1.1. Renewable and Sustainable Energy
Energy plays a pivotal role in propelling economic growth across agriculture, industries, and service sectors. Despite the growing emphasis on renewable energy sources, fossil fuels remain the dominant players in the global energy landscape. The combined contribution of oil, coal, and natural gas makes up approximately 80% of the world's energy usage. These fossil fuel-based systems face several drawbacks: (i) their finite nature, (ii) their depletion rate exceeding their replenishment rate, and (iii) their detrimental environmental impacts. Driven by an annual growth rate of 2.3%, global energy consumption has witnessed a significant rise, primarily met by increased consumption of fossil fuels [
1,
2,
3].
Climate change looms as one of the most pressing challenges facing our planet today. A significant portion of this crisis stems from the rampant consumption of fossil fuels across various sectors. Introduced during the industrial revolution, fossil fuels have driven the alarming accumulation of heat-trapping greenhouse gases (GHG) in the atmosphere. The 10 GtCO
2eq surge in annual anthropogenic GHG emissions observed in the past decade (2001-2010) was primarily driven by the non-renewable energy sector, which accounted for 47% of the increase. Industry, transport, and buildings contributed 30%, 11%, and 3%, respectively [
1]. Since 1970, industrial activities and fossil fuel combustion have been responsible for over three-quarters of the surge in GHG emissions. The compounding effects of GHG emissions on global climate change are expected to have irreversible consequences for agriculture, the healthcare sector, and the Earth's ecosystem as a whole [
1,
4,
5].
The urgency to harness renewable energy sources and curb fossil fuel consumption has intensified in recent years [
6,
7]. Sustainable energy surpasses mere renewable energy by emphasizing energy efficiency. Optimizing energy supply to minimize wastage enhances energy efficiency [
4]. Intertwining renewable energy sources and energy efficiency strategies forms the foundation of a sustainable energy paradigm, fostering economic progress, human well-being, and environmental protection. Sustainable energy solutions address climate change by reducing GHG emissions, ensure equitable energy access for all, and strengthen energy security [
4].
The proportion of renewable energy in the global energy mix is on an upward trajectory. In the year 2017, renewable energy sources represented 18.1% of global energy production, with modern renewables contributing around 10.6%, and the remaining portion sourced from traditional biomass (refer to
Figure 1) [
5]. Utilizing currently available technologies, it is anticipated that the global proportion of renewable energy will surpass 30% by the year 2030 [
8].
Renewable energy sources like solar and wind power are not always available, and their peak supply doesn't necessarily align with peak demand [
1,
9]. This intermittent nature necessitates the use of rechargeable batteries as backup energy storage during power fluctuations. Technological advancements have driven the demand for higher-performance batteries. The consumer electronics industry currently dominates the battery market, with LIBs holding a prominent position [
10]. However, LIBs have seen only marginal improvements in price and efficiency since their commercialization in 1991 [
10,
11,
12]. Therefore, novel developments are crucial to enhance battery reliability, safety, energy/power density, and reduce battery costs per kWh of energy storage.
1.2. Battery Technologies
Batteries, also known as electrochemical cells, are devices that harness chemical energy and transform it into electrical energy. Alessandro Volta pioneered the development of the first electrochemical battery in 1800, featuring copper and zinc electrodes submerged in a saltwater (NaCl solution)-soaked paper electrolyte. The year 1836 saw the introduction of the Daniell cell by John Frederic Daniell, a crucial milestone that set the stage for contemporary battery technologies. Subsequently, battery technology has experienced notable progress. Batteries can be broadly categorized into two types: primary and secondary. Primary batteries are designed for single use and cannot undergo recharging. In contrast, secondary batteries, also known as rechargeable batteries, are capable of being recharged and discharged repeatedly [
2,
11].
Gaston Planté's breakthrough in 1859 with the lead-acid battery paved the way for rechargeable batteries. It utilizes lead and sulfuric acid to store and release energy [
13]. Lead-acid batteries are known for their ability to handle high current loads and their suitability for both accessory batteries and power sources in cars. They are relatively inexpensive, reliable, and easily recyclable at the end of their lifespan. However, their disadvantage lies in their weight and bulkiness relative to their energy storage capacity. Nickel-cadmium (Ni-Cd) rechargeable batteries, pioneers of alkaline technology, were commercialized in the late 19th century. These batteries utilize nickel and cadmium electrodes immersed in a potassium hydroxide electrolyte. Ni-Cd batteries boast higher energy densities (60-100 Wh/L) than lead-acid batteries (50-80 Wh/L) [
11].
Emerging in 1989, the nickel-metal hydride (Ni-MH) battery employs a positive electrode of nickel oxyhydroxide (NiOOH) similar to Ni-Cd batteries but utilizes a metal hydride instead of cadmium for the anode material. Ni-MH batteries offer superior energy densities (170-430 Wh/L) and are environmentally preferable due to the absence of toxic cadmium electrodes (
Figure 2). These batteries have gained widespread adoption in mobile devices, electric vehicles, and hybrid electric vehicles. However, their high cost and heat generation at high current draws remain challenges for this technology [
11].
The debut of rechargeable LIBs by Sony in 1991 signified a crucial milestone in battery technology [
11]. Initially, graphite served as the anode material, while LiCoO
2 was employed as the cathode (
Figure 2). Graphite has since established itself as the preferred anode material due to its favorable properties, including safety, reversibility, and the ability to deliver high energy density for portable electronic devices. However, the widespread use of LiCoO
2 has been met with concerns regarding its high cost, toxicity, and safety limitations. To address these issues, alternative cathode materials, such as lithium iron phosphate (LiFePO
4), manganese spinels, and lithium nickel manganese cobalt oxides, are actively being explored [
11].
LIBs are rapidly gaining prominence as a versatile energy storage solution, catering to a diverse spectrum of devices ranging from compact appliances like watches and hearing aids to extensive applications like grid-scale energy storage systems and electric vehicles. To understand the operation of LIBs, it's essential to grasp the fundamental principles of battery technology. At the heart of any battery lie three key components: two electrodes, the cathode and anode, separated by a porous barrier called the separator, and an electrolyte solution.
Figure 3 illustrates the working mechanism of LIBs. The electrodes are composed of two distinct materials, separated by the separator. During discharge, a chemical reaction occurs between the electrodes, releasing electrons in the form of electricity. This reaction is facilitated by the electrolyte solution, which allows ions to flow between the electrodes. The electrolyte solution must be designed to conduct ions efficiently while preventing the flow of electrons. As the anode undergoes a chemical reaction, it releases electrons at the battery's negative terminal. These electrons travel through the external circuit, powering the connected device. Simultaneously, ions migrate through the electrolyte solution towards the cathode [
11]. At the cathode, these ions unite with electrons, giving rise to compounds that accrue on the surface of the cathode [
11]. This process is reversed during charging, where an external energy source drives the flow of electrons back into the battery, reversing the chemical reactions and restoring the initial state of the electrodes. The chemistry of LIBs revolves around the movement of lithium ions (Li+) within the battery. During charging, Li+ ions intercalate into the negative electrode, typically graphite, and during discharge, they react with Li+ ions at the positive electrode, where materials like LiCoO
2, LiMn
2O
4, or LiFeO
4 are commonly used [
11].
Electrifying vehicles with high-capacity rechargeable batteries has gained significant attention as a means to reduce or eliminate the reliance on internal combustion engines. In the realm of battery technologies, rechargeable lithium-air batteries have garnered attention as a promising alternative, exhibiting an exceptional energy density comparable to that of gasoline. While LIBs currently power most electric and hybrid electric vehicles (HEVs), their limited range and high cost pose challenges for widespread adoption. Affordable electric vehicles typically offer a driving range of around 120 miles, while luxury EVs like the Tesla S can reach up to 270 miles but come with a hefty price tag of nearly
$100,000 [
14]. Therefore, the development and implementation of safe, long-lasting, and cost-effective rechargeable batteries are crucial to meet the growing energy storage demands for various applications. The present accessible rechargeable LIB capacity is too limited to address these demands [
15]. This comprehensive review delves into the emerging field of MABs, a promising renewable energy technology that holds the potential to revolutionize energy storage solutions. Characterized by their exceptional theoretical specific energy, surpassing that of other battery types (
Figure 4) [
15,
16,
17], MABs, particularly Li-O
2 and Na-O
2 batteries (SOBs), offer the tantalizing prospect of powering vehicles for distances comparable to those achieved with fossil-fuel-based counterparts [
18].
MABs, including Li-, Na-, Al-, Mg-, Fe-, and Zn-O
2 batteries, have garnered significant attention as potential replacements for LIBs, especially in the transportation sector [
19]. Li-O
2 batteries (LOBs), with their exceptional specific capacity of ~3842 mAh/g, hold immense potential for revolutionary advancements in battery technology. These batteries are predicted to offer five to ten times’ higher energy density than currently available rechargeable LIBs. Primarily designed for vehicle electrification, LOBs could potentially provide a driving range comparable to gasoline vehicles (~500 kilometers per charge). In 1996, Abraham et al. introduced the first secondary LOB using a non-aqueous solvent, where lithium peroxide is formed during discharge at the cathode [
20]. While Li-O
2/Air batteries have demonstrated promising progress, significant improvements are still required before commercialization. These advanced battery technologies face several challenges that need to be addressed prior to widespread adoption. The practical application of rechargeable LOBs faces a notable obstacle known as "sudden death," attributed to the formation of an insulating layer on the cathode. In recent times, secondary SOBs have garnered significant attention, showcasing reported advantages over LOBs. With an impressive specific capacity of approximately 1500 mAh/g, which is more than double that of current Lithium-Ion Batteries (LIBs), SOBs emerge as a promising alternative to the prevailing LIBs. Rechargeable SOBs demonstrate low overpotentials even at high current densities (> 200 μA cm
-2) and achieve an energy efficiency of around 90% [
21]. However, SOBs also experience "sudden death" during discharge and recharge cycles. The occurrence of sudden death is evident in a pronounced surge in cell potential towards the conclusion of the charge cycle, particularly at elevated current densities [
21]. The fundamental cause of this abrupt failure during discharge is linked to restrictions in oxygen transport arising from the development of sizable NaO
2 crystals. Additionally, the buildup of these insulating NaO
2 crystals on the cathode surface hinders electrochemical reactions at the cathode-electrolyte interface [
22].
MABs, including Li-O2 and SOBs, face a multitude of challenges that hinder their widespread adoption. These challenges include complex parasitic side reactions, material instability across all battery components (anode, cathode, and electrolyte), poor conductivity, intricate interfacial phenomena, air impurities (particularly CO2 and water vapor), and safety concerns. Furthermore, the currently achievable power and current densities are limited, posing a significant constraint.