High safety separators for rechargeable lithium batteries

Lithium-ion batteries(LIBs)are presently dominant mobile power sources due to their high energy density,long lifespan,and low self-discharging rates.The safety of LIBs has been concerned all the time and become the main problem restricting the development of high energy density LIBs.As a significant part of LIBs,the properties of separators have a significant effect on the capacity and performances of batteries and play an important role in the safety of LIBs.In recent years,researchers devoted themselves to the development of various multi-functional safe separators from different views of methods,materials,and practical requirements.In this review,we mainly focus on the recent progress in the development of high-safety separators with high thermal stability,good lithium dendritic resistance,high mechanical strength and novel multifunction for high-safety LIBs and have in-depth discussions regarding the separator's significant contribution to enhance the safety and performances of the batteries.Furthermore,the future directions and challenges of separators for the next-generation high-safety and high energy density rechargeable lithium batteries are also provided.

An upgraded polymeric composite with interparticle chemical bonding microstructure toward lithium-ion battery separators with enhanced safety and electrochemical performances

A composite separator of SiC/PVDF-HFP was synthesized for lithium-ion batteries with high thermal and mechanical stabilities.Benefiting from the nanoscale,high hardness,and melting point of SiC,SiC/PVDFHFP with highly uniform microstructure was obtained.This polarization caused by barrier penetration was significantly restrained.Due to the Si-F bond between SiC and PVDF-HFP,the structural stability has been obviously enhanced,which could suppress the growth of lithium(Li) dendrite.Furthermore,some 3D reticulated Si nanowires are found on the surface of Li anode,which also greatly inhibit Li dendrites and result in irregular flakes of Li metal.Especially,the shrinkage of 6% SiC/PVDF-HFP at 150℃ is only 5%,which is notably lower than those of PVDF-HFP and Celgard2500.The commercial LiFePO_(4) cell assembled with 6% SiC/PVDF-HFP possesses a specific capacity of 157.8 mA h g^(-1) and coulomb efficiency of 98% at 80℃.In addition,the tensile strength and modulus of 6% SiC/PVDF-HFP could reach 14.6 and 562 MPa,respectively.And a small deformation(1000 nm) and strong deformation recovery are obtained under a high additional load(2.3 mN).Compared with PVDF-HFP and Celgard2500,the symmetric Li cell assembled with 6% SiC/PVDF-HFP has not polarized after 900 cycles due to its excellent mechanical stabilities.This strategy provides a feasible solution for the composite separator of high-safety batteries with a high temperature and impact resistance.

HIGH VOLTAGE SAFETY MANAGEMENT SYSTEM OF ELECTRIC VEHICLE

In order to improve the drivability and energy efficiency of electric vehicle （EV）, more and more batteries are connected in series with high voltage which makes it necessary to monitor the electric parameters of high voltage system （HVS） to ensure the high voltage safety. A high voltage safety management system is developed to solve this critical issue. Several key electric parameters including pre-charge, contact resistance, insulation resistance and remaining capacity are monitored and analyzed based on the presented equivalent models. An electronic unit called high voltage safety controller is developed. By the help of hardware-in-loop system, the equivalent models integrated in the high voltage safety controller are validated and the on-line electric parameters monitor strategy is discussed. The real vehicle experiment results indicate that the high voltage safety management system designed is suitable for EV application.

Incombustible solid polymer electrolytes:A critical review and perspective

Since the advent of the solid-state batteries,employing solid polymer electrolytes(SPEs)to replace routine flammable liquid electrolytes is regarded to be one of the most promising solutions in pursing highenergy-density battery systems.SPEs with superior thermal stability,good processability,and high mechanical modulus obtain increasing attentions.However,SPE-based batteries are not impenetrable due to their decomposition and combustibility under extreme conditions.Researchers believe incorporating appropriate flame-retardant additives/solvents/fragments into SPEs can intrinsically reduce their flammability to solve the battery safety issues.In this review,the recent research progress of incombustible SPEs,with special emphasis on flame-retardant structural design,is summarized.Specifically,a brief introduction of flame-retardant mechanism,evaluation index for safety of SPEs,and a detailed overview of the latest advances on diverse-types SPEs in various battery systems are highlighted.The deep insight into thermal ru naway process,the free-standing incombustible GPEs,and the ratio nal design of pouch cell structures may be the main directions to motivate revolutionary next-generation for safety batteries.

“Three‐in‐one”strategy:Heat regulation and conversion enhancement of a multifunctional separator for safer lithium-sulfur batteries

The safety problems encountered with lithium–sulfur batteries(LSBs)hinder their development for practical applications.Herein,a highly thermally conductive separator was constructed by cross‐weaving super‐aligned carbon nanotubes(SA‐C)on super‐aligned boron nitride@carbon nanotubes(SA‐BC)to create a composite film(SA‐BC/SA‐C).This separator was used to fabricate safe LSBs with improved electrochemical performance.The highly aligned separator structure created a uniform thermal field that could rapidly dissipate heat accumulated during continuous operation due to internal resistance,which prevented the development of extremely high temperatures.The array of boron nitride nanosheets endowed the composite separator with a large number of adsorption sites,while the highly graphitized carbon nanotube skeleton accelerated the catalytic conversion of high‐valence polysulfides into low‐valence polysulfides.The arrayed molecular brush design enabled the regulation of local current density and ion flux,and considerably alleviated the growth of lithium dendrites,thus promoting the smooth deposition of Li metal.Consequently,a battery constructed with the SA‐BC/SA‐C separator showed a good discharge capacity of 685.2 mAh g−1 over 300 cycles(a capacity decay of 0.026%per cycle)at 2 C and 60°C.This“three‐in‐one”multifunctional separator design strategy constitutes a new path forward for overcoming the safety problems of LSBs.

Safer Lithium-Ion Batteries from the Separator Aspect:Development and Future Perspectives

With the rapid development of lithium-ion batteries(LIBs),safety problems are the great obstacles that restrict large-scale applications of LIBs,especially for the high-energy-density electric vehicle industry.Developing component materials(e.g.,cathode,anode,electrolyte,and separator)with high thermal stability and intrinsic safety is the ultimate solution to improve the safety of LIBs.Separators are crucial components that do not directly participate in electrochemical reactions during charging/discharging processes,but play a vital role in determining the electrochemical performance and safety of LIBs.In this review,the recent advances on traditional separators modified with ceramic materials and multifunctional separators ranging from the prevention of the thermal runaway to the flame retardant are summarized.The component–structure–performance relationship of separators and their effect on the comprehensive performance of LIBs are discussed in detail.Furthermore,the research challenges and future directions toward the advancement in separators for high-safety LIBs are also proposed.

An all-organic aqueous potassium dual-ion battery

Benefiting from the environmental friendliness of organic electrodes and the high security of aqueous electrolyte,an all-organic aqueous potassium dual-ion full battery(APDIB) was assembled with 21 M potassium bis(fluoroslufonyl)imide(KFSI) water-in-salt as the electrolyte.The APDIB could deliver a reversible capacity of around 50 mAh g^(-1) at 200 mA g^(-1)(based on the weight of total active materials),a long cycle stability over 900 cycles at 500 mA g^(-1) and a high coulombic efficiency of 98.5%.The reaction mechanism of APDIB during the charge/discharge processes is verified:the FSI-could associate/disassociate with the nitrogen atom in the polytriphenylamine(PTPAn) cathode,while the K^(+) could react with C=O bonds in the 3,4,9,10-perylenetetracarboxylic diimide(PTCDI) anode reversibly.Our work contributes toward the understanding the nature of water-into-salt electrolyte and successfully constructed all-organic APDIB.

Freestanding and Ultra‑flexible PAN/ZIF‑67 Hybrid Membrane with Controlled Porosity for High‑Performance and High‑Safety Lithium Batteries Separator

Herein,a flexible ZIF-67/PAN hybrid membrane was successfully prepared by the incorporation of ZIF-67 nanoparticles and PAN nanofibers through electrospinning method.The hybrid membrane presented tomatoes on sticks structures with one single PAN fiber stringing series of ZIF-67 nanoparticles.The morphology,electrolyte wettability,heat resistance,flexibility,and electrochemical properties of the electrospun ZIF-67/PAN membranes were discussed.Among the membranes prepared with different percentage of ZIF-67,the 30% ZIF-67/PAN membrane exhibited outstanding heat shrinkage resistance(remained intact at 200℃ for 1 h),excellent electrolyte uptake(556.39%),wide electrochemical window(~5.25 V)and high ionic conductivity(2.98 mS cm^(−1)).When used as lithium-ion batteries(LIBs)separators,the cells assembled by 30% ZIF-67/PAN membrane presented excellent rate capacity and high capacity retention of 86.9% after 300 cycles at 1C.More importantly,the cells assembled with ZIF-67/PAN membranes repeated bent for 1000 times also exhibited high rate performance and maintained capacity retention of 92% after 100 cycles at 1 C.The characterization and the electrochemical testing suggest the electrospinning prepared ZIF-67/PAN flexible membranes can be expected to be used as potential separator for advanced batteries with high safety and high performance.

A nano fiber–gel composite electrolyte with high Li^(+) transference number for application in quasi-solid batteries

As their Liþtransference number(tLiþ),ionic conductivity,and safety are all high,polymer electrolytes play a vital role in overcoming uncontrollable lithium dendrites and low energy density in Li metal batteries(LMBs).We therefore synthesized a three-dimensional(3D)semi-interpenetrating network-based single-ion-conducting fiber–gel composite polymer electrolyte(FGCPE)via an electrospinning,initiation,and in situ polymerization method.The FGCPE provides high ionic conductivity(1.36 mS cm^(-1)),high t_(Li+)(0.92),and a high electrochemical stability window(up to 4.84 V).More importantly,the aromatic heterocyclic structure of the biphenyl in the nanofiber membrane promotes the carbonization of the system(the limiting oxygen index value of the nanofiber membrane reaches 41%),giving it certain flame-retardant properties and solving the source-material safety issue.Due to the in situ method,the observable physical interface between electrodes and electrolytes is virtually eliminated,yielding a compact whole that facilitates rapid kinetic reactions in the cell.More excitingly,the LFP/FGCPE/Li cell displays outstanding cycling stability,with a capacity retention of 91.6%for 500 cycles even at 10C.We also test the FGCPE in high-voltage NMC532/FGCPE/Li cells and pouch cells.This newly designed FGCPE exhibits superior potential and feasibility for promoting the development of LMBs with high energy density and safety.