Ever-increasing demands for lithium-ion batteries give rise to several safety concerns all over the world, capturing the attention of the scientific community and socio-economical stakeholders . This is largely due to frequent reports of combustion and explosion incidents, which are directly linked to the highly flammable nature of the electrolytes used in LIBs. Aqueous batteries appear to be a safer alternative but the limited energy density of aqueous batteries is a major drawback, resulting from the narrow electrochemical stability window (ESW) of water. This study aims to widen the ESW of aqueous electrolytes using innovative strategies such as super-concentration (SC) and hybrid electrolytes. New NASICON phases were also developed with the aim of minimizing the issues that may arise from the electrodes point of view. In the first part of the thesis SC approach was used to expand the ESW of aqueous electrolytes. Highly soluble CH3COOK was mixed with CH3COONa to form mixed-cation water-in-salt (WIS) electrolyte solutions with varying concentrations. The physicochemical and electrochemical properties of these solutions were thoroughly investigated using a range of techniques, including thermal, rheological, electrical, electrochemical, and spectroscopy measurements. The most concentrated solution, 20m CH3COOK+7 m CH3COONa, gives the best compromise between transport properties and stability, displaying a conductivity of 21.2 mS cm-1 at 25°C and ESW of 3.2 V. Raman spectroscopy allowed to gain insights into the interaction between phase components and emerging structural features in concentrated solutions. By analyzing the hydrogen bonding motifs in these solutions, we gained insight into their impact on the ESW. Raman analyses also helped to understand the interactions network, the phase evolution with temperature and the crystallization kinetics. As a proof-of-concept LiTi2(PO4)3 was shown to reversibly insert/de-insert Na+ ions at approximately -0.7 V vs. SHE in the 27m mixed cation electrolyte. In the second part of the thesis, we designed a hybrid organic/aqueous electrolyte that uses sulfolane (SL) as an organic co-solvent. Keeping the SL to water ratio fixed at 3:1 and increasing the ratio of LiFTFSI salt from 1 to 3, we studied the impact of salt content on the physicochemical properties. The evolution of solvation structure with concentration was analyzed via Raman spectroscopy and our findings revealed that the SL had a significant impact on the solvation structures of ionic species in the solution. SL acted as a coordinating agent for water and Li-ions, displacing water molecules in the primary solvation shell. Interestingly, at a moderate concentration below 8 mol kg-1, the FTFSI anion in the salt underwent reductive decomposition, forming a stable SEI layer in the presence of unique solvation structures, such as CIPs and AGGs. Using standard Al current collectors, we were able to increase the ESW beyond 3.5 V. The hybrid electrolyte enabled a full aqueous LTO/LMO cell with an average voltage of 2.4 V and specific energy of 156 Wh kg-1.The final section of the thesis discusses the synthesis and electrochemical characterization of NaAlNb(PO4)3 (NANP) and NaFeNb(PO4)3 (NFNP) NASICON phases. The materials were synthesized using a conventional solid-state route. Electrochemical characterization of NANP and NFNP was done in half cells with sodium, lithium, and potassium in organic electrolytes. The Na cells demonstrated the best electrochemical behavior with intercalation potentials corresponding to the reduction of Nb+5/Nb+4 and Nb+4/Nb+3 recorded at 2.1 V and 1.4 V vs Li/Li+ for NANP and 2.4 V and 1.4 V vs Li/Li+ for NFNP. For Na storage, NANP showed a capacity of 92 mAh g-1 at the 70th cycle at 0.1 C with a coulombic efficiency of 99.4%, while NFNP showed a capacity of 52.5 mAh g-1, with coulombic efficiency of 98.3% at 0.1 C.

Poiché le batterie agli ioni di litio (LIBs) continuano a essere utilizzate sempre più ampiamente nella nostra vita quotidiana, aumentano anche i problemi di sicurezza. Ciò è in gran parte dovuto alle frequenti segnalazioni di incidenti di combustione ed esplosione, che sono direttamente collegati alla natura altamente infiammabile degli elettroliti utilizzati nelle LIBs. Le batterie acquose sembrano essere un'alternativa più sicura, ma la loro limitata densità di energia, che deriva dalla stretta finestra di stabilità elettrochimica (ESW) dell'acqua, è un grosso limite. Lo studio qui presentato ha avuto come obiettivo l’ampliamento dell'ESW degli elettroliti acquosi, utilizzando strategie innovative come la super-concentrazione (SC) e l'approccio dell'elettrolita ibrido. Inoltre, sono state sviluppate anche nuove fasi NASICON con l'obiettivo di ottimizzare i materiali elettrodici. Nella prima parte della tesi CH3COOK, altamente solubile, è stato selezionato come principale componente e miscelato con CH3COONa per formare elettroliti acqua-in-sale (WIS) a cationi misti con concentrazioni variabili. Le proprietà fisico-chimiche ed elettrochimiche di queste soluzioni sono state studiate utilizzando misurazioni termiche, reologiche, elettriche, elettrochimiche e spettroscopiche. La soluzione più concentrata, 20m CH3COOK + 7m CH3COONa, è risultata fornire il miglior compromesso tra proprietà di trasporto e stabilità elettrochimica, mostrando una conducibilità di 21,2 mS cm-1 a 25°C e un ESW di 3,2 V. Analizzando i legami idrogeno in questi elettroliti concentrati, sono state ottenute informazioni riguardo il loro impatto sull'ESW. Le analisi Raman hanno anche aiutato a comprendere la rete di interazioni, le variazioni di stato con la temperatura e la cinetica di cristallizzazione. Come prova delle performances, LiTi2(PO4)3 è stato in grado di inserire e disinserire reversibilmente ioni Na+ a un potenziale di circa -0,7 V rispetto a SHE nell'elettrolita cationico misto 27m. Successivamente, un elettrolita ibrido organico/acquoso basato sul sulfolano (SL) è stato progettato come co-solvente organico. Mantenendo il rapporto SL/acqua fissato a 3:1 e aumentando il rapporto del sale LiFTFSI da 1 a 3, è stato studiato l'impatto del contenuto di sale sulle proprietà fisico-chimiche dell’elettrolita. La spettroscopia RAMAN ha rivelato che l'aggiunta di SL ha avuto un impatto significativo sulle strutture di solvatazione delle specie ioniche nella soluzione. SL ha agito come agente di coordinamento per l'acqua e gli ioni di litio, spostando efficacemente le molecole d'acqua nel guscio di solvatazione primario. È interessante notare che, a una concentrazione moderata inferiore a 8 mol kg-1, l'anione FTFSI nel sale ha subito una decomposizione riduttiva, portando alla formazione di uno strato SEI stabile in presenza di strutture di solvatazione uniche, come CIP e AGG. Utilizzando come collettore di corrente Al, l'ESW è stata aumentata a oltre 3,5 V. L'elettrolita ibrido ha consentito di realizzare una cella LTO/LMO completamente acquosa con una tensione media di 2,4 V e un'energia specifica di 156 Wh kg-1. La sezione finale del la tesi discute la sintesi e la caratterizzazione elettrochimica delle fasi NASICON NaAlNb(PO4)3 (NANP) e NaFeNb(PO4)3 (NFNP). I materiali sono stati sintetizzati utilizzando una sintesi convenzionale a stato solido. La caratterizzazione elettrochimica di NANP e NFNP è stata effettuata in semicelle con sodio, litio e potassio in elettroliti organici. Le celle Na hanno dimostrato il miglior comportamento elettrochimico con potenziali di intercalazione corrispondenti alla riduzione di Nb+5/Nb+4 e Nb+4/Nb+3 registrata a 2,1 V e 1,4 V vs Li/Li+ per NANP e 2,4 V e 1,4 V vs Li/Li+ per NFNP. Per lo stoccaggio di Na, NANP ha mostrato una capacità di 92 mAh g-1 al 70° ciclo a 0,1 C con

(2023). Innovative Strategies In The Development of High Energy Rechargeable Aqueous Batteries. (Tesi di dottorato, Università degli Studi di Milano-Bicocca, 2023).

Innovative Strategies In The Development of High Energy Rechargeable Aqueous Batteries

KHALID, SHAHID
2023

Abstract

Ever-increasing demands for lithium-ion batteries give rise to several safety concerns all over the world, capturing the attention of the scientific community and socio-economical stakeholders . This is largely due to frequent reports of combustion and explosion incidents, which are directly linked to the highly flammable nature of the electrolytes used in LIBs. Aqueous batteries appear to be a safer alternative but the limited energy density of aqueous batteries is a major drawback, resulting from the narrow electrochemical stability window (ESW) of water. This study aims to widen the ESW of aqueous electrolytes using innovative strategies such as super-concentration (SC) and hybrid electrolytes. New NASICON phases were also developed with the aim of minimizing the issues that may arise from the electrodes point of view. In the first part of the thesis SC approach was used to expand the ESW of aqueous electrolytes. Highly soluble CH3COOK was mixed with CH3COONa to form mixed-cation water-in-salt (WIS) electrolyte solutions with varying concentrations. The physicochemical and electrochemical properties of these solutions were thoroughly investigated using a range of techniques, including thermal, rheological, electrical, electrochemical, and spectroscopy measurements. The most concentrated solution, 20m CH3COOK+7 m CH3COONa, gives the best compromise between transport properties and stability, displaying a conductivity of 21.2 mS cm-1 at 25°C and ESW of 3.2 V. Raman spectroscopy allowed to gain insights into the interaction between phase components and emerging structural features in concentrated solutions. By analyzing the hydrogen bonding motifs in these solutions, we gained insight into their impact on the ESW. Raman analyses also helped to understand the interactions network, the phase evolution with temperature and the crystallization kinetics. As a proof-of-concept LiTi2(PO4)3 was shown to reversibly insert/de-insert Na+ ions at approximately -0.7 V vs. SHE in the 27m mixed cation electrolyte. In the second part of the thesis, we designed a hybrid organic/aqueous electrolyte that uses sulfolane (SL) as an organic co-solvent. Keeping the SL to water ratio fixed at 3:1 and increasing the ratio of LiFTFSI salt from 1 to 3, we studied the impact of salt content on the physicochemical properties. The evolution of solvation structure with concentration was analyzed via Raman spectroscopy and our findings revealed that the SL had a significant impact on the solvation structures of ionic species in the solution. SL acted as a coordinating agent for water and Li-ions, displacing water molecules in the primary solvation shell. Interestingly, at a moderate concentration below 8 mol kg-1, the FTFSI anion in the salt underwent reductive decomposition, forming a stable SEI layer in the presence of unique solvation structures, such as CIPs and AGGs. Using standard Al current collectors, we were able to increase the ESW beyond 3.5 V. The hybrid electrolyte enabled a full aqueous LTO/LMO cell with an average voltage of 2.4 V and specific energy of 156 Wh kg-1.The final section of the thesis discusses the synthesis and electrochemical characterization of NaAlNb(PO4)3 (NANP) and NaFeNb(PO4)3 (NFNP) NASICON phases. The materials were synthesized using a conventional solid-state route. Electrochemical characterization of NANP and NFNP was done in half cells with sodium, lithium, and potassium in organic electrolytes. The Na cells demonstrated the best electrochemical behavior with intercalation potentials corresponding to the reduction of Nb+5/Nb+4 and Nb+4/Nb+3 recorded at 2.1 V and 1.4 V vs Li/Li+ for NANP and 2.4 V and 1.4 V vs Li/Li+ for NFNP. For Na storage, NANP showed a capacity of 92 mAh g-1 at the 70th cycle at 0.1 C with a coulombic efficiency of 99.4%, while NFNP showed a capacity of 52.5 mAh g-1, with coulombic efficiency of 98.3% at 0.1 C.
RUFFO, RICCARDO
Acqua in elettroliti; Batterie acquose; Elettrolita ibrido; acetato di potassio; finestra di stabilit
Water in salt; Aqueous batteries; Hybrid electoryte; potassium acetate; stability window
CHIM/02 - CHIMICA FISICA
English
21-lug-2023
35
2021/2022
embargoed_20260721
(2023). Innovative Strategies In The Development of High Energy Rechargeable Aqueous Batteries. (Tesi di dottorato, Università degli Studi di Milano-Bicocca, 2023).
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Descrizione: Innovative Strategies In The Development of High Energy Rechargeable Aqueous Batteries
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Utilizza questo identificativo per citare o creare un link a questo documento: https://hdl.handle.net/10281/431798
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