Abstract
Li-O2 batteries offer a high theoretical discharge capacity due to the formation of light discharged species such as Li2O2, which fill the porous positive electrode. However, in practice, it is challenging to reach the theoretical capacity and completely utilize the full electrode pore volume during discharge. With the formation of discharge products, the porous medium evolves, and the porosity and tortuosity factor of the positive electrode are altered through shrinkage and clogging of pores. A pore shrinks as solid discharge products accumulate, the pore clogging when it is filled (or when access is blocked). In this study, we investigate the structural evolution of the positive electrode through a combination of experimental and computational techniques. Pulsed field gradient nuclear magnetic resonance results show that the electrode tortuosity factor changes much faster than suggested by the Bruggeman relation (an equation that empirically links the tortuosity factor to the porosity) and that the electrolyte solvent affects the tortuosity factor evolution. The latter is ascribed to the different abilities of solvents to dissolve reaction intermediates, which leads to different discharge product particle sizes: on discharging using 0.5 M LiTFSI in dimethoxyethane, the tortuosity factor increases much faster than for discharging in 0.5 M LiTFSI in tetraglyme. The correlation between a discharge product size and tortuosity factor is studied using a pore network model, which shows that larger discharge products generate more pore clogging. The Knudsen diffusion effect, where collisions of diffusing molecules with pore walls reduce the effective diffusion coefficients, is investigated using a kinetic Monte Carlo model and is found to have an insignificant impact on the effective diffusion coefficient for molecules in pores with diameters above 5 nm, i.e., most of the pores present in the materials investigated here. As a consequence, pore clogging is thought to be the main origin of tortuosity factor evolution.
Highlights
Lithium oxygen (Li-O2) batteries are potentially gamechanging energy storage systems with a very high theoretical capacity,[1] but they face a wide spectrum of very significant material challenges that need to be overcome before achieving commercialization.[2]
To help interpret the experimental observations, we investigate the effect of pore sizes, and pore clogging, on the tortuosity factor, using a kinetic Monte Carlo model and pore network model (PNM), respectively, as described in previous publications.[39,40]
To measure the diffusion of tetraglyme confined in the porous structure of a Super P electrode, it is important that all the solvent molecules are inside the porous structure and that there are no molecules on the surface of the electrode since the latter will have larger effective diffusion coefficients than the confined ones and will affect the average measured diffusion coefficient
Summary
Lithium oxygen (Li-O2) batteries are potentially gamechanging energy storage systems with a very high theoretical capacity,[1] but they face a wide spectrum of very significant material challenges that need to be overcome before achieving commercialization.[2] A Li-O2 battery consists of a lithiumcontaining anode (typically Li metal), a separator soaked with an ionically conducting but electronically insulating electrolytecontaining lithium ions (Li+), and a porous electronically conducting cathode (typically a porous carbon) in contact with oxygen gas. Lithium is oxidized at the anode and lithium ions migrate to the cathode where they react with oxygen (dissolved in the electrolyte) to form solid discharge products, predominantly Li2O2 (lithium peroxide). Li2O2 discharge products decompose and form Li+ and O2. The lithium ions migrate back to the anode where they are reduced. Oxygen molecules are released back to the oxygen gas esoleucrtcreo.niTc hinesuplaretodro.3mDinuanritngdidsicshcahragrege,pLroi+durceta,ctLs i2wOit2h, is O2 an to form LiO2 (lithium superoxide) close to the cathode surface
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