Rui Tan

Fe-Cluster Pushing Electrons to N-Doped Graphitic Layers with Fe3C(Fe) Hybrid Nanostructure to Enhance O2 Reduction Catalysis of Zn-Air Batteries

Non-noble metal catalysts with catalytic activity toward oxygen reduction reaction (ORR) comparable or even superior to that of Pt/C are extremely important for the wide application of metal-air batteries and fuel cells. Here, we develop a simple and controllable strategy to synthesize Fe-cluster embedded in Fe3C nanoparticles (designated as Fe3C(Fe)) encased in nitrogen-doped graphitic layers (NDGLs) with graphitic shells as a novel hybrid nanostructure as an effective ORR catalyst by directly pyrolyzing a mixture of Prussian blue (PB) and glucose. The pyrolysis temperature was found to be the key parameter for obtaining a stable Fe3C(Fe)@NDGL core-shell nanostructure with an optimized content of nitrogen. The optimized Fe3C(Fe)@NDGL catalyst showed high catalytic performance of ORR comparable to that of the Pt/C (20 wt %) catalyst and better stability than that of the Pt/C catalyst in alkaline electrolyte. According to the experimental results and first principle calculation, the high activity of the Fe3C(Fe)@NDGL catalyst can be ascribed to the synergistic effect of an adequate content of nitrogen doping in graphitic carbon shells and Fe-cluster pushing electrons to NDGL. A zinc-air battery utilizing the Fe3C(Fe)@NDGL catalyst demonstrated a maximum power density of 186 mW cm-2, which is slightly higher than that of a zinc-air battery utilizing the commercial Pt/C catalyst (167 mW cm-2), mostly because of the large surface area of the N-doped graphitic carbon shells. Theoretical calculation verified that O2 molecules can spontaneously adsorb on both pristine and nitrogen doped graphene surfaces and then quickly diffuse to the catalytically active nitrogen sites. Our catalyst can potentially become a promising replacement for Pt catalysts in metal-air batteries and fuel cells.

Fast Diffusion of O2 on Nitrogen-Doped Graphene to Enhance Oxygen Reduction and Its Application for High-Rate Zn–Air Batteries

N-doped graphene (NDG) was investigated for oxygen reduction reaction (ORR) and used as air-electrode catalyst for Zn-air batteries. Electrochemical results revealed a slightly lower kinetic activity but a much larger rate capability for the NDG than commercial 20% Pt/C catalyst. The maximum power density for a Zn-air cell with NDG air cathode reached up to 218 mW cm-2, which is nearly 1.5 times that of its counterpart with the Pt/C (155 mW cm-2). The equivalent diffusion coefficient (DE) of oxygen from electrolyte solution to the reactive sites of NDG was evaluated as about 1.5 times the liquid-phase diffusion coefficient (DL) of oxygen within bulk electrolyte solution. Combined with experiments and ab initio calculations, this seems counterintuitive reverse ORR of NDG versus Pt/C can be rationalized by a spontaneous adsorption and fast solid-state diffusion of O2 on ultralarge graphene surface of NDG to enhance effective ORR on N-doped-catalytic-centers and to achieve high-rate performance for Zn-air batteries.

A Metal–Organic‐Framework‐Based Electrolyte with Nanowetted Interfaces for High‐Energy‐Density Solid‐State Lithium Battery

Solid-state batteries (SSBs) are promising for safer energy storage, but their active loading and energy density have been limited by large interfacial impedance caused by the poor Li+ transport kinetics between the solid-state electrolyte and the electrode materials. To address the interfacial issue and achieve higher energy density, herein, a novel solid-like electrolyte (SLE) based on ionic-liquid-impregnated metal-organic framework nanocrystals (Li-IL@MOF) is reported, which demonstrates excellent electrochemical properties, including a high room-temperature ionic conductivity of 3.0 × 10-4 S cm-1 , an improved Li+ transference number of 0.36, and good compatibilities against both Li metal and active electrodes with low interfacial resistances. The Li-IL@MOF SLE is further integrated into a rechargeable Li|LiFePO4 SSB with an unprecedented active loading of 25 mg cm-2 , and the battery exhibits remarkable performance over a wide temperature range from -20 up to 150 °C. Besides the intrinsically high ionic conductivity of Li-IL@MOF, the unique interfacial contact between the SLE and the active electrodes owing to an interfacial wettability effect of the nanoconfined Li-IL guests, which creates an effective 3D Li+ conductive network throughout the whole battery, is considered to be the key factor for the excellent performance of the SSB.

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.

Effect of sulfur-containing additives on the formation of a solid-electrolyte interphase evaluated by in situ AFM and ex situ characterizations

An effective solid-electrolyte interphase (SEI) layer is the key to the stable operation of lithium-ion batteries by minimizing the loss of lithium and decomposition of the electrolyte. Sulfur-containing additives can facilitate the formation of SEI layers and the effect of two sulfur-containing additives was investigated in this work. The growth of the SEI layer on a highly oriented pyrolytic graphite substrate during a cyclic voltammetry scan was monitored in situ using an atomic force microscope for imaging and an electrochemical workstation for controlling the potential of the electrode simultaneously. Both the ethylene sulfite (ES) additive and prop-1-ene-1,3-sultone (PES) additive can promote the formation of an SEI layer at higher potentials. The ES additive started to form an SEI at a higher voltage than the PES additive and the surface of the SEI layer appeared to be smoother. The effect of the additives was further evaluated in coin cells and the ES additive was found to enable superior stability of the cell in comparison to the PES additive and the neat electrolyte. Examination of the multi-cycled graphite electrode showed that the SEI layer formed with the ES additive was thicker and denser with low resistance for Li-ion conduction, which explains the high performance of the coin cell.

Depolarization effects of Li2FeSiO4 nanocrystals wrapped in different conductive carbon networks as cathodes for high performance lithium-ion batteries

We report composite electrodes with Li2FeSiO4 (LFS) nanocrystals wrapped in three different types of conductive carbon such as Acetylene Black (AB), carbon nanotubes (CNT) and Ketjen Black (KB) to demonstrate depolarization effects on the electrochemical performance of Li-ions batteries. KB with a nanoporous structure and the largest surface area enabled the formation of the best electronic conductive-network with excellent capacity on the interface of LFS nanoparticles, showing reversible electrochemical activity. Compared to the electrodes of LFS wrapped in AB and CNT, the polarization of LFS particles wrapped in KB was reduced significantly due to high conductivity of the electrode, resulting in an increase of about 59.0% in the reversible capacity (269.0 mA h g−1, corresponding to 1.62 Li-storages) and obvious enhancement in the rate performance. By using the electrochemical analysis methods, we demonstrated the insight of discharge of more than one lithium ion at different voltages in the LFS@KB vs. LFS@AB and LFS@CNT electrodes, including interface capacity, Fe3+/Fe2+ and Fe4+/Fe3+ redox, respectively. The fundamental mechanism of enhanced electrochemical performance of LFS by creating a depolarization environment with optimized conductive carbon provides useful guidance to the future design of high performance LFS cathodes for LIBs.

Sn(II,IV) steric and electronic structure effects enable self-selective doping on Fe/Si-sites of Li2FeSiO4 nanocrystals for high performance lithium ion batteries

We report Sn(II) and Sn(IV) self-selective dual-doping, respectively, on Fe and Si sites of Li2FeSiO4 nanocrystals due to the steric and electronic structure effects of Sn(II,IV). Combined with experimental studies and theoretical calculations, we investigate the structure–property relationship of tin doped Li2FeSiO4 as the cathode material for high performance Li-ion batteries, in which the dual-doping enhances the electronic conductivity and lithium-ion diffusion coefficient. The doped sample with 5% Sn(IV) source shows the best electrochemical performance due to the improved electronic conductivity and Li-ion diffusivity. Density functional theory (DFT) calculations also reveal that tin dual-doped Li2FeSiO4 has better electronic conductivity and lower voltage of delithiation than that of the undoped Li2FeSiO4, which is in accordance with our experimental results.