Pepper stalk hard carbon anodes with temperature-tailored closed pores for high-performance sodium-ion batteries

Abstract

Biomass-based hard carbon is considered a highly promising anode for sodium-ion batteries. Nevertheless, its practical deployment is often impeded by excessive specific surface area and an abundance of structural defects, which inevitably lead to limited initial coulombic efficiency and unsatisfactory sodium storage capacity. Herein, we report a pepper stalk-derived hard carbon engineered via temperature-mediated closed-pore tuning, delivering a reversible capacity of 302.3 mAh·g−1 (initial coulombic efficiency of 86.7%) and remarkable cycling stability (87.6% capacity retention after 300 cycles). Systematic characterization reveals that carbonization at 1600°C optimally develops 3.48 nm closed pores and enhances graphite domains, achieving a high plateau capacity of 191.7 mAh·g−1, which constitutes 63.4% of the total capacity attributed to efficient Na+ ion filling. A full cell paired with a Na3V2(PO4)3 cathode achieves an energy density of 271.0 Wh·kg−1 based on the total mass of the active materials. This research provides a viable and industrial-scale methodology for pore-structure engineering, overcoming major hurdles to the market adoption of biomass-derived carbon materials.

1. Introduction

As an appealing alternative to lithium-ion batteries, sodium-ion batteries (SIBs) have attracted significant attention for large-scale energy storage, particularly in grid systems and electric mobility, due to their cost-effectiveness and abundant resources [1]. A pivotal bottleneck in advancing SIBs is the scarcity of high-performance anode materials, driving the quest for superior anodes as a critical research frontier [2]. Hard carbon (HC), as a preferred anode material for SIBs, is characterized by its distinctive “pseudo-graphite” layered structure and open nanopore architecture, and exhibits high theoretical capacity, a low operating voltage, and cost-effectiveness [3–4]. Nevertheless, the high density of defects adversely affects key performance metrics, notably resulting in low initial coulombic efficiency (ICE) and subpar cycling performance [5–6]. Consequently, the rational design of HC structures to optimize their performance is critical for advancing sodium-ion battery technology.

So far, HC derived from biomass has emerged as a cost-effective and environmentally sustainable synthesis strategy [7–8]. Currently, sources of plant-based HC materials can be primarily categorized into three types: fruit shells, sugar, and stalks and leaves [3,9]. The inherent layered porous architecture of the fruit shell endows the derived HC with superior long-term cycling performance, such as coconut shell, walnut shell, and peanut shell [10–13]. Sugar precursors, such as glucose and starch, possess stable compositions. After carbonization, they exhibit a relatively uniform morphology and a well-developed closed-pore structure, which contribute to their relatively high ICE and high reversible specific capacity. Nevertheless, their homogeneous composition hinders the formation of a hierarchical porous structure, resulting in suboptimal rate performance [14–16]. The stalk- and leaf-based materials develop a hierarchical porous structure following carbonization, owing to their abundant nutrient transport channels. This structure facilitates electrolyte penetration and sodium-ion transport while delivering a relatively high specific capacity of 300–420 mAh·g−1 [7,17–22]. However, the layered porous structure of these materials brings several critical challenges, including compromised ion transport kinetics caused by defective surfaces, and limited ICE. The combination of these fundamental shortcomings ultimately impedes the real-world implementation of HC anodes in full cells.

In this work, we propose using pepper stalks as a high-quality biomass precursor. As a widely cultivated crop, peppers yield large quantities of stalks as the main agricultural residue. Their management commonly entails discarding or burning, which inefficiently wastes biomass and thereby contributes to environmental pollution through the emission of harmful gases [23]. Such a practice facilitates the value-added repurposing of agricultural residues while mitigating the environmental impacts associated with traditional waste-disposal methods. Temperature-mediated closed-pore engineering enabled the successful synthesis of HC from pepper stalk waste. By leveraging the unique structure of pepper stalks, HC was synthesized, and its electrochemical properties were tailored by adjusting the pyrolysis temperature from 1300 to 1600°C. By systematically examining the interactions among closed pores, graphitization, and electrochemical kinetics, we demonstrate a scalable approach for op