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Liang et al. Energy Mater 2023;3:300006 https://dx.doi.org/10.20517/energymater.2022.63 Page 9 of 14

The conductivity of the separator is composed of the migration rates of cations and anions. Relatively
speaking, the migration rate of lithium ions is more important. Hence, it is necessary to increase the
mobility of lithium ions while increasing the ionic conductivity of the separator. The characterization of
lithium-ion migration number was achieved by applying a 10 mV initial voltage and dc polarization for
measuring Li | separator | Li battery, and the separator was soaked with electrolyte before testing. The
calculation for the lithium-ion migration number can be expressed as follows [Equation 2]:

where I is the steady-state current, I is the initial current, and T is the ion migration number. The T of
Li+
Li+
s
o
the GNF separator is higher than that of the PP separator. The T of the GNF separator is 0.79 [Figure 5B].
Li+
-
The amino acids with negative charges were used as coordination sites for attracting anions (TFSI ). The
main chain oxygen in the gelatin polypeptide chain also has a strong affinity towards lithium ions. The
result is that lithium ions can jump quickly between oxygen atoms. In consequence, the gelatin-based
separator has higher lithium-ion transport efficiency than the ordinary PP separator, which can effectively
avoid uniform lithium flux.
-2
-2
At the higher current density of 0.5 mA cm and an areal capacity of 0.5 mAh cm , the lithium ions cannot
be deposited on the surface of the lithium metal thanks to the low ionic conductivity of the PP separator
[Figure 6A]. The overpotential of the symmetric battery with PP separator increased rapidly after 400 h of
cycling. In contrast, the symmetrical batteries with the GNF separator and CZGNF separator had good
cycling stability. The polarization of symmetric batteries with CZGNF separator is lower than that of
symmetric batteries with PP separator for cycling beyond 400 h. The polarization voltage of symmetrical
batteries with PP separator and GNF separator is about 85 mV with cycling for 30-34 h. The polarization
voltage of the symmetric battery with PP separator and GNF separator is larger than that of the symmetric
battery with CZGNF separator [Figure 6B]. The reason is that the pore size of the GNF separator is too large
and not fully activated in the previous cycles. As the cycling progresses, however, the overpotential of the
symmetrical battery with GNF separator gradually decreases and tends to be stable. The polarization voltage
of symmetric batteries with GNF or CZGNF separators is about 29 mV over 500-504 h of cycling
[Figure 6C]. The polarization voltage of the symmetric batteries with the GNF separator and the CZGNF
separator is lower than that of the symmetric battery with PP separator. The impedance plots of Li||Li
symmetric batteries with PP, GNF and CZGNF separators after cycling for 100 h are also provided in
Supplementary Figure 2. In conclusion, gelatin rapidly and steadily deposits lithium ions during the battery
cycle and contributes to lithium nucleation.

Figure 7 displays the electrochemical performances of batteries assembled with different separators. The Li-
S batteries with CZGNF separator have the highest initial discharge specific capacity of 1358 mAh g . The
-1
initial discharge capacity of Li-S battery with GNF separator is 1334 mAh g . The initial discharge capacity
-1
of Li-S battery with PP separator is the lowest [Figure 7A]. In short, the Li-S batteries with CZGNF
separator had the highest utilization rate of sulfur active materials. The utilization rate of sulfur active
material was 81.1%. Therefore, the use of gelatin as the separator substrate can effectively reduce the loss of
sulfur in the active material. The conversion of soluble lithium polysulfide lithium was promoted by C and
60
ZIF-67 to form insoluble Li S /Li S. Figure 7B shows the second plateau of the discharge curves of Li-S
2
2 2
batteries with three separators. The overpotential was 20 mV for the GNF separator, 18 mV for the PP