Jiangtao Hu

Janus Solid–Liquid Interface Enabling Ultrahigh Charging and Discharging Rate for Advanced Lithium-Ion Batteries

LiFePO4 has long been held as one of the most promising battery cathode for its high energy storage capacity. Meanwhile, although extensive studies have been conducted on the interfacial chemistries in Li-ion batteries,1-3 little is known on the atomic level about the solid-liquid interface of LiFePO4/electrolyte. Here, we report battery cathode consisted with nanosized LiFePO4 particles in aqueous electrolyte with an high charging and discharging rate of 600 C (3600/600 = 6 s charge time, 1 C = 170 mAh g(-1)) reaching 72 mAh g(-1) energy storage (42% of the theoretical capacity). By contrast, the accessible capacity sharply decreases to 20 mAh g(-1) at 200 C in organic electrolyte. After a comprehensive electrochemistry tests and ab initio calculations of the LiFePO4-H2O and LiFePO4-EC (ethylene carbonate) systems, we identified the transient formation of a Janus hydrated interface in the LiFePO4-H2O system, where the truncated symmetry of solid LiFePO4 surface is compensated by the chemisorbed H2O molecules, forming a half-solid (LiFePO4) and half-liquid (H2O) amphiphilic coordination environment that eases the Li desolvation process near the surface, which makes a fast Li-ion transport across the solid/liquid interfaces possible.

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.

Storage and Effective Migration of Li-Ion for Defected β-LiFePO4 Phase Nanocrystals

Lithium iron phosphate, a widely used cathode material, crystallizes typically in olivine-type phase, α-LiFePO4 (αLFP). However, the new phase β-LiFePO4 (βLFP), which can be transformed from αLFP under high temperature and pressure, is originally almost electrochemically inactive with no capacity for Li-ion battery, because the Li-ions are stored in the tetrahedral [LiO4] with very high activation barrier for migration and the one-dimensional (1D) migration channels for Li-ion diffusion in αLFP disappear, while the Fe ions in the β-phase are oriented similar to the 1D arrangement instead. In this work, using experimental studies combined with density functional theory calculations, we demonstrate that βLFP can be activated with creation of effective paths of Li-ion migration by optimized disordering. Thus, the new phase of βLFP cathode achieved a capacity of 128 mAh g(-1) at a rate of 0.1 C (1C = 170 mA g(-1)) with extraordinary cycling performance that 94.5% of the initial capacity retains after 1000 cycles at 1 C. The activation mechanism can be attributed to that the induced disorder (such as FeLiLiFe antisite defects, crystal distortion, and amorphous domains) creates new lithium migration passages, which free the captive stored lithium atoms and facilitate their intercalation/deintercalation from the cathode. Such materials activated by disorder are promising candidate cathodes for lithium batteries, and the related mechanism of storage and effective migration of Li-ions also provides new clues for future design of disordered-electrode materials with high capacity and high energy density.

Core–Shell Sn–Ni–Cu-Alloy@Carbon Nanorods to Array as Three-Dimensional Anode by Nanoelectrodeposition for High-Performance Lithium Ion Batteries

We report the synthesis of a novel three-dimensional anode based on the core-shell Sn-Ni-Cu-alloy@carbon nanorods which was fabricated by pulse nanoelectrodeposition. Li ion batteries equipped with the three-dimensional anode demonstrated almost 100% capacity retention over 400 cycles at 450 mA g(-1) and excellent rate performance even up to 9000 mA g(-1) for advanced Li-ion battery. Insight of the high performance can be attributed to three key factors, Li-Sn alloys for Li-ion storage, Ni-Cu matrix for the electronic conductive and nanorods structure, and the carbon shell for the electronic/Li-ion conductive and holding stable solid electrolyte interphase (SEI), because these shells always kept stable volumes after extension of initial charge-discharge cycles.

2D amorphous iron phosphate nanosheets with high rate capability and ultra-long cycle life for sodium ion batteries

In our previous work, we reported the formation and mechanism of mono/bi-layer phosphate-based materials and their high performance as cathode materials for Li-ion batteries. In this work, we report that 2D amorphous nanosheets can be used as cathode materials to achieve outstanding performance for sodium ion batteries (SIBs) e.g. a high initial discharge capacity of 168.9 mA h g−1 at 0.1C, ultra-long life (92.3% capacity retention over 1000 cycles), and high rate capability (77 mA h g−1 at 10C) for Na-ion storage, whose electrochemical performance is also much superior to the reported amorphous FePO4 or olivine NaFePO4 with advantages of short paths and larger implantation surface areas for fast Na-ion diffusion and large specific surfaces with more interfacial capacitance. Interestingly, NaFePO4 nano-crystals with about 10 nm sizes are self-nucleated from amorphous 2D nanosheets in the charge/discharge process, which was verified by transmission electron microscopy (TEM) and in situ electrochemical impedance spectroscopy (EIS).

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.

The formation and mechanism of nano-monocrystalline γ-Fe2O3 with graphene-shell for high-performance lithium ion batteries

Using a sintering process with Prussian Blue (PB) and 20 wt% glucose at high temperature (950 °C for 6 hours in Ar/H2) with oxidation in the air at room temperature, we synthesized a nano-monocrystalline γ-phase iron oxide (γ-Fe2O3) compound coated with carbon comprising a number of graphene layers, which was named as core–shell nano-monocrystalline γ-Fe2O3@graphene. It can be noted that the formation of nano-monocrystal is different from forming core–shell nano-polycrystalline hollow γ-Fe2O3@graphene sintered at lower temperature (650 °C 6 hours in Ar) via a simple Kirkendall process with oxidation at room temperature as reported in our previous study. We further investigate how nano-monocrystalline γ-Fe2O3 is formed by controlling the synthesis process and testing with TEM and SEM. We confirmed that the nano-monocrystalline γ-Fe2O3 is grown from nano-monocrystalline Fe with interface catalysis of O2 and the related mechanism is discussed through comparing the structures of γ-Fe2O3 and the Fe crystals. The core–shell nano-monocrystalline γ-Fe2O3@graphene shows high performance as an anode material in Li-ions batteries (much better than nano-polycrystalline hollow γ-Fe2O3@graphene reported in previous study). For example, the cycling stability and rate performance are remarkable as an anode material for lithium ion batteries with a high reversible capacity of 848.08 and 782.54 mA h g−1 at 1C and 5C for 600 cycles, respectively, and a high rate performance (284.42 mA h g−1 at 20C). Another interesting performance is that during the first 80 cycles, the specific capacity increases, which may result from more interface area being generated by the γ-Fe2O3 nano-monocrystal crushing with protection of the graphene-shell during the initial charging/discharging cycles. This synthesis method and mechanism can be used as a guide to produce γ-Fe2O3 as an anode material for lithium ion batteries with high performance on a large scale.

Effects of Ga doping and hollow structure on the band-structures and photovoltaic properties of SnO2 photoanode dye-sensitized solar cells

The photon-to-electricity conversion properties of the prepared photoanode based on SnO2 nanocrystals, which are assembled as the rough hollow microspheres (RHMs), are improved by aliovalent Ga3+ doping. The conduction band (CB) of the doped SnO2 shifts negatively with increasing the Ga content from 1 to 5 mol% gradually. Moreover, the prepared Ga-doped SnO2 photoanode shows an advantage in repressing the charge recombination. As a result, both the negative shift of the CB and repressed charge recombination enhance the open-circuit photovoltage (Voc) and the short-circuit photocurrent (Jsc) of the DSSCs, and the power conversion efficiency (η) is increased by 80% at 3 mol% Ga-doping SnO2 to compare with the undoped SnO2 for DSSCs (AM 1.5, 100 mW cm−2). After treating the samples with TiCl4, an overall photoconversion efficiency (approximately 7.11%) for SnO2 based DSSCs is achieved.

Tuning Li-Ion Diffusion in α-LiMn1–xFexPO4 Nanocrystals by Antisite Defects and Embedded β-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 (β-LiMn1-xFexPO4, which is first reported in this work) embedded in α-LiMn0.5Fe0.5PO4. Because of the coherent orientation relationship between β- and α-phases, the β-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 β-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.