The traditional lithium ion battery material is mainly lithium-containing transition metal oxide, its capacity to play mainly depends on the transition metal elements in the redox process to release the number of electrons to the traditional LiCoO 2Materials, for example, completely delithiated when able to transfer 1mol of electrons, LiCoO 2The molecular weight is 97.8 g / mol, according to equation C 0= 26.8nm / M, LiCoO can be calculated 2The theoretical capacity of the material is 273.8mAh / g, which means that limiting the cathode material capacity to play a key factor in how to provide more electrons.Although the transition metal elements can provide electrons is limited, then the O element can provide part of In fact, the O element in lithium-rich materials is very prone to lose electrons during charging, but the resulting capacity is often irreversible, mainly because the oxidized O atoms eventually change O 2Lost, resulting in irreversible phase transition of lithium-rich material.
It is not hard to see from the above description that letting the O element participate in the reaction of the lithium-rich material can provide an extra 1-2 electrons, thereby doubling the capacity of the lithium-rich material even three times. However, Avoid stability problems 2-Change to O 2, Resulting in capacity loss recently, the United States Argonne National Laboratory Chun Zhan passed Li 5FeO 4The reaction mechanism of the material study found that the charging voltage control of the material at 3.8V, you can achieve O 2-Reversible oxidation, but will not release O 2, And to further improve Li 5FeO 4The stability of the material made recommendations.
In general, Li 5FeO 4Although the theoretical capacity of the material is as high as 700 mAh / g, it is hard to be used as a cathode material because of its poor reversibility, but some people also make full use of Li 5FeO 4The reversible capacity of the material is low, which is used as the cathode lithium supplement material, which remarkably improves the first efficiency of the lithium-ion battery.
Li 5FeO 4Materials as the cathode material must solve the problem of low reversible capacity, which requires understanding of the phase transition mechanism in the charging process.A above is Li 5FeO 4The crystal structure of the material, Figure b is the first charge and discharge curve of the material, we can see that during the first charge will appear at 3.5V and 4.0V near the two voltage platform, and in the discharge process of these two platforms Disappeared, and two very narrow voltage platforms appeared near 2.2V and 1.5V, indicating irreversible phase transition of the material during charging and discharging.The XRD analysis reduced the charge of Li 5FeO 4Material phase change process, about 3.5V Li 5FeO 4Take 2 Li off the material +After, Li 5FeO 4The crystal structure of the material changed from an anti-fluorite structure to a disordered rock salt structure, and the charge was continuously charged to 4.0V, and the Li +And the number of un-ordered rock salt structures continued to increase. When charge was continued, the amount of Li + removed continued to increase, and the structure of disordered rock salt began to disappear. Finally, the XRD diffraction curve was transformed into a smooth curve , All the characteristic peaks also disappeared.
High-resolution TEM images show that Li is not charged 5FeO 4The material is about 1 μm in diameter with good crystallinity, but eventually the large particles are transformed into smaller particles with a diameter of about 10 nm after charging is completed.
Using XANES analysis of the valence of Fe during the reaction process, it can be found that when charging to 3.5V, Li 5FeO 4Material exits two Li +, Fe 3+To Fe (3 + x) + (x is about 0.5), indicating Li 5FeO 4There are other elements in the material to participate in the reaction, otherwise the valence of Fe should be increased at this time 2. In the further charging process, the valence of Fe element does not rise, but instead appears to decrease, which also shows that the other Elemental oxidation reaction occurred (while in Li 5FeO 4The material except for Fe element will only be O element can be oxidized.) Analysis of gas generated during charging also shows that the O element involved in the reaction process, at about 3.5V, a slight increase in pressure, charging to 4.0V The pressure rises rapidly. DEMS data shows that 0.1E1 is released per electron on the 3.5V platform, but at 4.0V each electron can cause 0.3 O 2freed.
After analysis, Chun Zhan thinks Li 5FeO 4The reaction of material exiting four Li + is shown in the following formula
Calculations found when Li 5FeO 4Materials charged to 3.5V, part O 2-Will be oxidized to O. -, One O- and six Li +The formation of Li6-O space structure, further charge of this part of O-will be further oxidized to O 0, Which leads to the irreversible change of the whole reaction process 5FeO 4The reversibility of the material, it must limit the charging voltage.The following figure is the charge and discharge voltage is limited to 1-3.8V cycle between the battery charge and discharge curve (Li 5FeO 4Material only two Li +), you can see almost no gas at this time, but a large amount of gas will be generated when the charging voltage is raised to 4.0 V. When the charging voltage is limited to 3.8V, a relatively stable Cycle performance, but the charge to 4.7V will seriously affect the battery cycle performance.
Li 5FeO 4Material phase change during charging as shown below, when the charging voltage is controlled at 3.8V, part of the charging process Fe 3+And O 2-Reversibly oxidized to Fe 4+And O -When further charging, O -It will be further oxidized to O. 0, Resulting in O 2, Resulting in an irreversible loss of capacity.
Research work by Chun Zhan Let's talk about Li 5FeO 4The working principle of the material has a profound understanding, let us adjust according to different uses Li 5FeO 4The use of materials, for example, as a lithium supplement material, the charging voltage can be increased to 4.0V or more, so that the Li +Fully prolapse and make Li 5FeO 4The material loses its activity and no longer participates in subsequent reactions 5FeO 4When the material is used as a positive electrode material, the charging voltage needs to be controlled at 3.8V to prevent O - from being further reduced to OO, resulting in irreversible capacity loss. 5FeO 4The material indicates the direction - how to stabilize the Li6-O structure in the material to further enhance the material's capacity and cycle performance.
