Rechargeable Seawater Battery and Its Electrochemical Mechanism

Abstract

Herein, we explore the electrochemical mechanism of a novel rechargeable seawater battery system that uses seawater as the cathode material. Sodium is harvested from seawater while charging the battery, and the harvested sodium is discharged with oxygen dissolved in the seawater, functioning as oxidants to produce electricity. The seawater provides both anode (Na metal) and cathode (O 2 ) materials for the proposed battery. Based on the discharge voltage (~2.9 V) with participation of O 2 and the charge voltage (~4.1 V) with Cl 2 evolution during the first cycle, a voltage efficiency of about 73 % is obtained. If the seawater battery is constructed using hard carbon as the anode and a Na super ion conductor as the solid electrolyte, a strong cycle performance of 84 % is observed after 40 cycles.The requirements for energy-storage systems are vastly different from those for consumer electronics or electric vehicles (EVs) and vary widely depending on factors such as installation size and location. This makes it difficult for one type of battery systems to become a predominant solution satisfying all the requirements. Instead, a reasonable and practical strategy is to respond to diverse requirements of each application with various alternative battery systems considering the characteristics of each option. For example, while an advanced Li-ion battery, which has a high volumetric energy density, would be a right choice for the energy storage in densely populated areas considering its high volumetric energy density, other bulky but cheaper systems could become more competitive in remote areas where the size of systems can be easily expanded. Therefore, it is important to build a strong portfolio of novel battery systems to provide optimum solutions.The development of novel configurations of battery cells, such as Li-Air, [1] Li-liquid, [2] Na-air, [3] and Li-polysulfide batteries, [4] has been made possible by combinations of multi-phase electrolyte/electrode components such as liquid/solid electrolytes and liquid/solid/liquid (or gas) electrodes. Based on these battery designs, various interesting combinations of electrochemical reaction couples are being suggested and tested that were not previously considered due to the limitations of conventional battery cell design. Recently, we investigated the feasibility of a novel rechargeable battery system using seawater as an electrode material. [5] As the most abundant resource on Earth, seawater contains various chemical species released from the Earth's crust and living organisms. Examples of the major chemical constituents of seawater are sodium (Na) (~0.46 m) and chlorides (~0.54 m).[6] Detailed information on the chemical constituents of seawater is shown in Figure 1 a. Accordingly, we constructed a novel rechargeable battery cell that uses seawater and its chemical constituents as anode/cathode materials. The configuration of the seawater battery, composed of Na/non-aqueous liquid/solid electrolyte/seawater, is shown in Figure 1 b and Figure 1 c. 1 m NaClO 4 in et...

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