Self-actuating Thermal Protection Mechanism For Lithium-ion Batteries

In recent years, with the increasing interest in lithium ion batteries (LIBs) for electric vehicle (EV) applications, EV firing accidents caused by the hazardous behaviors of LIB power batteries happened one after another, raising the worries about the safety of LIBs. Safety concern has become a major obstacle retarding the commercial applications of high-rate and high-capacity LIBs in electric vehicles and renewable power stations. It is now well recognized that thermal runaway, which is usually induced by abusive conditions such as electrical overcharge, internal or external short-circuit, or high thermal impact, is a major cause for the hazardous behaviors of LIBs. Therefore, research and development of self-actuating thermal protection mechanism for LIBs to prevent the battery from going into thermal runaway is an urgent demand for a number of high-technology applications. This Ph. D work is aimed at exploring and developing such thermal protection mechanisms, so as to provide applicable principles for developing safety enhancing technologies for LIBs. The main contents and main results are as follows:1. Preparation and characterization of positive-temperature-coefficient (PTC) electrode with thermal cut-off mechanism. To develop a PTC electrode suitable for use in practical LIBs, we selected a thermoset epoxy resin and three kinds of soluble polymers including poly (vinylidene fluoride)(PVDF), polystyrene (PS), and poly (methyl methacrylate)(PMMA) as the insulating matrixto prepare plastic-carbon black (CB) PTC composites and investigated the PTC behaviors of these compounds at elevated temperatures, respectively. The results showed that the PMMA-Super P composite with a mass ratio of90:10has appropriate PTC transition temperature at～110℃, remarkable PTC effect and sufficient conductivity of～1.44*10-3S cm-1at room temperature, suitable for using as a PTC material to provide thermal cut off mechanism for safety control of the rechargeable Li batteries. Subsequently, we fabricated three kinds of PTC composite electrode, Al/PTC/LiCoO2, Cu/PTC/MCMB and PTC-LiCoO2, by using this PTC composite as the coating layer on the substrate or as the conductive agent, and investigated the electrochemical behaviors of these PTC electrodes at various temperatures. The experimental results from cyclic voltammetry, charge-discharge measurements and impedance spectrometry demonstrated that all three PTC electrodes have normal electrochemical performance at ambient temperature, but show an enormous increase in resistance at the temperature range of90～110℃. This PTC behavior greatly restrains or even cuts off the current passing through the electrode at elevated temperatures, capable of acting as a self-actuating safety mechanism to prevent the battery from thermal runaway.2. Preparation and application of temperature-sensitive electrode materials with thermal shutdown mechanism. Abovementioned PTC electrodes can restrain the current passage between the electrode substrate and the electro-active coating layer or between the particles of the electrode-active material at overheated conditions, but can not shut down the thermal runaway reactions directly occurring on the surface of highly active electrode material at elevated temperatures. For building thermally stable and safer LIBs, we proposed to develop novel temperature-sensitive electrode materials with self-activating thermal protection mechanism by coating a thin layer of conducting polymer with the required PTC effect on the surface of the conventional electrode-active particles. Since the outer PTC skin can sensitively respond to the temperature rise in the microenvironment of the electrode and transform promptly at risky temperature from an electrical conducting state to a highly resistive state with several orders of magnitude increase in the electric resistance, all the electrochemical reactions taking place on the surface of the electrode-active particles would be shut down, so as to prevent the thermal runaway from happening, thus ensuring the safety of lithium ion batteries at dangerous thermal abuse. In order to realize this concept, we synthesized a number of electro-active polymers, including poly (3-hexylpyrrole) and poly (3-alkylthiophene), by chemically oxidative polymerization, and investigated their PTC effects and electrochemical behaviors at various temperatures. The results showed that poly (3-decaylthiophene)(P3DT) has high conductivity of0.16S cm-1at ambient temperature, appropriate PTC transition temperatures at100～150℃, remarkable PTC effect, and good processability to dissolve in commonly used organic solvents, suitable for using as a PTC coating layer for constructing temperature-sensitive electrode material. Subsequently, we use P3DT polymer as PTC coating layer to prepare several typical temperature-sensitive electrode materials, including LiCoO2@P3DT, LiMn2O4@P3DT, LiNi0.33Co0.38Mno.3202@P3DT and MCMB@P3DT, and investigated the electrochemical behaviors of these composite materials in the battery environment at various temperatures. The experimental results demonstrated that, except for LiMn2O4@P3DT, other composite materials showed not only improved cycling performance at ambient temperature, but also a thermal shutdown action at elevated temperature of110℃, capable of providing self-activating thermal protection for the overheated cells. Further study on mechanism revealed that the temperature-sensitive characteristics of these composite electrode materials are resulted from the thermal de-doping behavior of the P3DT skin layer and the decomposition of PF6-dopant at elevated temperature.3. Polymerizable electrolyte additive with thermal shutdown mechanism for LIB. The polymerizable electrolyte additives for thermal shutdown protection of LIBs is based on their thermal polymerization reaction at an elevated temperature, which leads to a change of the electrolyte from liquid solution to a solid state, thus frustrating or even cutting off the ionic transport between the two electrodes so as to terminate the battery reactions to prevent thermal runaway from happening in the LIB at a risky temperature. To search for a thermally polymerizable additive with suitable polymerization properties, we selected a number of polymerizable monomers,1,3-dioxolane (DOL),1,1’-(Methylenedi-4,1-Phenylene) bismaleimide (BMI), Noepentyl glycol diacrylate (NPGDA) and Maleimide oligomers (BMI1), and compared their thermal polymerization behaviors as additives in electrolyte. The results showed that the BMI additive can polymerize to cause the rapid solidification of electrolyte at110℃, which can effectively block off the ion transport between electrodes, thus generating a thermal shutdown of the electrode reactions for safe control of lithium ion batteries. Meanwhile, the addition of BMI additive has no significant influence on the normal charge-discharge performance of rechargeable lithium batteries, demonstrating the suitability of BMI as a safety additive for thermal protection of LIBs. In addition, it was also found that BMI monomer can function as an anti-overcharge additive to provide effective overcharge protection for LiFePO4electrode, showing a great prospect for battery applications.