The Mechanistic Role of Lithium Salts in Aprotic Li-O2 Batteries
Authors: Aurbach Doron , Sharon Daniel , Hirshberg Daniel , Kwak Won-Jin , Sun Yang-Kook , Frimer Aryeh A , Afri Michal 
Published: February 2020

Lithium oxygen (Li-O2) research generates much interest and many expectations among electrochemists. The high theoretical specific energy, the simplicity of preparation and operation of electrochemical cells, the sizable commercial possibilities, and the ever-increasing number of new publications, has directed many groups to Li-O2 battery research. Over the past decade, significant progress in the study of possible Li-O2 battery systems has also prompted interest from the chemical and automotive industries. Nevertheless, practical Li-O2 batteries are far from realization.  

Many scientific challenges are related to the oxygen reduction reactions (ORR). In aprotic Li-O2 cells the general assumption is that during discharge, oxygen is reduced on the cathode to form insoluble lithium oxide compounds. Experimental evidence support that the major final product is lithium peroxide (Li2O2). Nevertheless, the electrolyte solution and the carbon cathode instability toward reduced oxygen species lead to intrinsic instability and irreversible formation of many side products.   

Electrochemical formation of Li2O2 can proceed in two ways: (i) One electron transfer to molecular oxygen to form LiO2, which continues to react by receiving another electron and Li ion to form Li2O2. (ii) Two electron transfer pathway that skip over the formation of LiO2, and forms directly Li2O2. Besides the electrochemical routes, Li2O2 can be formed chemically by disproportionation of Li-superoxide to form molecular oxygen as the second product. In fact, electrochemically formed LiO2 is unstable, its further reaction to form Li2O2 is fast1. The possibility of isolating lithium superoxide and to assess its lifetime in different aprotic media are still under investigation.

The rout which Li2O2 is being formed can affect the growth mechanism. Li2O2 is insoluble in aprotic solvents, thus during ORR, crystalline Li2O2 is deposit on the cathode surface. When fully covered by ORR products, the continuation of the process depends on electrons crossing the product layer to reach fresh oxygen molecules. Due to poor electronic conductivity of the oxygen reduction product layer, the cathode is deactivated and the ORR is terminated after the deposition of a few layers of Li-peroxide.

The stabilization of superoxide intermediate can change the growth mechanism of the Li2O2 layer. In contrast to lithium peroxide, superoxide radical can be soluble in aprotic solvents that contain lithium cations. During discharge, oxygen is reduce to superoxide through a free cathode site. Then, the charged radical can diffuse in the solution and transfer charge or to disproportionate in all dimensions, leading to a thick deposits of Li2O2. This mechanism can be considered as a top down growth, as superoxide can disproportionate on top of a Li2O2 particle.

Using aprotic solvents with high Guttman donor number such as DMSO, DMF, and DMA in Li-oxygen cells can help to extend the superoxide lifetime. The presence of high DN solvent reduces the electrophilicity (Lewis acidity) of the Li cations and thereby helping to extend the stability of O2
-· before disproportionating to Li2O2. In practice using high donor number solvents in Li-O2 cells is challenging due to their reactivity toward the lithium metal anode and reduced oxygen species.

Recently it was reported that superoxide radicals can be stabilized indirectly by the electrolytes’ counter anions. In highly associated electrolytes the counter anions are coordinated to the solvated lithium cations, these interactions practically reduce the lithium cations Lewis acidity and thereby temporarily preventing them to bind to the reduced oxygen moieties. As a result, soluble species such as superoxide can take part in the growth mechanism of Li-peroxide deposits on the cathode surface. In this manner we can stabilize superoxide moieties, lead to massive Li2O2 deposition by in a top-down mechanism (i.e. high ORR capacity) although using relatively low DN number solvents such as glymes - CH3O(CH2CH2O)nCH3. Glymes are suggested to be relatively stable towered lithium metal, lithiated carbons and reduced oxygen species.