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NaCrO2 is a promising cathode for Na-ion batteries. However, further studies of the mechanisms controlling its specific capacities and cycle stability are needed for real-world applications in the future. This study reveals, for the first time, that the typical specific capacity of ~110 mAh/g reported by many researchers when the charge/discharge voltage window is set between 2.0 and 3.6 V vs. Na/Na+ is actually controlled by the low electronic conductivity at the electrode/electrolyte interface. Through wet solution mixing of NaCrO2 particles with carbon precursors, uniform carbon coating can be formed on the surface of NaCrO2 particles, leading to unprecedented specific capacities at 140 mAh/g, which is the highest specific capacity ever reported in the literature with the lower and upper cutoff voltages at the aforementioned values. However, such carbon-coated NaCrO2 with ultrahigh specific capacity does not improve cycle stability because with the specific capacity at 140 mAh/g the Na deintercalation during charge is more than 50% Na ions per formula unit of NaCrO2 which leads to irreversible redox reactions. The insights from this study provide a future direction to enhance the long-term cycle stability of NaCrO2 through integrating carbon coating and doping. 

Applications of silicon as a high-performance anode material has been impeded by its low intrinsic conductivity and huge volume expansion (> 300%) during lithiation. To address these problems, nano-Si particles along with conductive coatings and engineered voids are often employed, but this results in high cost anodes. Here, we report a scalable synthesis method that can realize high specific capacity (~800 mAh g-1), ultrafast charge/discharge (at 8 A g-1 Si) and high initial Coulombic efficiency (~90%) with long cycle life (1000 cycles) at the same time. To achieve 1000 cycle stability, micron-sized Si particles are subjected to high-energy ball milling to create nanostructured Si building blocks with nano-channel shaped voids encapsulated inside a nitrogen (N)-doped carbon shell (termed as Si micro-reactor). The nano-channel voids inside a Si micro-reactor not only offer the space to accommodate the volume expansion of Si, but also provide fast pathways for Li ion diffusion into the center of the nanostructured Si core and thus ultrafast charge/discharge capability. The porous N-doped carbon shell helps to improve the conductivity while allowing fast Li ion transport and confining the volume expansion within the Si micro-reactor. Submicron-sized Si micro-reactors with limited specific surface area (35 m2 g-1) afford sufficient electrode/electrolyte interfacial area for fast lithiation/delithiation, leading to the specific capacity ranging from ~800 to 420 mAh g-1 under ultrafast charging conditions (8 A g-1), but not too much interfacial area for surface side reactions and thus high initial coulombic efficiency (~90%). Since Si micro-reactors with superior electrochemical properties are synthesized via an industrially scalable and eco-friendly method, they have the potential for practical applications in the future. 

In this study, we report the first investigation of the effectiveness of the partially converted carbon coating from polydopamine (PODA) to improve the cycle stability of Si anode for Li-ion batteries. It is hypothesized that by converting PODA to a partial carbonization condition, the resulting coating could have a higher electrical conductivity than PODA without carbonization, and at the same time may still contain some organic bonds and thus mechanical flexibility to accommodate the volume expansion of Si during lithiation. The results show that such a partial carbonization state can be obtained by carbonization of PODA at 400 °C. Furthermore, the partially converted carbon coating can offer sufficient electrical conductivity for lithiation and delithiation of Si anode while drastically reducing the charge transfer resistance for the redox reactions. In addition, the partially-converted‑carbon coated hollow Si nanospheres exhibit excellent cycle stability when the volume expansion of Si anode is not very large (~88%) even though this volume expansion is significantly larger than the engineered void space (47%of the Si volume) available inside the partially-converted‑carbon coated hollow Si nanospheres, unambiguously confirming the good tolerance of the partially converted carbon coating in withstanding some tensile strain without fracture. This study offers a new direction for systematic studies in the future as a means to provide a coating on Si material with sufficient electrical conductivity along with capability to withstand some tensile strain during the volume expansion of Si, thereby improving the cycle stability of Si anodes.