(104d) Investigating the Performance of Li2CO3 in the Li-Metal Solid Electrolyte Interphase Via Gas-Reacted Interphases

Transitioning from graphite to Li-metal anodes would be extremely desirable, offering a roughly 10x increase in the gravimetric energy capacity of Li-ion batteries. However, Li-metal anodes have yet to reach the 99.95+% Coulombic efficiencies (CE) required for long-term stable cycling (80-90% capacity retention for >1000 cycles), because the thermodynamic instability of Li-metal in practical electrolytes leads to parasitic reactivity at the Li surface (1). This reactivity generates the solid electrolyte interphase (SEI), a nanoscale, multicomponent passivation film that governs subsequent behavior of Li interfaces (2). Ideally, the SEI should permit facile conduction of Li⁺ ions while inhibiting further electrolyte decomposition, but in practice, electrolyte design for SEI improvement is complicated by the field’s lack of data on the conductivity and stability of common SEI materials. Experimental measurements are challenging because the complex nanoscale structure and extremely reductive conditions (-3.04 V vs SHE) of the SEI can dramatically change material properties from bulk behavior (3). In previous work, our group used reactions of Li metal with O₂ or NF₃ to synthesize single-component, nanoscale films of Li₂O or LiF, which enabled direct measurements of transport properties and reactivity of these SEI materials (4-6). Here, we applied similar techniques to study Li₂CO₃. Li₂CO₃ is of particular interest because previous literature has shown CO₂ to be a beneficial additive in certain electrolytes (7), but reductive instability (8) and other possible decomposition pathways (9) call into question whether Li₂CO­₃ is capable of effectively passivating Li. In this work, we investigated the functionality of Li₂CO₃ in the SEI using two platforms: synthetic Li₂CO₃-containing films on Li metal, and separately, Li-Cu half cells purged with either Ar or CO₂.

Li₂CO₃-containing films were prepared on Li via sequential reactions of O₂ and CO₂ at slightly elevated temperatures (175-200 °C). The films were confirmed to be conformal, smooth, and relatively pinhole-free using scanning electron microscopy (SEM) and air exposure experiments. A combination of Fourier-transform infrared attenuated total reflectance spectroscopy (FTIR-ATR) and titration-style quantification experiments were used determine the composition of the films. We found that the formation of Li₂CO₃ also generated Li₂O and Li₂C₂, likely due to reduction at the Li interface. The resulting films were stable and passivating in neat solvents and non-fluorinated electrolytes, confirming that Li₂CO₃ and its reduction products are electrically insulating and insoluble. However, the films were found to react in a variety of fluorinated electrolytes, generating materials chemically similar to the native SEI in each electrolyte. Impedance spectroscopy revealed that the Li₂CO₃-containing films were highly ionically conductive, with substantially greater ionic conductivity (~4-12 nS/cm) than previously measured in Li₂O (~1 nS/cm) and LiF (~0.5 nS/cm) (4). To understand the implications of these observations, Li-Cu cells were cycled after either Ar or CO₂ purging in a variety of electrolytes. Across both carbonate- and ether-based electrolytes, fluorinated and nonfluorinated, CO₂ led to substantial improvements in CE for all the electrolytes with the conventional 1 M salt concentration. The exception was local high concentration electrolytes, where CO₂ saturation had no effect. Titration gas chromatography (TGC) was performed to quantify species formed in the resultant SEIs, finding that improvements in CE with CO₂ purging are associated with decreases in accumulation of inactive Li in the SEI. Taken together, these results show that while Li₂CO₃ does exhibit reactivity with fluorinated electrolytes, it can improve the ionic conductivity of Li SEI, leading to more facile Li⁺ ion transport and better Li utilization. This work underscores the utility of Li₂CO₃ as an SEI phase and could help inform future electrolyte design for improvement of the Li-metal SEI.

References

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