Application And Research Of Conjugated Organic Materials In Aluminum Ion Batteries

Recently,the urgent demand for large-scale and low-cost energy storage devices promoted the rapid development of energy storage and conversion devices.As a new post-lithium battery,aluminum ion batteries（AIBs）have the advantages of large Al reserves,low cost and high safety of non-combustible electrolyte,which makes it stand out in many secondary battery systems.Based on the three-electron reaction,the Al anode can provide ultra-high theoretical volume specific capacity(8040 m Ah cm^-3)and theoretical mass specific capacity(2980 m Ah g^-1).Therefore,the suitable cathode material is the key factor restricting the performance of AIBs.The structure of organic electrode materials is diverse and flexible,and the selectivity of organic electrode materials to the radius of metal ions（or composite ions）is small,so it is suitable to be used in aluminum ion batteries.Herein,conjugated polycyclic aromatic hydrocarbons,conjugated conductive polymer polythiophene and polypyrrole are selected as cathode materials for AIBs,to explore their energy storage characteristics and energy storage mechanism.（1）Polycyclic aromatic hydrocarbons（PAHs）with a similar structure to graphite were selected as cathode materials for AIBs.Firstly,the theoretical feasibility of PAHs used in AIBs was analyzed by the DFT method.It was found that there was an electron acceptor-donor relationship between the conjugatedπbond in the molecular structure and Al Cl₄^-with charge transfer.Meanwhile,the results of theoretical calculation showed that there was a negative correlation between the theoretical specific capacity and the number of benzene rings in the PAHs structure.Because there was no strong interlayerπ-πinteraction between anthracene molecules,and the molecular structure was relatively small,so it is helpful to improve the utilization of the material and obtain the ideal capacity.Anthracene as the representative of PAHs to construct the anthracene//Al battery combining with Al.The battery provided high reversible specific capacity(157 m Ah g^-1 at 100 m A g^-1)and excellent cycle stability(130 m Ah g^-1 after800 cycles).（2）An organic conjugated polymer polythiophene（Pth）was proposed and prepared as a cathode material for AIBs in this work.In order to avoid the agglomeration of Pth,to expose more active sites and improve the utilization of active materials,carbon-based materials were introduced as the supporting skeleton to obtain Pth/carbon composites with uniform and stable structure.Pth/carbon composites with different micro-morphologies were synthesized by adjusting the types of carbon-based materials（carbon spheres,carbon tubes and graphene oxide）.Considering both the discharge specific capacity and cycle stability,two-dimensional layered Pth@GO composite showed the best electrochemical performance.The discharge capacity was up to 130 m Ah g^-1 at the current density of 1000 m A g^-1,and the capacity could still maintain more than 100 m Ah g^-1 after 4000 cycles,which indicates two-dimensional layered materials are more conducive to the energy storage of AIBs.Combined with the results of the DFT method and a series of ex-situ characterization techniques such as FTIR,Raman and XPS,the energy storage mechanism of Pth wasβ-H on the Pth ring acting as the active site with Al Cl₄^-through electrostatic interaction.（3）In order to respond to green chemistry,we proposed and prepared another conjugated organic polymer polypyrrole（PPy）,which synthesis conditions were milder than Pth.Graphene oxide was introduced into the polymerization process,and different proportions of PPy@GO composites were obtained by adjusting the amount of GO.Among them,PPy@GO-3 had a larger specific surface area and higher electrical conductivity,which can provide more active sites and faster charge transfer.Using PPy@GO-3 as the cathode material of AIBs,it showed excellent electrochemical performance.It could provide a high capacity of 235.5 m Ah g^-1 at the current density of 100 m A g^-1.Moreover,due to the interaction between PPy and GO,the electrical conductivity and structural stability of the composites had been improved,and excellent rate properties and excellent cyclic stability had been obtained.At a high current density of 5000 m A g^-1,the specific discharge capacity of PPy@GO-3 could be maintained at121.8 m Ah g^-1 after 15000 cycles,and there is almost no capacity attenuation compared with the initial capacity.Combined with DFT calculation and ex-situ characterization,it is proved that the energy storage mechanism of PPy was realized by Al Cl₄^-doping/de-doping on the active site ofβ-H in the PPy chain.