Graphite can be used as a good cathode because of its high conductivity and large surface. However, the large SEI layer that forms on the graphite surface limits its reversible capacity and results in unreliable battery performance. Recent studies found that the subsurface structure of graphite surfaces, known as gilding or blister bumps (GBs), could improve battery performance by providing stable Ion Channels and preventing SEI. This has opened new opportunities for GB research.
There is still a lack of understanding about the origins of blisters that appear on the surface graphite, however it is thought they may be due to chemical intercalation and mechanical impact. Recently, Sinitsyna et al. The blisters that are caused on graphite's surface by SO4-2 ions follow a similar trajectory to that of the grain boundaries. They suggest that they form from defects that already exist in graphite's atomic or molecular structure. A atomic force microscopy was also used to observe the blisters, which indicated that they were not simply topographic flaws.
The surface energy and graphite bonding of an electrode determine the blister sensitivity and the stability. The surface energy of the electrode determines how much blisters are dispersed during the deintercalation procedure. Lower surface energies, on the other hand, result in denser and more rigid blisters. Reversibility is dependent also on graphite-electrolyte interaction, and this depends in part on the ion channel sizes.
In addition, the blisters are prone to deformation under high voltage conditions. This is mostly due to blisters dissolving or adsorbing ions by the ion canals in the GBS. The change in electrolyte or temperature can trigger blister deformation.
The modification of graphite has taken many forms, including sonication or covalent functionalization. The use of polymers to modify graphite can help reduce SEI production and oxidative degradation. However, it is important to select the appropriate polymer for the intended application.
For example, a graphite electrode modified with polypyrrole/anthraquinonedisulphonate (PPy/AQDS) showed improved sensitivity and detection limit for determination of ST, DA, and AA in an organic solution. The electrode's sensitivity was increased by over 10 times and its peak separation improved. The electrode also demonstrated a superior electrocatalytic behavior for ORR in the presence of boron hydride.
The authors of a paper on the effects of ball milling found that graphite milled for 5 hours had a higher irreversible initial charge capacity (120 mAh/g) and a lower reversible discharging capacity (2 h). The authors explained that the difference in reversible and irreversible capacities can be attributed to two lithiation/de-lithiation mechanisms, one of which is surface storage. The other is intercalation/de-intercalation based on the size of graphene blisters, and this mechanism becomes dominant at high current rates.
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