Reversible Lithium Alloying States on a Red Phosphorus / Carbon Nanotube Composite Anode for Lithium Ion Batteries
Authors: Tojo Tomohiro , Ito Hiromitu 
Published: December 2020

Lithium ion batteries (LIBs) has been commonly used as a rechargeable energy device for mobile phones, laptop computers, and electric vehicles. These applications require high energy density to help reduce the cost per unit of energy capacity. In the last two decades, various electrode materials have been proposed to replace conventional electrode materials to others with a higher energy density. In the case of anode materials, lithium alloying materials including elemental silicon, tin and phosphorus have a huge theoretical capacity of approximately 4200, 1000 and 2600 mAh/g, respectively.[1] However, enormous volumetric change up to 400% during lithium alloying/dealloying process (Li+ insertion/extraction reaction) is known to be a main reason of rapid capacity fading, which hinders practical applications for LIBs.
Carbon composite materials have been proposed to circumvent the large volumetric change, maintaining a higher capacity than that of bare lithium alloying materials even though reversible capacities still showed low value of 1000 mAh/g at most.[2],[3],[4] In composite materials with lithium alloying materials, phosphorus encapsulated in cylindrical or porous carbon structures has been attracting intensive attention at present as the most promising alternative, showing a high reversible capacity of ~2000 mAh/g at only a few cycles.[5],[6] The improved capacity is thought to be due to suppressing large volumetric change of phosphorus. However, it has been reported that there was still volumetric change and capacity fading of 50%. This large capacity fading might be attributed to blocking an effective path of Li+ diffusion to carbon structures in which phosphorus was encapsulated.[5],[6] Up to now, we investigated Li+ insertion and extraction process on phosphorus-encapsulated carbon nanotubes, clarifying smooth diffusion on porous sidewalls of the nanotubes.[7] In this study, a relationship between BET surface area with changing amount of phosphorus and the reversible capacity was investigated to form the effective path and to encapsulate a suitable amount of phosphorus by using chemically-drilled carbon nanotubes as a starting material. The macro and micro morphologies were evaluated from TEM, SEM, EDX, XRD and Raman spectroscopy before and after Li+ insertion and extraction.
Phosphorus-encapsulated carbon nanotubes with nanopores on the sidewalls[7] were synthesized by changing an evaporation amount of phosphorus in the carbon/phosphorus ratio of 1:1, 1:0.5, 1:0.25, and 1:0. From the difference between sample weights before and after phosphorus encapsulation, the embedded amount of phosphorus was calculated to be 57.5, 36.9, and 18.7 wt.% in the ratio of 1:1, 1:0.5, and 1:0.25, respectively. BET surface areas of these samples were measured through N2 adsorption at 77K, showing the values of 408, 126, 10.4, and 5.5 m2/g in the ratio of 1:0, 1:0.25, 1:0.5, and 1:1, respectively. This tendency is evidence of phosphorus encapsulation into inner nanotubes, which is also observed in TEM images and EDX mapping images. When electron beam in TEM was irradiated on the samples during EDX measurements, cylindrical structures encapsulating a large amount of phosphorus were expanded along the radial direction. This expansion suggests that the phosphorus was almost encapsulated into the inner cavity of carbon nanotubes.
These samples were electrochemically evaluated in 2032 type coin cells using 1 mol/L LiPF6 EC/DMC as an electrolyte solution and lithium metal as a counter electrode. Charge-discharge profiles in each sample were recorded at a constant current of 25 mA/g, showing a high charge capacity (Li+ insertion capacity) of 2800 mAh/g-phosphorus at the 1st cycle beyond the theoretical capacity (2600 mAh/g) because of electrolyte decomposition on particle surfaces. Chemically-drilled carbon nanotubes without any phosphorus also had such high charge capacity at the 1st cycle. Therefore, the charge capacity includes an electrolyte decomposition to form a stable solid electrolyte interphase (SEI) on carbon surfaces. At the 2nd cycle, charge capacity was corresponded with discharge capacity (Li+ extraction), which resulted in high reversible capacity of 2000 mAh/g-phosphorus. Continuous cycles delivered approximately 100% of Coulombic efficiency, which indicates that the SEI formation was completed at the 1st cycle. After the 2nd cycle, the reversible capacity was estimated to be 2000~1600 mAh/g-phosphorus at each sample, decreasing with an increment in an encapsulation amount of phosphorus. However, volumetric capacity per phosphorus increased with increasing the amount.
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