WO2015009912A1 - LiNixFe1-xPO4 SOLID SOLUTION, COMPOSITES OF LiNixFe1-xPO4 SOLID SOLUTION, AND METHODS OF MAKING - Google Patents

Abstract

Embodiments of the present disclosure provide for electrodes, devices including electrodes, lithium ion batteries, methods of making electrodes, and the like. An embodiment of the present disclosure includes a composition, among others, that includes: a LiNixFe1-xP04 solid solution material, wherein 0 < x < 1. In an embodiment, the composition can also include a high electron conducting material such as amorphous carbon, a carbon nanotube, graphene, carbon black, and a combination thereof.

Description

LiNi_xFe_1-xP0₄ SOLID SOLUTION, COMPOSITES OF LiNi_xFe_1-xP0₄ SOLID SOLUTION, AND METHODS OF MAKING

CROSS-REFERENCE TO RELATED APPLICATION This application claims priority to co-pending U.S. Provisional Application entitled "LiNi_xFei__xP0₄ SOLID SOLUTION, COMPOSITES OF LiNi_xFei__xP0₄ SOLID SOLUTION, AND METHODS OF MAKING" having Serial No. 61/847,666, filed on July 18, 2013, which is incorporated herein by reference.

BACKGROUND

Since the discovery of electrochemical reactivity of LiFeP0₄, olivine structure materials have been under investigation by numerous researchers as cathodes for lithium ion batteries, especially considered as potential solution for electric vehicles (EV) and plug- in hybrid vehicles (PHV). These types of materials are typically phosphate compounds of lithium and transition metals (e.g.: LiMP0₄, M=Fe, Mn, Co, Ni) with space group Pnma. Among the four types of lithium transition metal phosphates, LiNiP0₄ has the highest operation voltage of 5.1V against lithium metal, according to computational results. The value is significantly superior to the currently commercialized LiFeP0₄ which has an operation voltage around 3.4 V. However, the extremely poor intrinsic electronic conductivity of LiNiP0₄ based materials has prevented their use. Thus, alternative materials are needed to overcome these deficiencies.

SUMMARY

Embodiments of the present disclosure provide for electrodes, devices including electrodes, lithium ion batteries, methods of making electrodes, and the like.

An embodiment of the present disclosure includes a composition, among others, that includes: a LiNi_xFei__xP0₄ solid solution material, wherein 0 < x < 1. In an embodiment, the composition can also include a high electron conducting material such as amorphous carbon, a carbon nanotube, graphene, carbon black, and a combination thereof. An embodiment of the present disclosure includes a cathode material, among others, that includes: a composition including a LiNi_xFei__xP0₄ solid solution material, wherein 0 < x < 1. In an embodiment, the composition can also include a high electron conducting material.

An embodiment of the present disclosure includes a lithium ion battery, among others, that includes: an anode; a cathode made of a composition a composition including a LiNi_xFei__xP0₄ solid solution material, wherein 0 < x < 1; and an electrolyte disposed between the anode and the cathode.

BRIEF DESCRIPTION OF THE DRAWINGS

Further aspects of the present disclosure will be more readily appreciated upon review of the detailed description of its various embodiments, described below, when taken in conjunction with the accompanying drawings.

Figure 1 illustrates a field-emission scanning electron microscopy picture for

LiNio.₆Feo.4PO4 (selected) nanocomposites without carbon coating in low and high magnifications.

Figure 2 illustrates X-ray diffraction patterns for LiNi_xFei__xP0₄ solid solution nanocomposites without carbon coating.

Figure 3 illustrates X-ray diffraction patterns for LiNi_xFei__xP0₄ solid solution nanocomposites with carbon coating.

Figure 4 illustrates the lattice parameter for LiNi_xFei__xP0₄ and LiNi_xFei__xP0₄/C nanocomposites.

Figure 5 illustrates charging/discharging profile for the comparison test of chemical delithiation of LiNio.₆Feo.4PO4 nanocomposites (inner graph showed the whole charging/discharging process).

Figures 6A-6D illustrate X-ray diffraction pattern for (A): pure phase

LiNio.₆Feo.4PO4 nanocomposites; (B): electrochemically charged LiNio.₆Feo.4PO4; (C): chemically delithiated LiNio.₆Feo.4PO4 by NO2BF4 in ratio 1 : 1; and (D): chemically delithiated LiNio.6Feo.4PO4 by N0₂BF₄ in ratio 1 :2 DETAILED DESCRIPTION

This disclosure is not limited to particular embodiments described, and as such may, of course, vary. The terminology used herein serves the purpose of describing particular embodiments only, and is not intended to be limiting, since the scope of the present disclosure will be limited only by the appended claims.

Where a range of values is provided, each intervening value, to the tenth of the unit of the lower limit unless the context clearly dictates otherwise, between the upper and lower limit of that range and any other stated or intervening value in that stated range, is encompassed within the disclosure. The upper and lower limits of these smaller ranges may independently be included in the smaller ranges and are also encompassed within the disclosure, subject to any specifically excluded limit in the stated range. Where the stated range includes one or both of the limits, ranges excluding either or both of those included limits are also included in the disclosure.

Embodiments of the present disclosure will employ, unless otherwise indicated, techniques of material science, chemistry, and the like, which are within the skill of the art. Such techniques are explained fully in the literature.

The following examples are put forth so as to provide those of ordinary skill in the art with a complete disclosure and description of how to perform the methods and use the compositions and compounds disclosed and claimed herein. Efforts have been made to ensure accuracy with respect to numbers (e.g., amounts, temperature, etc.), but some errors and deviations should be accounted for. Unless indicated otherwise, parts are parts by atomic ratio, temperature is in °C, and pressure is at or near atmospheric. Standard temperature and pressure are defined as 20 °C and 1 atmosphere.

Before the embodiments of the present disclosure are described in detail, it is to be understood that, unless otherwise indicated, the present disclosure is not limited to particular materials, reagents, reaction materials, manufacturing processes, dimensions, frequency ranges, applications, or the like, as such can vary. It is also to be understood that the terminology used herein is for purposes of describing particular embodiments only, and is not intended to be limiting. It is also possible in the present disclosure that steps can be executed in different sequence, where this is logically possible. It is also possible that the embodiments of the present disclosure can be applied to additional embodiments involving measurements beyond the examples described herein, which are not intended to be limiting. It is furthermore possible that the embodiments of the present disclosure can be combined or integrated with other measurement techniques beyond the examples described herein, which are not intended to be limiting.

It should be noted that, as used in the specification and the appended claims, the singular forms "a," "an," and "the" include plural referents unless the context clearly dictates otherwise. Thus, for example, reference to "a support" includes a plurality of supports. In this specification and in the claims that follow, reference will be made to a number of terms that shall be defined to have the following meanings unless a contrary intention is apparent.

Prior to describing the various embodiments, the following definitions are provided and should be used unless otherwise indicated.

Discussion

Embodiments of the present disclosure provide for electrodes, devices including electrodes, lithium ion batteries, methods of making electrodes, and the like. Embodiments of the present disclosure can be advantageous since they can be used in devices that require high energy density, high energy power, and/or long cycling life. Embodiments of the present disclosure can have superior electrical conductivity than LiNiP0₄. Embodiments of the present disclosure can be used in lithium ion batteries, energy storage devices, portable devices, power tools, electric vehicles, and the like.

An embodiment of the present disclosure includes a LiNi_xFei__xP0₄ solid solution, where 0 < x < 1, and specifically, x can be about 0.01 to 0.99 in any increment of about 0.01 and this includes all possible ranges of about 0.01 to 0.99, in increments of about 0.01 (e.g., any of about 0.1, about 0.2, about 0.3, about 0.4, about 0.5, about 0.6 about, 0.7, about 0.8, about 0.9 to any of about 0.2, about 0.3, about 0.4, about 0.5, about 0.6, about 0.7, about 0.8, about 0.9). An embodiment of the LiNi_xFei__xP0₄ solid solution can have an olivine-type structure with Pnma space group. In an embodiment, the LiNi_xFei__xP0₄ particles (e.g., spherical, semi-spherical, polygonal, non-spherical, and the like) can have a diameter (or longest dimension) of about 10 to 500 nm, about 10 to 100 nm, or about 40 nm, while other dimensions (e.g., length, width, height) can have the same or smaller ranges. Embodiments of the LiNi_xFei__xP0₄ solid solution can have an electrical conductivity of about 0.1 x 10^"09 S/cm to 9.9 x 10^"7 S/cm.

Embodiments of the LiNi_xFei__xP0₄ solid solution can have a lithium diffusion coefficient of about 1 x 10^"14 cm²/s to 2 x 10^"13 cm²/s. Another embodiment includes a composite of a LiNi_xFei__xP0₄ solid solution material and a high electron conducting material (e.g., LiNi_xFei__xP0₄/C

nanocomposite). In an embodiment, the high electron conducting material can be disposed on the surface of the particles of the LiNi_xFei-_xP04. In an embodiment, the LiNi_xFei__xP0₄ solid solution can be about 90 to 99.999 weight % of the composite. The LiNi_xFei__xP0₄ solid solution material can have the same characteristics and dimensions as described herein.

In an embodiment, the high electron conducting material can include a material having a high electron conducting characteristic, while not detrimentally reacting with LiNi_xFei__xP0₄ particles. In an embodiment, the high electron conducting material can include amorphous carbon, carbon nanotubes, graphene sheets, and combinations thereof. In an embodiment, the high electron conducting material is about 0.001 to 10 weight % of the composite.

In an embodiment, the LiNi_xFei__xP0₄/C nanocomposite particles (e.g., spherical, semi- spherical, polygonal, non-spherical, and the like) can have a diameter (or longest dimension) of about 10 to 500 nm, about 10 to 100 nm, or about 40 nm. Embodiments of the LiNi_xFei__xP0₄/C nanocomposite can have an electrical conductivity of about 0.1 x 10^~3 S/cm to 9.9 x 10^~3 S/cm.

Amorphous carbon is less ordered at the microscopic scale than crystalline graphite that includes a hexagonal or rhombohedral crystal structure. In an embodiment, amorphous carbon can include activated carbon, carbon black, charcoal, a combination thereof, and the like.

In an embodiment, the carbon nanotubes are generally described as fullerenes of closed-cage carbon molecules typically arranged in hexagons and pentagons. In an embodiment, the carbon nanotubes can be single wall nanotubes (SW T) or multi-walled nanotubes (MWNT). Embodiments of the MWNT can include 2 or more walls, 5 or more walls, 10 or more walls, 20 or more walls, or 40 or more walls. In an embodiment, the carbon nanotubes including SWNTs and MWNTs may have diameters from about 0.6 nanometers (nm) up to about 3 nm, about 5 nm, about 10 nm, about 30 nm, about 60 nm or about 100 nm. In an embodiment, the single-wall carbon nanotubes may have a length from about 50 nm up to about 1 millimeter (mm), or greater. In an embodiment, the diameter of the single-wall carbon nanotube is about 2 to 5 nm and has a length of about 50 to 500 nm. Embodiments of the LiNi_xFei__xP0₄ solid solution and the composite of a LiNi_xFei__xP0₄ solid solution material and a high electron conducting material can be made using solid state reaction methods. Exemplary methods of making LiNi_xFei__xP0₄ solid solution and the composite of a LiNi_xFei-_xP0₄ solid solution material and the high electron conducting material are described in Example 1.

In an embodiment, the LiNi_xFei__xP0₄ solid solution and the composite of a LiNi_xFei__xP0₄ solid solution material and a high electron conducting material can be used as a cathode material to form a cathode in a device, such as, a lithium battery, for example. In particular, the device can include an anode (e.g., graphite, multi-walled nanotube (MWNT), Ti0₂, Li₄Ti₅0i₂,and the like), an electrolyte (e.g., LiPF₆:EC-DMC (ethylene carbonate-dimethyl carbonate and other stable electrolytes), and the like), and a cathode made of the LiNi_xFei__xP0₄ solid solution and/or the composite of a

LiNi_xFei__xP0₄ solid solution material, a polymer binder (e.g., polyethlyene glycol, polypropylene, polyvinylidene fluoride (PVDF), and the like), and a high electron conducting material, where the electrolyte is disposed between the anode and cathode. In an embodiment the electrolyte can be a solid electrolyte or a liquid electrolyte.

Examples:

Example 1 :

Brief Introduction:

Nanosize LiNi_xFei__xP0₄ solid solution and Li i_xFei__xP0₄/C nanocomposites were prepared via a solid state reaction method under argon atmosphere. A single phase olivine-type structure with Pnma space group was determined by X-Ray diffraction. Crystallite sizes were found to be around 40 nm. Linear relationship was observed between lattice parameters and chemical composition. Synthesized materials displayed electronic conductivity similar to previous reported value of LiFePC^. Carbon coating also helps to increase the overall conductivity of nanocomposites to the order of 10^"3 S/cm. Chemical delithiation via NO₂BF₄ oxidant was able to extract more than 90% of lithium content from the solid solution material through which the lattice parameter was decreased. Electrochemical tests in combination with various high voltage electrolytes are ongoing to reveal the full potential of these solid solution materials. Introduction:

Since the discovery of electrochemical reactivity of LiFeP0₄ [1], olivine structure materials have been under investigation by numerous researchers as cathodes for lithium ion batteries, especially considered as potential solution for electric vehicles (EV) and plug-in hybrid vehicles (PHV) [2-4]. This type of materials are typically phosphate compounds of lithium and transition metals (e.g.: LiMP0₄, M=Fe, Mn, Co, Ni) with space group Pnma. One dimensional lithium transport channel is observed along [010] direction [5, 6]. Olivine type structure materials attracted common interests as cathode in lithium ion batteries due to their abundance in nature, low cost, non-toxicity and good electrochemical performance [7, 8]. A large reversible capacity between 160~170mAh/g has already been achieved in LiMnP0₄ and LiFeP0₄ [9, 10] . Among the four types of lithium transition metal phosphates, LiNiP0₄ has the highest operation voltage of 5.1V against lithium metal, according to computational results [11-14] . The value is significantly superior to the currently commercialized LiFeP0₄ which has an operation voltage around 3.4 V. However, it was accused that, the extremely poor intrinsic electronic conductivity of LiNiP0₄ based materials

(~10^"u-10^"14 S/cm), even 4-5 orders lower compared to LiFeP0₄ (~10^~7-10^~9 S/cm), prevented previous researcher from activating the Ni²⁺/Ni³⁺ redox pair and extracting any observable electrochemical reactivity from them [15-18].

Due to the common poor intrinsic conductivity of olivine type materials, researchers have developed several ways to improve their electrical performance. The most widely used method was to minimize the particle size of these cathode materials, which increased their specific surface area and ternary interface with electronic conductor and electrolyte [19, 20]. Nanosize synthesis also benefited the

charging/discharging processes by facilitating the lithium ion insertion/extraction as the overall ion migration length was reduced. Another way to do so was to apply a thin carbon coating on the lithium transition metal phosphates [9, 21]. In combination with reducing the particle size, LiFePOVC composites with near theoretical electrochemical capacity had been obtained. Additionally, Y.M. Chiang et al proposed that aliovalent ion doping on lithium sites within the lattice would promote the intrinsic electronic conductivity of LiFeP0₄ by an order of 8, but the overall feasibility and effectiveness of this aliovalent doping approach are still under debate and need further proof [22-27].

Besides the above mentioned approaches, there was another possible way to enhance the overall electronic conductivity as well as electrochemical performance of any olivine type materials. This involved the formation of a solid solution on the transition metal (M2) sites in the lattice (e.g.: Fe/Mn, Fe/Ni, Co/Ni, etc.) [6, 15, 17, 28-30] . By substituting M2 site transitional metal with another metal ion, it was expected that the conductivity of the low end member could be increased by the introduction of higher conductivity counterparts. Consequently the overall electrochemical performance was enhanced.

The solid solution between LiFeP0₄ and LiNiP0₄ had previously been of interest mainly because of the enhancement of LiFeP0₄'s cycling and rate performance by introducing nickel content into their lattice [28, 29]. Efforts were mostly made in Fe-rich region of the binary system (nickel content < 0.1). To date no publication had reported the synthesis and structural properties of the LiFeP0₄-LiNiP0₄ solid solution system with high nickel content, which would be crucial for the electrochemical activity on Ni²⁺/Ni³⁺ redox couple to be realized.

In this Example, we discuss the synthesis and structural properties of the whole series of LiNi_xFei-_xP0₄ solid solution material (x=0, 0.2, 0.4, 0.6, 0.8, 1). These solid solutions were synthesized with and without the additive of cellulose, which served as source for surface carbon coating. The difference in their electronic conductivity was characterized by 4 point probe test. Phase purity for these two series of solid solutions was confirmed by X-ray diffraction whereas crystallite sizes and lattice parameters were also calculated. Morphology of the materials was examined by field-emission scanning electron microscopy. Chemical delithiation was used to determine the lithium content that was able to be removed from the solid solution lattice and compared with electrochemically charged material (cycled to 4.3 V). Corresponding X-ray diffraction was conducted to examine the change in lattice with the removal of lithium content. This is the first time where the synthesis of phase pure LiNi_xFei-_xP0₄ solid solution nano-materials was reported. Also for the first time the removal of major part of lithium content within the crystal lattice in LiNiP0₄ based material was reported.

Experimental:

Synthesis

LiNi_xFei-_xP0₄ solid solution nanocomposites were prepared via solid state reaction method. L12CO₃ (lithium carbonate, 99.5+%, A.C.S certified, Fisher

Scientific), FeC₂0₄ (iron (ii) oxalate dehydrate, 99+%, Alfa Aesar), Ni(CH₃COO)₂ (nickel(ii) acetate tetrahydrate, 99+%, for analysis, Acros organics) and NH₄H₂P0₄ (ammonium dihydrogen phosphate, 99+%, for analysis, Acros organics) were used as raw material for synthesis. Cellulose (microcrystalline, Acros organics) was added to the solution for carbon coated nanocomposites. Stoichiometric amount of precursor materials were fully mixed by ball milling in acetone with zirconia media for 24 hours. The mixture was then ground with pestle and mortar and dried in air at 80 °C, followed by heat treatment at 350 °C for 8 hours in flowing argon for precursor decomposition. The calcinated powders were then ground and pressed into pellets in air before final firing and crystallization at 650 °C for 10 hours in argon. Different from the solid solution materials, pure phase LiFeP0₄ was synthesized by the same heat treatment route but with cellulose additive and a vacuum system because of the stricter atmosphere requirements. LiNiP0₄ powders were obtained through a sol-gel route developed by Gaugulibabu et al[31J. A few experiments were also done in nitrogen atmosphere and no observable difference between samples calcinated in different atmosphere was detected.

Analysis

Surface morphology of the nanocomposites was characterized by FEI XL-40 field emission scanning electron microscopy. Sample was coated with thin layer of Au-Pd before experiment for better resolution. The crystal phases of the synthesized series of materials were determined by Philips APD 3720 X-ray Diffractometer with CuKa source (λ=1.54178 A). The diffraction pattern was collected through a 2Θ angle from 10^° to 80^° at a speed of 0.04^° per second. A collinear 4 point probe setup was used to measure the electronic conductivity of disc-shaped fired samples by. Pellets used for the characterization were 1 cm in diameter and 1 mm in thickness. Samples were polished before the test to prevent any potential contact issue from uneven surface rendered from the firing process.

In order to evaluate the capability for insertion/removal of lithium in those nanocomposites, chemical delithiation experiments with strong oxidant 0₂BF₄ were conducted. The reaction was as follows: 0₂BF₄ oxidant was dissolved in 99% anhydrous acetonitrile (Acros Organics) before reaction. Selected composition of synthesized material was then mixed with the solution in atomic ratio 1 : 1 and 1 :2 against 0₂BF₄ to ensure that lithium content could be fully extracted from the lattice. After delithiation, the material was washed with acetonitrile repeatedly and left drying for 24 hours. Whole process was done in argon filled glove box to prevent any potential oxidation of the reactant. Finally the dry powder was characterized by X-ray diffraction for pattern analysis. For comparison purpose, electrochemically charged cathode material with same composition was also analyzed. Due to the limitation of the electrolyte used (LiPF₆ in ethylene carbonate: dimethyl carbonate), only low voltage electrochemical cycles to 4.3 V were able to be conducted. Three charging processes and two discharging processes were applied to the cathode in a CR2016 coin cell setup at a C rate of C/20. Lithium metal was used as the counter-electrode whereas separator was Celgard 260. Assembly and disassembly of the coin cell were both done in argon atmosphere. Upon completion of electrochemical cycles the coin cell was destroyed; cathode material was washed by acetonitrile (99%, anhydrous) and dried overnight before taking out for XRD analysis.

Results and Discussion:

Synthesis and characterization

Morphology for the solid solutions from SEM analysis was shown in Figure 1. Primary particle sizes ranged from 40 nm to 200 nm. Small particles were adhered to larger aggregates. For carbon coated nanocomposites the particles were embedded in carbon matrix and appeared in a relatively continuous manner.

Figure 2 shows the X-Ray diffraction patterns for LiNi_xFei__xP0₄ (x = 0, 0.2, 0.4, 0.6, 0.8, 1) series solid solution materials synthesized without carbon coating. It was found that all the peaks could be indexed to a single phase of ordered olivine type structure belonging to orthorhombic Pnma space group. No peaks related to alternative phases had been detected in the graph. Thus it was concluded that high phase purity series of LiNi_xFei__xP0₄ solid solution materials were synthesized with the solid state reaction routes described above. Similar to the pure LiFeP0₄ or LiNiP0₄ material, in the solid solution transition metal ion still occupied the octahedral M2 site

homogeneously in a ratio determined by the precursor materials. All diffraction peaks are sharp and distinguishable that no obvious phase separation could be found. All synthesized samples displayed a greyish color similar to the color of carbon-free LiFeP0₄ material as described in literature.

In Figure 3, selected X-Ray diffraction patterns were presented for

LiNi_xFei__xP0₄ series solid solution materials synthesized with surface carbon coating. Alternative phases evolved during the synthesis process, marked with a star label in the graph. The phases were labeled to be nickel phosphides such as ₃P, N1₇P₃, etc. These phases were believed to emerge from the reducing atmosphere created by the excessive carbon content from decomposition of the cellulose. Nickel ion was subjected to transition from [Ni³⁺] state to [Ni²⁺] state in reducing atmosphere, through which these off-stoichiometric impurity phases were formed. These impurity phases would decrease the overall electrochemical capacity, but were beneficial in overall electronic conductivity as proposed by Nazar et al[32]. Superior charge-discharge capacity and cycleability were observed for our carbon-coated sample tested towards a low voltage limit at 4.5 V as compared to the non-carbon coated ones.

Based on the XRD graphs, lattice parameters for as-prepared nanocomposites were calculated using least square method and summarized in Table 1. The results were plotted against composition in Figure 4, where a, b, c denoted the three dimensions of the orthorhombic cell. A linear correlation between lattice parameters and the content ratio of nickel in the composition was found. Lattice parameter a and b decreased linearly with the increasing ratio of nickel due to the smaller Shannon radii of [Ni²⁺] (0.69 A) compared to [Fe²⁺] (0.78 A). Lattice parameter c remained relatively unchanged. For the carbon coated LiNi_xFei__xP0₄ nanocomposites, lattice parameter a and b were slightly larger than that of carbon-free sample with the same composition. This could correspond to the fact that off-stoichiometric impurity phases of nickel phosphide ( ₃P, N1₇P₃) were formed due to the reducing atmosphere. Nickel content was partially consumed so the main phase switched towards the iron rich end. Selected samples had also been chosen to do multiple XRD tests in order to establish an error bar for the calculation. By repeat experiments the standard deviation for our test was determined to be ±0.5%.

Crystallite sizes for these nanocomposites were calculated by Scherrer equation:

where K is the shape factor, typically 0.9, λ is the X-ray wavelength, β is half maximum intensity broadening (FWHM) in radians, and Θ is the Bragg angle, τ is the calculated mean size of the crystalline domains, equal to the particle size of single crystallites. Crystal lengths along the three strongest peaks were calculated. No dendritic growth was observed. Typical crystallite size of 42+9 nm for as-prepared samples was obtained. No correlation between crystallite size and composition has been found for our synthesis route. Also no significant change in the crystallite size had been observed when introducing carbon coating into our solid solution system.

Due to the limitation of the equipment, we were only able to get rough numbers of electronic conductivity values through measurement using a 4 point probe setup. Solid solution without carbon coating have conductivity in the order of 10^~8~10^~9 S/cm, while conductivity for nanocomposites synthesized with carbon coating was determined to be 10^~2~10^~3 S/cm. The significant improvement in conductivity for these Li i_xFei-_xPC C nanocomposites could be attributed to a continuous phase of amorphous carbon content on particle surface, which helped to increase the conductivity extrinsically at the expense of minor impurity phases.

Chemical Delithiation Test

For evaluation of the lithium insertion/extraction capability of the solid solution, XRD diagrams for the selected composition (LiNio.₆Feo.4PO4)

nanocomposites after chemical delithiation with NO2BF4 oxidant and their comparison with electrochemically charged (delithiated) sample were shown in Figure 6.

Electrochemical delithiation process and its first cycle charge-discharge capacity were shown in Figure 5. Starting from the pure phase LiNio.₆Feo.4PO4 nanocomposite, peak shift towards high 2 theta angle was shown in the diagram. The shift of the peaks corresponding to each lattice plane indicated the contraction of the whole crystal lattice, induced by the removal of lithium content within oxygen octahedrons.

Using least square method, single cell parameters for electrochemically charged and chemically delithiated LiNio.₆Feo.4PO4 nanocomposites with lithium content partially removed were calculated and compared to the as-prepared sample. Significant contraction in lattice parameter a and b was found for both samples, while the change in lattice parameter for the chemically delithiated samples were much greater. This could be attributed to the fact that low voltage electrochemical cycles up to 4.3 V only removed the lithium content corresponding to Fe²⁺/Fe³⁺ redox couple, while strong oxidant NO2BF4 was capable of activating Ni²⁺/Ni³⁺ redox couple so more lithium were extracted. Applying Rietveld refinement method with Fullprof software, the lithium occupancy for the chemically delithiated samples was able to be simulated. Related information was available in the supplementary material section. It was found that more than 95% of lithium content could be extracted from the crystal lattice with a chemical ratio of 1 :2 between Li io.₆Feo.4PO4 and NO2BF4, while NO2BF4 with same chemical ratio as LiNio.₆Feo.4PO4 could remove 77% of lithium in the latter. These values far exceeded the theoretical value of lithium content (40%) corresponding to the Fe²⁺/Fe³⁺ redox couple and proved the activation of Ni²⁺/Ni³⁺ redox couple in the crystal lattice.

Based on the characterization data, it was concluded that two contributions from the iron substitution on nickel site helped the activation of Ni²⁺/Ni³⁺ redox couple in the solid solution material. The first enhancement was the increase in overall electronic conductivity whereas the electrons could be removed/added more freely for redox reactions, as presented in 4-point probe tests. The second factor was not as straightforward as the first one. The introduction of iron actually slightly decreased the redox voltage, through which the reactions on the higher voltage end could become more tolerable to electrolytes, or as presented in this manuscript, to N0₂BF₄ oxidant. The decrease in voltage would be less than 0.5 V depending on actual composition, so it didn't diminish the targeted voltage superior from LiNiP0₄ based material.

Having confirmed the reactivity of nickel content in our as-prepared

LiNi_xFei__xP0₄ solid solution materials, further electrochemical tests with high voltage electrolyte would be needed to further reveal the full potential of these series of materials to be used as cathodes for lithium ion batteries. Various electrolytes in combination with our LiNi_xFei__xP0₄ nanocomposites are ongoing and the results would be reported in our next paper.

Conclusion:

In this Example, a solid state reaction route was presented through which, for the first time, whole series of phase-pure LiNi_xFei__xP0₄ olivine type solid solution materials were synthesized and reported. The crystallite size for the as-prepared samples was 30-50 nm in aggregate form. Lattice parameters of the nanocomposites changed linearly with the content change of M2 site transition metal ion. The synthesized solid solutions displayed similar electronic conductivity of LiFeP0₄. The introduction of carbon source during synthesis could increase the overall conductivity to as high as 10^~2 S/cm. Using strong oxidant NO2BF4, we were able to prove the activation of the Ni²⁺/Ni³⁺ redox couple and remove more than 95% of the total lithium content in the material, which verified the feasibility of using iron solid solution to activate the LiNiP0₄ based material. Tests with the assistance of high voltage electrolyte are ongoing to reveal the full potential of these as-prepared solid solution nanocomposites.

Table. 1 Lattice parameter for LiNi_xFei__xP0₄ solid solution materials

Composition a (Angstrom) b (Angstrom) c (Angstrom)

LiFePC-4 10.328 6.003 4.692

LiNio.2Feo.8PO4 10.240 5.964 4.662

LiNio.4Feo.6PO4 10.178 5.922 4.663

LiNio.6Feo.4PO4 10.149 5.877 4.682

LiNio.8Feo.2PO4 10.096 5.846 4.676

LiNiPO 4 10.060 5.776 4.683

LiNioJeo_.sPO C 10.308 5.961 4.693

LiNio_.4Feo.6P0₄/C 10.213 5.891 4.663

LiNio.₆Feo.₄P0₄/C 10.154 5.869 4.681

Table. 2 Lattice parameter and lithium occupancy for charged and chemically delithiated LiNio.₆Feo.4PO4 solid solution materials

SampleYParameter a b c Li⁺ Occupancy

LiNi Fe PO sample

0.6 0.4 4 ^r 10.152 5.917 4.676 1.00

LiNi Fe PO 2.5 cycle charged sample

0.6 0.4 4 ^{J 0} 10.026 5.865 4.667 0.59

LiNi Fe PO 1 :1 NO BF 24hr

0.6 0.4 4 2 4 9.931 5.817 4.704 0.23 delithiation

LiNi Fe PO 1 :2 NO BF 24hr

0.6 0.4 4 2 4 9.908 5.806 4.703 0.05 delithiation

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It should be noted that ratios, concentrations, amounts, and other numerical data may be expressed herein in a range format. It is to be understood that such a range format is used for convenience and brevity, and thus, should be interpreted in a flexible manner to include not only the numerical values explicitly recited as the limits of the range, but also to include all the individual numerical values or sub-ranges encompassed within that range as if each numerical value and sub-range is explicitly recited. To illustrate, a concentration range of "about 0.1% to about 5%" should be interpreted to include not only the explicitly recited concentration of about 0.1 wt% to about 5 wt%, but also include individual concentrations (e.g., 1%, 2%, 3%, and 4%) and the sub-ranges (e.g., 0.5%, 1.1%, 2.2%, 3.3%, and 4.4%) within the indicated range. In an embodiment, the term "about" can include traditional rounding according to the measuring technique and the numerical value. In addition, the phrase "about 'x' to 'y'" includes "about 'x' to about 'y"\

While only a few embodiments of the present disclosure have been shown and described herein, it will become apparent to those skilled in the art that various modifications and changes can be made in the present disclosure without departing from the spirit and scope of the present disclosure. All such modification and changes coming within the scope of the appended claims are intended to be carried out thereby.

Claims

CLAIMS We claim at least the following:

1. A composition, comprising:

LiNi_xFei__xP0₄ solid solution material, wherein 0 < x < 1.

2. The composition of claim 1, further comprising: a high electron conducting material.

3. The composition of claim 2, wherein the high electron conducting material is about 0.001 to 10 weight % of the composite.

4. The composition of claim 2, wherein the high electron conducting material is selected from the group consisting of: amorphous carbon, carbon nanotubes, graphene, carbon black, a carbon nanotube, and a combination thereof.

5. The composition of claim 2, wherein the high electron conducting material is amorphous carbon.

6. A cathode material, comprising:

a composition including a LiNi_xFei__xP0₄ solid solution material, wherein 0 < x

< 1.

7. The cathode material of claim 6, further comprising: a high electron conducting material.

8. The cathode material of claim 7, wherein the high electron conducting material is selected from the group consisting of: amorphous carbon, a carbon nanotube, graphene, carbon black, and a combination thereof and wherein the high electron conducting material is about 0.001 to 10 weight % of the composite.

9. A lithium ion battery, comprising:

an anode;

a cathode made of a composition including a LiNi_xFei__xP0₄ solid solution material, wherein 0 < x < 1 ; and

an electrolyte disposed between the anode and the cathode.

10. The lithium ion battery of claim 9, further comprising: a high electron conducting material.

11. The lithium ion battery of claim 10, wherein the high electron conducting material is selected from the group consisting of: amorphous carbon, a carbon nanotube, graphene, carbon black, and a combination thereof and wherein the high electron conducting material is about 0.001 to 10 weight % of the composite.