In order to measure the ionic conductivities of batteries assembled with cellulose membranes and investigate the effect of nano-CaCO3 additions on electrochemical performances, the EIS of cellulose membranes before and after modification were tested at room temperature. The results are shown in Figure 5a . The PP separator has an ionic conductivity of 0.95 mS cm-1, while the cellulose membrane decreases to 0.91 mS cm-1. With the addition of CaCO3, the ionic conductivities of membranes increases. Specifically, they can reach 1.05 mS cm-1 and 1.08 mS cm-1at a value of 0.5 wt% and 1 wt% addition amount, respectively. This indicates that all three cellulose membranes have decent conductivities. Notably, nano-CaCO3 can effectively increase the conductivities of the cellulose membrane, which may be due to the crystalline structure of CaCO3. In addition, the effect of temperatures on conductivity is further investigated, and the impedances of membranes are tested at 25 °C, 35 °C, 45 °C, 55 °C and 65 °C. The results are shown in Figure 5c-d . it can be easily found that the resistance of membranes tends to decrease with increasing temperature. Specifically, the ionic conductivity of cellulose/CaCO3-1 reaches 1.49 mS cm-1.
The electrochemical window is an important index for membrane used in LIBs, which determines the feasibility of applying it in high-voltage cathode materials. To verify the electrochemical stability of cellulose membranes, the linear sweep voltammetry (LSV) tests of PP and cellulose membranes with stainless steel and Li electrodes were performed. The corresponding curves were shown in Figure 5b . All membranes show stable currents below 4.5 V. Compared with a conventional commercial PP separator, cellulose based membranes display no obvious decomposition until 4.8 V. Besides, with the addition of CaCO3, the oxidation peak currents drop. It is probably due to that CaCO3 can absorb trace water and hydrofluoric acid (HF) in the LiPF6 electrolyte, while these impurities affect oxidation reaction.