Ju-Hsiang Cheng

Organometal halide perovskite solar cells: degradation and stability

Organometal halide perovskite solar cells have evolved in an exponential manner in the two key areas of efficiency and stability. The power conversion efficiency (PCE) reached 20.1% late last year. The key disquiet was stability, which has been limiting practical application, but now the state of the art is promising, being measured in thousands of hours. These improvements have been achieved through the application of different materials, interfaces and device architecture optimizations, especially after the investigation of hole conductor free mesoporous devices incorporating carbon electrodes, which promise stable, low cost and easy device fabrication methods. However, this work is still far from complete. There are various issues associated with the degradation of Omh-perovskite, and the interface and device instability which must be addressed to achieve good reproducibility and long lifetimes for Omh-PSCs with high conversion efficiencies. A comprehensive understanding of these issues is required to achieve breakthroughs in stability and practical outdoor applications of Omh-PSCs. For successful small and large scale applications, besides the improvement of the PCE, the stability of Omh-PSCs has to be improved. The causes of failure and associated mechanisms of device degradation, followed by the origins of degradation, approaches to improve stability, and methods and protocols are discussed in detail and form the main focus of this review article.

Direct growth of high-rate capability and high capacity copper sulfide nanowire array cathodes for lithium-ion batteries

A general solution method for the growth of highly ordered large-scale Cu2S nanowire arrays onto the copper metal current collector substrates has been developed. The electrochemical behaviors of Cu2S nanowire array cathodes for lithium-ion battery applications reveal that they exhibit stable lithium-ion insertion/extraction reversibility, high reversible lithium storage capacity, long cycle life and outstanding rate capability. The superb electrochemical performance can be attributed to the nanowire arrays having increased reaction sites, improved cycle life in the face of mechanical strain and efficient charge transport. With the simplicity of fabrication and good electrochemical properties, the Cu2S nanowire arrays are promising cathode materials for the practical use in the next generation lithium-ion batteries.

Oriented growth of large-scale nickel sulfide nanowire arrays via a general solution route for lithium-ion battery cathode applications

A general solution method for the oriented growth of large-scale Ni3S2nanowire arrays has been developed. The controlled oxidation scheme by combining ethylenediamine-chalcogens and hydrazine in alkali solution has been shown to have great advantages for the fabrication of metal chalcogenides with fewer instrumental limitations. This method is reliable and works in mild template-free conditions for the production of single-crystalline nanowire arrays. It provides a convenient route for the large-scale growth of pure-phase metal chalcogenide nanowire arrays on metal substrates. The electrochemical measurement results of Ni3S2nanowire arrays for lithium-ion battery electrode applications reveal that they have high reversible lithium storage capacity, long cycle life, good cyclic stability and high charge/discharge rate. With the simplicity of fabrication and good electrochemical performance, Ni3S2nanowire arrays are promising cathode materials for lithium-ion batteries.

Controlled Synthesis of CdSe Quantum Dots by a Microwave-Enhanced Process: A Green Approach for Mass Production

A method that does not employ hot-injection techniques has been developed for the size-tunable synthesis of high-quality CdSe quantum dots (QDs) with zinc blende structure. In this environmentally benign synthetic route, which uses less toxic precursors, solvents, and capping ligands, CdSe QDs that absorb visible light are obtained. The size of the as-prepared CdSe QDs and thus their optical properties can be manipulated by changing the microwave reaction conditions. The QDs were characterized by XRD, TEM, UV/Vis, FTIR, time-resolved fluorescence spectroscopy, and fluorescence spectrophotometry. In this approach, the reaction is conducted in open air and at a much lower temperature than in hot-injection techniques. The use of microwaves in this process allows for a highly reproducible and effective synthesis protocol that is fully adaptable for mass production and can be easily employed to synthesize a variety of semiconductor QDs with the desired properties. Possible applications of the CdSe QDs were assessed by deposition on TiO(2) films.

Stabilizing Nanosized Si Anodes with the Synergetic Usage of Atomic Layer Deposition and Electrolyte Additives for Li-Ion Batteries

A substantial increase in charging capacity over long cycle periods was made possible by the formation of a flexible weblike network via the combination of Al2O3 atomic layer deposition (ALD) and the electrolyte additive vinylene carbonate (VC). Transmission electron microscopy shows that a weblike network forms after cycling when ALD and VC were used in combination that dramatically increases the cycle stability for the Si composite anode. The ALD-VC combination also showed reduced reactions with the lithium salt, forming a more stable solid electrolyte interface (SEI) absent of fluorinated silicon species, as evidenced by X-ray photoelectron spectroscopy. Although the bare Si composite anode showed only an improvement from a 56% to a 45% loss after 50 cycles, when VC was introduced, the ALD-coated Si anode showed an improvement from a 73% to a 11% capacity loss. Furthermore, the anode with the ALD coating and VC had a capacity of 630 mAh g(-1) after 200 cycles running at 200 mA g(-1), and the bare anode without VC showed a capacity of 400 mAh g(-1) after only 50 cycles. This approach can be extended to other Si systems, and the formation of this SEI is dependent on the thickness of the ALD that affects both capacity and stability.

Hierarchical Copper‐Decorated Nickel Nanocatalysts Supported on La2O3 for Low‐Temperature Steam Reforming of Ethanol

Copper/nickel nanocatalysts with a unique morphology were prepared by thermal reduction of a perovskite LaNix Cu1-x O3 precursor (x=1, 0.9, and 0.7). During thermal reduction, copper was first reduced and reacted with lanthanum to form metastable Cu5 La and Cu13 La. When the thermal reduction temperature was increased, the perovskite decomposed to Ni and La2 O3 , CuLa alloys disappeared, and Cu deposits on Ni nanoparticles were generated, thereby forming Cu/Ni nanocatalysts with hierarchical structures. Nanosized nickel, decorated with copper and supported on La2 O3 , could be produced at 520-550 °C. The steam reforming of ethanol was used as a model reaction to demonstrate the catalytic capability of the materials formed. The hierarchical structure of the Cu/Ni/La2 O3 catalysts confers synergetic effects that greatly favor the dehydrogenation of ethanol and which break the C-C bond to produce a higher yield of hydrogen at a low reaction temperature, whereas La2 O3 provides the required stability during the reaction. The reaction at 290 °C achieved almost 100 % conversion with a hydrogen yield reaching 2.21 molH2  mol(-1) EtOH thus indicating that this special structural feature can achieve high activity for the SRE at low temperatures. The proposed synthesis of nanocatalysts appears to be a good way to generate oxide-supported hierarchically structured nanoparticles that can also be applied to other reactions catalyzed by a heterogeneous metal oxide system.

Capacity retention of lithium sulfur batteries enhanced with nano-sized TiO2-embedded polyethylene oxide

The shuttle effect of polysulfides in Li/S batteries causes the loss of active materials and capacity decay. This phenomenon has retarded the practical application of Li/S batteries. Herein, we demonstrate that the undesired shuttle mechanism owing to dissolved lithium polysulfides can be effectively suppressed by the incorporation of TiO2 nanoparticles (NPs) within a solid nanocomposite polymer electrolyte of polyethylene oxide (PEO). The approach shows enhanced capacity retention, whereby a cell with a hybrid solid electrolyte of TiO2 NPs embedded in PEO has delivered more than 1450 mA h g−1 (first cycle discharge capacity) with ∼87% capacity retention after 100 cycles, compared to only 38% capacity retention in the absence of TiO2. Density Functional Theory (DFT) computational results based on bonding interactions and Raman characterization of the different components reveal that the undesired diffusion shuttle mechanism causing Li anode passivation can be effectively reduced by the use of nano-sized TiO2 and polyethylene oxide.

Facile synthesis of SnO2-embedded carbon nanomaterials viaglucose-mediated oxidation of Sn particles

An unusual oxidative reaction of micron-sized Sn particles to form highly dispersed SnO2 nanoparticles, embedded in a carbon matrix, during hydrothermal treatment in glucose solution is disclosed in this study. The reaction mechanism is proposed to result from the promotion of glucose decomposition by Sn. The work also discloses a green process with a yield of 5∼10-fold higher than the commonly employed hydrothermal process. The embedded SnO2 nanoparticles are electrochemically stable for Li storage during the charging–discharging process, which suggests a promised strategy for the stabilization of Sn-based anode materials for Li-ion batteries. It also implies that the unusual oxidative reaction holds great potential for the synthesis of various nano-sized inorganic oxides embedded in carbon matrices to specific applications.

Improved Interfacial Properties of MCMB Electrode by 1-(Trimethylsilyl)imidazole as New Electrolyte Additive To Suppress LiPF6 Decomposition

Trace water content in the electrolyte causes the degradation of LiPF6, and the decomposed products further react with water to produce HF, which alters the surface of anode and cathode. As a result, the reaction of HF and the deposition of decomposed products on electrode surface cause significant capacity fading of cells. Avoiding these phenomena is crucial for lithium ion batteries. Considering the Lewis-base feature of the N-Si bond, 1-(trimethylsilyl)imidazole (1-TMSI) is proposed as a novel water scavenging electrolyte additive to suppress LiPF6 decomposition. The scavenging ability of 1-TMSI and beneficiary interfacial chemistry between the MCMB electrode and electrolyte are studied through a combination of experiments and density functional theory (DFT) calculations. NMR analysis indicated that LiPF6 decomposition by water was effectively suppressed in the presence of 0.2 vol % 1-TMSI. XPS surface analysis of MCMB electrode showed that the presence of 1-TMSI reduced deposition of ionic insulating products caused by LiPF6 decomposition. The results showed that the cells with 1-TMSI additive have better performance than the cell without 1-TMSI by facilitating the formation of solid-electrolyte interphase (SEI) layer with better ionic conductivity. It is hoped that the work can contribute to the understanding of SEI and the development of electrolyte additives for prolonged cycle life with improved performance.

Polyethylene oxide film coating enhances lithium cycling efficiency of an anode-free lithium-metal battery

The practical implementation of an anode-free lithium-metal battery with promising high capacity is hampered by dendrite formation and low coulombic efficiency. Most notably, these challenges stem from non-uniform lithium plating and unstable SEI layer formation on the bare copper electrode. Herein, we revealed the homogeneous deposition of lithium and effective suppression of dendrite formation using a copper electrode coated with a polyethylene oxide (PEO) film in an electrolyte comprising 1 M LiTFSI, DME/DOL (1/1, v/v) and 2 wt% LiNO3. More importantly, the PEO film coating promoted the formation of a thin and robust SEI layer film by hosting lithium and regulating the inevitable reaction of lithium with the electrolyte. The modified electrode exhibited stable cycling of lithium with an average coulombic efficiency of ∼100% over 200 cycles and low voltage hysteresis (∼30 mV) at a current density of 0.5 mA cm-2. Moreover, we tested the anode-free battery experimentally by integrating it with an LiFePO4 cathode into a full-cell configuration (Cu@PEO/LiFePO4). The new cell demonstrated stable cycling with an average coulombic efficiency of 98.6% and capacity retention of 30% in the 200th cycle at a rate of 0.2C. These impressive enhancements in cycle life and capacity retention result from the synergy of the PEO film coating, high electrode-electrolyte interface compatibility, stable polar oligomer formation from the reduction of 1,3-dioxolane and the generation of SEI-stabilizing nitrite and nitride upon lithium nitrate reduction. Our result opens up a new route to realize anode-free batteries by modifying the copper anode with PEO to achieve ever more demanding yet safe interfacial chemistry and control of dendrite formation.