【作者】 赵雷；

【导师】 王家钧；

【作者基本信息】 哈尔滨工业大学 ， 化学工程与技术， 2022， 硕士

【摘要】 寻找价格低廉,高比能的新一代锂电池体系已经迫在眉睫。锂硫电池因其极高的理论容量和能量密度引起了人们的关注,却也因“穿梭效应”和氧化还原动力学缓慢的问题应用受限。除此以外,负极失效和活性物质的导电性及体积膨胀的问题也不容忽视,而这些问题也会使穿梭效应而加剧。因此,抑制锂硫电池中的穿梭效应即多硫化锂在正负极的来回扩散时锂硫电池改性的重中之重。目前,针对穿梭效应的抑制,常见的改性方法有物理限域和化学吸附两种。Fe-N-C材料作为单原子中的一种,其高原子利用率和良好的载体适用性使其在锂硫电池体系中应用广泛,也为锂硫电池的转化机理研究提供了绝佳的应用模板。本文将调控Fe-N-C材料的化学结构和物理结构（后者以ZIF8材料为碳基体模板）,研究结构变化对锂硫电池实际充放电能力的影响及其深层次原因。其中,通过将FePc担载在CNT上,通过热处理调控Fe-N的键长及局域结构,以分析Fe活性位点的局域环境变化对多硫化锂吸附-转化作用的改变和对锂硫电池氧化还原动力学的影响。结果表明,在热处理后酞箐环内的Fe N₄结构会发生收缩,Fe-N键将脱离酞箐环平面,吸附活性结构的非平面性和不对称性（通过N含量变化分析）乃至构型都会发生改变,这将影响材料对多硫化锂的锚定能力和其反应动力学。电化学测试表明,FePc@CNT-600/S正极拥有最佳的充放电性能,在0.2C下拥有1214 m Ah g^-1的初始比容量,循环100 cyc后仍保持779.3m Ah g^-1的比容量,并在300cyc的长循环基本不在衰减;在6.27 mg cm^-2的高载下也拥有不错的容量保持率,并表现出优秀的倍率性能。采用水热法并控制反应物浓度合成粒径为50nm,100nm,200nm和1000nm的Fe-N-C材料,通过构建有序微孔-介孔材料来研究介孔存在对锂硫电池实际性能的影响。结果表明,相较于纯微孔分布的1um材料,存在一定介孔分布的100nm材料隔膜电池拥有更好的电化学性能。其中,微孔中有丰富的Fe活性位点以抑制穿梭效应,而介孔的分布则有助于锂离子在正极界面的扩散以及在充放电过程中的电荷转移,从而提高了锂离子实际的氧化还原能力。在0.5C下,Fe-N-C-100nm隔膜组装的电池有1160 m Ah g^-1的首圈比容量,在循环100cyc后,电池仍有960 m Ah g^-1的比容量,并且有优异的倍率性能。更多还原

【Abstract】 It is urgent to find a new generation of lithium battery system with low price and high specific energy.Lithium-sulfur batteries have attracted attention due to their extremely high theoretical capacity and energy density,but their applications are limited due to the"shuttle effect"and slow redox kinetics.In addition,the problems of anode failure and the conductivity and volume expansion of active materials cannot be ignored,and these problems will also exacerbate the shuttle effect.Therefore,suppressing the shuttle effect in lithium-sulfur batteries,that is,the back-and-forth diffusion of lithium polysulfides in the positive and negative electrodes,is a top priority in the modification of lithium-sulfur batteries.At present,there are two common modification methods for the inhibition of shuttle effect:physical confinement and chemical adsorption.As a kind of single atom,Fe-N-C material is widely used in lithium-sulfur battery system due to its high atom utilization rate and good carrier applicability,and also provides an excellent application template for the study of the transformation mechanism of lithium-sulfur battery.In this paper,the chemical structure and physical structure of Fe-N-C materials（the latter using ZIF8 material as a carbon matrix template）will be regulated,and the effect of structural changes on the actual charge-discharge capacity of lithium-sulfur batteries and the underlying reasons will be studied.Among them,by loading FePc on CNTs,the bond length and local structure of Fe-N are regulated by heat treatment,In order to analyze the changes of the local environment of Fe active sites on the adsorption-conversion of lithium polysulfides and the effects on the redox kinetics of lithium-sulfur batteries.The results show that the Fe N₄ structure in the phthalocyanine ring will shrink after heat treatment,the Fe-N bond will be separated from the plane of the phthalocyanine ring,and the non-planarity and asymmetry of the adsorption active structure（analyzed by the change of N content）and even the configuration will be affected.changes,which will affect the material’s ability to anchor lithium polysulfides and its reaction kinetics.Electrochemical tests show that the FePc@CNT-600/S cathode has the best charge-discharge performance,with an initial specific capacity of 1214 m Ah g^-1 at 0.2C,and a specific capacity of779.3 m Ah g^-1 after cycling for 100 cyc,and there is basically no decay in the long cycle of 300cyc;it also has a good capacity retention rate under a high load of 6.27 mg cm^-2,and shows excellent rate performance.Fe-N-C materials with particle sizes of 50 nm,100 nm,200 nm and 1000 nm were synthesized by hydrothermal method and controlled reactant concentration,and the effect of mesoporous existence on the actual performance of lithium-sulfur batteries was studied by constructing ordered microporous-mesoporous materials.The results show that,compared with the 1um material with pure micropore distribution,the100nm material with a certain mesopore distribution has better electrochemical performance.Among them,there are abundant Fe active sites in the micropores to suppress the shuttle effect,and the distribution of mesopores facilitates the diffusion of lithium ions at the cathode interface and the charge transfer during charging and discharging,thereby improving the practical performance of lithium ions redox capacity.At 0.5C,the battery assembled with Fe-N-C-100nm separator has a specific capacity of1160 m Ah g^-1 in the first cycle,and after cycling for 100 cyc,the battery still has a specific capacity of 960 m Ah g^-1,and has excellent rate performance.更多还原