Periodic Reporting for period 1 - EXCELL (CELLulose nanocomposite separators for the nEXt generation of smart batteries)
Berichtszeitraum: 2023-06-01 bis 2025-02-28
As the popularity of electric vehicles continues to increase, so does the number of batteries reaching the end-of-life that are used to power them, with it expected that by 2030 it will reach 2 million tons worldwide. On top of this, the complexity of battery production results in very high scrap rates (about 10%-30%), especially during production ramp-up, while the scarcity of raw materials in Europe is intensifying EU regulations to localise supply chains and safeguard critical raw materials.
It is evident that there is a strong need to increase sustainability in the battery value chain, and contributions may come from improving both their lifetime and recyclability of the cell components. In EXCELL, it is proposed to prove a new concept for battery separators based on a 100% natural cellulose nanocomposite with tuneable mesopores obtained by a mixture of nanofibers and cellulose nanocrystals. Additionally, these new separators will be suitable for incorporating sensing elements to enable the new generation of smart battery cells. EXCELL will follow the outputs of NEWFUN-Stg, where it was demonstrated that cellulose-based ionic conductive materials can be recycled and reused while maintaining the electrochemical performance. The PI’s team has also shown that cellulose nanocrystals are able to create mesoporous ionic conductive channels that can be tuned to specific alkali ions, but proper functionalization of the crystals’ surface. EXCELL will now demonstrate the synergic effect of combining both to form hierarchical mesoporous membranes exhibiting a unique set of characteristics that can meet those ones expected for an “ideal” separator.
EXCELL will follow an approach of validating the new separator concept and then implement an IPR consolidation and a business case to attract the attention of battery market stakeholders on new opportunities for cell components based on abundant natural resources that are recyclable/biodegradable.
It is evident that there is a strong need to increase sustainability in the battery value chain, and contributions may come from improving both their lifetime and recyclability of the cell components. In EXCELL, it is proposed to prove a new concept for battery separators based on a 100% natural cellulose nanocomposite with tuneable mesopores obtained by a mixture of nanofibers and cellulose nanocrystals. Additionally, these new separators will be suitable for incorporating sensing elements to enable the new generation of smart battery cells. EXCELL will follow the outputs of NEWFUN-Stg, where it was demonstrated that cellulose-based ionic conductive materials can be recycled and reused while maintaining the electrochemical performance. The PI’s team has also shown that cellulose nanocrystals are able to create mesoporous ionic conductive channels that can be tuned to specific alkali ions, but proper functionalization of the crystals’ surface. EXCELL will now demonstrate the synergic effect of combining both to form hierarchical mesoporous membranes exhibiting a unique set of characteristics that can meet those ones expected for an “ideal” separator.
EXCELL will follow an approach of validating the new separator concept and then implement an IPR consolidation and a business case to attract the attention of battery market stakeholders on new opportunities for cell components based on abundant natural resources that are recyclable/biodegradable.
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.
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.
An innovative feature aimed to be demonstrated during the implementation of ECXCEL was the possibility of the separator membranes being compatible with the integration of sensors to monitor battery’s state of health. Two designs were produced and tested: loop antennas and LC sensors deposited using copper metal lines laminated on top of the cellulose separators. Membranes with sensors were used to assemble pouch cells with LFC and graphite electrodes, as seen in Figure 5. The main goal was to assess the compatibility with traditional battery assembly and the behaviour of the sensors in contact with the other materials existing inside a cell, including the liquid electrolytes used in conventional LFP pouch cells. The main conclusion was that the feasibility of developing separator membranes with embedded sensors was successfully demonstrated, and that those sensors could monitor the temperature inside a cell for one week. A post-mortem analysis demonstrated that the sensor structure was partially damaged, indicating that strategies of sensor passivation need to be implemented before moving to the next step of taking charge/discharge cycles with cells where sensors are inside.