CRYSTALLOGRAPHIC CONTROL AND INTERFACE DESIGN STRATEGIES 

FOR SODIUM-BASED SECONDARY BATTERY ANODES 

 

 

 

 

 

 

 

A Dissertation 

Presented to the Faculty of the Graduate School 

of Cornell University 

In Partial Fulfillment of the Requirements for the Degree of 

Doctor of Philosophy 

 

 

 

 

 

 

by 

Yue Deng 

August 2023



 

 

 

 

 

 

 

 

 

 

 

 

 

© 2023 Yue Deng



 

 

 

CRYSTALLOGRAPHIC CONTROL AND INTERFACE DESIGN STRATEGIES 

FOR SODIUM-BASED SECONDARY BATTERY ANODES 

 

Yue Deng, Ph. D.  

Cornell University 2023 

 
Rechargeable sodium batteries have garnered increasing interest as a promising 

pathway for low-cost, long-duration energy storage. With a high theoretical specific 

capacity of 1166 mAh/g and low reduction potential of −2.71 V, batteries 

incorporating metallic sodium as the anode hold great potential. However, the high 

reactivity and poor electrochemical reversibility of sodium anodes present significant 

challenges for practical implementation. 

 

We investigate the failure mechanisms of room-temperature, liquid electrolyte sodium 

metal anodes through in operando optical visualization and polarization 

measurements. Electronic disconnection of mossy metallic sodium deposits, known as 

"orphaning," is identified as a dominant source of anode irreversibility. To overcome 

this issue, nonplanar electrode architectures and thin metallic coatings are explored to 

accommodate fragile sodium deposits and promote good root growth, addressing poor 

reversibility and cell failure. Additionally, design principles for heterointerfacial 

alloying kinetics at metallic anodes are investigated. The crucial role of a moderate 

strength of chemical interaction between the deposit and substrate is revealed, 

enabling the highest reversibility and stability of plating/stripping redox processes. 



 

 

Crystallographic control is another key aspect for optimizing electrode performance. 

Textured electrodes formed via severe plastic deformation and optimized deposition 

conditions are explored to manipulate the built-in crystallographic heterogeneity of 

metal electrodes, resulting in significant improvements in plating/stripping 

performance. 

 

Collectively, this body of work contributes to the advancement of rechargeable 

sodium battery technologies by providing insights into alloying kinetics, electrode 

design strategies, and crystallographic control. The findings highlight the potential to 

enhance the performance of metal electrodes in sodium-based systems and beyond, 

fostering the development of high-performance, durable, and sustainable energy 

storage solutions. 

 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 

 



 

 v 

BIOGRAPHICAL SKETCH 

 
Yue Deng, born in Nanjing in Spring 1994, was raised in a family with a strong 

scientific and engineering background, thanks to her physician parents and civil 

engineer grandparents. This upbringing fueled her early passion for engineering, 

which she pursued by obtaining a major in chemical engineering at the University of 

California, Davis. It was during her time there that Yue discovered the captivating 

field of materials science, sparking a desire for further education in this domain. 

 

Driven by her thirst for knowledge and her aspiration to become a meticulous 

engineer, Yue embarked on a journey at Cornell University, pursuing both a Doctor of 

Philosophy and a Master of Science degrees in Materials Science and Engineering. 

Throughout her graduate studies, she received invaluable guidance from esteemed 

professors, including Professor Lynden Archer, Professor Lena Kourkoutis, Professor 

Christopher Ober, and Professor David Muller. Her research focused on 

crystallographic control and interface design strategies for sodium-based secondary 

battery anodes, with a particular emphasis on alkali-metal secondary batteries and the 

deep cycling of active metal anodes. 

 

Yue firmly believes that everyone possesses unique strengths and talents and is 

passionate about nurturing the growth and development of others through teaching. 

She envisions a greener and better future powered by science and technology and is 

committed to making a meaningful impact in the pursuit of sustainability. With her 

expertise and dedication, Yue eagerly anticipates contributing to the advancement of 

technology for a more sustainable and environmentally friendly world. 

 

 

 

 

 



 

 vi 

 

 

 

 

 

 

 

In loving memory of my dearest grandparents, 

Professor Qixian Luo and Ms. Mingzhu Zhou, 

whose unconditional love and unwavering support enriched my journey, 

 

and to the memory of Professor Lena Kourkoutis, 

whose guidance and mentorship shaped my graduate studies. 

 

This book is dedicated to you, 

with profound gratitude and everlasting love. 

 

In your memory,  

I am forever inspired to pursue knowledge and strive to make a lasting impact. 



 

 vii 

ACKNOWLEDGMENTS 

 

I would like to take this opportunity to express my sincere gratitude to the individuals 

who have played a pivotal role in my academic journey and the successful completion 

of this doctoral dissertation. 

 

First and foremost, I extend my deepest appreciation to my advisor, Professor Lynden 

Archer. From the moment we met, I was captivated by his wisdom and warmth, which 

led me to put him as my top choice for advisor though it meant transitioning into a 

completely new field. Now I can confirm that it has been one of the best decisions I 

have made in my life. I am glad that I was able to join the Archer Group and I am 

honored to graduate as a member of this exceptional group. Professor Archer's 

steadfast guidance, invaluable insights, and continuous support have shaped my 

research and personal growth. Our countless discussions, even amidst his busy 

schedule as the Dean of the College of Engineering, have been instrumental in the 

completion of this dissertation. His unwavering enthusiasm for science and 

commitment to excellence continue to inspire me to strive for the highest standards. 

 

I am also indebted to Professor Christopher Ober for his valuable guidance and 

assistance throughout my doctoral program. His expertise and thoughtful feedback 

have significantly contributed to the development and refinement of this dissertation. 

 

My sincere gratitude goes to Professor Lena Kourkoutis for her remarkable 

mentorship. Her belief in my potential, invaluable advice, and dedicated guidance 

have been pivotal throughout my graduate studies. I am especially thankful for her 

unwavering support during my final defense, which will forever serve as a guiding 



 

 viii 

light in my journey. I would like to express my sincere appreciation to Professor 

David Muller for graciously acting as my proxy during a challenging period and 

providing valuable insights on my dissertation. 

 

And I am super grateful for the firm support of my parents, whose love, 

encouragement, and sacrifices have been the foundation of my academic journey. 

Their belief in my abilities has been a constant source of inspiration. Additionally, I 

want to acknowledge my grandparents, Professor Qixian Luo and Mingzhu Zhou, 

whose unconditional love and support enriched my journey and hold a special place in 

my heart. 

 

Moreover, I extend my deepest gratitude to all my esteemed colleagues in the Archer 

Group, both present and past, as well as the invaluable assistance provided by the 

alumni who have accompanied me on this transformative path. Dr. Qing Zhao, Dr. 

Jingxu Zheng, Dr. Prayag Biswal, Dr. Jiefu Yin, Dr. Snehashis Choudhury, Dr. 

Zhengyuan Tu, Prof. Shuya Wei, Dr. Mun Sek Kim, Dr. Regina Garcia, Nyalalisk 

Utomo, Shuo Jin, and many others have collaborated with me, engaging in insightful 

discussions, and sharing our collective passion for research. It is through these 

collaborative endeavors that I have grown as a scientist, and their contributions have 

been truly invaluable. 

 

Finally, I would like to express my appreciation to my friends, including Dr. Yusuke 

Hibi, Kaiyang Wang, Dasol Yoon, Peilong Lu, Mengqi Duan, Dongyi Wu, Sidi Duan, 

Yichen Yan, Motoshi Horii, Grace Chen, Linda Jiang, Van Huynh, Faustine Wang, 

Karen Chiang, Shiling Cui, Dr. Chenyue Sun, Dr. Zhiyao Zhou, Jason Huang, Ziqiu 

Zhang, Yidan Zhang, Zhonghao Wang, Huilin Zhang, Shifeng Hong, and many others. 



 

 ix 

Their support and friendship have been a source of strength throughout this 

challenging yet rewarding endeavor. 

 

While the list of people I would like to thank goes on and on, the limitations of this 

space prevent me from acknowledging everyone. To everyone who have contributed 

to my journey, I offer my heartfelt gratitude. Your support, guidance, and belief in my 

abilities have been instrumental in reaching this significant milestone in my academic 

career. 

 

This dissertation represents the culmination of an extraordinary journey spanning over 

six years within a research group at an esteemed educational institution. I have poured 

all my love, effort, and creative passion into this work, collectively envisioning, 

creating, dreaming, and growing alongside exceptional individuals who have shaped 

my path. Together, we have overcome challenges, celebrated achievements, and 

pushed the boundaries of knowledge. It is with deep gratitude and a profound sense of 

accomplishment that I conclude this chapter of my academic journey, forever 

cherishing the invaluable contributions of each person who has played a role in this 

remarkable endeavor. 

 

 

 

 

 

 

 

 



 

 x 

TABLE OF CONTENT 

 

Chapter 1: Introduction .............................................................................................. 1 

1.1 Skyrocketing demand of secondary batteries  ...................................................... 2 

1.2 From lithium batteries to sodium batteries: chemistry and cost  ....................... 15 

1.3 Recent advances in high-performance sodium metal batteries (SMBs)  ........... 29 

Chapter 2: On the Reversibility and Fragility of Sodium Metal Electrodes  ....... 63 

2.1 Introduction  ....................................................................................................... 64 

2.2 Results and discuss ............................................................................................. 68 

2.3 Conclusions ........................................................................................................ 73 

2.4 Supporting information and additional discussions ........................................... 87 

Chapter 3: Design Principles for Heterointerfacial Alloying Kinetics at Metallic 
Anodes in Rechargeable Batteries  ......................................................................... 109 

3.1 Introduction  ..................................................................................................... 110 

3.2 Results and discuss ........................................................................................... 115 

3.3 Conclusions ...................................................................................................... 136 

3.4 Supporting information and additional discussions ......................................... 137 

Chapter 4: Highly Reversible Sodium Metal Battery Anodes via Alloying 
Heterointerfaces  ...................................................................................................... 194 

4.1 Introduction  ..................................................................................................... 195 

4.2 Results and discuss ........................................................................................... 198 

4.3 Conclusions ...................................................................................................... 215 

4.4 Supporting information and additional discussions ......................................... 217 



 

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Chapter 5: Textured Electrodes: Manipulating built-in crystallographic 
heterogeneity of metal electrodes via Severe Plastic Deformation  ..................... 244 

5.1 Introduction  ..................................................................................................... 245 

5.2 Results and discuss ........................................................................................... 249 

5.3 Conclusions ...................................................................................................... 268 

5.4 Supporting information and additional discussions ......................................... 269 

Chapter 6: Investigating the Influence of Electrolyte and Overpotential on 
Reversibility and SEI Formationof Sodium Metal Anodes  ................................. 291 

6.1 Introduction  ..................................................................................................... 292 

6.2 Results and discuss ........................................................................................... 296 

6.3 Conclusions ...................................................................................................... 316 

6.4 Supporting information and additional discussions ......................................... 318 

Chapter 7: CONCLUSION AND OUTLOOK  

Advancing Sodium Metal Anode Performance: Insights from Crystallographic 
Control, Interface Design, and Alloying Kinetics ................................................. 333 
 
 
 

 

 

 
 



 

 1 

 

 

 

 

 

 

 

 

 

 

 

 

CHAPTER 1 
 

INTRODUCTION 
 

 

 

 

 

 

 

 

 

 
 



 

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1.1 Skyrocketing demand of secondary batteries 

The need for energy has become increasingly critical in our modern world, and it is a 

vital factor for the growth and well-being of our society. From powering our homes and 

industries to fueling transportation and communication systems, energy plays a critical 

role in driving economic growth and sustaining our modern civilization. The U.S. 

Energy Information Administration (EIA)’s projects that the global energy consumption 

will come to about 910 quadrillion British thermal units (Btu), equivalent of 270 

quadrillion Wh, in 2025 (as shown in Fig 1.1.1a) 1; while the domestic demand for 

electricity in the United States will rise to 105.94 quadrillion Btu 2,3. The growth is most 

rapid in developing economies in the non-Organization for Economic Corporation and 

Development (non-OECD) countries. 

 

The global demand for energy, coupled with the finite supply of fossil fuels and the 

environmental consequences of their use, has led to an urgent need for a transition to 

cleaner and more sustainable energy sources. This transition is driven by the need to 

reduce carbon emissions and combat climate change, as exemplified by the landmark 

multilateral climate agreement. In 2015, the Paris Climate Agreement was adopted by 

196 nations and parties, with the goal of keeping a rise in global temperatures to below 

2°C from pre-industrial levels by the end of this century, while setting 1.5°C as the 

overarching target 4,5. More than 130 countries have set a target of reducing greenhouse 

gas emissions to 'net zero' by 2050 to achieve this goal 6. Implemented government 

policies and regulations aim to incentivize the adoption of clean energy technologies. 

Thus while fossil fuels still dominate the global energy mix, progress towards full 

deployment of renewable energy systems is accelerating and cost of these technologies 

is decreasing, making them progressively competitive with fossil fuels 7,8. These trends 

are illustrated in Fig. 1.1.1b-c, where USEIA projections show that the fraction of global 



 

 3 

power generated by renewables will grow significantly over the next thirty years 1,9. 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 
Figure 1.1.1 World energy consumption with historical data until 2020 and 

projections until 2050. (a) total consumption (b) consumption by energy source (c) 

share of energy consumption by source. 1 OECD refers to the countries that are members 

of the Organization for Economic Co-operation and Development (OECD); and non-

OECD includes all the countries and regions that are not in the Organization for 

Economic Co-operation and Development. Primary energy consumption measures the 

total energy input required to meet the energy needs of the economy, without accounting 

(a) 

(b) (c) 



 

 4 

for any energy losses during the conversion or distribution processes. It serves as a 

fundamental indicator for assessing energy demand and the overall energy footprint of 

a region, country, or sector. The U.S. Energy Information Administration (EIA) 

develops its energy projections through a comprehensive and data-driven approach. The 

EIA is the statistical and analytical agency within the U.S. Department of Energy 

responsible for collecting, analyzing, and disseminating energy-related information. 

The EIA regularly updates its projections as new data becomes available and adjusts its 

models to reflect changes in market conditions, policy developments, and technological 

advancements. It is important to note that projections inherently carry uncertainties, and 

the EIA's projections are subject to revisions as conditions evolve over time. 

 

 

 

 

 

 

With increasing attention and investment, renewable energy technologies such as solar, 

wind, hydro, and geothermal are experiencing significant improvements in efficiency, 

cost-effectiveness, and scalability.10–17 Solar photovoltaic (PV) technology, in 

particular, has undergone rapid advancements, leading to significant cost reductions and 

performance improvements. For instance, from 2010 to 2020, the cost of residential 

solar PV modules fell over 60%, and the cost of utility-scale solar PV has fell over 80%. 

18 Wind power technology is also experiencing significant advancements, with larger, 

more efficient turbines capable of generating more electricity and operating in lower 

wind speeds being developed.19–21 However, these sources still present unique 

challenges that need to be addressed. Solar energy is abundant and widely available, but 

it is dependent on daylight hours and weather conditions, which can limit its reliability. 



 

 5 

Wind energy, on the other hand, is dependent on wind speeds, which can vary 

considerably over time and location. Hydroelectric power can provide a consistent 

source of energy, but it is limited to areas with sufficient water resources and can be 

impacted by changes in precipitation patterns. In short, the intermittency of these 

sources makes it difficult to guarantee a steady and reliable energy supply, and therefore 

grid integration is another major challenge for renewable energy resources. Efficient 

energy storage systems are required to smooth the power output and to ensure the grid 

stability.  

 

Rechargeable batteries have emerged as a promising solution to overcoming many of 

the challenges associated with renewable energy sources, including their intermittency, 

and difficult grid integration. Secondary/rechargeable battery storage technologies 

address intermittency by providing a backup power supply during periods of low or no 

energy production. This feature of rechargeable batteries is particularly important for 

solar and wind energy systems, which are dependent on weather conditions. Battery 

storage technologies can also provide a flexible and scalable solution to balance energy 

supply on the grid by storing excess renewable energy during times of low demand and 

releasing it during peak demand periods. By utilizing rechargeable batteries, renewable 

energy systems can therefore become more reliable and stable, reducing the need for 

backup fossil fuel generators and the reliance on non-renewable energy sources. The 

increasing deployment of cost-effective rechargeable batteries in energy systems offer 

immense potential to accelerate the energy transition. 

 

In addition to their contribution to the energy transition, rechargeable batteries have also 

broadened energy access by offering off-grid power supply solutions. In this regard, the 

use of rechargeable batteries has become increasingly important in decarbonizing road 



 

 6 

transportation and portable devices. Electric vehicles (EVs) have the potential to 

significantly reduce carbon emissions and air pollution compared to conventional 

gasoline-powered vehicles. The use of rechargeable batteries in EVs is critical to their 

success, as they store and release the electrical energy that powers the vehicles. 

Furthermore, the development of rechargeable batteries has had an immense impact on 

society, as they have facilitated the creation of portable electronic devices such as 

laptops and smartphones. 

 

The growing demand for EVs and portable electronic devices has led to an increasing 

need for rechargeable batteries that are efficient, large in capacity, long-lasting, 

lightweight, and affordable. Due to its low gravimetric density and low reduction 

potential (-3.05 V vs standard hydrogen electrode), lithium batteries have gained 

significant attention since the first lithium-ion battery (LIB) was invented in the 1970s. 

In 1976, Stanley Whittingham developed the first functional lithium battery that used 

metallic lithium as the anode and various layered metal sulfides as cathode. Later, in 

1980, John Goodenough improved the battery's potential twofold by replacing the 

cathode material from a metal sulfide to lithium cobalt oxide, laying the foundation for 

a much more powerful and practical battery 22. In 1985, Akira Yoshino eliminated pure 

lithium from the battery, relying entirely on lithium ions, which are safer than pure 

lithium, making the battery more practical for use. Their collective efforts have paved 

the way for a LIB design that made it possible to fulfill the growing demand for portable 

electronic devices, electric vehicles, and renewable energy storage, offering a promising 

outlook for a sustainable future and significant benefits to humankind. Consequently, in 

2019, the Royal Swedish Academy of Sciences recognized the contributions of John B. 

Goodenough, M. Stanley Whittingham, and Akira Yoshino, awarding them the Nobel 

Prize in Chemistry 23.  



 

 7 

In the decades following the initial development of the LIB, researchers from around 

the globe have made significant contributions towards enhancing LIB performance 

through systematic exploration of the periodic table. Presently, most commercial LIBs 

incorporate a carbonate-based electrolyte solvent due to its high oxidation resistance up 

to approximately 4.5V. The inclusion of fluoride additives in the electrolyte aids in the 

formation of a fluorinated solid-electrolyte interface (SEI), which has been 

demonstrated to improve the anode reversibility, particularly in the case of lithium metal 

anodes. Additionally, ceramic coatings have been developed and employed on the 

separator and cathode to prevent dendritic deposition of lithium and internal short-

circuiting, which can occur on non-metallic lithium anodes and when discharging 

voltage is too near the reduction potential of lithium. Recent advancements in anode and 

cathode materials, such as the silicon anode and the lithium nickel-cobalt-oxide (NCA) 

cathode, have significantly improved both the specific capacity and kinetic performance 

of electrochemical Li+ cells, allowing for longer usage time and faster charging. To 

further increase the energy density and safety of current LIB systems, researchers are 

currently investigating solid-state electrolytes, examples including lithium superionic 

conductor (LISICON)-like structures, as such examples of LISICON, and garnet, such 

as Li7La3Zr2O12 (LLZO). 

 

To be widely used in the vast array of scenarios envisioned in the energy transition, 

materials and cell design strategies that lower battery cost are high priorities. Indeed, 

over the past three decades, the cost of lithium batteries has decreases significantly. 

When lithium-ion batteries (LIBs) were first commercialized in the early 1990s, their 

cost was over $1,000 per kilowatt-hour (kWh), which was prohibitively high for most 

applications. At that time, the batteries were mainly used in niche applications such as 

medical devices, military equipment, and satellites. Driven by technological 



 

 8 

advancements, increased production scale, and improvements in manufacturing 

processes, LIB costs have fallen steadily over the last three decades. For example, the 

introduction of cathode lithium iron phosphate (LFP) makes LIBs more affordable while 

also considerably extending the cell cycle life. Additionally, lithium nickel-cobalt-

manganese (NCM)24 is another LIB cathode that is widely used in electric vehicles 

(EVs) nowadays for its high specific capacity and high working voltage. The use of 

these two cathode materials contributed to a significant reduction in the cost of LIBs in 

terms of cost per kWh discharged during their lifetime. Furthermore, government 

policies and incentives have played an important role in driving down the cost of lithium 

batteries. For example, in 2009, the American Recovery and Reinvestment Act provided 

$2 billion in funding to support the development and production of advanced battery 

technologies, including lithium-ion batteries. By the mid 2010s, the cost of lithium-ion 

batteries had decreased to around $500 per kWh. And, in the last decade prices have 

fallen further to below $200 per kWh today, and projected to fall by at least another 

$100 per kWh in the next decade. 25  

 

 

 

 

 

 

 

 

 

 

 



 

 9 

 

 

 

 

 

 

 

 

 

 

 

 

 

 
Figure 1.1.2 Changes of LIB prices over time, with anticipated values for year 2017 

and beyond. 25 Here BNEF stands for BloombergNEF. which is a leading research 

organization that provides analysis, data, and insights on the global energy industry, 

including renewable energy, energy storage, electric mobility, and sustainable finance. 

BNEF projections are made using a combination of data analysis, modeling techniques, 

expert insights, and industry knowledge.  

 

 

 

 

 



 

 10 

It is understood that amortized rechargeable battery prices below the projected $100 per 

kWh would enhance the competitiveness of electrification in the transportation sector26. 

Naturally, the question now is, how much further can LIB costs fall? Canalys, a 

technology market analyst firm, recently reported a significant growth in global sales of 

electric vehicles (EVs), which rose annually by 55% to 10.1 million units in 2022 27. 

This impressive increase is projected to continue, with EVs predicted to account for 

48% of passenger car sales worldwide in 2030 28. This surge in demand has exposed 

concerns about the limited availability of raw materials required for LIB production. As 

shown in Table 1.1.1, LIB manufacturing involves several elements with scarcity, 

including lithium used in anodes, cobalt used in cathodes, and copper used for anode 

current collector. A standard 60 kWh LIB battery pack, with lithium/graphite as the 

anode and nickel-cobalt-manganese (NCM) as the cathode, would contain 

approximately 7.5 kg of lithium, 65 kg of nickel, cobalt, and manganese in total, 48 kg 

of copper, and 30 kg of aluminum 29. Among which, copper can be quite costly 30,31, as 

shown in Fig. 1.2.2. Mining, environmental, and geopolitical challenges associated with 

extraction of lithium and cobalt from known reserves distributed in regional pockets 

across the globe exacerbates these difficulties.   

 

Lithium is primarily sourced from brines 32,33 found under the salt crust of dry lakes in 

a handful of countries, mostly located in South America (as shown in Fig 1.1.3). 34 

Although providing the most economical means to access lithium, these deposits are 

situated in remote geographical areas exposed to severe weather Furthermore, lithium 

is usually obtained as a byproduct of mining other elements, with most brine operations 

primarily producing sodium and potassium (an example of brine composition is given 

in Fig. 1.1.4a), which makes the production scale of lithium not very sensitive to the 

demand, leading to price peaks from time to time.  



 

 11 

Similarly, cobalt, an essential element for LIB cathodes, as illustrated in Fig. 1.1.4b is 

mainly obtained as the byproduct of nickel and copper mining, with only about 15% 

produced through primary production 35. The dependence on co-elements renders cobalt 

mining less adaptable to swift market fluctuations, thereby jeopardizing supply 

reliability and pricing. Moreover, the Central African Copperbelt, located on the border 

between the Democratic Republic of the Congo and Zambia, is home to a substantial 

concentration of cobalt reserves but also known for regional political instability, which 

poses an additional risk to lithium production and mining conditions 36. As African and 

South American countries have a significant impact on raw mineral materials, 

industrialized Asian countries, such as Japan and South Korea, dominate the production 

of intermediate and finished products. Yet from a holistic view of the multilayer supply 

network, Germany, the USA, and China play vital roles 37,38. A recent study by Hu and 

Wang et al finds that critical stages of the industry are shifting from a focus on upstream 

mineral resources to a focus on intermediate components and finished products, and the 

varied importance of related commodities in the supply network redefines the influence 

of countries 39. Therefore, the production of LIBs, as an integrated final product, is under 

geopolitical risks and potential supply chain disruptions, which can result in high 

volatility in production volume and price 40–42. As the demand for secondary batteries 

continues to grow, it is essential to invest in research and development to mitigate 

supply chain risks and to explore alternative battery technologies, for promoting a more 

sustainable and resilient energy future. 

 

 

 

 

 



 

 12 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Figure 1.1.3 Geographical distribution of global lithium deposits and their forms. 

The circled region that covers parts of Chile, Argentina, and Bolivia is termed the 

“lithium triangle” where much of the world’s extractable lithium reserves are 

concentrated. Reproduced with permission. 40 Copyright 2016, Elsevier. 

 

 

 

 

 



 

 13 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Figure 1.1.4 Production scale and mining interdependencies of elements for LIB 

and SIB materials. (a) The elemental composition of brine from the Salar de Atacama 

in Chile. (b) Annual global production of transition metals used in LIBs and SIBs from 

mineral mining, with co-production amount included. 31 

 

 



 

 14 

 

 

 

 
Table 1.1.1 Earth Crust abundance of selected elements of interests 43. 

Atomic number Element Crustal abundance (%) 

3 Lithium (Li) 0.002 

8 Oxygen (O) 46.4 

11 Sodium (Na) 2.36 

13 Aluminum (Al) 8.23 

14 Silicon (Si) 28.15 

16 Sulfur (S) 0.026 

25 Manganese (Mn)  0.095 

26 Iron (Fe) 5.63 

27 Cobalt (Co) 0.0025 

28 Nickel (Ni) 0.0075 

29 Copper (Cu) 0.0055 

30 Zinc (Zn) 0.0070 

 

 

 

 
 



 

 15 

1.2 From lithium batteries to sodium batteries: chemistry and cost 

Based on the previous discussion, it is apparent that the increasing demand for 

rechargeable batteries necessitates the exploration of alternatives to LIBs, which are 

constructed from materials that are more earth-abundant and affordable. Rechargeable 

batteries based on sodium are leading candidates among the various so-called beyond 

Lithium batteries for several reasons, including: 1. The chemistry, solvation behaviors, 

and electrokinetics of the working Na+ ion in a sodium battery is similar to the Li+ ions 

in LIBs; 2. Sodium batteries offer energy densities that are comparable to LIBs; 3. 

Sodium’s high earth-abundance and availability world-wide position sodium batteries 

as cost-effective, scalable technology for addressing the global electrical energy storage 

needs associated with the energy transition. Additionally, various designs of sodium 

batteries are known 44,45, such as both aprotic and aqueous sodium-ion batteries (SIBs), 

as well as molten sodium-sulfur batteries, which have been commercialized by a number 

of companies 46–48. A consequence of these and other factors discussed later, is that the 

global sodium ion battery (SIB) market size is anticipated to reach 4368 million USD 

by 2030 45.Table 1.2.1 provides an overview of some of the commercial entities  that 

are already developing and producing SIBs. The following section of this chapter will 

explore the foundational chemistries, physical characteristics, and electrochemical 

phenomena that underpin present-day SIBs. And, on that basis we will explore the 

merits and drawbacks of the most promising of these technologies in comparison to 

LIBs. 

 

 

 



 

 16 

 

Table 1.2.1 Some leading companies engaged in the development and research of 
sodium batteries and the chemistry of their primary products. 

Company Chemistry 

NGK Insulators Ltd. 46 

Sodium-sulfur battery 

Anode: sodium (molten) 

Cathode: sulfur (molten) 

Electrolyte: sodium beta-alumina (solid electrolyte) 

Aquion Energy 49–51 

Aqueous sodium-ion battery 

Anode: NaTi2(PO4)3 

Cathode: Na0.44MnO2 

Electrolyte: < 5M NaClO4 

Faradion 52,53 

Aprotic sodium-ion battery 

Anode: hard carbon 

Cathode: mixed O3-and P2-type oxides 

Electrolyte: formulating PC-dominant electrolyte 

TIAMAT 54,55 

Aprotic sodium-ion battery 

Anode: hard carbon 

Cathode: sodium vanadium fluorophosphate (NVPF) 

Electrolyte: NaClO4/carbonate-based electrolyte 

Natron Energy 56,57 

Aprotic sodium-ion battery 

Anode: Prussian Blue 

Cathode: Prussian Blue 

Electrolyte: acetonitrile-based 

Contemporary Amperex 

Technology Co., Ltd. 

(CATL) 58,59 

Aprotic sodium-ion battery 

Anode: hard carbon 

Cathode: Prussian White 

Electrolyte: carbonate-based electrolyte 

 



 

 17 

Similarities and differences in chemistries of LIBs and SIBs 

 

In contrast to multivalent battery systems, such as zinc or aluminum battery systems, 

sodium batteries are a monovalent battery system that shares similar electrokinetic, 

transport, and interfacial characteristics as LIBs. Specifically, analogous to a lithium 

cell, during the charging/discharging process of a sodium-based electrochemical cell, 

only one stage of reduction/oxidation occurs at the anode. This similarity enables the 

use of most anode, cathode, and electrolyte materials that are commonly employed in 

lithium-ion batteries with only minor/obvious adjustments. For instance, the use of 

carbonate electrolytes is prevalent in both battery systems, while LiPF6 and NaPF6 serve 

as popular electrolyte salts for the respective battery systems. Additionally, layered 

oxides, take LiCoO2 and NaCoO2 as examples, are viable cathode material candidates 

for both battery types. Similarly, because of the low electrochemical potential at which 

Li+ and Na+ electroreduce at the anode, anions, solvent, and other components in the 

battery electrolyte are electrochemically unstable. The physical, mechanical, and 

electrochemical characteristics of the byproducts formed when these components 

degrade at the anode to forme new materials phases (interphases) are therefore a key 

determinant of long-term cyclability and self-discharge behaviors of Li- and Na-based 

batteries. Sacrificial electrolyte additives like fluoroethylene carbonate (FEC) and 

LiNO3/NaNO3 provide a straightforward strategy for manipulating the chemistry and 

properties of these interphases, making them popular, actively studied ingredients in 

electrolytes for both types of batteries.    

 

LIBs and SIBs share other features, including the fact that two main types of cell design 

are possible: intercalation-based and conversion reaction-based. As a result, this feature 

facilitates the transition to sodium batteries and makes it relatively easy to adopt them 



 

 18 

as a viable alternative to LIBs. However, it is noteworthy that the larger ionic radius of 

sodium ions compared to lithium ions results in weaker polarization of sodium ions (rNa+ 

= 116 pm, rLi+ = 90 pm). As a result, sodium ions exhibit different coordination number 

(for sodium, CN = 6; for lithium, CN = 4) and diffusion properties, leading to variations 

in lattice constants and crystal structures of both positive and negative electrodes used 

in LIBs and SIBs. Additionally, the desolvation energy of sodium ions differs from that 

of lithium ions. For instance, in organic solvents, sodium ions have approximately 30% 

lower desolvation energy than lithium ions 60, which can affect charge transfer 

resistance as well as the electrode and electrolyte kinetics in SIBs. 

 

One of the technical breakthroughs that have enabled wide-spread use of LIBs is the 

graphitic carbon host for the anode. Graphite serves as a near perfect host for Li+ which 

is able to intercalate between the molecular carbon layers with minimum mechanical 

strain and volume change of the carbon host. This is not true for SIBs. Due to its redox-

amphoteric nature and weak inter-graphene bonding, graphite can easily intercalate not 

only Li cations but also various anions, including popular counter-ions in LIB 

electrolytes like PF6- and TFSI- 61,62, producing a range of graphite intercalation 

compounds (GICs) 63,64. Graphite forms GICs at potentials below 0.25 V vs. Li/Li+, 

which results in a final composition of LiC6 and a capacity of 372 mAh/g 65,66. The 

suitable working potential and relatively large specific capacity give graphite a 

ubiquitous presence in commercialized LIBs. However, it has been found that sodium 

does not form sodium-rich GICs and therefore nearly not possible to be intercalated into 

graphite (with an intercalation capacity as low as 35mAh/g at a low current rate 67–69), 

making pristine graphite unsuitable as an anode material for sodium-ion batteries. 67,70,71 

The reason for sodium's low intercalation capacity with graphite is still unclear. While 

the small inter-plane spacing vs large ionic radius of Na+ appears to be a contributing 



 

 19 

factor, graphite has been shown to form GICs with larger metal ions such as K, Rb, and 

Cs. 29 Some theoretical studies suggest that most sodium-carbon compounds, such as 

NaC6 and NaC8, are unstable due to the stress built up during the stretching of C-C 

bonds. 64,71  

 

The instability of sodium GICs could be another mechanism leading to the low 

intercalation capacity of sodium in graphite. There are currently two primary methods 

being explored to address the challenge of using graphite as the anode host material for 

SIBs. The first is solvent co-intercalation, and the second is doping the graphene layers. 

Research has shown that several solvent molecules, including linear ethers and cyclic 

carbonates and their derivatives, can co-intercalate into graphite with sodium ions (as 

illustrated in Figure 1.2.1a-b). 70,72–74 Doping can increase the inter-layer spacing of 

graphite, which also helps to improve sodium ion intercalation. A widely used example 

of this approach is reduced graphite oxide (rGO). 75 Figure 1.2.1c demonstrates how 

larger inter-layer spacing of rGO can help to enable sodium ion intercalation. 

 

In addition, hard carbons are a promising alternative to graphite for energy storage 

applications. Unlike graphite, hard carbons are a type of disordered carbon that cannot 

be converted into highly ordered graphitic domains under heat treatment. Thermal 

processing of polymers without aromatic rings, such as polyvinylidene difluoride 

(PVDF), polyacrylonitrile (PAN), and biomass, produces mostly hard carbons. 

Biomass-derived hard carbons have been received a good amount of attention, because 

they are often inexpensive and naturally doped. Doping hard carbons with elements like 

boron (B), nitrogen (N), sulfur (S), or phosphorus (P) can improve their electronic 

conductivity, wettability, or capacity of the anode. 76–79 To further enhance ion 

transportation, hard carbons are typically processed into nanostructures. 80–83 In a recent 



 

 20 

study by Wang et al., dual doping with N and B resulted in a stable long-term capacity 

of around 250 mAh/g at a cycling rate of 1A/g, 84 which is comparable with graphite-

based anode materials for LIBs. 

 

At the cathode, transition metal oxides containing cations like Co, Mn, Fe, or Ni as 

redox-active elements are commonly used for both LIBs and SIBs. However, the crystal 

structure of LIBs and SIBs are usually different, as a result of the differences in 

coordination number and ionic radius. Typically, LIB cathodes usually have 𝛼-NaFeO2 

structure, where lithium ions occupy the interstitial octahedral sites. In contrast, 

transition metal oxide cathodes for SIBs typically have O3 and P2 structures, where O 

represents an octahedral insertion of sodium ions and P represents a prismatic insertion 

of sodium ions, and the number 2 or 3 indicates the packing number in a unit cell 85. 

 

As mentioned previously, in addition to the ion-insertion type of design, there exists 

another type of battery design based on conversion materials 86. Conversion-based 

electrodes rely on redox reactions that involve the conversion of one chemical species 

into another. Despite their high theoretical capacities, conversion-based electrodes are 

generally hindered by a significant volume change that occurs during charging and 

discharging processes, leading to mechanical degradation during repeated charging 

and discharging cycles. With the examples given in Fig. 1.2.2, in general, sodiation 

causes a larger volume expansion compared with lithiation for the same conversion 

material 87. Consequently, it presents a significant challenge to design a durable 

conversional SIB as compared to LIBs.  

 

 

 



 

 21 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 
Figure 1.2.1 Approaches to intercalate sodium ions into (modified) graphite. (a) 

Structure of common electrolyte solvents for Na rechargeable batteries. Reproduced with 

permission. 73 Copyright 2016, Wiley. (b) Schematic drawing showing the suggested 

mechanism of co-intercalation of solvent molecules into the graphite. Reproduced with 

permission. 73 Copyright 2016, Wiley. (c) Schematic drawing of Na intercalation into 

regular graphite, and modified graphite that has a larger layer spacing. 75 

(c) 

(b) 

(a) 



 

 22 

 

 

 

 

 

 

 

 

 

 

 

 

 
Figure 1.2.2 Volume expansion of various conversion electrodes under lithiation 

and sodiation at a temperature of 25°C. 87 Reproduced with permission.29 Copyright 

2018, Wiley. 

 

 

 

 

 

 

 

 

 



 

 23 

Comparing Energy density and cost  
 

In Section 1.1 we discussed the increased risks to the global supply chain of increasing 

demand for LIBs.  On the contrary, SIBs can be manufactured on a large scale without 

encountering raw material scarcity for anodes due to the high earth crust abundance of 

sodium, as demonstrated in Table 1.1.1. The abundance of sodium in the earth's crust, 

plus differences in electrochemical behaviors, make SIBs potentially a more sustainable 

and cost-effective alternative to LIBs. Fig. 1.2.3 presents a simplified material cost 

analysis of both LIBs and SIBs, focusing on the key components, including cathode, 

anode, current collectors (Al and/or Cu foil), and electrolyte. One notable advantage of 

SIBs is that they do not require the use of Cu foil, which significantly reduces the overall 

cost of the electrochemical cell. In a LIB, Cu foil is necessary for the anode side due to 

aluminum's capability to form an alloy with lithium. However, since aluminum does not 

form any alloy with sodium, SIBs can use cheaper Al foil as the anode current collector. 

Moreover, Fig. 1.2.3 also gives a low estimation for the cathode cost of a SIB than that 

of a LIB, which is which can be attributed to the advantages of Na-based layered oxides. 

Cathode materials of SIBs can be synthesized from a wide range of transition metals 

including Co, Ti, Cu, Fe, Ni, Mn, and V, for instance, Na1-xCoO2 88–92, Na1-xFeO2 76,93,94, 

Na1-xNiO2 95,96, Na1-xMnO2 47,97,98, and Na1-xVO2 99,100. In contrast, synthesis is limited to 

Mn, Ni, and Co for their Li-based counterparts 74,101. Additionally, Prussian Blue (PB) 

and its analogs (PBAs) are another category of SIB cathode materials that have been 

heavily studied in the current decade 57,97,102–105, and they are known to be low-cost and 

easy-to-synthesize; yet PBAs are not suitable cathode materials for LIBs. The flexibility 



 

 24 

in material selection for SIB cathodes presents a significant opportunity for further cost 

reduction in the production of SIBs. 

 

It is important to note that while Fig. 1.2.3 only provides a rough comparison of the 

material costs between LIBs and SIBs, it assumes similar values for anodes and 

electrolytes for both types of batteries. However, SIBs have the potential to further 

reduce costs in those areas as well. Take aqueous rechargeable alkali batteries as an 

example. Consider aqueous rechargeable alkali batteries, for instance. For a long time, 

organic electrolytes used in LIBs have faced criticism due to their flammable nature and 

safety concerns. It is possible to have aqueous LIBs, which would have much reduced 

risks for fire hazard, but the electrolytes require impractically high salt concentration 

106–110. Specifically, the electrolyte salts for LIBs (e.g., LiTFSI and LiFSI) that have 

high-enough solubility in water to achieve the high concentrations, are so costly that for 

all practical purposes, aqueous LIBs are commercially uninteresting. However, with the 

abundance of sodium, sodium analogs of these salts are less expensive and there are 

already efforts under way at at least one commercial concern (Aquion Energy) to 

produce aqueous SIBs. 49–51 

 

While the discussion so far has highlighted the potential for SIBs to provide more 

affordable energy storage systems compared to LIBs, the cost per unit of energy stored 

is a key variable in benchmarking electrochemical storage solutions against more 

established approaches (e.g., pumped hydroelectric storage or compressed air storage). 

In order to achieve a higher cell energy density, it is desireable that electrode materials 



 

 25 

have a larger specific capacity and for the cell to be able to operate at a higher potential. 

Despite the lower cost of SIB materials, currently SIBs do not offer significant economic 

advantages over LIBs when their cost per energy density is calculated, as SIB cathodes 

and anodes typically have lower specific capacities and lower working potentials 

compared to LIBs with similar formulas. For instance, the theoretical specific capacity 

and working potential of Na3V2(PO4)3 are about 107 mAh/g and 3.3 V respectively 111, 

but Li3V2(PO4)3 has a theoretical specific capacity at about 197 mAh/g and an average 

potential of 4.0 V. 112 More examples of such comparison can be found in Fig. 1.2.4.  

 

The longevity/cycle life of rechargeable batteries is an important determinant of the 

overall amount of energy stored during a battery’s lifetime relative to the cost to acquire 

and service the battery. Maximizing the cycle life of a battery cell is therefore a key area 

of activity in selecting the battery electrode and electrolyte chemistry, design of the 

battery system (including ancillary electronics and thermal management to minimize 

degradation), and the operating conditions. In comparison to lithium anodes, sodium 

anodes are more reactive and undergo a more complex redox chemistry at the electrode-

electrolyte interface. The byproducts of the reactions that form the solid electrolyte 

interphase (SEI) in sodium cells are more irregularly distributed, generally possess more 

variable chemistry, and are less able to protect the battery components from aggressive 

parasitic reactions, including higher rate of electrolyte decomposition at the anode. 

Consequently, the SEI in SIBs are typically richer in organic components and often less 

stable. 113–116 These factors reduce the reversibility of SIBs, resulting in a shorter 

lifespan than LIBs, which lowers the cost per unit of energy stored over the cell lifetime. 



 

 26 

 

A large body of work now exists that addresses the limitations of SIBs and advocate 

strategies for improving performance and lifetime from a range of perspectives. Sodium 

batteries that use the metallic sodium as the anode are for instance under active study 

because of the low potential vs Na/Na+ and high theoretical specific capacity of 1165 

mAh/g.117,118 This interest is tempered by the reactivity of the electroreduced sodium 

during battery charging,  which can cause a continuous reaction with organic liquid 

electrolytes, leading to the formation of a thick and inhomogeneous solid electrolyte 

interphase (SEI). This can cause hindered ion transport across the interface and lead to 

SEI-related problems, including poor anode material reversibility and dendritic Na 

growth, which can ultimately result in battery explosion or internal short circuit.119–121 

The next section will delve into possible strategies to address these issues. 

 

 

 

 

 

 

 

 

 

 

 



 

 27 

 

 

 

 

 

 

 

 

 

 

 

 

 

 
Figure 1.2.3 Raw material cost analysis of a typical LIB versus a typical SIB. 

Shaded rectangles indicates a noticeable cost reduction for that particular part when 

switching from a LIB to a SIB. 31 

 

 

 

 

 

 

 



 

 28 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 
Figure 1.2.4 Summary of the average potentials and specific discharging capacities 

of some Li and Na cathodes reported in literature. 24,29,111,112,122–133 Reproduced with 

permission.29 Copyright 2018, Wiley. 

 

 

 
 



 

 29 

1.3 Recent advances in high-performance sodium metal batteries (SMBs)  

 

Interface design for rechargeable batteries 

	

Over the past three decades, the composition and structure of the solid-electrolyte 

interphase (SEI) formed spontaneously on battery anodes has garnered significant 

attention among researchers world-wide. These efforts lartely emmanate from the 

groundbreaking work by Peled in 1979 that the existence of an SEI layer on alkali and 

alkaline earth metal anodes in nonaqueous battery systems was recognized. 134,135 

Peled's pioneering research established that alkali and alkaline earth metals, upon 

contact with the electrolyte, undergo an instantaneous reaction resulting in the formation 

of a surface layer.134,136,114 SEI is believed to consisting of both insoluble and partially 

soluble reduction products derived from the electrolyte components, resulting in a 

mixed layer of organic and inorganic phases (as illustrate in Fig. 1.3.1a-c). 137 

 

In the context of SMBs, the formation of a stable and well-formed SEI on the surface 

of sodium (Na) anodes is of paramount importance to ensure their reliable operation 

with extended cycle life. Well-formed interphases, particularly those possessing a high 

cation transport number approaching unity, have been shown both by theory and 

experiment 138–140 to be effective in suppressing concentration polarization and for 

facilitating reversible dissolution/deposition processes at a metallic anode. The 

mechanical stability and flexibility of the SEI layer are critical factors, as is its strong 

adhesion to the anode surface. 134 We emphasize here that the practical realization of 



 

 30 

primary or secondary alkaline or alkaline-earth batteries hinges on preventing anode 

dissolution or corrosion, thus necessitating the formation of a passivating SEI layer that 

provides comprehensive coverage across the entire anode surface. The SEI layer on Na 

anodes unfortunately exhibits nonuniformity, leading to uneven diffusion of ions and 

adversely affecting the initial nucleation of Na. As the deposition process progresses, 

the substantial volume expansion of the Na anode induces the formation of cracks in the 

SEI film. These cracks serve as preferential pathways for enhanced local Na ion flux, 

exacerbating the growth of dendritic structures. Consequently, the proliferation of 

dendrites not only compromises the electrochemical performance of SMBs but also 

poses significant safety concerns, including short circuits and thermal runaway. This 

process of the progress of dendrites in SMBs has been illustrated in Fig. 1.3.1d. It’s also 

noteworthy that the solubility of the SEI layer in sodium (Na) electrolytes is greater 

compared to lithium (Li) electrolytes, rendering the instability of the SEI layer on Na 

metal surfaces more pronounced than on Li surfaces. 141,142 These challenges 

significantly impede the practical utilization and reliability of Na anodes in SMBs, 

hindering their widespread application in diverse energy storage systems. 

 

The composition of SEI film formed on sodium metal anodes in carbonate-based 

electrolytes closely resembles those formed at lithium metal anodes. This SEI film 

consists of both inorganic and organic components, with distinct distribution within the 

film. The inorganic components, such as Na2O, NaF, and Na2CO3, are primarily 

concentrated in the inner layer of the SEI film. These inorganic species play a crucial 

role in passivating the metal surface and preventing further reactions with the electrolyte. 



 

 31 

On the other hand, the organic components, including RONa, ROCO2Na, and RCOONa, 

tend to be located closer to the electrolyte interface. 143 These organic species are 

believed to contribute to the flexibility and adhesive properties of the SEI film. 

 

Drawing inspiration from the success of using lithium fluoride (LiF) to enhance the 

stability and cycling performance of lithium metal anodes in LIBs, sodium fluoride 

(NaF) has emerged as a promising additive and SEI component for improving the 

performance of SMBs. Cui et al. first reported the favorable effects of NaF on the plating 

and stripping processes of sodium metal anodes at room temperature, enabling highly 

reversible and dendrite-free deposition. 144 Furthermore, the incorporation of 

fluoroethylene carbonate as an electrolyte additive has shown potential in reducing the 

irreversible capacity of Na-half cells. This additive promotes the formation of a NaF-

based interfacial layer on the surface of the sodium metal electrode. The NaF interface 

acts as a barrier, limiting the permeation of the electrolyte and effectively hindering 

undesired reactions between the sodium metal and the electrolyte. 145 

 

Additives that control the formation of inorganic phases of SEI, such as NaF, are one 

among a number of strategies being actively explored to improve the stability and 

performance of sodium-based battery systems. Organic or organic-inorganic hybrid 

SEIs have in fact been engineered in order to achieve the same goal. For example, Wu 

et al. recently reported a novel way to form sodium benzenedithiolate (PhS2Na2)-rich 

protection layer in-situ, which can stabilize the cycling of a SMB for more than 400 

cycles at a rate of 1 mA/cm2 and Na throughput of 1 mAh/cm2. 



 

 32 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Figure 1.3.1 SEI architecture and mechanism of dendrite formation. (a) Illustration 

of layered SEI structure on lithium metal anode. Reproduced with permission. 141 

Copyright 2000, Elsevier. (b) Electron-transparent cryo-FIB images showing type I (left) 

and type II (right) dendrites. 146 (c) HAADF cryo-STEM imaging revealing the structure 

of dendrites (upper), EELS elemental mapping shows the composition of the dendrites 

(lower). 146 (d) Illustration shows the mechanism of dendrite formation in SMBs. 144 

 

(a) 

(b) 

(c) 

(d) 



 

 33 

These findings underscore the critical importance of efforts that facilitate rational design 

and control of the composition, physical properties, and mechanics of the SEI on sodium 

metal anodes. By employing meticulous design and optimization strategies targeting the 

SEI, it becomes feasible to enhance the stability, cycling efficiency, and overall safety 

of sodium-based batteries. However, several challenges persist in the realm of SEI 

design and engineering for SMBs. One significant challenge lies in accurately 

characterizing the structure and composition of the SEI on sodium metal anodes, 

primarily due to its high chemical reactivity. Nonetheless, understanding the intricacies 

of the SEI is an essential initial step in tackling any issues. Therefore, there is a pressing 

need for the development of advanced characterization techniques tailored specifically 

for studying the SEI in SMBs. These methods will enable researchers to delve deeper 

into the SEI's properties and unravel its underlying mechanisms. Additionally, a 

systematic and comprehensive study is warranted to establish standardized parameters 

for the thickness, porosity, and mechanical properties of the SEI in SMBs. Such 

guidelines will facilitate the formulation of design principles that can be employed to 

engineer optimized SEI architectures in sodium-based battery systems.  

 

In conclusion, through the collective efforts of researchers, including advancements in 

SEI characterization techniques and the establishment of standardized design principles, 

it is now possinle to overcome a number of previously insurmountable challenges to 

broad-based use of sodium as a practical anode material in rechargeable batteries. 

Thanks to these efforts, we believe that it is now reasonable to anticipate that within the 

next ten years, scientists will be able to unlock the full potential of sodium batteries in 



 

 34 

energy storage applications where the low-cost and high natural abundance of Na are 

required for affordable storage of electrical energy.  

 

Non-planar or 3D electrode architectures 

 

While artificial SEI might require delicatedly-designed electrolyte or even preformation 

process, employing a 3D anode architecture can be an effective and straightforward 

solution to mitigate dendrite growth in SMBs. This 3D confinement strategy has 

emerged as an effective and straightforward solution to mitigate dendrite growth in 

SMBs. This approach has great research potential and practical applications in terms of 

the parameters that can be fine-tuned, including stabilizing 3D porous framework, 

optimizing electron/ion conduction, and improving the interface engineering.147,148  

Popular current 3D SMB anode designs can be categorized into three groups based on 

their materials: carbon-based frameworks (CBFs), metal-based frameworks (MBFs), 

and composite frameworks (CFs). While each of these frameworks has unique structural 

features and performance advantages and disadvantages, they all aim to reduce the local 

current density, provide more nucleation sites, and improve accessibility to the current 

collector to enhance the electrochemical deposition behavior of Na+ on the SMB anode. 

 

CBFs are an economical, durable, and flexible 3D framework that can withstand the 

significant volume changes that occur during the repeated stripping/plating of metallic 

sodium 149. In the investigation conducted by Chi, Fan et al., it is revealed that a Na/C 

composite anode, synthesized via a straightforward melt fusion process, demonstrates a 

more stable voltage profile with enhanced cycling performance.150 However, in 

comparison to metal-based frameworks (MBFs) and composite frameworks (CFs), 



 

 35 

unmodified CBFs typically exhibit lower sodiophilicity and may encounter wetting 

problems with certain electrolytes, resulting in a higher current flux at the interface 

instead of directing sodium ions into the anode. Nevertheless, these issues can be 

resolved by modifying the carbon fiber, either through doping or altering functional 

groups. Liu and Qu et al. report that current collectors produced using a simple 

electrospinning technique to create a uniformly nitrogen-doped porous carbon fiber 

skeleton, exhibit high specific surface area (1,098 m2/g) and strong binding to sodium 

metal. The nitrogen-doped porous carbon substrates were reported to enable Na 

deposition at low overpotentials and to facilitate stable cycling of Na anodes for 3,500 

hours with a high coulombic efficiency of 99.9% at 2 mA/cm2 and 2 mAh/cm2. 151 A 

related study by Sun and Wang et al. demonstrated that addition of N and S containing 

functional groups on carbon nanotubes produces nitrogen and sulfur co-doped carbon 

nanotube (NSCNT) paper with strong affinity for sodium (sodiophilicity), leading to a 

more evenly distribution of sodium deposits through the network. 152 

 

MBFs exhibit excellent electrical conductivity and mechanical properties. Sun, Jiang et 

al. demonstrated that 3D porous copper (Cu) achieves a high CE of 99.4% at 1 mAh/cm2 

cycling rate and supports highly reversible Na plating and stripping with a throughput 

of up to 6 mAh/cm2.153 However, due to their heavy weight, MBFs decrease the overall 

galvanostatic energy density of SMBs. Furthermore, unlike CBFs, modifying the 

surface of metal wires is challenging, and adding or altering any functional groups is 

even more difficult. Yet it is still possible to process MBFs in order to further tune their 

binding energy with sodium. For example, Shao, Wang, and Peng’s team achieved a 

stable host for dendrite-free Na metal anodes using a three-dimensional (3D) Cu foam 

skeleton with hierarchical ZnO nanorod arrays (CF@ZnO).154 They used chemical 

precipitation method to grow hierarchical ZnO nanorod arrays onto commercially 



 

 36 

available Cu foam. The highly "sodiophilic" ZnO nanorod arrays provided many Na 

nucleation sites and showed low nucleation over-potential, which helped to achieve 

uniform growth of Na on the electrode. 

 

CFs possess inherent advantages stemming from the synergistic integration of various 

components, wherein each contributes distinct merits. For instance, one component may 

offer commendable mechanical strength and conductivity, while another component 

enhances the sodiophilicity of the system. A notable illustration of such superiority is 

found in the 3D hydroxylated MXene/carbon nanotubes (h-Ti3C2/CNTs) composite, 

innovatively developed by Li and Niu et al. 155 Compared to bare Cu and CNTs anodes, 

the h-Ti3C2/CNTs anode exhibited a significantly reduced Na nucleation overpotential 

of merely 6 mV at a current density of 1 mA/cm2, attributed to the introduction of 

abundant sodiophilic sites enriched with oxygen and fluoride functional groups. 

Similarly, the Jiao Group recently reported a CF comprised of 3D hollow porous carbon 

nanofibers infused with Sb nanoparticles, showcasing remarkable effects of gradient 

sodiophility. 156 Upon initial sodiation, the Sb nanoparticles form Na3Sb, which exhibits 

heightened affinity towards sodium, thereby facilitating the homogeneous redistribution 

of Na+ ions throughout the entire anode structure. This Sb-based CF demonstrates 

excellent reversibility, even under the demanding conditions of a high cycling rate of 5 

mA/cm2. Though CFs entail more intricate synthesis pathways, they manifest 

outstanding comprehensive performance, adeptly balancing essential factors such as 

conductivity, mechanical strength, Na wettability, functional surface groups, and 

chemical activity through the thoughtful design and integration of diverse components. 

 

 

 



 

 37 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Figure 1.3.2 Summary of advantages and disadvantages of three types of 3D frameworks 

for SMBs. Reproduced with permission. 157 Copyright 2021, Wiley‐VCH GmbH. 
 

 

 

 

 

 



 

 38 

In conclusion, the use of 3D anode architectures has emerged as a promising strategy to 

address the challenge of dendrite growth in sodium metal batteries (SMBs). This 

approach offers the flexibility to fine-tune various parameters, including the 

stabilization of 3D porous frameworks, optimization of electron/ion conduction, and 

interface engineering. The three main categories of 3D anode frameworks, namely 

carbon-based frameworks (CBFs), metal-based frameworks (MBFs), and composite 

frameworks (CFs), each possess unique structural features and performance advantages. 

CBFs provide durability and flexibility, although their sodiophilicity and wetting 

properties may require modifications. MBFs exhibit excellent electrical conductivity 

and mechanical properties, although their weight may affect the overall energy density 

of SMBs. CFs, on the other hand, offer the opportunity to leverage the strengths of 

different components to achieve enhanced performance. With ongoing advancements in 

the synthesis and design of these 3D anode architectures, it is anticipated that the 

electrochemical deposition behavior of sodium ions on the anode can be significantly 

improved, leading to enhanced stability, cyclability, and performance of SMBs. 

Continued research efforts and innovation in this field will be crucial for realizing the 

full potential of 3D anode architectures in sodium-based battery technologies. 

 

 

 

 

 



 

 39 

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