Silicon waste from discarded solar cells is transformed into durable battery anodes through interface engineering, enabling high capacity and long cycle life using scalable, low-cost processes.
(Nanowerk Spotlight) Every time a lithium-ion battery charges, its electrode materials expand and contract. Inside the anode, where lithium ions are stored during charging, these shifts can be violent. In silicon, a material known for its exceptional ability to store charge, the expansion can exceed 300 percent. This internal swelling fractures particles, destabilizes interfaces, and eventually breaks down the battery itself.
Silicon promises vastly higher energy capacity than the graphite used in most commercial batteries, but its mechanical instability has kept it from delivering on that promise. Even small-scale prototypes struggle to survive more than a few hundred charge cycles.
The underlying problem is not just chemical but structural. Researchers have tried shrinking silicon to nanoscale dimensions, encasing it in carbon shells, and engineering porous or layered structures to absorb the stress. Many of these ideas have worked in principle. But translating them into industrial processes that are safe, consistent, and affordable has proven far more difficult. Most designs either require complex fabrication steps or introduce materials that are too expensive or unstable for real-world use.
Meanwhile, another kind of silicon is being wasted at industrial scale. During the manufacture of photovoltaic cells, silicon ingots are sliced into wafers using precision wire saws. This process produces enormous volumes of ultrathin silicon fragments, plate-like particles with high purity and nanoscale thickness.
Despite their structural complexity and material value, these fragments are treated as scrap. Recycling them typically involves melting and refining steps that consume significant energy and strip away their useful structural features.
Schematic of the fabrication process for the Si–TiO₂–CNT–C composite. Silicon fragments from photovoltaic wafer cutting are first combined with titanium dioxide through sand milling to form strong interfacial contact. Carbon nanotubes and a polymer precursor are then added, followed by spray drying and thermal treatment to produce dense, spherical particles with a conductive carbon coating. Also shown are the morphology of the waste silicon and the final composite, along with a demonstration of production at kilogram scale. (Image: Reprinted with permission by Wiley-VCH Verlag) (click on image to enlarge)
Their approach centers on controlling the interfaces between silicon and two key additives—titanium dioxide and carbon nanotubes. These additions stabilize the material, reduce stress concentrations, and create a continuous path for electrons to travel through the electrode. The result is a composite structure that maintains high capacity over thousands of cycles without the structural failure typical of silicon anodes.
The process starts with purified silicon fragments sourced from photovoltaic waste. These fragments are mechanically mixed with titanium dioxide using a sand milling technique that promotes strong contact at the interface between the two materials. Titanium dioxide acts as a mechanical buffer, helping distribute internal stress during battery operation. It also forms an electrically conductive compound when it reacts with lithium, which enhances the transport of charge through the electrode.
To improve performance further, the silicon–titanium dioxide particles are embedded in a conductive network of carbon nanotubes and a carbon shell. The carbon nanotubes form a flexible but interconnected framework that holds the material together and ensures that electrons can move efficiently across the structure. The outer carbon coating, produced from a polymer precursor, acts as a protective layer that prevents unwanted reactions between the silicon and the liquid electrolyte.
The final product, referred to as Si–TiO₂–CNT–C in the paper, consists of dense, spherical particles with a high packing density. This trait matters because it allows more active material to fit in a given volume, increasing the energy that can be stored.
The composite reached a discharge capacity of about 1100 milliamp-hours per gram after 2500 cycles at a current of 1 amp per gram. Even at high charging rates, where the material is under more strain, it retained more than 1000 milliamp-hours per gram. Commercial graphite typically reaches around 350.
The researchers also built a pouch cell using the new composite as the anode and a widely used commercial cathode made from nickel, cobalt, and manganese. The full battery achieved an energy density above 320 watt-hours per kilogram and maintained over 500 milliamp-hours of capacity after 500 cycles. That level of stability and performance, measured in a practical cell format, suggests that the material could be viable in commercial settings.
What makes this result particularly compelling is how the design prevents the material from falling apart under repeated use. When lithium enters and leaves silicon, it causes the particles to expand and contract significantly. In conventional designs, this expansion breaks apart the electrode, severs electrical contact, and damages the surface. In the new structure, the titanium dioxide and carbon nanotubes absorb and redistribute the stress.
Simulations show that the silicon–titanium dioxide composite reduces expansion by more than 30 percent compared to pure silicon. Microscopy images taken after 2000 charge cycles show that the particles remain intact and free of cracks.
Another key feature is the formation of a stable surface layer known as the solid electrolyte interphase. In most silicon-based anodes, this layer forms unevenly and breaks apart during expansion, consuming electrolyte and leading to rapid capacity loss. In the composite structure, the surface remains uniform and intact, with a higher proportion of lithium fluoride. This compound improves ion movement across the interface and prevents the build-up of unwanted reaction products.
The team also evaluated the material’s mechanical resilience under compression, which matters for manufacturing battery cells that are rolled and pressed. The particles remained stable under pressures up to 150 megapascals. Even at higher pressures, they resisted fragmentation, confirming the robustness of the structure.
The entire fabrication process was designed for scale. It uses commercially available methods such as sand milling, spray drying, and pyrolysis. These steps require less energy than traditional remelting, and they preserve the unique structure of the silicon waste rather than erasing it. A basic cost estimate showed that the composite could be produced for about 12 dollars per kilogram.
Considering its energy storage capacity, the cost per unit of performance compares favorably with graphite, even though the raw material cost is higher. The method also avoids approximately 12 kilograms of carbon dioxide emissions per kilogram of material produced, compared to conventional silicon recycling.
To test practical feasibility, the team blended the silicon composite with graphite and built a pouch cell with a commercial cathode. The device maintained 75 percent of its capacity after 500 cycles and delivered consistent performance across a range of charging speeds. It also demonstrated low internal resistance, suggesting strong compatibility between the new anode and other components of the battery.
By reusing silicon fragments that are otherwise discarded, this method turns waste into infrastructure. It shows that performance limits tied to material instability can be overcome not by redesigning the chemistry from scratch, but by controlling how materials interact at their boundaries.
The work provides a model for how structural engineering at small scales can be used to stabilize complex materials and extend their usefulness in demanding applications. It also presents a path toward more sustainable battery production, not through marginal gains but by rethinking how materials already in circulation can be redirected into systems where they are structurally and economically valuable.
For authors and communications departmentsclick to open
Lay summary
Prefilled posts
ORCID information
Fengshuo Xi (Kunming University of Science and Technology)
, https:0000-0003-219 corresponding author
Nanowerk Newsletter
Get our Nanotechnology Spotlight updates to your inbox!
Thank you!
You have successfully joined our subscriber list.
Become a Spotlight guest author! Join our large and growing group of guest contributors. Have you just published a scientific paper or have other exciting developments to share with the nanotechnology community? Here is how to publish on nanowerk.com.
