POWDEROUS SOLID ELECTROLYTE COMPOUND FOR SOLID-STATE RECHARGEABLE LITHIUM ION BATTERY

Abstract

A solid solution electrolyte suitable for solid-state rechargeable lithium ion battery comprising a compound having a general formula Li(3.5+L+x)Si(0.5+s−x)P(0.5+p−x)Ge2xO4+a wherein −0.10≤L≤0.10, −0.10≤s≤0.10, −0.10≤p≤0.10, −0.40≤a≤0.40, and 0.0<x≤0.30, preferably 0.05≤x≤0.30.

Description

TECHNICAL FIELD AND BACKGROUND

This invention relates to a solid electrolyte (SE) for solid-state rechargeable lithium-ion batteries suitable for electric vehicle (EV) applications. The solid electrolyte according to the invention has an improved lithium ionic conductivity.

Along with the developments of EVs, it comes also a demand for lithium-ion batteries as a possible constant source of power for such applications.

Aside the zero-emission aspect, additional requirements rendering a battery eligible for EV applications should therefore include: high capacity, longer cycle lives, lower cost, and better safety.

Most of the past and ongoing research on batteries usually emphasizes on the use of a liquid electrolyte, since such an electrolyte has a high lithium ionic conductivity of around 10⁻² S/cm at room temperature (measured for an electrolyte composed of ethylene carbonate/dimethyl carbonate EC/DMC 1M LiPF₆) and provides a good contact with electrodes through wetting. However, batteries containing liquid electrolytes imply concerns since liquid electrolytes are usually flammable.

Solid electrolyte-based batteries are considerably safer and can be an alternative to liquid electrolyte-based batteries for the power source of electric vehicles. Since the decomposition temperature of SE is higher than the liquid electrolyte, an EV comprising such a SE-based battery, instead of a liquid electrolyte-based battery, would be safer and its battery manufacturing would also be safer. Aside from the clear advantage that a SE-based battery is much safer than a liquid electrolyte-based battery, a SE-based battery is also more compact than liquid electrolyte-based batteries, and therefore lead to higher power densities

However, it is also true that, compared to liquid electrolyte, a SE has a lower lithium ionic conductivity (typically included between 10⁻⁷ to 10⁻⁶ S/cm). The lower lithium ionic conductivity in a SE is due to the higher migration energy of lithium ions.

Well-known (inorganic) types of SE suitable for solid-state rechargeable lithium-ion batteries (SSB) which have been mainly explored are the La, Zr comprising garnet and the LSPO (lithium silicon phosphorus oxide) LISICON (lithium superionic conductor) electrolyte having a general formula: Li_4±xSi_1−xP_xO₄. The La, Zr comprising garnet, especially the cubic garnet (Li₇La₃Zr₂O₁₂), shows a high conductivity. However, those materials are expensive due to the La and Zr content. This compound also needs to be synthesized at a temperature higher than 1000° C. which is not desired because of the volatility of lithium at the high temperature. The LSPO is well-known for its high chemical and electrochemical stability, high mechanical strength, and high electrochemical oxidation voltage, making such an electrolyte a promising candidate for EV applications.

LSPO compounds, like Li_3.5Si_0.5P_0.5O₄, have a relatively low lithium ionic conductivity. Li_3.5Si_0.5P_0.5O₄, is a solid solution of Li₄SiO₄ and Li₃PO₄ having a crystal structure of γ-Li₃PO₄ with orthorhombic unit cell and tetrahedrally coordinated cations. In the scope of the present invention, a solid solution (also called solid-state solution) refers to a multi-component solid-state solution as a result of a mixture of one or more solutes in a solvent. In particular, the solutes can be atoms or groups of atoms (or compounds). Such a multi-component system is considered a solution rather than a compound when the crystal structure of the solvent remains unchanged by addition of the solutes, and when the chemical components remain in a single homogeneous phase. In the system, the solvent usually is a component with the largest portion and in this case is Li₃PO₄. The crystal structure of the solvent component is maintained after blending whereas the other component (solute, e.g. Li₄SiO₄) dissolves in the solvent structure instead of forming a distinct compound having a structure that deviates from the structure of the solvent.

In Solid State Ionics (2015), 283, 109-114, Wang, Dawei et al. studied the enhancement of lithium ionic conductivity of LSPO by an addition of Li₃BO₃. The highest lithium ionic conductivity was 6.5×10⁻⁶ S/cm at 20% of Li₃BO₃ addition compared to 3.6×10⁻⁶ S/cm for 0% addition. Another study conducted by Choi, Ji-won et al. in Solid State Ionics (2016), 289, 173-179, explored the effect of Al cation substitution in 0.7Li₄SiO₄+0.3Li₃PO₄ solid solution system. The lithium ionic conductivity of 7.7×10⁻⁶ S/cm was achieved at room temperature by the addition of 10 mol % Al.

Sokseiha et al. in Chem. Mater. (2018), 30, 5573-5582 and Kamphorst et. al. in Solid State Ionics (1980), 1, 187-197 specify that LÍ4SÍO4-U3PO3 solid solutions have a lower conductivity than Li₄GeO₄—Li₃PO₃ teaching away from a possible substitution of Ge with Si.

Burmakin et. al. in Russian Journal of Electrochemistry (2010), 46, No. 2, 243-246 discloses a lithium germanium phosphate solid electrolyte doped with a tetravalent cation, Zr like: Li_3.75Ge_0.70Zr_0.05P_0.25O₄, Li_3.50Zn_0.125Ge_0.75P_0.25O₄, respectively. The Ge content in these formulation would not allow the obtention of a solid solution in a LSPO-based SE.

Although noticeable, these attempts to achieve a solid solution of a LSPO-based electrolyte are still below a desired threshold of at least 0.50×10⁻⁵ (or 5.0×10⁻⁶) S/cm required so that a solid-state battery made from said electrolyte is a tangible alternative to current liquid electrolyte-based batteries.

Certainly, there is a need for improving the lithium ionic conductivity of LSPO-based electrolyte so as to render the use of solid-state batteries made from such an electrolyte more performant and therefore more attractive in the field of EV applications.

It is therefore an object of the present invention to provide a LSPO-based SE having an improved lithium ionic conductivity whilst retaining a solid solution, which is a prerequisite for the use of such a SE in a solid-state secondary battery suitable for EV applications.

Metallic Li can be used in the scope of the present invention as an anode of a SSB comprising the electrolyte according to the invention.

Provided that the SE according to the present invention can be destined to be contacted to a Li metal-based anode, it must therefore be compatible to said Li metal-based anode of the SSB, meaning that the SE while contacting said anode must remain chemically stable.

SUMMARY OF THE INVENTION

A solid electrolyte having an improved lithium ionic conductivity whilst retaining a solid solution and being chemically and thermally stable while contacting a Li metal anode is achieved by providing a solid solution according to claim 1 which comprises a LSPO-based electrolyte comprising germanium (Ge) up to 60 mol %. A Ge doped LSPO is referenced hereunder as “LSPGO”.

If a LSPGO-based electrolyte comprises Ge of superior to 60 mol %, it comprises at least one impurity phase (i.e. Li₂SiO₃), and such a Li₂SiO₃-bearing LSPGO-based electrolyte is not a solid solution according to the present invention. A similar impurity phase is observed in Burmakin et. al. (Li₂ZrO₃ in this case) for a high content of Ge.

In the framework of the present invention, it has been observed that by integrating up to 60 mol % of Ge in a solid solution of LSPO-based electrolyte it was possible to preserve said solid solution property together with a noticeable improvement of the lithium ionic conduction to a minimal value of 0.50×10⁻⁵ S/cm, which constitutes a significant contribution to what is currently known from the prior art.

It has also been demonstrated that the electrolyte according to the present invention is stable in presence of a Li metal foil, confirming its suitability in a SSB wherein a Li metal foil is used as an anode.

The present invention concerns the following embodiments:

1. —A solid solution electrolyte suitable for solid-state rechargeable lithium ion battery comprising a compound having a general formula Li_(3.5+L+x)Si_(0.5+s−x)P_(0.5+p−x)Ge_2xO_4+a wherein −0.10≤L≤0.10, −0.10≤s≤0.10, −0.10≤p≤0.10, −0.40≤a≤0.40, and 0.00<x≤0.30.

In a first aspect of the embodiment 1, 0.05≤x≤0.30, preferably 0.10≤x≤0.30.

In a second aspect of the embodiment 1, 0.15≤x≤0.30, preferably 0.20≤x≤0.30.

2. —The solid solution electrolyte according to embodiment 1 wherein said compound has a general formula: Li_(3.5+x)Si_(0.5−x)P_(0.5−x)Ge_2xO₄.

3. —The solid solution electrolyte according to the embodiment 1 or 2, having a lithium ionic conductivity of at least 10⁻⁵ S/cm at 25° C.

4. —The solid solution electrolyte according to any of the preceding embodiments, wherein 0.25≤x≤0.30.

5. —The solid solution electrolyte according to any of the preceding embodiments, having a lithium ionic conductivity measured at 25° C. superior or equal to 2.0×10⁻⁵ S/cm and inferior or equal to 3.0×10⁻⁵ S/cm, preferably superior or equal to 2.5×10⁻⁵ S/cm and inferior or equal to 3.0×10⁻⁵ S/cm.

6. —The solid solution electrolyte according to any of the preceding embodiments, comprising a crystal structure having an XRD pattern measured at 25° C. and at a wavelength of 1.5418 Å comprising a first peak having a first intensity and a second peak having a second intensity, said first and second peaks being present in a range of 2θ superior or equal to 27.5 and inferior or to 30.0±0.5°, said XRD pattern being furthermore free of peaks at 2θ>37.0±0.5° having an intensity superior to said first or second intensity.

7. —The solid solution electrolyte according to any of the preceding embodiments, comprising a crystal structure having an XRD pattern measured at 25° C. and at a wavelength of 1.5418 Å comprising a first peak having a first intensity and a second peak having a second intensity, said first and second peaks being present in a first range of 2θ superior or equal to 27.5 and inferior or to 30.0±0.5°, wherein in a second range of 2θ superior or equal to 21.0 and inferior or to 25.0±0.5° said XRD pattern has no more than three additional peaks, each of said three additional peaks having an intensity superior to said first or second intensity.

8. —The solid solution electrolyte according to any of the preceding embodiments, comprising a crystal structure having an XRD pattern measured at 25° C. and at a wavelength of 1.5418 Å comprising a first peak having a first intensity and a second peak having a second intensity, said first and second peaks being present in a first range of 2θ superior or equal to 27.5 and inferior or to 30.0±0.5°, wherein in a second range of 2θ superior or equal to 34.0 and inferior or to 36.0±0.5°, said XRD pattern has no more than three additional peaks, each of said three additional peaks having an intensity superior to said first or second intensity.

9. —A solid-state rechargeable lithium ion battery comprising the solid solution electrolyte according to any of the previous embodiments.

10. —A solid-state rechargeable lithium ion battery comprising a negative electrode having a Li metal-base anode contacting the solid solution electrolyte according to any of the embodiments 1 to 9.

11. —Use of the solid-state battery according to the embodiment 9 or 10 in an electric vehicle, in particular in an electric car, wherein the operating voltage of said battery is superior or equal to 200 V and inferior or equal to 500 V.

12. —A catholyte comprising the solid solution electrolyte according to any of the preceding embodiments and a cathode material having the general formula: Li_1+kM′_1−kO₂ where M′=Ni_{1−x′−y′−z′}Mn_x′Co_y′A_z′ with −0.05≤k≤0.05, 0≤x′≤0.40, 0.05≤y′≤0.40, and 0≤z′≤0.05, wherein A is a doping element which is different to Li, M′ and O, said positive active material powder comprising particles having a layered R-3m crystal structure, said catholyte having a D99≤50 μm and an ionic conductivity of at least 1.0×10⁻⁶ S/m

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1. Comparison of Electrochemical Impedance Spectra (EIS) for sample with various Ge dopants (x in Li_(3.5+x)Si_(0.5−x)P_(0.5−x)Ge_2xO_4+a) in Example 1 and Comparative Example 1

FIG. 2. Variation of lithium ionic conductivity versus the amount of Ge dopants (x in Li_(3.5+x)Si_(0.5−x)P_(0.5−x)Ge_2xO_4+a) in Example 1 and Comparative Example 1 FIG. 3. Comparison of X-ray diffraction pattern of Example 1 and Comparative Example 1

FIG. 4. Fourier-transform infrared spectrum of EX1-A, EX1-B, EX1-C, and Comparative Example 1 showing transmittance in (%) and wavenumber in (cm⁻¹)

FIG. 5. Comparison of X-ray diffraction pattern of Comparative Examples 1 and 2

FIG. 6. Comparison of X-ray diffraction pattern of LSPO (a) before exposure to molten Li metal at 250° C. and (b) after 15 minutes exposure

FIG. 7. Comparison of voltage vs. capacity graph of EX2-CAT-A and CEX3-CAT-B

DETAILED DESCRIPTION

In the following detailed description, preferred embodiments are detailed so as to enable practice of the invention. Although the invention is described with reference to these specific preferred embodiments, it will be understood that the invention is not limited to these embodiments. To the contrary, the invention includes numerous alternatives, modifications and equivalents as will become apparent from consideration of the following detailed description.

This invention discloses a germanium bearing LSPO compounds having a general formula Li_(3.5+x)Si_(0.5−x)P_(0.5−x)Ge_2xO_4+a (LSPGO) wherein 0.00<x≤0.30 and −0.40≤a≤0.40. It is a solid solution of Li₄GeO₄, Li₃PO₄, and Li₄SiO₄. When Ge equally substitutes Si and P in a LSPO compound, it is observed that the lithium ionic conductivity of the compound according to claim 1 significantly increases (reaching at least 10⁻⁵ S/cm) for x values higher or equal to 0.10. Moreover, the conductivity of Li_(3.5+x)Si_(0.5−x)P_(0.5−x)Ge_2xO_4+a wherein 0.10≤x≤0.30 gradually increases and it is maximized when x is 0.30. In particular, the conductivity of the compound according to claim 1 is unexpectedly higher (higher than 2.0 10⁻⁵ S/cm) for the narrow range: 0.20<x≤0.30, in particular for 0.25≤x≤0.30. Compared to the broader range of Ge content of claim 1, this narrower range leads to an increase of the lithium ionic conductivity by a 1.5 to 3.0 factor, which is remarkable.

Such a high conductivity of at least 10⁻⁵ S/cm has never been reported yet for solid solution LSPO type of electrolytes.

In addition, compatibility of the electrolyte according to the present invention with a Li metal anode.

This invention is also inclusive of a catholyte compound made from a mixture of a NMC type of positive electrode active material and the germanium bearing LSPO compound according to the present invention.

Both polycrystalline and monolithic NMC can be used as a positive electrode active material in the catholyte according to the present invention. A “monolithic” morphology refers here to a morphology where a secondary particle contains basically only one primary particle. In the literature they are also called single crystal material, mono-crystal material, and one-body material. The preferred shape of the primary particle could be described as pebble stone shaped. The monolithic morphology can be achieved by using a high sintering temperature, a longer sintering time, and the use of a higher excess of lithium. A “polycrystalline” morphology refers to a morphology where a secondary particle contains more than one primary particles.

The (solid-state) catholyte material is prepared by mixing the germanium bearing LSPO compound with the NMC composition so as to produce the catholyte which is subjected to a heat treatment at 600° C.˜800° C. for 1˜20 hours under oxidizing atmosphere. In particular, the method for producing said catholyte is a co-sintering-based process wherein the germanium bearing LSPO and the NMC compositions are blended so as to provide a mixture which is then sintered.

There are several ways to obtain each of the compositions of the germanium bearing LSPO and NMC positive electrode active material in the catholyte. Whereas the difference of the median particle sizes (D50) between the solid electrolyte and positive electrode active material in the catholyte is superior or equal to 2 μm, they can be separated using a classifier such the elbow jet air classifier (https://elcanindustries.com/elbow-jet-air-classifier/). The compositions of separate particles are measured according to the protocol disclosed in the section F) Inductively Coupled Plasma method so as to determine each of the compositions of the germanium bearing LSPO material and NMC positive electrode active material in the catholyte.

The use of an Electron Energy Loss Spectroscopy (EELS) in a Transmission Electron Microscope (TEM) is another example for obtaining each of the compositions of the germanium bearing LSPO material and NMC positive electrode active material in the catholyte. The elements and their atomic amount can be obtained directly by measuring the EELS of cross-sectional germanium bearing LSPO particles and NMC positive electrode active material particles separately.

The following analysis methods are used in the Examples:

A) Electrochemical Impedance Spectroscopy (EIS)

A cylindrical pellet is prepared by following procedure. 0.175 g of a powderous solid electrolyte compound sample is put on a mold having a diameter of 1.275 cm. A pressure of 230 MPa is applied to the mold. The pellet is sintered at 700° C. for 3 hours in oxygen atmosphere. Silver paste is painted on both sides of the pellet to have a sample configuration of Ag/pellet/Ag in order to allow EIS measurements. Standard deviation of this measurement is 2.0×10⁻⁸.

EIS is performed using an Ivium-n-Stat instrument, a potentiostat/galvanostat with an integrated frequency response analyzer. This instrument is common to be used in the battery/fuel cell-testing to collect impedance response against frequency sweep. The measurement frequency range is from 10⁶ Hz to 10⁻¹ Hz. The setting point/decade is 10 and the setting voltage is 0.05V. Measurement is conducted at room temperature (at 25° C.). The lithium ionic conductivity is calculated by below equation:

$\sigma =\frac{L}{R\times A}$

where L is the thickness of the pellet, A is the area of the sample, and R is the resistance obtained by the electrochemical impedance spectroscopy.

B) X-Ray Diffraction Test

A cylindrical pellet is prepared by following procedure. 0.175 g of a powderous solid electrolyte compound sample is put on a mold having a diameter of 1.275 cm. A pressure of 230 MPa is applied to the mold. The pellet is sintered at 700° C. for 3 hours in oxygen atmosphere.

The X-ray diffraction pattern of the pellet sample is collected with a Rigaku X-Ray Diffractometer (D/MAX-2500/PC) using a Cu Kα radiation source emitting at a wavelength of 1.5418 Å. The instrument configuration is set at: 1° Soller slit (SS), 1° divergence slit (DS) and 0.15 mm reception slit (RS). Diffraction patterns are obtained in the range of 10-70° (2θ) with a scan speed of 4° per a minute. Obtained XRD patterns are analyzed by the Rietveld refinement method using X'Pert HighScore Plus software. The software is a powder pattern analysis tool with reliable Rietveld refinement analysis results.

C) Fourier-Transform Infrared (FTIR) Spectrometry

FTIR transmission spectrum for the LSPGO powder is collected using Thermo Scientific FTIR Spectrometer (Nicolet iS 50) in the wave number range of 1200 to 500 cm⁻¹, with a resolution of 4 cm⁻¹, and scan cycle of 32 scan.

D) Particle Size Distribution

The catholyte powder samples used in the particle-size distribution (psd) measurements are prepared by hand grinding the catholyte powder samples using agate mortar and pestle. The psd is measured by using a Malvern Mastersizer 3000 with Hydro MV wet dispersion accessory after having dispersed each of the catholyte powder samples in an aqueous medium. In order to improve the dispersion of the catholyte powder, sufficient ultrasonic irradiation and stirring is applied, and an appropriate surfactant is introduced. D50 and D99 are defined as the particle size at 50% and 99% of the cumulative volume % distributions obtained from the Malvern Mastersizer 3000 with Hydro MV measurements.

E) Coin Cell Test

For the preparation of a positive electrode, a catholyte containing 0.16 g of NMC, 0.03 g conductor (Super P), and 0.125 g of 8 wt % PVDF binder are mixed in NMP solvent using a planetary centrifugal mixer (Thinky mixer) for 20 minutes. The homogenized slurry is spread on one side of an aluminum foil using a doctor blade coater with a 15 μm gap. The slurry-coated foil is dried and punched as 8 mm diameter circular shape. A Swagelok cell is assembled in an argon-filled glove box with the configuration of positive electrode, separator having a diameter of 13 mm, and lithium foil having a diameter of 11 mm as a negative electrode. 1M LiPF₆ in EC/DMC (1:1 wt %) is used as electrolyte. Each cell is cycled at 25° C. using automatic battery cycler Wonatech-WBCS3000. The coin cell testing at 0.1 C in the 4.3˜2.5V/Li metal window range.

F) Inductively Coupled Plasma (ICP)

The composition of a positive electrode active material, a solid electrolyte, and a catholyte is measured by the inductively coupled plasma (ICP) method using an Agillent ICP 720-ES. 1 gram of a powder sample is dissolved into 50 mL high purity hydrochloric acid (at least 37 wt % of HCl with respect to the total amount of solution) in an Erlenmeyer flask. The flask is covered by a watch glass and heated on a hot plate at 380° C. until complete dissolution of the powder. After being cooled to room temperature, the solution from the Erlenmeyer flask is poured into a 250 mL volumetric flask. Afterwards, the volumetric flask is filled with deionized water up to the 250 mL mark, followed by a complete homogenization. An appropriate amount of solution is taken out by a pipette and transferred into a 250 mL volumetric flask for a second dilution, where the volumetric flask is filled with internal standard and 10% hydrochloric acid up to the 250 mL mark and then homogenized. Finally, this solution is used for ICP measurement.

The invention is further exemplified in the examples below. The following samples were prepared:

Comparative Example 1

CEX1 having a general formula Li_3.50Si_0.50P_0.60O₄ was prepared by the following steps:

1) Mixing: Li₂CO₃, SiO₂, and (NH₄)₂HPO₄ with total weight of around 6.0 g according to the corresponding molar ratio were put on a 250 ml bottle with 140 ml of deionized water and each 100 g of Y doped ZrO₂ balls having 3, 5, and 10 mm diameter. The bottle was rotated in a conventional ball mill equipment with 300 RPM for 24 hours. The homogeneously mixed slurry was dried at 90° C. for 12 hours.

2) Calcination: the dried mixture was calcined at 900° C. for 6 hours in Ar atmosphere.

3) Pulverization: 1.4 g of calcined powder was put on a 45 ml bottle with 30 ml of acetone and 3.4 g of Y doped ZrO₂ balls having a dimeter of 1 mm. The bottle was rotated in a conventional ball mill equipment with 500 RPM for 6 hours. The pulverized powder was dried at 70° C. for 6 hours.

4) Sintering: The dried powder was sintered at 700° C. for 3 hours in oxygen atmosphere to get the final LSPO solid electrolyte compound.

Example 1

Ge doped LSPO samples having a general formula Li_(3.5+x)Si_(0.5−x)P_(0.5−x)Ge_2xO₄ were prepared in the same manner as CEX1 except that different amount of GeO₂ was added and the amount of Li₂CO₃, SiO₂, and (NH₄)₂HPO₄ were adjusted in the mixing step according to the target molar ratios. EX1-A, EX1-B, EX1-C, EX1-D, EX1-E, and EX1-F had the x of 0.05, 0.10, 0.15, 0.20, 0.25, and 0.30, respectively.

Comparative Example 2

S doped LSPO samples CEX2-A1 and CEX2-A2 having a general formula Li_{(3.5−30 x)}Si_(0.5−x)P_(0.5−x)S_2xO_4+a were prepared in the same manner as samples in the Example 1 except that different amount of Li₂SO₄ was added instead of GeO₂. CEX2-A1 and CEX2-A2 had the x of 0.05 and 0.10, respectively.

Ga doped LSPO sample CEX2-B having a general formula Li_{(3.5−30 x)}Si_(0.5−x)P_(0.5−x)Ga_2xO_4+a was prepared in the same manner as samples in the Example 1 except that Ga₂O₃ was added instead of GeO₂. CEX2-B had the x of 0.05.

Explanatory Example 1

To examine the stability of electrolyte with Li metal, LSPGO samples of EX1-B is directly contacted with molten Li metal under Ar atmosphere. To this end, pellet of EX1-B is made by pressing 0.175 g of EX1-A powder under 2349.7 kgf/cm² pressure, followed by a sintering at 700° C. for 3 h under 02 atmosphere. A stripe of Li foil is placed on a stainless-steel plate heated on 250° C. hot plate (under controlled non-oxidizing atmosphere). The obtained molten Li is directly poured on the prepared EX1-B pellets and the pellets are observed for 15 minutes. This explanatory experiment is conducted in a glove box with Ar atmosphere. The pellet is visually observed/monitored so as to detect an eventual thermal runaway and the structure is examined using X-Ray Diffraction.

Results

TABLE 1
List of Examples and Comparative Examples with their properties
	Sample	Conductivity	Volume
Examples	ID	Composition	x -	(S/cm)	(As)
Comparative	CEX1	Li3.50Si0.50P0.50O4	0.00	4.4 × 10−6	324.15
example 1
Example 1	EX1-A	Li3.55Si0.45P0.45Ge0.10O4	0.05	7.2 × 10−6	327.11
	EX1-B	Li3.60Si0.40P0.40Ge0.20O4	0.10	1.0 × 10−5	329.96
	EX1-C	Li3.65Si0.35P0.35Ge0.30O4	0.15	1.4 × 10−5	332.93
	EX1-D	Li3.70Si0.30P0.30Ge0.40O4	0.20	1.5 × 10−5	335.73
	EX1-E	Li3.75Si0.25P0.25Ge0.50O4	0.25	2.7 × 10−5	338.83
	EX1-F	Li3.75Si0.25P0.25Ge0.50O4	0.30	2.4 × 10−5	340.76
Comparative	CEX2-	Li3.35Si0.45P0.45S0.10O4	0.05	2.3 × 10−6	324.27
	A1
example 2	CEX2-	Li3.20Si0.40P0.40S0.20O4	0.10	3.9 × 10−7	323.58
	A2
	CEX2-B	Li3.65Si0.45P0.45S0.10O4	0.05	3.5 × 10−6	No solid
					solution
(*x in CEX1 and EX1-A to EX1-F with formula Li(3.5x)Si(0.5-x)P(0.5-x)Ge2×04, in CEX2-A1 and CEX2-A2 with formula Li(3.5-3x)Si(0.5-x)P(0.5-x)S2xO4, and in CEX2-B with formula Li(3.5+303x)Si(0.5-x)P(0.5-x)Ga2xO4.)

Enhanced Conductivity of the LSPGO According to the Present Invention

Table 1 summarizes the list of example and comparative example samples. It can be seen that LSPGO samples at x=0.05-0.30 (EX1-A to EX1-F) have higher lithium ionic conductivity than the LSPO sample (CEX1). As for S and Ga bearing LSPO (CEX2-A1, CEX2-A2, and CEX2-B), the lithium ionic conductivities are lower.

FIG. 1 shows a typical Nyquist plot from EIS measurement Li_(3.5+x)Si_(0.5−x)P_(0.5−x)Ge_2xO_4+a (x=0.00 to 0.30). The plot is composed of real part of impedance in X axis and imaginary part in Y axis. High frequency measurement appears on the left side of the graph forms a semicircle that is followed by a low angle spike as the result of the low frequency measurement. The semicircle formation corresponded to the lithium conduction in SE. The lithium ionic conductivity of CEX1 is 4.4×10⁻⁶ at room temperature that is comparable with the earlier published works on LSPO. The reduction of the semicircle diameter with the higher Ge amount indicating lower resistance of sample suggesting a better conductivity.

FIG. 2 shows the improvement of lithium ionic conductivity where the value increases proportionally with more Ge in the structure. The highest lithium ionic conductivity is 2.7×10⁻⁵ at x=0.25 which is about 6 times of CEX1. When x is 0.30, the value reduces to 2.4×10⁻⁵, concluding that 0.25 is an optimal concentration.

Retention of the Solid Solution in the Ge-Doped LSPO According to the Invention

FIG. 3 shows the XRD patterns of CEX1 (undoped electrolyte) and EX1-A to EX1-F. It is observed that Ge bearing LSPO samples have a single LISICON phase without impurity, demonstrating that Ge is well doped. In all graphs, the γ-Li₃PO₄ structure is maintained, indicated by the characteristic double peak at 28°-29° representing (121) and (200) planes as well as four consecutive peaks at 34°-36° correspond to (220), (131), (211), and (002) planes. The shifted peak positions to the left related with the larger crystal volume showed in Table 1. The observed single phase also acts as an evidence of solid solution formation for the Ge bearing LSPO.

Additionally, FTIR measurement is displayed in FIG. 4 with the vibration bands of P—O, Si—O, and Ge—O marked. The vibration bands at 580 cm⁻¹, 910 cm⁻¹, and around 1050 cm⁻¹ correspond to the vibration modes of tetrahedron P—O while band in area of 985-1005 cm⁻¹ corresponds to the vibration modes of Si—O, also in tetrahedral structure. All bands show transmittance signal increase along with the addition of Ge in the structure. On the other hand, transmittance of 790 cm⁻¹ band which corresponds to Ge—O vibration decreases with the more Ge in the structure. It is concluded that the atomic substitution of Ge successfully occurred at Si and P sites without changing the structure. This observation matches with that of XRD where the structure of LSPO is maintained upon doping.

XRD of CEX2 are compared with CEX1 in FIG. 5. The S bearing LSPO structure (CEX2-A) is still well maintained while Ga bearing LSPO structure (CEX2-B) has multiple impurity peaks marked with (*) in the graph. This indicating Ga is not easy to be doped in the LSPO system and solid solution is failed to form for CEX2-B.

Chemical Stability of the LSPGO with a Li Metal Foil

Chemical stability of LSPGO with Li metal was examined through a direct contact with the molten Li metal.

LSPGO pellet XRD diffraction pattern after exposure with molten Li metal is compared with the original pattern before exposure as displayed in FIG. 6. Both measurement produced diffractogram with the same peak location indicating the structure remain the same (there is no structural change as the result of reaction with Li metal). The pellet is also observed to be very stable upon contact without thermal runaway or any exothermic reaction.

Example 2 (Catholyte)

A M-NMC622 compound having the target formula of Li(Ni_0.60Mn_0.20Co_0.20)O₂ and a monolithic morphology is obtained through a double sintering process and a wet milling process running as follows:

1) Co-precipitation: mixed transition metal hydroxides with D50 of around 4 μm are prepared by the process described in KR101547972B1 (from page 6 line 25 to page 7 line 32).

2) 1^st blending: to obtain a lithium deficient sintered precursor, Li₂CO₃ and the co-precipitation product are homogenously blended with a Li/M′ ratio of 0.85 in a Henschel mixer for 30 minutes so as to obtain a 1^st blend.

3) 1^st sintering: the 1^st blend is sintered at 935° C. for 10 hours under an oxygen containing atmosphere. The product obtained from this step is a powderous lithium deficient sintered precursor with Li/M′=0.85.

4) Blending: the lithium deficient sintered precursor is blended with LiOH.H₂O to correct the Li stoichiometry to Li/M′=1.01. The blending is performed in a mixer for 30 minutes so as to obtain a 2^nd blend.

5) 2^nd sintering: the 2^nd blend is sintered at 890° C. for 10 hours in an oxygen containing atmosphere in a roller hearth kiln (RHK). The sintered blocks are crushed by a jaw crushing equipment.

6) Wet milling: To break the agglomerated intermediate particles into monolithic primary particles, a wet ball milling process is applied. 5 L bottle is filled with 1 L of deionized water, 5.4 kg ZrO₂ balls, and 1 kg of 2^nd sintering product from process number 5. The bottle is rotated on a commercial ball mill equipment.

7) Healing firing step (3rd sintering): The wet milled product is heated at 750° C. for 10 hours under oxygen containing atmosphere in a furnace. The sintered compound is sieved.

A catholyte material EX2-CAT-A is made by mixing M-NMC622 and EX1-B (Li_3.60Si_0.40P_0.40Ge_0.20O₄) according to a mixing ratio of 1:1 by weight, followed by a heat treatment at 700° C. for 3 hours in an oxygen atmosphere (i.e. like air). EX1-B has a median particle size (D50) of 2 μm.

A catholyte material EX2-CAT-B is obtained through a similar manner as the preparation of EX2-CAT-A except that the mixture is heated at 600° C.

Comparative Example 3 (Catholyte)

CEX3-CAT-A and CEX3-CAT-B are obtained through a similar manner as the preparation of EX2-CAT-A except that the mixture is heated at 500° C., 900° C., respectively, as displayed in Table 2.

TABLE 2
Particle size distribution and ionic conductivity of EX2 and CEX3 material
	Co-sintering			Ionic	First discharge
Sample	temperature	D50	D99	conductivity	capacity
ID*	(° C.)	(μm)	(μm)	(S/cm)	(mAh/g)
EX2-CAT-A	700	10.2	34.7	1 × 10−5	176.4
EX2-CAT-B	600	10.3	31.0	2 × 10−6	_**
CEX3-CAT-A	500	10.4	28.1	1 × 10−7	_**
CEX3-CAT-B	900	10.5	143.0	9 × 10−4	133.8
*-CAT- stands for “catholyte”.
** - : not measured.

The results of the size distribution measurement as displayed in the Table 2 show that EX2-CAT-A, EX2-CAT-B, CEX3-CAT-A, and CEX3-CAT-B have a similar D50 at around 10.2-10.5 μm. However, the D99 values are larger at the higher co-sintering temperature. For instance, the D99 of EX2-CAT-A (resulting from a sintering at 700° C.) is 34.7 μm while the D99 of CEX3-CAT-B (resulting from a co-sintering at 900° C.) is 143.0 μm. The coin cell characterization of EX2-CAT-A and CEX3-CAT-B as displayed in FIG. 7 shows the effect of the co-sintering temperature to the electrochemical performance. Here, CEX3-CAT-B sintered at 900° C. clearly shows a lower discharge capacity comparing to EX2-CAT-A sintered at 700° C.

On the other hand, co-sintering temperature lower than 600° C. is also unpreferable. The ionic conductivity data provided in Table 2 also demonstrate that the ionic conductivity of the catholyte depends upon the co-sintering temperature, wherein the catholyte is more conductive at a higher co-sintering temperature. EX2-CAT-A which results from a co-sintering prepared at 700° C. has an ionic conductivity of 10⁻⁵ S/cm while CEX3-CAT-A (resulting from a co-sintering at 500° C.) has an ionic conductivity of 10⁻⁷ S/cm, i.e. a decrease of ×100 is observed for this sample with respect to CEX3-CAT-A's ionic conductivity.

Claims

1-12. (canceled)

13. A solid solution electrolyte suitable for solid-state rechargeable lithium ion battery comprising a compound having a general formula Li(3.5+L+x)Si(0.5+s−x)P(0.5+p−x)Ge2xO4+a wherein −0.10≤L≤0.10, −0.10≤s≤0.10, −0.10≤p≤0.10, −0.40≤a≤0.40, and 0.0<x≤0.30.

14. The solid solution electrolyte according to claim 13 wherein said compound has a general formula: Li(3.5+x)Si(0.5−x)P(0.5−x)Ge2xO4.

15. The solid solution electrolyte according to claim 13, having a lithium ionic conductivity of at least 10−5 S/cm at room temperature.

16. The solid solution electrolyte according to claim 13, wherein 0.15≤x≤0.30.

17. The solid solution electrolyte according to claim 13, having a lithium ionic conductivity at room temperature superior or equal to 2.0×10−5 S/cm and inferior or equal to 3.0×10−5 S/cm.

18. The solid solution electrolyte according to claim 13, comprising a crystal structure having an XRD pattern measured at a wavelength of 1.5418 Å comprising a first peak having a first intensity and a second peak having a second intensity, said first and second peaks being present in a range of 2θ superior or equal to 27.5 and inferior or equal to 30.0±0.5°, said XRD pattern being furthermore free of peaks at 37.0±0.5°≤2θ≤47.0±0.5° having an intensity superior to said first or second intensity.

19. The solid solution electrolyte according to claim 13, comprising a crystal structure having an XRD pattern measured at a wavelength of 1.5418 Å comprising a first peak having a first intensity and a second peak having a second intensity, said first and second peaks being present in a first range of 2θ superior or equal to 27.5 and inferior or equal to 30.0±0.5°, wherein in a second range of 2θ superior or equal to 21.0 and inferior or equal to 25.0±0.5°, said XRD pattern has no more than three peaks, each of said three peaks having an intensity superior to said first or second intensity.

20. The solid solution electrolyte according to claim 13, comprising a crystal structure having an XRD pattern measured at a wavelength of 1.5418 Å comprising a first peak having a first intensity and a second peak having a second intensity, said first and second peaks being present in a first range of 2θ superior or equal to 27.5 and inferior or equal to 30.0±0.5°, wherein in a second range of 2θ superior or equal to 34.0 and inferior or equal to 36.0±0.5°, said XRD pattern has no more than three peaks, each of said three peaks having an intensity superior to said first or second intensity.

21. A solid-state rechargeable lithium ion battery comprising the solid solution electrolyte according to claim 13.

22. A solid-state rechargeable lithium ion battery comprising a negative electrode having a Li metal-base anode contacting the solid solution electrolyte according to claim 13.

23. The solid-state battery according to claim 21 wherein the battery is configured to have an operating voltage superior or equal to 200 V and inferior or equal to 500 V such that the battery is suitable for use in an electric car.

24. A catholyte comprising the solid solution electrolyte according claim 13, and a cathode material having the general formula: Li1+kM′i−kO2 where M′=Ni1−x′−y′−z′Mnx′Coy′Åz′ with −0.05≤k≤0.05, 0≤x′≤0.40, 0.05≤y′≤0.40, and 0≤z′≤0.05, wherein A is a doping element which is different to Li, M′ and O, said positive active material powder comprising particles having a layered R-3m crystal structure, said catholyte having a D99≤50 μm and an ionic conductivity of at least 1.0×10−6 S/m.