In-Batteria Fabrication Method of a Lithiophilic LMA for Realizing Highly Stable Li Metal Batteries
Authors: Kim Subin , Eom Kwang Sup 
Published: December 2024

The lithium (Li) metal anode (LMA) has been considered as promising anode candidates to achieve high-energy-density batteries due to its remarkable capacity (3,860 mAh g-1) and low working potential (-3.04 V vs. S.H.E.) [1]. However, the commercial use of LMA is impeded by the problematic growth of lithium in the form of dendrites. This phenomenon leads to persistent issues such as cracking and reconstruction of the delicate solid electrolyte interphase (SEI), resulting in the formation of electrically isolated lithium covered by the SEI layer, commonly known as 'dead Li' [1]. These side reactions accelerate the consumption of electrolyte and the depletion of active lithium, thereby shortening the lifespan and reducing the Coulombic efficiency (CE) of lithium metal batteries (LMBs).
Among several approaches to address the challenge of lithium dendrite growth, the application of a 3D current collector (3D CC) for lithium hosts has gained significant attention [2-3]. This is because 3D CC can decrease effective current density by providing a large surface area, thereby mitigating lithium dendritic growth and accommodating lithium deposits to alleviate volumetric changes [2-3]. Various 3D CCs, such as Cu foam, Ni foam, and carbon cloth, have been explored for LMAs, showing promise for practical applications due to their cost-effectiveness. However, the inherent lithiophobic nature of these bare 3D CCs results in uneven lithium deposition. And moreover, the SEI formed on these bare 3D CC exhibits sluggish lithium ion (Li+) conductivity and weak mechanical strength, thus failing to effectively counteract lithium dendritic growth [3]. Consequently, it is crucial to consider methods that simultaneously impart good lithiophilicity to 3D CCs and enhance the SEI layer with excellent ionic conductivity and mechanical strength to achieve stable LMAs.
In this context, employing conversion-type materials such as metallic oxides and sulfides as coating layers for 3D CCs has emerged as a promising strategy [4-5]. These conversion-type materials possess good lithiophilicity due to their high affinity for Li+, and they can generate a stable SEI layer through conversion reactions. Notably, a coating of metal sulfide (MSx) has demonstrated highly effective in stabilizing LMAs due to the lithiophilic nature of MSx and the formation of a Li2S-rich SEI layer [4-5]. 1) Li2S promotes rapid Li+ transport and uniform Li growth, owing to its high ionic conductivity (~10-5 S cm-1) and low Li+ diffusion energy barrier. 2) Li2S helps mitigate electrolyte decomposition through its high mechanical strength and electrical insulating properties [4].
However, the fabrication of LMAs with surface-modified CCs typically involves a two-step process: ex situ surface treatment of CC and subsequent lithiation of CC. This process introduces challenges in handling and additional costs, considering the sensitivity of metallic Li to various chemicals, moisture, and oxygen [6]. Addressing these challenges, integrating CC surface treatment and lithiation into a one-pot process for LMA construction could offer an efficient solution, reducing time, cost, and the impact of adverse conditions on LMB operation. However, research on this method is currently limited.
In practice, we propose an electrochemical one-pot LMA fabrication method using thiourea (TU) as S precursor, and Cu foam as Cu precursor to form lithiophilic Cu2S layer on the Cu foam. As shown in Fig. 1a, Li/Cu cells with TU additive showed several oxidation peaks only on the Cu electrode, indicating possible oxidation reaction of TU by reacting with Cu electrode. This electrochemical oxidation of TU results in the formation of a lithiophilic Cu2S layer on Cu foam, as characterized by XPS and Raman spectra (Fig. 1b-c, CuxSCF). Next, we performed one-pot fabrication of lithiophilic LMA in one electrochemical cell. In detail, we applied constant potential of 2.6 V for electrochemical treatment (CuxSCF), followed by lithiation of CC through subsequent Li electrodeposition (Li@CuxSCF). The Cu2S on CuxSCF promotes uniform Li deposition, compared to bare Cu foam , by providing lithiophilic sites and transforms into a robust, ionic-conductive Li2S-rich SEI layer (Fig. 2).

References

[1] X.-B. Cheng et al. Chem. Rev, 117, 10403, 2017.
[2] Yang, Chun-Peng, et al. Nature communications
6.1 (2015): 1-9.
[3] Noh, J. et al. (2020). Nano Letters, 20(5), 3681-3687.
[4] Fan, Y. et al. (2022) Chemical Engineering Journal, 450, 138384.
[5] Yang, Z et al. (2023). Advanced Materials, 35(15), 2210130.
[6] Li, R et al. (2023). Small Methods, 7(1), 2201177.

Figure 1