Self-actuating Safety Protection For Li-ion Batteries

Safety issues have severely retarded the large-scale applications of lithium ion batteries(LIBs)in electric vehicles and renewable power stations.With the increasing of battery capacity and energy density,the safety issues of LIBs are becoming more prominent.Therefore,developing and building self-actuating safety protection technologies to enhance the intrinsic safety of LIBs are of vital importance for their large-scale applications.The safety issues arise from the temperature-induced thermal runaway.When the cell temperature is increased to a critical value dramatically due to the abusive conditions such as mechanical damage,overcharging or short-circuiting,the exothermic side reactions will be triggered to produce a large amount of heat accumulation in the internal cell,resulting in cell cracking,firing or even explosion.To prevent such a thermal runaway,the most effective strategy is to build a thermal shutdown mechanism for LIBs from the electrochemical point of view.As well known,an electrochemical reaction must involve the transports of both electrons and ions.Once the transports of either the electrons or ions are switched off before the internal temperature of the batteries approaches a requiring controlled value regardless of any reason,the battery reactions and the thermal accumulation will be terminated immediately,thus ensuring the battery safety.Based on this consideration,we chose polymeric materials with temperature-sensitive characteristics to develop two types of self-actuating thermal protection methods for LIBs in this PhD work.One is to cut off the electron transport inside the cell by constructing so called positive-temperature-coefficient(PTC)electrodes,which are fabricated by using conducting polymers with required PTC effects as the coating layer of electrode current collector,the conductive matrix of electrode or the surface coating layer of active material particles.Another is to cut off the Li+ ion transport between electrodes by developing a novel thermal shutdown separator,which is prepared simply by coating a thin layer of thermoplastic microspheres on the commercial separator.The main contents and results are as follows:1.Temperature-responsive cathodes based on conducting polymer coating layers.We synthesized a series of conducting polymers and tested their PTC behaviors.Based on the PTC characterization results,two polymers of poly(3-octylthiophene)(P30T)and poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate)(PEDOT:PSS)were chosen as the coating layer of current collector of LiCoO2 electrode to fabricate temperature-responsive cathodes of LCO-P30T and LCO-PEDOT,respectively.The temperature-responsive behaviors and application performances in practical pouch cells of the as-fabricated cathodes were also investigated.The experimental results demonstrated that P30T and PEDOT:PSS polymers have suitable transition temperature of 90-100 ? and 115-145 ?,remarkable PTC effect with three and two orders of magnitude variation in electrical resistivity,good film-forming processability,reversible redox behavior and are insoluble in electrolyte,making them highly suitable for using as coating layers to construct thermal stable cathodes.The LCO cathodes with a PTC layer of P30T or PEDOT:PSS exhibited not only a decent electrochemical performance at ambient temperature but also a reliable temperature-responsive characteristic.At elevated temperatures,the PTC layers can transform from a conductive state to a highly resistive state,which cuts off the electrode current and therefore switches off the cell reaction,thus protecting the cell from thermal runaway.More importantly,different from the conventional PTC material,these two polymers can form an ultra-thin PTC coating layer(?1 ?m)with a thickness less than 1 mm in between the A1 substrate and cathode-active LiCoO2 layer.Such an ultrathin layer can only occupy a very small part in the battery volume and therefore has almost indiscernible influence on the energy density of the cell.The experimental data from practical pouch cells showed that the temperature-responsive electrodes of LCO-P30T and LCO-PEDOT can effectively improve the safety performance of the cells under 150 ? thermal impact without compromising their normal electrochemical performance,showing great promise for use in building safer LIBs.2.Temperature-responsive cathodes based on conductive matrix of conducting polymer/carbon black composites.We selected two conducting polymer/carbon composites with required PTC effects,polypyrrole/carbon black(PPy/C)&PEDOT:PSS/carbon black(PEDOT/C)as the conductive matrixes of LiCoO2 electrode to fabricate temperature-responsive cathodes of LCO-PPy/C and LCO-PEDOT/C,respectively,and investigated the temperature-responsive characteristics and application performances in practical pouch cells of the as-fabricated cathodes.The experimental results demonstrated that PPy/C and PEDOT/C composites possess suitable transition temperature(100-120 ?,115-145?,respectively),decent dispersity(?30 nm,?50 nm),ultrahigh conductivity at ambient temperature(30.3 S cm-1,14.3 S cm-1),reversible p-doping/dedoping activity and high thermal stability(>270?,>450 ?),making them ideally suitable for constructing PTC electrodes.The electrochemical measurement results indicated that the temperature-responsive cathodes with a conductive matrix of PPy/C or PEDOT/C composite can exhibit not only excellent electrochemical performances at 25 ? but also a reliable thermal switching effect at elevated temperature of ?120 ?.Moreover,benefiting the good dispersity and high electrochemical activity,these composite conductive matrixes can build PTC cathodes but without sacrifice of the energy density of the cells.The application results from the practical pouch cells demonstrated that these two temperature-responsive cathodes can remarkably enhance safety performance of LIBs under abusive conditions such as overcharge,thermal impact and short-circuiting.Due to their small addition amount,good compatibility with other battery materials and particularly easy of large scale application,this new type of conductive composites shows promising commercial prospects for building thermal stable LIBs.3.Temperature-responsive electrode materials based on conducting polymer coatings.We selected polyaniline(PAni),PEDOT:PSS and PEDOT as the surface coating layer of electrode active materials to prepared three temperature-responsive electrode materials,LiCoO2@PAni,graphite@PEDOT:PSS and LiCoO2@PEDOT,and evaluated their electrochemical characterizations and application performances in practical pouch cells.The experimental results revealed that,benefiting the suitable transition temperature(110-150 ?),remarkable PTC effect(four orders of magnitude variation in electrical resistivity),high electrical conductivity and reversible p-doping/de-doping activity of PAni,LiCoO2@PAni composite electrode can exhibit not only enhanced cycling stability both at high temperature of 60 ? and at high cut-off voltage of 4.5 V,but also high efficient self-actuating thermal shutdown function at 120?.Also,the graphite@PEDOT:PSS composite can demonstarte a self-actuated thermal shutdown function at 110 ? due to the PTC effect of skin layer of PEDOT:PSS.This PTC effect can significantly improve the safety of pouch full cells under thermal impact at 150 ? and high-temperature storage stability of cells at 85?.Furthermore,the LiCoO2@PEDOT composite cathode,which is prepared by in-situ electro-oxidative polymerization of EDOT monomer additive in electrolyte during the first battery charging,can delay the thermal runaway onset time of LiCoO2-based pouch full cells for 13.8 min under high temperature impact at 150 ? without compromising the cell capacity and low temperature performance.4.Thermoplastic microspheres-coated thermal shutdown separator.We fabricated a new thermal shutdown separator with a more reasonable shutdown temperature of 90? by coating thermoplastic ethylene-vinyl acetate copolymer(EVA)microspheres onto a conventional polyolefin membrane film and tested it for thermal protection of LIBs.The experimental results demonstrate that owing to the melting of the EVA coating layer at a critical temperature,this separator can promptly cut off the Li+conduction between the electrodes and thus shut down the battery reactions,so as to protect the cell from thermal runaway.As a result,the safety performances of LiCoO2-based high capacity pouch full cells under abusive conditions such as intruding,short-circuiting and overcharging,were remarkably improved.In addition,this type of the separator has no negative impact on the normal battery performance,therefore providing an internal and self-protecting mechanism for safety control of commercial LIBs.