Peat-Derived Hard Carbons for High Capacity Sodium-Ion Batteries: Synthesis and Characterization
Authors: Adamson Anu , Väli Ronald , Paalo Maarja , Aruväli Jaan , Koppel Miriam , Palm Rasmus , Hark Eneli , Nerut Jaak , Romann Tavo , Lust Enn , Jänes Alar 
Published: December 2020

Hard carbons have emerged as one of the most promising candidates for high capacity anode materials in sodium-ion batteries (SIB). It has been demonstrated that the degree of graphitization, disorder, and porosity of obtained hard carbons have an enormous effect on capacity.1 Various biomass precursors (banana peels, peanut shells, leaves, etc.) have been used for obtaining hard carbons, which have shown promising results as SIB anodes.2–4

In this study, peat was used as the precursor for obtaining peat-derived hard carbon anode materials (PDCs). Peat has a distinctive advantage as a precursor for its ease of collection. It can be excavated in millions of tons, as opposed to collecting shells or peels of different fruit. Hard carbons were synthesized using three steps – pre-pyrolysis at 300-800 oC, base-acid treatment, and post pyrolysis at 1000-1500 oC (Fig. 1). Each step was deemed necessary to improve the capacity of obtained hard carbon material (Fig. 1a). The materials were labeled as follows: PDC – pre-pyrolysis temperature – post-pyrolysis temperature A (A appended if KOH-HCl treatment was used).5

The structure of hard carbon materials was investigated with XRD, SEM-EDX, TEM, Raman spectroscopy, and laser powder diffraction methods. The specific surface area was examined by gas sorption analysis (with N2 and Ar). XRF was used to determine the elemental composition of hard carbon materials before and after the KOH-HCl treatment. TGA was performed to evaluate the burn-off of peat’s organic content during pre-pyrolysis.
Cycling performance of hard carbon electrodes was investigated using the galvanostatic charge-discharge method at current densities ranging from 25 to 2000 mA g-1 and cyclic voltammetry at a potential sweep rate of 0.1 mV s-1. NaClO4 in EC:DEC (1:1) and NaPF6 EC:PC (1:1) were used as electrolytes to compare two rather common electrolytes and their effect on capacity (Fig. 1b).
The highest capacities were achieved for PDC-450-1400 A. The constant current measurements show very high capacities of 350 mAh g -1 with a plateau region (E < 0.2 V vs. Na/Na+) of 250 mAh g-1 at the current density of 25 mA g -1. At the current density of 50 mA g-1, a capacity of 328 mAh g-1 with a plateau region of 243 mAh g -1 was achieved. Long-term stability of PDC-450-1400 A was also demonstrated, after 130 charge-discharge cycles at the current density of 25 mA g-1, the capacity stabilized at 280 mAh g-1 with the initial capacity of 320 mAh g-1.
Overall, the highest capacities were achieved with materials that had the lowest impurity contents and specific optimal physical properties – low surface areas of around 6 m2 g-1, broad 002 reflexes in XRD diffractograms, interlayer spacing of about 3.9 Å and optimal I
D/I
G values of approximately 1.50. These characteristics might indicate the existence of local graphitic domains and therefore explain the high capacities.5

Acknowledgments

This work was supported by ‘Strengthening of sectoral R&D’ project LLTOM17351, by EU through the European Regional Development Fund project 2014-2020.3.01.15-0011 and Personal Research under Grant PRG676.

References

I. El Moctar et al., Functional Materials Letters, 11, 1830003 (2018).

E. M. Lotfabad et al., ACS Nano, 8, 7115–7129 (2014).

J. Ding et al., Energy Environ. Sci., 8, 941–955 (2015).

H. Li et al., ACS Appl. Mater. Interfaces, 8, 2204–2210 (2016).

A. Adamson et al., RSC Adv., 10, 20145–20154 (2020).

Figure 1