Thermodynamics and crystal structure anomalies in lithium-intercalated graphite

220

185

A mathematical model to simulate the generation of mechanical stress during the discharge process in a dual porous insertion electrode cell sandwich comprised of lithium cobalt oxide and carbon is presented. The model attributes stress buildup within intercalation electrodes to two different aspects: changes in the lattice volume due to intercalation and phase transformation during the charge/discharge process. The model is used to predict the influence of cell design parameters such as thickness, porosity, and particle size of the electrodes on the magnitude of stress generation. The model developed in this study can be used to understand the mechanical degradation in a porous electrode during an intercalation/deintercalation process, and the use of this model results in an improved design for battery electrodes that are mechanically durable over an extended period of operation.

Section: Resultsmentioning

confidence: 99%

Theoretical Analysis of Stresses in a Lithium Ion Cell

Renganathan

Sikha

Santhanagopalan³

et al. 2010

220

185

“…[12][13][14][15][16][17] In the case of Li-ion insertion into graphite, the experimental curve for entropy as a function of composition, Reference 12, shows a step in the transition of stage II to stage I that could not be explained in the terms presented there. In a subsequent work, 18 two of the previous authors, state that a possible mechanism of the stage I phase formation may involve a 'dilute lithium layer' (noted dil-Li) that would have an alternating 'normal' Li layer (Li) with a hexagonal structure and a dilute lithium layer following the sequence (Li)-G-(dil-Li)-G. However, in a more recent review, Fultz 19 stated that the vibrational entropy resulting from the insertion dominates the entropy, and also he added that there should be a small change in the configurational entropy when compounds are formed in stage I or II, as they are ordered.…”

mentioning

confidence: 86%

Shedding Light on the Entropy Change Found for the Transition Stage II→Stage I of Li-Ion Storage in Graphite

Leiva

Perassi

Barraco

2016

In order to examine the controversial hypothesis put forward to explain the entropy step experimentally observed for the stage II to stage I transition for lithium intercalation in graphite, a transparent statistical mechanical model is developed. The results obtained show that the entropy increase can be explained by the change of configurational entropy occurring at occupation of half of the lattice. Comparison with experimental data shows that attractive interactions between intercalated particles in the same layer must be assumed, in agreement with the ansatz made in the original experimental work. 5,6 and graphite, 7,8 are commonly found in electrodes of commercially lithium-ion cells. In addition to the mentioned intercalation materials, there are many others that are currently being studied or used as electrodes in lithium-ion cells. In particular, graphite is the most common material found in anodes of commercial lithium-ion cells.One of the most important features of the intercalation materials is that they can have ions stored in them with little changes in their crystalline structures. This feature allows a fast ion insertion and extraction and therefore high power density cells can be obtained when used as electrodes. However, a fast ion insertion and extraction generates heat in the cell, which can produce high temperature excursions of the cell and a premature aging.It has been demonstrated that a great portion of the heat that is generated in the intercalation materials comes from the intercalation entropy during a discharge. [9][10][11] In this sense, different methods to measure the intercalation entropy have been developed. [12][13][14][15][16][17] In the case of Li-ion insertion into graphite, the experimental curve for entropy as a function of composition, Reference 12, shows a step in the transition of stage II to stage I that could not be explained in the terms presented there. In a subsequent work, 18 two of the previous authors, state that a possible mechanism of the stage I phase formation may involve a 'dilute lithium layer' (noted dil-Li) that would have an alternating 'normal' Li layer (Li) with a hexagonal structure and a dilute lithium layer following the sequence (Li)-G-(dil-Li)-G. However, in a more recent review, Fultz 19 stated that the vibrational entropy resulting from the insertion dominates the entropy, and also he added that there should be a small change in the configurational entropy when compounds are formed in stage I or II, as they are ordered.In a previous work 20 we have proposed a theoretical approach to determine the intercalation entropy, and we applied this approach to the graphite/lithium compound. In order to clarify the origin of this entropy, in the present work we have used a simplified two-level lattice gas model to analyze the configurational contribution to the intercalation entropy of the graphite/lithium compound. The main features of the intercalation entropy are elucidated. z E-mail: eze_leiva@yahoo.com.ar Model and Statistical Mechanical Backgroun...

“…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.…”

mentioning

confidence: 99%

Model of Lithium Intercalation into Graphite by Potentiometric Analysis with Equilibrium and Entropy Change Curves of Graphite Electrode

Allart¹,

Montaru²,

Gualous³

2018

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 ...