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Cation Ordering and Oxygen Release in LiNi0.5-xMn1.5+xO4-y (LNMO): In Situ Neutron Diffraction and Performance in Li Ion Full Cells
Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Structural Chemistry.
Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Structural Chemistry.
Rutherford Appleton Lab, ISIS Pulsed Neutron & Muon Source, Harwell Campus, Didcot OX11 0QX, Oxon, England.
Inst Energy Technol, Dept Neutron Mat Characterizat, POB 40, NO-2027 Kjeller, Norway.
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2019 (English)In: ACS APPLIED ENERGY MATERIALS, ISSN 2574-0962, Vol. 2, no 5, p. 3323-3335Article in journal (Refereed) Published
Abstract [en]

Lithium ion cells utilizing LiNi0.5Mn1.5O4 (LNMO) as the positive electrode are prone to fast capacity fading, especially when operated in full cells and at elevated temperatures. The crystal structure of LNMO can adopt a P4(3)32 (cation-ordered) or Fd (3) over barm (disordered) arrangement, and the fading rate of cells is usually mitigated when samples possess the latter structure. However, synthesis conditions leading to disordering also lead to oxygen deficiencies and rock-salt impurities and as a result generate Mn3+. In this study, in situ neutron diffraction was performed on disordered and slightly Mn-rich LNMO samples to follow cation ordering-disordering transformations during heating and cooling. The study shows for the first time that there is not a direct connection between oxygen release and cation disordering, as cation disordering is observed to start prior to oxygen release when the samples are heated in a pure oxygen atmosphere. This result demonstrates that it is possible to tune disordering in LNMO without inducing oxygen deficiencies or forming the rock-salt impurity phase. In the second part of the study, electrochemical testing of samples with different degrees of ordering and oxygen content has been performed in LNMO vertical bar vertical bar LTO (Li4Ti5O12) full cells. The disordered sample exhibits better performance, as has been reported in other studies; however, we observe that all cells behave similarly during the initial period of cycling even when discharged at a 10 C rate, while differences arise only after a period of cycling. Additionally, the differences in fading rate were observed to be time-dependent rather than dependent on the number of cycles. This performance degradation is believed to be related to instabilities in LNMO at higher voltages, that is, in its lower lithiation states. Therefore, it is suggested that future studies should target the individual effects of ordering and oxygen content. It is also suggested that more emphasis during electrochemical testing should be placed on the stability of samples in their delithiated state.

Place, publisher, year, edition, pages
AMER CHEMICAL SOC , 2019. Vol. 2, no 5, p. 3323-3335
Keywords [en]
high-voltage spinel, neutron diffraction, LNMO, cation ordering, oxygen deficiency
National Category
Materials Chemistry
Identifiers
URN: urn:nbn:se:uu:diva-387975DOI: 10.1021/acsaem.8b02217ISI: 000469885300040OAI: oai:DiVA.org:uu-387975DiVA, id: diva2:1331610
Funder
Swedish Energy Agency, 42758-1Swedish Energy Agency, 39043-1StandUpAvailable from: 2019-06-27 Created: 2019-06-27 Last updated: 2019-07-29Bibliographically approved
In thesis
1. The Electrochemistry of LiNi0.5-xMn1.5+xO4-δ in Li-ion Batteries: Structure, Side-reactions and Cross-talk
Open this publication in new window or tab >>The Electrochemistry of LiNi0.5-xMn1.5+xO4-δ in Li-ion Batteries: Structure, Side-reactions and Cross-talk
2019 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The use of Li-ion batteries in portable electronic products is today widespread and on-going research is extensively dedicated to improve their performance and energy density for use in electric vehicles. The largest contribution to the overall cell weight comes from the positive electrode material, and improvements regarding this component thereby render a high potential for the development of these types of batteries. A promising candidate is LiNi0.5Mn1.5O4 (LMNO), which offers both high power capability and energy density. However, the instability of conventional electrolytes at the high operating potential (~4.7 V vs. Li+/Li) associated with this electrode material currently prevents its use in commercial applications.

This thesis work aims to investigate practical approaches which have the potential of overcoming issues related to fast degradation of LNMO-based batteries. This, in turn, necessitates a comprehensive understanding of degradation mechanisms. First, the effect of a well-known electrolyte additive, fluoroethylene carbonate is investigated in LNMO-Li4Ti5O12 (LTO) cells with a focus on the positive electrode. Relatively poor cycling performance is found with 5 wt% additive while 1 wt% additive does not show a significant difference as compared to additive-free electrolytes. Second, a more fundamental study is performed to understand the effect of capacity fading mechanisms contributing to overall cell failure in high-voltage based full-cells. Electrochemical characterization of LNMO-LTO cells in different configurations show how important the electrode interactions (cross-talk) can be for the overall cell behaviour. Unexpectedly fast capacity fading at elevated temperatures is found to originate from a high sensitivity of LTO to cross-talk.

Third, in situ studies of LNMO are conducted with neutron diffraction and electron microscopy. These show that the oxygen release is not directly related to cation disordering. Moreover, microstructural changes upon heating are observed. These findings suggest new sample preparation strategies, which allow the control of cation disorder without oxygen loss. Following this guidance, ordered and disordered samples with the same oxygen content are prepared. The negative effect of ordering on electrochemical performance is investigated and changes in bulk electronic structure following cycling are found in ordered samples, accompanied by thick surface films on surface and rock-salt phase domains near surface.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2019. p. 84
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1827
Keywords
LNMO, High-voltage spinel, FEC, Cross-talk, Cation ordering, Oxygen deficiency, Anionic redox
National Category
Inorganic Chemistry
Research subject
Chemistry with specialization in Inorganic Chemistry
Identifiers
urn:nbn:se:uu:diva-389848 (URN)978-91-513-0698-8 (ISBN)
Public defence
2019-09-13, Häggsalen, Ångströmlaboratoriet, Lägerhyddsvägen 1, Uppsala, 09:15 (English)
Opponent
Supervisors
Available from: 2019-08-23 Created: 2019-07-29 Last updated: 2019-09-17

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Aktekin, BurakValvo, MarioBrandell, DanielEdström, KristinaBrant, William

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