Study of the Electronic Structure of Alkali Peroxides and Their Role in the Chemistry of Metal–Oxygen Batteries

We use a multiconfigurational and correlated ab initio method to investigate the fundamental electronic properties of the peroxide MO2– (M = Li and Na) trimer to provide new insights into the rather complex chemistry of aprotic metal–O2 batteries. These electrochemical systems are largely based on the electronic properties of superoxide and peroxide of alkali metals. The two compounds differ by stoichiometry: the superoxide is characterized by a M+O2– formula, while the peroxide is characterized by [M+]2O22–. We show here that both the peroxide and superoxide states necessarily coexist in the MO2– trimer and that they correspond to their different electronic states. The energetic prevalence of either one or the other and the range of their coexistence over a subset of the MO2– nuclear configurations is calculated and described via a high-level multiconfigurational approach.


S2 Representative LiO 2 -wavefunctions
The optimized CASSCF wavefunctions of the LiO 2 -lowest electronic states (at selected geometries) are represented in terms of the CSFs active orbitals occupancy and their relative weights:  A 1 (b=1.60; h=1.50) Leading configuration:

S4 Higher spin multiplicities
For both the A 2 and B 1 singlet superoxide states, a corresponding triplet state is found to be very close in energy with respect to the open-shell singlet. Throughout the explored PES, the singlet-triplet splitting turned out to be within the range of 0.1 eV. In Figure S1 we report the energies of the singlet and triplet A 2 and B 1 PESs at the value b=1.30 Å. Since the reactants of (R4) can be an overall singlet or triplet, the same spin multiplicities can emerge in the products. This can be obtained either by combining a singlet O 2 with both singlet and triplet , or triplet O 2 with a singlet . Hence, the triplet product only MO enters in the reactive pathway towards singlet oxygen, which is energetically disfavored by ca. 0.9 eV with respect to triplet oxygen.

S5
Test on the size of the active space The choice of the active space is rather arbitrary. It should include all possible orbitals that give rise to significant occupations in the electronic states of interest for the chemical problem at hand. For our purposes, the choice of 7 active valence orbitals fulfils this obligation. However, the 2p orbitals of Li are close in energy to the 2s one and their inclusion is worth to be explored.
We have repeated some of the calculations using a larger [10,10] active space that includes the 2p orbitals of Li, to check whether their occupation gives rise to other electronic states which might be relevant for the reactions of superoxides. In Figure S3 we report three cuts of the resulting six lowest potential energy surfaces, at the same b values as in Figure 3 of the main text (b = 1.40 Å, 1.50 Å, 1.65 Å).
When we include the 2p orbitals, three new electronic states appear at low energies, one for each of the a 1 , b 1 and a 2 irreducible representation. They all correspond to superoxide states, with singly occupied p y , p x and p z lithium orbitals. We will design them as 2A 1 , 2B 1 and 2A 2 and we have indicated them using dot-dashed lines in Figure S3.
The ground superoxide state 1A 2 due to 2s occupancy (cyan, full line), is more than 0.5 eV lower in energy than the 2A 2 state due to the 2p occupancy (cyan, dashed line) which is characterized by a PES that appears to be simply shifted upwards in energy with respect to the former. The same happens to the 2A 1 state (violet, dashed line) that lies at higher energies, but has a PES that is almost parallel to the 1A 1 one.
Interestingly, the two B 1 states arising from either the 2s or 2p occupancy (green) are close to each other, and they clearly interact in the range of h = 1.45  1.65. However, the B 1 states represent electronically excited superoxide which we have included for sake of completeness but should play a minor role in the reactive behavior of (su)peroxides.

S8
Overall, the chemistry of the low-lying electronic configurations are mainly dominated by the Li 2s orbital, in agreement with the conclusions of ref. [27]. The little relevance of Li 2p orbitals in the active space of neutral LiO 2 was also noticed in ref. [20].
As the low-lying superoxide state 1A 2 , seems unaffected by an expansion of the active space, the discussion in the main text on the nature of the ground state of when varying the b LiO -2 and h parameters remains substantially unaltered and it is possible to limit the discussion to the electronic states arising from the 2s occupancy. scale is in eV.