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Artigo em Inglês | MEDLINE | ID: mdl-31961644


As a parent compound of Li-rich electrodes, Li2MnO3 exhibits high capacity during the initial charge; however, it suffers notoriously low Coulombic efficiency due to oxygen and surface activities. Here, we successfully optimize the oxygen activities toward reversible oxygen redox reactions by intentionally introducing protons into lithium octahedral vacancies in the Li2MnO3 system with its original structural integrity maintained. Combining structural probes, theoretical calculations, and resonant inelastic X-ray scattering results, a moderate coupling between the introduced protons and lattice oxygen at the oxidized state is revealed, which stabilizes the oxygen activities during charging. Such a coupling leads to an unprecedented initial Coulombic efficiency (99.2%) with a greatly improved discharge capacity of 302 mAh g-1 in the protonated Li2MnO3 electrodes. These findings directly demonstrate an effective concept for controlling oxygen activities in Li-rich systems, which is critical for developing high-energy cathodes in batteries.

ACS Appl Mater Interfaces ; 11(49): 45674-45682, 2019 Dec 11.
Artigo em Inglês | MEDLINE | ID: mdl-31714058


Cation-disordered rock-salt oxides with the O2-/O2n- redox reaction, such as Li1.2Mn0.4Ti0.4O2 (LMTO), are critical Li-rich cathode materials for designing high-energy-density batteries. Understanding the cationic-anionic redox accompanying the structural evolution process is really imperative to further improve the performance. In this work, the cationic-anionic redox and capacity degradation mechanism of carbon-coated LMTO during (dis)charge processes are elucidated by combining in situ X-ray diffraction, X-ray absorption near-edge spectroscopy, differential electrochemical mass spectrometry, transmission electron microscopy, and electrochemical analyses. It is concluded that the redox reaction of Mn2+/Mn4+ is quite stable, while the severe degradation is mainly caused by the O2-/O2n- redox process. Moreover, we clearly clarify how the cationic-anionic interplay governs sluggish kinetics, large polarization, and capacity fading in LMTO, and reveal for the first time that a certain amount of carbon coating is capable of suppressing the irreversible lattice oxygen loss and results in an encouraging cycling performance. In summary, we elucidate the degradation of cationic-anionic redox processes in cation-disordered cathode materials and propose strategies for adjusting the electronic/ionic conductivity of the electrodes to modulate the oxygen redox reactions, setting a new direction for the design of better cation-disordered oxides.

Angew Chem Int Ed Engl ; 58(50): 18086-18095, 2019 Dec 09.
Artigo em Inglês | MEDLINE | ID: mdl-31587462


Sodium layered P2-stacking Na0.67 MnO2 materials have shown great promise for sodium-ion batteries. However, the undesired Jahn-Teller effect of the Mn4+ /Mn3+ redox couple and multiple biphasic structural transitions during charge/discharge of the materials lead to anisotropic structure expansion and rapid capacity decay. Herein, by introducing abundant Al into the transition-metal layers to decrease the number of Mn3+ , we obtain the low cost pure P2-type Na0.67 Alx Mn1-x O2 (x=0.05, 0.1 and 0.2) materials with high structural stability and promising performance. The Al-doping effect on the long/short range structural evolutions and electrochemical performances is further investigated by combining in situ synchrotron XRD and solid-state NMR techniques. Our results reveal that Al-doping alleviates the phase transformations thus giving rise to better cycling life, and leads to a larger spacing of Na+ layer thus producing a remarkable rate capability of 96 mAh g-1 at 1200 mA g-1 .

ACS Appl Mater Interfaces ; 11(15): 14066-14075, 2019 Apr 17.
Artigo em Inglês | MEDLINE | ID: mdl-30801174


Silicon-based anodes have the potential to be used in next-generation lithium ion batteries owing to their higher lithium storage capacity. However, the large volume change during the charge/discharge process and the repeated formation of a new solid electrolyte interface (SEI) on the re-exposed Si surface should be overcome to achieve a better electrochemical performance. Fluoroethylene carbonate (FEC) has been widely used as an electrolyte additive for Si-based anodes, but the intrinsical mechanism in performance improvement is not clear yet. Here, we combined solid-state NMR, X-ray photoelectron spectroscopy, and X-ray photoemission electron microscopy to characterize the composition, structure, and inhomogeneity of the SEI on Si/C composite anodes with or without the FEC additive. Similar species are observed with two electrolytes, but a denser SEI formed with FEC, which could prevent the small molecules (i.e., LiPF6, P-O, and Li-O species) from penetrating to the surface of the Si/C anode. The hydrolysis of LiPF6 leading to Li xPO yF z and further to Li3PO4 could also be partially suppressed by the denser SEI formed with FEC. In addition, a large amount of LiF could protect the cracking and pulverization of Si particles. This study demonstrates a deeper understanding of the SEI formed with FEC, which could be a guide for optimizing the Si-based anodes for lithium ion batteries.