To Which Extent Is Paramagnetic Solid-State NMR Able To Address Polymorphism in Complex Transition-Metal Oxides? Chiara Ferrara, † Stefania Ferrari, † Marcella Bini, † Doretta Capsoni, † Guido Pintacuda, ‡ and Piercarlo Mustarelli* ,§ † Department of Chemistry, Section of Physical Chemistry, University of Pavia, Via Taramelli 16, 271001 Pavia, Italy ‡ Centre de RMN à Trè s Hauts Champs, Institut des Sciences Analytiques, Université de Lyon (ENS-Lyon, UCB Lyon 1, CNRS UMR 5280), 5 rue de la Doua, 69100 Villeurbanne, France § Department of Materials Science, University of Milano - Bicocca, and INSTM, Via Cozzi 55, 20125 Milano, Italy * S Supporting Information ABSTRACT: A detailed characterization of the polymorphs constituting cathode materials, both before and after cell cycling, is mandatory to develop more stable and powerful lithium batteries. In many cases, e.g., for transition metal lithium silicates, standard diffraction techniques cannot give a clear-cut response. Here we show that broadband adiabatic fast MAS NMR can give unique information in the case of model Li 2 (Mn,Fe)SiO 4 high-capacity cathode materials. By coupling 7 Li and 29 Si 1D and 2D spectra, we are able to address polymorphs speciation also in the mixed Mn/Fe compositions, which is a nearly impossible task for X-rays and neutrons diffraction. We finally discuss the conditions under which this approach is useful when applied to rare nuclei such as 29 Si. T here is increasing need of more performing rechargeable batteries for demanding applications, such as in automotive and smart grids. 1 At present, the energy storage market is dominated by Li-ion batteries, which, however, suffer of significant drawbacks, e.g. low volumetric/gravimetric energy density, and reduced cycling capabilities, which are chiefly related to the cathode compartment. 2,3 Polyoxyanion systems, such as the olivine LiMPO 4 with M transition metal (TM), have been proposed as valid alternative thanks to their structural stability (due to the strong P-O bonds network), the reduced costs and high performances. 4-6 In this frame, the family with general formula Li 2 MSiO 4 is particularly attractive for the enhanced stability related to Si- O bonds, the low cost and environmental benignity due to the presence of Si. 7 A key feature of these compounds is the possible extraction of two lithium ions per formula unit related to the M 2+ /M 3+ and M 3+ /M 4+ redox processes, which, in principle, should give capacity above 300 mAh g -1 . Within this family, Li 2 FeSiO 4 has been early studied thank to its promising electrochemical properties. 7 Li 2 MnSiO 4 is even more promis- ing thanks to the high potential of the Mn 2+/4+ couple with respect to the Li/Li + . 8,9 The major drawbacks of these oxides are low electronic conductivity, 10 and structural stability on cycling. To overcome these issues, several strategies were explored, including the synthesis of solid solutions Li 2 M x M′ 1-x SiO 4 with particular attention for the couple Fe/ Mn. 11,12 However, these materials suffer of extended polymorphism, due to the small formation energy differences between the different possible crystallographic structures. 13-15 Different polymorphs, with potentially different electrochemical behav- ior, can be prepared depending on synthetic route, composition, sintering temperature, heating/cooling rate, 8-10,16 and this issue is exacerbated in M-M′ solid solutions. 12 In these polymorphs all the cations occupy distorted tetrahedral positions. The polymorphs can be divided into two main classes, called β and γ, which are stable in different temperature ranges. 17 In the β structure (stable at lower temperature) all the tetrahedra point along the same direction, while in the γ phases (stable at higher temperature) the tetrahedra are organized in groups of three units with the central one pointing in the opposite direction with respect to the other two. Where both β and γ forms exist for a given composition, the phase transition from β to γ implies the inversion of half sites. For Mn- and Fe- based materials the most common polymorphs are β II (space group Pmn2 1 ), γ s (space group P2 1 /n) and γ II (space group Pmnb). 17 The crystal structures are reported in Figure S1. The main problem limiting complete structural character- ization is related to the strong similarities between these crystal Received: August 20, 2018 Accepted: October 2, 2018 Published: October 2, 2018 Letter pubs.acs.org/JPCL Cite This: J. Phys. Chem. Lett. 2018, 9, 6072-6076 © XXXX American Chemical Society 6072 DOI: 10.1021/acs.jpclett.8b02569 J. Phys. Chem. Lett. 2018, 9, 6072-6076 Downloaded via UNIV DEGLI STUDI DI PAVIA on October 5, 2018 at 06:41:34 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.