Low temperature performance of graphite electrode in Li-ion cells

We studied low temperature performance of Li/graphite cell. Results show that capacity of the graphite electrode falls significantly in the temperature range of 0 to −20 °C. When lithiation and delithiation are both carried out at −20 °C, graphite only retains 12% of the room temperature capacity. However, delithiation capacity of graphite increases to 92% of the room temperature value if the lithiation is carried out at room temperature. We believe that the poor low temperature performance of the cell is due to slow kinetics of lithium ion diffusion in graphite rather than low ionic conductivity of electrolyte and solid electrolyte interface (SEI) on the graphite surface. During lithiation and delithiation processes, lithium ion has the similar apparent chemical diffusion coefficient of 10⁻⁹-10⁻¹⁰ cm²/s at 20 °C, depending on the state of lithiation of graphite. We observed a dramatic decrease in lithium ion diffusivity in the temperature range of 0 to −20 °C, and that at low temperatures of <−20 °C, lithium ion has higher diffusivity in the delithiated graphite than in the lithiated one. We also observed that temperature dependence of cycling behavior of the Li/graphite cell follows the change of lithium ion diffusivity.     

The effect of electrolyte temperature on the passivity of solid electrolyte interphase formed on a graphite electrode

The effect of electrolyte temperature on the passivity of solid electrolyte interphase (SEI) was investigated in 1 M LiPF₆-ethylene carbonate/diethyl carbonate (50:50 vol.%) electrolyte, using galvanostatic charge-discharge experiment, and ac-impedance spectroscopy combined with Fourier transform infra-red spectroscopy, and high resolution transmission electron microscopy (HRTEM). The galvanostatic charge-discharge curves at 20 °C evidenced that the irreversible capacity loss during electrochemical cycling was markedly increased with rising SEI formation temperature from 0 to 40 °C. This implies that the higher the SEI formation temperature, the more were the graphite electrodes exposed to structural damages. From both increase of the relative amount of Li₂CO₃ to ROCO₂Li and decrease of resistance to the lithium transport through the SEI layer with increasing SEI formation temperature, it is reasonable to claim that, due to the enhanced gas evolution reactions during transformation of ROCO₂Li to Li₂CO₃, the rising SEI formation temperature increased the number of defect sites in the SEI layer. From the analysis of HRTEM images, no significant structural destruction in bulk graphite layer was observed after charge-discharge cycles. This means that solvated lithium ions were intercalated through the defect sites in the SEI, at most, into the surface region of the graphite layer.     

Compatibility of quaternary ammonium-based ionic liquid electrolytes with electrodes in lithium ion batteries

Electrochemical intercalation/deintercalation behavior of lithium into/from electrodes of lithium ion batteries was comparatively investigated in 1 mol/L LiClO₄ ethylene carbonate-diethyl carbonate (EC-DEC) electrolyte and a quaternary ammonium-based ionic liquid electrolyte. The natural graphite anode exhibited satisfactory electrochemical performance in the ionic liquid electrolyte containing 20 vol.% chloroethylenene carbonate (Cl-EC). This is attributed to the mild reduction of solvated Cl-EC molecules at the graphite/ionic electrolyte interface resulting in the formation of a thin and homogenous SEI on the graphite surface. However, rate capability of the graphite anode is poor due to the higher interfacial resistance than that obtained in 1 mol/L LiClO₄/EC-DEC organic electrolyte. Spinel LiMn₂O₄ cathode was also electrochemically cycled in the ionic electrolyte showing satisfactory capacity and reversibility. The ionic electrolyte system is thus promising for 4 V lithium ion batteries based on the concept of “greenness and safety”.     

Effect of surface modified porous inorganic-organic hybrid polyphosphazene nanotubes on the properties of polyethylene oxide based solid polymer electrolytes

2-(2-methyloxyethoxy)ethanol modified poly (cyclotriphosphazene-co-4,4′-sufonyldiphenol) (PZS) nanotubes were synthesized and solid composite polymer electrolytes based on the surface modified polyphosphazene nanotubes added to PEO/LiClO₄ model system were prepared. Differential Scanning Calorimetry (DSC) and Scanning Electron Microscopy (SEM) were used to investigate the characteristics of the composite polymer electrolytes (CPE). The ionic conductivity, lithium ion transference number and electrochemical stability window can be enhanced after the addition of surface modified PZS nanotubes. The electrochemical investigation shows that the solid composite polymer electrolytes incorporated with PZS nanotubes have higher ionic conductivity and lithium ion transference number than the filler SiO₂. Maximum ionic conductivity values of 4.95 × 10⁻⁵ S cm⁻¹ at ambient temperature and 1.64 × 10⁻³ S cm⁻¹ at 80 °C with 10 wt % content of surface modified PZS nanotubes were obtained and the lithium ion transference number was 0.41. The good chemical properties of the solid state composite polymer electrolytes suggested that the inorganic-organic hybrid polyphosphazene nanotubes had a promising use as fillers in solid composite polymer electrolytes and the PEO₁₀-LiClO₄-PZS nanotubes solid composite polymer electrolyte can be used as a candidate material for lithium polymer batteries.     

The cooperative effect of tri(β-chloromethyl) phosphate and cyclohexyl benzene on lithium ion batteries

Tri(β-chloromethyl) phosphate (TCEP) and cyclohexyl benzene (CHB) had been studied synchronously as the additives of the lithium ion batteries to improve the high temperature and overcharge safety. The study group used the cyclic voltammetry and environment scanning electron microscopy (ESEM) to investigate the oxidize potentials of the TCEP and CHB at normal environment and 150 °C and the surface characteristics of the positive electrode of the graphite/LiCoO₂(063465) batteries before and after overcharge. The self-extinguishing time (SET) tests were carried out to measure the effect of additives on the electrolyte combustibility. The oven and overcharge tests and other electrochemical testing methods were performed to test the reliability of the protection provided by the TCEP and CHB of the high temperature and overcharge safety and the effect of the additives on the electrochemical performance. The results show that the oxidize potential of the TCEP is about 4.75 V and the CHB about 4.6 V at normal environment and the oxidize potential of the TCEP drops to 4 V and CHB about 4.1 V at 150 °C; the TCEP has favorable effect on the self-extinguish time of the electrolyte; the surface of the LiCoO₂ positive electrode after overcharge formed a layer of CHB polymer; when the content of both additives are over 5 wt%, the batteries show improved safety through the oven tests of 150 °C and can stand 1.3 A (1 C) and constant 10 V overcharge tests; the adverse effect of TCEP and CHB on cycle performance of battery is relatively small. In sum, the cooperative use of these two additives can improve the safety of lithium ion battery greatly.     

The effects of oxidation on the surface properties of MCMB-6-28

Graphite is an important active material for lithium ion battery anode. In the present paper, a kind of graphitized MCMB (MCMB-6-28) was oxidized at 600 or 700 °C, and the effects of the oxidation were investigated by cyclic voltammetry (CV) method, electrochemical impedance spectroscopy (EIS), and rectangular potential technique. It was found that the specific surface area of the electrode made of the graphite can be increased notably by the oxidative treatment. Meanwhile, after oxidation, the electrochemical impedance of the graphite electrode is reduced, and the reaction reversibility is improved.     

Purification and carbon-film-coating of natural graphite as anode materials for Li-ion batteries

A process of modification of natural graphite materials as anode for lithium ion batteries was attempted. The process started with the treatment of natural graphite with concentrated hydrochloric acid and concentrated sulfuric acid in a thermal autoclave, followed by the in situ polymerization of resorcinol-formaldehyde resin to coat the graphite, then heat-treatment. SEM, XRD, Raman and electrochemical charge-discharge analysis showed that the surface defects and impurities on natural graphite were eliminated by purification of the concentrated acids, and carbon-film encapsulation modified the surface structure of the graphite and reduced its BET surface area. The as-obtained natural graphite sample presented an initial charge-discharge coulombic efficiency of 88.4% and a reversible capacity of 355.8 mAh g⁻¹. The proposed process paves a way to prepare a promising anode material with excellent performance with low cost of natural graphite for rechargeable lithium ion batteries.     

Comparisons of graphite and spinel Li1.33Ti1.67O4 as anode materials for rechargeable lithium-ion batteries

The aim of this work was to compare the electrochemical behaviors and safety performance of graphite and the lithium titanate spinel Li_1.33Ti_1.67O₄ with half-cells versus Li metal. Their electrochemical properties in 1 M LiPF₆/EC + DEC (1:1 w/w) or 1 M LiPF₆/PC + DEC (1:1 w/w) at room and elevated temperatures (30 and 60 °C) have been studied using galvanostatic cycling. At 30 °C graphite has higher reversible capacity than Li_1.33Ti_1.67O₄ when using the LiPF₆/EC + DEC as electrolyte. At 60 °C graphite declines in cell capacity yet Li_1.33Ti_1.67O₄ remains almost unchanged. In a propylene carbonate (PC) containing electrolyte, graphite electrode exfoliates and loses its mechanical integrity while Li_1.33Ti_1.67O₄ electrode is very stable. An accelerating rate calorimeter (ARC) and microcalorimeter have been used to compare the thermal stability of lithiated lithium titanate spinel and graphite. Results show that Li_1.33Ti_1.67O₄ may be used as an alternative anode material offering good battery performance and higher safety.     

Electrochemical intercalation of lithium into a natural graphite anode in quaternary ammonium-based ionic liquid electrolytes

Electrochemical intercalation of lithium into a natural graphite anode was investigated in electrolytes based on a room temperature ionic liquid consisting of trimethyl-n-hexylammonium (TMHA) cation and bis(trifluoromethanesulfone) imide (TFSI) anion. Graphite electrode was less prone to forming effective passivation film in 1 M LiTFSI/TMHA-TFSI ionic electrolyte. Reversible intercalation/de-intercalation of TMHA cations into/from the graphene interlayer was confirmed by using cyclic voltammetry, galvanostatic measurements, and ex situ X-ray diffraction technique. Addition of 20 vol% chloroethylenene carbonate (Cl-EC), ethylene carbonate (EC), vinyl carbonate (VC), or ethylene sulfite (ES) into the ionic electrolyte resulted in the formation of solid electrolyte interface (SEI) film prior to TMHA intercalation and allowed the formation of Li-C₆ graphite interlayer compound. In the ionic electrolyte containing 20 vol% Cl-EC, the natural graphite anode exhibited excellent electrochemical behavior with 352.9 mAh/g discharge capacity and 87.1% coulombic efficiency at the first cycle. A stable reversible capacity of around 360 mAh/g was obtained in the initial 20 cycles without any noticeable capacity loss. Mechanisms concerning the significant electrochemical improvement of the graphite anode were discussed. Ac impedance and SEM studies demonstrated the formation of a thin, homogenous, compact and more conductive SEI layer on the graphite electrode surface.     

Lithium/V6O13 cells using silica nanoparticle-based composite electrolyte

The electrochemical behavior of Li/V₆O₁₃ cells is investigated at room temperature (22 °C) both in liquid electrolyte consisting of oligomeric poly(ethyleneglycol)dimethylether+lithium bis(trifluoromethylsulfonylimide) and composite electrolytes formed by blending the liquid electrolyte with silica nanoparticles (fumed silica). The addition of fumed silica yields a gel-like electrolyte that demonstrates the desirable property of suppressing lithium dendrite growth due to the rigidity and immobility of the electrolyte structure. The lithium/electrolyte interfacial resistance for composite gel electrolytes is less than that for the corresponding base-liquid electrolyte, and the charge-discharge cycle performance and electrochemical efficiency for the Li/V₆O₁₃ cell is significantly improved. The effect of fumed silica surface group on the electrochemical performance is discussed; the native hydrophilic silanol surface group appears better than fumed silica that is modified with a hydrophobic octyl surface moiety.     

Electrochemical and thermal behavior of copper coated type MAG-20 natural graphite

Natural graphite powders, type MAG-20, were modified by electroless deposition of copper onto the graphite surface. The scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) of the modified graphite showed that almost all the copper covered the edge plane of graphite particles and the copper content increased from 6 to 17 at.% depending upon the plating time. Thermal gravimetric analysis (TGA) confirmed the copper composition obtained using EDS. TGA carried out under oxygen atmosphere showed that the combustion temperature decreased by 200 °C for a copper content from 0 to 16 at.%. Cycling the modified graphite at a C/10 rate showed that the initial large irreversible capacity typically present in untreated natural graphite was significantly reduced when the modified graphite contained 6-17 at.% copper coatings. A differential scanning calorimetry (DSC) comparison between fully lithiated copper coated MAG-20 with fully lithiated MCMB showed that the major exothermic peak for the MAG-20 modified graphite is lower in magnitude and is shifted to higher temperatures. However, both MAG-20 and MCMB types of lithiated graphite gave the same amount of total heat generation up to 350 °C.     

Enhanced thermal properties of the solid electrolyte interphase formed on graphite in an electrolyte with fluoroethylene carbonate

The influence of fluoroethylene carbonate (FEC) on the electrochemical and thermal properties of graphite anodes is examined. The dQ/dV graph of graphite/Li cells shows that the electrochemical reduction peak of an electrolyte shifts to higher potential in the presence of FEC. The DSC results for graphite anodes cycled in FEC-containing electrolytes clearly exhibit that an exothermic peak at around 120 °C mostly disappears. It is demonstrated by X-ray photoelectron spectroscopy (XPS) and electrochemical impedance spectroscopy (EIS) that SEI formed by the electrochemical reduction of FEC consists of a relatively high proportion of LiF and gives low interfacial resistance for graphite/Li/Li cells.     

Synthesis and electrochemical properties of lithium methacrylate-based self-doped gel polymer electrolytes

In this study, a strategy for synthesizing lithium methacrylate (LiMA)-based self-doped gel polymer electrolytes was described and the electrochemical properties were investigated by impedance spectroscopy and linear sweep voltammetry. LiMA was found to dissolve in ethylene carbonate (EC)/diethyl carbonate (DEC) (3/7, v/v) solvent after complexing with boron trifluoride (BF₃). This was achieved by lowering the ionic interactions between the methacrylic anion and lithium cation. As a result, gel polymer electrolytes consisting of BF₃-LiMA complexes and poly(ethylene glycol) diacrylate were successfully synthesized by radical polymerization in an EC/DEC liquid electrolyte. The FT-IR and AC impedance measurements revealed that the incorporation of BF₃ into the gel polymer electrolytes increases the solubility of LiMA and the ionic conductivity by enhancing the ion disassociations. Despite the self-doped nature of the LiMA salt, an ionic conductivity value of 3.0 × 10⁻⁵ S cm⁻¹ was achieved at 25 °C in the gel polymer electrolyte with 49 wt% of polymer content. Furthermore, linear sweep voltammetry measurements showed that the electrochemical stability of the gel polymer electrolyte was around 5.0 V at 25 °C.     

Effect of fluoroethylene carbonate on high temperature capacity retention of LiMn2O4/graphite Li-ion cells

In order to overcome severe capacity fading of LiMn₂O₄/graphite Li-ion cells at high temperature at 60 °C, fluoroethylene carbonate (FEC) was newly evaluated as an electrolyte additive. With 2 wt.% FEC addition into the electrolyte (EC/DEC/PC with 1 M LiPF₆), the capacity retention at 60 °C after 130 cycles was significantly improved by about 20%. To understand the underlying principle on the capacity retention enhancement, the electrochemical properties of the cells including cell performance, impedance behavior as well as the characteristics of the interfacial properties were examined. Based on these results, it is suggested that the improved capacity retention of LiMn₂O₄/graphite Li-ion cells with addition of FEC especially at high temperature is mainly originated from the thin and stable SEI layer formed on the graphite anode surface.     

Novel SEI formation of maleimide-based additives and its improvement of capability and cyclicability in lithium ion batteries

This investigation elucidates three maleimide (MI)-based aromatic molecules as additives in electrolyte that is used in lithium ion batteries. The 1.1 M LiPF₆ in ethylene carbonate (EC):propylene carbonate (PC):diethylene carbonate (DEC) (3:2:5 in volume) containing MI-based additives can prompt the formation of a solid electrolyte interface (SEI); and inhibit the entering into the irreversible state during lithium intercalation and co-intercalation. The reduction potential is 0.71-0.98 V versus Li/Li⁺ as determined by cyclic voltammetry (CV). The morphology and element analysis of the positive and negative electrode after the 100th charge-discharge cycle are examined by scanning electron microscopy (SEM), energy dispersive spectrometry (EDS) and X-ray photoelectron spectroscopy (XPS). Moreover, the MI was used in lithium ion batteries and provided 4.9% capacity increase and 16.7% capacity retention increase when cycled at 1C/1C. The MI-based additive also ensures respectable cycle-ability of lithium ion batteries. MI is decomposed electrochemically to form a long winding narrow SEI strip on the graphite surface. This novel SEI strip not only prevents exfoliation on the graphite electrode but also stabilizes the electrolyte. The MI-based additive also ensures respectable cycle-ability of lithium ion batteries.     

Electrochemical reaction of the SiMn/C composite for anode in lithium ion batteries

The SiMn-graphite composite powder was prepared by mechanical ball milling and its electrochemical performances were evaluated as the candidate anode materials for lithium ion batteries. It is found that the cyclic performance of the composite materials is improved significantly compared to SiMn alloy and pure silicon. The heat treatment of the electrodes is beneficial for enhancing the cyclic stabilities. The SiMn-20 wt.% graphite composite electrode after annealing at 200 °C has an initial reversible capacity of 463 mAh g⁻¹ and a charge-discharge efficiency of 70%. Moreover, the reversible capacity maintains 426 mAh g⁻¹ after 30 cycles with a coulomb efficiency of over 97%. The phase structure and morphology of the composite were analyzed by X-ray diffraction (XRD) and scanning electron microscopy. The lithiation/delithiation behavior was investigated by electrochemical impedance spectroscopy (EIS) and cyclic voltammetry. The composite materials appear to be promising candidates as negative electrodes for lithium rechargeable batteries.     

Correlations between surface properties of graphite and the first cycle specific charge loss in lithium-ion batteries

We report the influence of two surface parameters, the active surface area (ASA) and the surface chemistry (oxygen functional groups) on the first electrochemical reduction of graphite. The experimental results highlight two important points. One, the ASA is the determining parameter which controls the exfoliation during the first electrochemical cycle. An ASA limit (ca. 0.2 m² g⁻¹) above which the exfoliation is suppressed was experimentally found. Below or above this limit, the specific charge loss remains almost constant even if the ASA changes. Two, for a sample having an ASA value higher than 0.2 m² g⁻¹, it is shown that the presence of oxygen groups at the surface is critical for the formation of an efficient solid electrolyte interphase (SEI) layer. The lack of oxygen groups during the first electrochemical reduction cycle hinders the electrolyte reduction process, and consequently increases the specific charge loss via exfoliation of the graphite electrode. This was confirmed by TPD measurement where significant release of CO gas occured above 400 °C, suggesting the presence of high-thermal-stable surface oxygen-containing groups of different natures in the as-received - SLX50 sample. Finally, it was found that H₂ treatment avoids the formation of oxygen-containing groups during air contact leading to exfoliation of the graphite sample.     

Effects of oxidation and heat treatment of acetylene blacks on their electrochemical double layer capacitances

Correlations between the electrochemical double layer capacitances of various acetylene blacks modified by surface oxidation and heat treatment, and their morphologies are presented. The acetylene blacks were different from each other in primary structural unit size (equivalent to mean particle diameter). They were oxidized in air at 300 °C for 1 h to produce graphene sheets protruding from the surfaces of the spherical particles. In addition, the surfaces of the acetylene blacks were modified by heat treatments from 1000 °C to 2800 °C, which resulted in a morphological change from surfaces covered with protruding graphene sheets to ones wrapped with basal planes of graphite. Correlations between the capacitances of the acetylene blacks and the observed morphologies showed that the surface covered with protruding graphene sheets was roughly 10 times more effective in capacitive charging than the surface of graphite basal planes. Specifically, the surface specific capacitance of the edged-graphene-sheet-covering surface was 146 mF/m², while that of the basal-planes-wrapping surface was 16 mF/m². It was concluded that the capacitances of the acetylene blacks were mainly defined by surface morphology, which were in turn influenced by structural unit size and degree of oxidation.     

EPR study on petroleum cokes annealed at different temperatures and used in lithium and sodium batteries

EPR spectroscopy was used for characterisation of petroleum cokes heat-treated in the temperature range of 480-2800 °C and after their electrochemical reactions with lithium and sodium. Unpaired spins with localised character were detected for cokes treated below 1000 °C, while delocalised electrons were found to contribute to the EPR profile of cokes treated between 1400 and 2800 °C. After electrochemical interaction of cokes with lithium, the EPR signal due to delocalised electrons undergoes a strong line narrowing combined with an increase in signal intensity. Localised paramagnetic centres interact with Li and Na, as a result of which there is a line narrowing and decrease in the signal intensity. A new narrow signal is detected for low-temperature cokes after reaction with lithium. The origin of this signal could be associated with ‘near-metallic’ lithium, which is accommodated in non-ordinary carbon sites such as microcavities. The intensity of this signal is higher for samples treated at 600 °C, where a higher hysteresis in the voltage versus composition curve is observed.     

Nano/micro-hybrid NiS cathodes for lithium ion batteries

The present study highlights a low temperature process by which 1D stacked 3D microstructures of nickel sulfide comprised of nanospikes have been synthesized and assembled as cathodes for lithium chalcogenide batteries. These micro/nano-clusters were synthesized hydrothermally under different conditions. These clusters exhibited a surface area of 15 m² g⁻¹. The present study also provides the first reports on the electrochemical performance of these NiS microclusters as cathode materials in lithium fluoro-Tris-sulfonimide electrolyte for lithium ion batteries. A detailed study has been performed to elucidate how surface morphology and redox reaction behaviors underlying these electrodes impact the cyclic behavior and specific capacity. This electrode−electrolyte combination showed minimal dissolution of the electrode in the electrolyte which was confirmed by inductively coupled plasma atomic emission spectroscopy. From the electrochemical analysis performed an intrinsic correlation between the capacity, self-discharge property and the surface morphology has been deduced and explained on the basis of relative contributions from the redox reactions of nickel sulfide in lithium fluoro-Tris-sulfonimide electrolyte. A working model of lithium battery in a coin cell form is also shown exhibiting a specific capacity of 550 mAh g⁻¹.