Appendix For Setup Pourbaix Diagrams

A.1Pourbaix diagrams and MALT/CHD
At present, the thermodynamic database MALT does not contain any aqueous
species in its main database. Even so, the thermodynamic data for someaqueous species are available at 298.15 K and those data can be utilized inconstructing the Pourbaix diagram. On the other hand, the present CHD for
windows can provide a basis of constructing such Pourbaix diagrams, although
this has been developed originally for constructing high temperature chemical
potential diagrams.
Here, a method of constructing Pourbaix diagrams will be explained by using
the user’s data file, “FeSAqueous.txt” which store the thermodynamic data for
aqueous species in the Fe-S-H-O system. This appendix is based on thefollowing paper:
H. Yokokawa, N. Sakai, T. Kawada, and M. Dokiya, “GeneralizedElectrochemical Potential Diagrams for Complex Aqueous (M-X-H-O-e-)
Systems,” J. Electrochem. Soc. 137(2), 388 (1990).
Another example is the Li-Mn-O-H system which appears as the importantsystem for the positive electrode for lithium batteries. This will be based on
the user’s data file, “LiMnOHAqueous.txt.”
User’s DataBase: FeSAqueous.udb
In this appendix, the compounds and aqueous species listed in Table A. 1 areused in constructing Pourbaix diagrams. The condensed compounds are thoseretrieved from the thermodynamic database MALT. Data source “udb” means
the FeSAqous.udb which are compiled from the user data file
“FeSAqueous.txt.” The data source of the aqueous species are the NBS
thermodynamic properties published in 1982 which are the main parts of the
MALT database so that the consistency of the data in the FeSAqueous.udb
with the Main Database is obvious.

- 96 -

In the NBS tables, some data are lack in the absence of the experimental
data. For the construction of chemical potential diagram, only the Gibbs
energy data are used. On the other hand, the Gibbs energy in MALT
Direct is calculated from the enthalpy data and the entropy data. Whenenthalpy and entropy are lack but Gibbs energy is available in the NBS
table, the NBS data could not be used directly. In such a case, user data
are treated as follows as a convenient method although this has no
physical meaning, that is, the enthalpy value is assumed to be the same as
the Gibbs energy and the entropy is calculated to reproduce the Gibbs
energy value from thus assumed enthalpy.
In addition, the heat capacity of all aqueous species is assumed to be zero.
Apparently, this is not correct.
Thus, please note that the present data set of the aqueous species are notvalid at temperatures other than 298.15 K.
Table A.1 Gibbs energy change for formation of compounds and theirdata sources
Compoundstate∆fGº/RT
sourceH2S
g-13.53MALT
SO2
g-121.14
MALT
Fe2+
ao
-31.84
udb
3+
Fe
ao
-1.90
udb
HFeO2ao-176.82
udb
S2ao
34.63
udb
SO42ao-300.49
udb
HSao
4.87
udb
HSO4ao-305.11
udb
FeSO4ao-311.91udb
H2S
ao-11.24udb
H2O
l-95.72MALT
H2O2
l-48.69MALT
H2SO4*6.5H2O
l-922.66
MALT
Fe
c
0.00
MALT
Fe2O3
c
-299.55
MALT
Fe3O4
c
-409.85
MALT
FeS
c-40.52MALT
FeS2
c-67.36MALT
FeSO4
c-331.31MALT
Fe2(SO4)3c-909.78
MALT
FeSO4*7H2O
c-1012.99
MALT- 97 -

What are Pourbaix Diagrams
Usually, the Pourbaix diagrams can be used for different types of diagrams.
One is the predominance area diagram, in which the dominant aqueous species
will be given as a function of pH and pE, where pH means –log a(H+) and pE isthe corresponding variable for electrical potential. Resulting area of eachaqueous species can be expressed in terms of convex polygons.
The second, important diagram is the normal diagram in which the equilibria
between the aqueous species and the condensed phases are shown as chemical
potential diagrams.
Here, these two types of diagrams will be reproduced using the CHD forWindows.
Fe-O-H-e- system
In the MALT for Windows, the thermodynamic data should be retrieved in thefollowing manner.
1) User data file, FeSAqueous.txt, should be selected and compiled (menuTool/User data/compile user data text file).
2) In the menu of Tool/MALT options, check should be made on the option
for including aqueous species in the options in searching.
3) In the menu of “Get compounds by elements”, “Compounds consisting of
combinations of elements” should be selected as the mode of searchingcompounds. When the element sets of O,H, Fe will be selected, thecompounds and aqueous species in the Fe-O-H-e- system will be retrieved.
In CHD for Windows, these thermodynamic data will be transferred fromMALT by means of MALT Direct.
Note again here that the present thermodynamic data for aqueous species
stored in the user database: FeSAqeous.udb does not include the hightemperature heat capacity data. Calculation at temperatures other than298.15 K leads to serious errors so that this should never be attempted.

- 98 -

Fig. A1.1 Selection of Species to be included in the chemical potential
diagram.

Chemical System
The first diagram to be constructed is the Pourbaix diagram for the Fe-O-H-esystem. Species to be included are determined as followed:
1) Menu Chemical System is selected:
Selection of compounds is made by “selection by Default” with checking
on the “Condensed phases” and “Aqueous species.”
2) Then, “Reselection” button is clicked.
3) Among the listed species, the following species are excluded.
O2(a), O3(a), H2O2(a)Fig. A1.1 shows the resulting form for selecting compounds. The focused
species and compounds are to be included in the diagram construction.

- 99 -

Diagram Specification
Dimension Selection :No further specification is needed; that is, 298.15 K is selected.
Fixation
Since the Pourbaix diagram is constructed under the condition of a(H2O) =1. For this purpose, the following is made:1) Select the species, H2O a, in the list of species and compounds.
2) Select the coefficient of 1.
Diagram Selection
Here, the three dimension surface diagram is selected as diagram type.
As Coordinates, the followings are listed:(D:O)(D:Fe)(D:pE)
Here, (D:pE) is the coordinate corresponding to the electric charge. In thepresent treatment of CHD, the charge is treated as pseudo element. As a result,working variable for the electrical potential is defined as follows,
pE = φ / 2.303 kT(A1.1)

where φ is the electrical potential. This is well related with the pH defined as
pH = -µ(H+) / 2.303 kT = - log a(H+)(A1.2)

To modify the coordinates appropriate for the Pourbaix diagram,
1) pH is selected as X axis. For this purpose, first focus is moved on the firstline of (D:O) and double click on this line. A species, H+ (a), is first
selected and the coefficient of –1 is selected in the Coefficient Fom.
2) pE is selected as Y axis. For this purpose, focus is moved on the (D:pE)
and then click the upward arrow to move the (D:pE) to the second line.
3) The coordinate (D:Fe) is not changed.
Equilibria between condensed phases and aqueous species I generalized diagram- 100 -

Fig. A1.2 The generalized electrochemical potential diagram for the
Fe-O-H-e- system at 298.15 K under the condition of a(H2O) = 1.

Fig. A1.2 shows the generalized electrochemical potential diagram for the
Fe-O-H-e- system at 298.15 K under the condition of a(H2O) = 1. The
important point as the generalized diagram is that the chemical potential or theactivity of Fe is explicitly expressed in the diagram. In this diagram, thecondensed phase and the aqueous species are respectively shown as stable
polygon.
There are several points to be noted here:
1) Aqueous species are presented as the condition that their activity is equal tounity.
2) On the other hand, those species or compounds which do not contain theiron component form the frame within which the iron compounds can bedisplayed as stable compounds; namely, H+, H2O2(l), HO2-, and OH- form
such a frame as shown in Fig. A1.2.
Equilibria between condensed phases and aqueous species II Pourbaix diagram
Fig. A1. 3(a) and 3(b) are the diagram to reproduce the Pourbaix diagram.
Fig. A3(a) is the two dimensional chemical potential diagram which isconstructed under the same calculation conditions as Fig. A1.2. When
compared with the normal Pourbaix diagram, the regions beyond pH=0 or 14
are lacked. This is because the H+ and OH- species are explicitly included as- 101 -

Fig. A1.3 The generalized electrochemical potential diagram for the
Fe-O-H-e- system at 298.15 K under the condition of a(H2O) = 1. (a)with and (b) without H+, HO2-, and OH- species.

target compounds to be included as stable species and their activity is definedas unity.
To reproduce the Pourbaix diagram which shows the equilibria beyond theselimits, the following can be made.
1) In the menu of “Chemical System / Selection of compounds,”“ReSelection” button is clicked and the species, H+, HO2- and OH-, areexcluded.
- 102 -

2) Other conditions are the same as before.
Resulting diagram is shown in Fig. A1. 3(b). The species, Fe3+, appears as
stable species.
Predominant area diagram
Pourbaix also proposed to use the predominant area diagram which indicate
what aqueous species is predominant in the given thermodynamic conditionsin terms of pH and pE.
To reproduce the predominant area diagram, the following setting can be
made:1) In the menu of “Chemical System / Selection of Compounds, the default
setting is made only on the aqueous species.
2) Then, “ReSelection” button is clicked and those species which do notcontain the iron component will be excluded; namely, O2, O3, H+, HO2-,
OH-, H2O2 will be excluded.
Fig. A1. 4 shows the predominant area diagram in which only the ironcontaining aqueous species are presented as predominant species.

Fig. A1.4 The predominant area diagram for the Fe-containing species at
298.15 K.

- 103 -

S-O-H-e- System
Chemical System:
The same user database is used to retrieve the thermodynamic data for the
S-O-H-e- system.
The Pourbaix diagram and the predominant area diagram for the S-O-H-esystem can be constructed in the same procedures as those in the Fe-O-H-e-Fig. A1.5 The Pourbaix diagram for the S-O-H-e- system at 298.15 K.

Fig. A1.6 The predominant area diagram for the S-containing species at
298.15 K.

- 104 -

system.
Fig. A1.5 shows the two dimensional chemical potential diagram for the
S-O-H-e- system. As sulfur compounds, only S appears as stable compounds.
Fig. A1. 6 shows the predominant area diagram for the sulfur aqueous species.
This is very similar to Fig. A1. 5 where except S, aqueous species are the stableones.
Fe-S-O-H-e- System
The most powerful point of the present CHD program is to construct the
Pourbaix diagram for the multicomponent system without any difficulty. As
described above, we do not need to have any other algorithms for constructing
the Pourbaix diagram or the predominant area diagram. What is needed inconstructing such diagrams is only the following:
1) Select the species to be included in construction of diagram.
2) Specify the condition that a(H2O) = 1.
3) Select the coordinates appropriate for the Pourbaix diagrams. Particularly,
pH and pE should be correctly selected.
The same procedures will be adopted below to construct the Pourbaix diagram
for the Fe-S-O-H-e- multicomponent system.
Chemical System
The thermodynamic data for the Fe-S-O-H-e- system are retrieved from the
MALT main database and the user database: FeSAqueous.udb.
Three Dimensional Chemical Potential Diagram
Selection of Compounds
Condensed phases and also aqueous species are included, although species in
the O-H-e- system are excluded to eliminate the limits derived from H+ etc.
Fixation
Only the activity of water is fixed at unity.
Diagram Coordinates-log a(H+) (=pH)- 105 -

pE
log a(Fe) – log a(S)(D:Fe)
After execute the command Run, the range limits are modified as follows:-log a(H+)
pE
log a(Fe)-log a(S)
min
-2
-40
-100
max
16
40
150
step
2
10
50
factor
1
0.25
0.125
Fig. A1.7 shows the three dimensional Pourbaix diagram for the Fe-S-O-H-esystem ate 298.15 K under a(H2O)=1. As the third axis, we select the valuableof log a(Fe) – log a(S). The type of axis variable is convenient for treating two
metals in equivalent manner. Here, we are treating two elements, iron andsulfur.
In Fig. A1.7, the phase equilibrium information for the Fe-O-H-e- subsystemappears in front of the diagram. On the right hand side, the dissection at pH=16
appears as a function of log a(Fe)-log a(S).
Conventional diagram for the Fe-S-H2O system
Traditionally, the Pourbaix diagram for the Fe-S-H2O system has been
constructed under the assumption that the redox element of the target diagram
is Fe so that only species which contain the iron component will appears in the
diagram.
To reproduce such a diagram in CHD, the following should be made:1) the third axis is changed to – log a(Fe) + log a(S). This is to present thesulfur containing compounds before the iron containing compounds which
do not include sulfur.
2) In the menu of rotation of 3D diagram, the conversion matrix is changed to
as follows;
X-axisY-axisZ-axisHorizontal
1.0
0.0
0.0
Vertical
0.0
1.0
0.0
Depth
0.0
0.0
1.0

- 106 -

Fig. A1.7 The Three Dimensional Pourbaix diagram for the Fe-S-O-H-esystem at 298.15 K.

Fig. A1.8 The Three Dimensional Pourbaix Diagram projected on the
pH-pE plane.

3) In the menu of Graph Inspector / “Compounds” page, the transparency of- 107 -

the displayed compounds is changed in the following compounds:
H2O2 (l)S_Rhombohedral (cs)
H2S (a)S-2(a)
SO4- (a)HS-(a)
HSO4- (a)Fig. A1. 8 shows the three dimensional diagram which is rotated so as to givethe normal two dimensional Pourbaix diagram. There are several blue lines
which are not related with the stable polygons of the species. These areborderlines for species which are converted to be transparent.
The phase equilibria become complicated so that it is not easy to distinguishthe respective stable area. For example, in Fig. A1. 8, it is not easy where thestability region of Fe3O4. To make clear the relative relations among thestability areas, the option of zoom can be used. Fig. A1.9 shows the enlarged
diagram; here, the stability region can be well recognized in relations to otherstability areas.

Fig. A1.9 The Three Dimensional Pourbaix Diagram projected on the
pH-pE plane. This is enlarged diagram to show clearly the stability areaof Fe3O4.

- 108 -Dissections
There are many ways to specify another fixation to obtain the two dimensional
diagram or the three dimensional surface diagram. One of the most attractive
ways is to make a dissection on the three dimensional diagram.
Fig. 1A.10 shows the dissection diagram which was constructed on the basis of
the previously constructed three dimensional diagram (Fig.A1. 8,9).
The menu of Run / Dissection is selected. The dissection plane is defined by
selecting the variable of pH, that is, H+ is selected and the coefficient of –1 isselected.
“Dissection Swing” should be checked.
“Profile Diagram” should be also checked. The diagram type is specified as
dissected diagram(2D). The coordinates are selected as follows;

Fig. A1.10 The dissection diagram at pH=7 for the three Dimensional
Pourbaix Diagram at 298.at K.

- 109 -

1) The axis variable, –log a(Fe) + log a(S), is moved to the x-axis.
2) The axis variable, pE, is moved to the y-axis.
3) As a result, the working variable pH is moved to the z-axis that will not be
used in 2D dissection diagram.
4) When the Selection of Swing Values appears, the range limits are changed
by right-hand-mouse click; the new values should be changed from –2 to16.
Fig. A1. 10 shows the dissection at pH=7. In the left hand side, the phaserelations in the Fe-O-H-e- system are presented, whereas the phase relations in
the corresponding sulfur system are shown in the right hand side. In between,
those species which contain both iron and sulfur appear.
Relation among diagrams for dry, hydrated and wet systems
Since the chemical potential diagram for the Fe-S-O system and theelectrochemical potential diagram for the Fe-S-O-H-e- system can beconstructed by the same algorithm and the same program, it is very easy tocompare those diagrams under the same conditions. One interesting
comparison from the chemical point of view is the construction of diagramswith the same coordinates for the dry, the hydrated and the wet systems. By
hydrated system, the diagram is constructed for the Fe-S-O-H system without
any aqueous species. Even so, the activity of water is fixed at unity in thesystem, some hydrated compounds can be formed. By wet system, the diagram
can contain the aqueous species in addition to the compounds appearing in thehydrated system.
Fig.A1.11 compares three diagrams for the Fe-S-O (-H-e-) systems. As wet
systems, pH=7 is adopted.
In the hydrated system, FeSO4 disappears and instead, FeSO4*7H2O appears.
In addition, H2SO4*H2O appear in the upper-right corner.
In the aqueous system, the aqueous species, SO4(a), appears instead of FeSO4,
FeSO4*7H2O or H2SO4+H2O. In addition, the aqueous species, HS-, appears
in the left hand side. This diagram should be compared with the Pourbaix-type
dissection diagram in Fig. A2.10.

- 110 -

Fig. A1.11 The chemical potential diagrams for the Fe-S-O system: (a) wet
system, (b) hydrated system (c) dry system- 111 -

A2 Electrochemical potential diagrams for Li-Mn-O-H-esystem
The present electrochemical potential diagram can be applied to many
materials issues associated with the electrochemical devices. One of suchapplications is the lithium batteries.
Here, an attempt will be made to construct the electrochemical potential
diagram for the Li-Mn-O-H-e- system. Although the electrolyte for lithium
battery does not contain water in it, it will be interesting to know the phaseequilibria associated with the Li-Mn-O-H system because it has been alreadypointed out that the presence of a small amount of acidic water that is causedby the oxidation of the electrolyte may enhance the manganese dissolution.
Chemical System
The thermodynamic data for the Li-Mn-O-H-e- system can be completed when
the user database, LiMnOHAqueous.txt is used.
Pourbaix diagram for the Mn-O-H-e- system
The thermodynamic data for the Mn-O-H-e- system are retrieved as follows:
1) user database “LiMnOHAqueous.udb” is loaded in the menu “Options /User data /load user database.”
2) In the menu “Tools / Customize MALT Setting ,” check should be madefor the “including Aqueous species” in the Options for Search in“General” page.
3) In the menu “Search Compounds,” “Get compounds” with “Search by
Elements” is selected. In the form of “Search Compounds by Elements,”
the mode of “Compounds consisting of combination of the elements” ischecked and then “Mn,” “O,” and “H” are selected.
4) After preparing the thermodynamic data in MALT with above procedures ,the “MALT Direct” is used to transfer the data to CHD.
In a similar manner described in Appendix 1, the two dimensional Pourbaix
diagram for the Mn-O-H-e- system can be constructed as in Fig. A2.1.
5) In the chemical system, the selection of compounds is made by first
selecting the “Select by Default” with check on the condensed phases andthe aqueous species and then by clicking “ReSelction” button to remove
the species of O2(a), O3(a), H+(a), OH-(a), HO2-(a), H2O2(a).
- 112 -

Fig. A2.1 The Two Dimensional Pourbaix Diagram for the Mn-O-H-esystem at 298.15 K.

6) In the menu “Project / Specify Diagram,” the fixation is made to select thewater, H2O(a), with the fixed value at unity and the diagram type of 2D
chemical potential diagram with the coordinates of -log a(H+) as the x axis
and the (D:pE) as the y axis and (D:Mn) as the z-axis.
7) Since the resulting diagram has a display range wider than the normal, thedisplay range of diagram should be changed by selecting the menu
“Diagram / Range.” The proper range for the x- and the y-axes are thefollowings-log a(H+)
pE
min -2-40
max 1640
step 2
10
factor1
0.25
The chemical feature of the Pourbaix diagram for Mn is that the aqueous
species Mn2+ as well as MnO4- and MnO4-2 are present.

Pourbaix Diagram for the Li-O-H-e- system- 113 -

In the same manner, the Pourbaix diagram for the Li-O-H-e- system can beconstructed as given in Fig. A2.2. In this system, only Li+ species apears as theFig. A2.2 The Two Dimensional Pourbaix Diagram for the Li-O-H-esystem at 298.15 K.

stable species. However, the stability region of Li+ is quite wide comparedwith the corresponding species in the Mn-O-H system.
Three dimensional Pourbaix diagram for the Li-Mn-O-H e- system
The three dimensional Pourbaix diagram can be set up by the following
procedures:
1) Data retrieval for the Li-Mn-O-H-e- system from the MALT main database
and the user database: LiMnOHAqueous.udb.
2) Remove of aqueous species which do not include Li or Mn.
3) Fixation of water, H2O, at the activity of unity
4) The diagram type of 3D chemical potential diagram
5) Change of the coordinates as follows:
X axis : -log a(H+) (=pH)
Y-axis : pE

- 114 -

Z-axis : log a(Mn) – log a(Li)6) Diagram Range is defined as-log a(H+)
pE
log a(Mn)-log a(Li)
min
-2
-40
-90
max
16
40
60
step
2
10
10
factor
1
0.25
0.125
Fig. A2.3 shows the resulting diagram. Phase equilibria in the Mn-O-H-esubsystem shown in Fig. A2.1 appears in the front plane in Fig. A2.3, whereas
in the right hand side plane, the phase equilibria at pH=16 appear.
Since this is the three dimensional diagram so that some additional procedures
are needed to know more detailed information of the target chemical equilibria.
The followings are the possible procedures
1) To make some compounds transparent.

Fig. A2.3 The Three Dimensional Pourbaix Diagram for the
Li-Mn-O-H-e- system at 298.15 K.

- 115 -

Fig. A2.4 The Diagram Specification / One Dissection for the three
Dimensional Pourbaix Diagram for the Li-Mn-O-H-e- system at 298.15 K.

2) To make one dissection and to swing the dissection value.
3) To make a series of dissections
One dissection at log a(Li) = -60
Here, an attempt is made to make a dissection by fixing the lithium activity.
The menu “Dissection” is selected. Thus, the dialog for the Diagram
Specification / One Dissection page will appear as Fig. A2.4. One dissection
plane is selected by clicking the Add button and the Li(cs) is selected among
the Compound candidate. At this time, the fixed value is input as –60.
When profile diagram is checked, the diagram can be selected between thedissected diagram (2D)or the dissected diagram (3D). Here, the dissected
diagram (2D) is selected. In Fig. A2.4, the X-axis for the 2D dissection
diagram is defined as –log a(H+), the Y-axis being log a(Mn) – log a(Li).
A resulting dissection diagram is shown in Fig. A2.5. In this diagram, the- 116 -

display form for the dissection diagram is docked in the right-hand side
docking panel.
The CHD Main form has the two docking panels; that is, the right-handpanel and the bottom-side panel. When the top frame of the display formis dragged by putting the control key and left click of mouse, the positionof cursor appearing with the dragging square frame can be adjusted
between the Right-hand frame of the Main Form and the scroll bar of theMain Display Panel. Then, the Dissection Display Form will be docked tothe panel as shown in Fig. A2.5. The relative size of the main display
panel and the additional display panel can be adjusted by manipulating
the position of splitting bar adjacent to the vertical scroll bar of the main
display panel.

Fig. A2.5 The Three Dimensional Pourbaix Diagram for the Li-Mn-O-H-esystem at 298.15 K with the dissected diagram (2D) at log a(Li)=-60. In
the upper right corner, the Form for Selection of Swing values appears.
The dissected value can be changed by manipulating the scroll bar.

- 117 -

Fig. A2.6 The Three Dimensional Pourbaix Diagram for the Li-Mn-O-H-esystem at 298.15 K with the dissected diagram (2D) at -log a(H+)=6.3.

When the Dissection Swing option is checked, the small form for selecting theswing value appears as in Fig. A2.5. Whenever the dissection value ischanged, a new diagram is constructed and updated in the same display panel.
Fig. A2. 6 shows the dissection diagram for another dissection. Here, the pH,that is, -loga(H+), is defined as a dissection at 6.3. As the X-axis, thelogarithmic activity difference, log a(Mn)-log a(Li), is adopted. The equilibria
in the manganese system appear in the right-hand side, whereas those in thelithium system are given in the left-hand side.
In the chemical potential diagrams shown in Figs. A2.5 and 6, the chemical
stability of LiMn2O4 against the dissolution into aqueous solutions can be
discussed by using the following considerations.
First, the electrode reaction of LiMn2O4 can be written as follows;- 118 -

LiMn2O4 = Li+(a) + Mn2O4 + e-

(A2.1)The right-hand side direction is the charging process and the left-hand side
direction is the discharging process. In Fig. A2.5, the lithium activity is fixedat log a(Li)=-60, and the equilibria between LiMn2O4 and Li+(a) is shown.
Since E/V = -(2.303 RT / F) log a(Li), log a(Li)=-60 corresponds to E=3.55
VThis indicates that this lithium activity corresponds to the discharging process
where Li+(a) is inserted into Mn2O4 phase. In Fig. A2.6, the three phasecombination of Li+(a) / LiMn2O4/Mn2O4 corresponds well the above electrodereaction.
In addition to the normal electrode reaction, there is some possibility that side
reactions can occur during the charge/discharge processes. The manganesedissolution is one of the most important side reactions. It has been pointed outthat the manganese dissolution is enhanced by the acidic environments caused
from the oxidation of electrolyte component. Since LiMn2O4 is dissolved into
Li+ and Mn2+, this reaction can be written as follows:
LiMn2O4 + 8H+ + 3e- = Li+(a) + 2Mn2+(a) + 4 H2O(a)(A2.2)In Fig.A2.5, the stability regions of Li+ and Mn2+ appear in the low values of
pH=–log a(H+), whereas LiMn2O4 appears in the high pH region. This wellcorresponds to the fact that LiMn2O4 is dissolved under the acidic condition.
This is the reaction that consumes the electron so that this can be regarded asdischarge-process-enhancing reaction. Actually, during the discharge process,the lithium activity increases (the electrode potential value decreases); In Fig.
A2.5, the dissection value can be changed from –60 to –58. This corresponds
to a change in lithium activity on discharge. On this change in the lithium
activity, the stability area of LiMn2O4 becomes small. In other wards, thethree-phase point of Li+/LiMn2O4/Mn2+ shifts to the more basic side; that is,
the manganese dissolution is enhanced.

- 119 -

Fig. A2.7 The Three Dimensional Pourbaix Diagram for the Li-Mn-O-H-esystem at 298.15 K in the normal high temperature coordinates for the
Li-Mn-O systems.

In Fig. A2. 6, the interesting phase equilibria are plotted. Here, three-phase
point of Li+/LiMn2O4/Mn2+ is represented as the vertex point of their stability
polygons. There is however, a new feature appearing in this diagram. That is,
the spinel phase, Mn3O4, becomes very close to that three-phase point. This
thermodynamically implies that the dissolved manganese ions, Mn2+, can be
precipitated as the spinel phase. Since the LiMn2O4 itself has the spinel
structure, it is suggested that there is no kinetic barrier for Mn3O4 to beprecipitated.
3Mn2+ + 4H2O = Mn3O4 + 8H+ + 2e-(A2.3)

By combining with (A2.1), the following reaction can be written:
3LiMn2O4 + 8H+ + 5e-= 3Li+ + 2Mn3O4 + 4H2O

(A2.4)

By comparing Eqa. (A2.4) and (A2.1), the following is obvious:- 120 -

The

manganese ions remain without change in the ionic configuration after thelithium ion is removed in (A2.1), whereas the manganese ions in (A2.4) can be
regarded as reconstructed from aqueous solution; in such a case, there are nolithium sites in the new spinel precipitate.
Thus, these diagrams can give the reasonable explanation for the following
facts in the lithium batteries.
1) Manganese dissolution is enhanced in the acidic situation after theoxidation of electrolyte component.
2) Manganese dissolution becomes more severe on discharge process.
3) Even so, the manganese dissolution tends to be mild on heavily discharge
state.
4) The capacity loss is also associated with the manganese dissolution. The
loss of the lithium site due to the occupation of manganese ions should be
the reason for the capacity fading.

Relation among the Pourbaix diagram for the multi-component system and the hightemperature chemical potential diagram.
Since the Li-Mn-O system is one part of the subsystems of the Li-Mn-O-H-esystem, it can be possible to compare the phase relation appears in Fig. A2. 5
and 6 with those chemical potential diagrams proposed for high temperatureuse.
The main chemical potential diagram in Fig. A2.7shows the three dimensional
diagram for Li-Mn-O-H-e- system at a(H2O)=1.0. Although the aqueous
species are shown in this diagram, they are not well shown because the axis of
pH now disappears. In other wards, the phase equilibria developing along the
pH axis are degenerated into the non-pH axis. Actually, there should be many
species at one point in the three chemical potential diagram.
To display the diagram in Fig.A2.7, the species of MnO4- and MnO42- arechanged to be transparent.
The comparison is made in Fig. A2. 7 where the same phase equilibria as inFig. A2. 6 are plotted in the normal three dimensional diagram. In thisdiagram, the stable polygons for Li+ and Mn2+ appear in addition to the
non-aqueous species. This clearly shows how the aqueous species affect on
the normal phase equilibria.

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