Differential Thermal Voltammetry for Tracking of Degradation in Lithium-Ion Batteries

Over the lifetime of a lithium-ion battery, various irreversible degradation mechanisms such as for example excessive Solid Electrolyte Interphase (SEI) layer formation and mechanical fracture of the electrodes can lead to uneven capacity loss in the cathode and anode. This can result in a stoichiometric drift effect whereby the measured cell potential may be within the safe operating range specified by the cell manufacturer but the individual electrodes are at a thermodynamically unstable regime leading to an accelerated capacity fade (Figure 1).

Stoichiometric drift can be measured for individual electrodes using a reference electrode however this is not available in commercial cells. Information from Slow Rate Cyclic Voltammetry (SRCV) measurements can be used to observe stoichiometric drift when reference electrode is not present, however, this is a time consuming procedure which can take up to several hours or days to complete depending on the sweeping rate. Differential Thermal Voltammetry (DTV) has been proposed as a novel in-situ diagnosis technique to measure stoichiometric drift as a faster complement to the SRCV.

DTV uses the temperature profile of the cell to infer the same data as the SRCV but more applicable to real world systems (Figure 1). The peaks in the DTV curve represent phase transformation and in carefully controlled conditions, it may be possible give an indication of the entropy changes of the individual electrodes. DTV can be completed in minutes, can easily be computed in-situ, only requires voltage and temperature measurements and does not require iso-thermal conditions.

Simulations were carried out on a 1D thermally coupled electrochemical battery model with degradation modes. The cell used in this study was a 4.8 Ah lithium-ion polymer cell, with Nickel-Manganese-Cobalt oxide (NMC) based cathode and carbon/graphite anode. Results show that shifts in the location of the SRCV peaks can indicate stoichiometric drift and that the same information can be inferred through the novel in-situ diagnosis technique but at a rate of order of magnitude faster than the SRCV measurements.

The simulations were experimentally validated on commercial 4.8 Ah Kokam lithium-ion polymer cells placed through various accelerated aging processes: load cycling and constant voltage storage at high and room temperatures. It can be demonstrated that different operating/storage conditions induce various degradation mechanisms in the battery. For example, a high temperature operation will promote faster SEI layer growth hence loss of cyclable lithium and increased internal resistance associated with a thicker SEI and a high C-rate cycling will induce mechanical fractures of the electrodes resulting in loss of electro-active material. Throughout the experiment, the cells were regularly analysed using Electrochemical Impedance Spectroscopy, SRCV and DTV to characterise the degradation for each type of operating conditions and to check for the validity of the novel methodology.

By using the novel diagnosis technique, the stoichiometric drift of the battery can be measured sufficiently fast in order to provide useful information regarding the battery condition and to extend the battery life by reducing the operating voltage window according to stoichiometric drift of battery electrodes. 

Figure 1: (right) Potentials of a lithium-ion battery against stoichiometric ratios of lithiations for a carbon anode and NMC cathode. (left) Comparison of experimental SRCV and DTV results for a 4.8Ah lithium polymer Kokam cell at various stages of capacity fade.