Study on capacity fade factors of lithium secondary batteries using LiNi0.7Co0.3O2 and graphite-coke hybrid carbon

In our previous work, 10 Wh-class (30650 type) lithium secondary batteries, which were fabricated with LiNi_0.7Co_0.3O₂ positive electrodes and graphite-coke hybrid carbon negative electrodes, showed an excellent cycle performance of 2350 cycles at a 70% state of charge charge-discharge cycle test. However, this cycle performance is insufficient for dispersed energy storage systems, such as home use load leveling systems. In order to clarify the capacity fade factors of the cell, we focused our investigation on the ability discharge capacity of the positive and negative electrodes after 2350 cycles. Although the cell capacity deteriorated to 70% of its initial capacity after 2350 cycles, it was confirmed that the LiNi_0.7Co_0.3O₂ positive electrode and graphite-coke hybrid negative electrode after 2350 cycles still have sufficient ability discharge capacity of 86 and 92% of their initial capacity, respectively. Accompanied by the result for a composition analysis of the positive electrode material by inductively coupled plasma (ICP) spectroscopy and atomic absorption spectrometry (AAS), electrochemical active lithium decreased and the Li_xNi_0.7Co_0.3O₂ positive electrode could be charged-discharged in a narrow range of between x=0.41 and 0.66 in the battery, although it had enough ability discharge capacity that can use between x=0.36 and 0.87. It is predicted that solid electrolyte interface formation by electrolyte decomposition on the carbon negative electrode during the charge-discharge cycle test is a main factor of the decrease of electrochemical active lithium.     

Improvement on the electrochemical characteristics of graphite anodes by coating of the pyrolytic carbon using tumbling chemical vapor deposition

The electrochemical characteristics of graphite coated with pyrolytic carbon materials using tumbling chemical vapor deposition (CVD) process have been studied for the active material of anodes in lithium ion secondary batteries. Coating of pyrolytic carbons on the surface of graphite particles, which tumble in a rotating reactor tube, was performed through the pyrolysis of liquid propane gas (LPG). The surface morphology of these graphite particles coated with pyrolytic carbon has been observed with scanning electron microscopy (SEM). The surface of graphite particles can well be covered with pyrolytic carbon by tumbling CVD. High-resolution transmission electron microscopy (HRTEM) image of these carbon particles shows that the core part is highly ordered carbon, while the shell part is disordered carbon. We have found that the new-type carbon obtained from tumbling CVD has a uniform core (graphite)-shell (pyrolytic carbon) structure. The electrochemical property of the new-type carbons has been examined using a charge-discharge cycler. The coating of pyrolytic carbon on the surface of graphite can effectively reduce the initial irreversible capacity by 47.5%. Cyclability and rate-capability of theses carbons with the core-shell structure are much better than those of bare graphite. From electrochemical impedance spectroscopy (EIS) spectra, it is found that the coating of pyrolytic carbon on the surface of graphite causes the decrease of the contact resistance in the carbon electrodes, which means the formation of solid electrolyte interface (SEI) layer is suppressed. We suggest that coating of pyrolytic carbon by the tumbling CVD is an effective method in improving the electrochemical properties of graphite electrodes for lithium ion secondary batteries.     

The study on the conductivity and morphology of polyurethaneurea electrolytes based on poly(ethylene glycol) 2000 and LiClO4

Polyurethaneureas (PUU), which were synthesized from 4,4′-diphenylmethane diisocyanate (MDI), poly(ethylene glycol) (PEG, MW=2000), and 3,5-diaminobenzoic acid, were used as the matrix of the polyelectrolytes in this study. Differential scanning calorimetry (DSC), Fourier-transform infrared spectroscopy (FT-IR), and ⁷Li magic angle spinning (MAS) solid-state NMR were used to monitor changes in the morphology of PUU electrolytes corresponding to the concentration of lithium perchlorate (LiClO₄) dopants. The results of DSC and FT-IR indicate the different polymer complexes formed by the interaction of the Li⁺ ions with the different coordination sites of PUU. The ⁷Li MAS solid-state NMR investigation of the PUU electrolytes points out that two different Li⁺ environments exist at lower temperature. The results of DSC and the ⁷Li MAS solid-state NMR show that Li⁺ ions are preferentially coordinated to the ether oxygen of the PEG soft-segment when the salt concentration is below 0.1 mmol LiClO₄(gPUU)⁻¹. Impedance spectroscopy measurements show that the conductivity behavior followed the Arrhenius equation and was influenced by the hard-segment T_g. One of the PUU electrolytes under the investigation has an ionic conductivity as high as 3.0×10⁻⁵ S cm⁻¹ at 30 °C.     

Effect of electrochemical conditions on the performance worsening of Si/C composite anodes for lithium batteries

An effective and practical method for producing Si/C composites with 10-15 wt% of silicon nanoparticles embedded in a carbon matrix is developed. The procedure consists of mechanically mixing Si with pitch followed by dispersing in toluene and final heat-treatment between 1000 and 1100 °C. The homogeneity of the materials was confirmed by optical microscopy and HRTEM. X-ray photoelectron spectroscopy, X-ray diffraction and N₂ adsorption at 77 K were applied for determining the structural and textural characteristics. The lithium insertion/deinsertion performance was monitored from the galvanostatic charge-discharge characteristics using a Si/C-lithium two-electrode cell, and varying the electrochemical parameters. Silicon essentially enhances the electrode capacity (C_rev up to 600 mAh/g for 15% Si), the effect being proportional to the component content, but it affects the cycle life. The first cycle reversible capacity increases with the decrease of current density and discharge cut-off potential. However, using such conditions during cycling leads to rapid saturation of the silicon particles, from which the decay of the electrochemical performance starts. It is demonstrated that the evolution of reversible and irreversible capacity is strongly dependent on the kinetics of lithium diffusion in silicon particles and on the discharge potential cut-off.     

Effects of electrolytes on the electrochemical performance of Si/graphite/disordered carbon composite anode for lithium-ion batteries

The influences of LiBF₄, LiClO₄, lithium bis(oxalato) borate (LiBOB), LiPF₆ with VC and without VC, and the mixed electrolytes composed of different ratios of LiBOB and LiPF₆ or LiClO₄ on the electrochemical properties of Si/graphite/disordered carbon (Si/G/DC) composite electrode were systematically investigated by constant current charge-discharge and electrochemical impedance spectra (EIS) techniques. Scanning electron microscopy (SEM) was used to observe the change of electrodes in morphology after given cycle numbers. X-ray photoelectron spectroscopy (XPS) was employed to understand the influences of different mixed electrolytes on the composition of SEI layers. The results showed that Si/G/DC composite electrode in the mixed electrolytes presented better electrochemical performance than in single electrolyte. The compactness and compositions of SEI layers intensively influenced the cycle performance of Si/G/DC composite materials. LiBOB and additive VC had a good synergistic effect on the formation of the dense SEI layers. In particular, Si/G/DC in 0.5 M LiBOB + 0.38 M LiPF₆ electrolytes containing VC exhibited a high reversible capacity and excellent cycle performance.     

Influence of manganese(II), cobalt(II), and nickel(II) additives in electrolyte on performance of graphite anode for lithium-ion batteries

Manganese dissolution into an electrolyte from the spinel LiMn₂O₄ in the lithium-ion cell has been recently investigated. In order to study the influence of the dissolved manganese species on the lithium intercalation/deintercalation into a natural graphite electrode, the electrochemical behavior of graphite was investigated in 1 mol dm⁻³ LiClO₄ electrolyte solution containing a small amount of Mn(II) by the addition of manganese(II) perchlorate. During the charging process, Mn(II) ions were firstly electroreduced on the electrode around 1.0 V versus Li/Li⁺ followed by irreversible decomposition of the electrolyte and lithium intercalation into the graphite. By microscopic observation of the graphite surface, manganese deposition was confirmed after the charge/discharge test. Due to the manganese deposition, the reversible capacity of the graphite electrode was drastically decreased. Furthermore, the cyclability of the anode was degraded with the amount of the manganese additive increasing. We compared these results with those of the cobalt(II) and nickel(II) additives by dissolving the corresponding perchlorates. Furthermore, we discussed the influence in practical cells based on the consideration of electrochemistry of the deposited metals.     

In situ FTIR study of the decomposition of N-butyl-N-methylpyrrolidinium bis(trifluoromethanesulfonyl)amide ionic liquid during cathodic polarization of lithium and graphite electrodes

In this work we analyzed the cathodic reactions of an important ionic liquid (IL) based electrolyte solution, namely lithium bis(trifluoromethylsulfonyl)imide (LiTFSI)/N-methyl-N-methylpyrrolidinium (BMP) TFSI. In situ FTIR spectroscopy was used for the analysis of gaseous products of the electrochemical decomposition of this IL solution during cathodic polarization of lithium metal and graphite electrodes. The main volatile product of the reductive decomposition of the anion in these BMPTFSI solutions is trifluoromethane. BMP cations decompose to mixtures of tertiary amines and hydrocarbons. The composition of the products is influenced by the nature of the anode material. Graphite possesses a catalytic activity in the electroreduction process of BMP cations which occurs along with their intercalation into the graphite structure. The liquid phase after cathodic polarization of graphite electrodes was analyzed by multinuclear NMR spectroscopy coupled with FTIR spectroscopy. ¹⁵N NMR and FTIR spectra revealed an increase in the Li cations content in the electrolyte solution, as a result of BMP cations decomposition during repeated cycling of graphite electrodes.     

Structure and electrochemical performance of nanostructured Fe3O4/carbon nanotube composites as anodes for lithium ion batteries

Polyvinyl alcohol (PVA) was used as a hydrogen bond functionalizing agent to modify multi-walled carbon nanotubes (CNTs). Nanoparticles of Fe₃O₄ were then formed along the sidewalls of the as-modified CNTs by the chemical coprecipitation of Fe²⁺ and Fe³⁺ in the presence of CNTs in an alkaline solution. The structure and electrochemical performance of the Fe₃O₄/CNTs nanocomposite electrodes have been investigated in detail. Electrochemical tests indicated that at the 145th cycle, the CNTs-66.7 wt.%Fe₃O₄ nanocomposite electrode can deliver a high discharge capacity of 656 mAh g⁻¹ and stable cyclic retention. The improvement of reversible capacity and cyclic performance of the Fe₃O₄/CNTs nanocomposite could be attributed to the nanosized Fe₃O₄ particles and the network of CNTs.     

Synthesis and electrochemical performance of Li2FeSiO4/carbon/carbon nano-tubes for lithium ion battery

Li₂FeSiO₄/carbon/carbon nano-tubes (Li₂FeSiO₄/C/CNTs) and Li₂FeSiO₄/carbon (Li₂FeSiO₄/C) composites were synthesized by a traditional solid-state reaction method and characterized comparatively by X-ray diffraction, scanning electron microscopy, BET surface area measurement, galvanostatic charge-discharge and AC impedance spectroscopy, respectively. The results revealed that the Li₂FeSiO₄/C/CNT composite exhibited much better rate performance in comparison with the Li₂FeSiO₄/C composite. At 0.2 C, 5 C and 10 C, the former composite electrode delivered a discharge capacity of 142 mAh g⁻¹, 95 mAh g⁻¹, 80 mAh g⁻¹, respectively, and after 100 cycles at 1 C, the discharge capacity remained 95.1% of its initial value.     

Li NMR spectroscopy and ion conduction mechanism of composite gel polymer electrolyte: A comparative study with variation of salt and plasticizer with filler

Microporous composite gel polymer electrolyte (CGPE) has been prepared by incorporating the home-made silica aerogel (SAG) particles into the poly(vinylidene fluoride-co-hexafluoropropylene) (PVdF-HFP) copolymer/LiClO₄ matrix. The ionic transport behavior of the electrolyte is studied with various experimental techniques such as AC impedance, X-ray diffraction (XRD), infrared (IR) spectra, nuclear magnetic resonance (NMR), scanning electron microscopy (SEM), differential scanning calorimetry (DSC) and thermogravimetric analyzer (TGA), etc. The results reveal that the SAG particles are well dispersed in the electrolytes and incorporate with the other components of the CGPEs. The solid-state ⁷Li NMR study has confirmed the interactions of lithium ion with SAG, polymer and plasticizers, causing to form the microporous structure and reduce the glass transition temperature and crystallinity, resulting in an increase in ionic conductivity of the CGPE. The best ionic conductivity (1.04 × 10⁻² S/cm at room temperature) is obtained from the composite polymer electrolyte containing 4 wt% of SAG, which is approximately four times higher than the ionic conductivity of the electrolyte without the filler.     

A promising sol-gel route based on citric acid to synthesize Li3V2(PO4)3/carbon composite material for lithium ion batteries

Li₃V₂(PO₄)₃/carbon composite material was synthesized by a promising sol-gel route based on citric acid using V₂O₅ powder as a vanadium source. Citric acid acts not only as a chelating reagent but also as a carbon source, which enhance the conductivity of the composite material and hinder the growth of Li₃V₂(PO₄)₃ particles. The structure and morphology of the sample were characterized by TG, XRD and TEM measurements. XRD results reveal that Li₃V₂(PO₄)₃/carbon was successfully synthesized and has a monoclinic structure with space group P2₁/n. TEM images show Li₃V₂(PO₄)₃ particles are about 45 nm in diameter embeded in carbon networks. Galvanostatic charge/discharge and cyclic voltammetry measurements were used to study its electrochemical behaviors which indicate the reversibility of the lithium extraction/insertion processes. Li₃V₂(PO₄)₃/carbon performed in a voltage window (3.0-4.8 V) exhibits higher discharge capacity, better cycling stability and its discharge capacity maintains about 167.6 mAh/g at a current density of 28 mA/g after 50 cycles.     

Preparation and electrochemical performance of Li4Ti5O12/carbon/carbon nano-tubes for lithium ion battery

A Li₄Ti₅O₁₂/carbon/carbon nano-tubes (Li₄Ti₅O₁₂/C/CNTs) composite was synthesized by using a solid-state method. For comparison, a Li₄Ti₅O₁₂/carbon (Li₄Ti₅O₁₂/C) composite and a pristine Li₄Ti₅O₁₂ were also synthesized in the present study. The microstructure and morphology of the prepared samples are characterized by XRD and SEM. Electrochemical properties of the samples are evaluated by using galvanostatic discharge/charge tests and AC impedance spectroscopy. The results reveal that the Li₄Ti₅O₁₂/C/CNTs composite exhibits the best rate capability and cycling stability among the samples of Li₄Ti₅O₁₂, Li₄Ti₅O₁₂/C and Li₄Ti₅O₁₂/C/CNTs. At the charge-discharge rate of 0.5 C, 5.0 C and 10.0 C, its discharge capacities were 163 mAh/g, 148 mAh/g and 143 mAh/g, respectively. After 100 cycles at 5.0 C, it remained at 146 mAh/g.     

A study on the initiation of pitting corrosion in carbon steel in chloride-containing media using scanning electrochemical probes

Scanning electrochemical probes of corrosion potential and chloride ions were developed for the in situ monitoring of localized corrosion processes of reinforcing steel in NaCl-containing solution. The results indicated that the chloride ions (Cl⁻) preferentially adsorbed and accumulated at the imperfect/defective sites, resulting in initiation and propagation of pitting corrosion on the reinforcing steel surface. An electron microprobe analyzer (EMPA) was used to examine the corrosion morphology and elemental distribution at the corroded location to investigate the origins of the preferential Cl⁻ adsorption and pitting corrosion. By combining the in situ and ex situ images, we concluded that manganese sulfide inclusions in reinforcing steel are the most susceptible defects to pitting corrosion in chloride-containing solution.     

Study on the rapid synthesis of LiNi1−xCoxO2 cathode material for lithium secondary battery

LiNi_1−xCo_xO₂ (x = 0, 0.1, 0.2) cathode materials were successfully synthesized by a rheological phase reaction method with calcination time of 0.5 h at 800 °C. All obtained powders are pure phase with α-NaFeO₂ structure (R-3m space group). The samples deliver an initial discharge capacity of 182, 199 and 189 mAh g⁻¹ (25 mA g⁻¹, 4.35-3.0 V), respectively. The reaction mechanism was also discussed, which consists of a series of defect reactions. As a result of these defect reactions, the reaction of forming LiNi_1−xCo_xO₂ takes place in high speed.     

Comparative study of the preparation and electrochemical performance of LiNi<Subscript>1</Subscript><Subscript>/2</Subscript>Mn<Subscript>1/2</Subscript>O<Subscript>2</Subscript> electrode material for rechargeable lithium batteries

Positive electrode material LiNi_1/2Mn_1/2O₂ was synthesized via the carbonate co-precipitation method and the hydroxide precipitation route to study the effects of the precursor on its structural and electrochemical properties. The results of X-ray diffraction and Rietveld refinement show that the carbonate precursor of Ni²⁺ and Mn²⁺ exhibits one phase at a pH of 8.5, while the hydroxide deposit separates into Ni(OH)₂ and Mn(OH)₂ phases under the same experimental conditions. LiNi_1/2Mn_1/2O₂ material prepared from the hydroxide precursor shows 8.9% Li/Ni exchange and a large capacity loss of 11.3% in the first 10 cycles. By contrast, more uniform distribution of transition metal ions and stable Mn²⁺ in the carbonate precursor contribute to only 7.8% Li/Ni disorder in the obtained LiNi_1/2Mn_1/2O₂, which delivers a reversible capacity of about 182 mAh g⁻¹ at a current rate of 14 mA g⁻¹ between 2.5 and 4.8 V.     

The effects of oxygen flow rate and anion doping on the performance of the LiNio<Subscript>2</Subscript> electrode for lithium secondary batteries

This work presents the effects of O₂ flow rate and S-doping on structural and electrochemical properties of LiNiO₂. Layered LiNiO₂ were prepared using a sol-gel method. It was found that oxygen plays an important role in the crystallization of layered LiNiO₂. The deficiency of oxygen in the crystallization process induced the inclusions of impurities and cubic rock-salt structure in LiNiO₂ powders. For LiNiO₂ prepared at high O₂ flow rates, the electrode delivered high initial discharge capacity with a relatively good retention rate. S-doped LiNiO₂ not only stabilized the structural integrity of the electrode material, but also increased the electrode performance.     

Comparative study of Li(Li1/3Ti5/3)O4 and Li(Ni1/2−xLi2x/3Tix/3)Ti3/2O4 (x = 1/3) anodes for Li rechargeable batteries

A solid solution of spinel (2/3)Li(Li_1/3Ti_5/3)O₄–(1/3)Li(Ni_1/2Ti_3/2)O₄ was prepared, and its structural/electrochemical properties were compared with Li(Li_1/3Ti_5/3)O₄ to identify the effect of doping to the structural invariance of Li(Li_1/3Ti_5/3)O₄. The solid solution retained the zero strain characteristic of Li(Li_1/3Ti_5/3)O₄ during discharge/charge with an excellent cycle stability, while the rate capability was notably improved. However, a reversible broadening of the XRD peak was observed at the end of discharge, indicating some structural changes. XANES measurements showed that the oxidation state of Ti was +4 and that of Ni was +2 in the solid solution.     

A study on electrochemical characteristics of LiCoO2/LiNi1/3Mn1/3Co1/3O2 mixed cathode for Li secondary battery

In this study, the LiCoO₂/LiNi_1/3Mn_1/3Co_1/3O₂ mixed cathode electrodes were prepared and their electrochemical performances were measured in a high cut-off voltage. As the contents of LiNi_1/3Mn_1/3Co_1/3O₂ in the mixed cathode increases, the reversible specific capacity and cycleability of the electrode enhanced, but the rate capability deteriorated. On the contrary, the rate capability of the cathode enhanced but the reversible specific capacity and cycleability deteriorated, according to increasing the contents of LiCoO₂ in the mixed cathode. The cell of LiCoO₂/LiNi_1/3Mn_1/3Co_1/3O₂ (50:50, wt.%) mixed cathode delivers a discharge capacity of ca. 168 mAh/g at a 0.2 C rate. The capacity of the cell decreased with the current rate and a useful capacity of ca. 152 mAh/g was obtained at a 2.0 C rate. However, the cell shows very stable cycleability: the discharge capacity of the cell after 20th charge/discharge cycling maintains ca. 163 mAh/g.     

A review on the mechanical and electrical properties of graphite and modified graphite reinforced polymer composites

Carbon materials particularly in the form of sparkling diamonds have held mankind spellbound for centuries, and in its other forms, like coal and coke continue to serve mankind as a fuel material, like carbon black, carbon fibers, carbon nanofibers and carbon nanotubes meet requirements of reinforcing filler in several applications. All these various forms of carbon are possible because of the element's unique hybridization ability. Graphene (a single two-dimensional layer of carbon atoms bonded together in the hexagonal graphite lattice), the basic building block of graphite, is at the epicenter of present-day materials research because of its high values of Young's modulus, fracture strength, thermal conductivity, specific surface area and fascinating transport phenomena leading to its use in multifarious applications like energy storage materials, liquid crystal devices, mechanical resonators and polymer composites. In this review, we focus on graphite and describe its various modifications for use as modified fillers in polymer matrices for creating polymer-carbon nanocomposites.     

The study of lithium ion de-insertion/insertion in LiMn2O4 and determination of kinetic parameters in aqueous Li2SO4 solution using electrochemical impedance spectroscopy

In this work, we report a basic study on the mechanism of lithium ion de-insertion/insertion process from/into LiMn₂O₄ cathode material in aqueous Li₂SO₄ solution using electrochemical impedance spectroscopy (EIS). An equivalent circuit distinguishing the kinetic parameters of lithium ion de-insertion/insertion is used to simulate the experimental impedance data. The fitting results are in good agreement with the experimental results and the parameters of the kinetic process of Li⁺ de-insertion and insertion in LiMn₂O₄ at different potentials during charge and discharge are obtained using the same circuit. The results indicate that the de-insertion/insertion behavior of lithium ions at LiMn₂O₄ cathode in Li₂SO₄ aqueous solution is similar to that reported in the organic electrolytes. The charge transfer resistance (R_ct), warburg resistance, double layer capacitance and chemical diffusion coefficient (DLi+) vary with potentials during de-insertion/insertion processes. R_ct is lowest at the CV peak potentials and the important kinetic parameter, DLi+ exhibits two distinct minima at potentials corresponding to CV peaks during de-insertion–insertion and it was found to be between 10⁻⁸ and 10⁻¹⁰ cm² s⁻¹during lithium de-insertion/insertion processes.