Make the conversion battery module useful for multivalent cations (m…

…aterialsproject#985)

* Make the conversion battery module useful for multivalent cations

Applied the same treatment as we did for insertion electrodes. When the ion oxidation state is 2+, the voltage reduced to half, and the capacity multiplied by 2.

* Add unittest for multivalent working ion

Using Mg-MnO2 as the testing case.

* updated the Conversion battery

* testing file for the unittest of multivalent conversion battery module

Mg-Mn-O system

pymatgen/apps/battery/conversion_battery.py

@@ -359,9 +359,10 @@ def from_steps(step1, step2, normalization_els):
         """
         working_ion_entry = step1["element_reference"]
         working_ion = working_ion_entry.composition.elements[0].symbol
-        voltage = -step1["chempot"] + working_ion_entry.energy_per_atom
+        working_ion_valence = max(Element(working_ion).oxidation_states)
+        voltage = (-step1["chempot"] + working_ion_entry.energy_per_atom)/working_ion_valence
         mAh = (step2["evolution"] - step1["evolution"]) \
-            * Charge(1, "e").to("C") * Time(1, "s").to("h") * N_A * 1000
+            * Charge(1, "e").to("C") * Time(1, "s").to("h") * N_A * 1000*working_ion_valence
         licomp = Composition(working_ion)
         prev_rxn = step1["reaction"]
         reactants = {comp: abs(prev_rxn.get_coeff(comp))

pymatgen/apps/battery/tests/test_conversion_battery.py

@@ -36,7 +36,8 @@ def setUp(self):
         pass
 
     def test_init(self):
-        formulas = ['LiCoO2', "FeF3"]
+        # both 'LiCoO2' and "FeF3" are using Li+ as working ion; MnO2 is for the multivalent Mg2+ ion
+        formulas = ['LiCoO2', "FeF3", "MnO2"]
         expected_properties = {}
         expected_properties['LiCoO2'] = {'average_voltage': 2.26940307125,
                                          'capacity_grav': 903.19752911225669,
@@ -48,7 +49,11 @@ def test_init(self):
                                        'capacity_vol': 2132.2069115142394,
                                        'specific_energy': 1841.8103016131706,
                                        'energy_density': 6528.38954147}
-
+        expected_properties['MnO2'] = {'average_voltage': 1.7127027687901726,
+                                       'capacity_grav': 790.9142070034802,
+                                       'capacity_vol': 3543.202003526853,
+                                       'specific_energy': 1354.6009522103434,
+                                       'energy_density': 6068.451881823329}
         for f in formulas:
 
             with open(os.path.join(test_dir, f + "_batt.json"), 'r') as fid:
@@ -58,9 +63,12 @@ def test_init(self):
 
             # with open(os.path.join(test_dir, f + "_batt.json"), 'w') as fid:
             #json.dump(entries, fid, cls=MontyEncoder)
-
+            if f in ['LiCoO2', "FeF3"]:
+                working_ion = "Li"
+            elif f in ["MnO2"]:
+                working_ion = "Mg"
             c = ConversionElectrode.from_composition_and_entries(
-                Composition(f), entries)
+                Composition(f), entries,working_ion_symbol=working_ion)
             self.assertEqual(len(c.get_sub_electrodes(True)), c.num_steps)
             self.assertEqual(len(c.get_sub_electrodes(False)),
                              sum(range(1, c.num_steps + 1)))