Graphite For Lithium-ion Batteries Research Articles

Tailored N-doped porous carbon nanocomposites through MOF self-assembling for Li/Na ion batteries

Presently, carbon-based anodes for energy storage, such as graphite for lithium-ion batteries (LlBs) and hard carbon for sodium-ion batteries (SlBs), have low capacity and poor rate properties. However, the capacity and rate capability of these anodes can be improved via morphological control, doping and using nanostructures. In this report, a series of self-assembled N-doped porous carbon nanocomposites (NPCNs) were prepared via pyrolysis of metal-organic frameworks (MOFs)-ZIF-8/carbon nanocomposites grown on various carbon frameworks (1D CNT and/or 2D rGO). It was found that the NPC-CNT@G electrode significantly exhibits superior performance for lithium/sodium storage among the other NPCNs. NPC-CNT@G electrode delivers high initial reversible capacities (986 mAh g−1 at 0.1 A g−1 in LIBs; 315 mAh g−1 at 0.05 A g−1 in SIBs), excellent rate properties (443 mAh g−1 at 5 A g−1 in LIBs; 174 mAh g−1 at 1 A g−1 in SIBs) and durable cycle life (99% capacity retention after 2000 cycles at 5 A g−1 in LIBs, 80% capacity retention after 300 cycle at 1 A g−1 in SIBs). This ‘carbon-on-carbon’ approach described herein can be applied to make other interesting structures for high performance battery materials.

Three-dimensional lithium mapping of graphite anode using laser-induced breakdown spectroscopy

Herein, a method has been described to rapidly obtain a semi-quantitative three-dimensional lithium distribution of a lithium-ion battery graphite anode using laser-induced breakdown spectroscopy (LIBS) measurements. LIBS measurements of the graphite anodes with a diameter of 10 mm were performed in an argon atmosphere of 1000 Pa until the depth of 150 μm using the Li emission line of 610.4 nm. The emission intensity was measured for the pitch of 500 μm in plane. Homogeneous and inhomogeneous lithium distributions were observed in the anodes after both charge and charge-discharge processes. The inhomogeneous lithium distribution after the charge process was attributed to the preferentially reacted area in the anode, while that after the charge-discharge process was likely related to the low desolvation reaction rate of lithium ions at the solid electrolyte interphase. These inhomogeneous lithium distributions were consistent with the results from the charge-discharge curves and the lithium ion transfer mechanism. Thus, it was proven that it is possible to acquire a three-dimensional lithium distribution of a graphite anode of a lithium-ion battery by LIBS measurements.

Stress and Electrochemical Analyses of LiNixMnyCo1-x-YO2(NMC)/Graphite Lithium-Ion Battery with Real 3D Microstructures

LiNixMnyCo1-x-yO2 (NMC) is one of promising cathode materials in lithium-ion batteries (LIBs) for power tools and electric vehicles due to their higher specific capacity and energy density. However, it is observed that repeated cycling leads to micro-scale fracture along the grain boundaries in the secondary particles of NMC, and it could eventually cause low cycling stability and irreversible capacity retention. The aim of this study is to investigate the correlation of mechanical stresses and electrochemical battery degradation. Specifically, mechanical stress and strain caused by lithium intercalation and diffusion in both cathode and anode were calculated and crack initiation in particles was predicted. Furthermore, a modified Butler-Volmer equation, which accounts for the influence of mechanical stress on electrochemical reactions, is considered. In this study, we adopted real 3D microstructures of both anode (graphite) and cathode (NMC) based on over 100 images from a synchrotron radiation X-ray tomographic microscopy (SRXTM). The 3D finite element model (FEM) was developed by COMOSL Multiphysics 5.2a and followed by coupled electrochemical and mechanical analyses of a whole cell, including cathode, anode, and electrolyte. As the first step, the electrochemical model based on the microstructure was compared with results from the previous study with a simple geometry (e.g. spherical particles). Specifically, the real 3D microstructure model, concentration, polarization, over potential, and von-Mises stress distribution across both electrodes (e.g. graphite and NMC) were computed under different C-rates. We observed that effects of phase transformation of NMC on mechanical stress was significant during discharging processes. Moreover, the model revealed that at particle connections where exhibit complicated geometries and the bulged regimes of smaller curvature showed higher stress and were vulnerable to particle cracking. Finally, it is confirmed that mechanical stress plays important role in electrochemical performance. This computational model can make a solid foundation to understand the relationship between mechanical stress and electrochemical performance in NMC/graphite battery chemistry. It might be helpful to understand key factors in the deterioration of Lithium-ion batteries with higher C-rates and longer cycle life.

High-Capacity Silicon-Carbon Anodes Enabled By Dispersing Nano Silicon Onto Natural Graphite for Lithium Ion Batteries

Research on lithium ion batteries has attracted massive attention due to the development of electrical vehicles and portable devices as well as grid energy storage. Pursuit for higher capacity and better cycle life become crucial issue for lithium ion battery anode material. Among all the anode materials, silicon is one of the most promising candidate due to its high theoretical specific capacity (around 3579mAh/g). Graphite, which is the stae-of-the-art commercialized anode material, is known for its stable performance and high electrical conductivity. In this work, a high-capacity composite anode suitable for large-scale production has been synthesized and characterized by taking advantage of both silicon’s high capacity and graphite’s good stability. In this approach, maintaining good dispersion of nano-Si is a critical requirement, and the effects of different dispersants and polymeric binders, which also serve as carbon precursors, have been investigated. Silicon/carbon/graphite composites exhibit high capacity and better cycle life than simple mixing of silicon and graphite powder have been achieved. Figure 1

Recent progress on Ge oxide anode materials for lithium-ion batteries

In recent years, germanium oxides have attracted increasing attention as a new type of anode material to replace graphite for lithium-ion batteries because of their high capacity, appropriate voltage potential, and good safety properties. In this review, recent important advances for Ge oxide anode materials are summarized. The limitations of Ge oxide anode materials are discussed, and potential research directions are presented.

Derivation and tuning of a solvable and compact differential–algebraic equations model for LiFePO4–graphite Li–ion batteries

This paper presents a procedure for deriving and tuning a solvable and compact differential–algebraic equation (DAE) model for the LiFePO4–graphite lithium–ion battery cell. A reduced order model can drastically decrease the simulation time with a minimal loss of the prediction accuracy for the lithium–ion battery. This paper proposes a method based on a linear DAE theory for choosing a Galerkin formulation that will produce a solvable ROM from the original higher-order model of the lithium–ion battery cell. Moreover, a systematic tuning of the model parameters using the sensitivity study and the genetic algorithm is demonstrated by exploiting the computational efficiency of the simplified model. When coupled with the model for describing the hysteresis of the battery cell, the tuned model, consisting of 26 DAEs, shows a good agreement with the experimental data from a LiFePO4–graphite battery cell at rates up to 4 C.

Evolving affinity between Coulombic reversibility and hysteretic phase transformations in nano-structured silicon-based lithium-ion batteries

Nano-structured silicon is an attractive alternative anode material to conventional graphite in lithium-ion batteries. However, the anode designs with higher silicon concentrations remain to be commercialized despite recent remarkable progress. One of the most critical issues is the fundamental understanding of the lithium–silicon Coulombic efficiency. Particularly, this is the key to resolve subtle yet accumulatively significant alterations of Coulombic efficiency by various paths of lithium–silicon processes over cycles. Here, we provide quantitative and qualitative insight into how the irreversible behaviors are altered by the processes under amorphous volume changes and hysteretic amorphous–crystalline phase transformations. Repeated latter transformations over cycles, typically featured as a degradation factor, can govern the reversibility behaviors, improving the irreversibility and eventually minimizing cumulative irreversible lithium consumption. This is clearly different from repeated amorphous volume changes with different lithiation depths. The mechanism behind the correlations is elucidated by electrochemical and structural probing.

Sn Wears Super Skin: A New Design for Long Cycling Batteries.

Searching for new anode alternatives in lieu of graphite for lithium-ion batteries that can deliver better electrochemical performance to meet the emerging energy/power demands in electric vehicles becomes particularly challenging. We report a rationally designed hybrid composite as anode in LIB that exhibits a greatly improved gravimetric capacity of 727 mAh/g with a Coulombic efficiency of >99.8% after 3000 cycles at 1.0 C. A capacity of 662 mAh/g at a high rate of 5.0 C was obtained after impressively long 10 000 cycles. From the 50th to 10 000th cycle under 5.0 C, the capacity retention is >97% with a negligible decay of <0.00026% per cycle. The excellence in electrochemistry is attributed to the efficient stress relax, accommodable space, lack of agglomeration, and solid-electrolyte interphase consuming Li+ of a delicate composite configuration that is composed of a Sn kernel wearing adjustable TiO2 "skin".

Understanding the lithiation/delithiation process in SnP2O7 anode material for lithium-ion batteries

Anode materials based on silicon or tin, with distinctively higher theoretical specific capacities of 700mAh/g compared to the commonly used graphite in Lithium-ion batteries exhibiting limited capacity of 370mAh/g, are presently in the focus of intensive research studies as new anode materials. In the present work, coated and non-coated tin pyrophosphate materials were successively prepared and their electrochemical performance were compared. The use of carbon coating generally results in an improved cycling stability and a decrease in particles size. The X-ray diffraction patterns of the two samples are similar as indication of an amorphous character of the residual carbon. The scanning electron microscopy images of the both samples show a small particle size (nanometric) and the layers of the graphite are clearly observed. Raman spectroscopy was carried out to identify the D and G bands related to the graphite. X-ray photoelectron spectroscopy (XPS) was used to follow the Sn bonding/valence changes during discharge-charge process and also to identify the SEI layer which is formed at the last step of the discharge process.

Graphite for Energy Applications

INTRODUCTION Graphite is widely used as active material for anodes in lithium ion batteries (LIB), and by selection and processing, graphitic materials can be optimized for energy and power applications, respectively. We have looked at contributions of particle size and materials processing to capacity and discharge rate capabilities of natural and artificial graphite. This work continues research reported earlier4. METHODOLOGY First, we compared coated spherical natural graphite of regular and fine size at discharge rates from C/20 to 30C. Our interest was to assess diffusion as limiting factor1,2, We then looked at two kinds of artificial graphite of similar size which were derived from different processes. The investigation was done on half cells (G/EC-DMC-LiPF6/Li). The electrode composition was the same throughout: 91 % graphite, 7% binder PVDF 9305 from Kureha and 2 % carbon black Super P. Thin electrodes (≈30mm, 1 g/cc) were prepared for high and thick electrodes (≈60mm, 1.5 g/cc) for lower rates. Due to its mechanical properties, the desired electrode density of 1.5 g/cc proved impossible for artificial graphite #2. The two first cycles with the thick electrodes were done at constant current (CC) at C/20, and then the intercalation was done at C/5 followed by a step at constant voltage (CV) to reach full intercalation. For thin electrodes the intercalation was at done at C/5 with CV step throughout. RESULTS AND DISCUSSION Physical and chemical characterization: Table (1) presents physical and chemical characteristics of the materials. Figure (1) exhibits differences in texture with artificial graphite #2 having higher rugosity. Test Analysis Natural #1 Natural #2 Artificial #1 Artificial #2 Type of Graphite Coated NG Coated NG Synthetic Synthetic Ash (%) 0.02 0.024 < 0.01 < 0.01 True Density (g/cc) 2.18 2.2 2.25 2.13 Tap Density (g/cc) 1.05 1.14 1.21 0.34 Surface Area (m2/g) 2.06 1.09 1.15 6.45 Particle size (Microtrac, mm) D10 5.51 10.62 5.38 4.22 D50 8.34 16.9 13.22 10.46 D90 13.47 27.09 28.59 20.71 Table (1): Characteristics of two kinds each of coated natural and of artificial graphite Figure (1): SEM of artificial graphite #1 (left) and #2 (right). Electrochemical Characterization: Artificial Graphite Artificial graphite #1 and #2 have comparable discharge capacity of 330 mAh/g at C rate, but differ significantly at higher discharge rates (table 2). Although at same level of graphitization3 (86-87%), the differences in processing show in surface area, tap density (table 1) and surface rugosity (figure 1). The evolution of the discharge capacities as a function of discharge rate are presented in figure (2). Figure (2): Charge and discharge of artificial graphite #1 and #2 in half-cell with thin electrode. Artificial #1 Artificial #2 Rates (C) 1/2 discharge voltage (V) Capacity (mAh/g at 2 V) Capacity (% of capacity at C) 1/2 discharge voltage (V) Capacity (mAh/g at 2 Volts) Capacity (% of capacity at C) 1 0.184 332 100 0.170 331 100 5 0.362 330 99 0.305 331 100 10 0.579 323 97 0.467 329 99 20 0.946 282 85 0.796 318 96 30 1.135 228 69 0.960 289 87 Table (2): Gravimetric capacity and voltage vs. Li/Li+ at half discharge for artificial graphite #1 and #2 Spherical coated natural graphite Electrochemical results from spherical graphite #1 and #2 at high rate discharge are presented in figure (3) showing similar behaviors up to 10C discharge. At 20C and 30C we see a higher impedance for the finer spherical graphite #1. Figure 3: High rate discharges of coarse (right) and fine (left) spherical graphite CONCLUSION We have seen that differences in processing are reflected both in physical characteristics and in electrochemical behavior. The nature of graphite affects both the intrinsic capacity and the ability to sustain high discharge rates. The discharge capacity at 30C of artificial graphite reached 280 mAh/g, 85% of the capacity at C rate. The lower packing density could be an issue for energy applications, but may not be acceptable for power applications. REFERENCES 1) Persson, K. et al. The journal of physical chemistry letters 1(8) (2010) 1176-1180. 2) Yu, P. et al. Journal of The Electrochemical Society, 146(1) (1999)8-14. 3) J. Maire, J. Mering. Chemistry and physics of carbon 6 (1970): 125-190. 4) Henry FX. et al. (2006) “New developments in the advanced graphite for lithium-ion batteries”. In: NATO Science Series II: Mathematics, Physics and Chemistry, vol 229. Springer, Dordrecht Figure 1

Lithium loss in the solid electrolyte interphase: Lithium quantification of aged lithium ion battery graphite electrodes by means of laser ablation inductively coupled plasma mass spectrometry and inductively coupled plasma optical emission spectroscopy

Abstract In this work we present a new method using LA-ICP-MS to quantitatively determine the lithium content in aged graphite electrodes of a lithium ion battery (LIB) by performing total depth profiling. Matrix matched solid external standards are prepared using a solid doping approach to avoid elemental fractionation effects during the measurement. The results are compared and matched to the established ICP-OES technique for bulk quantification after performing a microwave assisted acid digestion. The method is applied to aged graphite electrodes in order to determine the lithium immobilization (= “Li loss”) in the solid electrolyte interphase after the first cycle of formation. For this, different samples including a reference sample are created to obtain varying thicknesses of the SEI covering the electrode particles. By applying defined charging voltages, an initial lithiation process is performed to obtain specific graphite intercalation compounds (GICs, with target stoichiometries of LiC30, LiC18, LiC12 and LiC6). Afterwards, the graphite electrode is completely discharged to obtain samples without mobile, thus active lithium in its lattice. Taking the amount of lithium into account which originates from the residues of the LiPF6 (dissolved in the carbon components containing electrolyte), it is possible to subtract the amount of lithium in the SEI.

LBM prediction of effective thermal conductivity of lithium-ion battery graphite anode

Study of thermal characteristics of a lithium-ion battery plays a vital role in determining and enhancing the performance and safety of the battery. This paper predicts the effective thermal conductivity of a graphite anode having microstructure reconstructed by an ellipsoid based simulated annealing method. A lattice-Boltzmann (LB) model is established for simulating the thermal diffusion process in the computer-generated 3D microstructure of graphite anode. The effective thermal conductivities derived from LB simulation results indicate evident anisotropic feature of the graphite anode. The numerical results show that the particle size does have some effects on the effective thermal conductivity, but the effects are generally not significant. The real graphite may have particles with particle size following a certain statistical distribution, very probably the normal distribution, which is found to weaken the anisotropy of the electrode. Comparing the numerical data with the theoretical predictions by effective media theory (EMT) suggests that the suitable value of the empirical correction factor (f) for the effective thermal conductivity of graphite anode in the electrode through-plane direction is about 6.0 and in the other direction about 4.5.

The effects of humidity on the self-discharge properties of Li(Ni1/3Co1/3Mn1/3)O2/graphite and LiCoO2/graphite lithium-ion batteries during storage

Exposure of battery tabs to humid air triggers self-discharge of batteries because of the induction effect of polar water molecules.

LBM prediction of effective electric and species transport properties of lithium-ion battery graphite anode

Numerical models play a vital role in the developing and performance optimization of lithium-ion batteries. The key factor to the prediction accuracy of macro-scale models is the specification of effective transport properties. This study, based on the anisotropic microstructure of graphite anode reconstructed by an ellipsoid-based simulated annealing method (SAM), established a mesoscopic model of diffusion process to predict the effective electric and species transport properties of lithium-ion battery graphite anode via lattice-Boltzmann (LB) method. The effect of particle size on the transport properties of graphite anode was discussed in detail. In the electrode through-plane direction, if the ellipsoidal particles are thinner and flatter, both the effective electric and species transport properties decrease; in the other two directions, the effective electronic charge transport properties barely change with the change of particle size while the effective species transport properties increase along with the increase of the size of particles. In addition, to get a more accurate replica of the real graphite anode, we assumed the sizes of solid particles follow a normal distribution and reconstructed the microstructure of electrode. The LB calculation results reveal that the normal distribution of particle size increases the electronic charge conductivity in the electrode through-plane direction and decreases in the other two directions, compared to the electrode of constant-sized particles; the effective species diffusivities (or ionic charge conductivities) in electrode through-plane direction for different microstructures are closer.

Li4Ti5O12 on Graphene for Lithium Ion Batteries with High Rate Performance

Spinel Li4Ti5O12 has become an alternative anode material for graphite in lithium ion batteries for large scale electrical energy storage in terms of solving safety issues and improving battery life-time. In order to form a much more bonding between the LTO and current collector, which is beneficial for stable cycling and high rate performance, we introduce a wrinkled graphene between the electrode layer and aluminum current collector to improve the rate performance and cycling performance of Li4Ti5O12 electrode. The introduced wrinkled graphene layer provides larger contact area, lower contact resistance, stronger adhesion and better electrode stability of Li4Ti5O12 electrode. This excellent electrochemical performance and very simple preparation method makes LTO@G-Al electrodes suitable for high-rate lithium ion batteries.

Cycle-life and degradation mechanism of LiFePO4-based lithium-ion batteries at room and elevated temperatures

Cycle-life tests of commercial 22650-type olivine-type lithium iron phosphate (LiFePO4)/graphite lithium-ion batteries were performed at room and elevated temperatures. A number of non-destructive electrochemical techniques, i.e., capacity recovery using a small current density, electrochemical impedance spectroscopy, and differential voltage and differential capacity analyses, were performed to deduce the degradation mechanism of these batteries. To further characterize their internal materials, we disassembled the batteries, and material analyses were performed. All results indicated that loss in active lithium was the main reason for battery aging, and the cells showed diverse recession of active materials at different temperatures. In addition, high discharge rate and growing impedance lead to a capacity fall down at 25 °C at approximately 300–500 cycles.

Li4Ti5O12 on Graphene for High Rate Lithium Ion Batteries

Spinel Li4Ti5O12 has been considered as a promising anode material to substitute graphite in lithium ion batteries (LIBs) for large scale electrical energy storage due to its high safety and long cycling stability. However, the drawback of its poor rate performance still hinders it from wide practical application. In this study, with the aim to solve this issue, we have designed a wrinkled graphene layer between the active spinel Li4Ti5O12 and aluminum current collectors by a blade coating method. This introduced wrinkled graphene layer provides larger contact area, lower contact resistance, stronger adhesion and better electrode stability of Li4Ti5O12 electrode. As a result, the rate performance of spinel Li4Ti5O12 has been significantly improved, without compromising its other properties.

Battery Fundamentals

The use of batteries as a portable power source has gained tremendous importance for powering mobile devices and for enabling greener and less energy-intensive transportation. Whether the demand is for a smart phone, laptop, tablet or the next wearable, the consumer demands a thinner and lighter battery while maintaining or increasing battery life at the lowest possible cost. This demand has caused many companies to increasingly consider the battery and its performance when designing their next device. While, in the past, it may have been acceptable to take an "off-the-shelf" battery that meets specifications and "plug it in", many companies are starting to implement "smart" designs that take into account specific battery chemistries and characteristics to improve overall performance. This requires a knowledge of how a battery works. With this in mind, this article gives an explanation of battery fundamentals, why batteries degrade over their life, what limits their performance, the trade-offs between high power and high energy, and current efforts to build a better battery. While many of the overall properties are similar for many battery chemistries, here we will focus on the lithium-ion battery, specifically the cobaltdioxide/ graphite lithium-ion battery, one of the most popular battery chemistries for mobile devices.

Role of Negative Electrode Porosity in Long-Term Aging of NMC//Graphite Li-Ion Batteries

Electrochemical energy storage associated to renewable energy sources requires devices with very long lifespan. In this work we simulated the long-term aging of Li(Ni2/5Mn2/5Co1/5)O2 (NMC)//graphite lithium-ion batteries by standardized accelerated aging tests in tough storage and cycling conditions. Aging mechanisms taking place at the graphite electrode/electrolyte interface were characterized by X-ray Photoelectron Spectroscopy (XPS) and Electrochemical Impedance Spectroscopy (EIS). Besides usual aging processes commonly admitted, we highlight the role of electrode porosity. A model is proposed to describe the electrical behavior of the electrodes taking into account the influence of electrochemically blocking pores. The blocking effect of pores is believed to be caused by confinement of Li2CO3 and/or gaseous CO2 inside the narrow pores, which do not allow lithium ions to diffuse and insert into graphite. In these pores, lithium can only participate to SEI growth which decreases the capacity by lithium consumption.

Numerical Reconstruction and Characterization Analysis of Microstructure of Lithium-ion Battery Graphite Anode

The real graphite anode of lithium-ion battery is of evident non-isotropic characteristic due to its cascading graphite flakes. An ellipsoid particle-based simulated annealing method is developed to numerically reconstruct the three-dimensional microstructure of graphite anode. The reconstructed anode is a composite of three clearly distinguished phases: pore (or electrolyte), graphite and solid additive, well representing the non-isotropic characteristic of real graphite anode. Characterization analysis of reconstructed electrodes gives information such as the connectivity, the specific surface area of solid or pore phase, and the pore size distribution. The results show that the size of graphite ellipsoids has important effects on the characteristics of electrode: 1) larger size graphite ellipsoids result in larger mean pore size and smaller specific surface area in the reconstructed electrode; 2) changing the polar radius of graphite ellipsoid particles has more pronounced influence on the characteristics of electrode than has changing its equatorial radius.