Compounds with mixed anions as solid Li-ion conductors

Abstract

A solid-state lithium ion electrolyte is provided which contains a composite material having at least 94 mole % lithium ions as cation component and multiple anions in an anionic framework capable of conducting lithium ions. An activation energy for lithium ion migration in the solid state lithium ion electrolyte is 0.5 eV or less. Composites of specific formulae are provided. A lithium battery containing the composite lithium ion electrolyte is also provided.

Claims

1. A solid-state lithium ion electrolyte, comprising: a composite material having lithium ions and a dopant cation M as cation component; and multiple anions; in an anionic framework capable of conducting lithium ions; wherein the composite material is at least one selected from the group of formulae consisting of (II), (III), (IV), (V) and (VI), a mole % content of the dopant cation is from 0 to 10 mole % based on total moles of lithium and the dopant cation, and an activation energy for lithium ion migration in the solid state lithium ion electrolyte is 0.5 eV or less,
Li.sub.10-nyM.sub.yN.sub.3Br  (II) wherein M is cation of n+ charge, y is a number from 0 to 1.0, and n is 2 or 3;
Li.sub.5-nzM.sub.zNCl.sub.2  (III) wherein M is cation of n+ charge, z is a number from 0 to 0.5, and n is 2 or 3,
Li.sub.4-nwM.sub.wNCl  (IV) wherein M is cation of n+ charge, w is a number from 0 to 0.4, and n is 2 or 3,
Li.sub.6-ntM.sub.tNBr.sub.3  (V) wherein M is cation of n+ charge, t is a number from greater than 0 to 0.6, and n is 2 or 3 and
Li.sub.6-nsM.sub.sNI.sub.3  (VI) wherein M is cation of n+ charge, s is a number from 0 to 0.6, and n is 2 or 3.

2. The solid-state lithium electrolyte according to claim 1 wherein a lithium ion (Li.sup.+) conductivity of the solid state lithium ion electrolyte is at least 10.sup.−6 S/cm at room temperature.

3. A solid state lithium battery, comprising: an anode; a cathode; and a solid state lithium ion electrolyte located between the anode and the cathode; wherein the solid state lithium ion electrolyte comprises the composite material of claim 1.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The FIGURE shows diffusivity of Li obtained from ab initio molecular dynamics simulation for selected composites of aspects of the first embodiment.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

(2) Throughout this description, the terms “electrochemical cell” and “battery” may be employed interchangeably unless the context of the description clearly distinguishes an electrochemical cell from a battery. Further the terms “solid-state electrolyte” and “solid-state ion conductor” may be employed interchangeably unless explicitly specified differently.

(3) Structural characteristics of effective Li.sup.+ conducting crystal lattices have been described by Ceder et al. (Nature Materials, 14, 2015, 1026-1031) in regard to known Li.sup.+ ion conductors Li.sub.10GeP.sub.2S.sub.12 and Li.sub.7P.sub.3S.sub.11, where the sulfur sublattice of both materials was shown to very closely match a bcc lattice structure. Further, Li.sup.+ ion hopping across adjacent tetrahedral coordinated Li.sup.+ lattice sites was indicated to offer a path of lowest activation energy.

(4) The inventors are investigating new lithium composite compounds in order to identify materials having the properties described above which may serve as solid-state electrolytes in solid state lithium batteries. In the course of this ongoing study and effort the inventors have developed and implemented a methodology to identify composite materials which have chemical and structural properties which have been determined by the inventors as indicators of lithium ion conductance suitable to be a solid state electrolyte for a lithium-ion battery.

(5) As described above, the inventors have recognized that a root cause of the instability of a lithium composite compound against a Li metal anode is the reduction of non-lithium cations (secondary cations) in the compounds. Accordingly in the developed methodology, (a) only composite lithium compounds having no secondary cation(s) may be considered. In this regard, materials containing only Li as cation specie may be included. In theory, any anion which meets the other criteria requirements described herein may be present in the composite structures. One of knowledge and skill in lithium composite chemistry will be able to identify anions suitable for study according to the method disclosed in this application.

(6) If an anion containing at least one of N.sup.3−, O.sup.2−, S.sup.2−, F.sup.−, Cl.sup.−, Br.sup.− and I.sup.− is selected as a basis for evaluation, seven compounds of formulae Li.sub.3N, Li.sub.2O, Li.sub.2S, LiF, LiCl, LiBr and LiI may be considered.

(7) A criterion of this methodology requires that to qualify as solid state electrolyte in practical application, the material must exhibit desirable Li-ion conductivity, usually no less than 10.sup.−6 S/cm at room temperature. Thus, ab initio molecular dynamics simulation studies are were applied to calculate the diffusivity of Li in the lattice structures of these seven materials. In order to accelerate the simulation, the calculation was performed at high temperatures and the effect of excess Li or Li vacancy was considered. In order to create excess Li or Li vacancy, aliovalent replacement of cation or anions was evaluated. Thus, Li vacancy was created by, for example, partially substituting N.sup.3− anion for the anions of the material. Li vacancies may also be created by partially replacing Li cation with higher valent cations such as Mg.sup.+2. The diffusivity at 300 K was determined according to equation (I)
D=D.sub.0 exp(−E.sub.a/k.sub.bT)  equation (I)
where D.sub.0, E.sub.a and k.sub.b are the pre-exponential factor, activation energy and Boltzmann constant, respectively. The conductivity is related with the calculated diffusivity according to equation (II):
σ=D.sub.300ρe.sup.2/k.sub.bT  equation (II)
where ρ is the volumetric density of Li ion and e is the unit charge.

(8) The only compound with room temperature Li-ion conductivity higher than 10.sup.−6 S/cm was determined to be Li.sub.3N, while the remaining compounds have conductivity lower than 10.sup.−10 S/cm.

(9) Accordingly, the inventors adapted the methodology to consider (b) compounds with two or more types of anions, defined as mixed anions, in the formula. Combining condition (a) and (b), the methodology limits the Li-ion conductors according to the first embodiment to compounds (a) not having a secondary cation and (b) to compounds having at least two different (mixed) anions.

(10) The anionic lattice of Li-ion conductors has been shown to match certain lattice types (see Nature Materials, 14, 2015, 2016). Therefore, in (c) the anionic lattice of the potential Li.sup.+ ion conductor is compared to the anionic lattice of Li.sup.+ ion conductor known to have high conductivity.

(11) Thus, lithium compounds (a) having no secondary cation and (b) having mixed anions of N.sup.3−, O.sup.2−, S.sup.2−, F.sup.−, Cl.sup.−, Br.sup.− and I.sup.− were compared to Li-containing compounds reported in the inorganic crystal structure database (FIZ Karlsruhe ICSD—https://icsd.fiz-karlsruhe.de) and evaluated in comparison according to an anionic lattice matching method developed by the inventors for this purpose and described in copending U.S. application Ser. No. 15/597,651, filed May 17, 2017, to match the lattice of these compounds to known Li-ion conductors.

(12) According to the anionic lattice matching method described in copending U.S. application Ser. No. 15/597,651, an atomic coordinate set for the compound lattice structure may be converted to a coordinate set for only the anion lattice. The anions of the lattice are substituted with the anion of the comparison material and the obtained unit cell rescaled. The x-ray diffraction data for modified anion-only lattice may be simulated and an n×2 matrix generated from the simulated diffraction data. Quantitative structural similarity values can be derived from the n×2 matrices.

(13) The purpose of anionic lattice matching is to further identify compounds with greatest potential to exhibit high Li.sup.+ conductivity. From this work, the compounds listed in Table 1 were determined to be potentially suitable as a solid-state Li.sup.+ conductor. Among the compounds identified, Li.sub.3OBr was reported as a solid Li conductor (Zhao et al., J Am. Chem. Soc., 2012, 134, 15042).

(14) TABLE-US-00001 TABLE 1 Compounds meeting requirement (a) and (b) with the lattice matching to known solid Li-ion conductors. Compound Li.sub.7Br.sub.3O.sub.2 Li.sub.10N.sub.3Br Li.sub.5NCl.sub.2 Li.sub.4NCl Li.sub.6NBr.sub.3 Li.sub.6NI.sub.3 Anions Br.sup.−, Br.sup.−, Cl.sup.−, Cl.sup.−, Br.sup.−, I.sup.−, O.sup.2− N.sup.3− N.sup.3− N.sup.3− N.sup.3− N.sup.3− Lattice LiZnPS.sub.4 Li.sub.10GeP.sub.2S.sub.12 Li.sub.6PS.sub.5Br Li.sub.3N Li.sub.3OBr Li.sub.3OBr matched to

(15) Ab initio molecular dynamics (AIMD) simulation was then applied to predict the conductivity of Li.sub.4NCl, Li.sub.7Br.sub.3O.sub.2 and Li.sub.10N.sub.3Br. The simulation was carried out with a small amount of Mg replacing Li to create the mobile Li vacancy. To accelerate the simulation, the calculation was performed at high temperatures. The FIGURE shows the calculated diffusivity for each of the three studied compositions. In the temperature range of 800-1650 K, the diffusivity for all compounds are in the order of 10.sup.−4 to 10.sup.−6 cm.sup.2/s, and shows good Arrhenius dependence on temperature.

(16) Table 2 lists the activation energy barriers and the conductivities at 300 K for these compounds. All three compounds have the conductivities above 10.sup.−6 S/cm, meeting the requirement of solid Li-ion conductor. More importantly, these compounds are stable when contacted with metal Li, indicating they can be used directly as solid state electrolyte with metal Li anode.

(17) TABLE-US-00002 TABLE 2 Activation energy and room temperature conductivity of Li.sub.4NCl, Li.sub.10N.sub.3Br and Li.sub.7Br.sub.3O.sub.2 from AIMD simulations. Compound Composition in AIMD simulation E.sub.a (eV) σ (S/cm) Li.sub.4NCl Li.sub.3.56Mg.sub.0.22NCl 0.26 2.1 × 10.sup.−3 Li.sub.10N.sub.3Br Li.sub.9.33Mg.sub.0.33N.sub.3Br 0.45 7.9 × 10.sup.−6 Li.sub.7Br.sub.3O.sub.2 Li.sub.6Mg.sub.0.5Br.sub.3O.sub.2 0.43 2.6 × 10.sup.−6

(18) As described the compounds are doped by replacing a maximum of 10 mole % of a total cation content of lithium with a dopant cation having a +2 or +3 charge in order to create vacancies for lithium mobility, while maintaining charge neutrality.

(19) Accordingly, in the first embodiment, the present application provides a solid-state lithium ion electrolyte, comprising: comprising: a composite material having at least 94 mole % lithium ions as cation component and multiple anions; in an anionic framework capable of conducting lithium ions; wherein the composite material is in the form of an anionic framework capable of conducting lithium ions, and an activation energy for lithium ion migration in the solid state lithium ion electrolyte is 0.5 eV or less.

(20) In an aspect of the first embodiment a lithium ion (Li.sup.+) conductivity of the solid state lithium ion electrolyte is at least 10.sup.−6 S/cm at room temperature.

(21) One aspect of the first embodiment includes a solid-state lithium ion electrolyte of formula (I):
Li.sub.7-nxM.sub.xBr.sub.3O.sub.2  (I)

(22) wherein

(23) M is cation of n+ charge,

(24) x is a number from 0 to 0.7, and

(25) n is 2 or 3.

(26) A second aspect of the first embodiment includes a solid state lithium ion electrolyte of formula (II):
Li.sub.10-nyM.sub.yN.sub.3Br  (II)

(27) wherein

(28) M is cation of n+ charge,

(29) y is a number from 0 to 1.0, and

(30) n is 2 or 3.

(31) A third aspect of the first embodiment includes a solid state lithium ion electrolyte of formula (III):
Li.sub.5-nzM.sub.zNCl.sub.2  (III)

(32) wherein M is cation of n+ charge,

(33) z is a number from 0 to 0.5, and

(34) n is 2 or 3.

(35) A fourth aspect of the first embodiment includes a solid state lithium ion electrolyte of formula (IV):
Li.sub.4-nwM.sub.qNCl  (IV)

(36) wherein M is cation of n+ charge,

(37) w is a number from 0 to 0.4, and

(38) n is 2 or 3.

(39) A fifth aspect of the first embodiment includes a solid state lithium ion electrolyte of formula (V):
Li.sub.6-ntM.sub.tNBr.sub.3  (V)

(40) wherein M is cation of n+ charge,

(41) t is a number from 0 to 0.6, and

(42) n is 2 or 3.

(43) A sixth aspect of the first embodiment includes a solid state lithium ion electrolyte of formula (VI):
Li.sub.6-nsM.sub.sNI.sub.3  (VI)

(44) wherein M is cation of n+ charge,

(45) s is a number from 0 to 0.6, and

(46) n is 2 or 3.

(47) Further to each of the six aspects a lithium ion (Li.sup.+) conductivity of the solid state lithium ion electrolyte may be at least 10.sup.−6 S/cm at room temperature.

(48) Synthesis of the composite materials of the first embodiment may be achieved by solid state reaction between stoichiometric amounts of selected precursor materials.

(49) For example, Li.sub.4NCl can be synthesized from Li.sub.3N and LiCl at 450° C. (Journal of Solid State Chemistry, 128, 1997, 241). Li.sub.10N.sub.3Br can be prepared from Li.sub.3N and LiBr at 500° C. (Zeitschrift für Naturforschung B, 50, 1995, 1061). Li.sub.5NCl.sub.2 can be prepared from Li.sub.3N and LiCl in a solid state reaction at 450° C. (Journal of Solid State Chemistry 130, 1997, 90). Li.sub.6NBr.sub.3 can be prepared from Li.sub.3N and LiBr at 430° C. (Journal of Alloys and Compounds, 645, 2015, S174). Li.sub.6NI.sub.3 can be prepared from Li.sub.3N and LiI at 490° C. (Z. Naturforsch. 51b, 199652 5)

(50) In further embodiments, the present application includes solid state lithium ion batteries containing the solid-state electrolytes described above. Solid-state batteries of these embodiments including metal-metal solid-state batteries may have higher charge/discharge rate capability and higher power density than classical batteries and may have the potential to provide high power and energy density.

(51) Thus in further embodiments, solid-state batteries comprising: an anode; a cathode; and a solid state lithium ion electrolyte according to the embodiments described above, located between the anode and the cathode are provided.

(52) The anode may be any anode structure conventionally employed in a lithium ion battery. Generally such materials are capable of insertion and extraction of Li.sup.+ ions. Example anode active materials may include graphite, hard carbon, lithium titanate (LTO), a tin/cobalt alloy and silicon/carbon composites. In one aspect the anode may comprise a current collector and a coating of a lithium ion active material on the current collector. Standard current collector materials include but are not limited to aluminum, copper, nickel, stainless steel, carbon, carbon paper and carbon cloth. In an aspect advantageously arranged with the solid-state lithium ion conductive materials described in the first and second embodiments, the anode may be lithium metal or a lithium metal alloy, optionally coated on a current collector. In one aspect, the anode may be a sheet of lithium metal serving both as active material and current collector.

(53) The cathode structure may be any conventionally employed in lithium ion batteries, including but not limited to composite lithium metal oxides such as, for example, lithium cobalt oxide (LiCoO.sub.2), lithium manganese oxide (LiMn.sub.2O.sub.4), lithium iron phosphate (LiFePO.sub.4) and lithium nickel manganese cobalt oxide. Other active cathode materials may also include elemental sulfur and metal sulfide composites. The cathode may also include a current collector such as copper, aluminum and stainless steel.

(54) In one aspect, the active cathode material may be a transition metal, preferably, silver or copper. A cathode based on such transition metal may not include a current collector.

(55) The above description is presented to enable a person skilled in the art to make and use the invention, and is provided in the context of a particular application and its requirements. Various modifications to the preferred embodiments will be readily apparent to those skilled in the art, and the generic principles defined herein may be applied to other embodiments and applications without departing from the spirit and scope of the invention. Thus, this invention is not intended to be limited to the embodiments shown, but is to be accorded the widest scope consistent with the principles and features disclosed herein. In this regard, certain embodiments within the invention may not show every benefit of the invention, considered broadly.