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LiFePO4 charging and discharging mechanism

by:dcfpower     2021-03-24
u003cpu003e According to the research of Padhi et al.: LiFePO4 charge-discharge reaction is carried out between LiFePO4 and FePO4. When charging, Li+ in LiFePO4 is deintercalated and converted into FePO4; when discharging, Li+ is embedded in FePO4 to form LiFePO4. The chargeu003c/pu003eu003cpu003edischarge reaction is as follows:u003c/pu003eu003cpu003e Charge reaction: LiFePO4-x Li+-x e- → x FePO4+(1-x) LiFePO4u003c/pu003eu003cpu003e Discharge reaction: FePO4 + x Li+ + x e- → x LiFePO4 + (1-x) FePO4u003c/pu003eu003cpu003e During the charging and discharging process of LiFePO4, the presence of P043- in the crystal limits the range of Li+ movement space, which makes Li+ release Embedding can only be a two-dimensional movement process. Therefore, the electronic conductivity and ion diffusivity of LiFePO4 are relatively low, and the capacity loss is relatively large during high current charge and discharge. In response to this phenomenon, researchers put forward different hypotheses to explain. Padhi et al. believed that the Li+ transfer rate per unit interface product during LiFePO4 charging and discharging is fixed, and the interface moves from the outside of the particles to the inside, and the area is shrinking. The discharge is terminated when the sum of all interface areas is not enough to support the discharge current. The greater the discharge current, the smaller the concentration of Li+ that can be embedded in the LiFePO4 particles, and the more serious the capacity loss of LiFePO4. Andersson et al. think it is related to the de-embedding mechanism of Li+. Two possible Li+ de-embedding models are given: 1) The radius model, the active material in the center of the large particles does not participate in the reaction, which causes the capacity loss. 2) Mosaic model, in the process of charging and discharging, many inactive areas are formed dispersed in the particles, and the amorphous film formed on the surface of these areas prevents them from participating in the reaction in the subsequent cycles, thereby causing capacity loss. Wang et al. believed that LiFePO4 stretched along the b-axis during the Li+ de-intercalation process, and eventually the excessive tension caused the crystal structure to deform and cracks. The occurrence of cracks leads to increased polarization, and is not conducive to electronic contact between particles and conductive agents or current collectors, resulting in capacity loss. u003c/pu003eu003c/pu003e
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