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.

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 NaCoTiO (x < 0.2) cathode with an extended potential range higher than 4.4 V to present a high specific capacity of 166 mAh g. 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. NaCoO 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, NaCoTiO exhibits excellent capacity retention (115 mAh g) even after 100 cycles, whereas NaCoO exhibits negligible capacity retention (<10 mAh g) at 4.5 V cutoff conditions. NaCoTiO also exhibits outstanding rate capabilities of 108 mAh g at a current density of 1000 mA g (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.

A breakthrough utilizing an anionic redox reaction (O 2− /O n− ) 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.

Unlike cathodes for lithium-ion batteries, oxygen redox (OR) processes at a high voltage (�4.2 V) during the first charge in sodium-ion batteries (SIBs) employ some Li-incorporated Mn oxides that is recovered during subsequent discharge. To determine the intrinsic origin, P2-type Na 0.6 [Li 0.2 Mn 0.8 ]O 2 exhibiting a reversible OR-induced two-phase reaction was investigated using experiments and first-principle calculations. First, operando X-ray diffraction results in reversible P2-Z phase transformations and thermodynamic analysis show the twophase reaction features Li migration into the tetrahedral sites from the transition-metal layer in the latter phase. Second, Liinduced decoupling of the oxygen 2p-electron led to selective anion redox activity depending on the oxygen sites that are Li-rich (redox-active) and Mn-rich (redox-inactive) environments. Third, redox-active oxygen coordinated to the Li vacancy predominantly participates in the formation of peroxo-like dimers with distortion of the MnO 6 octahedron, as observed in the reversible extended X-ray absorption fine structure spectra during the OR reaction. Considering three physicochemical perspectives, we reveal that Li ions play a role in activating OR reactions and control OR participation in the charge-compensation process. Our findings suggest that the Li/Mn ratio is a critical factor for achieving a reversible OR reaction, and broaden the possibilities of exploiting OR to reach high-energy densities in next-generation SIBs.

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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.

Sodium-ion batteries (NIBs) are promising alternatives to lithium-ion batteries for large-scale energy applications such as energy storage systems, owing to the earth-abundance and low cost of sodium resources. Among layered oxide cathode materials for NIBs, O3-type NaCrO 2 has attracted considerable attention owing to its electrochemically active Cr 3+/4+ , which is unlike that of LiCrO 2 , and potential for carbon coating with a high thermal stability. In this study, we propose a new facile and eco-friendly method for applying nitrogen-doped carbon to NaCrO 2 using coffee waste as a carbon source. The synthesized O3-type NaCrO 2 /coffee waste-derived N-doped carbon composite exhibits an outstanding electrochemical performance as an NIB cathode material. The sodium/composite cell achieved a 73.7% capacity retention after 500 charge/discharge cycles and an approximate 50% discharge capacity during 43 s of charge. The results demonstrate the potential use of coffee waste for battery materials with improved electrochemical performances.

Sodium-ion batteries (NIBs) for large power sources having kWh or MWh scale of energy are highly interesting alternatives for lithium-ion batteries because of abundant sodium resources in the earth and high chemical similarity between Li and Na ions. However, due to the high atomic weight of Na+ and the low working potential of NIBs, the energy and power densities of NIBs are also required to be improved for wide applications. Despite the similar chemical behavior of Na ions in the rechargeable batteries, the high performance of the active material in NIBs is an essential research- topic for the realization of NIBs. Especially, Na ion having a larger diameter than Li-ion in the P2 type layered structure has a distinct Na+/vacancy ordering behavior presenting multiple voltage plateaus during sodium ion insertion and extraction in the electrochemical cycling. This different behavior from lithium-ion batteries makes the sodium ion diffusion complex in the solid matrix due to the pass-through of various stable Na+/vacancy arrangements. This presentation covers an improvement in the electrochemical performance of the cathode material, Na0.67Co1-xTixO2 having P2 type crystalline structure by using various amounts of Ti doping to control Na/vacancy ordering for mitigating the Na ion diffusion in the solid matrix as well as for extending the de-sodiation range to 4.4 & 4.5 V vs. Na/Na+. The proposed material having Ti doping presents a highly relieved an electrochemical Na+/vacancy ordering behavior from the that of Na0.67Co1-xO2 having a severe capacity loss at high potential because of irreversible phase transitions. In particular, 10 % Ti-doping in the Co content highly advances the cycleability by showing 115 mAh g-1 at the 100th cycle, as well as the rate capability by reaching 108 mAh g-1 at a current density of 1000 mA g-1. The detail diffusion kinetics and phase behaviors from the prepared materials are investigated by a systematic analysis combination comprising TEM, in-situ XRD, and electrochemical methods.