Molecular Dynamics Simulations of Li Insertion in a Nanocrystalline V 2 O 5 Thin Film Cathode
The behavior of lithium ion diffusion from an electrolyte into a polycrystalline layered cathode has been studied using molecular dynamics computer simulations. Lithium silicate glass was the model solid electrolyte whilethe cathode was a nanocrystalline vanadia with amorphous V 2 O 5 intergranular films (IGF) between the crystals. Nanosized V 2 O 5 crystals were aligned with their (001) planes parallel to electrolyte/cathode interface, rotated 90° from each other around this interface's normal in order to present two different orientations between the crystal planes for lithium intercalation via the amorphous vanadia IGF. A series of nanocrystalline vanadia cathodes with different IGF thicknesses was simulated to examine the effects of the IGF thickness on lithium transport into the cathodes. Results showed preferential diffusion of Li from the electrolyte into the amorphous vanadia IGF, with some of those Li diffusing into the crystalline V 2 O 5 from the IGF. Results also showed easier lithium diffusion from the IGF into the V 2 O 5 crystal along the (010) direction than along the (100) direction. Additionally, an optimum IGF thickness of 2.5-3.0 nm is suggested as being neither too thick to decrease the capacity of the cathode nor too thin to impede the transport of lithium from glassy electrolyte into the cathode.
A facile and generic method to improve cathode materials for lithium-ion batteries via utilizing nanoscale surface amorphous films of self-regulating thickness.
As a facile and generic surface modification method, a unique class of surface amorphous films (SAFs) is utilized to significantly improve the rate performance and cycling stability of cathode materials for lithium-ion batteries. These nanoscale SAFs form spontaneously and uniformly upon mixing and annealing at a thermodynamic equilibrium, and they exhibit self-regulating or "equilibrium" thickness due to a balance of attractive and repulsive interfacial interactions acting on the films. Especially, spontaneous formation of nanoscale Li3PO4-based SAFs has been demonstrated in two proof-of-concept systems, LiCoO2 and LiMn1.5Ni0.5O4, which have an equilibrium thickness of ∼2.9 nm and ∼2.5 nm, respectively. At a high discharge rate of 25 C, these Li3PO4-based SAFs improve the discharge capacity by ∼130% for LiCoO2 and by ∼40% for LiMn1.5Ni0.5O4, respectively. Furthermore, these SAFs improve the cycling stability and reduce capacity fading of both LiCoO2 and LiMn1.5Ni0.5O4. At an elevated temperature of 55 °C, Li3PO4-based SAFs can help to maintain ∼90 mA h g(-1) discharge capacity of the high-voltage material LiMn1.5Ni0.5O4 after 350 cycles at a relatively high charge-discharge rate of 1 C. Further mechanistic studies suggest that these SAFs reduce the interfacial charge transfer resistance and suppress the growth of the solid-electrolyte interphase. This facile method can be utilized to improve a broad range of cathode and anode materials. A thermodynamic framework is proposed, which can be used to guide future experiments of other material systems.
Calcium Concentration Dependence of the Intergranular Film Thickness in Silicon Nitride
High-resolution electron microscopy and nano-beam analytical electron microscopy have been used to characterize both the intergranular silicate film thickness and its local composition in a series of high-purity Si3N4 ceramics doped with 0–450 at. ppm Ca. Calcium was detected at both two-grain junctions and triple junctions, even in the 80-ppm-Ca-doped specimen. The thickness of the intergranular film at two-grain junctions was found to depend sensitively on Ca content. In undoped material, the thickness was 1.0 ± 0.1 nm. With increasing Ca content, the thickness decreased in the dilute region (80 ppm Ca), but then increased. The variation in film thickness can be qualitatively understood in terms of the balance of three long-range forces acting normal to the film, namely the van der Waals dispersion force, a structural “steric” force, and an electrical-double-layer force. By comparing the measured thicknesses to those predicted, estimates for the structural correlation length and the inverse Debye length can be made. These estimates have values of ∼ 0.22 nm and approximately 0.3–0.5 nm, respectively, for the calcia-free and 80 ppm calcia materials.