Unravelling charge/discharge and capacity fading mechanisms in dual-graphite battery cells using an electron inventory model

Abstract

The dual-ion battery (DIB) and a subtype thereof, the dual-graphite battery (DGB), are considered as promising alternative options for stationary energy storage applications. Here, we show that not only the working principle of DGBs is fundamentally different from ion-transfer cells, such as the lithium ion battery (LIB), but the capacity fading mechanisms as well. In order to compare the charge/discharge mechanism and aging processes of LIBs and DIBs, which are associated with loss/change of the active species content, a unified “electron inventory model” applicable for both battery systems is introduced and comprehensively explained. This model regards all electron-consuming or -donating reactions in the cell at the respective electrodes. Using the model, two predominant aging variants of DGBs can be distinguished. Both of these originate from disparate (parasitic) electron consumption/donation at the negative or positive electrode, respectively. However, there is an “electron balance” or “electron charge neutrality” between the electrodes, which intrinsically does not allow this difference: This is a consequence of redox reactions that must take place simultaneously at both electrodes in order to guarantee electronic charge neutrality. For example, if electron-consuming reactions occur at the negative electrode that consume lithium cations, the positive electrode is forced to intercalate additional anions for charge compensation, i.e. to ensure charge neutrality in the electrolyte. These anions are irreversibly trapped in the positive electrode and can no longer contribute to reversible charge/discharge reactions. Suitable countermeasures including the pre-lithiation of the negative electrode are presented in this work.