Zhou Haoshen from Nanjing University and Chang Zhi from Central South University, "Nature Commun.: A new approach to improving the performance of high-voltage lithium metal batteries!

The rapid development of portable electronic devices and electric vehicles requires batteries with high energy density, long cycle life and fast charging performance. High-voltage positive electrode materials (such as LiNi₀.₈Co₀.₁Mn₀.₁O₂, i.e. NCM811) are expected to improve battery energy density, but they have poor stability and slow lithium ion diffusion in traditional electrolytes.

Recently, Zhou Haoshen from Nanjing University and Chang Zhi from Central South University introduced a metal organic framework (MOF, ZnPdmbIm) liquid infusion technology to completely infuse MOF liquid into the grain boundaries of NCM811 to form a thin and hard MOF glass layer that completely covers the positive electrode material. The surface non-conductive MOF glass layer has a pore window of 2.9 Å, which can promote lithium ion pre-desolvation and form a highly aggregated electrolyte in the glass channel, inhibiting the co-embedding and solvent decomposition of solvated lithium ions. The inner glass layer contains lithium ion conductive components, which significantly enhances lithium ion diffusion. This functional structure effectively prevents the cracking of positive electrode material particles, CEI rupture, oxygen loss and transition metal migration. Therefore, the Li||Glass@NCM811 battery exhibits good rate performance and cycle stability even at high charge rates (5 C) and high voltages (4.6 V). In addition, a soft-pack battery (19.579 g) with an energy density of 385 Wh/kg was successfully assembled, demonstrating the practical application potential of this method. This simple and general strategy can also be applied to other high-voltage cathode materials such as lithium-rich manganese oxides and LiCoO₂.

【Key points】

In this paper, the metal organic framework (MOF, Zn-P-dmbIm) liquid infusion technology is used to completely infuse the MOF liquid into the grain boundaries of high-voltage positive electrode materials (such as LiNi₀.₈Co₀.₁Mn₀.₁O₂, i.e. NCM811), forming a thin and hard MOF glass layer, thereby significantly improving the stability of the positive electrode material and the lithium ion diffusion performance.

 1. Double-layer structure of MOF glass layer

 Outer layer structure: The non-conductive MOF glass layer has a pore window of 2.9 Å, which is smaller than the size of the solvated lithium ions (about 7.0 Å), and can effectively promote the pre-desolvation of lithium ions. Pre-desolvation refers to the removal of solvent molecules around lithium ions before they reach the surface of the positive electrode, thereby reducing the transmission resistance of lithium ions.

 Inner layer structure: The inner glass layer contains lithium ion conductive components (such as LiP and OPLi), which can significantly enhance the diffusion performance of lithium ions and enable lithium ions to pass through the positive electrode material quickly.

 2. Suppressing the degradation of positive electrode materials

 Prevent particle cracking: The MOF glass layer can effectively prevent the particle cracking of the positive electrode material during the charging and discharging process. Particle cracking will lead to a decrease in the structural stability of the positive electrode material, thereby affecting the cycle life of the battery.

 Inhibit CEI rupture: The solid electrolyte interface (CEI) on the surface of the positive electrode material is prone to rupture during battery operation, resulting in direct contact between the electrolyte and the positive electrode material, triggering side reactions. The MOF glass layer can protect the CEI and prevent it from rupture.

 Reduce oxygen loss and transition metal migration: High-voltage cathode materials are prone to oxygen loss and transition metal migration during operation, which not only reduces battery performance but may also cause safety issues. The MOF glass layer can effectively inhibit these phenomena, thereby improving the safety and stability of the battery.

 3. Improve lithium ion transmission performance

 Pre-desolvation: The 2.9 Å pore window of the MOF glass layer can effectively promote the pre-desolvation of lithium ions, allowing lithium ions to remove the surrounding solvent molecules in advance before reaching the positive electrode surface, thereby reducing the transmission resistance of lithium ions.

 Formation of aggregated electrolyte: The channels inside the MOF glass layer can form a highly aggregated electrolyte, which has a low solvent coordination number, further promoting the rapid transport of lithium ions.

 Lower activation energy: Through experimental measurements, the MOF glass layer significantly reduces the activation energy of lithium ion desolvation and transport. Specifically, the activation energy of the Li||Glass@NCM811 battery is 45.7 kJ/mol, which is much lower than that of the uncoated Li||NCM811 battery (104.3 kJ/mol).

 4. Electrochemical performance

 Rate performance and cycle stability: Experimental results show that the Li||Glass@NCM811 battery exhibits good rate performance and cycle stability at high charge rate (5 C) and high voltage (4.6 V). Even at a high voltage of 4.6 V, the battery can still maintain a specific capacity of 180 mAh/g after 400 cycles.

 Practical application potential: The practical application potential of this technology was further verified by assembling a 385 Wh/kg soft-pack battery. The soft-pack battery can still maintain 86.9% of its capacity after 300 cycles, demonstrating the application prospects of this technology in high energy density batteries.

This MOF glass coating technology is not only applicable to NCM811, but can also be applied to other high-voltage positive electrode materials such as lithium-rich manganese oxide (LRMO) and LiCoO₂ (LCO). Experimental results show that these materials also show significant performance improvements after being treated with MOF glass coating.

In summary, MOF glass coating effectively inhibits the degradation of positive electrode materials through its unique double-layer structure, while significantly improving the transmission performance of lithium ions, thereby achieving the unity of high energy density, long cycle life and fast charging performance.

Figure 1 | MOF liquid infusion enables MOF glass impregnation of high voltage cathode to stabilize cathode stability and enhance fast lithium ion diffusion

a, b A double-layer coating is applied on the surface of high-voltage cathode materials. The outer layer is a non-conductive porous material with sub-nanometer channels that promote lithium-ion pre-desolvation, while the inner layer facilitates rapid lithium-ion conduction.

c Schematic diagram of the conversion of Zn-P-dmbIm MOF powder into MOF liquid and MOF glass, and the corresponding XRD patterns of Zn-P-dmbIm MOF powder and MOF glass.

d Schematic diagram of the preparation of MOF glass impregnated cathode using MOF liquid infusion strategy. Before the MOF liquid is vitrified into MOF glass, the MOF liquid is uniformly coated on the surface of NCM secondary particles and infused into the grain boundaries between NCM primary particles. e SEM of Zn-P-dmbIm MOF powder and f TEM image of MOF glass. g SEM image of bare NCM-811 and h SEM image of glass-coated NCM-811 (Glass@NCM-811). i Contact angle of MOF liquid on NCM-811 electrode. j, k TEM images of bare NCM-811 and l, m TEM images of Glass@NCM-811. The TEM image in m verifies the formation of uniform MOF glass outer and inner layers. n Deep etching XPS of Glass@NCM-811. XPS results confirm that the inner layer is composed of components that accelerate lithium ion conduction.

Figure 2 | Stabilization of NCM cathode by MOF glass coating and enhancement of lithium ion desolvation and transport

a XRD patterns of bare NCM-811 and Glass@NCM-811 cathodes. b Rate performance of cells based on bare NCM-811 and Glass@NCM-811 cathodes (defined as 1 C = 220 mA/g). c State of charge (SoC) versus time curves of Li||NCM-811 and Li||Glass@NCM-811 cells. This indicates that the Li||Glass@NCM-811 cells have faster lithium ion desolvation and transport due to the significantly reduced interfacial resistance. d Discharge curves measured by GITT, with the test cell being the same cell in Figure 2b after 100 cycles. Inset: Average voltage loss and its standard deviation at different GITT steps. e FTIR spectra of a typical electrolyte (LiPF₆-EC/DMC, bottom) and the electrolyte in the MOF glass layer (top). The aggregated electrolyte in the MOF glass layer indicates successful pre-desolvation through the sub-nano channels in the glass layer. f Comparison of activation energies during lithium ion desolvation and its migration across the cathode electrolyte interface (CEI) in Li||NCM-811 and Li||Glass@NCM-811 cells. g Comparison of the rates of desolvation/pre-desolvation and lithium ion transport across the cathode electrolyte interface (CEI) in bare NCM-811 cells (top) and Li||Glass@NCM-811 cells (bottom). h Schematic diagram of a typical CEI (mainly composed of solvent-derived organic matter) formed on a bare NCM-811 cathode (top) and the glass layer on the Glass@NCM-811 cathode after cycling (bottom).

Figure 3 | MOF liquid infusion strategy stabilizes NCM-811 cathode by effectively suppressing cathode cracks, CEI rupture, cation mixing, and side reactions

a SEM image of cycled NCM-811 after 200 cycles. b Cross-sectional SEM image of cycled NCM-811 after 200 cycles. c SEM image of cycled Glass@NCM-811 after 400 cycles. d Cross-sectional SEM image of cycled Glass@NCM-811 after 400 cycles. e–j TEM images of cycled Glass@NCM-811 and corresponding elemental distribution maps. k XRD of cycled Glass@NCM-811 and cycled bare NCM-811. l–n High-resolution transmission electron microscopy (HR-TEM) images of cycled bare NCM-811. o–q HR-TEM images of cycled Glass@NCM-811. r Deep etching FTIR of cycled bare NCM-811 cathode (top) and cycled Glass@NCM-811 (bottom).

Figure 4 | MOF liquid infusion strategy stabilizes NCM-811 cathode by significantly reducing oxygen loss, transition metal dissolution and migration

a, b Charging curves and corresponding DEMS data of bare Li||NCM-811 cells for in situ differential electrochemical mass spectrometry (DEMS) testing. c, d Charging curves and corresponding DEMS data of Li||Glass@NCM-811 cells for in situ DEMS testing. e SEM image of cycled Li anode collected from bare Li||NCM-811 cells after 400 cycles. f SEM image of cycled Li anode collected from Li||Glass@NCM-811 cells after 400 cycles. g Dissolved transition metals in cycled electrolyte and cycled Li anode in Li||Glass@NCM-811 (left) and Li||NCM-811 cells (right) detected by ICP-OES. h Ni XPS results of cycled Li anode collected from Li||Glass@NCM-811 cells (top) and Li||NCM-811 cells (bottom). Schematic diagram of the iMOF liquid infusion strategy to stabilize the NCM-811 cathode by suppressing electrolyte penetration and solvated lithium ion/solvent co-intercalation-induced problems, including cathode particle cracks, CEI rupture, oxygen loss, and transition metal migration. By implementing a perfect particle-level pre-desolvation method, the stability of both the cathode and the Li anode can be greatly improved.

Figure 5 | Superior electrochemical performance of Li||Glass@NCM-811 battery compared to bare Li||NCM-811 battery

Cycling performance of Li||Glass@NCM-811 cells (blue/green curves) and Li||NCM-811 cells (light grey curves) in the range of ac 2.7–4.4 V and d–f 2.8–4.6 V and corresponding charge/discharge curves (defined as 1 C = 220 mA/g for NCM-811-based cells). g Cycling performance of Li||Glass@LRMO cells and Li||LRMO cells at a cutoff voltage of 4.8 V (defined as 1 C = 280 mA/g for LRMO-based cells). h Cycling performance of Li||Glass@LCO cells and Li||LCO cells at a cutoff voltage of 4.6 V (defined as 1 C = 220 mA/g for LCO-based cells). i Cycling performance of 385 Wh/kg-class Li||Glass@NCM-811 pouch cells and pouch cells based on bare Li||NCM-811. Inset: Digital photograph of the Li||Glass@NCM-811 soft-pack battery.

【in conclusion】

Here, we present a simple and efficient metal-organic framework (MOF, Zn-P-dmbIm) liquid infusion strategy to fully infuse MOF liquid into the grain boundaries of high-voltage cathode materials such as NCM811, LRMO, and LCO, achieving complete MOF glass coverage (e.g., Glass@NCM811). The surface-nonconductive MOF glass layer has a pore window of 2.9 Å, which promotes lithium-ion pre-desolvation and forms a highly aggregated electrolyte within the glass channel, inhibiting the co-intercalation and solvent decomposition of solvated lithium ions. The inner glass layer contains lithium-ion conductive components, which significantly enhances lithium-ion diffusion. This MOF glass coating can prevent cathode particle cracking, CEI rupture, gas generation, and transition metal migration, while promoting fast lithium-ion transport. As a result, Li||Glass@NCM811 batteries exhibit excellent electrochemical performance with remarkable rate capability and cycling stability, even at high charge rates (5 C) and high voltages (4.6 V). Similarly, Li||Glass@LRMO and Li||Glass@LCO batteries exhibited good cycling stability for more than 400 cycles at 4.8 V and 4.6 V, respectively. The successful realization of 385 Wh/kg-level pouch cells using Glass@NCM811 further demonstrated the practical application potential of this strategy.

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