What is lithium metal nitride?

The research on nitrides mainly stems from the high ionic conductivity of Li₃N (10^-2S/cm), that is, lithium ions are prone to migration. However, the decomposition voltage of Li₃N is relatively low (0.44V), so it is not suitable to be used directly as an electrode material.

The high ionic conductivity of lithium metal nitride and the variable valence of transition metals make it possible to become a new type of negative electrode material for lithium-ion batteries. Lithium metal nitrides can be divided into inverse fluorite type and Li₃N type according to their structure.

Lithium nitrogen compounds belonging to the inverse fluorite structure include Li₇MnN₄ and Li₃FeN₂. Fluorite is usually called CaF₂, and its structure is shown in Figure 1. Fluorine is located in the face-centered cubic position, and calcium is located in the center of the tetrahedron with fluorine as the apex. In the periodic table, Ti to Fe can form compounds with the general formula Li_2n-1Mn. Among them, Li₅TN₃, Li₇VN₄, Li₅Cr₂N₉, Li₇MnN₄, Li₃FeN₂, etc. can exist stably. These nitrides correspond to the CaF₂ structure, which is equivalent to nitrogen at the calcium site. The fluorine position is composed of lithium and metal ions, and the arrangement of anions and cations is exactly opposite to that of CaF2, so it is called an inverse fluorite structure. The Ti, V, and Cr in the above-mentioned nitrides have reached the highest oxidation state. When Li is released, the valence state cannot be changed to maintain the electrical neutrality in the system. Therefore, only Li₇MnN₄ and Li₃FeN₂ may be used as electrode materials.

Figure 1 CaF2 crystal structure

Li₇MnN₄ and Li₃FeN₂ can be obtained from Li₃N and metals (Mn, Fe) or metal nitrides (Mn₄N, Fe₄N) in a certain proportion, heated at 600~700℃ for 8-12h in a nitrogen atmosphere. The synthesis reaction formula is as follows:
(3/7)Li₃N+Mn+(5/6)N₂=Li₇MnN₄
Li₃N+Fe+(1/2)N₂=Li₃FeN₂

In the Li₇MnN₄ structure, MnN4 exists independently as a tetrahedron, and the lithium-occupied points form a three-dimensional network. The valence state of manganese is +5, and its highest valence state is +7, so the theoretical maximum deintercalation amount of lithium ions is 2. The charging and discharging platform of Li7MnN4 is around 1.2V, and the capacity is 260mA·h/g when lithium is first removed to 1.7V, which is equivalent to 1.55 lithium removal. At this time, manganese presents two valence states of +6 and +7. Subsequently, the reversible lithium extraction capacity in the voltage range of 0.8~1.7V is about 300mA·h/g, and it has good cycle performance. Li₇MnN₄ has a second phase in the charging process, and it can return to the original phase after discharge, indicating that the material has good reversibility of lithium deintercalation. Unlike Li₇MnN₄, FeN₄ tetrahedrons in Li₃FeN₂ co-edge to form a one-dimensional chain structure along the c-axis. From the structural point of view, it has better electronic conductivity than Li₇MnN₄. The valence state of iron is +3, and it can be changed to +4 during delithiation. The theoretical maximum deintercalation amount of lithium ions is 1. When the delithiation amount is greater than 1, Li₃FeN₂ decomposes, and the decomposition voltage is about 1.5V. Li₃FeN₂ is charged The discharge platform is very flat (about 1.2V), the reversible lithium extraction capacity is about 150mA·h/g, and four different phases are generated during the charge and discharge process.

Li₃N has P₆ symmetry, and its structure consists of alternately arranged Li₂⁺ N^3-layers (A layer) and Li⁺ layers (B layer). The structure of lithium metal nitrides Li_3-xM_xN (M=Co, Ni, Cu) and Li₃N, in which Co, Ni, Cu partially replace the lithium in the B layer (Figure 2 takes Li_2.5Co_0.5 N as an example), Li_{3 -x}M_xN is usually prepared by using metal powder and Li₃N powder as reactants by high-temperature solid-phase method under nitrogen atmosphere. The composition range of Li_3-xM_xN solid solution synthesized by this method is: 0≤x≤0.5(Co); 0≤x≤0.6(Ni); 0≤x≤0.4(Cu). Since M²⁺ (especially Co²⁺, Ni²⁺) and M⁺ coexist in the Li_3-xM_xN system and form lithium defects of the same order of magnitude, the accurate expression for this type of nitride should be Li_3-x-y(M⁺_x-yM²⁺_y)N, where y represents a lithium vacancy.

Figure 2 Schematic diagram of the crystal structure of Li_2.5Co_0.5 N

Among nitrides, Li_2.6Co_0.4N material has the best electrochemical performance. The average charging and discharging voltage of Li_2.6Co_0.4N material is 0.6V, and the reversible lithium extraction capacity in the voltage range of 0~1.4V is 760~900mA·h/g, which is more than twice the theoretical capacity of graphite-like carbon materials. The density is comparable to graphite. When the Li_2.6Co_0.4N material is delithified for the first time, it is equivalent to about 1.6 lithium being removed, and the structural formula is changed to Li_1.0Co_0.4N, which means that all the lithium in the B layer and half of the lithium in the A layer are separated. When the upper limit voltage of delithiation exceeds 1.4V, the A layer may decompose due to excessive delithiation, and the structure will be destroyed, which will cause the material to lose electrochemical activity. During the first delithiation process, the material changes from a crystalline form to an amorphous form, and rearrangement of some elements occurs, and the amorphous form is maintained in the subsequent cycles. This amorphous form can allow a large amount of lithium ions to be deintercalated, which is the main reason for the high deintercalation capacity of Li_2.6Co_0.4N.

Lithium ions are the only ion that can be deintercalated during the charging and discharging process of the material, and the cobalt ions in the structure or the anions of the electrolyte components do not participate in it. Obviously, the cobalt in Li_2.6Co_0.4N is +1 valence, and the valence states of lithium and nitrogen are +1 valence and -3 valence respectively. For Li_1.0Co_0.4N, the product of delithiation, the valence of each element is more complicated. The stable charge caused by delithiation is compensated by the change in the valence of cobalt, and the valence of cobalt will change from +1 to +5. Cobalt does not have a high valence state of +5, so it is likely that part of the valence state of nitrogen has also changed during the delithiation process. This means that cobalt and nitrogen play a positive role in keeping the charge stable when the material is released from lithium. It is believed that Li_2.6Co_0.4N has strong covalent characteristics between cobalt and nitrogen, and it is not a compound with strong ionic characteristics. That is to say, the nitrogen in Li_2.6Co_0.4N is not all a trivalent, and the valence of cobalt is difficult to determine, and it may be between +2 and +3. Someone studied the structural change of Li_2.6Co_0.4N when lithium was deintercalated for the first time. The results showed that when the material was first delithified to 1.4V, the structure obviously changed. The nitrogen atom in the A layer deviated from the original position, and the material changed. It is amorphous. Then in the next lithium insertion process of the material, most of the lithium is re-inserted in the A layer, and this de-intercalation process is reversible from the short-range order.

Li_3-xCu_xN and Li_3-xNi_xN materials are far inferior to Li_2.6Co_0.4N materials in terms of reversible deintercalation capacity and other properties, so there is less research. The Li_2.6Co_0.4N material has a lithium insertion and desorption capacity of 650mA·h/g in the voltage range of 0~1.3V, and its cycle performance is relatively stable. The performance of Li_2.5Co_0.5N material is poor, and the lithium insertion and removal capacity is less than 250mA·h/g in the voltage range of 0~1.4V. Although these three materials also have a structure like Li₃N, their microstructure may exist There are differences. For example, the amorphous state formed after the material is delithified is different from the electronic environment around the lithium, which leads to different mechanisms for deintercalation of lithium, and thus exhibits completely different electrochemical properties.

Unlike other negative electrode materials, Li_3-xM_xN (M=Co, Ni, Cu) has a lithium-rich structure, and its charge and discharge voltage is several hundred millivolts higher than graphite-like carbon materials. Therefore, as a negative electrode material, a lithium-poor 5V positive electrode is required. The material corresponds to it; or it can be used in conjunction with lithium-rich LiCoO₂ and other positive electrode materials after removing part of the lithium in advance. Taking advantage of the characteristic that Li_2.6Co_0.4N's first lithium removal capacity is greater than the first lithium insertion capacity, it can be combined with some anode materials with higher initial irreversible capacity (such as SiO, Sn_xO, etc.) to form a high-performance composite electrode to improve the initial charge Discharge efficiency.

Among many lithium metal nitride materials, it is generally believed that only Li_3-xM_xN (M=Co, Ni, Cu) has a Li₃N structure. Research shows that pre-compacting Li₃N powder into a block, put it into a pure iron container filled with 300kPa nitrogen, and heat it for 12h at a temperature of 850 to 1050°C, and then undergo a quenching process to obtain Li_2.7Fe_0.3N material. The Li_2.7Fe_0.3N prepared by this method also has a structure like Li3N, and the reversible lithium extraction capacity is 550mA·h/g in the voltage range of 0.0~1.3V. Different from Li_2.6Co_0.4N, the first delithiation curve of Li_2.7Fe_0.3N has two voltage plateaus, which may be relative to the lithium in the A and B layers of the structure. Moreover, the material is also transformed into an amorphous form after the first delithiation, and the specific deintercalation mechanism needs to be further studied.