金锋，李静，胡晨吉,3，董厚才，陈鹏，沈炎宾,＊，陈立桅,3,＊

1中国科学技术大学纳米技术与纳米仿生学院，合肥 230026

2中国科学院苏州纳米技术与纳米仿生研究所国际实验室纳米卓越科学中心，江苏 苏州 215123

3 上海交通大学化学化工学院，上海 200240

1 Introduction

Lithium ion batteries (LIBs) have been successfully commercialized during the last two decades as energy-storage device for portable electronic devices thanks to their high energy density and long cycle life1-3. However, today’s LIBs are suffering from safety issues caused by the leakage, poor thermal stability and flammability of organic liquid electrolytes4. Solidstate lithium batteries (SSLBs), in which flammable organic liquid is replaced with solid-state electrolyte, are believed to be intrinsically safe and has higher energy density when a lithium metal anode is used5. The main challenge for the development of high performance SSLBs lies in the construction of the ionic conduction path throughout the battery, which require high lithium ionic conductivity solid state electrolytes (SSEs) and close contacts at electrode/electrolyte interfaces6. With the intensive efforts on the investigation of SSEs in the past a few years, the current ionic conductivity of many SSEs is approaching 10-3S·cm-1, which is equivalent to that of the liquid electrolyte7. However, high interface resistance between the electrodes and electrolyte still leads to problems, including weak reversibility, inferior rate capability and poor cycle life in SSLBs8.

Most causes of high interface resistance between the electrode and electrolyte for lithium ion conduction can be categorized into three aspects: poor solid-solid physical contact, existence of space charge layers, and chemical/electrochemical instability at the interface6,8. Especially when the highly reactive lithium metal is used as the anode, it can react with many SSEs, such as NASICON-type9and perovskite type SSEs10. These side reactions will generate uncontrolled interphase formation, which in many cases increase the interface resistance and block the transfer of the Li+. Several strategies have been reported to reduce the interface resistance between the SSE and the electrode. In the fabrication of thin film solid state batteries, SSE(LiPON) and cathode materials are deposited via magnetron sputtering to form a closely-contacted interface11. Hu’s group reported introduction of interfacial layers such as ZnO12, Si13,Ge14and Al2O315between a garnet-SSE and a lithium metal anode to improve their contact. Additives and buffer layers such as LiNbO316,17and CoS18were also investigated to relieve the high resistance caused by space charge layer between sulfide electrolytes and oxide cathodes. Introduction of interlayer such as PEO19and LiPON20between the lithium metal anode and NASICON-SSEs was found to be effective on preventing side reactions. Sun et al. reported using an ultra-thin Al2O3layer on the Li1.5Al0.5Ge1.5(PO4)3(LATP) SSE to realize a highly stable LATP/Li metal interface21. Other approaches, such as casting or spin-coating of cathode slurry on the SSE, is also proven to be effective on improving the interfacial contact due to the increased contact area between the cathode electrode and the SSE22,23.

Here, we construct a closely-contacted electrode/electrolyte interface through integrating the cathode and the electrolyte into one membrane by simultaneously electrospinning and electrospraying for the cathode, followed by electrospinning preparation of the SSE and explore the performance of the ICEM.

2 Experimental

2.1 Preparation of the ICEM

The ICEM was prepared via a two-step method, including the fabrication of a binder-free LiFePO4(LFP) cathode layer through simultaneous electrospinning and electrospraying,followed preparation of a polyacrylonitrile (PAN) SSE by electrospinning. 10% (w) PAN dissolved in a dimethyl-formamide (DMF) solvent was used for electrospinning. A 5%(w) suspension of LFP and acetylene black (AB) in anhydrous ethanol formed through ultrasound was used for electrospraying.

The binder-free LFP cathode layer was prepared through simultaneously electrospinning using the PAN-DMF solution and electrospraying using the LFP suspension against an Al foil for 4 h. A high voltage of 15 kV was set between the electrospinning/electrospraying needles and the Al foil. The rotation speed of the Al foil collector was controlled at 800 r·min-1. The flow speed of the PAN solution was 0.42 mL·h-1,and the flow speed of the LFP suspension was kept at 4.2 mL·h-1.Besides, the humidity of the environment was controlled to below 20% and the temperature was kept over 40 °C.

After the preparation of the binder-free LFP cathode by simultaneously electrospinning and electrospraying for 4 h, the electrospraying of the LFP suspension was stopped but the electrospinning of the PAN solution was continued with a high voltage of 12 kV for another 2 h, then the as resulted membrane was roll pressed to a total thickness of around 80 µm.

The obtained film was then filled with a succinonitrilebistrifluoromethanesulfonimide (SN-LiTFSI) solution that contains 2.87 g LiTFSI dissolved in 15.2 g SN under 75 °C and cooled to room temperature, resulting in a PAN-LFP ICEM.

2.2 Preparation a slurry coated LFP cathode

A slurry coated LFP cathode was prepared as a controlled sample. 60 mg of polyvinylidene difluoride (PVDF) binder was dissolved in a proper amount of N-methyl pyrrolidone (NMP)solvent and stirred for 30 min, then added with 60 mg of AB and 720 mg of LFP and stirring was continued for overnight. Then,the obtained cathode slurry was coated on an Al foil and dried at 80 °C for 12 h in an oven.

2.3 Assembly of the solid-state cell

The PAN-LFP ICEM was assembled with a lithium metal anode by simply pressed the two together in an Ar-filled glovebox, resulting in a Li|PAN-LFP cell.

3 Results and discussion

As shown in Fig. 1, the ICEM is prepared through a two-step process. Firstly, a binder-free cathode layer is made by a simultaneously electrospraying and electrospinning process using a suspension containing the cathode components (LFP and AB) and a polymer solution (PAN in DMF), respectively. After 4 h, when the intended thickness of the cathode layer is reached,the electrospraying process was stopped and the electrospinning of the PAN solution was continued. The electrospin only Step 2 resulted in a PAN electrolyte layer on top of the LFP cathode.The ICEM was then roll pressed and a SN-LiTFSI salt solution was added under 75 °C. The ICEM was then cooled to room temperature. As shown in bottom of Fig. 1, the cathode layer within the ICEM contains positive active material and conducting carbon particles that uniformly distributed in a 3D PAN fiber network; while only a PAN fiber network exists in the electrolyte layer. Because the 3D PAN network through the whole ICEM is prepared by continuous electrospinning, the cathode and the electrolyte layer is closely-contacted to each other.

Fig. 1 Schematic illustration for the fabrication process of the ICEM.

As shown in the digital images in Fig. 2a, the as-prepared ICEM is flexible and can be bent to a large angle. Fig. 2b, c show the morphology of the LFP cathode layer before being covered with the PAN electrolyte layer. The cathode is composed of large number of LFP particles that uniformly distributed in pores of the 3D PAN network (Fig. S1, Supporting Information), while the electrolyte layer only shows interlaced PAN nanofibers with an average diameter of ～460 nm (Fig. S2, Supporting Information). After the addition of the SN-LiTFSI salt, all the pores in the electrolyte layer were filled with SN-LiTFSI (Fig.2f), and the morphology of the PAN nanofibers in both the cathode and electrolyte changes from smooth to rough (Fig. 2e,f), which could be due to presence of the SN particles. Fig. 2d and 2g show the cross-section of the ICEM before and after adding SN-LiTFSI. It can be found that the electrolyte layer is porous while the cathode layer is pretty dense before adding the lithium salt (Fig. 2d), but it is difficult to distinguish the two layers using scanning electron microscope (SEM) after the addition of the SN-LiTFSI salt (Fig. 2g). This cross-sectional morphology is very different compared to that made by traditional coating process that showing obviously interface between the cathode and the electrolyte (Fig. S3, Supporting Information). Energy dispersive X-ray spectroscopy (EDX)mapping of N and P elements was conducted to distinguish the cathode and the electrolyte layer. It can be clearly seen that N(from PAN) can be detected through the whole ICEM (Fig. 2h)while P (from LFP) only appeared in the cathode layer (Fig. 2i).

Fig. 2 Optical image of the ICEM (a); SEM images of the cathode before (b) and after (e) adding SN-LiTFSI, and the electrolyte before (c) and after(f) adding SN-LiTFSI; Cross-sectional SEM images of the ICEM before (d) and after (g) adding SN-LiTFSI; The element maps of N from PAN (h)and P from LFP (i) for the SEM image showed at Fig. g.

Fig. 3 (a) Schematic of the Li|PAN-LFP cell. (b) Areal resistance of the Li|PAN-LFP cell at different storage time after the assembly.(c) A photograph showing the open circuit potential of the Li|PAN-LFP coin cell. (d) Rate capabilities of the Li|PAN-LFP cell. (f) Cycling performance of the Li|PAN-LFP cell at 0.2C rate and (e) its corresponding galvanostatic charge/discharge voltage profiles in different cycles.The Li|PAN-LFP cell can light up a LED under a bending angle (g) and after cutting into two parts (h).

The ICEM after addition of the SN-LiTFSI salt was assembled with a lithium foil anode to prepare a Li|PAN-LFP cell with the configuration showed in Fig. 3a, and the electrochemical performance was investigated. As shown in Fig. 3b, the cell impedance kept relatively stable (increased 104 Ω·cm-2to 110 Ω·cm-2) during 72 h storage after the cell assembly, indicating that no obvious chemical reaction that generated high impedance interfacial layer happened in the cell during the storage. The open circuit voltage of the Li|PAN-LFP cell was measured to be 3.403 V, which indicates that polarization of the cell is small.Fig. 3d shows the specific capacity of the Li|PAN-LFP cell measured at various C-rates. The measured capacity was approximately 160.8 mAh·g-1at 0.1Cand 109.8 mAh·g-1at 2C,which are pretty closed to the theoretical specific capacity (170 mAh·g-1) of the LFP cathode material, indicating a very good dynamic process thus good lithium ion conducting path through the solid-state Li|PAN-LFP cell. The voltage profiles at different cycles showed in Fig. 3e exhibits a small polarization of the cell between charge and discharge, which is similar to that of a liquid Li||LFP cell. The small polarization of the as-prepared Li|PANLFP cell could be attributed to the continuous ionic conducting path that ensured by the 3D PAN/LiTFSI electrolyte framework through the integrated cathode and electrolyte. Solid state cathode that prepared by mixing the active material with the solid-state electrolyte then slurry coating, will present numerous solid-solid contact impedance for Li ion transfer between the active material and the solid-state electrolyte thus has large polarization during charge/discharge. More importantly, the polarization did not get worse during long term cycling,indicating a stable electrode|electrolyte interface in the cell. The capacity retention of the Li|PAN-LFP cell is around 80.3% after 500 cycles at 0.2C(Fig. 3e), which is ranged one of the best in the reported cycle life of solid-state battery. More importantly, a small pouch Li|PAN-LFP cell can light up a LED even at a bent state (Fig. 3g) or after being cut (Fig. 3h), indicating excellent flexibility and safety of the cell.

4 Conclusions

In summary, an integrated ICEM membrane including a cathode and electrolyte has been successfully constructed using simultaneous electrospinning and electrospraying followed by continuous electrospinning. The resulted ICEM (PAN-LFP)showed closely-contacted interface between the cathode and the electrolyte. A Li|PAN-LFP cell made from a lithium metal anode and the as prepared PAN-LFP ICEM showed small polarization,similar to that of a Li||LFP cell with liquid electrolyte, but with much better cyclability and electrochemical performance. In specific, the Li|PAN-LFP cell exhibited approximately a discharge specific capacity of 160.8 and 109.8 mAh·g-1at 0.1C and 2C rates, respectively, and a capacity retention of 80.3%after 500 cycles at a rate of 0.2C. Generally, this work provides a new way to alleviate the interface problem between the electrode and electrolyte in solid state batteries.

Supporting Information:available free of charge via the internet at http://www.whxb.pku.edu.cn.