Size dependent electronic structure of LiFePO4 probed using X-ray absorption and Mössbauer spectroscopy.

We present the combined Mössbauer and X-ray absorption spectroscopy investigation of the electronic structure and local site symmetry of Fe in olivine structured LiFePO4 (LFP) with crystallite size (CS). The lattice parameters are found to contract with a decrease in CS, monotonously, whereas the electronic structural parameters exhibit two different regions with a threshold anomaly of around ≈30 nm. 57Fe Mössbauer studies reveal the coexistence of Fe2+ and Fe3+ sites and their relative concentrations are mainly determined by CS, which provides a comprehensive insight into the electronic structure of LFP at the mesoscopic level. The soft X-ray absorption unequivocally unravels the valence states of Fe 3d electrons in proximity to the Fermi level, which are prone to the local lattice distortion. The obtained spectra fingerprint the effect of CS supplying rich information on valency, lithium-ion vacancy concentration, covalency and crystal field. By comparing the spectra with the results of charge-transfer multiplet calculations, which include the full-atomic multiplet theory, we have found that the local symmetry of Fe ions is well described by the D4h point group with intermixing between eg and t2g orbitals. The unique structural and electronic properties of LFP are closely interlinked with changes in the bonding character, which shows the strong dependency on CS. The evolution of 3d states is in overall agreement with the local lattice distortion and provides the origin of the size effects on the electronic structure of olivine phosphate and other transition metal ion-containing materials.

Effect of crystallite size on the phase transition behavior of heterosite FePO4.

For advanced lithium-ion battery technology, olivine-based cathodes are considered to be the most dominant and technologically recognized materials. The extraction of lithium ions from olivine LiFePO4 results in the two-phase mixture with heterosite FePO4 exhibiting a deintercalation potential of 3.45 V vs. Li+/Li over a wide range of lithium content. Here, we report the synthesis and characterization of chemically deintercalated heterosite FePO4 with varying crystallite sizes using different analytical techniques. The decrease in the crystallite size of heterosite FePO4 leads to an increase in the lattice parameters including the unit cell volume. The characteristic behavior in the structural properties of heterosite FePO4 shows a strong dependency on the crystallite size which is correlated with the change in the chemical bonding. The volume expansion of the nano-sized heterosite FePO4 with respect to the bulk counterpart is suggested to be a direct consequence of reduced hybridization between the Fe3d and O2p states. Furthermore, the combined X-ray diffraction and Mössbauer spectroscopic studies reveal the appearance of a new phase namely trigonal FePO4 at the lower crystallite sizes due to the enhanced surface energy kinetics. We also find that the observed trigonal FePO4 phase is more magnetically active than the paramagnetic olivine FePO4. For the unique structural advantage of the heterosite phase as an electrode material, the change in bonding characteristics is very useful and can have strong implications on the electronic properties of heterosite FePO4 at the nanoscale level.

Direct evidence for the influence of lithium ion vacancies on polaron transport in nanoscale LiFePO4.

Improving the electronic conductivity in lithium-based compounds can considerably impact the design of rechargeable batteries. Here, we explore the influence of lithium ion vacancies on the electronic conductivity of LiFePO4, an active cathode material, by varying the crystallite sizes. We find that about 17% lithium ion vacancy concentration leads to an enhancement in electronic conductivity of about two orders of magnitude at 313 K with respect to our initial crystallite size. We attribute the enhanced electronic conductivity to the lithium ion vacancy concentration in addition to the reduced hopping length. The lithium ion vacancies are the source of polarons in LiFePO4, which increases with decreasing crystallite size due to the surface energy kinetics. The substantial increase in the polaronic sites (Fe3+) at a lower crystallite size leads to a reduction in lattice parameters including the unit cell volume. The analysis of temperature dependent dc conductivity within the framework of the Mott model of polaron conduction enables us to quantify the various physical parameters associated with polaron hopping in LiFePO4.

Size induced structural changes in maricite-NaFePO4: an in-depth study by experiment and simulations.

Rechargeable batteries based on the most abundant elements, such as sodium and iron, have a great potential in the development of cost effective sodium ion batteries for large scale energy storage devices. We report, for the first time, crystallite size dependent structural investigations on maricite-NaFePO4 through X-ray diffraction, X-ray absorption spectroscopy and theoretical simulations. Rietveld refinement analysis on the X-ray diffraction data reveals that a decrease in the unit cell parameters leads to volume contraction upon reduction in the crystallite size. Further, the atomic multiplet simulations on X-ray absorption spectra provide unequivocally a change in the site symmetry of transition metal ions. The high resolution oxygen K-edge spectra reveal a substantial change in the bonding character with the reduction of crystallite size, which is the fundamental cause for the change in the unit cell parameters of maricite-NaFePO4. In parallel, we performed first-principles density functional theory (DFT) calculations on maricite-NaFePO4 with different sodium ion vacancy concentrations. The obtained structural parameters are in excellent agreement with the experimental observations on the mesostructured maricite-NaFePO4. The volumetric changes with respect to crystallite size are related to the compressive strain, resulting in the improvement in the electronic diffusivity. The nano-crystalline maricite-NaFePO4 with improved kinetics will open a new avenue for its usage as a cathode material in sodium ion batteries.

Particle size dependent confinement and lattice strain effects in LiFePO4.

We report the intrinsic electronic properties of LiFePO4 (LFP) with different particle sizes measured by broad-band impedance spectroscopy and diffuse reflectance spectroscopy. The electronic properties show typical size-dependent effects with decreasing particle size (up to 150 nm). However, at the nanoscale level, we observed an enhancement in the polaronic conductivity about an order of magnitude. We found that the origin of the enhanced electronic conductivity in LFP is due to the significant lattice strain associated with the reduction of particle size. The observed lattice strain component corresponds to the compressive part which leads to a decrease in the hopping length of the polarons. We reproduce nonlinearities in the transport properties of LFP with particle size, to capture the interplay between confinement and lattice strain, and track the effects of strain on the electron-phonon interactions. These results could explain why nano-sized LFP has a better discharge capacity and higher rate capability than the bulk counterpart. We suggest that these new correlations will bring greater insight and better understanding for the optimization of LFP as a cathode material for advanced lithium ion batteries.

Tunable electronic structure of heterosite FePO4: an in-depth structural study and polaron transport.

The development of better electrode materials for lithium-ion batteries has been intensively investigated both due to their fundamental scientific aspects as well as their usefulness in technological applications. The present technological development of rechargeable batteries is hindered by fundamental challenges, such as low energy and power density, short lifespan, and sluggish charge transport kinetics. Among the various anode materials proposed, heterosite FePO4 (h-FP) has been found to intercalate lithium and sodium ion hosts to obtain novel rechargeable batteries. The h-FP has been obtained via the delithiation of triphylite LiFePO4 (LFP), and its structural and electronic properties have been investigated with different crystallite sizes. The synchrotron XRD measurements followed by Rietveld refinement analysis reveal lattice expansion upon the reduction of crystallite size of h-FP. In addition, the decrease in the crystallite size enhances surface energy contributions, thereby creating more oxygen vacancies up to 2% for 21 nm crystallite size. The expansion in the lattice parameters is reflected in the vibrational properties of the h-FP structure, where the red-shift has been observed in the characteristic modes upon the reduction of crystallite size. The local environment of the transition metal ion and its bonding characteristics have been elucidated through soft X-ray absorption spectroscopy (XAS) with the effect of crystallite size. XAS unequivocally reveals the valence state of iron 3d electrons near the Fermi level, which is susceptible to local lattice distortion and uncovers the detailed information on the evolution of electronic states with crystallite size. The observed local lattice distortion has been considered to be as a result of the decrease in the level of covalency between the Fe-3d and O-2p states. Further, we demonstrate the structural advantages of nanosized h-FP on the transport properties, where an enhancement in the polaronic conductivity with decreasing crystallite size has been observed. The polaronic conduction mechanism has been analyzed and discussed on the basis of the Mott model of polaron conduction along with an insightful analysis on the role of the electronic structure. The present study provides spectroscopic results on the anode material that reveal the evolution of electronic states for fingerprinting, understanding, and optimizing it for advanced rechargeable battery operations.

Effect of surfactant concentration on textural characteristics and biomineralization behavior of mesoporous bioactive glasses.

We report mesoporous bioactive glasses (MBGs) with different pore architecture synthesized using the supramolecular chemical approach using acid assisted sol-gel method followed by evaporation induced self-assembly (EISA) process. The surfactant of non-ionic block co-polymer used in the present work act as structure directing agent (SDA) with varying amount, which brings different textural properties. As prepared glasses possess wormhole-like mesostructure with different textural characteristics and it varies with surfactant concentrations. The pristine MBGs have been characterized by various analytical techniques before and after immersing in simulated body fluid (SBF). The textural properties show non-monotonous variations with varying the surfactant concentrations. We have quantified the various structural species present in the glass sample by using local structural probe such as nuclear magnetic resonance technique. Invitro studies on the MBGs revealed the influence of textural characteristics on the formation of nano-crystalline hydroxycarbonated apatite (HCA) layer. The biodegradability is largely related to the pore architecture and it affects the biocompatibility as well as bone formation. In-vitro studies reveals that there are serious negative effects that have been observed in the case of larger pore sized samples due to rapid degradation. We found that MBG with pore sizes of few nano meters exhibit favorable biocompatibility in vitro behavior and found to be promising candidate in the field of biomaterials including tissue regeneration and drug storage.

Nonideal Behavior of Glass and Crystal.

We report deviations from an ideal behavior of binary chalcogenide glass composition Ge20Te80 with respect to its quenching rate on mass density and thermal parameters, including glass transition temperature. In an ideal glass, the increase in quenching rate will decrease the characteristic relaxation time and correspondingly shift in the glass transition temperature (Tg) to higher temperature and result in lower density. This, however, holds only when the liquid structure remains the same as in equilibrium glass structure independent of their quenching rate. We find Ge20Te80 glass composition with higher quenching rate is found to possess larger density and lower Tg than the lower rate quenched or well annealed glass specimen. In contrast to conventional glass forming liquids, the anomalous behavior of Ge20Te80 glass with respect to quenching rate is closely related to the change in the local atomic structure with thermal history. Additionally, we found that crystal derived from the Ge20Te80 glass with different thermal history but with identical annealing conditions leads to different mass density, specific heat capacity, and local atomic structure. Thus, the observed unusual variations in the mass density and various thermal properties of Ge20Te80 glass and crystal are mainly determined by the resulting local atomic structure and concentration of defect states associated with each state.

Mesoporous 45S5 bioactive glass: synthesis, in vitro dissolution and biomineralization behavior.

The development of a new generation of biomaterials includes a sol-gel process to obtain glass foams, which is a well established method for CaO-SiO2-P2O5 compositions, but is not yet recognized for Bioglass® containing sodium oxide. In this study, we report, for the first time, the synthesis of a mesoporous 45S5 bioactive glass with superior textural characteristics and its in vitro dissolution and biomineralization behavior. Wormhole-like bioactive mesostructured 45S5 glass has been synthesized by an acid assisted sol-gel method followed by an evaporation induced self-assembly process. The virgin mesoporous 45S5 bioactive glass has been characterized by various analytical methods before and after soaking in simulated body fluid (SBF). The factors affecting the glass formation have been discussed in terms of the critical micelle concentration (CMC) at a particular temperature followed by a specified time interval. In vitro studies on the mesostructured 45S5 glass sample reveal the rapid formation of carbonated hydroxyapatite (HCA) with nano sized crystals. The mesostructured glass showed an excellent cell proliferation response without toxicity up to the concentration of 50 µg ml-1. Furthermore, we show that the 45S5 glass with superior textural parameters is extremely useful within the family of bioactive materials as it has accelerated formation kinetics of the apatite phase as compared to other bioactive glass compositions.

Alkali oxide containing mesoporous bioactive glasses: synthesis, characterization and in vitro bioactivity.

We report, for the first time, the synthesis of sodium oxide containing mesoporous bioactive quaternary glasses and compared with two different mesoporous ternary silicate systems by modified sol-gel process. With the aid of three different glass systems, a systematic analysis has been made on phosphorous-bearing (P-bearing) and phosphorous-free (P-free) mesoporous bioactive glasses to investigate the role of phosphorus on in vitro bioactivity of various silicate glasses with constant alkali oxide content. The combined use of multiple analytical techniques XRD, FTIR, SEM, nitrogen adsorption/desorption analysis before and after soaking in the SBF solution allowed us to establish strong correlation between composition, pore structure and bioactivity. We find that the P-bearing mesoporous glasses show the rapid hydroxycarbonate apatite (HCA) crystallization than P-free mesoporous glasses independent of calcium content. The present study reveals that the presence of phosphorous jointly with calcium in the bioactive glass system significantly enhances the rate of apatite formation as well as crystallization of apatite phase. Additionally, we find that a glass with sodium orthophosphate rich phase enhances the solubility when immersed in SBF and further accelerate the kinetics of apatite formation. The influences of the chemical composition and their superior textural properties on bioactivity are explained in terms of the unique structure of mesoporous bioactive glasses.

Bio-inspired synthesis of microporous bioactive glass-ceramic using CT-DNA as a template.

We report, for the first time, bio-inspired synthesis of a bioactive glass-ceramic with superior textural properties in atmospheric conditions using CT-DNA as template. The phase composition, structure, morphology, and textural properties of the bioactive glass sample were evaluated with various analytical techniques before and after in vitro tests. The BET surface area analysis of the obtained glass-ceramic sample reveals that it possesses a high surface area with a range of (micro- to meso-) pore sizes. The TEM analysis of the glass-ceramic phase indicates that the amorphous phase consists of spherical particles, whereas the crystalline phase is found to have needle-like shape. In the glass-ceramic, we find a new type of crystalline phase (Na0.11Ca0.89)(P0.11Si0.89)O3, which is different from the earlier observation on 45S5® glass-ceramic sample. The accelerated in vitro bioactivity of the glass-ceramic is evidenced based on the hydroxyl carbonate apatite (HCA) layer formation on the glass-ceramic surface after immersing the bioglass sample in simulated body fluid (SBF), by FTIR, SEM and EDX analysis. Additionally, the ion release kinetics of the bioglass sample in SBF is followed by ICP-AES with simultaneous pH measurements. The in vitro cytotoxicity experiments on the glass-ceramic sample using osteosarcoma cells by following the MTT assay method indicate that the sample has good biocompatibility and may serve as an effective biomaterial for bone tissue engineering.

Fast interfacial ionic conduction in nanostructured glass ceramics.

The hopping movements of mobile ions in a nanostructured LiAlSiO4 glass ceramic are characterized by time-domain electrostatic force spectroscopy (TDEFS). While the macroscopic conductivity spectra are governed by a single activation energy, the nanoscopic TDEFS measurements reveal three different dynamic processes with distinct activation energies. Apart from the ion transport processes in the glassy and crystalline phases, we identify a third process with a very low activation energy, which is assigned to ionic movements at the interfaces between the crystallites and glassy phase. Such interfacial processes are believed to play a key role for obtaining high ionic conductivities in nanostructured solid electrolytes.

Nanoscopic study of the ion dynamics in a LiAlSiO4 glass ceramic by means of electrostatic force spectroscopy.

We use time-domain electrostatic force spectroscopy (TD-EFS) for characterising the dynamics of mobile ions in a partially crystallised LiAlSiO4 glass ceramic, and we compare the results of the TD-EFS measurements to macroscopic electrical conductivity measurements. While the macroscopic conductivity spectra are determined by a single dynamic process with an activation energy of 0.72 eV, the TD-EFS measurements provide information about two distinct relaxation processes with different activation energies. Our results indicate that the faster process is due to ionic movements in the glassy phase and at the glass-crystal interfaces, while the slower process is caused by ionic movements in the crystallites. The spatially varying electrical relaxation strengths of the fast and of the slow process provide information about the nano- and mesostructure of the glass ceramic.