Patent Application
20200343539
The present invention relates to materials suitable for use as cathode materials in lithium-ion batteries. In particular, the present invention relates to particulate lithium transition metal oxide materials. The present invention also provides processes for making such materials, and cathodes, cells and batteries comprising the materials.
Layered nickel-containing lithium transition metal oxides, derivatives of LiCoO2, have been investigated due to their higher capacity, lower cost, better environmental benignity and improved stability compared with LiCoO2. These materials are considered promising candidates as cathode materials for a range of applications including full electric vehicles (EVs), hybrid electric vehicles (HEVs) and plug-in hybrid electric vehicles (PHEVs), in the face of the growing interest in higher capacity and energy density. However, to meet the demanding requirements in this area, some improvements in cycling stability, rate capability, thermal stability and structural stability are desired. Side reactions between electrode and electrolyte can result in increased electrode/electrolyte interfacial resistance and can lead to transition metal dissolution, particularly at elevated temperatures and under high voltage. These problems may be more severe with increased Ni content.
Recently, the surface modification of cathode materials has drawn attention with the aim of solving the above-mentioned problems. It has been demonstrated that surface modification with metal oxides [1-3], phosphates [4-6], fluorides [7-9], and some lithium conductive metal oxides [10-12] can improve cycling stability, rate capability, and, in some cases, even thermal stability.
U.S. Pat. No. 6,921,609 describes a composition suitable for use as a cathode material of a lithium ion battery which includes a core composition having an empirical formula LixM′zNi1-yM″yO2 and a coating on the core which has a greater ratio of Co to Ni than the core.
Cho et al [13] have described LiNi0.6Co0.2Mn0.2O2 with nano-sized crystalline Mn3(PO4)2 particles deposited on its surface, leading to improved thermal stability.
The present inventors have found that manganese phosphate is a promising candidate for depositing on the surface of particulate lithium nickel oxide materials, and have found that the nature of the manganese phosphate coating is important in providing advantageous physical and electrochemical properties to the lithium nickel oxide materials.
In particular, as demonstrated in the Examples, the present inventors have found that providing a continuous manganese phosphate coating on the surface of the particles can lead to one or more of decreased electrode polarisation, enhanced lithium ion diffusion, high rate capability, improved capacity retention and improved thermal stability.
Accordingly, in a first preferred aspect the present invention provides a coated lithium transition metal oxide material having a continuous coating of manganese phosphate provided on the surface of lithium transition metal oxide particles.
In a second preferred aspect, the present invention provides a process for providing a continuous coating of manganese phosphate on the surface of lithium transition metal oxide particles, the process comprising contacting particulate lithium transition metal oxide with a composition comprising Mn ions and phosphate ions, and heating to form the manganese phosphate coating.
Typically, the composition comprising Mn ions and phosphate ions has a Mn concentration in the range from 0.001M to 0.09M.
In a further preferred aspect, the present invention provides a coated lithium transition metal oxide material obtained or obtainable by a process described or defined herein. The material typically has a manganese phosphate coating provided on the surface of lithium transition metal oxide particles. The coating is typically continuous.
In a further preferred aspect, the present invention provides use of a coated lithium transition metal oxide according to the present invention for the preparation of a cathode of a secondary lithium battery (e.g. a secondary lithium ion battery). In a further preferred aspect, the present invention provides a cathode comprising coated lithium transition metal oxide according to the present invention. In a further preferred aspect, the present invention provides a secondary lithium battery (e.g. a secondary lithium ion battery) comprising a cathode which comprises coated lithium transition metal oxide according to the present invention. The battery typically further comprises an anode and an electrolyte.
Preferred and/or optional features of the invention will now be set out. Any aspect of the invention may be combined with any other aspect of the invention unless the context demands otherwise. Any of the preferred and/or optional features of any aspect may be combined, either singly or in combination, with any aspect of the invention unless the context demands otherwise.
The lithium transition metal oxide typically includes nickel. It may include one or more further transition metals, for example selected from the group consisting of cobalt, manganese, vanadium, titanium, zirconium, copper, zinc and combinations thereof. The lithium transition metal oxide may include one or more additional metals selected from the group consisting of magnesium, aluminium, boron, strontium, calcium and combinations thereof. The lithium transition metal oxide may comprise nickel and one or both of cobalt and manganese.
The lithium transition metal oxide may have a formula according to Formula I below:
LiaNixMyM′zO2+b Formula I
in which:
In Formula I, 0.8≤a≤1.2. It may be preferred that a is greater than or equal to 0.9, or 0.95. It may be preferred that a is less than or equal to 1.1, or 1.05.
In Formula I, 0.2≤x≤1. It may be preferred that x is greater than or equal to 0.3, 0.4, 0.5, 0.55 or 0.6. It may be preferred that x is less than or equal to 0.99, 0.98, 0.95, 0.9, 0.8 or 0.7.
In Formula I, 0<y≤0.8. It may be preferred that y is greater than or equal to 0.01, 0.02. 0.05 or 0.1. It may be preferred that y is less than or equal to 0.7, 0.6, 0.5, 0.4, 0.3, 0.2, 0.15, 0.1 or 0.05.
In Formula I, 0≤z≤0.2. It may be preferred that z is greater than 0, or greater than or equal to 0.005 or 0.01. It may be preferred that z is less than or equal to 0.15, 0.1 or 0.05. In some embodiments, z is 0 or is about 0.
Typically, 0.9≤x+y+z≤1.1. For example, x+y+z may be 1.
In Formula I, −0.2≤b≤0.2. It may be preferred that b is greater than or equal to −0.1. It may be preferred that b is less than or equal to 0.1. In some embodiments, b is 0 or about 0.
In Formula I, M′ is one or more selected from the group consisting of Mg, Al, V, Ti, B, Zr, Sr, Ca, Cu and Zn. It may be preferred that M′ is one or more selected from the group consisting of Mg and Al.
The lithium transition metal oxide may have a formula according to Formula II below:
LiaNixCovMnwM′zO2+b Formula II
in which:
In Formula II, 0.8≤a≤1.2. It may be preferred that a is greater than or equal to 0.9, or 0.95. It may be preferred that a is less than or equal to 1.1, or 1.05.
In Formula II, 0.2≤x≤1. It may be preferred that x is greater than or equal to 0.3, 0.4, 0.5, 0.55 or 0.6. It may be preferred that x is less than or equal to 0.99, 0.98, 0.95, 0.9, 0.8 or 0.7.
In Formula II, 0≤v≤0.8. It may be preferred that v is greater than 0, or is greater than or equal to 0.01, 0.02, 0.05 or 0.1. It may be preferred that v is less than or equal to 0.7, 0.5, 0.4, 0.3, 0.2 or 0.1.
In Formula II, 0≤w≤0.8. It may be preferred that w is greater than 0, or is greater than or equal to 0.01, 0.02, 0.05, 0.1 or 0.15. It may be preferred that w is less than or equal to 0.7, 0.6, 0.5, 0.45, 0.4, 0.3, 0.25, 0.2 or 0.1.
In Formula II, 0≤z≤0.2. It may be preferred that z is greater than 0, or greater than or equal to 0.005 or 0.01. It may be preferred that z is less than or equal to 0.15, 0.1 or 0.05. In some embodiments, z is 0 or is about 0.
Typically, 0.9≤x+v+w+z≤1.1. For example, x+v+w+z may be 1.
In Formula II, −0.2≤b≤0.2. It may be preferred that b is greater than or equal to −0.1. It may be preferred that b is less than or equal to 0.1. In some embodiments, b is 0 or about 0.
In Formula II, M′ is one or more selected from the group consisting of Mg, Al, V, Ti, B, Zr, Sr, Ca, Cu and Zn. It may be preferred that M′ is one or more selected from the group consisting of Mg and Al.
The lithium transition metal oxide may, for example, be doped or undoped lithium nickel cobalt manganese oxide (NCM), or doped or undoped lithium nickel cobalt aluminium oxide (NCA). The dopant may be one or more selected from Mg, Al, V, Ti, B, Zr, Sr, Ca, Cu and Zn, e.g. selected from Mg and Al.
The skilled person will understand that the features of the composition of the lithium transition metal oxide discussed herein relate to the composition of the lithium transition metal oxide independently of the manganese phosphate coating.
In some embodiments, the lithium transition metal oxide material is a crystalline (or substantially crystalline) material. It may have the α-NaFeO2-type structure. It may be a polycrystalline material, meaning that each particle of lithium transition metal oxide material is made up of multiple crystallites (also known as crystal grains or primary particles) which are agglomerated together. The crystal grains are typically separated by grain boundaries. Where the lithium transition metal oxide is polycrystalline, it will be understood that the particles of lithium transition metal oxide comprising multiple crystals are secondary particles. The manganese phosphate coating is typically formed on the surface of the secondary particles. It will be understood that the coated lithium transition metal oxide material is typically particulate.
The shape of the lithium transition metal oxide particles (e.g. the secondary particles) is not particularly limited. They may, for example be elongate particles (e.g. bar shaped particles), or they may be substantially spherical particles. The shape of the coated lithium transition metal oxide particles is not particularly limited. They may, for example be elongate particles (e.g. bar shaped particles), or they may be substantially spherical particles.
The lithium transition metal oxide particles have a continuous coating or film of manganese phosphate on the surface of the particles. The term continuous coating (or continuous film) is understood to refer to a coating covering each particle, the coating being formed from a layer of continuous manganese phosphate material. It is understood to exclude a coating made up from agglomerations of discrete particles, e.g. a coating where discrete particles are visible when viewed using TEM at a length scale of approximately 10 nm to 100 nm.
In some embodiments, the particles are entirely covered by the coating. It may be an MnPO4 coating. For example, it may be preferred that no more than 10%, 5%, 1% or 0.1% of the lithium transition metal oxide particle surface is exposed.
The coating layer may be substantially uninterrupted.
The coating layer may have a substantially uniform thickness. For example, the coating thickness at its thinnest point may be at least 15%, at least 25%, at least 50% or at least 75% of the average thickness of the coating layer. This may be determined by TEM, for example determining the thickness variation for ten representative particles.
The coating layer may be amorphous. The coating layer may be considered to be amorphous if no crystalline peaks representing manganese phosphate are visible by XRD analysis of the coated particles.
The continuous coating is a manganese phosphate coating. For example, it may comprise or consist essentially of MnPO4. The average oxidation state of the manganese in the manganese phosphate coating may be in the range 2.5-3.5, for example it may be 3.
Typically, the thickness of the continuous coating is less than or equal to 15 nm, 10 nm or 8 nm. The coating thickness may be greater than or equal to 0.5 nm, 1 nm, 2 nm, 3 nm or 4 nm. It may be particularly preferred that the coating thickness is in the range from 2 nm to 10 nm. The thickness may be determined using TEM. For example, the thickness may be determined for ten representative particles. The coating thickness may be the average (e.g. mean) coating thickness of the ten representative particles.
The manganese phosphate coating may be deposited from a composition comprising Mn ions and phosphate ions. The composition may be a solution, e.g. an aqueous solution.
The concentration of Mn ions in the composition may be in the range from 0.001M to 0.09M. It may be greater than or equal to 0.002, 0.003, 0.0035, 0.004, 0.0045, 0.005, 0.0055 or 0.006M. It may be less than or equal to 0.085, 0.08, 0.075 or 0.07M. (The concentration is calculated with reference to the total amount of Mn supplied and the total amount of liquid supplied to the lithium transition metal oxide material (i.e. Suspension C in the Examples below)).
The coated lithium transition metal oxide material may exhibit a capacity loss of less than 15%, less than 10%, less than 8% or less than 7% when cycled for 100 cycles at 1 C. The capacity loss may be determined using a Maccor series 4000 battery tester, and the cell may be cycled in galvanostatic conditions for 3 initial cycles at 0.1 C rate (activation of electrodes) followed by cycling at constant C-rate (1 C) for 100 cycles. The cell may be formed as follows:
The coated lithium transition metal oxide material may exhibit a lithium ion apparent diffusion coefficient on delitihation of at least 2×10−8 cm2s−1, e.g. at least 2.5×10−8 cm2s−1 or at least 3×10−8 cm2s−1. The lithium ion apparent diffusion coefficient may be determined by performing cyclic voltammogram (CV) scans at various scan rates from 0.1 to 1.5 mV s−1. The linear relationship of the peak current intensity as a function of square root of scan rate can be used to determine the apparent lithium ion diffusion coefficients according to the Randles-Sevcik equation.
The lithium transition metal oxide material may be obtained or obtainable by a process described or defined herein.
The present invention provides a process for providing a continuous coating of manganese phosphate on the surface of lithium transition metal oxide particles, the process comprising contacting particulate lithium transition metal oxide with a composition comprising Mn ions and phosphate ions, and heating to form the manganese phosphate coating.
The composition may be a solution, e.g. an aqueous solution.
The concentration of Mn ions in the composition may be in the range from 0.001M to 0.09M. It may be greater than or equal to 0.002, 0.003, 0.0035, 0.004, 0.0045, 0.005, 0.0055 or 0.006M. It may be less than or equal to 0.085, 0.08, 0.075 or 0.07M. (The concentration is calculated with reference to the total amount of Mn supplied and the total amount of liquid supplied to the lithium transition metal oxide material (e.g. Suspension C in the Examples below).)
The source of Mn ions is not particularly limited in the present invention. Typically, it is an Mn salt. Typically, the salt is soluble in water. The Mn ions may be Mn(II) or Mn(III) ions, typically Mn(II). Suitable Mn salts include Mn acetate (e.g. Mn(Ac)2), Mn chloride, Mn gluconate and Mn sulfate. Mn(Ac)2 may be particularly preferred.
The source of phosphate ions is not particularly limited in the present invention. Typically, it is a phosphate salt. Typically, the salt is soluble in water. Suitable phosphate salts include phosphate, hydrogen phosphate, dihydrogen phosphate and pyrophosphate salts. The counter ion is not particularly limited. It may be a non-metal counter ion, e.g. ammonium. NH4H2PO4 may be particularly preferred.
The particulate lithium transition metal oxide may be contacted with the composition comprising Mn ions and phosphate ions by a process comprising
The solution comprising phosphate ions may be added gradually, e.g. dropwise.
The concentration of Mn ions in the solution of Mn ions maybe less than or equal to 0.18M, 0.16M or 0.15M. It may be greater than or equal to 0.001M, 0.003M, 0.005M, 0.006M, 0.007M or 0.01M.
After contacting the particulate lithium transition metal oxide with the composition comprising Mn ions and phosphate ions, the mixture is typically dried.
The process comprises a step of heating the mixture (e.g. the dried mixture) to form the manganese phosphate coating. The heating step may involve heating to a temperature of at least 100° C., 150° C., 200° C., or 250° C. The temperature may be less than 800° C., 600° C., 400° C., or 350° C. The heating step may last for between 30 minutes and 24 hours. It may be at least 1, 2 or 4 hours. It may be less than 10 hours or 6 hours.
The heating step may be carried out in air. The Mn may be oxidised during the heating step, e.g. from Mn(II) to Mn(III). Alternatively, the heating step may be carried out in a different oxidising atmosphere, or in an inert atmosphere such as under nitrogen or argon.
The process of the present invention may further comprise the step of forming an electrode (typically a cathode) comprising the coated lithium transition metal oxide material. Typically, this is carried out by forming a slurry of the coated lithium nickel oxide, applying the slurry to the surface of a current collector (e.g. an aluminium current collector), and optionally processing (e.g. calendaring) to increase the density of the electrode. The slurry may comprise one or more of a solvent, a binder, carbon material and further additives.
Typically, the electrode of the present invention will have an electrode density of at least 2.5 g/cm3, at least 2.8 g/cm3 or at least 3 g/cm3. It may have an electrode density of 4.5 g/cm3 or less, or 4 g/cm3 or less. The electrode density is the electrode density (mass/volume) of the electrode, not including the current collector the electrode is formed on. It therefore includes contributions from the active material, any additives, any additional carbon material, and any remaining binder.
The process of the present invention may further comprise constructing a battery or electrochemical cell including the electrode comprising the coated lithium transition metal oxide material. The battery or cell typically further comprises an anode and an electrolyte. The battery or cell may typically be a secondary (rechargeable) lithium (e.g. lithium ion) battery.
The present invention will now be described with reference to the following examples, which are provided to assist with understanding the present invention, and are not intended to limit its scope.
1—Manganese Phosphate Coating of LiNi0.4Co0.2Mn0.4O2 Characterisation and Electrochemical Testing
Preparation of LiNi0.4Co0.2Mn0.4O2(Pristine NCM)
1.399 g LiAc, 1.991 g Ni(Ac)2.4H2O, 0.996 g Co(Ac)2.4H2O and 1.961 g Mn(Ac)2.4H2O were dissolved in 200 ml of deionised water and ethanol (volume ratio of water:ethanol was 1:5) under continuous stirring until the solution became transparent (solution A). 3.880 g oxalic acid was dissolved in 200 ml of deionised water and ethanol (volume ratio of water:ethanol was 1:5) under continuous stirring until it became transparent (solution B). Solution B was added into suspension A, drop by drop, under continuous stirring for 3 h. The suspension was then dried at 60° C.
The obtained dried material was heated to 450° C. for 10 h, and then heated up to 850° C. for 20 h in a muffle furnace (air atmosphere).
Preparation of Manganese Phosphate Coated LiNi0.4Co0.2Mn0.4O2(MP-NCM)
LiNi0.4Co0.2Mn0.4O2 (Pristine NCM) was prepared as described above. An appropriate amount of Mn(Ac)2.4H2O to give the desired manganese loading was dissolved in 10 ml of de-ionized water (DIW) under stirring, followed by the addition of 1 g of pristine NCM under continuous stirring for 30 min (suspension A). NH4H2PO4 (in the stoichiometric amount to give MnPO4) was dissolved in 10 ml of DIW (solution B). Solution B was added into suspension A, drop by drop, under continuous stirring for 3h. The resulting suspension (suspension C) was then dried at 60° C. while being stirred. The collected powder was then heated in a muffle oven (air atmosphere) at 300° C. for 5 h to form MnPO4-coated LiNi0.4Co0.2Mn0.4O2 (MP-NCM).
Three different amounts of MnPO4 were added, to prepare three different samples, as set out in Table 1 below;
This enabled the evaluation of the effect of coating thickness, and coating suspension composition, on the physical and electrochemical properties of the materials.
Characterisation
TEM images were collected. The samples were ground between two glass slides and dusted onto a holey carbon coated Cu TEM grid. The samples were examined in a JEM 2800 Transmission Electron Microscope using the following instrumental conditions: Voltage (kV) 200; C2 aperture (um) 70 and 40.
The TEM images are shown in
XRD patterns were identified using X-ray diffraction (Bruker D8 with Cu Kα radiation, λ=0.15406 nm).
X-ray photoelectron spectroscopy (XPS) measurements were performed with a PHI 5800 Multi-Technique ESCA system using a monochromatic Al Kα source (1486.6 eV) radiation. Charging effects at the surface were compensated for by low-energy electrons from a flood gun.
XPS was employed to investigate the effect of the coating on the oxidation states of the NCM material. The top line is pristine NCM, and the bottom line is MP-NCM-2 wt %.
The wide scan spectra in
Electrochemical Testing
Protocols
Cathode electrodes were fabricated by firstly dispersing/dissolving each of the active materials (80 wt %), C-NERGY Super C65 (IMERYS, 15 wt %) and poly-vinylidene fluoride (PVDF6020, Solvay, 5 wt %) in N-methyl-2-pyrrolidone (NMP, Aldrich). The slurries were intimately stirred to form a homogeneous dispersion, and then cast on Al foils by the doctor-blade technique. The wet electrodes were immediately dried at 60° C. to remove the
NMP. Afterwards, disc electrodes (12 mm in diameter) were punched and further dried under vacuum at 100° C. for 8 h.
CR2032 coin cells were assembled in an argon-filled glove box (with O2<0.1 ppm and H2O<0.1 ppm). Coin half cells were assembled using lithium metal as anode, 1M LiPF6 dissolved in ethyl carbonate-dimethyl carbonate (EC-DMC) (1:1 v/v) with 1 wt % of additive of vinylene carbonate (VC) as the electrolyte, single layer polyethylene membrane (ASAHI KASEI, Hipore SV718) as separator, and the cathodes prepared as described above. The average loading of the electrodes was ˜2.0±0.2 mg cm−2. For the cycling performance test with higher mass loading, electrodes were prepared with ˜4.0±0.2 and ˜6.0±0.2 mg cm−2 loading.
The electrochemical performance of the cells was tested using a Maccor series 4000 battery tester. The cells were cycled at different C-rates (from 0.1 C to 10 C) in the range of 3.0-4.3 V vs. Li+/Li to investigate the rate capability.
For the cycling stability test, the cells were cycled in galvanostatic conditions for 3 initial cycles at 0.1 C rate (activation of electrodes) followed by cycling at constant C-rates (0.1 C, 1 C, 2 C and 10 C) for 100 cycles.
Cyclic voltammetry (CV) measurements were performed using a multi-channel potentiostat (VMP Biologic-Science Instruments) within the voltage range between 2.5 and 4.5 V (vs. Li+/Li) at controlled temperature at 20° C. Initially three CV cycles were performed at a scan rate of 0.1 mV s−1 followed by other cycles at different scan rates (from 0.1 to 1.5 mV s−1).
For the evaluation of cycling performance at higher temperature, pristine NCM and MP-NCM-2 wt % electrodes were cycled at 10 C for 100 cycles at 60° C., following the initial three activation cycles at 0.1 C.
Cyclic Voltammograms
In order to investigate the effects of the manganese phosphate coating on the electrochemical performance of the active material (NCM), cyclic voltammograms of pristine NCM (
The anodic and cathodic peaks of pristine NCM in the first cycle are centred at 3.877 and 3.722 V with a peak separation of 0.155 V (see Table 2 below). The peak separation reduced to 0.1 V in the 3rd cycle. MP-NCM-1 wt % and MP-NCM-2 wt % showed even lower peaks separations, suggesting decreased electrode polarization, which indicates better electrochemical performance. MP-NCM-2 wt % displays the smallest peak separation, i.e., the smallest electrode polarization. On the other hand, MP-NCM-3 wt % showed an increased peak separation and poor reversibility upon the three voltammetric cycles.
To explore the effect of the manganese phosphate coating on the lithium ion transfer kinetics, cyclic voltammogram (CV) scans at various scan rates from 0.1 to 1.5 mV s−1 were collected. The linear relationship of the peak current intensity as a function of square root of scan rate can be used for the apparent lithium ion diffusion coefficients according to the Randles-Sevcik equation. The apparent lithium ion diffusion coefficients are set out in Table 3 below.
MP-NCM-2 wt % shows lithium ion apparent diffusivities of 3.28*10−8 and 7.64*10−9 cm2 s−1 for delithiation and lithiation processes, respectively. These values, almost twice those obtained with pristine NCM (ca. 1.85*10−8 and 4.85*10−9 cm2 s−1), clearly show that coating the NCM particles with a manganese phosphate layer of appropriate thickness enhances lithium ion insertion and extraction in the active material. MP-NCM-1 wt % showed improved extraction kinetics and acceptable insertion kinetics.
Cell Testing
Electrodes made from pristine NCM, MP-NCM-1 wt % and MP-NCM-2 wt % were subjected to galvanostatic charge-discharge cycles at various C-rates (from 0.1 C to 10 C) and then at constant rate (1 C for 100 cycles). The results are shown in
The initial capacities of pristine NCM, MP-NCM-1 wt % and MP-NCM-2 wt % were 166.2, 162.2, and 158.6 mAh g−1, respectively. At higher current rates, the capacity of the coated samples is greatly improved. At 10 C rate, MP-NCM-1 wt % and MP-NCM-2 wt % delivered capacities of 92.0 and 101.5 mAh g−1, respectively, which are higher than that of pristine NCM (70.5 mAh g−1). Additionally, the coated materials showed capacity losses of 6.3% and 3.3%, respectively, following 100 cycles at 1 C, while that of pristine NCM was 19.4%.
As seen in
The excellent performance of MP-NCM-2 wt % is even more obvious when comparing the charge/discharge profiles with pristine NCM upon cycling at 0.1 C, 2 C and 10 C rates. Pristine NCM electrodes showed lowest capacity retention values, ca. 89.7% (0.1 C), 78.2% (2 C) and 78.9% (10 C). At the highest rates, the pristine electrodes showed evidence of strong polarization due to the surface modification upon cycling. The same did not occur with the MP-NCM-2 wt % electrodes because of the effective manganese phosphate coating which protects the interface from side reactions. The results are shown in Table 4 below.
Although the initial capacity of pristine NCM at both 0.1 C and 2 C (165.4 and 139.1 mAh g−1) are slightly higher than those of MP-NCM-2 wt % (159.6 and 133.4 mAh g−1), the latter material performance surpasses that of the former after about 20 cycles. The difference becomes more prominent during following cycles. At 10 C, MP-NCM-2 wt % delivers superior capacity than pristine NCM from the initial cycle, and the capacity gradually increased during cycling which may be due to the activation of active material, yielding 102.7% capacity retention ratio (vs. 78.9% of pristine NCM). The significantly improved high rate capability and long-term cycling stability confirm the manganese phosphate coating of NCM material as a very successful approach.
Stress Conditions—Overcharge and Overdischarge
To evaluate the performance of the coated NCM upon cycling in more stressful conditions, further cycling tests were also performed.
Although the cycling stability is reduced, with this higher upper cut-off voltage (UCV) the material still provided 115.6 (at 4.4 V) and 129.2 (at 4.5 V) mAh g−1 capacity, i.e., higher than that of 107.5 mAh g-1 obtained upon charging up to 4.3 V. This shows that increased UCV provides higher capacity, but with a slight reduction in capacity retention and reversibility. The effect of the manganese phosphate coating layer was also investigated upon over-discharge. In particular, the MP-NCM-2 wt % electrode was subjected to 100 cycles (at 0.1 C) with the lower cut-off voltage set to 2.5 V to examine the cycling stability in case of over-discharge. From the charge-discharge profiles (
Thermal Stability of Pristine NCM and MP-NCM-2 wt % at Higher Operation Temperature (60° C.).
For the evaluation of thermal stability, pristine NCM and MP-NCM-2 wt % electrodes were cycled at 10 C in galvanostatic conditions for 100 cycles at 60 ° C. (
Excellent performance is also demonstrated for MP-NCM-2 wt % in cells using an ionic liquid electrolyte.
2—Manganese Phosphate Coating of LiNi0.6Co0.2Mn0.2O2 Characterisation and Electrochemical Testing
Preparation of LiNi0.6Co0.2Mn0.2O2(Pristine NCM622, P-NCM622)
LiCH3COO (22 mmol), Ni(CH3COO)2.4H2O (12 mmol), Co(CH3COO)2.4H2O (4 mmol) and Mn(CH3COO)2.4H2O (4 mmol) were dissolved in a mixture of deionised water (40 mL) and ethanol (160 mL) under continuous stirring until the solution became transparent (solution A). Oxalic acid (31 mmol) was dissolved in another mixture of deionised water (40 mL) and ethanol (160 mL) under stirring until transparent (solution B). After that, solution A was poured into solution B under vigorous stirring for 6 h. The mixture was then completely dried at 60° C. using a rotary evaporator.
The obtained dried material was heated to 450° C. for 10 h, then heated to 800° C. for 20 h in a muffle furnace (air atmosphere).
Preparation of Manganese Phosphate Coated LiNi0.6Co0.2Mn0.2O2 (MP-NCM622)
Manganese phosphate coating was carried out as described above for LiNi0.4Co0.2Mn0.4O2, to provide 1 wt % manganese phosphate coating (MP-NCM622-1 wt %).
Electrochemical Testing
Electrodes and cells were prepared as described above with respect to the LiNi0.4Co0.2Mn0.4O2 samples, and electrochemical testing was carried out according to the same protocols.
Cycling Performance
With the purpose of investigating the effects of coating material on the cycling performances, electrodes of P-NCM622 and MP-NCM622-1 wt % were tested at various C-rates (0.1 C, 2 C and 10 C) over 100 cycles.
This demonstrates that similar advantages of the manganese phosphate coating are achieved for different lithium transition metal oxide materials.
Thermal Stability
In order to evaluate thermal stability, both P-NCM622 and MP-NCM622-1 wt % electrodes were cycled at 10 C for 100 cycles at 40° C. (
Differential Scanning calorimetry (DSC) measurements were conducted to examine the thermal behaviour changes with and without manganese phosphate coating. P-NCM622 and MP-NCM622-1 wt % electrodes were charged to 4.3 V at delithiated state.
Cycling Stability at Higher Cut-Off Voltage
For the investigation of cycling stability at higher cut-off voltage, both P-NCM622 and MP-NCM622-1 wt % electrodes were tested at various C-rates (0.1-10 C), and subjected to 50 cycles at 0.1 C and 10 C, respectively.
| Number | Date | Country | Kind |
|---|---|---|---|
| 1800742.7 | Jan 2018 | GB | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/GB2019/050114 | 1/16/2019 | WO | 00 |
