Thermal Expansion Neutralization Enhancing the Cycling Stability of Ni-Rich LiNi0.6Co0.2Mn0.2O2 Cathode Material.

As promising cathode candidates with advantageous capacity and price superiority for lithium-ion batteries, Ni-rich materials are severely impeded in the practical application due to their poor microstructural stability induced by the intrinsic Li+/Ni2+ cation mixing and mechanical stress accumulation upon cycling. In this work, a synergetic approach is demonstrated to improve the microstructural and thermal stabilities of Ni-rich LiNi0.6Co0.2Mn0.2O2 (NCM622) cathode material through taking advantage of the thermal expansion offset effect of the LiZr2(PO4)3 (LZPO) modification layer. The optimized NCM622@LZPO cathode exhibits a significantly enhanced cyclability with a capacity retention of 67.7% after 500 cycles at 0.2 C and delivers a specific capacity of 115 mAh g-1 with a capacity retention of 64.2% after 300 cycles under 55 °C. Exploiting the chemical environment analysis of the Ni element detected by the synchrotron radiation technique, it is found that the mixing degree of Li+/Ni2+ cations in the bulk Ni-rich material can be effectively depressed through interfacial Zr4+ doping during the preparation of the LZPO-modified material. Additionally, time- and temperature-dependent powder diffraction spectra were collected to monitor the structure evolutions of pristine NCM622 and NCM622@LZPO cathodes in the initial cycles and under various temperatures, revealing the contribution of negative thermal expansion LZPO coating in promoting microstructural stability of the bulk NCM622 cathode. The introduction of NTE functional compounds might provide a universal strategy to address the stress accumulation and volume expansion issues of various cathode materials for advanced secondary-ion batteries.

Understanding the Effect of Top Electrode on Ferroelectricity in Atomic Layer Deposited Hf0.5Zr0.5O2 Thin Films.

It is commonly believed that the impact of the top electrodes on the ferroelectricity of hafnium-based thin films is due to strain engineering. However, several anomalies have occurred that put existing theories in doubt. This work carries out a detailed study of this issue using both theoretical and experimental approaches. The 10 nm Hf0.5Zr0.5O2 (HZO) films are prepared by atomic layer deposition, and three different top capping electrodes (W/MO/ITO) are deposited by physical vapor deposition. The electrical testing finds that the strain does not completely control the ferroelectricity of the devices. The results of further piezoelectric force microscopy characterization exclude the potential interference of the top capping electrodes and interface for electrical testing. In addition, through atomic force microscopy characterization and statistical analysis, a strong correlation between the grain size of the top electrode and the grain size of the HZO film has been found, suggesting that the grain size of the top electrode can induce the formation of the grain size in HZO thin films. Finally, the first-principles calculation is carried out to understand the impact of the strain and grain size on the ferroelectric properties of HZO films. The results show that the strain is the dominant factor for ferroelectricity when the grain size is large (>10 nm). However, when the grain size becomes thinner (<10 nm), the regulation effect of grain sizes increases significantly, which could bring a series of benefits for device scaling, such as device-to-device variations, film uniformity, and domain switch consistency. This work not only completes the understanding of ferroelectricity through top electrode modulation but also provides strong support for the precise regulation of ferroelectricity of nanoscale devices and ultrathin HZO ferroelectric films.

Dehydration of Electrochemically Protonated Oxide: SrCoO2 with Square Spin Tubes.

Controlling oxygen deficiencies is essential for the development of novel chemical and physical properties such as high-Tc superconductivity and low-dimensional magnetic phenomena. Among reduction methods, topochemical reactions using metal hydrides (e.g., CaH2) are known as the most powerful method to obtain highly reduced oxides including Nd0.8Sr0.2NiO2 superconductor, though there are some limitations such as competition with oxyhydrides. Here we demonstrate that electrochemical protonation combined with thermal dehydration can yield highly reduced oxides: SrCoO2.5 thin films are converted to SrCoO2 by dehydration of HSrCoO2.5 at 350 °C. SrCoO2 forms square (or four-legged) spin tubes composed of tetrahedra, in contrast to the conventional infinite-layer structure. Detailed analyses suggest the importance of the destabilization of the SrCoO2.5 precursor by electrochemical protonation that can greatly alter reaction energy landscape and its gradual dehydration (H1-xSrCoO2.5-x/2) for the SrCoO2 formation. Given the applicability of electrochemical protonation to a variety of transition metal oxides, this simple process widens possibilities to explore novel functional oxides.

Electronic structures and anisotropic carrier mobilities of monolayer ternary metal iodides MLaI5(M=Mg, Ca, Sr, Ba).

Exploiting two-dimensional (2D) materials with natural band gaps and anisotropic quasi-one-dimensional (quasi-1D) carrier transport character is essential in high-performance nanoscale transistors and photodetectors. Herein, the stabilities, electronic structures and carrier mobilities of 2D monolayer ternary metal iodides MLaI5(M = Mg, Ca, Sr, Ba) have been explored by utilizing first-principles calculations combined with numerical calculations. It is found that exfoliating MLaI5monolayers are feasible owing to low cleavage energy of 0.19-0.21 J m-2and MLaI5monolayers are thermodynamically stable based on phonon spectra. MLaI5monolayers are semiconductors with band gaps ranging from 2.08 eV for MgLaI5to 2.51 eV for BaLaI5. The carrier mobility is reasonably examined considering both acoustic deformation potential scattering and polar optical phonon scattering mechanisms. All MLaI5monolayers demonstrate superior anisotropic and quasi-1D carrier transport character due to the striped structures. In particular, the anisotropic ratios of electron and hole mobilities along different directions reach hundreds and tens for MLaI5monolayers, respectively. Thus, the effective electron-hole spatial separation could be actually achieved. Moreover, the absolute locations of band edges of MLaI5monolayers have been aligned. These results would provide fundamental insights for MLaI5monolayers applying in nano-electronic and optoelectronic devices.

Ideal two-dimensional solid electrolytes for fast ion transport: metal trihalides MX3 with intrinsic atomic pores.

Exploring ultrathin two-dimensional (2D) solid electrolytes with fast ion transport is highly desirable in nanoelectronics, ionic devices and various energy storage systems, following the rapid scaling of devices to the nanometer scale. Herein, two-dimensional (2D) metal trihalides MX3 (ScCl3, ScBr3, AsI3, ScI3, YBr3, SbI3, YI3 and BiI3) with intrinsic atomic pore structures have been examined and found to be promising as realistic 2D solid electrolytes. Through examining the binding interactions and the diffusion barriers of monolayer MX3-ion (Li+, Na+, K+, Mg2+, and Ca2+) systems by utilizing first principles calculations, it is found that MX3-ion complexes are energetically favorable and the energy barriers of some MX3-ion systems are comparable to or even smaller than those of the conventional solid electrolyte systems. More significantly, the short diffusion time of Na+ and K+ ions in some monolayers MX3 at the nanosecond (ns) or even at the sub-ns scale indicates fast ion transport. In terms of practical applications, ultrafast Li+ travelling in the timescale of sub-ns to ns and Na+ in several tens ns in few-layer MX3 is achieved. In addition, the insulating nature of wide band gaps for MX3 is maintained during the ion transport, which is essential for solid electrolytes. These theoretical results provide fundamental guidance that MX3 materials with natural atomic pores are realistic candidates for 2D solid electrolytes with broad applications in ionic devices and energy storage devices.

Molecularly Thin Electrolyte for All Solid-State Nonvolatile Two-Dimensional Crystal Memory.

A molecularly thin electrolyte is developed to demonstrate a nonvolatile, solid-state, one-transistor (1T) memory based on an electric-double-layer (EDL) gated WSe2 field-effect transistor (FET). The custom-designed monolayer electrolyte consists of cobalt crown ether phthalocyanine and lithium ions, which are positioned by field-effect at either the surface of the WSe2 channel or an h-BN capping layer to achieve "1" or "0", respectively. Bistability in the monolayer electrolyte memory is significantly improved by the h-BN cap with density functional theory (DFT) calculations showing enhanced trapping of Li+ near h-BN due to a ∼1.34 eV increase in the absolute value of the adsorption energy compared to vacuum. The threshold voltage shift between the two states corresponds to a change in charge density of ∼2.5 × 1012 cm-2, and an On/Off ratio exceeding 104 at a back gate voltage of 0 V. The On/Off ratio remains stable after 1000 cycles and the retention time for each state exceeds 6 h (max measured). When the write time approaches 1 ms, the On/Off ratio remains >102, showing that the monolayer electrolyte-gated FET can respond on time scales similar to existing flash memory. The data suggest that faster switching times and lower switching voltages could be feasible by top gating.

First-Principle Prediction on STM Tip Manipulation of Ti Adatom on Two-Dimensional Monolayer YBr3.

Scanning tunneling microscopy (STM) is an important tool in surface science on atomic scale characterization and manipulation. In this work, Ti adatom manipulation is theoretically simulated by using a tungsten tip (W-tip) in STM based on first-principle calculations. The results demonstrate the possibility of inserting Ti adatoms into the atomic pores of monolayer YBr3, which is thermodynamically stable at room temperature. In this process, the energy barriers of vertical and lateral movements of Ti are 0.38 eV and 0.64 eV, respectively, and the Ti atoms are stably placed within YBr3 by >1.2 eV binding energy. These theoretical predictions provide an insight that it is experimentally promising to manipulate Ti adatom and form artificially designed 2D magnetic materials.