EP4649055A1

EP4649055A1 - Lithium-ion conductor materials

Info

Publication number: EP4649055A1
Application number: EP24709154.9A
Authority: EP
Inventors: Guopeng HAN; Matthew Rosseinsky; Andrij VASYLENKO; Hongjun Niu; Luke DANIELS; John Claridge; Troy Manning; Ruiyong Chen; Matthew Dyer
Original assignee: University of Liverpool
Current assignee: University of Liverpool
Priority date: 2023-02-22
Filing date: 2024-02-21
Publication date: 2025-11-19
Also published as: GB202302541D0; WO2024175916A1; GB2627468A

Abstract

A solid crystalline material of formula (I): Li_aM2-bMʹ_bS_7-yY_y I_1-xX_x; wherein: a is from 5 to 9; M is selected from Si, Ge or Sn, or a mixture thereof; Mʹ is selected from Zn, B, Al, Ga, Sb, P, V, Nb, Ta, Mo, or W, or a mixture thereof; b is from 0 to 2; Y is selected from O, Se, Te, N, F, Cl, Br or I, or a mixture thereof; y is from 0 to 5; X is selected from O, S, Se, Te, F, Cl, Br, BH₄, OH, or NH₂, or a mixture thereof; and x is from 0 to 1. The solid crystalline material suitably provides a solid ionic conductor for use in a solid-state battery. A solid crystalline material comprising a solid solution, a solid-state battery comprising the solid crystalline material and a method of preparing the solid crystalline are also disclosed.

Description

Lithium-ion Conductor Materials

Field

The present invention relates to a solid crystalline material, a method of preparing the solid crystalline material, the use of the solid crystalline material as a solid ionic conductor and a solid- state battery comprising the solid crystalline material. In particular, the present invention relates to a new class of solid crystalline materials which may find utility as solid ionic conductors, for example as solid electrolytes in solid-state lithium-ion batteries and lithium metal batteries.

Background

Lithium-ion batteries comprise an anode, an ion-conducting electrolyte, a separator and a cathode. Organic electrolytes (primarily based on linear and cyclic alkyl carbonates) are typically used in lithium-ion batteries because of the wide operating voltage they provide. However, such organic electrolytes have high volatility and flammability which pose a serious safety issue for their use in the consumer electronics and transportation markets. When exposed to extreme conditions (such as elevated voltage and temperature), organic liquid electrolytes can react with the active electrode materials to release significant heat and gas, leading to fires and possibly explosions. There is therefore a need to provide safer, alternative electrolytes with comparable or superior ion conduction.

Recent advances in lithium-ion battery technology have provided inorganic solid ion conductors as replacements for organic liquid electrolytes. These inorganic solid ion conductors conduct charge by the passage of ions through an otherwise rigid crystal structure. Usually, only one type of ion (either cations or anions) is predominantly mobile and conducts charge through the solid electrolyte. Such ionic conductors are potentially useful in batteries, sensors and solid oxide fuel cells.

However, achieving conductivities in such solid electrolytes which are comparable to those of existing liquid electrolytes (~10² S cm ¹) remains a challenge due to the reduced mobility of ions in a solid electrolyte compared to a liquid electrolyte. One approach to achieve a desirable conductivity is based on doping various elements into the crystal structure of the solid ion conductor, for example to produce structures where oxide and sulfide are the anion. However, this often results in sub-optimal physical properties of the solid electrolyte which hinders processing of the material in the manufacture of batteries. Some current lithium ion conducting solid electrolytes have low ionic conductivity (< 10³ S cm ¹) and high elastic constants which hinder processing (e.g. lithium phosphorus oxynitride, Li?La3Zr2Oi2, L sAlosTii PC ), whilst others have poor stability against lithium metal and low elastic constants (e.g. LiwGeP2Si2) which hinders performance. The lithium-containing argyrodites are a family of lithium-ion conductors with potential application in solid-state batteries. Materials such as Lie-xPSs-xCh+x have high ionic mobility for such applications (Angew. Chem. Int. Ed. 2019, 58, 8681-8686). The highest ionic conductivity of 9.4 x i o ³ S cm ¹ has been observed for Li55PS45d1 5 at 298 K with an activation energy of 0.29 eV. Sb-based Li66Sio6Sbo4S5l with higher ionic conductivity of 2.4 x 10² S cm ¹ has been reported (J. Am. Chem. Soc. 2019, 141 , 19002-19013). However, the electronic conductivity (~ 10^-7 S cm^-1) of Sb-based solid electrolytes is about 2-3 orders of magnitude higher compared to related Li⁺ conductors (ACS Materials Lett. 2022, 4, 2187-2194) which is not desirable for the desired applications of these materials.

Solid electrolytes with liquid electrolyte like room temperature ionic conductivity (~ 10^-2 S cm^-1) have been reported to date. One such solid lithium-ion conducting solid electrolyte is Li9.54Sii.74Pi.44Sii.7Clo,3 (Nature Energy 2016, 1 , 16030), which is based on an established family of materials (LGPS - LiwGeP2Si2). This material exhibits an ionic conductivity of 2.53 x 10² S cm ¹ at room temperature. However, these materials are relatively unstable when in contact with lithium metal which may prevent the use of a high energy density lithium metal anode with the solid electrolyte, which would limit the performance of a lithium-ion battery incorporating these materials. LiwGeP2Si2 undergoes a continuous decomposition at the Li|LiwGeP2Si2 interface (Chem. Mater. 2016, 28, 2400-2407). For solid electrolytes that are thermodynamically unstable against reaction with Li metal, the formation of a mixed ionic-electronic conducting interphase as the case in LiwGeP2Si2 must be avoided. Similarly, Li9.54Sii.74Pi.44Sn.7Clo.3 showed severe interfacial reaction with Li metal with a low Coulombic efficiency of only 39% in a LiCoG2| Li954Si1 74P1 44S11 7CI03ILi cell (Nature Energy 2016, 1 , 16030).

The presence of impurities such as Li2S, Lil and LiCI in the as-prepared benchmark solid electrolytes such as Li55PS45d1 5 (Angew. Chem. Int. Ed. 2019, 58, 8681-8686), Li66Sio 6Sbo 4S5l (J. Am. Chem. Soc. 2019, 141 , 19002-19013) and unidentified impurities in Li9.54Sii.74Pi.44Sii.7Clo,3 (Nature Energy 2016, 1 , 16030) may lead to biased analysis of the solid electrolyte | Li interfacial stability. These impurities are known as main components responsible for a kinetically stable solid electrolyte|Li interface. Synthetic route that allows phase-pure materials is desirable to evaluate the intrinsic stability against Li metal.

However, there remains a need for further improved or alternative lithium ion-conducting solid electrolytes which have conductivities comparable to liquid electrolytes and that are stable with lithium metal electrodes and do not show significant electronic conductivity. Three-dimensional transport of the lithium ions is also desirable.

Summary of the Invention

It is one aim of the present invention, amongst others, to provide a solid crystalline material that addresses at least one disadvantage of the prior art, whether identified here or elsewhere, or to provide an alternative to existing solid crystalline materials. For instance, it may be an aim of the present invention to provide a solid crystalline material which can be used as a solid ionic conductor in lithium-ion batteries or lithium metal batteries.

It may be a further aim of the present invention to provide a solid electrolyte material which has high conductivity and exhibits sufficient stability when in contact with a lithium metal electrode.

It may be a further aim of the present invention to provide such a solid electrolyte material which comprises only earth-abundant, non-toxic elements or elements which are easily obtainable in large quantities.

According to aspects of the present invention, there is provided a solid crystalline material, a solid-state battery comprising such a solid crystalline material, a method of preparing such a solid crystalline material and use of the solid crystalline material, as set forth in the appended claims. Other features of the invention will be apparent from the dependent claims, and from the description which follows.

According to a first aspect of the present invention, there is provided a solid crystalline material of formula (I):

Li_aM₂-bM bSy-yYy h-xXx; wherein: a is from 5 to 9;

M is selected from Si, Ge or Sn, or a mixture thereof;

M' is selected from Zn, B, Al, Ga, Sb, P, V, Nb, Ta, Mo, or W, or a mixture thereof; b is from 0 to 2;

Y is selected from O, Se, Te, N, F, Cl, Br or I, or a mixture thereof; y is from 0 to 5;

X is selected from O, S, Se, Te, F, Cl, Br, BF , OH, or NH₂, or a mixture thereof; and x is from 0 to 1 .

Each of the M, M', X and Y atoms can be present in mixtures of atoms selected from the lists above and therefore are not necessarily present in stoichiometric amounts. As such b, y and x may not be integers. The inventors have found that the solid crystalline materials of this first aspect according to formula (I) may exhibit high conductivity (e.g. 1 .01 (4) x 1 o~² S cm^-1) which is comparable to the conductivity of the liquid electrolytes which these materials are intended to replace. The solid crystalline materials also suitably have a negligible electronic contribution to this conductivity (e.g. 8.1 (2) x 1 O ^W S cm ¹) and a low activation energy (e.g. 0.204(4) eV). The material also suitably exhibits good stability towards contact with lithium material and so may be incorporated into an all-solid-state lithium metal battery with a lithium metal anode.

Advantageously, phase-pure solid crystalline materials of this first aspect can be synthesised at a relatively low temperature (e.g. 723 K). Furthermore, in some embodiments, the materials of this first aspect may contain only earth-abundant or easily obtainable and non-toxic elements, which have the advantages of good availability, low cost of starting materials, reduced environmental impact and lower toxicity. This may provide significant advantages over current solid electrolyte materials, such as the LGPS materials discussed above.

Without being bound by theory, it is believed that the solid crystalline materials of this first aspect have a new structure type comprising considerable lithium site disorder which enables fast lithium ion transport. Variations of the chemical composition of the material appears to be possible within this new structure type with the aim of optimising the properties of the material.

The solid crystalline material of this first aspect suitably comprises a highly ordered structure forming a crystal lattice extending in three dimensions. The solid crystalline material is suitably formed from a unit cell which repeats in three dimensions to form a three-dimensional lattice structure. As will be known by the skilled person, a unit cell is characterized by six parameters. These parameters are three edges (a, b and c) and angles between them (a, /? and /). Suitably, using these parameters, a unit cell may be defined as cubic, tetragonal, orthorhombic, monoclinic, hexagonal or triclinic.

The solid crystalline material of the first aspect suitably has a monoclinic crystal structure with a P2i/n space group. For example, in an embodiment wherein the solid crystalline material of this first aspect is Li?Si2S7l , the material suitably has monoclinic P2 ln symmetry at 300 K with unit cell parameters of a = 14.9520(2) A, b = 6.04590(10) A, c = 15.1634(2) A and /3 = 105.6940(10)°.

The crystal structure of the solid crystalline material of this first aspect may be characterized by the anions S, Y, I and X, suitably S^2- and k, being ordered in 3³.4² nets which are stacked in an ABAB sequence along the b axis. This arrangement suitably provides anion coordination spheres of 13 and 11 . This structure is believed to provide chains of highly disordered Li⁺ sites with very short Li-Li distances which favours lithium ion migration and therefore facilitates high lithium ion conduction.

For the avoidance of doubt, the species mentioned above (Li, Si, Ge, Sn, Zn, B, Al, Ga, Sb, P, V, Nb, Ta, Mo, W, S, O, Se, Te, N, F, Cl, Br, I, BF , OH, or NH2) suitably have their normal charges in the solid crystalline material of this first aspect, for example Li⁺, Si⁴⁺, S²-, O²- and Ck. Suitably the solid crystalline material of the first aspect is overall charge neutral. The formula (I) is suitably charge balanced to provide such a charge neutral material. In particular, the value of “a” for the amount of lithium present in the formula may vary to account for and balance the charges of the species mentioned above when present in the material.

In some embodiments, the solid crystalline material has the formula (I):

Li_aM₂-bM bSy-yYy h-xXx; wherein: a is from 5 to 9;

M is selected from Si, Ge or Sn, or a mixture thereof;

M' is selected from Al, Ga, Sb or P, or a mixture thereof; b is from 0 to 1 ;

Y is selected from O, F, Cl, Br or I, or a mixture thereof; y is from 0 to 2;

X is selected from O, S, F, Cl or Br, or a mixture thereof; and x is from 0 to 0.5.

In the solid crystalline material of this first aspect, M in formula (I) is selected from Si, Ge or Sn, or a mixture thereof. By a “mixture thereof’ we mean that the material may contain at least two of these different species to provide the one “M” group in the formula (I). In such embodiments, the material may be considered to contain the different “M” species in a random orderthroughout the solid crystalline material with their relative abundance as indicated by the formulas disclosed herein. This meaning also applies to the species M', Y and X of the material of formula (I).

Suitably M comprises Si. M may comprise Si and Ge or Sn, or a mixture thereof. In such embodiments, M in formula (I) may be Si2-(c+d)Ge_cSnd, wherein c and d are each independently from 0 to 1 .

In such embodiments, M in formula (I) is suitably Si2-_cGe_c, wherein c is from 0 to 2. Therefore in such embodiments, the solid crystalline material of this first aspect has the formula (lib):

LiaSi2-cGecM bSy-yYy h-xXx; wherein c is from 0 to 2 and a, M', b, y, Y, x and X are as defined above. Suitably c is from 0 to 1 .5 or from 0 to 1 .

In some embodiments, M is Si. Therefore, in such embodiments, the solid crystalline material of this first aspect has the formula (III):

LiaSi2-bM'bS7-yYy h-xXx.

In the solid crystalline material of this first aspect, M' in formula (I) is selected from Zn, B, Al, Ga, Sb, P, V, Nb, Ta, Mo, or W, or a mixture thereof. Suitably M' is selected from Al, Ga, Sb or P, or a mixture thereof.

In such embodiments, suitably 0 < b < 2.

In some embodiments, b = 2 and therefore the solid crystalline material has the formula:

LiaM'2S7-yYy h-xXx; wherein M' is selected from Zn, B, Al, Ga, Sb, P, V, Nb, Ta, Mo, or W, or a mixture thereof.

For example, the solid crystalline material may be LigAl2S7l or LisSb2S7l.

Suitably b < 2.

Suitably b is from 0 to 1 .5 or 0 to 1 .

In some embodiments, M' is selected from Al, Ga or a mixture thereof. Therefore M' in formula (I) may be AleGaf, wherein e and f are each independently 0 to 2 and e+f = b, and therefore e+f

< 2. Suitably e and f are each independently 0 to 1 and e+f = b, and suitably e+f < 1 . In such embodiments, a in formula (I) is suitably 7+b.

In some embodiments, M' is selected from Sb, P or a mixture thereof. Therefore M' in formula (I) may be SbgPn, wherein g and h are each independently 0 to 2 and g+h = b, and therefore g+h

< 2. Suitably g and h are each independently 0 to 1 and g+h = b, and suitably g+h < 1 . In such embodiments, a in formula (I) is suitably 7-b.

M' in formula (I) may be represented by AleGafSbgPn, wherein e+f+g+h=b and is < 2, suitably < 1 . In such embodiments, the solid crystalline material of this first aspect may have the formula (IV):

LiaM2-bAleGafSbgPhS7-yYy ll-xXxJ wherein e, f, g and h are as defined above.

Suitably M is Si in such embodiments and the solid crystalline material has the formula (IVb): LiaSi2.bAleGatSbgPhS7.yY y h-xXx;

In such embodiments, a is suitably 7+(e+f)-(g+h).

In the solid crystalline material of this first aspect, Y in formula (I) is selected from O, Se, Te, N, F, Cl, Br or I, or a mixture thereof and y is from 0 to 5.

Suitably y is from 0 to 3 or from 0 to 2.

In some embodiments, y > 0. Therefore 0 < y < 5, suitably 0 < y < 3 or 0 < y < 2.

In such embodiments, Y may be selected from O, F, Cl, Br or I.

In such embodiments, Y may be O and the solid crystalline material may therefore have the formula (V):

LiaM₂-bM bSy-yOy h-xXx; wherein 0 < y < 5, suitably wherein 0 < y < 2.

In embodiments wherein Y is O, a in formula (I) is suitably 7.

In such embodiments, M is suitably Si and the solid crystalline material may therefore have the formula (Vb):

LiaSi₂-bM bSy-yOy li-xXxJ wherein 0 < y < 5, suitably wherein 0 < y < 2.

In embodiments wherein y > 0, Y may be selected from F, Cl, Br or I, or a mixture thereof, preferably F, Cl or Br, or mixtures thereof. The solid crystalline material may therefore have the formula (VI):

Li_aM₂-bM bSy-yFiCljBrklil 1-xXx; wherein 0 < y < 5, suitably wherein 0 < y < 2 and y=i+j+k+l.

In such embodiments, a in formula (I) is suitably 7-y. In such embodiments, M is suitably Si and the solid crystalline material may therefore have the formula (Vlb):

LiaSi2-bM'bSy._yFiCljBrklili-xXx; wherein 0 < y < 5, suitably wherein 0 < y < 2 and y=i+j+k+l.

Suitably Y is selected from O, F, Cl, Br or I, or a mixture thereof and y is from 0 to 2. In the solid crystalline material of this first aspect, X in formula (I) is selected from O, S, Se, Te, F, Cl, Br, BH4, OH, or NH2, or a mixture thereof.

In some embodiments, x > 0 and X is selected from F, Cl, Br, BH4, OH, or NH2, or mixtures thereof. In such embodiments, 0 < x < 1 , suitably 0 < x < 0.5.

Suitably X is selected from F, Cl, Br or a mixture thereof. The solid crystalline material may therefore have the formula (VII):

LiaM₂-bM bSy-yYyh-xF mClnBroJ wherein 0 < x < 0.5 and x = m+n+o.

In such embodiments, a in formula (I) is suitably 7. In such embodiments, M is suitably Si and the solid crystalline material may therefore have the formula (VII b) :

LiaSi2-bM'bS7-yYyll-xF mClnBTo; wherein 0 < x < 1 , suitably 0 < x < 0.5 and x = m+n+o.

In some embodiments, x > 0 and X is selected from O, S, Se, Te, or a mixture thereof. In such embodiments, 0 < x < 0.5.

Suitably X is selected from S or O or a mixture thereof. The solid crystalline material may therefore have the formula (VIII):

LiaM₂-bM bSz-yYyh-xSpOqJ wherein 0 < x < 0.5 and x = p+q.

In such embodiments, a in formula (I) is suitably 7+x. In such embodiments, M is suitably Si and the solid crystalline material may therefore have the formula (Vlllb):

LiaSi₂-bM bSz-yYyll-xSpOqJ wherein 0 < x < 0.5 and x = p+q.

In the solid crystalline material of this first aspect, b in formula (I) is from 0 to 2.

In some embodiments, b is 0. Therefore the solid crystalline material may have the formula (IX):

LialVhSz-yYy li-_xX_x; suitably wherein M is Si.

In such embodiments, a, Y, y, X and x are as defined above. In some embodiments, y is 0. Therefore the solid crystalline material may have the formula (X):

LiaM₂-bM bSyh-xXxJ suitably wherein M is Si.

In such embodiments, a, M', b, X and x are as defined above.

In some embodiments, x is 0. Therefore the solid crystalline material may have the formula (XI):

LiaM₂-bM'bS7-yYyl; suitably wherein M is Si.

In such embodiments, a, M', b, Y and y are as defined above.

In some embodiments, b is 0 and y is 0. Therefore the solid crystalline material may have the formula (XII):

Li_aM₂S7li-xX_x; suitably wherein M is Si.

In such embodiments, a, X and x are as defined above.

In some embodiments, b is 0 and x is 0. Therefore the solid crystalline material may have the formula (XIII):

LiaM₂S₇-yYy lj suitably wherein M is Si.

In such embodiments, a, Y and y are as defined above.

In some embodiments, y is 0 and x is 0. Therefore the solid crystalline material may have the formula (XIII):

Li_aM₂-bM'bS7l; suitably wherein M is Si.

In such embodiments, a, M' and b are as defined above.

In the foregoing embodiments, the number of Li ions present in the solid crystalline material of the first aspect is determined by the charge and abundance of the particular M', Y and X species in the material. The number of Li ions present is suitably sufficient to provide an overall charge neutral material. The term “a” represents the number of lithium ions present and is from 5 to 9. a is suitably 7 wherein b, y and x are all 0 and is adjusted from 7 wherein any of b, y and x are not 0 and the M', Y and/or X species which are present which differ in charge from Si⁴⁺, S^2- and I-, respectively.

In one embodiment, b, y and x are all 0 and therefore the solid crystalline material has the formula (II):

LiySi2S7l.

In embodiments wherein M is Si2-cGe_c and c is from 0 to 2, b is suitably 0. Therefore in such embodiments, the solid crystalline material suitably has the formula (He):

LiaSi2-cGe_cS7-_yYy li-xX_x; wherein c is from 0 to 2 and a, y, Y, x and X are as defined above, suitably wherein c is from 0 to 1 .5 or from 0 to 1 .

In embodiments wherein M is Si2-_cGe_c and c is from 0 to 2, y is suitably 0. Therefore in such embodiments, the solid crystalline material suitably has the formula (lid):

Li_aSi2-cGe_cM bSyh-xXxj wherein c is from 0 to 2 and a, M', b, x and X are as defined above, suitably wherein c is from 0 to 1 .5 or from 0 to 1 .

In embodiments wherein M is Si2-_cGe_c and c is from 0 to 2, x is suitably 0. Therefore in such embodiments, the solid crystalline material suitably has the formula (He):

Li_aSi2-cGe_cM bSy-yYylj wherein c is from 0 to 2 and a, M', b, y and Y are as defined above, suitably wherein c is from 0 to 1 .5 or from 0 to 1 .

In embodiments wherein M is Si2-_cGe_c and c is from 0 to 2, b is suitably 0 and y is suitably 0. Therefore in such embodiments, the solid crystalline material suitably has the formula (Ilf):

LiaSi2-cGecS7li-xX_x; wherein c is from 0 to 2 and a, x and X are as defined above, suitably wherein c is from 0 to 1 .5 or from 0 to 1 .

In embodiments wherein M is Si2-_cGe_c and c is from 0 to 2, b is suitably 0 and x is suitably 0.

Therefore in such embodiments, the solid crystalline material suitably has the formula (llg): LiaSi₂-cGecS7-yY_yl ; wherein c is from 0 to 2 and a, M', y and Y are as defined above, suitably wherein c is from 0 to 1 .5 or from 0 to 1 .

In embodiments wherein M is Si2-_cGe_c and c is from 0 to 1 , x is suitably 0 and y is suitably 0. Therefore in such embodiments, the solid crystalline material suitably has the formula (II h) :

LiaSi2-cGe_cM'bS7l; wherein c is from 0 to 2 and a, M' and b are as defined above, suitably wherein c is from 0 to 1 .5 or from 0 to 1 .

In embodiments wherein M is Si2-cGe_c and c is from 0 to 2, b, x and y are suitably 0. Therefore in such embodiments, the solid crystalline material suitably has the formula (Hi):

Li_aSi2-cGe_cS7l; wherein c is from 0 to 2 and a is as defined above, suitably wherein c is from 0 to 1 .5 or from 0 to 1.

In such embodiments, the solid crystalline material suitably has the formula (llj):

Li7Si2-_cGe_cS7l; wherein c is from 0 to 2, suitably from 0 to 1 .5 or from 0 to 1 .

In such embodiments, the solid crystalline material suitably has the formula (Ilk):

Li7SiGeS7l.

The solid crystalline material of this first aspect is suitably a solid-state lithium ion conductor. The solid crystalline material suitably has a room temperature ionic conductivity of at least 1 .0 x I O ⁴ S cm^-1, suitably at least 1 .0 x 10 ³ S cm^-1. The solid crystalline material may have a room temperature ionic conductivity of from 1.0 x 10⁴ S cm~¹ to 1 x 10 ¹ S cnr¹, suitably from 1.0 x 10-³ S cm ¹ to 2 x 10-2 _s cm ¹.

According to a second aspect of the present invention, there is provided a solid-state battery comprising a solid crystalline material of the first aspect.

The solid crystalline material is suitably a solid-state lithium ion conductor.

The solid-state battery may be a lithium-ion battery, a lithium-oxygen battery or a lithium-air battery. The solid crystalline material may be used as an electrolyte in the battery, separating cathode and anode materials, either by itself or as a component in a composite electrolyte.

The solid crystalline material may also be used as a component in a composite electrode in the battery. The solid crystalline material may be used in such a composite electrode to improve the lithium ion conduction of the electrode (suitably cathode).

In embodiments wherein the solid crystalline material is used as an electrolyte in a battery, the solid-state battery of this second aspect suitably comprises: an anode; a cathode; and an electrolyte comprising the solid crystalline material of the first aspect.

In such embodiments, the electrolyte is suitably arranged between the cathode and the anode. Suitably the electrolyte is a solid-state electrolyte. Suitably the solid-state battery is a primary battery or a rechargeable (or “secondary”) battery.

Suitable materials for the cathode and anode may be known in the art.

Suitable constructions of such a solid-state battery are known in the art.

The cathode is suitably a lithium host material capable of storing and releasing lithium ions. For example, the cathode may be a lithium metal oxide wherein the metal is one or more of aluminium, cobalt, iron, manganese, nickel and vanadium. Example lithium metal oxides are LiCo0₂ (LCO), LiFeO₂, LiMnO₂ (LMO), LiMn₂O₄, LiNiO₂ (LNO), LiNi_xCo_y0₂, LiMn_xC0yO₂, LiMn_xNi_yO₂, LiMn_xNi_yO₄, LiNi_xCo_yAl_z0₂ and LiNi_xMn_yC0zO₂ (NMC) amongst others.

Further examples of cathode materials are lithium-containing phosphates having a general formula LiM"PO₄ wherein M" is one or more of cobalt, iron, manganese, and nickel, such as lithium iron phosphate (LFP) and lithium iron fluorophosphates. Many different elements, e.g. Co, Mn, Ni, Cr, Al, or Li, may be substituted or additionally added into the structure to influence electronic conductivity, ordering of the layer and cycling performance of the cathode materials. The cathode active material can be a mixture of any number of these cathode materials.

In embodiments wherein the battery is a lithium-oxygen or lithium-air battery, the cathode is suitably oxygen or air, respectively.

In embodiments wherein the battery is a primary battery, the cathode may be a cathode suitable for use in such a primary battery. For example, the cathode in such a primary battery may be a fluorinated carbon cathode, a fluorinated carbon/S hybrid cathode, a fluorinated carbon/polymer hybrid cathode, an organic cathode or a metal sulphide cathode. Suitable materials for the anode are a lithium host material capable of incorporating and subsequently releasing the lithium ion such as graphite, a lithium metal oxide (e.g. lithium titanium oxide), carbon, a tin/cobalt alloy, a Li/ln alloy or silicon/carbon composite material. The anode material can be a mixture of any number of these anode materials. Alternatively, pure Li metal may provide the anode. Preferably, the anode is formed of lithium metal. The solid crystalline material of the first aspect may have good stability in contact with lithium metal and therefore the solid-state battery of this second aspect may comprise a lithium metal anode which may advantageously allow the solid-state battery to provide relatively high energy densities.

According to a third aspect of the present invention, there is provided a method of preparing a solid crystalline material according to the first aspect, the method comprising the steps of:

(a) admixing a source of Li, a source of M, a source of S, a source of I and optionally a source of M', a source of Y and/or a source of X;

(b) heating the mixture obtained in step (a); wherein:

M is selected from Si, Ge or Sn, or a mixture thereof;

Y is selected from O, Se, Te, N, F, Cl, Br or I, or a mixture thereof, suitably from O, F, Cl, Br or I, or a mixture thereof; and

X is selected from O, S, Se, Te, F, Cl, Br, BFL, OH, or NH2, or a mixture thereof, suitably from O, S, F, Cl or Br, or a mixture thereof.

The method may be a solid state (or dry) procedure, i.e. a method that does not require a solvent.

The solid crystalline material prepared by this third aspect may have any of the suitable features and advantages described above in relation to the first and second aspects.

The solid crystalline material prepared by this third aspect may be used in the solid-state battery of the second aspect.

Suitably the steps of the method of this third aspect are carried out in the order of step (a) followed by step (b).

Step (a) involves admixing a source of Li, a source of M, a source of S, a source of I and optionally a source of M’, a source of Y and/or a source of X.

The source of Li may be any suitable material comprising Li. In some embodiments, Li may also be present in the sources of I, Y and/or X. Suitably the source of M is selected from SiS2 and/or GeS2.

Suitably the source of Li is Li2S. Suitably the source of Si is SiS2. Suitably the source of I is Lil. Suitably the source of S is Li2S and/or SiS2.

In some embodiments, the source of S is elemental sulfur.

In some embodiments, the source of Si is elemental silicon.

In some embodiments, step (a) of the method involves admixing Li2S, SiS2 and Lil, with the sources of M', Y and/or X, when present.

In embodiments wherein M' is Zn, suitably the source of M' is ZnS

In embodiments wherein M' is B, Al or Ga, suitably the source of M' is M'2S3.

In embodiments wherein M' is P or Sb, suitably the source of M' is M 2S5.

In embodiments wherein M' is Nb, Ta, or V, suitably the source of M' is M'S2, M'Ss, or M'S4.

In embodiments wherein M' is Mo or W, suitably the source of M' is M'Ss or M'S2.

In embodiments wherein M' is Zn, B, Al, Ga, P, Sb, Nb, Ta, V, Mo, or W, suitably the source of

M' is the element.

The sources of Y and X suitably comprise lithium.

The sources of Y and X are suitably ionic salts of Li, suitably comprising one or more of F, Cl, Br, I, O and S. Therefore the source of Y may be selected from LiF, LiCI, LiBr, Lil and Li2O, or mixtures thereof, as appropriate to form the desired material. The source of X may be selected from LiF, LiCI, LiBr, Li2O, LiOH, LiBFU, LiNFh, Li2Se and Li2S, or mixtures thereof, as appropriate to form the desired material.

In some embodiments, the solid crystalline material is Li?Si2S7l and step (a) of the method involves admixing Li2S, SiS2 and Lil.

In some embodiments, step (a) of the method involves admixing Li2S, SiS2, GeS2 and Lil, with the sources of M', Y and/or X, when present.

In some embodiments, the solid crystalline material is LiySiGeSyl and step (a) of the method involves admixing Li2S, SiS2, GeS2 and Lil.

The method of this third aspect may be a solution-based synthetic or chemical vapour transport procedure. However, in preferred embodiments the method of the third aspect is a solid-based synthetic procedure. Suitably the sources of Li, M, S, I and optionally M', Y and/orX are provided as powders. Suitably in step (a) the sources of Li, M, S, I and optionally M', Y and/or X are ground together and then heated in step (b). The sources of Li, M, S, I and optionally M', Y and/or X may be ball-milled together.

Suitably, the method of the third aspect involves a step (a1) of drying the sources of Li, M, S, I and optionally M', Y and/or X, suitably under vacuum, suitably with heating to 150°C or above. Step (a1) suitably occurs before step (a).

Suitably step (b) involves heating the mixture obtained in step (a) to a temperature of from 300 to 600°C, suitably to a temperature of from 400 to 500°C.

Step (b) may involve heating the mixture to a set temperature in the ranges noted above at a rate of from 5 to 30°C/min, suitably from 10 to 25°C/min or from 15 to 25°C/min.

Following step (b), the mixture is suitably cooled at a rate of from 1 to 10°C/min, suitably from 2 to 8°C/min or approximately 5°C/min, suitably to room temperature.

Suitably step (b) involves heating, suitably at said temperatures, the sources of Li, M, S, I and optionally M', Y and/orX for at least 1 hour or at least 2 hours. Suitably step (b) involves heating for at least 1 day, at least 2 days or at least 3 days. Suitably step (b) involves heating for up to 10 days, up to 8 days or up to 6 days. For example, the source of the sources of Li, M, S, I and optionally M', Y and/or X may be heated to a temperature of from 300 to 600°C for from 1 to 8 days. Suitably the heating of step (b) is carried out under vacuum, for example a vacuum having a pressure of from 10⁴to 10⁶ mbar or approximately 10⁵ mbar.

Prior to the heating of step (b), the mixture obtained in step (a) may be consolidated under pressure, for example into pellets. The mixture obtained in step (a) may be pressed under a pressure of at least 100 MPa, at least 200 MPa or approximately 300 MPa, suitably to consolidate the powder material into pellets.

Suitably the heating of step (b) and the reaction to form the solid crystalline material provides the product as a powder. The product may then be manipulated into the desired format for the uses described herein, suitably using techniques known in the art.

Using this method of the third aspect, the materials of the first aspect may be prepared at a lower synthesis temperature than conventional materials used as solid-state electrolytes in solid-state batteries. The synthesis of the materials according to this third aspect may be simpler than the synthesis of such conventional solid-state materials.

Moreover, the method of this third aspect may utilise only elements with a high earth-abundance and/or which are easily obtainable in large quantities, suitably from extraction and production processes which are relatively low in energy consumption and which do not produce problematic waste materials.

According to a fourth aspect of the present invention, there is provided the use of a solid crystalline material according to the first aspect as a solid-state conductor.

The solid crystalline material of the first aspect may be used as a lithium ion electrolyte in primary and secondary electrochemical energy storage devices.

In this fourth aspect, the solid crystalline material is suitably used as a solid electrolyte, for example in a solid-state lithium-ion battery.

The solid crystalline material used in this fourth aspect may have any of the suitable features and advantages described in relation to the first aspect. For example, the solid crystalline material used in this fourth aspect may provide high ionic conductivity (suitably with low electronic contribution to conductivity) whilst also having mechanical properties (such as bulk modulus and shear modulus) which facilitate processing and manufacture, and also stability in use with lithium metal electrodes. The use of such solid crystalline materials of the first aspect may therefore provide an advantageous solid-state conductor for use as a solid-state electrolyte in batteries.

Example 1

Synthesis

Materials

Si (99.998%), S (99.998%), Lil (99.99%), Li₂CO₃ (99.99%), and CS₂ (99.9%) were purchased from Sigma-Aldrich. Lil, Si, and S were vacuum-dried overnight (at 200°C for Lil and Si; at room temperature for S) before use. The quartz tubes with graphite crucibles were flame-dried under vacuum before use. The precursors Lil, Si, and S and resulting powders were handled in an Ar- filled glove box (O₂ < 0.5 ppm, H₂O < 0.1 ppm).

Synthesis of Li₂S and SiS₂

Li₂S was prepared from sulfurization of Li₂CO₃ in an CS₂ vapor environment at 473 K for 1 h followed by a firing at 923 K for 6 h. In detail, 2 g of Li₂CO₃ was weighed and transferred in an alumina boat in a quartz tube installed in a clam-shell furnace. The quartz tube was then connected to an Argon outlet with input bubblers (the first one empty and the second one containing the CS₂ liquid reagent) and output bubblers containing oxidizing neutralizing solutions (the first one containing a sodium hypochlorite aqueous solution and the second one containing ~0.1 M KMnC aqueous solution). When the furnace has cooled to room temperature, the CS₂ bubbler was isolated to purge the quartz tube using pure Argon flow. The quartz tube was then opened in a glovebox to collect the IJ2S sample.

SiS2 was prepared through an elemental-synthesis gasifying-separation (ESGS) method (Mater. Lett. 266, 127508 (2020)). A mixture of silicon and sulphur powders with a molar ratio of Si:S = 1 :3.3 was ground together using a mortar and pestle. The mixture was sealed in an evacuated quartz ampoule (~10⁵ mbar) which was then placed in a furnace with sample-rich end downward in the furnace. The ampoule was then heated to 453 K for 10 h to make a homogeneous S- coated Si powder. The resulting product was then reground and sealed in an evacuated quartz ampoule which was then placed in a furnace in the same way and heated to 1073 K for 20 h. The quartz ampoule was then laid with the SiS2-rich end upward in the furnace. The excessive gaseous sulfur condensed to a liquid and flowed to the sample-poor end during cooling of the quartz ampoule to RT (at a rate of 5 K/min).

The synthesized Li2S and SiS2 were characterized by powder X-ray Diffraction (XRD), which confirmed that both products were free of impurities, which were supported by Raman spectroscopy and CHNS analysis. The synthesized Li2S and SiS2 were then used as precursors in the synthesis of LiySi2S7l.

Single crystal growth of Li₇Si?S₇l

Crystals of monoclinic Li?Si2S7l suitable for single crystal XRD were grown by crystallization of Li2S, Lil and elemental Si reagents from the flux of elemental S in a graphite crucible in an evacuated silica ampoule. Stoichiometric amount of Li2S (138 mg), Lil (134 mg), Si (56 mg) and S (128 mg) were ground in a mortar and pestle for 10 minutes. The resultant mixture was then pelletized and transferred into a graphite crucible and sealed in an evacuated silica ampoule (~10 ⁵ mbar). The ampoule containing the sample was heated to 723 - 773 K for 2-12 days and then cooled to room temperature. The crystals suitable for single-crystal XRD were isolated from the resulting crystalline product.

Synthesis of Li₇Si₂S₇l

Pure bulk powder of Li7Si2S7l was obtained from Li2S, SiS2 and Lil via solid-state reaction in a graphite crucible in an evacuated quartz ampoule by the following procedure. A mixture of 276 mg Li2S, 268 mg Lil and 387 mg SiS2 was thoroughly ground by a mortar and pestle for 20 minutes, and then pelletized and placed in a graphite crucible and inside a vacuum-sealed in a silica ampoule. The ampoule containing the sample was heat treated at 723 K for 2 days with the heating rate of 20 K/min and cooling rate of 5 K/min. The sample was then reground and reheated underthe same conditions but with a lower heating rate of 10 K/min, leading to a phase pure sample of Li7Si2S7l. The pure bulk powder of Li7Si2S7l can also be prepared from IJ2S, elemental Si, elemental S and Lil by the following procedure. A mixture of 138 mg Li2S, 56 mg Si, and 115 mg S and 134 mg Lil was thoroughly ground by a mortar and pestle for 20 minutes, and then pelletized and placed in a graphite crucible and vacuum-sealed in a graphite crucible in silica ampoule. The ampoule containing the sample was heat treated at 723 K for 1 week with the heating rate of 20 K/min and cooling rate of 10 K/min, leading to a phase pure sample of Li7Si2S7l.

Compositional analysis was performed via CHNS and ICP analysis and confirmed that the composition of synthesized pure powder sample Li₆₉(2)Si₂o5O(5)S₆₉₆(3)lo95(i) was consistent with the composition Li7Si2S7l determined from single crystal XRD.

Characterisation

Single crystal X-ray diffraction

Colourless, block shaped crystals with dimensions of 0.02x0.02x0.03 mm were isolated under a polarizing microscope and single crystal XRD data were recorded on beamline 119 at the Diamond Light Source (Oxfordshire, U.K.) using silicon double crystal monochromated synchrotron radiation (A = 0.6889 A, Pilatus 2M detector). To analyse structure transitions, the data were collected at five different temperatures, i.e., 100 , 240 , 300 , 330 , and 500 K. Data reduction was performed with the CrysAlis^Pro Version 171.40_64.53 software. The structures were solved using Intrinsic Phasing method provided by the SheIXT structure solution program, and refined with anisotropic displacement parameters for all atoms using the ShelXL leastsquares refinement package, interfaced through Olex2 program.

Powder X-ray diffraction

Synchrotron powder XRD was performed at the 111 beamline at Diamond Light Source (Oxfordshire, U.K.), with an incident wavelength of 0.825186 A using a wide-angle position sensitive detector and samples sealed in 0 = 0.5 mm silicate glass capillaries.

Figure 1 shows a plot of powder XRD data at 303 K.

The structure of Li7Si2S7l as determined by XRD adopts monoclinic P2dn symmetry with 300 K unit cell parameters of a = 14.9520(2) A, b = 6.04590(10) A, c = 15.1634(2) A and = 105.6940(10)°. The anion topology of the Li7Si2S7l is related to that of TH (B33) type structures, such as the NiZr binary intermetallic. The intermetallic net in NiZr is intermediate between the well-known hexagonal and square nets (3⁶ and 4⁴ in Schlafli notation, respectively) and is a 3³.4² semi-regular net. The anion topology of Li7Si2S7l, which is determined by the use of S²~ and |- as anions, has these species positionally ordered overthe nodes of the 3³.4² net (see Figure 2A). These layers stack in an ABAB sequence along the b direction, with the registry between successive layers directed by achieving the closest proximity for the large |- anions in neighbouring layers. This produces anion coordination spheres of 13 and 11 , in which squares and triangles, respectively, stack both above and below the anion concerned (Figure 2B). The larger |- occupies one of the 13 coordinate positions, while S^2- can be 11 or 13 coordinate. These coordination numbers are higher than 8 coordination found in bcc packings.

Alternating current (AC) impedance measurements

The conductivity of monoclinic LiySi2S7l pellets was measured by AC impedance spectroscopy. The pellets of LiySi2S7l were formed by uniaxially pressing the LiySi2S7l powder as synthesised at a pressure of 1 GPa using a cylindrical tungsten carbide die (5 mm in diameter). The pellets were then sintered at 723 K for 2 days in an evacuated, flame dried graphite crucible in a quartz tube. The experimental density of the sintered pellets was 96% of the theoretical maximum. A sputtered gold coating of ~2 nm thickness was used as the ion blocking electrodes. Sputtering was achieved using the sputter coater Q150R. The pellets were polished with sandpaper prior to gold sputtering. AC impedance measurements were performed using an impedance analyser (Keysight impedance analyser E4990A). Variable temperature conductivity measurements were performed in a frequency range of 10 MHz - 20 Hz with a voltage amplitude of 50 mV. Measurements were performed in the temperature range 237 - 373 K.

The AC impedance spectroscopy measurements on the sintered pellets (96% relative density) with blocking Au electrodes (Figure 3) show that LiySi2Syl has a total conductivity of 1.01 (4) x 10~² S cm^-1 at 303 K. Figure 3A shows the Nyquist plots measured between 303 and 373 K. Figure 3B shows the representative Nyquist plots at 237 K. Figure 3C shows the Arrhenius plots of the ionic conductivity values of LiySi2Syl between 373 and 237 K. The Arrhenius plots show two temperature ranges with different activation energies: 0.204(4) eV between 373 and 303 K, and 0.58(3) eV between 293 and 237 K. Between 237 K and 255 K, from the Nyquist plot of the Au|LiySi2Syl|Au ion-blocking electrode two semicircles at high frequency and medium frequency can be distinguished associated with bulk and grain boundary contributions. The extraction of the bulk conductivity through fitting the impedance data in the high frequency regions leading to an activation energy of 0.38(3) eV.

Direct current (DC) polarization measurements

The same Au|LiySi2Syl|Au ion-blocking electrode was then used for DC polarization measurements to determine the electronic conductivity. DC polarization data were collected at 303 K by applying constant bias voltages of 0.05, 0.1 , 0.2, and 1 V for 4, 8, 12, 20 h, respectively. The measurements were performed until the current reached a steady state. The electronic conductivity is then extracted through an Ohm’s law by fitting the voltage versus current curves.

Figure 4 shows the DC polarization measurements of sintered Li7Si2S7l pellet yield an electronic conductivity of o_e = 8.1 (2) x 1 O ^W S cm ¹ at 303 K. (a) Current-time curves measured at bias voltages 0.05, 0.1 , 0.2 and 1 V. (b) Linear fitting of voltage versus current, leading to the electronic conductivity based on the Ohm’s law.

The electronic conductivity of both Li?Si2S7l pellets is negligible compared to the total conductivity of 1.01 (4) x 10² S cm ¹ at 303 K. This data demonstrates that the Li?Si2S7l crystalline solid electrolyte exhibits liquid-electrolyte-like ionic conductivity (=10^-2 S cm^-1) at room temperature with minimal electronic conductivity contribution.

Cyclic voltammetry (CV) plating/stripping experiments

Cyclic voltammetry (CV) measurements were performed for a two-electrode “stainless steel (SS)|LiySi2S7l|Li” cell to show the plating/stripping behaviour of the solid electrolyte (Figure 5). The CV measurements were performed between -0.5 and 1 .0 V vs. Li⁺/Li at a potential sweep rate of 0.1 mV s ¹. Lithium foil (0.38 mm thickness) was prepared by scraping the surface with a plastic spatula. Around 0 V vs. Lr7Li, the Li plating/stripping can be observed. The CV plating/stripping experiments showed that Li?Si2S7l as a solid electrolyte allows reversible transport of lithium ions.

Linear sweep voltammetry (LSV) experiments

During the linear sweep voltammetry (LSV) measurements up to 10 V vs. LiVLi using the “stainless steel (SS) |LiySi2S7l|Li” cell (Figure 6), no oxidative decomposition of Li Si2Szl can be detected.

Linear sweep voltammetry (LSV) was also carried out on a “LizSi2Szl + carbon fiber (CF)|LizSi2Szl |Li” cell (Figure 6). The oxidation stability of LizSi2Szl was evaluated by using a carbon composite working electrode. A LizSi2Szl and CF composite with a weight ratio of 9:1 was used. The mixture was thoroughly ground in an agate mortar for 30 min. In contact with the CF, the oxidative decomposition of LizSi2Szl from the composite occurs at about 2.5 V vs. Li⁺/Li (Figure 6). The oxidative decomposition potential of 2.5 V vs. Li⁺/Li observed for LizSi2Szl is slightly higher than that for LiwGeP2Si2 (2.14 V vs. LiVLi) and LiePSsCI (2.3 V vs. Lr7Li).

The LSV was further scanned up to 8 V vs. LiVLi (Figure 6), showing a continuous increase in the decomposition current from about 12 to 22 pA. The use of the electronic conductive additive (carbon fiber) provides an electronic conductivity path for the solid electrolytes and accelerate their oxidative decomposition.

Galvanostatic Li plating/stripping experiments

To study the interfacial stability of the LizSi2Szl|Li electrode, time-resolved (240 h in total) electrochemical Li plating/stripping tests were performed on a Li|LizSi2Szl|Li symmetric cell. A cold-pressed LizSi2Szl pellet was used for these measurements. The diameter and thickness of the cold-pressed pellet was 5 mm and 1.1 mm, respectively. Two discs of Li (99.9%, 0.38 mm thickness) were mechanically pressed onto a LiySi2Syl pellet. The galvanostatic polarization measurements were carried out without any further thermal treatment of the cell, at room temperature under a constant pressure, at current density of 0.05 mA cm^-2 and 0.1 mA cm^-2 and areal capacity of 0.025 and 0.05 mAh cm^-2 per half-cycle, respectively. The overpotential and measurement time per half-cycle were controlled to prevent morphological instability at the Li|LiySi2S7l interface.

A steady Li plating/stripping was observed over 120 h at 0.05 mA cm^-2 and additional 120 h at 0.1 mA cnr²without increase in the overpotential, as shown in Figure 7. Such results indicate a stable Li| LiySi2S7l interface. It is believed that a kinetically stabilized solid electrolyte interphase (SEI) forms between LiySi2Syl and Li metal. Such a stable Li| LiySi2Syl interface is advantageous compared to the observed instability of Li|LiioGeP₂Si₂. A stable interface cannot be formed between LiwGeP2Si2 and Li metal. LiwGeP2Si2 undergoes a continuous decomposition at the Li|LiioGeP₂Si2 interface due to the formation of a mixed ionic-electronic conducting interphase (MCI). For solid electrolytes that are thermodynamically unstable against Li metal, the formation of a mixed ionic-electronic conducting interphase (MCI) must be avoided, as is the case in LiwGeP2Si2.

Ex situ XRD and Raman analysis

After the electrochemical tests, the cells were disassembled inside a glovebox and ex situ XRD and Raman characterization were performed for the bulk LiySi2Syl solid electrolyte pellet and the samples collected from the CF| LiySi2Syl and Li|LiySi2Syl interfaces.

After the LSV measurements, for the bulk LiySi2Syl solid electrolyte, no new diffraction peaks (Figure 8A) or Raman bands (Figure 8B) from possible oxidative (from LSV test for the LiySi2Syl + CF|LiySi2Syl|Li cell) and reductive (from the Li plating/stripping test for the Li|LiySi2Syl|Li cell) decomposition products of the LiySi2Syl can be identified, suggesting a good stability of the bulk LiySi2Syl solid electrolyte at high bias potentials and around 0 V vs. Lr7Li.

For the LiySi2Syl solid electrolyte at the CF|LiySi2Syl interface, the diffraction pattern and Raman bands of the LiySi2Syl remain largely unchanged after the LSV measurement. For the LiySi2Syl solid electrolyte at the Li|LiySi2Syl interface, Lil as the only resolvable phase was observed from the XRD pattern after the Li plating/stripping test (Figure 9). As such, Lil is the main component of the kinetically stabilized solid electrolyte interphase . The presence of impurities such as Li2S, Lil and LiCI in the as-prepared benchmark solid electrolytes such as Li55PS45CI1 5, Li67Sio7Sbo3S5l and unidentified impurities in Li9.54Sii.74Pi.44Sn.7Clo.3 may lead to biased analysis of the solid electrolyte | Li interfacial stability. These impurities are known as main components responsible for a kinetically stable solid electrolyte|Li interface. Example 2

Synthesis and crystal structure of Li SiGeS l

A solid-state reaction was performed with a combination of SiS2, GeS2, I 2S and Lil in graphite crucibles in vacuum-sealed quartz ampoules to synthesise LiySiGeSyl. A mixture of SiS2 (101 mg), GeS2 (150 mg), Li2S (138 mg), and Lil (134 mg) was thoroughly ground with a mortar and pestle for 20 min, and then pelletized, placed in a graphite crucible and vacuum-sealed in a quartz ampoule. The ampoule was heat treated at 723 K for 2 days with a heating rate of 20 K min ¹ and a cooling rate of 5 K min ¹. The sample was then reground and reheated under the same conditions but with a lower heating rate of 5 K min ¹, leading to a high purity sample of LiySiGeSyl, which was characterized by synchrotron powder X-ray diffraction (as shown in Fig. 10).

Ionic conductivity

AC impedance measurements of the LiySiGeSyl material prepared were performed as described above for Example 1 . The AC impedance spectroscopy measurements obtained (as shown in Fig. 11) on the sintered pellets (89% relative density) show that LiySiGeSyl has a very similar conductivity to LiySi2Syl at room temperature and above, and has significantly improved conductivity in the low temperature range (between 303 and 213 K).

These results demonstrate that the present invention provides solid crystalline materials with advantageous conductivities and stability which may be suitable for use as solid electrolytes in high-performance solid-state lithium-ion batteries. Furthermore, the achievement of these advantageous properties has proved to be possible using only low-cost and non-toxic elements (for example in the case of LiySi2Sy I and LiySiGeSyl) and a relatively low temperature synthesis procedure. Therefore, the solid crystalline materials of the present invention may provide such high-performance lithium-ion batteries whilst providing sustainability and cost benefits.

Although a few preferred embodiments have been shown and described, it will be appreciated by those skilled in the art that various changes and modifications might be made without departing from the scope of the invention, as defined in the appended claims.

Throughout this specification, the term “comprising” or “comprises” means including the component(s) specified but not to the exclusion of the presence of other components. The term “consisting essentially of’ or “consists essentially of’ means including the components specified but excluding other components except for materials present as impurities, unavoidable materials present as a result of processes used to provide the components, and components added for a purpose other than achieving the technical effect of the invention. Typically, when referring to compositions, a composition consisting essentially of a set of components will comprise less than 5% by weight, typically less than 3% by weight, more typically less than 1 % by weight of non-specified components.

The term “consisting of’ or “consists of’ means including the components specified but excluding addition of other components.

Whenever appropriate, depending upon the context, the use of the term “comprises” or “comprising” may also be taken to encompass or include the meaning “consists essentially of’ or “consisting essentially of’, and may also be taken to include the meaning “consists of’ or “consisting of’.

For the avoidance of doubt, wherein amounts of components in a composition are described in wt%, this means the weight percentage of the specified component in relation to the whole composition referred to.

The optional features set out herein may be used either individually or in combination with each other where appropriate and particularly in the combinations as set out in the accompanying claims. The optional features for each aspect or exemplary embodiment of the invention as set out herein are also to be read as applicable to any other aspect or exemplary embodiments of the invention, where appropriate. In other words, the skilled person reading this specification should consider the optional features for each exemplary embodiment of the invention as interchangeable and combinable between different exemplary embodiments.

Attention is directed to all papers and documents which are filed concurrently with or previous to this specification in connection with this application and which are open to public inspection with this specification, and the contents of all such papers and documents are incorporated herein by reference.

All of the features disclosed in this specification (including any accompanying claims, and drawings), and/or all of the steps of any method or process so disclosed, may be combined in any combination, except combinations where at least some of such features and/or steps are mutually exclusive.

Each feature disclosed in this specification (including any accompanying claims, and drawings) may be replaced by alternative features serving the same, equivalent or similar purpose, unless expressly stated otherwise. Thus, unless expressly stated otherwise, each feature disclosed is one example only of a generic series of equivalent or similar features.

The invention is not restricted to the details of the foregoing embodiment(s). The invention extends to any novel one, or any novel combination, of the features disclosed in this specification (including any accompanying claims, and drawings), or to any novel one, or any novel combination, of the steps of any method or process so disclosed.

Claims

1 . A solid crystalline material of formula (I):

LiaM₂-bM bSy-yYy ll-xXxJ wherein: a is from 5 to 9;

M is selected from Si, Ge or Sn, or a mixture thereof;

2. The solid crystalline material according to claim 1 , having a crystal structure comprising anions S, Y, I and X ordered in 3³.4² nets stacked in an ABAB sequence along the b axis.

3. The solid crystalline material according to any preceding claim, having a monoclinic crystal structure with a PQ n space group.

4. The solid crystalline material according to any preceding claim, wherein M is Si.

5. The solid crystalline material according to any preceding claim, wherein M' is selected from Al, Ga or a mixture thereof and a is 7+b.

6. The solid crystalline material according to any one of claims 1 to 4, wherein M' is selected from Sb, P or a mixture thereof and a is 7-b.

7. The solid crystalline material according to any one of claims 1 to 4, wherein y > 0, Y is O and a is 7.

8. The solid crystalline material according to any one of claims 1 to 4, wherein y > 0, Y is selected from F, Cl, Br or I, or a mixture thereof, and a is 7-y.

9. The solid crystalline material according to any one of claims 1 to 4, wherein x > 0, X is F, Cl, Br or a mixture thereof, and a is 7.

10. The solid crystalline material according to any one of claims 1 to 4, wherein x > 0, X is S or O and a is 7+x.

11 . The solid crystalline material according to any preceding claim, wherein b is 0.

12. The solid crystalline material according to any preceding claim, wherein y is 0.

13. The solid crystalline material according to any preceding claim, wherein x is 0.

14. The solid crystalline material according to any one of claims 1 to 3 having the formula

(XII):

Li_aM2S7li-xX_x; wherein: a is from 5 to 9;

M is selected from Si, Ge or Sn, or a mixture thereof;

X is selected from O, S, Se, Te, F, Cl, Br, BF , OH, or NH2, or a mixture thereof; and x is from 0 to 1 .

15. The solid crystalline material according to claim 14, wherein X is selected from F, Cl, Br or a mixture thereof.

16. The solid crystalline material according to any one of claims 1 to 3, having the formula (Hj):

Li7Si2-_cGe_cS7l; wherein c is from 0 to 2.

17. The solid crystalline material according to any one of claims 1 to 3 having the formula (II): Li7Si2S7l.

18. The solid crystalline material according to any one of claims 1 to 3 having the formula (Hk):

Li7SiGeS7l.

19. A solid-state battery comprising a solid crystalline material according to any of claims 1 to

20. The solid-state battery according to claim 19 comprising: an anode; a cathode; and an electrolyte comprising the solid crystalline material according to any of claims 1 to 18.

21 . A method of preparing a solid crystalline material according to any of claims 1 to 18, the method comprising the steps of:

(b) heating the mixture obtained in step (a); wherein:

M is selected from Si, Ge or Sn, or a mixture thereof;

M' is selected from Zn, B, Al, Ga, Sb, P, V, Nb, Ta, Mo, or W, or a mixture thereof;

Y is selected from O, Se, Te, N, F, Cl, Br or I, or a mixture thereof;

X is selected from O, S, Se, Te, F, Cl, Br, BF , OH, or NH2, or a mixture thereof; and

22. The method according to claim 21 , wherein step (b) involves heating the mixture obtained in step (a) to a temperature of from 300 to 600°C.

23. The method according to claim 21 or 22, wherein the solid crystalline material is Li?Si2S7l , and step (a) of the method involves admixing Li2S, SiS2 and Lil.

24. The use of a solid crystalline material according to any of claims 1 to 18 as a solid-state conductor.