Modification Of Several Organic Electrode Materials And Investigation Of Their Energy Storage Mechanisms

Lithium-ion batteries have been widely utilized in the fields of our social lives,and the commercial inorganic Li-ion electrode materials cannot satisfy the modern society maintaining “clean energy society” due to their high price as well as the usage of non-renewable transition metal oxides,et.al.However,organic small molecules have been widely concerned due to their green environmental friendliness,low cost,structural diversification and high theoretical specific capacity.Nevertheless,there are still some fundamental issues need to be solved,mainly including the dissolution into the common organic electrolyte resulting in capacity fading,the low real specific capacity,the poor rate performance and the unclear reaction mechanism.Based on theoretical and experimental study,this thesis researched on organic terephthalate(TP^2-)-based anode materials,through physical and chemistry property controls,material modifications,ion transport optimization and newly organic electrode materials designing to solve the mentioned issues.The main contents and results are listed as follows:?1?In our work,we synthesized organic terephthalate lithium?Li₂TP?and its ramification lithium salts by a simple one-step acid-base reaction and discussed the influences of electrochemical behaviors for electrode materials through the replacement of different substituent group.It was disclosed that these organic materials can reversibly insert/deinsert Li+,and Li₂ TP was a potential organic anode materials due to its high theoretical capacity,the obvious charge-discharge platform and the reversible insert/deinsert Li+ pathway.The incoming substituent group of organic electrode materials have effect on the molecular structure with inductive effect and conjugated effect,regulating and controlling the potential.Meanwhile,we took 2,5-dihydroxyterephthalic acid?DHTPA?and dilithium 2,5-dihydroxyterephthalate?Li₂DHTPA?as an example to investigate the reaction mechanism by XRD and FT-IR et.al.It suggested that the organic acid DHTPA demonstrated electrochemical activity toward Li+ storage and releases but with poor cyclic performance and experienced an ion exchange process in the first discharge process and the final product was Li₂ DHTA after the first cycle.On the contrary,the de-/lithiation process of chemically replaced Li₂ DHTP was highly reversible with good cyclic performance.We newly illustrated that either chemical replacement or electrochemical replacement can generate electrochemical active material.?2?Considering the dissolution issues for Li₂ TP in common organic electrolyte,we firstly reported that disodium terephthalate?Na₂TP?through replacement of cation on Li₂ TP could display as anode for Li-ion batteries and further offered a simple spray drying methodology.The results showed that the cyclic performance of Na₂TP was obviously improved compared to that of Li₂ TP due to its lower dissolution in organic electrolyte.Considering the poor electronic conductivity with low active site for organic small molecules,an organic/multiwall carbon nanotube?MCNTs?nanocomposite via a one-step spray drying methodology was synthesized.The electrochemical test showed that it can still maintain 214 mAh g^-1 after 50 cycles which delivered improved specific capacity and rate performance compared to the bulk microsized Na₂TP prepared by a conventional water-crystallization method and the pristine Na₂TP microspheres produced by the same spray drying method.This is mainly due to the as-prepared nanocomposite can shorten the Li-ion diffusion distance,form highly conductive network,stabilized the electrode structure and slow the dissolution rate in organic electrolyte.?3?Considering the electrochemical property of organic electrode materials also have some connection to the Li-ion pathway,we initially prepared potassium terephthalate?K₂TP?with open framework through replacement of cation on Li₂TP and further investigated the Li storage behaviors of terephthalate salts:Li,Na,K.It was disclosed the crowded anion packing in the crystal structures of Li₂TP and Na₂TP made it hard for ion migration due to the small radius of Li+/Na+ ions as well as short metal-oxygen bonds?1.96 ? /2.43 ??.Whereas in K₂TP crystal,K⁺ ion has a larger radius and displays better matching size with terephthalate anion,which results in much lower van der Waals repulsion during energy minimization,and thus leads to a more stable lattice structure.K₂TP indeed displays the best cyclic and rate capability and exhibited two-dimensional Li-ion and electronic transport way whereas Li₂TP and Na₂TP experienced one-dimensional Li-ion and electronic transport way.To improve the real specific capacity,we prepared a K₂TP/graphene nanocomposite which delivered a reversible capacity of 122 mAh g^-1 over 500 cycles at a current of 8 C.The main reasons are the as-synthesized nanosized K₂TP can shorten the Li-ion pathway,and the composite with graphene can construct a good conductive network and the stability of electroactive in organic electrolyte.?4?To extend the application fields of organic electrode materials,two potassium salts of para-aromatic dicarboxylates,namely potassium terephthalate?K₂TP?and potassium 2,5-pyridinedicarboxylate?K₂PC?,were newly exploited as the organic anodes in K-ion batteries.Both K₂TP and K₂PC exhibited the clear and reversible discharge and charge platforms in K-ion half cells,which were resulted from the redox behavior of organic para-aromatic dicarboxylates.The satisfactory and reversible specific capacities were realized in the K₂TP-and K₂PC-based K-ion cells,with average values of 181 and 190 mAh g^-1 for 100 cycles,respectively.The currently-obtained achievement of organic anodes could be the ideal anodes for rocking-chair K-ion batteries.?5?On the other hand,considering the richness of organic compound,a new redox-active family of aromatic dicyanides was developed as the organic electrode materials in rechargeable batteries.The detailed electronic structures of two primary representatives of 1,4-dicyanobenzene?DCB?and 9,10-dicyanoanthracene?DCA?were studied.The reversible reaction mechanism,electrochemical behavior and dissolution issues were systemically studied.Compared with DCB,DCA possessed better electronic conductivity and more stable reduced states of DCA^- and DCA^2- than DCB,ultimately resulting in the highly reversible and consecutive two-electron mechanism in secondary batteries.