RU2492557C1 - Composite cathode material - Google Patents

Abstract

FIELD: electricity.

SUBSTANCE: disclosed is a composite cathode material consisting of a mechanical mixture of nanocrystals of lithium-iron phosphate (LiFePO₄) and a phosphate with a Nasicon structure, specifically: either a double phosphate of the composition Li_xM₂(PO₄)₃, where: x=1 for M=Ti^lv, Zr^lv; x=3 for M=In^III, Cr^III, Fe^III; or a composite phosphate of the composition Li_1+yM^IV _2-yM^III _y(PO₄)₃, where: ; M^lv=Ti^IV Zr^lv; M^III=In^III, Cr^III, Fe^III; wherein the mixture is coated with carbon, with the following ratios of components, wt %: lithium - iron phasphate 82÷98; phosphate with a Nasicon structure 1÷15; carbon 1-6, wherein the size of lithium-iron phosphate crystals ranges from 20 to 100 nm, the size of crystals of the phosphate with a Nasicon structure ranges from 20 to 200 nm, thickness of the carbon coating ranges from 1 to 5 nm.

EFFECT: composite material significantly increases concentration of defects on the phase boundary and increases ion conductivity thereof.

4 dwg, 1 tbl

Description

The invention relates to the electrical industry and can be used for the production of improved cathode active material of lithium-ion batteries for powering portable electronics, power tools, electric vehicles.

Known lithium-ion batteries with active cathode material based on complex oxides LiCoO ₂ , LiMn ₂ O ₄ , LiCo _1/3 Ni _1/3 Mn _1/3 O ₂ . There are several general methods for producing such materials, among which the most common are methods involving the formation of intermediate products in solution, described in the patents [US 5135732] and [US 4246253]. The disadvantages of the materials obtained by these methods are distortion of the structure and the occurrence of adverse reactions during cycling of the battery, which leads to irreversible loss of capacity.

Also known is the cathodic active material LiFePO ₄ / C [US 5910382], which is lithium iron phosphate with a carbon-coated olivine structure. Lithium iron phosphate lithium-ion batteries have significant advantages over standard lithium-ion batteries. The structure of LiFePO _{4 is more} stable due to the stronger bonding of oxygen atoms, which leads to increased safety during operation, while the traditional cathode material LiCoO _{2 is} prone to decomposition with a high degree of charge, which can be accompanied by explosion or fire of the battery. In addition, LiFePO ₄ is charged and discharged at almost the same voltage of about 3.5 V. The disadvantage of LiFePO ₄ is a slightly lower operating voltage, which leads to a decrease in energy consumption and limitation of the scope of use of lithium-ion batteries based on it.

The technical solution of US Pat. No. 7,390,473 also relates to analogues of the invention. According to this patent, LiFePO _{4 is} prepared by mixing reagents in solution, followed by coprecipitation of the precursors or evaporation of the liquid phase. Nanoscale crystalline LiFePO ₄ obtained after exposure of the precursors at a temperature of from 600 to 800 ° C. A significant disadvantage of this method of obtaining the active material is its low electronic and ionic conductivity.

The analogues of the present invention also include the technical solution of the patent of Canada [CA 2307119]. The essence of the invention is to increase the surface electronic conductivity of lithium iron phosphate. An electrically conductive carbon layer is deposited _on LiFePO ₄ crystals. Despite the satisfactory electronic conductivity of such a composite material, it does not have sufficient electrochemical parameters due to the low ionic conductivity.

The closest technical solution [RU 2402114], (prototype), is to increase the ionic and electronic conductivity of lithium iron phosphate by doping the LiFePO4 structure with cations of polyvalent elements Co, Ni, Mg, Ca, Zn, Al, Cu, Ti, Zr, S , Si, V, Mo. The proposed material consists of particles of the composition Li _p Fe _x M _lx (PO ₄ ) _t (AO ₄ ) _lt and a carbon additive, where M = Co, Ni, Mg, Ca, Zn, Al, Cu, Ti, Zr, where A = S, Si, V, Mo, where 0 <p <2; 0 <x <1; 0≤t≤1, with a stated particle size of from 20 to 500 nm and a carbon coating thickness of up to 20 nm.

The disadvantage of the prototype, despite the increase in capacity of a lithium-ion battery based on it, is the relatively low ionic conductivity, which does not allow for a high speed of charging and discharging the battery, as well as a significant decrease in capacity at high currents. An additional increase in the capacity of a lithium-ion battery based on LiFePO ₄ can be achieved by reducing the particle size of the cathode material.

The technical task is to develop a method for producing a cathode material based on LiFePO ₄ with a crystal size of 20 ÷ 400 nm and a relatively higher ionic conductivity.

The invention is directed to finding a method for producing a composite cathode material based on LiFePO ₄ , with a crystal size of 20 ÷ 100 nm, with increased ionic conductivity and, as a consequence, a higher capacity.

The technical result is achieved by the fact that a composite cathode material is proposed, consisting of a mechanical mixture of nanocrystals of lithium iron phosphate (LiFePO ₄ ) and phosphate with a Nasikon structure, namely: either double phosphate composition Li _x M ₂ (PO ₄ ) ₃ , where: x = 1 for M = Ti ^IV , Zr ^IV ; x = 3 for M = In ^III , Cr ^III , Fe ^III ; or complex phosphate of the composition Li _{l + y} M ^IV _2-y M ^III _y (PO ₄ ) ₃ , where: y = 0.001 ÷ 1.999; M ^IV = Ti ^IV , Zr ^IV ; M ^III = In ^III , Cr ^III , Fe ^III , while the mixture is coated with carbon in the following ratios of components, wt.%:

lithium iron phosphate 82 ÷ 98 phosphate with Nasicon structure 1 ÷ 15 carbon 1 ÷ 6,

moreover, the particle size of lithium iron phosphate is from 20 to 100 nm, the particle size of phosphate with a Nasicon structure is from 20 to 200 nm, and the thickness of the carbon coating is from 1 to 5 nm.

The proposed composite cathode material is obtained by mechanochemical interaction of the components. A thin Debye layer is formed at the phase boundary, which contributes to an increase in ionic conductivity, acceleration of electrochemical processes, and an improvement in the characteristics of the cathode material as a whole.

The introduction of phosphate with a Nasicon structure in an amount of less than 1 wt.% Does not lead to an improvement in the characteristics of the composite cathode material due to insufficient phase boundary. The introduction of phosphate with a Nasicon structure in an amount of more than 15 wt.% Leads to a decrease in ionic conductivity and material capacity.

The essence of the invention lies in the fact that to create a cathode material based on LiFePO ₄ with increased ionic conductivity and small particle size, it is proposed to obtain a composite material containing phosphate with the Nasicon structure [Li _x M ₂ (PO ₄ ) ₃ (x = 1 for M = Ti ^IV , Zr ^IV ; x = 3 for M = In ^III , Cr ^III , Fe ^III ) or Li _{l + y} M ^IV _2-y M ^III _y (PO ₄ ) ₃ (y = 0.001 ÷ 1.999, M ^IV = Ti ^IV , Zr ^IV ; M ^III = In ^III , Cr ^III , Fe ^III )], which allows to increase the duration of cycling, to increase the capacity of lithium-ion batteries based on it.

The specified technical problem and the specified technical result are achieved due to the use of crystals with high ionic conductivity as an additional component.

The essence of the invention is illustrated by the following accompanying illustrations:

Figure 1. X-ray diffraction pattern of LiFePO ₄ + 5% Li _1.3 Ti _1.7 Fe _0.3 (PO ₄ ) ₃ + 4% C.

Figure 2. Micrograph of LiFePO ₄ + 5% Li _1.3 Ti _1.7 Fe _0.3 (PO ₄ ) ₃ + 4% C.

Figure 3. The charge-discharge curve of the first cycle for the LiFePO ₄ + 5% Li _1.3 Ti _1.7 Fe _0.3 (PO ₄ ) ₃ + 4% C sample during cycling with a current of 0.1C.

Figure 4. Change in discharge capacity for a LiFePO ₄ + 5% Li _1.3 Ti _1.7 Fe _0.3 (PO ₄ ) ₃ + 4% C sample upon cycling with a current of 0.1 ° C.

The invention is implemented as follows. The inventive composite cathode material based on LiFePO ₄ obtained by the method comprising the following stages:

- precursor synthesis for LiFePO ₄ : preparation of an aqueous solution of the main components containing cations of lithium, iron, phosphate anions; keeping the reaction mixture at temperatures of 150 ÷ 500 ° C.

- phosphate synthesis with the Nasikon structure [Li _x M ₂ (PO ₄ ) ₃ (x = 1 for M = Ti ^IV , Zr ^IV ; x = 3 for M = In ^III , Cr ^III , Fe ^III ) or Li _{l + y} M ^IV _2-y M ^III _y (PO ₄ ) ₃ (y = 0.001 ÷ 1.999, M ^IV = Ti ^IV , Zr ^IV ; M ^III = In ^III , Cr ^III , Fe ^III )]: preparation of a solution of reagents in ethylene glycol; keeping the reaction mixture at 150 ÷ 400 ° C; final annealing at temperatures of 650 ÷ 950 ° C and cooling.

- mixing the obtained powders in stoichiometric amounts and adding a precursor to produce carbon;

- carrying out mechanical activation of the mixture by grinding in a ball mill to improve contact between particles;

- maintaining the reaction mixture at a temperature of from 500 to 800 ° C in an inert atmosphere. The exposure time should be sufficient for the formation of reaction products, usually 8 ÷ 14 hours. The choice of the lower temperature limit is due to the insufficient degree of crystallization of the product at temperatures below 500 ° C. At temperatures above 800 ° C, the crystal growth rate of the reaction product becomes too high, particle agglomeration begins, which leads to the formation of a coarse-grained product with insufficient electrochemical activity.

The table shows examples of the implementation of the claimed invention. The examples illustrate but do not limit the proposed method.

Table Example No. Current capacity 0.1C / ionic conductivity of LiFePO ₄ / C / phosphate samples with a Nasicon structure with different contents and composition of a phosphate phase with a Nasicon structure and with a content of C = 4 wt.% one. LiFePO ₄ /1% Li _1.3 Ti _1.7 Fe _0.3 (PO ₄ ) ₃ 146 mAh / g / 5 · 10 ^-10 Ohm ^-1 cm ^-1

2. LiFePO ₄ /5% Li _1.3 Ti _1.7 Fe _0.3 (PO ₄ ) ₃ 154 mAh / g / 1 · 10 ^-9 Ohm ^-1 cm ^-1 3. LiFePO ₄ /15% Li _1.3 Ti _1.7 Fe _0.3 (PO ₄ ) ₃ 140 mAh / g / 9 · 10 ^-11 Ohm ^-1 cm ^-1 four. LiFePO ₄ /5% Li _1.1 Zr _1.9 Fe _0.1 (PO ₄ ) ₃ 150 mAh / g / 3 · 10 ^-10 Ohm ^-1 cm ^-1 5. LiFePO ₄ /5% Li _1.7 Zr _1.3 Fe _0.7 (PO ₄ ) ₃ 145 mAh / g / 1 · 10 ^-10 Ohm ^-1 cm ^-1 6. LiFePO ₄ /5% LiZr ₂ (PO ₄ ) ₃ 148 mAh / g / 2 · 10 ^-10 Ohm ^-1 cm ^-1 7. LiFePO ₄ /5% Li ₃ In ₂ (PO ₄ ) ₃ 146 mAh / g / 2 · 10 ^-10 Ohm ^-1 cm ^-1 Prototype LiFePO ₄ 138 mAh / g / 6 · 10 ^-12 Ohm ^-1 cm ^-1

Materials and methods

The following starting reagents can be used for the synthesis of LiFePO ₄ : lithium nitrate (LiNO ₃ , 99 +%, Aldrich), lithium carbonate (Li ₂ CO ₃ , Merck,> 99%), lithium hydroxide (LiOH, 99%, Acros), oxalate iron II (FeC ₂ O ₄ * 2H ₂ O, 99% Aldrich), iron nitrate III (Fe (NO ₃ ) ₃ 9H ₂ O,> 98%, Riedel-de Haen), iron oxide III (Fe ₂ O ₃ , > 98%, Sigma Aldrich), ammonium hydrogen phosphate ((NH ₄ ) ₂ HPO ₄ ,> 97%, Roth), iron phosphate III (FePO ₄ , 98%, Fluka), ammonium dihydrogen phosphate (NH ₄ H ₂ PO ₄ ,> 98%, Roth). For the synthesis of phosphates with the Nasicon structure, you can use: ammonium hydrogen phosphate ((NH ₄ ) ₂ HPO ₄ ,> 97%, Roth), lithium carbonate (Li ₂ CO ₃ , Merck,> 99%), citric acid (C ₆ H ₈ O ₇ , ChDA, Chemmed), titanium butoxide ((C ₄ H ₉ O) ₄ Ti, Alfa Aesar, 98 +%), zirconium oxychloride (ZrOCl ₂ , Merck,> 98%), indium oxide (In ₂ O ₃ ,> 99%, Merck), iron oxide III (Fe ₂ O ₃ ,> 98%, Sigma Aldrich), chromium III nitrate (Cr (NO ₃ ) ₃ · 9H ₂ O, 99%, Acros). As a carbon source, you can use activated carbon, citric acid (C ₆ H ₈ O ₇ , ChDA, Khimmed), sucrose (C ₁₂ H ₂₂ O ₁₁ , Alfa Aesar, ≥98%), hydroxyethylene cellulose (Merck), polypropylene ( [CH ₂ CH (CH ₃ )] _n , Sigma Aldrich).

The analysis of the obtained powders is carried out using scanning electron microscopy (using a Carl Zeiss NVision 40 scanning electron microscope at an accelerating voltage of 1 kV and determining the average particle size). X-ray phase analysis (XRD) is carried out on a Rigaku D / MAX 2200 diffractometer (CuK _α radiation). Laboratory tests of the obtained samples of composite cathode materials are carried out in test lithium cells and lithium-ion batteries with a capacity of 3 mAh using a charge-discharge stand of switchgear 50 mA - 10 V (LLC NTC Booster, Russia). The search for preferred material compositions was determined by discharge capacity.

Results and Conclusions

According to the XRD data, all the examples presented correspond to the structural parameters and phase composition indicated in the Table (Figure 1). The linear dimensions of single crystals of lithium iron phosphate are from 20 to 100 nm, preferably 30-50 nm; linear dimensions of phosphates with the Nasicon structure are 20–200 nm. These parameters are determined according to electron microscopy for each component separately (Figure 2, example 2). The specific capacity of the material is> 150 mAh / g at a current of 0.1C (Figure 3, example 2); > 95 mAh / g at 10C. Loss of capacity after 200 cycles do not exceed 3% (Figure 4, example 2).

A distinctive feature of the composite cathode material is an increased concentration of defects at the phase boundary and increased ionic conductivity. The synthesized composite cathode material has a high electrochemical capacity both at low and at high (up to 10 ° C) discharge rates in comparison with the known analogues and prototype. This set of essential features of the new composite cathode material expresses the essence of the invention, which can solve the problems of production and operation of lithium-ion batteries and expand the possibilities of their use.

Claims (1)

A composite cathode material consisting of a mechanical mixture of nanocrystals of lithium iron phosphate (LiFePO ₄ ) and phosphate with the Nasikon structure, namely: either double phosphate composition Li _x M ₂ (PO ₄ ) ₃ , where: x = 1 for M = Ti ^IV Zr ^IV ; x = 3 for M = In ^III , Cr ^III , Fe ^III ; or complex phosphate composition Li _{1 + y} M ^IV _2-y M ^III _y (PO ₄ ) ₃ , where: y = 0.001 ÷ 1.999; M ^IV = Ti ^IV , Zr ^IV ; M ^III = In ^III , Cr ^III , Fe ^III , while the mixture is coated with carbon at the following component ratios, wt.%:
lithium iron phosphate 82 ÷ 98 phosphate with Nasicon structure 1 ÷ 15 carbon 1 ÷ 6,
moreover, the size of the crystals of lithium iron phosphate is from 20 to 100 nm, the size of the crystals of phosphate with the Nasicon structure is from 20 to 200 nm, the thickness of the carbon coating is from 1 to 5 nm.

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