A synergetic promotion of surface stability for high-voltage LiCoO2 by multi-element surface doping: a first-principles study.

The utilization of high-voltage LiCoO2 is an effective approach to break through the bottleneck of practical energy density in lithium ion batteries. However, the structural and interfacial degradations at the deeply delithiated state as well as the associated safety concerns impede the application of high-voltage LiCoO2. Herein, we present a synergetic strategy for promoting the surface stability of LiCoO2 at high voltage by Ti-Mg-Al co-doping and systematically study the effects of the dopants on the surface stability, electronic structure and Li+ diffusion properties of the LiCoO2 (104) surface using first-principles calculations. It is found that Ti, Mg and Al dopants can be facilely introduced into the Co sites of the LiCoO2 (104) surface. Furthermore, the co-doping could significantly stabilize the surface oxygen of LiCoO2 at a high delithiation state. Particularly, by aggregating Ti-Mg-Al co-dopant distribution in the surface layer, surface oxygen loss is dramatically suppressed. In addition, analysis of the electronic structure indicates that Ti-Mg-Al co-doping can enhance the electronic conductivity of the LiCoO2 (104) surface and greatly inhibit the charge deficiency of the superficial lattice O atoms at a highly delithiated state. In spite of a negligible improvement in the surface Li+ diffusion kinetics, the Ti-Mg-Al surface-modified LiCoO2 is expected to exhibit improved electrochemical performance at high voltage due to its superior surface stability. Our results suggest that aggregating Ti, Mg and Al co-dopant distribution in the surface layer is a promising modulation strategy to synergistically promote the surface oxygen stability of LiCoO2 at high voltages.

The interfacial properties of 2D metal-monolayer blue phosphorene heterojunctions and transport properties of their field-effect transistors.

Monolayer blue phosphorene (BlueP) has attracted much interest as a potential channel material in electronic devices. Searching for suitable two-dimensional (2D) metal materials to use as electrodes is critical to fabricating high-performance nanoscale channel BlueP-based field effect transistors (FETs). In this paper, we adopted first-principles calculations to explore binding energies, phonon calculations and electronic structures of 2D metal-BlueP heterojunctions, including Ti3C2-, NbTe2-, Ga(110)- and NbS2-BlueP, and thermal stability of Ti3C2-BlueP heterojunction at room temperature. We also used density functional theory coupled with the nonequilibrium Green function method to investigate the transport properties of sub-5 nm BlueP-based FETs with Ti3C2-BlueP electrodes. Our calculated results indicate that Ti3C2-BlueP has excellent thermal stability and may be used as a candidate electrode material for BlueP-based FETs. The double-gate can more effectively improve the device performance compared with the single-gate. The estimated source leakage current of sub-5 nm transistors reaches up to 369µA µm-1, which is expected to meet the requirements of the international technology roadmap for semiconductors for LP (low-power) devices. Our results imply that 2D Ti3C2may act as an appropriate electrode material for LP BlueP-based FETs, thus providing guidance for the design of future short-gate-length BlueP-based FETs.

Hydrogeochemical changes during artificial groundwater well recharge.

Artificial groundwater recharge is a relatively economic and efficient method for solving shortages and uneven spatial-temporal distribution of water resources. Changes in groundwater quality during the recharge process are a key issue that must be addressed. Identifying the hydrogeochemical reactions that occur during recharge can be vital in predicting trends in groundwater quality. However, there are few studies on the evolution of groundwater quality during artificial recharge that comprehensively consider environmental, chemical, organic matter, and microbiological indicators. Based on an artificial groundwater recharge experiment in Xiong'an New Area, this study investigated the hydrogeochemical changes during groundwater recharge through a well. The results indicate that (1) as large amounts of recharge water (RW) were injected, the groundwater level initially rose rapidly, then fluctuated slowly, and finally rose again. (2) Water quality indicators, dissolved organic matter (DOM), and microbial communities were influenced by the mixture of RW and the background groundwater before recharge (BGBR), as well as by water-rock interactions, such as mineral dissolution-precipitation and redox reactions. (3) During well recharge, aerobic respiration, nitrification, denitrification, high-valence manganese (Mn) and iron (Fe) minerals reduction dissolution, and Mn2+ and Fe2+ oxidation-precipitation occurred sequentially. (4) DOM analysis showed that protein-like substances in the BGBR were the main carbon sources for aerobic respiration and denitrification, while humic-like substances carried by the RW significantly enhanced Mn and Fe minerals reduction dissolution. Therefore, RW quality significantly affects groundwater quality after artificial groundwater well recharge. Controlling indicators, such as dissolved oxygen (DO) and DOM, in the RW can effectively reduce harm to groundwater quality after recharge. This study is of theoretical and practical significance for in-depth analysis of the evolution of groundwater quality during artificial well recharge, prediction of trends in groundwater quality during and after recharge and ensuring groundwater quality safety.

A first-principles study of bilayered black phosphorene as a potential anode material for sodium-ion batteries.

Black phosphorene has attracted widespread attention because of its great potential as a high-performance anode material for sodium-ion batteries (SIBs). However, almost all theoretical studies on sodium (Na) atom adsorption and diffusion in it have not taken temperature into account. Actually, the structural stability of an anode material at room temperature is vital in practical applications. In this work, employing first-principles calculations, we investigate the stability of AA-, AB-, AC- and AD-stacked bilayered black phosphorene (BBP) at ground state, and Na adsorption and diffusion within BBPs. Using ab initio molecular-dynamics (AIMD) calculations, dynamic stabilities of pristine BBP and Na-adsorbed BBP systems at room temperature are discussed. Our calculations show that only AB-stacked BBP is stable. Na atoms generally prefer to intercalate within BBP, making all BBPs exhibit metallic properties, which provides good electrical conductivity required for an ideal anode of SIBs. In particular, our AIMD results indicate that the temperature effect on the structural stability of Na-adsorbed BBP could not be neglected. It increases Na capacity loss at room temperature. This provides an important reference for further theoretical and experimental exploration of anode materials for SIBs. Additionally, the AC-stacked structure facilitates Na intercalation within BBP, and Na diffusion exhibits a strong directional preference, diffusing very fast along the zigzag direction. Our results suggest that AC-stacked BBP is a potential anode material of SIBs.

Effect of non-magnetic doping on magnetic state and Li/Na adsorption and diffusion of black phosphorene.

Black phosphorene (BP) have aroused great concern because of its great potential for the application in nanoelectronic devices and high-performance anode materials for alkali metal ion batteries (AIBs). However, the absence of magnetism for an ideal BP limits its wide application in spintronic devices which is one of the important nanoelectronic devices, and its application as a high-performance anode material for AIBs is still to be explored. In this paper, we adopt first-principles calculations to explore the effects of B, C, N, O, F, Al, Si and S atom doping on the magnetic state of monolayer BP and Li or Na atom adsorption and diffusion on the BP. Additionally, the thermal stability of the doped BP systems at room temperature is revealed by theab initiomolecular-dynamics calculations. Our calculated results indicate that O and S doping can make the doped BP become a magnetic semiconductor, C and Si doping makes the doped BP be metallic, and B, N, F and Al doping preserves semiconductor property. Moreover, little structural changes and significant decreases of diffusion barriers in armchair direction and slight increases of diffusion barriers in zigzag direction make B-doped BP beneficial as an anode material for lithium-ion batteries (LIBs) and sodium-ion batteries (SIBs). It reveals that S-doping is suitable for improving the performance of SIBs rather than LIBs. Interestingly, it is found that magnetic states of O- and S-doped BP disappear when Li or Na atoms adsorb on them, whereas Li or Na adsorption on B- and Al-doped BP induces magnetic states of these systems. The analyses indicate that the distinct electron transfer between the dopant atom, adatom and neighboring P atoms, and specific electron configuration of dopant atoms cause the magnetism of the systems. Our results suggest that selecting appropriate composition to dope can effectively manipulate magnetic state and improve Li/Na adsorption and diffusion on the BP. These results may inspire further theoretical and experimental exploration on doped two-dimensional (2D) materials in spintronics and doped 2D promising anode materials for high-performance metal ion batteries.

Prediction and evaluation of groundwater level changes in an over-exploited area of the Baiyangdian Lake Basin, China under the combined influence of climate change and ecological water recharge.

Groundwater (GW) and surface water (SW) are important components of water resources and play key roles in social and economic development and regional ecological security. There are currently several stresses placing immense pressure on the GW resources of the Baiyangdian Lake Basin (BLB) in China, including climate change. A series of ecological and environmental challenges have manifested in the plain area of the BLB due to long-term over-exploitation of GW, including regional declines in GW level, aquifer drainage, land subsidence, and soil secondary salinization. Climate change may aggravate environmental challenges by altering GW recharge rates and availability of GW. This study applied the fully processed and physically-based numerical models, MODFLOW and the Soil & Water Assessment Tool (SWAT) in a semi-coupled modeling framework. The aim of the study was to quantitatively analyze changes to shallow GW levels and reserves in the plain area of BLB over the next 15 years (2021-2035) under climate change and different artificial recharge schemes. The results indicated that GW storage and levels are rising under the different GW recharge schemes. The maximum variation in the GW level was 20-30 m under a rainfall assurance rate of 50% and water level in the depression cone increased 14.20-14.98 m. This study can act as a theoretical basis for the development of a more sustainable GW management scheme in the plain area of the BLB and for the management and protection of aquifers in other areas with serious GW overdraft.

Nitrogen behavior during artificial groundwater recharge through ponds: A case study in Xiong'an New Area.

The Xiong'an New Area (XA) was established as a development hub in China. Excessive exploitation of groundwater has caused a series of environmental and geological problems, restricting further development of XA. The widely distributed ponds in this area have been targeted as convenient and efficient sites of artificial groundwater recharge. However, nitrogen accumulation in the shallow vadose zone associated with agricultural activities may pose environmental risks to groundwater during the recharge and infiltration process. Therefore, this study investigated the effects, transfer, and transformation of nitrogen during artificial groundwater recharge. The aeration zone is thick and the medium comprises fine particles, with total nitrogen and nitrate accumulation mainly in the shallow aeration zone. In indoor experiments, the nitrate removal rate reached 83.5% when organic carbon in the source water was increased by 10 mg/L. For Baigou diversion river water(BW) with slightly higher (14.46 mg/L) and lower (5.04 mg/L) nitrate contents, the nitrate content decreased by 26.0% (10.70 mg/L) and 26.8% (3.69 mg/L), respectively, after 150 days. When the water head was increased by 20 cm to increase the recharge rate, the time required for nitrate and ammonium to reach the maximum and equilibrium concentration was reduced by 50%. These findings indicate that nitrogen concentration in the source water, aeration zone media, and groundwater should be considered in pond replenishment. It is also necessary to control the concentration of organic carbon and the rate of recharge, which would provide guidance for other similar projects.

One- and two-dimensional electrical contacts and transport properties in monolayer black phosphorene-Ni interface.

Contacts between black phosphorene (BP) and metal electrodes are critical components of BP-based devices and can dramatically affect device performance. In this paper, we adopted first-principles calculations to explore binding energies, electronic structures, spatial potential distribution of monolayer BP-Ni interfaces in surface contact and edge contact types, and used density functional theoretical coupled with nonequilibrium Green's function method to investigate the electrical transport properties for transport systems of monolayer BP with Ni electrodes. Our calculated results indicate that contact type between monolayer BP and metal Ni electrodes may much affect the transport properties of monolayer BP-Ni devices. Interfacial interaction between Ni and monolayer BP in edge contact type is stronger than that in surface contact type. The potential distributions indicate that edge contact type is more beneficial for reducing contact resistance of monolayer BP-Ni contacts and conducive to improve the performance of BP-Ni electrode device.

Robustness of the electronic structure and charge transfer in topological insulator Bi2Te2Se and Bi2Se2Te thin films under an external electric field.

Using first-principles calculations based on density functional theory, we systematically investigated the electronic properties and charge transfer of topological insulator Bi2Te3-xSex thin films under an external electric field. As the selenium content in Bi2Te3-xSex thin films increases, the band gap is gradually opened, with changes in the charge distribution. In addition, the experimentally stable Bi2Te2Se and Bi2Se2Te thin films are extremely robust under vertical electric fields up to 0.2 V Å-1. The electronic structures of Bi2Te2Se and Bi2Se2Te thin films are insensitive to the electric fields and exhibit only a Rashba-like splitting pattern near the Fermi level. Remarkably, the charge transfer in Bi2Te2Se and Bi2Se2Te thin films under an external electric field is suppressed. We found that the robustness characteristic is inextricably linked to the strong covalent bonding of tellurium and bismuth atoms. These results indicated that Bi2Te2Se and Bi2Se2Te thin films are robust to the internal electrical field during growth on the substrate, which is beneficial for experimental studies as well as for the potential applications of spintronic devices.

In Situ Observation of the Insulator-To-Metal Transition and Nonequilibrium Phase Transition for Li1-xCoO2 Films with Preferred (003) Orientation Nanorods.

It is notoriously difficult to distinguish the stoichiometric LiCoO2 (LCO) with a O3-I structure from its lithium defective O3-II phase because of their similar crystal symmetry. Interestingly, moreover, the O3-II phase shows metallic conductivity, whereas the O3-I phase is an electronic insulator. How to effectively reveal the intrinsic mechanism of the conductivity difference and nonequilibrium phase transition induced by the lithium deintercalation is of vital importance for its practical application and development. Based on the developed technology of in situ peak force tunneling atomic force microscopy (PF-TUNA) in liquids, the phase transition from O3-I to O3-II and consequent insulator-to-metal transition of LCO thin-film electrodes with preferred (003) orientation nanorods designed and prepared via magnetron sputtering were observed under an organic electrolyte for the first time in this work. Then, studying the post-mortem LCO thin-film electrode by using ex situ time-dependent XRD and conductive atomic force microscopy, we find the phase relaxation of LCO electrodes after the nonequilibrium deintercalation, further proving the differences of the electronic conductivity and work function between the O3-I and O3-II phases. Moreover, X-ray absorption spectroscopy results indicate that the oxidation of Co ions and the increasing of O 2p-Co 3d hybridization in the O3-II phase lead to electrical conductivity improvement in Li1-xCoO2. Simultaneously, it is found that the nonequilibrium deintercalation at a high charging rate can result in phase-transition hysteresis and the O3-I/O3-II coexistence at the charging end, which is explained well by an ionic blockade model with an antiphase boundary. At last, this work strongly suggests that PF-TUNA can be used to reveal the unconventional phenomena on the solid/liquid interfaces.

Band-structure engineering of the magnetically Cr-doped topological insulator Sb2Te3 under mechanical strain.

Magnetic doping in topological insulator Sb2Te3 can produce very novel physical phenomena such as quantum anomalous Hall effect (QAHE). However, experimental observations of QAHE in the magnetic atoms doped Sb2Te3 have encountered significant challenges due to the complexity of the electronic structure and the relatively small band gap. Generally, mechanical strain can effectively modulate the band structure, thus we theoretically investigate the electronic structures of Cr-doped Sb2Te3 under mechanical strain using first-principles calculations within density functional theory. The band gap of Cr-doped Sb2Te3 is 0.031 eV. When the compressive strain η becomes as -2%, the band gap will be further enlarged to 0.045 eV, which is 45% larger than that of the unstrained material. However, as the compressive strain η exceed -2%, strong hybridization between Cr and Te atoms will cause the overlap of bands, which leads to the closure of band gap. In addition, when tensile strain is applied to Cr-doped Sb2Te3, the decrease in the spacing between quintuple layers can enhance the coupling between Te and Sb atoms, which can also result in the closing of the band gap. Finally, we used HSE06 to calculated the band gaps. The band gaps may be underestimated, but HSE06 and GGA have the same band structures evolution tendency under mechanical strain. Our calculated results provide a guideline for the modulation of band structure by mechanical strain, which pave the way for the observation of QAHE in Cr-doped Sb2Te3.

Strain-engineered indirect-direct band-gap transitions of PbPdO2 slab with preferred (0 0 2) orientation.

Layered transition metal oxide PbPdO2 has great potential application in electronic devices because of its unique electronic structure and large thermoelectric power at room temperature. In this work, strain effect on the electronic structure of PbPdO2 slab with preferred (0 0 2) orientation was systematically investigated using first-principles calculation. The calculated results indicate that PbPdO2 ultrathin slab possesses a small indirect gap while an indirect-direct band gap transition occurs when a moderate 2% compression or tensile strain is applied on the slab. Moreover, this strain induced indirect-direct band gap transition was analyzed in detail using the charge density difference at different point of valence band. The charge transfer and energy barrier with charge polarization resulting from the changes of bond length and angle for Pd-O bonding under the strain, have been accounted for this transition. Remarkablely, for the (0 0 2) preferred orientation PbPdO2 slab, the predicted carrier mobilities of electrons and holes are 11 645.31 and 694.60 cm2 V-1 s-1 along the x-axis direction, 935.05 and 16.05 cm2 V-1 s-1 along the y -axis direction, respectively. These calculated mobilities of electrons along the x-axis direction are larger than those for 2D MoS2 (~400 cm2 V-1 s-1), and being comparable to those for InSe (103 cm2 V-1 s-1) and black phosphorene (103-104 cm2 V-1 s-1). It is strong suggested that the (0 0 2) orientated PbPdO2 slab with high mobility should be an ideal candidate material for the application of electronics devices.

Strain Engineered Band Gaps and Electronic Properties in PbPdO2 and PbPd0.75Co0.25O2 Slabs.

Electronic structure and corresponding electrical properties of PbPdO2 and PbPd0.75Co0.25O2 ultrathin slabs with (002) preferred orientation were systematically investigated using first-principles calculations. The calculated results revealed the strain induced evidently the changes of band structure and carrier concentration in both slabs. It was also found that PbPdO2 and PbPd0.75Co0.25O2 ultrathin slabs exhibited evident differences in the external strain dependence of the band gap and charge carrier concentration, which was strongly dependent on bond angle and bond length induced by in-plane anisotropy strain. Interestingly, the carrier concentration of the PbPd0.75Co0.25O2 slab could increase up to 5⁻6 orders of magnitude with the help of external strain, which could explain the potential mechanism behind the observed colossal strain-induced electrical behaviors. This work demonstrated that the influence of the doping effect in the case of PbPdO2 could be a potentially fruitful approach for the development of promising piezoresistive materials.

Effects of graphene intercalation on dielectric reliability of HfO2 and modulation of effective work function for Ni/Gr/c-HfO2 interfaces: first-principles study.

We have investigated the effects of graphene intercalation on dielectric reliability of HfO2 for Ni/Gr/HfO2 interfaces, and the effects of graphene intercalation and interfacial atom vacancy on the effective work function (EWF) of Ni/Gr/HfO2 interfaces using first-principle calculation based on density functional theory. The calculated results indicate that graphene intercalation can improve dielectric reliability of HfO2 dielectric even for the interfaces having interfacial oxygen vacancy or a small amount carbon vacancy. Moreover, the calculated results indicate that, inserting graphene into Ni/HfO2 interface induces the EWF's to decline, and controlling interfacial oxygen or carbon vacancy can effectively tune the EWF of Ni/Gr/HfO2 interface. Our work strongly suggests that the use of graphene synthesized into Ni/HfO2 interface is a very effective way to improve the interface quality, and controlling interfacial oxygen or carbon vacancy is also an attractive and promising way for modulating the EWF of Ni/Gr/HfO2 interfaces.

An Ab Initio and Kinetic Monte Carlo Simulation Study of Lithium Ion Diffusion on Graphene.

The Li⁺ diffusion coefficients in Li⁺-adsorbed graphene systems were determined by combining first-principle calculations based on density functional theory with Kinetic Monte Carlo simulations. The calculated results indicate that the interactions between Li ions have a very important influence on lithium diffusion. Based on energy barriers directly obtained from first-principle calculations for single-Li⁺ and two-Li⁺ adsorbed systems, a new equation predicting energy barriers with more than two Li ions was deduced. Furthermore, it is found that the temperature dependence of Li⁺ diffusion coefficients fits well to the Arrhenius equation, rather than meeting the equation from electrochemical impedance spectroscopy applied to estimate experimental diffusion coefficients. Moreover, the calculated results also reveal that Li⁺ concentration dependence of diffusion coefficients roughly fits to the equation from electrochemical impedance spectroscopy in a low concentration region; however, it seriously deviates from the equation in a high concentration region. So, the equation from electrochemical impedance spectroscopy technique could not be simply used to estimate the Li⁺ diffusion coefficient for all Li⁺-adsorbed graphene systems with various Li⁺ concentrations. Our work suggests that interactions between Li ions, and among Li ion and host atoms will influence the Li⁺ diffusion, which determines that the Li⁺ intercalation dependence of Li⁺ diffusion coefficient should be changed and complex.

Electronic structure and its external electric field modulation of PbPdO2 ultrathin slabs with (002) and (211) preferred orientations.

The Electronic structure of PbPdO2 with (002) and (211) preferred orientations were investigated using first-principles calculation. The calculated results indicate that, (002) and (211) orientations exhibit different electric field dependence of band-gap and carrier concentration. The small band gap and more sensitive electric field modulation of band gap were found in (002) orientation. Moreover, the electric field modulation of the resistivity up to 3-4 orders of magnitude is also observed in (002) slab, which reveals that origin of colossal electroresistance. Lastly, electric field modulation of band gap is well explained. This work should be significant for repeating the colossal electroresistance.