Developing high-energy-density electrodes for lithium ion batteries (LIBs) is of primary importance to meet the challenges in electronics and automobile industries in the near future. Conversion reaction-based transition metal oxides are attractive candidates for LIB anodes because of their high theoretical capacities. This review summarizes recent advances on the development of nanostructured transition metal oxides for use in lithium ion battery anodes based on conversion reactions. The oxide materials covered in this review include oxides of iron, manganese, cobalt, copper, nickel, molybdenum, zinc, ruthenium, chromium, and tungsten, and mixed metal oxides. Various kinds of nanostructured materials including nanowires, nanosheets, hollow structures, porous structures, and oxide/carbon nanocomposites are discussed in terms of their LIB anode applications.

The intercalation of lithium ions into graphite electrode is the key underlying mechanism of modern lithium-ion batteries. However, co-intercalation of lithium-ions and solvent into graphite is considered undesirable because it can trigger the exfoliation of graphene layers and destroy the graphite crystal, resulting in poor cycle life. Here, it is demonstrated that the [lithium–solvent]+ intercalation does not necessarily cause exfoliation of the graphite electrode and can be remarkably reversible with appropriate solvent selection. First-principles calculations suggest that the chemical compatibility of the graphite host and [lithium–solvent]+ complex ion strongly affects the reversibility of the co-intercalation, and comparative experiments confirm this phenomenon. Moreover, it is revealed that [lithium–ether]+ co-intercalation of natural graphite electrode enables much higher power capability than normal lithium intercalation, without the risk of lithium metal plating, with retention of ≈87% of the theoretical capacity at current density of 1 A g−1. This unusual high rate capability of the co-intercalation is attributed to the (i) absence of the desolvation step, (ii) negligible formation of the solid–electrolyte interphase on graphite surface, and (iii) fast charge-transfer kinetics. This work constitutes the first step toward the utilization of fast and reversible [lithium–solvent]+ complex ion intercalation chemistry in graphite for rechargeable battery technology.

The effect of porous structures on the electrocatalytic activity of N-doped carbon is studied by using electrochemical analysis techniques and the result is applied to synthesize highly active and stable Fe-N-C catalyst for oxygen reduction reaction (ORR). We developed synthetic procedures to prepare three types of N-doped carbon model catalysts that are designed for systematic comparison of the porous structures. The difference in their catalytic activity is investigated in relation to the surface area and the electrochemical parameters. We found that macro- and mesoporous structures contribute to different stages of the reaction kinetics. The catalytic activity is further enhanced by loading the optimized amount of Fe to prepare Fe-N-C catalyst. In both N-doped carbon and Fe-N-C catalysts, the hierarchical porous structure improved electrocatalytic performance in acidic and alkaline media. The optimized catalyst exhibits one of the best ORR performance in alkaline medium with excellent long-term stability in anion exchange membrane fuel cell and accelerated durability test. Our study establishes a basis for rationale design of the porous carbon structure for electrocatalytic applications.

The impact of topological defects associated with grain boundaries (GB defects) on the electrical, optical, magnetic, mechanical and chemical properties of nanocrystalline materials^1,2 is well known. However, elucidating this influence experimentally is difficult because grains typically exhibit a large range of sizes, shapes and random relative orientations^3-5. Here we demonstrate that precise control of the heteroepitaxy of colloidal polyhedral nanocrystals enables ordered grain growth and can thereby produce material samples with uniform GB defects. We illustrate our approach with a multigrain nanocrystal comprising a Co₃O₄ nanocube core that carries a Mn₃O₄ shell on each facet. The individual shells are symmetry-related interconnected grains⁶, and the large geometric misfit between adjacent tetragonal Mn₃O₄ grains results in tilt boundaries at the sharp edges of the Co₃O₄ nanocube core that join via disclinations. We identify four design principles that govern the production of these highly ordered multigrain nanostructures. First, the shape of the substrate nanocrystal must guide the crystallographic orientation of the overgrowth phase⁷. Second, the size of the substrate must be smaller than the characteristic distance between the dislocations. Third, the incompatible symmetry between the overgrowth phase and the substrate increases the geometric misfit strain between the grains. Fourth, for GB formation under near-equilibrium conditions, the surface energy of the shell needs to be balanced by the increasing elastic energy through ligand passivation^8-10. With these principles, we can produce a range of multigrain nanocrystals containing distinct GB defects.