From Organic Small Molecules to Large Conjugated System: Advanced Synthetic Methodology to Regulate Carbon-based Energy Storage Materials


Student thesis: Doctoral Thesis

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Award date31 Jul 2023


High-performance secondary lithium-ion battery (LIB) technologies, which are becoming more and more desired, are highly dependent upon the development and evolution of novel advanced electrode materials. Particularly, the lifespan, safety, cycling performance, and rate capability are significantly affected by the anode materials. In comparison, carbon-based materials possess merits including low cost, high abundance, chemical inertness, high mechanical strength, and excellent electronic properties, which are manifestly promising substances for advanced anode materials. Tremendous works have been devoted to exploiting novel advanced carbonaceous materials with further modification to enhance the performance. However, current strategies to synthesize advanced carbon materials always required harsh conditions, limited quantity, and multiple procedures, prohibiting further industrial production. Accordingly, exploiting novel strategies to achieve large-scale fabrication of advanced carbonaceous materials is urgently required for the boosting expectation of high-performance LIB technologies.

Our work is committed to fabricating carbon-based materials, which are regarded as large conjugated systems for both elements (graphite, graphene, hard carbon et al.) and compounds (covalent organic framework, conjugated polymers et al.), through the intermolecular coupling of organic small-molecular compounds. The organic small molecules are required to possess an aromatic system, which can function as the building blocks for the architecture of the eventual carbonaceous materials. Thus, it is capable of precisely regulating the structure and composition of carbonaceous materials by intentional molecular engineering on the building blocks of organic small molecules. Specifically, we have achieved several methods from organic small molecules to prepare carbonaceous materials, exhibiting outstanding electrochemical performance.

Laser scribing technology has already been demonstrated as a straightforward approach to fabricating porous graphene, which was only conducted with the precursor of polymers before this work. Even though the polymers may also function as the building blocks, it is still difficult to conduct molecular engineering on polymers instead of organic small molecules. Herein, it is initially achieved the fabricating porous graphene via employing precursors of organic small molecules, pentacene quinone (PQ) and tetraazapentacene quinone (TAPQ). The graphene products here are noted as P-LIG (P for pristine) and N-LIG (N for nitrogen), corresponding to precursors of PQ and TAPQ (N-fused molecules), respectively. Several characterizations elucidated the individual N content in N-LIG, demonstrating the strategy to construct N-doped graphene by exploiting the N-fused precursors. Besides, the SAXS and SEM both revealed the porous for both P-LIG and N-LIG as we expected. When pairing with Li metal to fabricate half cells, the N-LIG and P-LIG both exhibit anomalously capacity self-enhancement upon long-term cycling, while the N-LIG delivers a reversible capacity of around 5800 mAh g-1 at a current density of 0.2 A g-1 and retains of around 1970 mAh g-1 at a high current density of 2 A g-1, which is still the topmost performance among graphene-type anode for LIBs. Moreover, the kinetical analysis and structural studies both verified the diffusion-controlled contributor and surface-controlled contributor are progressively enhanced for a higher Li storage capacity upon cycling. Furthermore, ex situ synchrotron SAXS elucidates the arising of the microporous level for both P-LIG and N-LIG upon cycling, which should also provide extra active sites to accommodate more Li storage. Generally, this work proposes a novel strategy to fabricate porous graphene for application as a high-performance LIB anode by laser scribing organic small molecules, and the expectation to regulate the composition of graphene via modification on precursors was also achieved. The mechanism studies provide original insights toward the Li storage capacity and the intrinsic structure of the graphene, which should contribute to the design and application of novel advanced carbonaceous anode materials.

On the other hand, another strategy to polymerize the conjugated organic small molecules through solid-state reactions was also achieved. The polymerization of organic small molecules was conducted by a facile ‘one-pot’ solid-state ionothermal Scholl reaction with the production of conjugated microporous polymers (CMPs). The CMPs are regarded as a three-dimensional (3D) carbonaceous host, offering function as the substrate for lithium metal anodes (LMAs) to alleviate the volumetric fluctuation and relieve the Li dendrite growth. Besides, the nature of CMPs endows the atomic-scale geometry of the distribution of the lithiophilic sites, which can be achieved through this synthetic strategy by employing specific precursors. In this case, a series of CMPs, denoted as pAQs, pPQs, and pTAPQs were synthesized from organic small molecules of anthracene quinone (AQ), pentacene quinone (PQ), and tetraazapentacene quinone (TAPQ), respectively. Both the polymeric samples display anomalously high intrinsic electrical conductivities, enabling the Li deposition behavior on the individual polymeric samples, which has never been achieved before. Benefiting from the homogeneous distribution of lithiophilic sites, both polymeric samples display superb Li deposition behavior with uniform morphology. Preferentially, pAQ350-Li displayed a stable Li stripping/plating cycling process for over 1500 h at a high areal capacity of 10 mAh cm-2 with a high current density of 5 mA cm-2, overwhelming the performance of the original metallic Li. When pairing with commercial LFP cathodes, the pAQ350-Li LMA also displayed superior performance to LFP//Li cells, elucidating the prospect for further practical application.

In conclusion, various synthetic routes from organic small molecules to large conjugated systems were proposed and achieved, which both elucidate the capability to regulate the composition and structure through molecular engineering on precursors. The merits of the synthetic approaches also are corresponding to their preferential electrochemical performance for advanced LIB anode techniques. These strategies unravel a novel insight for the design and synthesis of advanced carbonaceous materials, as well as guide the fabrication of next-generation high-performance LIBs.