Effect of Heat-Treated Temperature of Non-Graphitizable Carbon on Sodium-Ion Transfer Reaction
Authors: Tsujimoto Shota , Lee Changhee , Miyahara Yuto , Miyazaki Kohei , Abe Takeshi 
Published: December 2024

Introduction

Non-graphitizable carbon has attracted attention as the negative electrode of sodium-ion batteries (SIBs) due to its larger capacity among carbon materials [1]. Structure of non-graphitizable carbon changes depending on heat-treated condition, precursor, etc. [2]. Especially, heat-treated temperature (HTT) has a large impact on the structure of non-graphitizable carbon, such as graphene interlayer distance and pores [3]. Structure of non-graphitizable carbon has effects on sodium-ion storage behavior [4]. In particular, pore structure is the key to increasing capacity, and a large capacity exceeding 480 mAh g−1 was obtained by controlling the pores [5].

For practical use of SIBs, it is important not only to increase capacity but also to reduce resistance. The resistance of sodium-ion transfer reaction and its activation energy were larger than those of lithium-ion transfer reaction [6]. Our previous reports found that NaF in solid electrolyte interphase (SEI) had a significant resistance, but NaF affects only frequency factor [7]. In the lithium-ion transfer reaction on non-graphitizable carbon, the activation energy was reduced by modifying the surface of non-graphitizable carbon [8]. Therefore, the structure of non-graphitizable carbon may also affect the activation energy with sodium-ion.

Objective

In this work, we used three non-graphitizable carbons heat-treated at 1600, 2000, and 2400 ℃ (labeled as HC-1600, HC-2000, and HC-2400, respectively). The interfacial reaction of non-graphitizable carbon heat-treated at various temperature was investigated. In this paper, we reported about the structure of non-graphitizable carbon on the sodium-ion transfer reaction.

Experimental

A three-electrode cell was used for electrochemical measurements. Non-graphitizable carbon composite electrode was used as the working electrode, sodium metal used as the counter and reference electrode, and 1 mol dm−3 NaPF6/ethylene carbonate (EC)+diethyl carbonate (DEC) (1:1 by vol.) was used as electrolyte.

Cyclic voltammetry (CV) was performed to form SEI on non-graphitizable carbon. The scan rate and range were 0.1 mV s−1 and 0–2.5 V (Na+/Na), respectively. After CV, electrochemical impedance spectroscopy (EIS) was performed with an electrode potential of 0.2 V (Na+/Na) and AC amplitude of 10 mV in frequency range 100 kHz – 10 mHz.

Results and discussion

The specific surface areas of non-graphitizable carbons were around 6 m2 g−1, and this result indicated that non-graphitizable carbon used in this work had closed pore structure which could not be penetrated by gas. Graphene interlayers of non-graphitizable carbons calculated by 002 diffraction peak X-ray became smaller as HTT increased.

In Nyquist plots obtained by EIS, two semi-circles were observed, and they were corresponded the SEI resistance in high frequency region and the charge transfer resistance (R
ct) in middle frequency region. There were variations in the resistance, and this was due to factors such as the amount of slurry applied. Additionally, EIS analyses were conducted at 0.2 V (vs. Na+/Na) in the temperature range of 293 – 313 K. The activation energy (E
a) of the R
ct were evaluated by the Arrhenius plots (Figure). The values of E
a were 52±7 kJ mol−1 (HC-1600), 66±4 kJ mol−1 (HC-2000), and 64±4 kJ mol−1 (HC-2400), and E
a of HC-1600 was extremely small. E
a was larger for HC-2000 and HC-2400, which had small graphene interlayer distance, suggesting that the structure was involved in the rate-determining step, at least these two non-graphitizable carbons.

Conclusion

We investigated the sodium-ion transfer reaction at the interface of three non-graphitizable carbon heat-treated at various temperature. The values of the activation energy were larger for non-graphitizable carbon with small graphene interlayer. This result showed the relationship between structure of non-graphitizable carbon and rate-determining process of sodium-ion transfer reaction.

References

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Figure 1