Synthesis of Rock-Salt Type Lithium Borohydride and Its Peculiar Li+ Ion
Conduction Properties
Reona Miyazaki, Hideki Maekawa and Hitoshi Takamura
Supplemental material
Content
1. Identity of the secondary phase
2. Thermodynamic properties of KI - LiBH4 system
1
1.
Identity of the secondary phase
While the identity of the secondary phase observed in the SEM image of 3KI·LiBH4 remains uncertain, it is
possible that it is a LiBH4 - LiI solid solution.9, 10 Although the low atomic number of Li prevents it from being
detected by EDX analysis, the deficit in the K content suggests that the amount of Li in the secondary phase may
be substantial. Indeed, we can safely say that LiBH4 is enriched at the inter grain region. This assumption is also
supported by the high percentage of B fraction in the secondary phase. Furthermore, the considerable amount of I
detected suggests that a LiBH4 - LiI solid solution9, 10 may have formed during the sintering process.
Fig. S1 shows the SEM image for the fracture surface of sintered LiBH4·KI (50 mol% LiBH4). The results of
the EDX analysis are presented in Table S1. Point 001, which had almost the same K and I ratio, was confirmed
to be LiBH4 integrated in KI. The data from the EDX analysis clearly indicates that the B ratio was higher than
the blue point in Fig. 2(a): this can be explained by the enhancement of the formation of the rock-salt type solid
solution between KI and LiBH4 due to the increase in the amount of LiBH4 added. A secondary phase was also
observed for 3KI·LiBH4 (Point 002 for Fig. S1). The contrast in the visual provides information about the
distribution form of this phase. The appearance of LiI implies that an anion exchange reaction between KI and
LiBH4 occurred simultaneously with the formation of the rock-salt type solid solution. Another reaction product,
KBH4, which was not found in 3KI·LiBH4 was identified for LiBH4·KI due to the excess amount of K ratio at
Point 003 in Fig. S1. This excess amount of K is likely to be the result of the KBH 4 inclusion in the KI - LiBH4
rock-salt type solid solution.
2
003
001
002
×2000
10μm
I
Fig. S1. SEM image for the fracture surface of sintered LiBH4·KI (50 mol% LiBH4)
Table S1. The results of EDX analysis for three different measuring point in Fig. S1. Each composition is
described in units of at%.
B
Point001
Point002
Point003
2.
O
K
30
71
9
2
7
0
I
36
1
52
33
21
39
Thermodynamic properties of KI - LiBH4 system
The thermodynamic properties were investigated by DSC measurement (Exstar6100 SII. Co). A swaged
stainless steel capsule containing ~ 10 mg powder sample was measured with an empty capsule used as a
reference. The heating/cooling cycle (between 30˚C and 230˚C) was repeated twice at the rate of 1 K/min. Two
different phase transitions were observed in each distinct temperature range. Fig. S2(a) shows the DSC curves for
the obtained samples together with the XRD patterns of 3LiBH4·KI (75 mol% LiBH4) at 180˚C (Fig. S2(b)). In
the case of 3LiBH4·KI, one clear endothermic peak was observed around 160˚C, and is represented by the dark
yellow line. The lack of diffraction peaks above this transition point indicates that the sample had melted. The
endothermic peaks, which indicate the melting temperature of each sample, were lower with increasing amounts
of LiBH4 addition, as can be seen in Fig. S2(a). This variation in thermodynamic properties is explicit evidence of
the solid solution formation between KI and LiBH4. The gradual decrease in the heat of fusion (the surface area of
the DSC peaks) for LiBH4 poor samples indicates partial melting at the liquid/solid coexistence region, in
3
accordance with the phase rule. The large difference between the melting temperature between the obtained solid
solution and pure KI (~ 680˚C) suggests a significant change in the liquidus line in the low LiBH 4 content region
for the phase diagram of the KI and LiBH4 pseudo binary system.
The thermodynamic properties at lower temperature are indicated in Fig. S3. As can be seen in Fig. S3(a), two
additional phase transitions which would not have been observed if the samples were a single rock-salt phase
were identified at lower temperatures. As mentioned in the main text, SEM observations suggest the possibility of
a LiBH4 - LiI solid solution. It has been reported that the phase transition between H.T. and L.T. form was above
room temperature for the lightly LiI doped LiBH4 - LiI system.9, 10 Although the amount of LiI in the secondary
phase was not determined, it is reasonable to assume the secondary phase was the imperfectly stabilized H.T.
form of LiBH4. If this assumption is correct, these two endothermic peaks may be explained by the phase
transition behavior of the secondary phase. A comparison of the DSC curves of the second cycle with the previous
cycle in the same temperature range reveals that the endothermic peak around 90˚C disappeared at the second
cycle (Fig. S3(b)). This irreversible peak may be attributable to insufficient sintering. If the reaction was
incomplete, a certain amount of the L.T. form of LiBH4 would dissolve with KI and remain, as indicated by the
lever rule indicated by a black symbol in the estimated phase diagram of the KI - LiBH4 system (Fig. S4).
According to the phase rule, this unreacted phase should also exhibit transition behavior at a given temperature,
which would explain the existence of the other exothermic peak. Furthermore, the effect of heat treatment during
the DSC measurement can be expected to enhance the forming solid solution by eliminating the remaining phase,
which would lead to the disappearance of the DSC peak during the second cycle. Our results indicate that the
lattice parameter decreased along with sintering time, as indicated in Fig. S5, suggesting further progress in solid
solution formation due to the extra heat treatment.
4
0
(b)
Heat flow / Wmol-1
(a)
75 mol% LiBH4
(milling)
@180°C
-5
13 mol%
25 mol%
33 mol%
50 mol%
75 mol%
-10
120
140
160
180
200
220
Temperature / C
10
15
20
2deg.
25
30
Fig. S2. (a): DSC curves for each composition of KI - LiBH4 sample for the1st heating cycle. (b): XRD pattern for
3LiBH4·KI (75 mol% LiBH4) obtained by ball milling at 180˚C
(a)
(b)
Fig. S3. (a): DSC curves of 1st heating cycle between 40˚C and 120˚C for KI - LiBH4 samples. (b): DSC curves
for 2nd heating cycle for the same temperature range. Endothermic peaks observed around 90˚C during the 1st
cycle disappeared.
5
H.T.
H.P.
L.T.
L.T.
+ H.P.
KI
LiBH4
Fig. S4. Estimated phase diagram for KI - LiBH4 pseudo binary system. Certain degree of LiBH4 - KI (L. T. form)
is expected to exist followed by lever rule represented as black symbol on the dashed line.
As shown in Fig. S6, the conductivity behavior of LiBH4·KI (50 mol% LiBH4), whose main phase was
confirmed to be covered by the LiBH4 - LiI secondary phase (Fig. S1) differed from that of 3KI·LiBH4. The
results of DSC measurements are also supported by the conductivity measurement shown in Fig. S6. The
conductivity curve reflects the phase transition behavior discussed in the thermodynamic properties section, and
suggests that the secondary phase blocked the conduction pathway through the rock-salt type grains. It should be
noted that the core-shell like structure in the SEM image made it impossible to investigate the ion conductivity of
this rock-salt phase.
6
Fig. S5. Sintering time dependence of lattice parameter for 3KI·LiBH4. Decrease of a axis value means further
Electrical conductivity / Scm-1
enhancement of formation of solid solution between KI and LiBH4 along with sintering.
10-2
400
380
Temperature / K
360
340
320
300
10-3
10-4
10-5
10-6
10-7
10-8
2.4
2.6
2.8
3.0
1000 / T / K-1
3.2
3.4
Fig. S6. Temperature dependence of electrical conductivity for KI·LiBH4 (50 mol% LiBH4). Circle and triangle
symbol represent the result for heating and subsequent cooling cycle, respectively. Phase transition behavior is
clearly observed around 80˚C with certain degree of hysteresis.
7