Temperature-sensitive Electrodes With Thermal Protection Mechanism For Lithium-ion Batteries

The electric vehicle（EV） firing accidents happening one after another remind us that safety issues of lithium ion batteries（LIBs） have become the major obstacle to its application in many new technology fields. The mechanism of lithium-ion battery unsafe behaviors indicate that vast amounts of energy converting rapidly into heat in various forms in a limited volume is the main factor which eventually lead to cell cracking,firing or even explosion. Therefore, slowing down or even cuting off the energy release of LIBs in abuse states can improve the safety behavior of LIBs to a great extent. This Ph.D work is aimed at exploring and developing new temperature-sensive electrodes with thermal cut-off protection mechanism for LIBs, so as to provide theoretical basis for their commercial application. The main contents and results are as follows:1. Three kinds of plastic-carbon balck PTC composite materials were prepared with soluble polymers including PVDF, PMMA as the insulating matrixto, and were employed as a coating layer with a thinkness of less than 20 μm on Al foil substrate to fabricate sandwiched Al/PTC/LiFePO₄ composite electrodes. The temperature-sensitive properties and electrochamical performance of those composite electrodes were also tested. Then, a mathematical model of LIBs using LiFePO₄ as cathode material and mesocarbon microbeads（MCMB） as anode material was established and thus the relationship between the electrical conductivity, thinkness of PTC coating layers and the electrochemical performance of composite electrodes was also analyzed. The experimental results demonstrate that all these PTC electrodes have the normal electrochemical performance at ambient temperature but show a huge increase in resistance at the temperature of 160¹80℃. The simulation results show that the main difference of bare cells and PTC electrodes is their discharging voltage platform at high discharge current densities, and the voltage difference（ΔU） between them is proportional to the thickness of PTC layers（L）, the discharge current density（I/S）, and is inversely proportional to the conductivity（κ） of PTC layer. In a word, their voltage difference（ΔU） follows the Ohm’s law（ ΔU=IR, R=IL/κS）. When overheated, the conducivity of PTC layers will drop sharply, causing a huge ohmic polarization, and thus the charge/discharge capacity, discharge voltage（or current） of the PTC cells will decrease, protecting the LIBs from thermal runaway.2. The working mechanism of conductive polymer used as PTC material for thermal cut-off protection in LIBs is based on its intrinsic nature that it can transform from a conductive state to an insulating state upon thermal de-doping. In this part, the soluble poly（3-butylthiophene）（P3BT） polymer was employed as PTC material and a new positive temperature coefficient（PTC） electrode was prepared simply by coating a thin layer of P3 BT through spraying in between the electroactive Li[Ni_0.5Co_0.2Mn_0.3]O₂ [523] layer and the Al foil substrate. The electrochemical performance and conductivity-temperature dependence of the doped P3 BT were studied. And the discharge properties and thermal behaviors of the as-prepared Al/P3BT/523（P3BT-523） composite PTC electrodes were all tested in detial. The experimental results demonstrate that the electrical conductivity of the p-doped P3 BT decreases sharply at 135¹50℃, showing a remarkable PTC effect. As for the P3BT-523 composite electrodes, a rate capacity nearly reaching the reference electrode is obtained at less than 1C rate, while the rate discharge capacities increase obviously with the decrease of the P3 BT solution concentration at room temperature. When exposed to high temperature, owing to the thermal-dedoping behavior of doped P3 BT, the electrochemical reaction resistance of PTC electrodes increase significantly. For example, after being exposed to 150 ℃ for 5 min, the P3BT-523 composite electrodes are hardly discharged and the discharge voltage of the composite electrodes drop drastically below the cut-off voltage at a current density of 3C, suggesting an effective thermal shutdown switch for lithium-ion batteries.3. A polymer with self-crosslinking function was synthesized and its structure, molecular weight, thermal crosslinking properties were tested. Then this self-crosslinking polymer（S） was used as electrode additive in Li-rich solid solution materials（GRT）, and the electrochemical properties, morphologies and crystalline structure of the GRT-S composite electrodes were all investigated in detail. The experimental results demonstrated that the polymer solution can transform into gel state within 10 min at 150℃ or within 4 min at 170 ℃, showing an excellent self-crosslinking characteristic at elevated temperature. Therefore, when overheated, the ion transport resistance of GRT-S composite electrodes will increase signficantly, slowing down the electrochemical reaction and thus making a safer lithium-ion battery. Meanwhile, the addition of polymer S has little influence on the discharge capacity of GRT material. What’s more,compared with bare GRT electrodes, GRT-1%S composite electrodes show an excellent cycle performance, especially in the voltage range of 2.0-4.8V. At a current density of 200 mA g-1（1C）, GRT-1%S composite electrodes deliver a high capacity of 138.9 mAh g-1 after 100 cylces（73.8% of the first discharge capacity）, while only 89.5 mAh g-1（43.7% of the first discharge capacity） for bare GRT electrodes. Besides, results from SEM and XRD show that the polymer S in the composite electrodes can help to form a more stable solid electrolyte interphase（SEI） layer on the surface of GRT cathode material and suppress the structural change of GRT material during cycling to a certain extent.