NTU and A*STAR Breakthrough: Nucleophilic Fluorination Enables Self-Terminating SEI on TiO2 Anodes, Boosting Efficiency Over 90%

In a significant advancement for lithium-ion battery technology, researchers from Nanyang Technological University (NTU) Singapore and the Agency for Science, Technology and Research (A*STAR)'s Institute of Materials Research and Engineering (IMRE) have developed a novel nucleophilic fluorination method for titanium dioxide (TiO2) anodes. This breakthrough enables self-termination of solid electrolyte interphase (SEI) formation, achieving an initial Coulombic efficiency (ICE) exceeding 90%—a marked improvement over the typical 74% for unmodified TiO2. Published in Advanced Materials, the innovation addresses a key bottleneck in battery performance, paving the way for safer, more efficient energy storage solutions crucial for electric vehicles (EVs) and renewable energy grids.

This collaboration exemplifies Singapore's thriving higher education research ecosystem, where NTU's materials science expertise merges with A*STAR's applied engineering prowess to tackle global energy challenges. As Singapore pushes towards its 2050 net-zero emissions target, such innovations from local universities position the nation as a hub for sustainable battery technologies.

The Critical Role of SEI in Lithium-Ion Batteries

Lithium-ion batteries (LIBs) power everything from smartphones to EVs, relying on the movement of lithium ions between a cathode and anode through an electrolyte. During the first charge-discharge cycle, the electrolyte decomposes at the anode surface, forming a passivating layer called the solid electrolyte interphase (SEI). This SEI is essential—it prevents further electrolyte breakdown while allowing lithium ions to pass—but its formation consumes lithium irreversibly, reducing the battery's initial capacity.

In graphite anodes, the industry standard, SEI stabilizes quickly, yielding high ICE around 90%. However, alternative anodes like TiO2 struggle with 'non-terminating' SEI growth. Parasitic reactions, particularly between phosphorus pentafluoride (PF5)—a byproduct of electrolyte salt decomposition—and surface hydroxyl (–OH) groups on TiO2, generate hydrofluoric acid (HF). This corrodes the SEI, triggering continuous reformation and lithium loss, capping ICE at subpar levels.

Why TiO2 Anodes? Advantages and Persistent Challenges

Titanium dioxide (TiO2), particularly in its anatase form, stands out as a promising anode material due to several inherent benefits:

• Safety: High lithium insertion voltage (~1.7 V vs. Li/Li+) avoids lithium plating and dendrite formation, reducing fire risks.
• Structural stability: Minimal volume change (<4%) during cycling, enabling 10,000+ cycles.
• Abundance and cost: TiO2 is cheap, non-toxic, and earth-abundant, ideal for scalable production.
• Fast charging: Supports ultrafast rates, suitable for EVs needing quick top-ups.

Despite these strengths, low electronic conductivity and poor ICE from endless SEI evolution have sidelined TiO2. Traditional fixes like nanostructuring or electrolyte additives improve conductivity but exacerbate SEI issues, as they promote more decomposition.

The Nucleophilic Fluorination Innovation: A Step-by-Step Breakthrough

The NTU-A*STAR team shifted focus from SEI composition to its termination timing. Their nucleophilic fluorination pre-treats TiO2 surfaces, replacing reactive –OH groups with stable fluorine atoms. Here's how it works:

1. Surface activation: TiO2 particles are exposed to a fluorinating agent, enabling nucleophilic attack where fluoride ions displace hydroxyls.
2. Suppression of parasites: Fluorinated surfaces block PF5 adsorption and HF generation, halting SEI degradation.
3. Early stabilization: A thin, LiF-rich inner SEI forms quickly and self-terminates, preserving capacity.
4. Verification: Advanced techniques like STEM, XPS, ToF-SIMS, and DFT simulations confirmed reduced organic SEI outer layers while retaining protective inner structure.

This elegant chemistry ensures the SEI 'knows when to quit,' as lead researcher Xian Jun Loh puts it.

Impressive Results: Over 90% Efficiency and Beyond

Lab tests showcased transformative performance:

• ICE: 92.1% for fluorinated TiO2 vs. 74.1% pristine—a 24% leap.
• Cycling: Retained high capacity over hundreds of cycles at various rates.
• Pouch cells: Practical validation with stable operation at 0.2 mA/cm².
• No trade-offs: Rate capability and long-term stability intact.

Even with fluoroethylene carbonate (FEC) additive—meant to bolster SEI—the method outperformed, underscoring its robustness.

The Research Powerhouse: NTU and A*STAR Teams

Spearheaded by Prof. Xiaodong Chen (NTU School of Materials Science and Engineering, iFLEX-Max Planck-NTU Lab), Prof. Huarong Xia (NTU), and A*STAR's Xian Jun Loh (Executive Director, IMRE), Qiang Zhu, and Shengkai Cao (NTU alum). Song Yuan and Wei Zhang (NTU PhDs) contributed key experiments. This interdisciplinary effort leverages NTU's flexible electronics expertise and IMRE's materials characterization prowess. Detailed in A*STAR's highlight, their work reflects deep university-industry synergy.

NTU's Leadership in Energy Materials Research

NTU's Energy Research Institute @ NTU (ERI@N) drives battery innovation, from upcycled silicon anodes from solar panels to waste paper electrodes enduring 1,200 cycles. The iFLEX lab under Prof. Chen pioneers smart materials for energy storage. Collaborations like Singapore Battery Pack Programme (SGBP2) unite NTU, A*STAR, NUS, and SUTD for next-gen packs.

Singapore's Higher Education Battery Ecosystem

Singapore universities lead Asia in battery R&D. NUS develops battery-free RF harvesters; SUTD advances solid-state tech. National initiatives like A*STAR's Battery Materials Research and ERI@N's hydrogen-fuel cells programs align with the Green Plan 2030, fostering PhD training and startups. This TiO2 work boosts Singapore's EV ambitions, targeting 100% EV sales by 2040.

Industry Impacts and Global Relevance

High-ICE TiO2 anodes enable compact, fast-charging batteries for EVs and grids, cutting costs and extending life. Applicable to aqueous LIBs for stationary storage, it supports Singapore's renewable integration. Globally, it challenges graphite dominance amid supply shortages.

Challenges Ahead and Future Outlook

Scaling fluorination industrially and integrating with high-capacity cathodes remain hurdles. The team eyes aqueous systems and commercialization via SGBP2. As Prof. Loh notes, "Controlling SEI termination is key to unlocking anode potential." Singapore's higher ed will drive pilots, training talent for a battery-powered future.

Careers in Singapore's Battery Research Frontier

NTU and A*STAR seek materials scientists, electrochemists for PhDs/postdocs. With global demand surging, Singapore offers competitive salaries, funding, and networks—ideal for advancing clean energy careers.

Photo by Jakob Rosen on Unsplash