Femtosecond laser drilled micro-hole arrays in thick and dense 2D nanomaterial electrodes toward high volumetric capacity and rate performance

Highlights

• Femtosecond laser drills micro-hole arrays in thick and dense 2D material electrodes.

• Micro-hole arrays improve the volumetric performance of thick and dense electrodes.

• A wet pressing process is proposed to densify 2D material electrodes.

Abstract

Two dimensional (2D) nanomaterials have great application potential in developing the energy storage systems with high volumetric energy and power densities. However, due to the sluggish ion diffusion, achieving both high volumetric capacities and rate performance in thick and dense 2D nanomaterial electrodes is still challenging. Here, we report a femtosecond laser drilling method to introduce appropriate micro-hole arrays into the thick and dense electrodes for facilitated ion transport. These micro-hole arrays shorten ion transport paths and thus significantly reduce the ion transport resistance. More importantly, constructing micro-hole arrays with hole spacing higher than 40 μm rarely sacrifices the density of the electrode. The effectiveness of micro-hole arrays to improve capacities and rate performance of the thick and dense electrodes is evidenced by two model cases: a TiO₂-based electrode constructed with the upper-layer TiO₂-coated graphene hybrids and under-layer graphene (dTiO₂-G/G), and a graphene electrode. As presented, the micro-hole array with a hole spacing of 40 μm decreases the ionic resistance of the dTiO₂-G/G electrode, and consequently improves the volumetric capacity at 10C from 15.8 to 97 mA h cm⁻³. Such micro-hole array also improves the volumetric capacity and rate performance of the graphene electrode.

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.