Abstract
Hard carbon materials were prepared from different precursors (phenolic resin and commercially available cellulose and lignin) under different pyrolysis and processing conditions using industrially adapted syntheses protocols. The study of their microstructural features enabled to assess that the nature of the precursor and the temperature of pyrolysis are the major factors determining the carbon yield and the surface area, the latter one having a major effect on the electrochemical capacity. Finally, the presence of surface groups and physisorbed water can also play a role both on the maximum reversible capacity achievable (by influencing the interaction of sodium ions with the hard carbon surface) and the irreversible capacity. Phenolic resin combining high carbon yield (∼50%), tap density (0.7 g⋅cm−3) and reversible capacity (249 mAh/g) was found to be the precursor producing the most suitable hard carbon for practical use in Na-ion batteries. Cellulose can be a good candidate as well, the lower carbon yield being counterbalanced by its lower price and higher capacity (280 mAh/g).
Highlights
Sodium-ion batteries (SIBs) are based on a concept analogy with the ubiquitous lithiumion batteries (LIB) and have emerged in the last five years as a viable technical alternative holding promise for higher sustainability[1,2,3] and lower cost
The yield of pyrolysis was never 100%, which indicates partial carbon burn off. While this occurs to a rather small extent for phenolic resin and lignin the value is much larger for cellulose precursor is used. This can be related to the different structures of the precursors which in the case of phenolic resin and lignin are mainly composed of aromatic rings while in the case of cellulose are Dglucose (α-D-glucopyranose) units
Higher temperature pyrolysis results in hard carbons with lower values of BET specific surface area (Figure 2), which is in agreement with the expected closing of the microporosity at high temperature, but changes are much more significant in samples prepared from cellulose than in those prepared from the phenolic resin
Summary
Sodium-ion batteries (SIBs) are based on a concept analogy with the ubiquitous lithiumion batteries (LIB) and have emerged in the last five years as a viable technical alternative holding promise for higher sustainability[1,2,3] and lower cost. Pyrolysis was typically carried out using the as received precursors, in order to examine the possibility of improving the produced carbon properties further by reducing the surface area of the material at low carbonisation temperatures, compression of the precursor material before carbonisation was tested in some cases. Electrochemical tests were performed using a Bio-Logic VMP3 potentiostat in two-electrode Swagelok cells in galvanostatic mode with potential limitation (GCPL) between 2 and 0.03V vs Na+/Na at different rates, namely C/10, C/20, C/5, C and 2C (1C being one Na+ inserted in one hour), using sodium metal counterelectrodes. These enabled monitoring both the capacity evolution upon cycling and the rate capability, which is related to the power performances. The electrolyte used was 1M NaPF6 (STREM Chemicals 99%) dissolved in a mixture (0.45:0.45:0.1 in weight) of ethylene carbonate (EC, Aldrich anhydrous 99.0%), propylene carbonate (PC, Aldrich anhydrous 99.7%) and dimethyl carbonate (DMC, Aldrich anhydrous 99.0%).[24,25] The water content in all electrolytes was measured by Karl-fisher titration using a 899 Metrohm Karl-Fischer coulometer and found to be lower than 20 ppm in all cases
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