Phosphorus-Based Negative Electrode Materials for Na-Ion Batteries

Abstract

Introduction Na-ion batteries have recently received a lot of attention as a next-generation battery, because Na resources are inexhaustible and cheaper than Li resources. However, although Na-ion batteries have shown promising electrochemical performance including stable cycle performance and good rate performance, it should be mentioned that it is not easy for Na-ion batteries to replace Li-ion batteries owing to the critical issue of Na-ion batteries having slightly lower energy density than Li-ion batteries. Therefore, in order to successfully replace Li-ion batteries by Na-ion batteries, new anode materials having a high reversible capacity should be introduced. In this presentation, phosphorus-based negative electrode materials are introduced for Na-ion batteries. Phosphorus-based electrodes showed excellent electrochemical performance including a high reversible capacity (>700 mAh/g), excellent cycle performance, and an ideal redox potential (< 0.4 V vs. Na/Na+), thus making it a promising candidate for Na ion batteries. Experimental Two types of phosphorus-based active materials were prepared as follows. For red phosphorus/carbon composites, phosphorus powder and Super P carbon were mixed in a 7:3 weight ratio using a high energy mechanical ball-milling method. For the synthesis of tin phosphide, phosphorus and Sn powders were mixed using the same ball-milling method.Samples of active materials were mixed with carbon black (Super P) and polyacrylic acid (PAA) in a 7:1:2 weight ratio. The electrochemical performance was evaluated using 2032 coin cells with a Na metal anode and 1 M NaClO4in ethylene carbonate and diethyl carbonate (1:1 v/v) with or without the addition of 5% fluoroethylene carbonate (FEC).For the analysis of the reaction mechanism, ex-situ X-ray diffraction (ex-situXRD), high resolution transmission electron microscope (HR-TEM) and X-ray absorption spectroscopy (XAS) analysis were performed. Results and Discussion The amorphous red phosphorus/carbon composite exhibited promising electrochemical performance including i) high reversible capacity of 1890 mA h g-1 (equivalent to 2.18 mol of Na alloyed with one mole of P); ii) good cycle performance over 30 cycles; and iii) appropriate redox potential of ca.0.4 V vs. Na/Na+.Also, tin phosphide electrode showed excellent electrochemical performance including i) high reversible capacity of about 718 mA h g-1; ii) negligible capacity fading over 100 cycles in spite of the large particle size (> a few micrometer).Finally, the reaction and failure mechanisms of both electrodes were studied using ex situ instrumental analysis.