First-principles study of LiPON and related solid electrolytes

Lithium phosphorus oxynitride materials have been investigated for many years, especially in relation to the thin-film electrolyte LiPON, developed at Oak Ridge National Laboratory. We have carried out first-principles simulations of related crystalline materials as a first step toward understanding the sources of stability and mechanisms of Li-ion conductivity in these materials. In addition to a comprehensive survey of known crystalline materials related to LiPON, we have also predicted some materials. For example, starting with crystalline ${\text{LiPO}}_3$ which has twisted phosphate chains, we considered the possibility of modifying the structure by substituting N and Li for O. The optimized structures were computed to have regularized phosphate chains which form planar -P-N-P-N- backbones. To the best of our knowledge, the predicted crystals, which we call $s_1{\text{-Li}}_2{\text{PO}}_2\text{N}$ with a 24-atom unit cell and $s_2{\text{-Li}}_2{\text{PO}}_2\text{N}$ with a 12-atom unit cell, have not yet been observed experimentally. We suggest several possible exothermic reaction pathways to synthesize these crystals.

Figure 1

(Color online) Composition diagram based on ${\text{LiO}}_{1/2}$, ${\text{LiN}}_{1/3}$, ${\text{PO}}_{5/2}$, and ${\text{PN}}_{5/3}$ starting materials. Shown are labeled natural and synthetic crystalline materials [dark (blue) circles]. LiPON thin-film compositions [light (turquoise) circles] reported in the literature and examples of stable and metastable nitrided phosphate materials (◼) constructed in this work are also indicated.

Figure 2

(Color online) Ball and stick diagram of eight unit cells of ${\text{Li}}_4{\text{P}}_2{\text{O}}_7$ in the $P\overline{1}$ structure (using a nonstandard origin). The atomic sites are indicated with the following ball styles: Li (light), P [gray (yellow)], and O [dark (blue)].

Figure 3

(Color online) Partial densities of states plots for ${\text{Li}}_4{\text{P}}_2{\text{O}}_7$ and ${\text{Li}}_5{\text{P}}_2{\text{O}}_6\text{N}$ in the $P\overline{1}$ structure compared with that of $\gamma {\text{-Li}}_3{\text{PO}}_4$. The abbreviations “tetr.” and “br.” are used to distinguish sites with tetrahedral and bridge bond coordination of O or N with neighboring P, respectively.

Figure 4

(Color online) Raman-active (R) and infrared-active (I) zone-center lattice vibrations (in units of per centimeter) for ${\text{Li}}_4{\text{P}}_2{\text{O}}_7$. The lengths of the lines segments indicate the contribution of each atomic type as defined in Eq. (1). In this case, we distinguish the O sites having a tetrahedral configuration to P by “O (tet.)” and the O sites associated with the bridge bonds by “O (br.).”

Figure 5

(Color online) Raman-active (R) and infrared-active (I) zone-center lattice vibrations (in units of per centimeter) for ${\text{Li}}_5{\text{P}}_2{\text{O}}_6\text{N}$ plotted in the same scheme as in Fig. 4 except that bridge bonds are occupied by N’s and the amplitudes are labeled “N (br.).”

Figure 6

(Color online) Ball and stick model of natural ${\text{LiPO}}_3$ using the same ball styles as given in Fig. 2. The chain direction is perpendicular to the plane of the diagram.

Figure 7

(Color online) Single chain view of natural ${\text{LiPO}}_3$ shown in Fig. 6.

Figure 8

(Color online) Ball and stick model of $s_1{\text{-Li}}_2{\text{PO}}_2\text{N}$ structure using same ball styles as in Fig. 2 with the addition of N [dark gray (green)].

Figure 9

(Color online) Ball and stick model of $s_1{\text{-LiPO}}_3$ structure using same ball styles as in Fig. 2.

Figure 10

(Color online) Single chain views of $s_1{\text{-Li}}_2{\text{PO}}_2\text{N}$ (a) and $s_1{\text{-LiPO}}_3$ (b) with superimposed contours of valence densities plotted in a plane passing through each chain.

Figure 11

(Color online) Ball and stick model of $s_3{\text{-Li}}_2{\text{PO}}_2\text{N}$ structure using same ball styles as in Fig. 8.

Figure 12

(Color online) Partial densities of states plots for natural ${\text{LiPO}}_3$, $s_1{\text{-LiPO}}_3$, and $s_3{\text{-LiPO}}_3$ using the same notation as in Fig. 3.

Figure 13

(Color online) Partial densities of states plots for $s_1{\text{-Li}}_2{\text{PO}}_2\text{N}$, and $s_3{\text{-Li}}_2{\text{PO}}_2\text{N}$ using the same notation as in Fig. 3.

Figure 14

(Color online) Raman-active (R) and infrared-active (I) zone-center lattice vibrations for $s_1{\text{-Li}}_2{\text{PO}}_2\text{N}$ using the same notation as that given in Fig. 5.

Figure 15

(Color online) Raman-active (R) and infrared-active (I) zone-center lattice vibrations for $s_1{\text{-LiPO}}_3$ using the same notation as that given in Fig. 4.

Figure 16

(Color online) Raman-active (R) and infrared-active (I) zone-center lattice vibrations for natural ${\text{LiPO}}_3$ using the same notation as that given in Fig. 4.

Figure 17

(Color online) Ball and stick model of conventional unit cell $o{\text{-P}}_2{\text{O}}_5$ in its $Fdd2$ structure using ball styles of previous figures.

Figure 18

(Color online) Ball and stick model of conventional unit cell of $\alpha {\text{-P}}_3{\text{N}}_5$ in the $C2/c$ structure using the ball styles given in previous diagrams.

Figure 19

(Color online) Ball and stick model of several conventional unit cells of ${\text{LiPN}}_2$ in the $I\overline{4}2d$ structure using the ball styles of previous diagrams.

Figure 20

(Color online) Ball and stick models of multiple conventional cells of ${\text{LiP}}_4{\text{N}}_7$ using the ball styles of previous diagrams. The (a) diagram shows the $P{2}_1/c$ structure using the conventions of Ref. 25 and the (b) diagram shows $C2/c$ structure using the conventions of Ref. 26.

Figure 21

(Color online) Partial densities of states of some network structured materials. The notation is similar to that used in previous diagrams with the addition of “(trp.)” to indicate triply coordinated N site contributions.

Figure 22

(Color online) Raman-active (R) and infrared-active (I) zone-center lattice vibrations for $o{\text{-P}}_2{\text{O}}_5$ using scheme of previous diagrams.

Figure 23

(Color online) Raman-active (R) and infrared-active (I) zone-center lattice vibrations for $\alpha {\text{-P}}_3{\text{N}}_5$ using scheme of previous diagrams with the addition of the notation (trp.) to indicate triply coordinated N amplitudes.

Figure 24

(Color online) Raman-active (R) and infrared-active (I) zone-center lattice vibrations for ${\text{LiPN}}_2$ using scheme of previous diagrams.

Figure 25

(Color online) Raman-active (R) and infrared-active (I) zone-center lattice vibrations for ${\text{LiP}}_4{\text{N}}_7$ in the $P{2}_1/c$ structure using scheme of previous diagrams.

Figure 26

(Color online) Infrared spectrum of $\alpha {\text{-P}}_3{\text{N}}_5$ reproduced from Ref. 24 and superposed with the corresponding frequencies calculated in this work (also shown in Fig. 23).