WO2019100233A1

WO2019100233A1 - High voltage positive electrode material and cathode as well as lithium ion cell and battery including the same

Info

Publication number: WO2019100233A1
Application number: PCT/CN2017/112265
Authority: WO
Inventors: Peng Wu; Sung-Jin Kim; Wensheng Yang
Original assignee: Bayerische Motoren Werke Aktiengesellschaft
Priority date: 2017-11-22
Filing date: 2017-11-22
Publication date: 2019-05-31
Also published as: CN111316484A; CN111316484B

Abstract

The invention relates to a high voltage positive electrode material and a cathode and a lithium ion cell and/or a battery comprising the same. In particular, the invention relates to a high voltage positive electrode material with a coating containing composite transition metal oxides and the cathode and lithium ion cells and/or batteries comprising the same.

Description

High voltage positive electrode material and cathode as well as lithium ion cell and battery including the same

Field of the Invention

The invention relates to a high voltage positive electrode material and to a cathode and a lithium ion cell and/or a battery comprising the same. In particular, the invention relates to a high voltage positive electrode material with a coating containing composite transition metal oxides and to a cathode and lithium ion cells and/or batteries comprising the same.

Background of the Invention

In the light of a relatively high energy density, a relatively small volume, a relatively low internal resistance, and a low self-discharge rate when not in use, secondary or rechargeable lithium ion batteries are widely used in various stationary and mobile application areas, such as those in the consumer electronic, automobile and aerospace industries.

Generally, a battery refers to at least two connected cells. However, the term “battery” also refers to a sole or single cell. A lithium ion cell generally includes four components, i.e. cathode, anode, electrolyte and separator. In such system, the electrical energy is stored and/or released by means of lithium ions and usually transition metal oxides in a chemical process involving intercalation and/or deintercalation of lithium ions, during which process lithium can move back and forth in ionized form through the electrolyte between the two electrodes. The function of the separator is to electrically insulate the two electrodes from one another, so as to avoid short circuits.

This lithium ion flow is necessary to balance the external current flow during charging and discharging, so that the electrodes themselves remain (largely) electrically neutral. During discharging, lithium atoms each release an electron at the negative electrode and this electron flows via the external current circuit to the positive electrode. At the same time, the same number of lithium ions migrate through the electrolyte from the negative electrode (anode) to the positive electrode (cathode) . At the positive electrode, however, the electron is not taken up again by the lithium ions but instead by the transition metal ions which are present there and are strongly ionized in the charged state. In lithium ion systems, these transition metal ions can be cobalt, nickel, manganese, iron ions, etc. The lithium thus continues to be present in ionic form at the positive electrode in the discharged state.

Lithium-rich layered oxide (LLO) materials as cathode active material are increasingly of interest, since such materials may provide relatively high capacity. However, capacity loss of LLOs are relatively high and also the cycling performance of LLOs are relatively poor. In particular, lithium/manganese rich cathode materials show high irreversible capacity loss in the first cycle due to the activation of the Li₂MnO₃ component. During this process, the cathode material host structure is damaged and the lithium is not available anymore for further cycling.

For improving the cycling performance of LLOs, several approaching were addressed. For example, US 2014/0255603 A1 discloses a method for reducing irreversible capacity loss of a lithium rich transitional oxide electrode, which comprises the preparation of the active material in absence of air, forming a dispersion of a lithium rich transition metal oxide powder in a liquid, adding an oxide precursor to the dispersion and evaporating the liquid from the dispersion; hydrolyzing the oxide precursor, thereby forming an intermediate product; annealing the intermediate product, thereby forming an oxide coated lithium rich transitional oxide powder; and using the oxide coated lithium rich transition metal oxide powder to form the lithium rich transition metal oxide electrode.

However, absence of air is necessary to the method as disclosed in US 2014/0255603 A1, which increases the cost for preparing the electrode materials. There is a need for a method for preparing LLOs in a cost-efficient manner and the LLOs thus obtained having improved performances, in particular an improved cycling performance.

Summary of the Invention

As mentioned above, lithium/manganese rich cathode materials show high irreversible capacity losses in the first cycle, and a relatively poor cycling performance, whichshortens life of the cathodes and cells and batteries obtained therefrom. The inventors have now surprisingly found that, coating lithium/manganese rich cathode material particles with Li⁺ host materials, for example with composite transition metal oxides which have the same type and molar ratio transition metal elements as those in the lithium/manganese rich cathode materials or coating lithium/manganese rich cathode material particles with FePO₄ may improve the cycling performance of the materials and thereby prolong the life of cathodes and cells and batteries obtained therefrom.

Thus, in the first aspect, the invention relates to a high voltage positive electrode material comprising a coating containing composite transition metal oxides which have the same type and molar ratio transition metal elements as those in the lithium/manganese rich cathode material or containing FePO₄.

In the second aspect, the invention relates to a cathode comprising a high voltage positive electrode material, said positive electrode material comprises a coating containing composite transition metal oxides which have the same type and molar ratio transition metal elements as those in the lithium/manganese rich cathode material or containing FePO₄.

In the third aspect, the invention relates to a cell comprising at least one anode, a separator, electrolyte and at least one cathode comprising a high voltage positive electrode material, said positive electrode material comprises a coating containing composite transition metal oxides which have the same type and molar ratio transition metal elements as those in the lithium/manganese rich cathode material or containing FePO₄.

In the fourth aspect, the invention relates to a battery comprising the cell as defined above.

In the fifth aspect, the invention relates to a method for preparation of high voltage positive electrode material comprising a coating containing composite transition metal oxides which have the same type and molar ratio transition metal elements as those in the lithium/manganese rich cathode material or containing FePO₄. Said method comprises the steps of dissolving water soluble transition metal salts into an aqueous solvent or water to obtain a solution; adding a precipitating agent into the solution and optionally heating at a temperature of 70℃ up to the boiling point of the solution to obtain a precipitate; calcining the precipitate with a lithium compound to obtain a pristine LLOs; dispersing the pristine LLOs into a solution containing a water soluble transition metal compound; and drying the dispersion and calcining obtained materials.

In an alternative embodiment, the temperature for heating the solution is 70℃, preferably 80℃, more preferably 90℃ to the boiling point of the solution.

Preferably, the composite transition metal oxides of the invention comprise nickel, cobalt or manganese or the mixture thereof.

Preferably, the composite transition metal oxides of the invention comprise about 1 to about 10 wt. %of the coating, based on the total amount of the (pristine) LLOs, preferably about 2 to about 8 wt. %, more preferably about 3 to about 7 wt. %, most preferably about 5 wt. %.

More preferably, the soluble transition metal salts comprise Mn, Co, Ni with mole ratio of Mn: Co: Ni=0.54: 0.13: 0.13. In an alternative embosiment, the mole ratio of the total transition metal salts to complexing agent is in the range between 4 to 8. In another embodiment, after calcining the annealing temperature is between 800 to 1000℃ and/or the atmosphere is air or other oxygen-containing gases or oxygen and/or the mole ratio of the lithium compound to the precipitate is about 1.2. In yet another embodiment, the mass ratio of pristine LLOs to coating compound is about 10 to about 100 and the coating contains composite transition metal oxides which have the same type of the transition metal elements as those in the positive electrode material or contain FePO₄. In still yet another embodiment, the coating annealing temperature is between 500 to 600℃ and/or the atmosphere is air or other oxygen-containing gases or oxygen.

Definitions

As used herein, the term "lithium ion battery" , "rechargeable lithium ion battery" and "lithium ion secondary battery" are used synonymously. The terms also include the terms "lithium battery" , "lithium ion accumulator" and "lithium ion cell" , as well as all lithium alloy batteries, in particular Li-sulfur or alloy systems. Thus, the term "lithium ion battery" is used as a generic term for the abovementioned terms used in the prior art. It means both rechargeable batteries (secondary batteries) as well as non-rechargeable batteries (primary batteries) . It also includes the cell systems using liquid electrolytes, as well as all solid state cells. In particular, a "battery" for the purposes of the present invention also comprises a single or individual "electrochemical cell" . Preferably, two or more such electrochemical cells are connected together in a "battery" , either in series (i.e., successively) or in parallel.

The electrochemical cell according to the invention has at least two electrodes, i.e. a positive electrode (cathode) and a negative electrode (anode) . Both cathode and anode have at least one active material, which is capable of absorbing or discharging lithium ions and at the same time emitting or absorbing electrons. The term "positive electrode" means the electrode which, when the battery is connected to a load, absorbs electrons. It is the cathode in this nomenclature. The term "negative electrode" means the electrode which is capable of emitting electrons during operation. It represents the anode in this nomenclature. The electrodes have inorganic material or inorganic compounds or substances which can be used for or in or on an electrode or as an electrode. Under the operating conditions of the lithium-ion battery, these compounds or substances can accept (intercalate) or release (deintercalate) lithium ions or metallic lithium due to their chemical nature. Such a material is herein referred to as "active cathode material" or "active anode material" or generally "active material" or "active electrode material" . For application in an electrochemical cell or battery, such active material is applied to a carrier, preferably to a metallic carrier, preferably aluminum for the cathode or to copper for the anode. This carrier is also referred to as a "conductor" or also as a "collector" or collector film.

The active material for the positive electrode or the active cathode material can be any materials known from the related prior art. These include, for example, LiCoO₂ (LCO) , NMC111 (LiNi_1/3Mn_1/3Co_1/3O₂) , NMC622 (LiNi_0.6Mn_0.2Co_0.2O₂) , NCA (LiNi_0.8Co_0.15Al_0.05O₂) , high-energy NCM (HE-NCM) , lithium iron phosphate (LFP) or Li-Mn-spinel (LiMn₂O₄, LMO) . Therefore, there is no limitation with regard to the positive electrode in the sense of the present invention. In a preferred embodiment, as cathode material, a material selected from a group consisting of a lithium-transition metal oxide (hereinafter, also referred to as "lithium metal oxide" ) , layered oxides, spinels, olivine compounds, silicate compounds, and mixtures thereof is used. These active cathode materials are described, for example, in Bo Xu et al., "Recent progress in cathode materials research for lithium ion batteries" , Materials Science and Engineering R 73 (2012) 51-65. Another preferred cathode material is HE-NCM. Layered oxides and HE-NCM are also described in the patents US 6,677,082 B2, US 6,680,143 B2 and US 7,205,072 B2 of Argonne National Laboratory. Examples for olivine compounds are lithium phosphates of the general formula LiXPO₄, where X = Mn, Fe, Co or Ni, or combinations thereof. Examples of lithium transition metal oxide, spinel compounds and layered transition metal oxides are lithium manganates, preferably LiMn₂O₄, lithium cobaltates, preferably LiCoO₂, lithium nickelate, preferably LiNiO₂, or mixtures of two or more of these oxides, or mixed oxides thereof. The active material may also contain mixtures of two or more of the substances mentioned above. In order to increase the electrical conductivity, the active materials may comprise additional substances, preferably carbon-containing compounds, or carbon, preferably carbon black, in particular conductive carbon black or graphite. The carbon can also be introduced in the form of carbon nanotubes or graphene. These additives are preferably used in an amount of from 0.1 to 6%by weight, preferably from 1 to 3%by weight, based on the weight of the positive composition (without solvent) applied to the support.

The active material for the negative electrode or the active anode material can be any materials known from the prior art. Thus, there is no limitation with regard to the negative electrode in the sense of the present invention. The active anode material may be selected from the group consisting of lithium-metal oxides such as lithium titanium oxides, metal oxides (e.g. Fe₂O₃, ZnO, ZnFe₂O₄) , carbonaceous materials such as graphite (synthetic graphite and/or natural graphite) , graphene, mesocarbon, doped carbon, hard carbon, soft carbon, fullerenes, mixtures of silicon and carbon, silicon, tin, materials that may form alloy with lithium, and mixtures thereof. As electrode material for the negative electrode, niobium pentoxide, tin alloys, titanium dioxide, tin dioxide, silicon and silicon oxide may also be used. In some embodiment, graphite and its derivatives are preferred due to cost reasons. For application in hybrid vehicle, hard carbon may be preferred due to that its discharging curve may be suitable to such application.

It is also possible to use materials that may form alloy with lithium. These may be metallic lithium, a lithium alloy or a non-lithiated or partially lithiated precursor thereto, from which a lithium alloy is formed during formation. Preferred materials that may form alloy with lithium are lithium alloys selected from the group consisting of silicon-based, tin-based and antimony-based alloys. Such alloys are described, for example, in W. -J. Zhang, Journal of Power Sources 196 (2011) 13-24.

The active materials for the positive electrode or for the negative electrode are bonded together by one or more binders. The binder (s) may be selected from the group consisting of polyvinylidene fluoride (PVDF) , polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-HFP) , polyethylene oxide (PEO) , polytetrafluoroethylene, polyacrylate, styrene-butadiene rubber, carboxymethyl cellulose (CMC) and mixtures and copolymers thereof. The styrene-butadiene rubber and optionally the carboxymethyl cellulose and/or the further binders, such as PVDF, are preferably present in an amount of 0.5-8%by weight, based on the total amount of the binder used in the positive or negative electrode

The electrochemical cell according to the invention comprises materials which separate the positive electrode and the negative electrode from each other. This material is permeable for lithium ions, but is a non-conductor for electrons. Such materials used in lithium-ion cells/batteries are also referred to as separators. In a preferred embodiment, polymers are used as separators. In one embodiment, the polymers are selected from the group consisting of: polyester, preferably polyethylene terephthalate; polyolefin, preferably polyethylene or polypropylene; polyacrylonitrile; polyvinylidene fluoride; polyvinylidene-hexafluoropropylene; polyetherimide; polyimide; polyether; polyether ketone or mixtures thereof. The separator has porosity so that it is permeable to lithium ions. In a preferred embodiment, the separator comprises at least one polymer.

The term "electrolyte" preferably relates to a liquid in which lithium conductive salts are dissolved.

Preferably, the liquid is a solvent for the conductive salt. Preferably, the Li conductive salt is present as electrolyte solution. Suitable solvents are preferably chemically and electrochemically inert. Suitable solvents are preferably organic solvents such as, ethylene carbonate, propylene carbonate, butylene carbonate, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, sulfolane, 2-methyltetrahydrofuran and 1, 3-dioxolane. Organic carbonates are preferred.

In one embodiment of the invention, ionic liquids can also be used as solvents. Such "ionic liquids" contain exclusively ions. Preferred cations which can be alkylated are imidazolium, pyridinium, pyrrolidinium, guanidinium, uronium, thiouronium, piperidinium, morpholinium, sulfonium, ammonium and phosphonium cations. Examples of useful anions are halide, tetrafluoroborate, trifluoroacetate, triflate, hexafluorophosphate, phosphinate and tosylate anions.

Exemplary ionic liquids are N-methyl-N-propylpiperidinium bis (trifluoromethylsulfonyl) imide, N-methyl-N-butylpyrrolidinium bis (trifluoromethylsulfonyl) imide, N-butyl-N-trimethylammonium (Trifluoromethylsulfonyl) imide, triethylsulfonium bis (trifluoromethylsulfonyl) imide and N, N-diethyl-N-methyl-N- (2-methoxyethyl) ammonium bis (trifluoromethylsulfonyl) imide.

Preferably, two or more of the above-mentioned ionic liquids are used in the cells/batteries according to the invention. Preferred conductive salts are lithium salts having inert anions, preferably are non-toxic lithium salts. Suitable lithium salts are preferably lithium hexafluorophosphate (LiPF₆) , or lithium tetrafluoroborate (LiBF₄) and mixtures of one or more of these salts. In one embodiment, the separator is impregnated or wetted with the lithium salt electrolyte.

Alternatively, the electrolyte may also be a solid electrolyte. Said solid electrolyte includes but not limited to, polymeric electrolyte, e.g. those basing on PEO (poly ethylene oxide) , PMMA (poly methyl methacrylate) , etc. ; and inorganic electrolyte, e.g., crystal materials such as LISICON type, perovskite type and garnet type electrolyte and glasses state materials such as oxide electrolyte system, sulfide electrolyte system and LiPON/LiSiON electrolyte system.

Preparation of LLO

According to the invention, the pristine LLOs are prepared by co-precipitation followed by calcination with LiOH, i.e., lithiation. As used herein, the term “co-precipitation” means that transition metals are mixed in the form of ions in an aqueous solution and precipitated by a precipitating agent, preferably controlling the pH value of the solution, so that the transition metals homogeneously precipitate in a specific molar ratio. As used herein, the term “calcination” or “lithiation” refers to calcining a precipitate at high temperature with a lithium source.

For preparing the pristine LLO of the invention, the raw material including corresponding transition metals are dissolved in aqueous solvent or water. In a preferred embodiment of the invention, the raw material includes sulfates or acetates of nickel, manganese and/or cobalt. For dissolving the raw material in aqueous solvent or water homogenously and rapidly, an ultrasonic device may be used.

In a preferred embodiment of the invention, precipitating agents are used for precipitating the transition metals. Preferably, the precipitating agents are basic precipitating agents, which are preferably selected from the group consisting of alkali carbonates, alkali earth carbonates, ammonium carbonates, alkali bicarbonates, alkali earth bicarbonates, ammonium bicarbonates, alkali hydroxides, alkali earth hydroxides, ammonium hydroxides and ammonia, and the mixture thereof, more preferably selected from the group consisting of sodium carbonate, sodium bicarbonate, sodium hydroxide, potassium carbonate, potassium bicarbonate, potassium hydroxide and ammonia, and the mixture thereof, most preferably selected from the group consisting of sodium carbonate, sodium hydroxide and ammonia and the mixture thereof.

In a preferred embodiment of the invention, the pH of the solution containing the transition metal and the precipitating agents are adjusted in order to obtain the transition metal precursors. For the co-precipitation method, various types of technologies may be used, for example batch reactor (BR) technology, flow reactor (FR) technology and nucleation-aging separation (NAS) technology. In a preferred embodiment using carbonates as precipitating agent, corresponding carbonates containing transition metals are obtained. Similarly, when hydroxides are used as precipitating agent, transition metal hydroxides are obtained.

Then, the obtained precursors are homogenously mixed with a lithium source, such as Li₂CO₃ or LiOH and then calcined at high temperature (for example 900℃) in air or in oxygen atmosphere to yield layered cathode materials (LLOs) .

For obtaining the LLO coated with transition metal-containing coatings according to the invention, soluble transition metal salts are dissolved in aqueous medium, then the pristine LLOs are added into the solution thus obtained to produce a dispersion; the dispersion is dried and the obtained material is calcined to obtain LLOs coated with composite transition metal oxides or FePO₄, i.e. the LLOs of the invention.

In a preferred embodiment of the invention, the soluble transition metal salts include sulfates or acetates of nickel, manganese and cobalt, more preferably acetates of nickel, manganese and cobalt. For dispersing the soluble transition metal salts in aqueous medium homogenously and rapidly, an ultrasonic device may be used. In a preferred embodiment of the invention, spray drying is used for drying the dispersion. In another preferred embodiment of the invention, citric acid is used for preparing LLOs coated with FePO₄. Surprisingly, the inventors have found that, when citric acid is used as the complexing agent during the preparation of LLOs coated with FePO₄, the LLOs obtained are uniformly coated with FePO₄ on the surface thereof.

Description of accompanying figures

Figure 1A shows the SEM images of (a) pristine LLO and (b) LLO coated with 5wt% FePO₄;

Figure 1B shows the SEM images of (a) pristine LLO and (b) LLO coated with 5wt%MO_x;

Figures 2A and 2B show the surface of the prepared electrode;

Figure 3 shows the initial charge-discharge curves of LLOs and LLOs coated with 5wt%MO_x at 0.1 C;

Figure 4 shows the cycle performance of LLOs and LLOs coated with 5wt%MO_x at 1.0 C;

Figure 5 shows the cycle performance of LLOs and LLOs coated with 5wt%MO_x at 0.1 C;

Figure 6 shows the rate performance of LLOs and LLOs coated with 5wt%MO_x at 2.0-4.8V;

Figure 7 shows the initial charge-discharge curves of LLOs and LLOs coated with 5wt%FePO₄;

Figure 8 shows cycle performance of LLO and LLOs coated with 5wt%FePO₄ at 1C.

The invention will now be further illustrated by the following non limiting examples.

Examples

Example 1 -LLO baseline cathode active material

27.50 g MnSO₄·H₂O, 10.31 g NiSO₄·7H₂O and 11.02 g CoSO₄·7H₂O as raw material are dissolved in 160 mL deionized water, followed by adding 25.54 g Na₂CO₃ and 3.18 g NH₄HCO₃ to adjust the pH of the solution to 8.1 and to co-precipitate Ni²⁺, Co²⁺ and Mn²⁺ in the solution to yield 28.02 g Mn_0.54Ni_0.13Co_0.13 (CO₃) _0.8. The obtained Mn_0.54Ni_0.13Co_0.13 (CO₃) _0.8 is thoroughly mixed with 14.03 g Li₂CO₃, and then calcined at 500 ℃ for 6 h and calcined at 900℃ in air, so as to obtain layered Li_1.2Mn_0.54Ni_0.13Co_0.13O₂ cathode materials.

Example 2 -LLO coated with MO_x (M = Mn+Co+Ni) coating

Each of 2.7122 g of Mn (CH₃COO) ₂·4H₂O, 0.6638 g of Co (CH₃COO) ₂·4H₂O and 0.6624 g of Ni (CH₃COO) ₂·4H₂O are first dissolved in the aqueous solution by ultrasonication. Then the LLO obtained in Example 1 is added into the mixed solution to have a turbid liquid. The turbid liquid is spray dried, the obtained particulate material is calcined at different temperatures (400-600℃) for 6 h to obtain 19.5 g LLO powder coated with MO_x (M = Mn+Co+Ni) coating. Said LLO powder is collected and tested for its properties.

Example 3 -LLO coated with FePO₄ coating

0.6697 g of Fe (NO) ₃·H₂O and 0.2189 g of (NH₄) ₂HPO₄ are dissolved in water to obtain a solution, to which 0.3185 g of citric acid is added followed by adding 5 g pristine LLO under stirring. A turbid liquid is obtained and subsequently spray dried directly, the obtained particulate material is calcined at different temperatures (400～600 ℃) for 6 h to obtain LLO powder coated with FePO₄ coating. The obtained LLO powder is collected and tested for its properties.

Example 4-LLO coated with FePO₄ coating

0.6697 g of Fe (NO) ₃·H₂O and 0.2189 g of (NH₄) ₂HPO₄ are dissolved in water to obtain a solution, to which 0.3185 g of citric acid is added followed by adding 5 g pristine LLO under stirring. The mixture solution is heated at 80℃, and the liquid transfers to gel. And the gel was dried to powder. The obtained particulate material is calcined at different temperatures (400～600 ℃) for 6 h to obtain LLO powder coated with FePO₄ coating. The obtained LLO powder is collected and tested for its properties.

Example 5-LLO coated with MO_x coating

Each of 2.7122 g of Mn (CH₃COO) ₂·4H₂O, 0.6638 g of Co (CH₃COO) ₂·4H₂O and 0.6624 g of Ni (CH₃COO) ₂·4H₂O are first dissolved in the aqueous solution by ultrasonication. Then the LLO obtained in Example 1 is added into the mixed solution to have a turbid liquid. The turbid liquid is heated on 70℃ to get gel, and the gel is dried to powder. The obtained particulate material is calcined at different temperatures (400-600℃) for 6 h to obtain 19.5 g LLO powder coated with MO_x (M = Mn+Co+Ni) coating. Said LLO powder is collected and tested for its properties.

Physical &Chemical Properties Measurement and Results

The obtained powders were characterized using conventional methods including SEM (Scanning Electron Microscope) , and ICP-AES.

The ICP-AES results obtained from LLO coated with 5wt%FePO₄ in Table 1 shows that the coating amount of FePO₄ is 5wt%; similarly, the ICP-AES results obtained from LLO coated with 5wt%MO_x in Table 2 shows that the coating amount of MO_x is also 5wt%.

Table 1 -The metal contents of pristine LLO and LLO coated with 5wt%FePO₄

Table 2 -The metal contents of pristine LLO and LLO coated with 5wt%MO_x

As can be seen from Figures 1A and 1B, the crystal surface of uncoated LLO is smooth, while FePO₄ coated LLO has a rough surface; similarly, the crystal surface of MO_x coated LLO also has a rough surface. From the results in Tables 1 and 2, composition of MO_x is Mn: Co: Ni = 0.54: 0.13: 0.13 and the x value is in the range between 0.8 and 1.4.

Electrochemical Performance measurement

Electrochemical performance of LLO and coated LLO was measured in half cells (coin cell format) in terms of first cycle performance, cycle performance, and discharge rate performance.

The electrode film was made by coating a paste comprising the active electrode material, acetylene black and polyvinylidene fluoride (PVDF) binder (80: 10: 10 wt. %) on an aluminum foil current collector, and disks were punched from the film with a 1.0 cm diameter arch punch. Electrochemical behavior during discharge-charge cycles was examined using a two-electrode test cell with LLO electrode as the working electrode and lithium foil as the counter and reference electrode. Coin-type cells (CR 2032) were assembled in an argon filled glove box (H₂O <1 ppm, O₂ <1 ppm) with an electrolyte of 1 mol L^-1 LiPF₆ in ethylene carbonate–ethyl methyl carbonate–dimethyl carbonate (EC–EMC–DMC) (1: 1: 1 volume ratio) solution and a Celgard 2400 separator. The electrochemical data were collected using a LAND CT2001A test system within the voltage range 2.0 to 4.8V (vs. Li+/Li) . As shown in Fig. 2A and 2B, the electrode film shows a good distribution of the all components (active material, conductive agent, binder) .

As shown in Fig. 3, the initial discharge capacity of LLO and LLO coated with MO_x is 173.5 and 230.2 mAh·g^-1 respectively. The initial coulomb efficiency of LLOs and LLOs coated with MO_x is 59.8%and 71.5%respectively. LLOs coated with MO_x shows a better property over pristine LLOs.

In Fig. 4, the cycle performance of LLOs and LLOs coated with MO_x at 1.0 C is shown. The capacity of LLOs coated with 5 wt%MO_x is improved markedly. According to Figure 5, the cycle performance at 0.1 C does not change obviously.

Figure 6 shows the rate performance of LLOs and LLOs coated with MO_x at 2.0-4.8V, and the rate performance of LLOs is improved by coating with MO_x.

Figure 7 shows the initial charge-discharge curves of LLO coated with 5wt%FePO₄ at 0.1C. As can be seen, the initial specific discharge capacity and coulombic effeiciency of LLO coated with 5wt%FePO₄ is 237.6mAh/g and 80%, respectively, which are improved markedly compared with that of pristine LLO (Figure 3) .

Figure 8 shows the cycle performance of LLO and LLO coated with 5wt%FePO₄ at 1C. The cycle performance of LLOs coated with 5wt%FePO₄ is also improved. The capacity retention is above 80%even after 120 cycles.

It can be seen that the composite transition metal oxides coating has the same transition metal components with those of lithium/manganese rich cathode materials, therefore, have good compatibility with the LLOs and will not easily separate from the surface of lithium/manganese rich cathode materials. The composite transition metal oxides coating can work as a Li⁺ host material, which can partially compensate the loss of irreversible Li⁺ in the first cycle, which increases the initial Coulombic efficiency of lithium/manganese rich cathode materials.

Claims

A high voltage positive electrode material comprising lithium-rich layered oxides and a coating containing composite transition metal oxides which have the same type of the transition metal elements as those in the positive electrode material or containing FePO₄.
The positive electrode material as claimed in claim 1, wherein the composite transition metal oxides comprise nickel, cobalt or manganese or the mixture thereof.
The positive electrode material as claimed in claim 1, wherein the composite transition metal oxides comprise about 1 to about 10 wt.%of the coating, based on the total amount of the lithium-rich layered oxides.
The positive electrode material as claimed in claim 2, wherein the composite transition metal oxides MO_x comprise Mn, Co, Ni with mole ratio of Mn: Co: Ni=0.54: 0.13: 0.13 and the x value is in the range between 0.8 and 1.4.
A cathode comprising a high voltage positive electrode material, which comprises lithium-rich layered oxides and a coating containing composite transition metal oxides which have the same type of the transition metal elements as those in the positive electrode material or containing FePO₄.
A cell comprising the cathode as claimed in claim 5.
A battery comprising the cell as claimed in claim 6.
A method for preparation of high voltage positive electrode material as claimed in claim 1, said method comprises the step of dissolving water soluble transition metal salts into an aqueous solvent or water to obtain a solution; adding a precipitating agent and complexing agent into the solution to obtain a precipitate; calcining the precipitate with a lithium compound to obtain pristine LLOs; dispersing the pristine LLOs into a solution containing a water soluble transition metal compound; and drying the dispersion and calcining obtained materials.
The method as claimed in claim 8, wherein the soluble transition metal salts comprise Mn, Co, Ni with mole ratio of Mn: Co: Ni=0.54: 0.13: 0.13.
The method as claimed in claim 8, wherein the mole ratio of the total transition metal salts to complexing agent is in the range between 4 to 8.
The method as claimed in claim 8, wherein after calcining, the annealing temperature is between 800 to 1000℃, the atmosphere is air or other oxygen-containing gases or oxygen and the mole ratio of the lithium compound to the precipitate is 1.2.
The method as claimed in claim 8, wherein the mass ratio of pristine LLOs to coating compound is about 10 to 100 and the coating contains composite transition metal oxides which have the same type of the transition metal elements as those in the positive electrode material or contain FePO₄.
The method as claimed in claim 8, wherein the coating annealing temperature is between 500 to 600℃, and the atmosphere is air or other oxygen-containing gases or oxygen.
The method as claimed in claim 8, wherein the water soluble transition metal salts are sulfates or acetates or nitrates of iron, nickel, manganese and/or cobalt.
The method as claimed in claim 8, wherein the precipitating agent is a basic precipitating agent, preferably selected from the group consisting of alkali carbonates, alkali earth carbonates, ammonium carbonates, alkali bicarbonates, alkali earth bicarbonates, ammonium bicarbonates, alkali hydroxides, alkali earth hydroxides, ammonium hydroxides and ammonia, and the mixture thereof.