In Situ Lithiated Reference Electrode: Four Electrode Design for In-operando Impedance Spectroscopy

Summary

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.

Abstract

Extending operating voltage of Li-ion batteries results in higher energy output from these devices. High voltages, however, may trigger or accelerate multiple processes responsible for long-term performance decay. Given the complexity of physical processes occurring inside the cell, it is often challenging to achieve a full understanding of the root causes of this performance degradation. This difficulty arises in part from the fact that any electrochemical measurement of a battery will return the combined contributions of all components in the cell. Incorporation of a reference electrode can solve part of the problem, as it allows the electrochemical reactions of the cathode and the anode to be individually probed. A variation in the voltage range experienced by the cathode, for example, can indicate alterations in the pool of cyclable lithium ions in the full-cell. The structural evolution of the many interphases existing in the battery can also be monitored, by measuring the contributions of each electrode to the overall cell impedance. Such wealth of information amplifies the reach of diagnostic analysis in Li-ion batteries and provides valuable input to the optimization of individual cell components. In this work, we introduce the design of a test cell able to accommodate multiple reference electrodes, and present reference electrodes that are appropriate for each specific type of measurement, detailing the assembly process in order to maximize the accuracy of the experimental results.

Introduction

The demand for high energy densities from Li-ion batteries (LIBs) is driving research towards understanding fundamental factors that limit Li- ion cell performance¹. High voltage operation of cells containing a new generation of layered transition metal oxide cathodes, graphite anodes and organic carbonate electrolytes is associated with several parasitic reactions^2,3. Some of these reactions consume Li - ion inventory and often result in significant impedance rise of the cell^4,5,6,<....

Protocol

1. Stripping Copper/Tin Wires

1. Heat commercially obtained stripping solution.
   1. Pour commercial industrial grade stripping solution into a stainless steel beaker (7.6 cm in diameter and 8.5 cm in height) to a depth of about 5 mm from the bottom. Place the beaker on to a hot plate. Begin heating a slow rate of about 5 °C/min.
   2. Immerse a portable thermocouple into the solution to closely monitor the temperature ramp of the solution and adjust the heating rate of the hot plate to maintain at the required heating rate.
2. Setting up of the Cu/Sn wire on the jig for preparing reference wires.
   1. Wind a thick....

Results

Figure 2 is a representative profile of the voltages of individual electrodes with 1.2M LiPF₆ in (FEC):EMC (5:95 w/w) as the electrolyte during the first and second cycles of formation. Figure 3 shows the EIS spectra of the cell after three formation cycles and at the end of the cycle life ageing protocol. The ability to re-lithiate the RE to obtain EIS data aids in precise tracking of the impedance changes in individu.......

Discussion

Figure 2a is the voltage profile of the full cell while Figure 2b and 2c show voltage profiles corresponding to the positive and the negative electrode vs Li/Li⁺ couple while the full cell is cycled between 3 and 4.4 V. It can be seen that as the full cell scans between 3 and 4.4 V, the positive electrode experiences voltages between 3.65 V and 4.45 V and the negative electrode between 0.65 V and 0.05 V vs. Li/Li⁺ .......

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
Insulstrip 220	Ambion Corporation	081607-1
Sodium Hydroxide (23 wt%)	Ambion Corporation	1310-73-2	Contents of Insulstrip 220
Furfuryl Alcohol (10 wt%)	Ambion Corporation	98-00-0	Contents of Insulstrip 220
NCM523	TODA America	NM4100
C-45	Timcal Inc.
polyvinylidene fluoride (PVdF)	Sigma Aldrich	427152
Sn over Cu wire	Kanthal	MELT # 24633	Custom ordered
Battery cycler	Maccor USA	Series 2300
Potentiostat	Solartron Analytical	1470 E