Fast Lithium Ion Conduction in Lithium Phosphidoaluminates

Abstract Solid electrolyte materials are crucial for the development of high‐energy‐density all‐solid‐state batteries (ASSB) using a nonflammable electrolyte. In order to retain a low lithium‐ion transfer resistance, fast lithium ion conducting solid electrolytes are required. We report on the novel superionic conductor Li9AlP4 which is easily synthesised from the elements via ball‐milling and subsequent annealing at moderate temperatures and which is characterized by single‐crystal and powder X‐ray diffraction. This representative of the novel compound class of lithium phosphidoaluminates has, as an undoped material, a remarkable fast ionic conductivity of 3 mS cm−1 and a low activation energy of 29 kJ mol−1 as determined by impedance spectroscopy. Temperature‐dependent 7Li NMR spectroscopy supports the fast lithium motion. In addition, Li9AlP4 combines a very high lithium content with a very low theoretical density of 1.703 g cm−3. The distribution of the Li atoms over the diverse crystallographic positions between the [AlP4]9− tetrahedra is analyzed by means of DFT calculations.


Introduction
Thed evelopment of advanced energy-storage technologies plays ak ey role in realizing electric vehicles. [1][2][3][4] Nextgeneration high-energy-density storage systems require low flammability,good electrochemical stability,and fast charging times.L i-ion batteries based on organic electrolytes hinder the commercialization of long-range electric vehicles.A llsolid-state batteries (ASSBs) are promising candidates to overcome safety concerns of currently used Li-ion batteries with flammable organic liquid electrolytes. [5][6][7][8] Replacing the organic liquid electrolyte by an inorganic solid-state electrolyte (SSE), ASSBs offer high energy and power density, mechanical stability,and safety benefits. [9][10][11][12][13][14] However, ASSBs are limited by the slow ionic mobility of the SSE. [15] Hence, the discovery,c haracterization, and optimization of lithium superionic conducting solid phases are among the main aspects of todaysbattery material research. [8,[16][17][18] Despite the clear advantage of ASSBs,a chieving Li-ion conductivity in SSEs comparable to that in liquid electrolytes (> 10 mS cm À1 ) is ademanding task. [9] In the last decades,different crystalline materials have been proven to act as lithium conductors such as perovskite-type structures, [19][20][21][22] lithium superionic conductor (LISICON)-type structures, [23][24][25][26] thio-LISICON-type structures and thiophosphates, [27][28][29][30][31][32] sodium superionic conductor (NASICON)-type structures, [33,34] garnet-type structures, [35][36][37] lithium argyrodites, [38] lithium borohydrides, [39] lithium nitrides, [40][41][42] lithium hydrides, [43] and lithium halides. [44] Theb est lithium ion conductors currently known are rather complex systems such as Li 9.54 Si 1.74 P 1.44 S 11.7 Cl 0.3 and Li 6+x M x Sb 1Àx S 5 I(M= Si, Ge,Sn) with an ionic conductivity of 25 and 24 mS cm À1 ,r espectively,o utperforming the conductivity of liquid-based electrolytes. [45,46] By increasing the carrier densities,c hanging the diffusion pathways of the mobile species,c reating vacancies or increasing structural defects,t he ionic conductivity can be further optimized. [8,16] An effective way to increase the carrier density is especially the aliovalent substitution of cations:f or example,i nL i 3 PS 4 making af ormal substitution of "P 5+" with "Ge 4+ "r esults in Li 3.25 Ge 0.25 P 0.75 S 4 having af our times higher ionic conductivity. [28] On the way to new candidates with good lithium ion conducting abilities,w er ecently have investigated lithium phosphidosilicates and phosphidogermanates. [47,48] Thei dea of replacing S 2À by P 3À enables the accommodation of even more lithium ions in the structures.S ince our first report in 2016 on the aristo-type Li 8 SiP 4 , [47] also Li 14 SiP 6 was established as good lithium ion conductor. [49] Recently,w eh ave also shown that the system can be extended to the heavier homologue germanium and that for Li 8 GeP 4 two Li-ion conducting modifications exist which display ionic conductivities of up to 8.6 10 À5 Scm À1 at 298 K. [48] Structurally, phosphidosilicates and phosphidogermanates are built up by [TtP 4 ] 8À tetrahedra, where Tt denotes the respective tetrel atom (Si, Ge). Relating to the building principles of oxidosilicates,t hiosilicates,a nd thiophosphates,t hese tetra-Abstract: Solid electrolyte materials are crucial for the development of high-energy-density all-solid-state batteries (ASSB) using an onflammable electrolyte.I no rder to retain alow lithium-ion transfer resistance,fast lithium ion conducting solid electrolytes are required. We report on the novel superionic conductor Li 9 AlP 4 which is easily synthesised from the elements via ball-milling and subsequent annealing at moderate temperatures and which is characterized by singlecrystal and powder X-ray diffraction. This representative of the novel compound class of lithium phosphidoaluminates has,as an undoped material, ar emarkable fast ionic conductivity of 3mScm À1 and al ow activation energy of 29 kJ mol À1 as determined by impedance spectroscopy. Temperature-dependent 7 Li NMR spectroscopys upports the fast lithium motion. In addition, Li 9 AlP 4 combines avery high lithium content with avery lowtheoretical density of 1.703 gcm À3 .The distribution of the Li atoms over the diverse crystallographic positions between the [AlP 4 ] 9À tetrahedra is analyzed by means of DFT calculations.
hedra can be formally condensed and covalently connected by sharing edges or corners;h ence al arge variety of structure motifs can be gained. Theidea of phosphide-based structures as ionic conductors results from the aliovalent substitution of [TtS 4 ] 4À tetrahedra, which are an key building block in sulfidebased electrolytes,l eading to the analogous complex anions based on phosphorus.D ue to the resulting higher charge, [TtP 4 ] 8À tetrahedra allow for more lithium ions per formula unit for charge compensation when compared to [TtS 4 ] 4À .I n addition, the more negatively charged P 3À is more polarizable than S 2À .
Thecoincidence of ahigher charge carrier density due to more Li ions for charge compensation together with al arge number of vacancies is considered as an important prerequisite for ah igher lithium ion conductivity.C ertainly,t his aspect must be in line with alow activation energy for lithium mobility as it occurs in structures with an effective polyhedral connectivity. [50] An aliovalent substitution of formal "Si 4+" by "A l 3+ "and formation of [AlP 4 ] 9À instead of [SiP 4 ] 8À tetrahedra allow for the presence of an even higher number of lithium ions and as trong influence on the Li occupation in voids. Te rnary phases comprising the elements lithium, aluminum, and phosphorus were scarcely investigated. In the 1950s Juza et al. reported briefly the first lithium phosphidoaluminate, Li 3 AlP 2 ,a nd described its structure as as trongly distorted fluorite-type lattice in which Pa toms form ac lose-packed atom arrangement and Al and Li atoms are randomly distributed over the tetrahedral sites. [51] Occupation of Li ions in octahedral sites was,h owever,n ot considered. Tw o further publications on Li 3 AlP 2 were based on the poor structural characterization. [52,53] Here we report on the lithium-richest representative of the phosphidoaluminates,o btained by formal aliovalent substitution of "Tt 4+" by "A l 3+ "i nL i 8 TtP 4 .T he insertion of Al 3+ leads to formally ninefold negatively charged [AlP 4 ] 9À tetrahedra, resulting in an even higher lithium density per formula unit and achange in the spatial extent of the diffusion pathways.Due to the nature of the structure in which Patoms form acubic-close-packingarrangement, there are still ahigh number of unoccupied octahedral sites present. We expect that an increase of the carrier density will also lead to an increase of the ionic conductivity.I ndeed, only recently,a b initio simulations suggested that doping of the moderate lithium-ion conductor Li 2 SiP 2 with Al could enhance the ionic conductivity. [47,54]

Results and Discussion
Synthesis and Crystal Structure of Li 9 AlP 4 Li 9 AlP 4 was synthesized from the elements by atwo-step procedure.After ball-milling of stoichiometric amounts of Li, Al and P, the powder mixture was annealed at 973 Kfor one day yielding almost phase-pure Li 9 AlP 4 with 2.3(1) wt %LiP impurities (see Figure 1). Complete data of the Rietveld refinement are given in the Supporting Information (Tables S1 and S2). Single crystals of Li 9 AlP 4 were obtained by the direct reaction of the elements at 1073 Kw ith as light excess of Pu sing ar atio of 9:1:4.2 (Li:Al:P). Ther esulting product contains besides Li 9 AlP 4 also Li 3 Pa nd TaP( see Figure S4). Energy-dispersive X-ray spectroscopy (EDX) investigations of single crystals show an Al/P ratio which is in very good agreement with the expected values (see Table S6). Li 9 AlP 4 can also be synthesized starting from Li 3 P. However,a gain unreacted Li 3 Pr emains as ac ontamination (see Figure S5). According to EDX measurements,the single crystals were free of Ta.T he details of the structure refinement of the single-crystal X-ray diffraction of Li 9 AlP 4 are listed in Table 1( further data are given in Tables S3 and  S4 in the Supporting Information).
According to single-crystal and powder X-ray structure determination and refinement, Li 9 AlP 4 crystallizes in the cubic space group P " 43n (no.218). Thestructure of Li 9 AlP 4 is built up of isolated [AlP 4 ] 9À tetrahedra surrounded by Li + ions ( Figure 2a). TheAlP 4 tetrahedra are slightly distorted with P-Al-P angles ranging from 110.89(2)8 8 to 108.77(2)8 8 in comparison to the ideal tetrahedron angle of 109.748 8.The Al-P bonds at 2.425(1) and 2.433(1) are longer than the Al-P bonds observed in compounds with connected AlP 4 tetrahedra like AlP (2.360(1) ), Na 3 AlP 2 (2.376(4) ), and Sr 3 Al 2 P 4 (2.377-(3) -2.417 (2) ). [55][56][57] Fort he discussion of the coordination polyhedra of Li atoms,t he structure is considered as ad istorted cubic-facecentered packing of Patoms with Li and Al atoms filling voids in between and Wyckoff positions giving the multiplicity of the site.W ith Z = 8, there are 32 Pa toms per unit cell resulting in 64 tetrahedral and 32 octahedral voids.The atoms Al1 and Al2 occupy 1/8 of the tetrahedral voids on Wyckoff positions 2a and 6d, respectively.W elike to point out that the Al atoms are fully ordered, and no mixed occupancyofAland Li occurs,incontrast to the mixed Li/Si occupancyinthe Lirich phase Li 14 SiP 6 . [49] Theremaining tetrahedral voids in the title compound are occupied by Li atoms as follows:Li1, Li4, and Li5 (on Wyckoff positions 6b,1 2f,a nd 24i, respectively) are fully occupied, whereas Li2 and Li3 (on Wyckoff positions 6c and 8e,r espectively) are partially occupied with side occupation factors 0.50(5) and 0.71(4), respectively,based on single-crystal structure determination data. Hence,the overall occupation of tetrahedral positions by lithium is 91 %.
Theo ccupation of the 32 larger octahedral voids reveals two interesting aspects.T he voids are partially occupied and in addition, we see for the first time that the Li atoms are not necessarily located at the center of the P 6 octahedra (Figure 2c). The32octahedral voids can be distinguished by two crystallographically different Wyckoff positions that are both partially occupied with Li:L i7 on Wyckoff position 8e with site occupation factor (sof)0.25(4) and asplit position of Li6a and Li6b on Wyckoff position 24i with sof 0.46(2) and sof 0.23(2), respectively (Figure 2b). Theo verall occupation of the octahedral voids,therefore,is58%.
Due to the good quality of the crystallographic data it becomes obvious that Li6a, Li6b,a nd Li7 are not located in the centers of the octahedra, and the data also allow the determination of split positions in one octahedral void as anticipated for other lithium ion conducting materials that have Li ions in octahedral environments,a si ng arnets (Li 7 La 3 Zr 2 O 12 )and sulfide-based materials (Li 10 GeP 2 S 12 ). [58,59] Therefined positions of the octahedrally coordinated Li ions are shifted towards the triangular faces of the octahedra, where the face-sharing LiP 4 tetrahedra of the partially occupied Li sites Li2 and Li3 are located. All Li atoms exhibit large displacement ellipsoids (Figure 2a)which points towards astatic or thermal displacement indicative of ahigh lithium ion mobility.All atomic positions except Li1 were refined anisotropically.I nterestingly,t he octahedral positions Li6a and Li6b exhibit ellipsoids that point towards the center of the triangular faces of the neighboring tetrahedral voids that are only partially filled with Li2 and Li3 (Figure 2b).
Theh igh structural diversity of the Li atoms including disorder and split positions reflects the possibility of high lithium ion mobility.L i-P distances in the tetrahedral voids range from 2.50(1) to 2.73(1) ,a nd in the octahedral voids they range from 2.56(1) to 3.46(1) .T he bond lengths are similar to those in other ternary phases containing Li and P, such as Li 8 SiP 4 ,Li 2 SiP 2 ,L i 10 SiP 2 ,a nd Li 3 Si 3 P 7 . [47,60] According to the valence rules,L i 9 AlP 4 is an electronprecise compound and can be described as (Li + ) 9 [AlP 4 ] 9À , with formally two negative charges for the Pa toms and one negative charge for the Al atom, balanced by nine Li + ions.

DFT Calculations
DFT computations based on the experimentally derived structural information were performed to serve ad ouble purpose:f irst, to corroborate the refined structural model obtained from experiments;s econd, to obtain information about stability trends and the coordination of individual atoms in the presence of crystallographic disorder.W e constructed as eries of discrete atomistic models which approximate the disordered structure (making simplifications as detailed in the Supporting Information). Thee nergetic stability of the new compound as compared to competing phases is analyzed according to the line in the ternary composition (Gibbs) diagram. Since Li 9 AlP 4 is located on the line between the binaries Li 3 Pa nd AlP ( Figure S6), it is straightforward to inquire the formation energy according to 3Li 3 P + AlP (= Li 9 AlP 4 )a nd to assess whether the title compound is stable with regard to the constituent binaries. Thec omputed energies of ten randomized structural models were compared to those of the competing binary phases (3 Li 3 P + AlP) which are set as energy zero.The formation of the ternary title compound from the binaries is energetically favored by approximately 30 kJ mol À1 (Figure 3a and the Supporting Information), even without taking configurational entropy into account (which will further stabilize the ternary phase,a st here are no partially occupied crystallographic positions in either Li 3 PorA lP).
In addition to the energetic stability,h aving an ensemble of computationally optimized structural models allows us to quantify the distribution of interatomic distances and to compare with experimental results.S moothed histograms over all ten structures are given in Figure 3b.T he computed Al-P distances in the relaxed structures are 2.44(4) ,w ith anarrow distribution indicating the rigidity of the tetrahedral AlP 4 units.T he Li-P distributions peak at 2.6 ,b ut include as ubstantial amount of larger distances,r eflecting the more disordered nature of the Li atoms which are often shifted away from the centers of the octahedral voids.T he distribu-

Angewandte Chemie
Research Articles 5668 www.angewandte.org tion of Li···Li distances in the relaxed structures spans abroad range,s tarting at % 2.4 ,p eaking at % 2.6 ,a nd becoming significantly less pronounced beyond 3.0 ,i ng ood agreement with the experimental observations (Table S5).

Impedance Spectroscopy
Thel ithium ion conductivity of Li 9 AlP 4 was determined from impedance measurements in ab locking electrode configuration. Impedance spectra at different temperatures (273, 298, 313, 333, and 353 Ka ccording to the temperature profile shown in the inset) are displayed in Figure 4a and feature as emicircle at high frequencies and al ow-frequency tail. Thes emicircle can be described as parallel circuit element of ar esistor and ac onstant phase element (R/Q), with R representing both intragrain and grain boundary contributions to the lithium ion transport which could not be resolved, and thus only the total ionic resistance of the sample could be determined. Forthe constant phase element, the fit of the data acquired at 298 Kr esulted in a values of % 0.87 and Q parameters with avalue of % 17 10 À9 Fs (aÀ1) ;the ionic conductivity was determined to be s Li (Li 9 AlP 4 ) = (3.03 AE 0.16) 10 À3 Scm À1 at 298 K( obtained from three independently measured cells). Thea ctivation energy for lithium ion transport (Figure 4b)w as investigated by temperature-de-pendent impedance measurements in the range from 273-353 K, yielding E A PEIS of 28.5 AE 0.8 kJ mol À1 ( % 0.29 eV);this was determined from three independently measured cells, using the s Li T values of only the first heating and cooling cycle of each sample.T his activation energy is in very good agreement with the value obtained by NMR spectroscopy (shown below). Thet emperature ramp of ah eating and cooling cycle is displayed in the inset of Figure 4a.C olored dots indicate at which temperatures PEIS measurements were performed. In this context it should be mentioned that conductivities (and thus the product s Li T)f or heating and cooling differ by less than 6%.E rror bars are calculated separately for heating and cooling steps by taking the mean of three independent measurements obtained from three cells. DC polarization measurements in the range from 50-150 mV reveal an electronic conductivity of (2.0 AE 0.8) 10 À7 Scm À1 at 298 K(based on the standard deviation of two cells). 6

Li and 7 Li NMR Spectroscopy
The 6 Li MAS NMR spectrum at room temperature shows one signal at ac hemical shift of 4.19 ppm ( Figure 5). The fitting of ag eneralized Lorentzian function to the experimental data proves the presence of only one signal as observed before for related compounds. [47,48] Since the shifts  (Table S3). b) Unit cell with emphasis on the two crystallographically independentoctahedralv oids:p olyhedra of Li6a/Li6b and Li7 are shown in green and yellow,r espectively. c) Closeup of neighboring octahedral voids with partially occupied Li6a, Li6b, and Li7 positions. Li, Al, and Pare depicted in gray,o range, and purple, respectively (displacementellipsoids set at 90 %probability at 150 K). of Li atoms in tetrahedral and octahedral voids are not distinguishable,i tc an be assumed that all lithium ions are mobile at room temperature.
Static 7 Li NMR spectra were recorded as af unction of temperature to study the dynamic behavior of the lithium ions.T he central transition of the I = 3/2 nucleus 7 Li experiences ab roadening only from the homo-( 7 Li-7 Li)a nd heteronuclear (here: 7 Li-31 P) dipolar couplings.S ince both types of interactions scale with the second Legendrian (3 cos 2 bÀ1), any dynamic process should produce a( partial) averaging of the orientational dependence and hence entail anarrowing of the NMR signal.
At 290 K, the static 7 Li spectrum (Figure 6a  Thet emperature-dependent evolution of the linewidth (FWHH;f ull width at half height) of the 7 Li NMR signal is shown in Figure 6b.Arough estimation of the activation energy of lithium motion is possible employing the empirical Waugh-Fedin relation E A (kJ mol À1 ) = 0.156 T onset (K). [61] As the onset temperature,weidentified the temperature at which the linewidth is given by (n rigid lattice Àn mot. narrowing )/2. This leads to an approximate onset temperature of 160 Kw hich translates to an activation energy of 25 kJ mol À1 .

Conclusion
Recently,w ereported the synthesis and characterization of Li 14 SiP 6 ,for which we had optimized the ionic conductivity ( % 1mScm À1 )i nl ithium phosphidotetrelates by increasing the amount of lithium compared to Li 8 SiP 4 . [49] With Li 9 AlP 4 we now show that these materials can be further optimized, namely by full aliovalent replacement of the tetrelate atom. Compared to the ionic conductivities in Li 8 SiP 4 (4.5 10 À5 Scm À1 )a nd a/b-Li 8 GeP 4 (1.8 10 À5 Scm À1 ,8 .6 10 À5 Scm À1 ), via aliovalent substitution by aluminum and the associated higher lithium content in the structure alongside with ac hange in the distribution of vacancies (Table 2), the conductivity in Li 9 AlP 4 is strongly increased up to 3.0(2) mS cm À1 at room temperature.C oncomitantly,t he activation energy determined by impedance measurements drops significantly from 42 (a-Li 8 GeP 4 ), 39 (Li 8 SiP 4 ), and 38 kJ mol À1 (b-Li 8 GeP 4 )t o29kJmol À1 in Li 9 AlP 4 . Li 9 AlP 4 is easily accessible via ball-milling and crystallizes in the cubic space group P " 43n (no. 2 18). First-principles computations suggest that Li 9 AlP 4 is approximately 30 kJ mol À1 more stable than the constituent binary phosphides.T he structure of Li 9 AlP 4 shows aclose relationship to the recently characterized lithium ion conductors Li 8 SiP 4 , a-Li 8 GeP 4 ,a nd b-Li 8 GeP 4 .H owever,i nL i 9 AlP 4 the highly negatively charged, isolated [AlP 4 ] 9À tetrahedra make it possible to accommodate ah igher number of lithium ions per formula unit than in the phosphidotetrelates and therefore the new compound shows am uch better Li ion conductivity.Whereas Li 8 GeP 4 adopts both the a modification (space group Pa " 3, no.2 05) at lower temperature and the b modification at higher temperatures (space group P " 43n,n o. 218), Li 9 AlP 4 crystallizes exclusively in analogy to the b modification. These phosphidosilicates,-germanates,a nd -aluminates can be described as ac lose packing of Pa toms with four octahedral and eight tetrahedral voids per formula unit. Consequently,A l, Si, and Ge atoms occupy 1/8 of the tetrahedral voids forming covalently connected Al-P,S i-P, and Ge-P bonds,r espectively,a nd are located on fixed fully occupied positions in the structure.T he series of compounds now give insight into the occupancyo ft he lithium atoms which varies according to Table 2. By comparing the lithium Figure 3. a) DFT-computed energy of 10 randomized structuralm odels, compared to that of the competing binary phases (3 Li 3 P + AlP) which are set as energy zero. The models are ordered by ascending (unrelaxed) energy;l ines connectingsymbols are only guides to the eye. Static computations (using the experimental structure;open circles) indicate aclear preference for the Li6a over the Li6b site. DFT optimization of these structures (arrows) yields an ensemble of models that are all more favorable than the competing binaries and are essentially degeneratei nenergy (see the SupportingInformation for further details). b) Distribution of interatomicd istancesi nrandomized structural models of Li 9 AlP 4 ,optimized using DFT,asdetailed in the SupportingInformation.K ernel density estimates ("smoothed histograms") with ab andwidth of 0.05 are shown to characterize all relevant contacts collected over 10 structural models, which approximate the real structure within the limits of theory.
content, tetrahedral voids are slightly less occupied, whereas in the octahedral voids more lithium is found in Li 9 AlP 4 compared to the situation in b-Li 8 GeP 4 :i nL i 9 AlP 4 90 %o f the tetrahedral and 58 %ofthe octahedral voids are occupied compared to 93 %a nd 38 %, respectively,i nb-Li 8 GeP 4 .A t ah igher lithium content, the lithium ions are more evenly distributed over the tetrahedral and octahedral voids which might mainly result from al ower average electrostatic repulsion. Theh ere observed decrease of lithium content in the tetrahedral and the increase of lithium content in the octahedral voids in the presence of more lithium is in accordance with the observation made by aliovalent substitution in garnets and was well investigated by Cussen. [62] Moreover,t his increased population of octahedral voids contributes likely to the energy landscape flattening,a sa lso seen in further electrolytes like in Li 7 La 3 Zr 2 O 12 and argyrodites. [58,63] We found that the displacement of the Li + position from the center of the octahedral voids increases with an increasing lithium content per formula unit. And, as also observed in garnets,t his stronger displacement correlates with ah igher ion mobility.I nL i 9 AlP 4 we found, for the first time in ap hosphide-based material, as plit position in the octahedral voids.B oth positions are close to at riangular plane of the octahedra, supporting the hypothesis of lithium diffusion via face-sharing tetrahedra and octahedra. Lithium migration network analysis in phosphide-based materials like b-Li 8 GeP 4 ,h as already suggested that lithium diffusion is favored via face-sharing tetrahedra and octahedra. [48] Interestingly,t he split lithium positions in an octahedral environment, as observed here,has also been found in other fast ion conductors such as sulfide-based electrolytes and garnets:I n Li 10 GeP 2 S 12 (LGPS), one Li atom (Wyckoff position 8g) exhibits asplit position in aslightly distorted S 6 octahedron, [59] and in Li 7 La 3 Zr 2 O 12 (LLZO), one Li atom (Wyckoff position 96h) located in as trongly distorted O 6 octahedron appears with as plit position as well. [58] Ac hange of the metal atom and ah igher lithium concentration compared to the previously known compounds  ( Table 2) results in an extraordinarily high ionic conductivity of 3mScm À1 at room temperature,avery low activation energy of 29 kJ mol À1 ( % 0.29 eV), and ar oughly 4o rders of magnitude lower electronic conductivity of 2.0 10 À7 Scm À1 at room temperature for the title compound. This high ionic conductivity is achieved by the accommodation of more lithium ions in the octahedral voids.O ne further benefit of Li 9 AlP 4 is its very low density of % 1.7 gcm À3 ,which makes the compound very attractive for applications in all-solid-state batteries.
Even though the Li content itself is not ad ecisive parameter for increasing the Li mobility,t he possibility of aliovalent substitution of either Li 9Àx Tr 1Àx Tt x P 4 (Tr = triel element) allows for manifold optimization possibilities. Therefore,w ew ill further investigate the tetrahedral and octahedral occupancyi ns olid solutions Li 9Àx Tr 1Àx Tt x P 4 .F urther studies will also focus on the thermal behavior and the electrochemical stability of this material. ForL i 9 AlP 4 ,t em-perature-dependent powder neutron diffraction measurements in combination with MEM calculations are scheduled to localize the diffusion pathways. Figure 6. a) Temperature-dependent evolution of the 7 Li lineshape recorded in the temperature range from 132 to 290 K. b) Temperaturedependent linewidth of the 7 Li signal (full width at half height) recorded in the temperature range from 132 Kt oroom temperature. Table 2: Overview of lithium occupancy,c ell volume, lithium ion mobility at room temperature, and Li-P distances in Li 9 AlP 4, b-Li 8 GeP 4 , a-Li 8 GeP 4 , Li 8 SiP 4 ,a nd Li 14 SiP 6 .Data for the latter four are taken from the literature. [47][48][49]