Electrode performance of the Na 2 Ti 3 O 7 electrode. (d) Charge/ discharge curves of a P2-Na 0.66 [Li 0.22 Ti 0.78 ]O 2 electrode and Na 3 Ti 2 (PO 4 ) 3 electrodes cycled in the voltage range of (e) 2.5−2.0 and (f) 2.0 and 0.0 V. (a) Reprinted with permission from ref 320.

Rechargeable lithium batteries have risen to prominence as key devices for green and sustainable energy development. Electric vehicles, which are not equipped with an internal combustion engine, have been launched in the market. Manganese- and iron-based positive-electrode materials, such as LiMn(2)O(4) and LiFePO(4), are used in large-scale batteries for electric vehicles. Manganese and iron are abundant elements in the Earth's crust, but lithium is not. In contrast to lithium, sodium is an attractive charge carrier on the basis of elemental abundance. Recently, some layered materials, where sodium can be electrochemically and reversibly extracted/inserted, have been reported. However, their reversible capacity is typically limited to 100 mAh g(-1). Herein, we report a new electrode material, P2-Na(2/3)[Fe(1/2)Mn(1/2)]O(2), that delivers 190 mAh g(-1) of reversible capacity in the sodium cells with the electrochemically active Fe(3+)/Fe(4+) redox. These results will contribute to the development of rechargeable batteries from the earth-abundant elements operable at room temperature.

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.

Lithium-excess manganese layered oxides, which are commonly described by the chemical formula zLi(2)MnO(3)-(1-z)LiMeO(2) (Me = Co, Ni, Mn, etc.), are of great importance as positive electrode materials for rechargeable lithium batteries. In this Article, Li(x)Co(0.13)Ni(0.13)Mn(0.54)O(2-δ) samples are prepared from Li(1.2)Ni(0.13)Co(0.13)Mn(0.54)O(2) (or 0.5Li(2)MnO(3)-0.5LiCo(1/3)Ni(1/3)Mn(1/3)O(2)) by an electrochemical oxidation/reduction process in an electrochemical cell to study a reaction mechanism in detail before and after charging across a voltage plateau at 4.5 V vs Li/Li(+). Changes of the bulk and surface structures are examined by synchrotron X-ray diffraction (SXRD), X-ray absorption spectroscopy (XAS), X-ray photoelectron spectroscopy (XPS), and time-of-flight secondary ion mass spectroscopy (SIMS). SXRD data show that simultaneous oxygen and lithium removal at the voltage plateau upon initial charge causes the structural rearrangement, including a cation migration process from metal to lithium layers, which is also supported by XAS. This is consistent with the mechanism proposed in the literature related to the Li-excess manganese layered oxides. Oxygen removal associated with the initial charge on the high voltage plateau causes oxygen molecule generation in the electrochemical cells. The oxygen molecules in the cell are electrochemically reduced in the subsequent discharge below 3.0 V, leading to the extra capacity. Surface analysis confirms the formation of the oxygen containing species, such as lithium carbonate, which accumulates on the electrode surface. The oxygen containing species are electrochemically decomposed upon second charge above 4.0 V. The results suggest that, in addition to the conventional transition metal redox reactions, at least some of the reversible capacity for the Li-excess manganese layered oxides originates from the electrochemical redox reaction of the oxygen molecules at the electrode surface.

Highly reversible potassium intercalation into graphite in carbonate ester solution at room temperature is achieved byelectrochemical reductionat the potential approaching to K + /Kstandard potential which islower than that of Li + /Li. The intercalation results in formation of stage-1 KC 8 compound with delivering 244 mAh g-1 of reversible capacity. The initial irreversible capacity is suppressed by polycarboxylate bindercompared to poly(vinyledene fluoride) binder.The lower potential, good cyclabilty, andexcellent rate capabilityare first demonstrated forenergy storage applications. Because of the lowest potential andweakest solvation among Li + , Na + , K + , Mg 2+ , and Ca 2+ ion carriers, potassium shuttlecock mechanism between two insertion materials as "potassium-ion battery" is advantageous for higher-voltage/-power rechargeable batteries.The excellent rate performance is beneficial for the application to hybrid-type capacitor, "potassium-ion capacitor,"as an alternative to lithium-ion capacitors.

Li-ion batteries (LIBs), commercialized in 1991, have the highest energy density among practical secondary batteries and are widely utilized in electronics, electric vehicles, and even stationary energy storage systems. Along with the expansion of their demand and application, concern about the resources of Li and Co is growing. Therefore, secondary batteries composed of earth-abundant elements are desired to complement LIBs. In recent years, K-ion batteries (KIBs) have attracted significant attention as potential alternatives to LIBs. Previous studies have developed positive and negative electrode materials for KIBs and demonstrated several unique advantages of KIBs over LIBs and Na-ion batteries (NIBs). Thus, besides being free from any scarce/ toxic elements, the low standard electrode potentials of K/K + electrodes lead to high operation voltages competitive to those observed in LIBs. Moreover, K + ions exhibit faster ionic diffusion in electrolytes due to weaker interaction with solvents and anions than that of Li + ions; this is essential to realize high-power KIBs. This review comprehensively covers the studies on electrochemical materials for KIBs, including electrode and electrolyte materials and a discussion on recent achievements and remaining/emerging issues. The review also includes insights into electrode reactions and solid-state ionics and nonaqueous solution chemistry as well as perspectives on the research-based development of KIBs compared to those of LIBs and NIBs.

Fluoroethylene carbonate is an efficient electrolyte additive to improve the reversibility of electrochemical sodium insertion for hard-carbon and NaNi(1/2)Mn(1/2)O(2) electrodes in aprotic Na cells. The additive is also capable of the electrochemical deposition/dissolution of metallic Na with higher reversibility because of improved passivation and suppression of side reactions between Na metal and propylene carbonate solution containing Na salts.

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.