RU2556011C2 - Cathode material for lithium ion batteries based on modified phosphates - Google Patents

Abstract

FIELD: chemistry.

SUBSTANCE: invention relates to an active material based on carbon-coated lithiated vanadium phosphate for use in the positive active mass of lithium ion batteries. Lithiated vanadium phosphate crystals are further modified with a Na⁺ cation on the lithium sublattice, one or more cations from a group comprising Mg²⁺, Al³⁺, Y³⁺ and La³⁺ on the vanadium sublattice, and with a F^- or Cl^- anion on the phosphate sublattice, and are a compound of formula Li_3-xNa_xV_2-yM_y(PO₄)_3-zHal_z/C, where M is one or more metals from a group comprising Mg, Al, Y, La; Hal = F, Cl; 0 < x ≤ 0.1; 0 < y ≤ 0.2; 0 < z ≤ 0.16.

EFFECT: obtaining an active material for use in the positive active mass of lithium ion batteries with high capacitance and power, as well as acceptable cycling stability.

3 dwg

Description

The invention relates to the electrical industry and can be used in the production of lithium-ion batteries (LIA) and batteries based on them, intended for use as energy storage devices for electric vehicles, alternative energy, uninterruptible power supplies, energy recovery systems and balancing network loads.

Recently, the problem of developing energy storage devices for such applications as electric vehicles, uninterruptible power supplies, energy recovery systems and balancing network loads, as well as installations that use alternative (renewable) energy sources to generate electricity, has become particularly urgent. Today, lithium-ion batteries have established themselves as the most promising energy storage devices due to such advantages as high specific energy, low self-discharge, lack of memory effect and the need for periodic maintenance. LIAs are widely known in which lithium cobaltate LiCoO ₂ is used as the active material of the positive electrode [1]. Such batteries are widely used as power sources for portable electronics, but are unacceptable for applications such as transport and energy, requiring high power, cyclic and temperature characteristics, due to the following practical disadvantages:

- short service life (500 cycles);

- an irreversible decrease in capacitance at high potentials and temperatures due to phase transitions in the positive electrode material;

- insecurity during operation due to heat generation during the interaction of deliberate cobalt oxide with an electrolyte;

- the high cost and toxicity of cobalt.

A promising way to create high-power LIA with more stable characteristics is the use of alternative electrode materials. Known material of the positive electrode LIA based on lithium-manganese spinel LiMn ₂ O ₄ [2], non-toxic, cheaper and safe to use compared with lithium cobalt. The disadvantage of lithium-manganese spinel is an irreversible drop in capacity during cycling, especially at elevated temperatures, which makes this material unsuitable for the development of LIB for transport and energy.

Known material of the positive electrode LIA based on lithium ferrophosphate LiFePO ₄ [3], which has such advantages as non-toxicity, high capacity, stability and safety during cycling, due to the fact that oxygen is chemically more strongly bonded in the phosphate structure than in the oxide structure. On the other hand, the structure of lithium ferrophosphate molecules causes a number of inherent disadvantages. Due to the specific features of the crystal structure, lithium ions can move only in one dimension during charge and discharge of a battery, and not in three, as in traditional cathode materials based on transition metal oxides. This is the reason for the low conductivity of the cathode material, both ionic and electronic, which, in turn, leads to a lower value of specific energy (380 W · h / kg). In addition, due to the low conductivity of lithium ferrophosphate, there is a significant decrease in the capacity and power of LIB during operation at low temperatures.

The technical solution adopted for the prototype according to the patent [4] consists in the use of complex lithium-containing phosphates of the composition Li ₃ M ' _y M'' _2-y (PO ₄ ) ₃ , where M' and M '' are transitional as a positive electrode LIA metals selected from the series Fe, Ti, V, Cr, Mn, Co, Mo, Ni, Cu. The most promising possible combination seems to be lithiated vanadium phosphate (lithium vanadium phosphate, lithium phosphovanadate) Li ₃ V ₂ (PO ₄ ) ₃ , as safe to use as LiFePO ₄ , and at the same time having a higher capacity of 197 mA h / g versus 170 mAh / g in LiFePO ₄ due to its ability to reversibly intercalate not one, but three lithium ions. The crystal structure features of Li ₃ V ₂ (PO ₄ ) ₃ allow lithium to be reversibly introduced in not one but three dimensions, which, in turn, leads to higher values of the diffusion coefficient of lithium (five times higher than that of lithium ferrophosphate), energy density (530 W · h / kg). In addition, the discharge voltage of the LIA with positive electrodes based on Li ₃ V ₂ (PO ₄ ) ₃ is 4.1 V versus 3.3 V for LiFePO ₄ .

The practical use of Li ₃ V ₂ (PO ₄ ) ₃ as an active LIA material is still hindered by the drawback inherent in all polyanionic phosphate active materials, namely, low intracrystalline conductivity due to the placement of transition metal ions (vanadium) inside VO ₆ octahedra that are quite distant from each other . In addition, Li ₃ V ₂ (PO ₄ ) ₃ exhibits significant capacitance degradation when charged to a 4.8V potential, which is necessary to extract more than two Li ⁺ ions; in addition, vanadium dissolution is observed. These disadvantages cause degradation of the tank during the cycling process. One way to prevent vanadium from dissolving and increase electrical conductivity is by applying a carbon surface coating. However, the internal conductivity of the material both in lithium ions and in electrons remains practically unchanged, and the charge safety is unsatisfactory - even with a carbon coating, the capacity of the best samples after 50 charge-discharge cycles is 137.5 mAh / g or 94 , 6% of the initial discharge capacity. A more promising way of stabilizing the structure of the active material, increasing the internal conductivity and improving the electrochemical characteristics of Li ₃ V ₂ (PO ₄ ) ₃ is isovalent or alivalent doping along cationic or anionic sublattices.

The aim of the present invention is to develop a cathode active material for lithium-ion batteries, combining the advantages and at the same time devoid of the disadvantages of lithium vanadium phosphate Li ₃ V ₂ (PO ₄ ) ₃ . A distinctive feature of the proposed material is a stable discharge capacity during the cycling process. The introduction of the proposed active material eliminates the problems that impede the creation of safe, stable and durable lithium-ion batteries for transport and energy with high energy and power, workable for a long period of time in a wide range of temperatures and discharge currents.

The technical result is to obtain an active material based on lithiated vanadium phosphate with a surface carbon coating for use in the composition of the positive active mass of LIA. The material is characterized in that Li crystals₃V₂(PO_four)₃ additionally modified with Na cation⁺ on the lithium sublattice, one or more cations from the group containing Mg²⁺Al³⁺, Y³⁺and La³⁺, on the vanadium sublattice and anion F^- or Cl^- sublattice of phosphate, and are a compound of composition Li_3-xNa_xV_2-yM_y(PO_four)_3-zHal_z/ C, where M is one or more metals from the group consisting of Mg, Al, Y, La; Hal = F, Cl; 0 <x≤0.1; 0 <y≤0.2; 0 <z≤0.16.

The crystal lattice of lithium vanadium phosphate Li ₃ V ₂ (PO ₄ ) ₃ is a three-dimensional frame {[V ₂ (PO ₄ ) ₃ ] ^3- } _3∞ , composed of slightly distorted VO ₆ octahedra and PO ₄ tetrahedra with common oxygen vertices , in the voids (channels) of which Li ⁺ cations are located (Fig. 1). The purposeful introduction of various modifying components into the structure of lithium vanadium phosphate makes it possible to change the lattice parameters and thereby improve the electrochemical characteristics of the active material, for example, accelerate the diffusion of lithium ions and electrons or increase the resistance of the material to degradation during cycling.

Partial replacement of lithium with larger sodium ions contributes to an increase in the volume of unit cells, the expansion of intraframe channels, and facilitates the migration of both lithium ions and electrons. In addition, the addition of sodium leads to a decrease in the distance between neighboring lithium ions and an increase in the length of the Li – O bond, i.e. to her weakening. The latter contributes to a decrease in the activation energy of the process of extraction of lithium ions, to a decrease in polarization and resistance to charge transfer, increasing the reversibility of the main redox reaction and thereby improving cyclicity. An important point is that the amount of sodium x introduced should be no more than 0.1; with large amounts of sodium introduced, the monoclinic phosphate structure (α-phase) is transformed into a rhombohedral (n-phase) (Fig. 1), which can lead to an increase in the rate of capacity degradation during cycling. The addition of small amounts of sodium does not change the type of structure, but at the same time it increases the degree of disordering of atoms in the crystal lattice and causes the formation of electronic defects, increasing both the ionic and electronic components of the conductivity of the active material, which contributes to an increase in the value of the delivered capacitance (figure 2). An additional positive effect is a decrease in particle size and optimization of the morphology of the active material, which also increases its conductivity and capacity and improves cyclability.

The addition of Mg ²⁺ in an amount of no more than 0.2 does not change the structural type of the lattice, but contributes to the formation of electronic vacancies (holes) in the phosphate structure, which leads to the appearance of additional p-type conductivity and, thus, improves electrical conductivity. When small amounts of magnesium are introduced, the structural parameter from the crystal lattice increases, which facilitates the process of reversible intercalation of Li ⁺ ions. It should be noted that magnesium is electrochemically inactive, therefore, too high a magnesium content can lead to a decrease in the initial capacity due to an increase in charge transfer resistance and difficulty in lithium diffusion due to the fact that some of the lithium sites in the unit cell will be occupied by magnesium. In addition, the addition of magnesium in quantities of y to 0.2 contributes to a decrease in particle size with increasing y, however, at higher values of y agglomeration of particles of the active material occurs. At a high magnesium content, as well as at low (0.1 ° C) cycling rates, there is no significant improvement in performance, however, the addition of magnesium in the range indicated above helps to stabilize the structure, reduce particle size, reduce the resistance of the active material and increase the conductivity by more than 10 time. As a result, the cyclability improves and the LIA power increases (FIG. 3).

The addition of aluminum in an amount of no more than 0.05 causes the formation of intra-structural defects, which contribute to a decrease in charge transfer resistance, an increase in the electrical conductivity by two orders of magnitude, and facilitation of the transfer of lithium ions inside phosphate particles. As a result, the reversibility of the main electrode reaction and the specific capacitance given during the discharge increase (FIG. 2), the stability of the material and the preservation of the capacitance during cycling are improved, and the LIA power increases. In addition, the addition of Al ³⁺ effectively suppresses the degradation of capacity during the cycling of LIA, even at elevated temperatures. It should be noted that the introduction of aluminum in large quantities is undesirable, since with the increase in the aluminum content the formation of other phases is possible and, in addition, an increase in the particle size of the electrode material is observed, which affects the conductivity, reducing energy and power. The joint doping of lithium vanadium phosphate with Al + Mg promotes the formation of smaller crystals, which also favorably affects both the electrical conductivity of the electrode material and the electrochemical characteristics of the entire battery.

The introduction of yttrium additives in an amount of no more than 0.03 stabilizes the monoclinic structure (α-phase) of Li ₃ V ₂ (PO ₄ ) ₃ , which is more stable during cycling. In addition, partial vanadium substitution with yttrium improves the reversibility of the main electrode reaction, decreases the charge transfer resistance and increases the lithium transfer rate. As a result, the initial specific capacity of the electrode material increases and its safety during cycling, including at elevated temperatures.

Modification of Li ₃ V ₂ (PO ₄ ) _{3 with} lanthanum in an amount of y not more than 0.02 causes an increase in the unit cell volume in the monoclinic phosphate structure, facilitating the migration of lithium ions. This leads to a decrease in resistance to charge transfer and an increase in the diffusion coefficient of lithium. As a result, the initial specific capacity increases during charge and discharge, the Coulomb efficiency increases, and the process of capacity decrease during cycling slows down. The introduction of more lanthanum is undesirable, because with an increase in its content, the resistance to the transfer of Li ⁺ ions along the channels inside the phosphate structure increases, which leads to a decrease in the specific capacity and Coulomb efficiency.

The partial substitution of phosphate anions (PO ₄ ) by ^3- more electronegative halide ions F ^- and Cl ^- in an amount of z≤0.16 does not change the monoclinic structure of Li ₃ V ₂ (PO ₄ ) ₃ , but reduces the energy of Li-O bonds, which leads to facilitate the extraction of lithium ions. In addition, the crystal size uniformity increases and particle size decreases, which helps to reduce the diffusion path for reversibly implanted Li + ions, increase the contact area between the electrode and electrolyte, and facilitate the diffusion of lithium ions deep into the electrode. In this case, the Coulomb efficiency increases, the polarization decreases, the charge transfer resistance decreases, and the ionic and electronic conductivity both on the surface and inside the active electrode material increase. As a result, the discharge capacity and structural stability increase, which leads to an increase in the LIA energy, power and resource (Fig. 3).

Thus, the invention allows to solve the problem of developing LIA with high values of energy, power and resource, operable in a wide range of temperatures and load currents. The present invention is relevant and allows you to create energy storage for transport and energy based on batteries of new generation lithium-ion batteries.

Information sources

1. US patent No. 4,302,518 from November 24, 1981, an electrochemical cell with new fast ionic conductors.

2. US Patent No. 4,507,371 dated March 26, 1985. A solid-state element in which the anode, solid electrolyte and cathode have a structure that is a dense cubic package.

3. US patent No. 5,910,382 of June 8, 1999. Cathode materials for secondary (rechargeable) lithium current sources.

4. US patent No. 5,871,866 of February 16, 1999. Lithium-containing phosphates, the method of production thereof and the field of use.

Claims (1)

Carbon-coated lithium vanadium phosphate-based active material for use as part of the positive active mass of lithium-ion batteries, characterized in that the lithium vanadium phosphate crystals are additionally modified with a Na ⁺ cation in the lithium sublattice, by one or more cations from the group containing Mg ²⁺ , Al ³⁺ , Y ³⁺ and La ³⁺ , - on the vanadium sublattice, and anion F ^- or Cl ^- , - on the phosphate sublattice, and are a compound of the composition Li _3-x Na _x V _2-y M _y (PO ₄ ) _3-z Hal _z / C, where M is one or more metals from the group, containing Mg, Al, Y, La; Hal = F, Cl; 0 <x≤0.1; 0 <y≤0.2; 0 <z≤0.16.

Images