Nanocrystalline Li3V1.9Al0.1(PO4)3 / Multi-Walled Carbon Nanotubes Positive Electrode for Hybrid Supercapacitors
Authors: Okita Naohisa , Kisu Kazuaki , Fukami Taichi , Iwama Etsuro , Naoi Wako , Rozier Patrick , Simon Patrice , Naoi Katsuhiko 
Published: February 2020

1. Introduction

Monoclinic Li3V2(PO4)3 has promising features as a positive electrode for hybrid supercapacitors, such as high thermal stabilities, three-dimensional path for rapid ionic diffusion and high potential of 3.9 V vs. Li/Li+ with decent capacity (131 mAh g-1) compared with other polyanion-type materials such as olivine-type LiFePO4 (one-dimensional path for lithium ion diffusion, 3.4 V vs. Li/Li+,170 mAh g-1). One of the challenges associated with the use of Li3V2(PO4)3 cathodes is circumventing the limitation of their intrinsically low electronic conductivity. Designing nanostructure electrode materials1), establishment of the conductive carbon network2) and ion-doping3) are effective methods for enhancement of the power performance up to 100C. We have synthesized and characterized single-nanoscale Al-doped Li3V2(PO4)3 particles directly embedded onto the surface of multi-walled carbon nanotubes (MWCNT), using an original in-situ material synthesis method called ultracentrifugation (UC) treatment4). The obtained Al-doped Li3V2(PO4)3/MWCNT composites showed high power capability with high discharge capacity of 85 mAh g-1 at 480C compared to those of previous reports. Here, we report about the synthesis of Al-doped Li3V2(PO4)3, Li3V1.9Al0.1(PO4)3, directly attached on the surface of MWCNT via UC treatment. The influence of Al-doping on the important characteristics of Li3V2(PO4)3 (lithium diffusivity, electronic conductivity and cycleability) were also studied.

2. Experimental The synthetic procedure of the Li3V1.9Al0.1(PO4)3/MWCNT composite is achieved via UC treatment. NH4VO3, Al(NO3)3・9H2O, MWCNT, citric acid, ethylene glycol, CH3COOLi, and H3PO4 aqueous solution were mixed all together and UC treatment was carried out on the prepared mixture. The obtained sol was dried at 80oC for 12 hours, then heated at 900 oC with no holding time under N2 flow. The obtained composites were investigated by in-situ XRD, HRTEM and electrochemically tests.

3. Results and Discussion

The XRD pattern of Li3V1.9Al0.1(PO4)3/MWCNT composite indicates a monoclinic structure, without impurity phases. The decrease of refined cell parameters obtained from XRD pattern fitting for Li3V2-xAl0x(PO4)3/MWCNT composite with increasing Al content confirms that part of V3+ in Li3V2(PO4)3 was substituted with Al3+. The morphology of Li3V1.9Al0.1(PO4)3/MWCNT composite was investigated by a high resolution transmission electron microscope (HRTEM) observation. HRTEM images indicate that Li3V1.9Al0.1(PO4)3 particles of ca. 100 nm are highly dispersed and directly attached on the surface layers of MWCNT. The Li3V1.9Al0.1(PO4)3/MWCNT composite showed excellent discharge rate capability up to 85 mAh g−1 at a discharge rate of 480C, higher than undoped Li3V2 (PO4)3/MWCNT. This enhancement is attributed to an increase of both lithium diffusivity and electronic conductivity, associated to a smaller volumetric change in the unit cell during charging and discharging processes calculated from the in-situ XRD results. The results of electrochemical impedance spectroscopy (EIS) measurements using a symmetric cell showed a decrease in the charge transfer impedance of Li3V1.9Al0.1(PO4)3/MWCNT, in good agreement with better performances of Li3V1.9Al0.1(PO4)3/MWCNT than undoped one.

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