Interface Engineering with Formamidinium Salts for Improving Ambient-Processed Inverted CsPbI3 Photovoltaic Performance: Intermediate- vs Post-Treatment.

Inverted all-inorganic perovskite solar cells (PSCs) have attracted increasing attention owing to excellent thermal stability, easy fabrication, and adaptable application as top cell in tandem solar cells. Apart from efficiency, ambient processing is desirable for practical production. To avoid water invasion in ambient air, surface engineering for perovskites is reported as a valid approach. However, most were performed by post-treatment, which hardly regulates the formation process of perovskite crystals. This work demonstrates a simple but effective surface intermediate-treatment strategy to stabilize CsPbI3 perovskites fabricated in ambient air and compares the different effects yielded on the inverted PSCs. By using formamidinium (FA) salts for intermediate-treatment, the strong interaction between FA cation and [PbI6]4- octahedron improves the moisture resistance, and compared with the post-treatment strategy, the accelerated crystallization rate and the shortened exposure time to moisture reduce the devastation by water during film fabrication process further. Moreover, the greatly passivated defects and optimized energy level matching between perovskite and PCBM suppress the nonradiative recombination. Resultantly, the optimized device shows enhanced efficiency from 11.39% to 15.45%, and long-term stability is improved, with 97.6% efficiency remaining after storage for 1600 h. Therefore, we believe that this work can provide a promising guideline for fabricating all-inorganic inverted PSCs in a low-cost manufacturing scheme.

The Pillar Effect of Large-Size Alkaline Ions on the Electrochemical Stability of Sodium Manganese Hexacyanoferrate for Sodium-Ion Batteries.

Sodium manganese hexacyanoferrate (NaMnHCF) is an attractive candidate as a cathode material for sodium-ion batteries due to its low cost and high energy density. However, its practical application is hindered by poor electrochemical stability caused by the Jahn-Teller effect of Mn and the unstable structure of NaMnHCF. Here, this paper aims to address this issue by introducing highly stable AMnHCF (where A = K, Rb, or Cs) through a facile method to composite with NaMnHCF. The findings reveal that all AMnHCFs have a "pillar effect" on the crystal structure of NaMnHCF. It is observed that the degree of pillar effect varies depending on the specific AMnHCF used. The less electrochemically inactive the alkaline ion is and the greater the degree of compositing with NaMnHCF, the more dramatic the pillar effect. KMnHCF shows limited pillar effect due to its rough composition with NaMnHCF and the loss of K+ upon (de)intercalation. RbMnHCF has lower electrochemical activity and can be better composited with NaMnHCF. On the other hand, CsMnHCF exhibits the strongest pillar effect due to the inactivation of Cs+ and the excellent coherent structure formed by CsMnHCF and NaMnHCF. This research provides a new perspective on stabilizing NaMnHCF with other alkaline elements.

Ferroelastic Domains Enhanced the Photoelectric Response in a CsPbBr3 Single-Crystal Film Detector.

The ferroic domain, in metal halide perovskites (MHPs) at a low symmetry phase, was reported to affect optoelectronic properties. Building the relationship between ferroic domains and optoelectronic properties of MHPs will be a non-trivial task for understanding the charge transport mechanism. Here, high-quality CsPbBr3 single-crystal films (SCFs) were successfully grown by a cast-capping method. Through the phase transition process by heating and cooling the sample, dense domains in CsPbBr3 SCFs were formed and observed by an in situ polarized optical microscope. These domains were identified as 90° rotation twins by electron backscattered diffraction and transmission electron microscopy. Interestingly, the photocurrent response was dramatically enhanced after introducing ferroelastic domains. The highest responsivity, external quantum efficiency, and detectivity are 380 mA/W, 130%, and 12.9 × 1010 Jones, respectively, which are surprisingly 25.03, 25, and 7.8 times higher than those of the as-grown CsPbBr3 SCF, respectively, which may be attributed to the function of the domain wall of separating electrons and holes.

High-entropy materials for energy-related applications.

High-entropy materials (HEMs), including high-entropy alloys (HEAs), high-entropy oxides (HEOs), and other high-entropy compounds, have gained significant interests over the past years. These materials have unique structures with the coexistence of antisite disordering and crystal periodicity, which were originally investigated as structural materials. Recently, they have emerged for energy-related applications, such as catalysis, energy storage, etc. In this work, we review the research progress of energy-related applications of HEMs. After an introduction on the background, theory, and syntheses of HEMs, we survey their applications including electrocatalysis, batteries, and others, aiming to retrieve the correlations between their structures and performances. In the end, we discussed the challenges and future directions for developing HEMs.

Design of a High-Efficiency and -Gain Antenna Using Novel Low-Loss, Temperature-Stable Li2Ti1-x(Cu1/3Nb2/3)xO3 Microwave Dielectric Ceramics.

Microwave dielectric ceramics are vital for filters, dielectric resonators, and dielectric antennas in the 5G era. It was found that the (Cu1/3Nb2/3)4+ substitution can effectively adjust the TCF (temperature coefficient of resonant frequency) of Li2TiO3 and simultaneously increase its Q × f (Q and f denote the quality factor and the resonant frequency, respectively) value. Notably, excellent microwave dielectric properties (εr (permittivity) ≈ 18.3, Q × f ≈ 77,840 GHz, and TCF ≈ +9.8 ppm/°C) were achieved in the Li2Ti0.8(Cu1/3Nb2/3)0.2O3 (LTCN0.2) ceramic sintered at 1140 °C. Additionally, the sintering temperature of LTCN0.2 was reduced to 860 °C by the addition of 3 wt % H3BO3, exhibiting superior microwave dielectric properties (εr ≈ 21.0, Q × f ≈ 51,940 GHz, and TCF ≈ 1.4 ppm/°C) and being chemically compatible with silver. Moreover, LTCN0.2 + 3 wt % H3BO3 ceramics were designed as a patch antenna and a dielectric resonator antenna, both of which showed high simulated radiation efficiencies (88.4 and 93%) and gains (4.1 and 4.03 dBi) at the center frequencies (2.49 and 10.19 GHz). The LTCN0.2 + 3 wt % H3BO3 materials have promising future application for either 5G mobile communication devices and/or in low-temperature co-fired ceramic technology owing to their high Q, low sintering temperature, small density, and good temperature stability.

Direct Observation of Defect-Aided Structural Evolution in a Nickel-Rich Layered Cathode.

Ni-rich LiNi1-x-y Mnx Coy O2 (NMC) layered compounds are the dominant cathode for lithium ion batteries. The role of crystallographic defects on structure evolution and performance degradation during electrochemical cycling is not yet fully understood. Here, we investigated the structural evolution of a Ni-rich NMC cathode in a solid-state cell by in situ transmission electron microscopy. Antiphase boundary (APB) and twin boundary (TB) separating layered phases played an important role on phase change. Upon Li depletion, the APB extended across the layered structure, while Li/transition metal (TM) ion mixing in the layered phases was detected to induce the rock-salt phase formation along the coherent TB. According to DFT calculations, Li/TM mixing and phase transition were aided by the low diffusion barriers of TM ions at planar defects. This work reveals the dynamical scenario of secondary phase evolution, helping unveil the origin of performance fading in Ni-rich NMC.

In Situ Electron Microscopy Investigation of Sodiation of Titanium Disulfide Nanoflakes.

Two-dimensional (2D) metal sulfides show great promise for their potential applications as electrode materials of sodium ion-batteries because of the weak interlayer van der Waals interactions, which allow the reversible accommodation and extraction of sodium ions. The sodiation of metal sulfides can undergo a distinct process compared to that of lithiation, which is determined by their metal and structural types. However, the structural and morphological evolution during their electrochemical sodiation is still unclear. Here, we studied the sodiation reaction dynamics of TiS2 by employing in situ transmission electron microscopy and first-principles calculations. During the sodium-ion intercalation process, we observed multiple intermediate phases (phase II, phase Ib, and phase Ia), different from its lithiation counterpart, with varied sodium occupation sites and interlayer stacking sequences. Further insertion of Na ions prompted a multistep extrusion reaction, which led to the phase separation of Ti metal from the Na2S matrix, with its 2D morphology expanded to a 3D morphology. In contrast to regular conversion electrodes, TiS2 still maintained a compact structure after a full sodiation. First-principles calculations reveal that the as-identified phases are thermodynamically preferred at corresponding intercalation/extrusion stages compared to other possible phases. The present work provides the fundamental mechanistic understanding of the sodiation process of 2D transition metal sulfides.

Embedding laser generated nanocrystals in BiVO4 photoanode for efficient photoelectrochemical water splitting.

Addressing the intrinsic charge transport limitation of metal oxides has been of significance for pursuing viable PEC water splitting photoelectrodes. Growing a photoelectrode with conductive nanoobjects embedded in the matrix is promising for enhanced charge transport but remains a challenge technically. We herein show a strategy of embedding laser generated nanocrystals in BiVO4 photoanode matrix, which achieves photocurrent densities of up to 5.15 mA cm-2 at 1.23 VRHE (from original 4.01 mA cm-2) for a single photoanode configuration, and 6.22 mA cm-2 at 1.23 VRHE for a dual configuration. The enhanced performance by such embedding is found universal owing to the typical features of laser synthesis and processing of colloids (LSPC) for producing ligand free nanocrystals in desired solvents. This study provides an alternative to address the slow bulk charge transport that bothers most metal oxides, and thus is significant for boosting their PEC water splitting performance.

Bundled Defect-Rich MoS2 for a High-Rate and Long-Life Sodium-Ion Battery: Achieving 3D Diffusion of Sodium Ion by Vacancies to Improve Kinetics.

Molybdenum disulfide (MoS2 ), a 2D-layered compound, is regarded as a promising anode for sodium-ion batteries (SIBs) due to its attractive theoretical capacity and low cost. The main challenges associated with MoS2 are the low rate capability suffering from the sluggish kinetics of Na+ intercalation and the poor cycling stability owning to the stack of MoS2 sheets. In this work, a unique architecture of bundled defect-rich MoS2 (BD-MoS2 ) that consists of MoS2 with large vacancies bundled by ultrathin MoO3 is achieved via a facile quenching process. When employed as anode for a SIB, the BD-MoS2 electrode exhibits an ultrafast charge/discharge due to the pseudocapacitive-controlled Na+ storage mechanism in it. Further experimental and theoretical calculations show that Na+ is able to cross the MoS2 layer by vacancies, not only limited to diffusion along the layer, thus realizing a 3D Na+ diffusion with faster kinetics. Meanwhile, the bundling architecture reduces the stack of sheets with a superior cycle life illustrating the highly reversible capacities of 350 and 272 mAh g-1 at 2 and 5 A g-1 after 1000 cycles.

Metal-organic framework-derived mesoporous octahedral copper oxide/titania composites for high-performance lithium-ion batteries.

A mesoporous octahedral copper oxide@titania (CuO/TiO2) composites with core-shelled structure have been successfully fabricated via a facile and cost-effective approach, which involves two main steps: the creation of homogeneous TiO2 shell onto the octahedral Cu-based metal-organic frameworks (MOFs) template and thermal decomposition of the template at controlled temperature in the air. The design of combining CuO with the TiO2 layer within a porous octahedra structure is beneficial to integrate the advantages of different components and address the severe volume change associated with pulverization issue that exists in most metal oxides-based electrodes. When assembled as an anode material for lithium-ion batteries, the as-fabricated mesoporous CuO/TiO2 octahedra can achieve outstanding electrochemical performance in terms of a high reversible capacity (692 mAh g-1 at 100 mA g-1 for over 200 cycles) and exceptional rate capability (441 and 387 mA h g-1 at 1600 and 3200 mA g-1, respectively).

Multistep Lithiation of Tin Sulfide: An Investigation Using in Situ Electron Microscopy.

Two-dimensional (2D) metal sulfides have been widely explored as promising electrodes for lithium-ion batteries since their two-dimensional layered structure allows lithium ions to intercalate between layers. For tin disulfide, the lithiation process proceeds via a sequence of three different types of reactions: intercalation, conversion, and alloying, but the full scenario of reaction dynamics remains nebulous. Here, we investigate the dynamical process of the multistep reactions using in situ electron microscopy and discover the formation of an intermediate rock-salt phase with the disordering of Li and Sn cations after initial 2D intercalation. The disordered cations occupy all the octahedral sites and block the channels for intercalation, which alter the reaction pathways during further lithiation. Our first-principles calculations of the nonequilibrium lithiation of SnS2 corroborate the energetic preference of the disordered rock-salt structure over known layered polymorphs. The in situ observations and calculations suggest a two-phase reaction nature for intercalation, disordering, and following conversion reactions. In addition, in situ delithiation observation confirms that the alloying reaction is reversible, while the conversion reaction is not, which is consistent with the ex situ analysis. This work reveals the full lithiation characteristic of SnS2 and sheds light on the understanding of complex multistep reactions in 2D materials.

Enhanced Interfacial Kinetics and High-Voltage/High-Rate Performance of LiCoO2 Cathode by Controlled Sputter-Coating with a Nanoscale Li4Ti5O12 Ionic Conductor.

The selection and optimization of coating material/approach for electrode materials have been under intensive pursuit to address the high-voltage induced degradation of lithium ion batteries. Herein, we demonstrate an efficient way to enhance the high-voltage electrochemical performance of LiCoO2 cathode by postcoating of its composite electrode with Li4Ti5O12 (LTO) via magnetron sputtering. With a nanoscale (∼25 nm) LTO coating, the reversible capacity of LiCoO2 after 60 cycles is significantly increased by 40% (to 170 mAh g-1) at room temperature and by 118% (to 139 mAh g-1) at 55 °C. Meanwhile, the electrode's rate capability is also greatly improved, which should be associated with the high Li+ diffusivity of the LTO surface layer, while the bulk electronic conductivity of the electrode is unaffected. At 12 C, the capacity of the coated electrode reaches 113 mAh g-1, being 70% larger than that of the uncoated one. The surface interaction between LTO and LiCoO2 is supposed to reduce the space-charge layer at the LiCoO2-electrolyte interface, which makes the Li+ diffusion much easier as evidenced by the largely enhanced diffusion coefficient of the coated electrode (an order of magnitude improvement). In addition, the LTO coating layer, which is electrochemically and structurally stable in the applied potential range, plays the role of a passivation layer or an artificial and friendly solid electrolyte interface (SEI) layer on the electrode surface. Such protection is able to impede propagation of the in situ formed irreversible SEI and thus guarantee a high initial columbic efficiency and superior cycling stability at high voltage.