Electrolyte Optimization For Enhancing The Lithium/Sodium Storage Performance Of Quinone Cathode Materials

Present research on lithium-ion batteries(LIBs)and sodium-ion batteries(SIBs)are mostly focused on layered transition-metal oxides cathodes,they are difficult to break through their constraints of low theoretical specific capacity,complex manufacturing processes,and the environmental pollution caused by using heavy metals.In addition,the present LIBs using organic electrolytes have the risk of easily exploding when overcharged.In contrast,organic electrode materials have the merits of strong structural designability,easily accessible to raw materials,and environmental friendliness.Thus,they are much potential to be applied in large-scale energy storage systems.Quinone materials,as one of the organic electrode materials,whose carbonyl can be used as active sites for energy storage of multiple metal ions.As one of the organic electrode materials,quinone compounds can be used in various metal-ion batteries.Moreover,they are considered as the most suitable organic materials for LIBs and SIBs because of the superb redox reversibility.Unfortunately,their dissolution in organic electrolytes causes rapid capacity loss and poor cycle stability,the high capacity of quinones cannot be demonstrated and limits their applications in lithium/sodium secondary batteries.In this paper,Calix[4]quinone(C4Q)and Pillar[5]quinone(P5Q)are selected as the cathodes of LIBs and SIBs to study how to maintain their cycling stability.The method of changing electrolyte systems is adopted,and three different electrolyte systems of plastic crystal succinonitrile(SN-PCE),ionic liquid(IL)and high concentration lithium salt-acetonitrile(HCE-AN)are adopted to replace the traditional carbonate electrolyte.The issues mentioned above have been effectively solved,the cycle stability and rate performance of the battery are significantly enhanced.The main research contents and results are as follows:(1)The SIBs with P5Q cathode and SN-PCE system are firstly constructed.By passivation of sodium anode,the polymerization of SN can be effectively avoided,so that P5Q could present sound sodium storage performance with SN-PCE electrolyte system.The initial capacity of the P5Q SIBs is 412 mAh g^-1 at the current density of 0.2 C.After300 cycles,the capacity is stabilized at 287 mAh g^-1,and the capacity retention rate is up to 70%.At the same time,the lithium storage performance of C4Q in SN-PCE system is also systematically studied.It has been demonstrated that cycle stability of C4Q LIBs can be achieved without additional additives at the lower voltage window of 1.5-3.5 V.The LIBs obtain an initial capacity of 424 mAh g^-1 at 0.1 C,high specific capacities of and 254mAh g^-1 are remained after 1000 cycles.Even at a high current density of 2 C,the discharge capacity can still reach 140 mAh g^-1.These results indicate that the stable SN-PCE system can not only provide the high initial capacity of quinone electrodes,but also excellent cycle life.(2)The electrochemical properties of quinone cathodes in ILs electrolytes for LIBs and SIBs are studied.The cycle life of the quinone cathodes in the[PY13][TFSI]IL system is comparable to that of inorganic materials.The LIBs combined C4Q and 0.3 M Li[TFSI]/[PY13][TFSI](Li[TFSI]:lithium bis(trifluoromethanesulfonyl)imide,PY13:N-methyl-N-propylpyrrolidinium)IL electrolyte show great cycle stability and rate property.A high initial capacity of 363 mAh g^-1(82%of the theoretical capacity)is acquired at 0.1 C,it maintains at 280 mAh g^-1 after 100 cycles and even 262 mAh g^-1 after1000 cycles.When the current density gradually increases from 0.1 C to 1 C after 40cycles,the discharge capacity can still be maintained at 154 mAh g^-1.Remarkably,the combination of P5Q and 0.3 M Na[TFSI]/[PY13][TFSI]SIBs also obtain great achievement.At 0.2 C current density,the initial capacity is 388 mAh g^-1,which is 87%of the theoretical capacity,the capacity retention rate is still as high as 70%after 300 cycles.When the current density is increased to 1 C,a high reversible capacity of 225 mAh g^-1 is obtained.These results indicate that[PY13][TFSI]with a salt concentration of 0.3 M has appropriate viscosity coefficient and ionic conductivity to improve the cycle stability and rate performance of the batteries.(3)The P5Q-HCE-AN LIBs are constructed for the first time.The experimental results show that P5Q cathode exhibits excellent fast charging ability in HCE-AN system with a concentration of 4.2 M LiTFSI.At 0.2 C,the high specific capacity of more than300 mAh g^-1 is maintained after 500 cycles.The reversible capacity of P5Q can still exceed 150 mAh g^-1 at a large current of 2 C and even nearly 80 mAh g^-1 at 10 C.In the meanwhile,HCE-AN system can well inhibit the dissolution and deposition of P5Q,and greatly improve the cycle performance and rate property of P5Q LIBs.Furthermore,this study also broadens the ideas of how to design and use low-cost organic solvents as electrolytes,which has important practical significance for the build of high performance organic secondary batteries.