Three-Electrode Cells for Open Circuit Voltage Modelling of Commercial Lithium-Ion Batteries
Authors: McTurk Euan , Birkl Christoph Robert , Roberts Matthew , Bruce Peter G , Howey David Alastair 
Published: February 2020

We describe a method to experimentally obtain the State of Charge (SOC) and State of Health (SOH) of the positive and negative electrodes in commercial li-ion cells at key intervals during long-term degradation tests, with the aim to verify and improve our parametric Open Circuit Voltage (OCV) model [1] for lithium nickel manganese cobalt oxide (LiNiMnCoO2) cells.
In a li-ion cell, each electrode’s State of Health (SOH) is affected by a number of mechanisms that occur over the cycle life of the cell. These include the growth of a solid electrolyte interface (SEI), loss of active electrode material, and particle cracking, each of which affects the positive and negative electrodes differently [2, 3]. Due to the lack of a reference electrode, and therefore a constant potential against which to separately measure the potential of each electrode, the SOH and State of Charge (SOC) of the positive and negative electrodes respectively are not obtainable from a standard commercial two-electrode li-ion cell. If separate potentials for each electrode could be obtained, this would enable validation of battery management system (BMS) model-based estimation approaches, as well as provide new insights into degradation in commercial cells.
Previous work to incorporate reference electrodes into commercial cells has involved highly invasive surgery on 18650 cylindrical cells that altered their performance in comparison to unmodified cells [4, 5, 6, 7]. Changes were caused by the introduction of new electrolyte solutions [4]; the positioning of the reference electrode some distance from the cell in a large flooded chamber [5]; and drilling deep into the cell structure [4, 6, 7]. We have devised a minimally-invasive modification procedure for a pouch cell that introduces a lithium reference electrode to the system and provides a profile of the positive and negative electrode potentials whilst aiming to minimise the impact on cell performance.
The modification procedure (Figure 1) was as follows: within an argon-filled glove box, an incision was made in the pouch of a Kokam pouch cell (SLPB 533459H4; 740 mAh capacity) at the opposite end to the working and counter electrodes (WE and CE), exposing a U-shaped portion of the outermost separator. The exposed edges of the pouch foil were covered with pouch cell tape to prevent short-circuiting. A lithium reference electrode (RE) was placed on top of the separator, followed by a copper current collector with cable. The assembly was then held in place with an additional piece of polymer-laminated aluminium pouch material and several windings of pouch cell tape. The cell was placed inside a large outer pouch and the WE, CE and RE were connected to copper terminals that extended from the outer pouch. The outer pouch was then vacuum sealed.
Pseudo-OCV profiles for the WE and CE were obtained by cycling the cell at C/30. Galvanostatic Intermittent Titration Technique (GITT) tests were also performed on the cell. By performing the modification and cycling procedures on unmodified cells at various stages of long-term degradation, profiles of SOC and SOH for each electrode can be mapped throughout their cycle life (see example in Figure 2). The resultant OCV model can be integrated into the algorithm of a BMS, thus facilitating more accurate prediction of SOC, SOH and future cell behaviour as it degrades over its cycle life.

References:

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Figure 1