Lithium intercalation and interfacial kinetics of composite anodes formed by oxidized graphite and copper

The electrochemical behavior of composite anodes prepared either by mixing partially oxidized graphite and Cu powders or by coating the pristine partially oxidized graphite electrodes with few-nanometer-thick Cu layers has been studied by slow-scan-rate cyclic voltammetry (SSCV) and galvanostatic charge/discharge cycles over the temperature range of −30 °C to 20 °C. The interfacial intercalation/deintercalation kinetics has also been investigated using electrochemical impedance spectroscopy (EIS).     

Improvement of electrochemical characteristics of natural graphite negative electrode coated with polyacrylic acid in pure propylene carbonate electrolyte

In order to improve the negative electrode characteristics of a graphite electrode in a propylene carbonate (PC)-containing electrolyte, we have prepared a graphite negative electrode coated with a water-soluble anionic polymer as a binder for composite graphite electrodes. The electrochemical characteristics of the coated graphite were evaluated by cyclic voltammetry and charge–discharge cycle tests. The coated graphite negative electrode showed a stable Li⁺ ion intercalation/deintercalation reaction without the exfoliation of the graphene layers caused by the co-intercalation of the PC solvent in the LiClO₄/PC solution. The charge–discharge characteristic of the coated graphite negative electrode in a PC-containing electrolyte was almost the same as that in ethylene carbonate-based electrolyte.     

Dependence of the morphology of graphitic electrodes on the electrochemical intercalation of lithium ions

We have studied the effects of several parameters that influence the electrochemical intercalation of lithium ions into various carbonaceous materials: massive samples of pyrographite PGCCL (Le Carbone Lorraine), bulky pitch-based graphitized carbon fibres P100-S (Amoco) and divided natural graphite powder UF4 (Le Carbone Lorraine). The electrochemical Li⁺ intercalation has been achieved in electrolytic solutions composed of a solvent, ethylene carbonate and a conducting salt, LiClO₄. We have shown previously that such an electrolyte allows the intercalation of unsolvated lithium ions up to the richest stage-I LiC₆ composition without apparent solvent decomposition. The electrochemical behaviour of the electrodes in such electrolytes was followed either by chronopotentiometry (galvanostatic charge/discharge cycles) or by cyclic voltammetry. The use of micro-computers, able to conduct the experiments by imposition of charge or potential steps followed by cell relaxations, has allowed to obtain data on the kinetics of Li⁺ intercalation. The electrochemical behaviour of the graphitic electrode is strongly dependent on its morphology. Moreover, the decrease of the size of the crystalline domains during prolongated cyclings has been shown particularly in massive pyrographite samples. Such an electrochemical grinding of the electrode has obviously a positive effect on its performances characterized by a noticeable increase in the maximum x composition reached (x refers to the Li_xC₆ composition). It appears also that the use of poly(vinylidene difluoride) (PVDF) leads to side reactions that have a negative effect on the performances of the electrodes.     

Effect of solvents and solvent mixtures on intercalation/de-intercalation behaviour of Li+ and ClO4− ions in polypropylene–graphite composite electrodes

The intercalation of Li⁺ and ClO₄⁻ ions in polypropylene–graphite composite electrodes in different single solvents and 1:1 binary solvent mixtures is studied by means of cyclic voltammetry and scanning electron microscopy. The intercalation/de-intercalation efficiency as a potential dual-intercalation battery electrode for cationic intercalation (positive electrode) is found to be generally lower than that for anionic intercalation in most of the solvents. 1:1 solvent mixtures do not enhance intercalation/de-intercalation efficiency significantly beyond the values found in a single solvent. The mixed-solvent system leads, however, to less co-solvent intercalation and graphite exfoliation, and hence better cycle-life as a potential battery electrode.     

Utilization of (oxalato)borate-based organic electrolytes in activated carbon/graphite capacitors

The electrolyte salts composed of tetramethylammonium (TMA⁺) cation and difluoro(oxalato)borate (DFOB⁻) or bis(oxalato)borate (BOB⁻) anions have been proposed for the application in activated carbon (AC)/graphite capacitors. The electrochemical performance of AC/graphite capacitors has been studied using these electrolyte salts dissolved in propylene carbonate (PC). The intercalation behaviors of anions (BF₄⁻, DFOB⁻, and BOB⁻) at the graphite positive electrodes have been investigated by in situ XRD measurements. The bigger the anion is, the higher the cell voltage is where the intercalation happens. Accordingly, the bigger the anion is, the smaller discharge capacity delivered by an AC/graphite capacitor. The charge mechanism of TMA⁺ at the AC negative side has also been addressed. Compared with other bigger quaternary alkyl ammonium cations, the specific capacitance of the AC negative electrode towards TMA⁺ adsorption is somehow smaller as estimated.     

Influence of polymeric binder on the stability and intercalation/de-intercalation behaviour of graphite electrodes in non-aqueous solvents

Cyclic voltammetric and scanning electron microscopic investigations on a highly-packed, crystalline, graphite electrode (HPC) and on a polypropylene composite graphite electrode (CPP) containing 20 wt.% polypropylene binder indicate that the latter has higher mechanical stability and higher electrochemical intercalation/de-intercalation activity. This holds for the intercalation of lithium (Li⁺) and tetrabutyl ammonium (TBA⁺) cations from dimethyl sulfoxide (DMSO) and dimethyl formamide (DMF), as well as for the intercalation of perchlorate (Cl0₄⁻) and fluoroborate (Bf₄⁻) anions from propylene carbonate (PC) and acetonitrile (AN). There is a linear correlation between the threshold potential for the beginning of intercalation (E_th) and the intercalation/de-intercalation efficiency (IDE) for cationic intercalation. In the case of anionic intercalation, two distinct linear relationships for HPC and CPP electrodes are observed. Competitive oxidation processes reduce the IDE on the HPC electrode.     

Functional interface of polymer modified graphite anode

Graphite electrodes were modified by polyacrylic acid (PAA), polymethacrylic acid (PMA), and polyvinyl alcohol (PVA). Their electrochemical properties were examined in 1 mol dm⁻³ LiClO₄ ethylene carbonate:dimethyl carbonate (EC:DMC) and propylene carbonate (PC) solutions as an anode of lithium ion batteries. Generally, lithium ions hardly intercalate into graphite in the PC electrolyte due to a decomposition of the PC electrolyte at ca. 0.8 V vs. Li/Li⁺, and it results in the exfoliation of the graphene layers. However, the modified graphite electrodes with PAA, PMA, and PVA demonstrated the stable charge–discharge performance due to the reversible lithium intercalation not only in the EC:DMC but also in the PC electrolytes since the electrolyte decomposition and co-intercalation of solvent were successfully suppressed by the polymer modification. It is thought that these improvements were attributed to the interfacial function of the polymer layer on the graphite which interacted with the solvated lithium ions at the electrode interface.     

Surface structural disordering in graphite upon lithium intercalation/deintercalation

We report on the origin of the surface structural disordering in graphite anodes induced by lithium intercalation and deintercalation processes. Average Raman spectra of graphitic anodes reveal that cycling at potentials that correspond to low lithium concentrations in Li_xC (0 ≤ x < 0.16) is responsible for most of the structural damage observed at the graphite surface. The extent of surface structural disorder in graphite is significantly reduced for the anodes that were cycled at potentials where stage-1 and stage-2 compounds (x > 0.33) are present. Electrochemical impedance spectra show larger interfacial impedance for the electrodes that were fully delithiated during cycling as compared to electrodes that were cycled at lower potentials (U < 0.15 V vs. Li/Li⁺). Steep Li⁺ surface-bulk concentration gradients at the surface of graphite during early stages of intercalation processes, and the inherent increase of the Li_xC d-spacing tend to induce local stresses at the edges of graphene layers, and lead to the breakage of C-C bonds. The exposed graphite edge sites react with the electrolyte to (re)form the SEI layer, which leads to gradual degradation of the graphite anode, and causes reversible capacity loss in a lithium-ion battery.     

Electrochemical intercalation of cationic and anionic species from a lithium perchlorate–propylene carbonate system—a rocking-chair type of dual-intercalation system

The reductive and oxidative intercalation of ionic species of lithium perchlorate (LiClO₄) in propylene carbonate (PC) medium are carried out to develop a dual-intercalation battery system. Cyclic voltammetry (CV), potentiostatic transients (i-t), galvanostatic charging, thermogravimetry (TG) and differential thermal analysis (DTA) are performed to establish the intercalation behaviour of both lithium and perchlorate ionic species. A polypropylene graphite composite electrode material containing 20 wt.% polypropylene as a binder is found to be a suitable host material for dual intercalation studies. The intercalation/de-intercalation efficiency (IDE) increases with increasing sweep rate and reaches up to 90% for Li⁺ and 65% for ClO₄⁻ ions at a sweep rate of 40 mV s⁻¹. The formation of a passive film decreases the IDE during the first intercalation/de-intercalation cycle. The open-circuit potential for a battery assembly involving these two electrodes is in the range 3.8 to 4.0 V.     

Comparison of fluoride intercalation/de-intercalation processes on graphite electrodes in aqueous and aqueous methanolic HF media

The solvent can play a major role in the intercalation/de-intercalation process and the stability of graphite substrates towards this process. This fact is established in the present work that involves fluoride intercalation/de-intercatlation on graphite electrodes in aqueous and aqueous methanolic HF solutions where the HF concentration is varied between 1.0 and 18.0 M. In addition to cyclic voltammetry and potentiostatic polarization, open-circuit potential decay measurements, scanning electron microscopy and X-ray diffraction measurements have been employed. In general, addition of methanol and increasing concentration of HF raise the overall intercalation/de-intercalation efficiency. Methanol is adsorbed preferentially on the graphite lattice and, hence, suppresses both oxygen evolution and the formation of passive graphite oxides. In 15.0 M HF, the optimum methanol concentration is 5 vol.%. This suggests that, in addition to the adsorption effect, there is some weakening of the structured water molecules that facilitates the solvated fluoride ions for efficient intercalation.     

Lithium transport through graphite electrodes that contain two stage phases

The lithium transport through graphite electrodes that contain two stage phases has been investigated in 1 M LiAsF₆-EC/DEC (ethylene carbonate/diethyl carbonate) non-aqueous solution by using potentiostatic current transient technique supplemented by lithium charging/discharging experiments and a.c. impedance spectroscopy. An attractive interaction between the intercalated lithium ions and the graphite lattice is indicated from the decreased diffusivity value with increasing lithium content in the lithiated graphite electrode in the presence of a single stage phase. This attractive interaction gives rise to the stage transformation in the electrode, which is characterized by potential plateaus in the charge/discharge curves. From the results of the potentiostatic current transients, it is suggested that the stage transformation in the lithiated graphite electrode is accompanied by the limited transport of lithium through the electrode for which the stress generated by the stage phase boundary is responsible. The stress-controlled transport of lithium through the graphite structure is substantiated by the occurrence of hysteresis in the potential profile during the lithium intercalation and de-intercalation     

Suppression of Li deposition on surface of graphite using carbon coating by thermal vapor deposition process

10 wt.% carbon-coated natural graphite (NC-10) is prepared by thermal vapor deposition. The carbon coating is electrochemically investigated at −5 °C; it improves lithium intercalation in the graphite's interlayer spacing. NC-10 graphite clearly shows 3 voltage plateaus and a higher capacity during the first charge/discharge cycle at −5 °C than uncoated natural graphite. XRD study of the electrode after the first charging shows increased lithium intercalation into the graphite layers and also suppression of lithium deposition on the graphite's surface. Due to the homogeneous potential profile on the graphite surface, carbon coating enhance lithium intercalation at −5 °C. In addition, NC-10 shows less lithium deposition on the surface than bare natural graphite.     

Stability of poly(vinylidene fluoride-co-hexafluoropropylene)-based composite gel electrolytes with functionalized silicas

Various aspects of stability of composite polymer gel electrolytes based on poly(vinylidene fluoride-co-hexafluoropropylene) (PVdF/HFP) polymeric matrix and functionalized precipitated silicas have been studied. The silica fillers have been surface modified with methacryloxy or vinyl groups by partially replacing silanol groups, so that bi-functional (hydrophilic/hydrophobic) character of the inorganic fillers was created. Compatibility of the gel electrolytes with lithium electrode has been examined by means of EIS technique. Electrochemical stability window has been studied with the application of cyclic voltammetry technique with fast sweeping rate. Passive layer formation on graphite electrode has been investigated for all the gel electrolytes by means of cyclic voltammetry with slow scan rate and galvanostatic charging/discharging technique. It has been shown that stability of the interface between lithium and gel electrolyte is significantly improved when bi-functional silicas are used as fillers. The phenomenon has been ascribed to more effective scavenging of trace impurities as well as to better shielding of the electrode surfaces. Cyclic voltammetry on platinum has revealed excessive electrochemical redox processes upon prolonged cycling for all the gel electrolytes. It has been demonstrated that stable passive layers are formed on graphite electrodes upon electrochemical reduction in the presence of the studied composite polymer gel electrolytes.     

Irreversible capacity loss of graphite electrode in lithium-ion batteries

The irreversible capacity loss of the carbon electrode in lithium-ion batteries at the first cycle is caused mostly by surface film growth. We inspected an unknown irreversible capacity loss (UICL) of the natural graphite electrodes. The charge/discharge behavior of graphite and meso-phase carbon microbeads heat-treated at 2800°C (MCMB28) as the materials of the carbon anode in the lithium-ion battery were compared. It was found that the capacity loss of the natural graphite electrode in the first cycle is caused not only by surface film growth, but also by irreversible lithium-ion intercalation on the new formed surface at the potential range of lithium intercalation, while the capacity loss of the MCMB28 electrode is mainly originated from surface film growth. The reason for the difference of their irreversible capacity losses of these two kinds of carbon material was explained in relation to their structural characteristics.     

Functional binders for reversible lithium intercalation into graphite in propylene carbonate and ionic liquid media

Poly(acrylic acid) (PAA), poly(methacrylic acid) (PMA), and poly(vinyl alcohol) (PVA), which have oxygen species as functional groups, were utilized as a binder for graphite electrodes, and the electrochemical reversibility of lithium intercalation was examined in PC medium and ionic liquid electrolyte, lithium bis(trifluoromethanesulfonyl)amide dissolved in 1-butyl-1-methylpyrrolidinium bis(trifluoromethanesulfonyl)amide (BMP-TFSA). Columbic efficiency of 75–80% with more than 300 mAh g^?1 was achieved upon first reduction/oxidation cycle in both electrolytes using these binding polymers, which were significantly improved in comparison to a conventional PVdF binder (less than 45% of columbic efficiency for the first cycle). For the graphite-PVdF electrode, co-intercalation and/or decomposition of PC molecules solvating to Li ions were observed by the electrochemical reduction, resulting in the cracking of graphite particles. In contrast, the co-intercalation and decomposition of PC molecules and BMP cations for the first reduction process were completely suppressed for the graphite electrodes prepared with the polymers containing oxygen atoms. It was proposed that the selective permeability of lithium ions was attained by the uniform coating of the graphite particles with PAA, PMA, and PVA polymers, because the electrostatic interaction between the positively charged lithium ions and negatively charged oxygen atom in the polymer should modulate the desolvation process of lithium ions during the lithium intercalation into graphite, showing the similar functions like artificial solid-electrolyte interphase.     

Improving the electrochemical properties of graphite/LiCoO2 cells in ionic liquid-containing electrolytes

The electrochemical performance of graphite/lithium cobalt oxide (LiCoO₂) cells in N-methoxymethyl-N,N-dimethylethylammonium bis(trifluoromethane-sulfonyl) imide (MMDMEA-TFSI)-containing electrolytes is significantly enhanced by the formation of a fluoroethylene carbonate (FEC)-derived protective film on an anode during the first cycle. The electrochemical intercalation of MMDMEA cations into the graphene layer is readily visualized by ex situ transmission electron microscopy (TEM). Moreover, differences in the X-ray diffraction (XRD) patterns of graphite electrodes in cells charged with and without FEC in dimethyl carbonate (DMC)/MMDMEA-TFSI are clearly discernible. Conclusively, the presence of FEC in MMDMEA-TFSI-containing electrolytes leads to a remarkable enhancement of discharge capacity retention for graphite/LiCoO₂ cells as compared with ethylene carbonate (EC) and vinylene carbonate (VC).     

Improved electrochemical properties of amorphous Mg65Ni27La8 electrodes: Surface modification using graphite

Amorphous Mg₆₅Ni₂₇La₈ alloy is prepared by melt-spinning. The alloy surface is modified using different contents of graphite to improve the performances of the Mg₆₅Ni₂₇La₈ electrodes. In detail, the electrochemical properties of (Mg₆₅Ni₂₇La₈) + xC (x = 0–0.4) electrodes are studied systematically, where x is the mass ratio of graphite to alloy. Experimental results reveal that the discharge capacity, cycle life, discharge potential characteristics and electrochemical kinetics of the electrodes are all improved. The surface modification enhances the electrocatalytic activity of the alloy, reduces the contact resistance of the electrodes and obstructs the formation of Mg(OH)₂ on the alloy surface. An optimal content of graphite has been obtained. The (Mg₆₅Ni₂₇La₈) + 0.25 C electrode has the largest discharge capacity of 827 mA h g⁻¹, which is 1.47 times as large as that of the electrode without graphite, and the best electrochemical kinetics. Further increasing of graphite content will lead to the increase of contact resistance and activation energy for charge-transfer reaction of the electrode, resulting in the degradation of electrode performance.     

Lithium-ion rechargeable cells with polyacenic semiconductor (PAS) and LiCoO2 electrodes

Polyacenic semiconductor (PAS), heat-treated at 700°C, has a lithium intercalation capacity as high as 438 mAh g⁻¹ which is higher than the theoretical capacity of 372 mAh g⁻¹ for graphite. The electrochemical behaviour of PAS is examined by studying Li/PAS and Li/graphite cells. In a PAS or graphite anode, three reactions are distinguished: (i) reaction of lithium with the Teflon binder; (ii) decomposition of electrolyte, and (iii) intercalation of Li⁺ ions. Two laboratory cells with liquid organic electrolyte or polymer electrolyte and PAS as the anode demonstrate that PAS is a promising anode material for lithium-ion batteries.     

Low-temperature behavior of graphite-tin composite anodes for Li-ion batteries

The challenge of increasing low-temperature performances of anodes for Li-ion batteries is faced by preparing graphite-tin composite electrodes. The anodes are prepared by mixing partially oxidized graphite with nanometric Sn powder or by coating the oxidized graphite electrode with a thin Sn layer. Long-term cycling stability and intercalation/deintercalation performances of the composite anodes in the temperature range 20 °C to −30 °C are evaluated. Kinetics is investigated by cyclic voltammetry and electrochemical impedance spectroscopy, in the attempt to explain the role of Sn in reducing the overall electrode polarization at low temperature. Two possible mechanisms of action for bulk metal powder and surface metal layer are proposed.     

In situ analysis of the interfacial reactions between MCMB electrode and organic electrolyte solutions

The interfacial phenomena between graphite (mesocarbon-microbeads (MCMB)) electrode and organic electrolyte solution were analyzed by in situ atomic force microscopy (AFM) and Fourier transform infrared (FTIR) spectroscopy. The influence of lithium salts (anion species), LiPF₆, LiBF₄, and LiClO₄, on the interfacial reaction, including lithium intercalation into graphite, was investigated in EC+DMC solutions. In situ AFM observation disclosed that morphological changes are quite different from one another depending on the kind of lithium salt (anion). A large expansion of MCMB particle was observed particularly in LiPF₆/EC+DMC. An expansion of MCMB particle started above 1.0 V versus Li/Li⁺ and this expansion seemed to be caused by the decomposition of ternary graphite intercalation compound (GIC) (C_nLi(sol)_y), because the expansion remained after de-intercalation of lithium. IRAS spectra of each electrolyte solution showed different behaviors and different reduction products of solvents. double modulation FTIR (DMFTIR) spectra on graphite electrode, which emphasize the surface species, indicated relatively small changes after cathodic polarization. Therefore, the observed morphological changes were caused mainly by the expansion of graphene layers and not by the precipitation of reduction products.