Tracking of Cell Degradation
6.2 Application of characterisation methods
distinguishable in the new state, coalesce into a large, rough surface, whereas the single particles vanished. The structure of the LMO particles, identifiable by their symmetrical and well defined shape with a smooth surface exhibit large cracks, as highlighted in d.
6.2 Application of characterisation methods
The distinct features of the anode and cathode contributing to the full-cell profile are analysed using half-cell measurements of the electrodes, harvested from new cells. The half-cell and full-cell data are displayed in Fig. 6.6 and the anode and cathode profiles are shifted to match the full-cell features due to their higher utilisation range, when assembled in coin cells and cycled versus lithium metal.
To evaluate the ongoing degradation of the cell and to identify the sources of capacity fade, the changes and shift of the two electrodes are analysed. Three possible sources are identified to cause the capacity fade: I) Loss of active material (LAM) on the anode, e.g. delamination of active material as seen in Fig. 6.5b due to mechanical stress. II) LAM on the cathode, e.g. due to metal dissolution and cation mixing. III) Loss of lithium inventory (LLI) due to continuous side reactions, increasing the thickness of the SEI. A combination of LAM and LLI can be observed, if a fully lithiated particle loses electrical contact with the current collector and therefore its possibility to de-intercalate the lithium-ions.
The voltage of the cathode increases for low degrees of lithiation, causing the full-cell to reach the respective Ucco. Further lithium de-intercalation is either not possible or results in a significant harm of the material and is therefore prohibited. The anode remains at the stable voltage plateau at 80 mV for the stage 1 - stage 2 transition due to the higher capacity. Hence, the full-cell is limited by the upper voltage limit of the cathode, indicated asClimitin Fig. 6.6. During discharge, the anode is fully delithiated and a steep voltage increase is observed towards full delithiation, equivalent to a full discharge of the full-cell. The limiting properties of the anode on the full-cell are indicated by the label Alimit.
The various degradation mechanisms as LAM and LLI continuously decrease the cell’s performance by lowering the storable capacity and increasing the cell’s internal impedance. Fig. 6.6 gives a schematic overview of the effect of various degradation mechanisms occurring during the calendar ageing study, when the cell is stored at elevated temperatures at 100 % SoC.
In a, the loss of 20 % of lithium inventory (LLI) is shown. As a result, the maximum degree of lithiation in the anode is reduced, e.g. from Li0.9C6 to Li0.7C6and a shrinkage in the stage 1 - stage 2 voltage plateau is observed in the full-cell profile. The anode cut-off voltage during lithiation is constant, as long as the amount of lithium is sufficient to fill the anode to 50 %, at which point the phase transition occurs. For the discharge of the full-cell, the anode voltage during delithiation dominates the cell’s cut-off voltage, remaining constant at 1 V, when the anode is fully delithiated. The utilisation of the cathode is limited in the same manner as the anode and the degree of full lithiation is reduced by 20 %, e.g. from Li1Ni1/3Mn1/3Co1/3O2to Li0.86Ni1/3Mn1/3Co1/3O2(assuming a BoL utilisation of 0.3< x<1).
0
Discharge capacity / mAh cm-2
Cell
aChange in electrode utilisation due to 20 % LLI
0
Discharge capacity / mAh cm-2
Cell
bChange in electrode utilisation due to 20 % LAM of the lithiated anode
Cell / 20 % LAMAnode
Discharge capacity / mAh cm-2
Cell
Cell / 20% LAMCathode
LAMCathode
cChange in electrode utilisation due to 20 % LAM of the delithiated cathode
Figure 6.6–Exemplified effect of LLI, anode LAM and cathode LAM on the discharge perfor-mance of cell B
6.2. APPLICATION OF CHARACTERISATION METHODS 103 In b, a loss of 20 % lithiated anode material is depicted. A loss of the material on the anode side could partially be compensated due to the overbalancing of the anode, but as it is fully lithiated in this example, the resulting capacity fade is also 20 %. The utilisation range of the cathode is limited and the highest degree of lithiation decreased.
A loss of 20 % cathode material during the storage time is exemplified in Fig. 6.6c.
The material is considered to be delithiated in this state, therefore, no lithium is lost. This leads to a significant overbalancing of the anode. Due to the high degree of lithiation towards the end of the discharge, the voltage of the cathode decreases significantly. In contrast to the initial state, where the anode is the limiting electrode during the full-cell discharge, a limitation by the cathode is observed.
Due to the complexity of the electrochemical system, the separation of the different degradation is expected to be more difficult. Additional degradation such as loss of delithiated anode and lithiated cathode material, when the cell is fully discharged during the characterisation can occur. To track the impact of ageing mechanisms on full-cell level as well for the different cathode and anode materials, numerous characterisation methods are available as discussed in Sec. 3.6. Differential voltage analysis (DVA) and advanced thermodynamic measurements (ATM) were identified to be promising methods to reveal not only the remaining capacity of the cell but rather give an insight to the performance of each electrode. In addition, results of EIS measurements obtained during the ageing study are presented.
Using the data of the ageing study as presented in the previous section, the applicabil-ity and accuracy of the methods are compared. To achieve this, the ageing data are used to identify the different sources of capacity fade within the cell. In a final step, half-cells, built with harvested samples from the degraded cells, are used to verify the conclusions by comparing their electrochemical performance to half-cells built from new cells. The detailed assembly process is described in Sec. 3.6.1. The data sets for the cells B 4cyc
and B 4cal, which were characterised by DVA and the cells B 1cycand B 1cal, which were characterized by ATM are analysed in detail, while the peculiarities of cell A and cell C with a blended and a two-phase cathode material respectively, are addressed separately.
Tbl. 6.1 states the discharge capacities under different discharge conditions. For half-cell measurements using coin cells, the 0.2C discharge capacity determination is only applicable with restrictions due to high overpotential during the measurement. Especially in the case of aged anode half-cells, the cell voltage reaches quickly the discharge cut-off limit and the graphite material is not fully lithiated if the applied current is too high.
The capacity fade of the analysed cycled cells is 22.3 % and 24 %. The calendar aged cells lost 15.3 % and 16.3 %. The capacity fade determined during the DVA measurements is smaller as for the 0.2C characterisation due to the lower discharge current and the lower overpotential. The capacity fade during the thermodynamic characterisation is larger than the 0.2C characterization for the calendar aged cell. Possible reasons for this phenomenon are assumed to be the stepwise-discharge procedure and the self-discharge of the cell during the longer characterisation procedure.
Table 6.1–Various discharge capacities of different cells from type B, used for further analysis Cell B 4cyc Cell B 4cal Cell B 1cyc Cell B 1cal
0.2C BoL / mAh 2235.9 2235.4 2197.0 2203.2
0.2C EoL / mAh 1738.2 1893.0 1670.8 1844.2
Loss % 22.3 15.3 24.0 16.3
DVA BoL / mAh 2241.7 2242.4
DVA EoL / mAh 1793.4 1946.2
Loss % 20.0 13.2
ATM BoL / mAh 2225.1 2231.5
ATM EoL / mAh 1698.8 1845.0
Loss % 23.7 17.3
6.2.1 EIS of degraded lithium-ion cells
The EIS measurements during the ageing study were performed using a Soltaron 1470E multi-channel potentiostat. The cells were fully charged at 25◦C using a CC-CV protocol (0.1C CC, 4.2 V CV, I<0.01C CV) and subsequently stepwise discharged by applying a current of 0.1C. Every 5 % SoC, an EIS measurement with a current of 0.02C and a frequency range from 100 kHz to 10 mHz was performed. To compromise between the duration of the testing and the influence of relaxation effects on the spectra, a rest period of 30 min was included [107].
Fig. 6.7 depicts the resulting spectra for cell A every four weeks of storage at elevated temperature (60◦C, 100 % SoC), measured at 100 % a, 75 % SoC b and 15 % SoC c. To allow for a clearer distinction of the degradation effects on the spectra, Fig. 6.7d - f were aligned along theZ0-axis.
The shift towards a higher internal resistance (Z00 = 0) is observed for all SoCs. It is caused by an increasing SEI layer, the continuous consumption of electrolyte and its changes in properties due to a variation in salt concentration and the dissolution of further side reaction products. Consequently, the cell’s capability to deliver high currents is reduced. Furthermore, the increasing overpotential causes the cell to reach the respective charge and discharge cut-off voltage faster, limiting the deliverable capacity of the cell under the applied current. Aligning the Nyquist plots allows to analyse the effect of the degradation of the cell. In the spectra recorded at 100 % SoC, a second semi-circle occurs, representing a second dynamic process with a different time constant.
The characteristic of the local minimum is reduced and shifted towards higherZ0 and Z001. As stated in Sec. 6.2, the loss of inventory lithium leads to a misalignment of the electrodes. The impedance spectra are sensitive to the degree of lithiation of the respective electrode, leading to a variation in the observable time constants. With increasing cell degradation, the two semi-circles start to merge again, resulting in a more highlighted local minimum. The slope of the branch, attributed to the lithium-ion diffusion in the
1Please note thatZ00is plotted inversely, consequently the termsdecrease, increaseandminimumdescribe rather the graphical change ofZ00than the actual numerous values
6.2. APPLICATION OF CHARACTERISATION METHODS 105 Figure 6.7–Nyquist plots of cell A 25◦C for various storage durations at 60◦C and 100 % SoC
active material, remains constant, indicating no changes in the cell’s kinetic properties.
For the spectra measured at 75 % SoC, no second semi-circle appears but the semi-circle radii increase and the minimum is shifted linearly towards higherZ0 andZ00 values. For the nearly discharged cell at 15 % SoC a similar effect is observed in the beginning and the semi-circle radii increase. With continuous degradation, a second semi-circle occurs and for the last measurement set, the radius increases significantly, superimposing the linear slope at the low frequency range of the spectra.
The resistance of the cell (Z00 =0) varies with the SoC of the cell. While the variation is rather small at the BoL of the cell, considerable differences are observed after storing the cells for 16 weeks at elevated temperatures. Fig. 6.8 illustrates the increase in resistance during the ageing study, exemplified using the three different SoCs, namely 100 %, 75 % and 15 %. The resistances increase for all SoCs, but with a different slope and resistance increase of the fully charged cell is approximately 30 % higher as for the almost fully discharged cell. This leads to the assumption that the power capability of