Corresponding Author

Yan-Hui XU


In the present work the rate-limited step during Li intercalation/deintercalation processes for olivine-type phosphate has been discussed. It is found that the increase in the carbon content is effective to improve the electrode capacity and the electrochemical activity of the Mn element in the active materials. It is also found that the asymmetric phenomenon exists in the charge-discharging curves, especially when the carbon content is low. During charging the Fe2+/Fe3+ the plateau capacity is much less than the corresponding theoretical capacity, while the capacity that corresponds to Co2+/Co3+ couple is more than the theoretical capacity. The discharge capacity of Co2+/Co3+ couple is far less than the charged capacity, while the discharge capacity of Fe2+/Fe3+ couple is far larger than the corresponding charged capacity. The electrode is being charged with Co2+/Co3+, but discharged with Fe2+/Fe3+ couple. The internal interface is assumed to be presented during the charging and discharging. Accordingly, the compositions inside the internal interface are Fe2+, Co2+, Mn2 and Fe3+ of low concentration during the charging, while those outside the internal interface are Co3+, Mn2+, Fe3+ and Co2+ of low concentration. It should be concluded that the asymmetric phenomenon is attributed to the slow moving rate of the internal interface. The movement of the internal interface is proposed to be the rate-limited step of electrochemical lithium intercalation/deintercalation reaction.

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