The reported performance of the new Na-ion battery suggests that the sodium-ion system is a potentially promising power source for promoting the substantial use of low-cost energy storage systems in the near future, the team concluded.

… 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 Na_xVO₂.

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.

—Oh et al.

For the anode, they selected carbon-modified iron oxide (C-Fe₃O₄) conversion material. Graphite, they explained, cannot be used in sodium batteries due to the large difference in ionic radii between lithium and sodium, which induces exfoliation rather than intercalation with dendritic growth of reactive Na metal.

Hard carbon has a limited applied current density, reducing its utility in this application. However, the carbon-modified iron oxide has a theoretical capacity of about 920 mAh g⁻¹ when used in a lithium cell and undergoes a conversion reaction (Fe₃O₄ + 8Li⁺ + 8e⁻ → 4Li₂O + 3Fe); it undergoes a similar conversion reaction when used in a sodium cell.

Although the feasibility of Fe₃O₄ as the cathode or anode material in Na ion cells with sodium intercalation or conversion chemistry have been explored recently, the various analysis to identify the phenomenon for the anode with a conversion chemistry for Na-ion battery is needed. Therefore, our approach involving the conversion anode and EMS-based electrolyte for Na-ion cells is unprecedented and provides further incentives for exploration of advanced materials for Na-ion batteries.

—Oh et al.

The full battery operated reversibly around 2.4 V, delivering a capacity of about 130 mAh (g-NFM)⁻¹. Under test conditions, the battery showed 82.8% capacity retention at the 100^th cycle and 76.1% at the 150^th cycle with a Coulombic efficiency approaching 100%.

The rate capability varies from 130 mAh g⁻¹ at 0.1C,to 120 mAh g⁻¹ at 1C, to 72 mAh g⁻¹ at 10C. Such rate capability of the full cell is comparable to the advanced lithium-ion batteries, they noted.

… we reported a thermally stable NFM cathode material and improved electrochemical performance of Fe₃O₄/ NFM sodium-ion battery exploiting a unique combination of intercalation and conversion electrode chemistry in the presence of a novel electrolyte based on NaClO₄ in EMS/FEC. We are able to demonstrate that this battery pack offers very promising performance in terms of capacity and rate capability for a cycle life extending to over 150 cycles. Such performance is believed to associate with the structural stability and the high electric conductivity of the selected electrode materials, as well as the fast ion transport and wide electrochemical window of the EMS based electrolyte. In addition, the battery is based on sodium, an abundant, hence low cost, material. Overall, our sodium-ion system appears to be one of the potentially promising for low-cost energy storage in the near future.

—Oh et al.

Resources

• Seung-Min Oh, Seung-Taek Myung, Chong Seung Yoon, Jun Lu, Jusef Hassoun, Bruno Scrosati, Khalil Amine, and Yang-Kook Sun (2014) “Advanced Na[Ni_0.25Fe_0.5Mn_0.25]O₂/C–Fe₃O₄ Sodium-Ion Batteries Using EMS Electrolyte for Energy Storage,” Nano Letters doi: 10.1021/nl500077v