4.8 Article

Excavating Anomalous Capacity Increase of Li-S Pouch Cells by Electrochemical Oscillation Formation

Journal

ACS APPLIED MATERIALS & INTERFACES
Volume 14, Issue 19, Pages 22197-22205

Publisher

AMER CHEMICAL SOC
DOI: 10.1021/acsami.2c04284

Keywords

Li-S pouch cells; oscillation formation; cathode wettability; anode stability; capacity increase

Funding

  1. National Key Research and Development Program [2019YFA0705701]
  2. National Natural Science Foundation of China [22179149, 22075329, 51573215, 21978332]
  3. Guangdong Basic and Applied Basic Research Foundation [2021A0505030022, 2019A1515010803, 2020A1515011445]
  4. Guangzhou Scientific and Technological Planning Project [201904010271]
  5. Fundamental Research Funds for the Central Universities [19lgpy07, 20lgpy11]

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Insufficient activation of S/C cathode leads to low capacity of Li-S pouch cells, but electrochemical oscillation formation can increase capacity and restrict the formation of lithium polysulfides.
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 x 10(-3) Hz of oscillation frequency (similar to 1.4 mAh/g for SOS-500), in which the polysulfide dissolution was suppressed effectively. For ASOS formation, 100% discharge capacity (also similar to 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.

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