Silicon is a leading candidate for next-generation Lithium-Ion Batteries (LIBs) anodes due to its high theoretical capacity low lithiation voltage and natural abundance. These advantages offer a disruptive leap in energy density compared to conventional graphite, enabling lighter reduced volume batteries while increasing driving range. However, unlocking this potential requires overcoming major engineering challenges such as low electrical conductivity and silicon structural stability.

Technical comparison: silicon vs. graphite

The strategic advantage of silicon lies in its superior lithium storage capability, though it introduces extreme mechanical stress that compromises electrode integrity and cycle life.

Structural Challenges: Overcoming the “Expansion Challenge”

The volume expansion and electrode stablity remain the primary hurdle for commercial adoption, which induces severe mechanical and electrochemical degradation leading to:

• Particle Pulverization: Repeated swelling and shrinking fracture silicon particles causing structural integrity.
• Electrode degradation: Massive volume changes result in a loss of electrical contact between the active material, conductive network and current collector.
• Instable SEI Layer formation: Continuous cracking and surface reconstruction prevent the formation of a stable Solid-Electrolyte Interphase (SEI), leading to rapid electrolyte depletion, irreversible lithium consumption and causing rapid capacity fade.

Addressing these limitations required an integrated approach to silicon-based materials design and structuring. This fundamental advance was realized through synergistic material innovations pioneered by project partners, particularly the University of Liège (GREEnMat).

From EoL Silicon PV Panels to LIBs Anodes: The RESiLEX Silicon upcycling process

The RESiLEX project embraces “Urban Mining” by treating end-of-life (EoL) PV panels as a high-value secondary source of strategic raw materials. Recovering this material from the millions of panels reaching EoL status represents a major opportunity to strengthen European industry. Value chain and reduce dependence on primary raw materials.

Using secondary silicon recovered from EoL PV panels by Groupe Comet, and purified by GEMME Laboratory, the GREEnMat Laboratory developed a multi-step process to convert EoL PV silicon waste into battery-grade silicon anode material powder (Silicon-carbon (Si/C) composite). A scalable processing route combining milling, spray drying, and controlled pyrolysis:

1. Nanosizing via Wet Milling: The received Silicon material undergoes dry milling followed by wet milling in an organic solvent to produce nanosized particles.
2. Carbon Matrix secondary particle Stabilization: The nano-silicon is integrated into a matrix of carbon nanotubes (CNTs) and polymer-derived carbon, providing electrical connectivity and buffering the silicon 280% volume expansion.
3. Pilot-Scale Production: The engineered Si/C composite (GREEnSiBat) is produced at pilot scale using spray drying, followed by controlled pyrolysis to obtain battery-grade anode material with optimized morphology and particle size with an industrial yield of up to 80%.

By having a look at the performance metrics at the lab scale, the GREEnSiBat composite has demonstrated extraordinary stability. In half-cell configurations, the material delivered a specific capacity above 1000 mAh/g over 1200 cycles  and remaining electrochemically active beyond 1900 cycles. This demonstrates the technical feasibility of using recycled silicon that can meet the rigorous demands of next-generation energy storage.

Validating the breakthrough: Multi-layer pouch cell results

To bridge the industrial “valley of death,” RESiLEX project transitioned from laboratory-scale coin cells to Multi-Layer Pouch (MLP) cells, validating performance at the 2 Ah scale MLPs is a critical milestone for proving commercial feasibility for practical applications.

A pilot-scale MLPs manufacturing is established by CEA at its facility to produce MLP5050 cells. The resulting cells comprise 17 anodes / 16 cathodes stacked, exhibited high reproducibility, consistent quality and robust manufacturing under a closed-industrial relevant conditions.

Optimizing the electrochemical window: the validation phase highlighted the importance of managing the Depth of Discharge (DoD). Silicon is most volatile in lower voltage regions, necessitating an operational trade-off to ensure commercial longevity.

• The 3.5V cutoff solution: By raising the lower voltage cutoff from 3.3V to 3.5V, partners at CSEM and CEA successfully doubled the cycle life, reaching 120 cycles at 80% capacity retention.
• Maximizing stability: Further optimization to 55% DoD (at a 3.4V cutoff) allowed the cells to reach 400 cycles with 74% capacity retention.

While this represents a trade-off in total usable capacity, it is a crucial strategic step toward ensuring that silicon-based batteries can survive the multi-year lifecycles required by end-users.

Future horizons: scaling, policy, and the battery passport

Moving from kilogram-scale validation to tonne-scale production requires more than technological success; it demands a supportive socio-economic ecosystem and robust regulatory framework. During the final policy workshop organized by RESiLEX in Brussels, the experts and stakeholders identified key challenges and opportunities shaping the next phase of European clean-tech scale-up.

Policy recommendations

1. Targeted CapEx funding: European institutions should bridge the investment gap with dedicated funding, combining grants and blended finance to de-risk high-capital-intensive industrial deployments.
2. Global regulatory alignment: As emphasized by the Global Battery Alliance, the industry is shifting from voluntary ESG commitments to legally binding international market expectations. Europe should actively align with global standards to avoid regulatory fragmentation and compliance risks.
3. The battery passport for Material Traceability: This digital certification tool is essential for tracking material origin and environmental footprints, ensuring that the secondary silicon utilized in RESiLEX remains a verifiable asset in the global market.
4. Designing flexible recycling infrastructure for future chemistries: With battery chemistries evolving rapidly, European recycling infrastructure must remain flexible and adaptative. Flexible and modular processing technologies will be essential to minimize the risk of technological obsolescence and ensure long-term industrial resilience.

Future technical roadmap

• Morphology tuning: Tailoring particle surfaces to reduce initial irreversible capacity loss and improve cycling stability.
• Electrolyte and SEI engineering: Developing advanced electrolyte formulations to promote a stable SEI, improving silicon protection during cycling.
• Cell integration: Stack pressure is key, and optimal pressure should ensure long capacity retention and prevent lithium plating. Extra components, such as foam, should also be considered, as well as other compression modes, such as constant thickness.
• Industrial Scale-Up: Scaling Si/C composite to mass manufacturing, ensuring batch-to-batch consistency despite variability in recovered silicon feedstock and optimizing electrode formulations for high loading and fast charging.

A Blueprint for European Material Sovereignty

The RESiLEX project has demonstrated the technical feasibility of scaling up conversion of EoL PV low-value silicon into LIBs anode materials. By achieving 250 Wh/kg energy density in MLP cells, RESiLEX provides a compelling proof of concept for cross-sector circularity between the PV and battery fields.

Through the “European Matrix” collaboration between Groupe Comet, GREEnMat, CEA, and CSEM, the RESiLEX project has created a scalable blueprint for a European circular silicon value chain. This project has contributed to creating a cornerstone of the EU’s strategy to secure its clean-tech future, proving that the waste of today’s solar panels is indeed the material source of tomorrow’s batteries.

Read more

RESiLEX Factsheet #15 – From Solar to batteries – recycled silicon for energy storage

High–performance recycled silicon–based Li–ion battery anodes enabled by Si/C composites engineering and graphene oxide protective layer

Preparation-driven electrochemical performance of recycled-silicon-based Si/C:SnS2 hybrid anodes for lithium-ion batteries 

Batteries from waste materials? Opportunities and challenges in the EU