Graphite For Lithium-ion Batteries Research Articles

N-substituted phenothiazine derivatives: how the stability of the neutral and radical cation forms affects overcharge performance in lithium-ion batteries.

Phenothiazine and five N-substituted derivatives were evaluated as electrolyte additives for overcharge protection in LiFePO4 /synthetic graphite lithium-ion batteries. We report on the stability and reactivity of both the neutral and radical-cation forms of these six compounds. While three of the compounds show extensive overcharge protection, the remaining three last for only one to a few cycles. UV/Vis studies of redox shuttle stability in the radical cation form are consistent with the overcharge performance: redox shuttles with spectra that show little change over time exhibit extensive overcharge performance, whereas those with changing spectra have limited overcharge protection. In one case, we determined that a C-N bond cleaves upon oxidation, forming the phenothiazine radical cation and leading to premature overcharge protection failure; in another case, poor solubility appears to limit protection.

The Effects of Internal Pressure Evolution on the Aging of Commercial Li-Ion Cells

Utilizing a custom developed chamber, the internal pressure evolution of a commercial 2.6 Ah, LiFePO4//Graphite lithium-ion battery (LIB) was studied throughout elevated rate lifecycle testing. Verification tests confirmed the custom test chamber and puncture procedure resulted in minimal changes to the LIB performance. Capacity measurements and analysis confirmed the battery's accessible capacity faded 20% after 500 cycles as a result of the increase in DC Equivalent Series Resistance (ESR). Electrochemical Impedance Spectroscopy (EIS) measurements and modeling analysis also established that minimal material change occurred within the LIB during lifecycle testing and the decreased capacity is believed to primarily result from the increase in battery ESR. Analysis of the battery's internal pressure evolution during the baseline discharges yielded information elucidating apparent electrode volume changes during the 1 C discharge. A strong statistically significant correlation was identified between the 1C molar density delta and the battery capacity fade. Differential capacity analysis was used to examine the correlation of the behavior with the de-intercalation process. The pressure data and molar density analysis indicated a reduction in the amount of lithium de-intercalated during discharge as a result of the rise in ESR and thus a measurable shift in the volume as the LIB ages.

Vertical Distribution of Overpotentials and Irreversible Charge Losses in Lithium Ion Battery Electrodes

Porous lithium ion battery electrodes are characterized using a vertical distribution of cross-currents. In an appropriate simplification, this distribution can be described by a transmission line model (TLM) consisting of infinitely thin electrode layers. To investigate the vertical distribution of currents, overpotentials, and irreversible charge losses in a porous graphite electrode in situ, a multi-layered working electrode (MWE) was developed as the experimental analogue of a TLM. In this MWE, each layer is in ionic contact but electrically insulated from the other layers by a porous separator. It was found that the negative graphite electrodes get lithiated and delithiated stage-by-stage and layer-by-layer. Several mass-transport- as well as non-mass-transport-limited processes could be identified. Local current densities can reach double the average, especially on the outermost layer at the beginning of each intercalation stage. Furthermore, graphite particles close to the counter electrode act as "electrochemical sieve" reducing the impurities present in the electrolyte such as water.

Electrochemical Performance of Pulse Electrodeposited Sn-Ni/MWCNT Composite Anode for Lithium-Ion Batteries

Grain size, and MWCNT content in the electrolyte of nanocrystalline Sn Ni/MWCNT composite deposits produced from a solution containing nickel chloride and tin chloride at constant electrodeposition conditions (pulse on-time Ton at 2.5 ms and pulse o -time Toff at 7.5 ms) but with di erent peak current densities were investigated. The structure of the electroplated thin lm Sn Ni/MWCNTs composite electrode is investigated by X-ray di raction, scanning electron microscopy. Galvanostatic charge/discharge measurements were performed in the assembled CR2016 cells designed by using anode composite material produced by pulse electro co-deposition. These cells discharge capacities were cyclically tested by a battery tester at a constant current in voltage range between 0.02 and 1.5 V. The electrochemical results showed that the increase in the discharge capacity of the assembled cell was produced when compared with conventionally used graphite lithium-ion batteries.

3,7-Bis(trifluoromethyl)-N-ethylphenothiazine: a redox shuttle with extensive overcharge protection in lithium-ion batteries

Constant overcharging of LiFePO4/synthetic graphite lithium-ion batteries in which the electrolyte additives 1,4-di-tert-butyl-2,5-dimethoxybenzene (DBB), N-ethylphenothiazine (EPT), and 3,7-bis(trifluoromethyl)-N-ethylphenothiazine (BCF3EPT) limit battery voltage.

Interface Investigations of a Commercial Lithium Ion Battery Graphite Anode Material by Sputter Depth Profile X-ray Photoelectron Spectroscopy

Here we provide a detailed X-ray photoelectron spectroscopy (XPS) study of the electrode/electrolyte interface of a graphite anode from commercial NMC/graphite cells by intense sputter depth profiling using a polyatomic ion gun. The uniqueness of this method lies in the approach using 13-step sputter depth profiling (SDP) to obtain a detailed model of the film structure, which forms at the electrode/electrolyte interface often noted as the solid electrolyte interphase (SEI). In addition to the 13-step SDP, several reference experiments of the untreated anode before formation with and without electrolyte were carried out to support the interpretation. Within this work, it is shown that through charging effects during X-ray beam exposure chemical components cannot be determined by the binding energy (BE) values only, and in addition, that quantification by sputter rates is complicated for composite electrodes. A rough estimation of the SEI thickness was carried out by using the LiF and graphite signals as internal references.

Economical Synthesis and Promotion of the Electrochemical Performance of Silicon Nanowires as Anode Material in Li-Ion Batteries

Silicon is considered as one of the most promising anodes alternative, with a low voltage and a high theoretical specific capacity of ~4200 mAh/g, for graphite in lithium-ion batteries. However, the large volume change and resulting interfacial changes of the silicon during cycling cause unsatisfactory cycle performance and hinder its commercialization. In this study, electrochemical performance and interfacial properties of silicon nanowires (SiNWs) which are prepared by the Cu-catalyzed chemical vapor deposition method, with 1 M LiPF6/EC + DMC (1:1 v/v) containing 2 wt % or no vinylene carbonate (VC) electrolyte, are investigated by using different electrochemical and spectroscopic techniques, i.e., cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), scanning electron microscopy (SEM), and X-ray photoelectron spectroscopy (XPS) techniques. It is shown that the addition of VC has greatly enhanced the cycling performance and rate capability of SiNWs and should have an impact on the wide utilization of silicon anode materials in Li-ion batteries.

Capacity characteristics of carbon anodic materials based on Russian natural graphite for lithium-ion batteries

The effect of structural and surface properties of carbon anodic materials based on natural graphite of the Taiginka Deposit obtained using different technologies on the capacity characteristics of the negative electrode of a lithium-ion battery is studied. It is shown that the key factors determining the value of irreversible capacity of the negative electrode in the first cycle are the value of graphite specific surface area, the state of the surface, in particular, the content of disordered carbon, and functional groups on the surface of graphite particles, and also the composition of the active electrode layer. A change in the specific surface area value and content of functional groups is due to the efficiency of the pyrocarbon coating in the case of samples subjected to milling on vibration and cavitation mills, and also of finely dispersed samples obtained by milling on a jet mill. The observed decrease in the specific surface irreversible capacity at an increase in the specific surface area of carbon is apparently caused by inhomogeneity of the latter and nonparticipation of its microporous part in formation of the solid-phase surface passivating film. The minimum irreversible specific capacity of electrodes of the studied natural graphite for the optimum electrode material composition was about 20 mA h/g or 6%. The specific surface capacity changed as dependent on the value and state of the graphite sample surface more than threefold (from 2.5 to 7.7 mA h/m2) and by an order of magnitude at an additional change in the electrode composition (from 2.5 to 20.3 mA h/m2).

Lithium Ion Battery Graphite Solid Electrolyte Interphase Revealed by Microscopy and Spectroscopy

The surface reactions of electrolytes with the graphitic anode of lithium ion batteries have been investigated. The investigation utilizes two novel techniques, which are enabled by the use of binder-free graphite anodes. The first method, transmission electron microscopy (TEM) with energy dispersive X-ray spectroscopy, allows straightforward analysis of the graphite solid electrolyte interphase (SEI). The second method utilizes multi-nuclear magnetic resonance (NMR) spectroscopy of D2O extracts from the cycled anodes. The TEM and NMR data are complemented by XPS and FTIR data, which are routinely used for SEI studies. Cells were cycled with LiPF6 and ethylene carbonate (EC), ethyl methyl carbonate (EMC), and EC/EMC blends. This unique combination of techniques establishes that for EC/LiPF6 electrolytes, the graphite SEI is ∼50 nm thick after the first full lithiation cycle, and predominantly contains lithium ethylene dicarbonate (LEDC) and LiF. In cells containing EMC/LiPF6 electrolytes, the graphite SEI is nonuniform, ∼10–20 nm thick, and contains lithium ethyl carbonate (LEC), lithium methyl carbonate (LMC), and LiF. In cells containing EC/EMC/LiPF6 electrolytes, the graphite SEI is ∼50 nm thick, and predominantly contains LEDC, LMC, and LiF.

Decorated carbon nanotubes by silicon deposition in fluidized bed for Li-ion battery anodes

Multi-walled carbon nanotubes Graphistrength® were decorated with silicon by Fluidized Bed Chemical Vapor Deposition. The ability to fluidize of these nanotubes forming ball-shaped jumbles of several hundreds of microns in diameter and that of the final CNT-Si balls was first studied. These balls reveal to fluidize with characteristics of Geldart's group A particles, i.e. without bubbles and with high bed expansion. Coating experiments from silane SiH4 were performed at 500°C in the 30–60wt.% range of silicon deposited. SEM and TEM imaging reveals that the nanotubes are coated by silicon nanoparticles uniformly distributed from the periphery to the center of the balls for the whole conditions tested. On-line acquisition of key process parameters evolution shows that the material remains fluidizable, even for large proportions of silicon deposited. The Sauter diameter and the tapped, untapped and skeleton densities of balls increase with the percentage of silicon deposited, whereas their specific surface area decreases due to the progressive filling of the pores by the deposit. This composite material is a promising candidate as anode to replace graphite in lithium-ion batteries.

Effects of Polyacrylic Acid on the Suppression of Fe Deposition on Graphite for Lithium-Ion Batteries

In order to suppress the capacity fading of graphite negative-electrodes that is caused by the dissolution of LiFePO4 positive-electrode materials, the effects of the glue-like binder (polyacrylic acid) on the suppression of Fe deposition on the graphite electrodes were examined. The electrochemical properties of graphite electrodes using PAA as the binder were investigated by cycling test, cyclic voltammetry and electrochemical impedance spectroscopy in three kinds of different Fe content electrolyte solution. The graphite electrodes using PAA as the binder exhibit better cycle performance than those using PVDF as the binder in all electrolyte solution, and the reduction peaks occur at 1.8 V, which is corresponding to the Fe deposition process on the graphite electrodes, are remarkably weakened by using PAA as the binder. The growth rate of Rcell and Rct for graphite electrode after cycled in Fe-containing electrolyte are also significantly reduced. The SEM and XPS analysis show that the PAA binder can modify the surface film and block the electroreduction of iron ions on the graphite electrode.

Effects of Electrolyte Additives on the Suppression of Mn Deposition on Edge Plane Graphite for Lithium-Ion Batteries

To suppress the degradation of graphite negative-electrodes that is caused by the dissolution of Mn-containing positive-electrode materials, the effects of additives that should strongly interact with Mn ion in electrolyte solution were examined. The edge plane of highly oriented pyrolytic graphite (HOPG) was used as a model graphite electrode, and its electrochemical properties were investigated by cyclic voltammetry and electrochemical impedance spectroscopy in Mn-ion-containing electrolyte solution. 100 ppm Mn ion dissolved in the electrolyte solution caused a significant decrease in redox currents due to the intercalation and de-intercalation reactions of Li+ at an edge plane HOPG electrode, and a remarkable increase in interfacial resistance consisting of surface film resistance and charge transfer resistance due to Li+ transfer at an interface between the electrode and electrolyte solution. This performance deterioration could be effectively suppressed by the addition of a certain type of cyclic ether to the electrolyte solution. The influence of the additive concentration on interfacial resistance was also investigated.

Additives of Graphene Nanosheets on the Anode Performance of Spherical Natural Graphite for Lithium-Ion Batteries

Additives of Graphene Nanosheets on the Anode Performance of Spherical Natural Graphite for Lithium-Ion Batteries

Facile fabrication of reticular polypyrrole–silicon core–shell nanofibers for high performance lithium storage

Silicon is the most promising anode material to replace graphite in lithium ion batteries due to its high theoretical capacity of 4200 mAh g−1. However, the enormous volume expansion of bulk Si during the lithiation process results in severe electrode degradation and capacity decay. Extensive research effort has been devoted to fabricating nanostructured Si-based materials to improve the capacity cycling stability. Herein, a facile two-step approach is developed for the fabrication of novel three-dimensional (3D) nanoarchitectures composed of polypyrrole–silicon (PPy–Si) core–shell nanofibers. Electropolymerized PPy nanofibers are utilized as the flexible substrate for the deposition of Si thin films via a chemical vapor deposition (CVD) procedure. In this well-designed configuration, the PPy nanofibers are favorable for facile charge delivery and gathering, while the porosity of the electrode can efficiently cushion the volume expansion of Si. The electrode delivers a high reversible capacity above 2800 mAh g−1 with appealing cycling stability (∼91% capacity retained after 100 cycles). The rate capability of the electrode is also remarkable with a high capacity and stability. It is revealed that the reticular nanofibers’ morphology is well preserved after repeated lithium insertion and extraction, which certainly indicates the superiority of our electrode design. This fabrication approach can also be extended to other electrodes for electrochemical energy conversion and storage.

High-capacity potato peel-shaped graphite for lithium-ion batteries

Abstract

Phenolic Resin Coated Natural Graphite Oxide as an Anode Material for Lithium Ion Batteries

A core-shell structure of the carbon-coated natural graphite oxide composite was successfully prepared. Natural graphite was initially oxidized using concentrated sulfuric acid and then carbon coated by the carbonization of phenolic resin at high temperature. Scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), and Raman techniques were used to characterize the morphology and structure of the natural graphite materials before and after oxidation and carbon coating by the pyrolysis of the phenolic resin. The results showed that the surface of the natural graphite particles became smoother and the surface defects were effectively modified after oxidation and carbon coating. The electrochemical test results showed that the electrochemical performance of the natural graphite improved significantly by oxidation with sulfuric acid and by carbon coating. When the covering amount of phenolic resin was 9% the modified natural graphite material gave the best electrochemical performance. Its initial discharge capacity was 434.0 mAh·g-1 and it remained 361.6 mAh· g after 40 charge-discharge cycles. The discharge capacity of the untreated natural graphite was only 332.3 mAh·g-1. The modification approach that improved the capacity of the natural graphite effectively is of great significance for the application of natural graphite in lithium ion batteries. [Article] www.whxb.pku.edu.cn 物理化学学报(Wuli Huaxue Xuebao) Acta Phys. -Chim. Sin. 2011, 27 (9), 2129-2134 September Received: May 13, 2011; Revised: July 7, 2011; Published on Web: July 29, 2011. ∗Corresponding author. Email: asyu@fudan.edu.cn; Tel/Fax: +86-21-51630320. The project was supported by the Key Program of Basic Research of the Shanghai Committee of Science and Technology, China (10JC1401500) and Department of Chemistry and Shanghai Key Laboratory of Molecular and Innovative Materials, China (08DZ2270500). 上海市基础研究重点项目(10JC1401500)和上海市分子催化和功能材料重点实验室(08DZ2270500)资助 C Editorial office of Acta Physico-Chimica Sinica 2129 Vol.27 Acta Phys. -Chim. Sin. 2011

Local Tortuosity Inhomogeneities in a Lithium Battery Composite Electrode

While battery performance is well predicted by the macrohomogeneous model of Newman and co-workers, predicting degradation and failure remains a challenge. It may be that, like most materials, failure depends on local imperfections and inhomogeneities. In this work we use tomographic data previously obtained for a commercial lithium-ion battery graphite composite electrode to evaluate the homogeneity of the tortuosity of the electrode by directly integrating the transport equations through its pore space. We find that the tortuosity of two halves of the electrode, each roughly 250 × 350 × 50 μm differ by about 30%. On a smaller scale, 80 × 100 × 50 μm, local tortuosity variations up to a factor of 3 were observed. The Bruggeman relationship between porosity and tortuosity is also examined. We find that local porosity alone does not determine local tortuosity very well, and that the average relationship between porosity and tortuosity is not well predicted by the Bruggeman relationship for this electrode. We suggest that large local variations in tortuosity can lead to reduced capacity and life.

Effect of carbon nanotubes on the anode performance of natural graphite for lithium ion batteries

A comparative investigation was carried out on carbon black and multiwalled carbon nanotubes as conductive additives in spherical natural graphite for lithium ion batteries. Scanning electron microscopy images showed that carbon nanotubes interlaced graphite particles in series to form a three-dimensional network. The constant current charge–discharge experiments showed that carbon nanotubes were more effective in improving reversible capacity and cycle stability. The reversible capacity was improved to 366 mAh/g and the cycle stability was improved effectively when carbon nanotubes were used. The research is of potential interest to the application of carbon nanotubes as conductive additives in anode materials for high-power lithium ion batteries.

Dilation Control of Anode Materials Based on Spherical Natural Graphite for Lithium-Ion Batteries

not Available.

Modification of natural graphite for lithium ion batteries

A process was attempted to improve the performance of natural graphite for lithium ion batteries (LIB). The natural graphite was treated in a concentrated sulfuric acid solution at high temperature, followed by the coating of resorcinol–formaldehyde resin using in situ polymerization and heat-treatment over 800 °C. SEM, XRD, XPS and Raman spectroscopy were employed to investigate the samples. It was found that the sharp edges as well as other active sites “were repaired” and the thin pyrolytic carbon film coated on the surface of graphite led to the decrease of BET surface areas of coated graphite and depressed decomposition of electrolyte and co-intercalation of solvated ions, which led to an improved electrochemical performance of modified graphite. In fact, the modified natural graphite reached 90.3% of the initial coulombic efficiency, more than 350 mAh g −1 of the reversible capacity and 96.4% of the charge capacity retention reached after 70 cycles. The results show that the proposed process is a promising and practicable way to improve natural graphite for LIB.