Lithium ion batteries (LIBs) have find widespread use as power sources for electric vehicles, grids, and other large-scale energy storage systems. Polyolefin membranes, the most commonly used separators in LIBs, though capable of offering good electrochemical stability, suitable mechanical strength and pore size, suffer from large shrinkages at high temperatures due to the low glass transformation temperature and melting point of the polymer moiety. The incompatibility between the polar organic solvents and nonpolar polyolefin membranes results in poor electrolyte wettability and thus low ionic conductivity. Researchers at Shanghai Institute of Ceramics, Chinese Academy of Sciences, China and School of Materials Science and Engineering, Huazhong University of Science and Technology, China have designed and fabricated a highly flexible and porous separator by self-assembling hydroxyapatite nanowires (HAP NW) with cellulose fibers (CFs). The HAP/CF separator with a hierarchically cross-linked structure exhibits a good combination of high flexibility, robust mechanical strength, highly porous structure, superior electrolyte wettability, excellent thermal stability and fire resistance. Batteries fabricated using the HAP/CF separator exhibit enhanced cyclability and rate capability when bench marked against the commonly used polypropylene separator in LIBs.
HAP NWs were prepared by solvo-thermal method using calcium oleate as the precursor (Fig. 1(a)). The HAP NWs and CFs (Fig. 1(b)) were subjected to a self-assembly (hybridization) process (Fig. 1 (c)) in aqueous solution wherein the branched CFs were wrapped with network-structured HAP NWs through hydrogen bonding and van der Waals force. The self-assembled hierarchically cross-linked hybrid fibers were filtrated under vacuum suction (Fig. 1(d)) and dried. The HAP-CF separator was used to assemble the LiFePO4/separator/Li half cells (Fig. 1(e)).
Fig. 1 Various stages involved in the fabrication of HAP/CF separator
The morphological features of HAP NW, CF and HAP/CF separator (Fig. 2) indicate that the CFs are uniformly embedded in the porous HAP NW networks, leading to the formation of HAP/CF separator with open, continuous, and interconnected nanopores. The porosity of the HAP/CF separator is ~81% with an average pore size of 120.9 nm and a narrow pore size distribution ranging from about 110 to 130 nm, suggesting its suitability as separators for LIBs. The HAP/CF separator can be rolled, twisted, folded, scrunched and unscrunched with no visible damages (Fig. 3), which indicate its good strength and high flexibility. The synergistic combination of the van der Waals force and hydrogen bonding enables the HAP/CF separator to achieve a higher tensile strength of 13.21 MPa. The HAP/CF separator exhibits good mechanical strength even at 200 °C, demonstrating its excellent thermal stability.
Fig. 2 Morphological features of (a) HAP NW networks; (b) CFs; and (c, d) HAP/CF separator
Fig. 3 Flexibility of the HAP/CF separator under different bending conditions: (a) rolled; (b) twisted; (c) folded; and (d) scrunched.
The rapid penetration of the electrolyte droplet within 5 s indicates the high wettability, which enables the HAP/CF separator with an electrolyte uptake of 253%. The ability of HAP/CF separator to maintain 77% of the initial weight even at a high temperature of 900 °C, suggests its excellent thermal stability. When ignited, the HAP/CF separator wetted with electrolyte gets self-extinguished due to the non-flammable nature and strong affinity of HAP NW for the electrolytes. In contrast, the commonly used PP separator wetted with electrolyte gets ignited and continuously combusted (Fig. 4).
Fig. 4 Fire-resistant characteristics of (a, b) PP separator; and (c, d) HAP/CF separator; (a, c) before burning; and (b, d) after burning.
The cycling performance and rate capability of the LiFePO4/separator/Li half cells assembled using the HAP/CF separator and commercial PP separator (Fig. 5) indicate that the initial discharge capacity of the cell with HAP/CF separator (138 mAh/g) is higher than those obtained using the PP separator (130.1 mAh/g) at 0.5 C. Cells with the HAP/CF separator exhibit a higher discharge capacity of 135.4 mAh/g when compared to those obtained with the PP separator (129.5 mAh/g) after 145 cycles at 1 C. Cells with the HAP/CF separator exhibit a capacity retention of 85.7%, which is much higher than those exhibited by the cells with PP separators (65.7%).
Fig. 5 (a) Cycling performance; and (b) rate capability of theLiFePO4/separator/ Li half cells using the HAP/CF and PP separators
The electrochemical performance of the batteries prepared using HAP/CF and commercial PP separators, for the initial 5 cycles at room temperature and subsequent 20 cycles at 150 °C (Fig. 6 (a)) indicates that the battery constructed with the HAP/CF separator exhibits a good cycling performance with a higher discharge capacity of 157.8 mAh/g and an average Coulombic efficiency of >98%. In contrast, the battery constructed with PP separator fails to offer a good performance at 150 °C (Fig. 6(a)). The large shrinkage of the PP separator at 150 °C causes internal short circuit, which is evidenced by the sudden drop in open-circuit voltage (OCV) while the battery with the HAP/CF separator could maintain its initial voltage throughout the whole testing process (Fig. 6(b)). The battery equipped with HAP/CF separator can safely light up two 3.0 V LED lamps at a temperature as high as 150 °C, suggesting the extraordinary thermal stability of the HAP/CF separator and the great potential for its application in high-temperature-related batteries (Figs. 6 (c) and 6(d)).
Fig. 6 (a) Cycling performance of the batteries constructed using HAP/CF and PP separators at 2 C for the initial 5 cycles at room temperature and subsequent 20 cycles at 150 °C; (b) OCV curves of the LiFePO4/separator/Li batteries with the HAP/CF separator and the PP separator at 150 °C; (c, d) battery prepared using the HAP/CF separator working at 150 °C.
The superior electrolyte wettability, mechanical robustness, high thermal stability, and fire resistance of HAP/CF separator appears to be promising for LIBs with enhanced performance and safety.
T.S.N. Sankara Narayanan
For more information, the reader may kindly refer: Heng Li et al., Adv. Mater. 2017, 1703548, DOI: 10.1002/adma.201703548
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