Improved structural stability of Ni-rich cathode materials by a simple dry process for Li-ion batteries. A cobalt precursor was coated on the bare NCM surface with the aim of improving the integrity of the material by protecting the cathode surface from attack by acidic species. More importantly, the improved material integrity enabled a stable and uniform solid electrolyte interfacial layer (SEI) on the graphite anode, leading to unprecedented full cell performance.
After surface treatment, the high-temperature (45 oC) cycle performance improved from 20% for bare NCM to 50% for G-NCM. This finding could represent a breakthrough for LIB technology, as it would provide a rational approach to the development of advanced cathode materials. ii. Phase diagram of the pseudo∙binary system LiNiO2∙NiO2 from experimental data (XRD) and calculation (ab initio).
Electrochemical characterization of full-cell NCM and G-NCM a) cycling performance of NCM and G-NCM at room temperature (25 oC) (degree C charge and discharge: 0.5 C and 1 C) inner graph means columbic efficiency of each cycle. Cross-sectional SEM images of the NCM electrode and the G-NCM electrode. a) before the cycle b) after 500 cycles at 45 oC. FCC Face Center Cubic X-ray Diffraction Analysis XRD TGA Thermogravimetric Analysis DSC Differential Scanning Calorimetry CSTR Continuous Stirred Reactor SEM Scanning Electron Microscope TEM Transmission Electron Microscopy HR-TEM.
Introduction
Lithium-ion batteries
Ni-rich cathode materials for lithium-ion battery
- LiNiO 2
- Nonstoichiometry
Cathode is one of the most important components of the lithium-ion battery and usually has a metal oxide framework. The layered cathode materials such as LiMO2 consist of lithium ions, transition metal ions (M = Co, Ni, Mn, etc.) and oxygen ions.5 It has a -NaFeO2 crystal structure and belongs to the R3̅m space group with FCC structure (stacking of the ABC type) of the oxygen ion. The LiNiO2 material is attractive as a cathode material with higher theoretical capacity and lower cost than LiCoO2.8 The first report on the electrochemical properties of LiNiO2 was published in 1985 by M.
The electronic structure of LiNiO2 in Figure 3 is evidence that LiNiO2 has higher capacity than LiCoO2.9. Theoretically, only 50% of the theoretical capacity of LiCoO2 could be utilized in lithium ion cells. This corresponds to a reversible insertion and extraction of 0.5 lithium into LixCoO2, a significant overlap of the redox-active Co3+/4+ O2 :2p band. Phase diagram of the LiNiO2∙NiO2 pseudo∙binary system from experimental data (XRD) and calculation (ab initio).10c, 11.
According to crystal lattice theory, Ni3+ is unstable due to the unpaired electron spin of the e-orbitals. The non-stoichiometric structure leads to partial reduction of nickel ion, and the partial reduction causes structural collapse of the interlayer space and transition metal ions migrate from the transition metal sheet to the lithium sheet. Due to the smaller distance between the Li plate and the TM plate in the disordered phase, it has a higher activation energy barrier than the ordered phase for the diffusion of Li ion.
The disordered phase also has a lower Li-ion diffusion due to hindrance caused by TM in the Li slab.15 Therefore, increase in the cation mixture causes a decrease in the rate capability of the cathode material. Cation mixing leads to a partially destructive interference of constructive interference of the (003) plane at a Bragg angle of θd(003). Thermal stability is one of the most important considerations when choosing cathode material.18 The thermal stability of the cathode material is closely related to oxygen evolution.
The remaining lithium creates a larger amount of H2O in the electrolyte, which accelerates the decomposition of the salt. Electrochemical and physical properties of Ni-based cathode materials depend on the ratio of elements in mixed compositions. If the increase in the proportion of cobalt leads to a decrease in capacity, the increase in the proportion of nickel causes mixing of cations, and manganese causes a phase transition from a layered phase to a spinel phase.
It provides the lithium pathway in a layered structure and improves the conductivity of the materials. In this regard, LiNi1/3Co1/3Mn1/3O2 is considered to be one of the most promising alternative materials for LiCoO2.
Experiment
Introduction
Experimental method
This means small particles located on the surface of G-NCM are not a fragment of G-NCM but foreign coating material. As a result, coating (including electrochemically inactive phase) prevents the dissolution of transition metal. Figure 18 b) shows transition metal gradient on primary particle of G-NCM. Furthermore, the capacity of G-NCM from constant voltage mode was much lower than that of NCM.
The charge and discharge voltage profiles of the first cycle of NCM and G-NCM full cells are shown in Figure 21. The discharge capacity of NCM and G-NCM is 11 and 10 mAh and the coulombic efficiency of NCM and G-NCM was 89 and 88, respectively. The voltage profiles of NCM and G-NCM show that the average working voltage of NCM is much higher than that of G-NCM during charging.
Before cycling, both NCM and G-NCM had only 3 nm scale cation mixture layer on the surface of primary particle. The cross-sectional SEM images of NCM electrode and G-NCM electrode. a) Before and b) after 500 cycles at 45 oC. However, EDS scanning data of NCM indicate that TM was founded on the outside of the primary particle and that TM defect region was formed on the surface of NCM (~10 nm).
The SEI layer of the NCM/graphite sample was uneven, thick and dissolved transition metals were contained in the SEI layer. Furthermore, the EDS results of the NCM/graphite sample showed that excessive Ni was accumulated in the graphite. After the complete cell was disassembled, the anode surface of NCM/graphite sample changed color due to Li dendrite and Li dendrite was confirmed by SEM images.
Dissolved Ni ions appeared on the surface of NCM and G-NCM graphite samples. With the intensity of P and F elements in the scattering amount, we can predict that the thickness of the SEI layer of the NCM/graphite sample was greater than that of the G-NCM/graphite sample. Yamin, H., STUDY OF ELECTROLYTE SOLUTIONS BASED ON ETHYLENE AND DIETHYL CARBONATE FOR RECHARGEABLE LI BATTERIES .2.
Result & Discussion
Morphology and Surface Composition
Both NCM and G-NCM exhibit a similar spherical morphology with the primary particles clumping together to form large secondary clusters. The amount of free lithium (including both Li2CO3 and LiOH) associated with side reactions and gas evolution in G-NCM (0.15 wt%) decreases compared to NCM (0.19 wt%). All the peaks of the samples showed well a single phase with a layered structure without any impurities.
Transmission electron microscope (TEM) data show that there is no difference in particle size and morphology between NCM and G-NCM. Also, the thickness of the cation mixing layer on the NCM and G-NCM surface before the electrochemical test is almost the same (~3 nm). Two types of spinel phases are marked with the red line and the blue line respectively and Figure 16 d) shows that the red one is the Co3O4 phase which is full of cobalt ion and the blue one is the LixCo2O4 spinel structure (x < 1).
The coating layer is composed of a cobalt-rich phase (the ratio of cobalt is above 50. The EDS data proves that the red, which is located on the surface of the coating layer, is the Co3O4 phase, and the blue is the spinel structure of LixCo2O4, and the inner part of the coating layer is a layered structure, rich in cobalt The surface of the primary particle located on the surface of the secondary particle shows a higher cobalt concentration than that of the bare NCM, 10% (> 20.
The coating substance penetrates an internal primary particle which is located 3 ㎛ above the surface of the secondary particle. It means that the coating effect affects not only the surface primary particles, but also the inner primary particles.
Electrochemical results
- Half-cell results
- Full-cell results
Also, in Table 6, the DC-IR results measured after the forming process show similar resistance values.
Structural analysis
- Cathode
- Anode
To investigate the extent of transition metal (TM) decomposition, EDS scanning and TOF-SIMS (Time of Flight - Secondary Mass Spectrometry) were performed. The EDS scan was performed from the outside of the primary particle to the inside. The start point and the end point of scanning are marked by a white arrow in TEM images.
In Figure 26, the graphite paired with NCM had a higher concentration of Ni and Co ions than that of G-NCM. In addition, the accumulation of TM ions at the anode causes an unnecessary redox reaction and increases the resistance. The cross-sectional SEM and EDS images show that the SEI layer on the graphite (NCM sample) was not uniform and very thick.
The changed color of the anode was easily observed by the naked eye and distinguished from the G-NCM/graphite sample. For more accurate analysis of Li dendrite and material on the anode, ToF-SIMS depth profiling was performed. The blue peak of Li-ion in Figure 30 d) indicated a similar tendency towards P and F ions and was expected to show a SEI layer formed on the surface of the anode.
The sharp peak in Figure 30 d) indicated thin SEL layer while the broad peak in Figure 30 c) indicated thick SEI layer. The Li peak in Figure 30 c) is separated with P and F element and is expected as Li dendrite.
Conclusion
Lee, J.; Urban, A.; Li, X.; Su, D.; Hautier, G.; Ceder, G., Unlocking the potential of cation-disordered oxides for rechargeable lithium batteries. Kim, Y.; Cho, J., Nickel-Rich Layered Transition Metal Lithium-Oxide for High-Energy Lithium-Ion Batteries. Kang, K., Understanding the Degradation Mechanisms of LiNi0.5Co0.2Mn0.3O2 Cathode Material in Lithium-Ion Batteries.
W.; Obrovac, M.; Vonsacken, U., THERMAL STABILITY OF LIXCOO2, LIXNIO2, AND LAMBDA-MNO2 AND IMPLICATIONS FOR THE SAFETY OF LIION CELLS. Nishio, K.; Saito, T., ELECTROCHEMICAL CHARACTERISTICS OF LINIO2 AND LICOO2 AS POSITIVE MATERIALS FOR LITHIUM SECONDARY BATTERIES. Advanced energy materials b) Guilmard, M.; Rougier, A.; Grune, A.; Croguennec, L.; Delmas, C., Effects of aluminum on the structural and electrochemical properties of LiNiO2.
