Increase In Internal Resistance Research Articles

Prediction of Storage Ageing in a Lithium Ion Battery Using an Electrochemical Model

Hybrid and electric vehicles are becoming increasingly popular due to their benefits in lowering emissions and lower dependency on fossil fuels. A hybrid vehicle battery spends around 70% of its time in storage at different temperatures compared to being cycled. However there is less work on the effect of storage on battery life time than on cycling. Storage life time prediction is often complicated due to interdependency of ageing mechanisms. In particular, storage ageing is difficult to accelerate and moreover, the ageing is not linearly scalable with time. These characteristics are difficult to analyse using a model such as equivalent circuit models which depend heavily on experimental data. Electrochemical models are a feasible option to analyse the chemical and electrical response of a battery under storage conditions. In this work, the Pseudo Two Dimensional Model (P2D) equations are modified to include a continuous solvent reduction reaction responsible for capacity fade. A FORTRAN program is developed and finite volume based formulation is used to discretize the system of equations; electrolyte concentration, solid concentration, solid potential and electrolyte potential coupled with Butler-Volmer kinetics. An unconditionally-stable implicit method is adopted for time dependent calculations; the time discretization has first order accuracy. Mesh generation was done based on the initial radius and battery dimensions. The capability of this model to predict the SEI layer growth and internal resistance increase under different operating condition is used to analyse and quantify the parasitic ageing reactions. The over potential of the side reaction reduces to zero for a storage condition where no external current is applied. The capacity fading solvent reaction parameters are proportional to the interfacial surface area and the side reaction exchange current density. Thus the critical parameter controlling the rate of SEI layer growth is the side reaction exchange current density. Another important parameter is the temperature of the battery which is found to accelerate cell ageing. However, in this initial work, the analysis is limited to iso-thermal conditions since the dependency of temperature on cell performance is complex. The storage experiment can be divided into two main parts, the characterisation and the storage test. For the first one, an ESPEC thermal chamber has been used to control temperature and humidity level. For storage test, the cells are stored at 25°C at three different SoC values, 20%, 50% and 90% and the capacity characterisation test has been performed after 73, 139, 202 and 297 days to calculate the capacity of the cell. The side reaction coefficient in the electrochemical model was adjusted to match the experimental data for the first point after 73 days. The capacity at the remaining points are predicted based on this initial parameterisation. Conclusions about the parasitic reaction are made based on the variation side reaction exchange current density. The storage results are also used to fine tune the SEI properties and the influence of storage on capacity fading side reaction is investigated for the first time using an electrochemical model. This work establishes a guideline for calculating the SEI properties based on storage ageing experimentation where the driving potential is nearly zero. The results presented in this work can be used as a guiding tool for better battery design for different storage profiles. Figure 1

Simultaneous Wastewater Treatment, Algal Biomass Production and Electricity Generation in Clayware Microbial Carbon Capture Cells.

Performance of microbial carbon capture cells (MCCs), having a low-cost clayware separator, was evaluated in terms of wastewater treatment and electricity generation using algae Chlorella pyrenoidosa in MCC-1 and Anabaena ambigua in MCC-2 and without algae in a cathodic chamber of MCC-3. Higher power production was achieved in MCC-1 (6.4W/m3) compared to MCC-2 (4.29W/m3) and MCC-3 (3.29W/m3). Higher coulombic efficiency (15.23±1.30%) and biomass production (66.4±4.7mg/(L*day)) in MCC-1 indicated the superiority of Chlorella over Anabaena algae for carbon capture and oxygen production to facilitate the cathodic reduction. Algal biofilm formation on the cathode surface of MCC-1 increased dissolved oxygen in the catholyte and decreased the cathodic charge transfer resistance with increase in reduction current. Electrochemical analyses revealed slow cathodic reactions and increase in internal resistance in MCC-2 (55 Ω) than MCC-1 (30 Ω), due to lower oxygen produced by Anabaena algae. Thus, biomass production in conjunction with wastewater treatment, CO2 sequestration and electricity generation can be achieved using Chlorella algal biocathode in MCC.

Remaining Useful Life Prediction of Lithium-ion Battery Based on Discrete Wavelet Transform

Abstract The remaining useful life (RUL) and state-of-health (SoH) are critical to battery management system (BMS) to ensure the safety and reliability of electric vehicle (EV) operation. Recent literatures have reported various methods to estimate the RUL and SoH with a focus on the capacity loss, internal resistance increase, voltage drop, self-discharge, number of cycles, etc. However, most of the works only consider the battery in certain cases without thinking about the factors in real operation. In some cell internal parameters identification approaches, the accuracy of prediction results highly depends on the model and identification method. Aiming at these problems, a model free RUL prediction method is proposed in this work based on the discrete wavelet transform (DWT). The dynamic stress test (DST) schedule is conducted on the commercial 1665130-type lithium-ion battery and various outputs with non-stationary and transient phenomena are obtained and analyzed. Finally, experiments are conducted on the lithium-ion battery with different aging levels to verify the proposed method, and the results indicate that high accuracy of RUL prediction can be obtained by this quantitative correlation.

An Overview of Different Approaches for Battery Lifetime Prediction

With the rapid development of renewable energy and the continuous improvement of the power supply reliability, battery energy storage technology has been wildly used in power system. Battery degradation is a nonnegligible issue when battery energy storage system participates in system design and operation strategies optimization. The health assessment and remaining cycle life estimation of battery gradually become a challenge and research hotspot in many engineering areas. In this paper, the battery capacity falling and internal resistance increase are presented on the basis of chemical reactions inside the battery. The general life prediction models are analysed from several aspects. The characteristics of them as well as their application scenarios are discussed in the survey. In addition, a novel weighted Ah ageing model with the introduction of the Ragone curve is proposed to provide a detailed understanding of the ageing processes. A rigorous proof of the mathematical theory about the proposed model is given in the paper.

Analysis on Vibration of Li-ion Battery Module Used for Electric Vehicle

The vibration tests of Li-ion battery module used for electric vehicle were researched about open circuit voltage. The results showed that the structure of the lithium-ion power battery module remains unchangeable, and the voltage and temperature collection remains stable in the vibrating process. Before and after the vibration, the AC internal resistance of the lithium-ion battery module has no obvious change, and the active particles inside the battery is dispersed to move in high frequency vibration, so that its conductivity is significantly reduced, leading to the increase in DC internal resistance and decrease in charge capacity of battery module.

Degradation Study by Start-Up/Shut-Down Cycling of Superhydrophobic Electrosprayed Catalyst Layers Using a Localized Reference Electrode Technique.

Degradation of a polymer electrolyte membrane fuel cell (PEMFC) with electrosprayed cathode catalyst layers is investigated during cyclic start-up and shut-down events. The study is carried out within a single cell incorporating an array of reference electrodes that enables measurement of cell current as a function of local cathode potential (localized polarization curves). Accelerated degradation of the cell by start-up/shut-down cycling gives rise to inhomogeneous performance loss, which is more severe close to the gas outlet and occurs predominantly during start-up. The degradation consists primarily of loss of cathode catalyst activity and increase in cell internal resistance, which is attributed to carbon corrosion and Pt aggregation in both anode and cathode. Cells with an electrosprayed cathode catalyst layer show lower degradation rates during the first 100 cycles, compared with those of a conventional gas diffusion electrode. This difference in behavior is attributed to the high hydrophobicity of the electrosprayed catalyst layer microstructure, which retards the kinetics of corrosion of the carbon support. In the long term, however, the degradation rate is dominated by the Pt/C ratio in the cathode catalyst layer.

Needle age dependence of photosynthesis along a light gradient within an Abies alba crown

Age and shade effects on needle structure and photosynthesis were determined within a lower part of Abies alba crown along a horizontal increasing gradient of branches self-shading. It was hypothesized that a decrease in net CO2 assimilation rate with increasing needles’ age would be related to: (1) structural age-related changes in needles, (2) reduction of stomatal conductance, (3) nitrogen translocation from old to young needles, and (4) decrease in efficiency of photochemical processes. Leaf mass-to-area ratio increased non-linearly with needle age. In a needle cross section, distance between the vascular bundles decreased, and height of palisade parenchyma cells increased with age. The structural changes observed in our study might lead to an increase in internal resistance to CO2 with greater needle age. Total needle nitrogen concentration linearly decreased with age due to dilution and/or translocation to younger needles. When expressed per needle area, nitrogen content was reduced in 6-year-old needles compared with younger ones. Net CO2 assimilation rate per needle area decayed and was accompanied by a decrease in transpiration and water and photosynthetic nitrogen use efficiency. Old needles maintained high photochemical efficiency which compensated to some extent for light deficit in their micro-light environments. Our results have suggested that there is a mechanism controlling the relation between efficiency of light and dark photosynthetic processes along the needle age and shade gradient in A. alba crown.

Systematic aging of commercial LiFePO 4 |Graphite cylindrical cells including a theory explaining rise of capacity during aging

Abstract The contribution introduces a new theory explaining the capacity increase that is often observed in early stages of life of lithium-ion batteries. This reversible and SOC-depending capacity rise is explained by the passive electrode effect in this work. The theory assumes a slow, compensating flow of active lithium between the passive and the active part of the anode, where the passive part represents the geometric excess anode with respect to the cathode. The theory is validated using a systematic test of 50 cylindrical 8 Ah LiFePO4|Graphite battery cells analyzed during cyclic and calendaric aging. The cyclic aging has been performed symmetrically at 40 °C cell temperature, varying current rates and DODs. The calendar aging is executed at three temperatures and up to four SOCs. The aging is dominated by capacity fade while the increase of internal resistance is hardly influenced. Surprisingly shallow cycling between 45 and 55% SOC shows stronger aging than aging at higher DOD and tests at 4 C exhibit less aging than aging at lower C-rates. Aging mechanisms at 60 °C seem to deviate from those at 40 °C or lower. The data of this aging matrix is used for further destructive and non-destructive characterization in future contributions.

Evaluation of Thermoelectric Performance and Durability of Functionalized Skutterudite Legs

Thermoelectric generators are a promising technology for waste heat recovery. As new materials and devices enter a market penetration stage, it is of interest to employ fast and efficient measurement methods to evaluate the long-term stability of thermoelectric materials in combination with metallization and coating (functionalized thermoelectric legs). We have investigated a method for measuring several thermoelectric legs simultaneously. The legs are put under a common temperature gradient, and the electrical characteristics of each leg are measured individually during thermal cycling. Using this method, one can test different types of metallization and coating applied to skutterudite thermoelectric legs and look at the relative changes over time. Postcharacterization of these initial tests with skutterudite legs using a potential Seebeck microprobe and an electron microscope showed that oxidation and interlayer diffusion are the main reasons for the gradual increase in internal resistance and the decrease in open-circuit voltage. Although we only tested skutterudite material in this work, the method is fully capable of testing all kinds of material, metallization, and coating. It is thus a promising method for studying the relationship between failure modes and mechanisms of functionalized thermoelectric legs.

Ultra-Thin Rechargeable Lithium Ion Batteries on Flexible Polymer: Design, Low Temperature Fabrication and Characterization

In this work we developed flexible rechargeable thin film lithium ion batteries based on all-solid-state materials. The flexible batteries were fabricated using low temperature sequential physical vapor deposition techniques. A thin film battery (TFB) architecture consisting of Ti/V2O5/LiPON/Li/encapsulation multilayer was deposited on flexible polyimide substrate. To the best of our knowledge, the obtained TFBs are among the thinnest reported with an overall thickness of 50 μm including substrate, active layers and encapsulation stack. The fabricated TFBs delivered discharge capacity as high as 0.15 mAh (1.5 cm² of cathode area) upon galvanostatic cycling at 30 μA.cm−2 within 3.8–1.5 V voltage range. The capacity decreased during cycling with an average fading rate of 0.25% over 100 cycles. Electrochemical impedance spectroscopy (PEIS) measurements and modeling analysis revealed an increase of the internal resistance during cycling. Furthermore, it was shown that the internal resistance increase was induced by a progressive decrease of the electrolyte/electrodes contact area. TFBs having the same architecture and cell design were fabricated on two substrates: a flexible polyimide foil and a rigid silicon wafer. Electrochemical characterization showed a clear influence of substrate type on TFBs discharge capacity and internal resistance variations during cycling, the results are discussed within this paper

Non-Destructive Detection of Local Aging in Lithium-Ion Pouch Cells by Multi-Directional Laser Scanning

Understanding the mechanical activity of lithium-ion cells during cycling and its connection with aging phenomena is essential to improve cell design and operation strategies. Previous studies of lithium-ion pouch cells [B. Rieger et al., Journal of Energy Storage, 8, 1 (2016)] have shown non-uniform swelling with local displacement overshoots during charging. In this experimental work, a novel three-dimensional laser scanning method is used to investigate local reversible and irreversible thickness changes of six commercial LiCoO2/graphite cells during a cyclic aging experiment. Three cycle scenarios were included and two cells each were exposed to a specific temperature and charging rate. The cells showing local displacement overshoots also exhibit non-uniform distributions of irreversible thickness change. Post-mortem analysis showed largely inhomogenously degraded surfaces of the single anode layers. It is shown that the cells’ irreversible thickness change correlates with capacity fade and internal resistance increase monitored via electrochemical impedance spectroscopy.

Li-Ion Battery Deep Discharge Degradation

Deep discharging or over-charging of Li-ion cells must be avoided to ensure sufficient life-time and sometimes also the operation safety of these batteries. BMS circuits co-operating with the charger and forbidding deep discharging of the cells are necessarily present in battery packs. However, deep discharging can appear in a case of BMS failure or poor user care of the batteries. Then the battery can be discharged to zero voltage. Battery manufacturers do not allow such operation and battery behavior in this situation is not described in their datasheets. Experimental deep discharging was performed. After discharging, the battery was left in a fully discharged state (zero voltage) for various time intervals before it was fully recharged again. Consequently, capacity decrease or internal resistance increase was checked.

Chemical Analysis of Casual Elements for Deterioration of Cylindrical EDLC

SUMMARYChemical analysis was carried out before and after the constant voltage hold test that was an acceleration deterioration examination to clarify deterioration factors of electric double‐layer capacitor (EDLC). The results showed that the stress test slightly caused the increase of internal resistance. It was also confirmed that the range of fluorochemicals was formed on the electrode surface for approximately 10 nm in depth using electron spectroscopy for chemical analysis (ESCA). From the chemical analysis of the electrolyte using an inductively coupling plasma emission analyzer (ICP‐OES), it was confirmed that the electrolyte included silicon which is one of the ingredient elements of an electrode and that the increase in holding voltage in the stress test decreased the silicon density in the electrolyte.

The influence of battery degradation level on the selected traction parameters of a light-duty electric vehicle

The article describes results of an analysis of the impact of degradation level of battery made in lead-acid technology on selected traction parameters of an electric light duty vehicle. Lead-acid batteries are still used in these types of vehicles. They do not require complex systems of performance management and monitoring and are easy to maintaining. Despite the basic disadvantage, which is the low value of energy density, low price is a decisive factor for their use in low-speed electric vehicles. The process of aging of the battery related with an increase in internal resistance of the cells and the loss of electric capacity of the battery was considered. A simplified model of cooperation of the DC electric motor with the battery assuming increased internal resistance was presented. In the paper the results of comparative traction research of the light-duty vehicle equipped with a set of new batteries and set of batteries having a significant degradation level were showed. The analysis of obtained results showed that the correct exploitation of the battery can slow down the processes of degradation and, thus, extend battery life cycle.

Lithium Nitrate in Rechargeable Lithium-Sulfur Batteries: Catalytic Feature of Redox Shuttle Suppression

A big breakthrough in the study of rechargeable Li-S batteries was the discovery of LiNO3 additive in 2008, which effectively suppresses LiPSs redox shuttle and enables real rechargeable Li-S batteries. The role of LiNO3, according to conventional understandings, was to react with lithium anode in a controlled way to form a LiNxOy passiviation layer on the anode which prevents the reduction of polysulfide thereon. This understanding, however, is seeing more challenges. Recently, Dr. Sheng S. Zhang of US Army Research Lab detected progressive reduction of LiNO3 on Li anode accompanied by the gradual increase of internal resistance in Li-S batteries. This finding is a direct challenge to the claim that LixNOy layer on anode prevents the LiPSs reduction thereon, as LiPSs with higher reduction voltage (2.1 V) are more easily to be reduced than LiNO3 (1.7 V). The unclear mechanism of redox-shuttle suppression and limited understanding on the impact of LiNO3 have hampered the development of Li-S batteries with long-term cycling stability, despite huge efforts and encouraging progress made in the engineering of nanostructured sulfur cathode reported in a large number of high impact journal publications.In this manuscript, we revisit some key factors that influence LiPSs redox shuttles, clarify the roles of LixNOy passivation layers, and propose a new mechanism that redox shuttle suppression in the presence of LiNO3 is due to strong binding between the soluble high-order LiPSs and nitrate (NO3 –) anions adsorbed on carbon substrate, which promotes the oxidation of polysulfides to sulfur on charging. This is supported by density functional theory (DFT) calculations and NO3 –-induced low self-discharge rate in Li-S batteries. As the progressive reduction of LiNO3 compromises long-term cycling stability of Li-S batteries by gradual increase of the internal resistance, we propose the substitution of soluble LiNO3 with solid oxide catalyst of good polysulfide oxidation reaction (PSOR) activity, to which ruthenium oxide was found as a preferable choice. This new understanding on the role of LiNO3 is opening a new research path to the development of nanostructured polysulfide oxidation catalysts with fine-tuned composition and morphology to boost the performance of Li-S batteries holistically.

Influence of Porous Separators on the Ionic Conductivity of Nonflammable Liquid Perfluoropolyether Electrolytes

Conventional Lithium-ion coin cell batteries contain a volatile flammable liquid electrolyte, impregnated in a porous separator. This leads to a serious safety issue with, for example, the possibility of flammable reaction products and internal short-circuits.[1] Commercial microporous polyolefin membranes, made of semi-crystalline polymers as polyethylene (PE) and polypropylene (PP), have been widely used as battery separators. [2] The main functions of the separator are to act as a physical barrier to prevent short-cut between the two electrodes and to allow ionic transport. Ionic transport within the separator depends on several factors such as the separator geometry, porosity and tortuosity, the electrolyte resistance and wettability.[3] Thus, the presence of the separator leads to an effective ionic conductivity lower than the bulk electrolyte conductivity, and an increase of the Li battery internal resistance. The recent development of non-flammable liquid electrolytes based on functionalized perfluoropolyethers (PFPEs) opened the path of safe Li-ion batteries.[4, 5] PFPE is chemically resistant, non-crystalline, and nonflammable exhibit low glass transition temperature and low toxicity. It was demonstrated that a methyl carbonate-terminated perfluoropolyether (see Figure 1) is suitable for lithium battery operation, but optimization is necessary to improve the performance. So far little is known on the transport properties of this new class of non-flammable liquid electrolytes within the separator. As for any conventional liquid electrolyte, investigating the nature of the separator and its effect on the effective ionic conductivity is required to achieve an optimal battery design. We investigated the physico-chemical and electrochemical properties of three commercial separators: a PP monolayer, a trilayer PP-PE-PP and a polytetrafluoroethylene (PTFE). The ionic conductivities of low molecular weight PFPEs, with dimethyl carbonate or ethoxylated alcohol end-groups, doped with the lithium salt LiTFSI was measured as a function of the temperature either in the bulk or in the presence of the separators. For comparison, a conventional liquid electrolyte made of 1M LiPF6 in a mixture of ethylene carbonate and dimethyl carbonate was also studied. In addition, the effective ionic conductivity of each electrolyte impregnated in the separator is modeled based on the bulk electrolyte conductivity and a separator morphological form factor.

Temperature Effect on the Cell Life of Molten Carbonate Fuel Cell

This study proposes the temperature effect on the cell life and performance with coin type molten carbonate fuel cells (MCFC). The cells were operated at 600 oC, 700 oC and 800 oC under an atmospheric condition. The temperature effects were analyzed with electrochemical methods of steady state polarization, step chronopotentiometry, and electrochemical impedance spectroscopy. In general, the electrochemical reaction is accelerated by the temperature rising. Thus, higher exchange current is expected at higher temperature. The ionic conductivity in the carbonate electrolyte is also increased by the temperature increasing. Moreover, the gas solubility into molten carbonate has positive activation energies. Diffusion in the liquid and gas phases is also enhanced by the temperature. Therefore, higher performance is obtained at higher temperature. However, the carbonate has volatility, although it can be negligible below 600oC. The metal corrosion is also a function of temperature, higher temperature results in larger corrosion rate. Therefore, it could be guessed that reduction of electrolyte amount by the temperature rising makes higher internal resistance and mass transfer resistance. The OCV at 700 oC was higher than that at 800 oC because the absolute value of Gibbs energy is larger at 700 oC. The OCV at 700 oC continues for 1,200 h whereas 800oC continues only 250 h. Therefore, the cell life at 700 oC was much longer than that at 800 oC. At 150 mAcm-2, voltage of 700 oC was compared with that of 800 oC. It was comprehensible to know the temperature effect more clearly. The voltage at 800oC was initially higher than that at 700 oC. This is acceptable because the voltage at higher temperature has lower ionic and reaction resistances. However, the voltage at 800oC drops very quickly, indicating the electrolyte consumption was significant. On the other hand, the voltage at 700 oC is relatively stable and its internal resistance is slowly increasing. Therefore, lower temperature cell gives more stable and longer cell life. The cell performance of the electrolyte was analyzed at every 300 h of the cell at 700 oC and at every 60 h of the cell at 800 oC. From the steady state polarization, the current-voltage behaviors were measured while current density is applied from 0 mAcm- 2 to 150 mAcm- 2. The changes of the slope were increasing but not significant at 700 oC. However, the cell at 800oC shows severe slope change by the time. It means the resistance increasing becomes severe at higher temperature. At the step-chronopotentiometry, current steps were applied at every 60 second and following voltage relaxation was observed. All the cells showed step-voltage relaxation according to the current step. At early stage of operation, voltage relaxation was consistent. However at the cell of 800 oC, voltage relaxation was increased significantly at the end part of operation. It means mass transfer resistance becomes significant. The impedance behaviors were measured at the same time period. From the impedance results increasing internal resistance was observed at the cells. Since the internal resistance reflects ionic conductivity of the electrolytes, its enlargement represents electrolyte depletion. In particular, steeper internal resistance increase was observed at 800 oC than other temperature, representing significant electrolyte consumption existed at 800 oC. In addition, enlarging of the high-frequency half circle was obtained. Because the high frequency one represents cathodic mass transfer resistance, it means cathodic resistance becomes significant. It is plausible that the reduced electrolyte amount decreases active surface area of the cathode and enlarges its mass-transfer resistance. The anode has very high exchange current density, therefore the electrolyte reduction may not affect mass-transfer resistance significantly.

Optimal hybridization and amortized cost study of battery/supercapacitors system under pulsed loads

Hybrid electrical energy storage systems (HESS) show promise for solving the problems and exploiting the benefits of heterogeneous electrical storage systems (ESS). This paper compares the performance of a lead-acid battery/supercapacitors hybrid to that of a battery alone under pulsed loads. Two types of aging protocols, using pulsed and smoothed power, were carried out on VRLA batteries at 40°C. These protocols typically illustrate a single or hybrid battery’s use for an Uninterruptible Power Supply. In HESS, the supercapacitors smooth the power demand applied to the battery, and, when idle, the battery charges the supercapacitors. The same amount of energy is extracted from the battery during both protocols. The experiment indicated that the battery alone performs 150 ten-minute-cycles before reaching the end of its life. In the hybridized battery, the power dissipated was reduced to 36% of that used by the battery alone, and the number of cycles increased by 70%. The use of a hybrid also reduced the battery’s capacity fade by 60% and its increase of internal resistance by 83%. Finally, the amortized cost of the hybrid system was 17.6% less than that of the battery alone.

Activated Carbon Fiber Treated at Different Temperatures as Supercapacitor Electrodes: Electrochemical Characterization

Introduction Nowadays, carbon-based materials have been used as electrodes for supercapacitors due to their physical and chemical properties. Among the different carbon-based materials, carbon fiber (CF) offer the advantage for using as electrodes because is not necessary any substrates or binders. Besides, it is believed that may be a plausible strategy to maximize the energy density of supercapacitors. Currently, many researches reported that the incorporation of heteroatoms like oxygen and nitrogen into carbon-based materials improved the specific capacitance. These materials containing numerous electroactive heteroatoms may be appropriate for the production of electrodes with better capacitance because the heteroatoms are responsible to induce pseudocapacitive behavior and to increase their polar properties [1]. It is known that different oxygen-containing groups are produced using various techniques or exposures. The result of the oxidation process as a function of both the amount of oxygen as well as the type of carbon-oxygen groups may also depend on the CF surface nature. In this work were proposed two pre-treatments for the CF surface activation, electrochemical and chemical oxidation. The samples were evaluated electrochemically, aiming their performance as electrodes in supercapacitors. Experimental Part The CF samples were produced from polyacrylonitrile (PAN) precursor at different heat treatment temperature (HTT) of 1000 ºC, 1500 ºC and 2000 ºC using temperature steps of 330 ºC/h under inert atmosphere of nitrogen, reaching the maximum during 30 min up to its cooling down to room temperature. The CF samples were cut in size 1 cm2 and previously weighed. For the chemical treatment the CFs samples were dispersed in boiling acetone for 5 min. Subsequently, these were washed with distilled water, immersed in H2SO4/K2Cr2O7 and sonicated for 5 min. For electrochemical oxidation, the CFs were anodically polarized using as electrolyte 0.5 mol L-1 H2SO4 in a fixed potential of 2.0 V for 30min using a conventional three electrode cell. The electrochemical characterization of the electrodes was studied by Electrochemical Impedance Spectroscopy (EIE), Cyclic Voltammetry (CV), and Charge/Discharge tests. All measurements have been performed in 1.0 mol L-1 H2SO4 solution. Results and discussion The CV of all sample electrodes obtained between -0.1 and 0.8 V, recorded at scan rates of 1 at 100 mV s-1, using 1.0 mol L-1 H2SO4 as an electrolyte, showed a small distortion of the current response at the switching potential which may be attributed to electrolyte and electrode resistance. How this effect is usually dependent on the scan rate, this distortion is more pronounced between 50 and 100 mV s-1. However, the three materials exhibit the characteristic rectangular shape typical of the electric double-layer capacitors. The charge-discharge profile of the CF electrodes at HTT submitted at two treatment oxidation, carried out at a current density of 0.5, 0.75 and 1 mA cm-1, showed linear charge/discharge profile which indicates that capacitance is mainly due to electric double layer formation. The EIE analysis in the present work it is observed for all samples after the two pre-treatment that the vertical lines are close to the imaginary axis, which indicates supercapacitor behavior is very close to ideal capacitor. After two pre-treatment used it was observed an insignificant increase in internal resistance of materials. This behavior may be associated to the heteroatoms incorporation, becoming the ionic transport pathway more tortuous, which result in the diffusion phenomena more evidenced. Conclusion The CF samples submitted a different HTT were available for application as electrode materials in supercapacitors. The two pre-treatments contributed to the increase specific capacitance. Particularly, the CF 1000 ºC electrodes electrochemically treated exhibited better performance in general terms for possible application as electrodes for supercapacitors. Acknowledgments This work was supported by CNPq-170141/2014-4. References Yun,Y. S., Lee, M. E., Joo, M. J., Jin, H.-J. Journal of Power Sources 246 (2014) 540-547.

Design, Fabrication and Characterization of Flexible Thin Film Rechargeable Lithium Ion Batteries

The advent of fully flexible electronics is considered as a major technological leap forward and has attracted much attention during the last decade. Compared with conventional electronics, flexible electronics are lightweight, bendable and may be wearable or implantable. Hence the rapid and continuous growth in this field has stimulated the research and development of corresponding flexible energy storage systems and more particularly rechargeable lithium ion batteries [1-3]. In this work we developed flexible thin film lithium ion batteries based on all-solid-state materials. The flexible batteries were fabricated using sequential physical vapor deposition techniques. A thin film battery (TFB) architecture consisting of Ti/V2O5/LiPON/Li/encapsulation multilayer was deposited on flexible polyimide substrate. The fabricated flexible TFBs presented an overall thickness of 70µm (figure 1). The flexible TFBs delivered discharge capacity as high as 0.15mAh (1.5 cm² of cathode area) upon galvanostatic cyling at C/5 rate within 3.8-1.5V voltage range. The capacity decreased during cycling with an average fading rate of 0.25% over 100 cycles (figure 2). Electrochemical impedance spectroscopy (EIS) measurements and modeling analysis revealed a significant and continuous increase of the internal resistance during cycling. Furthermore, it was shown that the internal resistance increase was induced by a progressive decrease of the electrolyte/electrodes contact area. A strong correlation was identified between the variation of electrolyte/electrode contact area and variation of flexible TFB discharge capacity. TFBs having the same architecture and the same design were fabricated on two substrates: a flexible polyimide foil and a rigid silicon wafer. Electrochemical characterization showed a clear influence of substrate type on TFBs discharge capacity and internal resistance variations during cycling (figure 3). The electrochemical results variation with substrate type is discussed in terms of differences of TFB multilayer mechanical behaviour during cycling in both configurations. References 1- Y. Hu and X. Sun, J. Mater. Chem. A, 2, 10712 (2014). 2- L. Li, Z. Wu, S. Yuan, and X. Zhang, Energy Environ. Sci., 7, 2101 (2014) 3- M. Osiak, H. Geaney, E. Armstrong and C. O'Dwyer, J. Mater. Chem. A, 2, 9433 (2014). Figure 1