Investigation of Reference Electrodes Usable As an Understanding Tool of Aging Mechanisms in Lithium-Ion Batteries
Authors: Geniès Sylvie , Vidal Delphine , Vincens Christophe , Gutel Elise , Raccurt Olivier , Porcher Willy 
Published: May 2020

Fundamental electrochemical studies require to implement a reference electrode inside the cell to be able to record the potential profile and characterize the kinetics properties and the interface structure of each electrode individually [1]. Obtaining such data proves to be essential to better understand the electrochemical processes and the aging mechanisms that are generated by cycling. Moreover, the integration of reference electrode inside lithium-ion cells must be done by keeping the sealing of the cell.
Many studies are reported in the literature that characterize 3-electrodes lithium-ion cells in coffee-bag whom the design allows to exit a third tab easily through the heat-sealed laminated aluminum foil [2, 3, 4]. Such instrumentation can be realized when the cell is assembled or on commercial cell [5]. A few more recent studies gives results obtained in 3-electrodes cylindrical cells but hard packaging is much more delicate to instrument because it implies to overcome experimental difficulties for exit connections and sealing preservation [6, 7].
Several electrochemical couples can be used including metallic lithium (Li+/Li) [8], insertion materials Li(1-x)FePO4/ LiFePO4 (LFP), Li4Ti5O12/ Li(4+x)Ti5O12 (LTO) [4, 9] or alloy (LixAl/Al) [10]. They are identified as possible reference material because their insertion/desinsertion curves show a voltage plateau on a large lithiation range. The two last material families have to be partially delithiated (LFP) or lithiated (LTO, alloy) to place the insertion potential of the material on the plateau. That is performed in situ once the cell is assembled and charged [4]. Moreover, the design of the reference electrode and its placement inside the electrochemical cell are important to avoid potential shifts or artefacts on the impedance spectra [11, 12].
In the present study, the response of two reference electrochemical couples, Li+/Li and
Li(1-x)FePO4/LiFePO4 with a similar cell design and placement has been evaluated towards three criteria, (i) the potential profiles in function of the current rates, (ii) the impedance spectra shape and the stability in time. The stability in time appears more and more crucial in the perspective to develop innovative smart-cell. The comparative study that was performed in laboratory pouch-cells has allowed to discriminate the two electrochemical couples. The instrumentation of 18650 cell will be also presented in comparison.
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