The Ni-Mn-Co oxide can theoretically be synthesized to obtain a composite of these three transition metals, and adding cobalt to LiMn1-yNiyO2 can improve structural stability in a two-dimensional direction. It was found that the transition metal content in the lithium layer decreased from 7.2% in LiMn0.2Ni0.8O2 to 2.4% in LiMn0.2Ni0.5Co0.3O2, and the cobalt-doped compound had a capacity of 150mA·h after being intercalated with lithium. /g, the homogeneous compound LiNi0.33Mn0.33Co0.33O2 obtained at a synthesis temperature of 1000℃, charged and discharged at 0.17mA/cm2 at 30℃, the voltage range is 2.5~4.2V, and the specific capacity also reaches 150mA·h/g; Increasing the charge cut-off voltage to 5.0V can increase the capacity to 220mA·h/g, but the capacity attenuation is obvious. This material is also known as 333 material.
Someone used first-principles calculations and experiments to study the electronic structure of LiNi1/3Co1/3Mn1/3O2 and the valence distribution of transition elements. It is believed that the valences of nickel, cobalt, and manganese in LiNi1/3Co1/3Mn1/3O2 are +2, +3, and +4, respectively; comparing the bond length, bond energy and XPS of the M-O bond, XANES and other energy spectrum analysis show that the electronic structure of Co in LiNi1/3Co1/3Mn1/3O2 is consistent with that in LiCoO2, while the electronic structure of Ni and Mn is different from the electronic structure of Ni and Mn in LiNiO2 and LiMnO2.
LiNi1-x-yMnxCoyO2 was synthesized using Ni1-x-yMnxCoy(OH)2 and lithium salt in air and oxygen at 750℃, but the optimal synthesis temperature was 800~900℃. The discharge temperature has a great influence on the discharge capacity and rate characteristics of LiNi3/8Co2/8Mn3/8O2, lithium ion diffusion and charge transfer. Increasing the discharge temperature can significantly improve the discharge capacity and rate discharge performance of LiNi3/8Co2/8Mn3/8O2. This is because the increase in temperature helps to accelerate the charge transfer rate of the material and accelerate the electron insertion and escape reaction, so that its discharge capacity and rate discharge performance are significantly improved.
Figure 1 shows the XRD patterns of the precursors calcined at 920°C, 950°C and 980°C for 8 hours. It can be seen from the figure that the XRD diffraction line of Li1.15-xNi1/3Co1/3Mn1/3O2+δ synthesized at 920~980℃ is sharp, indicating that the crystal is complete and in a single phase, and all diffraction peaks can be indexed according to the layered α-NaFeO2 structure.
Figure 1 - XRD patterns of synthesized products under different temperature conditions
Figure 2 shows the SEM pictures of Li1.15-xNi1/3Co1/3Mn1/3O2+δ synthesized at different temperatures.
Figure 2 - SEM of Li1.15-xNi1/3Co1/3Mn1/3O2+δ synthesized at different temperatures; (a) 920℃; (b) 950℃; (c) 980℃
The first discharge capacity of LiNi1/3Co1/3Mn1/3O2 synthesized by spray pyrolysis at 0.2C and 2.8~4.6V voltage is 188mA·h/g, the capacity remained at 163mA·h/g and 173mA·h/g after 50 cycles at 30℃ and 55℃, and the high-current discharge performance was also better. The Li-Ni-Co-Mn-O cathode material has stable electrochemical performance, high discharge capacity and good discharge rate, and has a wide discharge voltage range and good safety. It is suitable for use in electric vehicles.
The typical preparation method of LiNi1-y-zMnyCozO2 is to compound Ni1-y-xMnyCoz(OH)2 with lithium salt and then react in air or oxygen. The optimum temperature is 800~900℃. Under these conditions, a layered O3 single-phase structure is obtained. The typical diffraction parameters conform to the uniform distribution of R3m of the α-NaFeO2 structure, as shown in Figure 3. Its structure is composed of a closed cube in which oxygen ions are arranged. In the structure, the transition metal ions occupy the interlayer in the octahedral position. The structure and properties of the hydroxide composite precursor have been studied; this is a CdI2 structure similar to TiS2, but the twisted layered structure is dislocated and transformed into a spinel structure when heated. The unit cell parameters of the structure are affected by the transition metal, as shown in Figure 4. When the manganese content is constant, the parameters a and c increase with the increase of the nickel content; and decrease with the increase of the cobalt content. For the LiNiyMnyCo1-2yO2 system, the parameters a and c obey Vegard's law, that is, they decrease linearly with the increase of cobalt content. When the nickel content is constant, the parameter a is directly proportional to the manganese content and inversely proportional to the cobalt content.
Figure 3 - Layered LiNi0.4Mn0.4Co0.2O2 material powder diffraction pattern
The lattice constant ratio c/3a can intuitively reflect the degree of deviation of the crystal lattice from the cubic lattice. The c/3a ratio of an ideal ccp lattice is 1.633, while the c/3a ratio of the LiNiyMnyCo1-2yO2 system is 1.672. For example, in TiS2, the molecular formula changes Li+iS2 after lithium insertion, the c/3a of its lattice will increase to 1.793, and the c/3a of CoO2 with a low ratio will also increase from 1.52 to 1.664 of LiCoO2. ZrS2 is also abnormally low. Similarly, its c/3a increased from 1.592 to 1.734 of LiZrS2. The closer the c/3a ratio is to 1.633, the greater the content of transition metal in the lithium layer. Therefore, the c/3a ratio of LiNiO2 is 1.639, which is close to that of spinel LiNi2O4. For LiNi0.5Ni0.5O2, the c/3a ratio is 1.644~ 1.649; when the second lithium is added, such as in Li2NiO2 and Li2Ni0.5Mn0.5O2, the c/a ratio changes very little, which are 1.648 and 1.6471 respectively.
It can be seen from Figure 4 that with the addition of cobalt, the c/3a ratio changes very significantly, indicating that cobalt provides layer-like characteristics. However, the curve of [Mn]=0.3 shows that as the cobalt content decreases (ie, the Ni content increases), the ratio is closer to the ideal cubic value of 1.633, and when [Ni]=0.4, the ratio hardly changes. It can be seen that the c/3a value of the compound depends on the concentration of nickel, and the presence of cobalt reduces the nickel in the lithium layer. Figure 4 (d) shows that when [Ni]=[Mn], the value of c/3a increases with the increase of cobalt content. When the manganese content is 0.1≤y≤0.5, the value of c/3a does not change, and its value is 1.644+0.005. The average c/3a value of LiNi0.33Mn0.33Co0.33O2 is 1.659, which is more lamellar than the average ratio of the compound LiNi0.5Mn0.5O2, which is 1.647. For the 333 material, the c/3a ratio decreases with the increase of the synthesis temperature of 900~1100℃, following c/3a=(1.680~2.35)10-5 T, indicating that heating will increase the nickel content in the lithium layer.
Figure 4 - The ratio of c/3a of the unit cell parameters of the layered compounds LiNiyMnzCo1-y-zO2 and LiNi1-yMn1-yCo2yO2
Figure 5 shows the relationship between the composition of the compound and its synthesis temperature. The data clearly shows that the increase in the cobalt content is beneficial to suppress the dislocation of the transition metal, but it is difficult to obtain the compound with the molecular formula LiNi1-yCoyO2. Among them, the dislocation of nickel can be observed only when y≤0.3, and this situation increases with the increase of nickel content. The synthesis temperature has the same significant effect as the compound composition. The example of LiNi0.4Mn0.4Co0.2O2 (hereinafter referred to as 442 material) is also shown in Figure 5. Among them, the sample synthesized at 1000°C and quickly cooled to room temperature has almost 10% nickel filled in the lithium layer. In the 333 material synthesized at 950℃, 5.9% of the nickel occupied the lithium site; only in the sample synthesized at 800℃, the dislocation of nickel can be reduced to zero with the increase of cobalt content. At 900℃, even if there is more cobalt than nickel, there will also be dislocations of nickel. All samples with a synthesis temperature of 900℃ have about 2% more nickel in the lithium layer than samples at 800℃. Obviously, high temperature increases the dislocation of nickel ions.
Figure 5 - LiNiyMnzCo1-y-zO2 materials synthesized at different temperatures are the relationship between the ratio of diamond to nickel and the percentage of lithium sites occupied
Although the above-mentioned materials have good electrochemical performance, their electronic conductivity is still very low as a large-rate positive electrode material. Therefore, it is necessary to find a way to increase the conductivity without adding a large amount of conductive agent such as carbon black, because carbon black will reduce the volume energy density. The conductivity of Li0.5Ni0.5Mn0.5O2 is 6.210-5S/cm; when cobalt is added to make LiNi0.4Mn0.4Co0.2O2, the conductivity of the sample will increase to 1.410-4S/cm. The conductivity value without cobalt compound is approximately equal to KMnO2 or LiMnO2, and the addition of 2% to 10% cobalt will increase by 100 times, about 10-3S/cm. Someone reported a conductivity value of (2~5) 10-4S/cm. When the cobalt content reaches 0.5, its conductivity does not change with the composition. Increasing the cobalt content can increase the rate, which may be due to the reduction of the Ni2+ limit in the lithium layer, which will reduce the diffusion coefficient of lithium ions.
There has been a lot of research on the physical properties and chemical bond properties of the multi-element transition metal oxide system. In the compound composed entirely of lithium oxide, cobalt is +3 valence, nickel is +2 valence, and manganese is +4 valence. Therefore, the electrochemically active element is mainly nickel, and cobalt is only active in the later stage of delithiation, and manganese does not play a role in the electrochemical reaction process. The magnetic study of this type of compound gives information on the position of nickel ions. When nickel ions are present in the lithium layer, it will cause a loop hysteresis phenomenon in transient magnetism. This phenomenon exists in both Li(NiMnCo)O2 and LiNi1-yAlyO2 systems. These magnetic laws are shown in Figure 6. The figure shows that under high temperature conditions, when the added amount of cobalt increases from 0.0 to 0.2, 0.33, they follow the Curie-Weiss law, and the hysteresis phenomenon decreases, which means that the Ni2+ content in the lithium layer decreases.
Figure 6 - Magnetic properties of LiNi0.5Mn0.5O2, LiNi0.4Mn0.4Co0.2O2 and LiNi0.33Mn0.33Co0.33O2 materials
Based on these transition metal oxides, a large number of XPS studies have been done, and it has been shown that Ni2+ plays a leading role. Therefore, for the 442 material, the spectrum of cobalt is undoubtedly the effect of Co3+, while about 80% of the spectrum of manganese is produced by Mn4+ and 20% is produced by Mn3+. The spectral characteristics of nickel are very strong and complex, with approximately 80% attributed to Ni2+ and 20% attributed to Ni3+. For the study of LiNi0.33Mn0.33Co0.33O2, LiNi0.5Mn0.5O2 and LiNiyCo1-2yMnyO2, it is found that when y=1/4 and 3/8, it also shows that the oxidation states of nickel and manganese with valences of +2 and +4 play a leading role, and the key element of electrochemical activity is nickel.
For the electrochemical properties of different component compounds under a series of current densities, pure Li1-yNi1-yO2 has the lowest capacity. For example, the electrochemical performance of a 442 compound synthesized at 900℃ is shown in Figure 7. When the test temperature is 22℃, the current is 1~2mA/cm2, and the voltage range is 2.5-4.3V; this is equivalent to the magnification of 44mA/g and 104mA/g, the initial voltage of all samples is about 3.8V, the synthesis temperature is very important for the capacity and capacity retention performance, and the best temperature is 800~900℃, and the synthesis sample capacity is extremely low at 1000℃. The specific capacity information of the 442 material is shown in Figure 7. It can be seen that the reproducibility of the sample structure obtained by different researchers is very good. For example, a 442 material with a constant capacity, charged and discharged at 0.2mA/cm2 (20mA/g or C/8), cycled 30 times, the voltage range is 2.8~4.4V, and its capacity is 175mA·h/g; when the current density increases to 40mA/g, 80mA/g, and 160mA/g(1.6mA/cm or 1C rate), the capacity is 170mA.h/g, 165mA.h/g, 162mA·h/g, respectively, which is slightly exhausted. LiNi0.375Mn0.375Co0.25O2 is a material with excellent performance, in which about 5.5% of the lithium sites are occupied by nickel, the capacity is 160mA·h/g at 30℃, and the rate is 40mA/g. After 50 cycles, the capacity decays to 140mA·h/g; when the temperature is raised to 55℃, the capacity increases to 170mA·h/g, and after 50 cycles, the capacity only decays to 160mA·h/g. When the sample contains only 3.2% nickel to occupy the lithium position, its capacity is lower, 30mA/g charge and discharge, the cut-off voltage is 4.2v, and the capacity is only 135~130mA·h/g after 50 cycles; when the cut-off voltage increases to 4.4V, its capacity increases by 20~30mA·h/g, which shows that the charging voltage is very critical.
Figure 7-Electrochemical performance of LiNi04Mn0.4Co0.2O2 material; (a) the relationship between capacity and cycle times; (b) the relationship between capacity and discharge rate
For the 333 material, increasing its charging voltage can increase its capacity. If someone reports that the synthesis temperature is increased from 800°C to 900°C, the initial capacity can be increased from 173mA·h/g to 190mA·h/g. 0.3C charge and discharge, the voltage range is 3.0~4.5V, after 16 cycles, the capacity increases from 160mA·h/g to 180mA·h/g. It has also been reported that the capacity is 150mA·h/g when the cut-off voltage is 4.2V, and 200mA·h/g when the cut-off voltage is 4.6V and 5.0V, that is, the increase in voltage can increase the specific capacity.
With the deintercalation of lithium atoms, the structure of the material changes accordingly. In LiNi0.4Mn0.4Co0.2O2, the unit cell volume shrinks by 2%, which is much lower than the 5% in LixNi0.75Co0.25O2 and LixTiS2, making it more difficult to undergo mechanical deformation and fragmentation during cycling. The unit cell volume shrinkage of 333 material during the delithiation process is also less than 2%. The reason for the small change in volume is that the changes in the unit cell parameters a and c cancel each other out during this process, and when c increases, a shrinks. In the 442 material shown in Figure 8, the X-ray diffraction parameters are forcibly adjusted to fit the hexagonal lattice of LiMO2.
Figure 8 - Unit cell parameters of LixNi0.4Mn0.4Co0.2O2
Research shows that 333 material is at least equivalent to or even better than pure LiCoO2 cathode material. In the prismatic battery structure, a constant capacity of 600mA.h can be maintained after 30 cycles at 1C. The thermodynamic stability of all these layered oxides in the delithiation process was studied. It is found that although MnO2 is stable at room temperature and in air, CoO2 and NiO2 are unstable. The stable oxide valence states of manganese, cobalt and nickel in the oxide system are 4, 2 and 2 respectively; when heating over 500°C, it becomes Mn2O3, and if the temperature continues to rise, it changes to Mn3O4. Therefore, in the battery, any external thermal deviation will cause the valence state of the elements to change and cause problems in dynamic stability.
For the following four oxides, LixNi1.02O2, LixNi0.89Al0.16O2, LixNi0.70Co0.15O2 and LixNi0.90Mn0.10O2, when x is less than or equal to 0.5, the structure will first transform into a spinel phase, and then into a rock salt structure. The second change is the loss of oxygen. The first step may be due to the composition, usually when x is less than 0.5; the release of oxygen in the second step occurs under low temperature conditions, such as Li0.3Ni1.02O2, which has a lower lithium content, when the temperature is lower than 190°C. The doping of aluminum and cobalt can improve its stability. The compound loses weight at 200°C to form a rock salt structure. Substituting nickel for manganese seems to increase the transformation temperature into the spinel phase. Therefore, Li0.5Ni0.5Mn0.5O2 is still layered even after being placed in an environment of 200°C for 3 days, and a spinel phase is formed above 400°C. And compared with compounds with a 1:1 ratio of nickel to manganese in the molecule, its high temperature stability is better. Spinel and nickel oxide composites will eventually be formed in the air, and NiO+Mn3O4 composites will exist in nitrogen.
The weight loss temperature of the compounds LiNi0.4Mn0.4Co0.2O2 and Li0.5Ni0.33Mn0.33Co0.33O2 is above 300℃, and the weight loss is 7%~8%; only when the temperature is above 450℃, cobalt will generate Co2+ from Co3+ and Ni2+ from Ni4+; manganese is still Mn, and the compound will become a spinel structure at 350°C, and the spinel structure will remain at 600°C.
Summarizing 550 solid solution materials and LiCoO2, we can see that they have the following properties:
①Charge and discharge under mild conditions, the capacity can reach 170mA·h/g after 50 cycles, and the capacity exceeds 150mA·h/g when charging and discharging 2mA/cm2. Increasing the charging cut-off voltage can increase the capacity;
②The synthesis temperature should be greater than 700°C and less than 1000°C, the best temperature is about 900°C;
③ Cobalt can reduce the nickel ions in the lithium layer, and the ratio of Co/Ni should be greater than 1, so as to exclude the nickel in the lithium layer;
④The final heating temperature cannot be higher than 800℃. If the final stable temperature is 900℃, there will be nickel ions in the lithium layer; a certain amount of nickel ions can prevent the formation of a block structure when the lithium concentration is low;
⑤The less likely it is to generate a block structure during the charging cycle, the better its capacity retention performance;
⑥ In Lix(NiMnCo)O2 system, when 0≤x≤1, all materials are not only single-phase; 442 compound will form a block structure only when x=0.05;
⑦The material structure at low x value needs to be studied;
⑧At low potential, nickel is the electrochemically active center ion
⑨The electronic conductivity needs to be improved;
⑩It is still necessary to determine the optimal relationship between the comprehensive performance of the material's capacity, power, and life.