Non-aqueous Metal–Oxygen Batteries: Past, Present, and Future

Abstract

A metal-oxygen battery (sometimes referred to as a 'metal-air' battery) is a cell chemistry in which one of the reactants is gaseous oxygen, O 2 . Oxygen enters the cell typically in the positive electrode-perhaps after being separated from an inflow of air-and dissolves in the electrolyte. The negative electrode is typically a metal monolith or foil. Upon discharge, metal cations present in the electrolyte react with dissolved oxygen and electrons from the electrode to form a metal-oxide or metal-hydroxide discharge product. In some chemistries the discharge product remains dissolved in the electrolyte; in other systems it precipitates out of solution, forming a solid phase that grows in size as discharge proceeds. In secondary metal-oxygen batteries the recharge process proceeds via the decomposition of the discharge phase back to O 2 and dissolved metal cations. In light of the processes associated with discharge and charging, reversible metal-oxygen batteries with solid discharge products are often referred to as precipitation-dissolution systems, a category that also includes lithium-sulfur batteries.The interest in metal-oxygen chemistries follows from their very high theoretical energy densities. Figure 1 summarizes the gravimetric and volumetric energy densities for several metal-oxygen couples, and compares these to the theoretical energy density of a conventional lithium-ion battery. On the basis of these energy densities, it is clear that many metal-oxygen systems hold promise for surpassing the state-of-the-art Li-ion system. Achieving this goal, however, remains a significant challenge when factors beyond energy density are accounted for: cycle life, round-trip efficiency, and cost

Intrinsic Conductivity in Magnesium–Oxygen Battery Discharge Products: MgO and MgO2

“…For example, a rechargeable battery based on a multi-valent Mg/O2 couple that discharges to magnesium oxide has a theoretical energy density that is nearly seven times (3.9 kWh/kg) that of conventional Li-ion batteries (0.57 kWh/kg), and even surpasses that of a "Li-air" cell (3.5 kWh/kg, assuming discharge to Li2O2). [1][2][3][4][5] Additional advantages of magnesium-based systems compared to Li analogues include an anode with higher volumetric capacity (3832 mAh cm -3 Mg vs. 2062 mAh cm -3 Li), suppressed dendrite formation, and lower cost. 6,7 Due to the nearly identical formation energies of magnesium oxide, ΔGf 0 (MgO) = -568.9 kJ/mol, 8 and magnesium peroxide, ΔGf 0 (MgO2) = -567.8 kJ/mol, 9 both compounds may be expected to participate in the cycling of non-aqueous Mg/O2 cells.…”

“…The demand for energy-dense batteries suitable for electric vehicle propulsion has sparked interest in metal–oxygen electrochemistry. For example, a rechargeable battery based on a multivalent Mg/O 2 couple that discharges to magnesium oxide has a theoretical energy density that is nearly seven times (3.9 kW h/kg) that of conventional Li-ion batteries (0.57 kW h/kg) and even surpasses that of a “Li–air” cell (3.5 kW h/kg, assuming discharge to Li 2 O 2 ). − Additional advantages of magnesium-based systems compared to Li analogues include an anode with higher volumetric capacity (3832 mA h cm –3 Mg vs 2062 mA h cm –3 Li), suppressed dendrite formation, and lower cost. , …”

Carbon-Loaded Flexible Electrode Films for Li-O2 Cells: Preparation, Porosity, Homogeneity and Electrochemical Characterization

“…Regarding Li-air systems, numerous modeling and theoretical works have been recently devoted to the prediction of the optimum electrode architecture enabling the maximum accessibility of the reactants (dissolved Li + and O2) to the electrode conducting surface (generally carbons), hence the maximum capacity resulting from the electrochemical formation of Li2O2 (2Li + + 2e -+ O2 → Li2O2). 6 In contrast, only a few works combine the experimental characterization of the electrodes texture/porosity and their electrochemical performances. 7,8 Anyway, from both aspects, a mixture of macro-and mesoporosity in the conducting electrode is considered as necessary to ensure good reactivity and acceptable capacity.…”

Mg/O2Battery Based on the Magnesium–Aluminum Chloride Complex (MACC) Electrolyte

“…Magnesium is an attractive negative-electrode material for battery applications due to its low cost, high energy density, and relative safety in comparison to alkali metals such as lithium. − Furthermore, across the many possible “metal/air” battery chemistries, the theoretical energy density provided by reacting Mg with O 2 is among the highest. , These advantages prompted the study of primary Mg/air batteries using aqueous electrolytes as early as the 1970s. − Despite decades of study since, several challenges remain . For example, aqueous electrolytes corrode Mg electrodes because Mg reacts with H 2 O spontaneously to form passivating Mg­(OH) 2 on the electrode surface .…”

“…1−10 Furthermore, across the many possible "metal/ air" battery chemistries, the theoretical energy density provided by reacting Mg with O 2 is among the highest. 11,12 These advantages prompted the study of primary Mg/air batteries using aqueous electrolytes as early as the 1970s. 13−17 Despite decades of study since, several challenges remain.…”