"Angew" electrolyte design achieves 4.8V NCM811-lithium metal battery

In August 2024, Zhou Haoshen's team published an online paper titled "Tailoring the Electrode-Electrolyte Interface for Reliable Operation of All-Climate 4.8 V Li||NCM811 Batteries" in the journal Angew (impact factor >16 ) . The study designed an ultra-oxidation-resistant electrolyte to achieve stable operation of the 4.8 V NCM811 lithium metal battery, and constructed an Ah-level high-energy-density 4.8 V lithium metal soft-pack battery.?

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Research Summary

Combining a high-voltage nickel-rich cathode with a lithium metal anode is one of the most promising approaches to achieve high-energy-density lithium batteries. However, most current electrolytes cannot simultaneously meet the compatibility requirements for lithium metal anodes and the tolerance requirements for ultra-high voltage NCM811 cathodes. This study designed a super-antioxidant electrolyte by adjusting the components of fluorinated carbonate electrolytes. The study found that a LiF- and Li2O - rich SEI was constructed on the lithium anode through the synergistic decomposition of fluorinated solvents and PF6 - anions, which promoted the smooth deposition of lithium metal. More importantly, the electrolyte has excellent antioxidant properties, allowing the Li||NCM811 button cell to maintain 80% of its capacity after 300 cycles at an ultra-high cut-off voltage of 4.8 V. In addition, under the harsh conditions of high cathode loading (30 mg cm -2 ), low N/P ratio (1.18) and poor electrolyte (2.3 g Ah -1 ), a 4.8 V-class lithium metal pouch cell with an energy density of 462.2 Wh kg -1 can be stably cycled for 110 times.? ?

Background

So far, quite a number of strategies have been developed to address the challenges of high-voltage lithium metal batteries, such as electrolyte engineering, lithium metal anode protection, current collector design, separator modification, cathode surface coating or doping, etc. Among them, electrolyte engineering can simultaneously enhance the interfacial stability of lithium metal anode and high-voltage cathode, and is the most direct and practical strategy to address the above challenges. Past electrolyte studies have found that high-concentration electrolytes, locally high-concentration electrolytes, fluorinated electrolytes, weakly solvated electrolytes, additives, etc. can significantly enhance the reversibility of lithium metal anodes. However, these advanced electrolytes have rarely been studied in ultra-high voltage (≥4.4 V vs Li + /Li) nickel-rich lithium metal batteries. Therefore, the development of a suitable electrolyte formulation is the key to promoting the development of ultra-high voltage nickel-rich lithium metal batteries.

Key Highlights

This work demonstrates a fluorinated carbonate-based electrolyte formulation consisting of 1.5 M LiPF 6 dissolved in a mixture of fluoroethylene carbonate (FEC), difluoroethylene carbonate (DFEC), and 2,2,2-trifluoroethyl methyl carbonate (FEMC) in a volume ratio of 2:1:7, denoted as FDF. On the lithium metal anode side, the designed FDF electrolyte generates a LiF- and Li 2 O-rich SEI via the synergistic decomposition of FEC, DFEC, and PF 6 - anions . The inorganic-rich interface ensures a dense metallic lithium deposition and inhibits the growth of lithium dendrites. Highly reversible lithium deposition/stripping with a Coulombic efficiency (CE) of 98.84% is achieved in Li||Cu cells using the FDF electrolyte. In addition, a robust and LiF-rich shielding layer is uniformly constructed on NCM811. In addition, the results show that DFEC exhibits the highest adsorption energy and the highest H transfer reaction energy on NCM811, protecting the dehydrogenation of other components. Therefore, this FDF electrolyte guarantees that the capacity of Li||NCM811 cells can be retained by 80% after 300 cycles at an ultra-high cut-off voltage of 4.8 V, making it one of the best performing ultra-high voltage Li||NCM811 cells to date. We further demonstrate the practical applicability of FDF electrolytes under realistic conditions. Multilayer lithium metal pouch cells achieve an energy density of up to 462.2 Wh kg -1 at a charge cut-off voltage of 4.8 V and operate stably for 110 cycles under high cathode areal capacity (6.8 mAh/cm2 ) , low anode/cathode capacity ratio (1.18) and low electrolyte (2.3 g Ah -1 ), which is also the first pouch cell verification of 4.8 V-class Li||NCM811 cells.

Graphical analysis

Fig. 1 Design principle and solvation structure analysis. (a) HOMO and LUMO energy levels of EC, EMC, FEC, DFEC, and FEMC. (b) Binding energy of different solvents with Li + . (c) FTIR spectrum of electrolyte. (d) 7 Li NMR spectrum of electrolyte. (e) Raman spectrum of electrolyte. (f) Radial distribution function g(r) and coordination number distribution n(r) of Base electrolyte and (g) FDF electrolyte. (h) LSV curves of Li|Al batteries under different electrolytes at a scan rate of 2 mV s -1 .

Figure 2 Electrochemical compatibility between lithium metal anode and different electrolytes. (a) Modified Aurbach measurement and (b) lithium metal CE cycling in Li|Cu cells using different electrolytes at 0.5 mA cm -2 and 1.0 mAh cm -2 . (c) Rate performance of lithium symmetric cells at current densities of 0.5 to 5 mA cm -2 . (d) Long-term cycling performance of lithium symmetric cells at 1.0 mA cm -2 and 1.0 mAh cm -2 . (e) F 1s XPS spectra of SEI on lithium metal anode after cycling with different electrolytes. (f) Deposition morphology of lithium metal in alkaline electrolyte and FDF electrolyte (0.5 mA cm -2 , 3 mA h cm -2 ). (g) Schematic diagram of SEI formation process.

Figure 3 Effects of different electrolytes on the electrochemical performance of Li| NMC811 batteries (a) Float test of different electrolytes at 4.8 V, 5.0 V and 5.2 V (b) Rate performance of Li| NCM811 batteries under different electrolytes. (c) Long-term cycle performance of Li| NCM811 batteries at 0.4 C with different electrolytes, (d) corresponding voltage distribution diagram. (e) Cycle performance of Li| NCM811 batteries at 0.5C and 50°C with different electrolytes. (f) Cycle performance of Li| NCM811 batteries at 0.2C-20°C using different electrolytes. (g) Comparison of high-voltage cycle stability with previous reports.

Figure 4. Interface dynamics and chemistry of the NCM811 cathode. (a) Electrochemical impedance spectroscopy (EIS) of a symmetric NCM811 cell. (b) In situ EIS of Li|NMC811 cells with different electrolytes after 60 cycles at 4.8 V, 0.4 C. (c) ICP-OES analysis of TMs deposited on the lithium anode with different electrolytes. Atomic content at different depths of the CEI for (d) Base electrolyte and (e) FDF electrolyte. (f) Corresponding F 1s XPS depth profile.

Figure 5 Morphological and structural evolution of NCM811 cathode after 60 cycles at 4.8 V and 0.4 C cutoff voltage. (a, c) SEM images of cathode cycled in Base electrolyte and (b, d) FDF electrolyte. (e) HRTEM images of cathode cycled in different electrolytes. (f) Raman spectra of cathode cycled in different electrolytes. (g) Schematic diagram of the role of electrolyte in stabilizing high-voltage NCM811 cathode. (h) XRD patterns of cathode cycled in different electrolytes.?

Figure 6 (a) Adsorption energies of EC, EMC, FEC, DFEC, and FEMC on the NCM811 (001) plane. (b) Calculation of H transfer reaction energies of EC, (c) EMC, (d) FEC, (e) DFEC, and (f) FEMC

Figure 7. (a) Structure of Li|NCM811 soft pack battery with 462.2 Wh kg -1 . (b) Voltage distribution of Li|NCM811 soft pack battery with FDF electrolyte at different cycles. (c) Cycling performance of Li|NCM811 soft pack battery at 0.1 C charge/0.2 C discharge after 1 cycle at 0.1 C charge/discharge with charge cutoff voltage of 4.8 V.

Summary and Outlook

In this work, we demonstrate an antioxidant electrolyte for 4.8 V-class Li|NCM811 cells by designing the fluorination of carbonate solvents. On the lithium metal anode side, a LiF, Li2O-rich SEI is generated , which exhibits remarkable reversibility in lithium deposition/stripping. On the cathode side, each component in the FDF electrolyte exhibits extraordinary antioxidant properties, building a uniform, dense, and robust CEI to protect the cathode. In addition, DFEC, which has the strongest dehydrogenation resistance, has the strongest adsorption tendency on NCM811, reducing the potential for oxidative decomposition of other components and further enhancing the high-voltage resistance of the electrolyte. 4.8 V Li//NCM811 button cells in FDF electrolyte provide 80% capacity retention after 300 cycles. 4.8 V lithium metal soft-pack cells have a high energy density of 462.2 Wh kg -1 and can be stably cycled for 110 times. This work demonstrates a concept of electrolyte design that addresses lithium metal anode and high-voltage cathode, providing an effective strategy for the practical application of ultra-high-voltage lithium metal batteries.

Original link: https://doi.org/10.1002/anie.202410893

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