Rational design of Fe/Co-based diatomic catalysts for Li–S batteries by first-principles calculations

Fe/Co-based diatomic catalysts decorated on an N-doped graphene substrate are investigated by first-principles calculations to improve the electrochemical properties of Li–S batteries.Our results demonstrate that Fe CoN8@Gra not only possesses moderate adsorption energies towards Li2Snspecies,but also exhibits superior catalytic activity for both reduction and oxidation reactions of the sulfur cathode.Moreover,the metallic property of the diatomic catalysts can be well maintained after Li2Snadsorption,which could help the sulfur cathode to maintain high conductivity during the whole charge–discharge process.Given these exceptional properties,it is expected that Fe CoN8@Gra could be a promising diatomic catalyst for Li–S batteries and afford insights for further development of advanced Li–S batteries.

The effect of electrochemically inactive Ti substituted for Ru in Li_(2)Ru_(1-x)Ti_xO_(3) on structure and electrochemical performance

The approach of substituting electrochemically active with inactive elements has widely been used to improve the electrochemical performance of Li-rich intercalation cathode materials. This especially is true for Li-rich compounds where almost all of the Li+ions are reversibly(de)intercalated during electrochemical cycling. The beneficial mechanism behind this substitution with electrochemically inactive elements is still not clear yet. Li_(2)RuO_(3) is chosen as basis for a model solid solution system to investigate the effect of electrochemically inactive elements owing to its high specific capacity of more than 300 m Ah g^(-1) and the significant contribution of anion redox mechanism. Herein, Li_(2)Ru_(1-x)Ti_xO_(3) solid solution series are synthesized and the effect of substituting with electrochemical inactive Ti for Ru on structure and electrochemical performance have been comprehensively investigated. The electrochemical performance is significantly improved, especially for Li_(2)Ru_(0.8)Ti_(0.2)O_(3), and the capacity retention after 50 cycles increases from 81% to 90%, as compared to the end member Li_(2)RuO_(3). Results of electrochemical impedance spectroscopy show that Ti substitution reduces the charge transfer impedance, which favors the Li+diffusion across the electrolyte–electrode interface and improves the electronic conductivity. For the first time,nuclear magnetic resonance was utilized to confirm that a small part of Ti ions exchange their position with Li ions in the Li layer. This research provides a better understanding of electrochemical inactive element substitution and strong insights for the functional design of the next generation of Li-rich cathode materials.

Role of Catalytic Materials on Conversion of Sulfur Species for Room Temperature Sodium–Sulfur Battery

Room temperature sodium–sulfur(RT Na-S)battery with high theoretical energy density and low cost has spurred tremendous interest,which is recognized as an ideal candidate for large-scale energy storage applications.However,serious sodium polysulfide shutting and sluggish reaction kinetics lead to rapid capacity decay and poor Coulombic efficiency.Recently,catalytic materials capable of adsorbing and catalyzing the conversion of polysulfides are profiled as a promising method to improve electrochemical performance.In this review,the research progress is summarized that the application of catalytic materials in RT Na-S battery.For the role of catalyst on the conversion of sulfur species,specific attention is focused on the influence factors of reaction rate during different redox processes.Various catalytic materials based on lightweight and high conductive carbon materials,including heteroatom-doped carbon,metals and metal compounds,single-atom and heterostructure,promote the reaction kinetic via lowered energy barrier and accelerated charge transfer.Additionally,the adsorption capacity of the catalytic materials is the key to the catalytic effect.Particular attention to the interaction between polysulfides and sulfur host materials is necessary for the exploration of catalytic mechanism.Lastly,the challenges and outlooks toward the desired design of efficient catalytic materials for RT Na-S battery are discussed.

MXenes for perovskite solar cells:Progress and prospects

Perovskite solar cells(PSCs)have been developed over the past decade as the forefront of the state-of-theart photovoltaic technologies owing to their high efficiency and low cost,where nanostructured functional materials play key roles in performance optimization.As a versatile class of two-dimensional(2D)materials,transition metal carbides/nitrides MXenes have gained enormous attentions in PSCs since 2018 due to their superior properties such as excellent metallic conductivity,abundant surface functional groups,tunable work functions,high optical transparency,and mechanical robustness.The explorations of MXenes are of significance in performance promotion and commercialization expansion of devices.As such,this review focuses on the diversified advantages of MXenes,comprehensively summarizing their applications and developments in PSCs as additives,electron/hole transporting layers,interfacial engineering layers,and electrodes in sequence and explaining the relevant mechanisms behind.Simultaneously,the problems emerged from the related studies are considered and the corresponding suggestions like opening up the type of MXenes usage,taking further insight of the modulation of surface termination groups on Fermi levels,understanding the effect on energy level structures of perovskite or other functional layers,and realizing commercialization,etc.are provided for the future in-depth explorations.This review is intended to provide overall perspective of the current status of MXenes and highlight the direction for the future advancements in MXenes design and processes towards efficient,stable,large-area,and low-cost PSCs.

Electronic structures and magnetic properties of Zn-and Cd-doped AlN nanosheets:A first-principles study

In this paper, the magnetic properties, electronic structures and the stabilities of Zn/Cd incorporated two-dimensional Al N nanosheets are investigated by the first-principles method. Numerical results indicate that Zn and Cd substituting Al atom in Al N nanosheets introduce some holes into the 2p orbitals of the N atoms, and the holes mainly come from spindown 2p orbitals of the N atoms. The magnetic moment of 1.0 μBis produced by Zn/Cd doping Al N nanosheets, and the main component of the magnetic moment of the system is contributed by the partially filled 2p states of the N atoms around the dopant. In particular, when Zn/Cd substituting Al atoms, the magnetic coupling is found to be ferromagnetic. We attribute the hole-mediated p–d interaction to the created ferromagnetic coupling. More importantly, the result of formation energy indicates that Al atom is more inclined to be replaced by Zn atom rather than Cd. This finding is beneficial to developing the spin electronic devices.

Forward-headed structure change of acetic acid–water binary system by stimulated Raman scattering

The acetic acid-water binary system is a classical hydroxy-carboxy mixed system, while new and interesting phenomena appear under stimulated Raman scattering(SRS).Compared with the weaker signal of the acetic acid-water binary system obtained in spontaneous Raman scattering, SRS provides a finer band and a relatively distinct structural transition point.The structural transformation points are respectively at 30% and 80% by volume ratio under the condition of spontaneous Raman spectroscopy, while they are respectively at 15% and 25% under the condition of SRS.This phenomenon is attributed to the generation of laser induced plasma and shockwave induced dynamic high pressure environment during SRS.

Elucidation of the sodiation/desodiation mechanism in Ca_(0.5)Ti_(2)(PO_(4))_(3)/C as promising electrode for sodium batteries: New insights into the phase transitions

The structure evolution and electrochemical performance of Na SICON-type Ca_(0.5)Ti_(2)(PO_(4))_(3) for sodium batteries are presented.This phosphate was synthesized by a solid-state method,and the obtained particles were coated with carbon using sucrose.This compound crystallizes in the rhombohedral system with space group R-3.The presence of carbon in the Ca_(0.5)Ti_(2)(PO_(4))_(3)/C composite was confirmed by Raman and Thermogravimetric analysis.The electrochemical performance of Ca_(0.5)Ti_(2)(PO_(4))_(3)/C was investigated in the potential window 1.5–3.0 V vs.sodium metal at different scan rates.The compound is able to initially intercalate/deintercalate 1.6/1.15 Na per formula unit,respectively.In operando synchrotron diffraction was done in the potential window 0.02–3.0 V vs.Na|Na+and revealed the occurrence of several reaction regions upon first discharge.Up to 4 Na+ion per formula unit can be inserted during the first discharge.An intensive refinement of the synchrotron X-ray diffraction(SXRD)patterns of discharged Ca_(0.5)Ti_(2)(PO_(4))_(3) evidenced the existence of five regions depending on the sodium content while the crystal structures of new phases were elucidated for the first time where sodium insertion occurs in the unusual M3 and M’3 sites of the Na SICON structure.

Scanning the energy dissipation process of energetic materials based on excited state relaxation and vibration–vibration coupling

The energy dissipation mechanism of energetic materials（EMs） is very important for keeping safety. We choose nitrobenzene as a model of EM and employ transient absorption（TA） spectroscopy and time-resolved coherent anti-stokes Raman scattering（CARS） to clarify its energy dissipation mechanism. The TA data confirms that the excited nitrobenzene spends about 16 ps finishing the twist intramolecular charge transfer from benzene to nitro group, and dissipates its energy through the rapid vibration relaxation in the initial excited state. And then the dynamics of vibrational modes（VMs） in the ground state of nitrobenzene, which are located at 682 cm^-1（v1）, 854 cm^-1（v2）, 1006 cm^-1（v3）, and 1023 cm^-1（v4）,is scanned by CARS. It exhibits that the excess energy of nitrobenzene on the ground state would further dissipate through intramolecular vibrational redistribution based on the vibrational cooling of vi and v2 modes, v1 and v4 modes, and v3 and v4 modes. Moreover, the vibration-vibration coupling depends not only on the energy levels of VMs, but also on the spatial position of chemical bonds relative to the VM.

Raman investigation of hydration structure of iodide and iodate

In the troposphere,the destruction of ozone and the formation of new particles are closely related to the iodine content,which mainly comes from iodide(I^(-)) and iodate(IO_(3)^(-)) in the seawater.Therefore,understanding the interactions between I^(-),IO_(3)^(-)and water molecules plays a certain role in alleviating the destruction of the ozone layer.Raman spectroscopy is commonly used to obtain the information of the interaction between I^(-),IO_(3)^(-)and water molecules quickly and accurately.Herein,the effect of I^(-)and IO_(3)^(-)on the change in Raman band characteristics of water is investigated to reflect the associated intermolecular interactions change.With the addition of the two ions,the Raman band corresponding to OH stretching vibration moves towards the high wavenumber,indicating the formation of hydration structure.The narrowing of the Raman band from OH stretching vibration under weak hydrogen bond agrees well with the hydrogen bond variation,while the abnormal broadening of the Raman band from OH stretching vibration under strong hydrogen bond indicates the formation of H-down structure.With the increase of ions concentration,the frequency shift of the Raman band from OH stretching vibration under both weak and strong hydrogen bonds becomes more apparent.Meanwhile,the frequency shift of I^(-)is more obvious than that of IO_(3)^(-),which indicates that I^(-)is more likely to form the hydration structure with water than IO_(3)^(-).These results contribute to analyzing the different interactions between I^(-)-water and IO_(3)^(-)-water,then helping to prevent ozone depletion.

A scalable one-pot strategy for the development of polymer electrolytes adaptable to room-temperature high-voltage lithium batteries

Poly(ethylene oxide)(PEO)polymer electrolytes(PEs)have been commercially applied in LiFePO_(4)||Li solid-state lithium batteries(SSLBs).However,it remains challenging to develop PEO-based PEs applicable to the high-voltage SSLBs with higher energy density,owing to the poor electrochemical stability of PEO.Herein,we report a scalable strategy for fabricating PEO-based PEs with high-voltage compatibility,by exploiting a new mechanism to stabilize the cathode-electrolyte interface in the highvoltage SSLBs.The protocol only involves a one-pot synthesis procedure to covalently crosslink the PEO chains,in the presence of high-content lithium bis(trifluoromethylsulphonyl)imide(LiTFSI)salts and N,N-dimethylformamide(DMF).LiTFSI-DMF supramolecular aggregates are formed and firmly embedded in the polymer network,endowing the PE with high room-temperature ionic conductivity.The dissociated and highly concentrated TFSI^-anions can enter the Helmholtz layer close to the high-voltage cathode,leading to the formation of a thin and homogeneous cathode electrolyte interface(CEI),mainly composed of LiF,on the cathode.The CEI with high electrochemical stability can effectively stabilize the cathode-electrolyte interface,enabling long-term stable cycling of the high-voltage LiCoO_(2)||Li and nickelrich NCM_(622)||Li batteries at room temperature.The simplicity and scalability of the strategy makes the reported PEO-based PE potentially applicable in high-voltage SSLBs in practice.

Efficient Two‑Dimensional Perovskite Solar Cells Realized by Incorporation of Ti_(3)C_(2)T_(x) MXene as Nano‑Dopants

Two-dimensional(2D)perovskites solar cells(PSCs)have attracted considerable attention owing to their excellent stability against humidity;however,some imperfectness of 2D perovskites,such as poor crystallinity,disordered orientation,and inferior charge transport still limit the power conversion efficiency(PCE)of 2D PSCs.In this work,2D Ti3C2Tx MXene nanosheets with high electrical conductivity and mobility were employed as a nanosized additive to prepare 2D Ruddlesden–Popper perovskite films.The PCE of solar cells was increased from 13.69(without additive)to 15.71%after incorporating the Ti_(3)C_(2)T_(x) nanosheets with an optimized concentration.This improved performance is attributed to the enhanced crystallinity,orientation,and passivated trap states in the 3D phase that result in accelerated charge transfer process in vertical direction.More importantly,the unencapsulated cells exhibited excellent stability under ambient conditions with 55±5%relative humidity.

High-Pressure Preparation of High-Density Cu_2ZnSnS_4 Materials

High-density Cu2ZnSnS4 （CZTS） materials are prepared via the mechanical alloying and high pressure sintering method using Cu2S, ZnS and SnS2 as the raw materials. The morphological, structural, compositional and electrical properties of the materials are investigated by using x-ray diffraction, scanning electron microscopy, and energy dispersive x-ray spectroscopy, as well as by the Raman scattering and the Hall EFfect measurements. The CZTS synthesized under 5 GPa and 800℃ shows a p-type conductivity, with a resistivity of 9.69 × 10^-2 Ω.cm and a carrier concentration of 1.45 × 10^20 cm-3. It is contributed to by the large grains in the materials reducing the grain boundaries, thus effectively reducing the recombination of the charge carriers.

Photoluminescence Characteristics of ZnCuInS-ZnS Core-Shell Semiconductor Nanocrystals

The photoluminescence （PL） characteristics of ZnCuInS quantum dots （Q, Ds） with varying ZnS shell thicknesses of O, 0.5, and 1.5 layers are investigated systemically by time-correlated single-photon counting measurements and temperature-dependent PL measurements. The results show that a ZnS shell thickness of 1.5 layers can effectively improve the PL quantum yield in one order of magnitude by depressing the surface trapping states of the core ZnCuInS QDs at room temperature. However, the PL measurements at the elevated temperature reveal that the core-shell nanocrystals remain temperature-sensitive with respect to their relatively thin shells. The temperature sensitivity of these small-sized single-layered core-shell nanocrystals may find applications as effective thermometers for the in vivo detection of biological reactions within cells.

Stress-assisted design of stiffened graphene electrode structure toward compact energy storage

The low spatial charge-storage density of porous carbons greatly limits volumetric performance in electrochemical capacitors.An increase of charge-storage density requires structural refinements to balance the trade-offs between the porosity and density of materials,but the limited mechanical properties of carbons usually fail to withstand effective densifying processes and obtain an ideal pore structure.Herein,we design the stiffened graphene of superior bending rigidity,enabling the fine adjustments of pore structure to maximize the volumetric capacitance for the graphene-based electrodes.The inplane crumples on graphene sheets are found to contribute largely to the bending rigidity,which is useful to control the structural evolution and maintain sufficient ion-accessible surface area during the assembling process.This makes the capacitance of stiffening activated graphene keep 98%when the electrode density increases by 769%to reach 1.13 g cm^(-3) after mechanical pressure,an excellent volumetric energy density of 98.7 Wh L^(-1) in an ionic-liquid electrolyte is achieved.Our results demonstrate the role of intrinsic material properties on the performance of carbon-based electrodes for capacitive energy storage.

Magnetic properties of AlN monolayer doped with group 1A or 2A nonmagnetic element: First-principles study

The electronic structure, magnetic properties, and mechanism of magnetization in two-dimensional（2D） aluminum nitride（AlN） monolayer doped with nonmagnetic elements of group 1A（Li, Na, K） or group 2A（Be, Mg, Ca） were systematically investigated using first-principles studies. Numerical results reveal that the total magnetic moments produced by group 1A and group 2A nonmagnetic doping are 2.0μB and 1.0μB per supercell, respectively. The local magnetic moments of the three N atoms around the doping atom are the primary moment contributors for all these doped AlN monolayers. The p orbital of the dopant atom contributes little to the total magnetic moment, but it influences adjacent atoms significantly, changing their density of states distribution, which results in hybridization among the p orbitals of the three closest N atoms, giving rise to magnetism. Moreover, the doped AlN monolayer, having half-metal characteristics,is a likely candidate for spintronic applications. When two group 1A or group 2A atoms are inserted, their moments are long-range ferromagnetically coupled. Remarkably, the energy of formation shows that, if the monolayer has been grown under N-rich conditions, substitution of a group 2A atom at an Al site is easier than substitution of a group 1A atom.

Observation of spin glass transition in spinel LiCoMnO_4

Spinel Li CoMnO4 is prepared by solid-state reaction and its magnetic properties are comprehensively studied by direct current（DC） and alternating current（AC） susceptibilities, isothermal remanent magnetizations, and magnetic hysteresis.Fitting to the Curie–Weiss law by using high-temperature zero-field-cooled susceptibility confirms a low-spin state of Co^3＋with S = 0. Both the fitting parameters first increase and then tend to be saturated at high magnetic fields through using isothermal remanent magnetizations, which suggests a spin glass transition at low temperature. AC susceptibility study also supports this conclusion since the frequency dependence of peak position and intensity follows the tendency of a spin glass transition. The origin of the spin-glass transition in Li CoMnO4 might be attributed to a spatial segregation between non-magnetic Co^3＋regions and spin glass ordered regions of Mn^4＋ions.

Electrochemical Behavior of Vanadium Carbide in Neutral Aqueous Electrolytes

The V_2C compound,belonging to the group of two-dimensional transition metal carbonitrides,or MXenes,has demonstrated a promising electrochemical performance in capacitor applications in acidic electrolytes;however,there is evidence to suggest that V_2C is unstable in an acidic environment.On the other hand,the performance of V_2C in neutral aqueous electrolytes is still moderate,and has not yet been systematically studied.The charge storage mechanism in a V_2C electrode,employed in neutral aqueous electrolytes,is investigated via cyclic voltammetry testing and in situ x-ray diffraction(XRD).Good specific capacitances are achieved,specifically208 F/g in 0.5 M Li_2SO_4,225 F/g in 1 M MgSO_4,120 F/g in 1 M Na_2 SO_4,and 104 F/g in 0.5 M K_2SO_4.Using in situ XRD,we observe that,during the charge and discharge process,the c-lattice parameter shrinks or expands by up to 0.25 A in MgSO_4,and 0.29 A in Li_2SO_4 which demonstrates the intercalation/de-intercalation of cations into the d-V_2C layer.

Rational design and synthesis of nanosheets self-assembled hierarchical flower-ball-like CuFeS_(2)for boosted wide temperature sodium-ion batteries

Nano-structure designs with conductive networks have been demonstrated as an efficient strategy to boost sodium storage properties for transition metal sulfides.Herein,an exquisite nanosheets self-assembled hierarchical flower-ball-like CuFeS_(2)embedded into the reduced graphene oxide(RGO)nanosheet matrix(F-CuFeS_(2)@RGO)is fabricated via a concise two-step solvothermal method.Such a well-designed architecture affords increased active reaction interfaces and enhanced mixed ionic/electronic conductivity.Meanwhile,the external RGO matrix can effectively alleviate the volume expansion and create a stable structure during long cycles.As a result,the composite material exhibits a high reversible capacity of 559 mAh·g^(-1)at 0.1 A·g^(-1),a superior rate capability of 455 mAh·g^(-1)at 5 A·g^(-1)and excellent cyclic stability with 96%capacity retention after 4800 cycles at 5 A·g^(-1),among the best in the state-of-the-art transition metal sulfide anodes.Especially,F-CuFeS_(2)@RGO delivers outstanding low-temperature performances with a high capacity retention of 100%and 91%at-20 and-40℃,respectively,over 200 cycles.The proposed hierarchical structure fabrication paves a new direction in the design of high-performance electrodes for all-temperature energy storage applications.

Simulation of the T-jump triggered unfolding and thermal unfolding vibrational spectroscopy related to polypeptides conformation fluctuation

We review in this article our recent simulation works on modeling peptide T-jump and thermal unfolding Fourier transform infrared spectroscopy(FTIR) and two-dimensional infrared(2DIR) spectra. The theoretical and computational techniques used,including Markov state model(MSM), integrated tempering sampling(ITS) and nonlinear exciton propagation(NEP), are first briefly introduced. The protocols for simulating the thermal unfolding as well as T-jump unfolding are then summarized in details. The simulated spectral features, such as the intensity and ellipticity, are demonstrated to agree well with the experimental observations.

ZnO/ZnS core-shell composites for low-temperature-processed perovskite solar cells

Electron transport layers （ETLs） in perovskite solar cells （PSCs） are a key factor to determine the photo- voltaic performance. Herein, we demonstrate preparation of ZnO/ZnS core-shell composites through di- rectly synthesizing ZnS on the ZnO nanoparticles in solution. We confirmed the formation of ZnO/ZnS core-shell composites by the uses of X-ray diffraction patterns and the Fourier transform infrared spec- troscopy. ZnO/ZnS composites exhibit much homogeneous surface morphology as compared with the bare ZnO as revealed in the scanning electronic microscopy. Moreover, the upper shift of conduction band level upon composition of the ZnO/ZnS film results in a better alignment of energy level, which facilitates cas- cade charge extraction and thus improves the current density of perovskite solar cell. The shift of conduc- tion band also improves the voltage of the PSCs. The photoluminescence （PL） spectroscopies measured in both steady and transient states were carried out to characterize the charge extraction at the interface between CH_3NH_3Pbl_3 and the electron transport layers of either ZnO or ZnO/ZnS composite. The ZnO/ZnS composite can more efficiently quench the PL signal of perovskite absorber than bare ZnO resulting in enhanced photocurrent generation in PSCs.