Lithium Hexamethyldisilazide Endows Li||NCM811 Battery with Superior Performance

High-energy-density lithium metal batteries (LMBs) pairing a Li metal anode with a Ni-rich LiNi0.8Co0.1Mn0.1O2 (NCM811) cathode are promising for advanced energy storage, especially at high cut-off voltages (≥4.5 V vs Li/Li+). However, NCM811 suffers from poor cycling stability due to particle breakage and interfacial degradation, exacerbated by trace H2O and HF in conventional electrolytes. Constructing a robust cathode–electrolyte interphase (CEI) and eliminating acidic/water impurities via electrolyte engineering are recognized strategies to mitigate these issues. This work summarizes He's group’s approach of using LiHMDS as an electrolyte additive to stabilize NCM811 and improve Li||NCM811 cell cyclability at room and elevated temperatures.

The article situates the study within efforts to stabilize electrode–electrolyte interphases and enable high-voltage operation in LMBs. Prior works include additive- and solvent-engineering strategies to optimize Li-ion solvation and interphase formation (e.g., perfluorinated sulfonate-enriched electrolytes; hydrophobic-solvation additives; heptafluorobutyric anhydride for dual interphases). Advances in high-voltage liquid electrolytes and understanding structural degradation in layered oxides are also referenced. Notably, a related study specifically reports LiHMDS as an effective additive for high-voltage non-aqueous LMBs, supporting the additive’s role in scavenging impurities and forming protective interphases on both electrodes.

Cells: Li||NCM811 cells were tested between 3.0 and 4.5 V using a baseline electrolyte (BE) of 1 M LiPF6 in EC/EMC/DMC (1:1:1 by volume, H2O < 20 ppm) and BE containing 0.6 wt% LiHMDS. Cycling protocols: At 25 °C, cycling was conducted at 90 mA g−1 for up to 1000 cycles; at 60 °C, at 180 mA g−1 for up to 500 cycles. Impurity scavenging tests: H2O and HF contents were quantified in BE and BE + 0.6 wt% LiHMDS, including conditions with an additional 1000 ppm H2O added to the electrolyte. Characterization: Ex-situ TEM was performed on NCM811 particles after cycling to assess structural preservation. Mechanistic analysis considered the preferential oxidation of LiHMDS relative to solvents, CEI composition, and suppression of layered-to–rock-salt phase transition. Proposed mechanism: LiHMDS scavenges H2O and HF to generate HMDS and inorganic products (LiF, LiOH), and is preferentially oxidized to form a thin, compact CEI on NCM811, mitigating side reactions, transition-metal dissolution, and detrimental phase transitions.

• Cycling performance at 25 °C (4.5 V cut-off, 90 mA g−1, 1000 cycles): BE retained 49.13% capacity; BE + 0.6 wt% LiHMDS retained 73.92%.
• Cycling performance at 60 °C (4.5 V cut-off, 180 mA g−1, 500 cycles): BE + 0.6 wt% LiHMDS retained 66.02% capacity with an average coulombic efficiency of 99.11%; BE failed after ~450 cycles.
• Impurity scavenging: In BE, H2O = 18.9 ppm; with 0.6 wt% LiHMDS, H2O ≈ 3 ppm and HF was undetectable. After adding 1000 ppm H2O, H2O rose to 969.2 ppm in BE but was limited to 23.3 ppm with LiHMDS; HF reached 17.58 ppm in BE and increased to 776.45 ppm after adding extra H2O, whereas HF was not detected with LiHMDS.
• Structural stabilization: Ex-situ TEM after 100 cycles at 60 °C (with LiHMDS) indicated a robust, uniform CEI and suppression of the layered-to–rock-salt phase transition, attributed to preferential oxidation of LiHMDS and reduced parasitic reactions.

The findings demonstrate that LiHMDS addresses two critical degradation pathways in high-voltage Li||NCM811 cells: (1) chemical attack from trace H2O/HF, and (2) interfacial and structural instability of NCM811 during cycling. By effectively scavenging H2O and HF, LiHMDS reduces electrolyte acidity and suppresses LiPF6 hydrolysis and transition-metal dissolution. Its preferential oxidation forms a compact, protective CEI on NCM811, limiting electrolyte oxidation, Ni4+ reduction, and the layered-to–rock-salt phase transition. These synergistic effects deliver markedly improved capacity retention and coulombic efficiency at both room temperature and 60 °C, extending cycle life and enabling high cut-off voltages in Ni-rich cathode LMBs. The results reinforce electrolyte-additive design as a practical route to stabilize both electrode interfaces and support high-voltage operation.

LiHMDS, as an electrolyte additive, simultaneously constructs a robust CEI on Ni-rich NCM811 and scavenges H2O/HF impurities, enabling long-term cycling of Li||NCM811 cells up to 4.5 V at both 25 °C and 60 °C with substantially improved capacity retention and efficiency. This strategy offers a feasible pathway for stabilizing high-voltage, Ni-rich cathodes in lithium metal batteries, particularly at elevated temperatures. Future research may optimize additive concentrations, probe CEI composition/thickness under varied conditions, assess compatibility with other high-voltage cathodes and advanced electrolytes, and evaluate scale-up and practical cell formats.