Lithium metal phosphates, their preparation and use

Abstract

The present invention provides a carbon-coated lithium metal phosphate that is aluminum-doped to an aluminum content of 300-5000 ppm and has a BET surface area of ​​15 m / g or less. The carbon-coated lithium metal phosphate finds use as a cathode active material and offers improved electrochemical performance at low temperatures.

Description

[Technical field]

[0001] The present invention relates to lithium metal phosphate materials, their preparation and use as cathode materials in secondary lithium ion batteries. [Background technology]

[0002] Lithium metal phosphate materials, such as lithium iron phosphate (LFP), have found widespread use as cathode materials in secondary lithium-ion batteries due to the advantageous properties of batteries incorporating such materials, including high power density and good safety profiles. The lithium metal phosphate materials used in such batteries are primarily in the form of electrically conductive carbon-coated particles and are typically produced by melt, hydrothermal, or solid-state processes.

[0003] The electrochemical performance of batteries incorporating lithium metal phosphate materials degrades significantly at low temperatures due to several factors, including increased internal resistance, which can be a significant problem for certain battery applications, such as batteries used to power starter motors, e.g., 12 / 48V starter batteries. Summary of the Invention [Problem to be solved by the invention]

[0004] There remains a need for enhanced lithium metal phosphate materials that exhibit improved electrochemical performance at low temperatures.

[0005] The present inventors have found that the electrochemical performance of lithium metal phosphate at low temperatures can be significantly improved by providing an at least partially agglomerated carbon-coated lithium metal phosphate having a low surface area and including aluminum as a dopant.

[0006] Thus, in a first preferred aspect, the present invention provides a carbon-coated lithium metal phosphate in the form of secondary particles formed, at least in part, by aggregation of primary particles, which carbon-coated lithium metal phosphate meets the following requirements: (i) Lithium metal phosphate has the formula: Li a (Fe 1-x M x )PO 4 wherein 0.8≦a≦1.2, 0≦x≦0.1, M is one or more selected from Mn, Ni, Co, Mg, and B, and the lithium metal phosphate is doped with aluminum such that the carbon-coated lithium metal phosphate has an aluminum content of 300-5000 ppm; (ii) Carbon-coated lithium metal phosphate is 15m 2 / g or less BET surface area.

[0007] The inventors have found that when such materials are incorporated into electrochemical cells, the internal resistance at low temperatures is significantly reduced compared to state of the art materials.

[0008] Such materials may advantageously be produced by hydrothermal methods. Thus, in a second preferred aspect, there is provided a process for preparing the carbon-coated lithium metal phosphates described herein, the process comprising: (i) combining an iron(II) source with at least one lithium source, at least one phosphate source, at least one aluminum source, and optionally at least one M source to form a precursor mixture; (ii) obtaining lithium metal phosphate from the precursor mixture under hydrothermal conditions; (iii) mixing the lithium metal phosphate with a carbon source and spray drying the mixture; (iv) heating the lithium metal phosphate and the carbon source to form a carbon-coated lithium metal phosphate.

[0009] The present invention further provides a carbon-coated lithium metal phosphate obtained or obtainable by the process of the second aspect.

[0010] In a further preferred aspect, the present invention provides the use of the carbon coated lithium metal phosphate of the present invention for the preparation of a cathode of a secondary lithium ion battery. In a further preferred aspect, the present invention provides a cathode comprising the carbon coated lithium metal phosphate of the present invention. In a further preferred aspect, the present invention provides a secondary lithium ion battery comprising a cathode comprising the carbon coated lithium metal phosphate of the present invention. The battery typically further comprises an anode and an electrolyte. [Brief description of the drawings]

[0011] [Figure 1] FIG. 1 shows the particle size distribution of the material of Example 2. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0012] Preferred and / or optional features of the invention will now be described. Any aspect of the invention may be combined with any other aspect of the invention unless the context requires otherwise. Any preferred and / or optional features of any aspect may be combined with any aspect of the invention, either singly or in any combination, unless the context requires otherwise.

[0013] The present invention provides a carbon coated lithium metal phosphate in the form of secondary particles formed, at least in part, by aggregation of primary particles, the lithium metal phosphate having the formula: Li a (Fe 1-x M x )PO 4 wherein 0.8≦a≦1.2, 0≦x≦0.1, M is one or more selected from Mn, Ni, Co, Mg, and B, and the lithium metal phosphate is doped with aluminum such that the carbon-coated lithium metal phosphate has an aluminum content of 300-5000 ppm.

[0014] The value of a is 0.8 or more. It may be 0.9 or more, or 0.95 or more. The value of a is 1.2 or less. It may be 1.1 or less, or 1.05 or less. Typically, 0.9≦a≦1.1, or 0.95≦a≦1.05. The value of a may be 1, or about 1.

[0015] The value of x is 0 or more, which may be 0.01 or more, or 0.02 or more. The value of x is 0.1 or less. It may be 0.075 or less, or 0.05 or less. It may be preferred that 0≦x≦0.05.

[0016] Further, the value of x may be preferably 0, and the lithium metal phosphate may have the formula Li a FePO 4 wherein 0.8≦a≦1.2, or LiFePO 4 It is.

[0017] M is one or more selected from Mn, Ni, Co, Mg, and B. It may be preferred that M is Mn, or M is one or more selected from Ni, Co, Mg, and B, or M is one or more selected from Ni and Co.

[0018] The lithium metal phosphate is doped with aluminum such that the carbon coated lithium metal phosphate has an aluminum content of 300-5000 ppm. The aluminum content is 300 ppm or greater. Surprisingly, the inventors have found that the inclusion of aluminum in an amount of 300 ppm or greater provides a lithium metal phosphate having reduced internal resistance when incorporated into an electrochemical cell. The aluminum content is 5000 ppm or less. It has been found that concentrations of aluminum greater than 5000 ppm result in a reduction in specific capacity.

[0019] The aluminum content may preferably be 500 to 5000 ppm, for example, 500 to 4500 ppm, 600 to 4000 ppm, 700 to 3500 ppm, 800 to 3500 ppm, 900 to 3500 ppm, 1000 to 3500 ppm, 1200 to 3500 ppm, or 1400 to 3500 ppm.

[0020] The aluminum content of the carbon-coated lithium metal phosphate can be measured, for example, by inductively coupled plasma optical emission spectroscopy (ICP-OES). Using the processes described herein, it has been found that at least a portion of the aluminum dopant is incorporated into the phospho-olivine crystal lattice of the lithium metal phosphate.

[0021] The carbon coated lithium metal phosphate is at least partially in the form of secondary particles formed by agglomeration of primary particles. Preferably, a majority or substantially all of the carbon coated lithium metal phosphate is in the form of secondary particles formed by agglomeration of primary particles. A majority of the carbon coated lithium metal phosphate means that the secondary particles are present in an amount greater than 50% by volume of the composition. The inventors have found that the formation of agglomerated particles in combination with the presence of aluminum results in enhanced internal resistance properties of the carbon coated lithium metal phosphate.

[0022] Carbon-coated lithium metal phosphate is 15m2 The materials of the present invention are surprisingly able to achieve low resistivity in combination with low BET surface area. 2 We have found that the resistance of at least partially agglomerated carbon-coated lithium metal phosphate with a surface area of ​​15 m / g or less is lower than that of non-agglomerated material with a similar Al-content but a higher BET surface area. 2 A surface area of ​​7 m / g or less is beneficial when using lithium metal phosphate to form an electrode because it reduces the amount of binder and solvent required to form a suitable electrode slurry. The lower limit of the BET surface area is not particularly limited in the present invention, but typically the BET surface area is 7 m 2 / g. Therefore, carbon-coated lithium metal phosphate has a solubility of 8-14m 2 / g etc. 7~15m 2 / g, or 7 to 13 m 2 / g, or 8 to 12 m 2 / g BET surface area.

[0023] The lithium metal phosphate may have a crystallite size of at least 100 nm as determined by Rietveld analysis of X-ray diffraction (XRD) data. The upper limit of the crystallite size is not particularly limited, but may be 500 nm or less, 250 nm or less, 200 nm or less, or 150 nm or less. Larger observed crystallite sizes indicate higher crystallinity and fewer crystal defects, which can enhance lithium ion conduction in the lithium metal phosphate material and thus enhance electrochemical performance.

[0024] Typically, the carbon coated lithium metal phosphate contains lithium phosphate in an amount of 0.25-3.5% by weight, based on the total weight of the carbon coated lithium metal phosphate. This helps ensure that the carbon coated lithium metal phosphate does not become lithium deficient. The presence of lithium phosphate can be determined by X-ray diffraction (XRD) and the amount of lithium phosphate determined by ICP-OES from a buffer solution.

[0025] Typically, the particle size distribution of the carbon-coated lithium metal phosphate is D 50 is greater than 8 μm, greater than 9 μm, or greater than 10 μm. D 50 may be 8 to 20 μm, or 8 to 15 μm. 50 The term D50 corresponds to the particle size value below which 50% by volume of the total particles in a particular sample reside. D50 can be determined using laser diffraction methods (e.g., using a Malvern Mastersizer2000).

[0026] It may be preferred that the carbon coated lithium metal phosphate is provided as a mixture of two particle size populations, a first population (coarse particles) in the range of 4-80 μm and a second population (fine particles) in the range of 0.1-4 μm. Providing the material with such a particle size distribution can allow closer packing of the secondary particles, resulting in improved electrode density. Typically the volume ratio of fine particles:coarse particles is 3:97 to 50:50, or preferably 20:80 to 40:60, such as about 30:70.

[0027] Carbon coated lithium metal phosphate is typically prepared by a hydrothermal process. Such methods include combining an iron(II) source with at least one lithium source, at least one phosphate source, at least one aluminum source, and optionally at least one M source, and obtaining a particulate lithium metal phosphate under hydrothermal conditions.

[0028] Suitable sources of iron(II) include iron sulfate (FeSO), typically in the form of a hydrate. 4 ), and iron oxalate.

[0029] Suitable lithium sources include lithium carbonate (Li 2 CO 3 ), lithium hydrogen phosphate (Li 2 HPO 4), lithium hydroxide (LiOH), lithium fluoride (LiF), lithium chloride (LiCl), lithium bromide (LiBr), lithium iodide (LiI), lithium phosphate (Li 3 PO 4 ), or mixtures thereof. Lithium hydroxide may be preferred.

[0030] Suitable phosphate sources include phosphoric acid, metaphosphoric acid, pyrophosphoric acid, triphosphoric acid, tetraphosphoric acid, hydrogen or dihydrogen phosphates, such as ammonium or dihydrogen phosphate, lithium or iron phosphate, or any desired mixtures thereof. Phosphoric acid is particularly preferred.

[0031] Suitable sources of M include, where applicable, sulfates and / or oxides of M, or mixtures thereof. Those skilled in the art will appreciate that M may also be present in the iron(II), lithium, phosphate, or aluminum source (typically the iron(II) source), and therefore no additional source of M needs to be added to achieve the desired concentration of M in the lithium metal phosphate.

[0032] Suitable aluminum sources include aluminum hydroxide (Al(OH) 3 ), aluminum chloride (AlCl 3 ), aluminum sulfate (Al 2 (SO 4 ) 3 * xH 2 O (typically 0≦x≦18), and aluminum oxide (Al 2 O 3 Aluminum hydroxide or aluminum sulfate may be particularly preferred.

[0033] In the context of the present invention, the term obtaining particulate lithium metal phosphate from a precursor mixture under hydrothermal conditions is to be understood as the treatment of the precursor mixture at a temperature above room temperature and a vapor pressure above 1 bar. The hydrothermal treatment can be carried out in a manner known to the skilled artisan, for example as described in WO 2005 / 051840, the contents of which are incorporated herein by reference. The hydrothermal treatment is preferably carried out at a temperature of 100 to 250° C., in particular at 100 to 180° C., and at a vapor pressure of 1 bar to 40 bar, in particular at a vapor pressure of 1 bar to 10 bar. The precursor mixture is typically reacted in a closed or pressure-resistant vessel. The reaction is preferably carried out in an inert or protective gas atmosphere. Examples of suitable inert gases include nitrogen, argon, carbon dioxide, carbon monoxide, or mixtures thereof. The hydrothermal treatment can be carried out, for example, for 0.5 to 15 hours, in particular for 6 to 11 hours. As a completely non-limiting example, the following specific conditions can be selected: heating time from 50° C. (temperature of the precursor mixture) to 160° C. for 1.5 hours, hydrothermal treatment at 160° C. for 10 hours, cooling from 160° C. to 30° C. for 3 hours. The lithium metal phosphate is carbon coated. To form the carbon coating, the particulate lithium metal phosphate formed by the hydrothermal process is typically mixed with a carbon source and then spray dried prior to the heating or calcination step.

[0034] The type of carbon source is not particularly limited in the present invention. The carbon source is typically a carbon-containing compound that decomposes to a carbonaceous residue when exposed to a calcination process. For example, the carbon source may be one or more of starch, maltodextrin, gelatin, polyols, sugars (such as mannose, fructose, sucrose, lactose, glucose, galactose, etc.), and carbon-based polymers such as polyacrylate, polyvinyl acetate (PVA), glucono delta-lactone (GDL), and polyvinyl butyrate (PVB). Alternatively, the carbon source may be elemental carbon, such as one or more of graphite, carbon black, acetylene black, carbon nanotubes, and carbon fibers (such as vapour grown carbon fibre, VGCF, etc.). Lactose and maltodextrin may be particularly preferred.

[0035] The amount of carbon source added is not particularly limited in the present invention. For example, the amount of carbon source added can be selected so as to obtain a carbon-coated lithium metal phosphate with a carbon content of 1-5 wt%, for example, 1.5-3.5 wt%. The amount of carbon source added can be in the range of 7-22 wt%, for example, 10-18 wt%, based on the weight of the particulate lithium metal phosphate, depending on the nature of the carbon precursor and its carbonization yield.

[0036] Those skilled in the art will appreciate that the carbon source may be combined with the particulate lithium metal phosphate by a number of means. For example, the lithium metal phosphate may be mixed with the carbon source in the presence of a solvent such as water, and then the mixture may be spray-dried. Those skilled in the art will also appreciate that in some cases it may be preferable to add a carbon source to the precursor mixture prior to the hydrothermal treatment. In such cases, it will be appreciated that the addition of a carbon source in step (iii) of the process is no longer required.

[0037] In the heating step (iv), the particulate lithium metal phosphate and the carbon source are heated to provide a particulate carbon-coated lithium metal phosphate. The heating step (v) serves two purposes. First, the carbon source is pyrolyzed to form a conductive carbon coating on the lithium metal phosphate particles. Second, it improves the crystallinity and / or heals potential defects in the lithium metal phosphate crystals. Typically, the heating is carried out in an inert atmosphere, for example an inert gas such as argon. Alternatively, it may be carried out in a reducing atmosphere. It is typically carried out at a temperature in the range of 550°C to 800°C, for example 630°C to 780°C, or 650°C or 700°C to 780°C. 750°C is particularly suitable. Typically, the calcination is carried out for 0.4 to 10 hours. The heating time depends on the scale of production (i.e., if larger quantities are prepared, longer heating times may be preferred). On a commercial scale, for example, 0.5 to 3 hours may be suitable.

[0038] Following the pyrolysis step, the carbon coated lithium metal phosphate may be subjected to a sieving, or grinding and / or screening step to provide material with the desired particle size distribution. Screening may be preferably carried out at a screener speed range of 500-10000 rpm and / or a pressure of 0.25-5 bar.

[0039] The process of the present invention may further include forming an electrode (typically a cathode) comprising the carbon-coated lithium metal phosphate. Typically, this is accomplished by forming a slurry of particulate carbon-coated lithium metal phosphate, applying the slurry to a surface of a current collector (e.g., an aluminum current collector), and optionally processing (e.g., calendaring) to densify the electrode. The slurry may include one or more of a solvent, a binder, and an additional carbon material.

[0040] The process of the present invention may further include constructing a battery or electrochemical cell that includes an electrode that includes the carbon-coated lithium metal phosphate. The battery or cell typically further includes an anode and an electrolyte. The battery or cell may typically be a secondary (rechargeable) lithium ion battery.

[0041] The present invention will now be described with reference to the following examples, which are provided to aid in the understanding of the invention and are not intended to limit its scope. EXAMPLES

[0042] Examples 1 and 2 and Comparative Examples 1-5 were produced according to the following general method of hydrothermal preparation.

[0043] General method As described in WO 2005 / 051840, FeSO in distilled water 4 (22kg), LiOH.H 2 O (10 kg), and H 3 PO 4 A mixture of (76%, 9 kg) was subjected to hydrothermal treatment at 160° C. for 10 hours. The resulting precipitate was filtered and the filter cake was washed with water. The resulting solid was mixed with lactose (10.5 wt %) and water, and then the mixture was spray dried. The spray dried material was calcined at 750° C. in a rotary kiln under nitrogen atmosphere.

[0044] Comparative Example 1 Carbon coated lithium metal phosphate was produced according to the general method described above. After calcination, the formed material was ground to obtain an unagglomerated powder.

[0045] Comparative Example 2 Carbon coated lithium metal phosphate was produced according to the general method described above. After calcination, the formed material was screened in a jet mill at 1 bar and 2100 rpm to obtain partially agglomerated material.

[0046] Comparative Example 3 Al(OH) before hydrothermal treatment 3 (0.23 kg) was added to obtain an aluminum doped material in the form of a non-agglomerated powder.

[0047] Comparative Example 4 Al before hydrothermal treatment 2 (SO 4 )3 .xH 2 The method of Comparative Example 1 was repeated with the addition of O (0.16 kg) to obtain an aluminum-doped material in the form of a non-agglomerated powder.

[0048] Comparative Example 5 Lithium carbonate, iron phosphate, polyvinyl butyral (PVB), and aluminum source (Disperal® OS-1 (boehmite modified with p-toluenesulfonic acid, Sasol)) were mixed in the desired proportions and then subjected to high energy milling in isopropanol for 8 hours (recirculating high energy mill with yttria stabilized zirconia (0.3 mm) charged at 75% charge ratio inside the mill). The milled slurry was spray dried and fired in a furnace (maximum temperature 710°C).

[0049] Example 1 Al(OH) before hydrothermal treatment 3 (0.23 kg) was added to obtain an aluminum doped material in the form of a partially agglomerated material.

[0050] Example 2 Al before hydrothermal treatment 2 (SO 4 ) 3 .xH 2 The method of Comparative Example 2 was repeated with the addition of O (0.16 kg) to obtain an aluminum-doped material in the form of a partially agglomerated material.

[0051] Analysis method BET surface area The specific surface area of ​​the carbon-coated lithium metal phosphate was determined using the Brunauer, Emmett and Teller (BET) method using a Gemini 2360 surface area analyzer (Micromeritics).

[0052] Carbon Content The carbon content of the carbon coated lithium metal phosphate was measured using a carbon / sulfur analyzer (Eltra CS2000).

[0053] Aluminum Content The aluminum content was measured by ICP-OES. 0.1 g of carbon-coated lithium metal phosphate was dissolved in 10 ml of HCl (18.5%, aqueous solution) and heated to 80-90 °C for 2 h. The dissolved carbon is filtered after 2 h through a Teflon filter (1 μm) by a vacuum system. Pure water is added to this filtrate up to a total volume of 100 mL. The solution is then analyzed for aluminum content by ICP-OES at wavelengths of 308.215, 394.401 and 396.152 (compared to a series of standard solutions with different Al concentrations). The aluminum content is calculated as the average of the aluminum content obtained by analysis of each wavelength.

[0054] Lithium Phosphate Content The lithium phosphate content was calculated based on ICP-OES measurements of lithium and phosphate in a buffer solution of lithium metal phosphate as follows:

[0055] The buffer solution is prepared by dissolving sodium acetate (49.3 g) and acetic acid (0.74 g) in water (950 mL). 50 mg of lithium metal phosphate is combined with 20 mL of the buffer solution. The solution is then kept in a water bath at 50° C. for 15 min. The sample is then treated in an ultrasonic bath for 1 h. The sample is filtered through a syringe filter. 0.2 mL of the filtered material is diluted with 1 mL of HCl (18.5%) and pure H 2 Fill to 10 mL with O. The solution was analyzed by ICP-OES (wavelengths 670.784 nm (Li) and 213.618 (P)).

[0056] The resulting value was used to calculate the weight percent of lithium phosphate in the lithium metal phosphate samples.

[0057] Particle size analysis The particle size distribution (PSD) of the carbon coated lithium metal phosphate was analyzed by laser diffraction using a MALVERN Mastersizer 2000.

[0058] For the non-agglomerated materials (Comparative Examples 1, 3 and 4), a sample of carbon coated lithium metal phosphate (approximately 50 mg) was added to ethyl alcohol (approximately 20 mL) and subjected to sonication for 5 minutes prior to PSD analysis.

[0059] For partially agglomerated materials (Comparative Example 2 and Examples 1 and 2), the samples were placed on a vibrating plate and sucked with an air pressure of 0.2 bar.

[0060] Crystallite Size The crystallite size was determined by X-ray powder diffraction analysis using a Bruker D8 advance diffractometer (Davinci design, radiation=Cu Kα, (λ=1.5406+1.54439 Å)) using the following parameters: Scan range 10-130°2θ, step size = 0.022°, scan mode = θ / θ coupling, tube voltage, current = 40 kV, 40 mA, temperature = ambient value, detector Lynxeye XE PSD, 0.0125° Ni filter, Measured crystallite size and lattice parameters; Software: Bruker-AXS TOPAS 5 (1999~2014) Rietveld analysis: full powder diffraction pattern fitting technique with complete structural model. Crystallite size calculated using LVol-IB method.

[0061] Analysis results The lithium metal phosphate materials were analyzed to determine the BET surface area, carbon content, and aluminum content. The results are shown in Table 1. The data show that the partially agglomerated materials (Comparative Example 2 and Examples 1 and 2) have a lower BET surface area than the powdered materials, and the carbon content of each sample is in the range of 2-3 wt.%. By including an aluminum source in the hydrothermal process, the aluminum content of the carbon-coated lithium metal phosphate is increased to within the range of 300-5000 ppm.

[0062] The material of Comparative Example 5, produced by the wet milling method, has a significantly higher BET surface area and lower crystallite size than the other materials tested.

[0063] [Table 1]

[0064] Further XRPD analysis of samples produced by the methods of Examples 1 and 2 indicates that at least a portion of the aluminum dopant is present within the phospho-olivine crystal lattice of the lithium metal phosphate.

[0065] PSD distribution Comparative Examples 3 and 4, and Examples 1 and 2 were analyzed to determine their particle size distribution. The results of this analysis were as follows: Comparative example 3-D 50 Particle size distribution of 0.4 μm Comparative example 4-D 50 Particle size distribution of 0.5 μm Example 1-D 50 Bimodal distribution with 11 μm Example 2-D 50 The particle size distribution is shown in Figure 1.

[0066] Electrochemical evaluation (1) Half-cell measurement values The electrochemical properties of the carbon-coated lithium metal phosphate materials were evaluated as follows: (1) An electrode slurry was prepared by combining the active material, binder (Solef 5130) and carbon black (Super P Li) in NMP in a weight ratio of 90:5:5. (2) The electrode slurry is coated onto an aluminum carrier foil that fits onto a surgeon's knife table to give a thickness of 11-12 mg / cm 2 A charge of the active material up to 10 ... was achieved and then dried. (3) The formed electrodes were tested against a lithium metal anode using a Basytec system at ambient temperature (25° C.). The electrolyte was 1:1 EC:DMC.

[0067] Half-cell test data is provided in Table 2. The data shows that the partially agglomerated materials (Examples 1 and 2) have improved polarization and rate performance compared to the non-agglomerated materials (Comparative Examples 3 and 4).

[0068] [Table 2]

[0069] (2) Full cell measurement The full-cell properties of the carbon-coated lithium metal phosphate materials were evaluated as described for the half-cells, but graphite was used as the anode instead of Li metal.

[0070] The DC resistance (DCR) was measured as follows: First, the full cells were subjected to a formation cycle using a Basytec battery testing system. After formation, the cells are connected to a Parstat MC 1000 (Ametek) potentiostat where a 1C cycle is performed and the SOC (state of charge) is adjusted to 50%. The cells are maintained at room temperature (25°C) by appropriately adjusting the temperature in the chamber. After leaving the cell for 20 minutes, a 10s 1C discharge pulse is applied at SOC50%. The cell voltage drops from V0 to V1 in a few milliseconds. This voltage drop (after 1ms) ΔV0 divided by the current ΔI represents the cell's ohmic resistance (RO). Furthermore, after a few hundred milliseconds to a few seconds, the voltage decays from V1 to V2, represented by ΔV1, due to the charge transfer reaction. In our experiment, the voltage (V2) is recorded after 10 seconds. This resistance (ΔV0+ΔV1) / ΔI is the DCR value.

[0071] To measure the DCR at low temperatures, the chamber with the cell is cooled to -20°C overnight. Another 1C discharge pulse is applied. The OCR value is calculated from the voltage drop and applied current after 1 ms (Ro) and 10 seconds (DCR).

[0072] [Table 3]

[0073] The full cell electrochemical data (Table 3) shows that increasing amounts of aluminum in Comparative Examples 3 and 4, and Examples 1 and 2, result in improved low temperature DCR values. Additionally, the formation of aggregates of the aluminum doped materials (Examples 1 and 2) further improves the low temperature DCR values ​​compared to the aluminum doped non-agglomerated materials (Comparative Examples 3 and 4). The data also shows that Examples 1 and 2 have significantly better low temperature DCR than Comparative Example 5 (high surface area aggregates produced by the grinding process).

Claims

1. A carbon-coated lithium metal phosphate in the form of secondary particles formed at least partially by aggregation of primary particles, wherein (i) the lithium metal phosphate has the following formula: Li ａ (Fe １－ｘ M ｘ )PO ４ wherein 0.8 ≤ a ≤ 1.2, 0 ≤ x ≤ 0.1, M is one or more selected from Mn, Ni, Co, Mg, and B, and the lithium metal phosphate is doped with aluminum such that the carbon-coated lithium metal phosphate has an aluminum content of 300 to 5000 ppm. (ii) The carbon-coated lithium metal phosphate satisfies the requirement of having a BET surface area of 15 m ２ / g or less, and is a carbon-coated lithium metal phosphate. The carbon-coated lithium metal phosphate, wherein the lithium metal phosphate has a crystallite size determined by Rietveld analysis of at least 100 nm.

2. The carbon-coated lithium metal phosphate according to claim 1, wherein the aluminum content is 900 to 3500 ppm.

3. The carbon-coated lithium metal phosphate according to claim 1 or 2, wherein 0 ≤ x ≤ 0.

05.

4. The carbon-coated lithium metal phosphate according to any one of claims 1 to 3, comprising lithium phosphate in an amount of 0.25 to 3.5% by weight based on the total weight of the carbon-coated lithium metal phosphate.

5. D ５０ The carbon-coated lithium metal phosphate according to any one of claims 1 to 4, wherein D is 8 μm or more.

6. A process for preparing a carbon-coated lithium metal phosphate according to any one of claims 1 to 5, comprising: (i) forming a precursor mixture by combining an iron (II) source with at least one lithium source, at least one phosphate source, at least one aluminum source, and optionally at least one M source; (ii) obtaining a lithium metal phosphate from the precursor mixture under hydrothermal conditions; (iii) mixing the lithium metal phosphate with a carbon source and spray-drying the mixture; and (iv) heating the lithium metal phosphate and the carbon source to form the carbon-coated lithium metal phosphate.

7. wherein the aluminum source is Al(OH) ３ or Al ２ (SO ４ ) ３ ＊ xH ２ O, the process according to claim 6

8. Use of a carbon-coated lithium metal phosphate according to any one of claims 1 to 5 in the preparation of an electrode for a secondary lithium-ion battery.

9. An electrode for a secondary lithium-ion battery, comprising a carbon-coated lithium metal phosphate according to any one of claims 1 to 5.

10. A secondary lithium-ion battery comprising the electrode according to claim 9.

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