Recently, lithium-ion batteries have been attracting more interest for use in automotive applications. Lithium resources are confi rmed to be unevenly distributed in South America, and the cost of the lithium raw materials has roughly doubled from the fi rst practical application in 1991 to the present and is increasing due to global demand for lithium-ion accumulators. Since the electrochemical equivalent and standard potential of sodium are the most advantageous after lithium, sodium based energy storage is of great interest to realize lithium-free high energy and high voltage batteries. However, to the best of our knowledge, there have been no successful reports on electrochemical sodium insertion materials for battery applications; the major challenge is the negative electrode and its passivation. In this study, we achieve high capacity and excellent reversibility sodium-insertion performance of hard-carbon and layered NaNi 0.5 Mn 0.5 O 2 electrodes in propylene carbonate electrolyte solutions. The structural change and passivation for hard-carbon are investigated to study the reversible sodium insertion. The 3-volt secondary Na-ion battery possessing environmental and cost friendliness, Na + -shuttlecock hard-carbon/NaNi 0.5 Mn 0.5 O 2 cell, demonstrates steady cycling performance as next generation secondary batteries and an alternative to Li-ion batteries.

Layered NaNi(0.5)Mn(0.5)O(2) (space group: R ̅3m), having an O3-type (α-NaFeO(2) type) structure according to the Delmas' notation, is prepared by a solid-state method. The electrochemical reactivity of NaNi(0.5)Mn(0.5)O(2) is examined in an aprotic sodium cell at room temperature. The NaNi(0.5)Mn(0.5)O(2) electrodes can deliver ca. 105-125 mAh g(-1) at rates of 240-4.8 mA g(-1) in the voltage range of 2.2-3.8 V and show 75% of the initial reversible capacity after 50 charge/discharge cycling tests. In the voltage range of 2.2-4.5 V, a higher reversible capacity of 185 mAh g(-1) is achieved; however, its reversibility is insufficient because of the significant expansion of interslab space by charging to 4.5 V versus sodium. The reversbility is improved by adding fluoroethylene carbonate into the electrolyte solution. The structural transition mechanism of Na(1-x)Ni(0.5)Mn(0.5)O(2) is also examined by an ex situ X-ray diffraction method combined with X-ray absorption spectroscopy (XAS). The staking sequence of the [Ni(0.5)Mn(0.5)]O(2) slabs changes progressively as sodium ions are extracted from the crystal lattice. It is observed that the original O3 phase transforms into the O'3, P3, P'3, and P3" phases during sodium extraction. XAS measurement proves that NaNi(0.5)Mn(0.5)O(2) consists of divalent nickel and tetravalent manganese ions. As sodium ions are extracted from the oxide to form Na(1-x)Ni(0.5)Mn(0.5)O(2), nickel ions are oxidized to the trivalent state, while the manganese ions are electrochemically inactive as the tetravalent state.

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Electrochemical activities of NaNi0.5Mn0.5O2 and NaCrO2, having the analogous layered structure to LiCoO2, were investigated in 1 mol dm-3 NaClO4 propylene carbonate at room temperature. Almost all sodium ions were extracted from the NaNi0.5Mn0.5O2 and NaCrO2 electrodes by galvanostatic oxidation to 4.5 V accompanied with several phase transitions. Layered NaNi0.5Mn0.5O2 electrode showed a highly reversible capacity of 185 mAh g-1 as positive electrode in Na cell in the potential region between 2.5 and 4.5 V versus Na. A NaCrO2 electrode was hardly electroactive after oxidation up to 4.5 V. When galvanostatic cycling was carried in the limited potential domain between 2 and 3.5 V, both electrodes showed discharge capacities of 100 - 120 mAh g-1 with satisfactory capacity retention. Layered LiCrO2 (R-3m) and NaCrO2 (R-3m) possess the quite similar crystal structures and the same transition metal, nevertheless they were inactive and active in Li and Na cells, respectively.

Electrochemical properties and structural changes during charge for NaCrO 2 , whose structure is classified as α-NaFeO 2 type layered polymorph (also O3type following the Delmas' notation), are examined as a positive electrode material for nonaqueous Na-ion batteries. NaCrO 2 delivers initial discharge capacity of 110 mAh g −1 at 1/20C rate in the voltage range of 2.5−3.6 V based on reversible Cr 3+ /Cr 4+ redox without oxidation to hexavalent chromium ions, while the initial discharge capacity is only 9 mAh g −1 when cutoff voltage is set to 4.5 V. Results from exsitu X-ray diffraction, X-ray absorption spectroscopy, and DFT calculations reveal that the irreversible phase transition occurs after sodium extraction by charging over a voltage plateau at 3.8 V associated with the lattice shrinkage along the c-axis in the case of x > 0.5 in Na 1−x CrO 2 , which originates from the migration of chromium ions from octahedral sites in CrO 2 slabs to both tetrahedral and octahedral sites in interslab layer. The irreversible structural change would disturb sodium insertion into the damaged layer structure during discharge, resulting in the loss of reversibility as electrode materials. Reversible cycle range with stable capacity retention is, therefore, limited to the compositional range of 0.0 ≤ x ≤ 0.5 in Na 1−x CrO 2 .