Concerned about the environmental pollutant such as greenhouse gas emission caused by extensive use of diesel, gasoline vehicle operation, being increased. Implementation of rechargeable battery in EV application because of EV’s zero emission has become very popular. Lithium ion battery (LIB) is one of the successfully commercialized system due to their high operating potential, high energy density and long cycle life [1]. However, high price and limited resources of lithium still makes hard to produce for reasonable price of EV. In contrast of LIB issue, sodium-ion batteries (SIBs) have drawn a considerable attention as an alternative to LIBs in the EV applications because of its relative abundance in the earth crust, global distribution, and drastically lower cost [2]. However, the fundamental differences between sodium and lithium make it challenging to develop a suitable anode material to host Na⁺ ions such as well-known graphite intercalation affair in NIB system [3]. Moreover, the higher reduction potential of sodium (–2.71 V vs. S.H.E.) as compared to lithium (–3.04 V vs S.H.E) inherently reduces the energy density of battery system. Therefore, there is still considerable efforts underway to develop a potential anode material that allows to host a large amount of Na⁺ ions at a low voltage potential. Among the various electrode material, sodium titanates (NTOs) have considered one of promising anode materials for SIBs because of their low starting material cost, environmental friendly, and abundance [4]. Among various type of sodium titanate, layered Na₂Ti₃O₇ is one of the most promising phase. It can uptake two Na⁺ ions per formula unit into its interlayer space at a low average potential of 0.3 V vs Na/Na⁺ , which could deliver a high theoretical capacity of 177 mAh g^-1 [5]. It makes particularly promising to design an anode material with high energy density. However, the poor electronic conductivity of Na₂Ti₃O₇ associated with its large bandgap (3.7 eV) and structural distortion upon Na⁺ ion uptake leds to sluggish Na⁺ ion diffusion and cycling stability [6]. In this study, ultrathin and uniform carbon layer-coated, layered Na₂Ti₃O₇ and tunnel Na₂Ti₆O₁₃ hybrids anode materials synthetic route was successfully developed using facile and fast supercritical methanol and subsequent carbon coating with low viscosity liquid carbon dioxide as a coating solvent. The deficiency of each material, e.g., poor rate performance and cyclability caused by sluggish Na⁺ ion diffusion and structural distortion of Na₂Ti₃O₇ and the low capacity of Na₂Ti₆O₁₃, could be overcome in the hybrid by taking advantages of low volume expansion and high electronic conductivity of Na₂Ti₆O₁₃ and high capacity of Na₂Ti₃O₇ with enhanced conductivity by carbon coating. Through the HR-TEM technique, conformal, uniform and ultrathin carbon layers on the NTO surface with an average thickness of 15 nm was observed. Moreover, significantly decreased charge transfer resistance was confirmed by way of the EIS measurement. A careful analysis of the cyclic voltammetry profiles of this sodium titanate hybrids revealed that the existence of two different Na⁺ ion diffusion pathways in the layered structure of Na₂Ti₃O₇ phase. Among two different Na⁺ diffusion pathways, one is kinetically more favorable but energetically less favorable site and the other is kinetically less favorable but energetically more favorable site. Under the high discharge–charge condition, some of Na⁺ ion uptake in the layered structure of Na₂Ti₃O₇ and structural integrity of tunnel structure of Na₂Ti₆O₁₃ resulted in excellent high-rate performance and long-term cyclability in the hybrid.

ACKNOWLEDGEMENTS

This research was supported by a National Research Foundation of Korea (NRF) grant provided by the Korean Government (MSIP) (No. 2016R1A2B3008800, NRF-2018R1A6A3A01012498).

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