Structural and Thermodynamic Understandings in Mn-Based Sodium Layered Oxides during Anionic Redox.

A breakthrough utilizing an anionic redox reaction (O2-/On-) for charge compensation has led to the development of high-energy cathode materials in sodium-ion batteries. However, its reaction results in a large voltage hysteresis due to the structural degradation arising from an oxygen loss. Herein, an interesting P2-type Mn-based compound exhibits a distinct two-phase behavior preserving a high-potential anionic redox (≈4.2 V vs Na+/Na) even during the subsequent cycling. Through a systematic series of experimental characterizations and theoretical calculations, the anionic redox reaction originating from O 2p-electron and the reversible unmixing of Na-rich and Na-poor phases are confirmed in detail. In light of the combined study, a critical role of the anion-redox-induced two-phase reaction in the positive-negative point of view is demonstrated, suggesting a rational design principle considering the phase separation and lattice mismatch. Furthermore, these results provide an exciting approach for utilizing the high-voltage feature in Mn-based layered cathode materials that are charge-compensated by an anionic redox reaction.

Operando Identification of the Chemical and Structural Origin of Li-Ion Battery Aging at Near-Ambient Temperature.

Integrated with heat-generating devices, a Li-ion battery (LIB) often operates at 20-40 °C higher than the ordinary working temperature. Although macroscopic investigation of the thermal contribution has shown a significant reduction in the LIB performance, the molecular level structural and chemical origin of battery aging in a mild thermal environment has not been elucidated. On the basis of the combined experiments of the electrochemical measurements, Cs-corrected electron microscopy, and in situ analyses, we herein provide operando structural and chemical insights on how a mild thermal environment affects the overall battery performance using anatase TiO2 as a model intercalation compound. Interestingly, a mild thermal condition induces excess lithium intercalation even at near-ambient temperature (45 °C), which does not occur at the ordinary working temperature. The anomalous intercalation enables excess lithium storage in the first few cycles but exerts severe intracrystal stress, consequently cracking the crystal that leads to battery aging. Importantly, this mild thermal effect is accumulated upon cycling, resulting in irreversible capacity loss even after the thermal condition is removed. Battery aging at a high working temperature is universal in nearly all intercalation compounds, and therefore, it is significant to understand how the thermal condition contributes to battery aging for designing intercalation compounds for advanced battery electrode materials.

Solvothermal-Derived S-Doped Graphene as an Anode Material for Sodium-Ion Batteries.

Sodium-ion batteries (SIBs) have attracted enormous attention in recent years due to the high abundance and low cost of sodium. However, in contrast to lithium-ion batteries, conventional graphite is unsuitable for SIB anodes because it is much more difficult to intercolate the larger Na ions into graphite layers. Therefore, it is critical to develop new anode materials for SIBs for practical use. Here, heteroatom-doped graphene with high doping levels and disordered structures is prepared using a simple and economical thermal process. The solvothermal-derived graphene shows excellent performance as an anode material for SIBs. It exhibits a high reversible capacity of 380 mAh g-1 after 300 cycles at 100 mA g-1, excellent rate performance 217 mAh g-1 at 3200 mA g-1, and superior cycling performance at 2.0 A g-1 during 1000 cycles with negligible capacity fade.

Na+/Vacancy Disordered P2-Na0.67Co1-xTixO2: High-Energy and High-Power Cathode Materials for Sodium Ion Batteries.

Although sodium ion batteries (NIBs) have gained wide interest, their poor energy density poses a serious challenge for their practical applications. Therefore, high-energy-density cathode materials are required for NIBs to enable the utilization of a large amount of reversible Na ions. This study presents a P2-type Na0.67Co1-xTixO2 (x < 0.2) cathode with an extended potential range higher than 4.4 V to present a high specific capacity of 166 mAh g-1. A group of P2-type cathodes containing various amounts of Ti is prepared using a facile synthetic method. These cathodes show different behaviors of the Na+/vacancy ordering. Na0.67CoO2 suffers severe capacity loss at high voltages due to irreversible structure changes causing serious polarization, while the Ti-substituted cathodes have long credible cycleability as well as high energy. In particular, Na0.67Co0.90Ti0.10O2 exhibits excellent capacity retention (115 mAh g-1) even after 100 cycles, whereas Na0.67CoO2 exhibits negligible capacity retention (<10 mAh g-1) at 4.5 V cutoff conditions. Na0.67Co0.90Ti0.10O2 also exhibits outstanding rate capabilities of 108 mAh g-1 at a current density of 1000 mA g-1 (7.4 C). Increased sodium diffusion kinetics from mitigated Na+/vacancy ordering, which allows high Na+ utilization, are investigated to find in detail the mechanism of the improvement by combining systematic analyses comprising TEM, in situ XRD, and electrochemical methods.