Development and Optimization of Rechargeable Batteries based on Aluminium

Programa de Doctorado de Electroquímica. Ciencia y Tecnología.

Universidad Autónoma de Madrid

DOCTORAL THESIS

Development and Optimization of Rechargeable Batteries based on Aluminium

A thesis submitted to Universidad Autónoma de Madrid in fulfilments of the requirements for the degree of Doctor of Philosophy by:

David Muñoz-Torrero Castaño

IMDEA Energy Institute, Electrochemical Processes Unit

Supervisors:

Rebeca Marcilla García Edgar Ventosa Arbaizar

Academic tutor:

Pilar Ocón Esteban

Universidad Autónoma de Madrid

Madrid, 2020

A mi familia, a los que están, y a los que desgraciadamente solo tenemos en nuestro recuerdo.

He decidido escribir estas líneas en castellano. Considero que la mejor forma de expresar mi gratitud hacia las personas que directamente o indirectamente me han ayudado en este periodo de más de cuatro años, es en mi propia lengua.

En primer lugar y como no podría ser de otra manera, quiero agradecerle a IMDEA Energía, y en concreto, a la Unidad de Procesos Electroquímicos la oportunidad que me brindaron en el 2015. En especial al Dr. Jesús Palma, aún recuerdo sus palabras: “yo confío en Rebeca, Rebeca confía en ti y por lo tanto, yo confío en ti”. Gracias. También quiero agradecer a mis directores de tesis, la Dra.

Rebeca Marcilla y el Dr. Edgar Ventosa, todo el apoyo ofrecido durante este proceso. Ellos han sido mi referencia y guía. Ojalá algún día pueda llegar a tener la mitad de las dotes que tienen ellos como investigadores. Ambos son grandes trabajadores y tienen características que son referencia para mí. De ellos destacaría su paciencia, su escucha, su alto grado de perfeccionismo y su precisión. También destacaría sus explicaciones y sobretodo sus preguntas, que me han ayudado a aprender, su ánimo en momentos difíciles, así como las conversaciones para generar ideas. Sin esas ideas esta tesis no hubiera sido lo mismo.

No me gustaría olvidar al Dr. Puiki Leung, por su gran estructuración de trabajos de investigación; al Dr. Enrique García-Quismondo, por su apoyo, su ayuda y su característico sentido del humor; al Dr.

Julio Lado y al resto de investigadores posdoctorales y titulares de la unidad. No quiero olvidar al Dr.

Afshin Pendashteh, un gran investigador y amigo, por su gran apoyo laboral y calidad humana. Quiero agradecerle al Dr. Girum A. Tiruye por enseñarme, por hacerme tan fácil el camino inicial en la investigación y por su positividad, que inundaba el laboratorio. También he de mencionar a mis excompañeros de trabajo y grandes amigos hoy en día: a la Dra. Cleis Santos, por su carisma, risas tanto dentro como fuera del laboratorio (habrá que volver a la Conchinchiña algún día) y por su don para llevar a la gente y organizar un laboratorio donde reine la paz y el “buen rollo”; a la Dra. Paula Navalpotro, ejemplo de trabajo y con esa capacidad de preguntarte, aconsejarte y apoyarte en lo humano justo cuando lo necesitas y sin tener que pedirlo; al Dr. Jaime S. Sánchez, mi compi y puedo decir “hermano de tesis”, no podré olvidar y no podré no echar de menos todos esos momentos divertidos compartidos contigo (los cafés, tus frases, el reparto de cápsulas de café, hasta limpiarnos los dientes era divertido). A Nachete, por su ayuda constante y su carisma, que le hace grande. A Mayte, eres una gran amiga y gran mujer, fuerte, luchadora y merecedora de un coche con etiqueta eco para poder ir al centro. A Antonio, llegaste un poco tarde, pero qué bien llegaste y cuántas risas y buenos momentos hemos pasado juntos. Tampoco quiero olvidar al resto, por hacerme mis días (y noches) más amenos y por vuestra gran calidad como personas: Dani, Bea, Carlos, Diana y el resto de investigadores y técnicos de laboratorio. Al resto de Unidades, sobre todo a los investigadores predoctorales, muchos de ellos ya doctores, con los he pasado grandes momentos y al equipo de laboratorios centrales.

adquirir una visión de la electroquímica diferente. Me hicieron muy fácil mi llegada y vida allí.

No puedo pasar por alto a mis amigos, que aunque no hayan estado allí en el día a día, también me han sufrido, escuchado y apoyado cuando hacía falta. Al grupo de “Correos”: Sergio, Carol, Yoli, Noe, Miguel, Roberto, Natalia, Dani, Pablo y los pequeños Gonzalo y Dani. Muy en especial a Sergio, Juanlu y Jesús, mis amigos de toda la vida, nos queremos incondicionalmente sin juzgarnos. A Inma, siempre pendiente y fiel a nuestra amistad. A mis amigos del pueblo, que me amenizan cada quedada, en especial a Sereno, Miguelito, Mansilla, Chemi, Muño, y Miguel, que siempre pregunta entre risas que si cuando sea doctor voy a poder recetarle cosas. A Leire, Marta y María, grandes apoyos en mi vida. En especial a María y a Dani, por ser uña y carne durante la carrera y porque cada vez que quedo con ellos es como si no hubiera pasado el tiempo.

A mis abuelos, Galo, Carmen, Mariano y Fermina. Toda mi infancia y parte de mi vida adulta está llena de recuerdos buenos a su lado. Quiero recordar en especial las palabras de mi abuela Fermina cuando le dije que quería estudiar un doctorado: “¡te vas a volver loco de tanto estudiar!”. Cuánta razón tenía.

A mis padres, Mª del Carmen y Francisco. Por darme la vida y convertirme en la persona que soy. Por ser ejemplo de trabajo y amor constantes. Porque no puedo estar más agradecido y orgulloso de tener los padres que tengo. Para vosotros es este libro. A mis hermanos. Por allanarme el camino. No me imagino mejores personas con las que crecer y aprender. Sois un pilar fundamental en mi vida y guía.

A Fran, mi fiel y generoso hermano, siempre a mi lado tanto de día como en noches en vela. A Gemma, mi consejera, siempre escucha y aconseja sin juzgar. A Nuria, que me ha visto crecer y me ha cuidado como si fuera una segunda madre. A mis otros hermanos: mis cuñados. A Javi que me ha visto crecer, a Cristina tan graciosa y tan buena como la persona que tiene al lado; a Hugo, cualquier conversación con él es interesante. A Antonio, siempre tan colaborador y encantador. A Miriam, por su generosidad durante más de diez años y sus tartas de queso. A mis sobrinos: mi Víctor, mi Patricia y Paulita, veros nacer y crecer es un regalo.

Al resto de mi familia de sangre y política. Todos de forma más o menos consciente me han acompañado en este camino.

Por último, quiero agradecer esta tesis a la persona más fuerte que conozco y la mejor mujer de todas, Marta. Ella es mi inspiración, mi apoyo, la que calma mis nervios, la que me ayuda a no tirar la toalla y la que me ha enseñado a trabajar duro y sacrificarme por lo que quiero. Ella es un gran ejemplo en todos los sentidos. Este libro tiene mucho de ella y sin ella no sería. Hemos vivido momentos muy felices y muy duros, pero siempre juntos. No la necesito, sino que la elijo día tras día. Por ti también va este libro.

Resumen ... v

Chapter 1. INTRODUCTION AND ALUMINIUM BATTERIES LITERATURE REVIEW ... 1

1.1. Energy System and Environmental Concerns ... 2

1.1.1. Energy Storage Systems ... 4

1.1.2. Electrochemical Energy Storage Systems: Batteries ... 6

1.2. Historical Development of Aluminium Batteries ... 11

1.2.1. Primary Aluminium Batteries: aqueous systems ... 11

1.2.2. Rechargeable Aluminium Batteries: non-aqueous systems ... 11

1.3. Electrolytes for Al-based batteries ... 14

1.3.1. Chloroaluminate Ionic Liquids (CILs) ... 14

1.3.2. Alternative electrolytes ... 15

1.4. Rechargeable Aluminium Batteries Classification ... 18

1.4.1. Metal Oxide/Sulphide-based Aluminium Batteries ... 18

1.4.2. Polymer-based Aluminium Batteries ... 22

1.4.3. Graphite-based Aluminium Batteries ... 22

1.5. Rechargeable Aluminium Batteries based on Graphitic Cathodes ... 23

1.5.1. Carbon Paper ... 24

1.5.2. Pyrolytic Graphite ... 26

1.5.3. Graphitic Foam ... 27

1.5.4. Graphene-based Cathodes ... 29

1.5.5. Graphite Flakes-based Cathodes ... 32

1.6. Competitiveness of Rechargeable Aluminium Batteries against other technologies ... 34

1.7. References ... 36

Chapter 2. OBJECTIVES ... 47

CHAPTER 3. CRITICAL ASSESSMENT OF RECHARGEABLE GRAPHITE-ALUMINIUM BATTERY 51 TECHNOLOGY ... 51

3.1. Introduction ... 53

3.2. Basic fundaments of a Graphite-Al Battery (GAB) ... 55

3.3. Battery Performance Analysis... 56

3.3.1. Specific Energy Density ... 56

3.3.2. Cycle Life and Efficiency ... 58

3.3.4. Battery Safety ... 60

3.3.5. Battery Cost ... 60

3.4. Comparison of Performance ... 62

CHAPTER 4. STUDY OF DIFFERENT MATERIALS AS NEGATIVE CURRENT COLLECTOR FOR

ALUMINIUM-BASED BATTERIES ... 69

4.1. Introduction ... 71

4.2. Results and discussion ... 73

4.2.1. Effect of Substrate Material on Aluminium Electrodeposition ... 73

4.2.2. Evaluation of Aluminium Electrodeposition in Substrate with 3D Surface Geometry ... 81

4.2.3. Polarization Testing in Aluminium-based Batteries ... 85

4.3. Conclusion ... 87

4.4. Experimental ... 89

4.4.1. Materials... 89

4.4.2. Electrolyte Preparation ... 89

4.4.3. Electrochemical Tests ... 89

4.4.4. Characterization of Aluminium Electrodeposits ... 90

4.4.5. Assembly and Electrochemical Performance of the Aluminium Battery ... 90

4.5. References ... 91

CHAPTER 5. STUDY OF ALTERNATIVE ELECTROLYTES FOR GRAPHITE-ALUMINIUM BATTERIES ... 95

5.1. Introduction ... 97

5.1.1. Chloroaluminate Ionic Liquids (CILs) ... 97

5.1.2. Air and Water Stable Non-haloaluminate Ionic Liquids ... 98

5.1.3. Organic-based Electrolytes ... 99

5.1.4. Deep Eutectic Solvents (DESs) ... 100

5.2. Results and Discussion ... 102

5.2.1. Study of the Anodic Reaction in Different Electrolytes ... 102

5.2.1.1 Chloroaluminate Ionic Liquids; AlCl3:BMImCl ... 102

5.2.1.2 Deep Eutectic Solvents; AlCl3:Urea ... 107

5.2.1.3 Comparison between Different Electrolytes ... 109

5.2.2. Evaluation of Electrolytes in full GAB ... 112

5.3. Conclusion ... 116

5.4. Experimental Section ... 118

5.4.1. Materials... 118

5.4.2. Electrolyte preparation ... 118

5.4.3. Electrochemical Electrolytes Characterization ... 118

CHAPTER 6. THE IMPACT OF THE POSITIVE CURRENT COLLECTOR IN THE COST OF

GRAPHITE-ALUMINIUM BATTERY ... 127

6.1. Introduction ... 129

6.2. Results and Discussion ... 131

6.2.1. Methodology for Cost Analysis ... 131

6.2.2. Evaluation of the Impact of Current Collector in the Battery Cost ... 133

6.2.3. Electrochemical Stability of the Proposed Current Collectors under Anodic Condition ... 137

6.2.4. Electrochemical Performance of Rechargeable Aluminium Batteries ... 140

6.3. Conclusion ... 143

6.4. Experimental ... 144

6.4.1. Materials... 144

6.4.2. Study of the Current Collector Corrosion ... 145

6.4.3. Graphite Cathode Preparation ... 145

6.4.4. Battery Assembling and Characterization ... 145

6.5. References ... 146

Chapter 7. WIDELY COMMERCIAL CARBONS AS CATHODE FOR GRAPHITE-ALUMINIUM BATTERIES ... 149

7.1. Introduction ... 151

7.2. Results and Discussion ... 154

7.2.1. Understanding Structural Requirements for AlCl4 - Intercalation... 154

7.2.2. Electrochemical Performance of Graphite-Al Batteries ... 159

7.2.2.4 Influence of Cathode Crystalline Structure on the Battery Performance ... 159

7.2.2.5 Effect of the Cathode Mass Loading on the Battery Performance ... 163

7.2.2.6 Powder Graphite Material as Cathode in GABs ... 165

7.3. Conclusion ... 171

7.4. Experimental ... 172

7.4.1. Materials... 172

7.4.2. Structural Characterization of Carbonaceous Materials ... 172

7.4.3. Electrochemical Evaluation of Cathodic Intercalation Process ... 172

7.4.4. Battery Assembling and Characterization ... 173

7.4.5. Structural Evolution of Cathodes During Intercalation Process ... 173

7.5. References ... 175

CHAPTER 8. PROOF OF CONCEPT OF A GRAPHITE-AL BATTERIES BASED SEMI-SOLID ELECTRODES ... 179

8.2.1. Semi-solid Cathode Enables High Mass Transport for High Areal Capacity

Graphite-Al Batteries ... 183

8.2.2. Optimization of Semi-solid Cathodes ... 185

8.2.3. Electrochemical Characterization of Semi-solid Graphite-Al Batteries ... 186

8.2.4. Versatility of Semi-solid GABs ... 190

8.2.5. Cost Breakdown for Conventional and Semi-solid Electrodes... 192

8.2.6. Evaluation of the Specific Energy and Energy Density in Semi-solid GABs ... 192

8.3. Conclusion ... 194

8.4. Experimental ... 195

8.4.1. Materials... 195

8.4.2. Preparation of Semi-solid Cathode ... 195

8.4.3. Electrochemical Characterization of Semi-solid Cathode ... 195

8.4.4. Electrochemical Characterization of Semi-solid GAB ... 196

8.4.5. Estimation of Ratio between Effective Diffusion Coefficients ... 196

8.4.6. Energy Density and Specific Energy Calculation ... 197

8.4.7. Molar Ratio of the Electrolyte used in Semi-solid GABs ... 197

8.5. References ... 198

Chapter 9. FINAL CONCLUSIONS AND FUTURE CHALLENGES... 201

9.1. Final Conclusions ... 202

9.2. Future Challenges... 205

Capítulo 9. CONCLUSIONES FINALES Y FUTUROS RETOS ... 207

9.1. Conclusiones Finales ... 208

9.2. Futuros retos ... 212

Appendix A. SCIENTIFIC CONTRIBUTIONS ... 215

Abstract

The energy demand has increased significantly during the last forty years. To respond to this demand in a sustainable and environmentally friendly way, the production of energy from on renewable sources has increased since the early 90´s. However, the intermittent character of renewable energies requires the use of energy storage systems to overcome the mismatching between energy production and demand. Amongst them, electrochemical energy storage systems, which are able to transform electrical energy from chemical energy and vice versa, stand out. In particular, lithium-ion batteries (LIBs), showing great properties in terms of energy density and efficiency as well as cycle life, have experienced great advances during the last years becoming the power of choice in portable electronics such as laptops and mobile phones amongst others. Moreover, LIBs are the most promising technology for electric vehicles. However, its high cost, the consumption of expensive and scarce metals and the associated safety issues hinder further market penetration, especially in stationary applications due to the massive energy storage capacity of each installation (MW). This has driven much scientific interest to new alternative post-lithium batteries based on cheaper metals such as sodium-ion batteries, zinc-based batteries or aluminium-based batteries. Aluminium (Al) stands out due to its high theoretical volumetric and gravimetric capacities (2.98 Ah g^-1 and 8.04 Ah cm^-2 respectively), its low cost and low reactivity. Nevertheless, it is less electronegative than other metals (-1.7 V for Al electrodeposition vs -3.04 V for Li, against SHE), resulting in batteries with a lower energy density.

Scientific interest in developing Al-based batteries has greatly increased during the last decade. In particular, Graphite-Al Batteries (GABs) comprising Al foil as anode, chloroaluminate ionic liquid (CIL) as electrolyte and graphite as cathode, have been investigated more intensively due to their high efficiency, long cycle life and high power rates. Most efforts have been focused on the development of new graphitic materials with higher charge storage capacity values while other aspects such as energy and cost are often overlooked. Therefore, the aim of this thesis is to perform a critical and complete study of the GAB technology in order to identify the main associated issues and propose scientific strategies to solve them, contributing to its commercial deployment.

This thesis is divided in 9 chapters that are structured as follows:

Chapter 1 comprises a general introduction to electrochemical energy storage systems focusing on the state-of-the-art of Al-based batteries. Chapter 1 clarifies the reason why the

development of energy storage systems such as Al-based batteries is necessary and what the future challenges of GABs are.

Chapter 2 details the main objectives, the key scientific questions and the starting hypotheses explored within this thesis.

In Chapter 3, the fundaments of GABs are discussed and compared to LIBs and lead-acid batteries attending to five parameters: energy density, specific power, cost, cycle life and safety. The energy storage mechanism, which is different from the rocking-chair mechanism present in LIB with Li⁺ moving from one electrode to the other during battery operation, is clarified. The weaknesses and strengths of GABs are defined and the critical aspects that require more intensive research efforts for the commercialization of GABs are brought under the spotlight. Power density, cycle life and safety can be considered as strengths whereas energy density and cost are weaknesses. The factors that influence the energy density of GABs are analysed, revealing the importance of exploring new electrolytes to enhance the battery energy density. Furthermore, the relevance of searching for a new and cheaper current collector material and enhancing the areal charge storage capacity to decrease the battery cost is highlighted.

Chapter 4 sheds light on the role and influence of the negative current collector in the anodic reaction and eventually in the battery performance. The Al electrodeposition is investigated by means of cyclic voltammetry (CV) and galvanostatic reduction-oxidation in AlCl₃:BMImCl (molar ratio, r=2) electrolyte using different substrates materials (aluminium, stainless steel and carbon) and 3D geometries (aluminium mesh and fibre-based carbon paper). The results confirm that the reversible electrodeposition of Al is feasible both in aluminium and carbon-based substrates, opening the possibility of using other types of substrates different from Al foil as anodic current collectors. However, the use of Al foil ensures the best reversibility of the anodic reaction characterized by the highest coulombic efficiency (CE), which is calculated as the ratio between the accumulated charge during the oxidation and the accumulated charge during the electrodeposition, and the lowest overpotential for plating/stripping reaction. The results also confirm that the use of 3D substrates is beneficial not only for the Al electrodeposition reaction, showing higher CE and lower plating/stripping overpotentials, but also in full batteries. In fact, Al-based batteries using 3D negative electrodes show higher discharge potential than Al-based batteries using flat electrodes in discharge polarization tests.

In Chapter 5, two families of electrolytes of great interest are investigated as electrolytes for GABs. On the one hand, chloroaluminated ionic liquid (CILs) electrolytes (AlCl₃:BMImCl and AlCl3:EMImCl) are selected because they are recognized electrolyte for Al electrodeposition and widely used in GABs. On the other hand, deep eutectic solvents (DESs)-based on AlCl3:Urea are studied due to their interesting properties for Al electrodeposition and low cost. The Al electrodeposition is investigated by means of CV and galvanostatic reduction-oxidation cycles in electrolytes based on AlCl3:BMImCl, AlCl3:Urea and AlCl3:EMImCl with different compositions. The results show that CILs electrolytes, especially the AlCl3:EMImCl r = 1.5 that displays the lowest overpotentials and the highest CE, show a better electrochemical performance for Al electrodeposition than the electrolytes based on AlCl3:Urea. GABs using AlCl3:EMImCl r = 1.5 show the best electrochemical performance reaching the highest values of specific capacity and energy efficiency.

In Chapter 6, several cheap materials are evaluated as positive current collector for GABs.

To date, only very expensive materials such as molybdenum (Mo), tantalum (Ta) and tungsten (W) are found to be stable under anodic conditions in CILs and can be used as positive current collector in GABs. However, a cost analysis performed reveals that the purchase cost per cycle (USD kWh^-1 cycle^-1) of the GABs using Mo, W or Ta is far to be economically viable and it is of crucial importance to found a cheaper alternative current collector. Therefore, the anodic dissolution of cheap materials such as titanium, nickel, stainless steel, copper and carbon-based GDL are investigated in AlCl₃:EMImCl r=1.5 electrolyte by means of linear sweep voltammetry (LSV). The results demonstrate that only carbon-based GDLs are stable under anodic conditions, being an excellent and cheap option for GABs. Finally, the performance of GABs with GDLs as current collectors are found to be comparable to those using more expensive state-of-the-art materials in terms of specific capacity and cycle life (60 mAh g^-1and 1600 cycles, respectively).

In Chapter 7, different commercial available carbons are characterized and studied as cathodes in GABs, since the majority of the materials reported in GABs literature require high cost manufacturing processes, increasing the total cost of the battery. The graphitic degree (g) and the amorphous proportion (Id/Ig) are determined by XRD and Raman spectroscopy respectively. The reversibility of the AlCl4-

intercalation is investigated by CV and the performance of the carbon-based cathodes is elucidated by charge-discharge in full batteries.

The results show that only carbons with high g and low Id/Ig are able to perform the reversible intercalation of AlCl4-

showing good electrochemical performance when they are used in

GABs. The nanoparticulated SGF powder presents the best characteristics to develop GAB cathodes due to the enhancement of the AlCl₄^- ion diffusion and the high flexibility in the manufacture processing. These properties result in batteries with low mass loading SGF cathodes (10 mg cm^-2) that have a high specific capacity and excellent rate capability. On the other hand, batteries with high mass loading SGF cathodes (100 mg cm^-2), reach areal capacities that are higher than those published for GABs in literature so far.

In Chapter 8, a novel Al-based battery technology, i.e. Semi-solid Al-based battery, is proposed and demonstrated. The aim of the semi-solid Al-based battery is to push the areal capacity of Al-based batteries, which is of key importance in reducing battery cost as our analysis reveals. The main novelty relies on the use of semi-solid cathode, consisting of graphite power and electrolyte in the absence of binder. This battery concept requires a new manufacturing process since cathode material is injected in a pre-assembled battery cell in the last step, in contrast to conventional manufacturing in which coating of current collectors with cathode materials is the first step. Different semi-solid cathodes are prepared and characterized and their influence in the GAB electrochemical performance by means of galvanostatic charge-discharge is evaluated. The semi-solid GABs show higher efficient diffusion coefficient than the batteries with conventional electrodes. This results in GAB with high areal capacity and good material utilization rate. This increase in the areal capacity decreases the total battery cost by 80%, dropping the weight contribution of the positive current collector. The use of semi-solid cathodes not only improves the mass transport through thick electrodes, but also enhances the poor wettability related to the high viscosity of the electrolyte. The versatility of the system opens the opportunity to design advanced materials to achieve further improvements in the battery performance in future.

Chapter 9 summarises the main conclusion of the thesis and present the identified future challenges of the technology.

Resumen

Durante los últimos cuarenta años, la demanda de energía ha crecido notablemente. Para responder a esta demanda de una manera sostenible y respetuosa con el medio ambiente, la producción de energía a través de energías renovables se ha incrementado desde el inicio de los años 90. Sin embargo, el carácter intermitente de las energías renovables hace necesario el uso de sistemas de almacenamiento de energía para poder ajustar la producción de energía con su demanda. Entre ellos, destacan los sistemas de almacenamiento electroquímico, capaces de transformar en energía eléctrica la energía química y viceversa. En concreto, las baterías de ion-Litio (BIL), que muestran grandes propiedades en términos de densidad de energía, eficiencia y ciclado de la batería, han experimentado grandes avances durante los últimos años y se han convertido en la fuente de energía de elección para dispositivos electrónicos portátiles como ordenadores portátiles y teléfonos móviles entre otros. Además, las BIL son la tecnología de almacenamiento más prometedora para vehículos eléctricos. Sin embargo, su alto coste, el consumo de metales caros y escasos y los problemas de seguridad asociados impiden una mayor implantación en el mercado, especialmente en aplicaciones estacionarias debido a la capacidad de almacenamiento masivo de energía en cada instalación (MW). Esto ha impulsado la investigación en nuevas baterías post-litio basadas en metales más baratos como las baterías ion-sodio, baterías basadas en zinc o baterías basadas en aluminio. El aluminio (Al) destaca debido a su alta capacidad teórica volumétrica y específica (2.98 Ah g^-1 and 8.04 Ah cm^-2 respectivamente), su bajo coste y su baja reactividad. Sin embargo, el Al es menos electronegativo que otros metales (-1.7 V para la electrodeposición de Al vs -3.4 V para la de Li). Esto conduce a baterías con menor densidad de energía.

El interés científico en desarrollar baterías basadas en Al se ha incrementado durante la última década. En concreto, las Baterías de Grafito-Al (BGA), que están formadas por una lámina de Al como ánodo, un líquido iónico cloroaluminato (LIC) como electrolito y grafito como cátodo, se han investigado más intensamente debido a su elevada eficiencia, su alto número de ciclos y su alta potencia. Los mayores esfuerzos se han centrado en el desarrollo de nuevos materiales grafíticos con alta capacidad mientras otros aspectos como la energía y el coste se han pasado por alto. Por lo tanto, el propósito de esta tesis es llevar a cabo un estudio profundo y completo de las BGA para identificar los principales problemas relacionados con esta tecnología y proponer una estrategia científica para su resolución y así contribuir a su desarrollo comercial.

Esta tesis está divida en 9 capítulos que se estructuran de la siguiente manera:

El Capítulo 1 comprende una introducción general de los sistemas de almacenamiento electroquímico y en particular, se centra en la revisión del Estado-del-arte de las baterías basadas en Al. El Capítulo 1 responde al porqué el desarrollo de la sistemas del almacenamiento de energía como las baterías basadas en aluminio es necesario y cuáles son los retos futuros para las BGA.

El Capítulo 2 detalla los principales objetivos, las preguntas científicas clave y las hipótesis iniciales exploradas en esta tesis.

En el Capítulo 3, se revisan los fundamentos de las BGA y se comparan con las BIL y las baterías plomo-ácido en relación a los siguientes cinco parámetros: densidad de energía, potencia específica, coste, número de ciclos y seguridad. Del mismo modo, se clarifica el mecanismo de almacenamiento de energía, que difiere del rocking-chair de las BIL en el que los Li⁺ se mueve desde un electrodo al otro durante el funcionamiento de la batería. Se definen las debilidades y fortalezas de las BGA y se ponen en el foco de atención los aspectos críticos que requieren unos esfuerzos de investigación más intensos para la comercialización de las BGA. Entre sus fortalezas destacan la densidad de potencia, el número de ciclos y la seguridad de la batería, mientras que la densidad de energía y el coste de la batería son sus principales debilidades. Se analizan los factores que influyen en la densidad de energía de las BGA y se muestra la importancia de explorar nuevos electrolitos para incrementar la densidad de energía de las baterías. Además, se subraya la importancia de buscar colectores de corriente nuevos más baratos así como de incrementar la capacidad areal para disminuir el coste de la batería.

El Capítulo 4 arroja más luz sobre el papel del colector de corriente negativo y su influencia en la reacción anódica y finalmente en el rendimiento de la batería. La electrodeposición de Al se investiga por medio de voltametrías cíclicas (VC) y ciclos de oxidación-reducción galvanostática en el electrolito AlCl3:BMImCl (proporción molar, r=2) en substratos de diferentes materiales (aluminio, acero inoxidable y carbón) y geometrías tridimensionales (malla de aluminio y papel de carbón basado en fibras de carbón). Los resultados confirman que la electrodeposición reversible de Al es posible tanto en substratos de aluminio como en substratos basados en carbón, lo cual abre la posibilidad de usar substratos diferentes de la lámina de Al como colector de corriente anódico. Sin embargo, la lámina de aluminio muestra la mayor eficiencia coulómbica (EC), que se calcula como el cociente entre la carga

acumulada durante la oxidación entre la carga acumulada en la reducción, y los menores sobrepotentiales para la reacción de reducción/oxidación. Los resultados también confirman que el uso de substratos tridimensionales es beneficioso no sólo para la reacción de electrodeposición de Al, ya que muestra mayores EC y menores sobrepotenciales en reacción, sino también para baterías completas. De hecho, las baterías basadas en Al que usan electrodos negativos tridimensionales muestran mayor voltaje de descarga que aquellas que usan electrodos planos en ensayos de polarización en descarga.

En el Capítulo 5, se evalúan dos tipos de electrolitos de gran interés como electrolitos para su aplicación en BGA. Por un lado, se seleccionan electrolitos basados en LIC (AlCl3:BMImCl and AlCl3:EMImCl con diferentes proporciones molares, r) debido a su alto reconocimiento como electrolitos para la electrodeposición de Al y su amplio uso en BGA. Por otro lado, se estudian un disolvente eutéctico profundo (DEP) basados en AlCl3:Urea debido a sus interesantes propiedades para la electrodeposición de Al y su bajo coste. La electrodeposición de Al se investiga mediante VC y ciclos de reducción-oxidación galvanostáticos en electrolitos basados en AlCl₃:BMImCl, AlCl₃:Urea y AlCl₃:EMImCl con diferentes composiciones. Los resultados muestran que los LIC, en concreto el AlCl₃:EMImCl r = 1.5, muestra los menores sobrepotenciales y la mayor EC en comparación con los electrolitos basados en AlCl₃:Urea. Las BGA que usan AlCl₃:EMImCl r = 1.5 como electrolito muestran el mejor rendimiento electroquímico con valores de capacidad específica y eficiencia energética altos.

En el Capítulo 6 se evalúan varios materiales baratos como colector de corriente positivo para BGA. Actualmente, sólo materiales de alto coste como el molibdeno (Mo), el tantalio (TA) y el tungsteno (W) son estables bajo condiciones anódicas en LICs y se pueden usar como colectores de corriente positivos en BGA. Sin embargo, el análisis de coste que se lleva a cabo revela que el coste de adquisición por ciclo (USD kWh^-1 cycle^-1) de la batería usando Mo, W o Ta está lejos de ser económicamente viable y es de crucial importancia encontrar un colector de corriente más barato. Por lo tanto, se investiga la disolución anódica de materiales más baratos como el titanio, el níquel, el acero inoxidable, el cobre o los GDL basados en carbón en AlCl3:EMImCl r=1.5 como electrolito mediante voltametría lineal de barrido (VLB). Los resultados demuestran que solo el GDL basado en carbón es estable bajo condiciones anódicas y es una opción excelente y barata para su uso en BGA. Finalmente, el rendimiento de las BGA que usan GDL como colector de corriente es comparable en términos

de capacidad específica, eficiencia y número de ciclos (60 mAh g^-1, 99% y 1600 ciclos, respectivamente) con aquellas baterías que usan materiales del Estado-del-arte más caros.

En el Capítulo 7 se caracterizan y estudian diferentes carbones comercialmente disponibles como cátodos en BGA, ya que la mayoría de materiales usados en las BGA que se muestran en la bibliografía requieren procesos de fabricación de alto coste, y esto incrementa a su vez el coste de la batería. El grado de grafitización (g) y la fracción amorfa (Id/Ig) se determina mediante DRX y espectroscopia Raman. La reversibilidad de la intercalación del AlCl4-

en estos materiales se estudia mediante VC y el rendimiento de los diferentes carbones estudiados como cátodos se dilucida mediante pruebas de carga-descarga en baterías completas. Las resultados muestran que sólo los carbones con un alto g y baja proporción Id/Ig

son capaces de llevar a cabo la intercalación de AlCl4-

con un buen rendimiento electroquímico cuando se usan en BGA. Los polvos de nanopartículas de SGF presentaron las mejores características para desarrollar cátodos de BGA debido a la mejora en la difusión de iones AlCl4-

y la alta flexibilidad en el proceso de fabricación. Estas propiedades dan como resultado a baterías con cátodos de SGF con baja carga másica (10 mg cm^-2) que tienen una alta capacidad específica y una excelente comportamiento en condiciones de alta demanda de corriente y a baterías con cátodos de SGF con alta carga másica (100 mg cm^-2) cuyas capacidades areales son mayores que las reportadas bibliográficamente para BGA hasta ahora.

En el Capítulo 8, se propone y demuestra una nueva tecnología de baterías basadas en Al:

Las Baterías de Al Semi-sólidas. El objetivo de las baterías semi-sólidas es incrementar la capacidad areal de las baterías de Al, que es clave para reducir el coste de la batería como revela nuestro análisis económico. La principal novedad se basa en el uso de un cátodo semi- sólido, que consiste en una mezcla de polvo de grafito y electrolito en ausencia de aglomerante. Este concepto de batería requiere un nuevo proceso de fabricación ya que el material catódico es inyectado en último lugar en una celda de una batería pre-ensamblada, en contraste con la fabricación convencional en el cual los colectores de corriente se revisten con el material catódico en el primer lugar. Diferentes cátodos semi-sólidos se preparan y se caracterizan y su rendimiento electroquímico en BGA se evalúa mediante cargas y descargas galvanostáticas. Las BGA semi-sólidas muestran un coeficiente de difusión mayor que las baterías con electrodos convencionales. Esto resulta en BGA con una capacidad areal y una tasa de utilización altas. Este incremento en la capacidad areal disminuye el coste de la batería en un 80% así como la contribución del colector de corriente positivo. El uso de cátodos semi-sólidos no sólo mejora el transporte de masas a través de electrodos de alto espesor si no

que aumenta la pobre mojabilidad relacionada con la alta viscosidad del electrolito. Por último en este capítulo se demuestra la versatilidad del sistema mediante el uso de diferentes tipos de grafito en polvo. Esto abre la posibilidad de diseñar materiales avanzados para alcanzar aún más mejoras en el rendimiento de la batería en el futuro.

El Capítulo 9 resume las principales conclusiones de la tesis y presenta los futuros retos identificados de esta tecnología.

Chapter 1. I NTRODUCTION A ^ND A ^LUMINIUM B ^ATTERIES

L ^ITERATURE R ^EVIEW

1.1. Energy System and Environmental Concerns

Nowadays, our society has to face two important issues: the population growth and the increase of the energy demand. On the one hand, the world population has risen from 3700 million people in 1970 to more than 7500 million in 2018, which is a twofold increase in the human population (Figure 1.1a).¹ On the other hand, between 1970 and 2018, the energy consumption per capita has increased by threefold from 1200 kWh per capita to 3100 kWh per capita (Figure 1.1b).² This is due to the technological and industrial development that the society has experienced during those years. Hence, the global energy demand is currently 6 times higher than 50 years ago.

Figure 1. 1. (a) Population growth from 1950 to 2018 and (b) energy consumption evolution from 1970 to 2018.^1,2

This energy demand can be easily supplied by means of fossil fuels. In fact, according to the 68^th BP Statistical Review of World Energy 2019,³ the majority of the energy produced is extracted by coal, oil and natural gas (Figure 1.2a). These types of resources have produced 69-80% of the total energy consumed between 1986 and 2018. However, there are several environmental issues associated to the energy production from fossil fuels due to the emission of CO2, SOx and NOx such as the global warming, acid rain or humane diseases.

Therefore, worldwide authorities are facing a dual challenge: the need of more energy and the use of lower amount of fossil fuels. This situation has promoted the use of alternative energy production systems like energy produced from renewable sources, also called renewable energies, which are generated from unlimited sources. Wind, geothermal or solar radiation energy sources fall into this category. The implementation of this type of energy in the energy system has been increasing year by year since the early nineties. Currently, the global production of renewable energy is close to 10% of the total energy produced, and this

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value reaches the 19% in Europe, (Figure 1.2b and Figure 1.2c). This value is expected to grow much more in the coming years. In fact, the total investment in renewable energy has increased from 177 billion USD in 2008 to 362 billion USD in 2017 according to the Renewables 2019. Global Status Report.⁴ However, these energy systems suffer from issues that should be overcome such as the intermittent character of sources like wind or solar radiation, which causes a mismatching between energy production and demand.

Figure 1. 2. (a) World energy consumption from different types of energy sources, (b) share of global electricity generation by fuel and (c) renewable share of power generation by region. Adapted with

permission from ref ³.

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1.1.1. Energy Storage Systems

The limitations derived from the intermittent character of renewable sources can be solved by using energy storage systems (ESSs). In this way, when the production exceeds the demand, the remaining energy is stored in ESSs. Consequently, when the energy demanded surpasses the energy produced, these systems can deliver the necessary energy to meet the demand. Furthermore, the use of ESSs softens the dependence between the energy demand and the production and decreases the installation costs. This is due to the fact that the power installed should answer the average of energy demand instead of the peaks of the demand.⁵ The ESSs can be classified into five categories depending on the way that energy is stored.

These categories are chemical, thermal, mechanical, electrical and electrochemical.

 Chemical Energy Storage Systems: the chemical ESSs store energy in molecules by reversibly forming chemical bonds. This energy is extracted only by means of chemical reactions which change the chemical nature of the substance used as storage system.

The most representative chemical ESSs are Hydrogen (H2), which is produced by water electrolysis and can be stored in gas phase at high pressure (350-700 bar), metal hydrides or other sorbent materials; and Synthetic Natural Gas (SNG), which is methane and can be produced from Syngas (O2 and CO) or hydrogasification and stored in tanks.⁶

 Thermal Energy Storage Systems: the thermal ESSs are based on a reversible chemical reaction in which there is a heat exchange. This chemical reaction leads to the transformation of reactants in products that absorb heat energy. During the reversible reaction, the products formed can change to the former reactant, releasing the heat energy stored.⁶ The dissociation reaction of ammonia (NH₃) in H₂ and N₂ is an example of this technology.⁷

 Mechanical Energy Storage Systems: the mechanical ESSs use the kinetic energy, potential energy or pressurized gases in order to store energy. One of the most relevant systems of this group are the flywheels, where energy is stored in as kinetic energy of a mass spinning at high speed and its power rate is in hundreds of kW.⁶ After that, the Compressed Air Energy Storage (CAES) stores energy by means of air compression using low consumption compressors. When energy is needed, the air is decompressed.

These systems have a power rating around 100-300 MW.⁸ Another alternative is the Pumped-hydropower storage (PHS) that can store energy based on the difference in

height between tanks of water. Energy is stored by pumping water from a lower tank to a higher one. The stored energy is delivered by means of pump turbines that transfer water from the higher tank to the lower one.⁶ The power range of this technology is higher than 1000 MW. Finally, it is noticeable to mention that, although it is in an early stage of research, Liquid Air Energy Storage (LAES) is a promising alternative, which is based on the compression of cooling air in a refrigerator plant and stores it in insulated tanks.⁹

 Electrical Energy Storage Systems: the most representative Electrical ESS is the Superconducting Magnetic Energy Storage (SMES). In this system energy is stored in a magnetic field creating a superconducting coil. Capacitors can be included in this group.

These devices are comprised by two metal plates separated by a dielectric material and store energy electrostatically through the dielectric polarization causing charge accumulation on opposite electrode plates.

 Electrochemical Energy Storage Systems (EESSs): the Electrochemical ESSs transform electrical energy from chemical energy and vice versa. In this group, supercapacitors and batteries stand out. On the one hand, supercapacitors, also called electrochemical double-layer capacitors (EDLCs), are systems comprised by two electrodes based on activated carbon with high surface area separated by a separator immersed in electrolyte. EDLCs store energy by the formation of a double electric layer in an electrode-electrolyte interface when the device is brought under an electrical driving force. These electrochemical devices are able to respond to high power density peaks.^10,11 On the other hand, batteries consist of two electrodes, which are composed by materials with different electrochemical potentials, and an electrolyte. These devices are able to transform reversibly energy released by an electrochemical reaction in electrical energy. There are different batteries in relation with the chemistry involved in the reaction.^12,13 The most important type of batteries will be explained in the next section.

In summary, each ESS can be defined by the energy that can store and the time for discharging this energy (Figure 1.3). According to these two parameters, each ESS will be suitable to be used in different applications.

Figure 1. 3. Relationship between energy capacity and time response for different ESSs.¹⁴

1.1.2. Electrochemical Energy Storage Systems: Batteries

As mentioned above, a battery is a device that can produce electrical energy by means of a reduction-oxidation reaction. These devices are comprised by an anode or negative electrode, which is oxidized yielding electrons to the external circuit during the discharge of the battery; a cathode or positive electrode, which is reduced accepting electrons during the battery discharge; and the electrolyte, which ensures ionic connection between cathode and anode maintaining the charge balance of the electrodes.¹²

Batteries can be generally classified by primary or secondary batteries. Primary batteries cannot be electrically recharged once energy stored is released. In other words, primary batteries are one-use devices that are disposed of after being depleted. These batteries were discovered two hundred years ago by Volta and Daniell (1800 and 1836 respectively), although its use was extended during the 1940s. The advantages of these batteries include their low maintenance, their high flexibility in terms of shape and size and their easy usage.

One of the first primary batteries technologies was based on Zinc and Carbon (Zn-C), although their energy performance was low. After that, the alkaline battery, which is comprised of Zn, manganese dioxide (MnO₂) and alkaline electrolyte, was introduced in the battery market. Other technologies such as those combining Zn or Cadmium (Cd) with

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mercury oxide (HgO) or silver oxide (Ag₂O) have fallen into disuse due to their high toxicity.

The use of lithium as anode has been explored due to the high energy density of the resulting battery. Lithium-based batteries can be classified according to the cathode composition (carbon monofluoride, iodide or copper oxide) or the type of the electrolyte (organic liquids, organics in polymeric matrixes or molten salts). Their battery voltage ranges from 1.5 to 3 V.

Another metal that has been widely used as anode in primary batteries is the Zn. Some Zn-Air primary batteries models have been developed being now commercially available. Finally, aluminium (Al) has been attracting increasing attention due to its low cost, its safety and its high theoretical energy density. Nevertheless, its high corrosion and polarization in aqueous electrolytes have hindered its commercialization.

Secondary batteries, in contrast to primary batteries, can be electrically recharged after their use. Their multiple uses and makes them more suitable for application in portable devices, such as laptops or cell-phones, and transportation, such as pure and hybrid electric vehicles. These batteries can be used in stationary applications as well. The main technologies of secondary or rechargeable batteries that are commercially available are described as follows:

 Lead-Acid Batteries: Lead-Acid Batteries consist of a lead anode, a lead oxide cathode and sulphur acid (H2SO4) solution as electrolyte. They possess a cell voltage of around ~2 V, a good battery efficiency but lower cyclability in comparison with other batteries (500 cycles). Although their specific energy is quite low (40 Wh kg^-1),¹⁵ they are widely used in some applications (stationary and star-up in vehicles) due to their low fabrication expenses.

 Nickel-Cd (Ni-Cd) Batteries: these batteries consist of a nickel oxide hydroxide (NiOOH) cathode and a metallic Cd anode, and alkaline potassium (KOH) as electrolyte. They show a nominal voltage of 1.2 V and a specific energy of 70 Wh kg^-1 and medium-high cycle life (1000 cycles).¹⁵ However, despite their low maintenance cost, their use was forbidden in the European Union (EU) due to the high toxicity of Cd and it was substituted by metal hydride (MH).

 Ni-MH Batteries: These batteries have the same structure of the batteries previously described but in this case the Cd metal is replaced by MH. These batteries have a specific energy of 150Wh kg^-1, a nominal voltage of 1.2 V and a life cycle of ~1000

cycles.¹⁶ These batteries have been used in consumer electronic equipments and hybrid electric vehicles.

 Redox Flow Batteries: these innovative rechargeable batteries are formed by anodes and cathodes which are molecules dissolved in the electrolyte. Therefore, the electrolyte that contains the cathode is called catholyte, and the electrolyte that contains the anode is called anolyte. Both anolyte and catholyte are stored in two external tanks. The anolyte and catholyte are pumped into the cell during the charge/discharge and the reactions take place on the surface of the electrodes. In this way, the power (cell size) and the energy (tank size) can be scaled independently. The most conventional electrolytes consist of soluble vanadium redox species. These batteries can afford an energy density of 20 Wh kg^-1 and a nominal cell voltage of 1.26 V.^17,18 However, due to the prices of the vanadium, other redox-flow technologies have been explored including aqueous solution of iron, chromium, copper or even organic molecules.^19,20

 Metal-air batteries: this type of batteries is based on redox reaction between a metal such as Li or Zn and the air. The main advantage of this technology is the high theoretical energy density, much higher in comparison with other technologies.²¹ Nevertheless, the development of these technologies is currently under research phase and there are several issues that have to be overcome before their commercial deployment.

 Lithium-ion Batteries (LIBs): these batteries are commercial since 1990 and, due to their good electrochemical properties, they can be found in a wide range of devices such as laptops, mobile phones or electric vehicles. S. Whittingham, J. B. Goodenough and A. Yoshiro are considered the “fathers” of LIBs. Recently, they have been awarded with the Nobel Prize of Chemistry (2019) for the important scientific breakthroughs achieved in this field, making possible the development of this important battery technology. In the 70s, S. Whittingham intensively studied the lithium intercalation in metal-based materials.^22,23 After that, in 1980, J. B. Goodenough explored the use of Lithium Cobalt Oxide (LCO) as cathode for high energy density LIBs for the first time.²⁴ Finally, in 1986, A. Yoshiro and co-workers developed the first viable and reliable LIB prototype.²⁵ Since then, scientific efforts have been performed in the development of novel materials in order to improve LIBs performance. LIBs are formed by graphite as anode and different intercalation materials as cathode such as Lithium iron phosphate (LFP), Lithium nickel cobalt manganese oxide (NMC) or Lithium Cobalt Oxide (LCO)

amongst others. The properties of LIBs vary depending on the cathode selected. For instance, the LFP-graphite battery has a nominal voltage of 3.2 V, a specific energy of 120 Wh kg^-1 and a cycle life of 3600 cycles. The NMC-graphite battery has 3.6 V as nominal voltage, a specific energy of 300 Wh kg^-1 and a cyclability of 2000 cycle.

Finally, the LCO-graphite batteries have a nominal voltage of 3.6 V, a specific energy of 250 Wh kg^-1 and a cyclability of 1000 cycle.^15,26,27 The LIBs that use Li metal as anode reach higher specific energies but their commercial deployment is hindered because of the high cost of Li metal, low stability of the Li metal and the associate safety issues of the battery.

In summary, batteries offer a wide range of different characteristics, e.g. specific energy and power, depending on the type of chemistry used. The most relevant battery technologies according to its specific energy and power can be seen in the Figure 1.4.

Figure 1. 4. Relation between specific power and specific energy for commercial secondary batteries.¹⁵

As it has previously mentioned, the lithium-ion battery (LIB) is nowadays one of the most mature and widely known energy storage technology offering high electrochemical performance. ^13,28 However, its massive use in stationary applications as well as in electric

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vehicles is under question due to the excessive consumption of high cost and scarce minerals containing Li and Co. Research on the development of batteries based on metals such as sodium (Na),²⁹ magnesium (Mg),³⁰ potassium (K),³¹ calcium (Ca),³² zinc (Zn),³³ and aluminium (Al)³⁴ has attracted increasing attention during the last decade (Figure 1.5 )

Figure 1. 5. Theoretical properties and abundances of metal proposed as anode for metal-based batteries.

Aluminium stands out amongst all of them in terms of volumetric capacity and abundance. Al presents a gravimetric charge storage capacity (2.98 Ah g^-1) which is comparable to Li (3.86 Ah g^-1). Furthermore, its volumetric capacity is four times higher than the volumetric density of Li (8.04 Ah cm^-3 and 2.06 Ah cm^-3, respectively). It is the third most abundant element in the Earth´s crust (8.1%) after oxygen (46.6%) and silicon (27.7%). Due to its abundance and the wide extension of bauxite deposits (Al(OH)₃) its commercialization is extended and its cost is quite low (thirty times lower than the price of metallic Li).³⁴ It is safe, environmentally friendly and easy to handle. However, the standard redox potential of Al (- 1.7 V vs SHE) is more positive than that of other metals, e.g. Li (-3 V vs SHE) and Na (- 2.8 V vs SHE) leading to lower cell voltage. Overall, the distinct features of Al-based batteries may be of high interest for a number of applications.

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