Binder-Free Electrodes and Their Application for Li-Ion Batteries

Abstract

Lithium-ion batteries (LIB) as energy supply and storage systems have been widely used in electronics, electric vehicles, and utility grids. However, there is an increasing demand to enhance the energy density of LIB. Therefore, the development of new electrode materials with high energy density becomes significant. Although many novel materials have been discovered, issues remain as (1) the weak interaction and interface problem between the binder and the active material (metal oxide, Si, Li, S, etc.), (2) large volume change, (3) low ion/electron conductivity, and (4) self-aggregation of active materials during charge and discharge processes. Currently, the binder-free electrode serves as a promising candidate to address the issues above. Firstly, the interface problem of the binder and active materials can be solved by fixing the active material directly to the conductive substrate. Secondly, the large volume expansion of active materials can be accommodated by the porosity of the binder-free electrode. Thirdly, the ion and electron conductivity can be enhanced by the close contact between the conductive substrate and the active material. Therefore, the binder-free electrode generally exhibits excellent electrochemical performances. The traditional manufacture process contains electrochemically inactive binders and conductive materials, which reduces the specific capacity and energy density of the active materials. When the binder and the conductive material are eliminated, the energy density of the battery can be largely improved. This review presents the preparation, application, and outlook of binder-free electrodes. First, different conductive substrates are introduced, which serve as carriers for the active materials. It is followed by the binder-free electrode fabrication method from the perspectives of chemistry, physics, and electricity. Subsequently, the application of the binder-free electrode in the field of the flexible battery is presented. Finally, the outlook in terms of these processing methods and the applications are provided.

Keywords: Binder-free electrode; Fabrication method; Flexible; Lithium ion batteries.

Fig. 1

The requirements, fabrication methods, advantages, and future development for binder-free electrode

Fig. 2

Thermal treatment for the commercially available structure (a) and fabricated structure (b). a1 Metallic oxide nanoparticles can be obtained on the surface of an metal structure via a simple thermal oxidation progress [34]. a2 The active materials can be synthesized on the surface of conductive structure by thermal treatment [35]. a3 Biomass can be carbonized to achieve the carbon architecture [32]. b1 The mixture of polymer and active materials can be carbonized to achieve the binder-free electrode [36]. b2 Hierarchical structure can be obtained by multiple processes [37]. b3 Binder-free electrode can be obtained by thermal treatment the electrospinning membrane [38]

Fig. 3

a The scheme of ZnCoO_x/CNF composite fabrication [47]. b The fabrication of cathode electrode using hydrothermal method [48]

Fig. 4

a Schematic illustration of the preparation of 3D graphene/MnO₂ hybrid, and illustrations of electrons transfer on 3D graphene/MnO₂ hybrid [59]. b CBD method for the fabrication of CNF@Ni(OH)₂ [60]. b1–3 Different hybrid membranes with increase of concentrations of Ni(NO₃)₂ solution. c Schematic of the synthesis procedure for the PVP@S-SACNT composite [61]

Fig. 5

a The ALD technique mechanism [81], and two examples for b surface coating [82] and c active materials fabrication [83]

Fig. 6

a Schematic of process for fabrication of binder-free, carbon-free film electrodes [101]. b Schematic fabrication process for the Fe₃O₄/CNTs/rGO composite electrode [102]. c Schematic illustration of the synthesis route for rGO/active materials/Ni foam [103]

Fig. 7

The schemes of a single axial and b coaxial electrospinning [111, 112]. c Inorganic fibers [113]. d Inorganic particles encapsulated carbon fibers [114]. e The modification of carbon fibers [115]. f Carbon fiber membrane with nanoparticles [38]. g Highly flexible carbon fiber membrane [116]

Fig. 8

The scheme of vacuum filtration process [136]

Fig. 9

a The scheme of fabrication of robust, freestanding, and conductive Ti₃C₂T_x/S paper. Photographs of freestanding Ti₃C₂T_x/S paper when bending b convexly and c concavely, showing good mechanical flexibility similar to that of the pure Ti₃C₂T_x paper [158]

Fig. 10

a Assembly and bending tests of flexible batteries with flexible electrodes [164]. b Electrical resistance change with folding cycles [165]. c Capacity retention of folded cells at different angles at 1 C [165]

Fig. 11

a Schematic illustration for the structural features of the flexible SnO₂ nanosheets on flexible carbon cloth electrode during the folding (I), the rolling (II), and twisting (III) tests. b Current-time curves of the composite samples at various bending angles of the 1st and 200th cycles, and the inset images show the corresponding bending angles for measurement and photographs [166]

Fig. 12

a Digital photographs of Zn(CH₃COO)₂-PAN film, which can be folded four times. LED lighting tests of a full battery when b flat, c folded once, and d folded twice; and e digital photographs of the electrode after the LED lighting test [130]