共价有机框架电极材料自由基调控及稳定化

It is a great challenge to prepare organic electrodes for sodium-ion batteries having long cycle life, high capacity, and excellent rate capability. The highly reactive radical intermediates generated during the sodiation/desodiation process could be a critical issue because of the undesired side reaction. Recently we have reported durable electrodes based on ketoenamine with a stabilized α-C radical intermediate. Through the π-π/p-π resonance effect as well as steric effect, the excessive reactivity of the unpaired electron in the radicals is successfully suppressed, thus stable cycling of over 2000 cycles with 96.8 % capacity retention was achieved. But the remaining issues are the low specific capacity and poor rate capability. In this project, we aim to develop high performance organic electrodes consisting of conjugated covalent organic frameworks (COFs) based on ketoenamine unit. A tri-carbonyl building block was constructed into the Schiff base COFs to produce α-C radical intermediates and aromatic amino linkers were adopted to construct large π-conjugated systems, which contribute to stabilize the radical intermediates and enhance the electronic conductivity. The stacking behaviors of the resultant COFs were systemically tuned to avoid the self-quenching behavior of the radical sites and improve the electrode-electrolyte interface contact. The high porous structure of COFs will also increase the specific surface area for electrolyte penetration. Therefore by this favorable combination of synergic effect, stable cycling, high specific capacity, and excellent rate capability could be expected in this COFs design. We will focus on investigating the effects of functionalized groups, conjugated properties, inter-layer distance, and pore size, etc. on the state of charge, energy level and electrochemical reactivity of the radicals. Special research attention will be directed to unravel the production, transformation and electrochemical reaction of radicals upon the electrochemical sodition and de-sodiation. The objective is to define the mechanism governing the stabilization of radicals and their correlation to electrochemical activities in the covalent organic frameworks. Outcomes of this project will be of great significance in guiding the design of next-generation high performance organic electrode for sodium ion batteries.

有机分子中C=O等不饱和基团在充放电过程中生成的•C-O-自由基中间体非常活泼，易偶合或发生副反应失活，这是钠离子电池有机电极材料失效的根本原因。最近我们提出了通过π-π/p-π联合共轭和位阻效应协同稳定自由基改善有机电极材料循环稳定性的有效策略，但仍存在容量低和导电差的关键问题。本项目拟在前期研究基础上，设计合成基于改性酮胺的共价有机框架(COFs)电极材料。利用COFs的离域大π共轭结构以及层层堆叠结构调控自由基稳定性。离域大π共轭骨架同时会形成电子输运网络；层层堆叠和多孔特性会加速钠离子扩散，同时改善电极倍率性能。将重点研究COFs成分(不同修饰基团)和结构(共轭体系程度、层间距和孔特性)对自由基电子状态、轨道能级及电化学活性的影响机制；COFs在电化学嵌脱钠过程自由基的形成和转移机制。目标是阐明COFs结构调控自由基稳定性和电化学活性的基本规律，为研发高性能电极材料提供理论指导。

自由基是有机电极材料充放电过程中最基本的中间态，是调控电子转移及电荷储存机制的关键因素。自由基中间体具有非常高的反应活性，容易发生耦合反应，导致电极逐渐丧失电化学活性。同时自由基中间体更易溶于电解液或与电解液发生副反应，这是有机电极材料失效的根本原因。因此自由基中间体的调控和稳定化研究，对于设计与开发新型高性能有机电极材料具有重要意义。通过分子内p-π/π-π共轭效应和位阻效应稳定α-C自由基, 首次揭示了自由基提供额外电荷存储位点的新机制，利用共价有机框架（COFs）的拓扑结构有效调控自由基中间体。通过调节COFs厚度调控自由基自旋电子重叠程度，提高离子扩散动力学。调节COFs砌块尺寸和共轭体系大小以及功能基团极性，可以有效调控自由基电子的离域范围和自旋电子密度。采用分子工程策略设计新型双极性有机电极材料，实现了同时对n-型和p-型双离子自由基的调控，为开发新型高性能有机电池提供了新方法。通过形成弱的Cu3+-O共价键，增加了氧的非键态，更加有效激发氧阴离子活性，降低氧离子中间体的充放电平台迟滞性，证实阴离子O2-部分氧化为O22-，材料存在由层状到尖晶石相的相转变过程，同时揭示了晶格氧反应机理。DSL水系粘结剂能够在钴酸锂表面强烈的氢键作用下增强钴酸锂表面晶格的Co-O键强度，从而避免高电压工况下形成H1-3亚稳态结构，提高其充放电的可逆性。通过价层电子轨道重构，d带空穴增加且价层电子逐渐向费米能级离散，增强了底物分子在催化位点上的吸附以及底物分子和催化位点之间的电子转移，并增强了催化剂电催化还原氧气的能力。.项目深入分析了电极的微观结构，组成成分，界面与电化学器件性能的构效关系。本项目研究结果为通过自由基中间体调控优化高性能超稳定电极材料提供了重要科学依据。基于本项目获得深圳市自然科学二等奖，共发表论文56篇，申请和授权技术发明专利7项，培养毕业博士研究生4名，硕士研究生6名，培养出站博士后2名。