Thermal Behavior of Lithiated Graphite with Electrolyte in Lithium-Ion Batteries

Figure 1 shows the first three cycling profiles of a graphite anode with a lithium foil as a counter electrode at a current of . A plateau observed near in the first discharge process is associated with the decomposition of the electrolyte on the surface of graphite in the formation of the passivating layer.^{12, 13, 28} This plateau disappears in subsequent cycling because the passivating layer suppresses further electrolyte decomposition. The irreversible capacity observed in the first three cycles for graphite electrode are 138, 19, and , respectively, suggesting that after the first three cycles, the formatted SEI film has been formed.

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Figure 2 presents the C80 heat flow curves obtained from the rinsed lithiated graphite anode sample, electrolyte and their coexisting system. This figure shows that the rinsed lithiated graphite, i.e., , starts to be exothermic at about , and reaches the first peak at , the mild second peak at with the total heat generation of . An endothermic peak at and an exothermic peak at are detected in the C80 scan of electrolyte. Three obvious exothermic peaks are detected at 102, 218, and , respectively, with total heat generation of (based on ) in the C80 experiment of coexisting with electrolyte.

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The rinsed lithiated graphite contains graphite, intercalated lithium, SEI layer, and PVDF. The SEI layer mainly consists of stable (such as LiF, ), and metastable components [such as polymers, , , and ROLi].^{6, 20, 21, 28, 33} The graphite, LiF, and are stable, and they do not decompose and produce heat below . The PVDF has a minimal effect on the thermal response of the anode reactions.²² Therefore, the peak of in C80 experiment is attributed to the SEI breakdown and its reaction with intercalated lithium as^{21, 28, 33}

Assuming the reaction mechanism is dependent on the Arrhenius law, based on the C80 data, the following equation 3 is obtained as^{43, 44}

where is over all heat flow, is heat of reaction, is initial mass of reactant, is activation energy, is gas constant, is temperature of system, and is pre-exponential factor. By plotting the curve of versus inverse temperature, the and can be easily calculated.⁴³ By this method, the SEI thermal decomposition activation energy is calculated as , and pre-exponential factor is .

With the presence of electrolyte together with , three exothermic peaks are detected in the C80 experiment. This coexisting system starts to decompose at , and reaches the first peak at with heat generation of , which is earlier than the rinsed alone. This first exothermic process should be the thermal decomposition of SEI layer judging from previous studies. It is reported^{23, 28} that was the major species responsible for the destruction of the SEI layer at high temperatures. Because is a powerful Lewis acid, which is produced by the equilibrium decomposition , and attacks the electron lone pair or the atom with a large electron density, it is likely to react with the oxygen atom of carbonyl groups and compounds possessing sites of increased electron density. In addition, the dominant component of the SEI is , indicating the C–O functional group. Hence, it is reasonable to propose that damages the SEI in a system at .^{23, 28} Using the above method, the activation energy is calculated as , and the pre-exponential factor is .

The breakdown of SEI layer makes the electrolyte easy to reach the lithiated graphite surface, furthermore, the intercalated Li ions move from the inner structure to the edge of graphite particles at high temperatures,^{23, 45} and accelerates the Li movement by removing electrons from graphite. Then, the intercalated lithium easily reacts with electrolyte directly. A mild heat generation continues from until an exothermic peak appears at in C80 experiment of -electrolyte in Fig. 2. This peak is attributed to the reaction of lithiated graphite with the electrolyte.^{24, 46} Several studies have concluded that intercalated lithium reacts with the electrolyte to form a new stable SEI film after SEI decomposition at the edge planes.^{24, 27, 28, 33} The formation of secondary SEI film reactions stops when either the new film is thick enough to insulate reactants or all the intercalated Li has been consumed. This new SEI film prevents the electrolyte from continued reaction with intercalated lithium. As this new SEI film could decompose with rising temperature, therefore, the peak at is contributed by new SEI film decomposition process. ARC results³⁵ disclosed that between 90 and , and EC/DEC react to form Li-alkyl carbonates, which are consistent with the coupled earlier exothermic processes in our experiments. It is reported^{22, 28} that PVDF has a minimal effect on the thermal response of the anode reactions; then, the peak at is attributed to overlapped reactions of PVDF, , Li-alkyl carbonates, and EC/DEC with the formation . Some researchers^{24, 26, 27} attributed this peak to the reaction of lithiated graphite with PVDF, Yamaki et al.²⁴ suggested that the sharp exothermic peak at (corresponding to the peak at in our experiments, as the heating rate, , is lower than Yamaki's, ) was the direct reaction of the lithiated graphite with electrolyte due to SEI breakdown. We believe that the salts and/or EC/DEC are also involved in these reactions.²⁸ The reaction heat of intercalated lithium with electrolyte (second exothermic process) is , and the activation energy is calculated as , and pre-exponential factor is .

Figure 3 presents the C80 heat flow curves obtained from lithiated graphite anode samples at a heating rate from . One main exothermic peak is detected in each sample. With the amount of intercalated lithium increasing in the graphite, the onset temperature presents a decreasing trend; however, the exothermic peak swing instead of adhering strictly to this decreasing trend. At , 0.52, and 0.76, small exothermic process presents before the main large exothermic peaks. These processes are according to the SEI decomposition in theirs coexisting system; at , a small peak at is observed too. In other situations, these processes are absent or not obvious. However, the exothermic curves are not smooth, which may be the overlapped results of different reactions, including SEI decomposition. These may be attributed to the effect of DMC, as DMC may dissolve parts of the organic SEI film. It is suggested that the prewashing process damages the SEI by dissolving some of its constituents (such as polymers and semicarbonates) and causing a fast vigorous reaction.²⁰ The heat generation presents an increasing trend, which is clearly exhibited in Fig. 4. Yang et al.²⁸ have proposed that the peak associated with the SEI film decomposition increases with the increasing intercalated lithium, but this trend is not obvious in their experiments. The heat generation of and decomposition dynamics parameters were calculated in Table I.

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Table I. The thermal characteristics and decomposition dynamics parameters of .

in	Onset temperature (°C)	Peak (°C)	Heat generation ()	Activation energy ()	Pre-exponential factor A()
0.18	136	207		115.3
0.22	65	247		103.8
0.32	47	204		105.8
0.45	50	166		86.6
0.52	44	213		96.5
0.68	42	219		68.8
0.76	41	234		82.0
0.84	47	181		101.3
0.92	42	224		77.0

With the presence of electrolyte in the lithiated graphite, in Fig. 5 all the samples show similar thermal behavior at elevated temperature. Obvious peaks can be identified and are designated peaks , , and in the text. A faint peak between peak and peak appears uncertain; it is coupled with other process and designated peak for unification. Judging from the above analysis, , , , and peaks are attributed to the SEI breakdown, lithium-electrolyte reaction, new SEI film breakdown, and formation reactions overlapped with PVDF reactions, respectively. The SEI breakdown peaks-remain around because the SEI layer's main compositions, LiF, , , , and ROLi, have been formed in the first cycle;^{20, 33, 45} therefore, its thermal stability is not influenced by the intercalated degree of lithium. After the breakdown of former SEI layer, the reaction peak , around , of electrolyte with intercalated lithium presents little increasing trend with intercalated lithium increasing. Peak is the breakdown of secondary SEI, and presents increasing trend of heat generation with the increasing intercalated degree of lithium. The breakdown of secondary SEI film permits the rest of the intercalated lithium to react with the electrolyte or its decomposition products, as the electrolyte decomposes below (Fig. 2). The electrolyte decomposition products are , , , HF, , , etc.^{47, 48} Then, the intercalated lithium will react with these products, which results in the peaks in Fig. 5. After , peaks become more sharp (Yang et al.²³ reported a sharper peak at using DSC at a heating rate); this sharper peak corresponds to the peaks of at heating rate. Aurbach et al.⁴⁹ and Chung et al.⁵⁰ noted that EC and can enter into the graphite sheets and react with intercalated lithium to form lithium carbonate and ethylene. This process is similar to the exfoliation in the first cycle of Li intercalation and can bring about significant changes in the surface structure, including the breakdown of the graphite powder into fragments. Therefore, the sharp peak detected for -electrolyte samples in Fig. 5 is partly attributed to the collapse of the graphite structure.²⁸ The released Li from the collapsed graphite particles can react with PVDF and electrolyte decomposition products at this high temperature and produce more heat. These reactions can be attributed to the dehydrofluorination of PVDF and formation of LiF and hydrogen, and so on.

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The thermal parameters and apparent activation energy were summarized in Table II. The activation energy of SEI decomposition (peak ) and Li-electrolyte reaction (peak ) was determined based on Arrhenius laws. The activation energy of SEI decomposition without much deviation with changing, which indicates the SEI is a stabilized unit formed by LiF, Li₂CO₃, ROCO₂LI, (CH₂OCO₂Li)₂, and ROLi. The apparent activation energy of Li-electrolyte reaction is without much changing too, possibly explained as follows. Before , Li is consumed completely before the new SEI film completely blocks the surface of edge planes, as Li is very active in high temperature; therefore, the more the Li in the lower the activation energy, and no sharp graphite structure collapse was detected in the last exothermic process in -electrolyte samples. At , the new SEI film prevents the Li from reacting with electrolyte, and there about 0.08 intercalated lithium per stays in the graphite sheet, which is not sufficient to generate a sharp peak from the reaction with PVDF and electrolyte decomposition by-products after the new SEI film breakdown. After , the more intercalated Li ( per ) remaining in the graphite, the more intercalated lithium remaining renders the structure more unstable and results in lower activation energy for this structural collapse reaction, which occurs at a lower temperature. The released lithium as a result of the structural collapse reacts with the PVDF binder and electrolyte decomposition by-products to form LiF, hydrogen, and .

Table II. The thermal characteristics and decomposition dynamics parameters of electrolyte.

in	Onset temperature (°C)	Peaks (°C)				Heat generation ()				Total	Activation energy ()		Pre-exponential factor A()
0.19	61	94	211	228	251						100.4	93.0
0.22	67	96	204	224	246						115.2	81.2
0.38	59	102	210	228	241						77.4	62.4
0.46	57	100	211	223	242						92.8	83.0
0.52	51	103	216	235	247						83.7	62.5
0.61	64	100	216	231	243						99.1	78.0
0.73	58	99	214	228	243						83.8	68.4
0.86	69	102	218	233	252						78.6	64.3
0.92	59	101	217	234	249						96.2	69.1