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

Abstract

A C80 calorimeter was used to study the thermal behaviors of Li x C 6 and Li x C 6 in 1.0 M LiPF 6 /EC + DEC electrolyte. C80 results show that Li x C 6 alone shows one exothermic peak, which is attributed to the solid electrolyte interphase ͑SEI͒ decomposition. Four exothermic peaks were detected in Li x C 6 -1.0 M LiPF 6 /EC + DEC electrolyte samples. These four peaks are attributed to SEI decomposition, Li-electrolyte reaction as well as new SEI film formation, new SEI film decomposition, and Li with PVDF/other products reactions. The apparent activation energy of Li x C 6 and Li x C 6 -electrolyte at different states of charge was calculated, and it was found that with intercalated lithium increasing, the activation energy shows a decreasing trend.The lithium-ion battery fulfills many of the demands made within the areas of portable electronics and electrical vehicles, and is superior in many ways to the more common nickel-cadmium and nickel-metal hydride batteries. 1-5 However, there are also potential safety problems in their use due to the occurrence of thermal runaway in abuse cases. 6 The safety is mainly related to the thermal reactivity of the cell materials. The exothermic reactions of materials in battery applications can cause thermal runaway in the cell, and thereby constitute a safety hazard. It has been shown that the thermal stability of anode is critical to the thermal runaway of battery. [6][7][8][9][10] Graphite remains the material of choice for anodes in rechargeable lithium-ion batteries because of its high capacity ͑372 mAh g −1 ͒, flat voltage, and low cost. 11 During the first charge, graphite-based electrolytes are reduced at the negative electrode. As a result, a surface film is formed consisting of a variety of solvent and salt reduction products. 12,13 This film functions as an ionic conductor that allows Li + ion to be transported through the film during the subsequent intercalation and deintercalation processes. The film is also an electronic insulator, which will prevent the continuous reduction of electrolyte as the film thickness reaches a certain limit. The film then functions as a passivating layer on the graphite surface. It is most often referred to as a solid electrolyte interphase ͑SEI͒. 11-15 Using ac impedance, the formation process of SEI film on graphite electrode during initial cycles was studied. [15][16][17][18][19] The results show that the SEI formation takes place through two major stages. The first stage takes place at voltages above 0.25 V ͑before lithiation of graphite͒, during which a loose and highly resistive film is formed. The second stage occurs at a narrow voltage range of 0.25-0.04 V, which proceeds simultaneously with lithiation of graphite electrode. In the second stage, a stable, compact, and highly conductive SEI film is produced. [11][12][13][14][15] Studies by means of differential scanning calorimetry ͑DSC͒ ͑Ref. 10 and 20-31͒ and accelerated rate calorimetry ͑ARC͒ ͑Ref. 26 and 32-38͒ have shown that the thermal stability of graphite electrodes...

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Improved thermal stability of graphite electrodes in lithium-ion batteries using 4-isopropyl phenyl diphenyl phosphate as an additive

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