Coupled LiPF6 Decomposition and Carbonate Dehydrogenation

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C: Surfaces, Interfaces, Porous Materials, and Catalysis 6

Coupled LiPF Decomposition and Carbonate Dehydrogenation Enhanced by Highly Covalent Metal Oxides in High-Energy Li-Ion Batteries Yang Yu, Pinar Karayaylali, Yu Katayama, Livia Giordano, Magali Gauthier, Filippo Maglia, Roland Jung, Isaac Lund, and Yang Shao-Horn J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b07848 • Publication Date (Web): 31 Oct 2018 Downloaded from http://pubs.acs.org on November 10, 2018

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The Journal of Physical Chemistry

Coupled LiPF6 Decomposition and Carbonate Dehydrogenation Enhanced by Highly Covalent Metal Oxides in High-energy Liion Batteries Yang Yua,†, Pinar Karayaylalib,†,*, Yu Katayamac,d, †, Livia Giordanob*, Magali Gauthierc, Filippo Magliae, Roland Junge, Isaac Lundf and Yang Shao-Horna,b,c

a

Department of Materials Science and Engineering,

b

Department of Mechanical Engineering,

c

Research Laboratory of Electronics, MIT, Cambridge, MA 02139, USA,

d

Department

of

Applied

Chemistry,

Graduate

School

of

Sciences

and

Technology for Innovation, Yamaguchi University, Ube 755-8611, Japan e

BMW Group, Petuelring 130, 80788 München, Germany

f

BMW Group Technology Office USA, 2606 Bayshore Parkway, Mountain View,

California 94043, United States

† These

authors contributed equally to this work.

Current address: M.G.: LEEL, NIMBE, CEA, CNRS, Université Paris-Saclay, CEA

ACS Paragon Plus Environment

1

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Saclay 91191 Gif-sur-Yvette, France. Corresponding Authors * Pinar Karayaylali ([email protected]) * Livia Giordano ([email protected])

ABSTRACT The (electro)chemical reactions between positive electrodes and electrolytes are not well understood. We examined the oxidation of an LiPF6-based electrolyte with ethylene carbonate (EC) with layered lithium nickel, manganese and cobalt oxides (NMC). Density functional theory calculations showed that the driving force for EC dehydrogenation on oxides, yielding surface protic species, increased with greater Ni in NMC. Ex-situ infrared and Raman spectroscopy revealed experimental evidence for EC dehydrogenation on charged NMC surfaces. Protic species on charged NMC surfaces from EC dehydrogenation


2

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could further react with LiPF6 to generate less-coordinated F species such as PF3O-like and lithium nickel oxyfluoride species on charged NMC particles, and HF and PF2O2- in the electrolyte. Greater salt decomposition was coupled with increasing EC dehydrogenation on charged NMC with increasing Ni or lithium de-intercalation.

An

oxide-mediated

chemical

oxidation

of

electrolytes

was

proposed, providing new insights in stabilizing high-energy positive electrodes and increasing Li-ion battery cycle life.


3


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1. Introduction

Understanding electrochemical and chemical reactions at the electrode-electrolyte interface is of fundamental importance to increase the safety and cycle life of Li-ion batteries.

1–4

Previous work has shown that increasing Ni in the layered structure from

LiNi1/3Mn1/3Co1/3O2 (NMC111), LiNi0.6Mn0.2Co0.2O2 (NMC622) to LiNi0.8Mn0.1Co0.1O2 (NMC811) leads to marked increase in the initial discharge capacity5,6 (Figure 1(a)) but greater capacity loss upon cycling5–7 as well as reduced thermal stability.6,8–10 The greater capacity loss and impedance growth of NMC with increasing Ni upon cycling5,6,11,12 has been attributed to the greater oxidation of carbonate-based electrolytes3–5,13. Unfortunately, the molecular or mechanistic details of the chemical reactivity between Ni-based high-energy electrodes and electrolytes are not fully understood.

Here we propose that the more significant capacity fade and impedance growth of NMC electrodes with increasing Ni can be attributed to increasing electrolyte oxidation associated with higher reactivity of oxygen in oxides3,5,14,15 with increasing metal-oxygen covalency.3,4,16,17 This hypothesis builds upon recent correlation of increasing catalytic activities of water oxidation to evolve molecular oxygen on late-transition-metal oxide surfaces with greater metal-oxygen covalency18. Upon the incorporation of late transition metals in metal oxides or the oxidation of the transition metal ions during de-intercalation upon charging, the covalency of the M-O bond increases with greater overlap between transition metal d states and oxygen 2p states, inducing more oxygen character at the Fermi level17–21. For example, upon charging, late transition metal redox couples such as Co3+/Co4+ and Ni3+/Ni4+ are pinned at the top of O-p band


4

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of layered oxides (e.g. Li1-xCoO2,17,19 Li1-xNiO2,17 Ni1-xCoO219 and Na1-xCoO220 ). The increase of M-O bond covalency can result in more facile electron transfer directly from the oxygen density of states (DOS) and easier transition metal reduction,17 which provides higher driving force for oxygen release22–25 (LixMO2 => LixMO2-2y + yO2). This argument is supported by reduced thermal stability of NMC with increasing Ni (Figure 1(a)). Moreover, greater metal-oxygen covalency can increase the thermodynamic driving force for the chemical oxidation of carbonatebased electrolytes by dehydrogenation as shown by recent density functional theory (DFT) studies.4,16,26 The thermodynamic driving force for the chemical oxidation of carbonate solvents on layered LixMO2 surfaces increases with later transition metal and less lithium contents, where the surface oxygen of LixMO2 can dehydrogenate carbonate solvents such as ethylene carbonate (EC)16,26 and ethyl methyl carbonate (EMC)27. Such dissociative adsorption would form surface protic species (e.g. C3O3H3+-Osurface and H+-Osurface), whereas transition metal ions are reduced (C3O3H4 + 2 O-M3+-O  *C3O3H3+ + *H+ + 2 O-M2+-O), as shown schematically in Figure 1(c). The dissociative adsorption16,28,29 is energetically more favorable than previously proposed electrophilic attack30,31, nucleophilic attack32, and EC dissociation with oxygen extraction from these oxide surfaces30.

Increasing Ni and/or lithium de-intercalation in NMC can increase the thermodynamic driving force for dehydrogenation of carbonate solvents, and thus lead to greater chemical oxidation of carbonate-based electrolytes. This argument is supported by the computed the O-p band center trend of NMC111, 622 and 811, closer to the Fermi level with increasing Ni. Projecting them onto the previous correlation between the computed energy of EC dissociative adsorption on LixMO2 surfaces and


5


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the O-p band center relative to the Fermi level16 (shown as dashed lines in Figure 1(c)) revealed higher driving force for EC dissociative adsorption with increasing Ni in NMC. Further support comes from recent on-line electrochemical mass spectrometry (OEMS) measurements3,5 showing that the onset potentials for the oxidation of carbonate-based electrolytes by NMC111, 622, and 811 to CO and CO2 are oxide-dependent, and decreasing with increasing

Ni

in

NMC

(Figure

1(a)).

The

dehydrogenated

carbonate

intermediates can form organic films and protic species on oxide surfaces such as LixCoO2 reported recently27

while protic species on the surface

can react with electrolyte salts such as LiPF6 to form oxidized products, such as PF3O (for NMC111 and Li-rich NMC33), which can be deposited on the electrode surface27. Both processes can become more prominent with

increasing

Ni

for

charged

NMC

and

contribute

to

increased

impedance growth and capacity loss during cycling. In this work, we seek experimental evidence for the dehydrogenation of carbonate solvents by charged NMC111, 622 and 811 as suggested by DFT and resulting


6

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further decomposition reactions with the salt (LiPF6). In order to avoid any ambiguities from the presence of carbon and binder in the reactivity at the oxide-electrolyte interface, where carbon often accounts for the largest surface area of the electrode, we examine oxide-only electrodes27 free of carbon and binder obtained at different voltages in the first charge. FT-IR, Raman as well as X-ray photoelectron spectroscopy (XPS) have been used to identify carbonate-derived and salt-related species on the NMC surfaces, and NMR analysis on the electrolyte solutions after charging is also conducted.

Figure 1: (a) The capacity fade (in red open circle) after 100 cycles5 increases while the thermal stability4 (in black closed circle), as well as the electrochemical oxidation onset voltage of electrolyte decomposition5 (in grey closed circle) decreases in the presence of LiNixCoyMn1-xyO2 (NMC) positive electrode materials with increasing nickel. (b) Schematic representation of the Li transition metal oxide projected density of states (DOS) showing the position of the O-2p


7


Page 8 of 69

and M 3d DOS relative to the molecular levels for EC (right) and dehydrogenated EC radical interacting with the oxide surface. (c) EC dissociative adsorption energy on NMC111, 622, and 811 (dashed lines) by projecting their computed O-2p band centers onto the trend established by layered oxides, LixMO2 (M= Co, Mn, Ni, Fe) from Giordano et al.16, where pushing the Fermi level (EF) closer to oxygen 2p band center increases the driving force for EC dissociative adsorption with increasing Ni in NMC. 2. Methods 2.1 Experimental methods

Oxide

synthesis

(LiNi0.33Mn0.33Co0.33O2)

was

and

Electrode

prepared

through

preparation solid-state

NMC

reactions

111

between

Ni0.33Mn0.33Co0.33(OH)2 and LiOH (Sigma Aldrich) precursors in stoichiometric ratio and sintered at 480 °C and 900 °C in air for 3 hours each, with intermediate grinding. The Ni0.33Mn0.33Co0.33(OH)2 precursors were prepared through co-precipitation process, where the 1 M solution comprises of 0.33M each

of

Co(NO3)2*6H2O

(Alfa

Aesar),

Mn(NO3)2*6H2O

(Alfa

Aesar),

and

Ni(NO3)2*4H2O (Sigma Aldrich). Then the nitrate solution was mixed with 2M LiOH H2O (Sigma Aldrich) dropwise and mixed for 30 minutes. The resulting solid product is then filtered and rinsed with deionized water and dry in air at 180

°C

for

12

hrs.

NMC

622

(LiNi0.6Mn0.2Co0.2O2)

and

NMC

811

(LiNi0.8Mn0.1Co0.1O2) were obtained from ECOPRO, South Korea.


8

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In this study, we employed oxide-only electrodes since using composite electrodes hinders extraction of useful information on solvent decomposition products in C1s (due to conductive carbon) and salt decomposition products in F1s (due to binder) in XPS. Moreover, the use of composite electrodes also poses difficulties on performing Raman and DRIFT measurements due to the high surface area of conductive carbon, which can significantly contribute to the signal. By using oxideonly electrodes, we can directly correlate the surface decomposition products and interfacial reactivity with the oxide electronic structure without the interference of carbon and binder. The pellet

electrode was prepared through pelletizing around 48 mg of active materials using 6 mm diameter pressing die set (Across International) for 15 mins. The pellets were then sintered under oxygen flow at 900 °C, 750 °C and 725 °C for 6 hours, for NMC 111, 622 and 811, respectively. The cooling and heating rates are controlled to be 2 °C/min. The pellet electrodes are then broken into pieces of around 3 mg each, then dry in vacuum under 120 °C overnight before transferred into Argon-filled glovebox ( H2O + Ovac; H2Ooxide + PF5 => PF3O + 2HF, PF3O + H2Ooxide => HPF2O2 + HF and 2HF + NMC => NMC-oxide/fluoride + H2O. This mechanism is different from previous works, which have reported that salt decomposition can proceed through hydrolysis through the residual H2O in the electrolyte and other cell components72 reacting with salt through H2O + LiPF6 => PF3O + 2HF27 or H2O + PF5 => PF3O + 2HF, PF5 being formed by thermal decomposition of PF6-73,74.


34

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Figure 5: XPS spectra of the Ni2p and F1s photoemission lines for (a) charged LixNi1/3Co1/3 Mn1/3O2 (NMC111),

(b)

LixNi0.6Co0.2Mn0.2O2

(NMC622)

and

(c)

LixNi0.8Co0.1Mn0.1O2 (NMC811) for pristine and charged carbon-free, binder-free electrodes to 4.1, 4.2, 4.4, 4.6 and 4.8 VLi with 1 M LiPF6 in EC: EMC (3:7 wt:wt) electrolyte (electrochemistry profiles are shown in Figure S11). All spectra were calibrated with the adventitious hydrocarbons at 285.0 eV and background corrected using a Shirley background. The F1s spectra were deconvoluted to three different species: lithium or metal fluoride species around 685 eV58, lithium

or

metal

fluorophosphate

around

686.5

eV58

and

lithium

hexafluorophosphate around 688 eV62. F1s spectra are compared with LiF and LiPF6 references and Ni2p spectra are compared with NiF264,75 reference. Co2p, Mn2p, P2p and Li1s/Co3p XPS spectra is shown in Figure S12-13, the


35


Page 36 of 69

quantification is shown in Figure S14 and Table S1-S3, and the reproducibility is shown in Figure S15-20.

Increasing reactivity towards LiPF6 from charged NMC111, NMC622 to NMC811 was further confirmed by XPS analysis of their F1s and Ni2p spectra and reference spectra of NiF2

64,75,

and LiPF6 at 688 eV62, as shown in Figure 5.

Additional spectra of Li1s, P2p, Mn2p and Co2p can be found in Figures S1213. The intensity of F1s collected from all charged NMC111 remained small, indicative of minimum decomposition of LiPF6 on charged NMC111 surfaces except

charged

NMC111

(xLi=0.1),

which

is

consistent

with

DRIFT

measurements in Figure 2(c). On the other hand, considerable intensities of F1s were observed for charged NMC 622 and 811, which include lithium or other transition metal fluorides (LiF and/or LiMxFyOz) at ~685.0 eV58, and lithiated or transition metal fluorophosphates (LixPFyOz) at ~686.5 eV58. This formation of LiMxFyOz-like species is accompanied with a positive shift in the Ni2p spectra from 854.5 eV in the pristine NMC to ~856 eV in the charged NMC111 (xLi=0.1 in Figure 5(a)), NMC622 (below xLi=~0.5 in Figure 5(b)) and


36

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NMC811 (between xLi=~0.5 and ~0.2 in Figure 5(c)) by comparing with reference NiF275. LiMxFyOz-like species can be formed by reacting HF with charged NMC622 and NMC811 as suggested previously76, where HF can be generated by reacting protic species from dehydrogenation of EC with LiPF6-like species.

F

in

LiMxFyOz-like

species

increased

with

decreasing

lithium

composition as revealed by Ni2p spectra, where charged NMC622 became NiF2-like on the surface at xLi = 0.17 while charged NMC811 had NiF2-like surface

between

xLi=~0.5

and

~0.2.

Unfortunately,

the

disappearance

of

LiMxFyOz-like species on the charged NMC811 surface (xLi=0.13), exposing the highly reactive oxide surface to the electrolyte for subsequent discharge and cycles, while LiMxFyOz-like species covered charged NMC622 (xLi=0.17) reducing the reactivity for subsequent discharge and cycles. The contribution of LixPFyOzlike species in the F1s spectra did not change significant relative to that of LiMxFyOz with decreasing lithium composition for charged NMC622, which is in agreement with the P2p spectra (Figure S13(a), (c), (e)). In contrast, the intensity of LixPFyOz-like species was increased considerably for charged


37


NMC811 greater

(below reactivity

xLi=~0.4) with

with

PF6-

decreasing

with

more

Page 38 of 69

lithium

protic

composition,

species

derived

suggesting from

EC

dehydrogenation than charged NMC111 and 622. The F1s intensities of charged NMC 622 and 811 were reduced at the lowest lithium composition examined, suggesting the dissolution of these F-containing species into the electrolyte, which will be discussed below. Evidence for the formation of soluble HF77 and oxidized salt species from the electrolyte chemical oxidation by charged NMC came from

19F

nuclear magnetic

resonance (19F-NMR) measurements of solution species collected from soaking charged carbon-free binder-free electrodes in EMC solvent for 10 minutes, 1 hour and 4 hours, as shown in Figure 6 and Figure S21. The presence of PF2O2-77 at ~83 ppm and HF at ~153 ppm was detected for charged NMC622 (below xLi=~0.3) while these two species appeared for charged NMC811 (below xLi=~0.4). This observation is consistent with greater reactivity of charged NMC811 to decompose PF6- as reflected by pronounced changes in the P-F region in the DRIFT spectra (Figure 2 (c)) as well as the F1s (Figure 5) and


38

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P2p (Figure S13(a), (c), (e)) XPS spectra. In contrast, PF2O2- and HF was barely detected for charged NMC111, which is in agreement with minimum salt decomposition on charge NMC111 surfaces, as shown in Figure 2(c) and Figure 5. In summary, the decomposition of LiPF6 salt on charged NMC surfaces with increasing nickel. The LiPF6 salt decomposes into LixPFyOz-like and LiMxFyOz- like species which is supported by F1s, O1s, Ni2p, Li1s and Ni2p spectra. While the ratio of LixPFyOz-like species to LiMxFyOz-like species relatively unchanged for charged NMC622, NMC 811 shows the growth of LixPFyOz-like at the expense of LiMxFyOz-like species, indicating increasing Ni inducing increasing salt decomposition triggered by protic species derived from EC dehydrogenation.


39


Page 40 of 69

Figure 6:19F-NMR spectra of carbon-free, binder-free LixNi1/3Co1/3 Mn1/3O2 (NMC111),


(NMC622),

and


(NMC811)

electrodes

charged to 4.1, 4.2, 4.4, 4.6 and 4.8 VLi with 1 M LiPF6 in EC: EMC (3:7 wt:wt) electrolyte, the example electrochemistry profile of those electrode upon charging is shown in Figure S11. The lithium content (x in LixNiyMnzCo1-y-zO2) of each sample is indicated under the spectrum. The NMC electrodes were soaked in EMC for 10 minutes before the NMR characterization. The spectra were normalized and referenced with the PF6- peak at -73.2 ppm chemical shift with phase and baseline correction.

19F-NMR


spectra of charged, carbon-free, 40

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binder-free NMC111, 622 and 811 electrodes with 1 M LiPF6 in EC: EMC (3:7 wt:wt) electrolyte and soaked in EMC for 1 hour and 4 hours are shown in Figure S21. 3.3

Proposed

Oxide-Mediated

Chemical

Oxidation

of

Carbonate-based

Electrolytes on NMC

Here we introduce an oxide-mediated mechanism for the chemical oxidation of LiPF6-based carbonate electrolytes by charged NMC in Li-ion batteries (Figure 7).

As we lower the Fermi level closer to O-p band through lithium de-

intercalation and/or increasing the Ni in NMC, the dissociative adsorption of EC on oxide surface become more energetically favorable as predicted by DFT. This prediction is experimentally supported by the observed blueshift of peaks at ~ 1804 and 1776 cm-1 demonstrated by DRIFT measurement of charged NMC811 (Figure 2(a)), as well as the emerging Raman peaks at 732 cm-1 and 907 cm-1 in the charged NMC811 (Figure 3). Both of those features can be attributed

to

the

dehydrogenation

of

spectroscopic EC

and

footprint

Li+-EC,

of

reaction

including

products

dehydrogenated

from

the

EC-based

oligomers with ring intact. The formation of dehydrogenated EC and oligomer


41


Page 42 of 69

species is further supported by the emerging broad features at ~ 1190 cm-1 and 1140 cm-1 from the C-O stretching region (Figure 2(c)) of DRIFT spectra. Furthermore, the formation of oxidized carbon species such as semicarbonates and polyethers, which can be produced through solvent oxidation derived from EC dehydrogenation, is observed by XPS C1s and O1s spectra (Figure 4) for charged NMC 622 and 811 at low lithium compositions. Those oxidized solvent species can further decompose by opening the ring and eventually form CO2 and

protons,

demonstrated

by

previous

OEMS

results

in

the

literature.5

Moreover, EC dehydrogenation generates surface protic species (Ooxide-H+)16 or H2Ooxide5 and H2O2oxide78 which can further attack LiPF6 salt. DRIFT spectra within the C-O stretching region (~1190 cm-1) have further shown that the formation of oligomers from solvent oxidation on the oxide surface (Figure 2(c)) is coupled with an emerging broad band from ~ 840 cm-1 and ~900 cm-1 in PF stretching region (Figure 2(c)) of charged NMC electrodes, corresponding to oxidized salt (PF3O-like) species, indicating that solvent and salt decompositions are coupled. This feature becomes pronounced at a higher lithium composition


42

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for charged NMC811 compared to NMC622 and 111, matching with the trend of observed solvent dehydrogenation on oxide surfaces, further supporting the salt decomposition is triggered by surface protic species generated through carbonates dehydrogenation. This salt decomposition process is also captured by XPS F1s and P2p spectra (Figure 5 and S12(a), (c), (e)) and NMR (Figure 6), where we observe more salt decomposition products on the oxide surface including LiMxFyOz and LixPFyOz, and in the electrolyte solutions including PF2O2- and HF as we increase Ni in NMC electrodes. HF generated through salt decomposition process can further attack oxides, forming metal oxyfluorides on oxide surfaces, as demonstrated by XPS (Figure S14). This coupled decomposition of solvent and salt is in agreement with the observed simultaneous decomposition of ethylene carbonate species and LiPF6 in the electrolytes of aged cells with NMC111, 442, 532 as positive electrodes characterized through machine-learning assisted FT-IR79 and differential thermal analysis80, where increasing degree of salt decomposition correlates with greater capacity loss.80 Lastly, charged NMC622 remain passivated by LiMxFyOz species up charging to xLi = 0.17 whereas LiMxFyOz built up on charged NMC811 was removed at lithium content lower than xLi = 0.29. Exposed oxide surface of charged NMC811 could further dehydrogenate carbonate solvents and decompose LiPF6,


43


Page 44 of 69

which could lead to greater capacity loss of NMC811 than NMC622. More in-depth investigations are needed to test this hypothesis.

Figure 7. Proposed decomposition mechanisms of ethylene carbonate (EC) solvent and LiPF6 salt upon oxidation in the presence the transition metal oxide, where EC molecules dissociate on oxide surface, forming dehydrogenated species and surface protic species16 or H2O5, proposed by Leung et al.30 and Giordano et al.16 The hydrogenated species can form oligomer with ring-opened EC molecules or deprotonated ethyl methyl carbonate (EMC). The proposed chemical decomposition of electrolyte on oxide surface presumably has an onset of around 4.1 VLi – 4.8 VLi, depending on the oxide chemistry, and this onset voltage is estimated through the emergence of spectroscopic footprint captured by FT-IR, Raman, and XPS. This mechanism of solvent decomposition on oxide surface is in parallel with the evolution of singlet oxygen from oxide5,78,81,

attacking

the

electrolyte

solution,

generating

dehydrogenated

carbonate species and protons, proposed by Jung et al.5 and Wandt et al.81 In


44

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contrast, the electrochemical oxidation of bulk electrolyte in the presence of salt species, accompanied by proton transfer to salt anion or other solvent molecules, is expected at much higher potential (> 5 VLi)82,83 compared to other two chemical oxidation pathways. The dehydrogenated species will further open the ring and eventually evolve CO2. The protic species originated by solvent oxidation can decompose the PF6-based salt species, eventually leading to formation of PF3O gas and PF2O2- soluble species in the electrolyte. HF generated in the salt decomposition process can further attack the oxides and form metal oxyfluorides, according to Aurbach et al.32.

The proposed oxide-mediated electrolyte decomposition mechanism in this study can

occur

in

parallel

with

previously

proposed

chemical

decomposition

mechanism of the electrolyte by Jung et al5 and Wandt et al.81 Highly reactive oxygen-species such as singlet oxygen81 evolved from charged NMC111, 622 and 811 (all at xLi=~-0.25) can attack carbonate molecules,78 and potentially deprotonates EC78 and decompose carbonate molecules to CO2 and CO. These two proposed mechanisms for oxide-dependent electrolyte chemical oxidation are supported by the lowered onset voltage for electrolyte decomposition in presence of spinel LixMn2O4 compared to glassy carbon previously reported by Xu et al.84 The chemical reaction pathways are in contrast with the electrolyte


45


Page 46 of 69

electrochemical decomposition pathway, where the carbonate oxidation can occur through either transferring the protons to PF6- salt species and forming HF or to other carbonate molecules at voltages greater than 5 VLi82,83. This electrochemical oxidation of electrolytes eventually form CO2 and protic species without the participation of oxides, and the onset voltage of this process is therefore not oxide-dependent, and much higher than that for chemical oxidation pathway.82,83

The oxide-surface mediated pathway of electrolyte oxidation for charged NMC via carbonate dehydrogenation in this study is different from pioneering work from Aurbach et al.32, where surface oxygen ions of charged LiNiO2 oxides is proposed to react with alkyl carbonate solvents through nucleophilic attack, leading to ring opening of EC32 and the formation of oligomers78 and polycarbonate species.32,85 The dehydrogenation of carbonate molecules via dissociative adsorption is shown by a number of DFT studies16,28,29 to be more energetically favorable than previously proposed nucleophilic attack32,85

(by more than 1 eV) and other mechanisms

including electrophilic attack on NMC and LixMn2O4 surfaces

30,31

and EC dissociation with

oxygen extraction from LixMn2O4 surfaces30. Although further work is needed to dive deeper,


46

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these nucleophilic attack reactions of electrolytes proposed earlier32 do not necessarily Therefore,

generate nucleophilic

surface attack

protic cannot

species explain

to

further the

decompose

observed

LiPF6.

coupled

salt

decomposition with carbonate oxidation observed in this studies unlike the mechanism proposed in this study.

This proposed oxide-mediated electrolyte degradation mechanism is discussed in the context of two general strategies to improve cycle life of Ni-rich NMC positive electrodes in Li-ion batteries, including oxide surface modifications86–90 and electrolyte additives91–97. By passivating the reactive surfaces of Ni-rich NMC electrodes with materials with much lower driving force towards solvent dehydrogenation and oxygen release, the oxide-facilitated solvent decomposition can happen at much higher voltage, reducing the salt decomposition and impedance growth on the oxide surface. This can explain the improved cell performance by coating the electrode with metal oxides (e.g. Al2O398–101, TiO2102– 104)

as well as fluorides (AlF3105) and phosphates (AlPO4101,106–108) where most

of those binary compounds have higher Fermi levels with regard to ligand p-


47


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band compared to those of the positive electrodes, thus decreasing the driving force for solvent dissociation on the electrode surface. This strategy can be further developed into a core-shell approach109–111 by coating Ni-rich core materials with Ni-poor oxide shell, which has much lower reactivity towards electrolyte decomposition, and as a result increasing the cycling performance of the positive electrodes. Altering the reactivity of electrode toward electrolyte can also be achieved through additives in the electrolyte. Previous study on LiCoO227 has demonstrated that diphenyl carbonate27,93,112, as a common additive, has an earlier onset for electrochemical oxidation compared to the electrolyte solvent. By sacrificially oxidizing the additive at low potential, it provides protic species and facilitates much earlier onset of LiF formation deposited on oxide surfaces, protecting the oxide towards further solvent and salt

decompositions,

improving

cycling

performance.

Another

category

of

additives includes the solvent and salt decomposition products, for example vinylene

carbonate

(VC)94,113–115

or

LiPO2F2116–118.

Through

increasing

the

concentration of decomposed products, we can mitigate the chemical equilibrium


48

Page 49 of 69 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


of

the

decomposition

reaction

going

forward,

decreasing

the

actual

decomposition products formed on the electrode surface. Currently, most of the works have been focusing on the impact of coating and additives on cell performance, and in-depth studies on the reaction mechanism of the electrolyte solvent and salt with positive electrode in the presence of different coating and additive chemistries are needed. This work has constructed a transferrable framework

with

techniques

including

Raman,

DRIFT,

XPS,

that

can

be

employed in those further studies to study the oxide surfaces, coupled with the analysis of the electrolyte to fully understand how coatings and additives alter the reaction mechanism of electrolyte decomposition on oxide surfaces for different chemistries. By connecting surface-sensitive techniques on oxides and bulk electrolyte studies with cycling performance, we can eventually pinpoint the key descriptors for effective coating and electrolyte chemistries, enabling rational design of more stable electrode electrolyte interface and better cycle life of high energy-density Li-ion batteries. 4. Conclusions


49


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This work combines DRIFT, Raman, XPS and NMR spectroscopies with DFT calculations to understand the electrolyte solvent and salt oxidation on charged positive electrode surfaces of NMC111, NMC622 and NMC811. By combining Raman and DRIFT spectroscopies and DFT, we show evidences for the chemical oxidation of electrolyte solvents by EC dissociative adsorption on charged NMC, where EC dehydrogenate and yield surface protic species. This process is facilitated by highly reactive surface oxygen associated with highly covalent Ni-O of charged NMC, where the driving force for electrolyte chemical oxidation via EC dehydrogenation is increased by lowering the Fermi level into the O-p band with increasing Ni and/or decreasing lithium content. Combining XPS (F1s and P2p spectra) and DRIFT measurements on C-O and P-F region, we show that the solvent oxidation and salt decomposition are coupled. As carbonate molecules dehydrogenate, the salt species also decompose to less coordinated P-F species by charging NMC622 and 811 but much less visibly for NMC111. These less coordinated P-F species include species such as PF3O-like (DRIFT) and lithium nickel oxyfluoride species (XPS), and soluble HF


50

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and

PF2O2-

in

the

electrolyte

solution

(NMR).

Charged

NMC622

remain

passivated by LiMxFyOz species up charging to xLi = 0.17 whereas LiMxFyOz built up on charged NMC811 was removed at lithium content lower than xLi = 0.29, exposing oxide surface to further dehydrogenate carbon solvents and decompose LiPF6 and leading to greater capacity loss of NMC811, which should be examined further. An oxide-mediated mechanism of electrolyte chemical oxidation was proposed and discussed in the context of previous work. Through those findings, we propose key reaction intermediates and an oxide-mediated decomposition pathway of carbonate solvents and PF6salt species in the electrolyte, which provide insights into laying the foundation

towards

the

rational

designs

of

coating

and

electrolyte

additives for more stable positive electrode-electrolyte interface. Supporting Information Supplementary electrochemical data, additional FT-IR and Raman spectra for pristine electrolyte and pellets charged in different electrolytes, additional computed FT-IR and Raman spectra, reproducibility of XPS data and details in deconvolution. This information is available free of charge via the Internet at http://pubs.acs.org


51


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Conflicts of interest There are no conflicts to declare.

ACKNOWLEDGMENT This work made use of the MRSEC Shared Experimental Facilities at MIT, supported by the National Science Foundation under award number DMR1419807. Research at MIT related to this work was supported financially by BMW. This research used resources of the National Energy Research Scientific Computing Center (NERSC), a DOE Office of Science User Facility supported by the Office of Science of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231. This work also used resources of the Extreme Science and Engineering Discovery Environment (XSEDE)119, which is supported by National Science Foundation grant number ACI-1548562. We would like to thank Jan Rossmeisl from University of Copenhagen, Hubert Gasteiger from Technical University of Munich for fruitful discussion.


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59


Page 60 of 69

TOC Graphic


60

Page 61 of 69 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60



61

LixPFyOz

ose mp co De

TheEthylene Journal of Physical Chemistry Page 62 of 69 Carbonate

Oxde-electrolyte interfacial reactivity

ss

oc

ia t

e

A tt a

ck

HF Attack

Di

1 2 3 4 5 6 7 8

PF6-

LiMxFyOz

LixNMC811

ACS Paragon Plus Environment Li NMC622 x

LixNMC111

O 2p-band center with respect to Fermi level

(a)

(b)


(c) 0.5

340 4.6 40

0.0

20 260 10 240

LiMnO2

EF a

M 3d

1.5

ΔE (eV)

280

LiCoO2

-1.0

Li0.5CoO2

-1.5

1.5

c

-2.0

O 2p

Oxide

1.0

1.0-2.5

0.5

0.5

LiNiO2 CoO2

-3.0 220

0

NMC111

NMC622

NMC811

nucleophilic attack@carbonyl C

ACS Paragon Plus Environment b Density of states 0.0

charge (e)

4.0

30

DE (eV)

4.2

300

LiFeO2

-0.5

Energy

4.4

Capacity fade (mAh/g)

320

Thermal Stability (oC)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17

Electrolyte decomposition voltage (Vgraphite)

Page 63 of 69

-3.5

0.0

-5.0

d

dissociation -4.5 -4.0

-3.5

-3.0

-2.5

O-2p band center (eV)

-2.0


(a)

NMC111

NMC622

LixNi0.33Mn0.33Co0.33O2


1 2 1776 3 1804 4 5 6 4.8 V 7 0.25 8 9 4.6 V 0.35 10 11 12 4.4 V 0.5 13 14 4.2 V 0.6 15 16 4.1 V 17 0.75 18 19 Pristine xLi = 1 20 21 22 LP57 23 24 1830 1800 1770 1740 25 26 (c) 27 NMC111 28 LixNi0.33Mn0.33Co0.33O2 29 30 C-O 31 P-F 32 33 34 4.8 V 35 0.25 36 37 4.6 V 0.35 38 39 40 4.4 V 0.5 41 42 4.2 V 0.6 43 44 4.1 V 45 0.75 46 Pristine 47 xLi = 1 48 49 50 LP57 51 52 12601170 910 820 730 53 54 55 56

Page 64 of 69

(b)

NMC811 LixNi0.8Mn0.1Co0.1O2

DFT Solvent

Solvent

Oligomer

1776

1776

1804

1804

1824 1811

0.03

0.05

1717 Li+-EMC

Li+-deH-EC 0.15

0.13 1782

0.25

0.18

0.35

0.3

0.45

0.38

xLi = 1

1830 1800 1770 1740

Li+-EC

EMC

1799 EC

1832

xLi = 1

1830 1800 1770 1740

1830 1780 1730

1830 1780 1730

1830 1780 1730

DFT

NMC811 LixNi0.8Mn0.1Co0.1O2

C-O

C-O

1791

VC

Wavenumber / cm-1

(d) LixNi0.6Mn0.2Co0.2O2

1799

1740

1828 deH-EC

Wavenumber / cm-1

NMC622

1806

1755 deH-EMC

Solvent

Oligomer

Salt

P-F

P-F

0.03

0.05

1250 deH-EMC

1129 910

EMC

PF3O

1260

0.15

1161

0.13 0.25

0.18

1150 Li+-deH-EC

896 PF5

0.35 0.3 0.45

0.38

xLi = 1

xLi = 1

1174

1192 Li+-EC

1149 deH-EC

780 1189

PF6-

EC 1154 12601170

910 820 730

Wavenumber / cm-1

12601170

910 820 730


1250 1150 1050

1250 1150 1050

990

Wavenumber / cm-1

900

810

720

(a) Page 65 of 69 NMC111

(b) The Journal of Physical Chemistry

NMC622


1 2 3 44.8 V 5 6 7 8 9 4.6 V 10 11 4.4 V 12 13 144.2 V 15 16 4.1 V 17 18 Pristine 19 20 21 22LP57 23 24 25

907 732

732

DFT

NMC811


907


Solvent

Solvent

Oligomer

907 732 0.03

0.25

0.05

753 917 Li+-deH-EC

794 + Li -EMC

944 720

959

0.13 0.35

0.5

0.6

0.15

0.25

0.35

0.18

740 897 deH-EC 0.45 xLI=1

xLi=1


400 600 800 1000

Wavenumber / cm-1

944 774 deH-EMC

794 EMC

730 929

944

715

895

0.38

715 EC 400 600 800 1000

911

0.3

0.75

xLi=1

739 Li+-EC

400 600 800 1000

900

700 800 900

748 911 VC

700 800 900

Wavenumber / cm-1

784 897

700 800 900


(a)

(b)

NMC 622

C1s

O1s

(c) C1s

NMC 811 O1s

0.10

0.17

0.13

0.30

0.28

0.19

0.36

0.31

0.47

0.40

0.39

0.51

0.49

0.48

4.4 V

4.6 V

4.8 V

C1s

0.29

x2

4.1 V

4.2 V

x2

x2

pristine

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41

NMC 111 O1s

Page 66 of 69

xLi=1

xLi=1

xLi=1


Binding Energy (eV)

Page 67 of 69

(a)

NMC 111

NMC 622

F1s 0.17

0.13

0.30

0.28

0.19

0.36

0.31

0.29

0.47

0.40

0.39

0.51

0.49

0.48

References

4.6 V

0.10

LiPF6

LiF

LiPF6

pristine

NiF2

LiF

Binding Energy (eV) ACS Paragon Plus Environment

Ni2p

LiPF6

pristine

NiF2

NMC 811

F1s

4.4 V

Ni2p

(c)

4.2 V

Ni2p

(b)

4.1 V

4.8 V

F1s

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47


LiF

pristine

NiF2

NMC111 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44

NMC622 The Journal of Physical Chemistry

NMC811 Page 68 of 69 PF2O2-

HF

4.8 V 0.15

0.1 PF2O24.6 V

0.14

HF 0.24

0.19

0.28

0.25

0.50

0.35

0.36

xLi = 0.55

xLi = 0.44

xLi = 0.42

0.33

4.4 V

0.35

4.2 V

4.1 V


Page 69 of 69

Moshkovich et al.

Ring Open

CO2+ H+

4.1 VLi - 4.8 VLi

Oligomers

DeH Giordano et al. Leung et al. (Spinel)

e-

Layered Oxides 1O

2

Attacking

?

Layered Oxides

Attacking

+PF5

LixPFyOz

Jung et al. Wandt et al.

+

Ethylene Carbonate (EC) >5.0 VLi Electrochemical oxidation Borodin et al.

DeH-EC solution ACS Paragon Plus Environment

H+

+H2O

4.3 VLi – 4.7 VLi

PF3O

Attacking oxides

HF Aurbach et al.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38


MF

PF2O2-

Coupled LiPF6 Decomposition and Carbonate Dehydrogenation

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