Oxidation-reduction reaction of cathode materials
For the cathode material LiCoO2, during the charging process, the lithium ions in LiCoO2 are extracted, electrons are transmitted through an external circuit, and at the same time, Co3+ is oxidized to Co4+. When the battery is discharged, Co4+ in Li1-x and CoO2 is reduced to Co3+ by electrons input from the external circuit. At the same time, lithium ions are embedded in the lattice. Therefore, during the spontaneous discharge, the electrode contains part of the reducible Li1-xCoO2 material.
Figure 1 shows the redox reaction of a lithium secondary battery with LiCoO2 as the positive electrode and carbon as the negative electrode. During the charging process, the lithium ions extracted from LiCoO2 and the released electrons respectively pass through the electrolyte and the external circuit, and recombine at the negative electrode. These non-spontaneous reactions (non-spontaneous oxidation reactions for the positive electrode) originate from the electrical energy provided by the external circuit. The electrons and lithium ions produced by the oxidation reaction are respectively transported to the negative electrode through the external circuit and the electrolyte, and the electrical energy is converted into chemical energy for storage through the non-spontaneous reduction reaction of the negative electrode material.
On the other hand, the discharge process is a spontaneous reaction. Due to the potential difference (or electromotive force) between the two charging electrodes, the negative electrode material undergoes an oxidation reaction, and the generated electrons flow through the external circuit to perform work on external equipment and cause the positive electrode material to undergo a reduction reaction. At the same time, lithium ions extracted from the negative electrode material are inserted into the positive electrode material through the electrolyte. As shown by the arrow in Figure 2, the potential of the positive electrode material drops, and the potential of the negative electrode material rises, making the battery voltage drop from 4.2V when fully charged to 3V.
Discharge voltage curve
Since the voltage value of the battery changes with the state of charge and discharge, the voltage value at the midpoint of the discharge is generally regarded as the nominal voltage. The battery voltage is affected by the arrangement of electrons and orbital energy when lithium ions are inserted and extracted from the character of the positive and negative materials. In other words, the voltage when the battery is discharged depends on the occupancy of lithium ions, and the occupancy of lithium ions is related to the Fermi energy level change of the electrode active material and the interaction of lithium ions, which can be expressed by the Ammand equation .
During the discharge, the voltage gradually drops (see Figure3)
. The gradient of the discharge curve change is affected by the diffusion rate of lithium ions on the surface of the positive electrode material, the phase change of the electrode active material, the destruction of the crystal structure and the migration of transition metal ions into the electrolyte. Under the same rate-limiting step, the discharge curve will be affected by factors such as the particle size and distribution of the cathode material, temperature, cathode material/conductive agent/binder mixture, electrolyte characteristics, and diaphragm pore structure.
The battery voltage can also be explained as the chemical potential difference of lithium ions in the positive and negative materials. The chemical potential (μ) of lithium ions can be defined as the partial differential of the free energy of the electrode material to the concentration of lithium ions. The equilibrium potential or open circuit voltage (OCV) can be obtained by the following equation. Among them, LixMX is a compound of lithium; z is the oxidation number of lithium ions in the electrolyte; e is the charge.
V(x) =-(μLicathode(x)－μLianode)/ze (1)
In the above equation, V(x) is the equilibrium potential; x is the lithium content; μ is the chemical potential of lithium in the electrode. Generally, the equilibrium potential of the battery can be derived as follows. Assuming that the negative electrode is lithium metal, the change in the lithium content of the positive electrode material from x1 to x2 can be expressed by the following formula:
Lix1MX (cathode) + (x2－x1) Li (anode) → Lix2MX (2)
Let △G be the Gibbs free energy, the balance potential (V) of the battery is
V=-△G/(x2-x1)ze (3)
The discharge voltage is directly affected by various factors that determine the chemical potential of lithium ions. For example, the change in the chemical potential of lithium caused by the staggered arrangement of lithium ions in the crystal lattice will affect the discharge voltage. In other words, the relative redox energy varies with the potential energy, and the potential energy is determined by the position of lithium in the electrode active material lattice. Therefore, the two lithium ions in different positions in the Li2Mn2O4/LiMn2O4 spinel structure have different potentials. The same Co ion has a potential of 3.6~3.7 V in layered LiCoO2, and a higher potential of 4.5 V in LiMnCoO4 with a spinel structure.
The distance between lithium ions in the crystal structure also changes the discharge voltage by affecting the chemical potential. For example, the repulsive force between lithium ions in the layered structure of Li2NiO2 and the Immm structure of Li2NiO2 is different, so they have different discharge voltages. Another factor that affects the discharge voltage is the redox potential of the transition metal element, which is produced by the interaction of the electrons on the p orbital of the oxygen element in the positive electrode active material and the electrons on the d orbital of the transition metal element. The electrons on the p orbital of the oxygen element make it appear -2 valence, and its energy is basically the same. However, the energies of the d orbital electrons of different transition metal elements are very different. It can be seen from Figure 4 that the potential of transition metal elements in the same period increases as the number of electrons on the d orbital increases. For elements of the same family, as the number of cycles increases, the binding energy of electrons decreases. The positive electrode material with the outermost shell layer of 3d has a higher potential than the material with the outermost shell layer of 4d.
For LiCoO2 and LiNiO2, although Ni has more d electrons than Co, LiCoO2 has a higher voltage. This is because the energy levels of Co3+ and Ni3+ are partially reversed. All the 6 electrons of Co3+ in LiCoO2 are in the low-spin t2g orbital, while the 7 electrons of Ni3+ in LiNiO2 are split into 6 t2g electrons and 1 eg electron. Since electrons in a high-energy state are easier to release, the potential of N3 is reduced.
The energy gap caused by the induction effect will also affect the discharge voltage. For example, in the Fe-O-P bond of LiFePO4, the PO42-strong P-0 covalent bond attracts the d electrons of iron ions and hinders the oxidation of iron ions. Therefore, LiFePO4 has a higher voltage than LiFePO2.
Unbalanced polarization is another important factor that affects battery voltage. In particular, when the lithium ion or electron transfer resistance of the positive electrode material is large, it is difficult to establish an electrochemical equilibrium, and the voltage will decrease as the iR drops. In order to overcome the iR drop, the resistance of potassium ions or electron migration of the cathode material must be minimized.
