Radicals are inevitable intermediates during the charging and discharging of organic redox electrodes. The increase of the reactivity of the radical intermediates is desirable to maximize the capacity and enhance the rate capability but is detrimental to cycling stability. Therefore, it is a great challenge to controllably balance the redox reactivity and stability of radical intermediates to optimize the electrochemical properties with a good combination of high specific capacity, excellent rate capability, and long-term cycle life. Herein, we reported the redox and tunable stability of radical intermediates in covalent organic frameworks (COFs) considered as high capacity and stable anode for sodium-ion batteries. The comprehensive characterizations combined with theoretical simulation confirmed that the redox of C−O• and α-C radical intermediates play an important role in the sodiation/desodiation process. Specifically, the stacking behavior could be feasibly tuned by the thickness of 2D COFs, essentially determining the redox reactivity and stability of the α-C radical intermediates and their contributive capacity. The modulation of reversible redox chemistry and stabilization mechanism of radical intermediates in COFs offers a novel entry to design novel high performance organic electrode materials for energy storage and conversion.
Phosphorus doping
is an effective strategy to simultaneously improve
the electronic conductivity and regulate the ionic diffusion kinetics
of TiO2 being considered as anode materials for sodium
ion batteries. However, efficient phosphorus doping at high concentration
in well-crystallized TiO2 nanoparticles is still a big
challenge. Herein, we propose a defect-assisted phosphorus doping
strategy to selectively engineer the surface structure of TiO2 nanoparticles. The reduced TiO2–x
shell layer that is rich in oxygen defects and Ti3+ species precisely triggered a high concentration of phosphorus doping
(∼7.8 at. %), and consequently a TiO2@TiO2–x
-P core@shell architecture was produced. Comprehensive
characterizations and first-principle calculations proved that the
surface-functionalized TiO2–x
-P
thin layer endowed the TiO2@TiO2–x
-P with substantially enhanced electronic conductivity and
accelerated Na ion transportation, resulting in great rate capability
(167 mA h g–1 at 10 000 mA g–1) and stable cycling (99% after 5000 cycles at 10 A g–1). Combining in/ex situ X-ray diffraction with ex situ electron spin resonance clearly demonstrated the
high reversibility and robust mechanical behavior of TiO2@TiO2–x
-P upon long-term cycling.
This work provides an interesting and effective strategy for precise
heteroatoms doping to improve the electrochemical performance of nanoparticles.
The
Ni-rich LiNi0.8Co0.1Mn0.1O2 (NCM811) cathode has attracted great interest
owing to its low cost, high capacity, and energy density. Nevertheless,
rapid capacity fading is a critical problem because of direct contact
of NCM811 with electrolytes and hence restrains its wide
applications. To prevent the direct contact, the surface inert layer
coating becomes a feasible strategy to tackle this problem. However,
to achieve a homogeneous surface coating is very challenging. Considering
the bonding effect between NCM811, polyvinylpyrrolidone
(PVP), and polyaniline (PANI), in this work, we use PVP as an inductive
agent to controllably coat a uniform conductive PANI layer on NCM811 (NCM811@PANI–PVP). The coated PANI layer
not only serves as a rapid channel for electron conduction, but also
prohibits direct contact of the electrode with the electrolyte to
effectively hinder side reaction. NCM811@PANI–PVP
thus exhibits excellent cyclability (88.7% after 100 cycles at 200
mA g–1) and great rate performance (152 mA h g–1 at 1000 mA g–1). In situ X-ray
diffraction and in situ Raman are performed to investigate the charge–discharge
mechanism and the cyclability of NCM811@PANI–PVP
upon electrochemical reaction. This surfactant-modulated surface uniform
coating strategy offers a new modification approach to stabilize Ni-rich
cathode materials for lithium-ion batteries.
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