Thermodynamics of Lithium Intercalation into Graphites and Disordered Carbons

Abstract

The temperature dependence of the open-circuit potential of lithium half-cells was measured for electrodes of carbon materials having different amounts of structural disorder. The entropy of lithium intercalation, ⌬S, and enthalpy of intercalation, ⌬H, were determined over a broad range of lithium concentrations. For the disordered carbons, ⌬S is small. For graphite, an initially large ⌬S decreases with lithium concentration, becomes negative, and then shows two plateaus associated with the formation of intercalation compounds. For all carbons ⌬H is negative, and decreases in magnitude with increased lithium concentration. For lithium concentrations less than x ϭ 0.5 in Li x C 6 , for the disordered carbons the magnitude of ⌬H is significantly more negative than for graphite ͑i.e., intercalation is more exothermic͒. The temperature dependence of the open-circuit voltage ͑OCV͒, in equilibrated half-cells can be used to determine the entropy, ⌬S, and enthalpy, ⌬H, of the lithium intercalation reactionFor an electrochemical system, the Gibbs free energy, ⌬G, can be related to the OCV, E 0 . 2,3 In this study ⌬G is defined as the OCV measured after at least 3 h restwhere F is the Faraday constant and T the absolute temperature. The entropy of formation isand enthalpy of formation is obtained from Eq. 2 and 3 asIn what follows, the variation of any state function, ⌬G, ⌬H, and ⌬S, relates to adding an incremental amount of lithium into the host material having lithium concentration x, taking metallic bcc lithium and the pure carboneous material as reference.The OCV method has been used to study some intercalation compounds, such as Li x TiS2 or manganese oxides.3,4 However, there is only a little prior work using this method for investigating lithium intercalation into carboneous materials, 5-9 or other graphite intercalation compounds.10 Takano et al. 11 studied full Li-ion batteries, which made it difficult to discuss the results. These previous investigations did not address the effect of disorder in the carbon materials, or the mechanisms of lithium intercalation. Here we report results of a temperature-dependent OCV study on how the degree of graphitization in carbon materials altered their dependence on ⌬S and ⌬H on lithium concentration. The effects of structural disorder on both entropy and enthalpy were large, and can be explained only in part by the configurational entropy of mixing. ExperimentalMesocarbon microbeads ͑MCMB͒ were provided by Osaka Gas, Japan, with different heat-treatment temperatures of 750, 1000, and 2800°C. Natural graphite from Superior Graphite, Chicago, IL, was also used for the electrochemical measurements ͑Ref. SO 3-24-1͒. The MCMB material heat-treated at 750°C was prepared from the so-called green powder by heating under nitrogen flow in a standard tube furnace. The temperature ramp was set to 10°C per hour to prevent sintering, and the final temperature was maintained for 2 h. The two other MCMB materials were heat-treated by Osaka Gas at 1000 and 2800°C.Coin cells of 20 mm diam an...

Li intercalation in graphite: A van der Waals density-functional study

“…6(f) is in line with what is found in experiments, where a small potential step is typically observed at x = 0.5, a larger one at or close to x = 0.2, and further increases of the potential for smaller x [12][13][14][15][16]22]. Evidently including entropy effects leads to a decent description of the voltage curve.…”

“…The stiffening of the Li vibrational modes in intercalated graphite (with respect to Li metal) reduces the entropy, resulting in a negative intercalation entropy. This effect has 155448-5 been observed experimentally [16,80]. Adding enthalpy and entropy contributions yields an intercalation free energy that is monotonically increasing with temperature.…”

“…The structure of the fully loaded stage 1 compound LiC 6 consists of graphene alternating with a layer of Li atoms. Controlling the Li content electrochemically and monitoring the Li x C 6 potential as a function of x shows a sequence of plateaus that is interpreted as subsequent phase equilibria [12][13][14][15][16][17]. Stage N (=2,3, .…”

The Science of Electrode Materials for Lithium Batteries

“…The method obtains the entropy and enthalpy of the lithiation reaction. natural graphite [71], and into meso carbon microbeads and cokes that were graphitized at various temperatures [72]. Some results are presented in Figure 4.…”

Thermodynamics of Lithium Intercalation in Randomly Oriented High Graphene Carbon

“…Temperature dependence of Open Circuit Voltage (OCV) (electromotive force (emf)) is reported for Li-Sn system in the literature to measure free energy components (entropy and enthalpy) associated with the alloy [12]. With an assumption that lithium intercalated in graphite (or any carbon) forms a graphite intercalation compound, the same concept is applied to Li-C systems and is reported in the literature by Reynier et al [13]. Using the fundamental laws of thermodynamics (1), the entropy (Δ ) and enthalpy (Δ ) associated with free energy of the lithium ion intercalation in the graphite structure is calculated from (2) and (3).…”