LiF Surface Research Articles

A synergistic modification strategy for enhancing the cycling stability and rate capacity of single-crystal nickel-rich cathode materials

Ni-rich layered LiNi0.8Co0.1Mn0.1O2 (NCM811) is considered to be one of the most promising cathode materials due to its advantages of high specific capacity, lower cost and environmental friendliness. However, the relatively high nickel content in the LiNi0.8Co0.1Mn0.1O2 layered cathode materials leads to the instability of the internal structure and surface of the materials during long-term cycling, which results in the deterioration of electrochemical performance. In this study, single-crystal NCM811 (SNCM811) was prepared using a convenient solid-state method. By doping Al into SNCM811 (SNCMAl) and purposefully low-temperature surface treatment with a small amount of LiPF6, a Ni-rich single-crystal NCM811 modified by both Al bulk doping and LiF surface coating (SNCMAl@LiF) was obtained. The reversible capacity of SNCMAl@LiF with a cutoff voltage of 2.7–4.3 V at 0.1C is 200.3 mAh·g−1. And the capacity retention of the single-crystal cathode materials is 92.4 % with capacity of 171.07 mAh·g−1 after 100 cycles at 1C. Even at high rate of 10C, the discharge specific capacity maintains 148.8 mAh·g−1, demonstrating excellent electrochemical performance. The dual-modified single-crystal material has the lowest potential difference and electrochemical impedance without intragranular cracks and crystal slips after 100 cycles. Stronger AlO bond in single crystals reduced the degree of cation mixing and maintained the stability of the layered structure. LiF-modified surface can promote the transfer of Li+, maintain the stability of the electrode/electrolyte interface and then improve the electrochemical performance of cathode materials. Al-doping and LiF-coating provides a guidance on how to enhance the electrochemical performance of Ni-rich single-crystal cathode materials.

Molecular Ion Desorption from LiF(110) Surfaces by Positron Annihilation.

We have studied the desorption of positive ions from a LiF(110) crystal surface using positron and electron irradiation at 500eV to examine the interaction between positrons and ionic crystals. Only monatomic ions, such as H^{+}, Li^{+}, and F^{+}, are detected under electron irradiation. However, positron irradiation leads to the significant desorption of ionic molecules, specifically, FH^{+} and F_{2}^{+}. Molecular ion yields are more sensitive to temperature than atomic ion yields. Based on the findings, we propose a desorption model in which positronic compounds are initially produced at the surface and subsequently desorbed as molecular ions via Auger decay following positron annihilation.

In Situ Ion‐Exchange Metathesis Induced Conformal LiF Surface Films on Cathode (NMC811) as a Cathode Electrolyte Interphase

AbstractHigh‐capacity cathodes (LiNi0.8Mn0.1Co0.1O2, NMC811) are promising for vehicle electrification because of their high gravimetric energy density. However, their electrochemical performance still relies upon the stability of the cathode electrolyte interphase (CEI). A highly reactive cathode interface leads to parasitic side reactions with electrolytes, resulting in accelerated capacity fading. Well‐developed LiF and LiF‐like inorganic compounds are believed to be good CEI components for stabilizing such reactive electrode interfaces. However, it is challenging to form an optimal surface sub‐nanolayer of LiF on the cathode surfaces because of the complexity of the electrochemical reaction during battery cycling. Herein, the formation of a conformal LiF layer on the NMC811 electrode surface via an in situ ion‐exchange metathesis process is reported, demonstrating a promising electrochemical performance because of a LiF‐stabilized CEI. In situ generated LiF‐coated NMC811 electrodes exhibit ≈97% capacity retention up to 100 cycles at a 0.3 C rate with average coulombic efficiency of ≈99.9% and ≈80% capacity retention up to 200 cycles at a 1 C rate with average coulombic efficiency of >99.6%. This finding may pave the way for reengineering the CEI to enhance the electrochemical performances and cycling stability of the high‐capacity cathodes.

Regulating Single-Crystal LiNiO2 Size and Surface Coating toward a High-Capacity Cathode for Lithium-Ion Batteries

Single crystals recently received a great deal of attention because the stabilities of cathode materials are improved. One of the major drawbacks of the single-crystal cathodes is that their achievable capacity is lower than that of the same composition polycrystalline cathodes. Although it is widely accepted that the large crystal size of single-crystal cathodes might be the main reason for their low capacity, a systematic study to verify all possible rationales is absent. In this work, we regulated the crystal size of a single-crystal LiNiO2 to investigate its relation to capacity for the first time. It was established that among the sizes studied, a 400 nm-sized single crystal LiNiO2 achieved high capacity, ∼240 mA h/g at 0.1 C, which is comparable to that of its polycrystalline counterpart. It is the first report that such a high capacity is obtained in a single crystal. Also, in our results, with increasing crystal size, a capacity decline was recorded as expected. Interestingly, it is first found that capacity loss occurs only in the high-lithium-composition region (x > 0.8 in LixNiO2), and polarization becomes high only in the same region upon increasing crystal size. This implies that kinetics of the region is significantly affected by the crystal size. Also, high capacity can be achieved in large single-crystal LixNiO2 once the region’s kinetics is optimized. In terms of capacity retention, large single-crystal LiNiO2 exhibits the highest stability. Accordingly, high capacity can be achieved when the crystal size is reduced by trading-off its cycling stability. In order to achieve both high capacity and stability, LiF surface coating was conducted on the small single-crystal LiNiO2. It was shown that the LiF coating can effectively protect against capacity degradation, and the capacity retention by such small single-crystal LiNiO2 can be made even better than that of large crystal LiNiO2. Therefore, both high capacity and cycle retention were achieved in single-crystal LiNiO2 by reducing its crystal size and LiF surface coating.

Improving the Electrochemical Performance of LiNi1/3Co1/3Mn1/3O2 Cathode Material by LiF Modification

LiNi1/3Co1/3Mn1/3O2 is a widely used commercial cathode material in the fields of consumer electronics and electric vehicles. However, its energy density still falls short of the standard and needs to be improved. The most effective method is to increase the cut-off voltage, but this will result in a drop in capacity. In this study, a LiF layer is coated on the surface of LiNi1/3Co1/3Mn1/3O2 via an in situ method. It is found that the LiF layer may protect materials from side reactions with electrolytes, improve the interfacial stability, and enhance the cyclic performance. The bare sample shows relatively poor cycling stability, with capacity retention rates of 65.9% (0.2 C) and 12.8% (5 C) after 100 cycles, while 1% LiF-coated NCM has higher cycling stability with capacity retention rates of 83.4% (0.2 C) and 73.3% (5 C) after 100 cycles, respectively. Our findings suggest that a LiF surface layer could be a useful means of boosting the electrochemical performance of NCM cathode materials.

Elastic and inelastic diffraction of fast neon atoms on a LiF surface.

Grazing incidence fast atom diffraction has mainly been investigated with helium atoms, considered as the best possible choice for surface analysis. This article presents experimental diffraction profiles recorded with neon projectile, between 300 eV and 4 keV kinetic energy with incidence angles θi between 0.3 and 1.5° along three different directions of a LiF(001) crystal surface. These correspond to perpendicular energy ranging from a few meV up to almost 1 eV. A careful analysis of the scattering profile allows us to extract the diffracted intensities even when inelastic effects become so large that most quantum signatures have disappeared. The relevance of this approach is discussed in terms of surface topology.

Commonality of structural transformations in LiF and MgF2 ionic crystals with Co and Ni impurities and metal crystals with metal impurities during heat treatment

The processes of formation of nickel and cobalt containing films on the surface of LiF:Co, LiF:Ni and MgF2:Co, MgF2:Ni single crystals due to high-temperature thermal annealing have been studied for the first time. The purpose of research was finding out the possibility of obtaining nanoscale metal containing films by intracrystalline diffusion of metal impurity from inside the ionic crystal to the surface. Methods of formation and properties of Ni and Co containing films on the ionic crystals is important due to the possibility of their application in optoelectronic devices, spintronics, for recording information with used of film as optical and magnetic media and in other fields. Thermal etching at 750–900 °C was carried out in vacuum and in air. The analysis of the surface of samples was carried out using an electron-ion scanning microscope. Elemental analysis of samples surface was carried out by mapping surface elements and at individual points. It was determined that impurities come out of dislocations in crystals and form metal containing film on the surface. The films contain predominantly Ni, Co, and O elements, as well as elements of the film interaction with air atmosphere. The value of the film thickness is estimated as lying in the range from 0.5 to 5.0 μm. After release of impurity from crystal dislocations, thermal etching pits of various shapes remain. An analogy is drawn between the processes of thermal etching of ionic crystals studied by us and metallic crystalline materials analyzed from scientific literature. General regularities are revealed and differences between two types of crystalline matrices due to high-temperature thermal annealing are noted.

Terrace effects in grazing-incidence fast atom diffraction from a LiF(001) surface

The effect produced by surface defects on grazing-incidence fast atom diffraction (GIFAD) patterns is studied by considering the presence of terraces in a LiF(001) sample. For helium atoms impinging along the $\ensuremath{\langle}110\ensuremath{\rangle}$ direction of the LiF surface, we analyze the influence of a monolayer terrace with its edge oriented parallel or perpendicular to the axial channel. We found that the presence of an outward transverse step introduces a diffuse background above the Laue circle, which displays additional peaked structures. For inward transverse steps, instead, such a background is placed below the Laue circle, showing a much weaker intensity. On the other hand, parallel steps give rise to asymmetric angular distributions, which are completely confined to the Laue circle. Therefore, these theoretical results suggest that GIFAD might be used to characterize terrace defects.

Evidence for the Formation of a Local Pre-Existing Surface Band-Gap Electronic State by F– Projectile Grazing Scattering from a LiF(001) Surface

Evidence for the Formation of a Local Pre-Existing Surface Band-Gap Electronic State by F^– Projectile Grazing Scattering from a LiF(001) Surface

Enhancing Cycling Stability and Capacity Retention of NMC811 Cathodes by Reengineering Interfaces via Electrochemical Fluorination

AbstractHigh‐capacity cathodes (LiNi0.8Mn0.1Co0.1O2) that can boost the energy density of lithium‐ion batteries are promising candidates for vehicle electrification. However, several factors specific to high energy density materials entailing electrode reactions inhibit their application. Fluorination has shown a promising ability to combat the detrimental electrochemical performances of cathode materials, however, it remains difficult to achieve the desired functionality. Herein, a novel electrochemical fluorination (ECF) that demonstrates a promising electrochemical performance enhancement via stabilization of the cathode–electrolyte‐interphase (CEI) by forming conformal LiF is proposed. Besides LiF surface layer formation, ECF reduces the degree of fluorination‐induced Ni/Li disordering and enhances the layered structural stability as probed by X‐ray diffraction. Because of the robust CEI, ECF‐NMC811 cathodes deliver 203.0 mAh g−1 first discharge capacity at the current rate of C/10, with ≈98% capacity retention up to 100 cycles. Similarly, it delivers ≈180 mAh g−1 capacity at a 1 C rate with 86.4% capacity retention up to 200 cycles with average coulombic efficiency of > 99.5%. Comprehensive characterization with a multitude of probes reveals that ECF enhances the cycling stability of the electrode without altering bulk structure and morphology.

Effect of FA: X+3 (X = B, Al, and Ga) Color Centers on the Electronic and Optical Properties of LiF (001) Surface

The density functional theory and configuration interaction single excitations methods were used to study the effect of FA color centers on the electronic and optical properties of the LiF (001) surface. Three types of FA color centers were used, FA: B+3, FA: Al+3, and FA: Ga+3. The evaluated values of the ionization potential, chemical hardness, and softness indicate the stability of the investigated color centers. FA: B+3 is the most stable one. While the electron affinity values reveal that the FA: Al+3 center is the highest reactive one. The Ultraviolet-Visible spectra for FA: B+3, FA: Al+3, and FA: Ga+3 centers showed that the scrutinized centers have two absorption peaks. The highest absorption peak recorded for FA: B+3 center is located at 285 nm, while the highest absorption peaks recorded for FA: Al+3, and FA: Ga+3 centers are located at 173 and 165 nm, respectively. According to the light-harvesting efficiency values, FA: Al+3, and FA: Ga+3 centers have the highest efficiency to gather the energy during the pumping process than FA: B+3 centers. Stokes-shift values for FA: B+3, FA: Al+3, and FA: Ga+3 centers were 0.16, 0.37, and 0.33 eV, respectively, consequently, FA: Al+3 and FA: Ga+3 centers, are more suitable for laser production than FA: B+3 center.

Atmospheric Photoionization Detector with Improved Photon Efficiency: A Proof of Concept for Application of a Nanolayer Thin-Film Electrode.

Atmospheric photoionization is a widely applied soft ionization mechanism in gas sensing devices for the detection of volatile organic compounds in ambient air. Photoionization is typically induced by low-pressure Vacuum Ultraviolet (VUV) lamps with MgF2 or LiF lamp surface windows depending on the gas fill and the required wavelength transmission window. These lamps are known to exhibit gradually reduced VUV transmission due to hydrocarbon contamination. LiF surface windows are known to be especially problematic due to their hygroscopic nature, reducing VUV lamp lifetime to a mere 100 h, approximately. Here, we present a new design for the electrode of a photoionization detector based on thin-film technology. By replacing the commonplace metal grid electrode’s VUV lamp surface window with a chromium/gold thin film we obtain a doubling of photon efficiency for photoionization. Replacing the hygroscopic LiF lamp window surface with a metallic layer additionally offers the possibility to vastly increase operational lifetime of low-pressure Argon VUV lamps.

Formation of LiF Surface Layer During Direct Fluorination of High-Capacity Co-Free Disordered Rocksalt Cathodes.

Disordered rocksalt (DRX) cathodes have attracted interest due to their high capacity and compositional flexibility (e.g., Co-free chemistries). However, the sloping voltage profile and gradual capacity fade during cycling have hindered the widespread adoption of these materials. Simulations predict that fluorine substitution in DRX cathodes will improve their capacity, rate performance, and cyclability. In this study, we use a fluidized bed reactor to fluorinate a model Li-rich DRX composition (Li1.15Ni0.375Ti0.375Mo0.1O2, NTMO) to investigate how fluorine content impacts the cathode's structure and electrochemical performance. Instead of substituting O with F to form oxyfluoride phases, direct fluorination of DRX cathodes leads to the formation of LiF surface films, which improves the specific energy and capacity retention. This study demonstrates the feasibility of direct fluorination to improve the electrochemical performance of high-voltage cathodes by tuning the material's surface chemistry.

Lif Deficiency Leads to Iron Transportation Dysfunction in Ameloblasts

Leukemia inhibitory factor (LIF), a member of the interleukin 6 family of cytokines, is involved in skeletal metabolism, blastocyst implantation, and stem cell pluripotency maintenance. However, the role of LIF in tooth development needs to be elucidated. The aim of the present study was to investigate the effect of Lif deficiency on tooth development and to elucidate the functions of Lif during tooth development and the underlying mechanisms. First, it was found that the incisors of Lif-knockout mice had a much whiter color than those of wild-type mice. Although there were no structural abnormalities or defective mineralization according to scanning electronic microscopy and computed tomography analysis, 3-dimensional images showed that the length of incisors was shorter in Lif−/− mice. Microhardness and acid resistance assays showed that the hardness and acid resistance of the enamel surface of Lif−/− mice were decreased compared to those of wild-type mice. In Lif−/− mice, whose general iron status was comparable to that of the control mice, the iron content of the incisors was significantly reduced, as confirmed by energy-dispersive X-ray spectroscopy (EDS) and Prussian blue staining. Histological staining showed that the cell length of maturation-stage ameloblasts was shorter in Lif−/− mice. Likewise, decreased expression of Tfrc and Slc40a1, both of which are crucial proteins for iron transportation, was observed in Lif−/− mice and Lif-knockdown ameloblast lineage cell lines, according to quantitative reverse transcription polymerase chain reaction, immunohistochemistry, and Western blot. Moreover, the upregulation of Tfrc and Slc40a1 induced by Lif stimulation was blocked by Stattic, a signal transducer and activator of transcription 3 (Stat3) signaling inhibitor. These results suggest that Lif deficiency inhibits iron transportation in the maturation-stage ameloblasts, and Lif modulates expression of Tfrc and Slc40a1 through the Stat3 signaling pathway during enamel development.

Negative ion formation by neutral hydrogen atom grazing scattering from a LiF(100) surface.

We present a theoretical description of negative ion conversion of the grazing scattering of neutral hydrogen atoms on LiF(100) surface. Here, in addition to the capture of a valence band electron near a surface F− site by an incident H atom, the Coulomb repulsive barrier tunneling behavior is also considered to treat the detachment of the affinity electron from the formed H− ion to vacuum through its interaction with the surface F− site. With incorporation of the image-attraction-induced increase on the vertical component of the projectile energy and the collective dielectric screening effect of surrounding anions and cations on the charge of surface F− site which participates in the electron detachment, the image attractive interaction was revealed to obviously increase the electron detachment probability, in turn obviously decreasing the final negative ion yield. Moreover, the sub-eV order of the energy defect between the H− affinity level and the unoccupied image state induced by the potential field of the H− image charge in LiF leads to an additional H− destruction channel in which H− affinity electron transfers to the unoccupied image state without electron emission by a nearly resonant charge transfer manner. A clear picture of this electron loss process is also presented. Our present results well reproduce the experimental observation in the whole velocity range and the high fraction of H− ion yield as an alternative solution can be used on the implantation of ITER device.

Lithium ion diffusion mechanism on the inorganic components of the solid–electrolyte interphase

The inorganic nanocrystals (LiOH, Li2O, Li2CO3, LiF) embedded in the amorphous organic phase forming a mosaic structure. We discovered that LiF surface presents the fastest diffusion rate for Li ions due to the relative lower energy barriers.

The interaction of several fluorinated ionic liquids on the LiF(001) surface

ABSTRACT LiF is a well-known constituent of the solid electrolyte interphase (SEI) layer in lithium metal batteries, which spontaneously forms in the presence of electrolytes and additives. The subsequent interaction of the electrolyte with this component at the atomic scale, however, is largely unreported. In this work, density functional theory (DFT) calculations were used to examine the interaction of several ILs, specifically 1-methyl-1-propylpyrrolidinium cation (Pyr13+) paired with the fluorinated anions bis(fluorosulfonyl)imide (FSI−), fluorosulfonyl-(trifluoromethanesulfonyl)imide (FTFSI−), and bis(trifluoromethylsulfonyl)imide (TFSI−) on a LiF(001) surface, to detail their adsorption characteristics. Each IL was shown to adsorb onto the LiF surface in different orientations via the anion O and F atoms and the cation H atoms, with binding energies ranging from -1.54 to -0.22 eV. These surface binding energies are significantly weaker than the corresponding anion-cation interaction energy. The thermal stability of each IL on the LiF surface was explored via AIMD simulations at 428 K, revealing that each of the examined IL pairs did not dissociate on the surface within the examined simulation time. This work further reinforces the stability of fluorinated ionic liquids on a LiF interface and provides a highly detailed account of the adsorption process.

(Invited) Insight into Aqueous and Non-Aqueous Electrolyte Structure, Transport and Interfacial Properties from Molecular Modeling

Insight into Aqueous and Non-Aqueous Electrolyte Structure, Transport and Interfacial Properties from Molecular Modeling A molecular scale insight into ion transport and decomposition is important for understanding deficiencies of the currently used aqueous and non-aqueous electrolytes. In this presentation I will summarize progress made towards improving molecular scale understanding of the structure and electrochemistry for a wide range of aqueous and non-aqueous electrolytes. Modeling of non-aqueous electrolytes will focus on the competitive solvent and salt reduction at the passivated electrochemical interfaces using Born Oppenheimer Molecular Dynamics (BOMD) simulations using DFT functionals. These BOMD simulations included critical factors needed to realistically represent electrolyte reactivity at electrodes such as explicit description of the substrate – electrolyte interactions; accurate representation of electrolyte structure, ion pairing and aggregation near an electrode; and collection of sufficient statistics from multiple unique simulations that were initiated with differing initial configurations. For example, 20 BOMD simulations starting from different initial conditions using 2.0M LiPF6 in THF tetrahydrofuran/2-methyl tetrahydrofuran showed no solvent decomposition nor HF formation while only LiF formation was observed as a result of LiPF6 salt decomposition.1 The most frequently observed reduction events included a PF6 − coordinated to Li+ cations from the electrolyte and LiF surface that lead to anion defluorination and formation of 3LiF and PF3 gas. The solvent separated LiPF6 and did not actively participate in reduction. When surface defects in LiF were present near a high population of PF6 − the anions there was a preference for the LiPF6 reduction and repair the SEI without ether solvent decomposition. Interestingly, a number of fast diffusion events for F- from the electrolyte | LiF interface to the LiF-lithium metal interface was observed that would be expected to occur during Li stripping indicating that F- re-arrangement in the thin LiF passivation films should be also considered. 1 Electrolyte reduction at the passivated interfaces from these simulations will be contrasted with other solvents ranging from ethers with mixed salts or carbonates and results from the representative quantum chemistry (QC) calculations performed on the small model electrolyte clusters to estimate oxidation and reduction.In the second part of the presentation the non-reactive molecular dynamics (MD) using APPLE&P polarizable force field to examine bulk and interfacial properties of the lithium and zinc aqueous electrolytes and electrochemical interfaces with the focus on the transport mechanism and relative contribution to charge flux from the anions and cations.2 Combined together information from these modeling scales suggests numerous strategies for stabilizing the electrolyte – electrode interfaces for numerous aggressive high energy density cathodes combined coupled with graphite and metal anodes. References Chen, J.; Li, Q.; Pollard, T. P.; Fan, X.; Borodin, O.; Wang, C., Electrolyte design for Li metal-free Li batteries. Materials Today 2020.Borodin, O.; Self, J.; Persson, K. A.; Wang, C.; Xu, K., Uncharted Waters: Super-Concentrated Electrolytes. Joule 2020, 4 (1), 69-100.

In Situ Designing a Gradient Li+ Capture and Quasi-Spontaneous Diffusion Anode Protection Layer toward Long-Life Li-O2 Batteries.

Lithium metal is the only anode material that can enable the Li-O2 battery to realize its high theoretical energy density (≈3500 Wh kg-1 ). However, the inherent uncontrolled dendrite growth and serious corrosion limitations of lithium metal anodes make it experience fast degradation and impede the practical application of Li-O2 batteries. Herein, a multifunctional complementary LiF/F-doped carbon gradient protection layer on a lithium metal anode by one-step in situ reaction of molten Li with poly(tetrafluoroethylene) (PTFE) is developed. The abundant strong polar C-F bonds in the upper carbon can not only act as Li+ capture site to pre-uniform Li+ flux but also regulate the electron configuration of LiF to make Li+ quasi-spontaneously diffuse from carbon to LiF surface, avoiding the strong Li+ -adhesion-induced Li aggregation. For LiF, it can behave as fast Li+ conductor and homogenize the nucleation sites on lithium, as well as ensure firm connection with lithium. As a result, this well-designed protection layer endows the Li metal anode with dendrite-free plating/stripping and anticorrosion behavior both in ether-based and carbonate ester-based electrolytes. Even applied protected Li anodes in Li-O2 batteries, its superiority can still be maintained, making the cell achieve stable cycling performance (180 cycles).

Negative Ion Conversion of Neutral Oxygen Atoms under Grazing Scattering from a LiF(100) Surface

We present a simple theoretical model study of negative ion conversion in neutral oxygen atom grazing scattering from a LiF(100) surface. A comparison of the calculated relative contributions of th...