posted on 2022-05-06, 21:10authored byYilong Lin, Sheng Huang, Min Xiao, Dongmei Han, Zhiheng Huang, Shuanjin Wang, Yuezhong Meng
The
insufficient activation of a S/C cathode makes insufficient
utilization of S in Li–S pouch cells, while the deep activation
of a S/C cathode in a formation process is time-consuming and produces
lithium polysulfides, which corrode a Li anode. Both situations lead
to a low actual capacity of the Li–S pouch cells with a high
S loading but are ignored for coin cells. In this work, electrochemical
oscillation (EOS) formation employing hundreds of shallow discharge/charge
cycles with high frequency was used to replace the resting and/or
one deep discharge/charge cycle of traditional (TD) formation protocols.
By controlling the discharge/charge capacity separately, symmetric
oscillation (SOS) and asymmetric oscillation (ASOS) protocols were
performed to facilitate the infiltration of electrolyte into the S
cathode and restrict the formed lithium polysulfide in the cathode
region. For SOS formation, the batteries were discharged/charged above
2.4 V with the same (symmetric) capacity with 2.78 × 10–3 Hz of oscillation frequency (∼1.4 mAh/g for SOS-500), in
which the polysulfide dissolution was suppressed effectively. For
ASOS formation, 100% discharge capacity (also ∼1.4 mAh/g for
ASOS-500) and 92% charge capacity are set in each oscillation period,
which leads to better activation effect but more shuttling polysulfides
than SOS. Compared with SOS protocol, for ASOS protocol, more oxidative
S (instead of polysulfides) inside original nonactivated cathode will
be preferentially reduced in the next discharging process, but all
the accumulated polysulfides during discharge of activation are oxidized
into elemental S in the final charging process. These efficient formation
protocols increase the practical capacity by up to 160% after 50 cycles
without any change in pouch cell assembly.