Although LiFePO4 has been widely studied and also used as a promising cathode material for Li ion batteries, its inferior tap density is still a big challenge for practical application. Well-defined three-dimensional porous LiFePO4 microspheres composed of nanosheets were successfully synthesized by a simple one-step solvothermal method. The porous spherical morphology could not only retain the excellent electrochemical performance characteristics of the LiFePO4 nanosheet but also meet the requirements of high tap density of the powder particles, leading to highly improved volumetric energy density. These micro-nano structured LiFePO4 microspheres have a high tap density of about 1.4 g cm−3. A growth mechanism is also proposed based on time-dependent experiments. This work provides an efficient route for designing a desirable micro–nano structure, which could also be extended to synthesize other hierarchical structures used in different fields.

•Porous sheet-like and sphere-like nano-architectures of SnO2 nanoparticles have been prepared.•A solvent-thermal approach without surfactant or polymer templates simply by changing the volume ratio of DMF to water.•The formation mechanism of nano-architectures is proposed in this article.•Porous sphere-like SnO2 nano-architectures exhibit good sensitivity to the reduce vapors tested.•Sheet-like materials show better selectivity to ethanol.Porous sheet-like and sphere-like nano-architectures of SnO2 nanoparticles have been prepared via a solvent-thermal approach in the absence of any surfactant or polymer templates by simply changing the volume ratio of DMF to water. The nano-materials have been characterized by FESEM, XRD, IR, TEM and BET. A mechanism for the formation of nano-architectures is also proposed based on the assembly behaviors of DMF in water. The gas sensors constructed with porous sphere-like SnO2 nano-architectures exhibit much higher sensitivity to the reduce vapors tested, compared to those from porous sheet-like SnO2 materials, while the sheet-like materials show better selectivity to ethanol. The nano-architectures fabricated with the facile method are promising candidates for building chemical sensors with tunable performances.

LiFePO4/C nanosheet composite has been prepared via a low-temperature solvothermal reaction followed by high-temperature treatment. The as-prepared sample is characterized by XRD, FTIR, Raman, SEM, and TEM. It is confirmed that the nanosheets are composed of ca. 50 nm thickness of crystalline LiFePO4-core coated with ca. 10 nm thickness of carbon-shell. The charge–discharge tests show that the as-fabricated LiFePO4/C nanosheet cathode in lithium-ion cell demonstrates high reversible capacity (164 mAh g−1 at 0.1 C) and good cycle stability (columbic efficiency 100% during 100 cycles). The cyclic voltammetric analysis indicates Li+ diffusion determines the whole electrode reaction kinetics, and the diffusion coefficient estimated by EIS is comparable to the reported data. The enhanced kinetic behavior of the as-fabricated cathode is actually originated from the nano-dimensional sheet-like morphology, which facilitates Li+ migration due to the shortened diffusion distance, and simultaneously increased exchangeable Li+ amount considering more accessible active surface. In addition, the uniformly coated thin conductive carbons contribute a lot for this enhancement because of considerably improved electronic conductivity.

In the work, two novel conceptions of “capacity quality” (CQ) and “capacity quality coefficient” (λ) were defined to evaluate cycling power capabilities of Ni–MH rechargeable batteries when considering the effect of the kinetic limitation. For convenient comparison, the capacity quality coefficient (λ) and the efficiency of charge/discharge (η) were in parallel applied to characterize cycling capabilities based on the data from BYD H-3/4AAA800 Ni–MH batteries at 1C–3.5C. The results show that there is an obvious difference between λ and η which served as evaluation indexes for rechargeable batteries, and that the secondary battery with good capacity quality also has a good cycling capability and rate capability, especially at high rate. The introduced capacity quality not only subtly covered kinetic information of the rechargeable batteries but also factually reflected stability of the electrode materials.

Capacity intermittent titration technique (CITT) was used to investigate the chemical diffusion coefficient (D˜) of lithium-ion in LiFePO4 cathode material. The values of D˜ at the galvano-charge current of 0.2 and 0.4 mA were respectively found to range from 8.8 × 10−16 to 8.9 × 10−14 cm2 s−1 and from 1.2 × 10−16 to 8.5 × 10−14 cm2 s−1 in the voltage range from 3.2 to 4 V (vs. Li+/Li). The transfer coefficients of cathode (0.32–0.42) and anodic (0.26–0.3), and the standard rate constant (1.58 × 10−9 to 1.30 × 10−8 cm s−1) were measured from the Tafel plots of LiFePO4 in the equilibrium potential range from 3.06 to 3.45 V. From these kinetic parameters, the finite kinetics at interface was taken into account to revise the above values of D˜. The revised values of D˜ at the galvano-charge current of 0.2 and 0.4 mA were respectively found to range from 2.44 × 10−15 to 2.21 × 10−13 cm2 s−1 and from 5.81 × 10−16 to 3.22 × 10−13 cm2 s−1 in the voltage range from 3.2 to 4 V. Results show that the approximation of infinite charge-transfer kinetics leads to a spurious value of D˜ which is lower than the revised value, and the spurious extent depends on the galvano-charge current of CITT experiment.

Traditionally, chemical reaction between solids has been considered to typically occur on a geological time scale without the benefit of high temperature, due to diffusion block in the solids. However, recent advancements have revealed that many solvent-free reactions between molecular crystals can quickly occur at room or near-room temperature. These reactions have raised a novel scientific question as to how the reactive species can overcome the diffusion-controlled kinetic limitations under such moderate conditions. From time-resolved powder UV−vis reflection spectra and optical micrographs with the reaction between dimethylglyoxime and Ni(Ac)2·4H2O and the reaction between hexamethylenetetramine and CoCl2·6H2O as models, we found that the solvent-free reaction really occurs at an intermediate state between the solid state and the liquid state. Formation of the liquid phase provides a convenient approach to diffusion of reactive species, whereas formation of a solid product layer hampered the transfer of reactive species. Both factors led to a broad reactive rate band in the long reaction region. The results have explained the diffusion mechanism of the fast reaction between the molecular crystals under moderate conditions.

Lithium-ion diffusion in insertion-host materials is of significant interest because of its importance in improving the power density of lithium-ion batteries. In this study, the dependence of the chemical diffusion coefficient (D) of lithium-ion in spinel LiMn2O4 cathode material on electrochemical cycling has been investigated by the capacity intermittent titration technique (CITT). Results show that there are two minimum peaks in the curves of D∼E respectively at ∼3.95 and ∼4.12 V in the voltage range from 3.85 to 4.30 V. The curves of D∼E at different cycles show an interesting phenomenon that the values of D tend to increase with the cycling numbers. This phenomenon indicates an enhanced diffusion of lithium-ion in LiMn2O4 cathode material induced by the electrochemical cycling.