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.