A safe and sustainable bacterial cellulose nanofiber separator for lithium rechargeable batteries

Abstract

Bacterial cellulose nanofiber (BCNF) with high thermal stability produced by an ecofriendly process has emerged as a promising solution to realize safe and sustainable materials in the large-scale battery. However, an understanding of the actual thermal behavior of the BCNF in the full-cell battery has been lacking, and the yield is still limited for commercialization. Here, we report the entire process of BCNF production and battery manufacture. We systematically constructed a strain with the highest yield (31.5%) by increasing metabolic flux and improved safety by introducing a Lewis base to overcome thermochemical degradation in the battery. This report will open ways of exploiting the BCNF as a "single-layer" separator, a good alternative to the existing chemical-derived one, and thus can greatly contribute to solving the environmental and safety issues.

Keywords: bacterial cellulose; cellulose separator; gene engineering; lithium batteries.

Fig. 1.

Overall process for cellulose nanofiber (CNF) production and battery manufacture, and the metabolic engineering of a CNF-producing strain. (A) Overall scheme for the construction of a bacterial CNF membrane for battery separators. (B) Schematic representation of the CNF production pathway in a metabolically engineered strain (S.Koma-pfkA/crp). The blue lines represent the CNF biosynthesis pathway from glucose 6-phosphate. The bold red arrows indicate the heterologous expression of pfkA and crp gene under the control of the tac promoter. Details for the construction of plasmids, abbreviations of genes and metabolites, and the primers used for gene modification are available in SI Appendix, Tables S1 and S2. (C) Intracellular adenosine triphosphate (ATP), adenosine diphosphate (ADP), and adenosine monophosphate (AMP) ratio in wild-type Koma and S.Koma-pfkA strains. The error bar represents the SD of 3 measurements. (D) CNF production and (E) yield of Koma, S.Koma-pfkA, and S.Koma-pfkA/crp, respectively. The yield of the by-product, gluconic acid (GA), is also shown in E.

Fig. 2.

CFD for optimizing the fermentation process. (A) Three-dimensional fluid models for turbulent energy and sheer stress using various impeller types: RT (Rushton type), P45b3 (pitch type, 45°, blade #3), P60b3 (pitch type, 60°, blade #3), P60b2 (pitch type, 60°, blade #2) at 10 cP, 250 rpm. k_La (volumetric oxygen transfer coefficient) = ln(∆DO)/(∆t) (DO, dissolved oxygen) at 40 cP, 250 rpm. The pitch-type impellers showed much higher turbulent energy with higher shear stress than those in the Rushton-type impeller. The P60b2 impeller showed highest TE/SS and k_La. (B) Dissolved oxygen vs. time; increasing the dissolved oxygen by optimizing the impeller configuration. The viscosity gradually increases over time, and in the case of RT, DO decreased rapidly after 6 h. The pitch-type impeller with 2 blades (P60b2) exhibited the highest dissolved oxygen amount as a function of time. (C) Plot demonstrating the improved CNF productivity and yield of the optimized strain, S.Koma-pfkA/crp (opt). The optimized S.Koma-pfkA/crp showed much higher CNF productivity (0.29 gCNF/L/h), which is 1.8 times higher, and CNF yield (31.5%), which is 3.3 times higher than wild-type Koma strain.

Fig. 3.

BCNF membrane and battery performances. (A) Side view (Left) and top view (Right) scanning electron microscopy (SEM) of the BCNF membrane. (B) Transmission electron microscopy (TEM) and distribution of the diameter of BCNF produced with 1% carboxymethylcellulose (CMC). (C) Large-area BCNF membrane and manufacture of a cylindrical LIB full cell via the roll-to-roll process. (D) Thermomechanical analysis (TMA) of the ceramic-coated separator (CCS) and BCNF separator. The BCNF exhibited much higher thermal tolerance up to 338 °C compared than that of 165 °C of the CCS. The Inset shows the thermal resistivity in a fixed frame at 160 °C (CCS) and 300 °C (BCNF), respectively. (E) Life cycle performances of CCS and BCNF in 18650 battery full cells. The Inset shows the relative direct current–internal resistance (DC-IR) increase ratios after every 100 cycles at 25 °C.

Fig. 4.

Thermal safety performances and characterization of BCNF. (A) Thermal safety performance of BCNF and polypropylene/polyethylene/polypropylene (PP/PE/PP) (purchased from Celgard). (B) Photographs showing the carbonization of BCNF at 170 °C. (C) BCNF with the electrolyte (EL) (without LiPF₆) at 170 °C. (D) BCNF with the EL (including LiPF₆) at 170 °C. The BCNF is carbonized only in the presence of LiPF₆. (E) Thermal safety performance of BCNF, enhanced thermal safety of PEG/BCNF, and ceramic-coated separator (CCS) (purchased from Toray) in the hot-box test. (F) Fourier-transform infrared (FTIR) spectra of BCNF and PEG/BCNF, heat-treated at 170 °C. The C–O, P–O–C, and PF₆ peaks in FTIR on the carbonized BCNF decreased on the PEG/BCNF. (G) Evolved gas analysis (EGA) of the EL, BCNF + EL, and PEG/BCNF + EL, respectively. The total amount of evolved gas was drastically reduced on the PEG/BCNF. (H) Solid-state ¹³C CP-MAS NMR spectra of heat-treated BCNF, BCNF + EL, and PEG/BCNF + EL at 170 °C. The BCNF showed typical peaks of cellulose, viz., distinguishable chemical shifts corresponding to the 6 carbon atoms in cellulose, according to their different bonding. However, these characteristic NMR signals decreased sharply and peaks related to C–H appeared simultaneously, when BCNF was heat-treated with an electrolyte containing LiPF₆, implying that it transformed to aliphatic chain forms owing to structural degradation. In contrast, the PEG/BCNF shows NMR peaks in keeping with the structural characteristics of cellulose.