Graphite Anodes For Li-ion Batteries Research Articles

Effective Measures of Thickness Evolution of the Solid Electrolyte Interphase of Graphite Anodes for Li-Ion Batteries.

Upon forming, the intensity or thickness of the solid electrolyte interphase (SEI) in a Li-ion battery (LIB) evolves to various states depending on the cell materials and operation conditions. Despite a crucial role in comprehending the behaviors of an LIB, its quantitative measure is far from satisfactory mainly because of the undue complexity of the concentration profiles of the comprising chemical species. Here, we calculate the depth profiles of atomic mole fractions of C and F and their ratio as RC/F = C/F of graphite anodes for LIBs in comparison to an X-ray photoelectron spectroscopy (XPS) experiment. To this end, we take a differential equation approach to dC/dt*, where t* is the reduced XPS etching time for depth. As a result, the respective analytical expression derived for C, F, and RC/F(t*) is verified to accurately account for the experiment. Moreover, we show that RC/F(t*) in the j state can be practically expressed in , where γ is a constant for a given anode. Based on this, we suggest ξj* = (αi + βi - βj)/αj as a measure of the SEI thickness evolution from the i to j state in terms of the cycle number. As an intriguing finding, the SEI thickness evolves up to about 3 times that of its initial state, beyond which it does not appear to grow any more.

Graphite Anodes for Li-Ion Batteries: An Electron Paramagnetic Resonance Investigation.

Graphite is the most commercially successful anode material for lithium (Li)-ion batteries: its low cost, low toxicity, and high abundance make it ideally suited for use in batteries for electronic devices, electrified transportation, and grid-based storage. The physical and electrochemical properties of graphite anodes have been thoroughly characterized. However, questions remain regarding their electronic structures and whether the electrons occupy localized states on Li, delocalized states on C, or an admixture of both. In this regard, electron paramagnetic resonance (EPR) spectroscopy is an invaluable tool for characterizing the electronic states generated during electrochemical cycling as it measures the properties of the unpaired electrons in lithiated graphites. In this work, ex situ variable-temperature (10-300 K), variable-frequency (9-441 GHz) EPR was carried out to extract the g tensors and line widths and understand the effect of metallicity on the observed EPR spectra of electrochemically lithiated graphites at four different states of lithiation. We show that the increased resolution offered by EPR at high frequencies (>300 GHz) enables up to three different electron environments of axial symmetry to be observed, revealing heterogeneity within the graphite particles and the presence of hyperfine coupling to Li nuclei. Importantly, our work demonstrates the power of EPR spectroscopy to investigate the local electronic structure of graphite at different lithiation stages, paving the way for this technique as a tool for screening and investigating novel materials for use in Li-ion batteries.

Enhancing the rate and cycle performance of graphite anode for Li-ion batteries by constructing a multidimensional conducting network

Enhancing the rate and cycle performance of graphite anode for Li-ion batteries by constructing a multidimensional conducting network

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.

Toward Li-ion Graphite Anodes with Enhanced Mechanical and Electrochemical Properties Using Binders from Chemically Modified Cellulose Fibers

Cellulose nanofibers (CNFs) are bio-sourced nanomaterials, which, after proper chemical modification, exhibit a unique ability to disperse carbon-rich micro- and nanomaterials and can be used in the design of mechanically strong functional nanocomposites. When used in the preparation of graphite anodes for Li-ion batteries, they have the potential to outperform conventional binders such as carboxymethyl cellulose (CMC) and styrene–butadiene rubber (SBR) both electrochemically and mechanically. In this study, cellulose-rich fibers were subjected to three different chemical modifications (including carbonyl-, carboxyl-, and aldehyde-functionalization) to facilitate their fibrillation into CNFs during the preparation of aqueous slurries of graphite and carbon black. Using these binders, graphite anodes were prepared through conventional blade coating. Compared to CMC/SBR reference anodes, all anodes prepared with modified cellulosic fibers as binders performed better in the galvanostatic cycling experiments and in the mechanical cohesion tests they were subjected to. Among them, the aldehyde- and carboxyl-rich fibers performed the best and resulted in a 10% increase in specific capacity with a simultaneous two- and three-fold increase of the electrode material’s stress-at-failure and strain-at-break, respectively. In-depth characterizations attributed these results to the distinctive nanostructure and surface chemistry of the composites formed between graphite and these fiber-based binders.

Accessing the solid electrolyte interphase on silicon anodes for lithium-ion batteries in-situ through transmission soft X-ray absorption spectroscopy

Silicon offers nine times higher theoretical storage capacity than commercial graphite anodes for Li-ion batteries. For cycling stability, the electrolyte needs to be kinetically stabilized by the so-called Solid Electrolyte Interphase (SEI), a layer which ideally forms once from decomposition products of the electrolyte. While it works for graphite, the SEI on silicon fails to stabilize the electrolyte sufficiently, partly due to the large volume changes upon de-/lithiation. To investigate the SEI on silicon anodes, we developed a novel approach for X-ray Absorption Spectroscopy (XAS), that puts a twist to conventional SiNx window-based liquid cells by utilizing a deliberately induced gas-bubble to form a soft X-ray transparent electrolyte layer. We demonstrate our approach to allow transmission XAS in the soft X-ray regime on liquids and electrode thin-film materials under in-situ conditions. In our case XAS study of the SEI on silicon anodes, we reveal the main SEI constituents as Li acetate, Li ethylene di-carbonate or Li ethylene mono-carbonate, Li acetylacetonate, LiOH, and LiF. Additionally, we see evidence for aldehyde species which we attribute to possible liquid inclusions within a porous SEI morphology. We consider our method an appropriate tool for the successful engineering of a stable, efficient SEI in the future.

Novel synthesis of porous Si-TiO2 composite as a high-capacity anode material for Li secondary batteries

Porous Si materials can be considered as a candidate for and an alternative to commercial graphite anodes for Li-ion batteries. In this study, we propose a modified magnesiothermic method to increase the materials’ yield and a design strategy to improve their electrochemical properties. For magnesiothermic solid-state reactions, Mg2Si was used instead of metallic Mg and amorphous nanoscale SiO2 was adopted. The two starting materials were converted into Si and MgO via a high-temperature solid-state reaction, resulting in a high-yield percentage of Si. Then, amorphous TiO2 layers were formed and the MgO was removed by chemical etching, forming a porous structure. The materials at each step were thoroughly characterized by using various analytical tools. Electrochemical test results demonstrated that the amorphous TiO2 coated porous Si composite electrode exhibited a high reversible capacity of 1965 mA h g−1 after 100 cycles and a greatly improved initial Coulombic efficiency of 90.1%. These results can be attributed to the controlled porous structure of the composite and amorphous TiO2 coating layer. The modified magnesiothermic method and porous material design could attract commercial use of Si-based materials as Li-ion battery anodes.

Diffusion kinetics of water in graphite anodes for Li-ion batteries

Since the loading of electrodes for Li-ion batteries with an excessive amount of moisture affects the performance of the cells, the moisture is partly removed in a post-drying process step. The drying time is thereby controlled by the amount of moisture that has to be removed as well as the mass transport kinetics of water. This mass transport kinetics is limited by various mass transport resistances, which in turn can be influenced by individual dryer settings. In this paper, we investigate water-based anodes for Li-ion batteries as well as selected materials, and distinguish between different possible mass transport resistances that affect the drying time. Determined diffusion coefficients can be used in future modeling of the post-drying process. Special consideration needs to be bestowed upon the mass transport of water in the polymeric binders, since earlier investigations revealed that the moisture uptake of the anodes is particularly caused by sodium carboxymethyl cellulose (Na-CMC). This high water uptake by the binder in water-based electrodes leads to a polymer film drying process within the porous structure of the electrode. The diffusion of water in layers of binder is particularly dependent on the concentration as well as the temperature of the binder.

Quantifying Immobile Lithium at the Graphite Anode Surface from Formation to Fast Charging

Engineering the solid electrolyte interphase (SEI) at the surface of graphite anodes in Li-ion batteries is crucial to optimizing the rate capability, cycle life, and calendar life of commercial cells. It has been reported that fast-forming SEIs on composite graphite electrodes in a high-potential region (~0.24 V vs. Li/Li+) has a positive influence on rate capability in half cell formats [1]. Potentiostatic EIS modulation applied during formation has shown similar results to the fast formation method. In this work, we evaluated the efficacy of various SEI formation methods on the rate and cycle life of full cell Li-ion batteries, and then analyzed electrode surfaces ex situ to locate and quantify immobile lithium. Lithium nuclear reaction analysis (Li-NRA) provided an effective non-destructive method to correlate irreversible electrochemistry with physical surface phenomena [2].It is well established that high current charging can induce Li plating on the graphite anode, leading to accelerated capacity fade. Several methods have been used to quantify this phenomena including titration and secondary ion mass spectroscopy (SIMS). Here, we employ Li-NRA and a suite of complementary techniques (i.e. XPS and SIMS) to show how the amount of immobile Li and its depth profile varies with charge rate (up to 5C) and cycle number.[1] B. K. Antonopoulos, C. Stock, F. Maglia, H. E. Hoster, Electrochim. Acta, 269 (2018), 331-339.[2] A. Schulz, D. DeRosa, S. Higashiya, M. Rane-Fondacaro, H. Bakhru, P. Haldar, J. Energy Storage 14 (2017), 106-111. A. Schulz, D. DeRosa, S. Higashiya, M. Rane-Fondacaro, H. Bakhru, P. Haldar, J. Power Sources 360 (2017), 129-135.

Electrochemical Nucleation of Silicon in Ionic Liquid-Based Electrolytes

Silicon is widely applied in a high number of industrial technologies, including different types of electronics, solar energy conversion and electrochemical energy storage. This extensively studied material has an exceptionally high theoretical capacity, more than ten times higher compared to the currently used graphite anodes in Li-ion batteries [1]. However, its main disadvantage is the huge volume expansion upon alloying with lithium, inducing disintegration of the electrode material, loss of electrical contact, poor reversibility and a rapid capacity fade [1]. It has been established that Si applied in a nanostructured or nanoparticulate form displays an enhanced electrochemical stability. Therefore, advanced production technologies have to be implemented in order to obtain cost-effectively the required electrochemically stable Si morphologies.Electrochemical methods offer a simple and low-cost deposition process. Furthermore, the electrodeposition is a versatile technique, allowing control of the morphology by adjusting the experimental conditions and obtaining deposits on different substrate geometries. However, due to the very low reduction potential and high reactivity of silicon precursors, it is impossible to deposit this element in aqueous electrolytes. Therefore, a broad variety of alternative media, including high temperature molten salts (HTMS) [2], conventional organic solvents [3] and ionic liquids (ILs) [4] has been researched.It is established that the initial stages of electrodeposition play an important role for the physical properties, stability and functional behavior of the final deposit. For example, homogeneous distribution of a high number density of small Si crystals would be beneficial for the electrochemical stability of the deposit during lithiation. Therefore, the nucleation and growth process of silicon deserves a special attention. Initial studies on the formation of Si nuclei in HTMS have been recently reported [2]. It was found that the diffusion limited nucleation process in HTMS (i.e. KF-LiF-K2SiF6) is instantaneous, followed by 3D growth [2]. However, due to the high temperatures (T > 800oC) and difficulties to perform the Si deposition economically in these electrolytes the research interest has turned to low temperature molten salts (LTMS).In our study we analyze the initial stages of Si electrodeposition in IL electrolytes by interpreting the chronoamperometric data (fig.1a) with a model accounting nucleation and growth under diffusion limitation, established by Scharifker et al. [5-7], eq.1. Vitreous carbon was selected as an inert substrate for the experiments and 1-butyl-1-methylpyrrolidinium bis (trifluoromethylsulfonyl) imide [BMP][TFSI] containing SiCl4 was chosen as an electrolyte.The current transients were quantitatively interpreted by a non-linear fitting procedure, applying a model equation for three-dimensional nucleation, eq.1. Eq.1 was modified with a term describing the surface charging phenomena, observed at the beginning of the silicon reduction, approximated to an exponential decay after Langmuir adsorption kinetics, eq.2. To prove the validity of this procedure, we extracted the charging currents (fig.1b) and fitted the resulting curves with the non-modified equation for 3D nucleation, where the respective output parameters remained nearly identical. According to eq.(3), the data for P1 should be constant for all fitted curves (i.e. potentials) since this parameter does not depend on other variables. This allows the value of the diffusion coefficient D =1.3×10−8 cm2 s−1 to be calculated. D was additionally confirmed by the analysis of the current maxima [5,6]. The data for the nucleation frequency, A and the number density of active sites, N0 obtained from the best fit are presented in Fig. 1c and show the typically observed linear increase as a function of the overpotential,η [7].Furthermore, we characterized the nucleation process by analyzing the current maxima in dimensionless coordinates (fig. 1d). According to the analysis the initial stages of the Si phase formation display an instantaneous behavior, showing a progressive character with decreasing the applied overpotential. The theoretically evaluated values for N0 correlate well with the crystal number density N, acquired by scanning electron microscopy. The talk will cover the main aspects of the electrochemical nucleation of Si in non-aqueous electrolytes and will provide a discussion on the major challenges towards understanding this process.

Understanding Graphite Impedance: Determining Solid Electrolyte Interphase, Charge Transfer, and Pore Resistance

Electrochemical Impedance Spectroscopy (EIS) is a powerful tool, as it is a non-invasive technique that allows to understand and deconvolute internal resistances of electrochemical systems. However, great caution has to be exercised when choosing a proper equivalent circuit for graphite anodes in Li-ion batteries and assigning resistances to specific mechanisms such as solid-electrolyte-interphase (SEI, at the electrolyte-SEI interface) or charge transfer resistance (at the SEI-graphite interface). The ionic resistance in the electrolyte of the porous electrode is hereby often overlooked or assumed negligibly small. Such assumptions lead to improper equivalent circuits and the desired analysis of a specific resistance is questionable.In this study we present a rigorous impedance analysis where the equivalent circuit assigned to the electrode is validated through measurements in multiple configurations. First, we show how the impedance response of a graphite electrode at standard loading (~3mAh/cm2) changes depending on its state of charge (SOC, see Fig. 1) and assign a preliminary equivalent circuit based on the transmission line model, similar to Refs. 1 and 2. We then validate the initial assumptions by changing the graphite loading - a simple but impactful procedure - as all resistances associated with the active material surface area are expected to directly correlate to the change in loading. Both SEI and charge transfer resistance are expected to increase with decreasing surface area, while the pore resistance that depends on electrode thickness, porosity, and tortuosity is expected to decrease. Eventually we will show a full impedance model for the graphite electrode and show how the SEI resistance is dominant compared to the charge transfer resistance, and that pore resistances are a major contribution to the overall resistance for typical battery graphite electrodes. Lastly, we will show the influence of additives on the SEI resistance in low-loaded graphite electrodes and how they affect the impedance spectra for graphite electrodes with a standard loading.Figure 1 Half-cell impedance of a graphite electrode (~3mAh/cm2) at varying state of charge (SOC) measured via a gold-wire µ-reference electrode before and after three formation cycles in 1M LiPF6 in EC:EMC 3:7. Measured at open circuit voltage with a voltage amplitude of 10 mV between 30 kHz and 0.1 Hz.Acknowledgements: This work was supported by the BMBF (Federal Ministry of Education and Research, Germany) under the auspices of the ExZellTUM II project (grant number 03XP0081) and by the BMWI (Federal Ministry for Economic Affairs and Energy) under the auspices of the SurfaLib project (grant number 03ET6103F).

Battery scientists tackle the lithium dendrite problem

Lithium-ion batteries have revolutionized the way we power our gadgets, and they even nabbed the Nobel Prize in Chemistry this year. But scientists want to make even more powerful versions. Replacing the graphite anodes in Li-ion batteries with lithium-metal ones could double the batteries’ charge capacity. But dendrites—spiky lithium structures that can accumulate on the anode during charging—can short-circuit batteries, posing a serious safety hazard. So scientists are working to master the dendrite problem by investigating how the structures grow and by developing solutions to suppress them, such as building physical barriers or using solid electrolytes to halt dendrite growth. Watch this video to learn more. Subscribe to our YouTube channel to catch all of our chemistry news videos. Read Mitch Jacoby’s story about dendrite research at Pacific Northwest National Laboratory here. Learn more about dendrite-related research at the links below: A solid new approach to sodium batteriesImaging lithium dendrite

Direct Observation of Carboxymethyl Cellulose and Styrene-Butadiene Rubber Binder Distribution in Practical Graphite Anodes for Li-Ion Batteries.

Despite the important role of carboxymethyl cellulose (CMC) and styrene-butadiene rubber (SBR) binders in graphite electrodes for Li-ion batteries, the direct analysis of these binders remains challenging, particularly at very low concentrations as in practical graphite anodes. In this paper, we report the systematic investigation of the physiochemical behavior of the CMC and SBR binders and direct observations of their distributions in practical graphite electrodes. The key to this unprecedented capability is combining the advantages of several analytic techniques, including laser-ablation laser-induced break-down spectroscopy, time of flight secondary ion mass spectrometry, and a surface and interfacial cutting analysis system. By correlating the vertical distribution with the adsorption behaviors of the CMC, our study reveals that the CMC migration toward the surface during the drying process depends on the degree of cross-linked binder-graphite network generation, which is determined by the surface property of graphite and CMC materials. The suggested analytical techniques enable the independent tracing of CMC and SBR, disclosing the different vertical distribution of SBR from that of the CMC binder in our practical graphite anodes. This achievement provides additional opportunity to analyze the correlation between the binder distribution and mechanical properties of the electrodes.

Abnormal electrochemical behavior of rounded graphite

Round-shaped natural graphite is commonly used as anode material for rechargeable lithium-ion batteries. We report atypical electrochemical behavior of round-shaped graphite anodes in Li-ion batteries: an intriguing phenomenon whereby substantial progressive increase in capacity is observed over tens of cycles. To understand the reasons underlying this abnormal behavior, we investigated the surface and bulk structure properties using HRSEM, XRD and Raman spectroscopy. Graphite particles with tense structure undergo exfoliation and fracture due to multiple transformations in intercalation/deintercalation processes. The increased capacity may result from enhanced particle exfoliation, compared with non-rounded graphite, which is accompanied by appearance of graphene sheets and fracture.

Kinetically Determined Phase Transition from Stage II (LiC12) to Stage I (LiC6) in a Graphite Anode for Li-Ion Batteries.

The electrochemical insertion of Li into graphite initiates a series of thermodynamic and kinetic processes. An in-depth understanding of this phenomenon will deepen the knowledge of electrode material design and optimize rechargeable Li batteries. In this context, the phase transition from dense stage II (LiC12) to stage I (LiC6) was comprehensively elucidated in a graphite anode via both experimental characterizations and first-principles calculations. The results indicate that, although the transition from stage II to stage I is thermodynamically allowed, the process is kinetically prohibited because Li ions tend to cluster into stage compounds rather than form a solid solution. Additionally, the phase transitions involve at least three intermediate structures (1T, 2H, and 3R) before reaching the LiC6 (stage I) phase. These findings provide new insights into the electrochemical behavior of graphite and the electrode process kinetics for rechargeable Li batteries.

Synthesis of Carboxymethyl Cellulose Lithium by Weak Acid Treatment and Its Application in High Energy-Density Graphite Anode for Li-Ion Batteries

Carboxymethyl cellulose lithium (CMC-Li) has recently been explored as a promising binder for Li-ion batteries because of enhanced Li+ ion flux. CMC-Li has been generally prepared by CMC acid form (CMC-H) as an intermediate product treated with a strong acid, which considerably causes a polymer degradation. Here, we report a synthesis method of CMC-Li through the use of a weak acid (acetic acid) and its application in a high energy-density graphite anode. CMC-Li synthesized by acetic acid (CMC-Li (A)) exhibits enhanced physicochemical properties including an appropriate viscosity of ∼3000 mPa·s at a shear rate of 10 s–1, good slurry stability, and strong adhesion force of 1.4 gf/mm compared to those of CMC-Li synthesized by hydrochloric acid. The high energy-density graphite anode prepared with CMC-Li (A) shows higher charge/discharge capacities and capacity retentions in various rates of 0.05–2 C than those of the electrode prepared with CMC-Na that might be due to the enhanced Li+ ion flux upon cycling.

Unraveling the role of LiFSI electrolyte in the superior performance of graphite anodes for Li-ion batteries

While lithium bis(fluorosulfonyl) imide (LiFSI) is widely used in current Li-ion batteries (LIBs), the role of LiFSI in the LIB performance remains elusive. We herein elucidate the effects of LiFSI on the electrochemical performance of graphite anodes in comparison with those of lithium hexafluorophosphate (LiPF6). An in-depth electrochemical analysis using graphite/Li half cells and graphite/graphite symmetric cells confirms that LiFSI provides little improvement to the cyclability of the graphite anode at 25 °C, but enables far better performance in cycle and storage tests at 60 °C. The superior thermal stability of the graphite anode in LiFSI electrolyte is attributed to the formation of a thin, inorganic-rich solid electrolyte interphase (SEI) layer as indicated by differential scanning calorimetry (DSC) and X-ray photoelectron spectroscopy (XPS) measurements.

KCl-Modified Graphite as High Performance Anode Material for Lithium-Ion Batteries with Excellent Rate Performance

Electrochemical properties of a graphite negative electrode of lithium-ion batteries have been enhanced by treating raw graphite with alkali molten salt. Here, the KCl-modified graphite is fabricated via a simple and low-cost technique in which graphite particles and potassium chloride are simply mixed with deionized water and sintered. Compared to the raw material, the rate capability of the graphite modified by potassium is enhanced significantly to 269.7 mAh g–1 at 1 C after 200 cycles which effectively solves the main problem to limit the application of the graphite anode for Li-ion batteries in the field of hybrid vehicles. First-principle calculations have been done to analyze the effect of K-ions on the lithium storage capacity of the graphite. This study provides a scientific basis for adopting the K-modified technique to enhance the electrochemical properties of the negative electrode materials.

Cross-linked poly(acrylic acid)-carboxymethyl cellulose and styrene-butadiene rubber as an efficient binder system and its physicochemical effects on a high energy density graphite anode for Li-ion batteries

An increase in graphite anode thickness is an effective way to achieve a high-energy density lithium ion battery by designing a compact cell with a high mass loading and density. However, this may cause critical problems in Li+ diffusivity due to drastic increases in length and tortuosity of the ionic migration path. A mixture of cross-linked poly (acrylic acid) (PAA)–carboxymethyl cellulose hydrogels (CMC) was developed using styrene-butadiene rubber (SBR) as an efficient binder system to increase Li+ diffusivity and adhesion strength. Electrolyte permeability and adhesion strength of the electrode were enhanced by 2.5 and 2 times, respectively, compared to a conventional CMC–SBR system, which led to excellent electrochemical properties due to enhanced affinity with electrolytic and interpolymeric properties. Particularly, capacity retention was improved from 81 to 91% at a high 1C rate.

Enhanced electrochemical properties of a natural graphite anode using a promising crosslinked ionomer binder in Li-ion batteries

A crosslinked ionomer binder was prepared and used in graphite anodes for Li-ion batteries. These binder-based anodes exhibit enhanced electrochemical performance due to the formation of hydrogen bonds and the release of conductive Li+.