A surface multiple effect on the ZnO anode induced by graphene for a high energy lithium-ion full battery

Highlights

• The ZnO and graphene composites were successfully prepared by eco-friendly method.

• The electrochemical properties can be revised by surface modification of graphene.

• The composites show enhanced cycling stability and rate capability in Li-ion battery.

• The pre-lithiation effect on initial irreversible capacity control was illuminated.

• A new concept of high energy full battery was proposed and assembled.

Abstract

A low-cost, environment friendly and scalable strategy was proposed to prepare ZnO@graphene (ZnO@G) composites, in which ZnO nanoparticles can be evenly modified by graphene without obvious agglomeration and the tap density can reach to 1.68 g cm⁻³. When employed as an anode for Li-ion battery, the as-prepared ZnO@G (15 wt% graphene) exhibited an excellent reversible capacity of 720 mAh g⁻¹ at 200 mA g⁻¹ and 480 mAh g⁻¹ even at 1600 mA g⁻¹. Furthermore, a concept of high energy Li-ion full battery configurated the pre-lithiation ZnO@G (10 wt% graphene) anode and commercial LiCoO₂ and LiNi_0.8Co_0.1Mn_0.1O₂ cathode was successfully assembled. Under the varying of pre-lithiation time to tune the appropriate compensating amount of initial irreversible capacity, one full battery of ZnO@G || LiCoO₂ delivered a reversible capacity around 400 mAh g⁻¹ (vs. anode) at 100 mA g⁻¹ with working potential around 3.8 V and a high energy density of 1478 Wh kg⁻¹ (vs. anode; 206.9 Wh kg⁻¹ vs. cathode); meanwhile, other full battery of ZnO@G || LiNi_0.8Co_0.1Mn_0.1O₂ exhibited a reversible capacity around 280 mAh g⁻¹ (vs. anode) at 400 mA g⁻¹, and it possessed a high energy density of 1787.2 Wh kg⁻¹ (vs. anode; 446.8 Wh kg⁻¹ vs. cathode) at 400 mA g⁻¹ and behaved superior rate capability. Furthermore, a proposition of surface multiple effect on the ZnO-based anode induced by graphene is demonstrated and it could be extended to designing some other advanced electrodes, benefiting from pre-lithiation process, the metal oxides electrode is identified to be a promising commercialized anode in high energy batteries.

Introduction

Lithium-ion batteries (LIBs) have been widely applicated in portable electronics and electric vehicles, however, the energy density of LIBs using the traditional graphite or other carbon-based anode would not meet the people’s increasing requirements since this kind of anode always only delivered a limited specific capacity around 372 mAh g⁻¹ [1]. Recently, transition metal oxides (TMOs, M = Fe, Co, Ni, Cu, Zn, etc) with higher theoretical specific capacities(≥600 mAh g⁻¹, twice times of graphite) have been proved to be an alternative anode in high energy LIBs [[2], [3], [4], [5], [6]]. Among them, zinc oxide (ZnO) demonstrates great potential because of its abundance, low cost, environmental benign and nontoxic [7]; especially as an anode, it possesses a high theoretical reversible capacity (978 mAh g⁻¹), lower intercalation potential (∼0.5 V vs. Li/Li⁺) and a larger Li-ion diffusion coefficient (10⁻¹⁴ to 10⁻¹² cm² s⁻¹) than most of transition metal oxides (eg. SnO₂ 10⁻¹⁵ to 10⁻¹³ cm² s⁻¹) [8,9]. However, the ointment is that ZnO anode still suffers from severe capacity fading during the long-term cycles, since its pulverization aroused from volume expansion (about 228%) not only lead to a weak electronic contact interface within electrodes [10], but also caused a large consumption of electrolyte for new solid-electrolyte interphase (SEI) formation [11]; coupled with its naturally poor electronic conductivity (10⁻³–10⁻⁵ S cm⁻¹) [12], the Li-ions storage performance of ZnO anode cannot reach the anticipated benchmark for its wide application in LIBs except the above mentioned drawbacks and challenges to be well resolved.

Therefore, some typical strategies of downsizing ZnO bulks into nanoscale (<1000 nm) [13] and/or combining it with conductive materials such as carbon matrixes (graphene, carbon nanotube, filers, porous carbon, etc) [[14], [15], [16]], metal carbide(Mxene) [17,18], nitride (TiN, C₃N₄, etc) [19], metal (Au, Ag, etc) [12,20] and polymer (dopamine, resin, polythiophene, etc) [11,21] have been developed to solve the above-mentioned key points. Generally, downsizing the ZnO-based active material into nanoscale will cause the increase of its specific surface area and then introduce a larger irreversible capacity or consume too much electrolyte enduring the SEI formation process [22]. And for the anodes composited from ZnO and conductive material, especially with an acceptable electrochemical performance in commercial LIBs, a complex synthesis procedure is unavoidable [23]. Especially for carbon modification, although combining ZnO with carbonaceous materials were surely identified to buffer its volume change and improve its electronic conductivity for excellent rate performance [24], there are some key points still need to be addressed. Supposed coating the ZnO with carbonaceous matrix which calcined from the organic compounds, the ZnO will easy to be reduced to metal Zn when the calcined temperature reached too high or the calcined time is too long, and there is no active for metal zinc to storing Li-ions [25]; Oppositely, if the carbonization conditions is insufficient, the conductivity of the carbon matrix is not enough to construct a faster electronic transfer network within the electrode [26]. Secondly, if the carbonaceous matrix in-situ formed on the surface of ZnO, the as-prepared carbon always causes low ionic diffusion coefficient since it is absence of enough pores and channels [27] and the carbonaceous matrix introduced in the ZnO-based composites could lead to agglomeration of active materials to some extent [28]. Finally, and the most importantly, the carbonaceous matrix from the insufficiently carbonization of organic compounds exists too much defects (the structural defects and disorders in the carbon matrix) and will significantly tone-up the irreversible capacity of the ZnO-based anode.

Based on the above consideration, graphene with excellent electrical conductivity (15000 cm² V⁻¹ S⁻¹), well mechanical strength (1,060 GPa) and high specific surface area (2630 m² g⁻¹) [29,30] is proposed to behave as coating layers or carbon matrix to form advanced structure with ZnO nanomaterials based on the following aspects (defined as surface multiple effect herein): (ⅰ) the surface area of graphene powders is large and the structure of the graphene is flexible compared with the traditional carbonized carbon, thus the graphene can effectively inhibit the volume expansion of ZnO during cycle process; (ⅱ) the layer structural graphene with long-range electrical conductivity will make each ZnO particles contacting closely and result a three dimensional conductive network within electrode [27]; (ⅲ) the tap density of the materials was significantly increased due to the controllable introduction of self-doping defect structures on graphene sheets during the ball milling [31], also the relative low physical defects rather than chemical defects in graphene will not cause the irreversible capacity increasing. Consequently, there is some typical works focused on combining the ZnO and graphene together through the hydrothermal method [32], atomic layer deposition (ALD) [33], sol-gel [34], ball-milled method [35] and solution-based method [36,37]. Among these methods and aiming to commercial production, choosing the ball-milled method is deemed to possible way to prepare ZnO-based composites due to its superior advantages with facile, time saving, waste free and low cost. Recently, a series of ZnO-graphene composites was prepared and the positive interactions between ZnO and graphene were reported [38]. For instance, Yu et al. [15] has synthesized ZnO-graphene hybrids via high energy ball milling of ZnO and reduced graphene oxide (RGO), the ZnO was homogeneously dispersed in the graphene matrix, and the nanocomposite maintained a stable capacity of 610 mAh g⁻¹ after 500 cycles at a current of 100 mA g⁻¹. However, the surface multiple effect on ZnO induced from the graphene are rarely discussed and proposed before, actually the superiority of ball-milled process for graphene surface modification can not only increase the tap density and long/short-range conductivity of ZnO-based anode, but also construct fast channels for Li-ions diffusion due to the cracks and physical defects produced during the balls-hit process. This shows, the similarly related effects and factors urgently need to be well addressed towards precisely designing an ZnO-based or graphene-modified electrodes in LIBs. Moreover, although enormous works on ZnO-based anodes have conducted, few works were indeed evaluated Li-ions storage properties of ZnO-based anodes in full battery due to its large initial coulombic efficiency (ICE) and inestimable side reaction enduring the cycles, which will consume the limited Li-ions in cathode and/or electrolyte and then accelerate a capacity degradation, the electrochemical properties of ZnO-based anode in full battery is urgently to be comprehensively known before it widely used in commercial LIBs.

According to the above key points and consideration, a typical composite of ZnO and graphene (ZnO@G) were prepared through solid-state ball-milling method, in which the ZnO nanoparticles were prepared directly through the precipitation of zinc acetate with sodium hydroxide and the graphene powders were purchased from commercial corporation, all the raw materials and the experimental conditions were settled to can be extend to commercial production. A rational design of ZnO@G compositive materials for practical application is proposed with different proportions of graphene powders and ZnO (10:90 and 15:85) for comparison. When employed as anode in LIBs, as we expected, both ZnO@G composites exhibited excellent cycling stability and high rate capability due to the surface multiple effect induced from the graphene modification with a rational design and the relationships between the proposed surface multiple effect and Li-ions storage properties were illuminated. Furthermore, a concept of high energy LIBs configurated with pre-lithiation ZnO@G anode and commercial LiCoO₂ and LiNi_0.8Co_0.1Mn_0.1O₂ (NCM811) cathode was developed. One full battery of ZnO@G || LiCoO₂ delivered a reversible capacity around 400 mAh g⁻¹ (vs. anode) at 100 mA g⁻¹ with working potential around 3.8 V and a high energy density of 1478 Wh kg⁻¹ (vs. anode; 206.9 Wh kg⁻¹ vs. cathode); meanwhile, other full battery of ZnO@G || NCM811 exhibited a capacity around 280 mAh g⁻¹ (vs. anode) at 400 mA g⁻¹ enduring 200 cycles, and it possessed a high energy density of 1787.2 Wh kg⁻¹ (vs. anode; 446.8 Wh kg⁻¹ vs. cathode) at 400 mA g⁻¹ and superior rate capability. Herein, the resultant conclusions will pave the facile way to the commercialized application of ZnO-based anodes in LIBs.