Análisis de los resultados del completamiento de frases (Rotter)

5.5.1.1 Theory

An apparatus was built to monitor and measure the change in volume of the pouch cells as a function of time in-situ during the overcharge regime. It relies on the

Archimedes’ principle for its operation. The weight of the cell submerged in silicone oil was measured to a very high accuracy using a load cell. The change in the volume of the cell was calculated as follows; the relationship between the buoyant force on a submerged object and its volume is given by the Archimedes’ Principle. The buoyant force (𝐹_𝑏) is equal to the weight of the fluid displace by the submerged object:

𝐹𝑏 = 𝜌𝑔𝑉 Equation 5-3

Where 𝜌 is the density of the fluid, 𝑔 is the gravitational acceleration and 𝑉 is the volume of the submerged object.

Figure 5-2 shows the forces acting on the submerged cell. The weight (𝑊) of the cell acting downwards, while the buoyant force (𝐹𝑏) and the tension (𝑇) in the string are

acting upwards. During equilibrium the forces are expressed as:

𝑇 + 𝐹𝑏 = 𝑊 Equation 5-4

As the volume of the cell changes, the buoyant force changes. Since the weight of the cell is constant, for the cell to stay in equilibrium, the tension must change to match the change in the buoyant force. Therefore:

𝛥𝑇 = −𝛥𝐹_𝑏 = −𝜌𝑔𝛥𝑉 Equation 5-5

Since the cell is hanging from the load cell, the change in tension can be accurately measured and the cell volume change can be calculated.

5.5.1.2 Equipment and Set-up

In order to make the 𝛥𝐹_𝑏 measurement required in Equation 5-5, highly sensitive 600 g load cells made by VPG Transducers were used. The data sheet of the load cells is in the Appendix Section (C). The calibration of the load cells was carried out using weights and a high precision scale. The calibration curves of the load cells are shown in Figure 5-3. It can be noticed that the change in their voltage output is linear with the force applied.

Figure 5-3: Calibration curves of load cells used

Figure 5-4 shows the components and the electrical connections of the set-up. A 12 V power supply was used to power the load cells. Voltage output signal was sent from load cell in response to the changes in the volume of the pouch cell. The signal output from the load cell was amplified using a purpose-built voltage amplifier before being connected to the auxiliary input of the same cycler channels used to cycle the cell, this means that both the cycling data and the load cell data were synchronised. The temperature of the cell surface and the oil were monitored and recorded every 30 seconds using a T-Type thermocouples and a PicoLog TC-08 thermocouple data logger. All data was recorded to a desktop computer.

Figure 5-4: Components and electrical connections of the in-situ cell volume measurement set-up

5.5.1.3 Volume Measurement Apparatus and Test Set-up

Figure 5-5 shows a photograph of the apparatus inside a thermal chamber. The apparatus was designed with two channels to run two tests in parallel. Two load cells were fixed to a plastic plate. The plastic plate was supported by a foam base to act as a shock absorber, in order to minimise vibration noise in the load cell reading. Each pouch cell was hung from the load cell using two connection points to ensure its stability. The connection between the load cell and the pouch cell was a polyester thread with a ring on one end and a hook on the other end. As shown in Figure 5-6, two small holes, approximately 3 mm in diameter, were punctured in the top corners of the pouch casing, above the seal of the cell to ensure that the cell stayed intact. A plastic bar with two points of attachment was fixed to the load cell. The hooks were then attached to the pouch cell and the rings to the plastic bar attached load cell. The use of such mechanical attachment allowed for easy changing of cells between tests. The electrical connections to the cell consisted of current carrying wires and fine voltage sense wires. Two 4 mm sockets, along with the voltage wire were soldered to the cell tabs as shown in Figure 5-6. The purpose of these sockets is to connect current carrying wires between the cell and the Bio-Logic VMP3 electrochemical cycler. The current carrying wires between the cell and the cycler were coiled to allow for the free movement of the cell and minimise any undesired influence on the load cell reading. A thermocouple was attached to the surface of the cell before being immersed in the silicone oil and another one dipped in the oil inside the container. Silicone oil was used due to its properties; electrically insulating, thermally stable and has low vapour

pressure. The data sheet of the silicone oil used can be found in the Appendix Section (D).

Figure 5-5: A photograph of the in-situ cell volume measurement apparatus and set-up

Figure 5-6: Pouch cell electrical connections and mechanical attachment points