Optimization of morphology, structure, and electrochemical properties of ball-milled graphene through modulating the ratio of grinding balls with different sizes

With the growing global emphasis on environmental protection and sustainable development, lithium-ion batteries (LIBs) have emerged as a highly efficient and environmentally friendly energy storage technology that is now used widely in various fields.¹ Pivotal in battery performance and lifetime, the anode material in such batteries is the primary medium for Li⁺ storage and release, which directly impacts the battery’s energy density, cycling performance, and charge/discharge rate.^2,3 As a two-dimensional single-layer carbon material characterized by sp² hybridization,⁴ graphene offers twice the lithium storage capacity of graphite (as high as 744 mA ⋅ h/g) because the former’s lithiation process occurs on both sides.⁵ Furthermore, graphene is a promising anode material for LIBs because of its exceptional characteristics, e.g., high specific surface area, superior electrical conductivity, and efficient Li⁺ intercalation and deintercalation,^6–8 and derived from graphene, graphene nanosheets have its structural and property advantages.⁹ 

Currently, graphene nanosheets are synthesized predominantly via either the redox method or chemical vapor deposition (CVD). The redox method is the most prevalent approach for laboratory-scale production of graphene nanosheets because of its simple operation, low cost, and minimal equipment requirement; however, the strong acids used in this process pose environmental risks during post-treatment, and graphene nanosheets prepared via this method often exhibit numerous defects, thus leading to compromised electrical and mechanical properties.^10–13 Another viable technique for synthesizing graphene nanosheets is CVD,^14–17 which produces individual graphene nanosheets with large size, minimal defects, and superior properties; nevertheless, that method has drawbacks including low yield and transferability challenges. Therefore, Tang et al. presented a method for the large-scale synthesis of graphene nanosheets using bubble CVD (B-CVD).¹⁸ This novel approach not only minimizes chemical usage but also utilizes natural gas as the primary raw material, thereby making it an eco-friendly and low-cost method. B-CVD results in graphene nanosheets with both high quality and high yield to facilitate the practical application of graphene in LIBs. However, there have been few studies to date of B-CVD graphene nanosheets applied to LIBs.

To enhance the performance of graphene-based anodes, research is currently focused on methods such as size adjustment,¹⁹ porous structure,^20,21 and introducing defects.^22,23 However, challenges remain in practical applications, including high cost, hazardous raw materials, technical complexity, and compatibility with current LIB production processes.

Ball milling (BM) creates reactions between solids via mechanical energy.²⁴ Because of its clean and simple operation and ability to process large quantities of diverse materials, BM has exhibited promising application prospects for material preparation and structural design in recent years.^25–28 Wang et al. discovered that after 150 h of BM, flake graphite fractured into fine (∼50 nm) particles,²⁹ and the increased surface energy caused the broken nanoparticles to agglomerate, giving an average particle size of 1 μm. This indicates that BM can be used to fragment particles and prepare materials with smaller size. Choi et al. reported that 8 h of BM enhanced the mesopore content, specific surface area, and specific capacity of activated carbon derived from walnut shells through KOH activation.³⁰ In other words, BM can regulate the pore size of a material to obtain a larger specific surface area. Zhang et al. obtained dense defect-rich graphene blocks from two-dimensional expanded graphene via crushing and reassembling during BM;³¹ the mechanical forces generated during BM introduced excessive defects into the carbon layers, resulting in capacitive behavior similar to that of a bilayer capacitor. This shows that BM can introduce defects and even special properties to materials. Liu et al. proposed nitrogen-doped graphene nanosheets for enhanced electronic conductivity, lithium-ion absorption, cycling stability, and rate capability by mixing and reacting materials in a jar during BM.³² Therefore, BM has the potential to improve the electrochemical performance of graphene anodes by optimizing the size, pore structure, specific surface area, and defect density of graphene nanosheets from B-CVD through the BM process, and this is likely to facilitate the practical use of graphene in LIBs.

The BM parameters determine the quality and performance of the final milled product, and it is known that the BM time and rotation speed directly affect the degree of material fracture and refinement.^33,34 The BM medium—i.e., water, air, alcohol, or organic solvent^35–38—is another important parameter that affects the degree of material infiltration and dispersion to thereby influence the movement state and crushing effect of the material. As another fundamental and crucial parameter, the ratio of grinding balls with different sizes is often ignored. It is common to use two sizes of grinding balls to ensure the efficiency of BM: large heavy balls have a strong impact on the material to facilitate material crushing, but they make fine grinding difficult, leading to the formation of dead zones in the grinding process; conversely, small balls have more-frequent impacts on the material and are particularly effective for fine grinding. Therefore, it is important to select the appropriate ratio of large and small grinding balls because their weights, volumes, and roles are different, but the ball ratio has received little attention to date.

In the work reported herein, B-CVD graphene nanosheets were milled by using different ratios of grinding balls to understand how the ball ratio affects the morphology, structure, and electrochemical properties. XRD, Raman spectroscopy, SEM, and low-temperature nitrogen adsorption testing were used to analyze the effects of different ball ratios on the structure and morphology of the graphene nanosheets. Furthermore, the electrochemical properties of the samples before and after BM with different ball ratios were evaluated, and the optimal ball ratio was determined.

The all-directional planetary BM device (QM-QX04) used in this work was produced by Nanjing Nanda Instrument Co., Ltd., with four 50-ml agate ball-milling jars operated at a rotation speed of 0–600 rpm and a flip cycle time of 1 min. Spherical agate grinding balls with diameters of 10 and 6 mm were used as the large and small balls, respectively, and the BM medium was air. The raw material for BM comprised graphene nanosheets prepared by B-CVD; see Ref. 18 for their detailed preparation procedure.

The crystal structure of the material was analyzed using powder XRD (MiniFlex 600, Rigaku), and the surface morphology of the samples was observed using SEM (Phenomeno Pharos). The density of defects characterized by I_D/I_G was measured using Raman spectroscopy (XproRA PLUS), and the pore size distribution and specific surface area of the samples were measured by means of N₂ adsorption–desorption isotherms (ASAP 2640 3.01).

From the original or BM1:x sample, 80 g were mixed with 20 g of polymeric binder (polyvinylidene fluoride) by usual grinding, then 400 μl of the organic solvent NMP (N-Methyl-2-pyrrolidone) were added to the mixture and stirred magnetically for 4 h to obtain a homogeneous slurry. The slurry was then coated uniformly onto a copper foil using a 4-μm-thick spatula followed by heating for 15 min to remove the organic solvent. The coated copper foil was then dried in a vacuum oven at 80 °C for 15 h, then the dried coated copper foil was cut into 10-mm-diameter electrode foils and assembled to form a CR2032-type half-cell in an argon glove box with a water and oxygen content of less than 1 ppm. In this setup, the electrode foil served as the anode and lithium metal acted as the counter electrode, with a polypropylene membrane (commercial Celgard 2500) serving as the separator. The electrolyte comprised 1M LiPF6 in a mixed solvent (ethyl carbonate: ethyl carbonate: dimethyl carbonate in a ratio of 1:1:1 by volume). Galvanostatic charge/discharge tests (GCD) and cycle stability tests were performed using a CT-4008T battery test system (Neware Technology) over a constant voltage range of 0.01–3.0 V.

The microscopic morphology of the samples after grinding with different ratios of grinding balls is shown in Fig. 1, and as can be seen, the morphology changes with the ball ratio. Figure 1(a) shows the morphology of the graphene nanosheets without BM; overall, numerous ribbons are coiled together with a distinctive folded structure. After 3 h of BM at the ratio of 1:2, the coiled ribbons fragment into smaller and flocculent graphene nanosheets, shown in Fig. 1(b). For sample BM1:3 [Fig. 1(c)], the size of the graphene nanosheets is reduced further with more small grinding balls; numerous stacked flakes are observed arising from the continuous extrusion and interconnections of the flocculated graphene nanosheets during BM, and the flake-shaped graphene nanosheets are thin and of uniform size. As shown in Fig. 1(d), the size of sample BM1:4 does not change, but the increased thickness of the nanosheets leads to the formation of loosely dispersed particles from flakes. When the number of small grinding balls reaches 40, the graphene morphology is dominated by large particles with relatively smooth surfaces, shown in Fig. 1(e).

The results indicate that BM with more small grinding balls is effective in reducing the size of the sample; however, once the size reaches a certain threshold, increasing the impact energy further no longer results in a size decrease but instead an increase. We suggest that further reduction of particle size is hindered for two reasons. (1) Graphene tends to fracture at its weak points, but these become fewer during BM. (2) As the graphene size decreases, the enhanced specific surface area leads to more surface activation points and increased surface free energy and van der Waals force between the graphene layers, so the graphene tends to agglomerate into larger particles. In summary, with more small milling balls, the original ribbon-like coiled graphene nanosheets are fractured into small flakes and then transformed into large particles because of aggregation.

In the Raman analysis of carbon materials, the area ratio I_D/I_G of the D peak to the G peak is used to quantitatively characterize the defect density of the material, and in some cases, higher levels of defects offer enhanced electrochemical performance.⁴³ (1) Defects can generate additional active sites on the surface of graphene to promote more reaction sites in the electrochemical process. (2) Defects can alter the electronic structure of graphene for improved charge-transfer kinetics; this can improve the efficiency of electron transfer in electrochemical reactions to achieve better electrochemical performance. (3) Defects can lead to structural disorder, increase the surface area of graphene, and provide more sites for electrochemical reactions; the increased surface area can enhance the electrode electrolyte interface, promote faster ion diffusion, and improve electrochemical performance. However, note that the relationship between defect density and electrochemical performance is not always linear; excessive defects can sometimes have harmful effects, such as increasing internal resistance, reducing mechanical stability, or causing unnecessary side effects. The Raman spectra in Fig. 3(a) show that both the original and ball-milled graphene nanosheets exhibit two distinct peaks at 1350 cm⁻¹ (D peak) and 1580 cm⁻¹ (G peak). The D peak corresponds to the defect peak generated by the A_1g symmetry vibration, indicating the presence of defects and amorphous carbon in the sample. Conversely, the G peak corresponds to the Raman peak generated by the symmetry vibration of the E_2g phonon in the center of the Brillouin zone, suggesting the presence of paired ordered carbon atoms in the hybridized orbitals of the aromatic ring sp². I_D/I_G was calculated using the LabSpec software, as shown in Fig. 3(b). The defect density of graphene nanosheets increases and then decreases with more small grinding balls, and the defect density reaches its maximum value when the ratio is 1:3.

Graphene nanosheets were subjected to low-temperature nitrogen adsorption tests before and after BM to obtain the specific surface area and pore structure data. The values of the specific surface area were analyzed using the Brunauer–Emmett–Teller (BET) method. The original sample exhibits a specific surface area of 94.96 m²/g, while the specific surface areas of samples BM1:2, BM1:3, BM1:4, and BM1:5 are 120.65, 122.01, 121.02, and 119.05 m²/g, respectively, shown in Fig. 3(c), which indicates that BM substantially increases the specific surface area of graphene nanosheets. Notably, the specific surface area shows a trend of increasing and then decreasing with more small grinding balls. Additionally, Fig. 3(d) shows the pore size distribution of the graphene nanosheets calculated by the Barrett–Joyner–Halenda method before and BM. The results indicate that the macropores and mesopores in graphene tend to transform into micropores with more small grinding balls.

Figure 4(a) shows the Li⁺ insertion/exfoliation performance of the original electrode and BM1:x electrodes through galvanostatic charge–discharge testing of lithium half-cells conducted at 100-mA/g current density within the voltage range of 0.01–3.0 V. After 80 cycles, the final discharge specific capacities of the original, BM1:2, BM1:3, BM1:4, and BM1:5 electrodes were recorded as 140.10, 246.23, 262.09, 253.22, and 231.06 mA ⋅ h/g, respectively. Obviously, the BM process greatly improves the specific capacity of the graphene anode, with the BM1:3 electrode exhibiting the highest specific capacity. Furthermore, it sustains a Coulombic efficiency above 98% across charge/discharge cycles. This is because the electrode surface maintains its integrity and does not form dead lithium or lithium dendrites during the low-current cycling.

Figure 4(b) shows the initial discharge specific capacities of lithium half-cells fabricated from the original and ball-milled electrodes. The initial discharge specific capacities are 344.4, 626.88, 704.83, 645.02, and 635.43 mA ⋅ h/g for the original, BM1:2, BM1:3, BM1:4, and BM1:5 electrodes, respectively. They present a trend of initial increase followed by a decrease with more small grinding balls. The initial coulomb efficiencies are 41.13%, 40.27%, 36.3%, 38.25%, and 38.99% for the original, BM1:2, BM1:3, BM1:4, and BM1:5 electrodes, respectively. This is the opposite trend to the initial discharge specific capacity, which decreases and then increases. The initial change is because the specific surface area of the ball-milled sample increases continuously [Fig. 3(c)] as the number of small grinding balls is increased to 24. The larger specific surface area not only provides more lithium storage sites to increase the specific capacity but also results in a larger number of irreversible reactions and the formation of a more extensive solid-electrolyte-interphase film, which reduces the initial Coulombic efficiency. Subsequently, with more small grinding balls, the specific surface area of graphene starts to decrease. This reduction results in a decrease of both the number of lithium storage sites and the occurrence of reactions on the electrode surface, leading to opposite trends in both specific capacity and Coulombic efficiency compared to the initial changes. These findings are consistent with the specific surface area results obtained through the BET method [Fig. 3(c)] and the morphology changes observed in SEM images (Fig. 1).

The curves in Figs. 4(c)–4(g) show the first, second, 20th, 50th, and 80th cycles of the galvanostatic charge/discharge tests for the original and ball-milled graphene electrodes, respectively. All graphene nanosheet electrodes exhibit a voltage plateau around 0.8 V during the initial discharge, where the electrolyte and solvent undergo a reduction reaction on the surface of graphene nanosheets to yield solid products and organolithium compounds. The solid compounds deposit onto the anode material surface to form a solid electrolyte interphase, which facilitates ion conduction while impeding electron conduction. Consequently, this prevents further electrolyte decomposition and thereby greatly reduces irreversible reactions in LIBs to ensure stable cycling performance. Moreover, the charge–discharge curves of all the materials can be divided into two parts, i.e., a plateau region below 0.2 V (low-potential platform zone) and a slope region above 0.2 V (high-potential slope zone), corresponding to the insertion and extraction of Li⁺ between graphene layers and reversible adsorption of Li⁺ at the edges of the graphene layer and defect sites, respectively.

To provide a clear observation of lithium storage behavior and capacity variations, Fig. 4(h) compares the first-cycle charging curves of the graphene before and after BM, and it determines the capacity contributions of platform and slope zones shown in Fig. 4(i). It is observed that the capacity contributed by the platform zone of the electrodes decreases gradually after BM, i.e., 61.48, 57.45, 52.64, and 40.02 mA ⋅ h/g for BM1:2, BM1:3, BM1:4, and BM1:5, respectively. This can be attributed to the increased graphitization of the samples after BM (Table I) to limit ion transport and reduce the platform capacity. Meanwhile, the capacity contributed by the slope zone exhibits a trend of initial increase followed by a decrease with more small grinding balls, consistent with the variation of defect density [Fig. 3(b)]. The BM1:3 electrode exhibits the highest slope zone capacity (200.92 mA ⋅ h/g), showing a significant increase of 121.1% compared to the original sample (90.79 mA ⋅ h/g).

The rate capability of the original, BM1:2, BM1:3, BM1:4, and BM1:5 electrodes is shown in Fig. 5(a). BM1:3 exhibits the highest reversible specific capacities of 171.19, 158.63, 125.2, and 70.55 mA ⋅ h/g at current densities of 500, 1000, 2000, and 5000 mA/g, respectively. When the current density returns to 100 mA/g, the specific capacity of the BM1:3 electrode even exceeds its initial value, suggesting that the electrode is thoroughly activated with the increasing cycle numbers, so the BM1:3 electrode exhibits higher specific capacities and maintains stable and reversible charging and discharging. The galvanostatic charge–discharge curves of the BM1:3 electrode were further tested at different current densities [Fig. 5(b)]. It is evident that the curves at different current densities have similar shapes, confirming the outstanding rate capability of BM1:3. Additionally, Fig. 5(c) shows the specific capacity and Coulombic efficiency of the BM1:3 electrode after 2000 cycles at a current density of 2000 mA/g. The specific capacity reaches its maximum value of 138.58 mA ⋅ h/g at about 400 cycles, which confirms again that the electrode has been activated completely as the number of cycles increases. After that, the specific capacity decreases slightly and ultimately stabilizes at 121.12 mA ⋅ h/g after 1200 cycles, about 87.4% of the maximum value. The Coulombic efficiency maintains nearly 100% during 2000 cycles, indicating the stable lithium storage capability of the electrode.

The focus herein has been on how the ratio of different grinding balls influenced the morphology, structure, and electrochemical performance of graphene nanosheets synthesized via B-CVD. As the number of small grinding balls was increased, the grain size of the graphene nanosheets decreased monotonically. However, small graphene nanosheets began to agglomerate into large particles at a certain threshold, as observed by SEM. The external force introduced by BM caused fragmentation and interlayer delamination of the graphene nanosheets to increase the defect density, specific surface area, and interplanar spacing. Simultaneously, the dominant impact force induced by more small grinding balls acted on both sides of the graphene to restack and reassemble nanosheets and result in decreased defect density, specific surface area, and interplanar spacing. Conversely, the graphitization degree of the samples decreased and then increased during BM. The final morphology and structure of the graphene nanosheets depended on the competition between the above factors. The electrochemical performance of lithium-ion half cells based on ball-milled graphene nanosheets was greater than that of the original electrode. Moreover, the specific capacity and rate capability of the ball-milled electrodes increased then decreased with more small grinding balls. The optimal electrochemical performance was achieved when the ratio of large to small balls was 1:3. The reversible specific capacity reached 262.09 mA ⋅ h/g at a current density of 100 mA/g and remained at 87.4% after cycling 2000 times at a current density of 2000 mA/g. Therefore, the ratio of grinding balls with different sizes is an important parameter that cannot be ignored during BM. Selecting an appropriate ball ratio is a simple and direct way to adjust the morphology and structure of graphene nanosheets to thereby improve the electrochemical performance of graphene-based lithium batteries.

This work was supported financially by the National Natural Science Foundation of China (Grant No. 12275047).

The authors have no conflicts to disclose.

The data that support the findings of this study are available from the corresponding authors upon reasonable request.

Ranlu Zheng was born in 2001 and is currently a second-year graduate student in electronic information engineering in the School of Physics at the University of Electronic Science and Technology of China (UESTC). Her research is focused on the preparation and optimization of graphene lithium-ion batteries.

Xiang Xia graduated from UESTC with a Ph.D. in optics in 2008 and is currently a professor in its School of Physics. She is mainly engaged in the preparation and characterization of nanostructures and optoelectronic functional materials, as well as research on the interactions among strong lasers, particle beams, and materials.

Bo Li received a Ph.D. degree in condensed matter physics from UESTC in 2019, followed by postdoctoral research in its School of Physics. Currently, he is an assistant research fellow at UESTC’s Yangtze Delta Region Institute (Huzhou). His research interests include laser and particle-beam surface treatment of materials, engaged mainly in ion-beam etching/polishing, material surface modification, material irradiation effects, and the preparation and characterization of thin-film materials.