Improved Thermal and Structural Stability of LiNi0.8Co0.1Mn0.1O2 cathode materials by Surface Modification

The Ni-based cathode materials have attracted much attention for the higher practical capacity than other cathode materials, but suffer from several inherent problems such as thermal instabilities and severe capacity fading. Herein, to overcome the structural instability problem of the LiNi0.8Co0.1Mn0.1O2, we propose a surface modification method that can reduce and slow down the heat generation and also improve the electrochemical performance at 60℃. The magnesium phosphate, Mg3(PO4)2, is coated on the surface of the LiNi0.8Co0.1Mn0.1O2 with the aim of improving the structural stability at high temperatures.

This method could remove the surface residual lithium by forming LixMgPO4 olivine using the remaining lithium ions with a nanometer scale. Since the structure of olivine shows exceptional thermal stability due to the strong binding of phosphorus and oxygen atoms, the thermal decomposition of the material can be slowed down. After surface treatment, the cyclability at high temperature (at 60℃) is improved up to 20%, while thermal stability is ensured.

Note the unique flexibility of the thin and flat plastic LiION configuration; unlike other configurations, PLiION technology does not contain free electrolyte.

Introduction

Introduction to Li-ion batteries

The lithium-ion battery is an electrochemical device which generates the electrical energy from the chemical energy. The structure of the active materials must be maintained stable, and the electrolyte must facilitate the delivery of the ions. A lithium-ion battery is in a state of discharge where lithium ions are present in the cathode crystal structure when it is first assembled.

In the charging process, lithium ions migrate through the electrolyte to the anode side, meanwhile generating electrons on the cathode electrode. This electron travels through the external circuit connected to the anode electrode, and eventually meets the lithium ions that have traveled through the electrolyte from the cathode. In this charging process, due to the difference of the Gibbs free energy, the cell voltage increases which is determined by the difference of the amount of lithium ions inside each electrode.

Most cathode materials are metal oxides that contain lithium ions in the crystal structure, such as LiCoO2 (layered structure), LiMn2O4 (spinel structure), and LiFePO4 (olivine structure). The most commonly used cathode material will be LiCoO2, but this is difficult to expect as an excellent cathode material due to its high cost and low practical capacity. In the case of the anode side, natural graphite is usually used in commercial lithium-ion batteries due to its structural stability.

The electrolyte, which consists of organic solvent and lithium salts, acts as an ionic conductor delivering the lithium ions between cathode and anode electrodes. Finally, the separator physically separates the cathode and anode electrodes to prevent the contact of the two electrodes. The most important properties of the lithium-ion batteries are the capacity of the battery, working voltage and power/energy density and these are closely related to the properties of cathode materials.

Figure 3 Schematic drawing showing the shape and components of various Li-ion battery configurations

Layered cathode material

• LiNiO 2
• LiNi 1-y-z Mn y Co z O 2

At the last phase transition, in the range of x=0.75, the phase transition from H2 to H3 is an irreversible reaction and the cause is distortion of the crystal lattice structure. The ideal compound for this binary transition metal oxide is LiNi0.5Mn0.5O2, which is located in the middle of the center in the mixed phase LiCoO2-LiNiO2-LiMnO2. The main problem for commercial Li-ion cells above 4.3V is electrochemical stability during the electrochemical process, with the cathode being the main source of thermal instability.

LiNi1/3Co1/3Mn1/3O2 ternary mixed phase LiCoO2, LiNiO2, LiMnO2 has an α-NaFeO2 structure and overcomes the disadvantages of thermal instability of LiCoO2 and LiNiO2 and spinel-like phase transformation of LiMnO2 with less atomic displacements, which leads to the final degradation of electrode performance. LiNi1/3Co1/3Mn1/3O2 has a multilayer structure and has shown a capacity higher than 160 mAhg-1 with full cycling ability without phase transition to the spinel phase throughout the electrochemical cycle. The Li-stoichiometric transition metal ternary oxide, LiNi1-x-yCoxMnyO2, has the advantages of good action rate of LiCoO2, high discharge capacity of LiNiO2, and structural stability affected by the presence of Mn4+ ion.

Because these systems differ from a mixed material of LiCoO2-LiNiO2-LiMnO2 mixed phase, due to the different oxidation numbers of transition metals in LiMO2 (M = Ni3+, Co3+, Mn3+) system. When the Ni and Mn contents are increased, it indicates that the Mn4+ contributes to the thermal stability. But Ni-rich sill appears to generate relatively more cation mixing between Li+ and Ni2+ and poor thermal stability, which is strongly dependent on the transition metal content.

As a representative example, LiNi0.8Co0.1Mn0.1O2 shows the highest reversible capacity with 200 mAhg-1, but shows poor capacity retention due to structural transition. In conclusion, the Ni content provided a large specific capacity, as the increase simultaneously worsens the thermal stability, which is attributed to the decrease in the Mn ion content. In summary, Ni-rich cathode materials exhibit high specific and volumetric energy density.

Until now, many research studies on Ni-rich have focused on improving thermal and structural stability. Performance of (a) composite phase diagram of layered LiCo1/3Ni1/3Mn1/3O2 .. Figure 60 a) DSC scans of anodic (negative) graphite and cathodic (positive) LixCoO2 after 100% charge [14] and b) accelerated rate calorimetry of different cathodes after 100% charge.

Figure 5 The crystal structures of (a) α-NaFeO2 type layered structure, (b) spinel structure, and olivine structure represented by MO6 polyhedral

Experimental

• Introduction
• Experimental
• Electrochemical measurement
• Instrumental analysis

This gas evolution is the reason for the heat generation of the material which is further related to the safety issues of the LIBs. In addition, the size of Ni2+ ions (0.7Å) is similar to that of the Li+ ions (0.78Å), thus causing the cation disorder and lithium residues on the surface of the material. In this study, to overcome the structural instability problem of the LiNi0.8Co0.1Mn0.1O2, we propose a surface modification method that can reduce and slow down the heat generation and also improve the electrochemical performance at 60℃.

In contrast, the distributions of Mg and P elements are mostly aggregated on the surface of the particles, and segregation can be observed, implying a greater enrichment of P element over Mg on the surface. The decrease in the initial irreversible capacity of NCM is thought to be due to the reduced amount of LiOH and Li2CO3 impurities. The voltage profile of the NCM shows a lower IR drop than that of the BNCM above the 3 C rate during the charge cycle, leading to a higher level capacity (Figure 20).

The initial charge capacity of NCM (144 mAh g-1) is much higher than that of BNCM, and the capacity retention gap between the two electrodes after 200 cycles is even more pronounced: the capacity retention is 83% and 56% for NCM-M and NCM, respectively. In the NCM sample, the outermost amorphous coating areas cannot be observed, but area 3 exhibits an identical FFT image to area 1 of the NCM sample. Because the intermittent high-rate discharge is also a critical condition for EVs, pulse frequency measurements of the NCM and NCM-M cathodes were performed in a lithium half-cell at 60 °C (Figure 27).

This difference in performance is related to the reduced amounts of lithium impurities and the formation of a solidified surface layer in the case of the NCM sample. The thermal instability of the cathode material during delithiation is an important concern for commercialization and is directly related to the phase transition due to lattice oxygen release. The thermal characteristics of the cathode materials were evaluated by high temperature (HT), XRD and differential scanning calorimetry (DSC) analysis after the samples were charged at 4.3 V (Figure 28), the latter being a powerful technique to investigate the structure, stability .

Because the continuous phase change from the layered to the cubic phase leads to oxygen evolution from the lattice, the delayed formation of spinel and rock salt phases indicates that the hardened bulk surface layer of the NCM sample is quite effective in resisting structural instability. In summary, to overcome the structural instability problem of the LiNi0.8Co0.1Mn0.1O2, we propose a surface modification method that can reduce and slow down the heat generation and also improve the electrochemical performance at 60℃.

Results and discussions

Morphology and Surface Composition

All secondary spherical particles are composed of densely packed primary particles with diameters in the range of 8-10um, while some mesopores can be clearly observed in the cross-section image. Elements in LiNi0.8Co0.1Mn0.1O2, including O, Mn, Co and Ni are uniformly distributed throughout the as-prepared spherical particles. As shown in Figure 18, the distribution of the coating material between the particles is not consistent, because the Mg content is higher than P inside the particle, but less than P near the surface.

Table 1 Powder XRD characterization of NCM and NCM-M

Electrochemical results

Structural analysis

Notably, the sudden drop in capacity after a pulse cycle was greater for the BNCA sample than for the NCM sample. The NCM sample sustained a capacity drop of 10 mAh g-1 during the pulse test, while the capacity drop for the NCM sample increased from 80 to 120 mAh g-1. The total amount of heat produced for the NCM sample was ~803 Jg-1, which is significantly lower than that measured for the NCM sample, ~1450 Jg-1.

Such heat generation is the result of continuous structural changes due to oxygen loss from their lattices. The NCM sample showed a different phase transition behavior, with a transformation from the R-3m phase to the Fd-3m phase at ~175 °C, a higher temperature than that required for the NCM sample. When the NCM-M samples are heated above 220 °C, the Fd-3m and Fm-3m phases coexist and are present in the sample up to 300 °C.

Figure 20 Cross-sectional TEM images of NCM and NCM-M

Conclusion