Conductive Polymer PTC Materials And Their Applications In Thermal Protection Of Lithium Ion Batteries

The safety concern has become a major obstacle that hinders the large-scale applications of lithium-ion batteries（LIBs）in electrical vehicles and energy storage stations,therefore,building a self-actuating thermal protection mechanism is of vital importance for enhancing the intrinsic safety of LIBs and promoting the development of new energy technologies.Our previous studies have demonstrated that the temperature-sensitive electrode based on conductive polymer PTC material can effectively switch off the electron transport on the electrode at risky temperatures,thus shutting down the electrode reactions and preventing the thermal runaway from happening.However,as a new type of PTC material,the temperature response mechanism of conductive polymer is still not very clear so far,which brings an uncertainty for the design of conductive polymer-based PTC materials and also a difficulty for the development of thermal shutdown cathodes.For this reason,in this Ph D work,we selected poly（3-alkylthiophene）s（P3ATs）as samples to investigate the structure-function relationship and temperature-responsive mechanism of conductive polymer PTC materials.Based on the research results,two conductive polymers,poly（3-dodecylthiophene）（P3DDT）and poly（3-octylpyrrole）（P3OPy）with suitable transition temperatures,were chosen as the PTC materials to fabricate the thermal shutdown cathodes,including the Li Co O₂（LCO）electrodes based on both P3DDT coating layer and composite conductive agents of P3DDT/carbon and P3OPy/carbon.The preparation conditions of PTC coating layer and composite conductive agents,as well as the electrochemical properties and the temperature-responsive behaviors of thermal shutdown cathodes were also investigated.The main contents and conclusions are as follows:1.The structure-function relationship and temperature-responsive mechanism of the conductive polymer PTC material.By investigating the influence of alkyl substituents,polymerization medium and doped anions on the temperature response behaviors of P3ATs,the structure-function relationship and temperature-responsive mechanism of the conductive polymer PTC material were discussed.It was found that the PTC transition temperatures of P3ATs are decreased with the increasing of chain length of alkyl substituents,and their PTC strength are enhanced with the increase of conjugation degree and reduction of doping anions volume.The PTC effect of p-doped conductive polymer arises from the thermal de-doping of anions from polymer chain at high temperatures,rather than the thermal decomposition of the polymer chain backbone.The conductive polymers have reversible PTC behaviors in some extent,but their PTC strength decreased rapidly with the increase of the thermal cycle numbers,possibly due to the violent thermal motion of the alkyl substituent at high temperatures,which causes the partially irreversible changes of polymer skeleton configuration.2.Temperature-responsive electrode with a P3DDT coating layer on current substrate.A thermal shutdown cathode of LCO-P3DDT was fabricated by coating P3DDT on the current collector of LCO electrode,and its temperature-responsive properties and heat evolution behaviors in the case of adiabatic short-circuiting were also investigated.The experimental results demonstrated that P3DDT have suitable transition temperature of 90-100°C,remarkable PTC effect with 4 orders of magnitude variation in electrical resistivity,highly reversible redox behavior,good film-forming processability and thermal stability,suitable for using as coating layer to fabricate shutdown cathode.The thermal response behaviors of LCO-P3DDT cathode reveal that the P3DDT coating layer can effectively disconnect the electrical contact between current collector and cathode-active layer to switch off the cell reaction at risky temperatures,thus protecting the cell from thermal runaway.3.Temperature-responsive cathodes using P3DDT/carbon composite as conductive agent.By comparing the structures of C@P3DDT composite conductive agents prepared from solvent-non-solvent method,in situ chemical oxidation polymerization and oxidative chemical vapor deposition（o CVD）,and investigating the influence of carbon matrixes on the thermal response behaviors of composite conductive agents,the preparation conditions of C@P3DDT composites were optimized.The results showed that the dispersity of conductive carbon matrixes and the uniformity of polymer coating on the carbon surface can be effectively improved by modifying the carbon matrixes with a N-doped pyrolytic carbon of polyvinylpyrrolidone（PVP）.The C@P3DDT composite conductive agent prepared by in situ chemical oxidation polymerization method has the optimal thermal response behavior.The electrochemical and thermal test results showed that the composite conductive agent of C@P3DDT can effectively shut down the cathode reaction at high temperatures,greatly reduce the heat generation of the electrode at risky temperatures,and meanwhile,have no adverse effect on the normal charging and discharging performance of the electrode.In addition,the thermal shutdown effect of SP@P3DDT is better than MCNTs@P3DDT.4.Temperature-responsive cathodes using P3OPy/carbon composite as conductive agent.Considering that polypyrrole and its derivatives have lower oxidation doping potential and higher room-temperature electric conductivity,poly（3-octylpyrrole）（P3OPy）was selected as temperature sensitive material to prepare C@P3OPy composite conductive agent using the in-situ chemical oxidation polymerization method.The temperature response behaviors of both P3OPy and its composite conductive agent were also investigated.The experimental results demonstrated that P3OPy possess suitable transition temperature of 100-120°C,remarkable PTC effect with 6 orders of magnitude variation in electrical resistivity and highly reversible redox behavior,and is therefore a more ideal PTC material.Benefitting the large specific surface area and high room-temperature electric conductivity,the BP2000@P3OPy composite can promptly cut off the electron transport on LCO cathode to switch off the electrode reaction at elevated temperatures,and meantime,ensure the electrochemical performance of LCO electrode at room temperature,showing a great promise for building safer LIBs.