Temperature Effect on Co-Based Catalysts in Oxygen Evolution Reaction.

Oxygen evolution reaction (OER), as the critical step in splitting water, is a thermodynamically "up-hill" process and requires highly efficient catalysts to run. Arrhenius' law suggests that the higher temperature, the faster the reaction rate, so that a larger OER current density can be achieved at a lower η. Herein, we report an abnormal temperature effect on the performance of Co-based catalysts, e.g., Co3O4, Li2CoSiO4, and Fe-doped Co(OH) x, in OER in alkaline electrolytes. The OER performance reached a maximum when the temperature increased to 65 °C, and the OER performance declined when the temperature became higher. The mechanism was investigated by using Co3O4 as a model sample, and we propose that at an optimal temperature (around 55-65 °C) the main rate-determining step changes from OH- adsorption dominant to a mixed mode and both the adsorption and the cleavage of the OH group can be rate-determining, which leads to the fastest kinetics.

Tuning Li-Ion Diffusion in α-LiMn1-xFexPO4 Nanocrystals by Antisite Defects and Embedded ß-Phase for Advanced Li-Ion Batteries.

Olivine-structured LiMn1-xFexPO4 has become a promising candidate for cathode materials owing to its higher working voltage of 4.1 V and thus larger energy density than that of LiFePO4, which has been used for electric vehicles batteries with the advantage of high safety but disadvantage of low energy density due to its lower working voltage of 3.4 V. One drawback of LiMn1-xFexPO4 electrode is its relatively low electronic and Li-ionic conductivity with Li-ion one-dimensional diffusion. Herein, olivine-structured α-LiMn0.5Fe0.5PO4 nanocrystals were synthesized with optimized Li-ion diffusion channels in LiMn1-xFexPO4 nanocrystals by inducing high concentrations of Fe2+-Li+ antisite defects, which showed impressive capacity improvements of approaching 162, 127, 73, and 55 mAh g-1 at 0.1, 10, 50, and 100 C, respectively, and a long-term cycling stability of maintaining about 74% capacity after 1000 cycles at 10 C. By using high-resolution transmission electron microscopy imaging and joint refinement of hard X-ray and neutron powder diffraction patterns, we revealed that the extraordinary high-rate performance could be achieved by suppressing the formation of electrochemically inactive phase (ß-LiMn1-xFexPO4, which is first reported in this work) embedded in α-LiMn0.5Fe0.5PO4. Because of the coherent orientation relationship between ß- and α-phases, the ß-phase embedded would impede the Li+ diffusion along the [100] and/or [001] directions that was activated by the high density of Fe2+-Li+ antisite (4.24%) in α-phase. Thus, by optimizing concentrations of Fe2+-Li+ antisite defects and suppressing ß-phase-embedded olivine structure, Li-ion diffusion properties in LiMn1-xFexPO4 nanocrystals can be tuned by generating new Li+ tunneling. These findings may provide insights into the design and generation of other advanced electrode materials with improved rate performance.

Excess Li-Ion Storage on Reconstructed Surfaces of Nanocrystals To Boost Battery Performance.

Because of their enhanced kinetic properties, nanocrystallites have received much attention as potential electrode materials for energy storage. However, because of the large specific surface areas of nanocrystallites, they usually suffer from decreased energy density, cycling stability, and effective electrode capacity. In this work, we report a size-dependent excess capacity beyond theoretical value (170 mA h g-1) by introducing extra lithium storage at the reconstructed surface in nanosized LiFePO4 (LFP) cathode materials (186 and 207 mA h g-1 in samples with mean particle sizes of 83 and 42 nm, respectively). Moreover, this LFP composite also shows excellent cycling stability and high rate performance. Our multimodal experimental characterizations and ab initio calculations reveal that the surface extra lithium storage is mainly attributed to the charge passivation of Fe by the surface C-O-Fe bonds, which can enhance binding energy for surface lithium by compensating surface Fe truncated symmetry to create two types of extra positions for Li-ion storage at the reconstructed surfaces. Such surface reconstruction nanotechnology for excess Li-ion storage makes full use of the large specific surface area of the nanocrystallites, which can maintain the fast Li-ion transport and greatly enhance the capacity. This discovery and nanotechnology can be used for the design of high-capacity and efficient lithium ion batteries.

The synergistic effect achieved by combining different nitrogen-doped carbon shells for high performance capacitance.

Ellipsoid nitrogen-doped hollow carbon shells with different nitrogen contents and electrical conductivities were prepared using a simple calcination method by regulating the calcination temperature. Although a high nitrogen content promotes pseudocapacitance, it reduces the electrical conductivity of carbon, causing loss of capacitance. The best rate performance was achieved by a mixture of two types of ellipsoid nitrogen-doped hollow carbon shells, in which one contains a higher level of nitrogen with lower conductivity and higher pseudocapacitance, while the other contains a relatively lower level of nitrogen with higher conductivity. The enhanced performance can be explained by the synergistic effect of one component providing high pseudocapacitance and the other component serving as a highly electrically conductive network, which leads to activation of "nitrogen" to enhance pseudocapacitance performance. The mixed material showed a specific capacitance of 156.9 F g-1 at a high current density of 10 A g-1, with no degradation after 10 000 cycles.

Self-assembled N-graphene nanohollows enabling ultrahigh energy density cathode for Li-S batteries.

Functional porous carbon materials are widely used to solve the low conductivity and shuttle effect of Li-S batteries; however, the common carbon/sulfur composite electrodes based on traditional technology (with conducting agents and binders) make it difficult for a battery to work stably at an ultra-high sulfur loading of 10 mg cm-2. Herein, an appropriate content of sulfur was injected into a pomegranate-like structure self-assembled with nanohollows (PSSN) of N-graphene. The Li-PSSN/S battery based on traditional technology displays a large-capacity, high-rate and long-life at an ultra-high areal-sulfur loading of 10.1 mg cm-2. The excellent performance with ultra-high areal-sulfur loading can be attributed to the hierarchal nanohollows with graphene-shells being in close contact to build a 3D-electronic conduction network and promoting electrolyte adsorption into the entire electrode to maintain rapid Li-ion transport, while stopping the shuttle-effect via the strong interaction of polysulfide with the doped N elements on graphene-shells. In addition, the exact sulfur content can provide just enough space to maintain the huge volume change and constant thickness of the S-electrodes during the charge-discharge process to enhance the cycling stability.