Extreme fast charging of lithium metal batteries enabled by a molten-salt-derived nanocrystal interphase

doi:10.21203/rs.3.rs-3662302/v1

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Extreme fast charging of lithium metal batteries enabled by a molten-salt-derived nanocrystal interphase

https://doi.org/10.21203/rs.3.rs-3662302/v1

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The extreme fast charging performance of lithium metal batteries (LMBs) with a long life is an important focus in the development of next-generation battery technologies. The friable solid electrolyte interphase and dendritic lithium growth are major problems. We report the formation of an inorganic nanocrystal-dominant interphase produced by pre-immersing the Li in molten lithium bis(fluorosulfonyl)imide that suppresses the overgrowth of the usual interphase. Its high surface modulus combined with fast Li⁺ diffusivity enables a reversible dendrite-proof deposition under ultrahigh-rate conditions. It gives a record-breaking cumulative plating/stripping capacity of >240,000 mAh cm⁻² at 30 mA cm⁻²@30 mAh cm⁻² for a symmetric cell and an extreme fast charging performance at 6 C for 500 cycles for a Li||LiCoO₂ full cell with a high-areal-capacity, thus expanding the use of LMBs to high-loading and power-intensive scenarios. Its usability both in roll-to-roll production and in different electrolytes indicates its scalable and industrial potential.

Physical sciences/Materials science/Materials for energy and catalysis/Batteries

Physical sciences/Chemistry/Electrochemistry/Batteries

Physical sciences/Energy science and technology/Energy storage/Batteries

The demand for high-performance lithium batteries is increasing with the rapid growth of long-use smart electronics and high-endurance, high-mileage electric vehicles¹. The increased use of these batteries depends on their energy density, charging time, safety, cycle life, and cost². In particular, being able to recharge the battery of an electric vehicle in almost the same time as it takes to fill a gasoline vehicle, the United States Department of Energy (US DOE) has set a goal of <15 min charge time to an 80% state of charge (SOC), with the battery having a capacity of >180 Wh kg⁻¹^{3, 4}. A more aggressive target is 240 Wh kg⁻¹ corresponding to an 80% SOC of a 300 Wh kg⁻¹ battery with a 5 min charging time⁵. Extreme fast charge (XFC) goals featuring a high energy density and fast charging rates have motivated intense research into the lithium metal anode and lithium metal batteries (LMBs)^{6, 7}, as the next technology beyond traditional lithium ion batteries (LIBs) and electrical double-layer capacitors (EDLCs) with lower maximum energy densities (Figs. 1a, b)^{8, 9}. Typically, the jump from LIBs to LMBs by replacing the graphite anode with lithium metal while coupling it with a high energy cathode would yield an energy density >400 Wh kg⁻¹ (Fig. 1c)¹⁰. At a certain current rate, the higher energy content in LMBs also favors a higher power output than LIBs (Fig. 1d). Its potential balance in power/energy performance is expected to help achieve the XFC goals.

When LMBs are cycled at a high rate, Li⁺ depletion at the near surface of lithium metal upon stripping and Li⁺ agglomeration around hotspots upon plating become inevitable¹¹. These undesirable issues may easily cause a nonuniform Li⁺ flux and unregulated Li deposition, which would lead to disastrous dendrite growth, “dead Li” formation and rapid capacity decay¹². To achieve the XFC goal without adversely impacting battery performance and safety has become a big challenge. Given the diffusion-determined nature of the Li deposition reaction, homogenizing the surface Li⁺ concentration provides an effective solution to these issues¹¹. In this context, controlling the solid electrolyte interphase (SEI) growth on the lithium metal surface can play a vital role in stabilizing it¹³. This requires an ideal SEI layer that acts not only as a barrier to prevent continuously repeated interfacial reactions between the liquid electrolyte (LE) and lithium metal, but also as a fast Li⁺ conductor to regulate the macroscopic Li⁺ concentration, particularly under high-rate conditions¹⁴. The direct surface engineering of a SEI with these features is needed^{15, 16}. In particular, inorganic-rich protective SEIs, which are dominated by LiF¹⁵, Li₂O¹³, Li_xPO₄¹⁶, and Li₃N¹⁷, have shown their effectiveness in providing stabilized and strengthened layers that suppress dendrite growth. More importantly, for fast-charging-oriented LMBs, the high ionic conductivities of these inorganics give the extra advantage of rapid homogenization of the Li⁺ flux¹⁴. However, performance gaps remain for practical applications, and substantial improvement is needed.

An ideal SEI for LMB XFC should be flat, compact and rich in ionically conductive domains. We report a molten salt immersion approach in which the lithium metal anode is immersed in molten lithium bis(fluorosulfonyl)imide (LiFSI) to construct an inorganic-dominant interphase prior to battery assembly. Replacing the redox reactions with LiFSI-containing LEs by those with LiFSI alone produces a dense protective layer composed of LiF, Li₂SO₄, Li₂O, and LiN_xO_y nanocrystal inorganics, which prevents the parasitic reactions that occur when lithium is combined with solvents. This strong ion-conducting layer allows rapid Li⁺ transport which favors uniform Li deposition during long-term and high-rate cycling. This modified Li anode has a stable cycle life of 8,000 h under the extremely harsh conditions of 30 mA cm⁻²@30 mAh cm⁻² and gives high-areal-capacity full cells with a greatly improved rate capability and cycling stability. Our process can also be used in roll-to-roll continuous production as well as in many different electrolytes, indicating its scalable and industrial potential.

A pre-formed interphase with good ionic transport and strength

Electro-chemo-mechanical modelling indicates that the most challenging feature in implementing high-energy and high-rate LMBs is the trade-off between the ionic conductivity and the mechanical strength of the formed SEI film¹⁸. An ideal SEI with sufficient strength is expected to prevent LE degradation of the Li metal surface while allowing rapid Li⁺ transport. Theoretical and experimental studies have shown that inorganic-rich SEIs are more likely to meet these requirements¹⁹. LiFSI-based LEs have been shown to improve lithium deposition homogeneity because they form an inorganic-rich SEI layer as the result of their reduction reactions with lithium^{20, 21}. Rupture of the S-F, S=O, and N-S bonds generates seeds of inorganic compounds that are dispersed throughout the entire SEI, which favor the formation of small, uniform nuclei that regulate Li⁺ diffusion and nucleation¹⁹. We therefore hypothesize that replacing the reduction reactions between Li and the LE with ones with LiFSI alone have a high probability of forming an inorganic-dominated SEI because it would avoid parasitic reactions between the solvents and lithium that generate undesirable organics. Based on the melting point difference of LiFSI (≈128°C) and lithium (≈180°C), we propose the immersion of the Li in molten LiFSI before battery assembly to promote the formation of a LiFSI-derived inorganic SEI. This strategy enables the intense reduction of LiFSI on the lithium metal surface to form a compact inorganic SEI.

The preparation of this pre-formed SEI-coated lithium (AS-Li) is depicted in Fig. 2a. Specifically, a piece of lithium foil is immersed in molten LiFSI at 150°C and then rinsed with diethyl carbonate (DEC) to remove any residues. The layer gives the Li anode good electrolyte wettability (Supplementary Fig. 1) and superb lithiophilic properties. Superior wettability of the LE enables a uniform Li⁺ flux rapidly arrives at the Li metal surface²². Cross-sectional focused ion beam-scanning electron microscope (FIB-SEM) images show that a flat layer ≈2 μm thick covers the Li metal (Fig. 2a, Supplementary Note 1 and Supplementary Figs. 2–5). In comparison, a control bare-Li anode has a rough surface with few small cracks and troughs (Supplementary Figs. 2a, b).

The pre-formed SEI is rich in uniformly-distributed F, N, and S (Supplementary Fig. 6). X-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectrometry (TOF-SIMS) were used to clarify the surface chemistry and quantify its composition. In the XPS spectra (Supplementary Fig. 7), AS-Li shows pronounced peaks for F, S, and N species including LiF, Li_xSO_y, LiN_xO_y, etc. The time-resolved top-down depth profile (Supplementary Fig. 8) and corresponding 3D images constructed from TOF-SIMS measurements (Figs. 2b–e) confirm these observations. The interphase is principally composed of F (represented by Li₂F⁺ and LiF₂⁻ fragments), S (LiSO₄⁻ and SO₂⁻), and N (NSO⁻) species, and other inorganics (Li₃O⁺). These results indicate that this reduction is a simple way of pre-forming an inorganic SEI on lithium metal.

Conductive atomic force microscopy (c-AFM) was used to measure the ability of ions to migrate across the SEI layer. The experimental setup is illustrated in Supplementary Figs. 9–10 and Supplementary Note 2. Sampling 50 scattered areas on the AS-Li, gives an average of 6.2×10⁻³ S cm⁻¹, suggesting the superior Li⁺ diffusivity in the SEI. This excellent ion conduction is predominantly caused by the large number of nano-sized domains and the derived interfaces in the SEI (Supplementary Note 3)^{17, 23-25}. The mechanical properties of bare-Li and AS-Li were also probed by AFM and maps of their Young's modulus distribution are shown in Figs. 2g and h. The mean moduli are 1.7 GPa for bare-Li and 18.6 GPa for AS-Li. The high mechanical modulus of the SEI in AS-Li is attributed to both the uniform interphase morphology and the homogeneous distribution of its inorganic nanocrystals. It is important to have the correct ionic conductivity and mechanical strength of the SEI to enable uniform deposition because both determine its stress distribution and local deposition rate and affect the Li electrodeposition¹⁸. A SEI ionic conductivity above a critical value of 1.5×10⁻³ S cm⁻¹ effectively eliminates stress concentrations and produces smooth Li deposition. A threshold of 4.0 GPa for the Young’s modulus has been suggested for the suppression of SEI breakdown^26-28. The ionic conductivity and Young’s modulus of proposed SEIs are summarized in Fig. 2i and Supplementary Table 1. The AS-Li SEI has a good combination of these two properties, and its strong, ion-conducting layer not only prevents parasitic reactions between a LE and the Li which would produce the overgrowth of the usual SEI, but also allows rapid Li⁺ transport to favor uniform lithium deposition.

Effectiveness in symmetric cells

To demonstrate the benefits of this inorganic-rich SEI on the electrochemical performance of LMBs, symmetric cells were assembled to determine its properties (Fig. 3 and Supplementary Fig. 11). An ether-based LE (1.0 M LiTFSI in 1,3-dioxolane (DOL):dimethoxy ethane (DME) = 1:1 vol.% with 2 wt.% LiNO₃) was used. The AS-Li anode clearly showed reduced plating/stripping overpotentials and a significantly improved cycling lifetime compared to bare-Li under all conditions (5 mA cm⁻²@5 mAh cm⁻², 20 mA cm⁻²@5 mAh cm⁻², 30 mA cm⁻²@3 mAh cm⁻², and 30 mA cm⁻²@30 mAh cm⁻²). The bare-Li overpotential had initial fluctuations for a short time followed by a sudden drop caused by a dendrite-induced short circuit, while at an ultrahigh current density of 30 mA cm⁻², the symmetric AS-Li cell had ultralong stability of over 8,000 h with an areal capacity of 30 mAh cm⁻² (Fig. 3b). The harsh deposition conditions combined with the excellent stability gave an unprecedented super-high cumulative capacity of >240,000 mAh cm⁻². This cell performance surpasses those of current state-of-the-art symmetric cells, particularly in terms of high current density and areal capacity, as summarized in Fig. 3d and Supplementary Table 2. These comparisons based on symmetric cells indicate that the LiFSI modification of the Li metal anode significantly improves both its cycling performance and rate capability.

Our SEI versus the usual SEI

To provide further correlation of the generated SEIs to overall battery performance in a more fundamental manner, and to interpret the exceptional stability in particular, morphology changes and composition variations are valuable. After 30 cycles at 1 mA cm⁻²@3 mAh cm⁻², significant differences are observed by FIB-SEM (Figs. 4a–d) and 3D atomic force microscopy (3D-AFM) (Supplementary Fig. 12). On bare-Li, a large amount of mossy Li and “dead” Li are inlaid in a layer of ≈10 μm (Fig. 4a). In this pattern, the Li anode readily reacts with the electrolyte, both chemically and electrochemically, resulting in the continuous depletion of active Li and the formation of a large volume of usual SEI. The porous and uneven microstructure, partial solubility, and poor strength of the SEI is not able to buffer the volume changes, inhibit crack formation, and restrain dendrite growth¹⁶. Moreover, the continuous reforming and accumulation of SEI on any freshly exposed Li surface slows Li⁺ diffusion across the thick SEI and therefore causes the rate performance of the LMB to deteriorate significantly¹³. In contrast, a flat thin (≈3 μm) SEI layer is observed in the cycled AS-Li (Fig. 4b). The pre-formed layer maintains superb coverage and close contact with the Li, giving a dense structure.

We analysed the components of the SEIs by XPS, TOF-SIMS and cryogenic transmission electron microscopy (cryo-TEM). In the pre-formed layer, abundant LiF typically remains after cycling (Figs. 4e, j, and k), providing good strength to suppress dendrite propagation^26-28. Similarly, lithiophilic species represented by LiN_xO_y and Li_xSO_y (Figs. 4f, g, j, and k) are also preserved, thus favoring excellent deposition homogeneity and reversibility. However, in the usual SEI, organics including C-F, C-C/C-H and C-O (Figs. 4e, h, and i, and Supplementary Fig. 13) are more prominent, indicating that a strong reaction has occurred between the Li and the electrolyte, depleting the active Li. Solvents in the solvation shell and adsorbed salt fragments (CF₃SO₂N⁻) are reduced to organic RCOLi accumulations and constitute the usual SEI²⁹. Repeated cracking and reconstruction increase the SEI thickness and cell resistance³⁰. To obtain further information on this process, we used a cryo-TEM to directly observe the atomic-scale fine structures and obtain local chemical information of the two SEIs (Extended Data Fig. 1, Supplementary Fig. 14 and Supplementary Note 4)³¹. The SEI on AS-Li is mostly inorganic with a mosaic structure whose nano-sized crystals are randomly joined to amorphous regions. This SEI appears to act as a proofing layer that inhibits side reactions between the LE and Li metal, and is therefore a stable interphase (Supplementary Fig. 15).

Long term-cycling and high-rate LMBs balancing energy and power

To evaluate the electrochemical performance of the AS-Li, both half and full cells were assembled. The AS-Li||LiFePO₄ (LFP) half cell showed stable cycling at 10 C for 1,200 cycles (Fig. 5a). In sharp contrast, the specific capacity of the bare-Li||LFP undergoes rapid decay accompanied by an unstable CE for only 250 cycles. For the AS-Li, current rates of 30 C (Supplementary Fig. 16) and 50 C (Fig. 5b), could be cycled up to 10,000 and 9,000 times, respectively, with accompanying high capacity retentions (94.4% and 68.4%) and ultralow fade rates (0.0006% and 0.0035%). This is the lowest capacity loss for LFP cells reported to date. After cycling, unlike the rough surface with cracks and exfoliation of the bare-Li, SEM imaging (Supplementary Fig. 17) confirms a flat surface for AS-Li, indicating uniform lithium plating/stripping and effective suppression of side reactions between AS-Li and the electrolyte. Fig. 5c and Supplementary Table 3 summarize the electrochemical performance of the Li||LFP half cells highlighting cycling stability under high-rate conditions. The data covers an extensive range of methods that have been used to stabilize the SEI, including surface decoration, artificial SEI construction, bulk alloying, and electrolyte engineering. It is clear that AS-Li is the best high-rate Li metal anode, with a robust cycle life of up to 10,000 cycles, superior specific capacity, and an outstanding rate capability of 50 C.

The use of the AS-Li anode not only gave a high specific capacity for LFP at 10 C but also enabled a superior rate capability far beyond 10 C in the half-cell configuration. Fig. 5d provides an illustration of the capacity and voltage characteristics under different rates. The theoretical energy density of LFP is 560 Wh kg⁻¹. At the material level³², LFP paired with AS-Li has an energy density of 365 Wh kg⁻¹ at 10 C, which is much higher than other reported values^{33, 34}. With an unprecedented superhigh rate of 50 C, an energy density of 208 Wh kg⁻¹ is retained. This represents a significant advantage when attempting to realize a high energy density of cells in power-intensive scenarios, thus overcoming the general trade-off between energy and power⁸. By pairing with AS-Li, LFP delivers a greatly increased power density from 486 W kg⁻¹ at 1 C, 3,650 W kg⁻¹ at 10 C, and up to 10,416 W kg⁻¹ at 50 C (increased by 21.4-fold), accompanied by relatively small drops in energy density from 486 and 365 to 208 Wh kg⁻¹ (reduced by 2.3-fold) (Fig. 5e and Supplementary Table 4). In contrast, when LFP is paired with bare-Li, a prominent reduction in specific capacity at high rates leads to substantial drops in both indices. The improvement from bare-Li to AS-Li boosts battery power while maintaining an excellent energy density (Fig. 5f).

The high-rate performance of AS-Li in half cells indicates the great potential of Li metal to balance energy and power. To evaluate this further, full cells paired with LFP and LiCoO₂ (LCO) cathodes with high areal capacities of ≈1.5 and ≈3.0 mAh cm⁻² were investigated. An AS-Li||LCO full cell with an areal capacity of 1.5 mAh cm⁻² and a low negative to positive (N/P) ratio of 2.5 (using a 20 μm Li foil of ≈4.0 mAh cm⁻²) showed good cycling stability for 150 cycles with negligible fade at both the 1 C and 3 C rates (Fig. 5g). A higher N/P ratio of 6 obtained by switching to a 50 μm Li foil of ≈10.0 mAh cm⁻² enabled more aggressive high−rate cycling at 3 C and 6 C for 500 cycles (Fig. 5h). We also assembled an AS-Li||LFP full cell with a high mass loading of 24.0 mg cm⁻² in the cathode and a 50 μm lithium foil in the anode. This cell with an N/P ratio of 3 could be operated for 200 cycles without an obvious capacity loss (97.5% retention) (Supplementary Fig. 18).

We have summarized the performance of reported high-loading (>10 mg cm⁻²) LMB full cells under high-rate conditions (≥1 C) since 2021 in Fig. 5i and Supplementary Table 5. This comparison shows that the performance of a battery based on AS-Li is the best in long-term cycling under service conditions including high loadings and high rates. The fast charging ability of a battery must be evaluated simultaneously using the three metrics of charging time, energy density, and number of cycles achieved under fast-charging conditions⁵. Our battery has a good balance (Fig. 5j), illustrating the great promise of AS-Li as a fast-charging anode material for practical applications.

Roll-to-roll demonstration and the use of carbonate LE

A knowledge gap exists between laboratory-level material research and practical industry manufacturing³⁷. For the latter, a roll-to-roll system for the continuous surface modification of lithium metal was developed (Fig. 6a). The single-sided or double-sided lithium foil on copper passes through a tank of molten LiFSI and then enters an outlet gas stream to blow off LiFSI residue. Fig. 6b and Supplementary Video 1 show the working conditions of the system, which demonstrates its scalable potential by continuous production as well as the good adaptability for the industry by matching the roll-to-roll battery manufacturing process³⁸. Furthermore, pouch cells based on the roll-to-roll produced lithium foil were assembled, and these have good cycling stability at both 1 C (Figs. 6c, d) and 2 C (Supplementary Fig. 19), confirming the effectiveness of the process.

Moreover, by virtue of the pre-formed SEI and reduced contact probability of Li metal with LEs, the AS-Li anode can also be used with the common carbonate LE (1.0 M LiPF₆ in ethylene carbonate (EC):DEC = 1:1 vol.%) instead of ether LE. A dramatic performance improvement has been demonstrated (Extended Data Fig. 2, Supplementary Note 5 and Supplementary Figs. 20−23). The pre-formed SEI modification has therefore proven its effectiveness in a carbonate LE, as in an ether LE. However, it must be noted that because of the significance of LE recipes for the Li metal anode (Extended Data Fig. 3), it is believed that the use of AS-Li with the latest electrolytes (Supplementary Note 6), would contribute to provision of a much more competitive electrochemical performance.

A simple and rapid continuous surface treatment was developed as an effective Li metal anode modification, and a compact but ion-conducting SEI layer was pre-formed. This multi-functional SEI improves electrolyte wettability, suppresses parasitic reactions, regulates Li⁺ deposition uniformity, and increases the surface modulus to jointly ensure dendrite-free deposition behavior. The interfaces between its various inorganic nanocrystals ensure significantly improved interfacial Li⁺ transport under high-rate conditions. Consequently, symmetric cells have shown exceptional lifespans of up to 8,000 h with an ultrahigh current density of 30 mA cm^− 2 and a record-breaking cumulative areal capacity of 240,000 mAh cm^− 2. High-load AS-Li||LCO full cells have a striking cycling performance at XFC conditions of 6 C for 500 cycles. This surface modification also dramatically reduces the need for high-cost ether electrolytes for LMBs, and its excellent performance is also demonstrated using a carbonate electrolyte. At a scientific level, we believe that our results not only offer a new way to pre-form a SEI for a Li metal anode with unmatched deposition reversibility, but also provide a reference for the design of strong but high-conductivity interphases that should be applicable to other metal anodes or even solid-state electrolytes for different applications. At a practical device level, we have demonstrated the high power density and long cycling life of high-energy LMBs.

Materials

Lithium bis(fluorosulfonyl)imide (LiFSI, 99.9%) was purchased from Suzhou Duoduo Chemical Technology Co., Ltd. Diethyl carbonate (DEC) and N-methyl-2-pyrrolidone (NMP) were obtained from Aladdin Co., Ltd. (Shanghai, China). Lithium iron phosphate (LFP) powder, conductive carbon (Timcal Super C45), and polyvinylidene fluoride (PVDF) binder were obtained from Kejing Co., Ltd. (Shenzhen, China). High-loaded LFP electrodes with ≈ 1.5 and ≈ 3.0 mAh cm^− 2 capacities and a lithium cobalt oxide (LCO) electrode with ≈ 1.5 mAh cm^− 2 capacity and Li foils with thicknesses of 20, 50, and 500 µm were purchased from Guangdong Canrd New Energy Technology Co., Ltd. Electrolytes, including ether-based (1.0 M LiTFSI in DOL:DME = 1:1 vol.% with 2 wt.% LiNO₃) and carbonate-based (1.0 M LiPF₆ in EC:DEC = 1:1 vol.%) were obtained from Suzhou Duoduo Chemical Technology Co., Ltd.

Preparation of AS-Li

The LiFSI powder was fused at 150°C and a piece of lithium foil was then immersed in the molten LiFSI for 15 min. The treated Li foil was then extracted and washed with DEC to remove any residues. The prepared AS-Li was used to perform the subsequent characterizations and electrochemical measurements. All preparation steps were performed in a glove box (H₂O ≤ 0.01 ppm, O₂ ≤ 0.01 ppm).

Physical characterizations

A scanning electron microscope (SEM; Hitachi S-4800) was used to image the surface and cross-sectional morphologies of both bare-Li and AS-Li. Energy-dispersive X-ray spectroscopy (EDS; Zeiss Supra® 55) was used to measure the contents and spatial distributions of the elements involved. X-ray photoelectron spectroscopy (XPS; Escalab 250Xi) was used to analyze the surface composition. Atomic force microscopy (AFM; Benyuan CSPM5500 system, China) was performed to measure the topography and the force curves for bare-Li and AS-Li. The AFM current-voltage (I-V) curves were obtained using an in-house-developed c-AFM setup using the Benyuan system, an electrochemical measurement unit (Keithley 2400 sourcemeter), and a 25PT300B Pt probe with a tip radius of 20 nm (Rocky Mountain Nanotechnology, USA). The entire setup was placed in a glove box (O₂ and H₂O < 1 ppm). Time-of-flight secondary ion mass spectrometry (TOF-SIMS) measurements were performed using a PHI nanoTOF II. Specifically, a Bi₃⁺⁺ beam (30 keV, 2 nA, 100×100 µm²) was used as the primary beam to detect the signals and an Ar⁺ beam (3 kV, 100 nA, 400×400 µm²) was used for the depth profiling analysis. An aberration-corrected FEI Krios G3i microscope was used to image the samples. Cells were disassembled and frozen instantly in an Ar-filled glove box before being transferred to a cryogenic transmission electron microscope (cryo-TEM). An automatic liquid nitrogen perfusion system was used to maintain the low temperature environment in the sample chamber and the lens barrel for days to ensure high imaging stability. FIB-SEM images were captured using a Thermo Scientific Helios 5 CX operating at 30 kV.

Electrochemical measurements

Symmetric lithium cells were used to perform the galvanostatic long cycling investigations. The measurements were performed at current densities of 1 to 30 mA cm^− 2 with areal capacities of 1 to 30 mAh cm^− 2. Before cycling, the cells were activated at a current density of 0.2 mA cm^− 2 for five cycles. Li||LFP and Li||LCO cells were assembled to study the cell performance of AS-Li. To prepare the cathode, LFP, conductive carbon, and PVDF were dispersed with 8:1:1 weight ratios in NMP. The mixture was stirred using a magnetic stirring bar at 800 rpm for 4 h to form a slurry that was then cast onto a carbon-coated aluminum foil. Subsequently, the electrodes were dried at 60°C in a vacuum oven for 12 h and cut into disks (diameter: 12 mm). The mass loading of the LFP was ≈ 2.0 mg cm^− 2. To provide a further demonstration of the advantages of the AS-Li anode in practical cells, high-loaded LFP electrodes of 11.9 and 24.0 mg cm^− 2 capacities (referring to their areal capacities of ≈ 1.5 and ≈ 3.0 mAh cm^− 2, respectively) and LCO electrodes with a capacity of ≈ 1.5 mAh cm^− 2 were assembled into CR2032-type coin cells. Thinner lithium foils with thicknesses of 20 and 50 µm were applied to assemble full cells with low N/P ratios. Charging and discharging of the cells were performed using a Land CT2001A battery testing system. The cut-off voltage ranges were set at 2.5–3.8 V for LFP and 2.5–4.2 V for LCO, and the cells were activated for five cycles at 0.2 C.

Roll-to-roll fabrication system

The roll-to-roll demonstration system was driven by two alternating current permanent magnet synchronous motors. Lithium foil on copper was passed through molten LiFSI in a U-tube to initiate the surface modification. Heater plates were placed on both the upper and lower surface of the U-tube. In the outlet, a gas stream was used to blow off the LiFSI residue.

Competing interests

The authors declare no competing interests.

Author contributions

W.W. and C.Y. directed the research. W.W. and C.S. conceived the idea and wrote the manuscript. C.S. and F.N. conducted material characterizations and electrochemical measurements. Q.W. helped with the cryo-TEM characterizations. L.W. and W.L. conducted the AFM characterizations. D.N., J.Z., M.C. and M.W. contributed to data discussion. Y.D., H.C. and J.W. gave advice to material characterizations and experimental design. All authors commented on the manuscript.

Acknowledgements

All authors thank Prof. Peter Thrower, the emeritus editor-in-chief of Carbon for editing the English text of this manuscript and the supplementary information. This work was supported by the National Natural Science Foundation of China (Grant Nos. 52173243, 52002393 and 52202331), Basic and Applied Basic Research Foundation of Guangdong Province (Grant No. 2022A1515110313) and Shenzhen Science and Technology Program (Grant No. JCYJ20230807140818040).

Data availability

The datasets analysed and generated during the current study are included in the paper and its Supplementary Information file. Source data are provided with this paper.

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Extreme fast charging of lithium metal batteries enabled by a molten-salt-derived nanocrystal interphase

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Preparation of AS-Li

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Roll-to-roll fabrication system

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