An Inorganic-Rich SEI Layer Enabled By Hydrogen-Bonding-Catalyzed LiNO3 Reduction for Stable Lithium Metal Batteries
Authors: Kim Subin , Cho Ki-Yeop , Eom Kwang Sup 
Published: July 2022

Lithium metal anodes(LMAs) have been considered as the most promising anode candidates due to its high gravimetric capacity (3860 mAh g-1) and low electrode potential (-3.04 V vs S.H.E.). However, uncontrollable Li dendrite growth and low coulombic efficiency hinder practical application of LMAs, causing safety hazard and short cycle life [1]. Among several strategies to prevent Li dendrite growth, introducing the electrolyte additives is one of the most attractive approaches considering its simplicity and cost-effectiveness. Construction of a stable solid electrolyte interphase layer (SEI) is a prerequisite for the stable LMA, hence traditional remedy was introducing the SEI-forming additives in the electrolyte to form stable and robust SEI layer. However, SEI-forming additives cannot guarantee prolonged cycle life after a depletion of the additives, so another strategy is urgently needed.
Recently, deposition-regulating additives show great opportunities in the LMA research field as shown in several researches [2,3]. These additives adsorb on the electrode surface and regulate Li deposition behavior to gain dendrite-free Li deposits. However, electrochemical working mechanism of these additives needs further investigation. These researches usually focus on their effect on the Li deposition kinetics, not on the SEI formation despite of the strong adsorption of additives on the electrode interface. Meanwhile, those deposition-regulating additives are not solely used, but often use LiNO3 as SEI forming co-additive. However, the interaction between those additives and LiNO3 is still veiled, despite the possibility of changed reduction behavior by additive-adsorbed surface. In this aspect, investigating interplay between deposition-regulating additive and LiNO3 is important for the deeper understanding of their working mechanism in the LMAs.
In this contribution, we first report the synergetic effect of thiourea (TU) with LiNO3 on the SEI formation, and we constructed inorganic-rich ASEI layer by using thiourea as a proof-of-study. Thiourea is known as one of the deposition-regulation additives, which adsorbs on the Li metal surface and help to gain smooth Li deposits [3]. TU shows adsorptive behavior on several metal electrodes by S atom, while two N-H hydrogen bonds are toward the electrolyte side which may interact with anions [4]. Additionally, thiourea has been utilized as an anion receptor and shows strong interaction with NO3
- anion, which is good hydrogen bonding acceptor [5]. In this aspect, we could guess that abundance of NO3
- anion can be achieved in the presence of thiourea on the electrode surface by forming hydrogen bonding, resulting in inorganic-rich SEI layer.
To investigate the adsorption behavior of TU on the Cu electrode, differential capacitance was measured and shown in Figure 1(a). In particular, with increment of TU concentration, PZC decreases, indicates more covered surface by TU. To investigate the interaction of TU with other electrolyte components, 1H NMR spectra of different components were conducted and shown in Figure 1(b). Addition of the other components resulted in upshift displacement of N-H bond of TU, implying weakened intramolecular hydrogen bonding of TU. In contrast, addition of LiNO3 resulted in downshift displacement meaning that NO3
- might form strong hydrogen bonding with TU. Also, linear scanning voltammetry (LSV) curves with different concentration of TU were conducted and shown in Figure 1(c) to investigate the effect of TU on electrolyte reduction. The characteristic peaks of LiNO3 and LiTFSI appear at 1.6 V and 1.3 V in the cell with 5wt% LiNO3. When TU is added in the electrolyte, those peaks shift negatively while showing higher current, might be due to catalytic contribution of TU on LiNO3 reduction.
To verify the effect of catalyzed LiNO3 reduction on SEI component, we constructed ASEI film on the Cu electrode and conducted XPS measurement. As shown in Figure 2(a-b), abundant LiNxOy and Li3N components are found on ASEI with TU compared with ASEI without TU. Consequently, compared to Li|Cu@NSEI and Li |Cu@ASEI w/o TU, Li|Cu@ASEI with TU shows better cyclability and higher average CE of 96.44% during 80 cycles, as shown in Figure 3. We presumably conclude that better electrochemical performance of Li|Cu@ASEI with TU is would be due to catalyzed reduction of LiNO3 with the aid of thiourea N-H bond. Detailed working mechanism of TU and property of thiourea-modified ASEI will be further presented.

References

[1] X.-B. Cheng et al., Chem. Rev, 117, 10403, 2017.
[2] Wang, Qian et al. ACS Applied Energy Materials 2.7 (2019): 4602-4608.
[3] Wang, Qian, et al. Advanced Materials 31.41 (2019): 1903248.
[4] Papapanayiotou et al. Journal of the Electrochemical Society 145.10 (1998): 3366.
[5] Nishizawa et al. Tetrahedron letters 36.36 (1995): 6483-6486.

Figure 1