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Cation-disordered Li-excess cathodes with oxygen redox reactions are promising candidates for high-energy-density Li ion batteries. Nevertheless, the oxygen redox process that is required for the high capacity often comes with the oxygen loss, which leads to severe capacity degradation and voltage decay. In this work, we have successfully synthesized a series of Li-excess cation-disordered cathodes (Li1.2Mn0.4+xTi0.4-xO2-xFx) (0 ​≤ ​x ​≤ ​0.2) with different fluorine (F) contents. The electrochemical performance results show that the Li1.2Mn0.55Ti0.25O1.85F0.15 (LMTOF0.15) exhibits the highest reversible capacity (275 mAh g-1, under 30 ​mA ​g-1), cyclability, and voltage retentions. The mapping of resonant inelastic X-ray scattering (mRIXS) and differential electrochemical mass spectroscopy (DEMS) results reveal that the fluorination enhances the reversible lattice oxygen redox reaction while suppressing irreversible gas release and surface reactions. The X-ray Absorption Spectroscopy (XAS) during the initial two cycles shows that F-substitution alleviates the reduction of the Mn valence state during the whole (dis)charge processes in the bulk and at the surface of the material, results in higher average discharge voltage. In addition, the introduction of F improves the structural stability and suppresses local lattice distortion of the material. Therefore, LMTOF0.15 is able to cycle with smaller polarization, less interfacial side reaction and Mn dissolution, and therefore results in enhanced cyclability. This work provides a comprehensive understanding of the fluorination effect on the cationic and anionic redox activities in cation-disordered Li-excess cathodes.

Voltage decay,i.e., the voltage decrease during electrochemical cycling, has been a decade-long challenge for lithium-ion batteries. This issue not only leads to a substantial loss of energy density, but also raises challenges for the battery management system, hindering the commercial application of high capacity lithium-rich oxide. Here, we show that through a combination of electrochemical conditioning and thermal treatment, Li2MnO3, the parent compound of lithium-rich oxide, which typically displays severe voltage and capacity decay, could be converted into a new phase that essentially suppresses the voltage decay with improved capacity retention and rate performance. By combining atomic-sensitive nuclear magnetic resonance, differential electrochemical mass spectrometry and synchrotron-based resonant inelastic X-ray scattering, we disclose that treatment triggers the formation of three-coexisting phases,i.e., the lithium-rich layered, spinel and defect spinel phases, which enables improved reversibility of the oxygen redox activity and enhanced manganese redox reactions in the initial cycle. Our findings suggest the key role of the local structure in the voltage decay problem and provide insights for material optimizations towards lithium- and manganese-rich cathodes without the voltage decay.

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.

The lattice oxygen redox (LOR) reaction, known for its potential to augment the capacity of layered transition metal oxides in sodium-ion batteries (SIBs), has attracted widespread attention. However, the utilization of the LOR reaction often leads to irreversible release of lattice oxygen, causing damage to the crystal structure and deterioration in electrochemical performance, thereby impeding its practical application. Here, we demonstrate a method to significantly enhance the electrochemical performance of P3-type Na0.6Li0.2Mn0.8O2 cathodes by concurrently applying surface NaBO2 coatings and boron doping. We reveal that the NaBO2 coating efficiently mitigates electrolyte corrosion and side reactions on the electrode surface. In addition, boron doping enhances the involvement of manganese in the redox process, thereby boosting capacity, while inhibiting the irreversible Li migration, thereby stabilizing the crystal structure of the material. The synergistic effect of surface NaBO2 coating and bulk phase boron doping enables 6%B-NLMO to exhibit improved electrochemical performance, achieving a discharge capacity of 98.7 mA h g−1 and 95.8% retention after 100 cycles at 20 mA g−1. This synergistic strategy, employing multiple modification techniques, offers a potential way to improve the performance of anion redox cathode materials. [ABSTRACT FROM AUTHOR]

Untethered magnetic millirobots with a characteristic length of a few millimeters can be wirelessly controlled. They exhibit promising potential in a wide variety of applications, particularly for tasks in clinic workspaces. However, magnetically controlling these robots is counter‐intuitive and requires a steep learning curve, hindering their wide adoption. Herein, a computer‐aided teleoperation platform is developed to operate a soft millirobot, with its feedback control being conducted behind‐the‐scenes, bridging the user's inputs directly with the millirobot's actions to offer an intuitive control. This system enables untrained users to conveniently control the position and actions of the millirobot inside a human stomach phantom by pointing‐and‐clicking on a real‐time video monitor or using a keyboard. The platform automatically materializes the user's instructions by maneuvering a robotic arm with a tip‐mounted magnet to exert a magnetic field to induce the desired response from the millirobot. Experiments show that the system allows the user to intuitively operate the millirobot and deliver its cargo without splitting their attention to monitor the workspace or to calculate the constantly changing control parameters. This platform can lower the barrier for healthcare practitioners without engineering expertise to adopt miniature robotic systems into their workflow and realize these systems' promising potential. [ABSTRACT FROM AUTHOR]

In this paper, a numerical model was built by ANSYS FLUENT to investigate the heat transfer performances of supercritical water in a circumferential non-uniformly heated vertical tube. The Shear Stress Transport (SST) k-ω model was adopted for describing turbulence. The operating parameters are chosen according to a 660 MW ultra-supercritical CFB boiler. The heat transfer performances under different operating parameters, such as boiler load, flow direction and heat flux distribution are analyzed. The temperature and heat flux on inner wall varies along the circumference and show symmetric distributions. The overall heat transfer performances at each cross section are better than the local heat transfer performance of midpoint of heating side. Flow direction has a great influence on heat transfer performance; it changes the radial distribution of axial velocity and then affects the turbulence distribution. Therefore, upward flow condition shows a better heat transfer performance. Smaller heat flux improves both the overall and local heat transfer performances. Reducing the heat flux area is not conducive to the overall heat transfer, but does not affect the local heat transfer at the midpoint of heating side. Finally, a new correlation is fitted based on the simulated results of supercritical water heat transfer with circumferential non-uniform heat flux distributions. [ABSTRACT FROM AUTHOR]