INTRODUCTION
Rechargeable batteries play a key role in liberating human production and life from dependence on fossil fuels and reducing greenhouse gas emissions.[1-3] Among them, lithium-ion batteries (LIBs) are generally composed of metal-containing active materials (Li, Co, Ni, etc.), flammable organic electrolytes and non-degradable polymer membranes (polyolefin).[4-6] However, the growing demand for LIBs is pushing up the price of raw materials and the existing battery materials cannot satisfy the energy demand. Accordingly, it is urgent to develop renewable, sustainable and green battery materials to realize sustainable development.[7-9] In this regard, natural biomaterials are increasingly attracting interest in green batteries due to the numerous advantages of biopolymers such as broad-resource, low-cost and environmental protection.[10, 11]
In the composition of LIBs, separator directly determines the electrochemical performance and the safety of the battery.[12-15] Although it does not directly participate in the electrochemical reaction of LIBs, its physical properties and structure play a key role in LIBs. Currently, widely used separators mainly include microporous polyolefin membranes formed by insulating polymer materials. However, due to its characteristics of large polarity difference with electrolyte and low thermal melting temperature, PP separators have some shortcomings such as poor infiltrative properties and poor thermal stability, which leads to a large safety hazard of LIBs.[16, 17] Therefore, it is urgent to develop next-generation separators to replace the existing polyolefin separator.
Cellulose and its derivatives are good alternatives to polyolefin-based materials because they are abundant in reserves, renewable, easy to access and low cost.[18-20] Till now, many researchers have reported novel cellulose-based membranes. For example, Du et al.[21] prepared a cellulose based gel membrane using epichlorohydrin (ECH) as a crosslinking agent. The ionic conductivity of the activated cellulose membrane with 5% ECH achieves 6.34 mS cm-1. What’s more, the assembled NCM523/Li battery can reach 145 mAh g-1 at 0.2 C, and the capacity retention rate is as high as 90% after 50 cycles. Liu et al.[19] prepared lignin-containing cellulose nanofibers (LCNFs) membranes from the unbleached pulp by a facile method. The LCNF porous membrane shows a high electrolyte uptake of 276% and high ionic conductivity of 1.86 mS cm-1. With the higher requirements for battery safety, the composite membrane combining ceramic particles and polymer has attracted more attention from researchers. Thereby, nano-inorganic particles such as Al2O3, SiO2 and TiO2 can significantly improve electrolyte wettability, thermal stability and ionic conductivity of polymer.[22-25] Recently, Cui et al.[26] prepared a polyvinylidene fluoride (PVDF)/cellulose acetate (CA) based membrane by using non-solvent induced phase separation (NIPS) method. Meanwhile, Al(OH)3 particles were used as added doping agents. The PVDF-CA/Al(OH)3 separator has high porosity (68.6%), high electrolyte absorption rate (403.9%), excellent dimensional stability (4.6% thermal shrinkage at 160°C) and high ionic conductivity (2.85 mS cm-1), endowing LIBs with good safety performance and cycling performance. However, batteries still suffer from a lack of cycle durability because LiPF6 in liquid electrolytes can form hydrofluoric acid (HF) to damage the electrolyte/elctrodes interfaces.[27, 28]
Herein, nano-CaCO3 is introduced into cellulose-based electrospinning membranes to reduce the crystallinity of polymer membrane, promote ion migration and neutralize acidic products. The morphology, thermal stability, mechanical properties and electrochemical properties of modified membranes are analyzed and explored in depth. Such inorganic particles with extremely high hydrophilicity and excellent surface properties can significantly improve the ionic conductivity of membrane. In addition, the rigidity and heat resistance of CaCO3 guarantee the mechanical strength and thermal stability of membrane. Consequently, the composite membrane is superior to polypropylene (PP) separator in electrolyte wettability and thermal stability, leading to the outstanding performance of LiFePO4/Li cells. We believe that cellulose/CaCO3 composite membrane with the above advantages is a promising candidate for practical applications in advanced LIBs.