Title: Structure of the high voltage phase of layered P2-Na_2/3-z[Mn _1/2Fe _1/2 ]O₂ and the positive effect of Ni substitution on its stability

Abstract

A combination of operando X-ray diffraction, pair distribution function (PDF) analysis coupled with electrochemical measurements and Mössbauer spectroscopy elucidates the nature of the phase transitions induced by insertion and extraction of sodium ions in P2-Na_0.67[Ni_yMn_0.5+_yFe_0.5-2_y]O₂ (y = 0, 0.10, 0.15). When phase transitions are avoided, the optimal cathode material – P2-Na_0.67Fe_0.2Mn_0.65Ni_0.15O₂ - delivers 25% more energy than the unsubstituted material, sustaining high specific energy (350 Wh kg^-1) at moderate rates and maintains 80% of the original energy density after 150 cycles - a significant improvement in performance vs. the unsubstituted analogue. The crystal structure of the high voltage phase is solved for the first time by X-ray PDF analysis of P2-Na_0.67-zFe0.5Mn_0.5O₂ (where z~0.5), revealing that migration of the transition metals – particularly Fe³⁺- into tetrahedral sites in the interlayer space occurs at high potential. This results in new short range order between two adjacent layers. Although the transition metal migration is reversible as proven by electrochemical performance, it induces a large disfavourable cell polarization. The deleterious high voltage transition is mitigated by substitution of Fe³⁺ by Mn⁴⁺/Ni²⁺, giving rise to better cycling performance. Moreover, as demonstrated by ⁵⁷Fe Mössbauer spectroscopy, the much lower ratio of Fe⁴⁺O6 to Fe³⁺O₆ observed systematicallymore » across the range of Ni content – compared to the values expected from a purely ionic model – suggests redox activity involves the O-2p orbitals owing to their overlap with the transition metal-3d orbitals.« less

Authors:

Talaie, Elahe ^[1]; Duffort, Victor ^[1]; Smith, Hillary L. ^[2]; Fultz, Brent ^[2]; Nazar, Linda F. ^[1]

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+ Show Author Affiliations

1. Univ. of Waterloo, ON (Canada). Waterloo Inst. for Nanotechnology
2. California Institute of Technology (CalTech), Pasadena, CA (United States)

Publication Date:

Fri Jul 03 00:00:00 EDT 2015

Research Org.:

Energy Frontier Research Centers (EFRC) (United States). Energy Frontier Research in Extreme Environments (EFree)

Sponsoring Org.:

USDOE Office of Science (SC), Basic Energy Sciences (BES)

OSTI Identifier:

1386015

Grant/Contract Number:  

SC0001057

Resource Type:

Accepted Manuscript

Journal Name:

Energy & Environmental Science

Additional Journal Information:

Journal Volume: 8; Journal Issue: 8; Related Information: EFree partners with Carnegie Institution of Washington (lead); California Institute of Technology; Colorado School of Mines; Cornell University; Lehigh University; Pennsylvania State University; Journal ID: ISSN 1754-5692

Publisher:

Royal Society of Chemistry

Country of Publication:

United States

Language:

English

Subject:

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY; catalysis (heterogeneous); solar (photovoltaic); phonons; thermoelectric; energy storage (including batteries and capacitors); hydrogen and fuel cells; superconductivity; charge transport; mesostructured materials; materials and chemistry by design; synthesis (novel materials)