Advanced Na[Ni0.25Fe0.5Mn0.25]O2/C Cathode / EMS Electrolyte / Fe3O4 Anode for Sodium-Ion Batteries

Lithium is an attractive battery material because it is the lightest metal, but its resources are concentrated in large quantities mainly in South America with an associated risk of dependence on a single geographic area.¹ In addition, the price of lithium-ion batteries is affected by the high cost of their active materials, commonly based on cobalt compounds. As a result, there is an increasing interest worldwide in developing low cost and sustainable energy storage systems that do not use lithium or cobalt. Among them, rechargeable sodium batteries, due to the almost infinite supply of the metal, are the most appealing as immediate alternatives to lithium batteries. Recently reported results^2-4 have triggered an increasing industrial and academic interest in sodium-ion batteries operating at room temperature. Pioneering work by Hagenmuller et al. and Yamamoto et al. introduced O3-type NaMO₂ (M = Ni, Co, and Fe) in which Na+ could be inserted into a host structure. Since then, interest in developing sodium batteries declined because of the fast development and market domination of lithium-ion batteries. However, recent increasing demands for low-cost energy storage have renewed interest in sodium-based batteries and, accordingly, the O3 type NaMO₂ compounds are being revisited as a cathode material due to the facile synthesis and structural stability, including Na-Ni_0.5Mn_0.5O₂, NaCoO₂, NaCrO₂, NaMn_xM_1−xO₂ (M =Co, Ni), Na[Ni_1/3Fe_1/3O_1/3]O₂, and NaxVO₂. At present, the electrochemical performance of these cathode materials has been limited due to the lack of electrolytes capable of withstanding voltages above 3.9 V versus Na/Na+. In response, we have developed a sodium-ion battery that has an electrolyte of NaClO₄ in a mixture of ethyl methanesulfonate electrolyte (EMS) and fluoroethylene carbonate (FEC) additive for increasing the stability and conductivity, along with an anode of carbon-coated Fe3O4 and a cathode of O3-type layered Na[Ni_0.25Fe_0.5Mn_0.25]O₂. The electrolyte is a key component if sodium-ion batteries are to become competitive with or surpass lithium-ion batteries. The main requirement is a wide anodic stability window in order to allow complete desodiation of the cathode. A common choice has been 1 M sodium perchlorate in propylene carbonate (NaClO₄−PC) with selected additives. For instance, as suggested by Komaba et al., the addition of a small amount of FEC to the PC-based solvent contributes to improved cycleability of sodium batteries, possibly because of a positive effect induced by the fluorination of the anode surfaces.

(1) Scrosati, B.; Garche, J. J. Power Sources 2010, 195, 2419−2430.

(2) Fouassier, C.; Matejka, G.; Reau, J.-M.; Hagenmuller, P. J. Solid State Chem. 1973, 6, 532−537.

(3) Delmas, C.; Fouassier, C.; Hagenmuller, P. Physica B+C 1980, 99,81−85.

(4) Braconnier, J.-J.; Delmas, C.; Fouassier, C.; Hagenmuller, P. Mater. Res. Bull. 1980, 15, 1797−1804.