Co/Li₂S of chemical confinement decorated electrochemically stable Co(OH)F nanoarrays enabling dendrite-free flexible 3D Li anodes

Abstract

Lithium (Li) metal is regarded as a promising anode for high-energy-density Li-ion batteries because of its high theoretical specific capacity and low potential. Unfortunately, the safety issues derived from Li dendrite formation have hindered their commercialization. Thus, fabricating a flexible, three-dimensional (3D) composite Li anode is an effective strategy to suppress Li dendrite growth. Lithiophilic nanoarrays are commonly constructed on a 3D skeleton to increase its surface area and lithiophilicity. However, the lithiophilic nanoarrays are unstable due to the conversion reaction during Li plating/stripping. Therefore, designing composite nanoarrays with an electrochemically stable inner core and a lithiophilic outer shell is conducive to providing a large surface area and promoting uniform Li deposition. Herein, CoS₂ shell–Co(OH)F core composite nanowire arrays were prepared on carbon cloth (CoS₂–Co(OH)F/CC) to fabricate a flexible 3D composite Li anode (Li@CoS₂–Co(OH)F/CC), in which the electrochemically stable Co(OH)F core ensures the stability of the nanoarray and the lithiophilic CoS₂ shell facilitates uniform Li adsorption. Moreover, the Co/Li₂S configuration derived from the lithiation of CoS₂ could promote homogeneous Li deposition, in which the lithiophilic Li₂S phase and electron-conductive Co phase jointly facilitate uniform Li nucleation and charge transfer. Consequently, the symmetric cells and full cells paired with a LiFePO₄ (LFP) cathode based on the flexible Li@CoS₂–Co(OH)F/CC anode presented superior cycling stability.

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1. Introduction

Lithium-ion (Li-ion) batteries have been widely utilized in energy storage systems. However, the commercial Li-ion batteries' energy density is approximately 250 Wh kg⁻¹, which poses a challenge to meet the emerging power battery system's demands.^1,2 The commercially available Li-ion batteries employ a graphite anode with a specific capacity of only 372 mA h g⁻¹, whereas the Li metal anode exhibits a significantly higher specific capacity of 3860 mA h g⁻¹ at a low potential (−3.04 V vs. the standard hydrogen electrode).^3,4 Therefore, the utilization of Li metal anodes exhibits promising potential for enhancing the energy density of Li-ion batteries. Nevertheless, safety concerns arising from Li dendrite formation have seriously hindered the progress of Li-metal batteries.^5–7

In addressing such intractable issues, many strategies have been developed, including electrolyte modification,^8,9 artificial solid–electrolyte interface construction,^10–12 separator modification^13–15 and 3D electrode fabrication.^16–18 According to the theory of “Sand's time,” the charge accumulation on some spots causes the local current density to exceed the limiting current density, which induces uncontrollable Li dendrite growth.^19,20 Among the strategies, fabricating a 3D Li metal anode can provide a large space to lower local current density and accommodate Li deposition, thereby decreasing the probability of Li dendrite formation.^21–23 Therefore, this strategy has great application potential, whereas the commonly used 3D skeletons are mostly lithiophobic, such as Ni foam,²⁴ Cu foam,²⁵ carbon nanotubes,²⁶ and carbon cloth (CC).^17,27 Introducing lithiophilic nanoarrays, including nanospheres, nanowires, and nanorods, onto the surface of a 3D skeleton is often performed to provide a larger surface area and physical space as well as to improve their lithiophilicity.^28–30 However, most lithiophilic nanoarrays consist of metallic compounds, which undergo a conversion reaction with Li during initial deposition and subsequently collapse in structure. As a result, they are unable to effectively regulate the subsequent Li plating/stripping process.^31,32 Therefore, constructing composite nanoarrays with an electrochemically stable inner core and a lithiophilic outer shell on a 3D skeleton is conducive to providing a stable structure and promoting uniform Li deposition.³³

Herein, composite shell–core CoS₂–Co(OH)F nanowire arrays were constructed on CC (denoted as CoS₂–Co(OH)F/CC) to fabricate a flexible 3D composite Li anode (denoted as Li@CoS₂–Co(OH)F/CC). The Co(OH)F core is electrochemically stable, which can maintain the structure of the nanoarray and the large space to decrease current density and accommodate Li deposition. The lithiophilic CoS₂ shell can facilitate Li adsorption, and it will be transferred into the Co/Li₂S configuration after lithiation, in which the lithiophilic Li₂S phase could regulate the oriented Li deposition and the electron-conductive Co phase could ensure efficient charge transfer, thereby synergistically promoting homogeneous Li nucleation and deposition. Consequently, the symmetric cells based on the flexible Li@CoS₂–Co(OH)F/CC anode achieved superior cycling stability with a low overpotential (13 mV) over 2500 h cycles. When assembled with a LiFePO₄ (LFP) cathode, the LFP//Li@CoS₂–Co(OH)F/CC full cell delivered outstanding cycling stability (1500 cycles at 1C).

2. Results and discussion

The CoS₂–Co(OH)F/CC sample was prepared by a hydrothermal method and vulcanization (Fig. 1a). The XRD results showed that the CoS₂–Co(OH)F/CC sample is composed of CoS₂ (PDF #89-1492) and Co(OH)F (PDF #50-0827, Fig. 1b). Co and S are detected in the XPS spectra, whereas F is absent (Fig. 1c). Furthermore, the Co 2p spectrum can be fitted into two spin orbit doublets (Co²⁺ and Co³⁺) and two satellite peaks (Fig. 1d). Two peaks of 163.1 eV and 164.3 eV are shown in the S 2p spectrograph (Fig. 1e), matching S 2p_3/2 and S 2p_1/2, respectively.^34–36 The Co 2p and S 2p core-level spectra confirm the presence of CoS₂. Considering the detection depth of XPS analysis, it indicated that the CoS₂ phase is on the outer surface, and Co(OH)F is in the inner core. The abovementioned results confirm the successful preparation of the shell–core CoS₂–Co(OH)F composite on CC.

Fig. 1 (a) Schematic illustration showing the preparation of the CoS₂–Co(OH)F composite on the surface of CC. (b) XRD pattern and XPS spectra of the CoS₂–Co(OH)F: (c) full, (d) Co 2p and (e) S 2p spectra.

The SEM images of the CoS₂–Co(OH)F/CC sample showed that its surface is covered with flower-like clusters that are composed of uniform needle-like nanorods (Fig. 2a–c). For comparison, the inadequate concentration of 1.0 mmol Co(NO₃)₂·6H₂O leads to uneven growth of nanowires (Fig. S1a†), while excessive concentration of 2.4 mmol Co(NO₃)₂·6H₂O results in agglomeration and accumulation (Fig. S1b†). An appropriate concentration of 1.7 mmol Co(NO₃)₂·6H₂O leads to uniform and dense nanowire clusters on the surface of the CC with each carbon nanofiber encapsulated by the Co(OH)F clusters. In addition, the SEM images of the pure CC and Co(OH)F precursor clearly illustrate the intricate CC structure formed by interweaved carbon fibers and the nanowire cluster structure of the Co(OH)F precursor indicates that the vulcanization reaction does not alter the overall 3D framework (Fig. S2†). Moreover, the impact of the vulcanization degree on the morphology of the samples was analyzed by regulating the mass ratios of sublimed sulfur to the precursor at 1 : 1, 6 : 1, and 30 : 1. The XRD patterns demonstrate a gradual decrease in the peak intensity of the Co(OH)F core and an increase in the presence of the CoS₂ shell as the vulcanization degree increases (Fig. S3†), indicating a progressive variation in the Co(OH)F/CoS₂ core–shell structure. Furthermore, CoS₂–Co(OH)F/CC-6:1 displays a dense layer of CoS₂ nanoparticles, indicating the proper vulcanization of the sample. In contrast, CoS₂–Co(OH)F/CC (1 : 1) and CoS₂/CC (30 : 1) exhibit insufficient and excessive vulcanization, respectively, which demonstrates the exceptional characteristics of the CoS₂–CO(OH)F/CC-6 : 1 structure (Fig. S4†). The microstructure of the nanorods was further characterized by TEM and selected area electron diffraction (SAED) (Fig. 2d–h), which further confirms the presence of CoS₂ on the outer surface of the composite nanorod.^34,37 The elemental mapping of Co and S atoms indicated that the CoS₂ layer is distributed homogeneously on the surface (Fig. 2d). TEM characterization reveals obvious CoS₂ shell and Co(OH)F core structures, as shown in Fig. 2e and f. The lattice fringe in zone I (Fig. 2g) corresponds to the (211) plane of CoS₂, while that in zone II corresponds to the (410) plane of Co(OH)F, which further confirms the presence of a shell–core structure of CoS₂–Co(OH)F. The width of a single nanorod is approximately 157 nm and sufficient space exists between the nanorods, which is conducive to providing a large surface area and promoting Li deposition.

Fig. 2 (a–c) SEM images of CoS₂–Co(OH)F/CC nanowire clusters at different magnifications. (d) Elemental mapping, (e–g) TEM images, and (h) SAED pattern of the CoS₂–Co(OH)F nanorod.

To reveal the effect of the CoS₂–Co(OH)F nanorod on the surface lithiophilicity of CC, Li adsorption energy (E_a) was calculated by using the density functional theory (DFT). E_a values on the different sites of C are −1.01, −2.12, and −1.02 eV, whereas E_a values on the different sites of CoS₂ are −2.86, −2.68, and −2.68 eV, indicating a stronger interaction between CoS₂ and Li than that between C and Li (Fig. 3a–c and S5†). Therefore, the surface lithiophilicity of CC is remarkably improved after being modified by the CoS₂–Co(OH)F nanorod, which could facilitate initial Li adsorption or nucleation.

Fig. 3 Li adsorption geometry on the (a) C layer and (b) CoS₂ (200) plane, and (c) the corresponding E_a values. XPS analysis of CoS₂–Co(OH)F/CC after discharging to 0.01 V: (d) S 2p, (e) Li 1s and (f) Co 2p spectra. The Li adsorption geometry on (g) Li₂S (111) and (h) Co (101) planes, and (i) the corresponding E_a values. (j) Charge density differences of Li on the surfaces of Li₂S (111) and Co (101). The blue and yellow spheres represent electron accumulation and depletion, respectively.

During the initial lithiation process, the discharge curve of the CoS₂–Co(OH)F/CC//Li half-cell presented an evident voltage plateau at approximately 1.5 V (Fig. S6†), which indicates the conversion reaction of CoS₂ into Li₂S and Co.³⁸ Moreover, the XPS analysis of the CoS₂–Co(OH)F/CC sample after the conversion reaction shows a peak at 162.2 eV in the XPS S 2p spectrum, a peak at 54.6 eV in the Li 1s spectrum, and a peak at 777.8 eV in the Co 2p spectrum, which indicates the presence of Li₂S and Co on the outer surface of the nanorod (Fig. 3d–f).^39,40 Furthermore, E_a values on Co and Li₂S are −2.69 and −4.28 eV, respectively, which indicates a stronger interaction between Li₂S and Li than that between Co and Li (Fig. 3g–i and S7†).⁴¹ The charge density difference (Fig. 3j) between Co and Li₂S also showed that the charge distribution within Li₂S compared with that of Co is more favorable for Li adsorption. Therefore, Li₂S within the Co/Li₂S configuration could regulate the preferential Li deposition on the Li₂S and Co phases to promote a uniform Li deposition. The electron-conductive Co network is conducive to charge transfer.

To reveal the effect of Co/Li₂S configuration–derived chemical confinement on regulating Li nucleation and deposition, the SEM images of CoS₂–Co(OH)F/CC and pure CC after electrochemical deposition of different capacities of Li were observed. First, after discharging to 1 and 0.01 V, the CoS₂–Co(OH)F/CC sample maintains a flower-like structure composed of nanorods (Fig. S8†), which can be attributed to the electrochemically stable Co(OH)F core within the CoS₂–Co(OH)F shell–core structure. Additionally, the peak density in the XRD pattern assigned to the Co(OH)F core in the CoS₂/CC-30 : 1 sample diminished with increasing degree of vulcanization, showing vulnerable structure stability after lithiation (Fig. S9†) and further validating the stable shell–core structure of CoS₂–Co(OH)F/CC (6 : 1). After depositing Li with a capacity of 2 mA h cm⁻², the CoS₂–Co(OH)F/CC sample presented a flower-like structure with Li deposits surrounding the nanowire clusters and filling in the space between nanorods (Fig. S10†). With the increase in the capacity of deposited Li to 5, 8, and 11 mA h cm⁻², the Li metal gradually fills in the gaps between the flower-like structure and covers the surface homogenously (Fig. 4a). Conversely, disordered mossy Li was formed on the surface of CC because of its lithiophobic properties, leading to non-uniform Li deposition (Fig. 4b). Hence, the Co/Li₂S-modified Co(OH)F nanoarrays could not only accommodate Li and but also facilitate rapid charge transfer, thus promoting the Li plating process. The structure stability of the CoS₂–Co(OH)F/CC framework during Li plating/stripping is further explored. The SEM images of the CoS₂–Co(OH)F/CC framework after plating 5, 8 and 11 mA h cm⁻² Li and stripping 5, 8 and 11 mA h cm⁻² Li showed that the decorated-Co(OH)F nanoarray ensures reversible Li plating/stripping and maintains stable structure integrity (Fig. 4c). In contrast, mossy Li deposits still exist on the CC surface because of the formation of “dead Li” (Fig. 4d). After 50 cycles of Li plating/stripping (5 mA h cm⁻²), the CoS₂–Co(OH)F/CC sample maintains a well-preserved flower-like structure, which greatly contributes to prolonged cycling stability (Fig. S11†). This finding indicated that the Co/Li₂S configuration-decorated electrochemically stable Co(OH)F nanoarray could effectively regulate the homogeneous Li deposition (Fig. 4e), in which the Co(OH)F nanoarray provides a “physical space” to accommodate Li and the Co/Li₂S-derived “chemical confinement” promotes Li adsorption and uniform Li distribution.^27,42

Fig. 4 SEM images of the (a) CoS₂–Co(OH)F/CC framework and (b) CC after plating different amounts of Li. SEM images of the (c) CoS₂–Co(OH)F/CC framework and (d) CC after Li stripping. (e) Schematic illustration showing the mechanism of the 3D CoS₂–Co(OH)F/CC skeleton in suppressing Li dendrite growth.

To assess the electrochemical characteristics of the CoS₂–Co(OH)F/CC sample, coulombic efficiency (CE) and nucleation overpotential based on the half-cells were tested. As shown in Fig. 5a, at 0.5 mA cm⁻² current density and 1 mA h cm⁻² capacity, the CoS₂–Co(OH)F/CC//Li half-cell exhibits a consistent operational stability for 250 cycles with an average CE exceeding 99%. When increasing the current density and capacity to 1 mA cm⁻² and 1 mA h cm⁻², 3 mA cm⁻² and 1 mA h cm⁻², and 3 mA cm⁻² and 3 mA h cm⁻², the CEs of half-cells remain over 99% (Fig. 5b and S12†). By contrast, the CEs of the CC//Li half-cell are markedly lower than that of CoS₂–Co(OH)F/CC, with inferior cycling stability. This impressive achievement can be credited to the Co/Li₂S-derived “chemical confinement” in facilitating Li plating and stripping as well as the stable Co(OH)F nanoarrays in ensuring structural stability. In addition, the overpotentials of the CoS₂–Co(OH)F/CC and CC samples at different current densities were investigated (Fig. 5c and S13†). The overpotentials of CoS₂–Co(OH)F/CC are lower than that of CC at all tested current densities (Fig. 5d), which can be attributed to the effect of the lithiophilic and electron-conductive Co/Li₂S configuration in promoting Li nucleation.

Fig. 5 (a) CE of CC//Li and CoS₂–Co(OH)F/CC//Li half cells at a current density of 0.5 mA cm⁻² with a capacity of 1 mA h cm⁻². (b) The CE values of the half cells at different current densities and capacities. (c) Voltage profiles for Li nucleation on the CC and CoS₂–Co(OH)F/CC electrodes at 3 mA cm⁻². (d) Li nucleation overpotentials on CC and CoS₂–Co(OH)F/CC electrodes at different current densities. Cycling performance of Li foil, Li@CC and Li@CoS₂–Co(OH)F/CC-based symmetric cells at (e) 1 mA cm⁻²/1 mA h cm⁻², (f) 3 mA cm⁻²/1 mA h cm⁻² and (g) 3 mA cm⁻²/3 mA h cm⁻². (h) Rate capability of Li foil, Li@CC and Li@CoS₂–Co(OH)F symmetric cells.

The assembled symmetric cells were utilized to explore the cycling stability and interface stability of the electrodes. The composite Li metal anode is obtained through the electrodeposition method. In contrast to the Li infusion method,⁴³ the electrodeposition method allows for precise control over the amount of deposited Li and ensures a uniform morphology, as shown in Fig. S14.† As shown in Fig. 5e, the Li@CoS₂–Co(OH)F/CC-based symmetric cells can operate stably for 2500 h with an overpotential of approximately 13 mV (1 mA cm⁻², 1 mA h cm⁻²). In contrast, Li foil symmetric cells presented a large overpotential (25 mV) and poor cycle performance (500 h) under the same conditions. The Li@CC symmetric cells show even worse performance with overpotential increasing from 78.9 to 128 mV, indicating highly unstable Li plating/stripping behavior. During the initial cycle, the symmetric cells based on Li@CoS₂–Co(OH)F/CC exhibited a higher overpotential compared to symmetric cells based on Li foil, which illustrates the longer formation period of the SEI film on the surface of Li@CoS₂–Co(OH)F/CC than on that of Li. This is because pure Li metal is more active and prone to react with electrolyte and form the SEI film,⁶ while the Co/Li₂S derived composite Li metal is more stable. Furthermore, the Li@CoS₂–Co(OH)F/CC symmetric cells still exhibited a prolonged cycle life of 1200 h and maintained an overpotential of 16 mV at 3 mA cm⁻²/1 mA h cm⁻² (Fig. 5f). However, the Li@CC and Li foil symmetric cells showed voltage fluctuations after 105 and 166 h, respectively. Moreover, the Li@CoS₂–Co(OH)F/CC-based cell still exhibited satisfactory cycling stability with a small overpotential for 500 h when the capacity further increases to 3 mA cm⁻²/3 mA h cm⁻² (Fig. 5g). The rate capability of Li foil, Li/CC, and Li@CoS₂–Co(OH)F/CC-based symmetric cells was also investigated (Fig. 5h). The Li@CoS₂–Co(OH)F/CC-based symmetric cell presented exceptional reversibility with a lower overpotential compared with the other two symmetric cells.

To reveal the cause of the remarkable cycling performance and rate capability of the Li@CoS₂–Co(OH)F/CC-based cells, the Nyquist plot of the EIS spectra of Li foil, Li@CC, and Li@CoS₂–Co(OH)F/CC-based symmetric cells at the 5th and 100th cycles was captured (Fig. 6a and b). At the 5th cycle, the charge transfer resistance (R_ct) of Li foil, Li@CC, and Li@CoS₂–Co(OH)F/CC symmetric cells is 44.78 Ω, 31.4 Ω, and 26.1 Ω, respectively. At the 100th cycle, the R_ct of Li foil and Li@CoS₂–Co(OH)F/CC symmetric cells is 5.5 Ω and 1.1 Ω, respectively (the Li@CC symmetric cells failed before the 100th cycle). The smaller R_ct of the Li@CoS₂–Co(OH)F/CC-based cell indicated faster Li⁺ transport and a more stable electrolyte/electrode interface. To reveal the relationship between microstructural stability and long-term cycling stability, the surface morphology of Li@CoS₂–Co(OH)F/CC and Li foil-based symmetric cells after the 20th, 50th, and 100th cycles was detected (Fig. 6c–h). The Li@CoS₂–Co(OH)F/CC electrode maintains a homogeneous and integral surface covered by Li metal (Fig. 6c–e). However, “dead Li” appeared on the surface of the Li foil even after 20 cycles, and severe cracks occurred after 100 cycles (Fig. 6f–h). Therefore, the Li@CoS₂–Co(OH)F/CC anode could maintain remarkable structure stability and effectively suppress Li dendrite formation during repeated Li plating/stripping.

Fig. 6 (a and b) Nyquist plot of Li foil, Li@CC and Li@CoS₂–Co(OH)F/CC-based symmetric cells at different cycles. SEM images of (c–e) the Li@CoS₂–Co(OH)F/CC and (f–h) Li foil electrode after different cycles.

To confirm the potential application of the Li@CoS₂–Co(OH)F/CC anode in full cells, the full cells with LFP as the cathode and Li foil and Li@CoS₂–Co(OH)F/CC as the anode were assembled (denoted as LFP//Li and LFP//Li@CoS₂–Co(OH)F/CC full cells). The discharge capacities of LFP//Li@CoS₂–Co(OH)F/CC full cells are 143.8, 135.5, 126.8, 117.4, 110.5 and 101.5 mA h g⁻¹ at 0.2, 0.5, 1, 2, 3, and 5C, respectively (Fig. 7a), which are significantly higher than those of LFP//Li full cells. The charge–discharge profiles of LFP//Li@CoS₂–Co(OH)F/CC and LFP//Li full cells at different rates are shown in Fig. 7b and c. The LFP//Li@CoS₂–Co(OH)F/CC cell presented a more stable charge–discharge platform and lower voltage polarization than those of the LFP//Li cell. In addition, the cycling performance of the full cells was tested at 1C (Fig. 7d). The LFP//Li@CoS₂–Co(OH)F/CC cell demonstrated excellent cycling stability for 1500 cycles at 1C with a discharge capacity of 119.3 mA h g⁻¹ compared with the LFP//Li full cell. Meanwhile, the Li@CoS₂–Co(OH)F/CC cell exhibited a more stable charge–discharge platform and lower voltage polarization than the LFP//Li full cell at different cycles (Fig. 7e). However, the discharge capacity of the LFP//Li full cell gradually decreased from the initial cycle because of the presence of Li dendrites. Moreover, the surface morphology of the anode in LFP//Li@CoS₂–Co(OH)F/CC and LFP//Li full cells was observed by SEM. As shown in Fig. 7g and S15a, b,† the Li@CoS₂–Co(OH)F/CC electrode still maintains a flat and uniform Li coating without any unwanted separation or smashing after 200 and 300 cycles. By contrast, unfavorable phenomena such as volume cracks and Li dendrites are observed in the Li foil of LFP//Li full cells (Fig. 7h and S15c, d†). The good cycling stability and rate performance of Li@CoS₂–Co(OH)F/CC-based symmetric cells and full cells are attributed to the Li₂S/Co configuration-decorated Co(OH)F nanoarray in stabilizing Li plating and stripping.

Fig. 7 (a) Rate capability and (b and c) charge–discharge profiles of LFP//Li@Li@CoS₂–Co(OH)F/CC and LFP//Li full cells at various rates ranging from 0.2C to 5C. (d) Cycle performance and (e and f) charge–discharge profiles of LFP//Li@CoS₂–Co(OH)F/CC and LFP//Li full cells at 1C. The SEM images of the corresponding anode for (g) LFP//Li@CoS₂–Co(OH)F/CC and (h) LFP//Li full cells after 200 cycles at 1C.

3. Conclusion

A highly stable lithiophilic and electron-conductive CoS₂–Co(OH)F shell–core nanoarray was successfully prepared on the CC skeleton to fabricate a flexible 3D Li metal anode. The electrochemically stable Co(OH)F core within the nanoarrays ensures structural stability during repeated Li plating/stripping, thereby accommodating Li deposition via the large area space between the nanoarrays. Meanwhile, the Co/Li₂S configuration formed on the outer surface of the nanoarrays could not only regulate the preferential Li plating on lithiophilic Li₂S sites and then Co sites but also ensure the rapid charge transfer. This derived chemical confinement synergistically promotes further homogeneous Li plating. Based on these advantages, the as-prepared Li@CoS₂–Co(OH)F/CC-based symmetric cells presented exceptional cycling stability with a prolonged lifespan of 2500 h and an overpotential of only 13 mV. Furthermore, the LFP//Li@CoS₂–Co(OH)F/CC full cells exhibited a stable cycling stability for 1500 cycles at 1C and superior rate capability. Therefore, the flexible 3D Li@CoS₂–Co(OH)F/CC anode could stabilize the Li plating/stripping via the functional Co/Li₂S configuration-modified Co(OH)F nanoarrays.

4. Experimental section

4.1 Experimental materials

Cobalt nitrate hexahydrate (Co(NO₃)₂·6H₂O), urea (CH₄N₂O) and ammonium fluoride (NH₄F) were acquired from Sinopharm Chemical Reagent Co., Ltd. Sublimed sulfur (S) purchased from Tianjin Yongsheng Fine Chemical Co., Ltd. The mentioned reagents are of analytical quality and can be used without further purification. CC with a thickness of 360 μm was purchased from CeTech Co., Ltd.

4.2 Preparation of the flexible Li@CoS₂–Co(OH)F/CC anode

4.2.1 Surface treatment of the CC substrate. To enhance the hydrophilicity of CC, it underwent an initial ultrasonic cleaning process using deionized water and anhydrous ethanol for a duration of 30 minutes to eliminate any surface impurities. Subsequently, it was fully immersed in a solution consisting of diluted nitric acid and sulfuric acid at room temperature for a period of 24 hours. Following this, multiple rinses were performed with deionized water and anhydrous ethanol before being subjected to drying at a temperature of 80 °C for 12 h.

4.2.2 Preparation of the Co(OH)F nanoarrays on CC. The hydrothermal method was used to synthesize the Co(OH)F array on CC. The procedure involved dissolving 1.7 mmol of Co(NO₃)₂·6H₂O, 20 mmol of NH₄F, and 20 mmol of CH₄N₂O in 40 mL of deionized water. The resulting mixture was stirred at room temperature for 2 h until a homogeneous pink solution was obtained. Then a pre-treated CC (3 cm × 3 cm) was added to the pink solution and transferred into a 50 mL polytetrafluoroethylene autoclave. The autoclave was placed in a 120 °C blast oven for 5 h. At the end of the reaction, the CC sample was repeatedly rinsed with deionized water and anhydrous ethanol. After drying, the Co(OH)F/CC sample was ultimately obtained.

4.2.3 Preparation of the shell–core CoS₂–Co(OH)F nanoarrays on CC. The shell–core CoS₂–Co(OH)F nanoarrays was prepared by a partial vulcanization reaction of the Co(OH)F array. The sublimed sulfur and Co(OH)F/CC precursor were placed in a tube furnace at a mass ratio of 6 : 1, with the sublimed sulfur upstream and the Co(OH)F/CC sample downstream. After annealing in an Ar environment at 400 °C for 1 h with a ramp rate of 5 °C min⁻¹ and cooling down, the CoS₂–Co(OH)F/CC sample was obtained.

4.2.4 Preparation of the flexible Li@CoS₂–Co(OH)F@CC anode. The Li@CoS₂–Co(OH)F/CC composite anodes were fabricated by electrodepositing 10 mA h cm⁻² of Li metal onto the CoS₂–Co(OH)F/CC sample at 1 mA cm⁻² current density.

4.3 Material characterization

The materials were subjected to X-ray diffraction (XRD) analysis using a Bruker D8 Advance instrument, with Cu-Kα radiation (λ = 0.15418 nm) at 40 kV and 40 mA. X-ray photoelectron spectroscopy (XPS) was performed on an Escalab 250xi device manufactured by Thermo Fisher Scientific. Scanning electron microscopy (SEM) examination was conducted using a GeminiSEM 500 instrument. Transmission electron microscopy (TEM) images were captured utilizing a JEM 2100 PLUS microscope. To acquire the XPS data of Li₂S and Co after the lithiation of CoS₂, the battery was disassembled within a glove box filled with Ar gas (with O₂ levels below 0.1 ppm and H₂O levels below 0.1 ppm). The target electrode was rinsed with glycol dimethyl ether and dried in the glove box. Then the electrode was transferred to the cavity of the XPS instrument using a transfer bunker under vacuum to prevent oxidation.⁴⁴

4.4 Electrochemical measurements

The electrochemical performance was evaluated by conducting tests on CR2032 coin cells that were assembled in the glove box. In the half-cell configuration, a working electrode of the CoS₂–Co(OH)F@CC sample with a 12 mm diameter was employed, while a counter electrode consisting of Li foil with a 14 mm diameter was utilized. Celgard 2300 was used as the separator and 1.0 M LiTFSI dissolved in 1,3-dioxolane/1,2-dimethoxyethane (DOL/DME, 1 : 1, v/v) with 2 wt% LiNO₃ was used as the electrolyte. The electrolyte volume in each cell was regulated to 60 μL. When testing the CE, the cell was charged to 1 V at a small current of 0.1 mA cm⁻² and cycled 5 cycles to form a stable solid electrolyte interface. The electrochemical impedance spectroscopy (EIS) test was conducted using an electrochemical workstation within the frequency range of 100 kHz to 10 mHz. The full cells were constructed using the Li@CoS₂–Co(OH)F/CC anode and LiFePO₄ (LFP) cathode with an active mass loading of ∼2 mg cm⁻². LFP powder, Super P and polyvinylidene fluoride (8 : 1 : 1 wt%) were dissolved in N-methylpyrrolidone (NMP) solvent, and the LFP cathode was prepared by a blade-casting method on Cu foil. The concentration of the electrolyte used was 1 M LiPF₆ in EC/DEC (1 : 1, v/v), with a controlled dosage of 80 μL for each cell. The voltage range for the full cells was maintained at 2.4∼4.2 V. The galvanostatic charge–discharge tests were conducted using a Neware battery test system (CT-4008T, Shenzhen, China).

4.5 Theoretical calculations

The GGA-PBE functional was utilized to depict the electron exchange and correlation.^45–47 The wave functions were expanded using the DNP-4.4 file, which represents a localized basis set incorporating a polarization d-function.^48,49 The spin unrestricted was used in all the calculations because the four models are open-shell systems.^47,50 The effective core potentials (ECP) were employed to treat the core electrons of the metal atoms,^51–53 while a uniform orbital cutoff of 5.0 Å was applied for all atoms. For the purpose of geometry optimization, the thresholds for achieving convergence were defined as 2 × 10⁻⁵ Ha for energy, 4 × 10⁻³ Ha Å⁻¹ for maximum force, and 5 × 10⁻³ Å for maximum displacement. Additionally, a SCF convergence criterion of 1.0 × 10⁻⁵ Ha was set to ensure accurate determination of electronic energy. The calculation of adsorption energy (E_ads) is performed using the equations provided below:⁴⁶

E_ads = (E_total − E_sub − E_Li)

where E_total, E_sub and E_Li represent the total energy of Li adsorption and non-adsorbed systems, and the energy of Li atoms in the metallic phase structure, respectively.

Author contributions

Liubin Song: investigation, methodology, writing − review & editing. Yixuan Wang: investigation, methodology, data curation, writing – original draft. Huaming Qian: data curation, investigation, writing – original draft, writing – review & editing. Mengxin Bai: data curation, methodology, visualization, writing − review & editing. Qinchuan Chen: data curation, validation, writing – review & editing. Minzhi Xiao: data curation, methodology, writing – review & editing. Zhongliang Xiao: writing – original draft, writing – review & editing. Tingting Zhao: visualization, writing – original draft, writing – review & editing. Jingjing Wang: validation, writing – original draft, writing – review & editing. Xifei Li: conceptualization, formal analysis, funding acquisition, project administration, resources, supervision, writing – original draft, writing – review & editing.

Conflicts of 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.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (21501015), Hunan Provincial Natural Science Foundation of China (2022JJ30604), Hunan Provincial Key Laboratory of Materials Protection for Electric Power and Transportation (2022CL01) and Foshan Science and Technology Innovation Team Project (1920001004098).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4ta02143f

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