Ultra High-Precision Studies of Degradation Mechanisms in Aged LiCoO2/Graphite Li-Ion Cells
Authors: Fathi Reza , Burns John Chris , Stevens David , Ye Hui , Hu Chao , Jain Gaurav , Scott Erik , Schmidt Craig , Dahn Jeff R 
Published: February 2020

The major causes of capacity loss in most Li-ion cells are the parasitic reactions between the charged electrodes and the electrolyte.  Examples of such reactions are the formation of the solid electrolyte interphase (SEI) at the negative electrode [1, 2], electrolyte oxidation at positive electrode [3], and loss of active materials due to metal dissolution [4, 5].  It is important to study Li-ion cells throughout their lifetime to be able to determine which failure mechanisms dominate so that appropriate changes to cell chemistry and cell design can be made to increase cell lifetime. Measuring coulombic efficiency (CE) using an ultra-high precision charger (UHPC) [6, 7] along with dVdQ analysis [8, 9] are powerful techniques to learn about capacity loss mechanisms of Li-ion cells.  This study will show results of Li-ion cells of exactly the same chemistry that were made recently, as well as 3 (G2), 8 (G3) and 12 (G4) years ago.  The aged cells were subjected to controlled cycle testing or used in human implants in the field for the majority of their life (at 37oC).  This set of cells, obtained from Medtronic, exhibit exceedingly stable performance over a long lifetime and represent an excellent opportunity to learn about aging in Li-ion cells.

The cells were charged and discharged between 3.4 V and 4.075 V at Medtronic. then were shipped to Dalhousie University for further studies.  The charge current was C/6 and the discharge was either C/24 (G2) or C/150 (G3 &G4).  The average discharge capacity losses for cells in groups G2, G3 and G4were ~12.3%, ~19.5% and ~21% of initial capacity after ~500 (3 years), ~435 (8 years), and ~700 cycles (12 years) respectively.

Figure 1 shows the fractional discharge capacity data versus time for two cells from each group versus calendar time (a) and cycle number (b).  The data indicates that calendar time and cycle # are both contributors to capacity loss; however, calendar fade is clearly the dominant factor.  In Figure 1(a), the fractional capacity versus time curves for each of the three groups have been fit (see the solid green lines in Figure 1a) using the expression:

q(t) = 1 – A t1/2           [1]                  

where q(t) is the fractional capacity at time, t.  A is a fitting parameter which has units of yr-1/2.  The t1/2 relationship given in equation 1 is characteristic of capacity loss due to SEI growth on the negative electrode where the fade rate slows down as the SEI film thickens with time following a parabolic growth  model [1, 2].  The reasonably good fit in Figure 1(a) is consistent with capacity fade that is dominated by SEI formation.  The data for group G2 clearly fades more rapidly than groups G3 and G4, as reflected by the larger value of A needed to fit the data.  This is likely caused by a greater number of cycles during the same time for the group G2cells, possibly leading to more expansions and contractions of the graphite particles and more rapid SEI growth.  With ageing, the loss of lithium to the SEI continually reduces, leading to more consistent performance with ageing.  However, UHPC results, obtained at Dalhousie University, show that parasitic reactions are still occurring in these cells and CE never reaches 1.0000.

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Figure 1