Bond Valence Energy Landscape Research Articles

New KRb2Sb4BO13 and Rb3Sb4BO13 compounds prepared by Rb+/K+ ion exchange from the K3Sb4BO13 ion conductor

Conduction pathways of Rb+/K+ ions in new ion conductors K3−xRbxSb4BO13 (0 ≤ x ≤ 3).

Influence of Temperature-Driven Polymorphism and Disorder on Ionic Conductivity in Li6Zn(P2O7)2.

Ionic conductivity in a compound is rooted in a delicate interplay between its crystal structure and its structural defects (vacancies, interstitials, etc.). Hence, understanding this interplay is of utmost importance to design new solid state electrolytes. To shed some light on the above query, we investigated the rich crystal chemistry of Li6Zn(P2O7)2. This compound undergoes multiple structural transitions under the influence of temperature, which increases the conductivity by several orders and lowers the activation energy. We explained this jump in conductivity by the increased disorder associated with cation mixing. Our structural exploration indicates that both the room-temperature α-polymorph and the high-temperature ζ-polymorph crystallize in a C2/ c space group but with a much smaller unit cell volume for the latter. While their structural framework based on P2O74- is similar, the ζ-polymorph presents a fully disordered Li/Zn sublattice, while it is fully ordered for the α-polymorph. Furthermore, the bond valence energy landscape calculations show that in the α-polymorph, the Li+ conduction is two-dimensional, whereas because of Li+/Zn2+ site mixing, Li+ can hop three-dimensionally in the ζ-polymorph.

Understanding migration barriers for monovalent ion insertion in transition metal oxide and phosphate based cathode materials: A DFT study

High ionic conductivity is a prerequisite requirement for materials used in monovalent metal-ion rechargeable batteries. The extensive search of new electrode materials for Na-ion and K-ion monovalent metal-ion batteries requires a deep understanding of structural and chemical details of cation migration through the crystal lattice. In the paper, we consider three classes of transition metal oxide and phosphate cathode materials: AMn2O4 spinels, AMPO4 olivines and AVPO4F tavorites (A = Li, Na, K, □; M = Fe, Mn), used for pragmatic applications for secondary (rechargeable) batteries. Herein we examine Na+ and K+ migration characteristics in comparison with that of Li+ by means of DFT+U, local energy calculations, empirical potentials, and bond valence energy landscape (BVEL). It is found that despite larger radii of Na+ and K+, the migration barriers are comparable with that of Li+. In several cases, we reveal that the migration barrier of K+ can be even lower than that of Li+. This behavior is explained through the interplay of site and lattice energies during cation migration. For automation of screening of migration properties via DFT calculations, a new Python-based framework (SIMAN) is developed and benchmarked across three cathode materials structures.

Structural, Electrochemical and Magnetic Properties of a Novel KFeSO4f Polymorph

Since the discovery of the promising electrode material LiFePO4, recent research has been focusing on the development of new iron-based polyanionic materials for next generations Li- and Na-ion batteries displaying better performances while still preserving cost and sustainability benefits.1,2 On this quest, our group explored the wide family of fluorosulfate-based compounds, where the most prominent member is LiFeSO4F crystallizing either in a tavorite or triplite crystal structure with the latter presenting a potential of 3.9 V vs Li+/Li0.3 In order to further investigate the rich crystal chemistry offered by 3d-metal-based fluorosulfates, we studied the effect of the replacement of Li by other alkali metals such as Na and K. One of the so discovered phases was KFeSO4F, which adopts an orthorhombic structure analogue to KTiOPO4 (KTP) and from which K+ ions can be extracted in a complex electrochemical process.4 During the synthesis of the KTP-like KFeSO4F, a secondary phase was observed at lower temperature, which turned out to be a new KFeSO4F polymorph.5 Using combined synchrotron and neutron powder diffraction as well as electron diffraction, it is shown that the new compound adopts a complex layered-like structure that crystallizes in a large monoclinic unit cell. Impedance measurements together with the Bond Valence Energy Landscape approach suggest that the K+ ions, which are located between the layers, are mobile within the structure and indeed, they can be electrochemically removed at an average potential of 3.7 V vs. Li+/Li0. Lastly, neutron diffraction experiments coupled with SQUID measurements reveal a long range antiferromagnetic ordering of the Fe2+ magnetic moments. These results confirm once more the richness of polymorphisms in sulfate-based materials and we hereby want to encourage the further exploration of their interesting electrochemical and physical properties. (1) Padhi, A. K.; Nanjundaswamy, K. S.; Masquelier, C.; Goodenough, J. B. Mapping of Transition Metal Redox Energies in Phosphates with NASICON Structure by Lithium Intercalation. J. Electrochem. Soc. 1997, 144 (8), 2581–2586. (2) Masquelier, C.; Croguennec, L. Polyanionic (Phosphates, Silicates, Sulfates) Frameworks as Electrode Materials for Rechargeable Li (or Na) Batteries. Chem. Rev. 2013, 113 (8), 6552–6591. (3) Ati, M.; Melot, B. C.; Chotard, J.-N.; Rousse, G.; Reynaud, M.; Tarascon, J.-M. Synthesis and Electrochemical Properties of Pure LiFeSO4F in the Triplite Structure. Electrochem. Commun. 2011, 13 (11), 1280–1283. (4) Recham, N.; Rousse, G.; Sougrati, M. T.; Chotard, J.-N.; Frayret, C.; Mariyappan, S.; Melot, B. C.; Jumas, J.-C.; Tarascon, J.-M. Preparation and Characterization of a Stable FeSO 4 F-Based Framework for Alkali Ion Insertion Electrodes. Chem. Mater. 2012, 24 (22), 4363–4370. (5) Lander, L.; Rousse, G.; Abakumov, A. M.; Sougrati, M.; Tendeloo, G. van; Tarascon, J.-M. Structural, Electrochemical and Magnetic Properties of a Novel KFeSO4F Polymorph. J. Mater. Chem. A 2015, 3 (39), 19754–19764.

Synthesis and Characterization of a Novel KFeSO4 F Polymorph: Structural, Electrochemical and Magnetic Properties

Since the discovery of the promising electrode material LiFePO4, recent research has been focusing on the development of new iron-based polyanionic materials for next generations Li- and Na-ion batteries displaying better performances while still preserving cost and sustainability benefits.1,2 Towards this quest, our group explored the wide family of sulfate-based compounds, with the most prominent members being monoclinic Li2Fe(SO4)2 and triplite LiFeSO4F having redox potentials of 3.83 V and 3.9 V vs. Li+/Li0, respectively.3,4 In order to further investigate the rich crystal chemistry offered by 3d-metal-based fluorosulfates, we explored the feasibility of using other alkali metals such as Na and K instead of Li. Through this approach we discovered a KFeSO4F phase, which adopts a KTiOPO4 structure and releases K+ ions via a complex electrochemical process.5 Knowing that sulfate-based compounds are prone to polymorphism, we recently unveiled a new low-temperature KFeSO4F polymorph.6 Using combined synchrotron and neutron powder diffraction as well as electron diffraction, it was shown that the compound adopts a complex layered-like structure that crystallizes in a large monoclinic unit cell. Impedance measurements together with the Bond Valence Energy Landscape approach show that the K+ ions, which are located between the layers, are mobile within the structure and can be electrochemically removed at an average potential of 3.7 V vs. Li+/Li0. Lastly, neutron diffraction experiments coupled with SQUID measurements reveal a long range antiferromagnetic ordering of the Fe2+ magnetic moments. These results confirm once again the richness of polymorphisms in sulfate-based materials, which, besides unusual electrochemical properties, show interesting physical properties. (1) Padhi, A. K.; Nanjundaswamy, K. S.; Masquelier, C.; Goodenough, J. B. Mapping of Transition Metal Redox Energies in Phosphates with NASICON Structure by Lithium Intercalation. J. Electrochem. Soc. 1997, 144 (8), 2581–2586. (2) Masquelier, C.; Croguennec, L. Polyanionic (Phosphates, Silicates, Sulfates) Frameworks as Electrode Materials for Rechargeable Li (or Na) Batteries. Chem. Rev. 2013, 113 (8), 6552–6591. (3) Reynaud, M.; Ati, M.; Melot, B. C.; Sougrati, M. T.; Rousse, G.; Chotard, J.-N.; Tarascon, J.-M. Li2Fe(SO4)2 as a 3.83 V Positive Electrode Material. Electrochem. Commun. 2012, 21, 77–80. (4) Ati, M.; Melot, B. C.; Chotard, J.-N.; Rousse, G.; Reynaud, M.; Tarascon, J.-M. Synthesis and Electrochemical Properties of Pure LiFeSO4F in the Triplite Structure. Electrochem. Commun. 2011, 13 (11), 1280–1283. (5) Recham, N.; Rousse, G.; Sougrati, M. T.; Chotard, J.-N.; Frayret, C.; Mariyappan, S.; Melot, B. C.; Jumas, J.-C.; Tarascon, J.-M. Preparation and Characterization of a Stable FeSO 4 F-Based Framework for Alkali Ion Insertion Electrodes. Chem. Mater. 2012, 24 (22), 4363–4370. (6) Lander, L.; Rousse, G.; Abakumov, A. M.; Sougrati, M.; Tendeloo, G. van; Tarascon, J.-M. Structural, Electrochemical and Magnetic Properties of a Novel KFeSO4F Polymorph. J. Mater. Chem. A 2015, 3 (39), 19754–19764.

Search for Li-electrochemical activity and Li-ion conductivity among lithium bismuth oxides

Previously reported lithium bismuth oxides, LiBiO2, LiBiO3, Li3BiO3, Li3BiO4, Li5BiO5, and Li7BiO6 were prepared either via ceramic or hydrothermal synthesis methods, and their feasibility as lithium battery cathode materials was tested. None of the oxides showed any desirable electrochemical activity at higher potentials, except for the limited capacity observed during the first discharge in LiBiO3. In contrast, these phases uptake large amounts of Li+via conversion reactions when cycled down to zero volt, but the reversibility is poor. The transformation of LiBiO3 to LiBiO2 and Li3BiO3 to Li3BiO4 observed via X-ray diffraction showed absence of intermediate phases with mixed Bi3+/Bi5+ oxidation states. Among the materials studied, the highest ionic conductivity of 3.8×10−8 S/cm at 25°C was measured for LiBiO2. The Li+ conduction pathways deduced from bond valence energy landscape approach suggest the best Li diffusion characteristics in Li7BiO6 among the Li-Bi-O phases studied.