When an external load is applied to the battery to discharge, due to the electrochemical potential difference between the electrodes, Li﹢ diffuses from the negative electrode to the positive electrode, and electrons flow from the negative electrode to the positive electrode through the external load. During this process, the battery voltage and electrode potential change with As the battery discharges, when Li﹢ is inserted into the positive electrode material, the electrochemical potential of the positive electrode relative to Li/Li﹢ decreases, and the electrochemical potential of the negative electrode increases with the release of Li﹢, resulting in a continuous decrease in the battery voltage until the end When the voltage (the positive LiCoO2 material reaches 3.0V vs. C), the discharge is stopped.
In summary, the charging and discharging process of lithium-ion batteries is realized by the shuttle of lithium ions between the positive and negative electrodes, so lithium-ion batteries are also called "rocking chair batteries." Taking LiCoO2 as the positive electrode material and graphite as the negative electrode material for a lithium-ion battery system as an example, the reaction of battery materials during charge and discharge is illustrated. When charging, Li﹢ is released from the LiCoO2 material and releases electrons. Co3﹢ is oxidized to Co4﹢ , and graphite also gets electrons when Li﹢ is inserted between its layers. Therefore, during the charging process, the positive electrode is in a lithium-poor state, and the negative electrode graphite is in a lithium-rich state. Correspondingly, during discharge, Li﹢ is released from the graphite layer and embedded in the positive electrode material, while the positive electrode material obtains electrons, Co﹢ is reduced to Co3﹢, the positive electrode becomes a lithium-rich state, and the graphite negative electrode returns to a lithium-poor state. The reaction equations of the positive and negative electrodes in the charge and discharge process are as follows;
Positive reaction: LiCoO2 ⇄ Li1-xCoO2 ＋ xLi﹢ ＋ xe
Negative reaction: Cn + xLi﹢ + xe ⇄ LixCn
Total battery reaction: LiCoO2 ＋ Cn ⇄ Li1-xCoO2 ＋ LixCn
In the lithium ion battery system, the transfer of lithium ions between the positive and negative electrodes is essentially a concentration difference diffusion. In theory, the insertion and extraction of Li﹢ will not cause damage to the crystal structure of the electrode material. In this sense From the above point of view, the lithium-ion battery reaction can be regarded as an ideal reversible reaction.