Femtosecond laser drilled micro-hole arrays in thick and dense 2D nanomaterial electrodes toward high volumetric capacity and rate performance
Introduction
The increasing demand in portable electronics and electrical vehicles promote the developing of the fast charge-discharge electrochemical energy storage (EES) devices. To meet this requirement, electrode materials should store and deliver sufficient charges in a short span of time [1,2]. However, for most micron-sized electrode materials, their specific capacities were severely reduced with the increased rate [3]. There have been considerable research interests in exploring new electrode materials with a high rate performance [3,4]. Nanomaterials possess shorter ion diffusion lengths, weaker electrochemical polarization and more paths for electronic transport. Benefiting from above advantages, nanomaterial electrodes can attain high rate performance, long cycling life and high gravimetric energy density, thus offering a great application potential [[5], [6], [7], [8]]. Unfortunately, nanomaterial electrodes usually have a low tap density, which causes the EES devices with a low volumetric energy density. In many practical applications, the volumetric energy density is more important than gravimetric energy density [9,10]. The low volumetric energy density is one of the major issues for nanomaterial electrodes to meet the application demands in EES devices [7,8,11,12].
In recent years, many efforts have been attempted to improve the volumetric energy densities of the nanomaterials-based EES devices [[13], [14], [15], [16], [17], [18], [19]]. However, the areal mass loadings of reported dense nanomaterial anodes for lithium-ion batteries are generally lower than 5 mg cm−2, as summarized in Fig. 1(a) [[20], [21], [22], [23], [24], [25], [26], [27], [28], [29]]. Such low mass-loaded electrodes lead to low energy densities of batteries [9,[30], [31], [32], [33]]. If the electrode thickness is increased, the rate performance will be degraded due to the long and tortuous ion transport pathways from the top to the bottom of the electrodes [30,34,35]. Additionally, regarding the reported high mass-loaded nanomaterial electrodes, their relative densities (the ratio of bulk density to theoretical density) usually stay below 50% (summarized in Fig. 1(a)), which is far from satisfactory for achieving a high volumetric energy density [[36], [37], [38], [39], [40], [41], [42], [43]]. Therefore, researchers need to make further efforts to attain both high volumetric capacity and rate performance in nanomaterial electrodes.
2D nanomaterials have shown promising potential toward high performance for electrochemical energy storage [44,45]. Numerous studies have been carried out on 2D nanomaterials such as graphene [46], transition metal dichalcogenides [47], metal-organic frameworks [48], hexagonal boron nitride [49], and the newly explored MXenes [50,51] as electrodes for batteries and supercapacitors. Moreover, 2D nanomaterials can be served as substrates for other nanomaterials, then offering the possibility to construct superstructures tailored for a variety of applications in energy storage [52,53].
Recently, 2D nanomaterials have attracted intensive interests in highly compact energy storage [18,[54], [55], [56], [57]]. Compared with other dimension nanomaterials, 2D nanomaterials can easily construct electrodes without excessive void space through layer-by-layer assembly [18]. However, for thick and dense 2D nanomaterial electrodes, the rate performance is reduced more seriously than other thick and dense electrodes because the complex U-shaped paths lead to the extremely sluggish ion transport, as shown in Fig. 1(b) [58,59].
Herein, we report a method to attain both high volumetric capacity and rate performance in thick and dense 2D nanomaterial electrodes. A femtosecond laser drilling technique is adopted to fabricate a micro-hole array in the thick and dense electrode constructed by parallelly stacked nanosheets (Fig. 1(c)). The hole diameter and spacing can be easily controlled at the micron scale in the drilling process. A rationally designed hole array with a mean hole diameter of 9 μm and a hole spacing of 40 μm only induces a 4.0% decrease in electrode density. The micro-hole array can effectively shorten the ion transport distance to facilitate ion transport, thus boosts the volumetric capacity and rate performance (Fig. 1(d)). This method adapted to favor ion transport is similar to a reported approach, in which the electrolyte-flow mass transport in carbon paper electrodes of vanadium redox flow batteries is improved by the holes with a diameter ranging from 171 to 421 μm [60].
Two thick and dense free-standing electrodes, a TiO2-based electrode and a graphene electrode were used as model cases in lithium-ion batteries to study the effectiveness of micro-hole arrays. The TiO2-based electrode is constructed by the upper-layer TiO2-coated graphene hybrids (TiO2-G) and under-layer graphene (denoted as dTiO2-G/G). A wet pressing process is employed to increase the density of the TiO2-based electrode. The dTiO2-G/G and graphene electrodes with a micro-hole array (hole spacing: 40 μm) possess a large relative density/thickness of 64.3%/85 μm and 69.2%/40 μm, respectively. Benefiting from the micro-hole array, the dTiO2-G/G electrode shows a high volumetric capacity and rate performance (97 mA h cm−3 at 10C). In addition, the dependence of the lithium ion transport kinetics, rate performance and cycling performance on the hole spacing of the dTiO2-G/G electrode was also systematically investigated.
Section snippets
Preparation of the fTiO2-G/G electrode
The preparation of the fluffy electrodes composed of the upper-layer TiO2-coated graphene hybrids and under-layer graphene (denoted as fTiO2-G/G) has been reported previously [61]. Briefly, the graphene and TiO2-G hybrid dispersions were separately prepared. A sodium lignosulfonate surfactant was used to prepare the graphene dispersion. The TiO2-G hybrid dispersion was synthesized through the hydrolysis of titanium glycolate with the graphene dispersion at the boiling point following a
Preparation and microstructure analysis of the dTiO2-G/G electrode
The rod-like titanium glycolate (Fig. S1a), synthesized through a one-step polyol-mediated reaction, was dissolved in sulfuric acid solution and then mixed with graphene sheets. In the subsequent hydrolysis, the graphene sheets (Figs. S1b and c) decorated by sodium lignosulfonate serve as the nucleation site for the growth of TiO2 nanoparticles to obtain the TiO2-G hybrids (Figs. S1d–f) [61]. Then, the graphene and TiO2-G hybrids were filtrated in sequence to obtain a fluffy free-standing
Conclusions
In conclusion, the method based on the femtosecond laser drilling is effective to improve the volumetric capacity and rate performance in thick and dense 2D nanomaterial electrodes, as evidenced by the laser-drilled dTiO2-G/G and graphene electrodes. By using a wet pressing, we fabricated a thick and dense electrode constructed of the layer-by-layer stacked TiO2-G hybrid nanosheets. Compared with traditional dry pressing, the wet pressing is more effective to increase the density of the
CRediT authorship contribution statement
Chunyang Xu: Visualization, Supervision, Writing - original draft. Qiang Li: Supervision. Qizhao Wang: Supervision. Xuandong Kou: Supervision. Hai-Tao Fang: Conceptualization, Investigation, Visualization, Writing - review & editing. Lijun Yang: Resources, Investigation.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This work was supported by the National Natural Science Foundation of China (No. 51272051, 50872026).
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