Oxygen Vacancy-Rich Ultrathin Co₃O₄ Nanosheets as Nanofillers in Solid-Polymer Electrolyte for High-Performance Lithium Metal Batteries

Abstract

The development of high-performance solid-polymer electrolytes (SPEs) is a key to the practical application of lithium metal batteries (LMBs). The use of two-dimensional (2D) inorganic nanofiller is an efficient way to build poly(ethylene oxide) (PEO)-based SPEs with high ionic conductivity and stability. Herein, a series of 2D oxygen vacancy-rich Co₃O_4-y−x (x = 1, 2 and 3) with well-defined 2D nanostructures, a high surface area and controllable oxygen vacancy contents (Co₃O_4-y) was synthesized via a facile self-assembly method and NaBH₄ reduction. When the 2D Co₃O_4-y−x (x = 1, 2 and 3) nanosheets are introduced as nanofillers in PEO-based SPEs, they can interact with the PEO to form a three-dimensional (3D) PEO/Co₃O_4-y film with uniform Li⁺ distribution and vertical diffusion channels, as well as strong adsorption of NO₃⁻ from LiNO₃ electrolyte salt at the defective sites. As a result, the PEO/Co₃O_4-y−2 film reached a high ionic conductivity of 4.9 × 10⁻⁵ S cm⁻¹, high Li⁺ a transference number of 0.51 and a wide electrochemical window over 4.6 V at 80 °C. The PEO/Co₃O_4-y−2 film enables the Li||PEO/Co₃O_4-y−2||LiFePO₄ cell to deliver a high reversible capacity of 117.7 mAh g⁻¹ at 2 C and to maintain 126.7 mAh g⁻¹ at 1 C after 250 cycles with an initial capacity retention of 87.9%.

1. Introduction

Owing to the use of metallic lithium metal as an anode with the ultrahigh theoretical specific capacity of 3860 mAh g⁻¹ and low redox potential of −3.04 V vs. Li⁺/Li, lithium metal batteries (LMBs) have been widely regarded as one of the most promising advanced energy storage systems [1]. The easy formation of lithium dendrites on the metallic lithium metal during cycling and their high sensitivity to traditional organic liquid electrolytes containing flammable and volatile organic solvents, however, can cause an explosion, thermodynamic instability and a shortened cycling life [2]. As expected, the development of alternative electrolytes to traditional liquid electrolytes is a key to the practical application of LMBs. The use of solid-state electrolytes (SSEs) for LMBs has been considered an effective way. This is because the SSEs with a wide electrochemical window and strong mechanical properties not only guarantee the delivery of high-energy density and excellent cycling stability, but also provide a strong physical block to suppress the growth of lithium dendrites [3].

Since the alkali metal salts were dissolved in polyethylene oxide (PEO) by Fenton in 1973 to form conductive complexes [4], using PEO as a polymer matrix to fabricate solid polymer electrolytes (SPEs) has been intensively investigated. This is mainly due to the PEO-based SPEs with good viscoelasticity, flexible geometry and high ion dissociation ability [5,6]. Despite these, the ionic conductivity of the PEO-based SPEs at room temperature is only 10⁻⁸–10⁻⁷ S cm⁻¹, which makes their practical applications difficult [3]. As a result, many strategies have been developed to improve the ionic conductivity of SPEs, as well as other properties of SPEs. For example, the use of inorganic nanofillers (e.g., Al₂O₃ [7,8], TiO₂ [9], CeO₂ [10], ZrO₂ [11]) into the SPEs can result in the reduced crystallinity of PEO, the decreased interfacial resistance between SPEs and electrodes and the improved ionic conductivity of SPEs. Moreover, the introduction of Lewis acid sites on the surface of nanofillers can adsorb and interact with the anions of the lithium salt, thus leading to an increased number of free Li⁺ for diffusion and an improved ionic conductivity [12,13]. Apart from the introduction of nanofiller or its surface modification, the morphology control of nanofillers with zero-dimensional (0D, e.g., Al₂O₃ [7,8] and ZnO [14]), one-dimensional (1D, e.g., CeO₂ nanotubes [10], carbon nanotubes [15] and Li_0.33La_0.557TiO₃ (LLTO) nanowires [16]), two-dimensional materials (2D, e.g., graphene oxide [17], g-C₃N₄ [18] and BN flakes [19,20]) or three dimensional (3D, e.g., LLTO frameworks and 3D interconnecting palygorskite network) patterns have also been considered as an efficient way to further enhance the electrochemical and physical properties of SPEs [21]. Among these, 2D nanofillers in SPEs with a large surface area and rich active sites easily interact with PEO so that it can enhance the amorphous phase of SPEs with high-ion mobility [22]. Further, the addition of 2D nanofiller leads to the formation of cross-linked PEO chains in the perpendicular direction, which could facilitate the fast transference of Li⁺ in the vertical direction [23]. Therefore, it is highly desirable to develop 2D nanofiller in SPEs with rich active sites for high-performance LMBs.

Owing to its exceptionally excellent redox properties and low cost, Co₃O₄ has been intensively investigated and regarded as one of the most important and popular transition metal oxides (TMOs) in various fields such as energy storage/conversion systems and ORR/OER/HER [24,25]. In addition, its intrinsic semiconductor nature further makes it an attractive inorganic nanofiller for SPEs. The defect engineering of Co₃O₄ can significantly optimize the electrocatalytic activities, such as OER [26,27] and CO₂ photoreduction [28]. However, studies on the role of oxygen vacancy in Co₃O₄ to enhance the SPEs properties remain rare and its mechanism also remains unclear [10,13]. Herein, we synthesized a series of Co₃O₄ with well-defined 2D nanostructures, a large surface area and controllable oxygen vacancy contents (Co₃O_4-y) via a facile self-assembly synthesis method and NaBH₄ reduction, as shown in Figure 1a. The oxygen vacancy contents (y) of Co₃O_4-y−x (x = 1, 2 and 3) nanosheets were controlled by controlling NaBH₄ with different molar concentrations (x = 1, 2 and 3 standing for 0.05, 0.1 and 0.2 M) in a Co₃O₄/NaBH₄ water solution. When the 2D Co₃O_4-y−x (x = 1, 2 and 3) nanosheets were used as nanofillers, they could interact with PEO polymer and LiNO₃ as an electrolyte additive to form 3D PEO/Co₃O_4-y films with uniform distribution of Li⁺ and their vertical diffusion channels. Moreover, the presence of oxygen vacancy in Co₃O₄ can strongly adsorb NO₃⁻ at the defective sites in order that it can liberate more free Li⁺ for diffusion [29]. As a result, the ionic conductivity and Li⁺ transference number of the PEO/Co₃O_4-y film with the optimized oxygen vacancy content (x = 2) could reach 4.9 × 10⁻⁵ S cm⁻¹ and 0.51 at 80 °C, respectively. When the PEO/Co₃O_4-y−2 film was applied in the Li||PEO/Co₃O_4-y−2||Li symmetric cell, the cell exhibited a low polarization voltage of <0.1 V over 800 h at a current density of 0.1 mA cm⁻² with a limited specific capacity of 0.1 mAh cm⁻². In addition, the PEO/Co₃O_4-y−2 electrolyte enabled the asymmetric cell of the LiFePO₄||PEO/Co₃O_4-y−2||Li to deliver high initial reversible capacities of 162.9 mAh g⁻¹ at 0.1 C and 117.7 mAh g⁻¹ at 2 C, and to maintain a high reversible capacity of 126.7 mAh g⁻¹ at 1 C after 250 cycles with an initial capacity retention of 87.9%.

2. Results and Discussion

The morphology and detailed structures of Co₃O₄ and Co₃O_4-y−x (x = 1, 2 and 3) nanosheets were characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). As shown in Figure 1b,c, the flower-like Co₃O₄ particle is composed of clear and curved two-dimensional (2D) nanosheets. After defect engineering by NaBH₄, the resultant Co₃O_4-y−x (x = 1, 2 and 3) particles preserve the 2D flower-like and the thin features of Co₃O₄. With the x increase, the Co₃O_4-y−x (x = 3) becomes agglomerated and even cracked (Figure S1). This is mainly attributed to the etching effect of excessive NaBH₄ [30]. Energy dispersive X-ray spectroscopy (EDS) mappings confirm the composition of Co and O as well as their uniform distribution through the nanosheets (Figure 1d–f). The ultrathin feature and crystal structure of Co₃O_4-y−2 nanosheets were also further confirmed by TEM (Figure 1g) and high-resolution TEM (HRTEM). A HRTEM image of exfoliated Co₃O_4-y−2 in Figure 1h indicates that the Co₃O_4-y−2 nanosheet has a high crystallinity with a clear interlayer distance of 0.23 and 0.46 nm between the fringes, which can be attributed to the d-space of (222) and (111) crystal plane of spinel Co₃O₄, respectively. Moreover, the ring feature from the corresponding selected area electron diffraction (SAED) pattern in Figure 1i is also well indexed to different crystal planes of spinel Co₃O₄, further confirming the well-defined crystallinity of Co₃O_4-y−2 (Figure S2). Figure 1j,k and Figure S3 compare the SEM images of PEO and PEO/Co₃O_4-y−2 electrolyte films. As can be seen, PEO/Co₃O_4-y−2 film has fewer grain boundaries than that of PEO film, indicating its enhanced amorphous feature and the stronger coordination effect between PEO/Co₃O_4-y−2 electrolyte film and lithium salts. Furthermore, the side-view SEM images reveal that PEO/Co₃O_4-y−2 film presents the well-aligned wrinkled structure perpendicular to its plane compared to the PEO film with a random and dense pattern (Figure 1k and Figure S3). It should be noted that the introduction of PVP to electrolyte film could provide extra porous structures to further enhance the amorphous feature of the electrolyte film, leading to the more flexible SPEs with faster ion transportation [31].

The phases of Co₃O₄ and Co₃O_4-y−x (x = 1, 2 and 3) were analyzed by an X-ray diffraction (XRD) technique. As shown in Figure 2a, the diffraction peaks at ~18.9, 31.3, 36.8, 44.8, 59.4 and 65.4° of Co₃O₄ and Co₃O_4-y−x (x = 1, 2 and 3) are well indexed into the (111), (220), (311), (400), (511) and (440) planes of cubic Co₃O₄ (JCPDS NO. 42-1467), respectively, indicating the formation of Co₃O₄ with a cubic crystal structure and its chemical stability after the NaBH₄ reduction [26]. It should be noted that the larger amorphous regions of Co₃O_4-y−x (x = 1, 2 and 3) than Co₃O₄ in the angle range of 11–28° are probably due to the generation of the rich surface edges of the Co₃O_4-y−x (x = 1, 2 and 3) during the reduction treatment. Raman spectra in Figure 2b also indicate that four peaks at ~187, 464, 506 and 662 cm⁻¹ of Co₃O₄ and Co₃O_4-y−x (x = 1, 2 and 3) correspond to the F¹_2g, E_2g, F²_2g and A_1g modes of the Co₃O₄, respectively [32]. As listed in Table S1, the increased full width at half maximum (FWHM) of Co₃O_4-y−x (x = 1, 2 and 3) with the x increase is attributed to the increased content of oxygen vacancy induced by NaBH₄ etching [33]. To further reveal the oxygen vacancy contents of Co₃O_4-y−x (x = 1, 2 and 3), electron paramagnetic resonance (EPR) spectra were carried out. As shown in Figure 2c, the g-value of 2.003 signals caused by the surface oxygen vacancies with an electron trapping of Co₃O_4-y−x (x = 1, 2 and 3) increase along with the x increase, indicating the higher x with the richer oxygen vacancies in Co₃O_4-y−x (x = 1, 2 and 3) [34]. Furthermore, the N₂ adsorption–desorption isotherm curve and its corresponding specific surface area (Figure 1d and Figure S6) reveal the surface area of Co₃O₄ with 232.6 m² g⁻¹, which is higher than that of Co₃O_4-y−x (x = 1, 2 and 3) with 167.9, 143.8 and 106.7 m² g⁻¹, respectively. As shown in Table S2, the pore volume of Co₃O_4-y−x (x = 1, 2 and 3) decreases with the x increase. The lower surface area and pore volume of Co₃O_4-y−x (x = 1, 2 and 3) than Co₃O₄ is attributed to the agglomeration of Co₃O_4-y−x (x = 1, 2 and 3) nanosheets during the strong NaBH₄ reduction.

X-ray photoelectron spectroscopy (XPS) was performed to further evaluate the elemental compositions and their oxidation states of Co₃O₄ and Co₃O_4-y−x (x = 1, 2 and 3). As shown in Figure 2e and Figure S5, all materials show the main peaks of Co 2p_1/2 and Co 2p_3/2 located at ~796 and 781 ev, respectively. The Co 2p_1/2 and Co 2p_3/2 peaks of Co₃O_4-y−x (x = 1, 2 and 3) have a slightly higher energy shift due to the formation of oxygen vacancies in Co₃O_4-y leading to the change in electronic structures of Co₃O₄ [35,36,37]. The high resolution XPS Co 2p_1/2 and Co 2p_3/2 spectra of all materials can be fitted into the pair peaks Co³⁺/Co²⁺ at ~780/795 and 781.6/796.8 eV, respectively [28,38]. As listed in Table S1, with the x increase, the intensity ratio of Co²⁺/(Co²⁺ + Co³⁺) in the Co₃O_4-y−x gradually increases from 58.1% (x = 1) to 68.9% (x = 3), which is much higher than Co₃O₄ of 37.7%, further supporting the formation of oxygen vaccancies in Co₃O_4-y−x. Moreover, the O 1s spectra of Co₃O₄ and Co₃O_4-y−x (x = 1, 2 and 3) can be deconvoluted into three peaks at ~529.9, 531.3 and 532.6 eV, which are attributed to the lattice oxygen species (O_Lat), surface adsorbed oxygen (O_Ads) and undesired water molecules (O_Res), respectively. As is shown in Table S1, the much higher ratio of O_Ads/(O_Lat + O_Ads) in Co₃O_4-y−x (x = 1, 2 and 3) than Co₃O₄, as well as its higher ratio in the Co₃O_4-y−x with higher x, once again supports the formation of oxygen vacancies.

Figure 3 shows the structural and morphological analysis of PEO, PEO/Co₃O₄ and PEO/Co₃O_4-y−x (x = 1, 2 and 3) electrolyte films. XRD patterns in Figure 3a and Figure S7 show the PEO/Co₃O₄ and PEO/Co₃O_4-y−x (x = 1, 2 and 3) films with weaker main characteristic peaks at ~19° and 23° than those of the PEO film, indicating the enhanced amorphous phases of PEO with the addition of Co₃O₄ and Co₃O_4-y−x as nanofillers. Thermogravimetric analysis (TGA) curves of the electrolyte films were measured under air in the temperature range from 100 to 700 °C. As shown in Figure 3b, the initial weight loss of all films below 120 °C is due to the evaporation of water. After 340 °C, the films experience two weight losses in the temperature ranges of 310–360 °C and 360–430 °C, which are assigned to the decomposition of PVP and PEO, respectively [39]. The more relatively smooth PEO/Co₃O₄ and PEO/Co₃O_4-y−x (x = 1, 2 and 3) films than the PEO film in these two regions suggest the effective interaction between PEO/PVP and the nanofillers. The difference in the residual mass of the films above 420 °C is mainly attributed to the final products of Co₃O₄ and Li₂CO₃, as evidenced by their XRD patterns (Figure S8). To determine the melting temperatures (T_m) of the electrolyte films, differential scanning calorimetry (DSC) curves were plotted in Figure 3c. Owing to the reduced crystalline and optimized cordinantion of components, the PEO/Co₃O_4-y−2 film displays the lower T_m of 59.8 °C, followed by PEO/Co₃O₄ (60.8 °C), PEO/Co₃O_4-y−1 (60.8 °C), Co₃O_4-y−3 (61.5 °C) and PEO (63.8 °C) films [23].

Raman spectra and Fourier transform infrared (FTIR) spectra were further carried out to investigate the interactions between the components of films. Raman spectra in Figure 3d present the increased contents of coordinated NO₃⁻ at ~1037 cm⁻¹ and decreased contents of free NO₃⁻ at ~1050 cm⁻¹ in the PEO/Co₃O_4-y−x (x = 1, 2 and 3) compared to those of PEO/Co₃O₄ and PEO [40,41]. With the x increase, the increased content of coordinated NO₃⁻ and decreased content of free NO₃⁻ were observed. Figure 3e shows the full-survey FTIR spectra of PEO, PEO/Co₃O₄ and PEO/Co₃O_4-y−x (x = 1, 2 and 3) films with characteristic vibration bands of CH₂ (ω) at 1310–1380 cm⁻¹, CH₂ (τ) at 1230–1300 cm⁻¹, C-O-C (ν_s) at 1000–1150 cm⁻¹ and CH₂ (ν) at 900–980 cm⁻¹ [39]. It should be noted that the FTIR spectra in the range of 1335–1350 and 815–840 cm⁻¹ (Figure 3f) are attributed to the signals of asymmetric and symmetric stretching of NO₃⁻ structures of LiNO₃ [42]. The peak position shift of asymmetric and symmetric stretching of NO₃⁻ indicates the easy dissociation of LiNO₃ in the electrolyte films, which is due to the rich oxygen vacancies with abundant Lewis acid sites on Co₃O₄ surfaces interacting with the free NO₃⁻ [8,13].

Figure 4 presents the ionic conductivity, Li⁺ transference number, electrochemical stability and mechanical properties of PEO, PEO/Co₃O₄ and PEO/Co₃O_4-y−x (x = 1, 2 and 3) films. The ionic conductivities of films were calculated based on the charge transfer resistance in the SS (stainless steel)||SPE||SS cells, which is fitted from the corresponding electrochemical impedance spectra (EIS) together with an equivalent model in Figure 4a and Figure S10. As shown in Figure 4b and listed in Table S3, all films display the improved ionic conductivity with the increasing temperature and the PEO/Co₃O_4-y−2 shows the highest ionic conductivity among the films at each temperature. At 80 and 90 °C, the ionic conductivities of PEO/Co₃O_4-y−2 reach 4.98 × 10⁻⁵ and 1.1 × 10⁻⁴ S cm⁻¹, respectively. The Li⁺ transference number (t⁺) is also a key indicator of the electrochemical kinetic. The t⁺ values were obtained from the chronoamperometry tests of the films in the Li||SPE||Li cells at 80 °C with a potential step of 20 mV [43]. Figure 4d shows the highest t⁺ value of 0.51 for the PEO/Co₃O_4-y−2 film. It should be noted that the lower t⁺ value of PEO/Co₃O_4-y−3 than PEO/Co₃O_4-y−2 is probably due to the overhigh NaBH₄ concentration destroying the 2D structure of Co₃O_4-y−3 (Figure S11). The electronic conductivity of PEO, PEO/Co₃O₄ and PEO/Co₃O_4-y−2 was measured by the DC polarization method at a constant voltage of 0.1 V for 1000 s and calculated by the steady-state current value and the applied external voltage value shown in Figure S12 [44]. As a result, the electronic conductivities of PEO, PEO/Co₃O₄ and PEO/Co₃O_4-y−2 films are 2.14 × 10⁻⁸, 2.27 × 10⁻⁹ and 3.03 × 10⁻⁹ S cm⁻¹, respectively, indicating that the introduction of Co₃O₄ and Co₃O_4-y−2 could reduce the electronic conductivity of the film. This is most likely attributed to the intrinsic semiconductor feature of Co₃O₄ and Co₃O_4-y−2 even with a low amount of 5 wt.%.

To evaluate the electrochemical stability of the electrolyte films, liner sweep voltammetry (LSV) curves of SS||SPEs||Li cells were conducted. As shown in Figure 4e and Figure S1, the cells using PEO/Co₃O_4-y−x (x = 1, 2 and 3) and PEO/Co₃O₄ films run stably with a higher oxidative stability >4.6 V than that using PEO film at ~4.4 V. The lower current density of the cells using PEO/Co₃O_4-y−2 than PEO/Co₃O₄ films at 5.2 V further supports its stronger high-voltage tolerance [45,46]. Figure 4f compares the mechanical properties of the electrolyte films with the stretch rate of 50 mm min⁻¹. As can be seen, all films exhibit the drecreased stress at initial states mainly due to the overcoming of the yield point. Despite the similar fracture limit length of the three electrolyte films, the yielding strength of PEO/Co₃O_4-y−2 and PEO/Co₃O₄ films can reach ~4.0 MPa, which is much higher than that of the PEO film with ~2.1 MPa. After ~1185% stretching, moreover, the tensile strength of PEO/Co₃O_4-y−2 and PEO/Co₃O₄ films can reach ~5.1 and 5.2 MPa, respectively, which is also much higher than that of the PEO film with ~4.7 MPa, further indicating its stronger mechanical strength.

To examine the electrochemical stability of the electrolyte films against lithium metal, the Li||SPE||Li symmetric cells using PEO, PEO/Co₃O₄ and PEO/Co₃O_4-y−2 as electrolyte films were assessed at various current densities from 0.02 to 0.30 mA cm⁻² at 80 °C. As shown in Figure 5a, the PEO/Co₃O_4-y−2 displays the lowest polarization at each current density among all the films. At 0.1 mA cm⁻², the PEO/Co₃O_4-y−2 can run more stably in a much lower polarization (<0.1 V) for 800 h without a short circuit (Figure 5b), compared to those of the PEO/Co₃O₄ (<0.16 V) and PEO (<0.2 V) with only 429 and 318 h, respectively. The enhanced cycling stability of the films can also be confirmed by the lithium metal of Li||PEO/Co₃O_4-y−2||Li cell after cycling for 100 and 318 h with much more smooth surfaces and uniform deposition of lithium dendrites than that of the Li||PEO||Li cell as evidenced by SEM images of the cycled lithium metal surface (Figure 5c–f).

In order to further evaluate the practical feasibility of the electrolyte films, the LiFePO₄||SPE||Li asymmetric cells were assembled by using commerical LiFePO₄ as the cathode, metallic lithium metal as the anode, and PEO, PEO/Co₃O₄ or PEO/Co₃O_4-y−x (x = 1, 2 and 3) as the electrolyte films. Figure 5g,h compares the rate performance of the LiFePO₄||SPE||Li cells at various C-rates from 0.1 to 2 C (1 C = 170 mA h g⁻¹) in the voltage range of 2.4–4.0 V. As can be seen, all the cells show the typical charge–discharge behaviours of LiFePO₄ with the characteristic flat plateaus at ~3.5 V. The LiFePO₄||SPE||Li cell using PEO/Co₃O_4-y−2 delivers a higher discharge capacity of 165.5 mAh g⁻¹ and lower electrode polarization but a lower initial Coulombic efficiency (CE, 78.6%), compared to those using PEO (132 mAh g⁻¹ and 92.5%) and PEO/Co₃O₄ (162.7 mAh g⁻¹ and 87.6%). The low initial CE is probably due to the formation of the irreversible interface between electrolytes and LiFePO₄. In the PEO/Co₃O_4-y−2 electrolyte film, additional Li⁺ probably take part in interface formation due to the exsistence of Co₃O_4-y nanoparticles [47]. Among the LiFePO₄||SPE||Li cells using the PEO/Co₃O_4-y−x, the PEO/Co₃O_4-y−2 exhibits the strongest rate capability and delivers the highest average reversible capacities of 169.9, 162.3, 153.8, 147.2 and 116.1 mAhg⁻¹ at 0.2, 0.5, 0.8, 1 and 2 C, respectively [48]. The relatively long-term cycling performance of LiFePO₄||PEO/Co₃O_4-y−2||Li cells at 1 C and 2 C in Figure 5j and Figure S14 shows the excellent cycling stability of the cell using PEO/Co₃O_4-y−2, as well as its superiority to that using PEO/Co₃O_4-y−1. This is also evidenced by the cell using PEO/Co₃O_4-y−2 with a high initial discharge capacity of 144.1 mAh g⁻¹ at 1 C with an initial capacity retention of 87.9% over 250 cycles, compared to that of PEO/Co₃O_4-y−1 with 142.8 mAh g⁻¹ and 68.8%. The typical and unchanged charge–discharge curves at different cycles in Figure 5k further imply the stability of the PEO/Co₃O_4-y−2 film during the charge–discharge process. Finally, We compared the electrochemical performance of previously reported literature as listed in Table S4.

3. Materials and Methods

3.1. Material Synthesis

Preparation of pristine Co₃O₄ nanosheets: The pristine Co₃O₄ nanosheets were prepared via a self-assembly hydrothermal synthesis method [49]. Typically, 0.4 g Pluronic P123 was dissolved in 6 g ethyl alcohol with a magnetic stirring for 15 min. Then, 16 mL deionized water and 24 mL ethylene glycol (98%, Aldrich) were added to the resultant solution to form an oil–water–surfactant equilibrium system [50]. After stirring for 30 min, 0.26 g Co(CH₃COO)₂·4H₂O and 0.14 g HMTA were added to the equilibrium system. After stirring for 1 h, the pink precursor was obtained and kept without stirring for 24 h, and was then transferred into a hydrothermal reactor and maintained at 160 °C for 15 h. The resultant sample was washed with deionized water five times and dried in a freezer dryer to form the Co₃O₄ nanosheets precursor. Finally, the Co₃O₄ nanosheets precursor was heated at 350 °C for 1 h to obtain the Co₃O₄ nanosheets.

Preparation of oxygen vacancy-rich Co₃O_4-y nanosheets: The Co₃O_4-y−x (x = 1, 2 and 3) nanosheets with different oxygen vacancy (y) were prepared by using a NaBH₄ solution with a different x molar concentration (x = 1, 2 and 3 standing for 0.05, 0.1 and 0.2 M) etching the obtained Co₃O₄ nanosheets. Typically, 0.07 g Co₃O₄ nanosheets were soaked in 30 mL x NaBH₄ solution for 20 min. The black powders (named Co₃O_4-y−x) were then collected by centrifugation, washed in deionized water and freeze-dried.

Preparation of the electrolyte films: The PEO/Co₃O₄ and PEO/Co₃O_4-y−x (x = 1, 2 and 3) films were fabricated by mixing PEO (M_w = 1 × 10⁶, Aldrich), LiNO₃ (99%, Aldrich), polyvinyl pyrrolidone (PVP, M_w = 5 × 10⁴, Aldrich) and Co₃O₄ (Co₃O_4-y−x (x = 1, 2 and 3)) in the methanol solvent in a weight ratio of 80:10:14:5 to form a homogeneous solution. The obtained solution was then cast onto a polytetrafluoroethylene (PTEE) plate and dried in a vacuum oven at 60 °C for 24 h. The collected electrolyte films were named as PEO/Co₃O₄ and PEO/Co₃O_4-y−x (x = 1, 2 and 3). For comparison, the PEO electrolyte was prepared by the same method without using Co₃O₄ and Co₃O_4-y−x (x = 1, 2 and 3). Before the mechanical and electrochemical test, all the electrolyte films were stored in an argon-filled glove box for seven days to remove the residual methanol.

3.2. Material Characterization

X-ray diffraction (XRD) patterns were measured on a Bruker D8 advance diffractometer with a Cu–Kα radiation (λ = 1.5406 Å). The morphologies and elemental mappings of the materials were characterized by field emission scanning electron microscopy (FESEM, Merlin, Zeiss) and transmission electron microscopy (TEM, FEI Talos-F200S). Raman spectra were recorded on a Raman spectrometer (Thermo Fischer DXR). Fourier transform infrared (FTIR) spectroscopy was recorded on a Thermo iS50 in the range of 2200–600 cm⁻¹. Nitrogen adsorption–desorption isotherms were obtained from BELSORP-max (Micro for Tristar II Plus 2.02) and analyzed by using the Barrett–Joyner–Halenda (BJH) method. X-ray photoelectron spectra (XPS) measurements were performed on an Escalab 250Xi X-ray photoelectron spectrometer using C 1s (B.E. = 284.8 eV) as a reference. Electron paramagnetic resonance (EPR) spectra were obtained using a JEOL (FA200) EPR spectrometer at 123.15 K. TGA was carried out on a STA 449 F5 TG analyzer at N₂ atmosphere in the temperature range of 100–700 °C with a heating rate of 5 °C min⁻¹. The melting temperature of the electrolyte films was determined by differential scanning calorimetry (DSC, DSC 214 Polyma, Bavaria, Germany).

3.3. Electrochemical Measurements

The electrochemical measurements of the symmetric (e.g., Li(electrode)||SPE (electrolyte)||Li(electrode) and stainless steel (SS)||SPE||SS) and asymmetric cells (e.g., LiFePO₄||SPE||Li and SS||SPE||Li) were fabricated in 2032-type coin cells. The LiFePO₄ cathode was prepared by mixing 80 wt.% LiFePO₄, 10 wt.% super P and 10 wt.% PEO binder in the methanol solution. The resultant slurry, after being stirred for 24 h, was coated in an aluminum foil. The foil was then dried in a vacuum oven at 70 °C for 12 h. Finally, it was punched to a diameter of 11 mm and then transferred into an Ar-filled glove box. The mass of the punched LiFePO₄ electrode was controlled with 1.2 mg cm⁻². The symmetric and asymmetric cells were assembled in an Ar-filled glove box.

The lithium transference number (t⁺) of the electrolyte films was measured in Li||SPEs||Li cells by combining AC impedance and chronoamperometry with 20 mV as the applied potential. Electrochemical impedance spectroscopy (EIS) spectra of the cells were measured from 0.1 to 10⁶ Hz before and after polarization. The t⁺ values of the electrolyte films were calculated according to the equation [51]:

$$t^{+}=\frac{I_{SS}\left(\mathrm{\Delta }V-I_0{R}_0\right)}{I_0\left(\mathrm{\Delta }V-I_{SS}{R}_{SS}\right)}$$

(1)

where I₀ and I_SS are the initial and steady-state current, and R₀ and R_SS represent the resistance before and after polarization (ΔV = 20 mV), respectively. The ionic/electronic conductivities of the electrolyte films were evaluated by EIS measurements with the frequency range of 0.1–10⁶ Hz in the temperature range of 30–90 °C. The electrolyte films were sandwiched between two stainless steel (SS) disks and the equation for calculating the ionic/electronic conductivity is described as follows:

$$\delta =\frac{l}{R_{b}S}$$

(2)

where l represents the thickness of the SPEs, R_b and S are the bulk resistance of the electrolyte films and the surface area of the electrodes, respectively. The electrochemical tests (EIS and chronoamperometry) of symmetric and asymmetric cells were carried out using a CHI 760C (CH Instruments, Shanghai, China). The coin cells were charged–discharged by using an automatic battery tester system (Land^®, Wuhan, China).

4. Conclusions

In summary, a series of two-dimensional (2D) Co₃O_4-y−x (x = 1, 2 and 3) with controllable oxygen vacancy contents were fabricated via a facile self-assembly synthetic method and NaBH₄ reduction. The 2D Co₃O_4-y−x (x = 1, 2 and 3) endow the PEO/Co₃O_4-y−x electrolyte films, the uniform distribution of Li⁺, its vertical diffusion channels to shorten the diffusion length, and the oxygen vacancy on Co₃O₄ surfaces to strongly adsorb NO₃⁻ at the defective sites in order that it can liberate more free Li⁺ for diffusion. As a result, the PEO/Co₃O_4-y−2 film reaches the high ionic conductivity of 4.9 × 10⁻⁵ S cm⁻¹, a high Li⁺ transference number of 0.51 and a wide electrochemical window over 4.6 V at 80 °C. The LiFePO₄||PEO/Co₃O_4-y−2||Li cell displays excellent lithium storage properties with a high initial discharge capacity of 143.9 mAh g⁻¹ at 1 C and an initial capacity retention of 87.9% after 250 cycles. This work develops the use of Co₃O_4-y nanosheets with oxygen vacancies as nanofillers to build strong mechanical and more flexible SPEs for high-performance lithium metal batteries.