Degradation of Ethylene Carbonate Electrolytes of Lithium Ion

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Article

Degradation of Ethylene Carbonate Electrolytes of Lithium Ion Batteries via Ring Opening Activated by LiCoO Cathode Surfaces and Electrolyte Species 2

Jonathon L. Tebbe, Thomas F Fuerst, and Charles B. Musgrave ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b06157 • Publication Date (Web): 09 Sep 2016 Downloaded from http://pubs.acs.org on September 17, 2016

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ACS Applied Materials & Interfaces

Degradation of Ethylene Carbonate Electrolytes of Lithium Ion Batteries via Ring Opening Activated by LiCoO 2 Cathode Surfaces and Electrolyte Species Jonathon L. Tebbe , Thomas F. Fuerst , Charles B. Musgrave*

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1̅ -

-

-

-

-

𝑅3̅𝑚 -

-

Figure 1. Side view of the 180 atom (101̅4) LiCoO2 surface model, with a 15 Å of vacuum gap between surface slabs, denoted 1̅4) LiCoO surface model is fully lithiated 2

and comprised of 12 Li, 12 Co, and 24 O surface atoms with hydroxyl groups terminating the surface Co atoms. Hydroxylation of the surface Co sites resulting in fractional coverages of 0.5 or greater were found to be energetically favorable by 1.43 eV on average. We examine EC ring opening reactions with this model surface that interact with bare surface

∆𝐻298𝐾 (= ∆𝐸0 + ∆𝐸𝑍𝑃𝐸 + ∆𝐸𝑡ℎ𝑒𝑟𝑚𝑎𝑙 + 𝑃∆𝑉)

*

Co sites without OH termination and CoOH* sites. The atoms shown are Co (blue), O (red), Li (green), and H (white).

∆𝐸0


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1̅ 1̅

1̅ -

1̅ 1̅

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1̅

1̅

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and species that comprise the SEI has been extensively studied because the unbiased standard bulk graphite anode potential (essentially, its Fermi level) lies above the LUMO of EC, and thus reduction of EC at the anode

is

expected

to

occur

based

on

thermodynamics2. Populating the EC LUMO leads to dissociation of the carbonate C and an ether O bond (101̅4)

to open the EC ring and form CO2 and acetaldehyde65. However, EC decomposition and CO2 formation activated by Lewis acids that oxidize EC remains relatively unexplored. Thus, we have examined ring opening reactions of EC to form CO2 activated by

-

various Lewis acids that are present in LiCoO2 LIBs with liquid EC-based electrolytes. We first calculated decomposition of a single, fully-oxidized EC+ molecule in PCM solvent, as shown in Figure 2, to form CO2 and the acetaldehyde cation through: EC+  CO2 + CH3CHO+

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Ring Opening of a Single EC+ in Implicit Solvent: Reductive decomposition of EC to form CO2 8 ACS Paragon Plus Environment

(2)

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(a)

Ethylene Moiety

-

-

Carbonate Moiety 1.21 Å

-

1.13 Å

-

1.33 Å



1.48 Å

(b)

-

1.21 Å

1.63 Å 2.43 Å +

Figure 2. (a) Structure of EC in PCM. Oxidation from EC to +

-

EC removes an electron from the carbonyl O lone pair HOMO of EC, but our calculations predict the carbonate half of EC only loses 0.85 e, demonstrating that the ethylene group donates charge to the electronegative carbonate group. The carbonyl O, carbonate C, and lower ether O dissociate from the EC molecule to form CO2 while the remaining atoms form the acetaldehyde cation. (b) The structure of the TS of Reaction (2). As the reaction approaches the TS, the C-O ether bond dissociates, while at the TS a H transfers to the dissociating CH2. The atoms

-

shown are C (brown), O (red), and H (white).

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EC Dimer Formation

Table 1: Summary of Reaction Barriers and Enthalpies for EC Ring Opening Lewis Acid

Electron Donation to

Ea

Constituent

Lewis Acid from EC (e)

(eV)

(eV)

None

+0.00

1.52

1.29

LiCoO2 Surface

+0.12

1.04

-0.13

PF5

+0.15

0.99

-0.16

+0.17

1.28

-0.26

+0.30

0.96

-1.02

None

+0.00

1.96

0.02

LiCoO2 Surface

+0.12

1.81

-0.50

PF5

+0.15

1.68

-1.38

EC+

+1.00

1.22

-1.61

LiCoO2 Surface with PF6Terminating at Surface CoOH*

CO2 Formation

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rxn

Ring opening of oxidized EC via dissociation

in the solvent phase, although in the gas phase Route 2

of the C-O bond between the carbonate C and ether O

was previously predicted to occur with a barrier of 0.81

atoms (Route 2) was predicted to occur in the gas

eV and a reaction energy that is exothermic by 1.45

phase41. However, in our examination of solvated EC+

eV41.

ring opening at the sp2-like carbonate C through

calculations, which avoid dissociation of the ethylene

Route 2, we were unable to locate a fully-optimized TS

C-O ether bond, identified an intermediate state 2.21

structure with a corresponding single imaginary

eV above the EC+ reactant, 0.99 eV above the TS for

frequency. We instead found a concerted dissociation

degradation via Route 1. Consequently, any TS to

of both the carbonate C-O ether bond and the

form this intermediate would lie at or above this

ethylene C-O ether bond, leaving a free O atom. This

energy and thus, although EC+ degradation was

suggests that a low lying TS for Route 2 does not exist

predicted to occur through Route 2 in the gas phase,

Furthermore,


constrained

optimization

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we calculate that this route is not competitive in

activation barriers, the lower exothermicity of ethylene

solvent and that instead formation of CO2 and

oxide

acetaldehyde cation occurs via Route 1.

concentration of ethylene oxide cation ~1010 lower

formation

results

in

an

equilibrium

Additionally, we explored EC+ ring opening

than that of acetaldehyde at 310 K. However, because

in solvent to form CO2 and the cyclic epoxide ethylene

the reverse barriers are -2.25 and -2.83 eV, both

oxide cation (see Figure 3) rather than an

reverse reactions will be extremely slow during LIB

acetaldehyde ion by:

operation and hence product formation will be under

EC+  CO2 + CH2CH2O+

(a)

kinetic and not thermodynamic control. Thus, more

(3)

ethylene oxide cation will likely be produced than its

(b)

equilibrium

concentration

suggests.

However,

ethylene oxide cation is predicted to isomerize and form the more stable acetaldehyde product with a reaction barrier of 0.92 eV41. Consequently, while

Figure 3. This cyclic (a) ethylene oxide cation is an intermediate of EC ring opening that is moderately stable. However, the epoxide will isomerize to form the (b) acetaldehyde cation, which we expect will be the primary ring opening product.

ethylene oxide is produced by ring opening, it is only a moderately stable intermediate and will isomerize into the acetaldehyde over relatively short time scales relative to LIB lifetimes and acetaldehyde cation will

We predict that degradation via Reaction (3) proceeds

thus be the primary decomposition product. Next, we

through a TS characterized by dissociation of the

examined the effect of a Lewis acid on EC ring

ethylene C-O ether bond with an activation barrier of

opening reactions.

1.22 eV. This reaction is analogous to ring opening by

Lewis Acid Activated EC Ring Opening

Route 2, and proceeds over a TS of the same energy.

Forming Organofluorines: In contrast to the anode,

However, progress along the ring opening reaction

electrolyte degradation is typically not expected at the

coordinate to form the epoxide cation does not involve concerted H

+

LiCoO2 cathode surface due to the electrochemical

transfer. Furthermore, we

stability window of the electrolyte - which lies between

calculate a reaction enthalpy that is exothermic by 1.03

approximately 1 and 5 V vs. Li+/Li0 relative to the bulk

eV, indicating that ethylene oxide cation formation is

chemical potential of LiCoO2, which lies between 3.7

0.58 eV less exothermic than acetaldehyde cation

and 4.7 V vs. Li+/Li0. However, cycled LIBs cathodes

formation. This is a consequence of the considerable

exhibit films on their cathode surfaces comprised of

strain in the three-member epoxide ring of ethylene

electrolyte

oxide. An equilibrium analysis suggests that although

decomposition

products

including

organics, organofluorines, lithium fluoride species, and

the two ring opening reactions have nearly identical 11 ACS Paragon Plus Environment


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other compounds31,36. This indicates that despite the

indicates that this adsorbing EC, referred to as ECads,

electrochemical stability between the EC electrolyte

donates 0.12 e to the LiCoO2 surface, demonstrating

and bulk LiCoO2 cathode material at the electrode

that two neighboring Co* sites oxidize an adsorbed EC

operating potentials, the LiCoO2 surface interacts with

more than a single Co* site. The resulting greater

EC to promote decomposition of the electrolyte that

degree of polarization of the adsorbed EC lowers the

forms the CEI. This section details the formation of

barrier to ring opening relative to EC adsorption at a

surface organofluorines in the CEI via EC ring

single Co* site. We found that the ethylene C-O ether

opening.

bond distance of ECads increases from 1.44 Å to 1.48 Å

We examined activation of EC ring opening

upon adsorption to two neighboring Co* sites,

by a partially hydroxyl-terminated (101̅4) LiCoO2

indicating weakening of this C-O bond. Furthermore,

surface (see Computational Details section). We

the applied bias during charging and overcharging of

found that EC adsorbs to LiCoO2 by forming a dative

LIBs results in greater positive fields at the cathode

bond between its carbonyl O and a single exposed Co*

that create a higher degree of polarization of the dative

surface atom with an adsorption energy of -0.66 eV.

bonds at the cathode surface and thus a greater degree

Bader charge analysis indicates that EC adsorption at a

of oxidation of adsorbed EC molecules than in the

surface Co* site only involves transfer of 0.04 e from

absence of an applied bias. Consequently, during

the adsorbing EC to the cathode surface, suggesting

charging and in an overcharged state we expect the

that the surface acts as a weak Lewis acid that does not

LiCoO2 surface to withdraw more than 0.12 e from

oxidize EC significantly. Furthermore, ring opening is

adsorbed EC, leading to greater distortion of the

not predicted to occur for EC adsorbed at a single

adsorbed EC and further weakening of the ethylene C-

Co* surface site in the absence of an applied bias

O ether bond.

during charging. However, our calculations of the

Electrolytes of commercial LIBs typically

LiCoO2 surface termination predict that the cathode is

contain 1 M LiPF6 and up to 13 M EC23,67,68. Thus,

covered by a distribution of both CoOH* and exposed

while an EC molecule adsorbed to the surface is most

Co* sites44. This motivated us to also examine EC

likely to interact with another EC molecule, we expect

adsorption at two neighboring exposed Co* sites,

that EC molecules adsorbed at the LiCoO2 surface

where the carbonyl O bonds to one of the Co sites and

also interact with PF6- counter ions in the electrolyte.

one of the ether O atoms bonds to a neighboring Co*

Observations of organofluorines on the cathode

site, as shown in Figure 4a.

surface and PF5 formation suggest that PF6- reacts with species

EC adsorption at two neighboring Co* sites is

in

the

electrolyte

and

LiCoO231,36.

Consequently, we have investigated ring opening

exothermic by 1.07 eV. Additionally, a charge analysis


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reactions of an EC molecule adsorbed at two

to form PF5 and an organofluorine species on the

neighboring surface Co* sites and reacting with PF6- via

surface (see Figure 4). The 1.28 eV barrier suggests a

the following reaction:

slow reaction rate at 298 K, requiring several months

Co*(s) + EC + PF6-  Co-OCOOCH2CH2F- + PF5

to develop the organofluorine film, which is consistent with observations of organofluorine film formation in

(4).

stored and aged LiCoO2 cathodes6,31,69.

Adsorbed EC interacting with PF6- (see Figure 4a) transfers 0.17 e to the surface according to Bader

At the TS of Reaction (4), we calculate an H-

charge analysis and exhibits a greater degree of

C-H angle of 118.2°, demonstrating that the

(a)

dissociating C transforms from being an sp3 hybridized

-

ECads

(b)

C to an sp2 hybridized, carbocation-like C with an

PF6

empty p orbital. Examination of the MEP indicates that Reaction (4) is characterized by simultaneous dissociation of a P-F bond of PF6-, where the

(c)

dissociating F- donates electron density to the empty

(d)

C p orbital to form the C-F bond. This leads to the

PF5

higher predicted barrier because while the dative bonds between the adsorbed EC and LiCoO2 surface weaken the C-O bond between the ethylene C and

Figure 4. Ring opening reaction of an adsorbed EC activated by the LiCoO2 surface acting as a Lewis acid. The electrophilic CH2

ether O atoms of ECads, the concerted dissociation of

-

group accepts charge from a dissociating PF6 . Degradation on the

both the P-F and the C-O bonds - rather than

cathode surface to form an organofluorine occurs with a barrier of 1.28 eV and a reaction energy of -0.26 eV. Shown above is: (a) EC

dissociation of just the C-O bond - produces a higher

*

-

adsorbed at neighboring Co sites with a solvating PF6 and (b) the

energy penalty along the reaction entrance channel

TS of EC ring opening, showing simultaneous dissociation of the C-O and P-F bonds and (c) the organofluorine on the cathode surface with PF5 and (d) a schematic of the TS, characterized by

and thus a higher reaction barrier than expected based on the degree of oxidation of the reacting EC. PF5

-

dissociation of F from PF6 , which donates charge from its lone

formation via Reaction (1) the decomposition of the

2

pairs to the sp , carbocation-like C atom. The atoms shown are Co (blue), O (red), Li (green), C (brown), H (white), F (light blue), and P (orange).

LiPF6 salt

polarization relative to ECads interacting with a

conditions with calculated activation energies of over

solvating EC.

Our calculations predict that ring

1 eV, negligible barriers to reverse reaction, and overall

opening via Reaction (4) occurs with an activation

endothermic reaction energies43. Based on the relative

barrier of 1.28 eV and a reaction enthalpy of -0.26 eV

barriers and reaction energies of Reactions (1) and

is predicted and observed to occur over

relatively long time scales at typical LIB operating



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(4), we predict that Reaction (4) is a competing

reduces the power density of LIBs. Thus, in addition

pathway for PF5 formation in LIBs.

to examining the formation of organofluorines, we

The ring opening reaction between EC and

have examined EC ring opening to form organic

PF6- at the LiCoO2 surface not only degrades the

oligomers. PF5 formed via Reaction (4) acts a Lewis

electrolyte solvent and cathode surface, but also the

acid72, and consequently, we expect PF5 to form a

counter ion of the electrolyte salt additive to form PF5.

Lewis acid-base complex with the highly polar EC

Generation of PF5 by either Reaction (1) or (4) in

molecules of the electrolyte in addition to reacting

the electrolyte is of particular concern because PF5

with H2O to form HF and POF3. We examined EC

reacts with H2O present in the electrolyte as a

ring opening reactions in the electrolyte activated by

contaminant at concentrations of ~20 ppm to generate

the PF5 Lewis acid. Additionally, the EC undergoing

HF and POF3 by the following reaction:

ring opening is solvated by a second explicit EC in



order to accurately model effects from neighboring

HF is expected to attack the LiCoO2 cathode,

solvent

molecules.

Thus,

initiating a deleterious cycle of capacity fading in

degradation occurs via:

PF5

activated

EC

PF5 + 2 EC  PF5-OCOOCH2CH2-OC(OCH2)2

LIBs18,48,49,67,68,70 where additional H2O forms as a result

(6).

of HF reacting with the cathode, which in turn reacts with PF5 to produce another two HF molecules, thus

PF5 and EC interact through Lewis acid-base

creating a cycle of cathode degradation18,49,71,72.

interactions where PF5 accepts electron density from the O lone pairs of the EC carbonyl group to form a

One advantage of organic liquid electrolytes over polymeric and solid-state electrolytes is that their

Lewis

acid-base

complex

with

a

calculated

higher Li+ conductivities enable rapid Li+ transport

complexation energy of -0.67 eV. An NBO population

between the electrodes, and thus, higher power

analysis estimates that EC donates 0.15 e to form the

densities in LIBs.

Organofluorine film formation,

PF5EC complex. Thus, while this complexation

however, is expected to increase electrolyte viscosity

energy is 0.40 eV less exothermic than the adsorption

and hinder Li+ migration5 7, reducing the effective

energy of EC on the LiCoO2 surface, PF5 more

power density of the battery and degrading

effectively polarizes EC than the Co* sites of the

performance.

cathode surface in the absence of an applied bias.


We predict that the PF5EC complex

Forming Organic Dimers: Oligomerization of the

undergoes a ring opening reaction to form an EC

liquid electrolyte has deleterious effects on battery

dimer (Figure 5) that is exothermic by 0.16 eV relative

operation; it hinders Li+ transport, and consequently

to the isolated reactants and with an activation barrier 14


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of 0.99 eV, suggesting both a sluggish rate and a small

Ring-opening of EC at the LiCoO2 surface could

concentration of EC dimers in the electrolyte. In

develop the CEI through the reaction: Co*(s) + 2 EC  Co-OCOOCH2CH2-

addition to the PCM implicit model of the electrolyte, we employed an explicit solvating EC in our model for

OC(CH2O)2

(7).

ring opening reactions in the electrolyte. The greater

We calculate that ring opening of ECads adsorbed at

degree of oxidation of EC by PF5 compared to the

two neighboring Co* sites occurs through a barrier of

LiCoO2 surface alone (vide infra) results in both a

1.04 eV and a reaction enthalpy of -0.13 eV relative to

lowering of the activation barrier and a more

the isolated reactants. Similar to ring opening

exothermic reaction enthalpy.

activated by PF5, this is a moderate activation barrier

(a)

ECsolv

at the operating temperatures for LIBs and suggests

(b)

that electrolyte degradation via ring opening at the

PF5 • EC

LiCoO2 surface will be sluggish in the absence of the positive applied bias used to charge LIBs. Similar to

(c)

the previous cases, donation of electron density from a

(d)

carbonyl O lone pair of ECsolv to the empty p orbital of the dissociating C atom reduces the partial positive charge on this carbocation-like C atom, as shown in

Figure 5. Ring opening reaction of EC activated by the PF 5 Lewis

Figure 6, which becomes a bond between the ring-

acid. Degradation of the electrolyte to form an EC dimer occurs with a barrier of 0.99 eV and a reaction energy of -0.16 eV. Shown above is: (a) the PF5 -base complex and (b) the TS

opened EC product and the solvating EC to form a

of EC ring opening, showing dissociation of the C-O and charge donation from the solvating EC and (c) the EC dimer product and (d) a schematic of the TS, characterized by dissociation of the

formation

dimer.

We suggest that this prediction of dimer describes

the

nucleation

of

EC

oligomerization that forms the CEI organic films

2

C-O bond and charge donation from lone pairs of EC to the sp , carbocation-like C atom. The atoms shown are O (red), C (brown), H (white), F (light blue), and P (orange).

observed on LiCoO2 cathodes. However, we

We also examined the ring opening reaction

a positive bias is applied across this interface of LIBs

of an EC adsorbed to and activated by the partially

during charging; we expect that the field at the

hydroxyl-terminated (10 1̅ 4) LiCoO2 surface44.

cathode surface will further polarize the dative bonds

Additionally, we added a second, solvating EC to the

between adsorbed EC and the cathode to increase the

model surface that provides electron density to the

degree of EC oxidation, consequently altering the

partially oxidized ethylene moiety of the adsorbed EC.

energetics of EC ring opening and promoting

calculated this reaction under no applied bias, whereas

formation of an organic film and the CEI.



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discussed in more detail in Section 2 of the SI. Bader charge analysis of the dimer formed by Reaction (7) shows that the carbonyl O of ECsolv

Beyond examining propagation of the EC

donates 0.3 e to the ring opened ECads to form a partial

oligomer by addition of EC to the growing oligomer,

C-O bond, as shown in Figure 6.

This partial

we investigated termination of the oligomerization by

oxidation of ECsolv suggests that it is also susceptible to

bonding its free end to the cathode surface, as shown

ring opening decomposition. Continued ring opening

in Figure 7. In this case, the resulting oligomer is

reactions into the bulk electrolyte occur with a

tethered to the cathode at both ends to produce an

calculated barrier of 0.84 eV but are endothermic by

organic film. Because this reaction occurs at the

0.80 eV, suggesting that addition of ring-opened

cathode surface, rather than continuing into the

ethylene monomers is quickly reversed by a nearly

electrolyte, the carbocation-like dissociating -CH2

barrierless back reaction and thus oligomerization into

group accepts electron density from the lone pairs of

the electrolyte is unlikely to occur. These results are

an O atom at a nearby surface CoOH* site, rather than from a solvating EC molecule. We calculate that ring

(a)

ECsolv

(b)

opening that terminates at the LiCoO2 surface occurs

1.90 Å

with a barrier of 0.96 eV and is exothermic by 1.02 eV

ECads

relative to the reactant shown in Figure 7. While this reaction is exothermic, tethering both ends of the oligomer to the cathode does involve an entropic

(c)

1.54 Å

penalty, but this should not significantly affect the

(d)

overall prediction of film formation. As the reaction proceeds from the TS to the product state, the proton from the reacting CoOH* group transfers to a neighboring CoOH* and forms an adsorbed H2O and Figure 6. EC ring opening reaction activated by neighboring

a C-O-Co group that anchors the dimer to the cathode

exposed Co sites on the (101̅4) LiCoO2 surface acting as a Lewis *

surface, as shown in Figure 7. We found that the C-O-

*

acid to form an EC dimer. Reaction at exposed Co sites occurs with a barrier of 1.04 eV and a reaction enthalpy of -0.13 eV.

Co group possesses a highly electronegative CoO*

*

Shown above is: (a) EC adsorbed at neighboring Co sites with a solvating EC and (b) the TS of dimer formation, showing dissociation of the C-O bond and (c) the EC dimer on the cathode surface and (d) a schematic of the TS, characterized by

terminating site, where 0.12 e of additional electron density localizes on the O* compared to other terminating O atoms, which further drives ring

2

donation of the solvating EC lone pairs to the sp , carbocation-like C atom. The atoms shown are Co (blue), O (red), Li (green), C (brown), and H (white).

opening

by


nucleophilic

attack.

The

greater

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exothermicity of the termination reaction relative to

Finally, we explored unactivated EC oligomer

the initial ring opening results not only from greater

formation via ring opening in the absence of a Lewis

polarization of the reactant EC, but also the formation

acid to determine the effect of EC Lewis acid-base

H2O and an additional C-O bond. Formation of H2O

activation. Without a Lewis acid present we calculate

at the cathode surface is undesirable because H2O

that the ring opening and dimer formation reaction

readily dissolves into the polar organic electrolyte and

proceeds with an activation barrier of 1.52 eV and is

reacts with PF5 to form HF, leading to the cycle of

endothermic by 1.29 eV.

degradation described above18,48,49,67,68,70.

organic and organofluorine oligomerization does not

Coupled with the initial ring opening via

occur without activation of EC by a strong Lewis acid.

Reaction (7), we calculate that the formation of short

(a)

chain organic films on the cathode surface will be

(b)

ECsolv ECads

exothermic by 1.15 eV overall, but will be limited by the initial ring opening barrier of 1.04 eV.

Thus, we predict that

The

moderate barrier to dimer formation on the surface suggests that this mechanism is active under an applied positive bias during charging

(c)

(d)

which at the

cathode further oxidizes the adsorbed EC

or at

elevated temperatures. However, we predict that at least half of the surface Co* sites of the LiCoO2 surface Figure 7. EC dimer formation on the LiCoO2 surface via ring

are terminated by OH groups and that OH

opening of EC, which has been activated by the complexing with CH2 group of the previously degraded EC. The CH2 group acts

termination persists under prolonged heating and ultrahigh vacuum conditions64.

as a Lewis acid by accepting charge from the carbonyl O lone pair

Based on this

*

of EC. A surface CoOH group donates charge to the dissociating ethylene group of the reacting EC along the reaction coordinate, stabilizing the dimer product. Dimerization terminating on the cathode surface occurs with a barrier of 0.96 eV and is exothermic by 1.02 eV. Shown above is: (a) a second EC dative bonded to the previously reacted EC and (b) the TS of EC dimer formation, showing dissociation of the C-O bond and (c) the EC dimer and H2O on the cathode surface and (d) a schematic of the TS,

fractional coverage, less than a quarter of the surface Co atoms will have neighboring exposed Co* sites to activate ring opening, although the degree of surface hydroxylation depends on the fabrication method of the cathode particles, which can be used to promote or

+

characterized by transfer of H to form H2O and donation of the

inhibit CEI formation by this mechanism. Thus, the

*

2

lone pairs from the CoOH group to the sp , carbocation-like C atom. The atoms shown are Co (blue), O (red), Li (green), C (brown), and H (white).

local surface composition reduces the rate of organic film growth by limiting the number of nucleation sites.



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Page 18 of 27


Our results predict that CO2 formation

Reactions Forming CO2 and Acetaldehyde: CO2

activated by PF5 involves an activation barrier of 1.68

formation resulting from electrolyte degradation is

eV and is exothermic by 0.72 eV, referenced to the

observed during LIB cycling6,9 which increases the

reactant complex, and thus 1.38 eV relative to the

internal pressure of the battery and can lead to

separated, solvated reactants. Compared to the

catastrophic failure.

Here, we investigated CO2

limiting case of fully oxidized EC+ decomposition

formation activated by PF5, the LiCoO2 cathode

described above, we calculate both a higher barrier and

surface, and neighboring EC solvent molecules acting

a less exothermic reaction enthalpy, due to the smaller

as Lewis acids.

extent of EC oxidation. Moreover, although CO2

PF5 acts as a Lewis acid72 and thus, in addition

formation is thermodynamically favored, we expect

to reacting with H2O to form HF and POF3, we expect

little CO2 formation by PF5 activation through the

PF5 to form a Lewis acid-base complex with the highly

PF5EC complex at typical LIB operating temperatures

polar EC molecules of the bulk electrolyte. We

due to the slow kinetics resulting from the high barrier.

examined EC ring-opening reactions in the electrolyte

Thus, we expect that CO2 and acetaldehyde formation

to form CO2 and acetaldehyde activated by the PF5

in the bulk electrolyte does not occur without

Lewis acid and solvated by a second EC via the

activation by a stronger Lewis acid than PF5.

reaction:

We also examined ring opening of an EC

ECPF5  CO2 + CH3CHO + PF5

adsorbed to and activated by the partially hydroxyl-

(8).

(a)

We calculate that EC complexation with PF5 is

ECsolv

PF5 • EC

exothermic by -0.67 eV, as shown in Figure 8. An

(b)

NBO population analysis suggests that PF5EC

(c)

complexation involves transfer of 0.15 e from EC to PF5, similar to the EC ring opening reaction activated by PF5 described above. We find that the TS is characterized by H+ transfer to the dissociating C from the neighboring ethylene C (see Figure 8). During the

Figure 8. CO2 formation via ring opening reaction of EC

proton transfer, hydrogen bonding between the

activated by the PF5 Lewis acid. EC degradation to form CO2 and

proton and the O lone pairs of the solvating EC

acetaldehyde occurs with a barrier of 1.68 eV and a reaction energy of -1.38 eV. Shown above is: (a) the PF5 -

stabilizes the TS for shuttling the H+ between the two

+

base complex and (b) the TS of EC ring opening, showing H transfer, stabilized by charge donation from the solvating EC and (c) the CO2 and acetaldehyde products. The atoms shown are O

C atoms.

(red), C (brown), H (white), F (light blue), and P (orange).


Page 19 of 27

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terminated LiCoO2 surface and solvated by a second

complex to activate EC, we calculate that the CO2 and

EC to form CO2 - similar to the case of dimer

acetaldehyde formation reaction proceeds with an

formation activated by the cathode surface described

activation barrier of 1.96 eV and is endothermic by

above. We calculate an activation barrier of 1.81 eV

0.02 eV. Thus, these results predict that CO2

and an exothermic reaction enthalpy of -0.50 eV

formation from EC degradation will not occur without

relative to solvated EC in the electrolyte. We predict

activation by a strong Lewis acid.

an analogous reaction mechanism to the case of CO2

For both CO2 and dimer formation, we

formation activated by the LiCoO2 surface (see SI

predict that the kinetics and thermodynamics of EC

Figure 1 for details of this reaction), where ring

degradation by ring opening reactions at the CEI and

opening involves H+ transfer and dissociation of a C-O

in the electrolyte are determined by the degree to

ether bond, but similar to the previous case, our results

which Lewis acids oxidize the reacting EC, polarizing

predict that CO2 formation will not be kinetically

and activating the EC for ring-opening. Our results

active. Consequently, we do not expect CO2 formation

show that an increase in the oxidation of the EC Lewis

as a result of EC degradation to occur at the cathode

base generally leads to a lowering of the activation

surface, except possibly over long time scales at

barrier and an increase in the exothermicity of the

elevated temperatures or under conditions of an

reaction.

applied bias, which is consistent with experimental observations

of

LIBs

operating

at

high

charge/discharge rates38. Finally, we examined CO2 and acetaldehyde

-

formation from an EC molecule without a Lewis acid present to determine the effect of EC Lewis acid-base interactions. In this case, we examined EC undergoing ring opening with two solvating EC molecules; one solvating EC provides electron density to the transferring H+ while the second EC interacts with the carbonate group of the EC undergoing ring opening. This is a likely configuration because EC in the

-

electrolyte interacts with other bulk electrolyte molecules more often than with the cathode surface or PF5. However, without formation of a Lewis acid-base



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Page 20 of 27

-

-

-

-

Supporting Information. Cathode Surface Model, Ethylene Carbonate Oligomerization, CO2 Formation at the Cathode Surface, and Atomic Coordinates

-

-

-


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-



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Degradation of Ethylene Carbonate Electrolytes of Lithium Ion

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