www.sciencemag.org/content/350/6260/530/suppl/DC1 Supplementary Materials for Cycling Li-O 2 batteries via LiOH formation and decomposition Tao Liu, Michal Leskes, Wanjing Yu, Amy J. Moore, Lina Zhou, Paul M. Bayley, Gunwoo Kim, Clare P. Grey* *Corresponding author. E-mail: [email protected]Published 30 October 2015, Science 350, 530 (2015) DOI: 10.1126/science.aac7730 This PDF file includes: Materials and Methods Supplementary Text Figs. S1 to S23 References (43–47)
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www.sciencemag.org/content/350/6260/530/suppl/DC1
Supplementary Materials for
Cycling Li-O2 batteries via LiOH formation and decomposition Tao Liu, Michal Leskes, Wanjing Yu, Amy J. Moore, Lina Zhou, Paul M. Bayley,
measurements on this sample show that a crystalline lithium hydroxide phase together with a
significant amount of other discharge (decomposition) products is present in the electrode (Fig.
S15). This higher level of decomposition products is ascribed to lower purity level (99% CP) of
this deuterated DME solvent (the deuterated solvent being difficult to distil due to the low
quantity purchased (due to the cost)). The 2H ssNMR spectrum (Fig. S16) of this sample
confirms the presence of LiOD in the electrode, which is more visible in the characteristic
26
spinning side band manifolds, rather than in the central band. However, LiOD is by no means the
dominant 2H signal in the discharged electrode produced in the presence of deuterated DME.
Fig. S15 1H and
7Li ssNMR and XRD of a Li-O2 cell discharged using an rGO electrode and a 0.05 M
LiI/0.25 M LiTFSI/DME electrolyte, prepared from as-received deuterated DME (D-10, 99.5 D%, 99%
CP). The NMR spectra of the discharged electrode and the reference compound are acquired at 11.7 T,
under (a, b) MAS speed of 60 kHz, and (c) static conditions. The 1H MAS spectrum shows a resonance at
-1.4 ppm that is assigned to LiOH indicating that it is present in the discharge products. The resonance at
8 ppm is attributed to lithium formate. The resonances at 1.4 and 3.9 ppm are due to saturated
hydrocarbons (1.4 ppm) and ethers (3.9 ppm). The XRD pattern (d) confirms that the major crystalline
phase in the discharged electrode is LiOH; the thin LiOH sheets observed by SEM (i.e., a short coherent
length along <001> direction) likely give rise to a weak/broad (001) reflection which is obscured by the
broad background from the Kapton film from the sample holder. The electrochemistry is given in Fig.
S22 (a, cell 1).
27
Fig. S16 2H ssNMR spectra of the rGO electrode from the same cell discussed in figure S15. The spectra
were acquired at 11.7 T, with a MAS speed of 60 kHz; magnified spectra of the isotropic and 1st spinning
sideband are shown for clarity. The major discharge product observed in the 2H spectrum is not LiOD at
-1.5 ppm but instead gives rise to another resonance at 3.5 ppm due to ethers.
To investigate whether water can serve as an alternative hydrogen source, we added D2O to the
0.05 LiI/0.25 M LiTFSI/non-deuterated DME electrolyte and discharged a Li-O2 cell using an
rGO electrode. The 7Li (b-c) and
2H (d) spectra all show that deuterated lithium hydroxide is the
dominant discharge product (Fig. S17). The observation that LiOD is now the only signal visible
in the intense 2H spectrum (d) while only a weak minor
1H resonance at -1.5 ppm due to LiOH is
seen in the 1H spectrum (a) of the same sample confirms that LiOD is formed from the deuterons
of D2O in the electrolyte; this result thus clearly shows that water is the major source of
hydrogen in the reaction to form LiOH.
28
Fig. S17. ssNMR spectra of an rGO electrode extracted from a Li-O2 cell, prepared using a 0.05 LiI/0.25
M LiTFSI/non-deuterated DME electrolyte but with added D2O (20,000 ppm, 15 mg). The ssNMR
spectra of LiOD reference compound is also shown for comparison: 1H (a),
7Li under MAS (b) and static
conditions (c), and 2H NMR spectra with a zoom (right) to show the isotopic region for clarity (d). The
resonances at 1.4 and 3.9 ppm in (a) are due to saturated hydrocarbons (1.4 ppm) and ethers (3.9
ppm).The electrochemistry data of the cell is given in Fig. S22(a, cell 2).
The observation that H2O supplies the H to the formed LiOH suggests that a significant amount
of H2O was introduced into the cell when deuterated DME was used (as discussed in figures S15
and 16). Experiments performed with deuterated DME dried by molecular sieves overnight still
showed that LiOH was the dominant phase and no trace of LiOD was observed (1H,
7Li and
2H
ssNMR measurements). H2O is still introduced from other sources. The capacity of the cell was
1.1 mAh. Assuming an electron/LiOH molar ratio of 1 and that all H in H2O goes to form LiOH
(see below), the amount of H2O needed was 0.37 mg, which corresponds to ~500 ppm of H2O in
the electrolyte.
29
One possible source of H2O comes from the O2 purge of our cells. To test this, 1 ml of nominally
dry DME (<5 ppm H2O as determined in a Karl Fischer test) was sealed in a Li-O2 cell chamber
and purged for 5 minutes using the current O2 line. The water content of the resulting DME was
measured as 47 ppm showing that one source of H2O comes from water in the lines. Further
water likely arises from water sorbed on other cell components such as the separator.
Fig. S18 1H (a),
7Li (b) and
2H (c) ssNMR spectra of a Li-O2 cell prepared using an rGO electrode and
0.05 M LiI/0.25 M LiTFSI/deuterated DME (dried by molecular sieves overnight). ssNMR spectra of
LiOD reference compound are shown for comparison. An intense 1H resonance at -1.5 ppm (a) and a
single 7Li resonance at 1 ppm (b) suggest LiOH is the dominant phase in the discharge product. No
signature due to LiOD was observed in the 2H spectra (c) of this discharge electrode. The electrochemical
data of the cell is given in Fig. S22 (a, cell 3).
In summary, although the DME solvent in the electrolyte is a potential H source for the formed
LiOH during discharge, it does not seem to be the dominant one. Water is shown to serve as an
30
alternative H source for LiOH in the current LiI mediated system. When water is intentionally
added to the DME-based electrolyte, it appears to preferentially supply H to form LiOH,
minimizing DME decomposition.
No appreciable amount of Li2CO3 or Li acetate was observed in our discharged or charged
electrodes. Only very little Li formate was observed in the 1H spectra of the discharged
electrodes. However, when Li2O2 forms as the dominant discharge product, a significant amount
of Li formate is present (13); this difference is probably related to the intrinsic reactivity of the
discharge product Li2O2 with a DME-based electrolyte, as suggested by previous studies (47).
Compared to Li2O2, the less reactive LiOH crystals appear to cause minimal chemical reactions
with the DME-based electrolyte. The reactivity of the intermediate LiO2, which will cause DME
decomposition has also been substantially reduced, which has been demonstrated by the Li-O2
cells cycled in the presence of additional water, as discussed in figures S17 and 18; this may also
be part of the reason why very little Li2CO3, Li acetate and formate were generated. LiOH does
not seem to react with the rGO electrode at the charge voltages (< 3.4 V) investigated here;
whether the side product/LiOH reacts with graphene to form Li2CO3 at higher voltages is
unclear. However, without the use of the mediator, we found that rGO oxidation is facile at
charge voltages beyond 4 V.
Finally, it should be emphasized that without the presence of LiI, DME electrolyte does not lead
to significant LiOH formation even in the presence of significant amounts of water; instead
Li2O2 is the dominant discharge product instead, large toroidal particles being observed. Of note,
this is consistent with prior work where water is shown to promote the formation of the Li2O2
toroidal particles, water contents of ≥ 500 ppm being required for toroid formation (42). The role
of water in the reaction helps explain why the kinetics (overpotential) of the 1st and 2
nd cycle are
essentially identical, but this may also be because water is not involved in the reaction that sets
the overpotential. When 0.05 M LiI is also added, no trace of Li2O2 is detected, LiOH becoming
the prevailing discharge product. Thus the LiI must play a role in promoting LiOH versus Li2O2
formation.
31
14.2. Electron/LiOH Molar Ratio during Discharge
In the main text of the manuscript we proposed that the first step during discharge is a
one-electron electrochemical process, i.e., Li+ + e
- + O2 → LiO2, because the observed discharge
voltages using rGO electrodes were the same with and without LiI in the electrolyte (Fig. 1A,
main text), i.e., the first step is identical to that observed in the standard Li-air cell (2-3). We also
proposed that the subsequent conversion of LiO2 to LiOH is a chemical process that occurs via a
solution mechanism. It is clear that I- is involved in changing the equilibia so that LiOH is
formed rather than Li2O2, since Li2O2 is still the major product under wet conditions (our work
and work of reference 8).
Formally the reaction can be written:
4Li+ + 4e
- + 4O2 → 4LiO2 Electrochemical, E [1]
4LiO2 + 2H2O → 4LiOH + 3O2 Chemical, C [2]
Where the chemical reaction [2] is mediated via reactions involving I-.
Potential competitive reactions are:
2Li+ + 2e
- + 2O2 → 2LiO2 Electrochemical, E [3]
2LiO2 → Li2O2 + O2 Chemical, C [4]
Li2O2 +H2O → 2LiOH + 1/2O2 Chemical, C [5]
or
2Li+ + 2e
- + 2O2 → 2LiO2 Electrochemical, E [6]
2LiO2 → 2Li2O2 + O2 Chemical, C [7]
We investigated the electron-to-LiOH molar ratio during discharge by quantifying the number of
moles of LiOH in several cells discharged to different extents (with added LiI mediator), and
comparing their NMR spectra with those of a reference LiOH sample to determine the number of
moles of LiOH versus the moles of electrons consumed in the electrochemical discharge (Fig.
S19). On the basis of the 1H LiOH signals derived from electrodes with different capacities, the
LiOH/electron molar ratio is equal to 0.76.
32
The method underestimates the LiOH content since we cannot pack all the LiOH within a
discharged cell into an NMR rotor (for example, rGO fragments are washed away during rinsing,
the rGO pieces stick in holes of the stainless steel mesh, LiOH is formed on the mesh itself and
on the glass fibre separators (as seen in the SEM), and LiOH is lost on rotor packing tools used
to pack NMR rotors etc.). We estimate we could lose as much as 20% of the sample in the
process. Given this, it appears reasonable that a LiOH:electron molar ratio close to 1 is observed
and the mechanism appears to occur via a reaction that consumes 1 Li+ per e
-. While the fit is not
perfect, our data does support a mechanism where each electron results in one LiOH and that
LiOH is the major product. We recognise that the parallel route to form Li2O2 results in a
LiOH/electron molar ratio of less than one (reactions [6] and [7]), but given that we do not detect
this product by XRD and NMR, we believe that the lower ratio originates from an inability to
measure all the LiOH. The current data clearly supports our proposal that LiOH is the major
product.
A major challenge is to identify the mechanism by which reaction [2] occurs. Given that
longevity of the DME electrolyte and the ability to cycle the battery for multiple cycles, we
speculate that I- must play a role in mopping up radicals, minimizing side reactions (noting that
some side reactions still occur, as seen in the 1H NMR above).
33
Fig. S19 A quantification plot of moles of LiOH versus moles of electrons obtained by analysing 1H
NMR spectra as a function of capacity. All 1H NMR spectra are acquired at 11.7 T, with a MAS
frequency of 60 kHz, with a recycle delay of 200 s. To avoid any saturation effects due to the long
spin-lattice relaxation times in this system, a small flip-angle pulse of 15º is used for all NMR
experiments. The number of scans varies between 16 and 176. Spectral deconvolutions are carried out
with DMFIT. The electrochemistry data and fitted spectra are presented in Fig. S22 (b) and Fig. S23.
34
15. Establishing the Charge Mechanism
In this work, we have demonstrated that LiOH can indeed be removed during charge at voltages
below 3.2 V using SEM, XRD and NMR (Fig. 2 in the manuscript). The fact that the cells are
able to cycle well without capacity fading also supports a reversible reaction involving LiOH.
Fig. S20 shows a Li-O2 cell cycled with a cut-off discharge voltage of 2.5 V. The end of
discharge in a Li-O2 battery is typically marked by the electrode surface being fully covered by
insulating discharge products or pores being so heavily clogged that diffusion of the electroactive
species becomes very sluggish (resulting in rapid polarization of the cell voltage). The
electrochemistry of the cell shown, which is discharged to the end, cycles with nearly an
identical electrochemical profile, demonstrating highly reversible electrochemistry (the 1st 10
cycles are shown here). The majority of the LiOH that forms during discharge must be removed
during the following charge, otherwise cell would polarize rapidly due to the blocking of its
pores and surface areas by LiOH. (This statement is supported by NMR spectra acquired at the
top of charge).
Fig. S20 The electrochemical profile of a Li-O2 cell discharged to the end. 2.5 V was set as the cut-off
voltage as side reactions become more severe below this point (see Fig. S2).
35
To verify the reaction mechanism proposed in figure 4 of the main text, it is necessary to detect
whether there is any O2 evolution during charge. Since water minimizes DME decomposition
and has no appreciable effect on the electrochemical profile, we investigated a cell made using
0.05 M LiI/0.25 M LiTFSI/DME electrolyte with a 9800 ppm level of water.
This Li-O2 cell was first discharged and then connected to a mass spectrometer (MS), with the
gas atmosphere in the cell then replaced by a high purity argon gas. We subsequently measured
the cumulative gas signal after charge (Fig. S21(a)) instead of the transient response, the low
sensitivity of our set-up and the large dead-space preventing the transient measurement. As can
be clearly in figure S21(b) and its inset, a peak for both m/z 16 and 32 (black and blue traces)
was observed, confirming that the initial sharp peaks indeed represent O2. Following the sharp
O2 peak, there is another broad peak associated with m/z 32, but no corresponding m/z 16 peak
was seen (inset in Fig. S21(b)); thus this second peak is not due to O2 and is likely associated
with the ionization of DME vapor at the MS head. Due to the presence of water in the electrolyte
and its reaction with lithium metal anode, the water (m/z=18) and hydrogen (m/z=2) signals also
rise as soon as the valves were opened. This result thus clearly shows that O2 is generated in the
cell after charging at 3.2 V.
In summary, electrochemistry, NMR, XRD and SEM results all suggest that LiOH is removed
during charge in the LiI mediated Li-O2 battery. The electrochemical profile during charge and
the observation that O2 is produced during charge by mass spectrometry indicate that the first
step during charge is the oxidation of iodide anions to triiodide anions and the second step is a
redox chemical reaction, where lithium hydroxide is decomposed by triiodide anions generating
water and oxygen.
36
Fig. S21 (a) The electrochemical profile of a Li-O2 battery made using an rGO electrode and 0.05 M
LiI/0.25 M LiTFSI/DME electrolyte with 9800 ppm of H2O content; the cell was discharged at a current
of 10 μA in O2 and charged at 15 μA under an Ar atmosphere. (b) Mass spectrometry measurement on the
gas atmosphere of the cell in (a) after it was charged under Ar. An initial polarization of the charge
voltage was observed for the cell in (a). This is occasionally observed for cells that have been subjected to
a very deep discharge, such as this one. (When a Li-O2 cell undergoes a deep discharge, most of the
electrode surface area is presumably covered by insulating discharge products and pores in the electrode
are clogged as well, making the diffusion of electro-active species very sluggish at the beginning,
resulting in the initial observed polarization). As charge continues, the voltage drops gradually and
eventually levels off, which is likely due to the removal of the discharge products.
37
15. Electrochemistry Data and Fitted NMR Spectra for Figure S15-19
Fig. S22 (a) Electrochemistry corresponding to Li-O2 cells discussed in Fig. S15-16 (cell 1), Fig. S17 (cell
2) and Fig. S18 (cell 3); the initial dip in the voltage profile (cell 2) is likely to be caused by the formation
of solid-electrolyte-interface at the Li metal anode (due to presence of added excess water 20,000 ppm, 15
mg). (b) Electrochemistry of Li-O2 cells corresponding to the samples used to derive LiOH/electron molar
ratio in Fig. S19; the capacities of the 6 lowest capacity electrodes are noticeably lower than our usual
cells, and consistent with this more impurities were detected by NMR. We tentatively ascribe this to
lower concentrations of the graphene oxide solution used (the electrodes having poorer structural integrity)
and impurities in the DME electrolyte. Increasing the GO concentrations removed the problems in
subsequent cells from a new batch (see the 1.99 mAh electrode as an example).
38
Fig. S23. Raw
1H NMR data for figure S19, used to quantify the moles of LiOH versus the moles of
electrons; an experimental spectrum, a total fit, and individual fits are shown as blue solid, red dashed,
and black lines, respectively. The intensity of the LiOH peak at approximately -1.5 ppm is used in the
calculations.
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