Surface-roughened current collectors for anode-free all-solid-state batteries
Graphical abstract
Roughening the current collector increases contact points with solid electrolyte, which promotes the Li precipitation at the anode interface; accordingly, it substantially improves the Coulombic efficiency and cycle stability of anode-free all-solid-state batteries.
Introduction
All-solid-state batteries (ASSBs) are considered as next-generation rechargeable batteries with high safety and energy density [1]. The safety is improved by replacing flammable organic-based liquid electrolytes with a solid-state electrolyte, and the energy density can potentially be increased by bipolar stacking and using Li metal [2]. In particular, coupling Li metal, which has a high theoretical capacity (3860 mAh g−1) and a lowest redox potential (−3.04 V vs. standard hydrogen electrode), with a solid-state electrolyte can be an alternative to conventional graphite anodes. This allows for a significant increase in the energy density (70%) of ASSBs [3]. To date, sulfide solid electrolytes (SSEs) have been the most promising candidates among various solid electrolytes (SEs), owing to their ductile nature and high Li-ion conductivity comparable to that of traditional liquid electrolytes at room temperature. This leads to simple cold pressing for manufacturing the electrolyte and electrode layers [4], [5]. However, SSEs and Li metal react easily upon contact with each other because of the narrow potential window of SSEs, leading to the continuous decomposition of electrolytes [6], [7]. Even argyrodite-type structures (Li6PS5Cl), which have a high Li-ion conductivity (3–10 mS cm−1) and inherent chemical and electrochemical stabilities, undergo slow reactions at the interface [7]. Although strategies have been reported, such as using lithium alloys [8], [9], introducing artificial layers at the interface [10], [11], [12], and doping electrolytes [13], resolving the interface problems between the Li metal and SSEs is still challenging.
At the laboratory scale, overbalanced thick Li foils (>200 μm) are commonly used to construct cells based on lithium metal anodes, leading to a decrease in the actual capacity and an increase in the cost of the cell. This is because most of the excess Li metal does not participate in the reaction during cycling [14]. For practical lithium-metal batteries (LMBs), a thin Li metal anode is required, where the negative/positive electrode areal capacity ratio should be 1 [15]. It was reported that a 50 μm thick Li anode could reach a high energy of 300 Wh kg−1 because of the use of limited lithium [15]. Therefore, to fabricate practical ASSBs having a specific energy of 500 Wh kg−1 or more, the thickness of the lithium metal anode must be reduced. However, although Li foils can be thinned down to less than ∼20 μm, the difficult and costly handling of free-standing Li foils remains an issue [16]. In addition, the integration of thin Li foils with SSEs without interface degradation is a hurdle in scalable cell fabrication processes [17]. As current commercial cathode materials, such as LiCoO2, Li(Ni1−x−yCoxMny)O2, and LiFePO4, are assembled in the cell in a fully lithiated state, Li foils of any thickness may have an irrelevant volume on the anode side. Thus, anode-free lithium-metal batteries (AFLMBs) have recently received attention because of their much higher energy density without excess lithium [18], [19]. In AFLMBs, the cell is assembled with a current collector (CC) without adding the anode part, where the Li metal is electrochemically formed on the CC by electroplating Li ions from the fully lithiated cathode during the first charge cycle [20]. The reduction in the manufacturing cost and the simplification of the cell fabrication procedure can be beneficial because of the absence of an anode [21].
However, AFLMBs still have a low coulombic efficiency, a short cycling lifespan, and an unstable interface. Hence, several improvement strategies have been suggested: (1) CC engineering [22], [23]; (2) optimizing the electrolyte [24]; and (3) designing optimal protocols [25], [26], [27], [28], [29]. The fundamental cause for the low coulombic efficiency in anode-free batteries is the reaction of the Li metal with the electrolyte and non-uniform lithium formation on CCs [28]. The solid-electrolyte interphase (SEI) formed by the high reactivity of Li metal continuously consume the electrolyte and Li, causing rapid capacity fading. In addition, the uneven initial lithium formation resulting from the large nucleation overpotential affects the subsequent Li deposition, decreasing the availability of deposited lithium and capacity retention. Therefore, the mechanism of Li metal formation through the CC must be understood. In several studies on AFLMBs, modifications to the CC, such as designing 3D structures [30] and coating function layers on CCs [31], have been proposed to improve the uniformity of the initial lithium deposition.
Recently, an anode-free concept has been studied for ASSBs. Lee and Han et al. [27] examined ASSBs by inserting a 10-μm-thick Ag-C composite layer between an argyrodite SSE and a stainless steel (SS) CC. While Cu foil reacted with SSE, they found that the SS CC was electrochemically inactive for Li deposition from the SSE and that the Ag nanoparticles provided Li nucleation sites and promoted Li plating onto the CC. Recently, Wang et al. [18] investigated the lithium plating-stripping behavior at the interface between a Li7La3Zr2O12 SE and a copper (Cu) CC. They revealed that the spatial change in interfacial resistance must be considered and the relevant mechanisms must be identified to improve the Li formation behavior at a solid-electrolyte/CC interface. Thus, the modification of the CC must be studies and the Li formation mechanism must be understood because these are essential for anode-free all-solid-state batteries (AFASSBs) and AFLMBs. Inspired by these pioneering works, we hypothesized that the surface roughness of the CC would affect the Li formation at the interface between the SE and CC and the optimized surface morphology could improve the Li plating/stripping in AFASSBs.
In this study, we fabricated surface-roughened SS CCs having different surface roughness by using a simple etching method. By comparing the results between the liquid and SE systems using these CCs, we found that the lithium plating/stripping mechanism in AFASSBs is different from that in AFLMBs. By conducting a systematic and integrated electrochemical analysis of the asymmetric and full cell, we demonstrated that the increased contact points between a SE and a roughened CC could provide multiple reaction sites and enable favorable Li formation at the interface. This was also supported by the fact that when the roughness was excessively increased, the performance deteriorated because of the limited SE/CC contact. In addition, a direct ex situ observation of the interfaces of the SE/Li/CC provided an indication to the cause of the performance degradation in AFASSBs, where interface degradation occurs during Li precipitation from the SSE. The SS foil with optimized roughness improved the contact and suppressed the interface degradation, thus enhancing the efficiency and cycle stability of AFASSBs. This model study can serve as a useful basis for a scientific understanding of solid-interface chemistry, thereby facilitating the performance improvement of AFASSBs.
Section snippets
Synthesis of SEs
Li5.5PS4.5Cl1.5 was prepared as a SE by employing a conventional planetary milling method. The raw materials were Li2S (>99.9%, Sigma-Aldrich), P2S5 (>99.9%, Sigma-Aldrich), and LiCl (>99.9%, Sigma-Aldrich), and these were placed, with an appropriate molar ratio, into ZrO2 ball-mill jars containing Ø3-mm ZrO2 balls, where the ball-to-powder weight ratio was 25:1. These procedures were performed in an Ar-filled glove box. The powders were mechanically mixed by applying planetary ball milling at
Results and discussion
Surface-roughened SS foils were fabricated as a CC by applying a conventional etching method with slight modifications (Fig. 1). The bare SS, initially having a mirror-like smooth surface, lost its shiny nature after being etched (Fig. 1a), thus indicating the roughened surfaces of WESS and SESS. The morphology and roughness of bare SS, WESS, and SESS were further investigated using SEM and AFM, as shown in Fig. 1(b–d). For the bare SS foil, the surface was very flat and smooth, as shown in the
Conclusions
We systematically investigated the effect of the surface roughness of CCs on the formation/utilization of Li metal layers in AFASSBs. Roughening the surface of the CC definitely improved the efficiency and cycle stability of Li plating/stripping at the interface between the liquid electrolyte and CC. However, there was an optimum surface roughness of the CC for lithium precipitation from SEs. In particular, the SS CC, with a roughness of 180 nm, significantly improved the performance of AFASSBs
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
This work was supported by the Institutional Program (2E31852) of Korea Institute of Science and Technology (KIST). This work was also supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT, 2022R1C1C1006019).
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