Alternative binder–free electrode based on facile deposition of carbon/graphene–TiO₂ on the coin cell anode for a lithium–ion battery

Highlights

• Direct deposition of C/GTiO₂ on the coin cell anode for binder–free electrode.

• The loading of C and TiO₂ determine the adherence of the deposited material.

• Graphene provides a conducting network, porosity and buffer effect for TiO_2.

• The drop test of the coin cell confirmed the good adherence of the C/GTiO₂.

Abstract

This paper introduces a binder–free electrode prepared by a facile deposition of C/TiO₂ (Carbon/TiO₂) or C/GTiO₂ (Carbon/Graphene–TiO₂) on the coin cell anode using poly (methyl methacrylate) (PMMA) as a cheap carbon and binding source without using binders and a current collector. The use of graphene not only presents a conducting support to TiO₂ particles, but also greatly improves the porosity of the electrode relative to pristine TiO₂ or C/TiO₂. In the lithium–ion battery (LIB) anode, the direct deposition enabled sufficient electrical conductivity of the electrode, the intercalated layer–by–layer (LBL) arrangement of the C/GTiO₂ allowed sufficient Li⁺ accessibility, and the resulting electrode exhibited a higher specific capacity and an excellent rate capability compared to conventional TiO₂ electrodes. Drop testing of the binder–free cells prior to electrochemical testing showed that all cells retained their original specific capacity, rate capability, and cycle stability, thus confirming the strong binding of the binder–free electrodes on the coin cell surface.

Introduction

In the development of electronic devices such as electric vehicles (EVs) and hybrid electric vehicles (HEV), rechargeable lithium–ion batteries (LIB) have attracted significant attention due to their light weight and high energy density [1], [2], [3]. However, high cost and poor cycle stability are the major problems associated with the electrode materials of LIB. The stability of the electrode is related to the migration of Li–ions through the electrode–electrolyte interfaces [4]. In traditional electrodes, the polyvinyledine fluoride (PVDF) binder is used to attach the electrode materials to the current collector. Inevitably, the binder and current collector have no influence on the electrochemical performances, and they are detrimental to the energy density of LIB [5], [6]. Therefore, designing new types of electrode materials without using the binders and current collector is an essential goal [5], [6], [7], [8].

One of the easiest and most frequently applied methods for the fabrication of a binder–free electrode is by the deposition or growing of metal oxides of desirable morphologies on the nickel (Ni) foam [9], [10], [11]. In comparison to a traditional electrode, the direct contact between the anode material and current collector in the three–dimensional (3D) Ni–foam facilitates sufficient electronic and ionic transfer kinetics. In another approach, a TiO₂ or CuO–based binder–free electrode is obtained by the engraving of their respective metal foils (Ti or Cu) [12], [13], [14]. The metal foils act as a continuous and fast conduction pathway for electrons. Due to the high cost and undesirable oxidation reactions of metal substrates, the flexible, high strength and electrically conductive carbon cloth is considered to be an alternative potential candidate for a binder–free electrode [15], [16], [17]. Barriers to the usage of metals or carbon–based binder–free electrodes have fallen dramatically since graphene has been developed as a processable colloidal suspension. Due to the excellent physical, mechanical and chemical characteristics of the 2–D G sheets, it allows the uniform deposition/growth of metal oxides (MO) on the basal plane and leads to a decrease in the aggregation/stacking of the MO/G nanocomposites [18], [19]. Recently, binder–free G/MO nanocomposites have been produced by layer–by–layer (LBL) assembly or by spin coating methods [7], [20], [21], [22], [23]. However, these processes are either complicated to scale up or have too high a cost for practical applications [24], [25]. In order to satisfy current energy needs, it is desirable to develop more effective and inexpensive synthesis methods for the binder and current collector in free porous LIB electrodes.

For the anode materials of LIB, the low cost, eco–friendly and highly stable structure of anatase TiO₂ has long been considered as a promising candidate [5], [26], [27], [28], [29], [30], [31], [32], [33]. The morphology, crystal structure, crystallinity, and surface area are the important factors that determine the electrochemical performances of TiO₂ [26], [34]. Nanostructured TiO₂ increases the contact areas between electrode and electrolyte and leads to a decrease in the diffusion length of the Li–ions. Further, the high surface area and narrow pore size distribution of nanosized features of TiO₂ increases the specific capacity, charge/discharge rates, and cycle stability of the LIB [34]. However, low electrical conductivity is a serious problem for TiO₂–based active materials, which limits the charge transport characteristics of the electrode. In order to increase the electrical conductivity, the use of conducting materials such as carbon, graphene (G), or doping with metal atoms to TiO₂ have been frequently studied [18], [35], [36], [37], [38], [39]. Among the conducting materials, 2D G has attracted a great deal of attention in LIB research due to its superlative, outstanding properties [18], [19]. 2D G acts as a strong conducting network for TiO₂ and provides an efficient electrical contact between the active material and current collector. Hybrid nanocomposites of porous TiO₂ and highly conductive G or carbon not only provide excellent performances in LIB, but also offer high performances in supercapacitor and photocatalytic activities [26], [40], [41]. Thus far, the binder–free TiO₂ electrode has been obtained by the engraving or anodization of Ti foil [12], [42], [43], production of 3D cross–linked electrospun TiO₂ nanofiber web [44], or with LBL assembly of GTiO₂ [45]. Inevitably, these methods have a high cost and have limited potential for large scale production. In order to solve these issues, we alternatively developed simple, binder–free C/TiO₂ and C/GTiO₂ electrodes by directly depositing the active materials on the coin cell anode without using the binder and current collector. It is believed that direct deposition enables sufficient charge transition between the coin cell anode and active materials, which results in an improvement in the electrochemical performances.

Herein, the binder–free C/TiO₂ or C/GTiO₂ nanocomposites were prepared through the simultaneous carbonization and direct deposition of respective PMMA/TiO₂ or PMMA/GO–TiO₂ precursors on the coin cell anode. The obtained C/TiO₂ and C/GTiO₂ nanocomposites were strongly attached to the coin cell anode. The resulting nanocomposites exhibited high surface area and a highly satisfactory LBL arrangement of GTiO₂ on the coin cell anode. The physical and electrochemical characteristics of the C/TiO₂ and C/GTiO₂ nanocomposites were thoroughly investigated. Drop testing of the fabricated electrode was conducted for systematically evaluating the binding efficiency of the C/TiO₂ and C/GTiO₂ nanocomposites on the coin cell anode. This binder–free electrode was fabricated with the use of cheap and readily available PMMA as a carbon source for attaching the TiO₂ or GTiO₂ on the coin cell anode. The fabrication method is simple, cost–effective, and could be useful to all MO based binder–free electrodes.