还原氧化石墨烯改性少层剥离石墨增强石墨基钾离子电池负极稳定性

钾在石墨中嵌入电位较低，因此石墨负极可使钾离子电池具有较高的能量密度，是一种理想的钾离子电池负极材料。然而，石墨嵌钾后的体积膨胀率高达60%，导致钾离子电池的循环稳定性较差。此外，钾嵌入石墨层间的动力学过程缓慢，制约了钾离子电池倍率性能的提升。在本工作中，我们用还原氧化石墨烯(rGO)包覆剥离石墨(EG)，得到一种具有协同效应的层状复合材料。一方面，以少层的EG代替石墨可以减少由于钾的嵌入/脱嵌所引起的体积膨胀和内部应力；另一方面，外层rGO可以避免EG的堆叠，这有利于加速动力学过程并在钾化/去钾化过程中稳定结构。当复合材料所用EG和GO的质量比为1 : 1时，其性能达到最优，在50 mA·g^-1的电流密度下能够提供443 mAh·g^-1的比容量；在电流密度为800 mA·g^-1时，比容量为190 mAh·g^-1，保持率为42.9%。相同测试条件下，纯EG和rGO的容量保持率仅为14.2%和27.2%。测试结果说明EG-1/rGO-1复合材料在比容量和倍率性能两个方面得到了提升。

Reduced Graphene Oxide Modified Few-Layer Exfoliated Graphite to Enhance the Stability of the Negative Electrode of a Graphite-Based Potassium Ion Battery

The intercalation of potassium in graphite provides high energy density owing to the low potential of 0.24 V vs. K/K⁺, thereby making it a promising anode material for potassium ion batteries. However, the high volume expansion (60%) of graphite after potassium intercalation induces significant stress and electrode pulverization. Additionally, the sluggish kinetics of potassium insertion undermine the rate capability of electrodes. Using few-layer exfoliated graphite (EG) as a negative electrode material effectively relieves expansion-induced stress. Unfortunately, the close stacking of ultra-thin two-dimensional EG impedes ion transport. Furthermore, EG with smooth surfaces lacks active sites to adsorb K⁺, which is unfavorable for intercalation reactions. To address these problems, in this study, we designed an rGO/EG/rGO sandwich that coats EG with reduced graphene oxide (rGO). This complex material has two main advantages: (1) its 3D network can effectively prevent EG from stacking and buffer the volumetric variation of EG to improve the cyclic stability of the electrode, and (2) the loose structure and rich functional groups of rGO can also enhance the kinetic of potassium intercalation. Through hydrothermal reduction, GO was coated onto the EG surface and cross-linked to form a 3D network, by which EG stacking could be effectively mitigated. The rGO : EG ratio was precisely controlled by modulating the amount of reactant GO and EG. Transmission electron microscopy and scanning electron microscopy images showed that the rGO was uniformly coated on the EG surface to form a sandwich structure. X-ray diffraction patterns and Raman spectra demonstrated that rGO was physically adsorbed on the EG surface without notable chemical interactions. The EG structure was retained to ensure that its characteristic electrochemical properties were unaffected. Cyclic voltammetry and galvanostatic cycling tests were performed on the complex material with various rGO : EG ratios, exhibiting that rGO : EG = 1 : 1 (w/w) was optimal with a specific capacity of 443 mAh·g^-1 at 50 mA·g^-1. Even when operated at a high current density of 800 mA·g^-1, a specific capacity of 190 mAh·g^-1 was achieved, retaining 42.9% of the low-rate capacity, far exceeding those of pristine EG (14.2%) and rGO (27.2%). These results demonstrate that the rGO coating indeed enhanced the kinetics of potassium intercalation and efficiently improved the capacity and rate capability compared to pristine EG. We hope this work sheds light on novel approaches to improving potassium intercalation mechanisms in graphite.