Research interest in Na-ion batteries has increased rapidly because of the environmental friendliness of sodium compared to lithium. Throughout this Perspective paper, we report and review recent scientific advances in the field of negative electrode materials used for Na-ion batteries. This paper sheds light on negative electrode materials for Na-ion batteries: carbonaceous materials, oxides/phosphates (as sodium insertion materials), sodium alloy/compounds and so on. These electrode materials have different reaction mechanisms for electrochemical sodiation/desodiation processes. Moreover, not only sodiation-active materials but also binders, current collectors, electrolytes and electrode/electrolyte interphase and its stabilization are essential for long cycle life Na-ion batteries. This paper also addresses the prospect of Na-ion batteries as low-cost and long-life batteries with relatively high-energy density as their potential competitive edge over the commercialized Li-ion batteries.

We report a coexistence of superconductivity and antiferromagnetism in five-layered compound HgBa2Ca4Cu5Oy (Hg-1245) with Tc = 108 K, which is composed of two types of CuO2 planes in a unit cell; three inner planes (IP's) and two outer planes (OP's). The Cu-NMR study has revealed that the optimallydoped OP undergoes a superconducting (SC) transition at Tc = 108 K, whereas the three underdoped IP's do an antiferromagnetic (AF) transition below TN ∼ 60 K with the Cu moments of ∼ (0.3 − 0.4)µB . Thus bulk superconductivity with a high value of Tc = 108 K and a static AF ordering at TN = 60 K are realized in the alternating AF and SC layers. The AF-spin polarization at the IP is found to induce the Cu moments of ∼ 0.02µB at the SC OP, which is the AF proximity effect into the SC OP.

For a nonaqueous sodium-ion battery (NIB), phosphorus materials have been studied as the highest-capacity negative electrodes. However, the large volume change of phosphorus upon cycling at low voltage causes the formation of new active surfaces and potentially results in electrolyte decomposition at the active surface, which remains one of the major limiting factors for the long cycling life of batteries. In this present study, powerful surface characterization techniques are combined for investigation on the electrode/electrolyte interface of the black phosphorus electrodes with polyacrylate binder to understand the formation of a solid electrolyte interphase (SEI) in alkyl carbonate ester and its evolution during cycling. The hard X-ray photoelectron spectroscopy (HAXPES) analysis suggests that SEI (passive film) consists of mainly inorganic species, which originate from decomposition of electrolyte solvents and additives. The thicker surface layer is formed during cycling in the additive-free electrolyte, compared to that in the electrolyte with fluoroethylene carbonate (FEC) or vinylene carbonate (VC) additive. The HAXPES and time-of-flight secondary ion mass spectroscopy (TOF-SIMS) studies further reveal accumulation of organic carbonate species near the surface and inorganic salt decomposition species. These findings open paths for further improvement for the cyclability of phosphorus electrodes for high-energy NIBs.

We report a site selective Cu-NMR study on underdoped Hg-based five-layered high-Tc cuprate HgBa2Ca4CU5O(12+delta) with a Tc = 72 K. Antiferromagnetism (AFM) has been found to take place at T(N) = 290 K, exhibiting a large antiferromagnetic moment of 0.67-0.69 microB at three inner planes (IP). This value is comparable to the values reported for nondoped cuprates, suggesting that the IP may be in a nearly nondoped regime. Most surprisingly, the AFM order is also detected with M(AFM)(OP) = 0.1 microB even at two outer planes (OP) that are responsible for the onset of superconductivity (SC). The high-Tc SC at Tc = 72 K can uniformly coexist on a microscopic level with the AFM at OP's. This is the first microscopic evidence for the uniform mixed phase of AFM and SC on a single CuO2 plane in a simple environment without any vortex lattice and/or stripe order.