Starting from the requirements aimed at separators targeting industrially relevant batteries (Li-ion and Li-metal), the cellulose-based separators were prepared at lab scale with the desired mesoporosity, tortuosity, and mechanical properties, through proper adjustment of the vol% of cellulose nanocrystals (CNCs) in nanofibrillated cellulose (NFCs) dispersion and drying conditions. During this stage, the incorporation of another natural-based polymer in the preparation of separators – lignin was also explored. Lignin-based separators can be engineered to control porosity, allowing for efficient ion transport while still maintaining their role as a physical barrier between electrodes. Additionally, lignin can be chemically modified to improve its compatibility with various electrolytes used in lithium and post-lithium batteries. The key findings are summarized below:
Regarding cellulosic separators, plating/stripping in symmetric Li/Li cells and assembled Li/LFP coin cells was done, after checking different swelling times in a standard liquid electrolyte. When immersed in the electrolyte, the membranes showed a mass increase, which technically demonstrates the electrolyte uptake. However, it was noted that when we pushed the swelling up to 48 h, the membrane became very rigid, which was not an expected behaviour since usually the swelling enlarges the membrane and improves its flexibility/manageability.
Theoretically, a separator does not necessarily have to swell, but it must entrap the liquid electrolyte inside: porosity and tortuosity must be engineered. The membrane increased in weight, but the liquid remained essentially on the surface (not inside). As a consequence, the electrochemical tests were not fully successful. In the plating/stripping test, a high resistance was detected (due to non-optimal ionic conduction within the membrane). This occurred for both 100% cellulose-based separator membrane types: fully NFC and NFC with CNC. Regarding the cycling test, it did not complete any cycles since the resistance was again too high.
To circumvent this, mixtures of cellulose fibres with lignin extracted from softwood and hardwood were introduced. The electrochemical performance of the separator/electrolyte was evaluated, revealing that PVA/lignin-containing membranes achieved an ionic conductivity of approximately 1.2 × 10^−3 S/cm. Including lignin enhanced electrolyte uptake and retention, and allowed for better porosity control. Impedance measurements were also performed at different DC potentials to evaluate the intercalation behaviour, showing more diffusive behaviour.
