Patent Application
20180269476
The invention relates to a lithium metal oxide material, in particular a doped lithium-manganese-nickel based oxide, the use thereof in a positive electrode of a secondary battery and a method for preparing such a lithium metal oxide material.
Commercially available lithium-ion batteries typically contain a graphite-based anode and cathode materials. A cathode material is usually a powderous material capable of reversibly intercalating and de-intercalating lithium. In modern rechargeable batteries LiCoO2 (LCO), Li1+a(NixMnyCoz)1−aO2 (NMC) with approximately similar amounts of Ni, Mn, Co and LiMn2O4 (LMO) are the dominant cathode materials. LCO was firstly introduced as a cathode material for Lithium-ion batteries in 1990 by Sony. Since then, LCO has become the most widely used cathode material. Especially after commercialization of high voltage LCO, it dominates the market for portable electronics, such as smartphones and tablets. NMC was developed around 2000, to replace LCO through substitution of Co by Ni and Mn, due to the high price of Co metal. NMC has a gravimetric energy density comparable to LCO, but a lower volumetric energy density, due its to lower product density. Nowadays, NMC is mainly used for automotive applications, for example electrical vehicles (EV) and hybrid electrical vehicles (HEV). This is because NMC is much cheaper than LCO, and the automotive application requires less volumetric density than portable electronics.
LMO materials have been developed since the middle of the 1990s. LMO has a spinel structure with a ‘3D’ diffusion path of Li ions. It has been widely used for various applications, such as power tools, E-bikes, and in automotive applications. Compared to LCO and NMC, LMO is much cheaper and has a high Li diffusion ability. However, LMO has a lower theoretical specific capacity of 140 mAh/g, compared to 280 mAh/g for LCO and NMC. Therefore, to improve the gravimetric energy density of LMO, the only known approach is increasing the operation voltage.
In 1995, Dahn et al. disclosed a new compound LiMn1.5Ni0.5O4 by substituting 0.5 Mn atom by 0.5 Ni atom in the formula of LiMn2O4. It was found that to fully delithiate LiMn1.5Ni0.5O4, a charge voltage of 4.9 V (vs. Li) should be applied. LiMn1.5Ni0.5O4 has a specific capacity similar to LiMn2O4. It also keeps the same crystal structure as LiMn2O4, hence its rate capability is very good. The gravimetric energy density of LiMn1.5Ni0.5O4 however is significantly improved compared to LiMn2O4, due to the higher operating voltage. Since then, spinel type LiMn1.5Ni0.5O4 (further referred to as “LMNO”) has become an important field of study and development of cathode materials.
However, the development of LMNO is facing several issues. Firstly, there is a lack of good electrolyte systems for very high voltage application, meaning circa 5V. Current applications of lithium-ion battery are focusing on an operating voltage below 4.5 V, for example, a lithium-ion battery for most smartphones operates at 4.35 V, and batteries for automotive application at about 4.1˜4.2 V. One of the main reasons for this low operating voltage is related to the electrolyte. Current organic solvents in the electrolyte, which are mainly linear and cyclic carbonates, start to decompose when the voltage is higher than 4.5 V, forming side products that negatively impact the cathode/electrolyte and anode/electrolyte interphase. Such side products deteriorate the electrochemical battery performance and cause a fast capacity fading. Research to improve the electrolyte stability at voltages >4.5 V is ongoing. Efforts include finding new solvents, inventing new salts, combining functional additives, etc.
Another critical issue for using LMNO is the problem of high voltage stability of the material itself. When charged to a high voltage, the dissolution of Mn becomes severe. Dissolved Mn migrates through the electrolyte and is deposited on the anode side, destroying the Solid Electrolyte Interphase (SEI) on the anode surface. During cycling of a battery, Mn continuously dissolves and destroys this SEI, thereby continuously consuming Li to form new SEI on the anode. This results in fast lithium loss and fast capacity fading in batteries.
An object of the present invention is therefore to provide LMNO cathode materials that are showing improved properties in terms of cycling stability, thermal stability, rate performance etc.
Viewed from a first aspect, the invention can provide the following product embodiments:
A powderous lithium metal oxide material having a cubic structure with space group Fd-3m and having the formula Li1−a[(NibMn1−b)1−xTixAy]2+aO4 with 0.005≤x≤0.018, 0≤y≤0.05, 0.01≤a≤0.03, 0.18≤b≤0.28, wherein A is one or more elements from the group of the metal elements excluding Li, Ni, Mn and Ti. It is needed to limit the Li/metal ratio (1−a)/(2+a) to avoid the formation of impurities or deteriorate the performance. A too low Li/metal ratio would result in the formation of impurities such as NiO, while a too high Li/metal ratio would result in increasing the ratio of Ni3+/Ni2+, which lowers the electrochemical reactivity of the material.
The lithium metal oxide material according to the invention, wherein 0<y, wherein A comprises one or more of Al, Mg, Zr, Cr, V, W, Nb and Ru, wherein preferably A consists of one or more elements from the group of Al, Mg, Zr, Cr, V, W, Nb and Ru. As is clear from the above formula, A is a dopant. A dopant, also called a doping agent, is a trace impurity element that is inserted into a substance (in very low concentrations) in order to alter the electrical properties or the optical properties of the substance.
In the lithium metal oxide material, x≤0.016. Up to a level of x=0.018, and more easily up to a level of x=0.016, Ti may be homogenously doped into the crystal structure of LMNO. This material shows improved cycle stability, rate capability, safety properties and high voltage stability when charged to 4.9V. Due to the improvements, such cathode materials show promising potential for various applications in lithium-ion battery, for example, power tools, E-bikes etc.
In the lithium metal oxide material, 0≤y≤0.02 and (y/x)<0.5.
The lithium metal oxide material according to the invention, wherein, in an X-ray diffractogram determined using Cu k-alpha radiation, the full width at half maximum of the peak with Miller index (111) and the full width at half maximum of the peak with Miller index (004) have a ratio of at least 0.6 and at most 1. In embodiment 5, the ratio of the full width at half maximum of the peak with Miller index (111) over the full width at half maximum of the peak with Miller index (004) is indicative for the strain inside the material. The bigger the ratio, the lower the strain inside of the material, but a certain strain is needed to achieve good electrochemical performance, while a too large strain indicates inhomogeneity inside of the material.
The lithium metal oxide material according to the invention is a crystalline single phase material. Preferably the material has a spinel structure.
The lithium metal oxide material according to the invention whereby Ti is homogeneously distributed inside the particles of the material.
It is clear that each of the individual product embodiments described hereabove can be combined with one or more of the product embodiments described before it.
Viewed from a second aspect, the invention can provide the following use embodiment 8: The use of the lithium metal oxide material according to the invention in a positive electrode for a secondary battery.
Viewed from a third aspect, the invention can provide the following method embodiments:
A method for preparing the powderous lithium metal oxide material according to the invention, the method comprising the following steps:
In the method the sources of Ni and Mn are formed by a coprecipitated Ni—Mn oxy-hydroxide or Ni—Mn carbonate, whereby the source of Ti is TiO2, and wherein the TiO2 is coated on the coprecipitated Ni—Mn oxy-hydroxide or Ni—Mn carbonate before the step of providing a mixture comprising sources of Ni, Mn, Li, Ti and the element or elements comprised in A. In a particular embodiment, the preferred source of Ti is a submicron-sized TiO2 powder having a BET of at least 8 m2/g and consisting of primary particles having a d50<1 μm, the primary particles being non-aggregated.
In the method the first temperature is at most 1000° C.
In the method the first time period is between 5 and 15 hrs.
In the method the second temperature is at least 500° C.
In the method the second time period is between 2 and 10 hrs.
The invention further provides an electrochemical cell comprising the lithium metal oxide material according to the invention.
Here it is appropriate to mention the following prior art:
Contrary to these documents, in the present invention the Li to metal ratio and the Ti content are selected to guarantee a homogeneous doping with Ti of the spinel structure that is phase-pure and has the space group of Fd-3m, and thus yielding an improvement of the electrochemical properties.
The authors discovered that LMNO cathode powders which contain Ti as a dopant have superior characteristics when used in Li-ion batteries. The existence of Ti doping can help to improve the cycle stability, rate capability, thermal stability and high voltage stability, which helps to promote the practical application of LMNO materials. Additional doping elements besides Ti may be optionally present.
The following characterization procedures were used:
X-Ray Diffraction (XRD)
X-ray diffraction was carried out using a Rigaku D/MAX 2200 PC diffractometer equipped with a Cu (K-Alpha) target X-ray tube and a diffracted beam monochromator, at room temperature in the 15 to 70 2-Theta (Θ) degree range. The lattice parameters of the different phases were calculated from the X-ray diffraction patterns using full pattern matching and Rietveld refinement methods. The FWHM of a selected peak is calculated using a software called “peak search” form Rigaku Corp with elimination of K-Alpha 2 diffraction.
Coin Cell Tests
A half cell (coin cell) was assembled by placing a Celgard separator between a positive electrode to be tested and a piece of lithium metal as a negative electrode, and using an electrolyte of 1M LiPF6 in EC/DMC (1:2) between separator and electrodes. The positive electrode was made as follows: cathode material powder, PVDF and carbon black are mixed with a mass ratio of 90:5:5. Sufficient NMP was added and mixed in to obtain a slurry. The slurry was applied to an Al foil by a commercial electrode coater. Then the electrode was dried at 120° C. in air to remove NMP. The target loading weight of the electrode was 10 mg cathode material/cm2. Then the dried electrode was pressed to obtain an electrode density of 1.8 g/cc, and dried again at 120° C. in vacuum before assembly of coin cells.
All coin cell tests in the present invention were performed using the procedure shown in Table 1, with the 1 C-rate being defined as 160 mAh/g. “E-Curr” and “V” signify the end current and cut-off voltage, respectively. At the first cycle, the DQ0.1 C (discharge capacity of the first cycle at a rate of 0.1 C) and IRRQ (irreversible capacity) were determined. The performance of cycle stability is obtained from cycle #7 to #60. The capacity fading at 0.1 C is represented by “Qfade0.1 C”. With DQ7 and DQ34 referring to the discharge capacity of cycle #7 and #34 respectively, Qfade0.1 C is calculated by the formula: Qfade0.1 C=(1−(DQ34/DQ7))/27*100*100 (in % per 100 cycles). The capacity fading at 1 C is represented by “Qfade1 C”. With DQ8 and DQ35 referring to the discharge capacity of cycle #8 and #35 respectively, Qfade1 C is calculated by the formula: Qfade1 C=(1−(DQ35/DQ8))/27*100*100. The capacity fading at 1 C/1 C (1 C charging and 1 C discharging) is represented by “Qfade1 C/1 C”. With DQ36 and DQ60 referring to the discharge capacity of cycle #36 and #60 respectively, the Qfade1 C/1 C is calculated by the formula: (1−(DQ60/DQ36))/24.
Float Charge Method
In a recent technical report of commercially available “3M battery electrolyte HQ-115”, a float charging method is used to test the stability of a novel electrolyte at high voltage. The method is carried out by continuously charging LCO/graphite pouch cells or 18650 cells at 4.2 V and 60° C. for 900 hours. The currents recorded under charge are compared. A higher current reflects more side reactions that occur, so this method is able to identify parasite reactions occurring in a battery at high voltage. In “Energy Environ. Sci., 6, 1806 (2013)”, a similar float charging method is used to evaluate the stability of electrolyte against oxidation under high voltage from 5V and up to 6.3V vs. Li metal.
Based on the above knowledge, by choosing a relatively stable electrolyte and anode material for the required charging voltage, a float charge method was used to study the stability of cathode materials under high voltage, where the metal dissolution from the cathode materials can be reflected by the leakage current. In addition, in “Nature Comm., 4, 2437 (2013)”, manganese dissolved from a lithium manganese oxide cathode is deposited on the surface of the anode in metal or metal alloy form, and the deposited amount can be detected by inductively coupled plasma-atomic absorption spectrometry (ICP-AAS). This ICP experiment on the anode can also be used to study the metal dissolution issue of LMNO, doped or not.
Therefore, the float charge method associated with ICP measurement (referred to hereafter as “floating experiment”) is a feasible way to evaluate the side reaction and metal dissolution of LMNO cathode materials at high voltage and elevated temperature. For the Examples and Counter Example, floating experiments are performed in order to evaluate the stability of the cathode materials at high voltage charging and at elevated temperature (50° C.).
The tested cell configuration was a coin cell assembled as follows: two separators (from SK Innovation) are located between a positive electrode and a negative graphite electrode (from Mitsubishi MPG). The electrolyte was 1M LiPF6 in EC/DMC (1:2 volume ratio) solvents. The prepared coin cell was submitted to the following charge protocol: the coin cell was firstly charged to a defined upper voltage (4.85V vs. graphite) at constant current mode with a C/20 rate taper current, and was then kept at constant 4.85V voltage for 144 hours at 50° C. The floating capacity was then calculated from the accumulated charge over these 144 hrs and the cathode material mass. After this procedure, the coin cells were disassembled. The anode and the separator in contact with the anode were analyzed by ICP-OES determine their Mn content, indicating Mn dissolved during the floating experiment.
DSC Measurements
Differential Scanning calorimetry (DSC) was performed by firstly making a coin cell as described above and charging it to 4.9 V vs. Li with a constant current of C/25. Then the coin cell was held at 4.9V with an end condition of current reducing to C/50. Then the coin cell was disassembled and the cathode electrode taken out. The cathode electrode was washed with dimethyl carbonate (DMC) twice to remove residual electrolyte, and dried at 120° C. for 10 minutes in vacuum. A 5 mm diameter round sample was punched from the electrode and used as a sample for DSC measurement, with circa 30% by weight of electrolyte added, using a closed DSC cell. A TA DSC Q10 instrument was used for the DSC test. The temperature range of test was from 50° C. to 350° C. using a temperature ramp of 0.5° C./min. Finally, the onset temperature of exothermic reaction and total heat generated are reported. They are indicative for the stability of the cathode when used in a battery.
The invention is further illustrated in the following Examples:
NiSO4.6H2O and MnSO4.1H2O, were dissolved in water to a summed total metal concentration of 110 g/L and having a Ni/Mn molar ratio of 0.21/0.79. An ammonia solution with NH3 concentration of 227 g/L was prepared by diluting a concentrated ammonia solution with water to reach the desired concentration. An aqueous nanoparticulate TiO2 suspension (385 g/L) was used as dopant feed and the concentration of NaOH solution was 400 g/L. The reactor was firstly charged with water and ammonia with the ammonia concentration of 15 g/L, and then heated up to 60° C. A Ti-doped metal hydroxide was then precipitated by continuously adding the Ni—Mn sulphate solution, the ammonia solution, the TiO2 suspension and the NaOH solution into a continuous stirring tank reactor (CSTR) through the control of mass flow controllers (MFC) under a N2 atmosphere. The precipitation process was controlled by changing the flow rate of the NaOH solution to reach the desired particle size, while the flow rates of the Ni—Mn sulphate solution, ammonia solution and the TiO2 suspension were kept constant. After the particle size of the precursor reached the target, the flow rate of NaOH solution was fixed. The resulting overflow slurry was collected and was separated from the supernatant by filtration. After washing with water, the precipitated solid was dried in a convection oven at 150° C. under N2 atmosphere. Chemical analysis of the obtained precursor material confirmed a composition consistent with [Ni0.21Mn0.79]0.985Ti0.015 metal atomic ratio. Oxygen and hydrogen level indicated the product to be a mixed metal oxyhydroxide, and SEM fotograph showed 1-15 μm particles with fine TiO2 particles embedded. Lithium carbonate and the obtained TiO2 coated Ni—Mn oxy-hydroxide precursor were homogenously blended a vertical single-shaft mixer by a dry powder mixing process. The blend ratio was targeted to obtain the following composition with respect to the elements Li, Ni, Mn and Ti: Li0.988[(Ni0.21Mn0.79)0.985Ti0.015]2.012 which was verified by ICP. The distribution of Ti in the powder was homogeneous, as can be easily verified.
The obtained powder mixture was heat-treated in a box furnace at a temperature of 980° C. for 10 hrs. Then the temperature was lowered to 700° C. for a period of 5 hrs. In both stages dry air was flowing through the box furnace, so that an oxidizing atmosphere was established. The product was cooled to room temperature and milled to a particle size distribution with D50=14 μm. The finally obtained material was Li0.988[(Ni0.21Mn0.79)0.985Ti0.015]2.012O4.
Example 2 was manufactured by the same method as Example 1, with the difference that the ratio of Li to the other elements was changed to result in a material with a composition of: Li0.971[(Ni0.21Mn0.79)0.985Ti0.015]2.029O4.
Counter Example 1 was manufactured by the following steps: Lithium carbonate and Ni—Mn oxy-hydroxide were homogenously blended in a vertical single-shaft mixer by dry powder mixing. The overall composition was targeted to obtain the following composition with respect to the elements Li, Ni and Mn: Li0.988[Ni0.21Mn0.79]2.012, which was verified by ICP. The same thermal treatment and milling treatment as for Example 1 was given to this blend.
Counter Example 2 was manufactured by the same method as Example 2, with the difference that the ratio of Li to the other elements was changed to result in a material with a composition of: Li0.971[(Ni0.21Mn0.79)0.98Ti0.020]2.029O4, having a Ti content outside the range of the invention.
Examples 1 and 2 and Counter Example 1 were submitted to the abovementioned characterizations, Counter Example 2 was only submitted to XRD and coin cell measurement, and the following results were obtained: Table 2 summarizes the ratios FWHM(111)/FWHM(004), and Table 3 summarizes the coin cell performance when the coin cells are charged to 4.9 V.
Example 1 and Example 2 show improved cycle stability compared to Counter Example 1 and Counter Example 2, as is particularly clear from the much lower Qfade values.
Example 1 and Example 2 have higher onset temperatures of the exothermic peaks, and their total heat values are smaller than for Counter Example 1. Overall this means that Example 1 and Example 2 show improved thermal stability compared to Counter Example 1, which is related to improved safety of the real cells using such cathode materials.
Table 5 shows the results of the floating experiments. Examples 1 and 2 show a significantly lower floating capacity and Mn dissolution than Counter Example 1. This indicates a better high voltage stability for Examples 1 and 2 compared to Counter Example 1.
| Number | Date | Country | Kind |
|---|---|---|---|
| 15184810.8 | Sep 2015 | EP | regional |
| 15186518.5 | Sep 2015 | EP | regional |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/IB2016/055143 | 8/29/2016 | WO | 00 |
