Lithium-graphite Intercalation Compounds Research Articles

Scalable Synthesis of Bilayer Graphene at Ambient Temperature.

In this work, we develop for the first time a facile chemical lithiation-assisted exfoliation approach to the controllable and scalable preparation of bilayer graphene. Biphenyl lithium (Bp-Li), a strong reducing reagent, is selected to realize the spontaneous Li-intercalation into graphite at ambient temperature, forming lithium graphite intercalation compounds (Li-GICs). The potential of Bp-Li (0.11 V vs Li/Li+), which is just lower than the potential of stage-2 lithium intercalation (0.125 V), enables the precise lithiation of graphite to stage-2 Li-GICs (LiC12). Intriguingly, the exfoliation of LiC12 leads to the bilayer-favored production of graphene, giving a high selectivity of 78%. Furthermore, the mild intercalation-exfoliation procedure yields high-quality graphene with negligible structural deterioration. The obtained graphene exhibits ultralow defect density (ID/IG ∼ 0.14) and a considerably high C/O ratio (∼29.7), superior to most current state-of-the-art techniques. This simple and scalable strategy promotes the understanding of chemical Li-intercalation methods for preparing high-quality graphene and shows great potential for layer-controlled engineering.

Electronic signatures of Lorentzian dynamics and charge fluctuations in lithiated graphite structures

Lithium graphite intercalation compounds (Li-GICs) are essential materials for modern day portable electronics and obtaining insights into their atomic structure and thermodynamics is of fundamental interest. Here we explore the electronic and atomic states of Li-GICs at varying degrees of Lithium loading (i.e., “staging”) by means of ab-initio molecular dynamics simulations and simulated X-ray adsorption spectroscopy (XAS). We analyze the atomic correlation functions and shows that the enhancements of the Li-ion entropy with increased staging result from Lorentzian lithium-ion dynamics and charge fluctuations, which activate low-energy phonon modes. The associated electronic signatures are modulations of the unoccupied π*/σ* orbital energy levels and unambiguous fingerprints in Carbon K-edge XAS spectra. Thus, we extend the canonical view of XAS, establishing that these “static” measurements in fact encode the signature of the thermodynamic response and relaxation dynamics of the system. This causal link between atomic structure, spectroscopy, thermodynamics, and information theory can be generally exploited to better understand stability in solid-state electrochemical systems.

The intrinsic electrostatic dielectric behaviour of graphite anodes in Li-ion batteries—Across the entire functional range of charge

Lithium–graphite intercalation compounds (Li-GICs) are by far the most common anode material for modern Li-ion batteries. However, the dielectric response of this material in the electrostatic limit and its variation depending on the state of charge (SOC) has not been investigated to a satisfactory degree – neither by means of theory nor by experiment – and especially not for the higher range of SOC. In this work, we – for the first time – predict a mostly linear dependency of the relative permittivity ϵr on the SOC, from ≈7 at SOC 0% to ≈25 at SOC 100%. This is achieved by making use of our recently published DFTB parametrization for Li-GICs based on a machine-learned repulsive potential in order to overcome the computational hurdles of sampling the long-ranged Coulomb interactions within this material. In doing so, we provide novel insight into a property which is highly desired, particularly as an input parameter for charged kinetic Monte Carlo simulations.

Accessing Structural, Electronic, Transport and Mesoscale Properties of Li-GICs via a Complete DFTB Model with Machine-Learned Repulsion Potential.

Lithium-graphite intercalation compounds (Li-GICs) are the most popular anode material for modern lithium-ion batteries and have been subject to numerous studies—both experimental and theoretical. However, the system is still far from being consistently understood in detail across the full range of state of charge (SOC). The performance of approaches based on density functional theory (DFT) varies greatly depending on the choice of functional, and their computational cost is far too high for the large supercells necessary to study dilute and non-equilibrium configurations which are of paramount importance for understanding a complete charging cycle. On the other hand, cheap machine learning methods have made some progress in predicting, e.g., formation energetics, but fail to provide the full picture, including electrostatics and migration barriers. Following up on our previous work, we deliver on the promise of providing a complete and affordable simulation framework for Li-GICs. It is based on density functional tight binding (DFTB), which is fitted to dispersion-corrected DFT data using Gaussian process regression (GPR). In this work, we added the previously neglected lithium–lithium repulsion potential and extend the training set to include superdense Li-GICs (LiC6−x; ) and lithium metal, allowing for the investigation of dendrite formation, next-generation modified GIC anodes, and non-equilibrium states during fast charging processes in the future. For an extended range of structural and energetic properties—layer spacing, bond lengths, formation energies and migration barriers—our method compares favorably with experimental results and with state-of-the-art dispersion-corrected DFT at a fraction of the computational cost. We make use of this by investigating some larger-scale system properties—long range Li–Li interactions, dielectric constants and domain-formation—proving our method’s capability to bring to light new insights into the Li-GIC system and bridge the gap between DFT and meso-scale methods such as cluster expansions and kinetic Monte Carlo simulations.

Synthesis of graphene and recovery of lithium from lithiated graphite of spent Li-ion battery

Recycling of spent Li-ion batteries is crucial for achieving sustainable development of battery industry. Current recycling processes mainly focus on valuable metals but less attention has been paid to spent graphite, which generally ends up as secondary waste. In this study, a process for preparing graphene and recovering Li in anode as a by-product from spent graphite was developed. The key point was to re-charge the spent LIBs to generate lithium graphite intercalation compounds. The lithium graphite intercalation compounds were then subjected to a hydrolysis procedure and graphene could be produced through ultrasonic treatment via the expansion/micro-explosion mechanism. Experimental results demonstrated that 1–4 layered graphene could be efficiently produced when spent Li-ion batteries with beyond 50% capacity were re-charged. The prepared graphene showed high quantity containing few defects (ID/IG = 0.33, C/O = 13.2 by energy dispersive spectroscopy and C/O = 8.8 by X-ray photoelectron spectroscopy). In addition, Li was simultaneously recovered in the form of battery-grade lithium carbonate in the above process. Economic analysis indicated that the graphene production cost was extremely low ($540/ton) compared to that of commercial graphene.

Operando X-Ray Diffraction during Battery Cycling at Low Temperatures

Lithium-ion batteries are nowadays widely used in consumer electronics and electric vehicles. However, the application in electric vehicles poses new challenges to this battery technology. High energy and power densities, a high degree of safety and a long lifetime are important to meet the requirements of the customers. To improve lithium-ion batteries, it is necessary to gain a deeper understanding of electrochemical processes occurring during battery cycling. Operando characterization techniques are used to shed light on these processes. In particular, operando X-ray diffraction (XRD) and neutron diffraction are applied for the observation of phase changes of battery materials during cycling. Thereby, the lithiation and delithiation mechanism can be investigated depending on different factors like C-rate or temperature. The gained deeper understanding of the structural behavior of battery materials under various operating conditions leads to the improvement of battery management systems by increasing the performance and reducing degradation processes of batteries. The phase changes of state-of-the-art battery materials at room temperature are well understood but the influence of temperature on the phase evolution is barely investigated. Recent studies show that the classical staging mechanism of graphite [1, 2] does not occur at higher C-rates [3, 4] and elevated or low temperatures [5, 6] and instead, several stages coexist. During a rest period after charge or discharge, these phases can equilibrate [6]. The lithiation and delithiation mechanism does not only influence the charge and discharge behavior, but also the dimensional changes concurrently with the degradation of the electrodes [5]. Therefore, a deeper understanding of the structural behavior of battery materials under various operating conditions is needed to improve the material properties and adapt battery management systems to increase the performance of the battery and reduce degradation. At DLR, a new set-up was developed to investigate with operando XRD the phase evolution of battery materials during cycling at temperatures below room temperature. This method allows a readily characterization of battery materials at various temperatures and the identification of the temperature dependent intercalation mechanisms. [1] J. Besenhard and H.P. Fritz, The Electrochemistry of Black Carbons, Angew. Chem. Int. Ed. 22 (1983) 950-975. [2] J.R. Dahn, Phase diagram of LixC6, J. Phys. Rev. B. 44 (1991) 9170-9177. [3] H. He, C. Huang, C.-W. Luo, J.-J. Liu and Z.-S. Chao, Dynamic study of Li intercalation into graphite by in situ high energy synchrotron XRD, Electrochim. Acta. 92 (2013) 148-152. [4] N. Sharma and V.K. Peterson, Current-dependent electrode lattice fluctuations and anode phase evolution in a lithium-ion battery investigated by in situ neutron diffraction, Electrochim. Acta. 101 (2013) 79-85. [5] N.A. Cañas, P. Einsiedel, O.T. Freitag, C. Heim, M. Steinhauer, D.-W. Park and K.A. Friederich, Operando X-ray diffraction during battery cycling at elevated temperatures: A quantitative analysis of lithium-graphite intercalation compounds, Carbon. 116 (2017) 255-263. [6] V. Zinth, C. von Lüders, J. Wilhelm, S.V. Erhard, M. Hofmann, S. Seidlmayer, J. Rebelo-Kornmeider, W. Gan, A. Jossen and R. Gilles, Inhomogeneity and relaxation phenomena in the graphite anode of a lithium-ion battery probed by in situ neutron diffraction, J. of Power Sources. 361 (2017) 54-60.

Grand Canonical Monte Carlo Study of Li Intercalation into Graphite

A computational model is proposed to study lithium-graphite intercalation compounds in the framework of Monte Carlo simulations in the Gran Canonical Ensemble. The results suggest the existence of fractional stages between stages II and I, which are rarely found in most experimental electrochemical measurements. These fractional stages seem to be part of a more general intercalation mechanism suggested for other intercalation compounds. The thermodynamic information obtained from the simulations for these fractional states at room temperature is discussed in the context of experimental data. In addition, clear evidence was found for the existence of a new stage, supported by recent experimental measurements.

Electrochemical lithium intercalation behavior of pristine and milled hexagonal boron nitride

Hexagonal boron nitride (h-BN) has been explored for lithium intercalation due to its similar crystal structure to graphite, a well-known anode material for Li-ion secondary batteries. Lithium h-BN intercalation compounds (Li-BNICs) have been successfully synthesized through heat treatment. In this study, potential physical properties related to electrical conductivity were investigated using pristine and milled h-BN, which is further mixed/combined with milled graphite. The electrochemical properties were characterized by cyclic voltammetry (CV) and galvanostatic cycling with potential limitation (GCPL). The chemical potential of h-BN was estimated about 1.0V versus Li/Li+ and a two-phase reaction was suggested. Accordingly, Li-BNIC is more stable than Li-GICs (lithium graphite intercalation compounds) in terms of thermal stability.

Applying Bayesian Approach to Combinatorial Problem in Chemistry.

A Bayesian optimization procedure, in combination with density functional theory calculations, was applied to a combinatorial problem in chemistry. As a specific example, we examined the stable structures of lithium-graphite intercalation compounds (Li-GICs). We found that this approach efficiently identified the stable structure of stage-I and -II Li-GICs by calculating 4-6% of the full search space. We expect that this approach will be helpful in solving problems in chemistry that can be regarded as a kind of combinatorial problem.

Phase formation and microstructure in lithium-carbon intercalation compounds during lithium uptake and release

Abstract The intercalation and de-intercalation behavior of lithium in graphite was investigated with respect to the formation of lithium graphite intercalation compounds (Li-GICs) using a combination of XRD measurements and metallography. Microstructural features and the evolution of phases and structural details of Li-GICs during Li uptake and release are correlated for the first time, giving new insight into the formation of Li-GICs. Lithium evaporation loss during annealing was exploited for studying the phase evolution during lithium de-intercalation, whereas phase formation during lithium uptake was studied in a sample exhibiting a lithium concentration gradient. The experiments indicate that growth and dissolution of the compound LiC6 do not occur by an inverse mechanism, as shown by the formation of different intermediate phases. The compound LiC12 was found to be a non-stoichiometric solid solution. LiC6 can be clearly identified in micrographs by the color and distinct phase boundaries, while LiC12 is situated in the vicinity of the LiC6 phase with varying coloration depending on the Li content.

Operando X-ray diffraction during battery cycling at elevated temperatures: A quantitative analysis of lithium-graphite intercalation compounds

The application of in situ or operando characterization techniques is highly important to gain a deeper understanding of the electrochemical and degradation processes that occur during battery cycling. This work presents the first results of operando X-ray diffraction (XRD) studies of a battery at elevated temperatures. Our in situ-cell was developed to simultaneously investigate the structural and electrochemical behaviors of batteries at room temperature and elevated temperatures. The cell design was applied to high temperature operando XRD measurements of graphite half-cells (vs. Li/Li+). New results on the structural behavior of graphite during lithium intercalation and deintercalation were obtained at elevated temperature (30 °C and 47 °C). At these temperatures, the classical staging mechanisms does not occur, and instead, several stages coexist. The average spacing between the graphene layers was calculated for phases in the single-stage region and those at which several stages coexist for different depths of charge and discharge and at different C-rates. Through this work, we aim to stimulate further operando XRD studies on battery materials' temperature behavior during cycling.

Intercalation of hexagonal boron nitride and graphite with lithium by sequential process of ball milling and heat treatment

Abstract Recently, intercalation compounds with various intercalants between hexagonal boron nitride (h-BN) layers have been studied, where lithium BN intercalation compound (Li-BNIC) is one of such compounds successfully synthesized. They are expected to exhibit similar properties to lithium graphite intercalation compounds (Li-GICs) that are known as the anode material for lithium ion batteries. It is difficult, however, to apply Li-BNIC for the batteries due to its returning to an insulator when Li is deintercalated. In this study a Li–BN–graphite ternary system has been focused because it is reported that graphite-like BC2N is a promising material for rechargeable Li batteries. The primary purpose of this study is thus to investigate combined reactivity of BN and graphite with Li through milling and heating processes, and possible intercalation of Li into the matrix: h-BN, graphite or B C N. The pieces of lithium metal, h-BN and graphite powders were ball-milled using a vibratory ball-mill machine and heat-treated at 700 °C for 2 h under argon atmosphere. The samples were then characterized by X-ray diffractometry, X-ray photoelectron spectroscopy, 7Li nuclear magnetic resonance and Differential thermal analysis study. Li-GICs were mainly produced by milling, while post-annealing caused their eliminations and instead produced Li-BNICs with small amount of other lithium compounds. In terms of thermal stability, Li-BNIC is more stable than Li-GICs. In a Li–BN–graphite system, an activation energy of Li-BNIC was estimated to be 119.6 kJ/mol, which is higher than reported activation energies for Li-GICs.

Anisotropic Tuning of Graphite Thermal Conductivity by Lithium Intercalation.

Understanding thermal transport in lithium intercalated layered materials is not only important for managing heat generation and dissipation in lithium ion batteries but also the understanding potentially provides a novel way to design materials with reversibly tunable thermal conductivity. In this work, the thermal conductivity of lithium-graphite intercalation compounds (LixC6) is calculated using molecular dynamics simulations as a function of the amount of lithium intercalated. We found that intercalation of lithium has an anisotropic effect on tuning the thermal conductivity: the thermal conductivity in the basal plane decreases monotonically from 1232 W/m·K of pristine graphite to 444 W/m·K of the fully lithiated LiC6, while the thermal conductivity along the c-axis decreases first from 6.5 W/m·K for graphite to 1.3 W/m·K for LiC18 and then increases to 5.0 W/m·K for LiC6 as the lithium composition increases. More importantly, we provide the very first atomic-scale insight into the effect of lithium intercalation on the spectral phonon properties of graphite. The intercalated lithium ions are found to suppress the phonon lifetime and to reduce the group velocity of phonons parallel to the basal plane but significantly to increase the phonon group velocity along the c-axis, which anisotropically tunes the thermal conductivity of lithiated graphite compounds. This work could shed some light on the search for tunable thermal conductivity materials and might have strong impacts on the thermal management of lithium ion batteries.

Lattice Energy Calculation for Li Inserted Graphite at Relaxation Process

Introduction Relaxation analysis makes transition of electrode material from kinetic state to equilibrium state clear [1]. We have applied the Relaxation analysis to various kind of cathode and anode materials, such as γ-Fe2O3[2,3], LiMn2O4[4], LiFePO4[5], LiCoO2[6], LiMnPO4[7] and so on. In the previous work, we conducted relaxation analysis to graphite, the widespread anode material for lithium ion rechargeable batteries[8]. We analyzed the variation of stage structure of lithium-graphite intercalation compounds (Li-GICs) based on the one-dimensional Rietveld method[9]. It was indicated that defective stage 1 was formed at the lithium insertion process, and it separated into stage 1 without defect and stage 2 during the relaxation. Stage 2 had two kinds of interlayer distance. The wider interlayer distance increased, and narrower one decreased with the relaxation time. Generally, the structure of stage 2 is presented as the stack of Li-inserted graphene layer and Li-not-inserted graphene layer. However, from the point of symmetry, stack of graphene layers with two different Li concentration at the interlayer makes stage 2 structure. It was considered that, at Li insertion process, Li-rich interlayer and Li-lean interlayer stacked to construct stage 2, and that, at the relaxation time, structure of stage2 changed to stack of Li–fully-inserted graphene layer and Li-not-inserted graphene layer. This means that the stack of Li-fully-inserted graphene and Li-not-inserted graphene has the minimum lattice energy. In the present study, we compared the lattice energy of various stage 2 stacking graphene layers with different Li concentrations by the first principle calculations. Calculation We used Advance/PHASE software developed by Advancesoft corporation. The Advance/PHASE is based on first principle calculation with using the DFT (Density-Functional Theory) adopting the LDA (Local Density Approximation) and GGA (Generalized Gradient Approximation). Advance/PHASE solve the Kohn-Sham equation in SCF (self-consistent field). First, we constructed a stage 2 model having two graphene layer with 24 C atoms each. Second, we set 4 Li atoms in the model with 4 kinds of distribution as Fig.1. Number 1: 4 Li atoms are located in one interlayer with LiC6 type structure (ideal stage 2). Number 2,3,4 : Three kinds of Li configuration at the Li-lean layer. Results and Discussion The calculated lattice energies increased with the order as Number 1 < Number 2 < Number 3 < Number 4. This means that Number 1 configurations, the stack of Li-fully-inserted graphene and Li-not-inserted graphene (ideal stage 2), is the minimum, most stable. It is consistent the previous result that the wider interlayer distance increased, and narrower one decreased with the relaxation time for stage 2 at relaxation process.

Relaxation Rietveld Stage Analysis of Li Inserted Graphite after Charge/Discharge Cycles

Introduction Relaxation analysis makes transition of electrode material from kinetic state to equilibrium state clear. We have applied the Relaxation analysis to various kind of cathode and anode materials, such as γ-Fe2O3[1,2], LiMn2O4[3], LiFePO4[4], LiCoO2[5], LiMnPO4[6] and so on. In the previous work, we conducted relaxation analysis to graphite, the widespread anode material for lithium ion rechargeable batteries[7]. We analyzed the variation of stage structure of lithium-graphite intercalation compounds (Li-GICs) based on the one-dimensional Rietveld method[8]. In the present study, relaxation analysis was performed on the Li inserted graphite after a charge/discharge cycle and the results were compared for the case of first Li-insertion. Experiment Working electrode was prepared by mixing natural graphite powder (LB-CG, Nippon Kokuen) and PVdF with a ratio of 93 : 7 (weight ratio). Lithium foil was employed as the counter electrode and EC/DMC (2:1 volume ratio) with 1 mol∙dm-3 LiPF6was used as the electrolyte. Lithium ion was inserted into graphite electrochemically using two electrode cell. After the insertion, we extracted lithium ion from the sample, then we inserted lithium ion again into the sample. Current rate and charge/discharge periods were all selected to be 0.1C and 10h, respectively. Then two types of Li-GICs were prepared; the samples of lithium insertion at the first time (named 1st) and the second time (named 2nd). After the termination of each sample preparation, we immediately removed the working electrode from the cell in a glove box to avoid the local cell reaction between the electrode material and the current collector. XRD patterns from 11° to 53° in 2θ were measured by using CuKα radiaton (UltimaIV, Rigaku, Japan) for various relaxation time. We analyzed XRD patterns by the one-dimensional Rietveld method [6] using RIEVEC program coded by T. Yao[1-8]. Interlayer distances and scale factors of stage1 and stage2 were obtained. Results and discussion XRD profile of the sample for each relaxation time after Li insertion was well fitted by the Rietveld calculation. Fig.1 shows the obtained mole fraction changes of stage 1 for samples of 1st and 2nd, indicating that mole fractions of stage1 decrease and those of stage 2 increase with the relaxation time for both samples. It is considered that defective stage 1 was formed at the lithium insertion process, and it separated into stage 1 without defect and stage 2 during the relaxation. The change in mole fraction became slower for the 2nd. Fig.2 shows lattice parameters of c-axis for the stages 1 and 2. The lattice parameters of stage 1 were almost constant and those of stage 2 decreased during the relaxation for both samples. Although the structure of stage2 is generally presented as the stack of Li-inserted graphene layer and Li-not-inserted graphene layer, the stack of graphene layers with two different Li concentration at the interlayer also makes stage2 structure from the point of symmetry. It is considered that, at Li insertion process, Li-rich interlayer (D w) and Li-lean interlayer (D n) stacked to construct stage 2, and that structure of stage2 changed into the stack of Li-inserted (D w) and Li-not-inserted (D n) graphene layers during relaxation. Fig.3 shows two kinds of interlayer distance of stage 2, D w and D n, for both samples, where D w increased and D n decreased with the relaxation time. The changing manner of D w and D nwere similar for 1st and 2nd samples. It is considered that the process of structure change was similar for 1st and 2nd during the relaxation process, however, that the driving force from kinetic state to equilibrium state was weaker for 2nd sample than that of 1st sample by the loose of graphen layer stacking due to lithium ion insertion and extraction.

Charge–discharge characteristics of natural graphite electrode in N, N-diethyl-N-methyl-N-(2-methoxyethyl)ammonium bis(trifluoromethylsulfonyl)amide containing lithium ion for lithium-ion secondary batteries

We investigated the electrochemical characteristics of a natural graphite electrode in room-temperature ionic liquids not containing additives. N, N-diethyl-N-methyl-N-(2-methoxyethyl)ammonium bis(trifluoromethylsulfonyl)amide (DEME-TFSA) containing lithium bis(trifluoromethylsulfonyl)amide (Li-TFSA) as the electrolyte and a natural graphite electrode as the negative electrode material were employed. The charge–discharge tests showed that the discharge capacity and the charge–discharge efficiency of the natural graphite electrode at the 1st cycle were 318 mAh g−1 and 75.6%, respectively. The cycle performance showed that the discharge capacity and the charge–discharge efficiency were stably maintained at ca. 320 mAh g−1 and 100%, respectively, until the initial 10th cycle. The ex-situ X-ray diffraction measurements showed that lithium–graphite intercalation compounds, such as LiC12 and LiC6, were formed after the 1st charge. The structural change in the natural graphite electrode was reversible because graphite recovered to its original structure after the 1st discharge. These results clarified that the graphite electrode could operate as a negative electrode for lithium-ion secondary batteries in DEME-TFSA containing Li-TFSA without organic solvents.

EC-Free Superconcentrated Electrolytes for Advanced Lithium-Ion Batteries

The development of a stable, functional electrolyte is urgently required for fast-charging and high-voltage lithium-ion batteries as well as next-generation lithium-oxygen batteries. Although there are numerous organic solvents with diverse characteristics used in the area of chemistry, lithium-ion batteries have exclusively employed an ethylene carbonate (EC) based electrolyte to ensure the reversibility of the graphite negative electrode reaction. Because of the limitation of electrolyte compositions, there has been no remarkable progress in commercial lithium-ion batteries despite active researches on positive electrode materials. Recently, we have presented a “salt-superconcentrating” strategy as a simple and effective method of expanding a graphite negative electrode reaction in a wide variety of organic solvents.1 This finding can break the battery-performance limitation based on an EC-based electrolyte to open up a way to advanced lithium-ion batteries. For example, acetonitrile (AN) solutions, having remarkably high chemical and oxidative stability as well as excellent ion transport property, are one of the most promising electrolytes to realize high-voltage and fast-charging lithium-ion batteries. Herein we report unusual reductive stability of a superconcentrated AN electrolyte and its origin based on spectroscopic analyses and first-principle density functional theory based molecular dynamics (DFT-MD) simulations.2 Figure 1 shows charge-discharge curves of a natural graphite/lithium metal coin cell with superconcentrated 4.2 mol dm-3 LiN(SO2CF3)2 (LiTFSA)/AN electrolyte. Several voltage plateaus were observed in 0.05 – 0.25 V, which are characteristic of sequential formation of several stage structures of lithium-graphite intercalation compounds. The obtained reversible capacity was ca. 350 mAh g-1, which is close to the theoretical capacity (372 mAh g-1) based on fully lithiated LiC6. This is the first to observe reversible operation of a graphite electrode in reduction-vulnerable AN solvent and clearly indicates enhanced reductive stability of superconcentrated electrolytes. X-ray photoelectron spectroscopy reveals the presence of a TFSA-based surface film on the cycled graphite electrode.To elucidate the origin of the unusual reductive stability with a TFSA-based surface film, DFT-MD was applied to dilute and superconcentrated LiTFSA/AN solutions. The superconcentrated solution has a unique networking structure of Li+ cations and TFSA- anions, which modifies frontier orbital characters in the solution. The projected density of states (PDOS) (Fig. 2) shows that the lowest unoccupied molecular orbitals (LUMOs), which dominate the behavior of its reduction reaction, shift from AN solvents to TFSA- anions. Hence, the TFSA- anions, instead of AN solvents, are predominantly reduced to form a TFSA-based surface film on a graphite electrode during first cycle, which kinetically suppresses further electrolyte decompositions. The modified surface film character, arising from the peculiar frontier orbitals, is the origin of the enhanced reductive stability of superconcentrated solutions that allow for reversible operation of a graphite negative electrode without EC. The salt-superconcentrating strategy, expanding the graphite electrode reaction for wide variety of organic solvents other than EC, will contribute to the development of advanced lithium-ion batteries with high-voltage and fast-charging characters based on new EC-free functional electrolytes.

Estimation of Reduced Partition Function Ratios of Lithium-Graphite Intercalation Compounds by Density Functional Theory

The reduced partition function ratio (RPFR) of lithium in lithium-graphite intercalation compounds (Li-GICs) was evaluated at the UB3LYP=6-311G(d) level of theory. The partition functions were written in the usual rigid-rotor harmonic oscillator approximation.With a C24H12coronene molecule as the model of graphene, lithium-coronene sandwich, and club sandwich compounds were considered as models of Li-GICs. The estimated value of the6Li-to-7Li RPFR was 1.0402 at 25 °C, which yielded 1.034 as the value of the equilibrium constant, K, of the lithium isotope exchange reaction between a lithium ion in an ethylene carbonate=ethylmethyl carbonate mixed solvent and a lithium atom in interlayer space of graphite. The estimated value of K was larger than the experimental value of 1.025. The unsatisfactory agreement between the estimated and experimental K values suggested that larger molecules should be used as models of graphene and that the vibrational anharmonicity should also be taken into consideration.

Theoretical Study of Li Migration in Lithium–Graphite Intercalation Compounds with Dispersion-Corrected DFT Methods

Structural, energetic, electronic, and defect properties of lithium–graphite intercalated compounds (LiC6n (n = 1, 2)) are investigated theoretically with periodic quantum-chemical methods. Calculated properties obtained with a gradient-corrected density functional theory (DFT) method and a dispersion-corrected DFT method (DFT-D3-BJ) are compared. The DFT-D3-BJ method gives better agreement with experiment for the structural parameters and Li intercalation energy compared to the uncorrected DFT approach, showing that interlayer interactions due to the van der Waals forces play an effective role in graphite and LiC6n compounds. In agreement with the literature, the calculated density of states (DOS) show that graphite is metallic with a low DOS at the Fermi level, whereas LiC6n compounds have a high DOS at the Fermi level. Between the considered point defects, VLi and Lii, the energy needed to form Li point defects is smaller if solid Li is used as reference. A moderate relaxation is observed for the atoms surrounding the Li defect. Competing pathways for Li diffusion in LiC6n compounds are investigated using the climbing-image nudged elastic band approach. Two different mechanisms for Li diffusion are observed, the vacancy mechanism and the Frenkel mechanism. For both cases, Li migration pathways along the ab plane and along the c crystallographic axis are investigated. A large activation barrier along the c crystallographic direction indicates that Li does not diffuse in the c direction. The calculated activation barriers for Li diffusion in the ab plane are consistent with previous experimental investigations.

Influences of Mild Oxidation Treatment on the Structure and Electrochemical Behavior of Spherical Graphite

The effects of mild oxidation treatment on the crystal structure,morphology and electrochemical behavior of spherical graphite were investigated.The structure and morphology of the samples were characterized by XRD,Raman spectra and SEM analysis,respectively.The electrochemical behavior of the samples was studied using galvanostatic charge/discharge test,powder cavity microelectrode technique and slow scan rate cyclic voltammograms.The results indicate that structural defects in graphite particles and disordered degree of near surface region of graphite increase after modification,while the average in-plane crystallite domain size La and content of rhombohedral phase decrease.The graphite particles present irregular flake shape and some graphene sheets show flexual and sculptural margin.Electrochemical measures show that reversible capacity of modified graphite increases from 345.5 mAh/g to 381.4 mAh/g,and cycling performance is improved.During deintercalation process of the modified graphite,stage transformation from stage 1 Li-GICs(lithium-graphite intercalation compounds) to stage 2 Li-GICs begins to happen at lower voltage,compared with that of the raw graphite.