Electronic and Structural Changes in Ni0.5TiOPO4 ... - ACS Publications

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Electronic and Structural Changes inTheNi TiOPO Journal of YORK PhysicalUNIV Subscriber access provided by NEW 0.5

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Li-Ion Battery Cells The Journal of YORK PhysicalUNIV Subscriber access provided by NEW Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no

upon First Lithiation and The Journal of YORK PhysicalUNIV Subscriber access provided by NEW Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no

Delithiation, Studied The Journal of YORK PhysicalUNIV Subscriber access provided by NEW Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no

by High-Energy Xrayaccess Spectroscopies The Journal of YORK PhysicalUNIV Subscriber provided by NEW Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no

Rickard Eriksson, Karima Lasri, Mihaela Gorgoi, Torbjorn

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Gustafsson, Kristina Edstrom, The Journal of YORK PhysicalUNIV Subscriber access provided by NEW Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no

Daniel Brandell, Ismael Saadoune, and Maria Hahlin

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J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp511170m • Publication (Web): 13 of Apr 2015 The Journal Physical Subscriber accessDate provided by NEW YORK UNIV Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no

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Page 1 of The 49Journal of Physical Chemistry

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ACS Paragon Plus Environment

b) F 1s

1

pristine

1

1st lithiation x=0.5

2

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st

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2nd fully lithiated 540 538 536 534 532 530 528 Binding energy / eV

694 692 690 688 686 684 682 680 Binding energy / eV

e) P 1s

(Li2O)

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c) O 1s

Intensity / arb.units


1 1st lithiation 2 x=0.5 2 3 4 1st lithiation 3 5 x=1 6 st 1 lithiation 7 x=2 4 8 9 fully lithiated 10 5 111st delithiation 12x=1 6 13 14fully delithiated 15 7 16 nd 172x=1lithiation 8 18 19 202nd fully 9 21lithiated 22 290 288 286 284 282 23296 294 292 Binding energy / eV 24 d)25 PF6 PO4/P-O/P=O P 2p 26pristine 1 27 28 st 1 lithiation 29x=0.5 2 30 st 1 lithiation 31 3 x=1 32 331st lithiation 34x=2 4 35 36fully lithiated 5 37 38 391st delithiation 40x=1 6 41 42fully delithiated 7 43 442nd lithiation 45x=1 8 46 472nd fully 48lithiated 9 49 140 135 130 50 145 Binding energy / eV 51


PF6 LiF

The Journal of Physical Chemistry

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Ti4+ Ti2+

b) Ti2p c) C1s 3+ The Journal of PhysicalTiChemistry

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pristine x=2 fully lithiated fully delithiated

Ni K-edge pristine

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1 2 3 4 5 6 7 8 9 10 11 b) 12 13 14 15 16 17 18 19 20 21 22 23 24 25 c) 26 27 28 29 30 31 32 33 34 35 36 37

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ACS Paragon Plus Environment Lithiation 0.0

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Electronic and Structural Changes in Ni0.5TiOPO4 Li-ion Battery Cells Upon First Lithiation and Delithiation, Studied by HighEnergy X-ray Spectroscopies Rickard Eriksson1, Karima Lasri1,2, Mihaela Gorgoi3, Torbjörn Gustafsson1, Kristina Edström1, Daniel Brandell1, Ismael Saadoune2, Maria Hahlin4* 1. Department of Chemistry - Ångström Laboratory, Uppsala University, Box 538, SE751 21 Uppsala, Sweden 2. LCME, FST Marrakech, Uni. Cadi Ayyad. Av. A: Khattabi, BP 549, 40000 Marrakech, Morocco 3. Helmholz-Zentrum Berlin für Materialien und Energie GmbH, 12489 Berlin, Germany 4. Department of Physics and Astronomy, Uppsala University, Box 516, SE-751 21 Uppsala

ABSTRACT Carbon coated Ni0.5TiOPO4, synthesized with a sol-gel method and used as a negative electrode material in a Li-ion battery, was examined with a range of surface and bulk sensitive high-energy spectroscopic techniques to reveal the mechanism for lithium insertion and electrode/electrolyte reactions as a function of battery cycling. 1 ACS Paragon Plus Environment


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Characterizing the electrode/electrolyte interface during the first lithiation with HAXPES showed a partially irreversible reduction of the Ni atoms while EXAFS results of the full bulk material revealed formation of Ni clusters. The large initial irreversible capacity loss is attributed to formation of the Ni clusters and SEI (Solid Electrolyte Interphase). The first lithiation process is found to be more complex than previously suggested, also incorporating partial reduction of Ti4+ to Ti2+. Moreover, XANES results also show the PO4 units in the redox reactions being partially participating in the irreversible reactions. Correlating the HAXPES results to the plateaus in the galvanostatic lithiation/delithiation voltage profile indicates a combined cooperative contribution from Ni and Ti in both plateaus; however, the first plateau is dominated by the reduction of Ti while the second is dominated by reduction of Ni.

INTRODUCTION The search for stable and reliable anode materials for Li-ion batteries that can react with lithium at a potential above Solid Electrolyte Interphase (SEI) formation (0.8 V vs. Li+/Li) has so far mostly been focusing on the lithium titanate (Li4Ti5O12). This is due to its high power performance; an important property for batteries to be utilized in electric vehicles (EVs) and especially in hybrid electric vehicles (HEVs). Li4Ti5O12 has intrinsic disadvantages of low electronic conductivity and slow lithium ion diffusion that partly limit its practical application in lithium-ion batteries

1-3

. In an attempt to

explore other possible titanium-based negative electrode materials metal oxyphosphates have been proposed where the metal can be nickel, iron, cobalt or others. 4-5

2 ACS Paragon Plus Environment

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It is already well known that phosphate based materials have interesting Li-ion battery properties. In the 1980s, Manthiram et al.

6-7

and Delmas et al.

8-9

showed

that electrode materials with polyanionic groups, XOym- (X = S, P, As, Mo, W...), possessed

promising

properties

in terms

of

stability

during

Li-ion

insertion/deinsertion. The open framework of these compounds allows fast Li-ion diffusion, and the strong covalent X-O bonds result in high intrinsic stability. Several families of polyanionic compounds were studied early on as cathode materials for Li or Na-ion batteries

10-12

, not least the NaSICON structure category,

6,13-18

which

displays a good electrochemical performance due to its high redox potential as compared to the corresponding oxides 19-20. In later years, materials with olivine structures have shown to be even more advantageous, not the least the LiFePO4 compound

21

. LiFePO4 is now a well-

established and well-studied commercial positive electrode material for Li-ion batteries, primarily due to an experimental specific capacity close to its theoretical value at high rates

22-23

. The good characteristics of LiFePO4 have motivated intense

research activities on transition metal (TM) phosphates with polyanionic structural frameworks to be used also as negative electrode materials for Li-ion batteries

24-30

,

which also this study is an example of. Among the possible titanium-based anodes, the titanium oxyphosphate compounds are particularly promising and a number of structurally related compounds have been reported by several groups

5,31-39

. This attention is justified by their good and stable

specific capacity (270 mAh/g for Ni0.5TiOPO4 cycled between 0.5 and 3.0 V vs. Li+/Li), their high rate performance and the safety related to the stability of the covalently bonded PO4 group.



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To date, most of the research in this field has been devoted to optimizing the electrochemical properties of these materials. The work has resulted in the identification of several critical aspects of the material preparation. For example, a significant increase in capacity, coulombic efficiency and rate capability was achieved by coating particles with carbon to overcome their poor electronic conductivity

31-32

.

The electrochemical behavior of Ni0.5TiOPO4 is strongly dependent on the synthesis method and an extra irreversible capacity was identified during the first lithiation process 4,31-32,34,37. Moreover, it was recently found that by limiting the potential range during cycling (between the OCV and 1.1 V, i.e., a cut-off above the voltages where the irreversible capacity is observed), the voltage profile was very similar to the behavior of the conversion reactions of the NiO anode material, but that the material resumed its oxyphosphate characteristics after the 8th cycle

37

. A limited cycling

window will, however, result in a lower energy density, but the operational voltage of these oxyphosphates is not significantly different from those of Li4Ti5O12. The lack of a clear voltage plateau might also restrict the application of the materials in some specific batteries where a well-defined voltage is necessary for implementation. Valuable information on the structural modifications occurring during the Li insertion process in Ni0.5TiOPO4 has been gathered by both in situ and ex situ X-ray diffraction (XRD)

31,33,37

, which has revealed a structural change – a decrease in crystallinity –

after the insertion of one Li+ ion per unit cell. Furthermore, investigation of the structurally related anode material Fe0.5TiOPO4 by Mössbauer spectroscopy did show the formation of metallic iron at the end of the first lithiation spectroscopy

(XAS)

and

resonant

inelastic

X-ray

36

. X-ray absorption

scattering

(RIXS)

of

Li0.5Ni0.25TiOPO4 have also shown changes in the nickel oxidation state from Ni2+ to


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metallic Ni at the end of the first lithiation process, but also showed that Ni changed its valency back to only Ni+ at the end of first delithiation process 40. However, despite the many efforts, the results found on Ni0.5TiOPO4 in literature are still unclear and contradictory, and a full picture of the complexity of the electrode reactions is missing. For example, two different potential plateaus can be observed during the first lithiation, indicating two transition-metal (TM) reduction processes, but it is not clear which element is responsible for each plateau. Neither is it known if the intensity decrease seen in the XRD peak patterns is due to amorphization of the compound or a radical decrease of the crystallite size, nor if the extra capacity during the first lithiation is due to a Solid Electrolyte Interphase (SEI) layer formation or due to a TM reduction to an irreversible form, as has been suggested for Li0.5Ni0.25TiOPO4 40

. In order to gain further insight into the reaction mechanisms of Ni0.5TiOPO4 and to

understand the origin of the detrimental loss of capacity after the first cycle, the goal of this study is to fully clarify these critical aspects and to solve the essential questions that have arisen. In the work presented here we have completed the electrochemical studies of the Ni0.5TiOPO4 material by investigating the chemical states of the surface and the bulk and their evolution during battery operation by means of synchrotron based Hard X-ray Photoelectron Spectroscopy (HAXPES) and X-ray Absorption Near Edge Structure (XANES) Spectroscopy. Furthermore, Extended X-ray Absorption Fine-Structure (EXAFS) measurements have been employed to obtain more details about the local structure around the Ni-atoms. These high-energy X-ray techniques enable a more careful analysis of the changes in composition and the oxidation states of the material components at different



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intercalation steps and during the first and second cycle of Ni0.5TiOPO4 in cells vs. lithium.

Experimental Material and electrodes preparation: Ni0.5TiOPO4 was prepared using a sol-gel method by mixing solutions of H3PO4 (98%, Fluka) and TiCl4 diluted in ethanol (98%, Prolabo) and then drop-wise adding the mixture onto a Ni(NO3)2·6H2O (97%, Prolabo) solution. Details of the synthesis and the method for carbon coating using a combustion synthesis are described in our previous paper 37. For all ex-situ HAXPES, XANES and EXAFS samples prepared in this study, ‘coffee-bag’ cell types (polymer coated aluminum pouch cells) were used following standard assembly procedures: Ni0.5TiOPO4/C electrodes were fabricated by mixing a 75:15:10 (w/w) ratio of active material, a carbon (Super P) electronic conductor and polyvinylidene fluoride (PVdF) binder, using N-methyl-2-pyrrolidine (NMP) as solvent. The mixture was thereafter casted as a thin layer (~50 µm) onto an Cu foil (samples for EXAFS measurements were casted on Al foil). After drying overnight at 60 °C, electrodes were cut into circular shapes (d=2cm) for cell assembly in an Ar-filled glove box (O2

Electronic and Structural Changes in Ni0.5TiOPO4 ... - ACS Publications

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