Lithium-ion modified cellulose as a water-soluble binder for Li-O₂ battery

Abstract

An environment-friendly, water-soluble, and cellulose based binder (lithium carboxymethyl cellulose, CMC-Li) was successfully synthesized by using Li⁺ to replace Na⁺ in the commercial sodium carboxymethyl cellulose (CMC-Na). Li-O₂ batteries based on the CMC-Li binder present enhanced discharge specific capacities (11151 mA·h/g at 100 mA/g) and a superior cycling stability (100 cycles at 200 mA/g) compared with those based on the CMC-Na binder. The enhanced performance may originate from the electrochemical stability of the CMC-Li binder and the ion-conductive nature of CMC-Li, which promotes the diffusion of Li⁺ in the cathode and consequently retards the increase of charge transfer resistance of the cathode during cycling. The results show that the water-soluble CMC-Li binder can be a green substitute for poly(vinylidene fluoride) (PVDF) binder based on organic solvent in the lithium oxygen batteries (LOBs).

Graphical abstract

Keywords

cellulose / binder / specific capacity / cyclabi- lity / lithium-oxygen batteries

1 Introduction

Rechargeable lithium oxygen batteries (LOBs) have attracted growing attentions owing to their ultrahigh theoretical specific capacity (3,500 W‧h/kg), which are considered as one of the new generation energy storage devices [1–3]. During the past decades, lots of efforts have been devoted to improving the performance of LOBs with major focuses on: efficient oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) catalysts [4–7], supporting materials such as porous conductive materials [8–10], electrolyte stability [11–14], anode protection [15,16], and reaction mechanism [17–21]. Significant progresses have been made in the above-mentioned aspects of LOBs. However, binder, one of the essential components of the oxygen cathode, is much less intensively reported. Binder plays a very important role in stabilizing the cycling performance of LOBs, because it maintains the integrity of the physical structure of the cathode to prevent it from falling apart. To date, the most widely used binders for oxygen cathodes in LOBs are derived from lithium-ion batteries (LIBs). For example, poly(vinylidene fluoride) (PVDF) with excellent electrochemical stability and high adhesion [22,23] is one of the alternative materials. However, PVDF is expensive and using PVDF as binder involves the environmentally harmful and toxic solvent N-methyl-2-pyrrolidone (NMP) as dispersant. PVDF is also very sensitive to trace amount of water or the residual NMP solvent [24–26], which will result in the degradation of PVDF during the cycling of LOBs [27]. Therefore, it is necessary to develop cost-effective, electrochemically stable, and environmentally friendly binders which are soluble in non-toxic solvent.

The water-soluble sodium carboxymethyl cellulose (CMC-Na) with eco-friendliness and low cost has been used as an alternative to PVDF in LIBs [28–32]. The previous study demonstrated that CMC-Na could also be an effective binder material for LOBs [33]. It is electrochemically stable between 2.2 and 4.5 V and easy to fabricate a 3D porous oxygen cathode, which shows the excellent rate performance and maintains stable more than 60 cycles. In this work, a Li-modified CMC-Na (CMC-Li) was prepared by replacing Na⁺ by Li⁺. The electrochemical performances of the oxygen cathode MnO₂/carbon nanotubes (MnO₂/CNTs) using the as-synthesized CMC-Li binder in LOBs were investigated and compared to those with the CMC-Na binder. The results show that the LOBs based on CMC-Li binder display an excellent discharge specific capacity (11151 mA·h/g at 100 mA/g) and a superior cycling stability (100 cycles at 200 mA/g). A few possible reasons for the enhanced performances induced by the CMC-Li binder were discussed. To the best of the authors’ knowledge, this is the first time that CMC-Li is used as a green binder for the LOBs.

2 Experimental

2.1 Synthesis of CMC-H and CMC-Li

In a typical process, a suspension was prepared by dispersing 10.0 g of CMC-Na into 150 mL ethanol/water mixed solution (V_ethanol:V_water = 95:5) and stirring for 30 min. Then, 6 mol/L of hydrochloric acid (HCl) solution (20 mL) was slowly poured into the resulting dispersion which was kept stirring for another 2 h at room temperature. The resulting white powder was washed repeatedly with absolute ethyl alcohol and deionized water several times and dried at 80°C overnight in a vacuum oven. The white powder obtained is labeled as CMC-H.

CMC-Li was synthesized by dispersing the as-synthesized CMC-H (3.7 g) into 45 mL of ethanol/water mixed solution (V_ethanol:V_water = 95:5) under sonication. Then, 15 mL of the aqueous solution of LiOH (1.02 g) was poured into the above mixture under vigorous stirring. Afterward, the mixture was kept stirring at 50°C for 2 h to finish the neutralization reaction in a water bath. After cooling to room temperature, the white product was collected by filtering and rinsing with ethanol several times, and dried at 80°C overnight in a vacuum oven. The water-soluble white powder obtained is denoted as CMC-Li.

2.2 Preparation of oxygen cathode

The cathode was prepared by mixing CNTs (70 mg) and α-MnO₂ (20 mg) in a water solution which contained CMC-Li or CMC-Na (10 mg) as the binder. After stirring for 10 h at room temperature, the resulting uniform slurry was coated on the carbon paper (TGP-H-060, Torray) which was used as a current collector. Then, the cathode was freeze-dried at − 40°C for 24 h to sublimate the ice template. Finally, the cathode was dried at 120°C for 12 h in a vacuum oven to remove the residual water and then transferred to the glovebox filled with pure argon gas. The mass loading of per cathode (CNTs+ α-MnO₂ + binder) was about 0.5–0.7 mg, which was used as the measure for calculating the specific capacities and current densities.

2.3 Electrochemistry test

The performance of the oxygen cathode based on the CMC-Li binder of the Li-O₂ battery was evaluated in 2032-type coin cells, including a pure lithium metal foil, a glass fiber (Whatman, GF/D) separator, and an as-prepared oxygen cathode. The coin cell was assembled in an argon-filled glove box (MIKROUNA, the volume fraction of water φ(H₂O)<0.1 × 10⁻⁶, the volume fraction of water φ(O₂)<0.1 × 10⁻⁶). The organic electrolyte used here was 1 mol/L of bistrifluoromethanesulfonimide lithium (LiTFSI) in tetra ethylene glycol dimethyl ether (TEGDME). The cells were stood in the glove box for 6 h to make sure the oxygen cathode was fully wetted and then transferred to a flowing oxygen atmosphere for another 2 h before testing. The electrochemical performance was evaluated on a Neware battery test system in 1 atm oxygen atmosphere (99.999% purity). For rate capability, the applied current densities were 100 mA/g, 200 mA/g, and 500 mA/g, respectively, and the potential window was from 2.2 to 4.4 V. The cycling performance was performed with a cut-off specific capacity of 1000 mA‧h/g at a current density of 200 mA/g.

2.4 Characterizations

Fourier transform infrared spectrometer (FTIR) measurements were employed to study surface functional groups through a Nicolet 6700 FTIR spectrometer. Morphological and structural observations were performed using a field emission scanning electron microscopy (FE-SEM, Hitachi S-4800) operated at 10 kV. Powder X-ray diffraction (XRD) patterns of the samples were investigated on a Bruker D8 Advance diffractometer with Cu Kα radiation.

3 Results and discussion

The formation process of CMC-Li was investigated by FTIR spectroscopy, as shown in Fig. 1. For the commercial CMC-Na, the absorption peaks near 3435.31 cm⁻¹, 2922.80 cm⁻¹, and 1507.08 cm⁻¹ correspond to the stretching vibration of the -OH, C-H and ether groups of cellulose, respectively. The peaks at 1618.95 cm⁻¹ and 1425.73 cm⁻¹ are attributed to the asymmetric and symmetric stretching vibrations of the carboxylic functional group. The characteristic peak at 1327.85 cm⁻¹ can be ascribed to the stretching vibration of C-H on the methyl groups adjacent to -COONa [34]. When CMC-Na is transformed to CMC-H, the two peaks at 1618.95 cm⁻¹ and 1425.73 cm⁻¹ are replaced by an absorption peak at 1752.92 cm⁻¹. It is a signature for the -COOH group, suggesting the formation of the intermediate product CMC-H. The FTIR spectrum for the final product CMC-Li is quite similar to that of CMC-Na. As LiOH is introduced to CMC-H to form the final product, the change of the spectrum gives evidence that CMC-Li is obtained.

To further confirm the complete transformation from CMC-Na to CMC-Li, the CMC-Li, CMC-H, and CMC-Na powders were calcined at 700°C for 1 h in air. For CMC-H, there was no residue, suggesting that the Na⁺ in CMC-Na was completely substituted by H⁺. The white powers left after heat treatment of CMC-Na and CMC-Li were identified by XRD as Na₂CO₃ and Li₂CO₃, respectively (Fig. 2(a) and 2(b)). The characteristic diffraction peaks on Fig. 2(b) can be indexed to Li₂CO₃ without any peak from Na₂CO₃, indicating that the -COONa group was completely substituted by the -COOLi group.

Furthermore, the linear sweep voltammetry (LSV) curves of CMC-Li and CMC-Na (Fig. 3) show an onset oxidation potential of 4.73 V for CMC-Li, which is higher than that of CMC-Na (4.65 V). The higher onset oxidation potential not only confirms the formation of CMC-Li, but also suggests a better electrochemical stability of CMC-Li than CMC-Na. The inset figure in Fig. 3 exhibits the solubility of CMC-Na, CMC-H, and CMC-Li in water. The transparent solutions are formed with CMC-Li and CMC-Na, while precipitates can be observed for CMC-H. The above information suggests that CMC-Li has been successfully synthesized and can be used as a green binder for Li-O₂ batteries.

Figure 4(a) demonstrates the structure of pure CMC-Li made by a lyophilization method. The scanning electron microscopy (SEM) image reveals that it has a channel-like interconnected architecture with macro porosity. The prepared oxygen electrode also possesses a similar three dimensional (3D) porous structure with continuously distributed pores ranging from 10 to 35 μm (Fig. 4(b)) which result from the sublimation of ice in the cathode during the lyophilization process. The macroporous structures not only act as pathways for the diffusion of oxygen and transportation of electrolyte channels, but also provide enough storage space for the discharge products in LOBs.

Rate capabilities of the cells based on the CMC-Li and CMC-Na binder were investigated at different current densities from 100 to 500 mA/g, as depicted in Fig. 4(c). The CMC-Li cell exhibits enhanced rate performances with higher discharge specific capacities (11151 mA‧h/g at 100 mA/g, 9189 mA‧h/g at 200 mA/g, and 6282 mA‧h/g at 500 mA/g) compared to the CMC-Na cell (10557 mA‧h/g at 100 mA/g, 8190 mA‧h/g at 200 mA/g, and 4701 mA‧h/g at 500 mA/g) to a terminal discharge voltage of 2.2 V. The discharge potentials decreased, reflecting that polarization increased rapidly at higher current densities for both kinds of cells [35]. It is worth noting that when the discharge current density increases from 100 to 500 mA/g, as displayed in Fig. 4(d), the capacity retention of the CMC-Li cathode is 56.3%, which is higher than that of the CMC-Na cathode (44.5%). Therefore, the substitution of Li⁺ for Na⁺ in the CMC can lead to a promotion of the discharge specific capacity and a better rate performance of LOBs.

The cycling stability of Li-O₂ cells based on the CMC-Li and CMC-Na binder was also investigated by curtailing the depth of discharging and charging specific capacity to 1000 mA‧h/g at a current density of 200 mA/g. As presented in Figs. 5(a) and 5(c), the terminal charge voltages for CMC-Li based cell increase slowly and the thermal discharge voltages decline slowly to the cut-off voltage. In contrast, the terminal discharge and charge voltages for CMC-Na based cell reach up to the cut-off voltage rapidly. Finally, the batteries based on the CMC-Li binder show a stable performance to ~ 100 cycles in comparison to ~ 68 cycles for cells based on CMC-Na (Figs. 5(b) and 5(d)) tested under the same electrochemical conditions, clearly demonstrating an enhanced cyclability.

The better cycling performance of CMC-Li based cell might be originated from a faster diffusion of Li⁺ ions in the cathode due to the ion-conductive nature of the CMC-Li binder in comparison with CMC-Na [36]. To clarify the effect of CMC-Li, electrochemical impedance spectra (EIS) of the cells based on the CMC-Li and CMC-Na binders were evaluated in different cycle stages (Fig. 6) where − Z” and Z represent the imaginary part and real part of the impedance respectively. The diameter of the semicircle represents the charge-transfer resistance (R_ct) of the electrode before cycling. The R_ct of the two cathodes is almost identical (36 Ω). After discharging to a curtailing specific capacity of 1000 mA‧h/g at a current density of 200 mA/g, the R_ct of CMC-Li based cathode increases to 86 Ω (Fig. 6(a)), which is closely associated with the formation of numerous insulating discharged products on the surface of the cathode. After the first and the 10th recharge, R_ct increases to 39 and 65 Ω, respectively, suggesting a good rechargeability. In contrast, the R_ct of CMC-Na based cathode increases to ~ 100 Ω after 10 cycles (Fig. 6(b)). Thus, it can be speculated that the Li⁺ in the CMC-Li binder cannot only enhance the cycling stability but also mitigate steric hindrance to improve the transportation of Li⁺ on the electrode/electrolyte interface. Therefore, Li⁺-modified binder has a positive effect on the performance of the Li-O₂ battery.

Finally, morphologies of the discharge product on both CMC-Li and CMC-Na binder oxygen electrodes were further investigated by SEM. After discharging the batteries to 2000 mA‧h/g at a current density of 200 mA‧h/g, a large number of toroidal-shaped discharge products (Li₂O₂) [37] are deposited onto the surface of the electrodes (Fig. 7). However, the sizes of the discharge products are different for the above two electrodes. For the CMC-Na based electrode, the toroidal discharge products with a diameter ranging from 350 nm to 450 nm are scattered on the electrode surface (Fig. 7(a)). For the CMC-Li based electrode, it is interesting to find that the toroidal particles become smaller with a diameter of 150–300 nm (Fig. 7(b)). This may be attributed to a faster Li⁺ transportation in the CMC-Li based electrode than that in the CMC-Na based electrode [38], which facilitates the nucleation of discharge products. In addition, the smaller Li₂O₂ particles are easier to be decomposed during the recharging process, which reveals a good cycling performance based on the new generation binders [39,40].

4 Conclusions

In summary, the Li⁺-modified binder (CMC-Li) has a positive effect on the electrochemical performance of Li-O₂ batteries. Batteries based on CMC-Li binder display excellent discharge specific capacities, reaching up to 11151 mA‧h/g, 9189 mA‧h/g, and 6282 mA‧h/g at current densities of 100 mA/g, 200 mA/g, and 500 mA/g, respectively. Meanwhile, the CMC-Li binder-based cell could operate almost 100 cycles, suggesting a good cycling performance. Compared to CMC-Na, the CMC-Li binder leads to an enhanced electrochemical stability and a faster Li⁺ transportation at the electrode/electrolyte interface, thereby promoting the rate and cycling performance.

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