Another important characteristic of lithium graphite interlayer compounds in lithium containing electrolytes is the formation of solid electrolyte (SEI) films. During the first cycle discharge of lithium graphite battery, some lithium atoms will react with non-aqueous solvent, resulting in the generation of initial irreversible capacity. The reaction product forms a lithium ion conductor and an electronic insulating layer on the carbon surface. Peled [24] named it SEI film. Once the SEI membrane is formed, lithium ions are reversibly embedded into carbon through the SEI membrane, even when the potential of the carbon electrode is always lower than the potential of electrolyte decomposition, which prevents the continuous decomposition of electrolyte in the carbon electrode.
By studying the intercalation of solvated lithium ions, besenhard et al. [25] proposed the formation mechanism of SEI film on graphite surface. Ogumi’s team subsequently confirmed this hypothesis in their systematic research [26 ~ 28]. It is generally believed that SEI film plays a very important role in the electrical properties of carbon (especially graphite carbon, which is more sensitive to electrolyte composition). Ethylene carbonate (EC) and propylene carbonate (PC) are high dielectric constant solvents widely used in lithium ion secondary batteries. It has been proved that PC based electrolyte has outstanding low-temperature performance than EC based electrolyte, mainly due to its different melting point (the melting point of PC based electrolyte is – 49 ℃, and the melting point of EC based electrolyte is 39 ℃). However, it is well known that EC based electrolyte is more suitable for graphite, while PC based electrolyte is incompatible with graphite negative electrode, because PC decomposes violently on the graphite surface and peels off graphite particles [29 ~ 32]. Therefore, how to successfully use graphite in PC based electrolyte has become a great challenge in the field of lithium-ion battery.
In fact, the chemical structure of EC and PC is very similar, with only one methyl difference. Why does the introduction of this methyl group into the cyclic carbonate lead to the exfoliation of graphite and the decomposition of electrolyte, thus causing the steric hindrance effect in solvent intercalation? Chung et al. [33, 34] added the second methyl group to the carbonate structure and obtained two geometric isomers, CIS and trans butene carbonate (BC). In the trans butene carbonate based electrolyte, the decomposition of electrolyte and the falling off of graphite are slight, but very intense in cis butene carbonate.
This experiment also proves the importance of SEI film formation mechanism through Li + solvent co embedding. Nakamura et al. [35] studied the properties of graphite carbon in nonaqueous electrolytes containing binary solvent mixtures PC / DEC, PC / DMC and PC / EMC. They found that once the PC concentration decreased to [PC]: [Li +] < 2, the decomposition of PC was basically inhibited. Xu et al. [36] recently found that lithium ion dihydrate (LiBOB) can stabilize graphite carbon in pure PC and support the reversible intercalation of lithium ions. Recently, Jeong et al. [37] successfully embedded lithium ions reversibly in 100% PC with 2.72mol/l Lin (so2c2f5) 2 dissolved, and speculated that the joint action of ion solvents would be the key factor for the formation of SEI membrane in PC based electrolyte. On the other hand, various additives (such as 1,2-vinylidene cyclic carbonate [38], crown ether [39,40], vinyl fluorocarbonate [41], vinyl sulfite [42], phenylphosphodiphenol carbonate [43,44], vinyl acetate [45]) have been used to form an effective SEI membrane in PC electrolyte. In most cases, when the voltage is higher than the decomposition voltage of PC on the graphite negative electrode, the additive will decompose. This means that before PC decomposition and graphite falling off, the decomposition products like vinyl sulfite additives can form a dense SEI film and wrap it on the surface of graphite negative electrode to protect it from direct reaction with PC based electrolyte. In addition, solvents such as crown ether, DMSO (dimethyl sulfoxide) and tetraethanol diethyl ether have stronger bonding force to Li + than PC.
Combined with the extensive study of lithium metal surface passivation film in non-aqueous electrolyte [47 ~ 49], aurbach team has conducted a lot of research on the electrochemical behavior of graphite in lithium ion battery [50 ~ 54]. The structure and chemical composition of SEI film are very important to the properties of graphite. For example, in addition to Li2CO3, roco2li and (ch2oco2li) 2 are the key components of the most effective SEI film of graphite electrode at room temperature. These conclusions were further confirmed by other teams through electron energy loss spectroscopy (EELS) [55], Auger electron spectroscopy (ASE) and temperature programmed decomposition mass spectrometry (tpamas) [56].
As the performance of lithium-ion battery at high temperature (50 ~ 70 ℃) is related to its use safety, recently, research on the performance of SEI membrane at high temperature has been carried out [57 ~ 62]. The results show that at high temperature, the metastable groups like roco2li in SEI films decompose into more stable products, such as lic2o3 and lif. This leaves more holes in the SEI membrane, exposing the graphite lithium surface to the electrolyte, resulting in more irreversible capacity during continuous circulation. In fact, according to the above phenomena, many companies generate stable SEI films on lithium-ion battery electrodes through the “aging” process. After assembly, the lithium-ion battery will be stored in high temperature for a certain time after charging. SEI film is mainly composed of stable molecules, such as lic2o3 and lif. It has been proved that it can compact and effectively passivate carbon negative electrode.
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