Investigation of the reactivity of Li- and Na- alanates as conversion anodes for lithium ion batteries

As for borohydrides, the alanate family promises very interesting capacities due to the light molar weight of aluminum and the high hydrogen content of these complex hydrides. In fact, in view of a full conversion reaction in LiH and metal elements, they can theoretically achieve more than 1000 mAhg-1 exchanging at least 3 electrons for redox center. Furthermore, their thermal stability makes these compounds feasible candidates for practical applications (details are in chapter 3). The interest in them has been supported from pioneering DFT calculation performed in our laboratory few years ago [25,26], with the task to theoretically demonstrate the feasibility of alanates conversion reaction and its exploitability for application in lithium ion batteries. Besides our investigations, also Trepovich et al. [24] recently reported the use of LiAlH4 and NaAlH4 as anodes in lithium cells, furnishing a reference for comparison of our results and conclusions. This PhD thesis provides the experimental evidences of the electrochemical activity of sodium and lithium alanates in lithium cells. The focus is on the properties and reaction mechanism of tetrahydro-alluminates (LiAlH4 and NaAlH4). Beside them, further three hexa-alanates phases (Li3AlH6, Na3AlH6 and Li Na2AlH6) have been investigated. In fact, the electrochemical reaction mechanism is expected to involve the formation of these compounds in the intermediate steps. Therefore, their behavior in electrochemical cells have been used to delineate a full picture of the conversion mechanism of the corresponding tetrahydro-alluminates compounds. The work has been structured in three sections. The first section is focused on lithium alanates (LiAlH4 and Li3AlH6). Chapter 4 reports the studies conducted on LiAlH4. After confirming the electrochemical activity of this compound, mechanochemical treatments have been used to improve its performance. It's well known that mechanical grinding causes the reduction of the particles and induces strains that could lead to a better diffusion of hydrogen and lithium by increasing the number of diffusion paths. Comparisons with pristine sample have been made to evaluate the effects of the performed treatments on the structure, morphology and electrochemical performance. The electrochemical reaction mechanism has been elucidated by ex-situ diffraction experiments on LiAlH4 based electrodes at different state of charge. The chapter ends with the study of the reactivity of LiAlH4 with the carbonate based electrolyte used for electrochemical tests. Chapter 5 provides the results for Li3AlH6. In this case, mechanochemistry has been used to synthesize the compound. Second section describes the sodium alanates compounds (NaAlH4, Na3AlH6 and LiNa2AlH6). As already did for lithium alanates, mechanochemical treatments have been used both to activate the bare NaAlH4 (chapter 6) and to synthesize hexa-alanates phases (chapter 7). Then, for the obtained samples the chemical-physical and the electrochemical properties have been studied. Finally, in the chapter 8, the conversion reactions of the three phases have been described by in situ diffraction experiments during discharge/charge cycling. Further investigations have been addressed on NaAlH4, in view of the encouraging results obtained. In conclusion, the last section is dedicated to the performance improvements of the alanates based electrodes. In fact, for all the samples poor cell efficiency and cyclability have been observed. This could be mainly ascribed to the big volumetric expansion observed with conversion reaction as well to the high reactivity of this materials with the common solvents used as electrolytes, due to the high reducing power of alanates. Two main strategies have been adopted to reduce these effects: the nanoconfinement in a nanoporous carbon matrix, as described in chapter 9 and the replacement of carbonates based electrolyte with an ionic liquid, as in chapter 10.