Functional Materials Letters

The paper presents basics of the lithium intercalation process into cathode materials used in lithium batteries. The ability and efficiency of lithium intercalation into transition metal compounds have been shown to depend strongly on their electronic structure. A correlation between chemical bonding, electronic structure and electrochemical properties of the cathode materials Li_xM_aX_b (M = transition metal; X=O, S, Se) has been pointed out.

The paper presents a review of the structural, electrical and electrochemical properties of LiFePO₄ pospho-olivine dopped with W, Ti, Al and manganese substituted LiFe_1-yMn_yPO₄, synthesized at high temperatures as well as preparation method of LiFePO₄–Fe₂P composite phases. The nature of the metallic type conductivity of doped phospho-olivine is discussed and some fundamental arguments against the bulk nature of the observed high electronic conductivity are presented.

Significant advantage could be achieved if mixed reactant fuel cells, MR-FC, were functioning. These cells are intended to operate on a mixture of air and fuel introduced into both the cathode and anode compartment. Symmetry is broken by using different electrode materials exhibiting special and different catalytic properties. No high temperature fuel cell was reported to date to function as a true MR-FC and only one, low temperature type, did function properly. We discuss the required catalytic properties which are unique in that they promote electrochemical reactions and suppress chemical ones as well as possible ways to search for them. The chemical reaction which has to be suppressed is the direct reaction of fuel and oxygen as the two components are premixed and the mixture is then introduced into the fuel cell at both electrode compartments. The electrochemical reactions that should be promoted are the reduction of oxygen at the cathode and the oxidation of fuel at the anode only by oxygen ions that emerge from the solid electrolyte. Conditions to promote this selectivity are discussed. These are derived from the theory of chemisorption as applied to heterogeneous catalysis.

The electronic structure and magnetic properties of Fe₂CrAl have been investigated theoretically and experimentally. The theoretical calculations show that there is an energy gap in the minority-spin band in the electronic structure of ordered Fe₂CrAl with the L2₁ structure and that the spin-polarization ratio is as high as 93%. The calculated total spin moment is 0.97 μ_B/f.u. However, in melt-spun Fe₂CrAl ribbons, the B2 type structure, an atomic-site disordered L2₁ structure, is found and this atomic site disorder gives rise to a magnetic moment of 2.02 μ_B in the ribbons which is much higher than the theoretical value. The calculations prove that the enhanced magnetic moment is mainly due to the Fe–Cr and Fe–Al types of disorder. In contrast, the Cr–Al type disorder has little influence on the total spin moment and the spin-polarization ratio.

Defects and electronic structures of the aliovalent acceptors substituted lead titanates were studied using density functional theory calculations. Three types of defect structures are identified in 3d transition metal and the group IIB and VB elements substituted systems. Cr substitute and oxygen vacancies are found to be dissociated from each other, forming an isolated point defect structure. In contrast, all other substitutes favor the immobile acceptor–oxygen–vacancy–acceptor defect clusters which weaken the space charge effects by limiting the motions of oxygen vacancies. Furthermore, two distinct defect-cluster structures (along the z direction and in the xy plane, respectively) are observed. We conclude that the defect structure in the xy plane induces head-to-head polarization patterns, which make the domain pinning effects even weaker. The electronic structures due to elements substitutions are also investigated and compared.

In the field of high-performance piezoelectric materials, PMN-PT single crystals and textured ceramics have been attracting increased research interest for several years. On the other hand, the growth of single crystals from melt for PZT-based compositions is impossible due to its incongruent melting behavior. In order to obtain the characteristics of pure single crystal PZT as closely as possible, the PZT must be textured by secondary recrystallization of introduced seeds in a fine-grained matrix. Zirconium was therefore added to a PMN-PT-ceramic with 32 mol% PT (Pb(Mg_1/3Nb_2/3)_0.68Ti_0.32O₃) in order to obtain a PMN-PZT-ceramic with 37 mol% PT and 21 mol% PZ (Pb(Mg_1/3Nb_2/3)_0.42(Ti_0.638Zr_0.362)_0.58O₃). Initially, the growth mechanism of (001)-oriented BaTiO₃ (BT) single crystals in those matrices was investigated. The piezoelectric single crystals were produced via a process that starts with the hot pressing of a BT single crystal in cold isostatically pressed ceramics, followed by an additional sintering step in order to achieve a secondary recrystallization. The measured growth lengths in PMN-PT and PMN-PZT matrices were up to 140 μm and 65 μm, respectively. Having developed this understanding, both ceramics were textured via the templated grain growth (TGG) process by using (001)-oriented BT templates. Sintering of templated grain bodies resulted in template growth into the matrix to produce textured ceramics with Lotgering factors up to 0.99 for both compositions. In textured samples unipolar strain s₃₃ was enhanced by a factor of up to 1.8 compared to randomly oriented ceramics. By contrast, BT single crystal growth in an alternative PZT matrix with NdMn doping was not successful. Hence, in the present work, growth experiments in this NdMn-doped PZT were first performed using PZT fibers of similar composition as seeds. Growth of the fiber diameter of up to 100 μm was observed in that matrix.

The scaling relations for the hysteresis loops of soft and hard PZT bulk ceramics were obtained in the forms of <A> ∝ f ^-0.25E₀ and , respectively, while those for the minor loops took the forms of and . The similarity of the scaling relations suggested that the scaling behaviors of the two types of bulk ceramics were in the same universality class and that the complex defects contributed only slightly to the dynamic behaviors. Finally, this study implied that the domain structures played significant role in controlling dynamic hysteresis behavior of ferroelectric materials.

A non-solvent induced phase separation (NIPS) process was used to prepare a new kind of porous nanocomposite polymer electrolyte using glycerol as non-solvent for poly(vinylidene difluoride-co-hexafluoropropylene) copolymer. TiO₂ nanoparticles were incorporated by in situ hydrolysis of Ti(OC₄H₉)₄ and dispersed uniformly in the polymer matrix. They affected the porous structure of the obtained nanocomposite polymer membranes (NCPMs). When the amount of TiO₂ arrived at 7.6 wt.%, the NCPM reached the maximum porosity of 70.5%. The resulting nanocomposite polymer electrolyte (NCPE) presents an ionic conductivity of up to 1.98 × 10^-3 S · cm^-1 at room temperature and an apparent activation energy for ion transport of 13.32 kJ · mol^-1, suggesting its promising application in lithium ion batteries.

Amorphous Ti₅₀Ni_(50-x)Cu_x (at.%) (x = 15, 20 and 25) alloy ribbons were prepared by melt spinning, and then their crystallization behavior was investigated by optical microscopy, transmission electron microscopy, X-ray diffraction and differential scanning calorimetry. Wavenumber (Qp) decreased from 29.40 nm^-1 to 29.29 nm^-1 and ΔT(T_g - T_x) increased from 31 K to 36 K with increasing Cu content from 15 at.% to 25 at.%, suggesting that glass forming ability of Ti–Ni–Cu alloy ribbons increased with increasing Cu content. Activation energy for crystallization decreased from 211.5 kJ/mol to 136.4 kJ/mol with increasing Cu content from 15 at.% to 25 at.%, suggesting that a stability of Ti–Ni–Cu amorphous decreased with increasing Cu content.

Highly ordered mesoporous iron oxide (α-Fe₂O₃) material has been successfully obtained from mesoporous silica template, KIT-6 (3-D Cubic Ia3d symmetry), through nano-replication method. The mesoporous α-Fe₂O₃ material thus obtained exhibits well-defined mesopores (2.7 nm in diameter), high surface area (148 m²/g), high pore volume (0.47 cm³/g) and crystalline frameworks. The morphology of the mesoporous α-Fe₂O₃ material is very uniform in spherical shape of which the average particle size is about 100 nm in diameter.

A first principle based mathematical model is presented to simulate the performance of a lithium-ion battery/electrochemical capacitor hybrid system. The advantages of the hybrid system over the battery system are discussed for pulse type of loads. The hybrid system predicted better utilization of the battery operated under high frequencies and low duty ratio pulse loads. Simulated specific energy and power relationships (Ragone plots) are also presented to identify regimes where the hybrid systems performed better than the battery system on a unit mass basis. The model was validated by comparing the simulations to the experimental data obtained from a Sony US 18650 lithium-ion battery/Maxwell PC100F electrochemical capacitor network hybrid system. Finally a case study comparing the response of the battery and a hybrid system to two different high power pulse profiles encountered in real time applications is presented. Simulation results indicate that hybrid systems exhibited superior performance over the battery system for pulse power profiles with sufficient relaxation times.