Regulating Interfacial Li‐Ion Transport via an Integrated Corrugated 3D Skeleton in Solid Composite Electrolyte for All‐Solid‐State Lithium Metal Batteries

Abstract Although solid composite electrolytes show tremendous potential for the practical solid‐state lithium metal batteries, searching for a straightforward tactic to promote the ion conduction at electrolyte/electrode interface, especially settling lithium dendrites formation caused by the concentration gradient polarization, are still long‐standing problems. Here, the authors report a corrugated 3D nanowires‐bulk ceramic‐nanowires (NCN) skeleton reinforced composite electrolyte with regulated interfacial Li‐ion transport behavior. The special and integrated NCN skeleton endows the electrolyte with fast Li‐ion transfer and solves the Li+ concentration polarization at electrode/electrolyte interface, thereby eliminating the energy barrier originated from the redistribution of charge carriers and offering homogeneous interfacial Li‐ion flux on lithium anode. As a “double insurance”, the bulk ceramic sheet in 3D framework enables the electrolyte to block the mobility of anions. The rational designed NCN composite electrolyte exhibits excellent ionic conductivity and the assembled all‐solid‐state battery possesses 90.2% capacity retention after 500 cycles. The proposed strategy affords a special insight in designing high‐performance solid composite electrolytes.


Regulating Interfacial Li-ion Transport via an Integrated Corrugated 3D Skeleton in Solid Composite Electrolyte for All-Solid-State Lithium Metal Batteries
Rong Fan, Wenchao Liao, Shuangxian Fan, Dazhu Chen, Jiaoning Tang, Chen Liu* and Yong Yang Experimental Section

Preparation of ceramic skeletons
Preparation of LLZTO thin ceramic sheet Li 6.5 La 3 Zr 1.5 Ta 0.5 O 12 (LLZTO) ceramic sheet was prepared by sol-gel method.
First, dimethylformamide (DMF) and acetic were mixed at a volume ratio of 5:1.
Afterwards, the solution was casted on a smooth aluminum foil to form a gel film.
The film was then placed in a vacuum oven at 80 °C for 6 h to evaporate the solvents.
Finally, the LLZTO ceramic sheet was obtained by calcining at 700 °C for 3 h at a rate of 1 °C min -1 .

Preparation of LLZTO network skeleton (NET-LLZTO)
NET-LLZTO was prepared by electrospinning process. Firstly, the sol-gel solution of LLZTO was prepared as same as the LLZTO ceramic sheet, and then electrospuned at the speed of 0.05-0.08 mL·min -1 under a voltage of 15~20 kV and collected on the aluminum foil. Finally, the nanofiber film was peeled off from the foil and calcined at 700 °C (1 °C min -1 ) for 3 h in the air and LLZTO network skeleton was attained after the annealing process.

Preparation of the nanowire-bulk ceramic-nanowire LLZTO skeleton (NCN-LLZTO)
The preparation of NCN-LLZTO skeleton combined the sol-gel method and electrospinning method. First, the gel film and nanofiber film were prepared according to the above-mentioned methods. Then the gel film was sandwiched between two nanofiber membranes and rolling pressed together. The films should be kept flat during the rolling process to avoid air bubbles and the gel film cannot be completely dry Thereafter, the prepared NCN-LLZTO precursor membrane was put in a muffle furnace at a rate of 1 °C min -1 and sintered at 700 °C for 3 h. The 3D nanowires-bulk ceramic-nanowires (NCN) skeleton was finally obtained.

Preparation of 3D composite electrolytes
The composite electrolytes were prepared via solution-casting method. For PEO-LiTFSI polymer matrix, PEO (Mn = 600000, Sigma-Aldrich) and lithium bis(trifluoromethane) sulfonimide (LiTFSI, Sigma-Aldrich) were dissolved in acetonitrile with an EO: Li + molar ratio of 8: 1 and kept stirring for 3 h. The polymer solution was casted on a Teflon mould and subsequently dried overnight under vacuum at 60 °C to remove the solvent. The prepared films were dried in a glove box for further use. For the polymer-ceramic-polymer structure composite electrolyte (PCP-CPE), the prepared LLZTO sheet was placed between two polymer films and rolling pressed together through a laminator. Taking care that the gas needs to be removed completely. For the 3D network structure composite electrolyte (NET-CPE) and integrate NCN-CPE, the active ceramic skeletons also coated the polymer matrix by solution-cast method. The ceramic skeletons were soaking in a small amount of polymer solution at first and dried in a vacuum oven for 2 h at room temperature and 12 h at 60 °C to remove the solvent. This process needs to be repeated several times until the ceramic was fully coated by the polymer. An extra heating process at 80 °C for 1 h under pressure was employed if the surface of CPEs is not flat. Finally, the obtained composite electrolyte membranes were stored in an argon-filled glove box with H 2 O and O 2 contents below 0.1 ppm to completely evaporate the solvent residue before use.

Characterization of 3D Structure Composite Electrolytes
X-ray diffraction patterns (XRD) of the LLZTO skeleton and NCN-CPE were examined by Brucker AXS D8 diffractometer with Cu Kα radiation (40 kV and 300 mA) in the 2θ range of 10°~70°, λ=1.5418Å. The morphologies of the LLZTO skeletons and three-dimension composite electrolytes were characterized by a field-emission scanning electron microscope (FESEM, Hitachi SU-70) with energy dispersive X-ray spectroscopy (EDS). Differential scanning calorimetry (DSC) measurements were conducted on a TA instrument (DSC 25) with a heating rate of 4 analysis (TGA 55, TA instruments) was conducted under an air atmosphere with a heating rate of 10 °C min −1 to determine the formation temperature of LLZTO ceramic skeleton for the calcination process.

Electrochemical properties of obtained composite electrolytes
The cells were all assembled in a glove box and tested by an American Princeton electrochemical impedance spectroscopy (EIS, PMC-1000). The ionic conductivity of the CPE was measured between two stainless steel (SS) blocking electrodes in a frequency range of 3 MHz to 0.5 Hz ranging from 25 °C to 80 °C. Electrochemical stability and cycling performance measurements were conducted by assembling instrument. For the pouch cell assembly, the preparation process of electrodes and composite electrolytes was similar to that for coin cells. An electrode welding operation was employed to bonding the solid electrolyte. Afterwards, the cell was sealed in an aluminum plastic package using a vacuum hot-pressing machine in glove box.