The research team of Professor Qiu Yongcai of South China University of Technology cooperated with Zhejiang University, Tsinghua University, South China Normal University and other research teams to break through the ceiling of energy density of lithium-ion batteries and overcome the problem of excessive volume fluctuation of silicon anode materials during charging and discharging. The relevant research results are framework by "High energy density lithium metal batteries enabled by a porous graphene/MgF2"(https://doi.org/10.1016/j.ensm. 2019.12.028) and "Thermal pyrolysis of Si @ ZIF-67 into Si @ N-doped CNTs towards highly stable lithium storage"(https://doi.org/10.1016/j.scib. 2019.12.005) was published in the authoritative journals Energy Storage Materials and Science Bulletin.

1. Breakthrough Energy Density Ceiling

Lithium metal anode is considered to be a good anode material and has been widely studied due to its high specific capacity (3860mAh · g-1), low negative electrochemical potential (-3.04V) and lighter density. Professor Qiu Yongcai's team from South China University of Technology, together with Zhejiang University, Tsinghua University and other units, reported in the Energy Storage Materials that a three-dimensional composite porous lithium metal anode is used to solve the problem of volume expansion of lithium metal anode during cycling and the problem of uncontrolled growth of lithium dendrites. The three-dimensional composite MgxLiy/LiF-Li-rGO lithium metal anode is not only simple and scalable, but also can meet the requirements of high energy density and long cycle of the actual battery. The soft-pack full battery assembled from the three-dimensional composite MgxLiy/LiF-Li-rGO anode and the high nickel (NCM811) cathode can provide an energy density of more than 350wh kg-1, and still has an energy retention rate of 85% after 150 cycles.

Figure: In-situ TEM Characterization and Verification of Nucleation Overpotential of MgF2 Electrode

PicoFemto produced by Zeyu Technology were used in this study®A series of in-situ sample rods were used to study the lithiation process of MgF2-rGO in-situ in transmission electron microscopy. A gold wire with a MgF2-rGO film and a tungsten wire plated with Li were used as both end electrodes, respectively, and a thin oxide layer on the surface of the lithium electrode was used as a solid electrolyte in the battery. When the lithium metal electrode is transferred to a TEM, the electrode is exposed to air for about 5 seconds to form a layer of Li2O about 20nm thick, which functions as a solid electrolyte. A relative bias of -3.4v was generated between the two electrodes, resulting in the transfer of Li + ions through the solid electrolyte to the MgF2-rGO electrode. During the experiment, the electron beam was widened to reduce the electron beam effect.

Figure: In-situ experimental flow

This study successfully designed a 3D-MgxLiy/LiF-Li-rGO composite anode, which showed excellent battery performance in both symmetrical and full batteries. This stems from the mechanically and chemically strong 3D porous structure and the lithophilic MgxLiy sites in the micropores. The graphene scaffold has a large specific surface area and rich microporous structure, which not only effectively reduces the current density, but also provides a free space for Li deposition to suppress the change of electrode size. The Lithiophilic properties of the MgxLiy functionalization provide a uniform distribution of active centers for uniform Li deposition, which is also beneficial to inhibit the uncontrolled growth of lithium dendrites during cycling. Therefore, the MgxLiy/LiF-Li-rGO anode with a reasonable combination of porous structure and lithium-affinity active center can significantly improve the electrochemical performance of the composite anode and can be applied to high-energy batteries.

2. Improved silicon-based anode material

Silicon, as an active anode material for advanced lithium-ion batteries, has attracted much attention because of its high theoretical capacity. However, the rapid capacity fading of silicon-based anode materials still hinders the reversible utilization of silicon-based anode materials due to the huge volume changes of silicon during cycling. Professor Qiu Yongcai's team from South China University of Technology, together with South China Normal University and other units, reported in Science Bulletin that a silicon-wrapped nitrogen-doped carbon nanotube (Si @ N-doped CNTs) was prepared by controlled thermal cracking using a co-zeolite imidazole framework (ZIF-67). The prepared nano-composite material can effectively prevent silicon powder and adapt to the volume fluctuation of silicon during the cycle.

Figure: In-situ Observation of Charge and Discharge Process of Si @ N-doped CNTs Anode

The PicoFemto produced by Zeyu Technology was used in this study.®The charging and discharging process of Si @ N-doped CNTs anode was observed in situ in transmission electron microscope. Specifically, a gold wire containing Si @ N doped carbon nanotubes and a tungsten wire containing Li are used as two end electrodes respectively, and the two end electrodes are loaded into the PicoFemto in a glove box filled with argon environment.®TEM-SEM on the in-situ sample bar. During the transfer process, the lithium counter electrode was deliberately exposed to air for about 5s, forming a thin Li2O layer as a solid electrolyte. During the experiment, a relatively constant bias of -3.4v was applied between the two electrodes to transfer Li + ions from the Si @ N-doped CNTs electrode to the Li electrode through the solid electrolyte.

Figure: In-situ experimental flow

The remarkable electrochemical performance of Si -N-doped CNTs was confirmed by in-situ transmission electron microscopy experiments due to the high electronic conductivity and flexibility of the N-doped CNT network that can act as a buffer to mitigate mechanical stress and volume expansion. In addition, a full cell consisting of a Si @ N-doped CNTs anode and a LiFePO4 cathode was also assembled, which can provide a high reversible capacity of 1264 mahg-1 in 140 cycles at a rate of 1/4c(1c = 4000ma g-1). And good cycle stability (>85% capacity retention).