Haibiao Chen

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.

Co3O4−δ Quantum Dots As a Highly Efficient Oxygen Evolution Reaction Catalyst for Water Splitting

Co3O4-δ quantum dots (Co3O4-δ-QDs) with a crystallite size of approximately 2 nm and oxygen vacancies were fabricated through multicycle lithiation/delithiation of mesoporous Co3O4 nanosheets. Used as an oxygen evolution reaction (OER) electrocatalyst for water splitting, the catalytic performance (an overpotential of 270 mV@10 mA cm-2 and no decay within 30 h) of Co3O4-δ-QDs is superior to that of previously reported Co-based catalysts and the state-of-the-art IrO2. Compared to that of the Co3O4 nanosheets, the enhanced OER activity of Co3O4-δ-QDs is attributed to two factors: one is the increased quantity of the Faradaic active sites, including the total active sites (q*Total), the most accessible active sites (q*Outer), and their ratio (q*Outer/q*Total); the other is the enhanced intrinsic electroactivity per active site evaluated by the turnover frequency and the current density normalized by the most accessible active sites (j/q*Outer) related to the OER. This multicycle lithiation/delithiation method can be applied to other transition metal oxides as well, offering a general approach to develop high-performance electrocatalysts for water splitting.

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.

Novel hybrid Si nanocrystals embedded in a conductive SiOx@C matrix from one single precursor as a high performance anode material for lithium-ion batteries

Silicon (Si) is a promising anode material for lithium-ion batteries (LIBs) owing to its very high lithium storage capacity; however, fragmentation of Si caused by drastic volume change during lithium insertion and extraction leads to serious capacity decay during cycling. In this work, we report a novel method to synthesize an in situ nanocomposite containing Si nanoparticles evenly embedded in an electrically conductive SiOx@C network (Si/SiOx@C) from one single polysiloxane precursor. In our process, Si nanocrystals were reduced from the polysiloxane precursor using a low temperature molten salt reduction method, and carbon segregated out in the SiOx@C phase which evolved during the subsequent pyrolysis. As an anode material for LIBs, the Si/SiOx@C nanocomposite showed a specific capacity up to 1292 mA h g−1 at a current density of 0.4 A g−1 and 81.84% capacity retention after 200 cycles. The high capacity and stable performance of Si/SiOx@C as an anode material can be attributed to the continuous SiOx@C matrix which provides reliable mechanical support, electronic and ionic conductivity, and a stable solid–electrolyte interphase (SEI). This work demonstrated the viability of deriving a homogenous nanocomposite from a single polymeric precursor, which creates a promising anode material for next-generation LIBs.

In situ atomic force microscope observing the effect of vinylene carbonate on the formation of solid-electrolyte interphase layer during the initial cycle

We used in situ atomic force microscope to observe the evolution of the solid-electrolyte interphase (SEI) layer on the graphite surface during the initial lithium intercalation process. We found that 1% vinylene carbonate (VC) in the electrolyte can promote the formation of an initial SEI at a higher potential by VC reduction. VC also restrained the reduction of ethylene carbonate (EC) and as a consequence, it can affect the morphology of the SEI formed.

Revealing the nanodomain structure of silicon oxycarbide via preferential etching and pore analysis

Preferentially etching either carbon or silica from silicon oxycarbide (SiOC) created a porous network as an inverse image of the removed phase. The porous structure was analyzed by gas adsorption, and the experimental results verified the nanodomain structure of SiOC. This work demonstrated a novel approach for analyzing materials containing nanocomposite structures.

In situ probing of interfacial kinetics for studying the electrochemical properties of active nano/micro-particles and the state of Li-ion batteries

It is critical to monitor the state of health (SOH) of the Li-ion batteries to ensure a safe operation and to extend the service life of the batteries in electric vehicles. In this work, we demonstrated that the equivalent capacitance (Cp) and resistance (Rp) of the electrode interface derived using a first-order RC equivalent circuit under a large galvanostatic pulse (LGPM) condition can be correlated with SOH. For both the cathode and the anode, the interfacial kinetics of Li-ions were analyzed to study the electrochemical properties of active particles. The RC parameters of the equivalent circuit were correlated with the diffusion kinetics of Li-ions near the interface between the electrolyte and the active nano/micro-particles during fast charging/discharging. For fresh LiFePO4 (LFP)/Li half-cells, the values and the change of Cp and Rp were explained using the hypothesis of interparticle ion transport under a non-equilibrium condition. For graphite/Li half-cells, the buffering of Li-ions by the solid-electrolyte interphase (SEI) layer was speculated to affect Cp and Rp under a non-equilibrium condition. In commercial LFP/graphite batteries, the Cp values of unhealthy batteries were found to be higher than those of healthy batteries. In further tests, the Cp values of the half cells with the graphite anode recovered from the unhealthy batteries were found to be higher than those of the half cells with graphite from the healthy batteries. The half cells with LFP from the unhealthy batteries behaved similarly to those with LFP from the healthy batteries. With additional analysis on the microstructure, we proposed that the deterioration of the LFP/graphite batteries was mostly due to the formation of a thicker SEI on the graphite anode. The method developed in this work can be integrated in EVs at a low calculation cost. More importantly, we gained a better understanding of the interfacial kinetics of Li-ions during a non-equilibrium process.

Self-Assembly of Antisite Defectless nano-LiFePO4@C/Reduced Graphene Oxide Microspheres for High-Performance Lithium-Ion Batteries

LiFePO4@C/rGO hierarchical microspheres with superior electrochemical activity and high tap density were first synthesized using a Fe-based single inorganic precursor (LiFePO4OH@RF/GO) obtained from a template-free self-assembly synthesis, following with direct calcination. The synthesis process requires no physical mixing step. The phase transformation path way from tavorite LiFePO4OH to olivine LiFePO4 upon calcination was determined by the in situ high temperature XRD technique. Benefitted from the unique structure of the material, these microspheres can be densely packed together, giving a high tap density of 1.3 g cm, and simultaneously, the defectless LiFePO4 primary nanocrystals modified with highly conductive surface carbon layer and ultrathin rGO provide good electronic and ionic kinetics for fast electron/Li ion transport. Lithium-ion batteries (LIBs), as one important technology for electric energy storage, have revolutionarily extend the battery life of portable electronic devices (e.g. smart phones, wearable devices, and laptop computers), and now stand as a promising candidate for high-power and long-life battery applications, typically for the electric transportation off the grid and the clean energy back-up and storage systems. [1-3] To date, olivine (e.g. LiFePO4), layer-type (e.g. LiNi1-x-yMnxCoyO2), and spinel (e.g. LiMn2O4) electrode materials are the three mostly used cathodes for high power LIBs. [1, 2, 4] Among them, LiFePO4 is more stable in thermodynamics and reaction kinetics than the other two owing to the presence of a robust polyanionic framework. [5-7] Moreover, LiFePO4 prevails in terms of natural abundance, cost, and environmental friendliness. The major issue to this material is the negative transport kinetics in both Li ion diffusion and electron delivery. Size tailoring, in conjunction with surface carbon (i.e. graphitic carbon, graphene, et al.) coatings to form LiFePO4/C nanocomposite is acknowledged as a technology of choice to alleviate this issue effectively. [6-9] However, when the particle size of LiFePO4/C is decreased down to nanoscale, thermodynamic instability and high risk of side reactions with the organic electrolyte become into new hazards needed to be faced with. [10] In addition, the processability and tap density (less than 1.0 g cm ) of the powders also require improvements when it comes to a practical point of view. [11] For that, small LiFePO4/C nanoparticles assembled into hierarchical architectures, ideally with a microsphere morphology, would naturally become more easily free-flowing and can be densely packed together, giving a higher tap density and hence a higher volumetric energy density. [12-14] Hydrothermal and solvothermal synthesis technologies or a combination of them are the methods commonly used to create LiFePO4/C microsphere with hierarchical architectures, the vast majority of which, however, required the use of instable and costly ferrous iron (Fe) salts as the raw material, and simultaneously a reducing agent and/or inert gas to avoid the oxidation of Fe ions during reactions. [13, 15] On top of that, most attempts to develop new routes, especially through ferric iron (Fe) based approaches, focused either on the “one-pot synthesis” or “in-situ carbon coating”, rather than considering both two concepts together. [12-14] Thus exploring a simpler synthesis scheme to produce LiFePO4/C microspheres with ideal structural features will be of great interest. In this communication, reduced GO-modified LiFePO4/C (i.e. LiFePO4@C/rGO) hierarchical microspheres were synthesized for the first time by a one-pot mixed-solvothermal process to form a ferric three-component precursor of LiFePO4OH@RF/GO, followed by direct calcination at high temperature, during which tedious physical mixing and grinding step can be totally avoided. The final LiFePO4@C/rGO microspheres created can be densely packed together, giving a high tap density of 1.3 g cm, and simultaneously, the small primary LiFePO4 nanoparticles (65 nm), in conjunction with the integrated carbonous conductive network can provide an adequate electrochemically available surface for enhancing the high-rate capability. The phase transformation mechanism from tavorite LiFePO4OH to the olivine LiFePO4 upon calcination was also determined by the in situ high temperature X-ray diffraction (XRD). [a] Dr. H. Wang, Dr. L. Liu, Prof. R. Wang, Dr. S. Jiang, L. Ni, H. Tang, Prof. Z. Zhang, Prof. S. Qiu State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, International Joint Research Laboratory of Nano-Micro Architecture Chemistry Jilin University Changchun 130012, P. R. China E-mail: zzhang@jlu.edu.cn [b] Dr. H. Wang, Dr. Z. Wang, J. Hu, Dr. H. Chen, Prof. F. Pan School of Advanced Materials Peking University Shenzhen Graduate School Shenzhen 518055, P. R. China E-mail: panfeng@pkusz.edu.cn [c] Dr. H. Qiu, Prof. Y. Wei, Prof. Z. Zhang Key Laboratory of Physics and Technology for Advanced Batteries (Ministry of Education) Jilin University Changchun 130012, P. R. China [d] Dr. X. Yan School of Chemistry and Materials Science Jiangsu Normal University Xuzhou 221116, P. R. China Supporting information for this article is given via a link at the end of the document. 10.1002/cssc.201800786 A cc ep te d M an us cr ip t ChemSusChem This article is protected by copyright. All rights reserved.LiFePO4 @C/reduced graphene oxide (rGO) hierarchical microspheres with superior electrochemical activity and a high tap density were first synthesized by using a Fe3+ -based single inorganic precursor (LiFePO4 OH@RF/GO; RF=resorcinol-formaldehyde, GO=graphene oxide) obtained from a template-free self-assembly synthesis followed by direct calcination. The synthetic process requires no physical mixing step. The phase transformation pathway from tavorite LiFePO4 OH to olivine LiFePO4 upon calcination was determined by means of the in situ high-temperature XRD technique. Benefitting from the unique structure of the material, these microspheres can be densely packed together, giving a high tap density of 1.3 g cm-3 , and simultaneously, defectless LiFePO4 primary nanocrystals modified with a highly conductive surface carbon layer and ultrathin rGO provide good electronic and ionic kinetics for fast electron/Li+ ion transport.