2.5 Electrochemical Tests
29
them to those found for single-phase samples. In the proposed phase diagram, tielines fan out from Li 2 MnO 3 up to an angle of about α = 70
◦ . This implies that up
to this angle, the value of the layered lattice parameters in the two-phase samples
should be equal to those of Li 2 MnO 3 . Figure 2.7 (b) illustrates the expected plot
of the c lattice parameter as a function of α based on the proposed tie-lines. Since
c = 14.23 Å for Li 2 MnO 3 when fitted as a hexagonal structure, as shown in Chap. 6,
such a graph would confirm that the tie-lines fan-out from Li 2 MnO 3 . Plots of this
type were extremely useful in demonstrating the directions of tie-lines in the Li–Mn–
Ni–O system and were used throughout this thesis. In three-phase regions, there is
no variation in the phases present; only the amount of each phase varies. As such, the
fitted lattice parameters should remain constant throughout the three-phase region.
In order to work out the boundaries of the single-phase regions, the lever rule was
used. Figure 2.7 (c) illustrates the use of the lever rule for sample X lying on a tie-line
between phases A and B. Extrapolating to where the integrated peak intensity of the
phase A peaks would be zero yields point B. If the proposed phase diagram is correct,
point B would coincide with Li 2 MnO 3 . In practice, the distance from A to B was
calculated using the mathematical form of the lever rule which states that the fraction
of phase A is equal to d(X,B)/d(A,B). This allows for the calculation of the position
of point B, which can then be compared to the position of Li 2 MnO 3 . This same
method was used to determine the positions of the corners of three-phase regions
as will be demonstrated in Chap. 6. It should be noted that microabsorption effects
were not taken into account in the calculations. This is justified for the combinatorial
samples with small particle sizes, as will be demonstrated in Sect. 5.4. Therefore,
even without Rietveld refinement, the XRD patterns can be used to identify the
boundaries of single-phase regions, the direction of tie-lines in two-phase regions
and the extent of three-phase regions. The large number of scans in the single-phase
regions also provided lattice parameters over wide composition ranges which were
expressed as contour plots in Chaps. 5, 6 and 7. These contour plots should be of
considerable use to researchers working in these systems.
2.5 Electrochemical Tests
Coin cells were made from a few bulk samples in order to determine the electrochemical performance of materials of interest. Figure 2.8 shows the parts involved
in assembling a standard coin cell. Electrochemical tests were carried out using a
standard 2325 coin cell with the positive electrode being cycled against a lithium
counter electrode (Chemetall Foote Corp.). The electrolyte used was 1 M LiPF 6 in
1:2 ethylene carbonate:diethyl carbonate (Novolyte Corp., now part of BASF). The
positive electrodes contained 5 % polyvinylidene-fluoride binder (Kynar 301P, Elf
Atochem) and 5 % carbon black (TIMCAL). Two identical cells were typically made.
The details of the coin cell design are included in Ref. [63]. The cycling conditions
varied from experiment to experiment and will be stated explicitly in the results
sections.
29
them to those found for single-phase samples. In the proposed phase diagram, tielines fan out from Li 2 MnO 3 up to an angle of about α = 70
◦ . This implies that up
to this angle, the value of the layered lattice parameters in the two-phase samples
should be equal to those of Li 2 MnO 3 . Figure 2.7 (b) illustrates the expected plot
of the c lattice parameter as a function of α based on the proposed tie-lines. Since
c = 14.23 Å for Li 2 MnO 3 when fitted as a hexagonal structure, as shown in Chap. 6,
such a graph would confirm that the tie-lines fan-out from Li 2 MnO 3 . Plots of this
type were extremely useful in demonstrating the directions of tie-lines in the Li–Mn–
Ni–O system and were used throughout this thesis. In three-phase regions, there is
no variation in the phases present; only the amount of each phase varies. As such, the
fitted lattice parameters should remain constant throughout the three-phase region.
In order to work out the boundaries of the single-phase regions, the lever rule was
used. Figure 2.7 (c) illustrates the use of the lever rule for sample X lying on a tie-line
between phases A and B. Extrapolating to where the integrated peak intensity of the
phase A peaks would be zero yields point B. If the proposed phase diagram is correct,
point B would coincide with Li 2 MnO 3 . In practice, the distance from A to B was
calculated using the mathematical form of the lever rule which states that the fraction
of phase A is equal to d(X,B)/d(A,B). This allows for the calculation of the position
of point B, which can then be compared to the position of Li 2 MnO 3 . This same
method was used to determine the positions of the corners of three-phase regions
as will be demonstrated in Chap. 6. It should be noted that microabsorption effects
were not taken into account in the calculations. This is justified for the combinatorial
samples with small particle sizes, as will be demonstrated in Sect. 5.4. Therefore,
even without Rietveld refinement, the XRD patterns can be used to identify the
boundaries of single-phase regions, the direction of tie-lines in two-phase regions
and the extent of three-phase regions. The large number of scans in the single-phase
regions also provided lattice parameters over wide composition ranges which were
expressed as contour plots in Chaps. 5, 6 and 7. These contour plots should be of
considerable use to researchers working in these systems.
2.5 Electrochemical Tests
Coin cells were made from a few bulk samples in order to determine the electrochemical performance of materials of interest. Figure 2.8 shows the parts involved
in assembling a standard coin cell. Electrochemical tests were carried out using a
standard 2325 coin cell with the positive electrode being cycled against a lithium
counter electrode (Chemetall Foote Corp.). The electrolyte used was 1 M LiPF 6 in
1:2 ethylene carbonate:diethyl carbonate (Novolyte Corp., now part of BASF). The
positive electrodes contained 5 % polyvinylidene-fluoride binder (Kynar 301P, Elf
Atochem) and 5 % carbon black (TIMCAL). Two identical cells were typically made.
The details of the coin cell design are included in Ref. [63]. The cycling conditions
varied from experiment to experiment and will be stated explicitly in the results
sections.