Developing single-site catalysts featuring maximum atom utilization efficiency is urgently desired to improve oxidation-reduction efficiency and cycling capability of lithium-oxygen batteries. Here, we report a green method to synthesize isolated cobalt atoms embedded ultrathin nitrogen-rich carbon as a dual-catalyst for lithium-oxygen batteries. The achieved electrode with maximized exposed atomic active sites is beneficial for tailoring formation/decomposition mechanisms of uniformly distributed nano-sized lithium peroxide during oxygen reduction/evolution reactions due to abundant cobalt-nitrogen coordinate catalytic sites, thus demonstrating greatly enhanced redox kinetics and efficiently ameliorated over-potentials. Critically, theoretical simulations disclose that rich cobalt-nitrogen moieties as the driving force centers can drastically enhance the intrinsic affinity of intermediate species and thus fundamentally tune the evolution mechanism of the size and distribution of final lithium peroxide. In the lithium-oxygen battery, the electrode affords remarkably decreased charge/discharge polarization (0.40 V) and long-term cyclability (260 cycles at 400 mA g−1).

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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...

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Alloy-type anodes in alkaline ion batteries have to face the challenges of huge volume change and giant structure strain/stress upon cycling. Here, reducing the structure stress for advanced performances by voltage regulation is demonstrated by using microsized Sn (μ-Sn) for sodium ion batteries as a model. Density functional theory and finite element analysis indicate that Sn/NaSn3 is the crucial phase transition highly responsible for inducing surface cracks, particle aggregations, and cell failures. Eliminating this phase transition by the control of cutoff voltages successfully extends the cycle life of μ-Sn to 2500 cycles at 2 A g–1, much longer than ∼40 cycles in a regular voltage window. The specific capacity is still retained at ∼455 mAh g–1, leading to a capacity decay of only ∼0.0088% per cycle. The results provide a simple way to achieve the outstanding performances without complicated preparation, expensive reagents, and laborious processing.