Liquid Ammoniates as Efficient Electrolytes for Room-Temperature Rechargeable Sodium-Metal Batteries Based on an Organic Cathode

Abstract

The development of room-temperature rechargeable sodium-metal batteries (SMBs) has gathered enormous interest as they are an attractive option to develop cost-effective devices to store energy from renewable sources. However, one of their main constraints is the high reactivity of sodium with most of the electrolytes commonly employed, which hampers finding an optimized SMB configuration. Here, we describe the use of a highly concentrated electrolyte based on liquid ammonia and sodium iodide. Its formulation is NaI·3.3NH3, and it confers metallic sodium high stability at room temperature. Moreover, the electrolyte is significantly more cost-effective than typical organic or ionic liquid electrolytes. Here, the advantages of this electrolyte are shown in a battery based on the combination of sodium metal as an anode and poly(anthraquinonyl sulfide) (PAQS) as a cathode. This combination leads to an experimental capacity of 218 mA·h·g–1 at 5C (reaching 97% of the theoretical capacity) at room temperature and maintains a Coulombic efficiency close to 100% over 300 cycles. In addition, the system described here attains, after 100 cycles, specific energies over 320 W·h·kgPAQS–1 and a specific power of 3500 W·kgPAQS–1, with an energy efficiency over 90%. These metrics, which compare favorably with those of typical organic electrolytes, are linked to an extremely low value of the charge-transfer resistance at the cathode/ammoniate interphase. Despite the dissolution of PAQS during the reduction process, the battery already presents a capacity retention over 80% after 150 cycles at 5C.