Due to the advantages of high specific capacity, various temperatures, and low cost, layered LiNi0.6Co0.2Mn0.2O2 has become one of the potential cathode materials for lithium-ion battery. However, its application was limited by the high cation mixing degree and poor electric conductivity. In this paper, the influences of synthesis methods and modification such surface coating and doping materials on the electrochemical properties such as capacity, cycle stability, rate capability, and impedance of LiNi0.6Co0.2Mn0.2O2 cathode materials are reviewed and discussed. The confronting issues of LiNi0.6Co0.2Mn0.2O2 cathode materials have been pointed out, and the future development of its application is also prospected.
To meet the continuously increasing demand of clean energy globally, the rechargeable Li-ion batteries have been used in various areas like electric vehicles, communications, military, energy, and other fields [
Due to the synergistic effect of the Ni, Co, and Mn, LiNi
Elementary composition and electrochemical properties diagram of Li[Ni
At present, many methods have been devoted to addressing synthesis obstacles, such as the coprecipitation method, spray drying method, high temperature solid state reaction, and combustion method. Different methods have great influences on the electrochemical properties of NCM622 cathode materials.
Coprecipitation method is a useful preparation process for the industrial production of cathode materials. This method can synthesize precursor with excellent spherical morphology and element mixing at an atomic level. The precipitation conditions such as coprecipitation temperature, pH value of solution, and stirring intensity play a decisive role in the performance of precursor.
The effects of hydroxide coprecipitation conditions of Ni0.6Co0.2Mn0.2(OH)2 were systematically studied by Liang et al. [
(a) Cycle performance and (b) rate capability of NCM622 with different synthetic methods [
The different crystals of precursor are formed under different coprecipitation conditions, such as pH values and temperatures. The (Ni0.6Co0.2Mn0.2)CO3 precursors were synthesized as the pH values varied from 7 to 9.5 [
(a) XRD diffraction patterns of (Ni0.6Co0.2Mn0.2)OH2 and (b) cyclic performance curve of NCM622 with different pH values [
The electrochemical properties of the cathode material are also influenced by the micromorphology of the precursor. Kim et al. [
The cathode materials are prepared by heating the precursor and lithium at high temperature in air. The cathode materials sintering at the different conditions have the different electrochemical properties. The sintering temperatures and sintering times of (Ni0.6Co0.2Mn0.2)CO3 precursors were studied in detail. When the optimum sintering temperature and sintering time were 850°C and 20 h, respectively, the corresponding cathode materials possessed the highest discharge capacity and capacity retention rate [
Spray drying is another useful method to synthesize the cathode materials with element mixing at the atomic level [
SEM of the (a) precursor powder and (b) NCM622 cathode powder prepared by ultrasonic spray drying [
As the improvements of technology, the oxidative precursors (a hybrid of NiO/MnCo2O4/Ni6MnO8) were prepared with ultrasonic spray thermal decomposition method [
For solid state reaction method, the metal salt and lithium salt are measured precisely and mixed evenly, and then the cathode materials are prepared through burning mixture directly at high temperatures. In comparison with other methods, the solid state reaction method has low requirements of equipment, and the reaction can be managed easily. However, there are technical problems such as uneven mixing of raw material and poor uniformity of particle size. In order to minimize the presence of impurity ions, the corresponding hydroxides and oxides are usually used. Due to the large segregation of compositions and the appearance of hetero phase, the cathode material prepared with solid state reaction method has poor electrochemical performance. For example, Xia et al. [
Particle size of NCM622 prepared by solid state reaction at (a) sintering temperatures and (b) sintering times [
The differences between solid state reaction and spray drying on the electrochemical performances of NCM622 cathode materials were studied at room and elevated temperatures [
Discharge capacity and fade rate of samples operated at different temperatures [
Temperature | prepared method | Discharge capacity of 1C (mAh/g) | Fading rate per cycle (mAh/g) |
---|---|---|---|
25°C | Spray-drying | 143.7 | 0.44 (0.31%) |
Solid -state | 138.4 | 0.48 (0.34%) | |
50°C | Spray-drying | 159.3 | 0.55 (0.35%) |
Solid -state | 156.1 | 0.94 (0.60%) |
In the combustion synthesis, the organic metal salts are usually first mixed with nitric acid/urea, and then the above mixture is heated directly to the ignition temperature. Finally the cathode materials are synthesized by the exothermic heat of the material in the chemical reaction. The combustion method is known for its main advantages of simple equipment, low cost, and being without external energy. But it is restricted because of the shortcomings of large particle size and poor controllability. Few researchers prepare the cathode material with this method. For example, Ahn et al. [
The discharge specific capacities and cyclic stabilities of NCM622 cathode material synthesized with different preparation methods were listed in Table
Electrochemical performance of NCM622.
Authors | Synthesis methods | Testing conditions | Initial discharge capacity (mAh/g) | Capacity retention rate (%) | Ref. |
---|---|---|---|---|---|
Liang et al. | Hydroxide co-precipitation | 1C, 2.8–4.3 | 172.1 | 94.3 (100 cycles) | [ |
Li et al. | Hydroxide co-precipitation | 30 mA/g, 2.8–4.3 | 172.8 | 71.8 (50 cycles) | [ |
Zhang et al. | Carbonate co-precipitation | 0.2C, 2.8–4.3 | 180.0 | 82.4(30 cycles) | [ |
Xu et al | Hydroxide co-precipitation | 0.1C, 3.0–4.3 | 201.6 | 90.1 (100 cycles) | [ |
Zhong et al. | Carbonate co-precipitation | 0.2C, 3.0–4.3 V | 148.0 | 91.8 (30 cycles) | [ |
Yue et al. | Spray-drying | 1C, 3.0–4.3 V | 160.8 | 93.7 (40 cycles) | [ |
Yue et al. | Spray-drying | 80mA/g,3.0–4.3V | 155.7 | 89.0 (50 cycles) | [ |
Li et al. | Spray-drying | 1C, 2.8–4.3 V | 160.8 | 90.8 (100 cycles) | [ |
Xia et al. | Solid state | 1C, 2.8–4.3 V | 156.3 | 102.9 (100 cycles) | [ |
Yue et al. | Solid state | 1C, 2.8–4.3 V | 138.0 | 82.9 (100 cycles) | [ |
Ahn et al. | Combustion | 0.1C, 3.0–4.3 V | 170.0 | 98.2 (30 cycles) | [ |
It is imperative to further improve the electrochemical performance of cathode material for the next generation electric vehicles. The long-term cycling is directly correlated with the capacity loss. The mechanical stress heterogeneity is ultimately attributed to intergranular fracturing that degrades the connectivity of subsurface grains and causes the capacity loss [
The capacity fading of the nickel-rich cathode material has become serious during charge-discharge. At present, the surface coating as a mainly modification method is used to improve the cycle stability and thermal stability of the cathode materials. The commonly used coating materials for cathode materials are oxides, mineral salts, and active electrode materials.
When TiO2 is used for coating, the inert TiO2 coating on the cathode surface can protect the active material from reacting with the electrolyte and inhibit the cathode materials dissolution during charge-discharge cycles. For instance, Chen et al. [
(a) Initial charge-discharge, (b) cycle performance, and (c) rate capability of TiO2 coated NCM622 [
As is known, SiO2 is corrosion resistant and has a poor conductivity. When the nano-SiO2 was coated on the surface of NCM622 cathode materials, the thick SiO2 coating could improve the cycle performance of NCM622 but reduce the conductivity at a high current density [
The nanosized Mn3(PO4)2 coating was proposed to enhance the electrochemical and thermal properties of NCM622 [
Electrochemical properties of Mn3(PO4)2 coated and uncoated NCM622 cathode materials at different temperatures [
Samples | 25°C | 60°C | ||
---|---|---|---|---|
1st cycle/mAh−1 | 50th cycle/mAh−1 | 1st cycle/mAh−1 | 50th cycle/mAh−1 | |
Pristine NCM622 | 165 | 153 | 175 | 142 |
0.5wt% NCM622 | 160 | 149 | 176 | 160 |
To improve the interface reaction and reduce the interfacial resistance, the Li-ion conductor Li3PO4 is used for the modification of cathode materials. When the Li3PO4 coated NCM622 was prepared with sol-gel method using citric acid as complexing agent [
Some cathode materials with positive electrochemical properties are also used as coating materials. For example, the Li1.3Al0.3Ti1.7(PO4)3 coated NCM622 cathode materials were prepared with a sol-gel method and the effect of coating amount on the electrochemical properties of cathode materials was studied [
(a) Cycle life performance and (b) impedance curves of Li1.3Al0.3Ti1.7(PO4)3 coated NCM622 [
The electronic conductivity and the ionic conductivity of the NCM cathode material were increased via doping metal or nonmetal ions into the crystal lattice of the NCM cathode material. Meanwhile, the structural stability and thermal stability of the cathode materials were improved [
For the NCM622 cathode materials, the doping elements should be aimed at improving the electronic conductivity and rate performance, such as Mg [
Rietveld refinement results for the prepared samples: (a) Mg-0, (b) Mg-1, (c) Mg-3, and (d) Mg-5 [
(a) Li+ diffusion coefficient, (b) cyclic performance at 160mAg−1, and (c) rate capability of Na+ doped-NCM622 [
In fact, the battery will produce traces of HF during the circulation process of charge/discharge. If some doping elements could help to resist HF erosion, the electrochemical performance could be enhanced [
The electrochemical properties of cathode materials before and after modification with various materials were listed in Table
Electrochemical performance of NCM622 before and after modification.
Authors | Modification methods | Testing conditions | Pristine NCM622 | Modified NCM622 (mAh/g) | Ref. |
---|---|---|---|---|---|
Chen et al. | 1wt% TiO2 coating | 1C, 2.8–4.5V | 175.1(initial); | 177.3(initial); | [ |
Chen et al. | 1wt% Al2O3 coating | 1C, 3.0–4.5 V | 176.8 (initial); | 197(initial); | [ |
Li et al. | 2.5wt% MDA coating | 1C, 3.0-4.5 V | 173.5(initial); | 196.5(initial); | [ |
Tao et al. | 0.5wt% ZrO2 coating | 0.1C, 2.8–4.3V | 133.7(initial); | 146.6(initial); | [ |
Cho et al. | 1wt% SiO2 coating | 0.5C, 2.8–4.3V | 168.1 (initial); | 167.9 (initial); | [ |
Fu et al. | 2wt% Li2SiO3 coating | 1C, 2.8–4.6 V | 180(initial); | 191.7(initial); | [ |
Wang et al. | 3wt% Li2SiO3 coating | 0.2C, 2.8–4.3V | 196 (initial); | 199(initial); | [ |
Liu et al. | 1wt% Li2Si2O5 coating | 5C, 3.0–4.3 V | 171.7 (initial); | 182.4 (initial); | [ |
Cho et al. | 0.5wt% Mn3(PO4)2 coating | 0.5C, 3.0–4.3V | 153 (initial); | 149 (initial); | [ |
Choi et al. | 1wt% Li | 0.5C, 3.0–4.3V | 161.8 (initial); | 162.4 (initial); | [ |
Ju et al. | PEDOT-Co-PEG coating | 0.5C, 2.8-4.3 V | 10.7% capacity loss | 6.1% capacity loss (100 cycles) | [ |
Fu et al. | 3% Mg doping | 5C, 3.0–4.3 V | 126 (initial); | 148 (initial); | [ |
Huang et al. | 1% Mg doping | 1C, 2.8–4.3V | 162.6(initial); | 169.7 (initial); | [ |
Huang et al. | 1% Na doping | 1C, 3.0–4.3 V | 158.2(initial); | 162 (initial); | [ |
Kaneda et al. | 3% Nb doping | 2C, 3.0–4.1 V | 150 (initial); | 139 (initial); | [ |
NCM622 cathode materials with the excellent electrochemical properties are promising in next generation lithium-ion batteries. Their cyclic stability and thermal stability have been significantly improved after surface coating or element doping. The various modification mechanisms for NCM622 mainly focus on the following aspects: The coating layer provides the isolation protection layer on the cathode material surface and greatly weakens the gap between the electrolyte and the electrode material. Meanwhile, the surface coating material enhances the ionic conductivity of the cathode material particles and shortens the Li+ diffusion aisle during the charge-discharge cycles. The coating effectively reduces the electrochemical impedance and improves the battery cycle performance. The doping ions reduce the mixing of Ni2+ and Li+ in NCM622 cathode material and meliorate the aisle of Li+ diffusion and then increase the stability of the cathode material and impede the structural damage of NCM622 during the cycle of charging/discharging. Based on the structure and properties of cathode materials, the surface coating and ion doping are designed to improve their electrochemical and structural stability.
The research of Ni-rich cathode material (Ni content ≥ 60%) has been gradually paid more attention for the past few years. A high performance ternary material still faces many challenges related to structural instability and reaction mechanism at harsh temperatures. The future modification work should be carried out from the following aspects: Firstly, the energy density and safety of cathode material are supposed to be improved during the charge-discharge cycles. Secondly, the cyclic stability should be improved under the harsh environment conditions. Finally, in order to reduce the cost of production for commercial promotion, the preparation process of NCM622 should be optimized.
The authors declare that they have no conflicts of interest.
This work was financially supported by the Funding of Jiangsu Innovation Program for the Graduate Education (no. KYLX16_0325).