathodes that release lattice oxygen such as Li-/Mn-rich and disordered rocksalt cathodes) and motivates further work on wider classes of electrolytes and additives.

Multiscale Electrochemomechanics Interaction and Degradation Analytics of Sn Electrodes for Sodium-Ion Batteries

Sodium-ion batteries have emerged as a strong contender among the beyond lithium-ion chemistries due to elemental abundance and the low cost of sodium. Tin (Sn) is a promising alloying electrode with high capacity, redox reversibility, and earth abundance. Tin electrodes, however, undergo a series of intermediate reactions exhibiting multiple voltage plateaus upon sodiation/desodiation. Phase transformations related to incomplete sodiation in tin during cycling, in the presence of a frail solid electrolyte interphase layer, can quickly weaken the structural stability. The structural dynamics and reactivity of the electrode/electrolyte interface, being further dependent on the size and morphology of the active material particle in the presence of different electrolytes, dictate the electrode degradation and survivability during cycling. In this study, we paint a comprehensive picture of the underpinnings of the electrochemical and mechanics coupling and electrode/electrolyte interfacial interactions in alloying Sn electrodes. We elicit the fundamental role of electrode/electrolyte complexations in the Sn electrode structure-property-performance relationship based on multimodal analytics, including electrochemical, microscopy, and tomography analyses.

Investigation of Water-Soluble Binders for LiNi0.5 Mn1.5 O4 -Based Full Cells

Two water‐soluble binders of carboxymethyl cellulose (CMC) and sodium alginate (SA) have been studied in comparison with N‐methylpyrrolidone‐soluble poly(vinylidene difluoride–co‐hexafluoropropylene) (PVdF‐HFP) to understand their effect on the electrochemical performance of a high‐voltage lithium nickel manganese oxide (LNMO) cathode. The electrochemical performance has been investigated in full cells using a Li₄Ti₅O₁₂ (LTO) anode. At room temperature, LNMO cathodes prepared with aqueous binders provided a similar electrochemical performance as those prepared with PVdF‐HFP. However, at 55 °C, the full cells containing LNMO with the aqueous binders showed higher cycling stability. The results are supported by intermittent current interruption resistance measurements, wherein the electrodes with SA showed lower resistance. The surface layer formed on the electrodes after cycling has been characterized by X‐ray photoelectron spectroscopy. The amount of transition metal dissolutions was comparable for all three cells. However, the amount of hydrogen fluoride (HF) content in the electrolyte cycled at 55 °C is lower in the cell with the SA binder. These results suggest that use of water‐soluble binders could provide a practical and more sustainable alternative to PVdF‐based binders for the fabrication of LNMO electrodes.

Electrolyte Reactivity at the Charged Ni-Rich Cathode Interface and Degradation in Li-Ion Batteries

The chemical and electrochemical reactions at the positive electrode-electrolyte interface in Li-ion batteries are hugely influential on cycle life and safety. Ni-rich layered transition metal oxides exhibit higher interfacial reactivity than their lower Ni-content analogues, reacting via mechanisms that are poorly understood. Here, we study the pivotal role of the electrolyte solvent, specifically cyclic ethylene carbonate (EC) and linear ethyl methyl carbonate (EMC), in determining the interfacial reactivity at charged LiNi_0.33Mn_0.33Co_0.33O₂ (NMC111) and LiNi_0.8Mn_0.1Co_0.1O₂ (NMC811) cathodes by using both single-solvent model electrolytes and the mixed solvents used in commercial cells. While NMC111 exhibits similar parasitic currents with EC-containing and EC-free electrolytes during high voltage holds in NMC/Li₄Ti₅O₁₂ (LTO) cells, this is not the case for NMC811. Online gas analysis reveals that the solvent-dependent reactivity for Ni-rich cathodes is related to the extent of lattice oxygen release and accompanying electrolyte decomposition, which is higher for EC-containing than EC-free electrolytes. Combined findings from electrochemical impedance spectroscopy (EIS), TEM, solution NMR, ICP, and XPS reveal that the electrolyte solvent has a profound impact on the degradation of the Ni-rich cathode and the electrolyte. Higher lattice oxygen release with EC-containing electrolytes is coupled with higher cathode interfacial impedance, a thicker oxygen-deficient rock-salt surface reconstruction layer, more electrolyte solvent and salt breakdown, and higher amounts of transition metal dissolution. These processes are suppressed in the EC-free electrolyte, highlighting the incompatibility between Ni-rich cathodes and conventional electrolyte solvents. Finally, new mechanistic insights into the chemical oxidation pathways of electrolyte solvents and, critically, the knock-on chemical and electrochemical reactions that further degrade the electrolyte and electrodes curtailing battery lifetime are provided.

Engineering and characterization of interphases for lithium metal anodes

Lithium metal is a very promising anode material for achieving high energy density for next generation battery systems due to its low redox potential and high theoretical specific capacity of 3860 mA h g-1. However, dendrite formation and low coulombic efficiency during cycling greatly hindered its practical applications. The formation of a stable solid electrolyte interphase (SEI) on the lithium metal anode (LMA) holds the key to resolving these problems. A lot of techniques such as electrolyte modification, electrolyte additive introduction, and artificial SEI layer coating have been developed to form a stable SEI with capability to facilitate fast Li+ transportation and to suppress Li dendrite formation and undesired side reactions. It is well accepted that the chemical and physical properties of the SEI on the LMA are closely related to the kinetics of Li+ transport across the electrolyte-electrode interface and Li deposition behavior, which in turn affect the overall performance of the cell. Unfortunately, the chemical and structural complexity of the SEI makes it the least understood component of the battery cell. Recently various advanced in situ and ex situ characterization techniques have been developed to study the SEI and the results are quite interesting. Therefore, an overview about these new findings and development of SEI engineering and characterization is quite valuable to the battery research community. In this perspective, different strategies of SEI engineering are summarized, including electrolyte modification, electrolyte additive application, and artificial SEI construction. In addition, various advanced characterization techniques for investigating the SEI formation mechanism are discussed, including in situ visualization of the lithium deposition behavior, the quantification of inactive lithium, and using X-rays, neutrons and electrons as probing beams for both imaging and spectroscopy techniques with typical examples.

Sacrificial Agent Gone Rogue: Electron-Acceptor-Induced Degradation of CsPbBr3 Photocathodes

Lead halide perovskites (LHPs) have emerged as perspective materials for light harvesting, due to their tunable band gap and optoelectronic properties. Photocatalytic and photoelectrochemical (PEC) studies, employing LHP/liquid junctions, are evolving, where sacrificial reagents are often used. In this study, we found that a frequently applied electron scavenger (TCNQ) has dual roles: while it leads to rapid electron transfer from the electrode to TCNQ, enhancing the PEC performance, it also accelerates the decomposition of the CsPbBr₃ photoelectrode. The instability of the films is caused by the TCNQ-mediated halide exchange between the dichloromethane solvent and the LHP film, during PEC operation. Charge transfer and halide exchange pathways were proposed on the basis of in situ spectroelectrochemical and ex situ surface characterization methods, also providing guidance on planning PEC experiments with such systems.

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.

Inhomogeneities and Cell-to-Cell Variations in Lithium-Ion Batteries, a Review

Battery degradation is a fundamental concern in battery research, with the biggest challenge being to maintain performance and safety upon usage. From the microstructure of the materials to the design of the cell connectors in modules and their assembly in packs, it is impossible to achieve perfect reproducibility. Small manufacturing or environmental variations will compound big repercussions on pack performance and reliability. This review covers the origins of cell-to-cell variations and inhomogeneities on a multiscale level, their impact on electrochemical performance, as well as their characterization and tracking methods, ranging from the use of large-scale equipment to in operando studies.

Lower than low: Perspectives on zero- to ultralow-field nuclear magnetic resonance

The less-traveled low road in nuclear magnetic resonance is discussed, honoring the contributions of Prof. Bernhard Blümich, aspiring towards reaching 'a new low.' A history of the subject and its current status are briefly reviewed, followed by an effort to prophesy possible directions for future developments.

Accessing copper oxidation states of dissolved negative electrode current collectors in lithium ion batteries

A novel capillary electrophoresis (CE) method with ultraviolet-visible spectroscopy (UV-Vis) detection for the investigation of dissolved Cu⁺ and Cu²⁺ in lithium ion battery (LIB) electrolytes was developed. This method is of relevance, as the current collector at the anode of LIBs may dissolve under certain operation conditions. In order to preserve the actual oxidation states of dissolved copper in the electrolytes and to prevent any precipitation during sample preparation and CE measurements, neocuproine (NC) and ethylenediamine tetraacetic (EDTA) were effectively applied as complexing agents for both ionic copper species. However, precipitation and loss of the Cu⁺ -NC-complex for quantification occurred in presence of the commonly applied conducting salt lithium hexafluorophosphate (LiPF₆ ). Therefore, acetonitrile (ACN) was added to the sample in order to suppress this precipitation. Dissolution experiments with copper-based negative electrode current collectors in a LIB electrolyte were conducted at 60°C under non-oxidizing atmosphere. First findings regarding the copper species via CE revealed dissolved Cu⁺ and mainly Cu²⁺ . However, primarily Cu⁺ dissolved from the passivating oxide layer (Cu₂ O and CuO) of the current collector due to the formation of acidic electrolyte decomposition products. Due to the instability of Cu⁺ in the electrolyte a further disproportionation reaction to Cu⁰ and Cu²⁺ occurred. The results show the high potential of this CE method for prospective investigations regarding the current collector stability in new battery electrode formulations and correlations of dissolution events with dissolution mechanisms and battery cell operation conditions.

Quantitative spatially resolved post-mortem analysis of lithium distribution and transition metal depositions on cycled electrodes via a laser ablation-inductively coupled plasma-optical emission spectrometry method

Diminishing the loss of performance of lithium ion batteries (LIBs) is a challenge that is yet to be fulfilled. Understanding of deterioration processes and mechanisms (i.e., so-called aging) requires analytically accurate examination of aged cells. Changes in the distribution of lithium or transition metals in the LIB cells can influence their cycle and calendar life significantly. As electrochemically treated cells and especially their electrodes do not age homogeneously and the local electrochemistry (e.g. deposition patterns) is strongly dependent on surface properties, bulk analysis is not a satisfactory investigation method. Therefore, a surface sensitive method, namely laser ablation-inductively coupled plasma-optical emission spectrometry (LA-ICP-OES) is presented. LIB cells with lithium metal oxide LiNi_1/3Co_1/3Mn_1/3O₂ (NCM111) as cathode material and graphite as anode material are investigated using a 213 nm Nd:YAG laser.

An LA-ICP-OES method was developed and applied to investigate the transition metal dissolution in lithium batteries as well as lithium deposition e.g. in case of short circuits.

The influence of hydrofluoric acid etching processes on the photocatalytic hydrogen evolution reaction using mesoporous silicon nanoparticles

H2 has been identified as one of the potential energy vectors that can provide a sustainable energy supply when produced through solar-driven water-splitting reaction. Si is the second most abundant element in the Earth's crust and can absorb a significant fraction of the solar spectrum while presenting little toxicity risk, making it an attractive material for photocatalytic H2 production. Hydrogen-terminated mesoporous Si (mp-Si) nanoparticles can be utilized to effectively drive the hydrogen evolution reaction using UV-to-visible light. In this work, the response of the photocatalytic activity of mp-Si nanoparticles to a series of HF acid treatments was investigated. A two-step magnesiothermic reduction method was used to prepare crystalline mp-Si nanoparticles with a specific surface area of 573 m2 g-1. The HF etching process was optimized as a function of the amount of acid added and the reaction time. The reaction time did not influence the H2 evolution rate substantially. However, the amount of HF used did have a significant effect on the photocatalytic activity. In the presence of ≥1.0 mL HF acid per 0.010 g of Si, morphological damage was observed using electron microscopy. N2 adsorption measurements indicated that the pore size and surface area were also altered. Solution-phase 19F{1H} NMR studies indicated the formation of SiF5- and SiF62- when larger volumes of HF were used. Both factors, morphological damage and the presence of byproducts in the pores, likely result in a lowering of the photocatalytic H2 evolution rate. Under the optimized HF treatment conditions (0.5 mL of HF per 0.010 g of Si), a H2 evolution rate of 1398 ± 30 μmol g-1 h-1 was observed.

Kerr gated Raman spectroscopy of LiPF6 salt and LiPF6-based organic carbonate electrolyte for Li-ion batteries

Fluorescent species are formed during cycling of lithium ion batteries as a result of electrolyte decomposition due to the instability of the non-aqueous electrolytes and side reactions that occur at the electrode surface. The increase in the background fluorescence due to the presence of these components makes it harder to analyse data due to the spectroscopic overlap of Raman scattering and fluorescence. Herein, Kerr gated Raman spectroscopy was shown to be an effective technique for the isolation of the scattering effect from the fluorescence enabling the collection of the Raman spectra of LiPF6 salt and LiPF6-based organic carbonate electrolyte, without the interference of the fluorescence component. Kerr gated Raman was able to identify POF3 on the LiPF6 particle surface, after the addition of trace water.

NMR Study of the Degradation Products of Ethylene Carbonate in Silicon-Lithium Ion Batteries

Ethylene carbonate (EC) is the most widely used electrolyte solvent in lithium ion batteries, but it fails to form a stable passivation layer on materials such as Si and Li metal, which will enable the long-term cycling of the next-generation high-capacity lithium ion batteries containing these anode materials. High concentrations of soluble degradation products are detected in the electrolyte after prolonged cycling, but the chemical structures of these species remain unclear. Here, we used 1D, 2D, and diffusion NMR techniques combined with mass spectrometry to analyze electrolyte-containing ¹³C-labeled EC, and we report on the formation of a series of linear oligomers consisting of ethylene oxide and carbonate fragments with methoxide end groups as the major soluble degradation products of EC. Oligomers with methoxide terminals are likely to have weak interaction toward the electrode; thus, they easily detach from the electrode and are unable to passivate the surface, which may explain the origin of the capacity fade for high-capacity Si-based or Li metal anodes.

A Bifunctional Electrolyte Additive for High-Voltage LiNi0.5Mn1.5O4 Positive Electrodes

4-(Trimethylsiloxy)-3-pentene-2-one (TMSPO) is tested as an electrolyte additive to enhance Coulombic efficiency and cycle retention for the Li/LiNi_0.5Mn_1.5O₄ (LNMO) half-cell and graphite/LNMO full-cell. TMSPO carries two functional groups, siloxane (-Si-O-) and carbon-carbon (C═C) double bonds. It is found that the siloxane group reacts with hydrogen fluoride (HF), which is generated by hydrolysis of lithium hexafluorophosphate (LiPF₆) by impure water in the electrolyte solution, to produce 4-hydroxypent-3-ene-2-one (HPO). The as-generated HPO, as well as TMSPO itself, is electrochemically oxidized to form a protective surface film on the LNMO electrode, in which it is inferred that the carbon-carbon (C═C) double bond initiates radical polymerization. The surface film derived from the TMSPO-added electrolyte shows a superior passivating ability to that generated from the pristine (TMSPO-free) electrolyte. The suppression of electrolyte oxidation enabled by the superior passivating ability offers two beneficial features to the half-cells and full-cells: the suppression of both HF generation and deposition of the resistive surface film on LNMO. As a result, the metal dissolution by HF attack on LNMO appears to be smaller by the addition of TMSPO. The cell polarization is also less significant because of the latter beneficial feature. In short, the bifunctional activity of TMSPO (HF scavenger and protective film former) allows an enhanced Coulombic efficiency and cycle retention to the half-cell and full-cell.

A new HILIC-ICP-SF-MS method for the quantification of organo(fluoro)phosphates as decomposition products of lithium ion battery electrolytes

The lithium ion battery (LIB) is the most popular choice for powering consumer electronics, grid storage and electric vehicles. Decomposition reactions in LIBs, leading to so-called aging, are the main reason for loss of capacity and power and will affect LIB safety. Organo(fluoro)phosphates (O(F)Ps) as decomposition products of LIB electrolytes have been identified in several studies in the literature but quantitative data of O(F)Ps in LIBs are only scarcely available. In terms of toxicity, this substance class is highly relevant as it shows structural similarities to chemical warfare agents. Thus, approaches that can deliver quantitative data are in need. In this study, acidic O(F)Ps were quantified with an inductively coupled plasma-sector field-mass spectrometer (ICP-SF-MS) after separation of species with hydrophilic interaction liquid chromatography (HILIC). The formation of OFPs exceeds the amount of non-fluorine containing OPs by a factor of up to 15. A total of 16 different O(F)P compounds could successfully be quantified. Organic mass spectrometry was used for the assignment of quantitative data.

The lithium ion battery (LIB) is the most popular choice for powering consumer electronics, grid storage and electric vehicles.

The Role of Cations on the Performance of Lithium Ion Batteries: A Quantitative Analytical Approach

Lithium ion batteries are nowadays the state-of-the-art power sources for portable electronic devices and the most promising candidate for energy storage in large-size batteries, e.g., pure and hybrid vehicles. However, the degradation of the cell components minimizes both storage and operation lifetime (calendar and cycle life), which is called aging. Due to the numerous different aging effects, in either the single constituents or their interactions with each other, many reports about methodologies and techniques, both electrochemical and analytical, can be found in the literature. However, quantitative data about the degradation effects were seldom stated. One important effect is the cation distribution and migration during operation. Metal dissolution and metal migration of the cathode and the corresponding deposition of these metals on the graphitic anode are known harmful degradation effects, especially for the formed solid electrolyte interphase on the surface of the anode. Depending on the applied cell chemistries and therefore the cathode material, different mechanisms were reported so far. For lithium manganese oxide based cells, the acidification of the electrolyte due to composition of the conduction salt is attributed as the main source of metal migration. Due to subsequent loss of manganese from the cathode, the overall performance of the cell is seriously impaired. Based on the obtained observations, this degradation mechanism was adapted to lithium nickel cobalt manganese based cells as main cause of the capacity fading. However, with the help a developed total X-ray fluorescence method and additional surface and electrolyte investigations, the proposed HF based mechanism was disproven. Instead, the migration was directly associated with material defects or mechanical spalling of the particles. Furthermore, with the obtained quantitative data of the migrated transition metals on the anode and separator, the contribution on the capacity fade was determined. It ranged only the ‰ region and could therefore be excluded as the main source of the capacity in these lithium ion batteries. Nevertheless, the oxidation state of the cations is hardly accessible; but would provide further information about the exact migrating mechanisms. In addition, lithium can be "lost" or immobilized during charge/discharge and is therefore no longer available as an electrochemically active cation. For example, the formation, reformation, and growth of the solid electrolyte interphase and cathode electrolyte interphase leads to an increased active lithium loss during cycling. The investigations on this topic are frequently reported in literature; however, quantitative data on the actual lithium distribution throughout the cell are relatively few. Furthermore, the exact amount of lost lithium in the in the respective interphases is so far not available. In order to determine quantitatively the lithium distribution within the cell, inductively coupled plasma-based method was applied. For laboratory test cells, the lithium that was lost to the housing of the cell was 32 times higher than that for pouch bag cells. Furthermore, the determined concentration of lithium in the interphases ranged only from 2 to 4%. However, the investigations need to be repeated with isotope labeled material (⁶Li) in order to obtain statements that are more precise.

The Role of Sub- and Supercritical CO2 as "Processing Solvent" for the Recycling and Sample Preparation of Lithium Ion Battery Electrolytes

Quantitative electrolyte extraction from lithium ion batteries (LIB) is of great interest for recycling processes. Following the generally valid EU legal guidelines for the recycling of batteries, 50 wt % of a LIB cell has to be recovered, which cannot be achieved without the electrolyte; hence, the electrolyte represents a target component for the recycling of LIBs. Additionally, fluoride or fluorinated compounds, as inevitably present in LIB electrolytes, can hamper or even damage recycling processes in industry and have to be removed from the solid LIB parts, as well. Finally, extraction is a necessary tool for LIB electrolyte aging analysis as well as for post-mortem investigations in general, because a qualitative overview can already be achieved after a few minutes of extraction for well-aged, apparently "dry" LIB cells, where the electrolyte is deeply penetrated or even gellified in the solid battery materials.

A battery cell for in situ NMR measurements of liquid electrolytes

Development of an in situ battery cell to monitor the liquid electrolyte by means of NMR spectroscopy.

Investigating the lithium ion battery electrolyte additive tris (2,2,2-trifluoroethyl) phosphite by gas chromatography with a flame ionization detector (GC-FID)

The quantification of lithium ion battery electrolyte additives like flame retardants is both important and challenging. Here, different analytical methods were applied to investigate detection phenomena when applying GC-FID for the quantification.

Quantification of ionic organo(fluoro)phosphates in decomposed lithium battery electrolytes

Simultaneous identification and quantification of organofluorophosphates via the developed setup.

Fast screening method to characterize lithium ion battery electrolytes by means of solid phase microextraction – gas chromatography – mass spectrometry

Several electrolytes of commercially available lithium ion batteries (LIBs) were analyzed by solid phase microextraction – gas chromatography – mass spectrometry (SPME-GC-MS).