Li Salt Research Articles

Nb/Al co‐doped Li7La3Zr2O12 Composite Solid Electrolyte for High‐Performance All‐Solid‐State Batteries

AbstractAll‐solid‐state Li batteries (ASSLBs) are considered suitable candidates to replace conventional batteries utilizing liquid electrolytes. However, the applications of this type of batteries are limited owing to their narrow operating temperature range, low ionic conductivity, poor long‐term stability, and complex production process. Herein, a simple approach that combines all potential battery candidates that have been investigated over the last few years, including polyethylene oxide (PEO), Li7La3Zr2O12 (LLZO), succinonitrile (SN), and Li salt (LiTFSI), is employed to solve the limitations of ASSLBs. LLZO codoped with Al3+ and Nb5+ (NAL) is synthesized at a low temperature using a modified sol‐gel Pechini method. NAL, along with SN, plays a critical role in improving the performance of the resultant solid polymer electrolyte, which can be operated at room temperature. The integrated electrolyte PEO/LiTFSI‐SN‐NAL (PLS‐NAL) delivers a high ionic conductivity of 3.09 × 10−4 S cm−1 and an excellent Li‐ion transference number of 0.75 at room temperature. ASSLBs combining LiFePO4 and PLS‐NAL exhibit excellent cycling stability at both room temperature and 45 °C with a high capacity retention of ≈90% after 200 cycles and cycle life of up to 400 cycles.

Understanding the Degradation of a Model Si Anode in a Li-Ion Battery at the Atomic Scale.

To advance the understanding of the degradation of the liquid electrolyte and Si electrode, and their interface, we exploit the latest developments in cryo-atom probe tomography. We evidence Si anode corrosion from the decomposition of the Li salt before charge–discharge cycles even begin. Volume shrinkage during delithiation leads to the development of nanograins from recrystallization in regions left amorphous by the lithiation. The newly created grain boundaries facilitate pulverization of nanoscale Si fragments, and one is found floating in the electrolyte. P is segregated to these grain boundaries, which confirms the decomposition of the electrolyte. As structural defects are bound to assist the nucleation of Li-rich phases in subsequent lithiations and accelerate the electrolyte’s decomposition, these insights into the developed nanoscale microstructure interacting with the electrolyte contribute to understanding the self-catalyzed/accelerated degradation Si anodes and can inform new battery designs unaffected by these life-limiting factors.

Constructing Low‐Solvation Electrolytes for Next‐Generation Lithium‐Ion Batteries

AbstractThe realization of high‐concentration electrolytes (HCEs) is a benchmark breakthrough attributed to the modification of cation aggregation, which owns technical advantages over the widely used conventional electrolytes. Due to the high cost brought by the large amount of lithium (Li) salts, high viscosity, low Li+ conductivity, and wettability of the HCE system, the concept of localized HCEs (LHCEs) is proposed to improve the aforementioned shortcomings without affecting the performances of high‐energy‐density batteries. Nevertheless, the effect factors of the weakly solvated structure of LHCEs have not been summarized, so it is urgent to conclude this direction to further survey the low‐solvation structure and properties. Hence, for the first time we offer a comprehensive and distinct overview on the electrolyte development, strategies for constructing low‐solvation structure, and scientific perspectives for lithium‐ion batteries, especially by focusing on the binding energies between cation and solvents, solvation and de‐solvation process, structure of novel solvents, and selection of Li salts. Emphasis is placed on the relevance of constructing low‐solvation structure and forming electrode/electrolyte interphases, and new insights into weakly solvated electrolytes associated with efficient solid electrolyte interphase and cathode electrolyte interphase layers are presented for developing next‐generation lithium‐ion batteries.

Self-Healability of Poly(Ethylene-co-Methacrylic Acid): Effect of Ionic Content and Neutralization.

Self-healing polymers such as poly(ethylene-co-methacrylic acid) ionomers (PEMAA) can heal themselves immediately after a projectile puncture which in turn lowers environmental pollution from replacement. In this study, the thermal-mechanical properties and self-healing response of a library of 15 PEMAA copolymers were studied to understand the effects of the ionic content (Li, Na, Zn, Mg) and neutralization percentage (13 to 78%) on the results. Differential scanning calorimetry (DSC), dynamic mechanical analysis (DMA), and tensile testing were used to study the thermo-mechanical properties of PEMAA copolymers while the self-healing response was studied using the projectile test. Puncture sites were observed using scanning electron microscopy (SEM) and the healing efficiency was quantitatively measured using the water leakage test. Five different self-healing responses were observed and correlated to ionic content and neutralization. At high neutralization, divalent neutralizing ions (Zn and Mg) that have stronger ionic interactions exhibited brittle responses during projectile testing. PEMAA samples neutralized with Mg and Li at low concentrations had a higher healing efficiency than PEMAA samples neutralized with Zn and Na at low neutralization. The PEMAA copolymers with higher tensile stress and two distinct peaks in the graph of loss factor versus temperature that indicate the presence of sufficient ionic aggregate clusters had improved healing efficiency. By increasing the neutralization percentage from 20% to 70%, the tensile strength and modulus of the samples increased and their self-healability generally increased. Among the investigated samples, the copolymer with ~50% neutralization by Li salt showed the highest healing efficiency (100%). Overall, the strength and elastic response required for successful self-healing responses in PEMAA copolymers are shown to be governed by the choice of ion and the amount of neutralization.

Engineering the interface of organic/inorganic composite solid-state electrolyte by amino effect for all-solid-state lithium batteries

Composite solid-state electrolyte (CSSE) with integrated strengths avoids the weaknesses of organic and inorganic electrolytes, and thus become a better choice for all-solid-state lithium battery (ASSLB). However, the poor dispersion of inorganic fillers and the organic/inorganic nature difference leads to their interface incompatibility, which greatly destroys the performance of CSSE and ASSLB. Herein, silane coupling agent (SCA) aminopropyl triethoxysilane (ATS) is introduced to tailor the organic/inorganic interfaces in CSSE by the common chemical bridging effect of SCA and the special amino effect (hydrogen bond and lone pair electron effects). It is found that the hydrogen bond interaction between –NH2 and polyethylene oxide (PEO) enhances their interface interaction. And the lone pair electrons on nitrogen atom allow it to react with solvent acetonitrile and promote the uniform dispersion of ceramic fillers. Moreover, the lone pair electrons can complex with Li+, which promotes the dissociation of Li salts, uniforms Li+ diffusion and inhibits the Li dendrite. Thanks to the above merits, the interface compatibility and stability of organic/inorganic CSSE are much enhanced by innovatively introducing ATS, showing high ionic conductivity and superior mechanical/thermal stability. The ASSLB with this modified CSSE exhibits excellent electrochemical performance with a reversible capacity of 140.9 mAh g−1 and a capacity retention of 94.4% after 280 cycles. These achievements offer a new insight into improving the stability of organic/inorganic CSSE interface and promoting their applicability into ASSLB.

In Situ Derived Bi Alloys for High-Performance Li-Ion Batteries: Effect of Conversion Chemistry, Mesomatrix, and Electrolyte

LixBi alloys are a unique path to enable extraordinarily high rate, safer, and high volumetric energy density batteries. The effectiveness of BiF3 as an electrochemical precursor to a LixBi nanocomposite with excellent cycling efficiency extending beyond 250 cycles is explored to identify key factors critical to future successful development. The relationships between the converted crystallite size and subsequent electrochemical properties were reported with a specific focus on cycling efficiency. Through electrochemical and physical characterization of post-converted BiF3, Bi2O3, and Bi2S3, the size of the post-conversion Bi product was directly correlated with the ionic conductivity of the in situ formed Li salt matrix and subsequent cycling stability. Further key areas for development were introduced, including volumetrically dense conductive alternatives to C and electrolyte formulations, which demonstrate significant improvements in cycling stability. In addition, we demonstrate the ability of BiF3-derived LixBi alloy thin films to delithiate with an 80% utilization at >100 C rate despite the presence of a LiF nanomatrix.

Lithium salts: assessment of their chronic and acute toxicities to honey bees and their anti-Varroa field efficacy.

Varroa control is essential for the maintenance of healthy honey bee colonies. Overuse of acaricides has led to the evolution of resistance to those substances. Studies of the short-term acaricidal effects and safety of various lithium (Li) salts recently have been reported. This study examined the long-term in vitro and in vivo bee toxicities, short-term motor toxicity to bees and long-term anti-Varroa field efficacy of several Li salts. In an in vitro chronic-toxicity assay, lithium citrate (18.8mm) was the most toxic of the examined salts, followed by lithium lactate (29.5mm), and lithium formate (32.5mm). In terms of acute locomotor toxicity to bees, all of the Li salts were well-tolerated and none of the treatment groups differed from the negative control group. In an in vitro survival study, all of the Li treatments significantly reduced bee life spans by a factor of 1.8-7.2, as compared to the control. In terms of life expectancy, lithium citrate was the most toxic salt, with no significant differences noted between lithium formate and lithium lactate. In the bee-mortality field study, none of the examined treatments differed from the negative control. Amitraz and lithium formate exhibited similar acaricide effects, which were significantly different from those observed for lithium lactate and the negative control. In light of lithium formate's honey bee safety and efficacy as an acaricide, additional sublethal toxicity studies in brood, drones and queens, as well as tests aimed at the optimization of administration frequency are warranted. © 2022 Society of Chemical Industry.

Physicochemical Interplay in Solid Polymer Electrolytes: Benchmarking, Prospects and Limits in High Voltage Batteries

A systematic R&D of solid electrolytes (SEs) requires reasonable benchmark systems for the intra- and interlaboratory comparison and evaluation. Given its abundance, costs, compatibility with Li and a relative simplicity in processing, the poly(ethylene oxide)-based solid electrolyte (PEO-based SE) is a reasonable benchmark SE system for solid-state lithium batteries.(1)On the basis of recent progress in cell design and methodology,(2-5) the physicochemical properties of PEO-based SE are elaborated as a function of Li salt concentration and related with the performance in LiNi0.6Mn0.2Co0.2O2 (NMC622)||lithium cells. The overall aim is to unravel the apparently complex interplay of relevant parameter and finally to demonstrate the prospects and limits of the SPE benchmark in practical lithium-based batteries.(6)For instance, despite the decrease of the crystalline phases with a Li salt in a plasticizing manner leading to SE membrane softening, the accompanied increase in amorphous phases enhances the Li+ diffusion coefficient, which can be easily obtained from the analysis of Li||Li cells with the Sand equation. Both, the increased diffusivity of Li+ and the overall amount of charge carriers leads to improved ionic conductivities with higher Li salt concentration, particularly below the melting point (Tm < 60 °C). In terms of anodic behavior, neither SE decomposition nor Al current collector dissolution is visibly affected by the Li salt concentration, revealing a surprisingly high bulk electrolyte stability of 4.6 V vs. Li|Li+ on practical, i.e. composite electrodes; and an Al dissolution tendency as low as in conventional LiPF6/carbonate-based liquid electrolytes. Finally, at an operation temperature below Tm, Li salt concentration is demonstrated to have a direct link with characteristic performance aspects of e.g. NMC622||Li cells. J. Mindemark, M. J. Lacey, T. Bowden and D. Brandell, Prog. Polym. Sci., 81, 114 (2018).L. Stolz, G. Homann, M. Winter and J. Kasnatscheew, Materials Advances, 2, 3251 (2021).G. Homann, L. Stolz, M. Winter and J. Kasnatscheew, iScience, 23, 101225 (2020).L. Stolz, G. Homann, M. Winter and J. Kasnatscheew, Materials Today, 44, 9 (2021).G. Homann, L. Stolz, J. Nair, I. C. Laskovic, M. Winter and J. Kasnatscheew, Sci Rep, 10, 4390 (2020).L. Stolz, S. Röser, G. Homann, M. Winter and J. Kasnatscheew, The Journal of Physical Chemistry C, 125, 18089 (2021). Figure 1

High-Performance Liquid Electrolytes for Lithium Metal Batteries Discovered By Machine Learning and High-Throughput Experimentation

High-performance liquid electrolytes for lithium metal batteries were discovered via a closed-loop machine learning and high-throughput robotic experimentation workflow. The novel workflow iterates: (1) machine-learning property prediction of a given electrolyte formulation based on a training dataset; (2) machine-learning electrolyte search for optimal candidates; (3) robotic high-throughput sample preparation and testing; and (4) training dataset updates based on new results. As a result, for the electrolytes, the lithium coulombic efficiency at 3 mA/cm2 was increased to 98.2% from the baseline of 76.2%. The full cell capacity was 98.6% at cycle 373 with one improved electrolyte. In comparison, a baseline electrolyte showed a cycle life of 96 cycles. In addition, for machine learning, the electrolyte prediction showed promising prediction accuracy vs. lab-verified data.The advancement of electric drive vehicle batteries is crucial for improving transportation energy efficiency. Batteries using high-capacity lithium (Li) metal anode hold promise due to their high energy densities to extend the driving range and lower costs. Li metal batteries (LMBs) are a technological pathway to below $100/kWh, but with high risk. Currently, an advanced electrolyte is urgently needed to stabilize lithium metal anode for longer battery life.Liquid electrolyte formulation development that discovers effective electrolyte components (including solvents, Li salts, and additives) and optimizes their contents in the electrolyte is a promising approach. The combination of machine learning and robotic high-throughput experimentation shows promise as an effective and efficient development methodology, to tackle the electrolyte formulation search space >1020 formulation candidates too large for researchers to manually study in a timely fashion. Natural language processing was developed to capture latent knowledge from materials science literature without human labelling and demonstrated the capability to recommend materials several years before their discovery. An early prediction model and a Bayesian optimization algorithm were developed to perform closed-loop optimization of fast-charging protocols for LIBs. Using machine learning coupled to a robotic test-stand, hundreds of sequential experiments were performed on aqueous electrolytes containing 6 lithium and sodium salts for conductivities, pHs, and electrochemical responses on platinum, with non-intuitive discoveries.In this work, by using a novel closed-loop machine learning and high-throughput robotic experimentation workflow, liquid electrolytes for lithium metal batteries were optimized. The primary Li CE was improved to 99.7% from the baseline performance of 76.7%. The optimized electrolyte showed high ionic conductivity and oxidation voltage as well. Coin cell validation showed that the cell capacity retention was 98.6% at cycle 373, with the coulombic efficiency of around 100%. Furthermore, 200 mAh NCA pouch cell with the optimized electrolyte showed improved cycle life and thermal stability. Regarding the machine learning algorithm, the correlation between the actual values and the predicted values for the unseen data achieved a high value of 0.986, and the r2 value was 0.79.This material is based upon work supported by the U.S. Department of Energy, Office of Science, Office of Energy Efficiency and Renewable Energy under Award Number DE-SC0020872. This research used resources of the Molecular Foundry at Lawrence Berkeley Nation Lab, which is a DOE Office of Science User Facility.

Role of Agent Molecules for Low-Temperature Activation of Lithium-Ion Transport for Solid-State Polymer Electrolytes

The realization of all-solid-state lithium-ion batteries (LIBs) is often considered as the final challenge in the development of LIBs. Replacing Li-ion conductive liquid electrolytes with high-performance solid-state electrolytes is indispensable for the development of all-solid-state LIBs. Solid-state electrolytes under development fall into three classes: polymers, oxides, and sulfides. Polymer-based electrolytes have advantages over oxides and sulfides, such as formation of low-resistance electrolyte/electrode interfaces, good processability, and high energy-density owing to low density. Therefore, polymer-based solid-state electrolytes are being developed in both industry and academia as a practical route for realizing high-capacity LIBs.[1]For application in commercial LIBs, the electrolyte should have an ionic conductivity higher than 10-4 S/cm at room temperature. Conventional solid polymer electrolytes, such as polyethylene oxide (PEO)-based electrolytes, do not meet the performance requirements due to insufficient ionic conductivity in the range of 10-6 to 10-5 S/cm. Recently, polyphenylene sulfide (PPS)-based polymer electrolytes have been reported to yield ion conductivities as high as those of liquid electrolytes over a wide temperature range (> 1.0 × 10-4 S/cm at 25 °C, > 1.0 × 10-3 S/cm at 80 °C, and > 1.0 × 10-3 S/cm at −40 °C).[2] These electrolytes consist of base polymer chains containing PPS, Li salts that can dissociate into cations and anions, and neutral agent molecules. However, the detailed Li-ion transport mechanism in terms of the respective roles of the molecular components of PPS electrolytes is yet to be determined. This limited understanding hinders the further improvement of PPS-based electrolytes.In this study, we perform a series of first-principle calculations and demonstrate that certain types of neutral molecules (so-called agent molecules) accelerate solid-state lithium-ion migration when mixed with lithium salts.[3] We find that the intermolecular interaction in a selected agent-molecule/lithium-salt binary system is governed by the strong coupling between lithium and oxygen atoms. Upon the addition of agent molecules, the anionic species surrounding the lithium of lithium salts is replaced by the agent molecules. The resulting weakened Coulomb energy coupling between lithium and oxygen atoms is determined to be a key factor in enabling fast lithium-ion migration via facile dissociation of lithium salts and subsequent formation of ion-hopping sites in the form of lithium-free oxygen-cages. The structure-based interpretation of agent molecules suggests that neutral molecules with functional groups which enhance chemical resonance can be selected as potential agent molecules. We believe that the results obtained in this study serve as a theoretical basis for the future development of solid-state polymer electrolytes, particularly toward mitigating the dependence of lithium-ion transport on the movement of polymer chains.

Enhancement of Lithium-Mediated Ammonia Synthesis By Addition of Oxygen

Ammonia is one of the most produced chemicals worldwide and is currently synthesized by the Haber-Bosch process, which is a thermally catalyzed method that requires high pressures and temperatures. These harsh conditions, in addition to the prerequisite steam reforming process, leads to about 1 % of the annual energy consumption and 1.4 % of the global CO2 emission. One way to mitigate some of the Haber-Bosch process is to produce ammonia electrochemically, utilizing renewable energy sources.The electrochemical synthesis of ammonia faces several big issues. One being the selectivity, since the more facile hydrogen evolution reaction (HER) will always dominate over the nitrogen reduction reaction (NRR), and another issue being the activity, given that the nitrogen triple bond is very stable and therefore hard to split. Hence, most often the reported ammonia contents are in the low ppm regime, which makes it very susceptible to contaminations both from the gas stream (NH3 and NOx impurities) and the system itself (catalyst, cell, chemicals, nitrile gloves, etc.). To avoid misleading results, several protocols have been published on how to correctly perform NRR experiments [1, 2]. One of these confirm that only the Li-mediated ammonia synthesis (LiMeAS) is currently able to produce ammonia electrochemically [3]. The actual mechanism is not fully understood, but it is generally believed that the first step is Li plating from a Li salt containing non-aqueous electrolyte. The very reactive Li will then react with N2 solvated in the electrolyte to form Li3N, which is believed to hydrolyze to ammonia when in contact with a proton source. The currently highest archived faradaic efficiency (FE) is at 69 % at 20 bar N2 pressure when applying an ionic liquid as a proton shuttle [4].In this work, we achieved up to 79 % FE at 20 bar N2 by the addition of 0.8 mol. % O2 in the reaction atmosphere. The positive effect of O2 is a very counterintuitive observation, since the original work by Tsuneto et al. [5] showed that the use of synthetic air significantly hindered the reaction, as it was postulated that O2 inhibits the reaction due to LiO2 formation and/or leading primarily to the oxygen reduction reaction (ORR). We will present experimental results obtained at 10 and 20 bar with varying O2 contents, which were measured accurately by a mass spectrometer probing the atmosphere just above the electrolyte inside the pressure vessel. By combining experimental observations with theoretical modelling, we conclude that the unexpectedly beneficial role of small O2 concentrations has a positive influences the solid electrolyte interface (SEI), which is of great importance in our system. Additional ex-situ X-Ray diffraction (XRD) and X-Ray photoelectron spectroscopy (XPS) measurements were conducted without exposure to air and moisture, to analyze the SEI layer and deposition after electrochemistry. We believe that this study will not only be beneficial for industrializing the LiMeAS, but will also bring us a step further in understanding the complex mechanism behind this process.[1] S. Z. Andersen et al., "A rigorous electrochemical ammonia synthesis protocol with quantitative isotope measurements," Nature, vol. 570, pp. 504-508, 2019.[2] H. Iriawan et al., "Methods for nitrogen activation by reduction and oxidation," Nature Reviews Methods Primers, vol. 1, no. 1, pp. 1-26, 2021.[3] J. Choi et al., "Identification and elimination of false positives in electrochemical nitrogen reduction studies," Nature communications, vol. 11, no. 1, pp. 1-10, 2020.[4] B. H. Suryanto et al., "Nitrogen reduction to ammonia at high efficiency and rates based on a phosphonium proton shuttle," Science, vol. 372, no. 6547, pp. 1187-1191, 2021.[5] A. Tsuneto, A. Kudo, and T. Sakata, "Lithium-mediated electrochemical reduction of high pressure N2 to NH3," Journal of Electroanalytical Chemistry, vol. 367, no. 1-2, pp. 183-188, 1994.

Chemical and Morphological Changes of Graphite Electrodes at Low Cathodic Potentials and Its Relevance to Li-Ion Batteries

Lithium-ion (Li-ion) batteries have become ubiquitous to modern-day electronics and electric vehicles. A detailed understanding regarding the reversible and irreversible nature of Li-ion electrode processes is a prerequisite to rationally delineate the factors affecting device performance and clarify degradation mechanisms. This includes, for instance, the role of electrode cracking and solid solid-electrolyte-interphase (SEI) formation which can contribute toward irreversible capacity loss. In terms of experimental approaches, aside from the high surface area electrodes used in practical systems, one powerful approach is to incorporate the study of well-defined electrodes and surface science techniques.1 Herein, we employ highly oriented pyrolytic graphite (HOPG) as a well-defined model anode in Li-ion batteries and reveal the chemical and morphological changes at low cathodic potentials prior to the potentials of bulk SEI formation.2 To achieve this, we employ the use of X-ray photoelectron spectroscopy (XPS) and ultrahigh vacuum scanning tunneling microscopy (UHV-STM). We utilize a so-called UHV-EC setup to provide a snapshot-like analysis of the electrode surface without the involvement of electrode washing or exposure to air.Upon cathodic polarization at ca. 1.75V (Figure 1), STM imaging shows the onset and occurrence of graphite exfoliation which can originate from the intercalation of solvated Li+ ions. Meanwhile, XPS shows a minimal amount of residual electrolyte along with the observation of LiF indicating the decomposition of the Li salt (LiPF6). The future extensions of our methodology and additional details will be provided at the presentation. Figure 1. (a) Cathodic linear sweep voltammogram in 1.1 M LiPF6 EC/DMC. Following polarization at ca. 1.75 V (b) STM imaging, and (c) XPS F 1s and Li 1s spectra.

Critical Percolation Threshold for Solvation Site Connectivity in Polymer Electrolytes Mixtures

To address the tradeoff between mechanical strength and Li+ conductivity in Poly(Ethylene Oxide) (PEO)-based electrolytes, a rigid nonconductive polymer is frequently added to the electrolyte via blending or copolymerization. The ionic conductivity of mixed PEO electrolytes is generally lower than that of unmixed PEO electrolytes. The suppressed ionic conductivity is attributed to the reduced segmental mobility and connectivity of the conductive PEO cites. Most experimental systems make it difficult to decouple the two mechanisms and accurately examine their impact on conductivity. We compare two symmetric polymer mixtures (50:50 wt%): a miscible polymer blend PEO/PMMA and a disordered block copolymer (BCP) PEO-b-PMMA, both with the same amount of Li salt. Because their chemical and physical properties are the same, changes in ionic conductivity can be attributed solely to local changes in PEO network connectivity. We discover that the mixtures' immediate Li+ solvation sites (<5 Å) are identical to those of unmixed PEO electrolytes. The presence of non-conducting PMMA near the PEO, on the other hand, causes local concentration changes at longer range scales. The BCP is more mixed than the blend electrolyte at these length scales, resulting in a factor of two drop in conductivity. To that end, we propose a quantitative computational model that considers Li+ transport within and across PEO clusters at the appropriate length scales. This new understanding of network connectivity in polymer electrolyte mixtures is critical for the design of future multiphase polymer electrolyte systems.

Effects of Molecular Weight on the Electrochemical Properties of Poly(vinylidene difluoride)-Based Polymer Electrolytes.

Polymer-based electrolytes have attracted ever-increasing attention for solid-state batteries due to their excellent flexibility and processability. Among them, poly(vinylidene difluoride) (PVDF)-based electrolytes with high ionic conductivity, wide electrochemical stability window, and good mechanical properties show great potential and have been widely investigated by using different Li salts, solvents, and inorganic fillers. Here, we report the influence of the molecular weight of PVDF itself on the electrochemical properties of the electrolytes by using two kinds of common PVDF polymers, i.e., PVDF 761 and 5130. Our results demonstrate that the electrolyte with a larger molecular weight (PVDF 5130) has a denser structure and lower crystallinity, and thus much better electrochemical performance, than one with a smaller molecular weight (PVDF 761). With PVDF 5130, the LiFePO4-based solid-state cells present a steady cycling performance with a capacity retention of 85% after 1000 cycles at 1 C and 30 °C. The cycle life of the LiCoO2-based solid-state cells is also extended by using PVDF 5130.

Dual In Situ Polymerization Strategy Endowing Rapid Ion Transfer Capability of Polymer Electrolyte toward Ni‐Rich‐Based Lithium Metal Batteries

Poly(vinylidenefluoride-co-hexafluoropropylene) (PVDF-HFP) is one of the most promising candidate electrolyte matrices for high energy batteries. However, the spherical skeleton structure obtained through the conventional method fails to build continuous Li ion transmission channels due to the slow volatilization of high boiling solvent, leading to inferior cycling performance, especially in a Ni-rich system. Herein, a novel strategy is presented to enrich the Li ion transfer paths and improve the Li ion migration kinetics. The tactic is to prepare cross-linked segments through the PVDF-HFP matrix by adopting free radical polymerization and Li salt induced ring-opening polymerization. Most significantly, the visualization of the structure of as-prepared electrolyte is innovatively realized with the combination of polarization microscopy, transmission electron microscopy, scanning electron microscope-energy dispersive spectroscopy, PVDF-HFP, and cross-linked network form interconnected microstructures. Therefore, poly(glycidyl methacrylate and acrylonitrile)@poly(vinylidene fluoride-hexafluoropropylene) electrolyte presents a high ionic conductivity (1.04mScm-1 at 30°C) and a stable voltage profile for a Li/Li cell after 1200h. After assembly with a LiNi0.8 Co0.15 Al0.05 O2 cathode, a high discharge specific capacity of 190.3mAhg-1 is delivered, and the capacity retention reaches 88.2% after 100 cycles. This work provides a promising method for designing high-performance polymer electrolytes for lithium metal batteries.

Structure, Diffusion, and Stability of Lithium Salts in Aprotic Dimethyl Sulfoxide and Acetonitrile Electrolytes

The development of lithium batteries, including lithium-ion batteries and lithium-air batteries, is the key technological breakthrough in the field of renewable energy storage. However, to date, it is still challenging to design a lithium battery with both high specific power and stability. Specifically, the solvation structure and thermodynamic stability of various electrolytes, mostly Li salts and aprotic solvents, have not been systematically studied from an atomic viewpoint. In this paper, we studied the solution chemistry of three most common inorganic Li salts (LiClO4, LiBF4, and LiPF6) and O2 in two aprotic (dimethyl sulfoxide (DMSO) and acetonitrile (CH3CN)) solvents. The Born–Oppenheimer molecular dynamics simulations and enhanced free energy samplings are employed to obtain their solvation structures, diffusion coefficients, and stability performances at room temperature. As a result, the tetrahedral Li(DMSO)4+ and Li(CH3CN)4+ are obtained as the stable solvation shells of Li+ in DMSO and CH3CN solvents, respectively. Among the three inorganic Li salts, the stability performances are found in the order of LiClO4 > LiBF4 > LiPF6 in both DMSO and CH3CN solvents. Compared with CH3CN, DMSO provides a more stable environment for the long-term usage of Li salts for that increases the energetic barriers of the degradation reactions of solvated Li+ components. However, DMSO shows a weaker ability (than CH3CN) to transport the main redox species (solvated Li+ and O2) in the electrolyte, which limits the discharging and charging rate in the batteries.

Eutectic Electrolytes Composed of LiN(SO2F)2 and Sulfones for Li-Ion Batteries

Sulfones are polar molecules that can be used as thermally stable electrolyte solvents for Li-ion batteries (LIBs). Li salts form stoichiometric solvates with sulfones in the electrolytes. The melting points of the solvates tend to be higher than room temperature, thereby limiting the operating temperature range of the batteries. In this study, the applicability of ternary eutectic mixtures of LiN(SO2F)2 (LiFSA), sulfolane (SL), and dimethyl sulfone (DMS) as LIB electrolytes was assessed. Relative to the binary LiFSA–sulfone electrolytes, the ternary eutectic electrolytes remained liquid over a wide temperature range due to the increased entropy of mixing. Sulfone-bridged Li+–sulfone–Li+ and anion-bridged Li+–FSA––Li+ network structures were formed in the eutectic electrolyte with a composition of [LiFSA]/[SL]/[DMS] = 1/1.5/1.5. Pulsed-field gradient NMR measurements revealed that the Li+ ion dynamically exchanges sulfones and anions and diffuses more rapidly than these ligands, resulting in the relatively high Li+ transference number of the electrolyte. Highly reversible charge–discharge processes of the LiCoO2 and graphite electrodes were attained using the ternary eutectic electrolyte. The rate capability of the Li/LiCoO2 cell in the eutectic electrolyte was comparable to that of the cell in the conventional 1 M LiPF6 in an ethylene carbonate/dimethyl carbonate solution despite its lower ionic conductivity.

Designing Anion-Derived Solid Electrolyte Interphase in a Siloxane-Based Electrolyte for Lithium-Metal Batteries.

The rational electrolyte design with weak solvation is regarded as an effective way to regulate the electrolyte/electrode interface (SEI) that profoundly affects the performance of Li-metal batteries. Herein, we propose a newly developed siloxane-based weakly solvating electrolyte (SiBE) with contact ion pairs (CIPs) or aggregates (AGGs) dominating the solution structure, which enables the dendrite-free Li deposition and long cycle stability of Li-metal batteries. By altering the combination of Li salts, the SiBE leads to the formation of an inorganic anion-derived solid electrolyte interphase, which is highly stable and Li+-conductive. Based on SiBE, the Li||LiFePO4 (LFP) full cell can stably cycle for 1000 cycles at a 2C rate with a capacity retention of 76.9%. Even with a limited Li-metal anode, it can maintain a capacity retention of 80% after 110 cycles with a high average Coulombic efficiency of 99.8%. This work reveals that siloxane can be a promising solvent to obtain weakly solvating electrolytes, which opens a new avenue for SEI composition regulation of Li-metal batteries.

Effects of Carbonate Solvents and Lithium Salts in High-Concentration Electrolytes on Lithium Anode

Lithium (Li) metal is considered an ideal anode material for Li-ion batteries. However, traditional carbonate-based solvents exhibit poor compatibility with the Li anode. High-concentration electrolytes (HCEs) are promising in the improvement of the behavior of the Li anode. To determine suitable HCE formulations, we revealed the effects of various carbonate solvents and Li salts in HCEs on the Li anode in terms of electrochemical performance, morphology, and surface chemical components. After screening six carbonates and four Li salts, the results suggested that ethylene carbonate (EC) and lithium bis(fluorosulfonyl)imide (LiFSI) were suitable in HCEs for the Li anode. The EC1–2 (molar ratio of LiFSI to EC is 1:2) electrolyte exhibited great cycling stability for up to 250 cycles at a high average Coulombic efficiency of 97.1% at a current density of 1 mA cm−1 with a fixed capacity of 0.5 mAh cm−2. This was demonstrated as the formation of large Li with uniform nodule-like morphology and dense structure. In addition, the surface components on the Li anode were observed to have been highly contributed by the FSI-anion decomposition with the least EC reduction, providing an anion-derived surface with rich Li-F content.

Lithium difluoro(bisoxalato) phosphate-based multi-salt low concentration electrolytes for wide-temperature lithium metal batteries: Experiments and theoretical calculations

Current knowledge and works on high-energy-density Li metal batteries (LMBs) mainly focus on their room-temperature performances. However, the wide-temperature properties of LMBs manifesting greater significance in their large-scale applications are rarely explored. In this work, two LiDFBOP-based multi-salt low-concentration electrolytes (LCEs) are proposed and further explored by experiments and theoretical calculations for wide-temperature LMBs. Molecular dynamics (MD) simulations reveal the weaker attractive interactions between solvent molecules in LCEs, thus resulting in the lower viscosity and freezing point. Specially, the Li+ in representative solvation structures of LCEs possesses accelerated desolvation behavior with low charge-transfer impedance in Li||Li symmetric cells. Furthermore, the thermally stable Li salts in LCEs manifest obvious effect in stabilizing Li metal anode, which contributes to forming a compact solid electrolyte interphase (SEI) layer with good mechanical properties and high ionic conductivity. Ultimately, the Li||LiNi0.7Co0.1Mn0.2O2 battery exhibits extraordinary electrochemical performances over a wide temperature range (−25 °C to 70 °C). This work provides a facile and practical design strategy for the wide-temperature LMBs.