Controlling Reversible Expansion of Li2O2 Formation and Decomposition by Modifying Electrolyte in Li-O2 Batteries

锂空电池分别使用空气中的氧气和金属锂作为正负极活性材料，具有极高的能量密度。但是，这一体系尚不能实现商业化的应用，其中一些关键问题未能解决。由于其正极活性材料是气体，使得电化学反应涉及气-液-固三相界面，电极过程十分复杂。与其它二次电池相比，空气电极需要考虑结构因素和催化因素。不仅要改善氧气电化学反应的动力学迟缓问题，还要考虑放电产物的驻留空间问题。董全峰教授课题组在前期开展了基于空气电极固相表面电催化研究，并结合电极结构方面的问题，构筑了有利于氧气发生反应的仿生开放式结构电极。 该研究工作主要由化学化工学院2015级iChEM直博生林晓东（第一作者）在董全峰教授、郑明森副教授和龚磊副教授的共同指导下完成，理论计算由袁汝明助理教授（共同第一作者）完成，曹勇、丁晓兵、蔡森荣、韩博闻等学生参与了部分工作。周志有教授和洪宇浩博士生在电化学微分质谱方面给予大力的帮助与支持。【Abstract】The aprotic lithium-oxygen (Li-O2) battery has attracted worldwide attention because of its ultrahigh theoretical energy density. However, its practical application is critically hindered by cathode passivation, large polarization, and severe parasitic reactions. Here, we demonstrate an originally designed Ru(Ⅱ) polypyridyl complex (RuPC) though which the reversible expansion of Li2O2 formation and decomposition can be achieved in Li-O2 batteries. Experimental and theoretical results revealed that the RuPC can not only expand the formation of Li2O2 in electrolyte but also suppress the reactivity of LiO2 intermediate during discharge, thus alleviating the cathode passivation and parasitic reactions significantly. In addition, an initial delithiation pathway can be achieved when charging in turn; thus, the Li2O2 products can be decomposed reversibly with a low overpotential. Consequently, the RuPC-catalyzed Li-O2 batteries exhibited a high discharge capacity, a low charge overpotential, and an ultralong cycle life. This work provides an alternative way of designing the soluble organic catalysts for metal-O2 batteries.This work was supported by the National 973 Program (2015CB251102), the Key Project of National Natural Science Foundation of China (21673196, 21621091, 21703186, 21773192),and the Fundamental Research Funds for the Central Universities (20720150042,20720150043). The authors thank Prof. Eric Meggers at Philipps-Univeristaet Marburg for his discussion about the synthesis of RuPC complex; Prof. Gang Fu at Xiamen University for his instructive discussions in DFT calculations; Lajia Yu and Dandan Tao at Xiamen University for their assistance in EPR experiments and UV-Vis spectroscopy experiments, respectively; and Yu Gu and Tao Wang at Xiamen University for their discussions in XPS results and CV data,respectively. 该工作得到科技部重大基础研究计划（项目批准号：2015CB251102）、国家自然科学基金（项目批准号：21673196、21621091、21703186、21773192）和中央高校基本科研业务费专项资金（项目批准号：20720150042、20720150043）的资助。 此外，感谢傅钢教授在理论计算方面的讨论和建议，Eric Meggers教授在配合物合成上的讨论，泉州师范学院吴启辉教授和化学化工学院谷宇博士生在X射线光电子能谱方面的帮助，于腊佳老师在电子顺磁共振实验上的帮助，陶丹丹博士生在紫外可见光谱测试上的帮助以及王韬博士生在循环伏安方面的讨论

Computational Insight into the Mechanism of Nickel-Catalyzed Reductive Carboxylation of Styrenes using CO2

DFT calculations have been carried out to study the detailed mechanisms for the nickel-catalyzed reductive carboxylation of ester-substituted styrenes H2C=CHAr using CO2 to form alpha-carboxylated products. Two possible mechanisms, the oxidative coupling mechanism and the nickel hydride mechanism, were calculated and compared. Our calculations show that, for the oxidative coupling mechanism, a metallacycle thermodynamic sink is generated from oxidative coupling between CO2 and a styrene substrate molecule on the nickel(0) metal center, which should be avoided in order for smooth reductive carboxylation of styrenes. For the nickel hydride mechanism, a nickel hydride species is the active species, from which styrene insertion into the NiH bond followed by reductive elimination produces the a-carboxylated product. Calculations show that either of these two steps (insertion and reductive elimination) can be the rate-determining step, and both transition states are only slightly more stable than the oxidative coupling transition state leading to the thermodynamic sink. Because of the competitive nature between the two mechanisms, the reaction conditions and other factors (substituent, pressure, and ligand) significantly affect the reaction outcome, all of which have been discussed in detail

Mechanism for the Carboxylative Coupling Reaction of a Terminal Alkyne, CO2, and an Allylic Chloride Catalyzed by the Cu(I) Complex: A DFT Study

DFT calculations have been carried out to study the detailed mechanisms for carboxylative-coupling reactions among terminal alkynes, allylic chlorides, and CO2 catalyzed by N-heterocyclic carbene copper(I) complex (IPr)CuCl. The competing cross-coupling reactions between terminal alkynes and allylic chlorides have also been investigated. The calculation results show that a base-assisted metathesis of (IPr)CuCl with PhC CH occurs as the first step to give the acetylide (IPr)Cu-C CPh, from which CO2 insertion and reaction with an allylic chloride molecule, respectively, lead to carboxylative-coupling and cross-coupling reactions. It was found that both the reactions of (IPr)Cu-C CPh and (IPr)CuOCOC CPh (a species derived from CO2 insertion) with an allylic chloride molecule occur through an S(N)2 substitution pathway. The two S(N)2 transition states (calculated for the carboxylative coupling and cross coupling) are the rate-determining transition states and show comparable stability. How the reaction conditions affect the preference of one pathway over the other (carboxylative coupling versus cross coupling) has been discussed in detail

Computational Insight into the Mechanism of Nickel-Catalyzed Reductive Carboxylation of Styrenes using CO₂

DFT calculations have been carried out to study the detailed mechanisms for the nickel-catalyzed reductive carboxylation of ester-substituted styrenes H₂CCHAr using CO₂ to form α-carboxylated products. Two possible mechanisms, the oxidative coupling mechanism and the nickel hydride mechanism, were calculated and compared. Our calculations show that, for the oxidative coupling mechanism, a metallacycle thermodynamic sink is generated from oxidative coupling between CO₂ and a styrene substrate molecule on the nickel(0) metal center, which should be avoided in order for smooth reductive carboxylation of styrenes. For the nickel hydride mechanism, a nickel hydride species is the active species, from which styrene insertion into the Ni–H bond followed by reductive elimination produces the α-carboxylated product. Calculations show that either of these two steps (insertion and reductive elimination) can be the rate-determining step, and both transition states are only slightly more stable than the oxidative coupling transition state leading to the thermodynamic sink. Because of the competitive nature between the two mechanisms, the reaction conditions and other factors (substituent, pressure, and ligand) significantly affect the reaction outcome, all of which have been discussed in detail

Mechanism for the Carboxylative Coupling Reaction of a Terminal Alkyne, CO₂, and an Allylic Chloride Catalyzed by the Cu(I) Complex: A DFT Study

DFT calculations have been carried out to study the detailed mechanisms for carboxylative-coupling reactions among terminal alkynes, allylic chlorides, and CO₂ catalyzed by N-heterocyclic carbene copper­(I) complex (IPr)­CuCl. The competing cross-coupling reactions between terminal alkynes and allylic chlorides have also been investigated. The calculation results show that a base-assisted metathesis of (IPr)­CuCl with PhCCH occurs as the first step to give the acetylide (IPr)­Cu–CCPh, from which CO₂ insertion and reaction with an allylic chloride molecule, respectively, lead to carboxylative-coupling and cross-coupling reactions. It was found that both the reactions of (IPr)­Cu–CCPh and (IPr)­CuOCOCCPh (a species derived from CO₂ insertion) with an allylic chloride molecule occur through an S_N2 substitution pathway. The two S_N2 transition states (calculated for the carboxylative coupling and cross coupling) are the rate-determining transition states and show comparable stability. How the reaction conditions affect the preference of one pathway over the other (carboxylative coupling versus cross coupling) has been discussed in detail

Mechanistic Insight into the Gold-Catalyzed Carboxylative Cyclization of Propargylamines

DFT calculations have been carried out to study the detailed mechanisms for the carboxylative cyclization of propargylamine using CO₂ catalyzed by NHC-gold­(I) complexes. The calculation results indicate that the reaction starts with an N-coordinated species, [(NHC)­Au­(propargylamine)]­Cl, which undergoes isomerization to an alkyne-coordinated species. An amine–carbon dioxide interaction gives a carbamate ion species, from which a nucleophilic attack of the in-plane lone pair of electrons in the carbamate anion moiety on one of two coordinated alkyne carbons leads to formation of a five-membered-ring intermediate. The final product is generated through deprotonation and protonation processes. Through a detailed mechanistic study, we found that the substrate propargylamine assists (catalyzes) the deprotonation and protonation processes. Careful study of the solvent effect indicates that solvents, which are polar and capable of hydrogen bonding, promote the catalytic reactions through stabilizing the carbamate ion intermediate species

Computational Insight into the Mechanism of Nickel-Catalyzed Reductive Carboxylation of Styrenes using CO₂

DFT calculations have been carried out to study the detailed mechanisms for the nickel-catalyzed reductive carboxylation of ester-substituted styrenes H₂CCHAr using CO₂ to form α-carboxylated products. Two possible mechanisms, the oxidative coupling mechanism and the nickel hydride mechanism, were calculated and compared. Our calculations show that, for the oxidative coupling mechanism, a metallacycle thermodynamic sink is generated from oxidative coupling between CO₂ and a styrene substrate molecule on the nickel(0) metal center, which should be avoided in order for smooth reductive carboxylation of styrenes. For the nickel hydride mechanism, a nickel hydride species is the active species, from which styrene insertion into the Ni–H bond followed by reductive elimination produces the α-carboxylated product. Calculations show that either of these two steps (insertion and reductive elimination) can be the rate-determining step, and both transition states are only slightly more stable than the oxidative coupling transition state leading to the thermodynamic sink. Because of the competitive nature between the two mechanisms, the reaction conditions and other factors (substituent, pressure, and ligand) significantly affect the reaction outcome, all of which have been discussed in detail

How the Coordinated Structures of Ag(I) Catalysts Affect the Outcomes of Carbon Dioxide Incorporation into Propargylic Amine: A DFT Study

Density functional theory calculations have been carried out to explore the detailed mechanisms for carbon dioxide incorporation of N-unsubstituted propargylic amine catalyzed by Ag­(I) catalysts. We show that the reaction undergoes substrate adsorption or displacement, isomerization from amine-coordinated species to the alkyne-coordinated species, CO₂ attack, and proton transfer, giving the carbamate intermediate. Subsequently, the reaction would bifurcate at the intermolecular ring-closing step, which produces five-membered ring (5MR) and six-membered ring (6MR) products at the same time, thus raising a regioselectivity issue. Our calculations reveal that the outcomes of the reaction critically depend on the coordination number and the basicity of the ligands. Higher coordinate number and stronger basicity of the ligands would stabilize the 5MR transition state over the 6MR counterpart. Such a preference can be rationalized by using transition state energy decomposition. All of these results could promote the rational design of noble metal/organic base combined catalysts with higher selectivity

Tradeoff between reliability and security in block ciphering systems with physical channel errors

In this paper, we study the effects of channel errors on security and decoding error probability of three encryption systems where encrypted message is sent and eavesdropped over binary symmetric channels (BSC). The three systems are all based on Data Encryption Standard (DES) in cipher feedback (CFB) mode. They are DES only (DC), DES concatenated with Reed Solomon encoding (DCRS), and DES concatenated with RS coding and S-box diffusion (DCRSS). We adopt linear cryptanalysis to quantitatively measure the effects of channel errors on the security of these systems. We have found the required known cipher-plain text pairs in each system for linear attack launched by Eve, an eavesdropper. In addition, performance analysis in terms of decoded information bit error probability (IBER) for Bob, the legitimate receiver, has been conducted for each system, whose accuracy is later verified by simulation results. Our results suggest there exists tradeoff between communication reliability and security. More security level can be attained by sacrificing the accuracy at the legitimate receiver end, which can be captured by the relationship between IBER and our proposed novel metric, security improvement factor (SIF). ©2010 IEEE