Biomass-based hydrochars for hard-carbon for Na-ion batteries

Résumé

Li-ion batteries are the most suitable for energy storage technology, which makes them the most used in electronics and mobility. Yet, lithium availability on Earth is very limited in quantity and place. Sodium is abundant and presents a similar reactivity to lithium. Despite being similar in many points, their reactivity are still different: unlike lithium, the insertion of sodium ions between the graphite layers at the negative electrode is limited. Hard-carbon is used instead, however its price is three times higher and it is usually petro-based. To solve this problematics, it can be synthetized from waste biomass after high-temperature pyrolysis. Many studies report pyrolysis of dry matter, but few deal with wet biomass, which is an underestimate carbon resource because of its high moisture content (>60%). Moreover, to obtain a hard carbon with interesting performances, a low ash content is required, which limit even more the choice of the feedstock. It limits the suitable materials.In order to open the possibility of biomass as the same time as reducing the cost and environmental impact, hydrothermal carbonisation (HTC) is performed before the pyrolysis. This thermal treatment takes place in a sealed vessel, in which the sample is directly immerged in water and hence doesn’t require any drying step. With the temperature, the pressure autogenously increases: the water reach the subcritical state and directly react with the feedstock to provide a carbonaceous material called hydrochar. In addition, during the process the inorganics in the biomass leach in the liquid phase, preventing graphitisation and limiting the surface area development during the pyrolysis step, key parameter for hard carbon electrodes.The objective of this thesis is to characterise the influence of the HTC pre-treatment on the resulting hard carbon, by investigated the reactions occurring depending on the operation conditions. A complete investigation on both the composition and structure of each starting, intermediate and final materials are performed, to conclude on the influence of each element on the resulting hard carbon. At last, both composition and structure are linked to the battery performance, offering a screening of the most suitable biomass as well of the best conditions to provide promising hard carbon for Na-ion batteries.The HTC temperature was highlighted to impact greatly the reactions occurring, both regarding the biomass degradation and secondary reaction between the hydrochar and the liquid, particularly regarding the inorganics. Each behave differently: while alkali mainly leach and heavy metals remain no matter the conditions, Ca, Mg, P and Si behave differently depending on the chemical form they are in the biomass. Furthermore, even if they are first solubilised, the increase of the temperature favour their reprecipitation, leading them back in the hydrochar.The volatilisation of these elements are limited during the high temperature pyrolysis, their quantity being mostly removed after the pre-treatment. Particularly K, Ca and Si have been linked to the resulting hard carbon properties. K and Ca, in addition to most of the heavy metal traces, tend to catalyse graphitisation. Moreover, K seems to favour its porosity. At last, all the hydrochars containing important Si contents, SiC whiskers have been observed in the resulting material, increasing the surface area in contact with the electrolyte and decreasing the cell performances.

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