INTRODUCTION
Lithium metal batteries (LMBs) have attracted considerable attention due to the exceptional theoretical capacity of lithium metal, which reaches 3,860 mAh/g, along with its notably low electrochemical potential of –3.04 V versus the standard hydrogen electrode [1–3]. These attributes make LMBs highly promising candidates for future energy storage solutions, especially in sectors that demand high energy density, such as electric vehicles and portable electronics [4–6]. To achieve these high energy densities, the use of ultra-thin lithium metal anodes is essential, underscoring their critical function in such advanced applications.
The conventional production method for thin lithium metal foils at an industrial scale generally involves extrusion followed by precision rolling. These processes require meticulous regulation of pressure and tension to minimize the occurrence of defects, such as tearing, and to maintain dimensional integrity. The high adhesiveness and inherent ductility of lithium, however, often lead to challenges, such as sticking to rolling equipment or damage during processing. Additionally, due to the limited reduction ratio achievable in a single pass, multiple rolling passes are necessary, which heightens the risk of defects and compromises the consistency of the final product, leading to elevated production expenses [7].
Dry processes, such as physical vapor deposition (PVD), have also been employed for manufacturing ultrathin lithium metal layers. These methods provide precise control over thickness and uniformity, producing high-purity films that improve battery performance by reducing dendrite growth during cycling. However, PVD suffers from inherent limitations, including low deposition rates, high costs associated with vacuum systems, and inefficiencies for large-scale production. These challenges underline the need for alternative methods that are more suitable for high-throughput manufacturing.
Electrochemical deposition has been investigated as a promising alternative to address the limitations inherent in both extrusion-rolling and dry processes. This method facilitates the direct deposition of lithium metal onto substrates, allowing precise control over film thickness and uniformity. Unlike mechanical processes that incrementally reduce thickness or dry processes with slow deposition rates, electrochemical deposition builds thickness progressively, enabling high-speed and cost-effective production of ultra-thin layers, making it advantageous for scalable manufacturing [8]. However, the high chemical reactivity of lithium poses significant challenges; interactions with the electrolyte result in the formation of an uneven solid electrolyte interphase (SEI), promoting dendritic growth that compromises both performance and safety due to potential internal short circuits [9–11].
Research aimed at mitigating dendritic growth has often focused on tailoring the electrolyte composition to form a stable SEI. This involves manipulating variables such as solvent types, lithium salt concentrations, and the introduction of additives to improve SEI properties [12–14]. Ren et al. demonstrated that electrolytes with high lithium salt concentrations promote a more uniform SEI, effectively reducing dendrite formation [15]. Perez et al. expanded on this by utilizing localized high-concentration electrolytes (LHCEs), which maintain a low overall viscosity while ensuring high local lithium concentrations to enhance SEI uniformity [16]. Electrolyte additives such as fluoroethylene carbonate (FEC), tris(2,2,2-trifluoroethyl) borate, CsPF6, vinylene carbonate (VC), trace water and silicon tetrachloride have been shown to facilitate the formation of lithium halides within the SEI, which promotes uniform nucleation and suppresses dendritic growth [17–22].
Despite significant advances in electrolyte optimization for SEI stabilization, other methods, such as the application of mechanical pressure during the electrodeposition process, have demonstrated potential in influencing lithium deposition behavior. Our previous studies have investigated the application of controlled pressure during lithium electrodeposition to suppress dendritic growth, promoting the formation of a dense and uniform lithium layer [23]. This approach directs lithium growth laterally to fill surface voids before any vertical expansion occurs, leading to a more compact and uniform deposition.
Nonetheless, many of these investigations have primarily focused on low current density conditions (generally below 1 mA/cm2). For the development of commercially viable lithium metal foils through electroplating, processes at higher current densities (≥10 mA/cm2) are required. Research specifically addressing dendrite suppression at such high current densities remains limited. Expanding on our earlier work with pressure-assisted electrodeposition, this study explores the challenges and deposition behavior of lithium under high current density conditions with applied mechanical pressure. Furthermore, we examine the incorporation of ultrasonic assistance as a strategy to strengthen the structural integrity of the lithium film and enhance its charge-discharge stability. The outcomes of this study aim to advance the optimization of lithium metal electroplating processes under high current density conditions, contributing to the commercial viability of LMBs.
EXPERIMENTAL
Lithium Electrodeposition Preparation
The working electrode (20 μm thick copper, 60 mm × 60 mm) was prepared and treated with an atmospheric pressure plasma device (iREV, API) to remove impurities and enhance surface activity. Lithium electrodeposition was conducted using a custom-designed split cell equipped with a load cell for uniaxial pressure control. The cell structure is schematically illustrated in Fig. 1, which provides an overview of the key components, including the working and counter electrodes, separator, electrolyte chamber, ultrasonic device, and the load cell positioned at the top to apply precise vertical pressure for controlling deposition uniformity. The counter electrode was a 600 μm thick lithium metal sheet (50 mm × 50 mm), and a 25 μm thick PP separator was used. The electrodeposition took place in a glovebox with moisture and oxygen levels maintained below 0.5 ppm. The electrolyte used was 3M LiFSI in DME. Post-deposition, the cells were disassembled, and the electrodes were rinsed with DME inside the glovebox to remove residual electrolyte. Preliminary experiments revealed that insufficient pressure had little to no effect on suppressing dendritic growth, whereas excessively high pressure caused electrolyte squeezing through the separator, leading to inadequate ion supply and a significant increase in deposition voltage. Based on these findings, a pressure range of 0.8–1.3 MPa was established as the optimized condition for the main experiments.
Ultrasonic-Assisted Lithium Electrodeposition Setup
To promote uniform lithium deposition, an ultrasonic device (40 kHz, 200 W) was used throughout the electrodeposition process. The ultrasonic generator, positioned at the bottom center of the cell and maintained at a 2 cm distance from the electrode, enhanced ion mobility and reduced the concentration gradient at the electrode surface, facilitating electrochemical activation. The ultrasonic waves were continuously applied for the duration of each experiment to ensure consistent conditions. Additionally, the temperature of the electrolyte was monitored in real-time using a temperature sensor, and a cooling system was employed to prevent the temperature from rising above 25°C, thereby maintaining optimal deposition conditions.
Morphological Analysis and Electrochemical Performance Evaluation
The morphology of the electrodeposited lithium was analyzed using a scanning electron microscope (SEM, Thermo Fisher Scientific, Sirion TM). Cross-sectional analysis was conducted using focused ion beam (FIB) milling, followed by SEM observation, to assess the uniformity and density of the deposited lithium layer. These analyses provided insight into the physical characteristics of the lithium layer that could influence its electrochemical behavior.
The electrochemical performance of the electrodeposited lithium electrodes was evaluated using 2032-type coin cells assembled with the plated lithium as the working electrode, a 200 μm thick lithium counter electrode (15 mm in diameter), and a 25 μm thick polypropylene (PP) separator (19 mm in diameter). The electrolyte used was 3 M LiFSI in DME, supplemented with 3 wt.% LiNO3 and 1 wt.% FEC to enhance stability and facilitate the formation of a uniform solid electrolyte interphase (SEI). The stripping capacity was measured at a current density of 1 mA/cm² until the cell voltage reached 1.0 V, and the utilization efficiency of the electrodeposited lithium was calculated as the ratio of the stripping capacity to the plating capacity. For charge-discharge stability evaluation, Li|NCM811 half-cells were used. The lithium anode was prepared using either lithium foils electrodeposited at various current densities or commercial rolled lithium foil for comparison. The NCM811 electrodes were prepared by mixing NCM811 active material, Super-P carbon black, and PVDF binder in an 8:1:1 weight ratio to create a slurry in NMP solvent, which was then uniformly coated onto aluminum current collectors and dried under vacuum at 120°C for 12 hours. The half-cells, assembled with a 40 μm thick lithium foil as the anode, were cycled at a rate of 0.5 C within a voltage range of 2.8–4.3 V to assess long-term charge-discharge performance. All cycling and electrochemical tests were conducted using the WBCS3000 battery testing system, providing precise monitoring and comprehensive analysis of the electrochemical stability and overall performance.
RESULTS AND DISCUSSION
Previous findings from our team demonstrated that the pressure applied during lithium electrodeposition significantly affects lithium growth behavior, aligning with the observations reported by C. Fang et al. [23,24]. Under high pressure, lithium initially grows horizontally to fill surface voids and then transitions to vertical growth, forming a dense columnar structure. In contrast, at lower pressures, lithium grows preferentially in directions where ion supply is more favorable, leading to non-uniform and low-density dendritic deposits. Such pressure-dependent growth behavior has primarily been studied at current densities below 1 mA/cm², prompting our investigation into the effects of the interaction between pressure and higher current densities on lithium growth.
Firstly, electrodeposition experiments were conducted under pressures ranging from 0.8 to 1.3 MPa and current densities from 1 to 15 mA/cm2, with the results presented in Fig. 2. At a pressure of 1.3 MPa, stable columnar structures were observed at a current density of 1 mA/cm² and maintained up to 10 mA/cm2. At 15 mA/cm2, the growth transitioned to a dendritic structure. Columnar structures are characterized by vertically aligned, dense, and uniform growth, whereas dendritic structures exhibit irregular, porous cross-sections with complex branching. To clearly differentiate these structural patterns, conditions where dendritic structures were observed are highlighted with a yellow background in Fig. 2, enabling readers to easily compare columnar and dendritic growth. Under 1 MPa pressure, the columnar structure was maintained up to 5 mA/cm2, while dendritic growth began at 10 mA/cm2. At the lowest pressure of 0.8 MPa, the columnar structure was only observed at 1 mA/cm2, with dendritic growth appearing immediately upon an increase in current density. These results indicate that there is a critical threshold pressure required to maintain a columnar growth structure at each current density, and this threshold increases as the current density rises. Notably, at a high current density of 15 mA/cm2, even 1.3 MPa was insufficient to suppress dendritic growth, suggesting that pressure alone is inadequate for preventing dendritic formation under high-current conditions.
The differences in lithium growth behavior can significantly impact the composition of the electrodeposited lithium layer. The contrast observed in Fig. 2 indicates that, alongside pure lithium, SEI (solid electrolyte interphase) and byproducts are interspersed within the deposit. These byproducts do not contribute to the active material of the anode and can degrade battery stability and energy density. To quantify how the deposition rate affects the amount of usable pure lithium, utilization efficiency was evaluated using lithium films plated at a constant pressure of 1.3 MPa and current densities ranging from 1 to 15 mA/cm2.
Fig. 3 shows the stripping capacity results after depositing a capacity of 2 mAh/cm2 at current densities of 1, 5, 10, and 15 mA/cm2 under a constant pressure of 1.3 MPa. The samples were stripped at a constant current density of 1 mA/cm², yielding stripped capacities of 1.988, 1.970, 1.894, and 1.751 mAh/cm2, corresponding to utilization efficiencies of 99.4%, 98.5%, 94.7%, and 87.6%, respectively. Fig. 2 and 3 collectively demonstrate that lithium films electrodeposited at higher current densities exhibit uneven growth and increased SEI formation. This leads to a reduction in the amount of usable lithium. Therefore, utilization efficiency is lower in lithium films deposited under high current density conditions.
The variation in utilization efficiency due to changes in lithium deposition structure has direct implications for the stability of LMBs. To verify this quantitatively, charge-discharge testing was performed using 40 μm thick electrodeposited lithium films produced at current densities of 1 to 15 mA/cm² under 1.3 MPa pressure, compared against commercial rolled foils (Honjo metal, Japan). As shown in the results presented in Fig. 4a, lithium anodes electrodeposited at higher current densities exhibited reduced cycle stability. Lithium deposited at 1 mA/cm2 showed a cycle life of 248 cycles, surpassing the 213-cycle life of commercial rolled foils, while lithium deposited at 5 mA/cm² achieved 181 cycles and at 10 mA/cm², 160 cycles, indicating a progressive decrease in stability. Notably, the lithium deposited at 15 mA/cm2 with dendritic growth recorded only 64 cycles, representing a reduction of more than 70% compared to commercial rolled foils. To further evaluate the long-term performance and reversibility, Coulombic efficiency (CE) data over extended cycles were analyzed and are presented in Fig. 4b. Across all conditions, initial CE values were maintained above 99%, demonstrating high reversibility and stability during early cycles. However, as capacity degradation became more pronounced, CE also exhibited a decline.
Referring to fig. 3 and 4, as the current density increased from 1 mA/cm2 to 10 mA/cm2, the utilization efficiency slightly decreased from 99.4% to 94.7%. However, this minor change resulted in a significant reduction of approximately 36% in cycle life. The reason why a small difference in efficiency can result in a substantial change in stability can be illustrated through a simple calculation. This indicates that even minor differences in efficiency can have a significant impact on long-term battery performance. At 1 mA/cm2, about 54.8% (0.994100) of the initial lithium would remain usable after 100 cycles. In contrast, at 10 mA/cm², only approximately 0.43% (0.947100) of the initial lithium would be retained after 100 cycles. This highlights the critical importance of forming uniform and high-purity lithium films during electrodeposition to optimize long-term battery performance. From this perspective, it is essential to develop processes that can suppress dendritic structures and promote uniform columnar growth even at current densities of 15 mA/cm2 or higher, to enhance electrochemical stability and the usability of lithium. To achieve this, strategies are needed that can efficiently supply lithium ions to counter the rapid depletion that occurs during high-rate deposition.
In electrodeposition processes, stirring or bubbling is commonly employed to overcome insufficient ion supply. However, due to spatial constraints in pressurized lithium electrodeposition setups, such agitation techniques are challenging to implement. Therefore, we proposed applying ultrasonic waves during pressurized lithium electrodeposition to promote uniform mixing of the plating solution. Ultrasonic waves typically induce microstreaming in the liquid, enhancing solution flow and ensuring an even distribution of ions within the electrolyte. This continuous ion supply at the lithium/electrolyte interface is expected to facilitate uniform lithium growth. To verify the effect of ultrasonic application in pressurized lithium electrodeposition, we conducted experiments to compare lithium deposition behavior with and without ultrasonic assistance. Fig. 5a illustrates voltage profiles during lithium electrodeposition at a current density of 15mA/cm2 and 1.3 MPa, with and without the application of ultrasound. Before applying ultrasound, the average deposition voltage was approximately 43.8 mV, which significantly dropped to around 6.4 mV after applying ultrasound. To further confirm the voltage enhancement effect of ultrasound, we conducted an experiment where ultrasound was turned off after 2 hours (indicated by a black arrow) and then turned back on (indicated by a white arrow). The voltage increased to pre-ultrasound levels when ultrasound was turned off and dropped immediately when ultrasound was reapplied.
In order to investigate the mechanism behind the observed voltage change, electrochemical impedance spectroscopy (EIS) analysis was performed, and the spectra were quantitatively interpreted using ZView software for equivalent circuit fitting (Supplementary Fig. S1). In the high-frequency region, the first semicircle represents the solution resistance (R1), which reflects the ionic conductivity of the bulk electrolyte, and the interface resistance (R2), which is primarily influenced by SEI formation and interfacial properties at the electrode/electrolyte interface [25]. According to the ZView fitting results, R1 was measured at 12 Ω without ultrasound and 10.9 Ω with ultrasound, while R2 was measured at 158.3 Ω and 164 Ω, respectively. These results indicate that ultrasound has minimal impact on the intrinsic ionic conductivity of the electrolyte or the stability of the SEI layer. In contrast, the charge transfer resistance (R3), predominantly observed in the low-frequency region, decreased significantly from 494.1 Ω without ultrasound to 381 Ω with ultrasound. R3 represents the resistance associated with the charge transfer process at the electrode/electrolyte interface [26]. This reduction strongly suggests that ultrasound facilitates charge transfer by promoting ionic mobility and reducing the activation energy of interfacial reactions.
The reduction in activation energy due to ultrasound is attributed to the enhancement of ionic mobility and redistribution caused by microstreaming and cavitation effects near the electrode surface [27–30]. According to previous studies, cavitation induced by ultrasound generates microjets and turbulence that disrupt the stagnant boundary layer near the electrode surface, while oscillating microbubbles produce vortex flows that enhance mixing and improve ionic distribution [27,28]. Additionally, selective cavitation at specific interfaces alters surface properties, further promoting ion redistribution and enhancing electrochemical reactions [29,30]. The application of ultrasound improves ionic supply near the interface, preventing localized ion depletion and enabling the formation of stable and uniform lithium deposition structures.
Fig. 6 shows the structure of lithium layers electrodeposited under 1.3 MPa pressure at current densities from 1 to 15 mA/cm² with a total capacity of 2 mAh/cm2, under ultrasonic application. Yellow dotted lines were added to clearly visualize the interface between lithium columns and SEI layers, facilitating the interpretative analysis of these regions. To further enhance data interpretation, the original image without graphical markings has been included as Supplementary Fig. S2, allowing the lithium layer structure to be examined without additional annotations. Compared to lithium layers deposited without ultrasound (Fig. 2), the column width increased significantly after ultrasound application. Particularly at 15 mA/cm², where dendritic structures were observed before ultrasound application, stable columnar structures were maintained with ultrasound.
The impact of structural changes due to ultrasound on charge-discharge stability is presented in Fig. 7a, while the coulombic efficiency (CE) data over extended cycles is shown in Fig. 7b. Charge-discharge testing was performed on lithium anodes electrodeposited at current densities of 1 to 15 mA/cm², following the same conditions as in Fig. 4. The cycle life of lithium anodes deposited without ultrasound is indicated at the bottom of the graph with identical symbols for comparison, and the results for commercial rolled foils are shown as gray dotted lines. Fig. 7a reveals that charge-discharge stability improved at all current density conditions after ultrasonic application. At 1 mA/cm2, a cycle life improvement of approximately 27.8% was observed, while at 15 mA/cm2, where dendritic structures had formed previously, stability increased by over 3.6 times after ultrasound. Additionally, the lithium anodes produced with ultrasound at all current densities showed superior stability compared to commercial rolled foils. The CE results (Fig. 7b) demonstrated consistently high reversibility, maintaining an average CE above 99% prior to significant capacity fade, further supporting the observed stability improvements.
These results indicate that using ultrasonic technology enables the production of dense and high-purity lithium films that exhibit greater charge-discharge stability than commercial rolled foils, even under high current density conditions. This finding suggests the potential for mass production of lithium metal films via electrodeposition, contributing significantly to the commercialization and performance enhancement of lithium secondary batteries. However, large-scale implementation introduces additional challenges, such as integrating pressurized deposition with roll-to-roll processes and maintaining uniform ultrasonic wave distribution across large areas. Addressing these challenges will require extensive research on equipment design and process optimization, including the development of double-belt press systems for continuous pressurized deposition. These aspects are critical for advancing the commercial viability of ultrasound-assisted electrodeposition technology and will be explored in future studies. Furthermore, the findings of this study demonstrate that ultrasound-assisted electrodeposition technology could be applied to other metal secondary battery systems, offering broad potential for improving material uniformity and energy storage device performance. This technological advancement could mark a pivotal step in enhancing the consistency of battery materials and the performance of energy storage systems.
CONCLUSIONS
This research applied a pressurized electrodeposition process combined with ultrasonic agitation technology to enhance the fabrication of lithium metal battery anodes. The pressure required to transition dendritic structures into columnar formations was analyzed across various current densities. Results confirmed that under low-current conditions, pressurized deposition enabled the growth of uniform, high-density columnar lithium. However, at high current densities exceeding 15 mA/cm2, even applying a maximum pressure of 1.3 MPa proved insufficient, leading to dendritic formation.
To overcome this limitation, ultrasonic agitation was employed to improve ion mobility, ensuring consistent ion supply to the electrode surface and reducing charge transfer overpotential. Consequently, the application of ultrasound facilitated stable columnar growth even under high current density conditions, preventing dendritic formation and significantly enhancing charge-discharge stability.
These findings demonstrate that incorporating ultrasonic technology in the lithium electrodeposition process can suppress dendritic growth and promote the formation of dense columnar structures, thereby improving the stability and performance of lithium secondary batteries. This approach is expected to provide a robust technological foundation, offering high reliability even at high current densities, and advancing next-generation energy storage systems.
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