Hard carbon has been shown in many studies to be one of the best candidates for an anode material for the development of sodium ion batteries. Lignin is the second most abundant biopolymer¹ and a byproduct of pulp and paper industry². Due to its high content of aromatic rings (polyphenols), lignin gives a high char yield (~40% wt.) after pyrolysis³. Lignin is therefore a good candidate for being used as a hard carbon precursor for sodium ion batteries (NIBs). Its processability by melt spinning and electrospinning makes lignin a good material for producing carbon fibers from renewable in comparison to PAN-based carbon fibers. This is shown by our previous study where we successfully used lignin based melt-spun carbon fibers as an anode material in lithium ion batteries⁴.

The aim of this work is to study the potential of using a byproduct of the pulp and paper industry as a precursor for anode materials in NIBs. A second motivation is to study the sodium insertion mechanism and effects on changes in the material structure by varying the properties of the precursor and the carbonization temperature. Finally, the motivation for using the ether-based diethylene glycol dimethyl ether (diglyme) electrolyte is to study whether the problem of the low first coulombic efficiency and the thick SEI layer formation generally observed by using carbonate solvents can be solved by using an ether based electrolyte instead⁵.

The results of this study show that the first coulombic efficiency is as high as 90% with a specific capacity reaching 310 mAh/g after the first cycle due to a thin SEI layer created at the surface of the material. The carbonization temperature has a big impact on the electrochemical performance of the carbon fibers due to the structural changes occurring during carbonization. The stability of the electrolyte on the lignin carbon fibers has been investigated by cyclic voltammetry and high precision coulometry. The results display a coulombic efficiency of 99.9% after 15 cycles at C/10 in a half cell setup.

In conclusion, the results of the study so far show that lignin carbon fibers have a high potential for being used as an anode material in NIBs due to their high specific capacity, low cost as well as being a renewable material. The ether based electrolyte showed promising results in terms of first coulombic efficiency and thin SEI layer. Its reliability for long term cycling however, must be studied further.

1. G. Milczarek, O. Inganas, Science 2012, 335, 1468.

2. T. Saito, R. H. Brown, M. A. Hunt, D. L. Pickel, J. M. Pickel,J. M. Messman, F. S. Baker, M. Keller, A. K. Naskar, Green Chem. 2012.

3. R. K. Sharma, J. B. Wooten, V. L. Baliga, X. H. Lin, W. G. Chan, M. R. Hajaligol, Fuel 2004, 83, 1469.

4. Nowak, A. P., et al. (2017). "Lignin-based carbon fibers for renewable and multifunctional lithium-ion battery electrodes." Holzforschung 0(0).

5. Weadock, N., et al. (2013). "Determination of mechanical properties of the SEI in sodium ion batteries via colloidal probe microscopy." Nano Energy 2(5): 713-719.