Correlation between the local scale structure and the electrochemical properties in lithium orthosilicate cathode materials

Abstract

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.

Recent advances in the application of total scattering methods to functional materials

“…Copyright 2013 American Chemical Society. Among others, relevant examples are Idemoto et al28 where the correlation between the synthesis and the local structure of the xLi(Li reported; Adipranoto et al29 where the crystal and local effects of the Ni doping in LiCo 1-x Ni x O 2 system is carefully examined, leading to the conclusion that the distance of diffusion pathways of oxide ions around Co atoms does not change when it is substituted with Ni; Moriya and coworkers30 which reported a study of the charged/discharged structures of carbon hybridized Li 2 MnSiO 4 nanoparticles in which is evident the repetitive disappearance/appearance of the long range order of the lithium manganese orthosilicate during the charge/discharge processes, respectively, with the consumption of the long range order due to distortions of the [MnO 4 ] octahedra; and the neutron PDF investigation of the rich polymorphism showed by the Li 2 MSiO 4 family (with M = Fe, Mn), reported in ref 31,. showing that all the polymorphs investigated have the same monoclinic distortion in the short range, thus suggesting a possible explanation for the similar electrochemical behavior of the different average crystallographic structures.…”

Electrochemical characterization of monoclinic and orthorhombic Li3CrF6 as positive electrodes in lithium-ion batteries synthesized by a sol–gel process with environmentally benign chemicals

“…The local arrangements around these octahedra are quite similar and have only a slight effect on the electrochemical potential profiles of the M III /M II redox couples. Such behavior was also observed for the different Li 2 MnSiO 4 polymorphs (P21/n, Pn, Pmnb, and Pmn21) [47]. Similar discharge potentials for the electrochemical redox couple Fe III /Fe II were recently reported for quaternary lithium transition metal fluorides LiNiFeF 6 [10], LiMgFeF 6 (trirutile-type) [9] and LiMnFeF 6 (Na 2 SiF 6 -type) [8].…”

Strategies and Considerations for Least-Squares Analysis of Total Scattering Data

“…Many examples of this approach can be found in the literature, − particularly in describing local distortions in perovskite materials. Perovskites have the advantage of being thoroughly characterized through crystallographic − and computational , methods, and the various lower-symmetery distortions and phase transitions are well-classified, providing a library of structures to fit data against.…”