Lithium Ion Battery Safety Strategies Based On Poly(3-octythiophene) And Related Mechanism Research

Whereas the energy density,power density and cycling life of LIBs have been significantly improved in the past two decades,battery safety remains an important and unresolved issue.Safety issues have become a major obstacle impeding the large-scale application of high-energy-density LIBs.The root of safety concerns for LIBs is the catastrophic thermal runaway.The"electrochemical reactions switch"mechanism is introduced into LIBs.When the battery is abused,it can cut off the electron or ion transport of the electrochemical reactions,and consequentially switch off the battery in time to avoid the continuous damage of the battery,preventing the occurrence of thermal runaway.Overheat and overcharge are the most common abuses in LIBs.Based on the thermal dedoping and overoxidation behaviors of poly（3-alkylthiophene）,this study has developed it as the electrochemical reactions switch of LIBs to prepare the safer battery materials,including the Ni-rich cathode materials with temperature-responsive coating,the organic cathodes with overheating self-protection function,the all-organic batteries with overheating self-protection function and the overheat-overcharge-responsive switching.（1）Ni-rich cathode materials with temperature-responsive coating.Poly（3-octythiophene）with positive temperature coefficient was selected as the surface coating of Nickel-rich cathode material.Coating of Li Ni_0.8Co_0.1Mn_0.1O₂materialswithtemperatures-sensitive poly（3-octylthiophene）was prepared via a facile electrostatic self-assembly method.The charge/discharge behaviors of NCM811 and NCM811@P3OT cathode are compared under normal and overheat conditions.The experimental results indicate that NCM811@P3OT cathode exhibits a superior cycling stability than NCM811 cathode under room temperature.When the battery temperature reached to 110°C,NCM811@P3OT cathode is switched off spontaneously,shutting down the battery reactions.And the NCM811 cathode can still continue to work,which causes the serious concerns.Neutral P3OT coating is electrochemically oxidized in the charging process,inducing the doping of PF₆^-into P3OT to maintain charge neutrality.Anions-doped P3OT has a good electronic conductivity,which does not affect the electrochemical reactions of NCM811 material.P3OT coating can isolate the bulk cathode materials from the electrolyte,alleviate the structure destroy and suppress the side reactions on the cathode electrolyte interface,enhancing the cyclability of NCM811.At risky temperature,doped PF₆^-is thermally dedoped from the P3OT matrix due to the violently accelerated thermal motion of the polymer chains and the thermal decomposition of doped anion at elevated temperature,which leads to a transition of P3OT from conductive state to an insulating state and therefore halts the transport of electrons between NCM811 particles and neighboring particles followed by switching off battery reactions,thus preventing battery from further generating heat and avoiding the occurrence of thermal runaway.（2）Organic cathode materials with overheating self-protection function.Poly（3-alkylthiophenes）show a reversible electrochemical PF₆^--doping/dedoping behavior in battery environment,which enables it as a cathode material for LBs.We constructed a P3BT@CNT and P3OT@CNT nanocomposites via a simple dissolution-precipitation method and investigated their electrochemical performances as cathode materials for LIBs.The results indicate both P3BT@CNT and P3OT@CNT exhibit a good electrochemical performance under room temperature.When the internal temperature is abnormal,the battery with P3BT@CNT or P3OT@CNT electrode maintains a low state-of-charge in the whole charge process,which can offer an early warning of battery overheating.In the discharge the battery is rapidly switched off,providing overheat protection for LBs and preventing the occurrence of thermal runaway.The overheat protection function is assigned to the thermal dedoping of PF₆^-from poly（3-alkylthiophenes）matrix.（3）All-organic battery with overheating self-protection function.Firstly,the lithium storage behaviors of P3BT@CNT and P3OT@CNT as anode materials for lithium-ion battery are explored.The test results show that both P3BT@CNT and P3OT@CNT anode materials show an excellent lithium storage capacity.The thiophene rings act as the redox-activesitesduringLi⁺storageprocessof poly（3-alkylthiophenes）.The experiments and theoretical calculations reveal CNT makes important contributions to the high performance of the anode materials.CNT acts as the skeleton with high specific surface area and excellent electronic conductivity,which can provide better electron transfer and enhance the utilization of P3AT.In addition,the strongπ-πinteraction between poly（3-alkylthiophenes）and CNT is beneficial to the stabilization of charged and discharged states of poly（3-alkylthiophenes）for enhancing cyclability.We report a symmetric all-organic battery with overheating self-protection function using poly（3-octylthiophene）（P3OT）as both cathode-and anode-active materials.The assembled symmetric all-organic battery delivered a reversible capacity of 97.2 m Ah/g at 50m A/g within a voltage window of 0.01-2 V.Furthermore,this all-organic battery exhibited a superior cycling stability.When the temperature is increased to 110°C,the voltage of the all-organic battery maintained at around 0.3 V during the whole charging process.The abnormal charge voltage signal offered an early warning of battery overheating,allowing the timely handling.In the discharge process,the discharge voltage quickly dropped down to the lower cut-off voltage of 0.01 V,delivering little capacity.It indicates that the all-organic battery was rapidly switched off during the discharging process.It is deduced that the overheating self-protection function of the all-organic battery is ascribed to the thermal-dedoping of PF₆^-anions from the cathode-active P3OT.The results indicate that Li⁺is not dedoped from the host skeleton of the P3OT at 110°C.（4）Overheat/overcharge-responsive switch.We design a sandwiched current collector/electroactive conductive polymer/cathode materials layer by inserting P3BT between Al foil and NCM811 cathode materials（denoted as NCM811-P3BT cathode）.The electrochemical behaviors of NCM811 and NCM811-P3BT cathodes are compared under normal and abuse conditions.Under normal operations,there was no clear difference in specific capacity and cycling performance between NCM811-P3BT or and NCM811 cathodes.Once the battery is overcharged to 4.9 V,P3BT switch starts immediately,cutting off the electron transport between the active mass and Al current collector,as a result,switching off the battery permanently.The timely shutdown of overcharged cell can avoid continuous cell damage and prevent the occurrenceofthermalrunaway.P3OTasthe overheat/overcharge-responsive switch was inserted into between current collector and cathode materials layer to form NCM811-P3OT cathode.Once the battery is overcharged to 4.9 V,P3OT switch starts immediately,cutting off the electrochemical reactions.When the temperature is increased to 110°C,P3OT switch also starts immediately,switching off the battery reactions.However,NCM811 cathode still work properly when the battery was abused,which causes the serious concerns.The overcharge protection function is assigned to the overoxidation behavior of poly（3-alkylthiophenes）,which causes that poly（3-alkylthiophenes）lose their electroactivity and conductivity.The overheat protection function is assigned to the thermal dedoping of PF₆^-from poly（3-alkylthiophenes）matrix.There are 70 Figures,5 Tables and 208 References in this thesis.