Key Takeaways & Executive Findings
- •• This review establishes a unified conceptual framework for closed pores in hard carbon anodes, clarifying their origin and structural characteristics. • It systematically correlates closed pore features with sodium storage behavior, proposing design principles for directional pore regulation. • Advanced modification strategies integrating molecular-level design and kinetic/thermodynamic analyses are highlighted for performance optimization. • The insights aim to accelerate commercialization of hard carbon anodes for high-energy-density sodium-ion batteries.
Abstract
Hard carbon (HC) is considered the most promising anode material for sodium-ion batteries (SIBs) due to its high cost-effectiveness and outstanding overall performance. However, the amorphous and intricate microstructure of HC poses significant challenges in elucidating the structure–performance relationship, which has led to persistent misinterpretations regarding the intrinsic characteristics of closed pores. An irrational construction methodology of closed pores inevitably results in diminished plateau capacity, which severely restricts the practical application of HC in high-energy-density scenarios. This review provides a systematic exposition of the conceptual framework and origination mechanisms of closed pores, offering critical insights into their structural characteristics and formation pathways. Subsequently, by correlating lattice parameters with defect configurations, the structure–performance relationships governing desolvation kinetics and sodium storage behavior are rigorously established. Furthermore, pioneering advancements in structural engineering are critically synthesized to establish fundamental design principles for the rational modulation of closed pores in HC. It is imperative to emphasize that adopting a molecular-level perspective, coupled with a synergistic kinetic/thermodynamic approach, is critical for understanding and controlling the transformation process from open pores to closed pores. These innovative perspectives are strategically designed to accelerate the commercialization of HC, thereby catalyzing the sustainable and high-efficiency development of SIBs.
1. Introduction
The development of efficient and cost-effective energy storage technologies is crucial for improving energy utilization and achieving sustainable development goals [1]. As global demand for renewable energy increases, the intermittency and instability of sources such as wind and solar energy present significant challenges. As a result, large-scale energy storage systems have become increasingly important, particularly battery energy storage systems [2–4]. Compared to lithium, sodium is much more abundant in the crustal content (Na: 2.36% vs. Li: 0.0065%) and has a lower production cost [5–9]. Therefore, sodium-ion batteries (SIBs) have become the preferred choice for meeting grid-scale energy storage requirements [10].
While SIBs offer advantages, including excellent low-temperature performance and high safety, challenges such as lower energy density, lower initial Coulombic efficiency (ICE), and poor rate capability continue to pose considerable barriers to further application [11]. Due to the large atomic radius of Na, there is a shortage of high-performance electrode materials for the anode, which remains a critical area in need of improvement and breakthroughs [12]. Among the reported anode materials for SIBs, organic-type materials are characterized by significant capacity decay, while conversion-type and alloy-type materials are hindered by substantial volume changes, both of which present clear limitations. Therefore, intercalation-type materials with stable cycling performance have garnered widespread attention. Among these, carbon-based materials are favored due to their higher theoretical capacities and greater abundance relative to titanium-based materials [13]. The sodium storage behavior across different carbon structures varies significantly. Na has limited intercalation ability in graphite, which features narrow spacing and an absence of defects. In contrast, Na easily intercalates into amorphous carbon, which possesses defects and expanded interlayer spacing, thereby forming stable compounds. Among these, hard carbon (HC) is considered to be the most promising and commercially viable anode materials, typically demonstrating a reversible sodium storage capacity of 300 mAh g−1 without modification [7, 14–16].
The complex charge–discharge profiles of HC can be divided into two distinct regions: the surface-controlled high-potential slope region (>0.1 V) with excellent reaction kinetics and the diffusion-controlled low-potential plateau region (<0.1 V) with relatively slower reaction kinetics [17]. The graphite layers, defects, and nanopores are considered key active sites in the microstructure, closely associated with electrochemical behavior.
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Siyang Gan, Yujie Huang, Ningyun Hong, Yinghao Zhang, Bo Xiong, Zhi Zheng, Zidong He, Shengrui Gao, Wentao Deng, Guoqiang Zou, Hongshuai Hou, Xiaobo Ji (2025). Comprehensive Understanding of Closed Pores in Hard Carbon Anode for High-Energy Sodium-Ion Batteries. Nano-Micro Letters. https://doi.org/10.1007/s40820-025-01833-x
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Frequently Asked Questions
What are closed pores in hard carbon anodes?
Closed pores are nanoscale voids within the hard carbon structure that are not accessible from the external surface. They play a crucial role in sodium-ion storage, particularly in the low-potential plateau region, by providing additional active sites for sodium ion insertion and desolvation.
Why are closed pores important for sodium-ion batteries?
Closed pores significantly enhance the plateau capacity of hard carbon anodes, which is essential for achieving high energy density in sodium-ion batteries. Properly engineered closed pores improve sodium storage capacity and cycling stability, making hard carbon a more viable anode material for commercial applications.
How can closed pores be controlled in hard carbon?
Closed pores can be controlled through precursor selection, pyrolysis temperature, and activation processes. The review suggests that a molecular-level understanding and a combined kinetic/thermodynamic approach are critical for directing the transformation from open to closed pores, enabling rational design of pore structures.
What are the challenges in studying closed pores?
The amorphous and complex microstructure of hard carbon makes it difficult to characterize closed pores accurately. Misinterpretations of their intrinsic characteristics have led to irrational construction methods, resulting in diminished plateau capacity. Advanced characterization techniques and theoretical modeling are needed to better understand and optimize closed pores.
What is the future direction for hard carbon anodes?
Future research should focus on integrating advanced modification strategies with molecular-level design and dynamic/thermodynamic hybrid analyses to optimize closed pore structures. This will accelerate the commercialization of hard carbon anodes for high-energy-density sodium-ion batteries, contributing to sustainable energy storage solutions.
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