A Highly Reversible Room-Temperature Lithium Metal Battery Based on Cross-Linked Hairy Nanoparticles

Abstract

Rough electrodeposition, uncontrolled parasitic side reactions with electrolytes, and dendrite-induced short circuits have hindered development of advanced energy storage technologies based on metallic Li, Na, and Al electrodes. Solid polymer electrolytes and nanoparticle-polymer composites have shown promise as candidates to suppress Li dendrite growth, but the challenge of simultaneously maintaining high mechanical strength and high ionic conductivity at room temperature has so far been unmet in these materials. Here, we report a facile and scalable method of fabricating tough, freestanding membranes that combine the best attributes of solid polymers, nanocomposites, and gel polymer electrolytes. Hairy nanoparticles are employed as multifunctional nodes for polymer cross-linking, which produces mechanically robust membranes that are exceptionally effective in inhibiting dendrite growth in a lithium metal battery. The membranes are also reported to enable stable cycling of lithium batteries paired with conventional intercalating cathodes. Our findings appear to provide an important step towards room-temperature dendrite-free batteries.

Appendix: Supplementary Information

3.1.1 Supplementary Figures

Supplementary Fig. 3.1

Fourier transform infrared spectroscopy (FTIR) characterization of the reaction scheme: the reaction process is confirmed by the IR-peak transition involved in urethane reaction. The –NCO peak of the PPO diisocyanate disappears, while characteristic peaks of –NH vibration and shift in the –C=O peak appear because of the urethane bond formation in the final product

Supplementary Fig. 3.2

TGA analysis: the initial silica content in the hairy nanoparticle (Si-PEO) was 83%, which reduces to 6% silica in the final product of cross-linked film. Thus, the nonconducting entity in the entire material is remarkably low, in spite of having decent mechanical strength. Further soaking the CNPC in 1 M electrolytes comprising of PC-LiTFSI reduces the silica content close to 2%, as shown in the inset of the figure. Also, it is evident that the film is remarkably stable, such that there is degradation of the product up to 120 °C, where PC starts to degrade

Supplementary Fig. 3.3

DSC characterization: the glass transition temperature of the PPO polymer increases from −63 °C to −42 °C due to the reduction of free volume as the chains are constricted due to the cross-linking. However, the cross-linked film is still in amorphous state at room temperature where it is used as electrolyte in battery systems. The amorphous nature of the polymer membrane enables higher ionic conductivity compared to other high MW PEO based electrolyte at room temperature

Supplementary Fig. 3.4

TEM analysis: the interparticle spacing in the cross-linked polymer is estimated by graphical analysis of TEM micrograph

Supplementary Fig. 3.5

Equivalent electric circuit for the impedance spectroscopy results: the Nyquist plot obtained by the measurement of the impedance at a wide range of frequency can be fitted by an equivalent circuit shown above. The bulk resistance and interfacial resistance obtained are plotted against temperature; and it is seen that the interfacial resistance is always higher than the bulk resistance, which indicates interface limited ion transfer

Supplementary Fig. 3.6

Strip-plate measurement of a symmetric lithium cell without CNPC separator: the voltage profile of a control cell (Li/PC + LiTFSI/Li) is plotted against time. (a), It is seen that at current density of 0.20 mA/cm² about 55 h of charging-discharging, the voltage profile gets distorted and the voltage range drops down which is a signature of dendrite-induced short circuit. (b), At current density of 1.00 mA/cm², the voltage profile is unstable even at the start of cycle

Supplementary Fig. 3.7

SEM image of pristine lithium: surface of lithium is presented in order to compare the changes in surface morphology after cycling using neat and cross-linked gel-based electrolytes. (Scale bar is 10 microns)

Supplementary Fig. 3.8

Coulombic efficiency test using Li| electrolyte| stainless steel configuration showing 40th cycle: batteries with cross-linked gel electrolyte and pristine PC-LiTFSI were cycled at 0.25 mA/cm². It is seen that at the 40th cycle, the neat electrolyte exhibits deposition at a much lower voltage and also low coulombic efficiency compared to the cross-linked gel electrolyte

Supplementary Fig. 3.9

Polarization curve of symmetric lithium cell with cross-linked gel electrolyte: a symmetric lithium cell consisting of cross-linked gel electrolyte is charged constantly at a current density of 0.12 mA/cm². It is seen that the cell successfully depositing Li ion onto the anode surface for about 400 h before failing is indicated by drop in the voltage profile

Supplementary Fig. 3.10

Cycling performance using LTO cathode: it is seen that using the cross-linked gel electrolyte, a LTO-based battery cycles well for over 150 cycles at a high current density of 1 mA/cm²

Supplementary Fig. 3.11

Cycling performance of LiFePO₄ battery using cross-linked gel electrolyte: it is seen that at a C-rate of C/3, the battery cycles with minimum fade for at least 100 cycles

3.1.2 Supplementary Tables

Supplementary Table 3.1 Content of different components at successive synthesis stage: the weight percent of the silica, PEO, PPO, and electrolyte is given in the table. It is seen that ultimately in the cross-linked gel, electrolyte contains as low as 2% silica, still having a relatively high mechanical modulus

Supplementary Table 3.2 VFT parameters of the different electrolyte configuration: σ = Αexp(−B/(T−T_O)), where A is the pre-exponential factor corresponding to conductivity at infinite temperature, B is the activation energy, and T_O is the reference temperature

3.1.3 Supplementary Methods

The size of clusters was obtained by fitting the SAXS data with Beaucage unified Eq. (3.1) as shown below.

$$ I(q)=A\exp \left(-\frac{q^2{R}_p^2}{3}\right)+B \operatorname {erf}{\left(\frac{{\left(\frac{qR_{\mathrm{p}}}{\sqrt{6}}\right)}^3}{q}\right)}^{p_1}+\sum \limits_i{C}_i\exp \left(-\frac{q^2{R}_{\mathrm{p}}^2}{3}\right)\operatorname{erf}{\left(\frac{{\left(\frac{qR_{\mathrm{c},i}}{\sqrt{6}}\right)}^3}{q}\right)}^{p_{2,i}} $$

(3.1)

First two terms (Guinier and power-law) contribute to the scattering for spheres in a dilute suspension with radius $a=\sqrt{\frac{5}{3}}{R}_{\mathrm{p}}$~ 4–5 nm and power-law exponent p₁(~4). A and B are the Guinier and Porod scaling factors. The last term contributes to the scattering from the fractal objects in the low q regime with ${R}_{\mathrm{fractal}}=\sqrt{\frac{5}{3}}{R}_{\mathrm{c}}$ ~ 51–72 nm with a power exponent p₂~2, indicating the fractals to be mass fractals. Since the low q regime has only the power-law scattering, no Guinier term has been included suggesting that the R_fractal obtained from the fitting will be the lower bound as exact dimension cannot be determined. C is the prefactor for the power-law scattering in the low q. Absence of any additional structure contribution in intermediate to high q suggests that the particles are reasonably far apart.

3.1.4 Supplementary Reference

• Beaucage, G.: Small-angle scattering from polymeric mass fractals of arbitrary mass fractal dimension. J. Appl. Crystallogr. 29(2), 134–146 (1996)