Bifunctional Catalytic Activity Guided by Rich Crystal Defects in Ti₃C₂ MXene Quantum Dot Clusters for Li–O₂ Batteries

Abstract

lithium-ion counterparts. This technologically favorable Li-O 2 battery works on the following principle: 2Li + + O 2 + 2e -⇔Li 2 O 2 (2.96 V), involving with oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) processes, respectively. [1][2][3] Despite admitted competitiveness, there still exists serious bottleneck and key scientific issues to be solved. Cumbersome overpotential and thereby induced detrimental parasitic reaction during repeated cycles cast a shadow over the real-life implementation of Li-O 2 batteries. In terms of cathode side, solid discharge species, namely Li 2 O 2 , with inherent poor conductivity and insolubility in electrolyte, are tough to be decomposed. Intrinsically, electrochemical reaction kinetics is undesirably retarded, giving rise to large potential hysteresis between ORR and OER. [4][5][6][7] To circumvent this notorious dilemma, the prevailing strategy lies on the exploitation of efficient catalysts (precious metals, carbonaceous materials, metallic oxides, sulfides, etc.) with synergistically tailored geometric constructions and electronic characteristics to manipulate Li 2 O 2 accommodations with optimized morphology and structure. [8][9][10][11][12] As a future-proof approach, seeking for efficient catalysts is still a rigorous challenge for Li-O 2 chemistry. In response, defect engineering (vacancies, edges, boundaries, dislocations, and doping) shows huge potential in tuning the electronic structure and thus facilitates the electrochemical characteristic of active catalysts. [13][14][15] For instance, J. O-Medina et al. demonstrated that the defective sites such as, heteroatom doping, vacancies, and edges could tailor the initial sp 2 -hybridized networks of nanocarbons, leading to enhancement of electrocatalysis activity. [16] Wang's group also proposed that the introduction of various defects played a crucial role in modifying electron delocalization structure within carbon and metallic compounds for various rechargeable batteries, suggesting its superiority in accelerating charge transfer kinetics and optimizing intermediate adsorption behavior during cycling. [13] Wang et al. reported that superior catalytic activity and long-term stability towards electrocatalytic reactions could be realized by introducing doping, edge, and grain boundary defects. [14] Ameliorating round-trip efficiency and mitigating parasitic reaction play a key role in enhancing the activity and durability of lithium-oxygen batteries. Herein, it is first reported that Ti 3 C 2 MXene quantum dot clusters full of rich crystal defects anchored on N-doped carbon nanosheets (Ti 3 C 2 QDC/N-C) can operate well as bifunctional catalyst for Li-O 2 batteries. The well-defined grain boundary and edge defects make crucial contributions in modulating the local unsaturated coordination state of active titanium atoms and thus the electronic structure of Ti 3 C 2 QDC/N-C, greatly enhancing the catalytic capability. Furthermore, density functional theory calculations disclose that the fruitful cryst...

Quantum Dots Compete at the Acme of MXene Family for the Optimal Catalysis

“…Moreover, the defect-rich MQDs cluster/N-doped carbon nanosheet nanocomposites were prepared by using electrostatically self-assembly for Li–O 2 batteries (Fig. 10 d) [ 137 ]. TEM image shows the uniform distribution of MQDs in the N–C nanosheets (Fig.…”

“…Copyright @2020, American Chemical Society. d Synthesis process schematic of Ti 3 C 2 QDC/N-C nanocomposites and e TEM image of Ti 3 C 2 QDC/N-C, inset illustration is SAED pattern; f HRTEM image of Ti 3 C 2 QDC/N-C [ 137 ]. Copyright @2021, Wiley–VCH.…”

“…Copyright @2019, Elsevier. e AC-STEM image of Ti 3 C 2 nanosheet; f AC-STEM of Ti 3 C 2 MQDs; g Initial deep discharge–charge curves of the three samples at 200 mA g −1 ; h Cycling stability and terminal discharge–charge voltages of Ti 3 C 2 QDC/N–C electrode at 200 mA g −1 [ 137 ]. Copyright @2021, Wiley–VCH …”

“…So, it leads to reducing the redox energy barriers during Li 2 O 2 formation and decomposition, achieving enhanced electrocatalytic kinetics. Wang et al [ 137 ] prepared Ti 3 C 2 MXene quantum dot clusters (Ti 3 C 2 QDC) with rich grain boundaries and edge defects through hydrothermal thermal-shearing reaction method. The defects were firstly characterized by using AC-STEM.…”

Recent progress in quantum dots based nanocomposite electrodes for rechargeable monovalent metal-ion and lithium metal batteries

“…In addition, owing to a large number of abundant catalytic active sites and tuneable structural defects, QDs also possess catalytic properties towards the kinetic conversion between sulfur and polysulfides and inhibiting polysulfide shuttling in Li–S batteries 48–50 or kinetic conversion between O 2 and Li 2 O 2 in Li–air batteries. 51–53 QDs have also shown promising results in terms of regulating uniform and dendrite-free metal deposition at the anode 54–56 and improving the lithium-ion transport properties when used as fillers in solid-state electrolytes. 57,58…”