【Research Background】
High-pressure, high-nickel, low-cobalt-lithium layered oxide ternary cathode (LiNixCoyMn1-x-yO2) is one of the most popular cathode materials for high-energy-density lithium-ion batteries due to its high specific capacity and low cost. However, high voltage means deep redox reactions and more lithium de-intercalation, which poses more serious challenges to the structural and chemical stability of the material. The removal of a large number of lithium ions will cause more lithium vacancies, causing transition metal ions to migrate to the lithium site and form cation mixture, which will reduce the lithium diffusion kinetics of the material and cause local lattice distortion and intragranular microcracks of primary particles. Further, deep lithium deintercalation will also cause a greater degree of expansion and contraction of the layered structure, and will produce greater anisotropic internal stress inside the secondary particles, which will form intergranular microcracks, which will cause particle crushing after long-term cycling. In addition, deep redox reactions will produce more highly active and unstable Ni4+, and Ni4+ tends to spontaneously seize electrons in adjacent lattice oxygen and transform into more stable Ni2+. Due to the partial pressure of oxygen, it mainly occurs on the surface of the particle and extends inward, which not only causes the release of interfacial lattice oxygen, catalyzes the decomposition of the electrolyte and aggravates thermal runaway, but also promotes the occurrence of harmful interfacial phase transitions. The problems caused by the above high pressure will cause the deterioration of the bulk phase structure of the material and the deterioration of the electrode/electrolyte interface environment, thereby endangering the cycle life and safety of the high-pressure, high-nickel and low-cobalt ternary cathode materials. Therefore, effective modification strategies must be adopted to alleviate the above problems in order to make the practical application of high-pressure, high-nickel and low-cobalt cathodes possible.
【Job Introduction】
Recently, researchers Huang Gang, Cheng Yong and Wang Limin of Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, and Professor Zhang Xiuyun of Yangzhou University prepared LiNi0.6Co0.05Mn0.35O2(NCM60535) ternary cathode material with multi-electron and magnetic-rich properties by using a simple liquid-solid phase uniform mixing and re-roasting method. Rare earth oxide functional coatings can not only act as a "physical passivation film" to constrain particle volume changes and prevent direct contact between electrodes/electrolytes, but also play the role of "electronic library" to inhibit the loss of interface lattice oxygen and the conversion of harmful interfacial phase transitions through interfacial charge compensation. These two aspects greatly improve the bulk structure and stability of the electrode/electrolyte interface of NCM60535 at high voltage of 4.5V, making it exhibit excellent high-pressure lithium storage performance. At the same time, the functional coating is universal, scalable to all kinds of lanthanide rare earth oxides, and their coated modified NCM60535 material shows significantly improved high-pressure cycle stability and rate performance. The article was published in the internationally renowned journal Energy Storage Materials with the title "A universal multifunctional rare earth oxide coating to stabilize high-voltage lithium layered oxide cathodes", and doctoral student Shen Yabin is the first author of this paper.
【Content Statement】
A 7 nm thick uniform and dense Gd2O3 rare earth oxide film coated modified NCM60535 cathode material was prepared by a two-step synthesis method of liquid phase homogeneous mixing and solid phase mixing roasting. Theoretical calculations show that the relative energy required for the migration of Gd elements to the inner layer of the particles gradually increases with the increase of the number of layers, which confirms that Gd mainly exists in the form of surface coating and is difficult to dope into the NCM lattice phase, which is mainly caused by the relatively large ion radius of Gd. The theoretical calculation of electron state density shows the increased electron transfer channel, indicating that Gd2O3 coating modification can enhance the electronic conductivity of the material.
Figure 1. (a) Schematic diagram of the preparation of Gd2O3 coated modified NCM60535. 1%-Gd2O3@NCM samples (b) SEM, (c) TEM, (d) HRTEM, (e-h) EDS, and (i) XRD data. (j) theoretical computational structural model and (k) results of GD migration to the inner layer of the bulk phase. 1%-Gd2O3@NCM (l)Gd 3d XPS results and (m)DOS electronic state density calculations.
The prepared 1%-Gd2O3@NCM samples showed significantly improved cycle stability and rate performance at a high voltage of 4.5 V: after 100 cycles at a current density of 0.5C, they had a higher capacity retention rate than the original NCM (95.3% vs. 81.1%); It can still maintain 88.1% of the capacity after 400 cycles at a current density of 1.0C, and the specific capacity released at a high rate of 5.0C is still as high as 88 mAh g-1.
Figure 2. (a-b) CV curves, (c) initial charge-discharge curves, (d) cycle performance, (e-f) charge-discharge curves of different cycles, (e) long cycle performance, and (f) rate performance of 1%-Gd2O3@NCM and original NCM samples at 4.5 V.
Improvement of bulk phase structural stability: Battery in situ XRD shows that after Gd2O3 coating modification, it can convert the two phase reactions of harmful H1 to H2 into quasi-single phase reactions, and inhibit the formation of harmful H3 phases, which greatly improves the bulk phase structural stability of NCM under high pressure. Different from the original NCM, which had obvious intergranular microcracks after charging and discharging, no intergranular microcracks were observed in the modified NCM, which was attributed to the rigid constraint ability of Gd2O3 rare earth oxide functional coating and the improved lithium-ion diffusion migration environment. The high-temperature in-situ XRD surface coating modification can delay the temperature of NCM high-temperature phase transition, indicating that the cladding layer inhibits the loss of lattice oxygen at NCM high temperature, and then inhibits the occurrence of phase transition.
Figure 3. (a-b) cell in situ XRD data from 1%-Gd2O3@NCM and original NCM samples, particle cross-sectional SEM plots after (c-d) 100 cycles, and (e-f) high-temperature in situ XRD data.
Improvement of interfacial stability: 1%-Gd2O3@NCM samples exhibited smaller high-temperature self-discharge behavior, weaker interfacial polarization voltage, smaller interfacial charge transfer impedance and less electrolyte decomposition, and the theoretical calculation results showed that Gd2O3-coated NCM had greater oxygen vacancy formation energy, which indicated that the loss of lattice oxygen was inhibited. These can be attributed to the fact that Gd2O3 rare earth oxide coating with multi-electron properties can inhibit the loss of interfacial lattice oxygen by charge compensation of metal ions in the interfacial active material by electron transfer, and the loss of lattice oxygen is the root cause of the interfacial instability of NCM materials under high pressure.
Figure 4. (a-c) high-temperature self-discharge test, (d) polarization voltage change, (e) interfacial charge transfer impedance, (f) interfacial chemical environment after cycling, theoretical calculation of lattice oxygen release (g-h) structural model and (i) oxygen vacancy formation energy, and (j-k) two-dimensional electron slice plot of 1%-Gd2O3@NCM and original NCM samples.
Rare earth oxide functional coatings mainly play two roles in physical passivation and charge compensation: as a passivation film, it can physically block the direct contact of electrode/electrolyte, prevent acid HF from attacking the cathode material, and then inhibit the dissolution of active transition metal ions and the oxidative decomposition of electrolyte; As an electron library, it can enhance the electron and ion transport capacity of the material, improve the rate performance, and at the same time compensate for the charge of the interface unstable Ni4+ to prevent it from preying on the electrons of the lattice oxygen, thereby inhibiting the loss of interfacial lattice oxygen and the occurrence of harmful interfacial phase transitions. The designed high-voltage full battery with graphite as the negative electrode shows good cycle stability. Except for Gd2O3, NCM60535 of all lanthanide rare earth oxide coating modifications exhibited excellent high-pressure cycle stability and rate performance, which fully demonstrated the universality of rare earth modified cladding layers.
Figure 5. (a) Schematic diagram of the mechanism of action of Gd2O3 rare earth oxide coating. (b-d) Cycle performance and energy density calculation of high-voltage full battery with graphite as negative electrode designed. (e) High-pressure lithium storage properties of all kinds of lanthanide rare earth oxide coated modified NCM60535 materials.
【Conclusion】
In general, the physical passivation and charge compensation functions of rare earth oxide functional coatings can greatly improve the bulk phase structure stability and electrode/electrolyte interface stability of NCM materials under high pressure. The multi-electron characteristics make it have the electron transfer ability that other coating materials do not have, so it has a fundamental inhibitory effect on the severe lattice oxygen release problem faced by high-pressure and high-nickel cathodes, which greatly enhances the cycle stability of high-pressure and high-nickel cathodes, and further develops lithium-ion batteries with high energy density and long cycle life.