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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 -
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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
-
1̅
1̅
(101̅4) 7 ACS Paragon Plus Environment
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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
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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
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-
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,
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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,
PF5EC 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.
Lewis Acid Activated EC Ring Opening
We predict that the PF5EC 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
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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
Lewis Acid Activated EC Ring Opening
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
PF5EC 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
ECPF5 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 PF5EC
(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).
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ACS Applied Materials & Interfaces
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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