Lithium-Ion Dynamic Interface Engineering of Nano-Charged Composite Polymer Electrolytes for Solid-State Lithium-Metal Batteries

Abstract

Composite polymer electrolytes (CPEs) offer a promising solution for all-solid-state lithium-metal batteries (ASSLMBs). However, conventional nanofillers with Lewis-acid-base surfaces make limited contribution to improving the overall performance of CPEs due to their difficulty in achieving robust electrochemical and mechanical interfaces simultaneously. Here, by regulating the surface charge characteristics of halloysite nanotube (HNT), we propose a concept of lithium-ion dynamic interface (Li⁺-DI) engineering in nano-charged CPE (NCCPE). Results show that the surface charge characteristics of HNTs fundamentally change the Li⁺-DI, and thereof the mechanical and ion-conduction behaviors of the NCCPEs. Particularly, the HNTs with positively charged surface (HNTs⁺) lead to a higher Li⁺ transference number (0.86) than that of HNTs^- (0.73), but a lower toughness (102.13 MJ m^-3 for HNTs⁺ and 159.69 MJ m^-3 for HNTs^-). Meanwhile, a strong interface compatibilization effect by Li⁺ is observed for especially the HNTs⁺-involved Li⁺-DI, which improves the toughness by 2000% compared with the control. Moreover, HNTs⁺ are more effective to weaken the Li⁺-solvation strength and facilitate the formation of LiF-rich solid-electrolyte interphase of Li metal compared to HNTs^-. The resultant Li|NCCPE|LiFePO₄ cell delivers a capacity of 144.9 mAh g^-1 after 400 cycles at 0.5 C and a capacity retention of 78.6%. This study provides deep insights into understanding the roles of surface charges of nanofillers in regulating the mechanical and electrochemical interfaces in ASSLMBs.

Keywords: Charged nanofillers; Dynamic lithium ion interface; Nanocomposite polymer electrolyte; Solid ion-conductors; Solid-state lithium-metal battery.

Fig. 1

The concept of lithium-ion dynamic interface (Li⁺-DI) engineering in nano-charged composite polymer electrolytes (NCCPEs). The mechanical and electrochemical interfaces of NCCPEs are rationally regulated by the surface charge characteristics of halloysite nanotubes (HNTs) for all-solid-state lithium-metal batteries (ASSLMBs) (for details, see the text)

Fig. 2

Surface charge regulation for controlling the interactions between charged HNTs and the ions of lithium salt. a Schematic illustration of the negatively charged HNT (HNT⁻) treated by sodium hexametaphosphate (SHMP) [35]. b Schematic illustration of the positively charged halloysite (HNT⁺) treated by PDDA. c TEM image and the element mapping for HNT⁺. d XPS of N 1 s for HNT⁺. e Zeta potential for the HNT, HNT⁻, and HNT⁺. f SEM images for HNT⁺. g Comparison of the binding energy of Li⁺ with PVDF, HNT inner surface, and outside charged surface of HNT⁻ and HNT⁺ by DFT calculations (Li⁺: purple; C: gray; F: cyan; H: white; O: red; N: blue; Si: yellow; and Al: rose red). h Comparison of the binding energy of TFSI⁻ with PVDF, HNT inner surface, and outside charged surface of HNT⁻ and HNT⁺ by DFT calculations

Fig. 3

Studies on the mechanical reinforcement by the Li⁺-DI of the charged HNT in NCCPEs. Polarized light microscopy (PLM) images of the surface and SEM image of the cross-section for a HNT-PVDF, b HNT-PE, c HNT⁻-PVDF, d HNT⁻-NCCPE, e HNT⁺-PVDF, and f HNT⁺-NCCPE. g Strain–stress curves for PVDF-HNT nanocomposites with or without lithium salt. h Comparison of tensile strength of the HNT-CPE and NCCPEs with different surface charges as compared with the control samples without lithium salt. i Comparison of the elongation at break for the HNT-CPE and NCCPEs with different surface charges as compared with the control samples without lithium salt. j Schematic of possible different mechanical responses for the dynamic Li⁺-interfaces in HNT⁻-NCCPE and HNT⁺-NCCPE

Fig. 4

Experimental studies on the ion transport and solvation structures of the HNT-supported Li⁺-DI. a Ionic conductivity and active energy of the NCCPEs as compared with the conventional CPEs. b Arrhenius plots of the NCCPEs as compared with the conventional CPEs (the data for HNT⁻ are from Ref. [35]). c Comparison of the Li⁺-transference number of the NCCPEs with conventional CPEs. d Solid-state nuclear magnetic resonance (ss-NMR) spectra for ⁷Li and ¹⁹F. e Raman spectra of HNT⁻-NCCPE and HNT⁺-NCCPE, respectively. f Ion coordination state of the NCCPEs obtained from the Raman spectra results. g Radar diagram comparing the electrochemical performances of the NCCPEs with conventional CPEs. h Comparison of the binding energy of DMF with PVDF, HNT, inner surface, HNT⁻, or HNT⁺ from DFT calculation. Colors: Li⁺: purple; C: gray; F: cyan; H: white; O: red; N: blue; Si: yellow; and Al: rose red

Fig. 5

Simulation studies of the effects of surface charge characteristics on the lithium-ion transport dynamics inside the Li⁺-DI. a Migration energy barriers for [Li⁺(DMF)] at the Li⁺-DI between PVDF and charged HNT along with the axial pathways. b-d Simulation snapshots of the [Li⁺(DMF)] migration behaviors inside the PVDF/HNT, PVDF/HNT⁻, and PVDF-HNT⁺ interfaces, respectively. e Migration energy barriers for [Li⁺(DMF)] and Li⁺ inside the inner surface of the charged HNT along the axial pathways. f, g Simulation snapshots of the Li⁺-hopping and [Li⁺(DMF)] hopping behaviors inside inner surface along the axial pathways, respectively. The initial, transition, and final states are abbreviated as IS, TS, and FS, respectively. Colors: Li⁺: purple; C: gray; F: cyan; H: white; O: red; N: blue; Si: yellow; and Al: rose red

Fig. 6

Analysis of the characteristics of the solid–electrolyte interphase (SEI) derived from the NCCPEs. a Coulombic efficiency of the Li|NCCPEs|Cu cells as compared with the control samples at a current density of 0.2 mA cm⁻². b Cyclic voltammetry curves of Li|NCCPEs|Cu cells at a scan rate of 0.5 mV s⁻¹ from 2.5 to 0 V. c Comparison of the LUMO energy level of the free TFSI⁻ and HNT⁺-adsorbed TFSI⁻. d-g SEM images of the Li-metal anode disassembled from Li||Li symmetric cells after 30 cycles at an areal capacity of 0.5 mAh cm⁻² at different current densities (0.5 and 1 mA cm⁻²) for pure PVDF-based PE, PVDF/HNT-based CPE, PVDF/HNT⁻-based NCCPE, and PVDF/HNT⁺-based NCCPE, respectively. h XPS curves of F1s of the cycled Li-anode in Li||Li cells with pure PVDF-based PE, PVDF/HNT-based CPE, PVDF/HNT⁻-based NCCPE, and PVDF/HNT⁺-based NCCPE. i Scheme illustration of the effects of HNT surface charges on the formation of SEI at Li-metal surface

Fig. 7

Electrochemical performance of NCCPE-based ASSLMB. a Galvanostatic cycling curves of Li||Li cells with HNT⁺-NCCPE at a current density of 0.2 mA cm⁻². b Cycling stability of the Li|HNT⁺-NCCPE|LFP half-cell as compared with other solid polymer electrolytes. c The corresponding charge–discharge voltage profiles of Li||LFP cell at the 1st cycle. d The corresponding charge–discharge voltage profiles of Li||LFP cell at the 200th cycle. e Cycling stability of the Li| HNT⁺-NCCPE |NCM811 half-cell as compared with other solid polymer electrolytes. f The corresponding charge–discharge voltage profiles of Li|HNT⁺-NCCPE|NCM811 at different cycle numbers. g The corresponding charge–discharge voltage profiles of Li|HNT-CPE|NCM811 cell at different cycle numbers