No abstract

No abstract

No abstract

One of the interests in studying the intercalation phenomenon of Li-ion is to explain the hystereses which are observed on the open circuit voltage curve of the graphite electrode during the charge and discharge. We investigated a potentiometric method to obtain the equilibrium curve and entropy change curve of the graphite electrode in charge and discharge. These curves lead to the analysis of the intercalation compounds in the graphite electrode. The results show a high hysteresis between the lithiation and delithiation in region II on the entropy curve. We do not observe the formation of LiC 18 compound which would be observed at filling fraction x = 0.33 and there is no clear evidence of the LiC 27 at x = 0.22. Based on our observations, we propose an intercalation model of Li into graphite in an attempt to explain the hysteresis phenomenon which was observed during charge and discharge in region II due to the possible presence of the compound LiC 24 . We have also observed a possible compound for the LiC 24 by XRD post-mortem analysis. With the significant growth of renewable energies, Li-ion batteries are going to play an increasing important role in energy storage systems. In Li-ion batteries, the material used for the anode is mainly graphite carbon for the following reasons: low cost, safety, capacity, cyclability and redox potential. The graphite of the negative electrode is prone to degradations such as solid electrolyte interphase formation (SEI), dendrites formation and Li plating on the surface. These mechanisms increase battery ageing and seriously reduce the performance and lifespan.1 It is interesting to study intercalation effects of Li-ion to understand the Li intercalation mechanisms in the crystal structure of the graphite and to explain the hystereses which are observed between the charge and discharge processes.2,3 The analysis of the open circuit voltage (OCV) curve and entropy curve of the electrode material highlights the crystallographic phases of the compounds inserted into graphite. In the following study, we focus our investigations on the transition phase in the regions II and III (Figure 7) in an attempt to explain the high hysteresis which was more specifically observed on the entropy curve of the negative electrode.The graphite presents a planar hexagonal structure and the stacking is A-B-A-B. 4 The dynamic of intercalation of Li-ions into graphite creates disorder within the crystal structure. The entropy variation S during the chemical reaction is reflected in the degree of disorder in the active material. When a crystal structure is achieved, it corresponds to a crystallographic phase. 5 The electrode charges and discharges according to the various insertion stages (Figure 1), this representation is based on the Daumas-Hérold intercalated model. 6 The graphite stage corresponds to the natural carbon graphite when the electrode is empty of Li. During the insertion reaction, the graphite is loading in Li through several stages. The stages are defined according to the number of ...

No abstract