Multiple applications of lithium‐ion batteries in energy storage systems and electric vehicles require highly stable electrode materials for long‐term battery operation. Among the various cathode ...materials, high‐Ni cathode materials enable a high energy density. However, cathode degradation accompanied by complex chemical and structural changes results in capacity and voltage fading in batteries. Cathode degradation remains poorly understood; the majority of studies have only explored the oxidation states of transition‐metal ions in localized areas and have rarely evaluated chemical degradation in complete particles after prolonged cycling. This study systematically investigates the degradation of a high‐Ni cathode by comparing the chemical, structural, and electrical changes in pristine and 500 times‐cycled cathodes. Electron probe micro‐analysis and X‐ray energy dispersive spectroscopy reveal changes in the Ni:O ratio from 1:2 to 1:1 over a large area inside the secondary particle. Electron energy loss spectroscopy analysis related to structural changes is performed for the entire primary particle area to visualize the oxidation state of transition‐metal ions in two dimensions. The results imply that the observed monotonic capacity fade without unusual changes is due to the continuous formation of the Ni2+ phase from the surface to the bulk through chemical and structural degradation.
The degradation of the high‐Ni layered oxide cathode is systematically investigated by comparing a pristine and 500‐cycled full‐cell through analyzing the microstructural visualization of the structural, chemical, and electrical changes from micrometer‐scale to the nanometer‐scale. This reveals that the monotonic capacity fade is due to the continuous formation of Ni2+ phase at the surface and bulk via chemical and structural degradation.
Non‐toxic InP‐based nanocrystals have been developed for promising candidates for commercial optoelectronic applications and they still require further improvement on photophysical properties, ...compared to Cd‐based quantum dots (QDs), for better device efficiency and long‐term stability. It is, therefore, essential to understand the precise mechanism of carrier trapping even in the state‐of‐the‐art InP‐based QD with near‐unity luminescence. Here, it is shown that using time‐resolved spectroscopic measurements of systematically size‐controlled InP/ZnSe/ZnS core/shell/shell QDs with the quantum yield close to one, carrier trapping decreases with increasing the energy difference between band‐edge and trap states, indicating that the process follows the energy gap law, well known in molecular photochemistry for nonradiative internal conversion between two electronic states. Similar to the molecular view of the energy gap law, it is found that the energy gap between the band‐edge and trap states is closely associated with ZnSe phonons that assist carrier trapping into defects in highly luminescent InP/ZnSe/ZnS QDs. These findings represent a striking departure from the generally accepted view of carrier trapping mechanism in QDs in the Marcus normal region, providing a step forward understanding how excitons in nanocrystals interact with traps, and offering valuable guidance for making highly efficient and stable InP‐based QDs.
In InP/ZnSe/ZnS quantum dots with near‐unity luminescence, carrier trapping into shallow defects at a picosecond timescale is a key mechanism to reduce the quantum yield. The carrier trapping assisted by ZnSe phonons can slow down with increasing the energy gap between the band‐edge and shallow trap states.
Robust multi‐level spin memory with the ability to write information electrically is a long‐sought capability in spintronics, with great promise for applications. Here, nonvolatile and highly ...energy‐efficient magnetization switching is achieved in a single‐material device formed of van‐der‐Waals (vdW) topological ferromagnet Fe3GeTe2, whose magnetic information can be readily controlled by a tiny current. Furthermore, the switching current density and power dissipation are about 400 and 4000 times smaller than those of the existing spin‐orbit‐torque magnetic random access memory based on conventional magnet/heavy‐metal systems. Most importantly, multi‐level states, switched by electrical current are also demonstrated, which can dramatically enhance the information capacity density and reduce computing costs. Thus, the observations combine both high energy efficiency and large information capacity density in one device, showcasing the potential applications of the emerging field of vdW magnets in the field of spin memory and spintronics.
The highly energy‐efficient nonvolatile magnetization switching and multi‐level states by current are achieved in single van‐der‐Waals (vdW) topological ferromagnet Fe3GeTe2. The switching current density and power dissipation are about 400 and 4000 times smaller than conventional magnet/heavy‐metal‐based spin orbit torque magnetic random access memory. The observations combine high energy‐efficiency and large information‐capacity‐density in one device, bringing the nascent vdW magnets toward spin memory and spintronics.
Concepts of non-volatile memory to replace conventional flash memory have suffered from low material reliability and high off-state current, and the use of a thick, rigid blocking oxide layer in ...flash memory further restricts vertical scale-up. Here, we report a two-terminal floating gate memory, tunnelling random access memory fabricated by a monolayer MoS2/h-BN/monolayer graphene vertical stack. Our device uses a two-terminal electrode for current flow in the MoS2 channel and simultaneously for charging and discharging the graphene floating gate through the h-BN tunnelling barrier. By effective charge tunnelling through crystalline h-BN layer and storing charges in graphene layer, our memory device demonstrates an ultimately low off-state current of 10(-14) A, leading to ultrahigh on/off ratio over 10(9), about ∼10(3) times higher than other two-terminal memories. Furthermore, the absence of thick, rigid blocking oxides enables high stretchability (>19%) which is useful for soft electronics.
Printing techniques could offer a scalable approach to fabricate thermoelectric (TE) devices on flexible substrates for power generation used in wearable devices and personalized thermo-regulation. ...However, typical printing processes need a large concentration of binder additives, which often render a detrimental effect on electrical transport of the printed TE layers. Here, we report scalable screen-printing of TE layers on flexible fiber glass fabrics, by rationally optimizing the printing inks consisting of TE particles (p-type Bi
Sb
Te
or n-type Bi
Te
Se
), binders, and organic solvents. We identified a suitable binder additive, methyl cellulose, which offers suitable viscosity for printability at a very small concentration (0.45-0.60 wt.%), thus minimizing its negative impact on electrical transport. Following printing, the binders were subsequently burnt off via sintering and hot pressing. We found that the nanoscale defects left behind after the binder burnt off became effective phonon scattering centers, leading to low lattice thermal conductivity in the printed n-type material. With the high electrical conductivity and low thermal conductivity, the screen-printed TE layers showed high room-temperature ZT values of 0.65 and 0.81 for p-type and n-type, respectively.
For developing the industrially feasible Ni-rich layered oxide cathode with extended cycle life, it is necessary to mitigate both the mechanical degradation due to intergranular cracking between ...primary particles and gas generation from the reaction between the electrolyte and residual Li in the cathode. To simultaneously resolve these two issues, we herein propose a simple but novel method to reinforce the primary particles in LiNi0.91Co0.06Mn0.03O2 by providing a Li-reactive material, whose spinel interphase is coherent with the surface of the cathode. The modified structure significantly outperforms analogous bare samples: they conserve more than 90% of the initial capacity after 50 cycles and also exhibit a greater rate capability. By tracking the same particle location during cycling, we confirmed that the current method significantly reduces crack generation because the provided coating material can penetrate inside the grain boundary of the secondary particle and help maintain the volume of the primary particle. Finally, first-principles calculations were implemented to determine the role of this spinel material, i.e., having intrinsically isotropic lattice parameters, superior mechanical properties, and only a small volume change during delithiation. We believe that the proposed method is straightforward and cost-effective; hence, it is directly applicable for the mass production of Ni-rich cathode material to enable its commercialization.
Layered lithium transition‐metal oxide materials, e.g., Li(Ni1−x−yCoxMny)O2 (NCM) and Li(Ni1−x−yCoxAly)O2, are the most promising candidates for lithium‐ion battery cathodes. They generally consist ...of ≈10 µm spherical particles densely packed with smaller particles (0.1–1 µm), called secondary and primary particles, respectively. The micrometer‐ to nanometer‐sized particles are critical to the battery performance because they affect the reaction capability of the cathode. Herein, the crystal structure of the primary particles of NCM materials is revisited. Elaborate transmission electron microscopy investigations reveal that the so‐called primary particles, often considered as single crystals, are in fact polycrystalline secondary particles. They contain low‐angle and exceptionally stable special grain boundaries (GBs) presumably created during aggregation via an oriented attachment mechanism. Therefore, this so‐called primary particle is renamed as primary‐like particle. More importantly, the low‐angle GBs between the smaller true primary particles cause the development of nanocracks within the primary‐like particles of Ni‐rich NCM cathodes after repetitive electrochemical cycles. In addition to rectifying a prevalent misconception about primary particles, this study provides a previously unknown but important origin of structural degradation in Ni‐rich layered cathodes.
The crystal structure of primary particles in layered lithium transition‐metal oxide materials is investigated by transmission electron microscopy. The micrometer‐ to nanometer‐sized particles, the so‐called primary particles, are in fact polycrystalline secondary particles containing low‐angle and special grain boundaries. The low‐angle grain boundaries in the so‐called primary particles can cause nanocrack development after repetitive electrochemical cycles in lithium‐ion batteries.
A stretchable resistive pressure sensor is achieved by coating a compressible substrate with a highly stretchable electrode. The substrate contains an array of microscale pyramidal features, and the ...electrode comprises a polymer composite. When the pressure‐induced geometrical change experienced by the electrode is maximized at 40% elongation, a sensitivity of 10.3 kPa−1 is achieved.
Herein, we probed the microstructure of Tb-diffused Nd-Fe-B magnets to investigate the relationship between Tb-diffused area and coercivity enhancement, employing prolonged stepwise heat treatment to ...ensure sufficient diffusion of Tb in relatively large-size magnets and revealing that this stepwise annealing generated core-shell structures. Quantitative compositional changes pertaining to individual phases of the multiphase system in each heat treatment process were analyzed by constructing ternary diagrams based on electron probe microanalysis compositional maps. During the grain boundary diffusion process, coercivity increased from 15.28 to 24.86 kOe, while only negligible remanence and energy product decreases were concomitantly observed. Microstructure analysis suggested that coercivity was closely related to the concentration and distribution of Tb; more precisely, the abovementioned core-shell structures successfully suppressed the nucleation of reverse domains at Nd-rich phase/main phase interfaces and therefore enhanced magnet coercivity without decreasing remanence and energy product.
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•We employed grain boundary diffusion to enhance the coercivity using TbH in large-sized magnets.•The stepwise annealing for large-sized magnets were introduced for enhancing the coercivity.•The relationship between Tb-diffused area and coercivity was revealed by probing microstructure.•EPMA investigations including ternary diagram confirmed with the XRD analysis.•We enhanced coercivity without decreasing their remanence and energy product.
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