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The role of semi-labile oxygen atoms for intercalation chemistry of the metal-ion battery polyanion cathodes Ivan V. Tereshchenko, Dmitry A. Aksyonov, Oleg A. Drozhzhin, Igor A. Presniakov, Alexey V. Sobolev, Andriy Zhugayevych, Daniil Striukov, Keith J. Stevenson, Evgeny Antipov, and Artem M. Abakumov J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.7b12644 • Publication Date (Web): 23 Feb 2018 Downloaded from http://pubs.acs.org on February 23, 2018
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Journal of the American Chemical Society
The role of semi-labile oxygen atoms for intercalation chemistry of the metal-ion battery polyanion cathodes Ivan V. Tereshchenko†, ‡, Dmitry А. Aksyonov†, Oleg A. Drozhzhin†, ‡, Igor A. Presniakov‡, Alexey V. Sobolev‡, Andriy Zhugayevych†, Daniil Striukov§, Keith J. Stevenson†, Evgeny Antipov‡, Artem M. Abakumov†* †
Skoltech Center for Electrochemical Energy Storage, Skolkovo Institute of Science and Technology, Nobel str. 3, 143026 Moscow, Russian Federation ‡
Department of Chemistry, Lomonosov Moscow State University, 119991 Moscow, Russian Federation
§
Faculty of Physics, Southern Federal University - 5, Zorge str., Rostov-on-Don, 344090, Russian Federation
KEYWORDS: metal-ion batteries, Na2FePO4F, intercalation mechanism, lithium, sodium ABSTRACT: Using the orthorhombic layered Na2FePO4F cathode material as a model system we identify the bonding of the alkali metal cations to the semi-labile oxygen atoms as an important factor affecting electrochemical activity of alkali cations in the polyanion structures. The semi-labile oxygens, bonded to the P and alkali cations, but not included into the FeO4F2 octahedra, experience severe undercoordination upon alkali cation deintercalation, causing an energy penalty for removing the alkali cations located in the proximity of such semi-labile oxygens. Desodiation of Na2FePO4F proceeds through a two-phase mechanism in the Na-ion cell with a formation of an intermediate monoclinic Na1.55FePO4F phase with coupled Na/vacancy and Fe2+/Fe3+ charge ordering at 50% SOC. In contrast, desodiation of Na2FePO4F in the Li-ion cell demonstrates sloping charge profile suggesting a solid-solution mechanism without formation of a charge ordered intermediate phase. A combination of a comprehensive crystallographic study and extensive DFT-based calculations demonstrate that the difference in electrochemical behavior of the alkali cation positions is largely related to the different number of the nearest neighbor semi-labile oxygen atoms, influencing their desodiation potential and accessibility for the Na/Li chemical exchange, triggering coupled alkali cation – vacancy ordering and Fe2+/Fe3+ charge ordering, as well as switching between the “solid solution” and “two-phase” charging mechanistic regimes.
INTRODUCTION The average intercalation potential is one of the key parameters influencing energy density of cathodes (positive electrodes) for metal-ion batteries. Within the same structural environment, the redox potential of the Mn+/M(n+1)+ couple depends on the formal oxidation state of the M cation, its electronegativity and ionization energy, and on the details of its electronic configuration.1-3 For polyanion cathodes (i.e. comprising the XO3n- or XO4n- polyanion groups, such as borates, carbonates, sulphates, phosphates) the anion sublattice plays a decisive role. By changing the X cation in the XO4npolyanion group one can adjust the redox potential of the Mn+/M(n+1)+ couple within the same type of the polyanionic structure (experimentally demonstrated in the NASICONtype LixFe2(XO4)3 structures with X = As, P, Mo, W, S)1,4,5 and raise its average potential significantly compared to the redox potential exhibited in oxide structures.6 This mechanism, termed an “inductive effect”, is confined to tuning the covalency/ionicity of the M-O bond by varying charge transfer through the M-O-X bond via changing electronegativity of the X cation. According to density functional theory (DFT) computations, the potential of Li+ de-intercalation for the polyanion compounds such as LiCoXO4 (X = P, As, Sb), Li2VXO4O (X = P, As, Si, Ge) and
Li2MXO4 (M = Mn, Fe, Co, Ni; X = P, Si, Ge) increases within the row GeO4
