Aqueous sodium ion batteries (ASIBs) are a promising alternative to fulfil the requirements of low-cost and high safety needed for a new battery technology. The main issue that has to be addressed for this technology is the limited operating potential window, typical of energy storage systems based on aqueous solvent that limits the choice of electrode materials and most importantly lowers the energy density of the final device. In this work, NaTi2(PO4)3 (NTP) is studied as an anodic material for Na-based aqueous energy storage. The low electronic conductivity of NTP is a limiting factor for high-rate operations, and so a conductive additive is needed for practical applications. In this work, carbon nanotubes (CNTs) are selected for this purpose, and as a matter of facts the main aim of the thesis is the study of the effect of CNTs concentration to the electrical and electrochemical properties of the NTP/CNT composite electrode. NTP is synthesized by solvothermal technique followed by calcination for final crystallization. This method is considered one of the easiest, economic, and relatively fast way to obtain NTP, since it is based on the reaction in water or organic solvent at moderate temperature under high pressure that eventually produces a nanocrystalline material. Moreover, through the solvothermal synthesis, CNTs can be added in the precursors’ mixture, leading to the formation of composite NTP/CNT. The morphology, crystal structure and phase composition of the obtained powders are characterized by performing XRD, SEM, TEM and TGA analysis. The electrical conductivity of compacted powder of NTP and mixture of NTP/CNT is also investigated. Cyclic voltammetry and charge/discharge tests in aqueous electrolyte (1 M Na2SO4) are used to assess the electrochemical behavior of composite electrodes. This work demonstrates that a moderate content of CNTs (around 18 wt%) is helpful to enhance the NTP capacity, especially at low C rate (120 mAh g-1 in discharge). On the other hand, by further increasing the CNT content (23 wt%), lower performances are achieved at low rates (98 mAh g-1 in discharge), but the obtained capacity is retained more as the charge/discharge current is increased (72 mAh g-1 in discharge at 5C). This can be attributed to a partial CNTs segregation for the high-content electrodes, resulting on the one hand on a relatively lower utilization of active material and on the other hand in enhanced rate capability. Therefore, this study highlights opportunities and limitations of the CNTs composite strategy for the enhancement of NTP electrode conductivity and performances.
Le batterie agli ioni di sodio acquose (ASIBs) potrebbero in linea di principio soddisfare i requisiti di basso costo e di sicurezza richiesti per un tipo di sistema di accumulo che si proponga come credibile alternativa alle tecnologie a base litio. Tuttavia, la ricerca per questo sistema di accumulo è ancora agli inizi. La principale limitazione dei sistemi acquosi consiste nella bassa densità di energia di questi sistemi rispetto ai dispositivi a base di elettroliti organici, dovuta principalmente ai limiti di stabilità elettrochimica del solvente H2O. Questo lavoro ha come obiettivo lo studio di un materiale anodico per batterie a ioni sodio, NaTi2(PO4)3 (NTP), prodotto mediante sintesi solvotermale seguita da una fase di calcinazione in forno tubolare. Questo metodo è considerato uno dei modi più facili, economici e relativamente veloci per ottenere questo materiale. Tuttavia, la bassa conduttività elettronica dell'NTP è un fattore limitante per la capacità ad alta velocità. L'obiettivo principale della tesi è lo studio dell'effetto dei nanotubi di carbonio (CNTs) sul comportamento elettrochimico dell’NTP. Questa tecnica si basa sulla sintesi di materiale in acqua o solvente organico a temperatura moderata e ad alta pressione e viene ampiamente utilizzata per sintetizzare monocristalli e nanomateriali. Inoltre, i CNTs possono essere aggiunti direttamente nella miscela di reazione solvotermale, portando alla formazione di elettrodi compositi NTP/CNT. La morfologia, la struttura cristallina e la composizione delle polveri ottenute sono state caratterizzate tramite analisi SEM, EDS, TEM, XRD e TGA. Inoltre, è stata investigata la conducibilità elettrica delle polveri sia di NTP che delle miscele NTP/CNT. Prove di voltammetria ciclica e di carica/scarica in elettrolita acquoso (1 M Na2SO4) sono state utilizzate per valutare le proprietà dell'elettrodo composito. I risultati ottenuti dimostrano che un moderato contenuto di CNT (circa il 18% in peso) è utile per aumentare la capacità, specialmente a bassi C rate (120 mAh g-1 in scarica). Al contrario, aumentando ulteriormente il contenuto di CNT (23% in peso), si ottengono prestazioni inferiori (98 mAh g-1 in scarica), ma la capacità ottenuta rimane costante all'aumentare della corrente di carica/scarica (72 mAh g-1 in scarica a 5C). Questo studio evidenzia quindi che i nanotubi possono costituire una rete interna di interconnessione tra le particelle di NTP, riducendo significativamente la resistenza interna dell’elettrodo e incrementando la frazione di NTP attiva per la reazione di intercalazione.
Synthesis and electrochemical study of NaTi2(PO4)3/CNT composite electrode material as anode for aqueous sodium ion batteries
Gerocarni, Oreste
2020/2021
Abstract
Aqueous sodium ion batteries (ASIBs) are a promising alternative to fulfil the requirements of low-cost and high safety needed for a new battery technology. The main issue that has to be addressed for this technology is the limited operating potential window, typical of energy storage systems based on aqueous solvent that limits the choice of electrode materials and most importantly lowers the energy density of the final device. In this work, NaTi2(PO4)3 (NTP) is studied as an anodic material for Na-based aqueous energy storage. The low electronic conductivity of NTP is a limiting factor for high-rate operations, and so a conductive additive is needed for practical applications. In this work, carbon nanotubes (CNTs) are selected for this purpose, and as a matter of facts the main aim of the thesis is the study of the effect of CNTs concentration to the electrical and electrochemical properties of the NTP/CNT composite electrode. NTP is synthesized by solvothermal technique followed by calcination for final crystallization. This method is considered one of the easiest, economic, and relatively fast way to obtain NTP, since it is based on the reaction in water or organic solvent at moderate temperature under high pressure that eventually produces a nanocrystalline material. Moreover, through the solvothermal synthesis, CNTs can be added in the precursors’ mixture, leading to the formation of composite NTP/CNT. The morphology, crystal structure and phase composition of the obtained powders are characterized by performing XRD, SEM, TEM and TGA analysis. The electrical conductivity of compacted powder of NTP and mixture of NTP/CNT is also investigated. Cyclic voltammetry and charge/discharge tests in aqueous electrolyte (1 M Na2SO4) are used to assess the electrochemical behavior of composite electrodes. This work demonstrates that a moderate content of CNTs (around 18 wt%) is helpful to enhance the NTP capacity, especially at low C rate (120 mAh g-1 in discharge). On the other hand, by further increasing the CNT content (23 wt%), lower performances are achieved at low rates (98 mAh g-1 in discharge), but the obtained capacity is retained more as the charge/discharge current is increased (72 mAh g-1 in discharge at 5C). This can be attributed to a partial CNTs segregation for the high-content electrodes, resulting on the one hand on a relatively lower utilization of active material and on the other hand in enhanced rate capability. Therefore, this study highlights opportunities and limitations of the CNTs composite strategy for the enhancement of NTP electrode conductivity and performances.| File | Dimensione | Formato | |
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https://hdl.handle.net/10589/177390