Designable Synthesis And Lithium Storage Properties Of Carbonyl-Containing Polymer Electrode Materials

The ever-growing demand for energy,the excessive consumption of fossil fuels,the rising environmental issues,and the worsening global climate,have brought about an urgent requirement to develop non-fossil energy routes,and seek high efficiency,clean,low carbon,and safe energy techniques.Lithium ion batteries（LIBs）,as important clean energy storage devices,have been gradually applied from consumer electronics to large energy storage systems such as electric vehicles and smart grids,occupying more than 60%of the market share of the battery industry.The commercial LIBs mainly use metal oxides as the cathode active materials,and graphite or silicon carbon as the anode active materials.However,such electrode material sources,and battery performances are currently facing many challenges.Metal compounds come from mineral sources which have the limited reserves,high energy consumption,difficult recovery,and heavy metal pollution risk.Meanwhile,commercial cathode materials such as Li Co O₂ and Li Fe PO₄ exhibit actual specific capacities of only140～170 m A h g^-1,and the specific capacity of graphite anode has reached the theoretical limit.Silicon carbon anodes can deliver a high capacity but demonstrate large volume changes during the charge and discharge processes,resulting in poor rate and cycle performance.Moreover,inorganic materials possess intrinsic brittle characteristics,and are difficult to meet the rapid development of flexible electronic requirements.In view of the above problems in inorganic electrode materials,it is very important to design and develop new green electrode materials for high-performance LIBs.The intrinsic properties and microstructures of electrode materials are the key factors to determine the performance of LIBs.In this regard,organic polymers as electrode materials bring a new dawn to green batteries in the future.Compared with traditional inorganic materials,polymer electrode materials have high theoretical capacities,designable structures,adjustable potentials,abundant resources,low cost,good toughness and easy processing.In particular,carbonyl-containing polymers have been one of the most promising organic electrode materials due to their high electrochemical activity,fast redox kinetics,tailored structures and facile synthesis.The dissertation focuses on the choice of active materials,structure regulation,and designable synthesis of carbonyl polymer electrodes including polyamides,polyimides（PI）,and polymethyl methacrylate（PMMA）.The electrochemical performances and lithium storage mechanisms of such polymer electrodes are systematically investigated using cyclic voltammetry,galvanostatic charge-discharge,electrochemical impedance spectroscopy,ex-situ Fourier transform infrared,and X-ray photoelectron spectroscopy techniques.This thesis includes the following five experimental chapters:（1）Commercial Nylons were for the first time,used as the anode materials including PA6,PA66,PA12,and PA1212.Four polyamides exhibit a pair of stable redox peaks at 0.83/1.06 V,due to their highly-reversible lithiation/delithiation of C=O groups in the amide units.At a current density of 20 m A g^-1,the reversible capacity of PA6 is 207.4 m A h g^-1,higher than those of PA66(172.6 m A h g^-1),PA12(166.4 m A h g^-1),and PA1212(165.6 m A h g^-1)due to the relatively low molecular weight per carbonyl group within PA6.After increasing to 500m A g^-1,the capacity retention of PA6（37.5%）is lower than those of PA66（62.5%）,PA12（66.6%）,and PA1212（73.9%）,showing a gradually-increasing rate capability.After 400cycles at 200 m A g^-1,the capacity retentions of PA6,PA66,PA12,and PA1212 are 98.6%,99.6%,99.7%,and 99.8%,respectively,suggesting high cycling stability for all PI.（2）A facile solid-phase polycondensation method was developed to prepare three PI by reacting melamine with 3,3’,4,4’-benzophenonetetracarboxylicdianhydride（BTDA）,3,3’,4,4’-biphenyl tetracarboxylic diandhydride（BPDA）,and 4,4’-（hexafluoro isopropylidene）diphthalic anhydride（6FDA）,respectively,abbreviated as PI-MT,PI-MP,and PI-MF.When used as the anode materials,three PI show two oxidation peaks～0.37 V,and 0.98 V,corresponding to the reversible intercalation/de-lithiation of carbonyl groups in the imide group.At 0.1 A g^-1,the reversible capacity of PI-MT is 251.0 m A h g^-1,higher than those of PI-MP(242.8 m A h g^-1)and PI-MF(219.0 m A h g^-1).After increasing to 5 A g^-1,the capacity retentions of PI-MT,PI-MP,and PI-MF are 57.8%,45.1%,and 37.6%,respectively.The first coulomb efficiency of PI-MT（40.8%）is also higher than that of PI-MP（31.3%）and PI-MF（38.5%）,implying that the carbonyl group in BTDA can inhibit the irreversible lithium consumption during the first discharge.Compared to PI-MP and PI-MF,the better performance of PI-MT is attributable to the bridged C=O group in BDTA for an additional contribution to the capacity,the inactive-（CF₃）₂junction unit in PI-MF for hindering the charge transport,and the irregular bulk microstructures of PI-MP and PI-MF.（3）A hydrothermal polymerization approach was developed to prepare four PI by polycondensation of 1,4,5,8-naphthalenetetracarboxylic dianhydride（NTCDA）and4,4’-methylenedianiline（MDA）,pyromellitic dianhydride（PMDA）and MDA,PMDA and4,4’-aminophenyl disulfide（DTDA）,PMDA and 4,4’-diamino-p-terphenyl（DAPT）,respectively,abbreviated as PI-1,PI-2,PI-3 and PI-4 in turn.When used as the cathode materials,PI-1 shows two pairs of redox peaks at 2.32/2.45 V,and 2.50/2.65 V,corresponding to the reversible intercalation/de-lithiation of two para-position C=O groups in NTCDA units.PI-2 also gives two pairs of redox peaks at 1.95/2.15 V and 2.25/2.35 V due to its higher LUMO energy level in PMDA compared to that in NTCDA.PI-3 only presents a pair of redox peaks at 1.98/2.45 V mainly due to the repaid redox reaction caused by the easily-accepted electron ability of S-S linker in DTDA.However,there is no discernible redox peaks possibly due to its less exposure of carbonyl sites embedded in the micron-scale thick lamellar structure.At 25 m A g^-1,the reversible capacity of PI-1(87.2 m A h g^-1)is higher than those of PI-2(74.6 m A h g^-1),PI-3(66.8 m A h g^-1),and PI-4(25.5 m A h g^-1).After increasing to 1 A g^-1,the capacity retentions of PI-1,PI-2,and PI-3 are 48.3%,28.4%,and 38.6%respectively,while the specific capacity of PI-4 approaches zero.PI-1 has more excellent lithium storage performance due to the synergistic effect of highly-active NTCDA units and 3D nanofiber porous network structures.（4）Carbon nanotube/polyimide（PI@CNT）composites were prepared by hydrothermal polymerization of pre-formed NTCDA and ethylenediamine salts.Four polyimides of pure PI,PI@CNT-1,PI@CNT-2,and PI@CNT-3 were obtained by adding CNTs of 0.0 wt%,6.4 wt%,10.8 wt%,and 18.5 wt%,respectively.Pure PI is stacked in micron-scale flakes.PI@CNT composites are core-shell nanostructures,and PI nanoprotuberances are uniformly coated on the surface of CNTs to form a worm-like network structure.The average thickness of PI layer is gradually decreased from 20.5 nm（PI@CNT-1）,17.3 nm（PI@CNT-2）to 15.2 nm（PI@CNT-3）with the increase of CNTs content.When used as the cathode materials,all four electrodes show significant redox peaks in the range of 2.0～3.0 V,corresponding to the lithiation/de-lithiation of two carbonyl groups on the opposite site of NTCDA units.The reversible capacity of PI at 50 m A g^-1 is 116.2 m A h g^-1,lower than those of PI@CNT-1(161.6 m A h g^-1),PI@CNT-2(152.8 m A h g^-1),and PI@CNT-3(160.8 m A h g^-1).After increasing to 10 A g^-1,the capacity retention of PI is only 14.4%,much lower than those of PI@CNT-1（73.2%）,PI@CNT-2（86.4%）,and PI@CNT-3（88.3%）.PI retains 89.8%of stable capacity after 500 cycles in 1 A g^-1,while PI@CNT-1,PI@CNT-2,and PI@CNT-3 remain almost unchanged.PI@CNT composites have better lithium storage performance because the CNTs core improves the electronic conductivity and structural stability,and the 3D network structure shortens the ion transport path.With increasing the CNT content,carbonyl groups in the PI shell are more exposed,and the carbonyl utilization ratio and capacity increase.This results in a rapid surface-mediated pseudocapacitive process.（5）A simple one-pot solvothermal polymerization approach was developed to produce graphene/chemically-crosslinked poly（methyl methacrylate）（Gr/c-PMMA）composites in the presence of methyl methacrylate,1,6-hexanediol diacrylate as a crosslinking agent,2,2’-azobis（2-methylpropionitrile）as an initiator,and graphene oxide as graphene precursor.As control samples,linear PMMA and pure c-PMMA are also solvothermally polymerized without and with the addition of crosslinker,respectively,in the absence of graphene oxide.Such thermoplastic PMMA-based materials as anodes for LIBs exhibit a pair of stable redox peaks at 0.79 and 1.02 V over a potential range of 0.01 to 2.5 V,due to their highly-reversible lithiation/delithiation of the in-situ generated 1,2-cyclopentanedione active units.The reversible capacities of PMMA,c-PMMA,and Gr/c-PMMA anodes are 97,147,and 206 m A h g^-1 at 20 m A g^-1,respectively.After increasing to 400 m A g^-1,the Gr/c-PMMA anode manifests a capacity retention of 82%,higher than those of c-PMMA（72%）and PMMA（38%）anodes.Moreover,Gr/c-PMMA delivers a high capacity of 159 m A h g^-1 after 1,000 cycles at400 m A g^-1.Notably,compared to PMMA,the better performance of c-PMMA can be attributable to the chemical crosslinking induced-formation of porous networks consisting of smaller nanoparticles（～50 nm）for c-PMMA than that of PMMA（～100 nm）.The incorporation of uniformly-dispersed graphene into c-PMMA enables the creation of 3D conductive networks with interconnected porous channels,thereby rendering for the efficient electrochemical activity of Gr/c-PMMA while retaining the structural integrity during cycling.This,in turn,results in a large specific capacity,a high rate capability,and a long cycling stability.These findings may provide new insights into the exploration of low-cost commercial plastics for rechargeable batteries as well as the possibility of other high added-value energy applications by capitalizing on recyclable plastics.