Abstract
Thermodynamic and kinetic modification for LiBH4 and its composite with MgH2（LiBH4 および MgH2 との複合化材料の熱力学的および動力学的改良）

Pratibha PalThe present study was focused on to lower the decomposition temperature of LiBH4 and its
composite system 2LiBH4-MgH2 by tuning their thermodynamics and kinetics. According to
the issues associated with LiBH4 (hard thermodynamics) and 2LiBH4-MgH2 (sluggishkinetics), the thesis was structured with two objectives.
First objective was to focus on the 2LiBH4-MgH2 composite, where the thermodynamics ofLiBH4 was modified by the addition of MgH2 but this composite system possess serious kinetic
issues. In this work, KH was added as catalyst to the 2LiBH4-MgH2 system in order to enhance
the kinetics. A reduction in the temperature was observed for the first hydrogen desorption
reaction, but no effect could be seen in the second dehydrogenation. However, an interesting
phenomenon of eutectic melting was observed during the thermal heating of this system. By
addition of 5 mol% KH, the melting point was shifted to lower temperature, which wascontinuously decreased with increase in the amount of KH from 0 to 55 mol%. For 55 mol%
KH i.e. at eutectic composition 0.45(2LiBH4-MgH2)-0.55KH the lowest melting temperature
was observed as 79oC. Eutectic melting is an interesting phenomenon and happens if two or
more components sustain their liquid state below the melting points of their pure components.
In this work, kinetic enhancement was achieved due to the occurrence of eutectic melting onadding KH as additive. The enhanced ionic mobility must be the driving force to desorb
hydrogen at lower temperature in comparison of 2LiBH4-MgH2 composite, thus caused akinetic enhancement on addition of KH. Reduced melting point for some complex hydrogenstorage systems behaves as eutectic melts and can be utilized for refueling of vehicles. Apseudo-binary phase diagram between the 2LiBH4-MgH2 composite and KH was discussed
and was explained in terms of degree of freedom.
The second objective of the thesis was to modify the thermodynamics of pure LiBH4 by theaddition of Bi2X3 chalcogenides and study their decomposition properties. This work wasinitiated with a curiosity during an electrochemical experiment on Bi2X3 chalcogenides as
anode materials for all-solid-state Li-ion battery in a recent work by our group. LiBH4 was usedas solid electrolyte in the cell with Bi2Te3 nanostructures as anode material. An interesting
issue of cell opening during the charging – discharging motivated us to investigate the effectof Bi2X3 (X = S, Se and Te) addition on the decomposition properties of LiBH4. An efficientdestabilization of LiBH4 could be achieved by addition of chalcogenides (Bi2S3, Bi2Se3 andBi2Te3), which could tune the thermodynamic properties of LiBH4 decomposition by alloying
with Li. The hydrogen desorption temperature of LiBH4 was reduced to below 150 ℃ by theaddition of bulk Bi2X3. The detailed mechanism was established for the reaction of LiBH4 withthe Bi2Te3 (similar reaction steps were found for other two composites as well, i.e. LiBH450wt%Bi2Se3 and LiBH4-50wt%Bi2S3) using several techniques and was proposed as follows:
38LiBH4 + Bi2 𝑇𝑒3 → Li2 𝑇𝑒 + 25LiBH4 + 7𝐿𝑖𝐻 + 2Bi + 12𝐵 + 22.5H2→ 2Li3 Bi + 17LiBH4 + 2𝐿𝑖𝐻 + 8𝐵 + 15H2
→ 13LiH + 4Li + 17B + 27.5H2
A further reduction in the decomposition temperature was achieved using nanostructured
Bi2X3, which must be due to kinetic enhancement due to better solid-solid contact between
LiBH4 and Bi2X3 that promotes a faster reaction between the components.

All the work was compiled in Chapter 1 to 6. Chapter 1 describes the introduction of the field,
need of research, and associated challenges. Based on the background discussed in chapter 1,the objectives are described in chapter 2, which comprises the aim of the thesis and motivation
behind the research work. All the methods and materials adopted for the preparation as well ascharacterization of composite materials used in this work are covered in chapter 3. Chapter 4
is the explanation of our first objective i.e. improving the kinetics of 2LiBH4-MgH2 system by
the addition of potassium hydride (KH) as a catalyst. The results on the destabilization ofLiBH4 by the infusion of Bi2X3 (X = S, Se, Te) are presented in chapter 5. At last, chapter 6
summarizes the results and suggests overall conclusion of the thesis and proposes the future
direction of research.