Identifying Capacity Limitations in the Li/Oxygen Battery Using Experiments and Modeling

Abstract

The Li/oxygen battery may achieve a high practical specific energy as its theoretical specific energy is 11,400 Wh/kg Li assuming Li 2 O 2 is the product. To help understand the physics of the Li/oxygen battery we present the first physics-based model that incorporates the major thermodynamic, transport, and kinetic processes. We obtain a good match between porous-electrode experiments and simulations by using an empirical fit to the resistance of the discharge products ͑which include carbonates and oxides when using carbonate solvents͒ as a function of thickness that is obtained from flat-electrode experiments. The experiments and model indicate that the discharge products are electronically resistive, limiting their thickness to tens of nanometers and their volume fraction in one of our discharged porous electrodes to a few percent. Flat-electrode experiments, where pore clogging is impossible, show passivation similar to porous-electrode experiments and allow us to conclude that electrical passivation is the dominant capacity-limiting mechanism in our cells. Although in carbonate solvents Li 2 O 2 is not the dominant discharge product, we argue that the implications of this model, ͑i.e., electrical passivation by the discharge products limits the capacity͒ also apply if Li 2 O 2 is the discharge product, as it is an intrinsic electronic insulator.

A reversible long-life lithium–air battery in ambient air

“…However, Li-air batteries represent a distinct reaction mechanism in comparison with Li-ion batteries 35 . The primary discharge product (Li 2 O 2 ) is almost completely insoluble and insulate in aprotic electrolyte.…”

Electrical conductivity in Li2O2 and its role in determining capacity limitations in non-aqueous Li-O2 batteries

“…Since the reaction product Li 2 O 2 is completely insoluble in the nonaqueous electrolyte, it builds up as a solid in the porous C cathode where it is formed. It has previously been argued 3 that the limited cell capacity is a consequence of electrical passivation of the cathode by the discharge products rather than pore a) Author to whom correspondence should be addressed. Electronic mail:…”

“…Using an empirically defined electrical resistance through the deposited film as a function of average discharge product thickness fit to the discharge behavior of cells with the GC electrodes, a continuum model (including thermodynamics, transport, and kinetic processes) also described the capacity limitations of Li-O 2 batteries with porous cathodes. 3 In these early experiments, carbonate based electrolytes were used, so that the reaction products were undoubtedly predominately insoluble carbonates rather than Li 2 O 2 . 3 In this paper, we investigate electrical conductivity through films of Li 2 O 2 both experimentally and theoretically and demonstrate conclusively that electrical conductivity is responsible for the capacity limitation of Li-O 2 batteries.…”

“…3 In these early experiments, carbonate based electrolytes were used, so that the reaction products were undoubtedly predominately insoluble carbonates rather than Li 2 O 2 . 3 In this paper, we investigate electrical conductivity through films of Li 2 O 2 both experimentally and theoretically and demonstrate conclusively that electrical conductivity is responsible for the capacity limitation of Li-O 2 batteries. In the experiments, we use cyclic voltammetry and impedance measurements of a well known redox couple (ferrocene/ferrocenium) to probe electron (or hole) transport through films of Li 2 O 2 grown on flat GC electrodes at various stages of Li-O 2 discharge using dimethoxyethane as the electrolyte solvent (which produces principally Li 2 O 2 on discharge 6 ).…”

Rate-Dependent Morphology of Li2O2 Growth in Li–O2 Batteries

“…Existing models of Li-O 2 batteries are either macroscopic or atomistic. Cell-level models propose pore blocking due to reaction products [34][35][36][37][38] and surface passivation [24,39]. Atomistic models discuss the surface structure of Li 2 O 2 crystals [40][41][42][43][44], the kinetics of the oxygen reduction/evolution in aprotic electrolytes [40,44,45], and the electron conductivity of Li 2 O 2 [24,41,46].…”