Construction on Dual-ion Batteries Based on Ionic Liquid Electrolyte and Exploration of Negative Electrode Materials

Author:Fan Jia Zuo

Supervisor:yuan wen hui


Degree Year:2019





In order to solve the current problems of environmental pollution and energy shortage,and promote the sustainable development of society,it is necessary to accelerate the utilization of renewable energy and promote the development of large-scale energy storage technology.Electrochemical energy storage systems,especially lithium-ion batteries,have been rapidly developed in portable electronic devices and electric vehicles.However,the shortage of lithium resources,uneven distribution,high prices and potential safety hazards have limited the large-scale applications of lithium-ion batteries.Therefore,the development of new battery systems with low-cost and good performance is a hot topic in the field of energy science research.Based on the above research background,this paper aims to explore cheap,safe,environmentally friendly and well-performing secondary dual-ion battery systems.The specific research contents are as follows:(1)A dual-graphite battery based on pure ionic liquid electrolyte was constructed.The composite graphite was used as the positive and negative electrodes,and pure ionic liquids were used as the electrolyte.By comparing dual-ion batteries with different ionic liquid electrolytes,the following experimental results are obtained:hexafluorophosphate-containing ionic liquid(BMImPF6)has a higher viscosity,which results in serious polarization of the battery and lower specific discharge capacity;tetrafluoroborate-based ionic liquid(BMImBF4)has a lower viscosity,but the battery discharge platform is lower and the cycling performance is limited;the trifluoromethanesulfonate ionic liquid(EMImTfO)battery exhibits a higher discharge platform and better cycle performance,but the electrochemical window of the liquid is relatively narrow;the bis(trifluoromethanesulfonyl)imide(EMImTFSI)battery has good discharge capacity and cycling performance.Furthermore,three kinds of bis(trifluoromethanesulfonyl)imide ionic liquids were compared.The results show that the PP14TFSI dual-graphite battery exhibits even better electrochemical performance:at a current density of 30 mA/g,the discharge platform is between 3.3 and 4.3 V,the discharge specific capacity is 82.0 mAh/g and the energy density is 256 Wh/kg.At a current density of 300mA/g,the battery exhibits high cycling stability with almost no capacity attenuation after 600cycles.However,since both PP14+and TFSI-in the ionic liquid have a large ion size,the graphite is expanded due to the intercalation reactions.In particular,as the PP14+ion has a three-dimensional(3D)annular stereo configuration and even larger size,the expansion of the negative electrode graphite is even more serious,resulting in the exfoliation of the graphite particles.(2)In order to improve the volume expansion of graphite anode in pure ionic liquid electrolyte,a dual-ion battery system with molybdenum disulfide(MoS2)anode material was constructed.MoS2 has a graphite-like multilayer stack structure and high crystallinity with a layer spacing of 0.62 nm,which is larger than 0.355 nm of graphite.Two types of MoS2anode materials were prepared by hydrothermal method and high temperature pyrolysis method,respectively.The hydrothermal-MoS2 is flowerlike,with thin and soft petals,and agglomeration occurs.The pyrolysis-MoS2 has a flaky structure,with uniform particle dispersion.The strong(002)peak tested by XRD indicates highly stacked S-Mo-S layers.Galvanostatic charge-discharge,Raman and SEM tests show that the flake-MoS2 has even better discharge platform,specific capacity and cycling stability than those of the flowerlike-MoS2.At a voltage window of 0.5-3.6 V and a charge-discharge current of 50mA/g,the flake-MoS2/EMImTFSI/graphite dual-ion battery exhibits two higher discharge platforms between 3.2-3.3 V and 2.2-3.0 V,with a specific discharge capacity of 77.0 mAh/g and a corresponding energy density of 176 Wh/kg.At 400 mA/g,the battery exhibits good stability of 300 cycles,with a high capacity retention of 84.3%and a high coulombic efficiency of 96.1%.MoS2 has a larger interlayer spacing than that of graphite,which can effectively accommodate the reversible insertion/extraction of large EMIm+cations during charge-discharge.Meanwhile,the unique"sandwich"layer structure of MoS2 can provide sufficient active sites and intercalation spaces,which can not only achieve rapid ion diffusion,but also withstand the mechanical stress and volume change during the charge-discharge process,ensuring good electrochemical performance and high structural stability.(3)In order to reduce the battery self-discharge rate and increase the discharge specific capacity,a dual-ion battery with an iron ferricyanide FeFe(CN)6 anode and a NaTFSI/EMImTFSI(1.0 mol/L)electrolyte was constructed.Prepared by a simple liquid reaction method,the FeFe(CN)6 nanoparticles have a cubic frame structure,in which the low-spin Fe1 atoms are attached to six carbon atoms,while the high-spin Fe2 atoms are attached to six nitrogen atoms.Cyclic voltammetry(CV)and constant current charge-discharge tests show that Na+in electrolyte can reversibly insert into FeFe(CN)6crystal,while EMIm+in EMImTFSI ionic liquid cannot intercalate in FeFe(CN)6.The FeFe(CN)6-graphite dual-ion battery has a maximum discharge specific capacity of 93.0mAh/g between 0.1-2.0 V,which is higher than the dual ion batteries described above.The stoichiometry of Na+-embedded FeFe(CN)6 corresponding to 93.0 mAh/g capacity is Na1.08FeFe(CN)6.At a current of 0.4 mA/cm2,the battery exhibits an extremely high 99.9%Coulombic efficiency.Even at a lower current of 0.05 mA/cm2,the battery still has a higher98.5%Coulombic efficiency,indicating its lower self-discharge rate.In the self-discharge tests,after resting for 110 h,the capacity retention is 63.6%,and the average self-discharge rate is as low as 0.32%/h,which is mainly related to the high binding force between Na+and FeFe(CN)6 crystals during the intercalation process,thus reducing the self-discharge rate of the battery.(4)In order to further increase the specific discharge capacity and reduce the negative electrode expansion in dual-ion batteries,metal negative electrodes were introduced,which involve reversible deposition of metal cations on anode from the electrolyte.Firstly,the zinc-graphite dual-ion battery was studied.Three-electrode cyclic voltammetry(CV)tests show that Zn2+can be reversibly deposited from the 0.2 mol/L Zn(TfO)2/EMImTfO electrolyte,and the TfO-can be reversibly embedded into the graphite electrode.The zinc-graphite dual-ion battery exhibits a medium discharge platform of 2.0 V and a specific discharge capacity of 33.7 mAh/g.To further improve the discharge capacity,the sodium based dual-ion battery with tin anode was explored,which exhibits a higher discharge platform of 4.5-3.5 V,with a high discharge capacity of 99.0 mAh/g.The capacity retention after 100 cycles is better than that of the zinc-graphite battery.The stoichiometry of TfO-graphite compound corresponding to 34 mAh/g capacity is C66TfO,while that of TFSI-graphite composition to 99.0 mAh/g is C22.5TFSI.Although TfO-contains fewer atoms than TFSI-,graphite has a higher amount of TFSI-intercalation,which can be related to the lower oxidation stability of TfO-,leading to decomposition reaction under high voltage.On the other hand,it can be assigned to the stereo 3D structure of the two ions,as well as the repulsive forces between the ions and graphite cathodes.The SEM tests show that after repeated cycles,the deposits on the metal negative electrodes are evenly distributed,without the hint of dendrites,indicating a feasible process.