Computational Insights into Li_xO_y Formation, Nucleation, and Adsorption on Carbon Nanotube Electrodes in Nonaqueous Li–O₂ Batteries

Abstract

Recent theoretical and experimental studies have shown that the formation of Li2O2, the main discharge product of nonaqueous Li–O2 batteries, is a complex multistep reaction process. The formation, nucleation, and adsorption of Li x O y (x and y = 0, 1, and 2) and (Li2O2) n clusters with n = 1–4 on the surface of carbon nanotubes (CNTs) were investigated by periodic density functional theory calculation. The results showed that both Li2O2 and Li2O on CNT electrodes are preferentially generated by lithiation reaction rather than disproportionation reaction. The free energy profiles demonstrate that the discharge potentials of 2.54 and 1.29 V are the threshold values of spontaneous nucleation of (Li2O2)2 and (Li2O)2 on a CNT surface, respectively. The electronic structure indicates that Li2O2 is a p-type semiconductor, while Li2O exhibits the properties of an insulator. Interestingly, once Li2O2 molecules condense into large clusters, they will be repelled away from the CNT surface and continue to grow into large-sized Li2O2. Our results provide insights into the full understanding of the electrochemical reaction mechanism and product formation processes of lithium oxides in the cathodes of Li–O2 batteries.

Understanding the Catalytic Activity of the Preferred Nitrogen Configuration on the Carbon Nanotube Surface and Its Implications for Li–O2 Batteries

“…As for pyridinic N4 (pyriN4) shown in Figure a5, the four N atoms are in their respective hexagons, and the area occupied by the vacancies on the NCNT surface is significantly larger than that of pyriN3 and pyrroN3. As shown in Figure c, both O 2 and lithium oxides (Li x O y with x and y = 1 and 2) have a planar structure, which is in accordance with the reported results. , The bond lengths of Li–O and O–O bonds have been verified in our previous work …”

“…57,58 The bond lengths of Li−O and O−O bonds have been verified in our previous work. 7 The bond lengths of the C−C and N−C bonds of the optimized geometries of PCNTs and NCNTs are presented in Figure 2. It can be seen that the distribution of C atoms on the surface of the PCNTs (Figure 2a) is relatively regular, and the average bond length of the C−C bonds alternates between 1.42 and 1.43 Å from the top to bottom, which is in agreement with the calculations of Yuan and Huang 59 and Peralta-Inga et al 60 For comparison, the bond lengths of C−C and N−C bonds around N atoms on the NCNT surface were analyzed.…”

“…As described in our previous letter, Li 2 O 2 and Li 2 O generated during discharge will inevitably aggregate into clusters if they cannot be decomposed in time, which will increase the difficulty of product decomposition during charging. 7 After Lau et al 58 determined the structure of (Li 2 O 2 ) n clusters, the interaction between (Li 2 O 2 ) n and NCNTs was explored, as shown in Figure 7. It can be seen that both the PCNTs and the NCNTs exhibit the performance of adsorption and repulsion to monomer Li 2 O 2 and (Li 2 O 2 ) n clusters with n > 1, respectively.…”

“…4,5 Undoubtedly, these problems are closely related to the sluggish kinetics of the oxygen reduction reaction (ORR) and evolution reaction (OER). 6,7 In particular, the generation of insoluble lithium peroxide (Li 2 O 2 ), which is the main discharge product with extremely poor conductivity, not only blocks the active site with limited catalytic activity but also severely hinders electron transport, inducing capacity loss and even cell death. 8,9 Therefore, the detailed formation and growth mechanism of Li 2 O 2 behind the physical and chemical parameters urgently need to be clarified.…”

“…Rechargeable Li–O 2 batteries (LOBs) first proposed by Littauer and Tsai are considered as next-generation batteries and have attracted much attention due to their superior theoretical energy density (∼3400 W h/kg) in comparison to the state-of-the-art Li-ion batteries (LIBs). , Nevertheless, the development of LOBs currently faces huge challenges that hinder commercial applications, such as poor rate capability, high charging overpotential, and low Coulombic efficiency. , Undoubtedly, these problems are closely related to the sluggish kinetics of the oxygen reduction reaction (ORR) and evolution reaction (OER). , In particular, the generation of insoluble lithium peroxide (Li 2 O 2 ), which is the main discharge product with extremely poor conductivity, not only blocks the active site with limited catalytic activity but also severely hinders electron transport, inducing capacity loss and even cell death. , Therefore, the detailed formation and growth mechanism of Li 2 O 2 behind the physical and chemical parameters urgently need to be clarified.…”