Journal of The Electrochemical Society, 160 (5) A3077-A3081 (2013) A3077 0013-4651/2013/160(5)/A3077/5/$31.00 © The Electrochemical Society JES FOCUS ISSUE ON INTERCALATION COMPOUNDS FOR RECHARGEABLE BATTERIES Electrochemical and Thermal Properties of α-NaFeO 2 Cathode for Na-Ion Batteries Jie Zhao, a Liwei Zhao, b Nikolay Dimov, b Shigeto Okada, b, z and Tetsuaki Nishida c a Interdisciplinary Graduate School of Engineering Sciences, Kyushu University, Japan b Institute for Materials Chemistry and Engineering, Kyushu University, Japan c School of Humanity-Oriented Science and Engineering, Ninki University, Japan α-NaFeO 2 is promising as minor-metal free cathode materials for low cost sodium-ion batteries. It has a flat voltage plateau at 3.3 V vs. Na metal and a stable reversible capacity of 85 mAh g −1 . Fe 3+ /Fe 4+ redox reaction on charge/discharge cycle was confirmed by 57 Fe M ¨ ossbauer spectrometer. The thermal stability of NaFeO 2 cathodes with/without 1 mol dm −3 NaClO 4 /EC-DMC electrolyte was investigated by DSC measurements. The fully-charged Na 0.58 FeO 2 powder decomposed thermally at a temperature higher than 300 ◦ C, with Fe 2 O 3 as a possible product. On the other hand, the mixture of Na 0.58 FeO 2 powder and electrolyte showed exothermic heat in a temperature range of 220–300 ◦ C. However, NaFeO 2 showed better thermal stability in the electrolyte than LiCoO 2 counterparts in Li ion battery systems, including less heat generation and higher exothermic onset temperature. This indicated that the Na-ion batteries might have comparable thermal stability with Li-ion batteries. © 2013 The Electrochemical Society. [DOI: 10.1149/2.007305jes] All rights reserved. Manuscript submitted January 1, 2013; revised manuscript received February 19, 2013. Published March 13, 2013. This paper is part of the JES Focus Issue on Intercalation Compounds for Rechargeable Batteries. In recent years, ambient temperature sodium-ion batteries have drawn much interest as a power source for large-scale grid energy storage, because of the low cost and abundant resources of sodium. As sodium intercalation host materials, some of layered sodium transition metal oxides have received considerable attention as cathode materials for sodium-ion batteries. 1–4 Among them, minor-metal free NaFeO 2 has important advantages for practical use, because iron-based cathode and sodium anode is the most ideal combination for large scale battery system in view point of cost and environmental impact. It is well known that LiCoO 2 and LiNiO 2 have layered rocksalt structure as stable phase and they show good cathode utilization, because the layered rocksalt structure has two dimensional lithium diffusion layers in the matrix. However, we cannot obtain layered rocksalt LiFeO 2 as stable phase, because Fe 3+ and Li + cations have similar ion size and they are prone to cation mixing. 5 Fortunately, this is not the case for NaFeO 2 , because Na + cation is larger than Fe 3+ . NaFeO 2 has two polymorphs. β-NaFeO 2 is an orthorhombic crystal and has not been reported to have electrochemical activity. Rhom- bohedral α-NaFeO 2 , which is the focus of this paper, is classified as O3-type layered structure following Delmas’ notation. 6 Na chemical deintercalation from α-NaFeO 2 was first reported by Kikkawa et al. in 1985. 7 The O3 structure was preserved during this process and the product had a composition about Na 0.9 FeO 2 . But under electro- chemical process, a new monoclinic phase of Na 0.5 FeO 2 was obtained from α-NaFeO 2 after Na deintercalation. 8 Although α-NaFeO 2 had been demonstrated to be electrochemically active, there are few re- ports about its application in batteries. Our group has firstly reported the cathode properties of α-NaFeO 2 against Na anode. 9 Recently, Yabuuchi reported the cutoff voltage dependency on the cathode per- formance of α-NaFeO 2 in sodium-ion batteries. 3 By the recent devel- opments of sodium-ion battery, the reality of sodium-ion battery as post lithium-ion battery has been rapidly increased. Thermal stability of Li-ion batteries has caused serious concern especially for large scale application such as EV. Considering the high activity of sodium metal, Na-ion batteries was suspected to be more danger than Li-ion batteries. Therefore, the investigation of thermal properties for Na-ion batteries is important for the development and practical application of Na-ion batteries. However, few attentions have been paid on the safety issue of Na-ion batteries. In this study, electrochemical properties of α-NaFeO 2 as cath- ode material in Na-ion batteries were investigate by charge/discharge measurements. The behavior of iron valence states in α-NaFeO 2 cath- z E-mail: s-okada@cm.kyushu-u.ac.jp odes on charging was studied with 57 Fe M ¨ ossbauer spectroscopy. The thermal stabilities of fully-charged α-NaFeO 2 in Na secondary battery were investigated by thermogravimetry coupled with differential scan- ning calorimeter (TG-DSC) and compared with that of fully-charged LiCoO 2 counterparts in Li secondary battery. Experimental α-NaFeO 2 was synthesized by firing a mixture of Na 2 O 2 (Wako Pure Chemical Industries, Ltd.) and Fe 2 O 3 (Aldrich) at 650 ◦ C for 12 h in air. 70 wt% fabricated α-NaFeO 2 , 25 wt% AB (Acetylene Black, Denki Kagaku Kogyo) and 5 wt% PTFE (Polytetrafluoroethy- lene, Daikin Industries, Ltd.) were mixed and then made into pellets in an Ar glove box. LiCoO 2 pellets were prepared in air with a weight ratio of LiCoO 2 (Aldrich):AB:PTFE = 70:25:5. The NaFeO 2 and LiCoO 2 pellets were then fabricated in the form of disks (10-mm diameter) with a typical active material loading of 18 mg cm −2 and dried at 110 ◦ C under a vacuum for 12 h. Two-electrode coin-type cells were used here for evaluation of the electrochemical properties. Each cell consisted of a working electrode, a polypropylene separator (Celgard 3501), and a sodium or lithium metal foil as a counter electrode, respectively. All cells were assem- bled in the Ar glove box with a dew point below −80 ◦ C. 1 mol dm −3 NaClO 4 /ethylene carbonate (EC) : dimethyl carbonate (DMC) (1 : 1 v/v) solution (Kishida Chemical Co., Ltd.) was used as the electrolyte for Na cells, while 1 mol dm −3 LiClO 4 /EC-DMC (1 : 1 v/v) solution (Tomiyama Pure Chemical Industries, Ltd.) for Li cells. The electro- chemical properties of the cathodes were studied by charge/discharge measurements at a rate of 0.2 mA cm −2 . The α-NaFeO 2 cells were cycled between 1.5 and 3.6 V versus Na, and the LiCoO 2 cells were cycled between 3.0 and 4.2 V versus Li. After charge/discharge cycles, the cells were disassembled in the Ar glove box. The test cathodes were rinsed and soaked with DMC for 4 h in Ar glove box, and then dried under vacuum at room temper- ature for 12 h. Most of the low-molecular-weight components were dissolved away from the electrodes in the rinsing step and evacu- ated during the vacuum drying. For M¨ ossbauer measurement, the initial cathode and the cathode after the first charge process were laminated with aluminum bag in Ar glove box, respectively. A 57 Fe M¨ ossbauer spectrometer (Laboratory Equipment Corp.) was used with α-Fe foil as reference for isomer shift measurement. For thermal anal- ysis, given amounts of the cathode powder was hermetically sealed into a stainless-steel pan together with/without electrolyte in Ar glove box. 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