Preconcentration of lithium salt in nanoporous alumina on Cu foil as a concentrated lithium semi-solid layer for anode-free Li-metal batteries

Abstract

We preconcentrate 2 M LiTFSI in 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide inside a nanoporous alumina (Al₂O₃) layer as a so-called concentrated lithium semi-solid layer on a Cu current collector for anode-free Li-metal NMC90 batteries. This concept lowers the activation energy for lithium-ion transport and reduces the nucleation overpotential, enhancing the cycling stability.

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Lithium metal batteries (LMBs) have emerged as one of the most promising solutions for next-generation energy storage. LMBs offer a high theoretical capacity (3860 mA h g⁻¹), low density (0.534 g cm⁻³), and low electrochemical potential (−3.040 V vs. SHE). Despite these advantages, the practical application of LMBs faces significant challenges due to the highly reactive nature of lithium metal, substantial volume changes, uncontrollable lithium metal plating/stripping, and the spontaneous continued formation of a solid electrolyte interphase (SEI) on the electrode surface leading to irreversible losses of active lithium and electrolyte. These problems result in fast capacity fading during cycling and low coulombic efficiency (CE).^1–3

To address the challenges faced by LMBs, minimizing the use of lithium metal can maximize energy density by configuring anode-free lithium metal batteries (AFLBs). However, AFLBs still encounter practical issues due to low CE and poor cycling performance.^4–6 One promising approach to mitigating these problems is constructing a stable ex situ protective coating film or artificial solid electrolyte interphase (ASEI) layer on the Cu electrode.⁷ Significant progress has been made in researching ASEI coatings on Cu substrates, including inorganic-based materials, polymer-based materials, and organic–inorganic composites.^8–14

Here, this study introduces a new preconcentration concept of lithium salt, 2 M LiTFSI in 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide or [EMIM][TFSI], inside a nanoporous Al₂O₃ layer as a so-called concentrated lithium semi-solid layer (CLSSL) on a Cu current collector for anode-free Li-metal NMC90 batteries. We anticipated that the Cu substrate coated with the CLSSL would exhibit exceptional overall electrochemical performance. This is due to the dual functionality of the coating layer: the Al₂O₃ component is expected to provide a stable protective layer, while high ionic conductivity from the LiTFSI is expected to reduce energy barriers for lithium nucleation. Consequently, this design is anticipated to promote uniform Li deposition on the Cu electrode and enhance reversible lithium plating/stripping during cycling. This approach holds promise for addressing the challenges associated with lithium metal usage in next-generation AFLBs.

The morphology of the Cu electrode coated with CLSSL was characterized by field emission scanning electron microscope (FESEM), as illustrated in Fig. 1. Fig. 1a shows the bare Cu foil's top-view, and the foil's thickness is approximately 6 μm, as depicted in Fig. 1d. The Cu electrode coated with a mixture of 2 M LiTFSI in [EMIM][TFSI] and integrated into a nanoporous Al₂O₃ structure, termed as Al₂O₃-IL@Cu, demonstrated a predominantly smooth surface with a coating layer of around 5 μm (see Fig. 1b and e). The Cu foil coated solely with Al₂O₃ (Al₂O₃@Cu) for a controlled experiment presented a rough surface with a similar thickness of approximately 5 μm, as illustrated in Fig. 1c and f.

Fig. 1 FESEM images at the top-view of the (a) Cu electrode, (b) Al₂O₃-IL@Cu electrode, and (c) Al₂O₃@Cu electrode, and cross-sectional view of the (d) Cu electrode, (e) Al₂O₃-IL@Cu electrode, and (f) Al₂O₃@Cu electrode.

The alteration in electrode structure following modification was analyzed using X-ray diffraction (XRD), as depicted in Fig. 2a. The XRD patterns of all samples reveal the characteristic peaks corresponding to Cu's 111, 200, and 220 planes, suggesting that the coating layer, ASEI, does not induce any structural change of the Cu. In Fig. S1a (ESI†), the XRD patterns of the Al₂O₃-IL powder reveal alumina peaks consistent with those observed in the Al₂O₃ powder, confirming that the structure of Al₂O₃ remains intact. In Fig. S1b (ESI†), the XRD patterns of the Al₂O₃@Cu and Al₂O₃-IL@Cu electrodes, characterized using thin-film mode, display only characteristic peaks of Cu. However, at higher magnification, as shown in Fig. S1c (ESI†), the Al₂O₃@Cu and Al₂O₃-IL@Cu electrodes exhibit alumina peaks obviously at (440), clearly indicating the presence of alumina in both electrodes.

Fig. 2 (a) XRD patterns of Cu, Al₂O₃@Cu, and Al₂O₃-IL@Cu electrodes, (b) TGA curves of Al₂O₃-IL powder (black), dried Al₂O₃-IL powder after soaking in solvent for 1-week (red) and Al₂O₃ powder (green), and (c) digital photo for 1 ml of pure ionic liquid mixed in 1 ml of solvent (FEC : DMC, 1 : 4 v/v).

XPS analysis was conducted to investigate the coating layer on the Cu electrode. The wide-scan XPS spectra of the Cu, Al₂O₃@Cu, and Al₂O₃-IL@Cu electrodes are shown in Fig. S2 (ESI†). In the narrow-scan XPS spectra (Fig. S3, ESI†), both Al₂O₃@Cu and Al₂O₃-IL@Cu electrodes exhibited the Al 2p peak, confirming the successful deposition of the coating material on the Cu electrode. Additionally, EDX mapping was conducted to further validate the coating, as shown in Fig. S4 and S5 (ESI†), where the presence of Al was detected in both Al₂O₃@Cu and Al₂O₃-IL@Cu samples. Furthermore, the FTIR spectra of all electrodes, presented in Fig. S6 (ESI†), show the presence of Al–O bond vibrations in both the Al₂O₃@Cu and Al₂O₃-IL@Cu electrodes, indicating the presence of alumina. In the Al₂O₃-IL@Cu sample, additional C=N, N–S, and S=O bands were observed, corresponding to the ionic liquid. These findings further confirm the successful deposition of the coating layer on the Cu electrode.

Furthermore, the stability of CLSSL has been investigated. We immersed 50 mg of semi-solid Al₂O₃-IL powder in 5 ml of FEC : DMC solvent (1 : 4 v/v) for a week, followed by a drying process. Subsequently, thermogravimetric analysis (TGA) was conducted on this prepared powder and compared with the unsoaked Al₂O₃-IL powder, as shown in Fig. 2b. Both samples exhibited nearly identical weight loss in their TGA curves, indicating that the LiTFSI in [EMIM][TFSI] or IL does not dissolve into the FEC : DMC solvent. Another experiment conducted to verify this observation involved mixing 1 ml of 2 M LiTFSI in [EMIM][TFSI] with 1 ml of the mixed solvent, as shown in Fig. 2c. Observation revealed two distinct layers in the mixture, with the lower layer composed of 2 M LiTFSI in [EMIM][TFSI] and the upper layer comprising the FEC : DMC solvent.

Electrochemical impedance spectroscopy (EIS) was then applied in a half-cell coin cell setup to examine the charge transfer resistance (R_ct), which represents the resistance to lithium-ion movement across the electrode/electrolyte interface. EIS measurements with non-blocking conditions were conducted on Cu|Li, Al₂O₃@Cu|Li, and Al₂O₃-IL@Cu|Li CR2025 coin cells at temperatures ranging from −10 to 30 °C, with a potential of 0.001 V vs. Li/Li⁺. The impedance data for those cells are presented in Fig. S7a–c (ESI†), respectively. Across all samples, a consistent trend appeared: lower temperatures correlated with increased impedance. To determine R_ct, an equivalent circuit was employed for EIS fitting, outlined in Fig. S8a (ESI†). Fig. S8b (ESI†) illustrates the example of a Nyquist plot from the experiment with the fitted data. The fitted parameters for all samples, including R_s, R_contact, R_ct, and specific R_ct, are summarized in Tables S1–S3 (ESI†). Moreover, the activation energy values (E_a) were determined by analyzing the logarithmic relationship between R_ct and the inverse of temperature using the Arrhenius equation, as depicted in Fig. S7d (ESI†). These values indicate the energy associated with lithium ions traversing sites within the ASEI layer.¹⁵ The calculated E_a values for Cu, Al₂O₃@Cu, and Al₂O₃-IL@Cu cells are 0.719, 0.650, and 0.644 eV, respectively, as shown in Table S4 (ESI†). Notably, the Al₂O₃-IL layer exhibited an 11% lower activation energy than bare Cu, suggesting that the ionic liquid in the coating layer can mitigate the barrier for Li⁺ hopping within the SEI layer.¹⁶

For a relative comparison due to the lack of a previous report determining the activation energy using EIS together with the Arrhenius equation¹⁷ for Li⁺ transport in anode-free Li-metal battery systems, the Al₂O₃-IL@Cu system demonstrates an activation energy of 0.644 eV, which is consistent with the activation energy range (0.31–0.94 eV) of practical Li-ion batteries using liquid-based electrolytes.^18,19

To further investigate the impact of the CLSSL or Al₂O₃-IL layer on the Cu electrode, we assessed the rate capability and stability in half-cell coin cell configurations. For the rate capability test, all cells were evaluated at various current densities for Li plating: 0.25, 0.5, 1.0, 2.0, 3.0, 4.0, and 5.0 mA cm⁻², before returning to 0.25 mA cm⁻², with a fixed Li deposition capacity of 1.0 mA h cm⁻². Fig. 3a presents the CE of Al₂O₃@Cu|Li and Al₂O₃-IL@Cu|Li cells, compared to the control Cu|Li cell. At low current densities (0.25 to 1.0 mA cm⁻²), all samples showed similar %CE values. It is important to note that increasing the current density directly intensifies non-uniform lithium deposition, which results in a greater formation of lithium dendrites within the cell.²⁰ However, at higher current densities (2.0 to 5.0 mA cm⁻²), the Al₂O₃-IL@Cu|Li cell outperformed the others, especially at 5.0 mA cm⁻². Additionally, upon reverting to 0.25 mA cm⁻², the Cu electrode with CLSSL or Al₂O₃-IL layer demonstrated exceptional performance with a high CE of up to ∼73%, indicating that this layer can enhance ionic conductivity and reduce ongoing Li consumption within the SEI layer.^21,22

Fig. 3 Electrochemical performance in a half-cell coin cell of (a) CE of rate capability of Cu|Li (black), Al₂O₃@Cu|Li (red), and Al₂O₃-IL@Cu|Li (blue) with various current densities from 0.25 to 5.0 mA cm⁻² at 1.0 mA h cm⁻², (b) CE of stability test for 200 cycles with a current density of 0.25 mA cm⁻² at 0.5 mA h cm⁻² with an upper cutoff potential of 1.0 V, (c) plating profiles at the 1st cycle, and (d) CE of the stability test for 100 cycles with a current density of 1.0 mA cm⁻² at 1.0 mA h cm⁻² with an upper cutoff potential of 1.0 V.

Additionally, a stability test was conducted on all samples at a low current density of 0.25 mA cm⁻² for Li plating, with a fixed Li deposition capacity of 0.5 mA h cm⁻². The CE values of Li plating/stripping for all asymmetric cells in this test are shown in Fig. 3b. After 200 cycles, the %CE for the bare Cu, Al₂O₃@Cu, and Al₂O₃-IL@Cu cells were 96.9%, 95.9%, and 98.6%, respectively. In the first cycle, the voltage profiles during Li plating, depicted in Fig. 3c, revealed that the Al₂O₃-IL@Cu electrode exhibited an overpotential of 96 mV, compared to 109 mV for the bare Cu electrode. Despite the anticipated improvement in ionic conductivity, the Al₂O₃@Cu electrode exhibited a higher overpotential than bare Cu. This observation can be attributed to the lower intrinsic conductivity of Al₂O₃ compared to Cu.¹⁷ However, the Cu electrode coated with CLSSL or Al₂O₃-IL layer significantly reduced the initial Li plating overpotential (13% lower, from 109 mV to 96 mV). This finding suggests that the CLSSL, with its lithiophilic properties, effectively reduces the energy barrier for lithium nucleation, promoting more uniform Li deposition on the Cu electrode surface.^23,24 Fig. S9 (ESI†) illustrates the voltage hysteresis at the 200th cycle for all samples. The voltage polarization values for Cu, Al₂O₃@Cu, and Al₂O₃-IL@Cu cells were 29.6, 32.8, and 33.7 mV, respectively. The voltage hysteresis in the Al₂O₃-IL@Cu cell was lower by ∼13% compared to bare Cu owing to the lithiophilic nature of the coating layer.²⁵ Furthermore, cycling stability was evaluated at a higher current density of 1 mA cm⁻² with a fixed Li deposition capacity of 1 mA h cm⁻². After 100 cycles, the %CE values for bare Cu, Al₂O₃@Cu, and Al₂O₃-IL@Cu cells were 28.5%, 63.5%, and 86.5%, respectively, as shown in Fig. 3d. The voltage profiles for all cells at the 1st, 2nd, and 50th cycles are depicted in Fig. S10 (ESI†). The potential polarization values after 50 cycles for the Cu, Al₂O₃@Cu, and Al₂O₃-IL@Cu cells were 213 mV, 124 mV, and 96 mV, respectively. Notably, the Cu electrode coated with CLSSL or Al₂O₃-IL layer exhibited voltage polarization that was 2 times lower than that of the bare Cu cell. This reduction in nucleation barrier during the initial cycle, along with the lower voltage polarization after prolonged cycling, contributed to the enhanced electrochemical performance observed in the Al₂O₃-IL@Cu cells.

The stability evaluation was extended to full-cell coin cell configurations, where Cu, Al₂O₃@Cu, and Al₂O₃-IL@Cu were paired with a LiNi_0.9Co_0.05Mn_0.05O₂ (NMC90) cathode. The cells underwent a formation step for two cycles at a current density of C/20 with an upper potential of 4.2 V, followed by cycling at 0.25C for 50 cycles. The potential profiles during the formation step are shown in Fig. S11 (ESI†). The initial coulombic efficiencies (ICE) for Cu|NMC90, Al₂O₃@Cu|NMC90, and Al₂O₃-IL@Cu|NMC90 CR2025 coin cells were 83%, 78%, and 73%, respectively. In the second cycle, the discharge capacities for those cells were 145, 141, and 150 mA h g⁻¹, respectively. Fig. S12a (ESI†) displays the long-term cycling performance of Al₂O₃-IL@Cu|NMC90, Al₂O₃@Cu|NMC90, and Cu|NMC90 cells, with capacity retention of 45.1%, 32.8%, and 17.3%, respectively. After 50 cycles, the Al₂O₃-IL@Cu|NMC90 cell demonstrated a specific discharge capacity of 51 mA h g⁻¹, which is 2.5 times higher than that of bare Cu, as shown in Fig. S12b (ESI†). This outcome indicates that the CLSSL or Al₂O₃-IL layer with a dual-functional protective layer from Al₂O₃ and lithiophilic properties from 2 M LiTFSI in [EMIM][TFSI] or IL enhances reversible Li plating/stripping.²⁶ Therefore, these results suggest that the CLSSL or Al₂O₃-IL layer on the Cu electrode significantly improves electrochemical performance. Furthermore, the rate capability of all samples in the full-cell configuration was investigated, as shown in Fig. S13 (ESI†). The cells underwent a two-cycle formation step at a current density of C/20 with an upper cutoff potential of 4.2 V, followed by rate capability testing at various current densities ranging from 0.1C to 2.0C. The Cu electrode with the Al₂O₃-IL layer exhibited a higher discharge capacity at 1.0C. However, none of the cells could operate at 2.0C due to the excessive current density, which led to the formation of lithium dendrites in the cells. This issue will be addressed in future studies.

The post-mortem analysis after cycling was conducted to evaluate the effect of the coating layer on the Li plating behaviour. All cells were disassembled after 10 cycles at a current density of 0.25 mA cm⁻² with a fixed capacity of 1.0 mA h cm⁻², focusing on the Li plating step. As shown in Fig. 4a and d, the bare Cu cell exhibited more severe Li deposition and non-uniform Li plating compared to the coated samples. In contrast, the Cu electrode with a coating layer demonstrated more uniform Li plating as shown in Fig. 4b and e for Al₂O₃@Cu and displayed a denser and flatter structure for the Al₂O₃-IL@Cu electrode (Fig. 4c and f) indicating that the ionic liquid within the coating layer promotes uniform Li plating on the Cu electrode. Additionally, XPS was employed to investigate the Li plating behaviour of all electrodes after 10 cycles. The Li 1s region for all samples is shown in the wide-scan XPS spectra (Fig. S14, ESI†) and the corresponding elemental quantification is provided in Fig. S15 (ESI†). From the XPS fitting results in Fig. S16 (ESI†), Li–O and Li–F bonds were identified, contributing to the SEI composition. Notably, the Al₂O₃-IL@Cu electrode exhibited a lower quantity of Li metal compared to the other samples, suggesting that Li plating beneath this coating resulted in reduced Li dendrite formation during cycling.

Fig. 4 FESEM images of top-view at different magnifications of all electrodes in the Li plating step after 10 cycles at 0.25 mA cm⁻² and 1.0 mA h cm⁻² of the (a) and (d) Cu electrode, (b) and (e) Al₂O₃@Cu electrode, and (c) and (f) Al₂O₃-IL@Cu electrode.

In conclusion, our study has demonstrated that a dual-functional CLSSL or Al₂O₃-IL layer on a Cu electrode significantly enhances the electrochemical performance of anode-free lithium metal batteries. The as-fabricated cells in this work exhibit superior rate capability and stability in both half-cell Al₂O₃-IL@Cu|Li and full-cell Al₂O₃-IL@Cu|NMC90 CR2025 coin-cell configurations. This remarkable performance is attributed to the dual-functional protective layer and lithiophilic properties of the CLSSL or Al₂O₃-IL layer, which reduce the energy barriers for Li nucleation and promote uniform Li deposition on the Cu electrode. Consequently, this enhancement leads to improved reversible Li plating/stripping during cycling. Our findings have important implications for the design of next-generation anode-free lithium metal batteries.

This work was financially supported under the Program Management Unit for National Competitiveness Enhancement (PMU-C) by the Office of National Higher Education Science Research and Innovation Policy Council (NXPO) and Thailand Science Research and Innovation (TSRI) and under the Fundamental Fund by TSRI and VISTEC and as well as Energy Policy and Planning Office (EPPO), Ministry of Energy, Thailand. Additional support was received from the Frontier Research Centre at VISTEC.

Data availability

The data that support the findings of this study are available from the corresponding author, upon reasonable request.

Conflicts of interest

The authors declare that they have no known competing financial interest or personal relationships that could have appeared to influence the work reported in this paper.

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Footnote

† Electronic supplementary information (ESI) available: Experimental details, supporting figures and tables. See DOI: https://doi.org/10.1039/d4cc03946g

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