Three-electrode Coin Cell Preparation and Electrodeposition Analytics for Lithium-ion Batteries

Summary

Three-electrode cells are useful in studying the electrochemistry of lithium-ion batteries. Such an electrochemical setup allows the phenomena associated with the cathode and anode to be decoupled and examined independently. Here, we present a guide for construction and use of a three-electrode coin cell with emphasis on lithium plating analytics.

Abstract

As lithium-ion batteries find use in high energy and power applications, such as in electric and hybrid-electric vehicles, monitoring the degradation and subsequent safety issues becomes increasingly important. In a Li-ion cell setup, the voltage measurement across the positive and negative terminals inherently includes the effect of the cathode and anode which are coupled and sum to the total cell performance. Accordingly, the ability to monitor the degradation aspects associated with a specific electrode is extremely difficult because the electrodes are fundamentally coupled. A three-electrode setup can overcome this problem. By introducing a third (reference) electrode, the influence of each electrode can be decoupled, and the electrochemical properties can be measured independently. The reference electrode (RE) must have a stable potential that can then be calibrated against a known reference, for example, lithium metal. The three-electrode cell can be used to run electrochemical tests such as cycling, cyclic voltammetry, and electrochemical impedance spectroscopy (EIS). Three-electrode cell EIS measurements can elucidate the contribution of individual electrode impedance to the full cell. In addition, monitoring the anode potential allows the detection of electrodeposition due to lithium plating, which can cause safety concerns. This is especially important for the fast charging of Li-ion batteries in electric vehicles. In order to monitor and characterize the safety and degradation aspects of an electrochemical cell, a three-electrode setup can prove invaluable. This paper aims to provide a guide to constructing a three-electrode coin cell setup using the 2032-coin cell architecture, which is easy to produce, reliable, and cost-effective.

Introduction

Although the origin of lithium-batteries can be traced arbitrarily far back into the past, the large-scale production and commercialization of many of today's commonly found lithium-ion batteries began in the 1980s. Many of the materials developed during this era, one example being Lithium Cobalt Oxide (LiCoO₂), are still commonly found in use today¹. Many current studies have been focused towards the development of various other metal oxide structures, with some emphasis placed towards reducing or eliminating the use of cobalt in place of other lower cost and more environmentally benign metals, such as manganese or nickel². The continuously changing landscape of materials used in lithium-ion batteries necessitates an effective and accurate method of characterizing both their performance and safety. Because the operation of any battery involves the coupled electrochemical response of both the positive and negative electrodes, typical two electrode batteries fall short of being able to characterize the electrodes independently. Poor characterization and the subsequent lack of understanding may then lead to dangerous situations or poor overall battery performance due to the presence of degradation phenomena. Previous research has been aimed at standardizing the processing techniques for typical two-electrode cells³. One method that improves upon the shortcomings of standard cell configurations is the three-electrode cell.

A three-electrode setup is one method to decouple the two electrodes' responses and provide a greater insight into the fundamental physics of the battery operation. In a three-electrode setup, a reference electrode is introduced in addition to the cathode and anode. This reference electrode is then used to measure the potential of the anode and cathode dynamically during operation. No current is passed through the reference electrode and hence, it provides a singular, and ideally stable, voltage. By using a three-electrode setup, the full cell voltage, the cathode potential, and the anode potential can be collected simultaneously during operation. In addition to potential measurements, the impedance contributions of the electrodes can be characterized as a function of the cell state of charge⁴.

Three-electrode setups are very useful for studying degradation phenomena in lithium-ion batteries, such as the electrodeposition of lithium metal, also known as lithium plating. Other groups have proposed three-electrode setups^{5,6,7,8,9,10,11,12,13} but they often use the inherently unstable lithium metal as a reference and include custom, difficult to assemble setups leading to reduced reliability. Lithium plating takes place when instead of intercalating into the host electrode structure, lithium is deposited on the surface of the structure. These deposits commonly assume the morphology of either a (relatively) uniform metallic layer (plating) or small dendritic structures. Plating can have effects ranging from causing safety issues to impeding cycling performance. From a phenomenological perspective, lithium plating occurs due to an inability of lithium to intercalate into the host electrode structure effectively. Plating tends to occur at low temperature, high charging rate, high electrode state of charge (SOC), or a combination of these three factors¹². At low temperature, the solid-state diffusion inside the electrode is reduced, due to the Arrhenius diffusivity dependence on temperature. The lower solid-state diffusion results in a buildup of lithium at the electrode-electrolyte interface and a subsequent deposition of lithium. At a high charging rate, a similar phenomenon occurs. The lithium attempts to intercalate into the electrode structure very quickly but is unable to and thus is plated. At a higher SOC, there is on average less available space for the lithium to intercalate into the structure, and thus it becomes more favorable to deposit on the surface.

Lithium dendrites are of particular importance due to the safety concern they cause. If dendrites form inside a cell, there is a potential for them to grow, pierce the separator, and cause an internal short between the anode and cathode. This internal short can lead to very high-localized temperatures in the flammable electrolyte, often resulting in thermal runaway and even in an explosion of the cell. Another issue related to dendrite formation is the increased surface area of the reactive lithium. The newly deposited lithium will react with the electrolyte and cause increased solid electrolyte interphase (SEI) formation, which will lead to increased capacity loss and poor cycling performance.

One issue associated with the design of a three-electrode system is the selection of the appropriate reference electrode. Logistics relating to the location and size of the reference, positive, and negative electrodes can play an important role in acquiring accurate results from the system. One example is that the misalignment of the positive and negative electrodes during the cell construction and the resulting edge effects can introduce error in the reference reading^14,15. In terms of material selection, the reference electrode should have a stable and reliable voltage and have a high non-polarizability. Lithium metal, which is often used as a reference electrode by many research groups, has a potential that depends on the passive surface film. This can produce issues because cleaned and aged lithium electrodes display different potentials¹⁶. This becomes a problem when long-term aging effects are studied. Research by Solchenbach et al. has attempted to eliminate some of these instability issues by alloying gold with lithium and using it as their reference¹¹. Other research has looked at different materials including lithium titanate, which has been studied experimentally and shows a large electrochemical potential plateau range around 1.5 – 1.6 V¹⁷ (~50% SOC). This plateau helps to maintain a stable potential, especially in the event of accidental perturbation to the electrode's state of charge. The potential stability of LTO, including carbon-based conductive additives, is maintained even at different C-rates and temperatures.¹⁸ It is important to emphasize that the selection of the reference electrode is an important step in the three-electrode cell design.

Many research groups have proposed experimental three-electrode cell setup. Dolle et al. used thin plastic cells with a lithium titanate copper wire reference electrode to study changes in impedance due to cycling and storage at high temperatures¹⁹. McTurk et al. employed a technique whereby a lithium plated copper wire was inserted into a commercial pouch cell, with the main goal being to demonstrate the importance of noninvasive insertion techniques⁹. Solchenbach et al. used a modified Swagelok-type T-cell and a gold micro-reference electrode (mentioned earlier) for impedance and potential measurements.¹¹ Waldmann et al. harvested electrodes from commercial cells and reconstructed their own three-electrode pouch cells for use in studying lithium deposition¹². Costard et al. developed an in-house experimental three-electrode cell housing to test the effectiveness of different reference electrode materials and configurations¹³.

Most of these research groups use pure lithium metal as the reference, which can have concerns with stability and SEI growth, especially with long-term use. Other issues involve complicated and time-consuming modifications to existing or commercial setups. In this paper, a reliable and cost-effective technique for constructing three-electrode Li-ion coin cells for electrochemical tests is presented, as shown in Figure 1. This three-electrode setup can be constructed using standard coin cell components, copper wire, and lithium titanate-based reference electrode (see Figure 2). This method does not require any specialized equipment or elaborate modifications and follows standard laboratory scale electrochemical procedures and materials from commercial vendors.

Protocol

1. Reference Electrode and Separator Preparation Reference electrode preparation Wire preparation Cut one 120-mm length of size 32 AWG (0.202 mm diameter) enameled copper wire. NOTE: Each wire will become 1 reference electrode and will be used inside 1 three-electrode cell. Place one end of the wire in a laboratory press. Gently press approximately 10 mm of wire at one end to a pressure of about 4 MPa. Cut the excess wire off the…

Representative Results

Typical results for the voltage and potential profiles for the three-electrode cell can be seen in Figure 7. In an ideal setup, the full cell voltage should be identical to that produced from a two-electrode cell using the same electrode couple. This is one method to determine whether the insertion of the reference electrode modifies the performance of the cell. If there is a significant difference between the two- and three-electrode full cell performance (f…

Discussion

Cell crimping pressure plays an important part in the success rate of both the preparation and working cells. If the cell is crimped at too high a pressure (>800 psi), the reference electrode can become shorted with the cell cap due to the reference wire position in-between the cap and the gasket. Note that the wire crossing this interface is a requirement in order to connect the reference electrode reading to an external measurement device. If the cell pressure is too low (<700 psi), the cell can have issues wit…

Materials

Agate Mortar and Pestle	VWR	89037-492	5 in diameter
Die Set	Mayhew	66000
Laboratory Press	MTI	YLJ-12
Analytical Scale	Ohaus	Adventurer AX
High-Shear Mixing Device	IKA	3645000
Argon-filled Glovebox	MBraun	LABstar
Hydraulic Crimper	MTI	MSK-110
Battery Cycler	Arbin Instruments	BT2000
Potentiostat/Galvanostat/EIS	Bio-Logic	VMP3
Vacuum Oven and Pump	MTI	–
Copper Wire	Remington	PN155	32 AWG
Glass Balls	McMasterr-Carr	8996K25	6 mm borosilicate glass balls
Stirring Tube	IKA	3703000	20 ml
Celgard 2500 Separator	MTI	EQ-bsf-0025-60C	25 μm thick; Polypropylene
Stainless Steel CR2032 Coin Cell Kit	Pred Materials		Coin cell kit includes: case, cap, PP gasket
Stainless Steel Spacer	Pred Materials		15.5 mm diameter × 0.5 mm thickness
Stainless Steel Wave Spring	Pred Materials		15.0 mm diameter × 1.4 mm height
Li-ion Battery Anode – Graphite	MTI	bc-cf-241-ss-005	Cu Foil Single Side Coated by CMS Graphite (241mm L x 200mm W x 50μm Thickness)
Li-ion Battery Cathode – LiCoO2	MTI	bc-af-241co-ss-55	Al Foil Single Side Coated by LiCoO2 (241mm L x 200mm W x 55μm Thickness)
Polyvinylidene Difluoride (PVDF)	Kynar	Flex 2801
N-Methyl-2-Pyrrolidinone Anhydrous (NMP), 99.5%	Sigma Aldrich	328634
CNERGY Super C-65	Timcal
Electrolyte (1.0 M LiPF6 in EC/DEC, 1:1 by vol.)	BASF	50316366
Lithium Titanate (Li4Ti5O12)	Sigma Aldrich	702277
KS6 Synthetic Graphite	Timcal
Lithium Metal Ribbon	Sigma Aldrich	320080	0.75 mm thickness
Epoxy Multipurpose	Loctite
Electrical Tape	Scotch 3M Super 88
Isopropyl Alcohol (IPA), ACS reagent, ≥99.5%	Sigma Aldrich	190764