Nitrogen Doped/Carbon Tuning Yolk‐Like TiO2 and Its Remarkable Impact on Sodium Storage Performances

Yolk‐like TiO₂ are prepared through an asymmetric Ostwald ripening, which is simultaneously doped by nitrogen and wrapped by carbon from core to shell. It presents a high specific surface area (144.9 m² g^?1), well‐defined yolk‐like structure (600–700 nm), covered with interweaved nanosheets (3–5 nm) and tailored porosity (5–10 nm) configuration. When first utilized as anode material for sodium‐ion batteries (SIBs), it delivers a high reversible specific capacity of 242.7 mA h g^?1 at 0.5 C and maintains a considerable capacity of 115.9 mA h g^?1 especially at rate 20 C. Moreover, the reversible capacity can still reach 200.7 mA h g^?1 after 550 cycles with full capacity retention at 1 C. Even cycled at extremely high rate 25 C, the capacity retention of 95.5% after 3000 cycles is acquired. Notably, an ultrahigh initial coulombic efficiency of 59.1% is achieved. The incorporation of nitrogen with narrowing the band gap accompanied with carbon uniformly coating from core to shell make the NC TiO₂‐Y favor a bulk type conductor, resulting in fast electron transfer, which is beneficial to long‐term cycling stability and remarkable rate capability. It is of great significance to improve the energy‐storage properties through development of the bulk type conductor as anode materials in SIBs.     

Extraordinary Performance of Carbon‐Coated Anatase TiO2 as Sodium‐Ion Anode

The synthesis of in situ polymer‐functionalized anatase TiO₂ particles using an anchoring block copolymer with hydroxamate as coordinating species is reported, which yields nanoparticles (≈11 nm) in multigram scale. Thermal annealing converts the polymer brushes into a uniform and homogeneous carbon coating as proven by high resolution transmission electron microscopy and Raman spectroscopy. The strong impact of particle size as well as carbon coating on the electrochemical performance of anatase TiO₂ is demonstrated. Downsizing the particles leads to higher reversible uptake/release of sodium cations per formula unit TiO₂ (e.g., 0.72 eq. Na⁺ (11 nm) vs only 0.56 eq. Na⁺ (40 nm)) while the carbon coating improves rate performance. The combination of small particle size and homogeneous carbon coating allows for the excellent electrochemical performance of anatase TiO₂ at high (134 mAh g^?1 at 10 C (3.35 A g^?1)) and low (≈227 mAh g^?1 at 0.1 C) current rates, high cycling stability (full capacity retention between 2nd and 300th cycle at 1 C) and improved coulombic efficiency (≈99.8%).     

Transition Metal Oxide Anodes for Electrochemical Energy Storage in Lithium‐ and Sodium‐Ion Batteries

Lithium‐ion batteries (LIBs) with outstanding energy and power density have been extensively investigated in recent years, rendering them the most suitable energy storage technology for application in emerging markets such as electric vehicles and stationary storage. More recently, sodium, one of the most abundant elements on earth, exhibiting similar physicochemical properties as lithium, has been gaining increasing attention for the development of sodium‐ion batteries (SIBs) in order to address the concern about Li availability and cost—especially with regard to stationary applications for which size and volume of the battery are of less importance. Compared with traditional intercalation reactions, conversion reaction‐based transition metal oxides (TMOs) are prospective anode materials for rechargeable batteries thanks to their low cost and high gravimetric specific capacities. In this review, the recent progress and remaining challenges of conversion reactions for LIBs and SIBs are discussed, covering an overview about the different synthesis methods, morphological characteristics, as well as their electrochemical performance. Potential future research directions and a perspective toward the practical application of TMOs for electrochemical energy storage are also provided.     

Sodium‐Ion Batteries Paving the Way for Grid Energy Storage

The recent proliferation of renewable energy generation offers mankind hope, with regard to combatting global climate change. However, reaping the full benefits of these renewable energy sources requires the ability to store and distribute any renewable energy generated in a cost‐effective, safe, and sustainable manner. As such, sodium‐ion batteries (NIBs) have been touted as an attractive storage technology due to their elemental abundance, promising electrochemical performance and environmentally benign nature. Moreover, new developments in sodium battery materials have enabled the adoption of high‐voltage and high‐capacity cathodes free of rare earth elements such as Li, Co, Ni, offering pathways for low‐cost NIBs that match their lithium counterparts in energy density while serving the needs for large‐scale grid energy storage. In this essay, a range of battery chemistries are discussed alongside their respective battery properties while keeping metrics for grid storage in mind. Matters regarding materials and full cell cost, supply chain and environmental sustainability are discussed, with emphasis on the need to eliminate several elements (Li, Ni, Co) from NIBs. Future directions for research are also discussed, along with potential strategies to overcome obstacles in battery safety and sustainable recyclability.     

Anodes and Sodium‐Free Cathodes in Sodium Ion Batteries

The increase in electricity generation poses growing demands on energy storage systems, thus offering a chance for the success of the reliable and cost‐effective energy storage technologies. Sodium ion batteries are emerging as such a technology, which is however not yet mature enough to enter the market. At the crux of building practical sodium ion batteries is the development of electrode materials that promise sufficient cost‐ and performance‐competitiveness. As such, herein, all typical sodium storage materials are discussed, considering their fabrication methods and sodiation mechanisms in detail. A comprehensive cross‐literature and cross‐material comparison, which also includes the related thermodynamic analysis of their sodiation products, is also provided. The review focusses particularly on anodes and sodium‐free cathodes, as they both play the role of the acceptor rather than the donor of sodium ions in their operation in batteries; their difference lies in the (de‐)sodiation voltage. In the discussion, special attention is paid to contradictory observations and interpretations in contemporary sodium ion battery research, since debates on these controversies are likely to fuel future sodium battery research.     

Chemical Synthesis of 3D Graphene‐Like Cages for Sodium‐Ion Batteries Applications

Sodium (Na) super ion conductor structured Na₃V₂(PO₄)₃ (NVP) is extensively explored as cathode material for sodium‐ion batteries (SIBs) due to its large interstitial channels for Na⁺ migration. The synthesis of 3D graphene‐like structure coated on NVP nanoflakes arrays via a one‐pot, solid‐state reaction in molten hydrocarbon is reported. The NVP nanoflakes are uniformly coated by the in situ generated 3D graphene‐like layers with the thickness of 3 nm. As a cathode material, graphene covered NVP nanoflakes exhibit excellent electrochemical performances, including close to theoretical reversible capacity (115.2 mA h g^?1 at 1 C), superior rate capability (75.9 mA h g^?1 at 200 C), and excellent cyclic stability (62.5% of capacity retention over 30000 cycles at 50 C). Furthermore, the 3D graphene‐like cages after removing NVP also serve as a good anode material and deliver a specific capacity of 242.5 mA h g^?1 at 0.1 A g^?1. The full SIB using these two cathode and anode materials delivers a high specific capacity (109.2 mA h g^?1 at 0.1 A g^?1) and good cycling stability (77.1% capacity retention over 200 cycles at 0.1 A g^?1).