AGENTES COMUNITARIOS CAPACITADOS PARA PROMOVER EL CUIDADO INFANTIL Se debe desarrollar las siguientes tareas:

3.1. Impedance spectroscopy

In this thesis, Electrochemical Impedance Spectroscopy (EIS) was used to study the interfaces, i.e. electrode-electrolyte and the ionic conductivity of the electrolytes. A wide range of frequencies from 1 MHz to 10 mHz was carried out using an impedance meter Hewlett Packard 4192A, for the ionic conductivity measurement, and a VMP3 potentiostat from Biologic for the observation of electrolyte- electrode interface. The results were fitted by Zview software.

 Sample preparation and EIS program

For the ionic conductivity measurement of solid membrane electrolytes, the coin cells (Fig. 2), and Swagelok cells (Fig. 3) were prepared in a glove box. The membrane of 10 mm of diameter were

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sandwiched between two stainless steel electrodes (d = 9 mm). The cells were submitted to two heating/cooling cycles, from 20 °C to 90 °C, with a scan rate of 5 °C/ hour. An EIS was measured every hour during the heating/cooling processes. For the PEO/salt membranes, a PTFE ring with a definite thickness was placed around the membranes to maintain the shape of the sample even above the melting point.

The conductivity of ILs were carried out with the micro conductivity cells (Fig. 4). The sample were prepared in a glove box and PTFE tap was used to ensure an anhydrous environment. The temperature was performed from 90 to -10 °C. The cell constant (κ) determined by the value of the conductance for 0.1 M KCl (κ KCl = 12.85 mS.cm-1, at 25 °C) and its resistance, following the equation below:

𝜎 =

𝑙

𝑅 ∗ 𝑆

=

𝜅

𝑅

Where

𝜎

l: distance between two electrodes (cm)

R: electrolyte resistance (Ω) S: electrode surface (cm2₎ And

𝜅 =

188 Fig. 3 The picture of Swagelok cell, two

stainless steel electrodes, and spring.

Fig. 4 Cell 2-Pole Stainless steel Suit Micro-Samples for ionic conductivity measurement of ionic liquids

3.2. Cyclic voltammetry (CV)

CV is a popular electrochemical technique, in which the oxidation and reduction processes of a system can be investigated. The current density was recorded during the applying of a potential sweep as a function of time. In this work, CV method was used to determine the electrochemical stability, the presence of the intermediates or impurities in oxidation-reduction reactions and the reversibility of a reaction. The trace in Fig. 5 describes an example of a cyclic voltammogram where the horizontal axis represents the applied potential (E), and the vertical axis records the response of the current density (i). The arrows indicate the direction of applied potential.2 _{The CV technique was carried out} using a VMP3 potentiostat from Biologic. Data were treated using an EC-lab version 10.40 software.

Fig. 5 An example of a cyclic voltammogram

 Sample preparation and CV program

The membrane was sandwiched between a stainless steel electrode and a lithium metal, in a coin cell. These cells were prepared in an anhydrous glove box and the performed temperature was controlled in an oven. An applied potential range starts from OCV up to 4.1 V vs Li+_{/Li in the anodic sweep} and reverses to -0.1 V vs Li+_{/Li in the cathodic scan, at a scan rate of 0.1 mV.s}−1_.

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3.3. Chronopotentiometry (CP)

In this thesis, CP technique was used to study the plating/stripping processes of Li+ _{in a symmetric} Li-metal cell. The method was carried out using a VMP3 potentiostat from Biologic. Data were treated with an EC-lab version 10.40 software. A constant current was applied across the symmetric cell while the potential was recorded as a function of time. The cell was prepared in a glove box following the same concept with the conductivity cell, but the two disks of lithium metal were used in place of stainless steel electrodes.

 CP program

An EIS was applied from 1 MHz to 10 mHz across the cell during 24 h, following a rest of 2 h between each scan. The cell stability was checked by obtaining identical Nyquist plots after 1 day operation at 80 °C. Then 10 pre-conditioning cycles at 10 µA.cm-2 _{were submitted to the cell. The} current was imposed for 4 h in each direction, followed by 45 minutes of rest between each side. The strong and constant currents density increasing from 50, 100, 120, 150, up to 200 µA.cm-2 _were further applied across the cell and the evolution of over-potential as a function of current were recorded.

3.4. Chronoamperometry (CA)

This method was used to measure the transference number of Li+ _{ions between the two Li-metal} electrodes. In CA technique, a constant potential was applied across the cell and the current response was recorded versus time. The cell using for this test was a symmetric lithium metal coin cell. At first, an EIS was performed to obtain the initial resistance interface (Ri), then a 40 mV polarization (ΔV) was applied to the cell in order to reach the initial current (Ii) value. Next, the EIS was performed by applying a 40 mV perturbation from 1 MHz to 1 Hz in every 10 minutes, at open circuit conditions to observe the cell’s resistance until the polarized current reached constant. The resistance and current after polarization were noted as Rs and Is, respectively.

3.5. Batteries cycling

The batteries cycling was investigated using the Galvanostatic Cycling with Potential Limitation (GCPL) technique, with a VMP3 potentiostat from Biologic. The performance of batteries was presented as a function of their charge/discharge conditions. The galvanostatic rate expresses as C/h, which means the time that the charged species passed through the two electrodes. The galvanostatic rate can be calculated by the specific capacity of electrode material (mA.h.g-1_{) i.e. LiFePO}

4 (170 mA.h.g-1_{). The cells were cycled at 80 °C at different current regimes from low to fast constant} current charge/discharge between 2.5 and 3.8 V vs Li+_{/Li. The cell for this test was an asymmetric} coin cell combined a solid electrolyte sandwiched between a cathode LiFePO4 and a disk of lithium