Flame-made Lithium Transition Metal Orthosilicates

Journal of Physical Chemistry C, 2016

Vanadium substitution is an interesting approach to manipulate the properties of the poor electronic and ionic conducting lithium transition metal orthosilicates. Especially, if incorporated on the Si-site it could alter the highly insulating character of the SiO 4 framework. This study addresses the feasibility and limitations of V substitution in Li 2 MnSiO 4. Nominal compositions of Li 2 Mn 1-x V x SiO 4 (0 ≤ x ≤ 0.2) and Li 2 MnSi 1-x V x O 4 (0 ≤ x ≤ 0.3) were synthesized by a sol-gel method, and the structural evolution was analyzed by X-ray diffraction and transmission electron microscopy (TEM) coupled with electron energy loss spectroscopy (EELS). While the solid solubility of V on tetrahedral Mn-sites was shown to be limited, substantial amounts of V entered the structure when intended to substitute Si. Elemental mapping by TEM showed that V was highly inhomogeneously distributed and high energy resolution EELS demonstrated that the

The Journal of Physical Chemistry C

Vanadium substitution is an interesting approach to manipulate the properties of the poor electronic and ionic conducting lithium transition metal orthosilicates. Especially, if incorporated on the Si-site it could alter the highly insulating character of the SiO 4 framework. This study addresses the feasibility and limitations of V substitution in Li 2 MnSiO 4. Nominal compositions of Li 2 Mn 1-x V x SiO 4 (0 ≤ x ≤ 0.2) and Li 2 MnSi 1-x V x O 4 (0 ≤ x ≤ 0.3) were synthesized by a sol-gel method, and the structural evolution was analyzed by X-ray diffraction and transmission electron microscopy (TEM) coupled with electron energy loss spectroscopy (EELS). While the solid solubility of V on tetrahedral Mn-sites was shown to be limited, substantial amounts of V entered the structure when intended to substitute Si. Elemental mapping by TEM showed that V was highly inhomogeneously distributed and high energy resolution EELS demonstrated that the

Improving cathode materials is mandatory for next-generation Li-ion batteries. Exploring polyanion compounds with high theoretical capacity such as the lithium metal orthosilicates, Li 2 MSiO 4 is of great importance. In particular, mixed silicates represent an advancement with practical applications. Here we present results on a rapid solid state synthesis of mixed Li 2 (FeMnCo)SiO 4 samples in a wide compositional range. The solid solution in the P2 1 /n space group was found to be stable for high iron concentration or for a cobalt content up to about 0.3 atom per formula unit. Other compositions led to a mixture of polymorphs, namely Pmn2 1 and Pbn2 1. All the samples contained a variable amount of Fe 3+ ions that was quantified by Mössbauer spectroscopy and confirmed by the T N values of the paramagnetic to antiferromagnetic transition. Preliminary characterization by cyclic voltammetry revealed the effect of Fe 3+ on the electrochemical response. Further work is required to determine the impact of these electrode materials on lithium batteries. Promising cathode materials for lithium ions batteries have recently emerged belonging to the Li 2 MSiO 4 (M = Fe, Mn, Co) orthosilicates family 1–3. These compounds have attracted great interest due to their high safety and, especially for those based on Fe and Mn ions, also for their low cost, low toxicity and environmental friendliness 4,5. In addition, they all appear particularly interesting due to the theoretical possibility to reversibly de-intercalate two lithium equivalents from the structure, so increasing the overall electrode capacity. In fact, Li 2 MnSiO 4 could in theory deliver 333 mAhg −1 , Li 2 CoSiO 4 325 mAhg −1 , while Li 2 FeSiO 4 166 mAhg −1 for the extraction of one Li ion per formula unit 6. However, the low electronic conductivity of silicates has to be overcome in order to reach the theoretical capacity: different approaches have been tried, e.g. the doping with Cr, V, Mg, Zn, Cu and Ni 7–10 , the carbon-coating 11 or the preparation of composites with carbon nanotubes 12. Another critical feature of the orthosilicates, also reported as tetrahedral structures, is their rich polymor-phism with numerous crystal structures that, having similar lattice energies, can be stabilized depending on subtle differences in the synthesis conditions 6,13–14. Usually, the monoclinic P2 1 /n and the orthorhombic Pmn2 1 or Pmnb space groups are reported for both the Li 2 MnSiO 4 and Li 2 FeSiO 4 compounds 15,16. For Li 2 CoSiO 4 , three stable polymorphs were prepared and structurally characterized: the orthorhombic β II (Pmn2 1) and β I (Pbn2 1) and the monoclinic γ 0 (P2 1 /n or P2/n) 6,17. The differences among these structures are mainly due to different arrangements of the cation tetrahedra. The polymorphism, with the associated small transition energies, is one of the factors affecting the long-term cyclability of these materials 18. To gain new insights in this field the relationships between structural and electrochemical properties have been studied by using in situ X-ray diffraction measurements during the cell cycling 19. The formation of either a disordered orthorhombic or monoclinic structure was observed during the lithium extraction, but more studies are needed to better explain the electrochemical behaviour of these compounds.

Silicates, 2016

Improving cathode materials is mandatory for next-generation Li-ion batteries. Exploring polyanion compounds with high theoretical capacity such as the lithium metal orthosilicates, Li 2 MSiO 4 is of great importance. In particular, mixed silicates represent an advancement with practical applications. Here we present results on a rapid solid state synthesis of mixed Li 2 (FeMnCo)SiO 4 samples in a wide compositional range. The solid solution in the P2 1 /n space group was found to be stable for high iron concentration or for a cobalt content up to about 0.3 atom per formula unit. Other compositions led to a mixture of polymorphs, namely Pmn2 1 and Pbn2 1. All the samples contained a variable amount of Fe 3+ ions that was quantified by Mössbauer spectroscopy and confirmed by the T N values of the paramagnetic to antiferromagnetic transition. Preliminary characterization by cyclic voltammetry revealed the effect of Fe 3+ on the electrochemical response. Further work is required to determine the impact of these electrode materials on lithium batteries. Promising cathode materials for lithium ions batteries have recently emerged belonging to the Li 2 MSiO 4 (M = Fe, Mn, Co) orthosilicates family 1-3. These compounds have attracted great interest due to their high safety and, especially for those based on Fe and Mn ions, also for their low cost, low toxicity and environmental friendliness 4,5. In addition, they all appear particularly interesting due to the theoretical possibility to reversibly de-intercalate two lithium equivalents from the structure, so increasing the overall electrode capacity. In fact, Li 2 MnSiO 4 could in theory deliver 333 mAhg −1 , Li 2 CoSiO 4 325 mAhg −1 , while Li 2 FeSiO 4 166 mAhg −1 for the extraction of one Li ion per formula unit 6. However, the low electronic conductivity of silicates has to be overcome in order to reach the theoretical capacity: different approaches have been tried, e.g. the doping with Cr, V, Mg, Zn, Cu and Ni 7-10 , the carbon-coating 11 or the preparation of composites with carbon nanotubes 12. Another critical feature of the orthosilicates, also reported as tetrahedral structures, is their rich polymor-phism with numerous crystal structures that, having similar lattice energies, can be stabilized depending on subtle differences in the synthesis conditions 6,13-14. Usually, the monoclinic P2 1 /n and the orthorhombic Pmn2 1 or Pmnb space groups are reported for both the Li 2 MnSiO 4 and Li 2 FeSiO 4 compounds 15,16. For Li 2 CoSiO 4 , three stable polymorphs were prepared and structurally characterized: the orthorhombic β II (Pmn2 1) and β I (Pbn2 1) and the monoclinic γ 0 (P2 1 /n or P2/n) 6,17. The differences among these structures are mainly due to different arrangements of the cation tetrahedra. The polymorphism, with the associated small transition energies, is one of the factors affecting the long-term cyclability of these materials 18. To gain new insights in this field the relationships between structural and electrochemical properties have been studied by using in situ X-ray diffraction measurements during the cell cycling 19. The formation of either a disordered orthorhombic or monoclinic structure was observed during the lithium extraction, but more studies are needed to better explain the electrochemical behaviour of these compounds.

Solid State Ionics, 2020

With the aim of probing the influence of the highly oxidizable selenium in the electrochemically inactive cathode Li 2 MnO 3 material, samples were prepared with selenium substitution in the manganese coordination according to the general formula of Li 2 Mn 1-x Se x O 3. In Li-ion batteries, oxygen instabilities are one of the major problems confronted that effect the performances of the cathode materials. The crystal and electronic structure properties of the materials were studied with x-ray absorption techniques. Selenium atoms were determined to build Li 2 SeO 4 crystal and due to the oxygen removal during sample preparation mechanisms were determined to cause lower ionic conductivity than the parent Li 2 MnO 3 oxide. The atomic distances in the materials were determined by the fits performed by the commercial code FEFF 8.2. Li 2 SeO 4 crystal was determined as stacked between manganese and lithium atoms and isolated with each other.

J.Mat.Chem A, 2013

The search for new low-cost and safe cathodes for next-generation lithium batteries has led to increasing interest in silicate materials. Here, a systematic comparison of crystal properties, defect chemistry and Li-ion migration behaviour of four polymorphs of Li 2 MnSiO 4 is reported based on the results of atomistic simulations. The four polymorphs examined have Pmn2 1 , Pmnb, P2 1 /n, and Pn symmetry. Lattice energies of all four polymorphs are very similar, with only a small energy preference for the two orthorhombic phases over the monoclinic phases, which explains the difficulty experimentalists have had preparing pure-phase samples. Defect formation energies of the polymorphs are also similar, with antisite Li/Mn defects the most energetically favourable. Detailed analysis of the Li-ion migration energy surfaces reveals high activation energies (around 0.9 to 1.7 eV) and curved trajectories. All four polymorphs are thus expected to be poor Li-ion conductors, requiring synthesis as nanoparticles to facilitate sufficient Li transfer. The results accord well with experimental reports on the structure, relative phase stabilities and electrochemical performance of materials in this system.

J. Mater. Chem. A, 2014

Lithium metal orthosilicates with general formula Li 2 MSiO 4 (M ¼ Mn, Fe and Co) are among the most attractive new materials as potential high-specific-energy cathodes for lithium batteries. All the members of this family present a rich polymorphism with at least three clearly identified crystal structures of each Li 2 MSiO 4 compound. Several theoretical investigations have highlighted that the energy stability of the different polymorphs is very close to each other irrespective of their average crystal structures. At the same time, the calculated and experimental electrochemical performances are again very similar among different polymorphs. By means of neutron total scattering investigation of different polymorphs (monoclinic and orthorhombic) of Li 2 FeSiO 4 and Li 2 MnSiO 4 orthosilicates coupled to Pair Distribution Function (PDF) analysis we showed that, at the local scale, all the polymorphs have the same structure (in particular the structure of the monoclinic polymorph) irrespective of the average structure they possess. This experimental evidence of a strong similarity at the local scale can be correlated with the observed electrochemical similarity (such as the lithium extraction voltages) among the different orthosilicate polymorphs, thus providing an approach to elucidate the relevance of local versus longrange structure.

Computational Materials Science, 2017

The phase stability of the Li 2 MnSiO 4 during Li insertion/extraction, is a key requirement for acceptable cyclability and practical application as a cathode material for lithium batteries. Here we present firstprinciples calculations used to study the phase stability of Mg substituted Li 2 MnSiO 4. The 137 structures of Li 2 Mn 1Àx Mg x SiO 4 (x = 0.25-0.50) were calculated based on 5 known polymorphs of Li 2 MnSiO 4. Using three different functionals (PBE, PW91 and PBEsol), it is shown that the total-energy vs. distance between layers in the layered Pmn2 1 curve has a clear minimum and does not demonstrate the exfoliation of layers found previously. The amorphlization of Li 2 MnSiO 4 is explained by high value of energy above Hull of its fully delithiated form. Crystal orbital Hamiltonian populations (COHP) revealed that the strength of MnAO and SiAO bonds unchanged during the substitution with Mg, thus eliminating the concern about the safety. The pure Li 2 MnSiO 4 in the P2 1 /n form was suggested as the most stable upon cycling. In the Mg substituted Li 2Àx MnSiO 4 case, the Mg substitution is more beneficial for x in the range (0.0-2.0) than in the range (0.0-1.0). The increase in the performance for the x = 0.0-1.0 region, can be explained by the small particle size and the uniformity of nanoparticles distribution rather than the enhancement of the thermodynamic stability.

ECS Transactions, 2015

Lithium transition-metal silicate materials are promising candidates for next generation Li-ion batteries since they allow Li extraction/insertion beyond one Li ion per formula unit. They consist of cheap, non-toxic and abundant elements. Here we focus on the synthesis and electrochemical performance of Fe and V substituted Li 2 MnSiO 4. Cations were substituted to overcome poor stability of the undoped compound and increase conductivity, thus expected to increase the electrochemical performance. Up to 20 mole % Fe and 5 mole % V were incorporated into orthorhombic Pmn2 1 Li 2 MnSiO 4 while keeping a high phase purity and the desired porous nano-sized structure. For materials with 20 mole % Fe substitution, the reversible Li intercalation capacity during slow cycling (C/33) was increased from 0.63 Li per formula unit for the undoped to 0.81 for the doped sample. A 5 mole % V substitution increased the reversible Li capacity from 0.65 Li per formula unit to 0.73.

Chem Mater

"A new family of silicate materials has attracted interest for potential use in rechargeable lithium batteries. The defect chemistry, doping behavior, and lithium diffusion paths in the Li2MnSiO4 cathode material are investigated by advanced modeling techniques. Our simulations show good reproduction of both monoclinic and orthorhombic structures of Li2MnSiO4. The most favorable intrinsic defect type is found to be the cation anti-site defect, in which Li and Mn ions exchange positions. The migration energies suggest differences in intrinsic Li mobility between the monoclinic and orthorhombic polymorphs, which would affect their rate capability as rechargeable electrodes. The results indicate curved Li diffusion paths and confirm the anisotropic nature of Li transport, which is probably general for the Li2MSiO4 (M = Mn, Fe, Co) family of compounds. Subvalent doping by Al on the Si site is energetically favorable and could be a synthesis strategy to increase the Li content."