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1 Supporting Information Mg 2 B 2 O 5 Nanowires Enabled Multifunctional Solid-State Electrolyte with High Ionic Conductivity, Excellent Mechanical Properties and Flame-retardant Performance Ouwei Sheng, Chengbin Jin, Jianmin Luo, Huadong Yuan, Hui Huang, Yongping Gan, Jun Zhang, Yang Xia, Chu Liang, Wenkui Zhang, and Xinyong Tao, * College of Materials Science and Engineering, Zhejiang University of Technology, Hangzhou , People s Republic of China * Correspondence should be addressed to tao@zjut.edu.cn Supplementary methods Synthesis of Mg 2 B 2 O 5 nanowires: The nanowires were synthesized via a KCl-NaCl flux method. First of all, MgCl 2 6H 2 O and Na 2 B 4 O 7 10H 2 O were mixed with a molar ratio of n(b)/n(mg)=3:1. Then, KCl and NaCl of the same molar ratio were introduced as cosolvent and the adding quality was equal to 80 wt% total quality of MgCl 2 6H 2 O and Na 2 B 4 O 7 10H 2 O mixture. After grinding uniformly, the mixture was then calcined in muffle furnace at 800 o C for 6 h with a heating rate of 5 o C min -1. The obtained samples were dissolved in 80 o C hot water and filtered, dried to obtain the final product. Preparation of SSEs film and assembly of the SSLIBs: Beforehand, PEO (M v =10 6 g mol -1, Aldrich), Mg 2 B 2 O 5 and LiTFSI (Aldrich) were dried at 50, 80, 120 o C respectively. Then the Mg 2 B 2 O 5, LiTFSI, PEO were dissolved in acetonitrile, stirred for 24 h to form a homogeneous solution. The amount of LiTFSI was determined by

2 the molar ratio of (EO)/Li + (EO/Li + =20), and the quantity of Mg 2 B 2 O 5 was decided by the mass ratio of Mg 2 B 2 O 5 /LiTFSI and PEO (0, 5, 10, 15, 20 wt%). The homogeneous solution was casted in polytetrafluoroethylene (PTFE) base and dried at 60 o C for 24 h. The obtained thin film with thickness of 0.22 mm was used in performance characterization. The PEO based SSLIBs were assembled in 2025-type coin cells with lithium metal anode, PEO-LiTFSI-Mg 2 B 2 O 5 composite electrolyte and LiFePO 4 cathode. The cathode was composed of LiFePO 4, conductive additive and PEO-LiTFSI-Mg 2 B 2 O 5 electrolyte binders with a weight ratio of LiFePO 4 : super P: binder=7:1:2. The slurry was coated on Al foils and dried at 60 o C for 16 h. Then a certain amount of PEO-LiTFSI-Mg 2 B 2 O 5 electrolyte was coated onto cathode directly and dried at 60 o C for 24 h in order to make the complete volatilization of acetonitrile. This strategy can effectively reduce the interfacial impedance between cathode and electrolyte. The battery was housed and sealed in a 2025 coin cell for electrochemical measurements and these above process were all performed in an argon filled glove box. Characterization: X-ray diffraction (XRD) analysis aimed to investigate the crystalline phase of the Mg 2 B 2 O 5 samples and composite SSEs films was conducted on an X'Pert Pro diffractometer using Cu K α radiation (λ= nm). The data were recorded in the 2 theta range of o with a 20 o stepwise per minute. The scanning electron microscopy (FE-SEM, NanoSEM450) was used to investigate the morphology of Mg 2 B 2 O 5 powder and composite SSEs films. Elemental analysis was conducted on an energy dispersive X-ray (EDX) spectrometer attached to SEM. The

3 transmission electron microscopy (TEM, FEI, Tecnai G2 F30) was also used to observe the morphology of Mg 2 B 2 O 5 nanowires. Chemical bonds of PEO, LiTFSI, Mg 2 B 2 O 5, PEO-LiTFSI, PEO-LiTFSI-Mg 2 B 2 O 5 were determined by the Fourier transform infrared spectrum (FT-IR). Differential scanning calorimetry (DSC) was carried out to investigate the melting transition of SSEs with and without Mg 2 B 2 O 5 nanowires at a heating rate of 10 o C min -1 from o C in the heating cycle under nitrogen atmosphere. In the DSC test, the weights of SSEs were maintained in the range of 7-8 mg. The thermogravimetry (TG) analysis was conducted to test the thermal stability of SSEs under N 2 atmosphere from 25 to 780 o C with a heating rate of 10 o C/min. The mechanical stability of the SSEs films was measured by high and low temperature tensile test machine. The composite SSEs were made into long strips with length of 18.7 mm, width of 10 mm, thickness of 0.22 mm and the stretching speed is fixed at 100 mm min -1. Flammability tests of liquid electrolytes (1M LiPF 6 dissolved in a mixture of ethylene carbonate (EC)/dimethyl carbonate (DMC) (v/v=1:1)) and composite SSEs with or without Mg 2 B 2 O 5 were compared using a burning torch heater. Electrochemical measurements: The ionic conductivities of composite SSEs were tested via a SS/SSEs/SS (SS= stainless steel) cell and evaluated by electrochemical impedance spectroscopy, in which the temperature range was set between 0-80 o C with the frequency range between Hz. The Li-ion transference number (t + Li ) of SSEs was measured in Li/SSEs/Li cell with DC polarization. Voltage of 10 mv was applied and EIS spectra of cell before and after polarization were obtained from

4 Hz. The calculated of t + Li is based following equation: t + Li = I ss R ss b ( V-I o R o i )/ I o R o b ( V-I ss R ss i ) Where I o and I ss are the initial and steady state currents, V is a polarization potential at 10 mv. R o b and R ss b are the initial and steady state bulk resistances, and R o ss i and R i are Li/electrolyte interfacial resistances. Pulse gradient field nuclear magnetic resonance (PFG NMR) was used to measure the diffusion coefficient of Li + in SSEs. Linear sweep voltammetry (LSV) were conducted on the electrochemical workstation using Li/SSEs/SS 2025 coin cells from V. The stability of lithium anode against these composite SSEs was tested by Neware multichannel battery cycler using Li/SSEs (with or without Mg 2 B 2 O 5 )/Li symmetric cells, proceeding the stripping/plating test at a current density of 0.1, 0.2, 0.3, 0.5 ma/cm 2. Rate performance of the SSLIBs with PEO-LiTFSI electrolyte and PEO-LiTFSI-Mg 2 B 2 O 5 electrolyte at 50 o C was evaluated at 0.2, 0.4, 0.8, 1.0, 2.0 C for 5 cycles respectively and finally back to 0.2 C for 25 cycles. The C-rates in all of the electrochemical measurements are defined based on 1.0 C=174 mah g -1. Cycle performance of SSLIBs using LiFePO 4, LiCoO 2, LiNi 1/3 Co 1/3 Mn 1/3 O 2 cathode was tested by Neware multichannel battery cycler at o C, and the SSLIBs were charged and discharged between V at 0.2 and 1.0 C. The electrochemical impedances of the SSLIBs with PEO-LiTFSI-Mg 2 B 2 O 5 electrolyte and PEO-LiTFSI electrolyte before and after rate capability tests were measured using CHI660 Chenhua electrochemical workstation with the frequency range between Hz.

5 Supplementary figures Figure S1. (a) SEM image of Mg 2 B 2 O 5 nanowires. (b) Corresponding diameter distributions and average diameters. Figure S2. (a) The conductivity of composite SSEs with different Mg 2 B 2 O 5 additives at 40, 50 o C. The inset is equivalent circuit model for the composite SSEs film. (b) Impedance spectroscopy of PEO-LiTFSI-10 wt% Mg 2 B 2 O 5 film at different temperatures. (c) The mechanical property of different composite SSEs with various Mg 2 B 2 O 5 additives. (d) Digital photograph of the PEO-LiTFSI-10 wt% Mg 2 B 2 O 5 film at initial and final stretch state. Figure S2b is typical AC impedance spectra of the composite SSEs containing 10 wt% Mg 2 B 2 O 5 nanowires measured at various temperatures, which is fitted by the

6 Vogel-Tamman-Fulcher (VTF) equation at high temperature (Amorphous) and Arrhenius equation at low temperature (Crystalline). These spectra consist of a semicircle at high frequencies area and a line at low frequencies area. The semicircle results from the migration of the lithium ions, which is contributed to the bulk resistance (R b ) and bulk capacitance (CPE). The straight line is a reference of the interface impedance between stainless steel electrodes and electrolyte which is corresponding to Z w (Figure S2a inset). The digital photos of composite SSEs containing 10 wt% Mg 2 B 2 O 5 nanowires before and after stretch are shown in Figure S2d. From these figures, it is confirmed that even after stretch with nearly 1000% elongation, no cracks are observed on the composite SSEs, illustrating the advanced flexibility of this electrolyte. Figure S3. The enlarged stress-strain curves of different composite electrolyte with various Mg 2 B 2 O 5 additives.

7 Figure S4. Flammability tests of liquid electrolyte (1M LiPF 6 dissolved in a mixture of ethylene carbonate (EC)/dimethyl carbonate (DMC) (v/v=1:1)). Figure S5. The AC impedance spectra of the Li/ SSEs/ Li cell with different electrolytes before and after polarization at 50 o C, (a) PEO-LiTFSI electrolyte, (b) PEO-LiTFSI-Mg 2 B 2 O 5 electrolyte. Figure S6. PFG NMR measured results of (a) PEO-LiTFSI electrolyte and (b) PEO-LiTFSI-Mg 2 B 2 O 5 electrolyte.

8 Figure S7. TG curves of the thermal property for PEO-LiTFSI electrolyte and PEO-LiTFSI-Mg 2 B 2 O 5 electrolyte. In Figure S7, we can see that the thermo-gravimetric curves of PEO-LiTFSI electrolyte, PEO-LiTFSI-Mg 2 B 2 O 5 electrolyte from 25 to 780 o C. The 0.6 wt% weight loss could be observed below 100 o C, which is due to the loss of water absorbed from the atmosphere. And the maximum weight loss occurs at o C, which corresponds to the damage of PEO and LiTFSI. Eventually, the remaining amount of PEO-LiTFSI electrolyte and PEO-LiTFSI-Mg 2 B 2 O 5 electrolyte is about 4 and 13 wt%, respectively. The difference of 9 wt% is due to thermal stability of Mg 2 B 2 O 5. This results show that the addition of Mg 2 B 2 O 5 to SSEs is beneficial to increase the thermal stability of composite materials.

9 Figure S8. Voltage-time profiles of Li/ SSEs (with or without Mg 2 B 2 O 5 ) /Li cells at 0.1, 0.2, 0.3, 0.5 ma cm -2 and 50 o C. Figure S8 shows the Li symmetric test of PEO-LiTFSI electrolyte and PEO-LiTFSI-Mg 2 B 2 O 5 electrolyte at different current densities. Obviously, the PEO-LiTFSI-Mg 2 B 2 O 5 electrolyte shows a much lower Li stripping/plating overpotential at all current densities (65, 100, 140, 200 mv at 0.1, 0.2, 0.3 and 0.5 ma cm -2 ) compared with PEO-LiTFSI electrolyte (100, 175, 220, 330 mv at 0.1, 0.2, 0.3 and 0.5 ma cm -2 ).

10 Figure S9. The cross-sectional enlarged SEM image of SSEs with Mg 2 B 2 O 5 additive. Figure S10. (a) Electrochemical performance at 0.2 C for LiFePO 4 based SSLIBs at different temperature (30, 40, 50 o C). (b) Charge and discharge performance at 0.2 C for LiFePO 4 based SSLIBs at different temperature.

11 Figure S11. Electrochemical characterization of PEO-LiTFSI-Mg 2 B 2 O 5 electrolyte in solid-state batteries. (a,c) The first three charge-discharge curves of LiCoO 2 /SSEs/Li battery and LiCo 1/3 Ni 1/3 Mn 1/3 O 2 /SSEs/Li battery at 50 o C and 0.2 C. (b,d) Cycling performance of two batteries at 50 o C and 0.2 C. We fabricated batteries using LiCoO 2 cathode, LiCo 1/3 Ni 1/3 Mn 1/3 O 2 cathode and PEO-LiTFSI-Mg 2 B 2 O 5 electrolyte. As can be seen in Figure S11, LiCoO 2 and LiCo 1/3 Ni 1/3 Mn 1/3 O 2 all have their characteristic charging and discharging platform. And the batteries respectively present stable capacities with a value of 130 and 140 mah g -1 in the 50 cycles at 50 o C and 0.2 C. And high efficiency of 99% can be obtained for both batteries at each cycle expect first three cycles. Moreover, capacity retention close to 73% (LiCoO 2 cathode) and 93% (LiCo 1/3 Ni 1/3 Mn 1/3 O 2 cathode) compared to the discharge capacity in the second cycle can be maintained.

12 Figure S12. (a) Mg 2 B 2 O 5 nanowires with a length-diameter ratio of 12. (b) Mg 2 B 2 O 5 nanowires with a length-diameter ratio of 4. (c) Cycle performance at 0.2 C for LiFePO 4 based SSLIBs at 50 o C using different length-diameter ratio Mg 2 B 2 O 5. (d) The ionic conductivity of solid state electrolyte using different length-diameter ratio Mg 2 B 2 O 5 nanowires. The morphology and performance of Mg 2 B 2 O 5 nanowires with different length-diameter ratio was compared. In Figure S12a,b, The length-diameter ratio of Mg 2 B 2 O 5 nanowires is near 12 and 4. In Figure S12c, we can see that there is a better capacity and cyclic stability of Mg 2 B 2 O 5 nanowires with length-diameter ratio of 12 than that of 4. Compared with the 10th cycle, about 81 % of the capacity was maintained after 90 cycles for SSEs adding Mg 2 B 2 O 5 nanowires with length-diameter ratios of 12, which are better SSEs adding Mg 2 B 2 O 5 nanowires with length-diameter ratios of 4 (60% capacity retention). In addition, the ionic conductivity of Mg 2 B 2 O 5 nanowires with length-diameter ratio of 12 is also higher than that of 4 (Figure S12d).

13 Table S1. The activation energy of the different solid-state electrolytes Ea 1 (ev) Ea 2 (ev) 0wt% wt% wt% wt% wt% Ea 1 is activation energy at low temperature, and Ea 2 is activation energy at high temperature