Key Takeaways & Executive Findings
- •• • Triple-functionalization (dichlorination, sulfonation, N-doping) of PVC yields hard carbon with 25% carbon yield, avoiding toxic dioxin emissions, addressing both waste management and resource recovery. • • The engineered hard carbon exhibits a reversible capacity of 355 mAh g−1 at 0.1 A g−1, equivalent to 95.4% of graphite's Li-ion capacity, demonstrating commercial viability for sodium-ion anodes. • • Long-term cycling stability: 216 mAh g−1 retained after 1000 cycles at 1.0 A g−1 (70.1% capacity retention), indicating robust structural integrity for grid-scale storage. • • High-rate capability: 188 mAh g−1 at 5.0 A g−1, enabled by closed pores (~2.55 nm) and expanded interlayer spacing (0.382 nm), facilitating rapid Na+ transport.
Abstract
The global surge in polyvinyl chloride (PVC) waste demands urgent technological solutions that address both environmental persistence and resource recovery. Here, we present a triple-functionalization strategy that converts chlorinated plastic waste into high-performance sodium-ion battery anodes through molecular-level control of carbon architectures. Sequential dichlorination, sulfonation, and N-doping collaboratively reconfigure precursor reactivity, steering pyrolysis toward hierarchically porous hard carbon with tailored defect chemistry. Sulfonic groups stabilize 3D carbon skeletons during carbonization, enabling closed-pore formation with an average diameter of ~2.55 nm while N-doping expands interlayer spacing (0.382 nm) and creates adsorption-active pyrrolic-N sites. This defect-engineered synergy delivers unprecedented sodium storage metrics: 355 mAh g−1 reversible capacity at 0.1 A g−1 (95.4% of graphite’s Li-ion capacity), a capacity retention of 216 mAh g−1 after 1000 cycles at 1.0 A g−1 (70.1% capacity retention), and 188 mAh g−1 even at a high current density of 5.0 A g−1. Operando analyses reveal a potential-dependent storage hierarchy: surface-dominated adsorption transitions to intercalation/filling-dominated behavior with defect-buffered structural integrity. The process simultaneously achieves 25% carbon yield from PVC and avoids toxic dioxin emissions, establishing a scalable prototype for sustainable energy storage systems.
1. Introduction
Sodium-ion batteries (SIBs) have emerged as a cost-effective complement to lithium-ion technologies, leveraging Na's crustal abundance (2.83% vs. Li's 0.0065%) and redox similarity. However, the 34% larger ionic radius of Na+ (1.02 Å vs. Li+ 0.76 Å) critically limits ion accommodation in conventional graphite anodes, manifesting as inadequate capacities (31 mAh g−1 in ester electrolytes; <150 mAh g−1 even with ether optimization). This fundamental mismatch drives urgent demand for anode materials with tailored diffusion pathways and expanded host structures. Hard carbon (HC) stands as the most viable SIB anode candidate due to its pseudo-graphitic domains and adaptable interlayer spacing (0.37–0.42 nm vs. graphite's 0.34 nm). Current precursors face three key limitations: (1) biomass-derived carbons (coconut shells, wood) exhibit desirable porosity but suffer from seasonal variability and uncontrolled impurity profiles; (2) synthetic organics (e.g., phenolic resin at $1363.9 per tonne) deliver superior performance (~300 mAh g−1) yet prove economically prohibitive for scalable production; (3) petroleum-based precursors (coal tar, pitch) offer cost advantages but suffer progressive graphitization during pyrolysis, degrading capacity retention. These constraints necessitate precursor innovation balancing structural control, economic viability, and supply chain stability.
Polyvinyl chloride (PVC) presents a transformative opportunity as an SIB anode precursor, aligning global production scales with circular economy imperatives. With a large amount PVC waste generation currently landfilled or incinerated, its high chlorine (Cl) content (56.8 wt%) and catastrophic carbon loss during pyrolysis (>90% mass loss at 600°C) have historically precluded utilization. Recent breakthroughs in chemical pretreatment, particularly KOH-mediated dechlorination, enable chlorine removal while activating crosslinkable carbon sites. Hu et al. demonstrated that chlorine-depleted PVC chains undergo spontaneous structural reorganization during pyrolysis, yielding carbonaceous frameworks with promising sodium storage. However, these approaches lack molecular-level control over functional groups, resulting in suboptimal pore architecture and surface chemistry. The present work introduces a sequential dichlorination, sulfonation, and N-doping strategy that precisely engineers precursor reactivity, steering pyrolysis toward hierarchically porous hard carbon with tailored defect chemistry. This triple-functionalization not only stabilizes the carbon skeleton via sulfonic groups but also expands interlayer spacing and creates adsorption-active pyrrolic-N sites, directly addressing the bottlenecks of capacity, cycling stability, and rate performance in SIB anodes.
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ZHAO Qianqian, MENG Wei, LIU Haizhou, XIAO Shuhao, ZHU Kai, CAO Dianxue, ZHANG Ying (2026). Recycling polyvinyl chloride plastics into hard carbon: influence of functional groups on structural and electrochemical properties. SCIENCE CHINA Materials. https://doi.org/10.1007/s40843-025-3778-8
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Frequently Asked Questions
What is the carbon yield from PVC after the triple-functionalization process, and how does it compare to conventional pyrolysis?
The process achieves a 25% carbon yield from PVC, significantly higher than the >90% mass loss observed in conventional pyrolysis at 600°C. This improvement is attributed to the stabilizing effect of sulfonic groups, which prevent excessive volatilization and promote crosslinking, making the process economically viable for large-scale waste upcycling.
How does the interlayer spacing and closed-pore structure contribute to the high reversible capacity of 355 mAh g−1?
The N-doping expands the interlayer spacing to 0.382 nm, facilitating Na+ intercalation, while sulfonation-induced stabilization enables the formation of closed pores with an average diameter of ~2.55 nm, which act as reservoirs for Na+ adsorption and filling. This dual mechanism, confirmed by operando analyses, supports a capacity that is 95.4% of graphite's Li-ion capacity, demonstrating the effectiveness of the structural engineering.
What is the long-term cycling stability of the hard carbon anode, and what mechanisms ensure capacity retention?
The anode retains 216 mAh g−1 after 1000 cycles at 1.0 A g−1, corresponding to 70.1% capacity retention. This stability is attributed to the defect-buffered structural integrity, where pyrrolic-N sites and closed pores accommodate volume changes and prevent structural degradation, ensuring prolonged cycle life suitable for grid-scale energy storage.
How does the material perform at high current densities, and what makes it suitable for fast-charging applications?
At a high current density of 5.0 A g−1, the anode delivers 188 mAh g−1, indicating excellent rate capability. The hierarchically porous structure and expanded interlayer spacing facilitate rapid Na+ diffusion, while the surface-dominated adsorption at high rates ensures minimal diffusion limitations, making it promising for applications requiring fast charge/discharge.
What is the environmental impact of this PVC recycling process, particularly regarding dioxin emissions?
The process avoids toxic dioxin emissions, a critical advantage over incineration or uncontrolled pyrolysis of PVC. By employing sequential dechlorination and controlled carbonization, chlorine is removed in a safe manner, preventing the formation of dioxins. This aligns with circular economy principles and regulatory requirements, making the process environmentally sustainable.
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