Effect of Surface Roughness Evolution of a Metal Current Collector Using a Pulse-Reverse Polarization Method on the Stability of the Anode-Free Lithium Metal Batteries
Authors: Cho Ki-Yeop , Eom KwangSup 
Published: December 2020

Recently, Lithium(Li) metal has been considered as a “ideal” anode material due to the extra-high capacity (3,860 mAh g-1), the lowest electrochemical equilibrium potential (-3.040 VSHE) and low density (0.534 g cm-3). Despite its ideal characteristics, Li metal anode(LMA) has a critical drawback that Li deposits grow in the form of dendrite due to its ununiform plating/stripping behaviors. The undesired dendritic growth of Li results in the detrimental effects on the cell performance, such as cell short-circuit, low coulombic efficiency(CE), and dead lithium, which have hampered widespread exploits of lithium metal-based batteries. Therefore, in recent years, numerous strategies to prevent or mitigate the problematic dendrite growth, including construction of artificial solid electrolyte interphase (SEI), lithiophilicity/morphology control of current collector, modification of electrolyte, and so forth [1,2].
Meanwhile, by virtue of quite successful improvement in performance of LMAs, the battery system using in-situ formed LMAs, called anode-free lithium metal batteries (AFLMBs) have gained much interests [3]. In AFLMBs, the anode compartment has only a current collector at the initial state and then lithium metal anode is formed by electroplating with lithium ions provided from the cathode through initial charging step. The absence of corrosive/reactive Li metal in assembly process decreases the production cost and the energy density of a full-cell can be increased due to additional loading of cathode active materials. However, the dendritic growth of lithium could become more problematic because the cyclable lithium ions are fully limited to those provided from the cathode active materials. For solving notorious dendrite problem in AFLMBs, many strategies such as a use of highly concentrated salt electrolytes [3], the addition of functional electrolyte additives [4] and the introduction of lithiophilic elements on the current collector have been reported [5]. Unfortunately, however, it still seems difficult to nurture all strengths of AFLMBs related to the cost-effectiveness, simple cell design and high-energy density.
One of the effective ways to regulate electroplating behavior of lithium is to introduce the surface-roughed metal current collector. Using such the structures, the charge accumulation sites on the microscopic structure of current collector can be randomly distributed, resulting in uniform electric field on the surface of current collector. Additionally, the localized current density can be reduced, and problematic volume changes of lithium can be buffered [2].
In this context, we introduced the surface-roughed metal current collector as a substrate of AFLMBs to improve their stability. The surface roughness was controlled by an electrochemical method using a pulse-reverse polarization [6]. In particular, we focused on studying the effect of various pulse parameters on the morphological evolution of a metal current collector. To investigate their effects on the stability of AFLMBs, the CR 2032-type coin cells composed of the roughly surface-treated metal foil as a current collector and high-loading Li-Ni0.5Co0.2Mn0.3 (≈ 12.1 mg/cm2) as a cathode were assembled in an Ar-filled glove box.

Figure 1-(a) presents an example of pulse signal used to modify roughness of metal surfaces. First, to investigate effect of pulse current density(iox=ire=i), we fixed the amount of metals, which were electrochemically oxidized and reduced by giving different pulse duration time (tox = tre = t,). Figure 1-(b) presents the effect of pulse current on the morphology of metal foils. After pulse with 0.5 mA/cm2, the surface of metal foil is covered by the clusters of metal/metal oxide particles. With increasing current density, the diameter of the clusters decreased and the number of clusters increased. Figure 2 shows the effect of surface-roughed metal surface on the electroplating behavior of lithium metals. As can be seen in Figure 2-(a), the morphology of electrodeposited lithium on the surface-treated metal foil has a denser and flatter Li deposit than that on pristine metal foil, where a needle-like dendritic structure is shown. Figure 2-(b) presents the CE of the repeated plating/stripping process of lithium metal. All the cells adopting the surface-treated foils showed a higher cycling stability compared to a pristine metal foil.
Additionally, in this presentation, the correlation between pulse-reverse parameters and morphological evolution of a metal current collector, and the effect on the electrochemical long-term stability of AFLMBs will be discussed in more detail

References

[1] X.-B. Cheng et al., Chem. Rev, 117, 10403, 2017.
[2] J. Zheng et al., Chem. Soc. Rev., 49, 2701, 2020.
[3] J. Qian et al., Adv. Funct. Mater.,
26, 7094, 2016.
[4] N. A. Sahalie et al., J. Power Sources, 437, 226912, 2019.
[5] S. S. Zhang et al., Electrochimica Acta, 258, 1201, 2017.
[6]M. S. Chandrasekar and M. Pushpavanam, Eletrochimica Acta, 53, 3313, 2008

Figure 1