On the incompatibility of lithium-O2 battery technology with CO2

Abstract

When solubilized in a hexacarboxamide cryptand anion receptor, the peroxide dianion reacts rapidly with CO₂ in polar aprotic organic media to produce hydroperoxycarbonate (HOOCO₂^-) and peroxydicarbonate (^-O₂COOCO₂^-). Peroxydicarbonate is subject to thermal fragmentation into two equivalents of the highly reactive carbonate radical anion, which promotes hydrogen atom abstraction reactions responsible for the oxidative degradation of organic solvents. The activation and conversion of the peroxide dianion by CO₂ is general. Exposure of solid lithium peroxide (Li₂O₂) to CO₂ in polar aprotic organic media results in aggressive oxidation. These findings indicate that CO₂ must not be introduced in conditions relevant to typical lithium-O₂ cell configurations, as production of HOOCO₂^- and ^-O₂COOCO₂^- during lithium-O₂ cell cycling will lead to cell degradation via oxidation of organic electrolytes and other vulnerable cell components.

Fig. 1. The reaction scheme of peroxide cryptate 1 with CO₂ and a line drawing of [O₂⊂mBDCA-5t-H₆]^2– and [CO₃⊂mBDCA-5t-H₆]^2–.

Fig. 2. Addition of CO₂ to 1 in the presence of an oxygen-atom acceptor.

Fig. 3. Variable temperature ¹³C NMR (left) and ¹⁷O NMR (right) analysis of the reaction between ¹³CO₂ and 1.

Fig. 4. Possible intermediates during the conversion of 1 and CO₂ to 2 (top) and formation of symmetric and unsymmetric peroxydicarbonate (bottom).

Fig. 5. Experimental (black trace) and simulated (red trace) solid-state ¹⁷O NMR spectra of (a) 1-¹⁷O₂, (b) 2-CO¹⁷O₂, and (c) product resulting from the treatment of solid 1-¹⁷O₂ with CO₂. All solid-state ¹⁷O NMR experiments were performed on a Bruker Avance-600 (14.1 T) spectrometer under static conditions. A Hahn echo sequence was used for recording the static spectra to eliminate the acoustic ringing from the probe. A 4 mm Bruker MAS probe was used without sample spinning. The effective 90° pulse was of a duration of 1.7 μs. High power ¹H decoupling (70 kHz) was applied in all static experiments. A liquid H₂O sample was used for both RF power calibration and ¹⁷O chemical shift referencing (δ = 0 ppm).

Fig. 6. Proposed mechanistic pathways for CO₂-mediated solvent decomposition in lithium–O₂ batteries. OER is an “oxygen evolving reaction”, HAT is a “hydrogen atom transfer” oxidative process involving hydrogen atom abstraction by carbonate radical anion, and OAT is “oxygen atom transfer” to a substrate, S.

Fig. 7. (a) X-Band EPR spectra of: pristine BMPO in DMF (red), BMPO + 1 without adding CO₂ in DMF (purple), and exposure of BMPO + 1 to CO₂ in DMF (black). (b) Simulation of the EPR spectra of BMPO + 1 + CO₂ in DMF by linear combination of the contribution from: [BMPO–OCO₂]˙^– (green), [BMPO–OH]˙ (red), and [BMPO–O]˙ (yellow). (c) Formation of [BMPO–O]˙ from [BMPO–OH]˙ and an oxidant “[O]”.

Fig. 8. CO₂-mediated oxidation of organic solvents by Li₂O₂. Addition of excess CO₂ to solid Li₂O₂ in the organic solvent 1,2-dimethoxyethane (DME) generates an oxidizing equivalent “O”, which converts to O₂ (74%) and methyl methoxyacetate (15%) with the remainder unidentified. A similar reaction performed in dimethylsulfoxide (DMSO) generated dimethylsulfone (DMSO₂) in a 90% yield.