RU2453950C1 - Cathode active material based on lithiated iron phosphate with manganese modifying additive - Google Patents

Abstract

FIELD: physics.

SUBSTANCE: invention relates to electrical industry and can be used in production of cathode active material for lithium-ion accumulators and batteries based thereon, particularly meant for powering electrical vehicles, electrical tools and uninterrupted power devices in conditions with high-energy consuming loads. The disclosed cathode material has the composition LiFe_1-xMn_xPO₄/C, where 0.01≤x≤0.6. Modification of lithiated iron phosphate via partial substitution of iron with manganese increases internal conductance and discharge capacity of the cathode active material and also increases voltage and specific energy of the accumulator.

EFFECT: improved capacitance and energy characteristics of lithium-ion accumulators with LiFePO₄ based cathode material.

5 dwg

Description

FIELD OF TECHNOLOGY

The invention relates to the electrical industry and can be used in the production of cathode active material of lithium-ion batteries and batteries based on them, intended, in particular, for powering electric vehicles, power tools and uninterruptible power supplies under high energy-intensive loads.

BACKGROUND

As the cathode active material of lithium-ion batteries, [1] lithium cobaltate LiCoO ₂ is known. The disadvantages of this active material include the high price and toxicity of the raw materials, insecurity during operation and an irreversible decrease in capacity at high temperatures. Known [2] lithium-manganese spinel LiMn ₂ O ₄ , less toxic and cheaper and safer to use. The disadvantage of this cathode material is the drop in capacity during the cycling process, especially at elevated temperatures.

A number of compounds of the general composition LiMPO _{4 are} known, where M is a transition metal [3], the most studied of which is lithium iron phosphate LiFePO ₄ , which has such advantages as high capacity, environmental safety, low toxicity, low cost and stability during cycling. The disadvantages of this cathode material are its low conductivity and reduced operating voltage (3.4 V).

Known [4] is a method of modifying LiFePO ₄ , which allows to increase the surface conductivity of the cathode material through the use of surface carbon-containing coatings. The disadvantage of this method is that the internal conductivity of phosphate crystals remains virtually unchanged.

The closest technical solution adopted for the prototype, according to the patent [5], is to modify the nanosized carbon-coated lithium iron phosphate by doping it at the positions of Fe ²⁺ and (PO ₄ ) ^3- . The resulting compounds are nanosized crystals of the composition Li _p Fe _x M _1-x (PO ₄ ) _t (AO ₄ ) _1-t with a carbon additive, where M = Co, Ni, Mg, Ca, Zn, A1, Cu, Ti, Zr, A = S, Si, V, Mo, 0 <p <2, 0 <x <1, 0≤t≤1, while the average size of the crystals of the modified phosphate is from 20 to 500 nm, and the average thickness of the carbon coating is from 1 to 20 nm. The disadvantage of this solution is that the operating voltage of a battery with a modified cathode material is practically no different from the voltage of batteries with a cathode based on pure LiFePO ₄ .

SUMMARY OF THE INVENTION

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 lithiated iron phosphate. An advantageous feature of the proposed material based on lithiated iron phosphate is the improved energy characteristics, which allows to expand the scope of lithium-ion batteries.

The technical result is to obtain a cathode active material, which is a lithiated iron phosphate with a surface carbon coating and a modifying additive of manganese, having the formula LiFe _1-x Mn _x PO ₄ / C, where 0.01 ≤ x ≤ 0.6. The advantage of this material is that when iron is replaced with manganese, the electrode potential shifts to a region of more positive values, due to which it is possible to increase the voltage of the battery with cathode active material based on lithiated iron phosphate.

In the olivine-like structure of lithiophilite Li (Fe ²⁺ , Mn ²⁺ ) PO _4, larger Fe ²⁺ (Mn ²⁺ ) cations and smaller P ⁵⁺ cations populate the octahedral and tetrahedral voids, respectively, in a distorted hexagonal dense packing formed by О ² anions ^- . In the voids of the frame structure formed by the tetrahedra of PO ₄ and octahedra of FeO ₆ (MnO ₆ ), Li ⁺ cations are located (Fig. 1). The manganese ion Mn ²⁺ has a larger radius than the iron ion Fe ²⁺ (respectively, 0.084 nm and 0.074 nm in octahedral coordination); therefore, with an increase in the manganese content in the structure of lithiophile, a linear increase in the lattice parameters a, b and c occurs, resulting in an increase unit cell volume V (FIG. 2). An increase in the b parameter improves the transport of Li ⁺ ions in the direction of the b axis. Structural changes caused by the introduction of manganese affect the activation energy of lithium diffusion and, therefore, the lithium diffusion rate. The activation energy of lithium diffusion is directly related to the energy needed to break the lithium-oxygen bonds. Lithium in the structure of lithium LiFe _1-x Mn _x PO _{4 is} less stable than in the structure of trifilite LiFePO ₄ , therefore, the breaking of lithium-oxygen bonds is energetically easier in lithophytes, which leads to an increase in the diffusion rate of lithium and makes lithophyllite a promising cathode material for lithium-ion batteries.

In addition to the size factor, the electronic structure of the replacement element contributes significantly to the conductivity improvement. Substitution of Fe ^{2+ ions} (electronic configuration of the valence shell of 3d ⁶ ) with Mn ^{2+ ions} (3d ⁵ ) causes the formation of additional vacancies localized on manganese, activating electronic conductivity due to local charge transfer between iron and manganese. For example, the specific conductivity of a compound of the composition LiMn _0.01 Fe _0.99 PO ₄ is 2.8 × 10 ^-6 S / m, which is two orders of magnitude higher than the specific conductivity of pure LiFePO ₄ (7.7 × 10 ^-8 S / m). As a result, due to the increased mobility of Li ⁺ ions, the specific capacity of doped phosphoolivine increases. The increase in specific capacity is especially significant with additional modification of phosphate crystals due to the deposition of surface carbon layers (figure 3). The optimal manganese content in the composition of LiFe _1-x Mn _x PO ₄ / C is in the range of 0.01 ≤ × ≤0.6, because at x> 0.6, Mn ³⁺ ions pass from the high-spin to low-spin state with a decrease in the ionic radius, the MnO ₆ octahedra are compressed (Jan-Teller distortion), as a result of which the percentage of lithium extraction from the cathode decreases and a noticeable decrease in discharge capacity occurs.

Due to the coexistence of Fe and Mn in the 4c octahedral interstice in the lattice of the LiFe _1-x Mn _x PO ₄ compound, the potential of the lithiated iron phosphate-based electrode rises to 4.1 V relative to lithium, which is accompanied by an increase in the redox potential of the Fe ³⁺ redox pair / Fe ²⁺ at 0.1 V (FIG. 4). The overall increase in the volume of the lattice with the introduction of manganese contributes to an increase in the length of the transition metal-oxygen bonds (Fig. 5). Due to the redistribution of electron density (inductive effect), the covalent and ionic nature of the iron-oxygen bond is weakened, resulting in an observed increase in the redox potential of the Fe ³⁺ / Fe ²⁺ pair to 3.5 V, which allows to achieve a higher energy density for a battery with cathode material based on LiFe _1-x Mn _x PO ₄ . The indicated voltage shift is negligible for one electrochemical cell, but may be high enough for a battery with a large number of batteries connected in series. Thus, the use of a material of the composition LiFe _1-x Mn _x PO ₄ / C, where 0.01 х x ,6 0.6, allows to increase the discharge capacity and voltage of a lithium-ion battery with a cathode based on lithiated iron phosphate.

Information sources

1. US patent No. 4302518. Electrochemical cell with new superionic conductors. Stated March 21, 1980

2. Canadian Patent No. 2137081. The process of obtaining LiM ³⁺ O ₂ or LiMn ₂ O ₄ and LiNi ³⁺ O ₂ for use in the composition of the positive electrode of a secondary chemical current source. Stated March 31, 1994

3. US Patent No. 5910382. Cathode materials for secondary (rechargeable) lithium current sources. Stated April 21, 1997

4. US patent No. 6962666. Electrode materials with high surface conductivity. Declared December 22, 2003

5. RF patent No. 2402114. Nanoscale composite material containing modified nanoscale lithium iron phosphate and carbon. Announced August 18, 2009

Claims (1)

Lithium iron phosphate cathode active material for lithium-ion batteries and secondary batteries, characterized in that the crystals of lithium iron phosphate with a carbon coating are additionally modified with manganese at the iron positions and are a compound of the composition LiFe _1-x Mn _x PO ₄ / C ( 0.01 х x 0 0.6).

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