Under Pressure: Mechanochemical Effects on Structure and Ion Conduction in the Sodium-Ion Solid Electrolyte Na₃PS₄

Abstract

Fast-ion conductors are critical to the development of solid-state batteries. The effects of mechanochemical synthesis that lead to increased ionic conductivity in an archetypical sodium-ion conductor Na 3 PS 4 are not fully understood. We present here a comprehensive analysis based on diffraction (Bragg, pair distribution function), spectroscopy (impedance, Raman, NMR, INS) and ab-initio simulations aimed at elucidating the synthesis-property relationships in Na 3 PS 4. We consolidate previously reported interpretations about the local structure of ball-milled samples, underlining the sodium disorder and showing that a local tetragonal framework more accurately describes the structure than the originally proposed cubic one. Through variable-pressure impedance spectroscopy measurements, we report for the first time the activation volume for Na + migration in Na 3 PS 4 , which is ~30% higher for the ball-milled samples. Moreover, we show that the effect of ball-milling on increasing the ionic conductivity of Na 3 PS 4 to ~10-4 S/cm can be reproduced by applying external pressure on a sample from conventional high temperature ceramic synthesis. We conclude that the key effects of mechanochemical synthesis on the properties of solid electrolytes can be analyzed and understood in terms of pressure, strain and activation volume. File list (2) download file view on ChemRxiv Na3PS4_mechanical_v10.1.pdf (2.18 MiB) download file view on ChemRxiv SI_Na3PS4_mechanical_v10.1.pdf (2.38 MiB)

The Devil is in the Defects: Electronic Conductivity in Solid Electrolytes

“…Unsurprisingly, our results suggest that both Li and Na vacancies as well as their interstitials are among the favorable defects that form in both Li 6 PS 5 X and Na 3 PS 4 , thereby enabling their high ionic conductivities. 16,18,38,56,59,76,77 In the case of the Li 6 PS 5 X, we find that the chemical environments, either Li-poor or Li-rich do not alter substantially these defects concentrations. Importantly, these chemical environments can also be accessed at specific electrochemical conditions: a Li(Na)-rich condition represents the situation of the electrolyte placed near a highly reducing materials and mimicked by the low voltage of the Li(Na)metal electrode (see Figure 2(a)).…”

“…The Na 3 PS 4 samples were synthesized identically to a previous study. 77 Stoichiometric amounts of Na 2 S and P 2 S 5 (Sigma-Aldrich) were utilized as received. The high-temperature tetragonal Na 3 PS 4 phase was synthesized by a solid-state synthesis route.…”

On the underestimated influence of synthetic conditions in solid ionic conductors

“…sintering, or from pressure applied either prior to a measurement or during measurement. 13,74,75 For applied pressure, the elastic modulus, fracture toughness of the material are important. By use of soer materials, like the thiophosphates better connectivity with the adjacent phases like electrode materials can be achieved by allowing elasticdeformation as opposed to oxides; nevertheless, both the materials are prone to crack formation under stress due to low fracture toughness.…”

“…124 Recent studies show an effective change in conductivity of c-Na 3 PS 4 , which can be attributed to a combination of crystal defect formation and its nanocrystalline nature as (discussed in Section 3.1). 75 Another important parameter is milling time. The yield increases with milling time due to the gradual rise in the collision number and the total energy transfer.…”

Polymer-based hybrid battery electrolytes: theoretical insights, recent advances and challenges

“…sample and electrode geometry, pelletizing pressure, the pressure applied during the measurement, temperature ranges, applied frequency, the way the electrodes have been attached to the pellet, cell conguration, and even the sample history. 167 A recent inter-laboratory reproducibility study via round robin test of thiophosphate-based electrolytes suggests a signicant variation in the measured conductivity and E a . 168 Thus, it is worth noting that there may be a convoluted effect leading to considerable uncertainty in the results.…”