Design of Electrolyte for Achieving 4.8V-Class NCM811-Lithium Metal Batteries in Angew

1.Research Abstract

Combining high-voltage nickel-rich cathodes with lithium metal anodes is one of the most promising approaches to achieving high-energy-density lithium batteries. However, most current electrolytes cannot simultaneously meet the requirements for compatibility with lithium metal anodes and tolerance for ultra-high-voltage NCM811 cathodes. In this study, by adjusting the composition of fluorinated carbonate-based electrolytes, an ultra-anti-oxidative electrolyte was designed. The research found that through the synergistic decomposition of fluorinated solvents and PF6- anions, an SEI (solid electrolyte interphase) rich in LiF and Li2O was constructed on the lithium anode, which facilitated smooth deposition of lithium metal. More importantly, this electrolyte exhibited excellent antioxidant properties, enabling Li||NCM811 coin cells to maintain 80% of their capacity after 300 cycles at an ultra-high cut-off voltage of 4.8 V. Furthermore, under harsh conditions of high cathode loading (30 mg cm-2), low N/P ratio (1.18), and lean 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 could stably cycle for 110 times.

 

 

2.Background Introduction

So far, considerable 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, and so on. Among them, electrolyte engineering is the most direct and practical strategy to address the aforementioned challenges by simultaneously enhancing the interfacial stability of both lithium metal anodes and high-voltage cathodes. Past research on electrolytes has found that various advanced electrolytes, including high-concentration electrolytes, locally high-concentration electrolytes, fluorinated electrolytes, weakly solvating electrolytes, and additives, 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 suitable electrolyte formulations is crucial for advancing the development of ultra-high-voltage nickel-rich lithium metal batteries.

 

3.Key Highlights

This work presents a fluorinated carbonate-based electrolyte formulation, denoted as FDF, consisting of 1.5 M LiPF6 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. On the lithium metal anode side, the designed FDF electrolyte forms an SEI (solid electrolyte interphase) rich in LiF and Li2O through the synergistic decomposition of FEC, DFEC, and PF6- anions. This inorganic-rich interface ensures dense 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 batteries using the FDF electrolyte. Additionally, a robust and LiF-rich shielding layer is uniformly constructed on NCM811. Furthermore, the results indicate that DFEC exhibits the highest adsorption energy and the highest H-transfer reaction energy on NCM811, protecting other components from dehydrogenation. Therefore, this FDF electrolyte enables Li||NCM811 batteries to maintain 80% of their capacity 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 batteries to date. We further demonstrate the practical applicability of the FDF electrolyte under realistic conditions. A multilayer lithium metal pouch cell achieves an energy density of up to 462.2 Wh kg-1 at a charging cut-off voltage of 4.8 V and operates stably for 110 cycles under high cathode areal capacity (6.8 mAh/cm2), low anode/cathode capacity ratio (1.18), and lean electrolyte (2.3 g Ah-1). This is also the first pouch cell validation for 4.8 V-class Li||NCM811 batteries.

 

4.Graphic and Text Analysis

 

Design Principles and Solvation Structure Analysis. (a) HOMO and LUMO energy levels of EC, EMC, FEC, DFEC, and FEMC. (b) Binding energies of different solvents with Li+. (c) FTIR spectra of the electrolytes. (d) 7Li NMR spectra of the electrolytes. (e) Raman spectra of the electrolytes. (f) Radial distribution function g(r) and coordination number distribution n(r) of the base electrolyte, and (g) the FDF electrolyte. (h) LSV curves of Li|Al batteries with different electrolytes at a scan rate of 2 mV s-1.

 

 

 

Electrochemical Compatibility Between Lithium Metal Anode and Different Electrolytes. (a) Modified Aurbach measurement and (b) lithium metal coulombic efficiency (CE) cycling in Li|Cu batteries 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 ranging from 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 the SEI on lithium metal anodes after cycling with different electrolytes. (f) Deposition morphology of lithium metal in base electrolyte and FDF electrolyte (0.5 mA cm-2, 3 mAh cm-2). (g) Schematic illustration of the SEI formation process.

 

 

Impact of Different Electrolytes on the Electrochemical Performance of Li|NMC811 Batteries. (a) Float charge tests of different electrolytes at 4.8 V, 5.0 V, and 5.2 V. (b) Rate performance of Li|NCM811 batteries with different electrolytes. (c) Long-term cycling performance of Li|NMC811 batteries at 0.4 C with different electrolytes, along with (d) the corresponding voltage profiles. (e) Cycling performance of Li|NMC811 batteries at 0.5 C and 50°C with different electrolytes. (f) Cycling performance of Li|NMC811 batteries at 0.2 C and -20°C with different electrolytes. (g) Comparison of high-voltage cycling stability with previously reported results.

 

NCM811 Cathode Interface Dynamics and Chemistry.

(a)Electrochemical impedance spectroscopy (EIS) of NCM811 symmetric cells.

(b)In-situ EIS plots of Li|NMC811 batteries after 60 cycles at 4.8 V and 0.4 C using different electrolytes.

(c)ICP-OES analysis of transition metals (TMs) deposited on the lithium anode with different electrolytes. (d) Atomic content at different depths of the cathode electrolyte interface (CEI) for the base electrolyte

and

(e) the FDF electrolyte. (f) Corresponding F 1s XPS depth profiles.

 

 

Morphological and Structural Evolution of NCM811 Cathode after 60 Cycles at 4.8 V and 0.4 C Cut-off Voltage. (a, c) SEM images of cycled cathodes in base electrolyte and (b, d) FDF electrolyte. (e) HRTEM images of cathodes cycled in different electrolytes. (f) Raman spectra of cathodes cycled with different electrolytes. (g) Schematic illustration of the role of electrolytes in stabilizing high-voltage NCM811 cathodes. (h) XRD patterns of cathodes cycled in different electrolytes.

 

 

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

 

 

1. (a)Structure of a Li|NCM811 pouch cell with a capacity of 462.2 Wh kg-1. (b) Voltage profiles of a Li|NCM811 pouch cell with FDF electrolyte at different cycle numbers. (c) Cycling performance of the Li|NCM811 pouch cell at 0.1 C charge/0.2 C discharge after one formation cycle at 4.8 V cut-off voltage with 0.1 C charge/discharge. 

 

5.Summary and Outlook

By fluorinating carbonate solvents, an antioxidant electrolyte for 4.8 V-class Li|NCM811 batteries has been demonstrated. On the lithium metal anode side, an SEI rich in LiF and Li2O is formed, exhibiting remarkable reversibility during lithium deposition/stripping. On the cathode side, each component in the FDF electrolyte exhibits exceptional antioxidant performance, constructing a uniform, dense, and robust CEI to protect the cathode. Furthermore, DFEC, which has the strongest antidehydrogenation ability, exhibits the strongest adsorption tendency towards NCM811, reducing the potential for oxidation and decomposition of other components, further enhancing the electrolyte's high-voltage tolerance. The 4.8 V Li//NCM811 coin cell with FDF electrolyte maintains 80% capacity retention after 300 cycles. The 4.8 V lithium metal pouch cell has a high energy density of 462.2 Wh kg-1 and can stably cycle for 110 times. This work presents an electrolyte design concept addressing both lithium metal anodes and high-voltage cathodes, providing an effective strategy for the practical application of ultra-high-voltage lithium metal batteries.

 

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