Transition Metal Silicate Research Articles

Fe and V Substituted Li2MnSiO4/C As Potential Cathode Material for Li-Ion Batteries

Rechargeable batteries can store energy in the form of chemical energy and facilitate it with a high conversion rate when needed. Moreover, rechargeable batteries are used in almost all kinds of portable consumer electronics, hybrid and pure electric vehicles. Thus, the development of advanced battery technologies is a major field of scientific focus. Li-ion technology has been shown to be superior compared to other battery concepts in performance, cycling stability, self-discharge and expected lifetime.[1] However, commercially used cathode materials (LiMO2) exhibit low capacities compared to the graphite based anodes (~300 mAh/g) and thus limits the battery performance. In the last decade polyanion based materials gained interest and in 2005 Nyten et al. reported on Li2FeSiO4as a new Li-battery cathode material [2]. Lithium transition-metal silicate based materials are promising candidates for next generation’s Li-ion batteries since they allow Li extraction/insertion beyond one Li ion per formula unit. Furthermore, they consist of cheap, non-toxic and abundant elements [3].This work focuses on Li2MnSiO4 (LMS), which can theoretically deliver two Li-ions per formula unit since the transition metal ion possesses two redox couples (Mn3+/Mn2+ + Mn4+/Mn3+). Synthesis of phase pure LMS material has, however, turned out to be challenging. Here, synthesis of LMS with high phase purity was demonstrated by an acidic, PVA assisted sol-gel method using metal nitrates and TEOS as precursors, which is also suitable for upscaling. The dried precursor was pre-calcined and then mixed with a given amount of corn-starch as carbon source. To obtain the desired phase and coat the material in a single step, the powder/starch mix was then carbothermally reduced at 700 °C for 10 h. The atmosphere and starch content are seemingly crucial in both heat treatments to achieve high phase purity in the final cathode powder. Best results were achieved when both heat treatments were carried out in 95% Ar 5% H2 and with starch contents ≥ 25 wt-%. Figure 1 shows powder XRD patterns of LMS with optimized parameters and different starch contents and a full pattern refinement.The synthesized materials were micro- and meso-porous powders with a thin uniform carbon coating (confirmed by TEM), offering high external surface areas of more than 30 m2g-1 (excluding micropore area which is inaccessible to the electrolyte). In addition to pure Li2MnSiO4, this work also focuses on the synthesis and electrochemical performance of Fe and V substituted LMS. Fe substitution should provide increased cycling stability since Li2FeSiO4 has shown relatively good long-term stablility [2,3]. The doping of V on either the Mn or the Si site of LMS is currently under investigation. V doped LMS is believed to offer improved electrochemical performance since V offers 3 redox couples in the accessible voltage window of the electrolyte of a Li-ion battery [4].[1] Tarascon, J. M., Armand, M, Issues and challenges facing rechargeable lithium batteries, Nature, 2001, 414, 359-367.[2] Nyten A., Abouimrane A., Armand M., Gustafsson T., Thomas J. O., Electrochemical performance of Li2FeSiO4 as a new Li-battery cathode material, Electrochemistry Communications, 2005, 7, 156-160.[3] Saiful Islam M., Dominko R., Masquelier C., Sirisopanaporn C., Armstrong A. R., Bruce P. G., Silicate cathodes for lithium batteries: alternatives to phosphates?, J. Mater. Chem., 2011, 21, 9811-9818[4] Li, Y, Cheng, X, Zhang, Y, Achieving High Capacity by Vanadium Substitution into Li2FeSiO4, Journal of The Electrochemical Society, 2012, 159 (2) A69-A74.

Solution Based Methods for Fast Determination of Lithium Insertion/Extraction Kinetics

Arguably, the electric and hybrid electric vehicles are the new frontiers in lithium batteries. As a new market it is distinctly different from mass produced microelectronics, not only in the energy per battery unit but also in the power performance requirements. Especially the required charging rates during deceleration and emergency charging, as well as, the discharge rates during startup from standstill are exciting challenges.[1,2] Consequently new chemistries and new materials morphologies like nanowires, shape controlled nanoparticle and size/shape controlled meso-scale agglomerates are being developed at an impressive rate. [3,4]The standard methodology to characterize the rate performance of these new materials is to fabricate composite electrodes that combine the electroactive materials with a conductive matrix and polymer binder.[5] This porous electrode structure is filled with electrolyte and thus electron and electrolyte transfer paths to the active material is established (Figure:Left). This brings about an important question inherent to the composite electrode, i.e. is the electrochemical performance limited by the active material or by the transport of electrons and/or ions in the electrolyte filled electrode structure?In response to this type of question we developed a series of techniques that allow the study of the redox process kinetics without the use of composite electrodes.[6-8] The key to our methodology is that the electrons are delivered and retrieved not by a solid-solid contact to the active material, but via a solution based redox couple (Figure:Right). As such our techniques are similar to the common potential step electrochemical approach while providing two unique advantages: A) the complete surface of the active particle experiences the same electrochemical potential at all times. B) the flux of lithium ions to and from the particle is unobstructed. In turn, this allows the study of materials like transition metal phosphates and silicates without the conductive coating required in the standard electrochemical analysis. Consequently, and possibly for the first time, the effect of the coating and its fabrication process on the lithium insertion/extraction kinetics can be studied. Another advantage of electron transport via redox species is that nanoparticles can be studied without concerns that they due to their small nature have not been properly incorporated in the electrode matrix.In this presentation we will display the analytical techniques that we have developed over the electron transport via solution based redox species theme. This includes in situ and ex situ detection of the reaction progress, as well as, several different redox systems appropriate for different positive electrode materials. Importantly, we will show that sub-second time resolution is possible using a photometric technique and that this approach yields considerably faster results, compared to the procedure required to produce test batteries. Further the reaction uniformity throughout the sample provides new kinetic insight, which can help to distinguish between possible reaction mechanisms, thus providing better fundamental understanding of both new and existing redox chemistries. References. [1] Baisden, A.C., Emadi, A. (2004) IEEE Trans Veh Tech, 53, 199. [2] Zaghib, K., et al. (2013) Materials, 6, 1028. [3] Bi, Z., et al. (2013) RSC Adv, 3, 19744. [4] Bresser, D., et al. (2012) J Power Sources, 219, 217. [5] Park, M., et al. (2010) J Power Sources, 195, 7904. [6] Trinh, N.D., et al. (2012) J Power Sources, 200, 92. [7] Kuss, C., et al. (2013) Chem Sci, 4, 4223. [8] Lepage, D., et al.(2014) J Power Sources, DOI: 10.1016/j.jpowsour.2013.12.054. Figure: Left: Standard battery. Right: Electron transport via solution based redox species. Arrows indicate transport (white: Electron; gray: Lithium) to the active material particle in the center of the image.

Substitution Chemistry of Li2MSiO4: A Highly Flexible Framework

Lithium transition metal silicates, Li2MSiO4(M=Fe, Mn, Co)[1], have attracted a lot of interest recently as positive electrodes for Li-ion batteries due to their potentially high theoretical capacities and to their rich crystal chemistry as they undergo complex polymorphism. One of the drawbacks of the silicates is their modest rate capability, particularly at room temperature. The present work addressed this problem.The polymorphs of Li2MSiO4can be classified into low (β) and high (γ) temperature forms, which differ in the ordering/distribution of cations within tetrahedral sites of an hcp based packing of oxygen. β forms exhibit only corner sharing and γ polymorphs both corner and edge sharing. It has been shown by us and by others that Li2FeSiO4 exists in at least 3 different polymorphic forms and that. in the case of Li2MnSiO4two stable polymorphs are observed [1-3].The silicates are analogous to the LISICON materials (Lithium SuperIonic Conductors), of the form Li2+2xZn1-xGeO4, which adopt the γ structure with interstitial and highly mobile lithium. Therefore, stabilization of the γ polymorph to room temperature represents a potentially attractive route to enhancing the rate performance thus overcoming one of the major barriers to adoption of Li2MSiO4 cathodes. With appropriate substitutions we have been able to stabilise the γ polymorph for both Li2FeSiO4 and Li2MnSiO4systems. We will report the effects on structure and electrochemistry of the various substitutions. In the iron system good cycling stability and improved rate capability are observed (fig. 2).

Electrochemical Properties of Cu Doped Lithium Manganese Silicate/PANI Composite As Cathode Material for Lithium-Ion Batteries

Development of cathode materials for lithium battery application like electric vehicle (EV) and plug-in hybrid electric vehicle (PHEV) requires high capacity materials[1]. At the same time, cost, environmental concerns and safety issues cannot be excluded. Recently, lithium transition-metal silicates such as Li2MSiO4 (M=Fe, Mn and Co) have been extensively studied as cathode materials for lithium ion batteries, due to high theoretical capacity and better thermal stability than lithium transition metal oxides such as LiCoO2. Among them, Li2MnSiO4 cathode could be a better choice as a high capacity( ~330 mAh/g) cathode for lithium ion batteries than Li2FeSiO4 cathode material. Due to Mn2+/Mn3+ and Mn3+/Mn4+ redox couples, extraction of two lithium ions can be achieved from Li2MnSiO4. However, due to defects such as structure change after the first charge and low electronic conductivity(~ < 10-14S/cm), this material has poor cycle performance and lower capacity than the theoretical prediction [2-3].In this regard, the current work is aimed on the improvement of capacity and cycle stability of Li2MnSiO4 cathode active material by applying Cu doping and poly-aniline composites. The Li2Mn0.95Cu0.05SiO4were prepared using the sol-gel synthesis. PANI was synthesized by oxidative polymerization.XRD patterns as shown in Figure 1 exhibit that the Cu doping and PANI composite does not contribute to structure change of Li2MnSiO4. From the cycle performances as shown in Figure 2, the Li2Mn0.95Cu0.05SiO4/PANI cell shows a high discharge capacity, as well as improved cycle retention compared with pristine Li2MnSiO4 material. Synthesis methodology of Li2Mn0.95Cu0.05SiO4/PANI materials along with their physical, morphological and electrochemical characteristics will be discussed in detail. References : [1] R.J. Gummow, N.Sharma, V.K.Peterson, Y.He, J. solid state chem., 188 (2012) 32–37.[2] R. Dominko, M. Bele, A. Kokalj, M. Gaberscek, J. Jamnik , J. Power Sources., 174 (2007) 457–461.[3] D. M. Kempaiah, D. Rangappa, I. Honma, Chem. Commun.,48 (2012) 2698–2700.

Polymorphism in Li2MSiO4 (M = Fe, Mn): A Variable Temperature Diffraction Study

AbstractLithium transition metal silicates, Li2MSiO4 (M = Fe, Mn, Co), have attracted much interest as positive electrodes for Li‐ion batteries due to their high theoretical capacities and their rich crystal chemistry. Low (β) and high (γ) temperature forms of these tetrahedral structures differ in the ordering/distribution of cations within tetrahedral sites of an hcp based packing of oxygen. We have carried out VT powder neutron and X‐ray diffraction on Li2MSiO4 (M = Fe, Mn) to characterize the rich polymorphism observed across the Li2MSiO4 (M = Fe, Mn) system as a function of temperature. For Li2FeSiO4 below 500 °C a βII phase was observed with slight Li/Fe disorder. Above this temperature there was a sluggish transition to the γs phase, which was observed up to ca. 820 °C, with the γII polymorph seen at highest temperatures. Excellent agreement with phases obtained by quenching was observed, except that the γ phases exhibit a statistical distribution of Li/Fe, in contrast to the well‐ordered structures of quenched phases. The Li2MnSiO4 polymorphism is less complex, the βII phase is stable from room temperature to 750 °C, at which temperature it transforms to the γ structure with disordered Li/Mn.

Chromatographic Separation Studies of Penicillins, Cephalosporins and Carbapenems on Transition-Metal Silicate Modified Silica Layers

The chromatographic behavior of penicillins, cephalosporins and carbapenems has been studied on the thin layers of transition-metal ion (viz. Ni(2+)/Zn(2+)/Cu(2+)/Co(2+)) silicate modified silica. Transition-metal silicate (3.92%) and silica (96.08%) were found to be optimum and resulted in spherical-compact spots and improved resolution of the analytes. The effect of various mobile phases was also investigated. The chromatograms were visualized as yellow spots by placing in an I(2)-chamber. The method has been found to be reproducible and convenient for routine analysis.

First-Principles Study of the Electronic and Dielectric Properties of Group IVB Transition Metal Oxides and Silicates.

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Magnetic excitations and heat capacity of fayalite, Fe2SiO4

We have used inelastic neutron-scattering measurements to study the magnetic excitations in the antiferromagnetic and paramagnetic phases of polycrystalline fayalite, Fe2SiO4. Sharp, nondispersing excitations are found in the ordered state, at 3.3, 5.4, 5.9, and 11.4 meV, and are interpreted as arising from the spin-orbit manifold of the Fe2+ ions. These excitations are increasingly damped with increasing temperature, merging into a quasielastic continuum near the 65 K Neel temperature, although their energy does not vary with temperature. We have calculated the contribution of the heat capacity arising from these magnetic excitations and found that it compares favorably with the magnetic heat capacity deduced experimentally. Our analysis indicates that the M1 and M2 sites behave distinctly. The M1 site behaves quasi-locally and appears in the heat capacity as a Schottky anomaly that explains the shoulder in the heat capacity curve near 20 K, while the M2 site contributes predominantly to the critical lambda anomaly. The behavior of fayalite illuminates the nature of magnetic states in several related minerals, including others that also show shoulders and lambda anomalies in the heat capacity (tephroite), those that show only lambda anomalies (cobalt olivine and liebenbergite), and those that show only non-lambda anomalies (bronzite, anthophyllite, and almandine). We find no evidence to support the recent claim that some transition metal silicate and germanate olivines exhibit strong geometric frustration.

Salt‐Inclusion Synthesis of Two New Polar Solids, Ba6Mn4Si12O34Cl3 and Ba6Fe5Si11O34Cl3.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Salt-Inclusion Synthesis of Two New Polar Solids, Ba6Mn4Si12O34Cl3 and Ba6Fe5Si11O34Cl3

A new family of salt-containing, mixed-metal silicates (CU-14), Ba6Mn4Si12O34Cl3 (1) and Ba6Fe5Si11O34Cl3 (2), was synthesized via the BaCl2 salt-inclusion reaction. These compounds crystallize in the noncentrosymmetric (NCS) space group Pmc2(1) (No. 26), adopting 1 of the 10 NCS polar, nonchiral crystal classes, mm2 (C2v). The cell dimensions are a = 6.821(1) A, b = 9.620(2) A, c = 13.172(3) A, and V = 864.4(3) A3 for 1 and a = 6.878(1) A, b = 9.664(2) A, c = 13.098(3) A, and V = 870.6(3) A3 for 2. The structures form a composite framework made of the (M(4+x)Si(12-x)O34)9- (M = Mn, x = 0; M = Fe, x = 1) covalent oxide and (Ba6Cl3)9+ ionic chloride sublattices. The covalent framework exhibits a pseudo-one-dimensional channel where the extended barium chloride lattice (Ba3Cl1.5)(infinity) resides, and it consists of fused eight-membered meta-silicate rings propagating along [100] via sharing two opposite [Si2O7]6- units to form an acentric lattice. Single-crystal structure studies also reveal the ClBa4 unit adopting an interesting seesaw configuration, in which the lone pair electrons of chlorine preferentially face the oxide anions of the transition metal silicate channel, thus forming the observed polar frameworks. Similar to the synthesis of organic-inorganic hybrid materials, the salt-inclusion method facilitates a promising approach for the directed synthesis of special framework solids, including NCS compounds, via composite lattices.

Final state effects in VUV and soft X-ray absorption spectra of transition metal oxides and silicate alloys: comparisons between experiment and ab initio calculations

This paper uses X-ray absorption spectroscopy and vacuum ultra-violet spectroscopic ellipsometry to study the electronic structure of high- k transition metal (TM) oxide gate dielectrics. The results are applicable to TM and rare earth (RE) silicate and aluminate alloys, as well as complex oxides comprised of mixed TM/TM and TM/RE oxides. These studies identify the nature of the lowest conduction band d * states, which define the optical band gap, including their relationship to the band gap, E g, of the oxide.

Spectroscopic studies of metal high-k dielectrics: transition metal oxides and silicates, and complex rare earth/transition metal oxides

This paper uses X-ray absorption spectroscopy to the study of electronic structure of the transition metal oxides TiO2, ZrO2 and HfO2, Zr and Hf silicate alloys, and the complex oxides, GdScO3, DyScO3 and HfTiO4. Qualitative and quantitative differences are identified between dipole allowed intra-atomic transitions from core p-states to empty d*- and s*-states, and inter-atomic transitions from transition metal and oxide 1s states to O 2p* that are mixed with transition metal d*- and s*-states for transition metal oxides and silicate alloys. The complex oxide studies have focused on the O K1 edge spectra. Differences between the spectral peak energies of the lowest d*-features in the respective O K1 spectra are demonstrated to scale with optical band gap differences for TiO2, ZrO2 and HfO2, as well as the complex oxides providing important information relevant to applications of TM oxides as high-k gate dielectrics in advanced Si devices. This is demonstrated through scaling relationships between (i) conduction band offset energies between Si and the respective dielectrics, and the optical band gaps, and (ii) the optical band gaps, the conduction band offset energies, and the electron tunneling masses as functions of the atomic d-state energies of the transition metal atoms.

Electronic structures of high-k transition metal silicates: first-principles calculations

The energy band gaps and total density of states of different transitional metal (Sc, Zr) silicates have been studied using density functional theory and local density approximation. The problem of a decreasing band gap in Zr silicate predicts the band offset reduction from the introduction of 4d state below the conduction band edge. While, in case of Sc silicate, there is no such decrease in the band gap and it becomes more suitable for the device performances.

Open Framework and Microporous Transition Metal Silicates

AbstractFor Abstract see ChemInform Abstract in Full Text.

Synthesis and characterization of neutral cis-hexacoordinate bis(beta-diketonate) silicon(IV) complexes.

Three new neutral cis-hexacoordinate bis(beta-diketonato) silicon(IV) complexes, (thd)2SiX2, where X = Me (1), tBuO (2), and tAmO (3), and thd = 2,2,6,6-tetramethyl-3,5-heptanedionato, were synthesized in high yield. Single crystal X-ray crystallographic analysis revealed that 1 was monomeric with cis-hexacoordinate octahedral geometry on the silicon and oxygen atoms. Crystal data: empirical formula C24H46O4Si, crystal system monoclinic; space group P2(1)/n; unit cell dimensions a = 10.4195(5) A, b = 19.7297(10) A, c = 13.6496(7) A; beta = 102.6590(10) degrees; Z = 4. Variable temperature NMR confirmed (thd)2SiX2 maintained cis-geometry in solution by observing two distinct methyl proton resonances (of thd) at room temperature or low temperatures. These compounds show potential for use as low temperature silicon oxide CVD precursors for transition metal silicate high kappa gate dielectrics.

Microscopic bonding and macroscopic strain relaxations at Si-SiO 2 interfaces

Optical second harmonic generation studies of the temperature dependence of the phase angle differences from contributions of terrace and edge bonding at silicon–silicon dioxide interfaces prepared on vicinal Si(111) wafers have revealed an interface relaxation process at an annealing temperature between 850 and 900 °C. Complementary studies by synchrotron soft X-ray photoelectron spectroscopy have established that this relaxation is associated with changes in the concentration and composition of local suboxide bonding environments in an ultrathin, interfacial transition region that is approximately one molecular layer thick. This relaxation occurs at a significantly lower temperature than an approximately 990±10 °C relaxation of macroscopic compressive strain in the bulk of the SiO2 film. This paper establishes an analogy between i) the Si–SiO2 interface in which there is a transition from a ‘rigid’ substrate, Si, to an ‘ideal continuous random covalent network’, SiO2, in which the average number of bonding constraints/atom matches the network dimensionality, and ii) a concentration dependent transition between ‘under-’ and ‘over-constrained’ local bonding in non-crystalline glass alloys such as GexSe1-x. The interfacial suboxide transition region is demonstrated to have properties in common with a regime of alloy compositions in which ‘self-organization’ reduces bond constraint induced strain, thereby stabilizing these compositions against ‘aging’ as for example in time-dependent changes in the glass transition temperature. These comparisons provide important new insights into defect formation at Si–SiO2 interfaces, as well as interfaces between Si and alternative high-k dielectrics being considered for advanced Si devices including Al2O3 and transition metal silicate alloys, e.g., (ZrO2)x(SiO2)1-x. This new perspective is also extended to interfaces between GaN and SiO2, where the interfacial transition region is a suboxide of Ga, GaOx, with x<1.5.

Band offset energies in zirconium silicate Si alloys

Transition metal silicates, (ZrO 2) x (SiO 2) 1− x , with dielectric constants, k>10 have been proposed as alternative dielectrics for advanced Si devices. Studies by X-ray absorption, X-ray photoelectron and Auger electron spectroscopy are combined to identify the compositional variation of the valence and conduction band offset energies with respect to Si in Zr silicate alloys. The minimum conduction band offset energy, associated with localized Zr 4d ∗-states, is ∼1.4 eV, and is independent of alloy composition, while valence band offsets decrease monotonically with increasing ZrO 2 content. Differences between the coupling of tunneling electrons to localized Zr 4d ∗ and extended Si 3s ∗ states, characterized by respective tunneling masses of ∼0.5 m o and ∼0.2 m o, combine to contribute to a minimum in the direct tunneling current in the mid-silicate-alloy composition range, x∼0.4–0.6.

Electronic structure of transition metal high-k dielectrics: interfacial band offset energies for microelectronic devices

Transition metal silicates, (ZrO2)x(SiO2)1−x, have dielectric constants k>10 that make them attractive for advanced Si devices. Band offset energies relative to Si are an important factor in determining tunneling leakage current, and internal photoemission. Studies by X-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES) and X-ray absorption spectroscopy (XAS) are combined with ab initio calculations to identify the compositional variation of the band-gap, and valence and conduction band offset energies of Zr silicate alloys with respect to Si. The minimum conduction band offset, due to Zr 4d∗ states, is shown to be independent of alloy composition, while valence band offsets decrease monotonically with increasing ZrO2 content. The implications of these results for direct tunneling are discussed.

A molecular orbital model for the electronic structure of transition metal atoms in silcate and aluminate alloys

Applied to transition metal oxides and silicate and aluminate alloys, a classification scheme that separates non-crystalline dielectrics into three groups with different amorphous morphologies, demonstrates a direct correlation between stability against crystallization and oxygen atom coordination. It also provides a local bonding model for molecular orbital (MO), calculations that are based on the coordination and symmetry of transition metal atoms and the orbital energies of their oxygen neighbors. These calculations provide important insights into the electronic structure of transition metal dielectrics, e.g. the role of anti-bonding d-states in determining conduction band offset energies with respect to Si.

Correlations between electronic structure of transition metal atoms and performance of high-k gate dielectrics in advanced Si devices

This paper develops a classification scheme for non-crystalline dielectrics that separates them into three groups with different amorphous morphologies, and identifies a linear scaling relationship between average bond ionicity and oxygen atom coordination. The classification scheme is applied to transition metal silicate and aluminate alloys and provides a structural model for molecular orbital, MO, calculations that are based on the coordination and symmetry of transition metal atoms and the orbital energies of their oxygen neighbors. The MO calculations show that conduction band offset energies with respect to Si scale inversely with the energy difference between transition metal atomic n+1 s- and n d-states providing an important insight into the choice of alternative gate dielectrics for advanced Si devices.