high energy and power density, long cycle life, low cost, and environmental benignity. [3] Due to the high capacity of the currently used graphite anode −372 mAh g-1 , [1b,4] commercial cathodes have become the bottleneck for improving the energy density. [5] In addition, cathode materials take up about 40% of the total material cost of typical LIB cells. It is therefore crucial to develop cathode materials with high energy density and low cost, while maintaining superior safety features. [6] Driven by the wide deployment of electric vehicles in recent years, the demand for safer cathode materials with higher capacity and lower cost has become imperative. [2b] The specific capacity depends on the inherent physical properties of the cathode materials. Commercial materials such as LiCoO 2 , Li(Ni x Mn y Co z)O 2 , Li(Ni x Co y Al z)O 2 , LiMn 2 O 4 and LiFePO 4 all possess discharge capacities below 200 mAh g-1. [7] Ni-rich NCM, NCA, and Li-rich oxides are all prospective cathodes for high-energy LIBs as they can exhibit high specific discharge capacities above 200 mAh g-1. [6c] As compared with Ni-rich NCM and NCA, Li-rich Mn-based layered oxide (LMLO) cathode materials are cheaper and have higher specific capacity. They can deliver an initial specific discharge capacity that approaches 300 mAh g-1 , nearly doubling the capacity of commercially used cathodes and close to the limit for lithiated transition metal oxides. [8] Figure 1 lists the main development milestones of LMLO. In 1997 a novel material, LiCoO 2-Li 2 MnO 3 , was found by Numata et al., [9] a discovery that initiated intensive work on high energy density cathode materials. This group studied these materials intensively and demonstrated their cycling stability in the range of 3.0-4.3 V versus Li. [10] In 1999, Kalyani et al. reported that Li 2 MnO 3 can undergo electrochemical activation at potentials >4.5 V versus Li. [11] Dahn et al. described the charge compensation mechanism of LMLO, [12] and these cathodes were shown to provide high capacity at high operational voltage (>4.5 V). [13] In 2004, Thackeray et al. explored xLi 2 M′O 3-(1-x)LiMn 0.5 Ni 0.5 O 2 cathode materials with M′ = Zn, Ti, or Mn, concluding that Li 2 M′O 3 and LiMn 0.5 Ni 0.5 O 2 in these composite materials were integrated by short-range interactions. [14] The following Rechargeable lithium-ion batteries have become the dominant power sources for portable electronic devices, and are regarded as the battery technology of choice for electric vehicles and as potential candidates for grid-scale storage. Commercial lithium-ion batteries, after three decades of cell engineering, are approaching their energy density limits. Toward continually improving the energy density and reducing cost, Li-rich Mn-based layered oxide (LMLO) cathodes are receiving more and more attention due to their high discharge capacity and low cost. However, commercialization has been hampered by severe capacity and voltage decay, sluggish rate capability, and poor safety performance during charge/discharge cyc...

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Single-crystalline cathodes are the most promising candidates for highenergy-density lithium-ion batteries (LIBs). Compared to their polycrystalline counterparts, single-crystalline cathodes have advantages over liquid-electrolyte-based LIBs in terms of cycle life, structural stability, thermal stability, safety, and storage but also have a potential application in solid-state LIBs. In this review, the development history and recent progress of single-crystalline cathodes are reviewed, focusing on properties, synthesis, challenges, solutions, and characterization. Synthesis of single-crystalline cathodes usually involves preparing precursors and subsequent calcination, which are summarized in the details. In the following sections, the development issues of single-crystalline cathodes, including kinetic limitations, interfacial side reactions, safety issues, reversible planar gliding and micro-cracking, and particle size distribution and agglomeration, are systematically analyzed, followed by current solutions and characterization techniques. Finally, this review is concluded with proposed research thrusts for the future development of singlecrystalline cathodes.

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Low-temperature anion exchange membrane direct ammonia fuel cells (AEM-DAFCs) have emerged as a potential power source for transportation applications with the recognition that liquid ammonia is a carbon-free hydrogen carrier and facilitates storage, refill, and distribution. However, ammonia crossover from the cell anode to cathode can decrease the fuel efficiency, drop the voltage, and poison the cathode catalysts. In this work, the Mn−Co spinel on three different carbon supports [BP2000, Vulcan XC-72R, and multiwalled carbon nanotubes (MWCNTs)] has been successfully synthesized and demonstrated a high oxygen reduction reaction (ORR) activity with good ammonia tolerance. The structure and composition of the obtained Mn−Co−C catalysts were characterized by high-angle annular dark-field scanning transmission electron microscopy, X-ray diffraction, energy-dispersive X-ray spectroscopy, and X-ray photoelectron spectroscopy. All three catalysts exhibit superb ammonia tolerance, and Mn−Co−BP2000 demonstrates the highest ORR activity, even better than the commercial Pt−C in the presence of ammonia. When paired with the commercial PtIr−C anode, the Mn−Co−BP2000 cathode improved the peak power density of single cells from 100.1 mW cm −2 for the Pt−C cathode to 128.2 mW cm −2 under a 2 bar backpressure in both electrodes at 80 °C. All the results have manifested that Mn−Co−BP2000 is a good cathode catalyst for low-temperature AEM-DAFCs.