Lithium metal batteries have attracted significant attention due to their high energy density. However, their development has been hindered by issues such as uncontrollable dendrite growth of lithium metal anodes, unstable solid electrolyte interphase (SEI), and poor cycling stability. Solid-state electrolytes (SSEs) are attractive due to their inherent safety, with polymer solid-state electrolytes (SPEs) receiving particular attention for their processability, cost-effectiveness, and good contact with electrodes. However, traditional SPEs (such as poly(ethylene oxide) (PEO)-based electrolytes) have low ionic conductivity (typically <10⁻⁵ S cm⁻¹ at room temperature), limiting their application at room temperature.
Recently, Kang Feiyu, Lu Wei, Liu Ming, and He Yanbing’s team from Tsinghua University’s Shenzhen International Graduate School proposed a 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether (TTE) diluent to significantly modulate dipole-dipole interactions in polymer ionic solvate electrolytes (TPISEs). TTE can encapsulate ionic solvents, reducing dipole-dipole interactions between ionic solvents and the polymer matrix, thereby promoting their uniform distribution and forming a continuous ionic percolation network within the polymer matrix. As a result, the ionic conductivity of TPISEs was enhanced to 1.27×10⁻³ S cm⁻¹ at 25°C. Meanwhile, TTE induces ionic solvents to transform from contact ion pairs to aggregates, which helps form a stable Li/electrolyte interface, with an exchange current density 190 times higher than without TTE. The Li||LiNi₀.₈Co₀.₁Mn₀.₁O₂ (NCM811) full cell exhibits good cycling stability across a temperature range of 30°C to 60°C. The practical pouch cell, using a 50 μm thick lithium metal foil and high areal capacity cathode (3.58 mAh cm⁻²), achieved a high specific energy of 354.4 Wh kg⁻¹ and maintained 78.1% capacity after 450 cycles at 25°C and 54 mA g⁻¹. This study provides a design strategy for polymer electrolytes to overcome the ionic conductivity bottleneck in practical solid-state batteries.
This achievement was published in “Nature Communications” under the title “Homogeneous polymer ionic solvate electrolyte with weak dipole-dipole interaction enabling long cycling pouch lithium metal battery,” with first authors Chen Likun, Gu Tian, and Mi Jinshuo.
【Key Points of the Work】
This study significantly enhances the ionic conductivity and interface stability of polymer-ionic solvate electrolytes (TPISEs) by introducing 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether (TTE) to modulate dipole-dipole interactions.
Mechanism of Ionic Conductivity Enhancement
Construction of Weak Dipole-Dipole Interactions: TTE can encapsulate ionic solvents, reducing dipole-dipole interactions between ionic solvents and the polymer matrix. This weak interaction promotes uniform distribution of ionic solvents in the polymer matrix, forming a continuous ionic percolation network. Compared with traditional polymer electrolytes, this uniform distribution allows lithium ions (Li⁺) to move more freely in the polymer matrix, significantly improving ionic conductivity. At 25°C, the ionic conductivity of TPISEs reaches 1.27×10⁻³ S cm⁻¹, far exceeding traditional polymer electrolytes (such as PEO-based electrolytes, typically <10⁻⁵ S cm⁻¹ at room temperature).
Optimization of Ionic Transport Pathways: The introduction of TTE transforms ionic solvents from contact ion pairs (CIPs) to aggregates (AGGs), which facilitates rapid lithium ion transport. In traditional polymer electrolytes, lithium ions mainly transport through coordination with polymer chain movement, while strong ion-dipole interactions limit the migration rate of lithium ions. In TPISEs, the TTE-encapsulated ionic solvents reduce interactions with polymer chains, allowing lithium ions to migrate more quickly in the polymer matrix, achieving efficient ion transport.
Interface Stability
Formation of Stable Solid Electrolyte Interface (SEI): TTE induces ionic solvents to transform from CIPs to AGGs, helping form an SEI layer rich in lithium fluoride (LiF). This SEI layer has high mechanical modulus and uniformity, effectively suppressing lithium dendrite growth. Experiments show that the average Derjaguin-Müller-Toporov (DMT) modulus of the SEI layer formed by TPISEs is 9.46 GPa, 8 times that of the SEI layer formed by traditional polymer electrolytes (1.18 GPa). This high-modulus SEI layer not only physically blocks lithium dendrite penetration but also reduces side reactions of the lithium metal anode, significantly improving battery cycling stability and Coulombic efficiency.
Formation of High-Quality Cathode-Electrolyte Interface (CEI): The introduction of TTE also optimizes the chemical composition and structure of the cathode-electrolyte interface (CEI). In TPISEs, preferentially oxidized FSI⁻ anions in the CEI layer produce more LiF, reducing side reactions between the electrolyte and cathode material. This high-quality CEI layer not only effectively prevents electrolyte decomposition but also promotes rapid lithium ion migration in the cathode material, improving battery charging-discharging efficiency and cycling stability.
Battery Performance
Wide Temperature Range Cycling Stability: Due to the high ionic conductivity and stable interface characteristics of TPISEs, Li||NCM811 full cells exhibit good cycling stability across a wide temperature range of 30°C to 60°C. At low temperatures, TPISEs maintain high ionic conductivity to ensure normal battery discharge; at high temperatures, stable SEI and CEI layers effectively suppress electrolyte decomposition and side reactions, extending battery life.
High Specific Energy and Long Cycle Life: Pouch cells using TPISEs perform excellently in practical applications. Using a 50 μm thick lithium metal foil and high areal capacity cathode (3.58 mAh cm⁻²), the battery achieves a high specific energy of 354.4 Wh kg⁻¹ and maintains 78.1% capacity after 450 cycles at 25°C and 54 mA g⁻¹. This high performance is attributed to the significant advantages of TPISEs in ionic transport and interface stability, meeting the practical application requirements of high-energy-density lithium metal batteries.
Figure 1 | Design principle of TPISEs for high-performance, high-safety lithium metal batteries
- Illustrates the restricted ion transport pathways and ion solvent structure dominated by contact ion pairs (CIPs) in PISEs.
- Illustrates the rapid ion transport pathways achieved through uniformly distributed ion solvents and the structure of TPISEs dominated by aggregates (AGGs).
- Illustrates the ion solvent structure dominated by solvent-separated ion pairs (SSIPs) and lithium ion transport pathways in PISEs using traditional solvents.
Figure 2 | Properties and solvation structure of electrolytes based on PISEs
- HAADF image of PISEs and corresponding elemental distribution map of electron energy loss spectrum.
- HAADF image of TPISEs and corresponding elemental distribution map of electron energy loss spectrum.
- Arrhenius plot of PISEs and TPISEs.
- 2D ¹H¹⁹F heteronuclear correlation spectrum of PISEs.
- 2D ¹H¹⁹F heteronuclear correlation spectrum of TPISEs.
- Schematic diagram of TPISEs solvation structure.
- Lithium ion self-diffusion coefficient and corresponding diffusion mode derived from MD simulation mean square displacement.
- Raman spectra of PISEs and TPISEs.
- PDOS curve of HCEs.
- PDOS curve of PISEs.
- PDOS curve of TPISEs.
Figure 3 | Lithium reversibility and SEI microstructure
- Coulombic efficiency of Li||Cu batteries using HCEs, PISEs, and TPISEs at 0.1 mA cm⁻² and 0.2 mAh cm⁻² for lithium deposition/stripping.
- Modified Aurbach Coulombic efficiency test of Li||Cu batteries using HCEs, PISEs, and TPISEs.
- Tafel plots of lithium symmetric cells using PISEs and TPISEs.
- Galvanostatic charge-discharge curves of lithium symmetric cells using HCEs, PISEs, and TPISEs at 0.5 mA cm⁻² and 0.5 mAh cm⁻², with inset showing scanning electron microscopy images of lithium metal anodes after 100 hours of cycling.
- Galvanostatic charge-discharge curves of lithium symmetric cells using HCEs, PISEs, and TPISEs at 1 mA cm⁻² and 1 mAh cm⁻².
- Atomic force microscopy images of lithium metal anodes after cycling using PISEs (top) and TPISEs (bottom).
- Three-dimensional views of LiF and C₂H₆O in the sputtered volume of SEI formed using HCEs, PISEs, and TPISEs as characterized by TOFSIMS.
- Atomic ratio of F element in SEI formed using HCEs, PISEs, and TPISEs as characterized by XPS.
Figure 4 | Cycle stability of Li||NCM811 full cells and cathode structure characterization
- Rate performance of Li||NCM811 full cells using PISEs and TPISEs.
- Long-term cycle stability of Li||NCM811 full cells using PISEs and TPISEs at 25°C (cathode mass loading = 2 mg cm⁻², lithium metal thickness = 450 μm).
- Long-term cycle stability of Li||NCM811 full cells using PISEs and TPISEs at 30°C (cathode mass loading = 2 mg cm⁻², lithium metal thickness = 450 μm).
- Long-term cycle stability of Li||NCM811 full cells using PISEs and TPISEs at 60°C (cathode mass loading = 2 mg cm⁻², lithium metal thickness = 450 μm).
- Insitu XRD characterization of NCM811 cathode using TPISEs during initial charge-discharge cycles.
- HAADF-STEM image of NCM811 particles after cycling with PISEs.
- HAADF-STEM image of NCM811 particles after cycling with TPISEs.
Figure 5 | Cycle stability and safety tests of pouch cells
- Long-term cycle stability of Li||NCM811 pouch cells using PISEs and TPISEs at 25°C (cathode mass loading = 2 mg cm⁻², lithium metal thickness = 50 μm).
- Charge-discharge curves of Li||NCM811 pouch cells using TPISEs at different cycle numbers.
- Long-term cycle stability of high-energy Li||NCM811 pouch cells using TPISEs at 25°C (cathode mass loading = 20 mg cm⁻², lithium metal thickness = 50 μm). Inset shows the Li|TPISE|NCM811-based pouch cell for abuse testing.
- Charge-discharge curves of high-energy Li||NCM811 pouch cells using TPISEs at different cycle numbers.
- Infrared thermal imaging photos of Li|TPISE|NCM811 pouch cells under abuse conditions.
[Conclusion]
In this work, we designed a homogeneous polymer-ion solvent electrolyte (TPISEs) with weak dipole-dipole interactions, achieving a high ionic conductivity of 1.27×10⁻³ S cm⁻¹ at 25°C. The TTE-induced aggregate-dominated ion-solvent structure helps form stable LiF-rich SEI and CEI, stabilizing both the lithium metal anode and NCM811 cathode interfaces simultaneously. Notably, the exchange current density at the Li/TPISEs interface is 190 times higher than without TTE addition. As a result, solid-state Li|TPISE|NCM811 batteries exhibit excellent cycling stability over a wide temperature range from -30°C to 60°C. Excitingly, practical lithium metal pouch cells using TPISEs achieve 78.1% capacity retention after 450 cycles at a specific energy of 354.4 Wh kg⁻¹.
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