Thermal Behavior of Ionic Liquid Electrolytes in Lithium-ion Cells

Abstract

The thermal behavior of ionic liquids in lithium-ion cells was examined by a Calvet-type calorimeter using typical ionic liquid electrolytes and the charged electrodes (a Li x C 6 anode and a Li y Ni 1/3 Mn 1/3 Co 1/3 O 2 cathode) obtained by disassembling the lithium-ion cells with a conventional carbonate solvent electrolyte. Although ionic liquid electrolytes showed a little bit different thermal behavior compared with the conventional electrolyte, general features were almost the same. Cell safety does not directly connect to non-flammability of electrolytes based on the thermal runaway mechanism. Due to the lack of carbonate structure, ionic liquids produce much less CO 2 , which might be an advantage of ionic liquids for the cell safety. IntroductionLithium-ion cells are now widely used as power sources for portable electronic devices such as laptop computers and cellular phones due to their high energy density. Large-sized lithium-ion cells are also very attractive for pure or hybrid electric vehicle applications. However, lithium-ion cells intrinsically have serious safety problems, and the cell safety becomes the most important issue for the large batteries. When they are abused, several exothermic reactions occur inside the cell and the cell temperature increases rapidly, which results in fire or explosion. Because the heat generation and dissipation are related to the volume and surface area of the cell, respectively, the heat generation surpasses the heat dissipation as the cell size increases. Based on this thermal runaway mechanism, cell safety can be controlled by the suppression of the heat generation. Therefore, it is very important to understand the thermal behavior of nonaqueous electrolytes in lithium-ion cells.There are many reports on the thermal behavior of lithium-ion cells and their components by using accelerating rate calorimetry (ARC) and differential scanning calorimetry (DSC). We also have investigated quantitatively the thermal behavior of a graphite/LiCoO 2 cell, its components, and their combinations by a Calvet-type calorimeter, and confirmed that the following four reactions are important as main exothermic factors (1):(1) Electrolyte reduction by the anode (2) Electrolyte oxidation by the cathode (3) Combustion reactions of electrolyte and separator based on the released oxygen from the cathode. (4) Chemical shuttle reactions between the anode and the cathode.Since flammable organic solvents such as ethylene carbonate (EC) and ethyl methyl carbonate (EMC) are generally used for their nonaqueous electrolyte solutions, alternative non-flammable solvents are desired to enhance the cell safety. Ionic liquids have been expected as promising electrolyte materials due to their non-flammability, nonvolatility, and high thermal stability (2).It has long been believed that no ionic liquid is compatible with graphite anodes, because ammonium cations can destroy the graphite layer structure due to the cointercalation during charge. Therefore, the research using ionic liquids was oriented t...

Challenges and Opportunities to Mitigate the Catastrophic Thermal Runaway of High‐Energy Batteries

“…[41] Also, replacing the electrolyte to ionic liquids only puts off the intense heat release for ≈30 °C higher according to full-cell DSC tests (Figure 15f). [279] To mitigate the crosstalk reactions, introducing electrolyte additive to capture the active oxygen is verified to slightly decrease Figure 13. Safety functional materials/structures.…”

“…[ 41 ] Also, replacing the electrolyte to ionic liquids only puts off the intense heat release for ≈30 °C higher according to full‐cell DSC tests (Figure 15f). [ 279 ]…”

“…[274,275] Moreover, when paired with the cathode or anode materials, most of the ionic liquid chemistries do not display better thermal stability than carbonate electrolytes, especially the ones containing [EMim] + and [FSI] − . [276][277][278][279] SSE can be divided into polymer SSE and inorganic ceramic SSE (sulfide, oxide, etc.). The individual polymers and ceramics Reproduced with permission.…”

“…[ 274,275 ] Moreover, when paired with the cathode or anode materials, most of the ionic liquid chemistries do not display better thermal stability than carbonate electrolytes, especially the ones containing [EMim] + and [FSI] − . [ 276–279 ]…”

“…Reproduced with permission. [ 279 ] Copyright 2009, The Electrochemical Society. g) DSC curves of carbonate electrolyte and β‐carotene‐containing electrolyte without or with cathode materials (Li‐rich magnesium‐based cathode charged to 4.6 V).…”