We report the designed synthesis of Co 1,3,5-benzenetricarboxylate coordination polymers (CPs) via a straightforward hydrothermal method, in which three kinds of reaction solvents are selected to form CPs with various morphologies and dimensions. When tested as anode materials in Li-ion battery, the cycling stabilities of the three CoBTC CPs at a current density of 100 mA g(-1) have not evident difference; however, the reversible capacities are widely divergent when the current density is increased to 2 A g(-1). The optimized product CoBTC-EtOH maintains a reversible capacity of 473 mAh g(-1) at a rate of 2 A g(-1) after 500 galvanostatic charging/discharging cycles while retaining a nearly 100% Coulombic efficiency. The hollow microspherical morphology, accessible specific area, and the absence of coordination solvent of CoBTC-EtOH might be responsible for such difference. Furthermore, the ex situ soft X-ray absorption spectroscopy studies of CoBTC-EtOH under different states-of-charge suggest that the Co ions remain in the Co(2+) state during the charging/discharging process. Therefore, Li ions are inserted to the organic moiety (including the carboxylate groups and the benzene ring) of CoBTC without the direct engagement of Co ions during electrochemical cycling.
Monovalent
Li-substitution has been proven to be an effective strategy
to resolve the pivotal problems confronted with P2-type layered Mn
oxides, such as cooperative Jahn–Teller distortions of Mn3+ ions and drastic P2-(OP4)-O2 phase transformations occurring
during desodiation. However, the cycling stability of most Li+-substituted P2-Na
x
Li
y
Mn1–y
O2 remains far from satisfactory. We herein develop
a facile Ti-substitution method to improve the cyclability by taking
Na0.72Li0.24Mn0.76O2 (NLMO)
as an example. As expected, the novel layered oxide cathode Na0.72Li0.24Ti0.10Mn0.66O2 (NLMTO-0.1) is able to deliver a very high reversible capacity
of 165 mA h g–1 for over 80 cycles within the voltage
range of 1.5–4.5 V (vs Na metal), which is among the best for
the reported Na-storage cathode materials. Moreover, the structure–property
relationship of Ti4+ substitution is scrutinized by an
arsenal of 23Na/7Li solid-state nuclear magnetic
resonance, dual-mode electron paramagnetic resonance, and synchrotron
X-ray diffraction techniques. The results unequivocally substantiate
that Ti substitution can effectively reduce the Li+/Mn4+ ordering in TMO2 slabs, assist the reversible
migration of Li+ during Na+ extraction/intercalation,
and ultimately enhance the reversibility of the oxygen redox process.
This work provides a comprehensive insight into the structure chemistry
in developing high-capacity and high-stability layered oxide cathodes.
We herein demonstrate the fabrication of Mn- and Ni-based ultrathin metal-organic framework nanosheets with the same coordination mode (termed "Mn-UMOFNs" and "Ni-UMOFNs", respectively) through an expedient and versatile ultrasonic approach and scrutinize their electrochemical properties as anode materials for rechargeable lithium batteries for the first time. The obtained Mn-UMOFNs with structure advantages over Ni-UMOFNs (thinner nanosheets, smaller metal-ion radius, higher specific surface area) exhibit high reversible capacity (1187 mAh g at 100 mA g for 100 cycles), excellent rate capability (701 mAh g even at 2 A g), rapid Li diffusion coefficient (2.48 × 10 cm s), and a reasonable charge-discharge profile with low average operating potential at 0.4 V. On the grounds of the low-cost and environmental benignity of Mn metals and terephthalic acid linkers, our Mn-UMOFNs show alluring promise as a low-cost high-energy anode material for future LIBs. Furthermore, the lithiation-delithiation chemistry of Mn-UMOFNs was unequivocally studied by a combination of magnetic measurements, electron paramagnetic resonance, and synchrotron-based soft X-ray spectroscopy (O K-edge and Mn L-edge) experiments, the results of which substantiate that both the aromatic chelating ligands and the Mn centers participate in lithium storage.
Oxygen redox has recently emerged
as a lever to boost the specific
energy density of layered sodium transition metal oxide cathode materials.
However, the oxygen redox reaction is universally confronted with
concomitant issues such as irreversible lattice oxygen loss and parasitical
electrolyte degradation, thus debilitating cycling stability. Herein,
a novel F-substituted layered structure P2-Na0.65Li0.22Mn0.78O1.99F0.01 cathode
is designed, which exhibits superb capacity retention (183.6 mAh g–1 after 50 cycles at 0.05C, 87.8% of the highest discharge
capacity) and rate capability (105.5 mAh g–1 at
5C) in Na half-cells. Such results are nontrivial as this system only
contains the low-cost Mn transition metal element. Moreover, by systematic
bulk/surface spectroscopy evidence (hard and soft X-ray absorption
spectroscopy, electron paramagnetic resonance, and operando differential
electrochemical mass spectrometry), we explicitly corroborate that
the irreversible oxygen evolution and notorious Jahn–Teller
distortion are effectively subdued by trace F-substitution. In addition,
a higher oxygen vacancy formation energy for the F-substituted structure
was demonstrated via density functional theory calculations. Anionic
substitution could therefore be an impactful solution to boost reversible
oxygen redox chemistry for layered sodium oxide cathodes.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.