Introduction: Due to its high energy density, power density, long cycle life and high safety, the nickel rich layered ternary material LiNi0.8Co0.1Mn0.1O2 (NCM) is considered to be one of the most ideal cathode materials for the next generation of lithium ion batteries.
However, the stability of NCM materials faces many challenges in the process of high voltage and high rate electrochemical cycling:
1. The phase transformation of the bulk and surface leads to intergranular cracking of secondary particles along grain boundaries (GBs);
2. Formation and growth of cathode electrolyte mesophase (SEIs);
3. The side reaction will not only consume electrolyte, but also produce gas and cause the dissolution of transition metal (TM), which may then migrate and precipitate at the anode side, thus affecting the anode stability;
4. The most serious is that the surface oxygen of NCM becomes unstable and easy to escape under high voltage. This oxygen loss will not only oxidize organic electrolytes and release gas, but also lead to cationic reduction and phase change of densification disease, which may lead to other degradation processes in the form of chain reaction.
The application of thin and electrochemical inert coating on the surface of NCM cathode material is considered to be an effective solution to improve the stability. However, it is usually difficult to achieve 100% coverage in synthesis due to the wetting problem of solids to solids and the need to maintain shape in the electrochemical cycle. Therefore, it is an arduous challenge to construct high-quality coatings at the level of primary and secondary particles that are firmly bonded with surface oxygen.
In view of this, Jaephil Cho of the Ulsan National Institute of Science and Technology (UNIST) in South Korea and Professor Li Ju of Massachusetts Institute of Technology (MIT) put forward a "coating plus injection" strategy of synthesizing at room temperature. A uniform layer of CoxB metallic glass coating was loaded on the NCM surface, which not only achieved the complete surface coverage of NCM particles, but also injected the grain boundary of secondary particles. The introduction of CoxB coating not only effectively inhibits the degradation of microstructure of NCM intergranular crack, but also reduces the side reaction with electrolyte, greatly enhancing the electrochemical performance of NCM at high temperature and high magnification. The atomic simulation results confirmed that the CoxB coating formed a strong selective interface binding with the NCM surface, which not only provided a huge chemical driving force to ensure uniform reactive wetting of the NCM electrode and convenient injection of GBs, but also reduced the oxygen activity of the surface/interface, thus obtaining excellent mechanical and electrochemical stability.
The research results are entitled "Reactive border investment stabilizes Ni rich categories for lithium ion batteries" and published on the top energy journal Nature Energy.
Part 1: Why Cobalt Boride (CoxB) Metallic Glass Coating?
Why did the author choose Cobalt Boride (CoxB) as the coating? The reasons are as follows:
1. CoxB is a metal compound, which has no direct connection with oxygen, and will react with oxygen thermodynamically to form stable compounds, such as B2O3, Co3O4 and Co4B6O13. This means that there is a strong reactivity between CoxB and NCM surface oxygen.
2. CoxB has excellent oxidation resistance even at high temperatures of 850 – 950 ℃. Even if CoxB reacts with oxygen, a dense self-healing passivation layer will be formed at the interface. The passivation layer can not only dynamically inhibit oxygen penetration or loss caused by flowing through the coating, but also may maintain the properties of metal glass and combine with Li alkali metal to become a mixed conductor of ions and electrons.
3. CoxB coating can be synthesized at room temperature, thus eliminating the complexity of subsequent high-temperature treatment.
4. CoxB has long been widely used in the coating of metal parts to improve its corrosion resistance and wear resistance. Therefore, it is not easy to crack or crack on the nanometer scale, and has good mechanical properties.
It is worth noting that the CoxB metallic glass coating synthesized at room temperature not only completely covers the surface of NCM secondary particles, but also injects into GBs between primary particles at zero equilibrium wetting angle (Fig. 1). This is similar to the complete wetting of GBs by liquid metals (such as liquid Ga in aluminum GBs) and intergranular amorphous nano films in ceramics.
Fig. 1. Microstructure diagram of CoxB of "coating plus injection" NCM.
Part 2. Synthetic CoxB coating at room temperature, completely covering the surface of NCM particles and injecting GBs
Key point 1: preparation of coating. In order to obtain the NCM (CoxB – NCM) injected with CoxB, the author added the NCM synthesized by coprecipitation to the ethanol solution of cobalt nitrate, and then added the ethanol solution of NaBH4 under the protection of argon at room temperature to obtain CoxB – NCM through reduction reaction.
Key point 2: microstructure of the coating. The synthesized NCM has a typical polycrystalline microstructure, consisting of spherical secondary large particles composed of primary fine particles (Fig. 2a, median diameter D50=12 μ m）。 SEM and TEM results show that CoxB – NCM not only retains the microstructure of NCM well, but also has a continuous uniform coating with a thickness of about 5 nm on the surface (Fig. 2b-d).
Key point 3: Although the room temperature synthesis route was adopted, XPS data confirmed that CoxB coating not only achieved uniform 100% surface coverage, but also injected into GBs at the depth of secondary particles in the contact mode of zero equilibrium wetting angle (Fig. 2g-n).
Figure 2. Characterization of amorphous CoxB coating on NCM surface and GBs
Part 3. Enhancement of CoxB coating on the performance of NCM positive electrode
Key point 1: The author compared the half cell performance of NCM and CoxB – NCM positive electrodes under high load (10.5 ± 0.2 mg cm-2, area capacity 2.05 mAh cm-2) and high electrode density (3.20 ± 0.03 g cm-3). The results show that CoxB – NCM shows higher magnification performance (Fig. 3a, b) and cycle stability. In contrast, the polarization of pure NCM is more serious, and the average voltage loss is 3.75 times that of CoxB – NCM (Figure 3c).
Key point 2: In order to highlight the excellent performance provided by CoxB coating, the author conducted tests under more stringent conditions (45 ° C, 7C ratio). The results show that pure NCM has almost no capacity after 60 cycles, while CoxB – NCM can not only stabilize the cycle by 200, but also maintain the capacity by 82.2% (Fig. 3d), showing higher coulomb efficiency, energy efficiency and stable average discharge voltage.
Key point 3: Combined with spherical graphite (Gr) cathode, the author has assembled a 400 mAh soft packed full battery. The test results show that CoxB – NCM shows an impressive 95.0% capacity retention and multiplying performance (Figure 3e), while the pure NCM is only 79.2%. Therefore, CoxB coating and injection can greatly enhance the magnification performance and cycle stability of NCM, and can inhibit the impedance growth and voltage loss at high magnification and high temperature.
Figure 3. Electrochemical Performance of Pure NCM and CoxB NCM Cathode Materials
Part 4. Mechanism of strengthening NCM performance by CoxB coating
Key point 1: SEM and TEM images of electrodes after cycling show that after 200 cycles at 45 ° C and 7C discharge rate, pure NCM has serious intergranular cracks (Fig. 4a), resulting in more fresh surfaces exposed to electrolyte and the formation of SEI film. In contrast, the secondary particles of CoxB – NCM remain intact without breaking (Fig. 4d), while producing less electrolyte penetration and SEI formation.
Key point 2: interestingly, the intergranular cracking of nickel rich cathode materials is usually due to the expansion or contraction of anisotropic lattice and the heterogeneous charge discharge dynamics in the electrochemical cycle, which is difficult to remove through the anode coating. This shows that the introduction of CoxB coating can inhibit the cracking caused by mechanical stress or strain, and also can inhibit the side reaction at GB, namely stress corrosion cracking (SCC). TEM and EDS confirmed that the introduction and implantation of GB into CoxB coating effectively inhibited surface cation reduction, cation densification and surface phase transition (Fig. 4b-f).
Key point 3: The surface of circulating samples was characterized by XPS. The results showed that even after a long cycle, the CoxB surface coating would not be covered by thick SEI, which confirmed that the growth of SEI in CoxB – NCM was inhibited (Fig. 4j, k).
Key point 4: The results of in situ differential electrochemical mass spectrometry (DEMS) show that the gas emission (CO2 and O2) in the first charging cycle of CoxB – NCM is less than that of pure NCM (Fig. 4l). This shows that CoxB implantation effectively reduces the oxygen activity of the surface and GB, and inhibits the oxidation of the electrolyte. At the same time, it is further found that TM solubility (Ni, Co and Mn) in circulating CoxB – NCM is also less than that in NCM.
Therefore, CoxB coating not only alleviates the degradation of microstructure, but also effectively alleviates the side reaction between cathode and electrolyte.
Figure 4. The introduction of CoxB coating inhibits the degradation of microstructure and side effects at the same time.
Part 5. Strong interface combination can inhibit oxygen activity.
In order to understand the reaction mechanism at the atomic scale, the author carried out first principles calculations on the (104) surface of LiNiO2 and the interface between LiNiO2 (104) surface and amorphous CoxB.
Key point 1: The calculation results show that at the interface between LiNiO2 (104) surface and amorphous CoxB (Fig. 6a), Co and B preferentially bind to the surface O of LiNiO2, but not to Li or Ni, because the interface Co-O and B-O bonds are stronger than the lattice Ni-O bonds (Fig. 6b, c). Strong covalent B – O bond can achieve more significant energy level reduction, which greatly stabilizes the interfacial oxygen species.
Key point 2: Reactive wetting is due to the selective formation of Co-O and B-O bonds at the CoxB/NCM interface, and the excellent stability is due to the electronic structure with effective inhibition of interfacial oxygen activity. This shows that the rational design of reactive wetting metal quasi metallic glass coating materials with selective binding can not only ensure uniform and complete coverage, but also stabilize the surface oxygen.
Fig. 5. The first principle calculation confirms that strong interface binding can inhibit the surface oxygen activity.
Section 6 General
In summary, the authors present a simple room temperature synthesis and "coating+injection" strategy to build high-quality CoxB metallic glass coatings through reactive wetting. Under the strong driving force of interface chemical reaction, nano CoxB metallic glass is not only completely wrapped on the surface of secondary particles, but also injected GBs between primary particles. At the same time, because it is synthesized at room temperature, the secondary particles will not change the crystal, but will produce dramatic changes in GBs and penetrate through reactive wetting. Therefore, CoxB NCM shows excellent electrochemical performance, such as long cycle stability and high rate performance at 45 ℃ and 7C high speed. In the actual soft packed full battery, the capacity retention rate after 500 cycles is 95.0%. In addition, better safety can be provided by mitigating the cross effects of intergranular stress corrosion cracking (SCC), microstructure degradation and positive side effects, as well as negative TM. Atomic simulation revealed the strong selective interface binding between CoxB and NCM, which provided a consistent explanation for the active wetting and inhibition of oxygen activity observed in the experiment.
