The Li-Mn-Cr-O2 system has a NaFeO2 structure. When part of the transition metal is replaced by Li ions, such as Li3CrMnO5 or layered Li[Li0.2Mn0.4Cr0.4]O2, its cycle performance is very good. The discharge curve shows its single-phase rather than two-phase characteristics. The lithium ion clusters in the transition metal layer structure are around manganese ions, such as in Li2MnO3.
In the LiNi1-yMnyO2 system. The composition with the best electrochemical performance is LiNi0.5Mn0.5O2. The XPS and magnetic data are consistent with the current interpretation. The valence states of the elements in the compound are NP+ and Mn+ instead of Ni3+ and Mn3+. It has been reported that it has better electrochemical cycling performance than LiNiO2, and this compound is called 550 material (that is, the molecular ratio of the compound is 0.5Ni, 0.5Mn, 0.0Co). In the LiNiyMnyCo1-2yO2 system, when 0.33≤y≤0.5, it is found that this is a solid solution of LiNi0.5Mn0.5O2 and LiCoO2.
When LiNi0.5Mn0.5O2 is a hexagonal lattice, a=0.2894nm, c=1.4277 nm, a=0.2892nm, c=1.4301nm, c/3a=1.647. Excessive insertion of lithium can cause slight expansion of the hexagon of the unit cell, and its lattice constant is a=0.2908nm, c=1.4368nm; this may be a compound with the molecular formula Li2Ni0.5Mn0.5O2. This raises another question, that is, if lithium occupies a tetrahedral position, what position does nickel occupy in the lithium layer?
The capacity of 550 material decreases from 150mA·h/g to 125mA·h/g after 25 cycles, and to 75mA·h/g after 50 cycles; when the synthesis temperature rises from 450°C to 700°C, the capacity and capacity retention of the material increase, which is the lowest temperature known so far for materials with suitable electrochemical properties. Someone prepared 550 materials at 1000°C, with a current of 0.1mA/cm2, a charge cut-off voltage of 4.3V, and 30 cycles to obtain a constant capacity of 150mA·h/g, the battery's operating voltage range is 4.6~3.6V, as shown in Figure 1; the figure shows the operating voltage data in LixNi0.5Mn0.5O2 as the value of x changes. The reaction is a single camera. For LiNi0.25Mn0.75O4, its structure is closer to the spinel structure, which is a typical two-phase reaction mechanism, and there are two discharge platforms.
Figure 1 - The working voltage curve of the layered and spinel lithium nickel manganese oxide with different contents of lithium
The 550 material, sintered at 900°C and annealed at room temperature, has a capacity of more than 150mA·h/g after being made into a thin-film electrode after 50 cycles. After charging to 4.4V with constant current and then overcharging with 4.4V constant voltage, the first overcharge capacity reached 170mA·h/g, but after 20 cycles, the capacity failure was less than 150mA·h/g. Like most lithium battery materials, the lithium ion diffusion coefficient of the 550 sample is about 3×10-10cm2/s. The 550 material can insert the second lithium, especially after a certain amount of titanium is doped, so that Mn(IV) is reduced to Mn(II), and the material with the molecular formula yLiNi0.5Mn0.5O2·(1-y)Li2TiO3 is obtained; Li[Ni1/3Mn5/9Li1/9]O2 can be obtained by replacing part of the transition metal with lithium, and its capacity can be increased from 160mA·h/g to 200mA·h/g, and the assembled battery is charged and discharged at a current of 0.17mA/cm2 and a voltage of 4.5V and maintained for 19h; the cathode active material content is about 15mg/cm2. These data can be seen in Figure 2, the discharge current can exceed 10mA/cm2.
Figure 2 - Ragone curve between the discharge rate of LiNi0.5Mn0.5O2 material and its capacity
In this type of compound, the electrochemically active element is nickel, whose valence is between +2 and +4, and manganese is always +4 regardless of the lithium content. The application of first-principles quantum theoretical calculations and structural determinations confirmed this redox mechanism. Since the manganese in it maintains a +4 valence and does not generate Mn3+, there is no Jahn-Tell-er effect. In order to verify this model, the ion exchange method was used to react with similar Na compounds to synthesize the compound Li0.9Ni0.45Ti0.55O2 with a-NaFeO2 structure; it was found that the dislocation of the cations was found to form a rock salt structure at high temperatures. About half of the lithium in the battery assembled from such materials can be extracted, and only 50% of the lithium can be re-inserted during discharge; this is due to the substitution of cations, which may be the migration of titanium ions into the lithium layer. Obviously, the manganese ion at this time plays a key role in the stability of the α-NaFeO2 structure.
It is very complicated to determine the precise information of LixNi0.5Mn0.5O2 structure by X-ray diffraction. The long-range order has been determined by the emission electron microscope, and with the increase of the lithium content, the 550 material gradually transformed into Li2MnO3, and its sequence size increased from 1nm to 2nm. Obviously, 8%-10% nickel is always present in the lithium layer in the structure, and correspondingly, the transition metal also contains a certain amount of lithium. NMR studies have shown that lithium is surrounded by six manganese ions in transition metals, such as Li2MnO3. Through calculation and experimental verification, about 8% of lithium is required in the transition metal layer; manganese ions are sequentially surrounded by nickel ions on the hexagonal lattice. During the charging process, lithium is first extracted from the lithium layer, but when two adjacent lithium sites in a certain layer are all vacant, the lithium ions in the transition metal layer will move down from the octahedral position to the empty tetrahedral position. This is consistent with the results observed in NMR that lithium can be reversibly released from the transition metal layer during the charging of the material. The lithium in the tetrahedral position can only be released under the highest voltage state, and when all the octahedral lithium has been released, it can be released. This reaction mechanism is consistent with the lithium deintercalation mechanism of the lithium-deficient tetrahedron Li0.5Ni0.5Ni0.5O2 material.
In summary, 550 material has the following electrochemical properties:
①Under mild cycle conditions, at least 50 cycles, its specific capacity is higher, about 180mA·h/g
②Overcharging will accelerate the capacity decay, but the surface coating can help prevent its capacity decay;
③The synthesis temperature is 700~1000℃, and the best temperature is generally about 900℃;
④ A certain amount of nickel ions in the compound exist in the lithium layer, the content is about 10%, which will limit the specific capacity of the material and reduce its energy density;
⑤The addition of cobalt can help reduce the nickel in the lithium layer, such as the compound LiNi1-yCoyO2;
⑥Lithium in the transition metal layer is an inevitable component of the material structure;
⑦Ni is the electrochemically active element in the compound.