Binuclear Cu complex catalysis enabling Li-CO2 battery with a high discharge voltage above 3.0 V

MOF-Derived CoSe2 Nanoparticles/Carbonized Melamine Foam as Catalytic Cathode Enabling Flexible Li-CO2 Batteries with High Energy Efficiency and Stable Cycling

Rechargeable aprotic Li-CO2 batteries have aroused worldwide interest owing to their environmentally friendly CO2 fixation ability and ultra-high specific energy density. However, its practical applications are impeded by the sluggish reaction kinetics and discharge product accumulation during cycling. Herein, a flexible composite electrode comprising CoSe2 nanoparticles embedded in 3D carbonized melamine foam (CoSe2/CMF) for Li-CO2 batteries is reported. The abundant CoSe2 clusters can not only facilitate CO2 reduction/evolution kinetics but also serve as Li2CO3 nucleation sites for homogeneous discharge product growth. The CoSe2/CMF-based Li-CO2 battery exhibits a large initial discharge capacity as high as 5.62 mAh cm-2 at 0.05 mA cm-2, a remarkably small voltage gap of 0.72 V, and an ultrahigh energy efficiency of 85.9% at 0.01 mA cm-2, surpassing most of the noble metal-based catalysts. Meanwhile, the battery demonstrates excellent cycling stability of 1620 h (162 cycles) at 0.02 mA cm-2 with an average overpotential of 0.98 V and energy efficiency of 85.4%. Theoretical investigations suggest that this outstanding performance is attributed to the suitable CO2/Li adsorption and low Li2CO3 decomposition energy. Moreover, flexible Li-CO2 pouch cell with CoSe2/CMF cathode displays stable power output under different bending deformations, showing promising potential in wearable electronic devices.

Cross-linked K0.5MnO2 nanoflower composites for high rate and low overpotential Li-CO2 batteries

Rechargeable Li-CO2 batteries are deemed to be attractive energy storage systems, as they can effectively inhale and fix carbon dioxide and possess an extremely high energy density. Unfortunately, the irreversible decomposition of the insoluble and insulating Li2CO3 results in awful electrochemical performance and inferior energy efficiency of Li-CO2 batteries. Furthermore, the low energy efficiency will exacerbate the extra waste of resources. Therefore, it is vital to design novel and efficient catalysts to enhance the battery performance. Herein, a facile, one-step strategy is introduced to design cross-linked, ultrathin K0.5MnO2 nanoflowers combined with CNTs (K0.5MnO2/CNT) as a highly efficient cathode for Li-CO2 batteries. Impressively, the Li-CO2 battery based on the K0.5MnO2/CNT cathode achieves a low overpotential (1.05 V) and a high average energy efficiency (87.95%) at a current density of 100 mA g-1. Additionally, the K0.5MnO2/CNT cathode can steadily run for over 100 cycles (overpotential < 1.20 V). Moreover, a low overpotential of 1.47 V can be obtained even at a higher current density of 1000 mA g-1, indicating the superior rate performance of K0.5MnO2/CNT. This strategy offers new insight and guidance for the development of low-cost and high-performance Li-CO2 batteries.

High Entropy Alloys Enable Durable and Efficient Lithium-Mediated CO2 Redox Reactions

Designing electrocatalysts with high activity and durability for multistep reduction and oxidation reactions is challenging. High-entropy alloys (HEAs) are intriguing due to their tunable geometric and electronic structure through entropy effects. However, understanding the origin of their exceptional performance and identifying active centers is hindered by the diverse microenvironment in HEAs. Herein, NiFeCoCuRu HEAs designed with an average diameter of 2.17 nm, featuring different adsorption capacities for various reactants and intermediates in Li-mediated CO2 redox reactions, are introduced. The electronegativity-dependent nature of NiFeCoCuRu HEAs induces significant charge redistribution, shifting the d-band center closer to Fermi level and forming highly active clusters of Ru, Co, and Ni for Li-based compounds adsorptions. This lowers energy barriers and simultaneously stabilizes *LiCO2 and LiCO3+CO intermediates, enhancing the efficiency of both CO2 reduction and Li2CO3 decomposition over extended periods. This work provides insights into specific active site interactions with intermediates, highlighting the potential of HEAs as promising catalysts for intricate CO2 redox reactions.

Mechanistic Insights into the Discharge Processes of Li-CO2 Batteries

Li-CO2 batteries facilitate renewable energy storage in a cost-effective, eco-friendly manner. However, an inadequate understanding of their reaction mechanism severely impedes their development. Here we outline recent mechanistic advances in the discharge processes of Li-CO2 batteries, particularly in terms of the theoretical aspect. First, the vital factors affecting the formation of discharge intermediates are highlighted, and a surface lithiation mechanism predominantly applicable to catalysts with weak CO2 adsorption is proposed. Subsequently, the modeling of the chemical potential of Li++e-, which is crucial for the evaluation of the theoretical limiting voltage, is detailed. Finally, challenges and future directions pertaining to the further development of Li-CO2 are discussed. In essence, this concept article seeks to inspire future experimental and theoretical studies in advancing the development of Li-CO2 electrochemical technology.

Reversible Carbon Dioxide/Lithium Oxalate Regulation toward Advanced Aprotic Lithium Carbon Dioxide Battery

Li-CO2 batteries have received significant attention owing to their advantages of combining greenhouse gas utilization and energy storage. However, the high kinetic barrier between gaseous CO2 and the Li2CO3 product leads to a low operating voltage (<2.5 V) and poor energy efficiency. In addition, the reversibility of Li2CO3 has always been questioned owing to the introduction of more decomposition paths caused by its higher charging plateau. Here, a novel "trinity" Li-CO2 battery system was developed by synergizing CO2, soluble redox mediator (2,2,6,6-tetramethylpiperidoxyl, as TEM RM), and reduced graphene oxide electrode to enable selective conversion of CO2 to Li2C2O4. The designed Li-CO2 battery exhibited an output plateau reaching up to 2.97 V, higher than the equilibrium potential of 2.80 V for Li2CO3, and an ultrahigh round-trip efficiency of 97.1 %. The superior performance of Li-CO2 batteries is attributed to the TEM RM-mediated preferential growth mechanism of Li2C2O4, which enhances the reaction kinetics and rechargeability. Such a unique design enables batteries to cope with sudden CO2-deficient environments, which provides an avenue for the rationally design of CO2 conversion reactions and a feasible guide for next-generation Li-CO2 batteries.

Facile fabrication of Ni, Fe-doped δ-MnO2 derived from Prussian blue analogues as an efficient catalyst for stable Li-CO2 batteries

Rechargeable Li-CO2 batteries are regarded as an ideal new-generation energy storage system, owing to their high energy density and extraordinary CO2 capture capability. Developing a suitable cathode to improve the electrochemical performance of Li-CO2 batteries has always been a research hotspot. Herein, Ni-Fe-δ-MnO2 nano-flower composites are designed and synthesized by in situ etching a Ni-Fe PBA precursor as the cathode for Li-CO2 batteries. Ni-Fe-δ-MnO2 nanoflowers composed of ultra-thin nanosheets possess considerable surface spaces, which can not only provide abundant catalytic active sites, but also facilitate the nucleation of discharge products and promote the CO2 reduction reaction. On the one hand, the introduction of Ni and Fe elements can improve the electrical conductivity of δ-MnO2. On the other hand, the synergistic catalytic effect between Ni, Fe elements and δ-MnO2 will greatly enhance the cycling performance and reduce the overpotential of Li-CO2 batteries. Consequently, the Li-CO2 battery based on the Ni-Fe-δ-MnO2 cathode shows a high discharge capacity of 8287 mA h g-1 and can stabilize over 100 cycles at a current density of 100 mA g-1. The work offers a promising guideline to design efficient manganese-based catalysts for Li-CO2 batteries.

Boosting a practical Li-CO2 battery through dimerization reaction based on solid redox mediator

Li-CO2 batteries offer a promising avenue for converting greenhouse gases into electricity. However, the inherent challenge of direct electrocatalytic reduction of inert CO2 often results in the formation of Li2CO3, causing a dip in output voltage and energy efficiency. Our innovative approach involves solid redox mediators, affixed to the cathode via a Cu(II) coordination compound of benzene-1,3,5-tricarboxylic acid. This technique effectively circumvents the shuttle effect and sluggish kinetics associated with soluble redox mediators. Results show that the electrochemically reduced Cu(I) solid redox mediator efficiently captures CO2, facilitating Li2C2O4 formation through a dimerization reaction involving a dimeric oxalate intermediate. The Li-CO2 battery employing the Cu(II) solid redox mediator boasts a higher discharge voltage of 2.8 V, a lower charge potential of 3.7 V, and superior cycling performance over 400 cycles. Simultaneously, the successful development of a Li-CO2 pouch battery propels metal-CO2 batteries closer to practical application.

Porous Carbon Foam with Carbon Nanotubes as Cathode for Li-CO2 Batteries

With the extensive use of fossil fuels, the ever-increasing greenhouse gas of mainly carbon dioxide emissions will result in global climate change. It is of utmost importance to reduce carbon dioxide emissions and its utilization. Li-CO2 batteries can convert carbon dioxide into electrochemical energy. However, developing efficient catalysts for the decomposition of Li2 CO3 as the discharge product represents a challenge in Li-CO2 batteries. Herein, we demonstrate a carbon foam composite with growing carbon nanotube by using cobalt as the catalyst, showing the ability to enhance the decomposition rate of Li2 CO3 , and thus improve the electrochemical performance of Li-CO2 batteries. Benefiting from its abundant pore structure and catalytic sites, the as-assembled Li-CO2 battery exhibits a desirable overpotential of 1.67 V after 50 cycles. Moreover, the overpotentials are 1.05 and 2.38 V at current densities of 0.02 and 0.20 mA cm-2 , respectively. These results provide a new avenue for the development of efficient catalysts for Li-CO2 batteries.

Boosted Mg-CO2 Batteries by Amine-Mediated CO2 Capture Chemistry and Mg2+ -Conducting Solid-electrolyte Interphases

Mg-CO2 battery has been considered as an ideal system for energy conversion and CO2 fixation. However, its practical application is significantly limited by the poor reversibility and sluggish kinetics of CO2 cathode and Mg anode. Here, a new amine mediated chemistry strategy is proposed to realize a highly reversible and high-rate Mg-CO2 battery in conventional electrolyte. Judiciously combined experimental characterization and theoretical computation unveiled that the introduced amine could simultaneously modify the reactant state of CO2 and Mg2+ to accelerate CO2 cathodic reactions on the thermodynamic-kinetic levels and facilitate the formation of Mg2+ -conductive solid-electrolyte interphase (SEI) to enable highly reversible Mg anode. As a result, the Mg-CO2 battery exhibits boosted stable cyclability (70 cycles, more than 400 h at 200 mA g-1 ) and high-rate capability (from 100 to 2000 mA g-1 with 1.5 V overpotential) even at -15 °C. This work opens a newly promising avenue for advanced metal-CO2 batteries.

Recycled Tandem Catalysts Promising Ultralow Overpotential Li-CO2 Batteries

Lithium-carbon dioxide (Li-CO2 ) batteries are regarded as a prospective technology to relieve the pressure of greenhouse emissions but are confronted with sluggish CO2 redox kinetics and low energy efficiency. Developing highly efficient and low-cost catalysts to boost bidirectional activities is craved but remains a huge challenge. Herein, derived from the spent lithium-ion batteries, a tandem catalyst is subtly synthesized and significantly accelerates the CO2 reduction and evolution reactions (CO2 RR and CO2 ER) kinetics with an in-built electric field (BEF). Combining with the theoretical calculations and advanced characterization techniques, this work reveals that the designed interface-induced BEF regulates the adsorption/decomposition of the intermediates during CO2 RR and CO2 ER, endowing the recycled tandem catalyst with excellent bidirectional activities. As a result, the spent electronics-derived tandem catalyst exhibits remarkable bidirectional catalytic performance, such as an ultralow voltage gap of 0.26 V and an ultrahigh energy efficiency of 92.4%. Profoundly, this work affords new opportunities to fabricate low-cost electrocatalysts from recycled spent electronics and inspires fresh perceptions of interfacial regulation including but not limited to BEF to engineer better Li-CO2 batteries.

Efficient Rechargeable Li-CO2 Battery with a Liquid Electrolyte-Soluble CuCl2 Electrocatalyst

We demonstrate here a simple liquid electrolyte soluble Cu-compound, viz., cupric chloride (CuCl2) as an alternative electrocatalyst for nonaqueous Li-CO2 batteries. The key point behind the selection of CuCl2 is that the theoretical potential of Li-CO2 batteries (≈2.8 V; Li+|Li) lies within the Cu1+|Cu0 redox couple (2.3-3.3 V; Li+|Li). The presence of CuCl2 in the liquid electrolyte near to the carbon nanotubes (≡ coelectrocatalyst)-loaded porous-CO2 cathode led to efficient electrocatalysis of CO2 and superior Li-CO2 battery performance. The cell overpotential in the presence of CuCl2 is 0.65 V, which is less than half compared to the one without it (≈1.7 V). Extensive investigations precisely elucidate the electrocatalytic mediation of CuCl2 with the redox characteristics of CO2. Additionally, only in the presence of CuCl2, the existence of Li-oxalate (Li2C2O4) is detected, which is a seldomly reported intermediate preceding the formation of Li2CO3.

Understanding Catalytic Mechanisms and Cathode Interface Kinetics in Nonaqueous Mg-CO2 Batteries

We leverage first-principles density functional theory (DFT) calculations to understand the electrocatalytic processes in Mg-CO2 batteries, considering ruthenium oxide (RuO2) as an archetypical cathode catalyst. Our goal is to establish a mechanistic framework for understanding the charging and discharging reaction pathways and their influence on overpotentials. On the RuO2 (211) surface, we found reaction initiation through thermodynamically favorable adsorption of Mg followed by interactions with CO2. However, we found that the formation of carbonate (CO32-) and oxalate (C2O42-) intermediates via the activation of CO2 at the catalytic site is thermodynamically unfavorable. We predict that MgC2O4 will form as the discharge product due to its lower overpotential compared to MgCO3. However, MgC2O4 is thermodynamically unstable and is expected to decompose into MgCO3, MgO, and C as final discharge products. Through Bader charge analysis, we investigate the covalent interactions between intermediates and catalyst sites. Moreover, we study the electrochemical free energy profiles of the most favorable reaction pathways and determine discharge and charge overpotentials of 1.30 and 1.35 V, respectively. Our results underscore the importance of catalyst design for the cathode material to overcome performance limitations in nonaqueous Mg-CO2 batteries.

Bifunctional electrolyte additive MgI2 for improving cycle life in high-efficiency redox-mediated Li-O2 batteries

Here, MgI2 is introduced as a bifunctional self-defense redox mediator into dimethyl sulfoxide-based Li-O2 batteries. During charging, I- is first oxidized to I3-, which facilitates the decomposition of Li2O2, and thus reduces overpotential. In addition, Mg2+ spontaneously reacts with the Li anode to form a very stable SEI layer containing MgO, which can resist the synchronous attack by the soluble I3- and improve the interface stability between the Li anode and the electrolyte. Therefore, a Li-O2 battery containing MgI2 exhibits an extended cycling life span (400 cycles) and a quite low overpotential (0.6 V).

Recent Advances in Rechargeable Metal-CO2 Batteries with Nonaqueous Electrolytes

This review article discusses the recent advances in rechargeable metal-CO2 batteries (MCBs), which include the Li, Na, K, Mg, and Al-based rechargeable CO2 batteries, mainly with nonaqueous electrolytes. MCBs capture CO2 during discharge by the CO2 reduction reaction and release it during charging by the CO2 evolution reaction. MCBs are recognized as one of the most sophisticated artificial modes for CO2 fixation by electrical energy generation. However, extensive research and substantial developments are required before MCBs appear as reliable, sustainable, and safe energy storage systems. The rechargeable MCBs suffer from the hindrances like huge charging-discharging overpotential and poor cyclability due to the incomplete decomposition and piling of the insulating and chemically stable compounds, mainly carbonates. Efficient cathode catalysts and a suitable architectural design of the cathode catalysts are essential to address this issue. Besides, electrolytes also play a vital role in safety, ionic transportation, stable solid-electrolyte interphase formation, gas dissolution, leakage, corrosion, operational voltage window, etc. The highly electrochemically active metals like Li, Na, and K anodes severely suffer from parasitic reactions and dendrite formation. Recent research works on the aforementioned secondary MCBs have been categorically reviewed here, portraying the latest findings on the key aspects governing secondary MCB performances.