Electrolytes For Li-ion Batteries Research Articles

Solvation Structures and Transport Mechanisms of Cations in Water-in-Bisalt Electrolytes for High-Concentration Lithium-Ion Batteries Revealed by Molecular Dynamics Simulations.

The development of safe and cost-effective electrolytes for rechargeable batteries is currently underway. While water-based electrolytes hold promise, their restricted electrochemical stability window poses a challenge. Combining multiple ionic species emerges as a promising strategy to broaden this stability window and optimize Li-ion battery performance. This study focuses on dual-cation electrolytes, which blend lithium and potassium acetates to enhance the electrochemical characteristics of the solution at high concentrations. We investigated the solvation structure of each ion and its interactions on a molecular level. Our analysis reveals that ion clusters and aggregates are formed through shared acetate and water molecules at high salt concentrations. Furthermore, the residence time analyses of atom pairs indicate that cations diffuse in vehicular mode at low concentrations. In contrast, they switch to a structural mode at high concentrations due to diminishing water content. This study offers a comprehensive model for exploring diverse solvation structures of cations and gaining insights into their diffusion mechanisms within water-in-bisalt electrolytes for aqueous Li-ion batteries.

Designing Nonflammable Liquid Electrolytes for Safe Li-Ion Batteries.

Li-ion batteries are essential technologies for electronic products in the daily life. However, serious fire safety concerns that are closely associated with the flammable liquid electrolyte remains a key challenge. Tremendous effort has been devoted to designing nonflammable liquid electrolytes. It is critical to gain comprehensive insights into nonflammability design and inspire more efficient approaches for building safer Li-ion batteries. This review presents current mechanistic understanding of safety issues and discusses state-of-the-art nonflammable liquid electrolytes design for Li-ion batteries based on molecule, solvation, and battery compatibility level. Various safety test methods are discussed for reliable safety risk evaluation. Finally, the challenges and perspectives of the nonflammability design for Li-ion electrolytes are summarized.

Understanding the Solid-Electrolyte-Interface (SEI) Formation in Glyme Electrolyte Using Time-Of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS).

Lithium-ion batteries are commonly used for energy storage due to their long lifespan and high energy density, but the use of unsafe electrolytes poses significant health and safety concerns. An alternative source is necessary to maintain electrochemical efficacy. This research demonstrates new safe glyme-based electrolytes for silica/carbon (SiOx/C) nanocomposite derived from Australian rice husk (RH). The quality of SiOx/C was preserved by using deep eutectic solvent-based pre-treatment and single-step carbonization, which was confirmed through the X-ray analysis of the crystalline phase of silica. The electrochemical assessment of SiOx/C anode using various glyme-based electrolytes for LIBs was carried out. Among them, the resultant half cells based on diglyme electrolyte is superior to others with the first discharge capacity at 1274 mAh/g and a reversible discharge capacity of 759.7 mAh/g. Ex-situ SEM and Time-of-Flight Secondary Ion Mass Spectrometry (ToF- SIMS) analysis of the electrode indicated that diglyme not only improves the capacity but also sustains the electrode architecture for longer cycle life with more LiF-based components and also showed the absence of HF components. Importantly, the addition of fluoroethylene carbonate (FEC) additive enhanced the cycling stability. These results provide a new perspective on developing advanced SiOx/C anode using glyme electrolytes for Li-ion batteries.

Electrochemical Removal of HF from Carbonate-based LiPF6-containing Li-ion Battery Electrolytes

Due to the hydrolytic instability of LiPF6 in carbonate-based solvents, HF is a typical impurity in Li-ion battery electrolytes. HF significantly influences the performance of Li-ion batteries, for example by impacting the formation of the solid electrolyte interphase at the anode and by affecting transition metal dissolution at the cathode. Additionally, HF complicates studying fundamental interfacial electrochemistry of Li-ion battery electrolytes, such as direct anion reduction, because it is electrocatalytically relatively unstable, resulting in a LiF passivation layer. Methods to selectively remove ppm levels of HF from LiPF6-containing carbonate-based electrolytes are limited. We introduce and benchmark a simple yet efficient electrochemical method to selectively remove ppm amounts of HF from LiPF6-containing carbonate-based electrolytes. The basic idea is the application of a suitable potential to a high surface-area metallic electrode upon which only HF reacts (electrocatalytically) while all other electrolyte components are unaffected under the respective conditions.

Effect of Ca doping on Li ion conductivity of Ge and Ta doped garnet LLZO

The series Li6.55+xGe0.05La3-xCaxZr1.75Ta0.25O12 (x = 0, 0.05, 0.10, 0.15, 0.20) was prepared by conventional solid state reaction method with the sintering temperature of 1050 °C for 7.30 h by substituting Ca at the La site to increase the Li ion conductivity. The synergistic effects of Ca incorporation on Li6.55Ge0.05La3Zr1.75Ta0.25O12 were studied using various structural and electrochemical analyses. X-ray diffraction, scanning electron microscopy, and Impedance analysis were used to determine the crystal structure, morphology, and AC conductivity of the prepared ceramic samples, respectively. The highest conductivity of 9.95 x 10−4 S/cm was obtained for a 0.05 Ca ceramic sample with minimum activation energy of 0.23 eV. The DC polarization measurements confirmed the dominance of ionic conduction in 0.05 Ca ceramic. The results obtained make the 0.05 Ca ceramic sample a promising candidate as solid electrolytes for all solid state Li-ion batteries (ASSLIBs).

Effect of Li6.4La3Zr1.4Ta0.6O12 Fillers on the Interfacial Properties between Composite PEO-LiTFSI Electrolytes with Li Metal during Cycling.

PEO-LiX solid polymer electrolyte (SPE) with the addition of Li6.4La3Zr1.4Ta0.6O12 (LLZTO) fillers is considered as a promising solid-state electrolyte for solid-state Li-ion batteries. However, the developments of the SPE have caused additional challenges, such as poor contact interface and SPE/Li interface stability during cycling, which always lead to potentially catastrophic battery failure. The main problem is that the real impact of LLZTO fillers on the interfacial properties between SPE and Li metal is still unclear. Herein, we combined the electrochemical measurement and in situ synchrotron-based X-ray absorption near-edge structure (XANES) imaging technology to study the role of LLZTO fillers in directing SPE/Li interface electrochemical performance. In situ XRF-XANES mapping during cycling showed that addition of an appropriate amount of LLZTO fillers (50 wt %) can improve the interfacial contact and stability between SPE and Li metal without reacting with the PEO and Li salts. Additionally, it also demonstrated the beneficial effect of LLZTO particles for suppressing the interface reactions between the Li metal and PEO-LiTFSI SPE and further inhibiting Li-metal dendrite growth. The Li|LiFePO4 batteries deliver long cycling for over 700 cycles with a low-capacity fade rate of 0.08% per cycle at a rate of 0.3C, revealing tremendous potential in promoting the large-scale application of future solid-state Li-ion batteries.

Co-Intercalation-Free Graphite Anode Enabled by an Additive Regulated Interphase in an Ether-Based Electrolyte for Low-Temperature Lithium-Ion Batteries.

Graphite (Gr) anode, which is endowed with high electronic conductivity and low volume expansion after Li-ion intercalation, establishes the basis for the success of rocking-chair Li-ion batteries (LIBs). However, due to the high barrier of the Li-ion desolvation process, sluggish transport of Li ions through the solid electrolyte interphase (SEI) and the high freezing points of electrolytes, the Gr anode still suffers from great loss of capacity and severe polarization at low temperature. Here, 1,2-diethoxyethane (DEE) with an intrinsically wide liquid region and weak solvation ability is applied as an electrolyte solvent for LIBs. By rationally designing the additives of electrolytes, an intact SEI with fast Li-ion conductivity is constructed, enabling the co-intercalation-free Gr anode with long-term stability (91.8% after 500 cycles) and impressive low-temperature characteristics (82.6% capacity retention at -20 °C). Coupled with LiFePO4 and LiNi0.8Mn0.1Co0.1O2 cathodes, the optimized electrolyte also demonstrates low polarization under -20 °C. Our work offers a feasible approach to enable ether-based electrolytes for low-temperature LIBs.

Structure and molecular interactions in 1‑ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ion pair conformers: abinitio DFT based study

Density functional theory (DFT) calculations with Grimme’s B97D functional including dispersion are employed to provide both structural and electronic insight into the multifold interactions occurring in 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ionic liquid [EMI][TFSI], which is currently being targeted for applications in next-generation Li-ion battery electrolytes. The geometric structure, interaction energy and natural bond orbital analysis of the ion-pair conformers of [EMI][TFSI] show differences from the conventional H-bonds. A total of six stable [EMI][TFSI] ion pair conformers were observed. Our results show that a doubly ionic H-bond exists between [EMI]+ and [TFSI]− ion pair conformers. The greater the number of multiple H-bond type interactions, the greater the absolute value of the interaction energy. Moreover, results from the NBO analysis show that, for [EMI][TFSI] ion pair conformers, charge transfer occurs mainly from the lone pairs of oxygen and nitrogen atom to the σ-type anti-bonding orbital of the C–H and π-type anti-bonding orbitals of N–C bonds. This was evident from the values of the stabilization energy E(2) associated with each electron delocalization from the donor to acceptor orbitals. There exist large numbers of multiple concomitant hydrogen bonds, but the values of the stabilization energy E(2) are generally small (E(2)n→\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\ o$$\\end{document}σ* < 3 kcal/mol) for the individual H-bond interactions. The shorter the C–H···O and/or C–H···N bond is, the larger charge transfer, and the larger the stabilization energy E(2) associated with electron delocalization from donor to acceptor. Generally, the [EMI][TFSI] ion pair conformers tend to form multiple but bent H bonds, reducing the strength of the individual H bonds from a potential (linear) maximum.

Phonon DOS-Based Machine Learning Model for Designing High-Performance Solid Electrolytes in Li-Ion Batteries

In the present work, a machine learning (ML) model was built to design solid electrolytes with improved ionic conductivity for Li-ion batteries (LIBs), and the model was based on the phonon density of states (PhDOS). Compounds with PhDOS calculations were collected from the Materials Project (MP) and processed to obtain frequencies and PhDOS to calculate the total phonon band center, a proxy for ionic conductivity. Total phonon band centers were involved in a learning process using four ML algorithms (extra random trees (XT), gradient boosting (GB), extreme gradient boosting (XGB), and decision trees (DT)). The cross-validation results from the algorithms showed that the performance of the XT-model was superior and confirmed through density functional theory- (DFT-) based phonon calculations conducted on LiYO3, Li4CO4, LiNiO3, LiGeO3, and LiSiO3. The XT-model was then used to predict the total phonon band centers of new compounds where these had no phonon calculations beforehand. Experimental validation of the XT-model involved electrochemical impedance spectroscopy (EIS) measurements on two compounds: Li2CO3 from the high range and Li6PS5Cl from the low range. Additionally, LiBiO2 that is predicted to have low total phonon band center, according to the XT-model, was considered further to estimate its potential as solid electrolyte in LIBs.

Non-flammable electrolyte for large-scale Ni-rich Li-ion batteries: Reducing thermal runaway risks

We present a novel, non-flammable electrolyte for Ni-rich 18650 cylindrical lithium-ion batteries (LIBs) based on a blend of triethyl phosphate (TEP) and fluorinated ethylene carbonate (FEC). We conducted a comprehensive analysis of the electrolyte, including electrochemical tests, safety evaluations, electrode surface characterizations, and quantum calculations, to elucidate its property-function relationship. We found that combining TEP with a concentrated FEC as a co-solvent (1.2 M LiPF6 in TEP/FEC, 30/70 %v) resulted in a robust solid electrolyte interface (SEI) predominantly composed of lithium fluoride (LiF). This innovation improved the SEI's stability and suppressed electrolyte decomposition during extended cycling, leading to consistent battery performance. Our findings introduce a promising non-flammable electrolyte solution for LIBs that offers both thermal runaway prevention and seamless integration into existing battery manufacturing processes. This positions it as a potential alternative to solid-state electrolytes in advanced battery technology, with the potential to revolutionize the field.

Decoupling of ion-transport from polymer segmental relaxation and higher ionic-conductivity in poly(ethylene oxide)/succinonitrile composite-based electrolytes having low lithium salt doping.

Only limited enhancement in room-temperature ionic-conductivity for poly(ethylene oxide), PEO, based electrolytes is possible due to coupling between ionic-conductivity and segmental relaxation. In the present study, we have achieved ionic-conductivity of 1.07 × 10-3 and 6.20 × 10-4 S cm-1 at 313 and 298 K, respectively, by adding 45 wt% of succinonitrile (SN) in PEO having low LiTFSI loading (Li : EO = 1 : 20). This enhancement in the ionic-conductivity is attributed to faster ion transport (diffusion coefficient, D = 3.63 × 10-5 cm2 s-1) occurring through the ion-transport channels as confirmed by positron annihilation lifetime spectroscopy. The ionic-transport through these channels is observed to be highly decoupled from the segmental relaxations as confirmed using broadband dielectric spectroscopy through Ratner's approach. The observed decoupling of ionic-conductivity from PEO segmental relaxation in PEO-SN composite-based electrolytes would be useful to design rather inexpensive all solid-state polymer electrolytes for Li ion batteries.

Boron-containing fullerene-based salts with cyclic carbonate solvents as electrolytes for Li-ion batteries and beyond.

Replacement of carbon atoms by a heteroatom in fullerene is a promising route that enhances the electronic properties of fullerenes and results in hetero fullerene-based effective agents ensuring applications in vivid fields of the solar cell, cathode materials for batteries, etc. Towards the development of new electrolyte salts, attention has been paid to facilitating ion mobility in particular and moderate stability of the anions in addition. From the atomistic molecular dynamics simulation studies, for the first time, we uncover that the boron-containing hetero fullerene, C59B- anion-based LiC59B, and NaC59B salts in cyclic carbonate solvents can act as efficient electrolytes by improving the transport phenomenon of the metal ions in solution, importantly for Li+ and satisfactorily for Na+ as compared to their commonly used BF4- anion based salts. Additionally, our study revealed that apart from LiC59B, and NaC59B salts, C58B22- based MgC58B2 salt can facilitate the ionic conductivity of the electrolyte. The properties of the proposed electrolyte under an electric field and different temperatures were investigated. Some of the bulk properties of the used electrolytes to some extent were found to be improved in the presence of these salts. The first principle-based electrochemical calculations further justify the stability of the proposed anions. The initial investigation from the Reactive force-field (ReaxFF) based atomistic simulations study elucidates that LiC59B reduces the decomposition of the EC solvent compared to LiBF4 and facilitates solvent stability.

A structural study on a specific Li-ion ordered complex in dimethyl carbonate-based dual-cation electrolytes.

Dimethyl carbonate (DMC) is a linear carbonate solvent commonly used as an electrolyte for electric double-layer capacitors (EDLCs) and Li-ion batteries. However, there are serious problems with the use of DMC as an electrolyte solvent: (1) low ionic conductivity when using Li salts (e.g. LiBF4) and (2) liquid-liquid phase separation when using spiro-type quaternary ammonium salts (e.g. SBPBF4). Dual-cation electrolytes, i.e., bi-salt (SBPBF4 and LiBF4) in DMC, are promising candidates to avoid the phase separation issue and to enhance the total and Li+ conductivities. Herein, we reported a specific Li-ion structure in DMC-based dual-cation electrolytes by combining high-energy X-ray total scattering (HEXTS) and all-atom molecular dynamics (MD) simulations. Quantitative radial distribution function analysis based on experimental and simulation results revealed that the phase-separated SBPBF4/DMC (i.e., the bottom phase of 1 M SBPBF4/DMC) forms long-range ion ordering based on the structured SBP+-BF4- ion pairs. When adding LiBF4 salt into SBPBF4/DMC (i.e., dual-cation electrolyte), the ordered SBP+-BF4- structure disappeared owing to the formation of Li-ion solvation complexes. We found that in the dual-cation electrolyte Li ions form multiple Li+-Li+ ordered complexes in spite of relatively low Li-salt concentration (1 M), being a promising Li+-conducting medium with reduced Li salt usage and low viscosity.

Machine-learning assisted high-throughput discovery of solid-state electrolytes for Li-ion batteries

The integration of machine learning with high-throughput computation accelerates the precise prediction of novel battery materials.

First-Principles study on Li-ion migration properties in Li&lt;sub&gt;1+&lt;i&gt;x&lt;/i&gt;&lt;/sub&gt; Al&lt;sub&gt;&lt;i&gt;x&lt;/i&gt;&lt;/sub&gt;Ti&lt;sub&gt;2-&lt;i&gt;x&lt;/i&gt;&lt;/sub&gt;(PO&lt;sub&gt;4&lt;/sub&gt;)&lt;sub&gt;3&lt;/sub&gt; (LATP)

NASICON type materials are specific skeleton structures in which ions move in three dimensions. Li&lt;sub&gt;1+&lt;i&gt;x&lt;/i&gt;&lt;/sub&gt;Al&lt;sub&gt;&lt;i&gt;x&lt;/i&gt;&lt;/sub&gt;Ti&lt;sub&gt;2-&lt;i&gt;x&lt;/i&gt;&lt;/sub&gt;(PO&lt;sub&gt;4&lt;/sub&gt;)&lt;sub&gt;3&lt;/sub&gt; (LATP) is a promising NASICON type solid-state electrolyte for Li-ion batteries, due to its relatively high Li-ion conductivity, chemical stability to air and moisture and mechanical strength. Motivated by this, we studied the doping and electronic properties of Li&lt;sub&gt;1+&lt;i&gt;x&lt;/i&gt;&lt;/sub&gt;Al&lt;sub&gt;&lt;i&gt;x&lt;/i&gt;&lt;/sub&gt;Ti&lt;sub&gt;2-&lt;i&gt;x&lt;/i&gt;&lt;/sub&gt;(PO&lt;sub&gt;4&lt;/sub&gt;)&lt;sub&gt;3&lt;/sub&gt; (&lt;i&gt;x&lt;/i&gt; = 0.00、0.16、0.33、0.50) and the transport properties of Li&lt;sup&gt;+&lt;/sup&gt; in them using first-principles calculations based on density functional theory as implemented in Vienna ab initio Simulation Package (VASP). The results indicate that Al can substitute Ti to form stable structures. When the Al doping concentration is &lt;i&gt;x&lt;/i&gt; = 0.16, the average bond length of Li-O bonds is the longest and the bonding strength is the weakest, This may lead to the expansion of channels for Li&lt;sup&gt;+&lt;/sup&gt; migration, which facilitates the diffusion of Li&lt;sup&gt;+&lt;/sup&gt;. With the increase of Al doping concentration, the strength of Ti-O bond remains almost unchanged. The electronic structure calculations exhibit that with the increase of Al doping concentration, the bandgap of LATP does not change much, and LATP all shows semiconductor characteristics. The differential charge results indicate that more electrons are localized on O-atoms surrounding the Al-dopant, causing the AlO&lt;sub&gt;6&lt;/sub&gt; groups to form polarization centers. The study on the migration properties of Li&lt;sup&gt;+&lt;/sup&gt; indicates that Li&lt;sup&gt;+&lt;/sup&gt; exhibits different migration characteristics when it adopts one of three different migration modes (vacancy migration, interstitial migration and cooperative migration) : With the increase of Al doping concentration, the migration barriers of Li&lt;sup&gt;+&lt;/sup&gt; increase in migration via vacancies involving only lattice site migration, And the migration barrier for LATP-0.16 is the lowest (0.369 eV). While in interstitial migration involving only interstitial sites, the migration barrier of Li&lt;sup&gt;+&lt;/sup&gt; decreases accordingly. When the Al doping concentration is x=0.50, the migration barrier is the lowest (0.342 eV). In terms of cooperative migration, this migration mode involves both vacancy and interstitial sites, so the migration barrier first decreases and then increases with the increase of Al doping concentration. Thus, our study suggests that by varying the concentration of Al doping, the interstitial Li&lt;sup&gt;+&lt;/sup&gt;content, migration channel structure, and the migration performance of Li&lt;sup&gt;+&lt;/sup&gt; can be changed favorably. Our results provide a theoretical basis for improving the ion conductivity of Li in LATP with varying Al doping concentration in experiments.

Design and Investigation of Superatoms for Redox Applications: First-Principles Studies.

A superatom is a cluster of atoms that acts like a single atom. Two main groups of superatoms are superalkalis and superhalogens, which mimic the chemistry of alkali and halogen atoms, respectively. The ionization energies of superalkalis are smaller than those of alkalis (<3.89 eV for cesium atom), and the electron affinities of superhalogens are larger than that of halogens (>3.61 eV for chlorine atom). Exploring new superalkali/superhalogen aims to provide reliable data and predictions of the use of such compounds as redox agents in the reduction/oxidation of counterpart systems, as well as the role they can play more generally in materials science. The low ionization energies of superalkalis make them candidates for catalysts for CO2 conversion into renewable fuels and value-added chemicals. The large electron affinity of superhalogens makes them strong oxidizing agents for bonding and removing toxic molecules from the environment. By using the superatoms as building blocks of cluster-assembled materials, we can achieve the functional features of atom-based materials (like conductivity or catalytic potential) while having more flexibility to achieve higher performance. This feature paper covers the issues of designing such compounds and demonstrates how modifications of the superatoms (superhalogens and superalkalis) allow for the tuning of the electronic structure and might be used to create unique functional materials. The designed superatoms can form stable perovskites for solar cells, electrolytes for Li-ion batteries of electric vehicles, superatomic solids, and semiconducting materials. The designed superatoms and their redox potential evaluation could help experimentalists create new materials for use in fields such as energy storage and climate change.

Design of High-Entropy Electrolytes Enabled By the High-Throughput and Autonomous Procedure

Electrolytes in Li-ion batteries play a significant role as they influence different aspects directly related to the battery performance, such as safety, voltage window, electrochemical stability, and the formation of solid-electrolyte interphase (SEI). Conventionally, these electrolytes are composed of a lithium salt dissolved in an organic solvent such as ethylene carbonate and propylene carbonate. Regarding safety, these organic electrolytes can be replaced by room-temperature ionic liquids (RTILs), which present lower vapor pressure and non-flammability.1 Moreover, adding polymer to these liquid electrolytes to form polymer gel electrolytes has proven to be a good strategy to avoid dendritic growth and increase electrolyte stability.2 The properties of RTIL electrolytes containing Li salts have been studied in the last decade. Still, the scope has been limited primarily to simple RTILs consisting of one cationic and one anionic species. Inspired by the success of high-entropy alloys (HEAs) for forming stable solid solutions with excellent mechanical properties,3 the concept of high-entropy materials has been recently transferred to electrolytes, where the effect of mixing multiple Li salts is explored.4 However, the number of compositional combinations increases rapidly when multiple salts, cationic and anionic species are introduced, and the explorable compositional space increases even more when polymers are added to the mixture.Automating the electrolyte preparation and characterization can be the key feature to allow the exploration of all the possible electrolyte combinations. In this work, we explain the first preliminary results obtained from an autonomous preparation and characterization of high-entropy electrolytes composed of a mixture of two different lithium salts in ionic liquids consisting of multiple anionic and cationic species. In addition, the adaptation of the autonomous system of preparation and characterization of liquid electrolytes to polymer gel electrolytes is also proposed and explained. References. (1) Niu, H.; Wang, L.; Guan, P.; Zhang, N.; Yan, C.; Ding, M.; Guo, X.; Huang, T.; Hu, X. Recent Advances in Application of Ionic Liquids in Electrolyte of Lithium Ion Batteries. J. Energy Storage 2021, 40, 102659. https://doi.org/10.1016/j.est.2021.102659.(2) Chen, J.; Wu, J.; Wang, X.; Zhou, A.; Yang, Z. Research Progress and Application Prospect of Solid-State Electrolytes in Commercial Lithium-Ion Power Batteries. Energy Storage Mater. 2021, 35, 70–87. https://doi.org/10.1016/j.ensm.2020.11.017.(3) Yeh, J.-W.; Chen, S.-K.; Lin, S.-J.; Gan, J.-Y.; Chin, T.-S.; Shun, T.-T.; Tsau, C.-H.; Chang, S.-Y. Nanostructured High-Entropy Alloys with Multiple Principal Elements: Novel Alloy Design Concepts and Outcomes. Adv. Eng. Mater. 2004, 6 (5), 299–303. https://doi.org/10.1002/adem.200300567.(4) Wang, Q.; Zhao, C.; Wang, J.; Yao, Z.; Wang, S.; Kumar, S. G. H.; Ganapathy, S.; Eustace, S.; Bai, X.; Li, B.; Wagemaker, M. High Entropy Liquid Electrolytes for Lithium Batteries. Nat. Commun. 2023, 14 (1), 440. https://doi.org/10.1038/s41467-023-36075-1.

Investigation of the Solid Electrolyte Interphase of Silicon Wafers Using a Fluorine-Free Electrolyte

Lithium-ion (Li-ion) batteries have become an important solution for energy storage owing to their high gravimetric and volumetric energy density. The electrolyte solution of the battery has a large effect on the stability and performance of the cell. During the first cycles of a battery, the solid electrolyte interphase (SEI) layer is formed due to the degradation of the electrolyte. Hence the composition of this layer, and the stability, are dependent on the electrolyte composition.The most commonly used electrolyte for Li-ion batteries is the salt lithium hexafluorophosphate LiPF6 dissolved in organic solvents together with additives, both containing fluorinated compounds.1 However these fluorinated compounds are susceptible to release HF which is toxic and corrosive which could be detrimental for the cell performance but also hinder the recycling process. Therefore, fluorine-free electrolytes could be a promising alternative. While this type of electrolytes has been investigated with graphite anodes, when moving to higher energy densities, such as silicon, their performance and degradation mechanism is not yet clear.2 Previous research has shown increased stability for Li-ion batteries using a fluorine-free electrolyte based on the salt lithium bis(oxalato) borate (LiBOB).2 Furthermore, operando X-ray reflectometry (XRR) measurements on silicon wafers half-cells with the same electrolyte have shown the build-up of multiple layers on the anode, providing the thicknesses and densities of the different layers. This work presents the chemical composition of these different layers formed on the silicon wafers at different potentials when using a fluorine-free electrolyte based on LiBOB in ethylene carbonate and methyl ethyl carbonate. These results are also compared with the effect of adding vinylene carbonate VC as an additive. The different components in these layers are correlated to the previous XRR data to explain the SEI-formation mechanism with fluorine-free electrolytes. Xu, K. Nonaqueous liquid electrolytes for lithium-based rechargeable batteries. Chem Rev 104, (2004).Hernández, G. et al. Elimination of fluorination: The influence of fluorine-free electrolytes on the performance of LiNi1/3Mn1/3Co1/3O2/silicon-graphite li-ion battery cells. ACS Sustain Chem Eng 8, (2020).

Improving the Performance of Flame-Retarding Sodium Bis(Oxalato)Borate in Triethyl Phosphate Electrolyte for Sodium Ion Batteries

An ever-growing demand for energy storage has highlighted potential future resource constraints for Li-ion batteries (LIBs), in particular with respect to Li.1-3 With this in mind Na-ion batteries (NIBs) hold promise as a complementary technology to LIBs, particularly for stationary energy storage.4 To be competitive with LIBs, NIBs need to become fully competitive on cost and on capacity density.5-7 NIBs can however offer more than simply a cost competitive option to LIBs, with potential to offer improvements on both safety and sustainability. A number of cathode materials exist that can avoid many of the unanswered question around the sustainability of state-of-the-art LIB cathodes.3 However, to address safety concerns of LIBs it is impossible to ignore the electrolyte. State of the art electrolyte in LIBs, regularly LiPF6 in a mix of carbonates, is both toxic (by virtue of LiPF6 and to a limited extent ethylene carbonate) and flammable (by virtue of the electrolyte solvents). Importing the Na analogue of state-of-the-art LIB electrolytes (i.e. NaPF6 in carbonates) creates the same problems of flammability and toxicity currently seen in LIBs (albeit the toxicity of NaPF6 is lower than that of LiPF6).Recently Mogensen et al.8 proposed sodium bis(oxalato)borate (NaBOB) in trimethyl phosphate (TMP) electrolyte that overcame the flammability concerns of NaPF6 in carbonate electrolytes. In addition to non-flammability, the use of NaBOB instead of NaPF6 allowed for higher temperatures to be used and produced an electrolyte that was halogen free. Switching from TMP to triethyl phosphate (TEP) allowed for a relatively less-toxic electrolyte (NaBOB and TEP are listed by the European Chemical Agency as ‘harmful if swallowed’ in comparison to ‘harmful if swallowed, in contact with skin or inhaled’ for NaPF6 in carbonates, although limited research has been conducted on the toxicity of NaBOB and so this should be received with limited certainty) without sacrificing cell performance.9 However, hurdles remain for NaBOB in organophosphate electrolytes. Principal amongst these has been the poor solubility of NaBOB in most solvents and the difficulties transferring promising low mass-loading full cell results to higher mass-loading full cells. The low solubility of NaBOB limits the concentration of NaBOB in organophosphate electrolytes to < 0.4 M, producing electrolytes with relatively low conductivities (for NaBOB in TMP, 4.5 mS/cm, and in TEP, 5 mS/cm). N-methyl pyrrolidone (NMP) has previously been shown to increase the solubility of NaBOB when used as a cosolvent with TMP, with conductivity improved to 6.5 mS/cm without sacrificing non-flammability and promising cycling shown in low mass-loading full-cells.10 A range of alternative cosolvents were here investigated to increase the solubility of NaBOB in TEP. Of these, NMP, N,N-dimethyl formamide (DMF) and N,N-dimethyl acetamide (DMAc) were evaluated as cosolvents with TEP for near-commercial mass loading full-cells (Prussian white||hard carbon at ~ 2 mAh/cm2). To maintain non-flammable characteristics, the cosolvents had to be kept below 40% (v/v) for DMAc and DMF and below 60% (v/v) for NMP, resulting in electrolyte conductivities of 5.7, 6.9 and 6.0 mS/cm respectively. These nearly doubled the conductivity we observed for NaBOB in TEP of 3.5 mS/cm. However, all electrolytes examined showed large polarisation, suspected to be due a thick, resistive SEI. A range of strategies have been employed in order to reduce this polarisation, with particular focus on improved formation protocols (studying the effects of temperature, current density and electrochemical method of formation) and their use in concert with film forming additives. Future work will aim to understand the interplay between formation conditions, the resultant SEI formation and the cell polarization in an effort to translate promising low mass-loading full cell results to near-commercial mass-loading full cells.

Improving Extreme Temperature and Charge Rate Performance in Li-Ion Batteries through Addition of Fluorinated Electrolyte Additives

Despite advances in Li-ion battery electrolytes, commercial electrolytes continue to include organic solvents that are unable to withstand many of the arduous conditions demanded by the industry today, such as extreme temperatures and charge rates. The stability limits of these solvents damper the advancement of Li-ion batteries needed to sustain automotive electrification and other industrial applications. Further efforts are needed to enable electrolyte formulations to withstand the operational demands of Li-ion batteries in the market today.Koura has developed a series of fluorinated materials for use in lithium-ion electrolytes that demonstrate improved stability and safety over conventional commercial electrolytes. In a recently completed study, the impact of Koura’s fluorinated solvents on performance in 4.3V Gr/NMC811 was examined, with a particular focus on benefits seen on extreme temperature and rate performance. Testing included cycling and rate performance across a range of temperatures from -20 °C to 45 °C and cycling conditions from C/2 to 4C. It has been found that these materials exhibit improved performance in low temperature testing, as well as reduced gassing and increased performance in high-temperature and fast charge cycling. To investigate the mechanisms responsible for the observed improvements, comprehensive post-test analysis has been conducted, including gas identification and quantification via gas chromatography, bulk electrolyte composition analysis via NMR spectroscopy, and surface layer characterization via XPS and SEM. Bulk properties of the materials and electrolytes are correlated to performance benefits where appropriate.This presentation will summarize Koura’s efforts to fill a major gap in the demands of the battery industry today through the development of fluorinated electrolyte materials. Specifically, we will present data highlighting the performance of fluorinated compounds on improving battery performance under strenuous conditions with respect to both temperature and cycling rate. Results from post-test analyses will be discussed highlighting the mechanisms underlying the observed performance improvements.