# Introduction¶

This notebook demonstrates how to perform phase and electrochemical assessments starting from a VASP calculation using Python Materials Genomics (pymatgen) and the Materials Project database (via the Materials API). These notebooks are described in detail in

Deng, Z.; Zhu, Z.; Chu, I.-H.; Ong, S. P. Data-Driven First-Principles Methods for the Study and Design of
Alkali Superionic Conductors. Chem. Mater. 2017, 29 (1), 281–288 DOI: 10.1021/acs.chemmater.6b02648.



If you find these notebooks useful and use the functionality demonstrated, please consider citing the above work.

Let's start by importing some modules and classes that we will be using.

# Preparation¶

We will first read the results from the vasprun.xml output file from our VASP calculations. Only the lowest energy result is used here.

To construct the phase diagram, we need all entries in the Li-P-S-Cl chemical space. We will use the MPRester class to obtain these entries from the Materials Project via the Materials API.

In addition to all the MP entries, here we also load the computed entries of O/S substituted Li-P-O tenary compounds.

Next, we need to combine all the entries and postprocess them using MaterialsProjectCompatibility. This postprocessing step corrects the energies to account for well-known DFT errors, e.g., in the sulfur binding energy.

# Phase diagram construction¶

The phase diagram can then be constructed using the PhaseDiagram class, and plotted using the PDPlotter class.

We may observe from the above phase diagram that Li6PS5Cl is not a stable phase (red nodes) in the calculated 0K phase diagram.

The pseudo-ternary Li2S-P2S5-LiCl is constructed using the CompoundPhaseDiagram class.

# Calculating $E_{\rm hull}$ of Li6PS5Cl¶

We may evaluate the $E_{\rm hull}$ of Li6PS5Cl using the PDAnalyzer.

# Electrochemical Stability¶

The electrochemical stability can be assessed using a similar phase diagram approach, but using the lithium grand potential instead of the internal energy.

First, we need to identify a reference for lithium chemical potential using the bulk Li energy $\mu_{\rm Li}^0$.

The PDAnalyzer class provides a quick way to plot the phase diagram at a particular composition (e.g., Li6PS5Cl) as a function of lithium chemical potential called get_element_profile.

This element profile can be plotted as a Li evolution versus voltage using matplotlib as follows.