Research Progress in Cathode Catalysts for Li-CO 2 Batteries

. The need for environmentally friendly and effective energy storage technologies is growing urgently in response to the rising energy demand and the seriousness of the environmental issues, in order to meet the Double Carbon objective. Li-CO 2 batteries are a newer battery technology that has drawn a lot of interest. Its distinctiveness comes from the utilization of CO 2 as a key component in energy storage, which can efficiently transform CO 2 supplies into energy preservation and potentially be sustainable and environmentally friendly. This article examines the Li-CO 2 battery's internal working mechanisms, delves into the selection and development of positive electrode catalysts, and contrasts several materials to list their benefits and drawbacks. Carbon-based materials, precious metals, and their compounds, and transition metals and their complexes are the main areas of emphasis. Carbon materials' exceptional conductivity, enormous specific surface area, and high commercial viability are underlined. The performances of precious metals and transition metals paired with carbon materials are compared in detail. Finally, suggestions for further research into potential cathode catalytic materials for Li-CO 2 batteries are provided based on the effectiveness and practical utility of various materials. In an attempt to combat global warming and environmental pollution, new methods for converting waste gases, such as carbon dioxide, into energy will be developed thanks to research on Li-CO 2 batteries. The features of this battery technology, such as its high density of energy and lengthy cycle life, are projected to make it more practical to store and use renewable energy


Introductio
*Corresponding author: hekai@tjkjdx.wecom.workn Currently, new energy vehicles are gradually entering the public's field of vision.Driven by policies, the comprehensive development of new energy vehicles is the general trend and has gained a place in the market.Among them, lithium-ion batteries are very suitable as energy storage devices for new energy vehicles.Although traditional lithium ion batteries (LIBs) are already stable, they have a significant drawback, which is short battery life.The creation of renewable energy vehicles will suffer as a result.The introduction of Li-CO2 batteries, nevertheless, has the potential to solve this problem as lithium battery technology advances.Li-CO2 batteries have not yet been widely commercialized, and battery issues such as battery loss and efficiency in electric vehicles have not been resolved yet Compared to conventional lithium-ion batteries, Li-CO2 batteries have an energy density of over 7 times higher than that of traditional LIBs, with advantages such as higher energy storage capacity and stronger endurance.Furthermore, they convert energy using carbon dioxide, highlighting the advantages of new energy and breaking through development bottlenecks.
Li-CO2 is a fantastic energy storage option that may effectively reduce the greenhouse effect by turning carbon dioxide into carbonate.Because Li-CO2 batteries have a greater conceptual effective energy density (1876 Wh/kg) and a higher discharge voltage (2.8 V) than older lithium-ion batteries, they are superior energy storage technologies for long-distance vehicles with sustainable power supply [1,2].Li-CO2 batteries offer great potential for growth in capturing CO2 and energy storage, particularly in their incorporation of CO2 [1,3].A lithium metal anode, an electrolyte, an electrolyte separator, and a CO2 cathode make up a Li-CO2 battery.Under the driving voltage, metallic lithium migrates to the cathode as a result of electrons discharging during discharge.At the cathode, the dissolved CO2 simultaneously absorbs electrons and reacts with lithium ions to create Li2CO3.The actual discharge product Li2CO3 is considered a type of solid-state insulating material, which, although thermally stable, is not easily decomposed and can block porous electrodes, leading to the deactivation of active sites [4].
Both the negative and positive substances for electrodes have been essentially established since the creation of Li-CO2 batteries.However, there is still significant room for development in positive electrode catalysts.By carefully evaluating the charge-discharge process of Li-CO2 batteries, providing relevant positive electrode catalysts, and weighing their advantages and disadvantages, this article gives references for the decision-making process of favorable electrode catalysis for upcoming Li-CO2 batteries.

Overview of Li-CO batteries 2.1 Structure of Li-CObatteries
Lithium-air batteries were used before Li-CO2 batteries, hence they share many structural characteristics, including semi-open designs.(According to Figure 1) Lithium metal serves as the negative electrode in Li-CO2 batteries, while a porous structure serves as the positive electrode and liquid electrolyte fills the middle electrode.

The way Li-CO2 batteries function
Li-CO2 batteries are batteries that use CO2 in the air as a reactant, which have a higher energy density than ordinary lithium-ion batteries.When a Li-CO2 battery is discharged, lithium loses electrons and becomes lithium ions that detach from the positive electrode.The electrolyte moves to the positive electrode and reacts with CO2 dissolved near the positive electrode to generate a solid carbon discharge product of Li2CO3.The discharging process, transforming Li2CO3 back to the ions that make up lithium and CO2, is contrary to the recharging process.Since Li-CO2 batteries were first discovered, there hasn't been much time for investigation, but it has been established that their reaction is roughly 4Li+3CO2 2Li2CO3+C (E0=2.80V versus Li/Li + ) [5].Li2CO3, the discharge outcome of Li-CO2 battery packs, is more reliable than Li2O2 when compared to Lithium-air batteries, but it also results in slower breakdown during charging, necessitating greater operating temperatures to optimize performance.

Positive electrode catalyst materials
Positive catalyst materials are essential to the efficiency of Li-CO2 batteries for the purpose of energy storage and CO2 collection by accelerating the electrochemical response kinetics, particularly by lowering the Overpotential.In the past few years, the cathode catalyst materials discovered by researchers mainly include carbon materials, precious metals and their compounds, transition metals and their oxygen/Carbon compounds, and metal-organic frameworks.

Carbon materials
Porous carbon materials have excellent conductivity, low density, and large contact surfaces with porous structures.They are often used as catalyst carriers, electrode materials, and agents in electrochemical energy storage.These include carbon black, nanostructured carbon, functionalized carbon, and Graphene.For instance, carbon black has received a lot of scientific attention because of its affordability, high specific surface area, and high conductivity [6].Table 1 below provides an analysis of the advantages and disadvantages of carbon black, nanocarbon materials, heteroatom doped carbon materials, and multi atom co-doped carbon materials.

Precious metals and their compounds
Precious metals are widely used in the field of new energy due to their high activity, selectivity, and stability [7].Among them, Ru noble metal is a potential noble metal catalyst on the market at present, which has an outstanding ability to promote the redox of CO2.To create Ru NG, Yang et al. added Ru nanoparticles to N-doped graphene.It can perform steadily for 50 cycles at current densities of 200 mA/g and 400 mA/g and had a discharge capacity of 16743 mAh/g when applied to Li-CO2 batteries [7].The advantages of Ru-NG are its good reversibility and electrocatalytic activity.
Although single metal catalysts can optimize the electrocatalytic activity of the positive electrode and improve its structure, they still have certain limitations.So Yang et al. dispersed Ru and Cu onto carbon materials to form (Ru-Cu-G).Testing indicates that when used with Li-CO2 batteries, it can cycle steadily for 100 cycles at current densities of 200 mA/g and 400 mA/g and has a discharge capacity of 13698 mAh/g at a current density of 200 mA/g [7].This not only preserved the high discharge capacity of single metal catalysts but also achieved better stability because of the synergistic effect between metals which made their nanoparticles more dispersed and less prone to aggregation.In addition to Ru and Cu, there are also excellent precious metal-based catalytic materials such as metal Ir, Pt, Au, Ag, etc.

Transition metals and their complexes
Transition metals, with their affordable prices and abundant natural reserves, have a greater potential for application in Li-CO2 batteries compared to precious metals.Therefore, developing inexpensive non-precious metal catalysts from them is an important strategy for solving the commercialization of Li-CO2 [8].Among them, there are cheap metals such as Fe, Ni, Co, Mn, Fe2O3, Fe3O4, NiO, CuO, and CoFe2O4 as cathode catalysts [9].By altering the ligand, their framework, and the redox state of transitional metal complexes, it is possible to maximize the efficiency and selectivity of catalysts as well as the performance and longevity of lithium carbonate batteries.Since Cu's exceptional catalytic capacity to remove CO2, Cu-NG onto Li-CO2 batteries was developed employing this material.This led to a discharge capacity of 14864 mA/g at an average current density of 200 mA/g and steady cycling at low overvoltage densities of 200 mA/g and 400 mA/g for 50 cycles [7].Moreover, a layer of CuO formed on the surface of Cu during the reaction, enhancing the stability of the positive electrode and protecting its structure.At the same time, transition metal complexes can also promote the embedding and de-embedding processes of lithium electrons in electrode materials, improving the cycling and energy storage capacity of batteries.Based on a report, Hu et al. embedded extremely thin Ni nanoparticles on graphene using 3D printing and thermal shock technology, and as a result, the lithium-ion carbon dioxide battery with its effect produced a specific area capacity of 14.6 mAh/cm 2 , and steadily cycled for 100 times when the limited capacity was 1000 mAh/g and the battery's overpotential was lower than 1.05 V [10].Cu, in alongside Ni, has the ability to chemisorb and generate CO2 spontaneously.Additionally, the long-term stability of the positively charged electrode can be increased by the dispersion of nanoparticles of Cu on the outer layer of N-doped graphene.
The positive electrode of a Li-CO2 battery is promoted by all of the aforementioned transition metal oxides, which are also the most attractive beneficial electrode building blocks for Li-CO2 batteries for the reason of their inexpensive cost and extensive availability.

Research directions and application prospects
The most mainstream research directions for positive electrodes currently include the following aspects.
(1) The design and synthesis of transition metal oxide materials with high catalytic activity, stability, and cycle life are of great significance.At the same time, the catalytic performance of the material is further improved , 03003 (2023) through surface modification, nanostructure regulation, and other means.
(2) The current research mainly focuses on the conductivity, electrochemical activity, and catalytic reduction performance of carbon materials for carbon dioxide.By controlling the structural and functional alterations made to carbon materials, researchers hope to increase their capacity for catalysis and cyclic stability.
(3) Researchers are creating composite substances to serve as anode components for Li-CO2 batteries by harnessing the synergistic effects of various elements.For example, transition metal oxides are combined with carbon materials, conductive polymers, etc. to improve catalytic performance and electrochemical performance.
As a novel form of energy storage technology, lithium dioxide batteries have numerous potential uses.It can convert greenhouse gases such as carbon dioxide into useful chemicals and simultaneously store electrical energy.The following are some application prospects for Li-CO2 batteries.
(1) Energy storage and electric vehicles: Li-CO2 batteries are expected to become an important technology in the field of energy storage, used in energy storage systems, electric vehicles, and other fields.It can effectively convert carbon dioxide into electricity, achieving carbon dioxide reduction and sustainable resource utilization.
(2) Renewable energy integration: To achieve effective energy storage and use, Li-CO2 battery packs can be used in conjunction with systems that produce electricity from renewable sources (such as solar systems and wind-powered systems).This will promote the stable supply of renewable energy and promote the development and application of clean energy.
In terms of real-world implementation, Li-CO2 batteries have just recently developed, and research into them is still at their early stages with no existing commercial applications.The Chinese Academy of Engineering Physics' study group has currently created graphene carbon materials with a lot of sp-hybridized carbon atoms through coupling processes.Numerous research teams investigate nanoparticles, multi-atom doping, and other porous carbon materials as cathode catalysts.They also undertake theoretical research on mechanisms in conditions with only CO2 and build paired catalysts, electrolytes and electrodes.The slow decomposition kinetics of the discharge products, high overpotential, short cycle life, and the high cost associated with known high-performance cathode materials hinder the practical application of batteries.However, their theoretically higher energy density and environmentally friendly materials and mechanisms have a broad potential market in the context of dual carbon goals.It is believed that with continuous technological development, Li-CO2 batteries can overcome various challenges and accompany people's green lives.

Conclusion
According to comparison, carbon materials can be used as electrode materials to provide an active site for lithium insertion/delithiation reaction due to their excellent conductivity and chemical stability.At the same time, it also has the advantages of a large contact surface with a porous structure and low cost, making it very suitable as a carrier material for positive electrode catalysts.Precious metal materials, such as platinum and palladium, have excellent catalytic activity and can be doped onto carbon materials in the form of nanoparticles, which can improve the catalyst structure, increase battery discharge capacity, optimize catalytic activity, and improve battery stability.Compared to precious metals, transition metals have the following advantages, making them more likely to combine with carbon materials to become an ideal material for Li-CO2 battery cathode catalysis.
(1) Rich catalytic activity: In order to accomplish efficient catalysis, transition metallic oxide materials may offer rich locations for reactions and centers of action in the catalytic process due to their rich electronic makeup and chemical reactivity.
(2) Controllability: Transition metal dioxide materials' catalytic performance can be improved and altered by modifying their structure of crystals, surface anatomy, and oxidation state, along with other components.This controllability enables researchers to design and synthesize more efficient and stable catalytic materials.
(3) Abundant resources: Transitioning metal oxide materials are more cheap and sustainable than precious metal materials since they are more common in the crust and cost less.
(4) Durability: Because transitioning metal oxide materials typically have great chemical stability and endurance and can maintain permanent catalytic activity under harsh electrochemical circumstances, they increase the battery life of Li-CO2 batteries.
In conclusion, carbon materials and transition metal oxide materials have great potential to become ideal candidates for cathode catalytic materials of Li-CO2 batteries.However, precious metal materials still have certain application scenarios, and materials should be selected based on the requirements of catalysts and actual application situations to make their properties more suitable and achieve a greater effect of one plus one than two.

Table 1 .
Analysis of the Advantages and Disadvantages of Four Carbon Materials