Structural and Compositional Factors That Control the Li-Ion Conductivity in LiPON Electrolytes

Abstract

Amorphous Li-ion conductors are important solid-state electrolytes. However, Li transport in these systems is much less understood than for crystalline materials. We investigate amorphous LiPON electrolytes via ab initio molecular dynamics, providing atomistic-level insight into the mechanisms underlying the Li+ mobility. We find that the latter is strongly influenced by the chemistry and connectivity of phosphate polyanions near Li+. Amorphization generates edge-sharing polyhedral connections between Li­(O,N)4 and P­(O,N)4, and creates under- and overcoordinated Li sites, which destabilizes the Li+ and enhances their mobility. N substitution for O favors conductivity in two ways: (1) excess Li accompanying 1­(N):1­(O) substitutions introduces extra carriers; (2) energetically favored N-bridging substitutions condense phosphate units and densify the structure, which, counterintuitively, corresponds to higher Li+ mobility. Finally, bridging N is not only less electronegative than O but also engaged in strong covalent bonds with P. This weakens interactions with neighboring Li+ smoothing the way for their migration. When condensation of PO4 polyhedra leads to the formation of isolated O anions, the Li+ mobility is reduced, highlighting the importance of oxygen partial pressure control during synthesis. This detailed understanding of the structural mechanisms affecting Li+ mobility is the key for optimizing the conductivity of LiPON and other amorphous Li-ion conductors.

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“…The study considered both small DFT-generated structure models of stoichiometric Li 3 PO 4 and large models (up to ∼1000 atoms) with slightly Li-deficient compositions directly from ANN-potential calculations (figure 8), all of which were obtained from melt-quench simulations. The activation energies for Li diffusion were obtained from MD simulations using the ANN potential and were estimated to be ∼0.55eV, in good agreement with experiment [101,102] and ab initio MD simulations [70]. Li et al also used an ANN potential to model Cu diffusion in amorphous Ta 2 O 5 , though the potential in this later work was trained only on the energy differences caused by Cu intercalation, thereby reducing the complexity of the potential energy surface [103].…”

“…The authors compared the amorphous structures obtained from computational delithiation (electrochemical amorphization) with those obtained from melt-quench MD simulations (thermal amorphization) and found slight differences in the pair distribution functions which they attributed to the quench-rate in the MD simulations. In subsequent work, the same group applied the GA-ANN formalism to amorphous N-substituted lithium phosphate (LiPON), a solid electrolyte for Li-ion batteries, to generate amorphous structure models as input for ab initio MD simulations in order to investigate the effect of nitrogen doping on the local atomic structure and the Li diffusivity [70].…”

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“…10 −6 S/cm), numerous studies have been devoted to the improvement of the ionic conductivity through different means. Similar to the findings pertaining to the structure of LPS discussed in the previous section, the conductivity of LiPON benefits from increasing disorder (Lacivita et al, 2018a). The increases in network former disorder can be accomplished by processing (e.g., higher deposition temperature) or compositional (introduction of non-bridging anions) means.…”

“…However, a number of competing explanations for the effect of N-substitution on O-sites have been proposed in recent years. Two papers by Lacivita et al (2018a) extensively modeled (DFT, MD) and characterized (neutron scattering, infrared spectroscopy) to show that the increase in conductivity (from 10 −6 to approximately 10 −5 S/cm) with increasing Li and N content is potentially due to a number of factors, including orientational disorder of the PO 4 units, excess Li + introduced at high Li/N compositions, and bridging Q 2 nitrogen lowering the electrostatic interaction between the anionic units and the mobile Li ions, with some of these factors also being identified by other workers (Pichonat et al, 2010;Fleutot et al, 2011;Famprikis et al, 2019b). Additionally, their extensive structural modeling and characterization showed no evidence for Q 3 nitrogen, calling into question the previous notion of a positive correlation between of these hypothetical species to the increase in conductivity (Lacivita et al, 2018b).…”

“…LiPON is most commonly fabricated by radio frequency magnetron (RFM) sputtering at elevated temperatures (T sputter > 200 • C) in a pure nitrogen, or N 2 mixed with Ar 2 /O 2 , atmosphere with a Li 3 PO 4 target (Bates et al, 1992). The formation of a non-crystalline Li x PO 4−y N y structure is thought to proceed by the partial nitridation of Li 3 PO 4 (Lacivita et al, 2018a); however there is an ongoing debate regarding the process which will be summarized later. The precise structure over the wide composition range also remains a somewhat open question, but it is agreed upon that the material is mainly comprised of randomly oriented, corner-sharing, PO 4−y N y tetrahedra, where the nitrogen can be non-bonding to reduce the connectivity of the network.…”