Rolling Press Enables Electrospun Fibers Based Binder-Free Electrodes with High Stability for Lithium Ion Batteries

Rolling Press Enables Electrospun Fibers Based Binder-Free Electrodes, Batteries Abstract With the demand for higher energy density and smaller size lithium-ion batteries (LIBs), the development of high specific capacity active materials and the reduction of the usage of inactive materials are the main directions. Herein, a universal method is developed for binder-free electrodes for excellent stable LIBs by rolling the electrospun membrane directly onto the commercial current collector. The rolling process only makes the fiber web denser without changing the fiber structure, and the fiber web still maintains a porous structure. This strategy significantly improves the structural stability of the membrane compared to the direct carbonized electrospun membrane. Moreover, this method is suitable for a variety of polymerizable adhesive polymers, and each polymer can be composited with different polymers, inorganic salts, etc. The electrode prepared by this method can be stably cycled for more than 2000 cycles at a current density of 2500 mA g-1. This study provides a cost-effective and versatile strategy to design the LIB electrode with high energy density and stability for experimental research and practical application.


Introduction
Lithium-ion batteries (LIBs) are widely applied in portable devices, electric vehicles, and stationary energy storage systems [1,2]. Energy density is one of the most important parameters for LIBs. Though much effort has been made to improve the specific capacity of the anode and/or cathode materials, the research of reducing the electrochemicallyinactive component in the electrode materials is limited. State-of-the-art battery preparation process with ~ 10 wt.% polyvinylidene fluoride (PVDF) and carbon materials as the binder and conductive additives, respectively, limits the specific capacity and energy density of LIBs [3]. The reduction of the amount of inactive materials in the 3 electrode is an effective method to improve energy density. Therefore, the binder-free electrode, which only consists of active materials and conductive substrate, offers a new opportunity to enhance the energy density of electrodes [4].
Nowadays, the methods to prepare the binder-free electrode are mostly hydrothermal synthesis, vapor deposition, etc. [5][6][7][8], which operate generally under harsh conditions in a limited scale. Although binder-free electrodes can be easily fabricated by electrospinning technique with a simple, versatile and cost-effective way [9], the asprepared membranes often become brittle after carbonization [10], thus, the electrodes have to be prepared by mixing and grinding the carbonized materials with PVDF in organic solution, which is not only time-consuming but also inefficient. The grinding process could lead to the decrease of particle size, the increase of surface area and the exposure of active materials to the electrolyte, all of which will result in poor electrochemical performance [11]. Therefore, it is extremely important to design the stable electrospun membrane for advanced binder-free electrodes.
Here, a universal method is developed for binder-free electrodes for stable LIBs by rolling the electrospun membrane directly onto the commercial current collector. The porous structure of the fiber network can be maintained after the rolling process. This method significantly improves the structural stability of the membrane compared to the direct carbonized membrane. The power and energy density of the active materials can be significantly enhanced by the unique binder-free process. Besides, a variety of polymerizable adhesive polymers can be used as the electrospun membrane sources for this study, and inorganic salts or particles can be added into the polymers to fabricate high performance electrodes. The electrode prepared by this method can be stably cycled for more than 2000 cycles at a current density of 2500 mA g -1 .

Fabrication of fiber membranes
The coaxial electrospinning needles were purchased from Changsha Nanoapparatus China.
The core-shell fiber membranes were obtained by extruding 10 wt.% polyacrylonitrile

Membrane characterization
The morphology of the binder-free electrodes was characterized by Scanning Electron Microscopy (SEM, Hitachi, SU-8010). The crystalline structure of the membranes was examined by X-Ray Diffraction (XRD, SmartLab, Rigaku) and Raman spectroscopy (Horiba, HR-800). 5

Electrochemical characterization
The electrochemical performance was evaluated using coin cells with fiber membrane discs as working electrode and lithium foil as the counter electrode. The electrolyte contained 1 mol L -1 LiPF 6 in a mixture of ethylene carbonate (EC) and dimethyl carbonate (DMC) (v/v = 1:1). The galvanostatic discharge-charge cycling was examined in Land system (CT2001A, BTRBTS) in the voltage range of 0.01-3 V, and the current densities are set at 250 mA g -1 in the first 5 cycles for activation and gradually increased to 2500 mA g -1 in the following cycles.

Results And Discussion
Pressing process is just the physical combination of electrospun membrane and Cu foil.
When pressing, the solvent-containing electrospun fibers are similar to the binder and adheres strongly to the current collector. The pressing process did not damage the porous structure of the materials (Figure 2). After carbonized, the Cu foil will form a strong connection with the polymer. It is worth noting that this method is suitable for a variety of electrospun fibers, and here only three representative materials are demonstrated, namely: pure polymer (Figure 2a), polymeric composite (Figure 2b), and inorganic and polymeric composite (Figure 2c). PMMA@PAN@Cu material smoothly remains on the Cu foil after the same testing process (Figure 3e, f). The ultrasonic treatment and adhesion test clearly demonstrate that the carbon material of the PMMA@PAN@Cu has a strong adhesion to the Cu foil [12].
The crystal structure of PMMA@PAN and PMMA@PAN@Cu is characterized by Raman spectroscopy and XRD to observe the differences after pressing the polymer fibers onto the Cu foil (Figure 3g, h). The first peak of Raman spectra at about 1350 cm -1 and the second at 1590 cm -1 corresponds to the D band of defect-induced mode and the G-band of E 2g graphitic mode, respectively [13]. The intensity ratios between the D and G band indicating the disorder degree of carbon materials. It shows the same value of 1. This featured peak is corresponding to layers of the graphite structure [14]. In short, the carbonization process of the electrospun membrane has not changed after being composited with Cu foil.

Electrochemical performance
The electrochemical performances of various binder-free electrodes are examined using a CR2032 coin-type half-cells. The rate performances at current densities ranging from 250 to 2500 mA g -1 are displayed in

Conclusion And Perspective
A universal method is developed for binder-free electrodes for LIBs with stable electrochemical performance. This method is not only suitable for the preparation of binder-free electrodes, but also has the potential to be a current collector protection strategy. A thin layer of active carbon material can be coated on the surface of the current collector to avoid the contact of current collector and electrolyte without increasing the content of inactive materials. It is believed that not only Cu foil but also Al foil can achieve similar functions. In addition, the adhesion between the binder and the current collector can be enhanced by coating the carbon onto the current collector.
Therefore, it is more convenient to develop high energy density electrode by utilizing this strategy.  13 Figure 4 (a, b) Cycling performances of different binder-free electrodes, and the corresponding rate performances showed in the insert images.