Electrode Characteristics And Lithiation Mechanism Of Iron-phthalocyanine

Because of the advantages of high energy density,high operating voltage and long service life,lithium ion batteries?LIBs?are widely used in energy storage systems for portable electronic devices and electric vehicles.At present,graphite based carbon materials play an important role as an anode material for LIBs.However,its low theoretical capacity severely limits the development of LIBs with high energy density.Non-carbon anode materials such as metal oxide,alloy,silicon have been investigated owing to their high capacity.However,the large volume variation leads to the degradation of capacity in the process of lithium ion intercalation and deintercalation,thereby affecting the cycling stability of the battery.Therefore,many researchers are actively working on seeking environmentally friendly organic electrode materials with low energy consumption and flexible design.Iron phthalocyanine?FePc?is a two-dimensional planar organometallic compound with a large ?-conjugated structure of polycyclic aromatic rings and considered to be a very attractive candidate for LIBs electrode materials.It has been proved that each C6 ring in the FePc molecule can reversibly accept six lithium ions to form a Li6/C6 structure.In addition,redox reactions can occur on atoms with lone pair electrons,such as unsaturated nitrogen atoms,resulting in an unexpected theoretical capacity.At the same time,the iron-phthalocyanine is a two-dimensional planar molecule,arranging in parallel to form a layered structure.Lithium ions can easily embed into the gap between the layers.Although there is a large ?-conjugated structure with a high density of electron cloud,however,the electronic conductivity of iron-phthalocyanine is still poor.On the other hand,graphene?GN?,a two-dimensional carbon nanomaterial,has aroused widespread attention due to its superior intrinsic properties,especially high electronic conductivity and flexibility.Such excellent features renders the GN an appropriate electrode material for LIBs.In order to enhance the electronic conductivity of the whole electrode system,GN is used as a conductive agent and also as one of the active materials in LiB electrodes in this paper.The work of this thesis mainly includes the following parts:?1?A series of FePc/GN composites were prepared by mixing of FePc and graphene as the anode material for LIBs.The effect of GN content on the electrochemical performance of FePc/GN electrode was explored.When the content of GN is sufficient,the composite electrode system has excellent electric conductivity.The FePc molecules can sufficiently contacted with GN to eliminate the influence of the poor conductive FePc on the conductivity of the electrode system and to lead to that the FePc molecules maximize their own capacity for the battery performance.It is found that the capacity of the composite electrode reaches an optimum value of 1200 mAh g-1 at the mass ratio of FePc/GN=1/2.25.The high power density performance is excellent at the large current densities of 300-2000 mA g-1,and the capacity retention rate is 45% at the current density of 2000 mA g-1.As the current density dropped back to 300 mA g-1,a reversible capacity around 1185 mAh g-1 can be recovered.The excellent rate capability is mainly attributed to the stability of FePc conjugated structure.?2?The capacity contribution of FePc in the FePc/GN composite electrode system was calculated and plotted.It is found that the capacity contribution of FePc increases with the increase of GN content.When the ratio of FePc/GN is 1/2.25,the capacity contribution value of FePc reaches the maximum value of 2275 mAh g-1.By electrochemical measurements and corresponding calculations,it can be concluded that the specific capacity corresponding to the FePc chemical structure is 1792 mAh g-1,and the specific capacity corresponding to FePc molecular layers is 483 mAh g-1.Such excellent electrochemical performance is attributed to the fact that the presence of large ?-conjugated structure,rich N atoms with one pair electrons as well as aromatic rings in the FePc molecule.Meanwhile,FePc molecules are stacked in parallel with each other to form a layered structure.In addition,the GN provides good charge conduction in the electrode due to its excellent electrical conductivity and flexibility.The synergistic effect of FePc and GN may also be one of the reasons for the excellent electrochemical performance of FePc/GN composite electrode.?3?Comparing the cycling performance of FePc and H₂ Pc in the composite electrode,the results indicate that the capacity of H₂ Pc in 1/2.25-H₂Pc/GN composite electrode is 1503 m Ah g-1,while the corresponding capacity of FePc in1/2.25-FePc/GN composite electrode is 2275 m Ah g-1,significantly higher than the capacity of H₂ Pc.The AC impedance spectra shows that the impedance of the 1/2.25-FePc/GN composite electrode is about 90 ?,while the impedance of the 1/2.25-H₂Pc/GN composite electrode is about 200 ?.This shows that the conductivity of the metal complex FePc molecule is better than that of the H₂ Pc molecule without metal complexation.That is,the transmission resistance of electrons in FePc molecules is much smaller than that in H₂ Pc molecules.