The incorporation of reference electrodes in a Li-ion battery provides valuable information to elucidate degradation mechanisms at high voltages. In this article, we present a cell design that accommodates multiple reference electrodes, along with the assembly steps to assure maximum accuracy of the data obtained in electrochemical measurements.
The demand for better performing lithium ion batteries is driving research towards understanding factors causing degradation. The method described here can precisely guage the behavior of individual battery components. The wealth of information that this technique provides amplifies the reach of diagnostic analysis in lithium ion batteries, and provides valuable input into the optimization of individual cell components.
To begin, obtain a four centimeter wide seven centimeter long jig made of two millimeter thick copper wire. With flat loops to stabilize the jig at one end, and a handle at the other end. Then, pour about 23 milliliters of industrial grade stripping solution into a 7.6 centimeter diameter stainless steel beaker to obtain a solution depth of about five millimeters.
Immerse a thermo couple in the solution, keeping the tip away from the bottom of the beaker. Start heating the beaker on a hot plate to 85 degrees celsius. While the solution heats, carefully wind 60 centimeters of 25.4 micron polyurethane coated tin plated copper wire length wise around the jig.
Gently tape the ends of the wire to the handles. Once the solution reaches 85 degrees celsius, turn off the heat and place the jig vertically in the solution to immerse one side of the wrapped wire. Leave the jig in the solution for 15 seconds, and then rinse the wire and jig in deionized water for 15 seconds.
Inspect the wire for exposed tin, which will appear as a silvery white material. Repeat the immersion in stripping solution and rinsing until one side of the wrapped wire has been completely stripped to the tin layer. Be careful not to etch away the plated tin.
Then, rinse the jig and wire in deionized water and allow them to dry in air at room temperature. Once dry, cut the wire in the center of each stripped area to obtain approximately 10 centimeter long wires with exposed tin plated ends. To begin preparing a reference wire, cut about 10 centimeters of insulated two millimeter wide electrical circuit wire.
Strip about two centimeters of insulation from each end of the wire. Then, gently lay one stripped end of a tin plated copper wire on the stripped electrical wire. Solder the junction to form an electrical contact between the wires.
Confirm that the resistance across the newly formed tin reference electrode is between six and eight ohms. Assemble a second reference electrode with an exposed copper end in the same way, and bring the wires into an argon filled glove box. Next, cut a piece of 200 micron thick lithium metal no larger than five millimeters by five millimeters, and place it on a PTFE platform.
Use a roller covered with polymer tape to flatten the metal to 25 microns thick. Then, bend the metal into a U shape, and lay the exposed tip of the copper wire in the bend. Carefully press the lithium around the wire to completely encapsulate the exposed copper.
This will be the lithium metal reference electrode. Place an anode in a lithium cell fixture so that the center of the electrode is slightly offset from the center of the fixture. Place an insulating separator made of a trilayer of polyethylene and polypropylene on the anode.
Use a PTFE sweep to gently remove trapped air bubbles between the anode and the separator. Ensure that the separator is firmly seated in the fixture. Then, apply a 10 microliter drop of the electrolyte on the separator two millimeters form the edge of the anode.
Apply a second drop over the center of the anode. Position the exposed tin plated tip of the tin reference electrode in the drop at the center of the anode. Position the tip of the lithium reference electrode in the drop two millimeters away from the anode.
Apply another 10 microliter drop of electrolyte to the lithium encapsulated tip, and remove air bubbles between the lithium metal and the separator with the sweep. Then, apply 400 microliters of electrolyte to the separator. Place a second separator on the assembly so that the reference electrode wires are sandwiched between the separators.
To avoid applying excessive tension to the wires, leave some slack in the wires between the separators. Remove air bubbles between the separators with the sweep. Then, wet a cathode with 400 microliters of the electrolyte solution.
Place the cathode on the upper separator aligned with the anode. Carefully place a stainless steel spacer on the cathode without disturbing the alignment of the cell stack. Put two stainless steel wave springs on the spacer to accommodate cell volume changes and pressure build up, and then close the fixture.
Connect the cathode and anode of the battery cell to the corresponding battery cycler terminals. Connect the lithium reference electrode to the auxiliary terminal. Generate voltage profiles for each electrode with respect to the lithium couple.
Apply a constant current of five microamperes for six hours between the cathode and the tin reference electrode with an upper voltage cutoff of four volts to electrochemically lithiate the tin. When finished, disconnect the cell from the battery cycler terminals, and allow the lithiated wire to equilibrate for two hours. To confirm successful lithiation, connect the lithium reference electrode to the auxiliary terminal, and the lithiated tin electrode to the negative terminal.
The potential difference between them should be close to zero volts. To obtain spectra for the full cell, first, connect the negative voltage and current terminals of a potentia stat to the anode, and the positive terminals to the cathode. Cycle the electrochemical couple over a small voltage amplitude below five millivolts using alternating current or varying frequencies, with the impedance response plotted as the imaginary component versus the real component.
Next, switch the negative voltage and current terminals from the anode to the lithium tip alloy electrode, and acquire a spectrum of the cathode versus the lithium tin alloy electrode. Lastly, connect the negative voltage and current terminals to the anode, and the positive terminals to the lithium tin alloy electrode. Voltage profiles were obtained of the full cell, and of each electrode with respect to the lithium couple.
The increasing potential of the positive electrode during charge indicated delithiation. And the decreasing potential of the negative electrode correspondingly indicated lithiation. Electrochemical impedance spectroscopy showed a significant increase in impedance of the positive electrode with respect to the lithiated tin reference electrode after the battery had been cycled 100 times, but a minimal increase in impedance of the negative electrode.
This implied that the overall increase in impedance was predominantly driven by the increase in positive impedance. Cycling at extreme voltage conditions increases the chances of lithium plating onto the negative electrode, causing intense safety challenges. Additional experiments are under way to understand the occurrence of lithium plating by developing protocols to probe the onset of lithium deposition.
This approach can be used to obtain insider information about electrode behavior during the aging of a battery. Align tin wire with metals such as sodium or magnesium can widen the application of this technique to new generation batteries.
