Exploring the Role of Additives in Enhancing the Performance of Limestone-Based Thermochemical Energy Storage: A Review

: This review article explores the critical role of additives in enhancing the performance and durability of thermochemical energy storage (TCES) materials, particularly in limestone-based systems. It evaluates various strategies, including hydration and the use of fine particles, along with additives like Al 2 O 3 and ZrO 2 , to address challenges like performance degradation and sintering over multiple cycles. Additionally, the review examines how multicyclic stability and material activity toward CO 2 are related. It emphasizes the importance of selecting support materials that optimize both stability and reactivity. Furthermore, it highlights the need for systematic investigation into the selection, synthesis methods, and additive percentages to identify optimal formulations for improved multicyclic stability. Finally, it underscores the importance of understanding the mechanisms of interaction between additives and CaO/CaCO 3 matrices to guide the design of effective additive-integrated systems. This comprehensive analysis provides valuable insights into current methodologies, emerging trends, and future directions for advancing sustainable energy storage technologies.


Introduction
In our ongoing efforts to have a cleaner and longer-lasting energy future, the need for better ways to store energy is a crucial topic in scientific discussions [1].The increasing use of renewable energy sources like solar and wind power is a positive step in fighting climate change and reducing our dependence on limited fossil fuels [2][3][4].However, there's a big challenge-these eco-friendly sources do not generate energy all the time.This on-and-off pattern, called intermittency, poses a significant hurdle to relying solely on renewables [5].
Renewable energy generation, influenced by variable weather conditions, requires a seamless mechanism to address the fluctuations between surplus and deficit [6].Energy storage emerges as an integral component in the pursuit of a resilient and dependable energy grid [7][8][9].Its primary function involves the accumulation of excess energy during peak production and its subsequent release during periods of low generation, thereby stabilizing the intermittent nature of renewable energy availability [10].This operational feature not only alleviates issues related to intermittency but also establishes a critical foundation for the widespread acceptance and integration of sustainable energy sources [11].
The collaboration between energy storage and grid resilience becomes evident when we consider incorporating renewable energy into our current power systems [12].The adaptable characteristics of energy storage systems play a crucial role in maintaining a balance between the ever-changing dynamics of energy production and consumption.This proves instrumental in providing a defense against disruptions and preventing blackouts [13].As energy storage alleviates the burden on traditional power plants, it becomes a pivotal element in strengthening the fundamental structure of our energy infrastructure [14].
Energies 2024, 17, 2572 2 of 20 Essentially, energy storage is not just a temporary solution; it is the essential element driving the shift towards a sustainable energy environment [15].With nations worldwide pledging to achieve ambitious renewable energy goals, the scalability and effectiveness of energy storage technologies take center stage [16].It serves as the channel through which intermittent renewables smoothly blend into the energy mix, propelling us towards a future marked by low carbon emissions [17].The necessity for energy storage becomes the guiding principle leading us through the intricate process of deploying renewable energy sources [18].
In summary, the requirement for energy storage in the scientific discourse goes beyond being a mere technological necessity; it represents the central point for the sustainable energy transition.From addressing the intermittency of renewable energy sources to strengthening grid resilience and expediting the shift towards a cleaner energy framework, energy storage emerges as a game changer in this defining era.The ongoing dedication to advancing energy storage technologies is not solely a scientific pursuit; it is a clear call for the harmonious coexistence of environmental responsibility and global energy security [19].

Types of Energy Storage
As mentioned in the previous section, energy storage is crucial for maintaining a stable and reliable energy supply, especially as we integrate more renewable energy sources with intermittent generation patterns [20].Energy storage technologies can be categorized into five types based on the process through which energy is stored: mechanical [21,22], electrochemical (or batteries) [23,24], thermal [25,26], electrical [26,27], and hydrogen storage technologies [28][29][30].Figure 1 summarizes these types of energy storage technologies along with their sub-categories.
Energies 2024, 17, x FOR PEER REVIEW 2 of 21 becomes a pivotal element in strengthening the fundamental structure of our energy infrastructure [14].Essentially, energy storage is not just a temporary solution; it is the essential element driving the shift towards a sustainable energy environment [15].With nations worldwide pledging to achieve ambitious renewable energy goals, the scalability and effectiveness of energy storage technologies take center stage [16].It serves as the channel through which intermittent renewables smoothly blend into the energy mix, propelling us towards a future marked by low carbon emissions [17].The necessity for energy storage becomes the guiding principle leading us through the intricate process of deploying renewable energy sources [18].
In summary, the requirement for energy storage in the scientific discourse goes beyond being a mere technological necessity; it represents the central point for the sustainable energy transition.From addressing the intermittency of renewable energy sources to strengthening grid resilience and expediting the shift towards a cleaner energy framework, energy storage emerges as a game changer in this defining era.The ongoing dedication to advancing energy storage technologies is not solely a scientific pursuit; it is a clear call for the harmonious coexistence of environmental responsibility and global energy security [19].

Types of Energy Storage
As mentioned in the previous section, energy storage is crucial for maintaining a stable and reliable energy supply, especially as we integrate more renewable energy sources with intermittent generation patterns [20].Energy storage technologies can be categorized into five types based on the process through which energy is stored: mechanical [21,22], electrochemical (or batteries) [23,24], thermal [25,26], electrical [26,27], and hydrogen storage technologies [28][29][30].Figure 1 summarizes these types of energy storage technologies along with their sub-categories.Technologies used for mechanical energy storage, store energy in the form of gravitational potential energy, kinetic energy (associated with motion), or potential energy resulting from compression.These technologies can be further categorized as Pumped Hydro Storage (PHS) [31], Gravity Energy Storage [32], Compressed Air Energy Storage (CAES) [33], and Flywheel Storage [34] technologies.In PHS, surplus electricity is used to pump water from a lower reservoir to a higher one during periods of low demand.When electricity demand is high, the stored water is released and used to generate electricity through turbines.CAES stores energy by compressing air and storing it in underground Technologies used for mechanical energy storage, store energy in the form of gravitational potential energy, kinetic energy (associated with motion), or potential energy resulting from compression.These technologies can be further categorized as Pumped Hydro Storage (PHS) [31], Gravity Energy Storage [32], Compressed Air Energy Storage (CAES) [33], and Flywheel Storage [34] technologies.In PHS, surplus electricity is used to pump water from a lower reservoir to a higher one during periods of low demand.When electricity demand is high, the stored water is released and used to generate electricity through turbines.CAES stores energy by compressing air and storing it in underground caverns.During peak demand, the compressed air is released and expanded through turbines to generate electricity.Whereas, in Flywheel Energy Storage, energy is stored in the form of kinetic energy by spinning a rotor (flywheel) at high speeds.When energy is needed, the kinetic energy is converted back to electricity [21].In an electrical energy storage system, electricity is stored by using an electric charge/ electric field, or magnetic field.It includes supercapacitors [35] and superconducting magnetic energy storage [36].A supercapacitor, also known as an ultracapacitor, is a powerful type of capacitor.It has much higher capacitance than regular capacitors but operates at a lower voltage.It acts as a bridge between electrolytic capacitors and rechargeable batteries.Superconducting Magnetic Energy Storage (SMES) systems store energy by utilizing the magnetic field generated through the flow of direct current in a superconducting coil.This coil is cooled to a temperature below its superconducting critical temperature using cryogenic methods [37].
Hydrogen energy storage is a form of chemical energy storage in which energy is used to produce hydrogen [38].Hydrogen storage technologies operate on the fundamental idea of using electricity (from renewable sources) to carry out water electrolysis, generating hydrogen and oxygen [39].The produced hydrogen is stored [40] and can be utilized either for direct combustion in applications like heating or for electricity generation through polymer electrolyte fuel cells [41].
Electrochemical energy storage includes a range of secondary batteries that convert the chemical energy within their active materials into electrical energy through a reverse electrochemical oxidation-reduction reaction [24,42].Secondary batteries are typically classified into distinct groups based on the electrochemical system they employ.These include standard batteries such as lead-acid [43] and Ni-Cd [44], as well as modern batteries like Ni-MH [45], Li-ion [46], and Li-polymer [47].Special batteries, such as Ag-Zn and Ni-H 2 , represent another category, while technologies like flow batteries (e.g., Br 2 -Zn, vanadium redox) and high-temperature batteries (e.g., Na-S, Na-metal chloride) constitute additional segments [48].This diverse range of commercially available secondary batteries caters to various applications and technological requirements.
Thermal Energy Storage (TES) allows the storage of heat energy in the form of temperature, phase change, or a reversible chemical reaction [49].Multiple case studies have been performed for better conversion of solar to heat energy for TES, such as optimized solar air collectors, PCM nanoadditive solar stills, and improved energy storage in permanent magnets and Mg 2+ -doped CaCO 3 /PEG composites [50][51][52][53].TES are categorized into sensible heat storage (SHS) [54], latent heat storage (LHS) [55], and thermochemical energy storage (TCES) [56].SHS involves storing heat in a material by changing its temperature, like water or oil, that can later be released when needed.LHS involves the storage and release of heat during the phase transition of materials called phase change materials (PCMs) (e.g., salts, alkali nitrates, hydroxides, or chlorides) [57].TCES stores energy through reversible chemical reactions.During charging, the chemical reaction absorbs heat, and during discharging, the reverse reaction releases heat, for example, calcination (endothermic) and carbonation (exothermic) of calcium carbonate [58].TCES not only possesses the highest energy densities but also offers other benefits over SHS and LHS technologies.The characteristics comparison of all three types of TES is summarized in Table 1.
Each type of energy storage has its advantages and disadvantages, and the choice depends on factors such as the application, duration of energy storage, scale, and specific requirements of the energy storage system.As technology continues to advance, there's ongoing research to improve the efficiency, cost-effectiveness, and sustainability of these energy storage methods.Energy storage technologies are also categorized based on storage duration: shortduration energy storage (SDES) and long-duration energy storage (LDES) [60].SDES systems store energy for up to a few hours, while LDES systems can store energy from hours to days, or in some cases, from days to months [61].Both of these energy storage systems are essential to balance the grid on hourly, daily, weekly, and even seasonal timescales.The requirement for longer energy storage increases as the percentage share of renewable energy increases in the grid [62].Figure 2 illustrates the relationship between the fraction of annual energy sourced from variable renewable generators (specifically wind and solar) at a regional or local level and the maximum duration of electricity storage necessary to ensure continuous demand fulfillment, presented on a logarithmic scale.Arrows are employed to denote differing levels of stringency regarding curtailment, transmission, and grid flexibility assumptions, with a leftward arrow indicating more restrictive conditions and a rightward arrow indicating more lenient conditions.For instance, in scenarios where curtailment is minimized (arrow pointing left), a longer storage duration is required compared with scenarios where greater curtailment is permitted (arrow pointing right) [63].Beyond a 60% threshold for variable renewables, there is a notable escalation in the demand for both daily and weekly storage [63].The necessity for seasonal storage (LDES for months) becomes particularly pronounced beyond an 80% penetration of variable renewables [63].Consequently, the imperative role of energy storage in facilitating system flexibility and energy management emerges as crucial for the effective integration of substantial proportions of wind and solar energy sources.Energy storage technologies are also categorized based on storage duration: shortduration energy storage (SDES) and long-duration energy storage (LDES) [60].SDES systems store energy for up to a few hours, while LDES systems can store energy from hours to days, or in some cases, from days to months [61].Both of these energy storage systems are essential to balance the grid on hourly, daily, weekly, and even seasonal timescales.The requirement for longer energy storage increases as the percentage share of renewable energy increases in the grid [62].Figure 2 illustrates the relationship between the fraction of annual energy sourced from variable renewable generators (specifically wind and solar) at a regional or local level and the maximum duration of electricity storage necessary to ensure continuous demand fulfillment, presented on a logarithmic scale.Arrows are employed to denote differing levels of stringency regarding curtailment, transmission, and grid flexibility assumptions, with a leftward arrow indicating more restrictive conditions and a rightward arrow indicating more lenient conditions.For instance, in scenarios where curtailment is minimized (arrow pointing left), a longer storage duration is required compared with scenarios where greater curtailment is permitted (arrow pointing right) [63].Beyond a 60% threshold for variable renewables, there is a notable escalation in the demand for both daily and weekly storage [63].The necessity for seasonal storage (LDES for months) becomes particularly pronounced beyond an 80% penetration of variable renewables [63].Consequently, the imperative role of energy storage in facilitating system flexibility and energy management emerges as crucial for the effective integration of substantial proportions of wind and solar energy sources.Figure 3 presents the storage duration of some energy storage technologies against the size of energy storage along with their function on top [59].All kinds of electrochemical batteries (lead acid, li-ion batteries) and capacitors lie under the category of SDES [64].CAES, PHS, and thermal energy storage (molten salts) are considered LDES and scalable Figure 3 presents the storage duration of some energy storage technologies against the size of energy storage along with their function on top [59].All kinds of electrochemical batteries (lead acid, li-ion batteries) and capacitors lie under the category of SDES [64].CAES, PHS, and thermal energy storage (molten salts) are considered LDES and scalable to GWs of energy storage, can be used for bulk power management [65].TCES, which is a sub-category of TES, is not present in Figure 3, but it can be placed alongside the molten salts (LHS).Because TCES possesses the highest energy densities among the other TECs (8-10 times higher than SHS and two times higher than LHS) [26], TCES will be the focus of this review.to GWs of energy storage, can be used for bulk power management [65].TCES, which is a sub-category of TES, is not present in Figure 3, but it can be placed alongside the molten salts (LHS).Because TCES possesses the highest energy densities among the other TECs (8-10 times higher than SHS and two times higher than LHS) [26], TCES will be the focus of this review.

Introduction to Thermochemical Energy Storage
TCES is a promising technology that involves the use of chemical reactions to store and release energy.TCES systems utilize reversible chemical reactions to store thermal energy, which can be later retrieved for various applications.These systems typically involve the conversion of a material between two chemical states, one absorbing energy during an endothermic reaction (charging) and the other releasing energy during an exothermic reaction (discharging).
An ideal TCES system should possess several key attributes.Firstly, it should exhibit high energy storage density to minimize the system's weight and volume requirements, with target values set at 300 kWh m −3 or >1000 kJ kg −1 [66].Secondly, high working temperatures are essential for maximizing power generation efficiency, with the charging step falling between 700 °C and 1500 °C [66].Additionally, materials must demonstrate high cycling and chemical/thermal stability to continue prolonged operation, with a focus on maintaining chemical reaction reversibility and multicyclic stability [66].Strong mechanical properties are crucial for ensuring long-term integrity during repeated cycles, as solid materials are prone to morphological degradation at high temperatures.Moreover, storage materials should be inexpensive, abundant, and low in toxicity (targeted cost of $4.2 MJ −1 , DOE 2020) [66].Fast and stable kinetics of both forward and reverse reactions are necessary to facilitate energy charging and discharging steps, while favorable reaction thermodynamics play a decisive role in selecting suitable reactions based on working temperatures and reaction enthalpy.
Finally, high solar absorptivity and thermal conductivity are vital for maximizing heat storage efficiency, which is crucial for driving endothermic reactions in TCES systems.Therefore, improving these properties represents a significant challenge in the context of TCES [67].
Table 2 summarizes the properties of popular TCES systems, along with their reversible chemical reactions, phase-phase interaction types, and material types.Three types of

Introduction to Thermochemical Energy Storage
TCES is a promising technology that involves the use of chemical reactions to store and release energy.TCES systems utilize reversible chemical reactions to store thermal energy, which can be later retrieved for various applications.These systems typically involve the conversion of a material between two chemical states, one absorbing energy during an endothermic reaction (charging) and the other releasing energy during an exothermic reaction (discharging).
An ideal TCES system should possess several key attributes.Firstly, it should exhibit high energy storage density to minimize the system's weight and volume requirements, with target values set at 300 kWh m −3 or >1000 kJ kg −1 [66].Secondly, high working temperatures are essential for maximizing power generation efficiency, with the charging step falling between 700 • C and 1500 • C [66].Additionally, materials must demonstrate high cycling and chemical/thermal stability to continue prolonged operation, with a focus on maintaining chemical reaction reversibility and multicyclic stability [66].Strong mechanical properties are crucial for ensuring long-term integrity during repeated cycles, as solid materials are prone to morphological degradation at high temperatures.Moreover, storage materials should be inexpensive, abundant, and low in toxicity (targeted cost of $4.2 MJ −1 , DOE 2020) [66].Fast and stable kinetics of both forward and reverse reactions are necessary to facilitate energy charging and discharging steps, while favorable reaction thermodynamics play a decisive role in selecting suitable reactions based on working temperatures and reaction enthalpy.
Finally, high solar absorptivity and thermal conductivity are vital for maximizing heat storage efficiency, which is crucial for driving endothermic reactions in TCES systems.
Table 2 summarizes the properties of popular TCES systems, along with their reversible chemical reactions, phase-phase interaction types, and material types.Three types of phasephase interaction are mentioned in Table 2: solid-gas, liquid-gas, and gas-gas.Among the three, solid-gas systems (carbonates, hydroxides, hydrides, and metal oxides) are considered the most promising TCES because the primary products of these reactions are solids that can be conveniently stored [68].Carbonates offer low material costs and abundance, making them suitable for largescale use, although they face challenges with limited reversibility and cyclic stability [91].Hydroxides, also economically viable and abundant, demonstrate good reversibility but are susceptible to agglomeration and side reactions [92].Metal hydrides display high volumetric energy density but are hindered by hydrogen embrittlement and high material costs [93].Metal oxides exhibit good reversibility yet suffer from toxicity and high costs [94].Ammonia synthesis, while easy to control, encounters issues with low operating temperatures and storage costs [89].Methane reforming, despite high operating temperatures, lacks volumetric energy density and poses toxicity concerns [90].The SO 3 /O 2 /SO 2 system operates at high temperatures but has a low energy density and requires a catalyst [90].

Limestone-Based Thermochemical Energy Storage
One of the most promising classes of gas-solid systems for TCES consists of those relying on the reversible decomposition of limestone, also known as calcium carbonate.Limestone is decomposed into CaO and CO 2 by absorbing heat (calcination), while during the reverse reaction the heat is released (carbonation).This approach offers numerous advantages, including low material cost (approximately $10 per tonne for CaCO 3 /CaO), widespread availability, nontoxicity, exceptionally high energy density (reaching up to 4 GJ m −3 ) and reaction enthalpy (∆H 860 • C = 165.9kJ) [95], operation at very high temperatures (exceeding 800 • C), catalyst-free operation, byproductfree reactions, and extensive experimental validation from Carbon Capture and Storage (CCS) applications [58].
Despite the advantageous characteristics of limestone-based TCES, it is afflicted by significant drawbacks, particularly concerning the multicyclic stability of CaCO 3 /CaO.Specifically, the reduction in CaO reactivity necessitates the use of oversized equipment due to the substantial presence of unreactive solids within the system.As CaO deactivation progresses, the volume of inert solid material requiring transportation, preheating, cooling, and processing throughout the plant escalates, resulting in reduced overall efficiency.
The multicycle stability and performance degradation of the CaCO 3 /CaO system have been extensively reported and studied for CO 2 storage and capture (CCS) [96][97][98].The multicycle performance of limestone strongly depends on the operating conditions of the process.The operating conditions for CCS are different from those for TCES (Table 3).It has been widely reported that the calcination reaction goes to completion under appropriate conditions, while the carbonation reaction often does not.The carbonation reaction is divided into two stages.Initially, carbonation occurs rapidly as CO 2 molecules are adsorbed onto the exposed surface of CaO particles.This early stage is not only influenced by the intrinsic reaction kinetics but also by factors such as mass and heat transfer efficiency to the particle surface, which can significantly affect the carbonation rate.As the reaction progresses, a thin layer of CaCO 3 forms on the CaO surface, leading to a transition to a diffusion-controlled stage.Here, the rate of carbonation becomes limited by the diffusion of CO 2 molecules through the formed CaCO 3 layer before reaching the underlying CaO surface for further reaction.This shift highlights the importance of considering both reaction kinetics and diffusion phenomena in understanding the overall carbonation process.This incomplete carbonation affects the overall efficiency of the energy storage system.
The observed decline in carbonation activity of CaO over multiple cycles is primarily attributed to CaO deactivation mechanisms, including sintering, pore plugging, and agglomeration.Sintering refers to the process where CaO particles coalesce due to high temperatures, reducing the available surface area and slowing carbonation reactions.Pore plugging occurs when pores within the CaO material become filled with CaCO 3 , hindering the diffusion of CO 2 into the material.Agglomeration involves the formation of larger CaO particles, which reduces the overall active surface area of the material.This issue has been extensively studied in the context of CO 2 capture applications and, more recently, in thermochemical energy storage (TCS) applications, where it directly impacts the energy density of the system.

Strategies to Improve Energy Storage in Limestone-Based Systems
Some strategies have been devised to address the issue of multicycle performance degradation of limestone, including hydration [102], Ca-based sorbents/additives, and the use of fine CaO/CaCO 3 particles.
Hydration involves the exposure of CaO to water vapor, leading to the formation of calcium hydroxide (Ca(OH) 2 ).This process can help mitigate CaO deactivation by forming a protective layer of Ca(OH) 2 on the CaO surface, which can inhibit sintering and enhance the regeneration ability of CaO during carbonation.Specifically, upon hydration, CaO undergoes conversion into Ca(OH) 2 , resulting in an expansion of molar volume (CaO = 16.9 cm 3 mol −1 ; Ca(OH) 2 = 33.7 cm 3 mol −1 ) and a reduction in density (CaO = 3.32 g cm −3 ; Ca(OH) 2 = 2.20 g cm −3 ) [103].This hydration process leads to swelling of the CaO, followed by the decomposition of the formed hydrate, inducing the development of fractures within the material's core.Consequently, the material's porosity and surface area are increased, facilitating enhanced diffusion of CO 2 during carbonation reactions and mitigating pore blockage phenomena [103].Research studies have demonstrated the effectiveness of hydration in improving the cyclic stability and reactivity of CaO-based sorbents for CO 2 capture and thermochemical energy storage [103][104][105].
The size of particles plays a significant role in the multicyclic performances of limestone [106,107], alongside factors like crystallinity [108], morphology [109], and the inclusion of additives [110].Large or highly crystalline particles exhibit slower reaction kinetics and suffer from plugging during carbonation.Conversely, smaller particles enable faster carbonation kinetics at lower temperatures, leading to improved multicyclic stability by minimizing sintering and pore-plugging issues [111,112].
Research indicates that reducing limestone particle size below 45 µm can notably increase residual CaO multicyclic conversion [58].This effect is attributed to the relative thickness of the CaCO 3 product layer compared with pore size within the CaO skeleton, where coarse particles are prone to severe pore-plugging and subsequent sintering, decreasing the available CaO surface area.Fine particles (<45 µm) mitigate pore-plugging phenomena due to their higher surface-to-volume ratio, enhancing CO 2 accessibility [113].
Despite benefits in multicyclic stability, fine particle usage in large-scale reactors poses challenges such as agglomeration and poor gas/solid contact [114,115].However, studies employing high-intensity acoustic waves demonstrate enhanced carbonation performances of fine limestone particles in fluidized bed reactors, mitigating sintering-induced CaO deactivation [116].
The vulnerability of limestone-based materials to sintering over multiple cycles, where particles fuse together, is due to the low Tammann temperature (Tt) of CaCO 3 .The Tammann temperature is the temperature at which sintering begins [117].To overcome this challenge and maintain the reactivity of CaCO 3 /CaO over multiple cycles, a promising solution is the doping or mixing of an additive with heat-resistant properties such as Al 2 O 3 , SiO 2 , TiO 2 , ZrO 2 , Y 2 O 3 , MgO, etc., which have high Tt values [105,[118][119][120][121].These additive materials help with sintering resistance, ensuring better stability over repeated carbonation Energies 2024, 17, 2572 9 of 20 and calcination cycles.These additives can be classified into two groups based on their interaction with CaO: reactive additives and non-reactive additives.Reactive additives include Al 2 O 3 , SiO 2 , ZrO 2 , etc.; they react with CaO and form ternary oxides (Table 4).Non-reactive additives do not react with CaO; examples of such additives are Y 2 O 3 , MgO, CeO 2 , etc. (Table 5) [117].
Al 2 O 3 emerges as a prominent choice among materials for creating ternary oxides with CaO, owing to its impressive attributes such as remarkable thermal stability (notably demonstrated by a Tt of 900 • C), cost-efficiency, and strong mechanical resilience [117].Recent advancements in the multicyclic performances of CaO-based sorbents supported on Al 2 O 3 are outlined in Table 4. Incorporation of Al 2 O 3 into CaO-based sorbents leads to the formation of diverse Ca-Al ternary oxides, such as Ca 3 Al 2 O 6 [122], Ca 5 Al 6 O 14 [123], Ca 9 Al 6 O 18 [124], and Ca 12 Al 14 O 33 [125].Consequently, these mixed oxides contribute to the stabilization of CaO particles, enhancing their resistance to sintering during repeated operational cycles [126].
It is crucial to emphasize that the multicyclic performance achievable by Al 2 O 3stabilized sorbents is heavily influenced by the synthesis methodology employed, as it dictates the distribution of CaO, Al 2 O 3 , and Ca-Al ternary oxides within the final material [117].In this context, an array of synthesis techniques has been proposed in the scientific literature for the fabrication of Al 2 O 3 -supported CaO-based sorbents, including wet mixing, coprecipitation, sol-gel, chemical vapor deposition, atomic layer deposition, and others.These methods offer distinct advantages in terms of control over material morphology, porosity, and chemical composition, thus influencing the sorbents' performance characteristics [127].
ZrO 2 -supported CaO-based sorbents have been extensively investigated using various synthesis techniques, primarily due to the high thermal stability of ZrO 2 (as high as 1221 • C).In contrast to Al 2 O 3 and SiO 2 supports, the interaction between CaO and ZrO 2 is relatively straightforward, resulting mainly in the formation of CaZrO 3 [126,128].CaZrO 3 is a perovskite with a high melting point of 2345 • C, a small thermal expansion coefficient, high strength, and chemical and thermal stability at elevated temperatures [129,130].A thorough examination of Table 4 reveals that ZrO 2 -supported CaO-based sorbents exhibit notable multicyclic stability, particularly when a high mass ratio of ZrO 2 is utilized.For instance, research conducted by Koirala et al. found that a ZrO 2 -stabilized sorbent containing 76 wt.% of CaZrO 3 maintained its CO 2 uptake (0.11 g CO2 /g sorbent ) over 1200 cycles without any significant decline [131].However, it is worth noting that while these sorbents demonstrate impressive multicyclic stability, the higher amount of ZrO 2 will reduce the amount of CaO by reacting with it to form CaZrO 3 , which may lead to reduced CaO reactivity, thereby limiting the overall CO 2 uptake capacity [128].
SiO 2 stands out as another widely accessible and cost-effective support material extensively utilized in the production of stabilized CaO-based sorbents [130].Nanostructured silica plays a crucial role in enhancing the dispersion of CaO agglomerates and mitigating sintering phenomena [132].It has been demonstrated that this beneficial effect stems from the interaction between SiO 2 and CaO, leading to the formation of calcium silicates (Ca 2 SiO 4 ), which impart greater thermal stability to the CaO framework.Consequently, this results in a higher effective conversion over multiple cycles compared with raw limestone [133].Notably, Ca 2 SiO 4 exhibits higher thermal stability (characterized by a Tt of 929 • C) compared with SiO 2 (which has a Tt of 664 • C) [134].During the multicycle process, Ca 2 SiO 4 undergoes a phase transformation, causing a volume expansion of the sorbent, which counteracts sintering phenomena [134].
The incorporation of MgO in CaCO 3 /CaO has been extensively explored [135,136].Huang et al. homogeneously dispersed and mixed MgO nanoparticles (ranging from 50 to 100 nm) with CaO grains on the particle surface, resulting in improved sintering resistance and enhanced multicyclic stability [137].The cyclic stability of MgO-stabilized sorbents is notably influenced by the MgO amount, as reflected in the MgO-to-CaO weight ratio [105].It has been established that an adequate amount of MgO is essential for achieving effective and stable reactive performance.Regardless of the MgO/CaO ratio, MgO-stabilized sorbents typically exhibit a higher specific surface area and pore volume compared with the original material [138].However, an increase in MgO content leads to a reduction in specific surface area.There exists an optimum MgO/CaO ratio that maximizes multicyclic CO 2 uptake; beyond a certain MgO amount, sintering resistance diminishes [139].
Apart from MgO, Y 2 O 3 (with a Tt of 1083 • C) has been identified as an effective stabilizer for CaO-based materials, mitigating sintering tendencies [117].Y 2 O 3 nanoparticles were evenly distributed on CaO particles by Zhang et al., resulting in enhanced reaction kinetics and a linear correlation between the maximum carbonation rate and specific pore volume (less than 220 nm) [140].Different synthesis methods, such as thermal decomposition and wet impregnation, have been employed to synthesize Y 2 O 3 -stabilized CaO-based materials, with no significant difference observed in multicyclic performances, indicating minimal impact of the synthesis procedure [141].Additionally, CeO 2 (with a Tt of 1064 • C) has been successfully integrated as a stabilizer in CaO-based materials, attributed to its high thermal stability and ability to facilitate oxygen mobility and vacancy generation, thereby enhancing CO 2 diffusion and O 2− mobility [171,172].Various Ca/Ce molar ratios have been explored in CeO 2 -supported CaO-based material synthesis, revealing a loose-shell-connected cross-linking structure with CeO 2 acting as a physical barrier to prevent CaO crystallite growth and sintering [170].
Moreover, the simultaneous incorporation of multiple inert materials has been investigated to exploit synergistic effects on multicyclic stability.For instance, TiO 2 and Al 2 O 3 were simultaneously used to fabricate hierarchical core-shell microarchitectures.CaObased materials are enriched in Ca, supported by Al 2 O 3 , and stabilized by TiO 2 [173].Similarly, a Y 2 O 3 /MgO-stabilized CaO-based material was synthesized via a sol-gel method to enhance cyclic stability and CO 2 uptake capacity [174].Table 6 summarizes the performance of multiple additives used in pairs.In summary, the selection of support material for stabilizing CaO-based materials must consider both multicyclic stability and material activity towards CO 2 .While metal oxide-stabilized materials typically exhibit better multicyclic stability due to the stable framework formed, the presence of metal oxides may reduce CaO activity due to the formation of ternary oxides, necessitating a balance between mitigating CO 2 uptake (carbonation) decline and minimizing CaO activity reduction.Moreover, the utilization of novel sintering-resistant materials necessitates consideration of higher material costs and environmental impacts associated with synthesis processes.Furthermore, the majority of synthetic materials in the literature are produced as fine powders, posing handling and processing challenges.
The selection and characterization of additives, including their synthesis methods and doping ratios, require systematic investigation to identify optimal formulations for enhanced multicyclic stability.Additionally, the mechanisms underlying the interaction between additives and CaO/CaCO 3 matrices need to be elucidated to guide the design of tailored additive-integrated systems.

Challenges and Limitations
The exploration of additives in limestone-based thermochemical energy storage (TCES) systems opens up avenues for further research and development in several critical areas.Understanding the interaction between additives and the primary material, such as calcium carbonate (CaCO 3 ) or calcium oxide (CaO), is essential for optimizing the performance and durability of TCES systems.
Future research should focus on systematically evaluating a broader range of additives beyond those already investigated, such as Al 2 O 3 , SiO 2 , MgO, Y 2 O 3 , CeO 2 , and TiO 2 .This exploration could involve novel materials with potentially superior properties in terms of sintering resistance, thermal stability, and catalytic effects.Additionally, detailed characterization studies, such as in-situ synchrotron experiments like powder X-ray diffraction (PXRD) or Small/Wide Angle X-ray Scattering (SAXS/WAXS), are necessary to clarify the mechanisms underlying the interaction between additives and the primary material, including surface morphology, chemical bonding, and phase transformations.
The effectiveness of additives in enhancing the multicyclic stability of TCES systems is highly dependent on their concentration and uniform dispersion within the material matrix.Further research is needed to optimize these parameters to achieve the maximum benefit while minimizing costs and processing complexity.Advanced synthesis techniques, such as atomic layer deposition, sol-gel methods, and mechanochemical activation, offer precise control over additive distribution and morphology and warrant investigation for their applicability in large-scale TCES systems.On the other hand, the environmental impact of such synthesis methods should be carefully evaluated, and perhaps greener alternative synthesis methods should be considered.
In-depth mechanistic studies are required to unravel the specific roles played by additives in mitigating sintering, pore plugging, and agglomeration phenomena in TCES materials.Computational modeling and simulation techniques can provide valuable insights into the thermodynamic and kinetic aspects of additive interactions, for example, how additives can enhance ion mobility at high temperatures, facilitate the carbonation reaction, and aid in the rational design of optimized materials with tailored properties for enhanced TCES performance.
As TCES technologies transition from laboratory-scale research to practical implementation, considerations of scalability, cost-effectiveness, and sustainability become paramount.Future research activities should address the scalability of additive synthesis and integration processes, as well as the economic feasibility of large-scale production and deployment.Life cycle assessments and techno-economic analyses can help identify the most promising additive formulations and processing routes for commercialization, taking into account factors such as raw material availability, energy consumption, and environmental impact.
The successful deployment of TCES systems hinges on their seamless integration with renewable energy sources, such as solar and wind power, to enable grid stabilization and energy arbitrage.Research efforts should focus on developing synergistic approaches that optimize the coupling between TCES and renewable energy technologies, leveraging advances in control strategies, predictive modeling, and system optimization algorithms.Collaborative research initiatives involving academia, industry, and government stakeholders can facilitate the rapid translation of fundamental research findings into practical solutions for real-world energy challenges.
By addressing these research implications, the field of limestone-based TCES can advance towards the realization of cost-effective, high-performance energy storage solutions that play a pivotal role in enabling the widespread adoption of renewable energy and achieving global sustainability goals.

Conclusions
In conclusion, this review has underscored the critical role of additives in enhancing the performance and durability of limestone-based thermochemical energy storage (TCES) systems.By evaluating various strategies such as hydration, fine particle utilization, and the incorporation of additives like Al 2 O 3 and ZrO 2 , this paper has addressed challenges such as performance degradation and sintering over multiple cycles.It is evident from the discussion that the selection of support materials must carefully balance multicyclic stability with material activity towards CO 2 .While metal oxide-stabilized materials offer better multicyclic stability, their potential to reduce CaO activity necessitates a nuanced approach to optimize performance.
Future research should explore a wider range of additives, prioritize detailed characterization studies, and employ advanced synthesis techniques to optimize additive distribution and morphology while minimizing costs.Mechanistic studies and computational modeling offer avenues to better understand the specific roles of additives in TCES materials.As TCES technologies advance towards practical use, scalability, cost-effectiveness, and sustainability considerations must be addressed.By embracing these implications and fostering collaboration, the field can move towards cost-effective, high-performance energy storage solutions.

Figure 1 .
Figure 1.Types of energy storage-process basis.

Figure 1 .
Figure 1.Types of energy storage-process basis.

Figure 2 .
Figure 2. Maximum duration of electricity storage needed versus fraction of annual energy from variable renewable generators (wind and solar) [63].

Figure 2 .
Figure 2. Maximum duration of electricity storage needed versus fraction of annual energy from variable renewable generators (wind and solar) [63].

Figure 3 .
Figure 3. Storage duration of some energy storage technologies against the size of energy storage technologies [59].

Figure 3 .
Figure 3. Storage duration of some energy storage technologies against the size of energy storage technologies [59].

Funding:
M.V.S. acknowledges the financial support from the UCD Ad Astra Fellowship Program and the Technology Transfer Strengthening Initiative Program from Enterprise Ireland (reference grant number 68001).R.A. acknowledges the financial support from the UCD Ad Astra Studentship Program.

Table 2 .
Properties of popular TCES systems.

Table 4 .
The performance of reactive additives and their operating conditions.

Table 5 .
The performance of non-reactive additives and their operating conditions.

Table 6 .
The performance of multiple additives used in pairs and their operating conditions.