The Construction Of High-performance Anion-type Secondary Battery Based On Layered Double Hydroxides (LDHs)

Rechargeable batteries are one of the most important types of electrochemical energy storage and conversion devices.Currently,lithium-ion batteries(LIBs)have performed prominently in the past two decades and dominate the portable electronics and electric vehicle markets.In recent years,researchers have developed a type of anionic energy storage system,which used halide ions such as F-and Cl-as the charge carriers.The electrochemical performance of this kind of anionic batteries is comparable to that of traditional cationic batteries.However,exploiting suitable anion-hosting cathode materials with a simultaneous high capacity and long cycling life is a top priority for the development of rechargeable anion-type battery.Layered double hydroxides(LDHs)are rare two-dimensional materials with anions as interlayer guests.Due to their tunable interlayer spacing,high anionic conductivity and unique topological transformation characteristic,transition metal-based LDHs can in principle serve as a potential cathode materials for rechargeable batteries based on anion transfer.In this dissertation,a family of room temperature anion-type"rocking-chair" batteries by employing halide ions(F-,Cl-or Br-)intercalated transition metal-based LDHs as cathode materials was demonstrates for the first time.A series of LDHs electrode materials were synthesized by the one-step co-precipitation method or hydrothermal synthesis/topological oxidation followed by acid exchange method.Different LDHs-based halogen anion batteries were constructed.Through the optimization of electrochemical performance and investigation of energy storage mechanism,some new insights were provided for the design,assembly and performance enhancement for the new LDHs-based anionic energy storage system.The detailed researches are as follows:1.A new type of chloride ion battery(CIB)was constructed by realization of Cl-shuttling,using LDH as cathode materials.A one-step co-precipitation method was used to synthesize the CoFe LDH nanoplates with chloride anions intercalation.First principle calculations demonstrated that there are 6 different diffusion paths with extremely low energy barriers(0.12-0.25 eV)for Cl-diffusion in the gallery of LDH.Using the CoFe-Cl LDH as cathode and metal lithium as anode,we assembled the LDH-based CIB system for the first time.This CIB shows a maximum discharge specific capacity of 239.3 mAh g-1 and a stable capacity of?160 mAh g-1(after 100 cycles)at 100 mA g-1,which is the highest level compared with previously reported CIB cathode materials.The XRD results proved that there is no phase transformation for CoFe-Cl LDH cathode and the layered structure was well maintained during the whole charge/discharge process with negligible expansion/contraction of LDH interlayer spacing.The characterization results of XAFS,STEM-EELS,etc.indicated that all transition metals in the LDH octahedral host layers were involved into the charge compensation and undergo a reversible oxidation-reduction reaction in the battery system.According to the results of experiments and theoretical calculations,we proposed the "rocking-chair" energy storage mechanism for the LDHs-based CIB.During the discharge,Cl-ions are released from the interlayer space of LDH and migrate to the surface of the metal lithium anode through the Cl-containing ionic liquid electrolyte,accompanied by the reduction of Co3+/Fe3+ to Co2+/Fe2+;while the charging reverse the process.The work laid a solid foundation for the further performance optimization of CIB systems.2.The Cl--storage performance was optimized based on the synergistic effect of metal elements in the LDHs host layer.In the above work,we achieved the transition metal-based LDHs CIB,but there is still a gap between the commercial LIBs and such CIBs in terms of the cycling life.Therefore,in this part,we selected the electrochemically inert metal and synthesized a NiVA1 ternary LDH with interlayer Cl-as cathode material of CIB by a combined hydrothermal and anion exchange method.The content of Al3+in the LDH host layer was changed to obtain a series of Ni2V1-xAlx-Cl LDH(x=0,0.01,0.05,0.15 and 0.2)LDH cathode materials.Then,we systematically investigated the effects of different V/Al ratio on the morphology,crystallinity,water content and electrochemical energy storage performance of Ni2V1-xAlx-Cl LDH.The optimized Ni2V0.9Al0.1-Cl LDH electrode can deliver a maximum discharge capacity of 312.2 mAh g-1 and even remain stability with a reversible capacity of?113.8 mAh g-1(at 200 mA g-1)after 1000 charge/discharge cycles.The energy storage mechanism of the battery system is as follows.With the reversible shuttling of Cl-between the LDH cathode and lithium anode,vanadium in Ni2V0.9Al0.1-Cl LDH undergoes multivalent state change(V3+/V5+).During the entire operation of this CIB,layered structure of LDHs electrode remains unchanged and there is also no electrolyte ion intercalation,material aggregation or phase transformation.Through experiments and theoretical calculation,it was proved that the synergistic effect of Vm+(the valence variability),Ni2+(appropriate electronic structure)and inactive Al3+(inhibiting the local structural distortion of MO6)contribute to the remarkable cycling performance.It is worth noting that the amount of crystal water in the Ni2V0.9Al0.1-Cl LDH interlayer is obviously more than that in the Al-free sample,which resulted in the difference in terms of the DCl-.This part of work provides a brand new strategy for the future development of CIB cathode materials with enhanced electrochemical performance and structural stability.3.The reversible storage of F-and Br-in LDHs was also been realized,which enriched the cathode material system of anion batteries.After repeatedly proving the universality of transition metal-based LDHs with Cl-in the interlayer space as cathode materials for CIBs,we further prepared hexagonal CoNi LDH nanoplates of high crystallinity with F-,Cl-or Br-in the LDH gallery by topological oxidation and anion exchange methods.Then,for the first time,a kind of bromide ion battery(BIB)was presented by using CoNi-Br LDH as cathode,lithium foil as anode and 1-butyl-1-methylpyrrolidinium bromide(Bpyl4Br)-propylene carbonate(PC)as electrolyte.The galvanostatic charge/discharge showed that a discharge capacity of 338.9 mAh g-1(coulombic efficiency:96%)was obtained at a current density of 200 mA g-1.Moreover,after 50 cycles,this BIB could also deliver a reversible capacity of?100 mAh g-1 with a coulombic efficiency higher than 95%.The energy storage mechanism was explored subsequently,which was proved that our LDHs BIB system is a "rocking chair" type battery based on the reversible transfer of the Br-between two electrodes.On the one hand,the hexagonal morphology of CoNi-Br LDH was maintained without any agglomeration or structural cracking.On the other,the charge compensation of this battery involved both Co2+/Co3+and Ni2+/Nii3+,but the Co element in the electrochemical cycle contributed more capacity than Ni element.In the same way,F-and Cl-intercalated CoNi LDH were synthesized to construct the fluoride ion battery and CIB system,which proved the universality of our strategy.Considering the wide range of precursors sources and the highly flexibility in chemical composition of LDHs,this part of work provide an exciting opportunity in terms of large-scale,low-cost and safe anion-type energy storage systems.