Cathode architecture and active site engineering in lithium-CO₂ batteries

Mechanisms of aprotic CO₂RR

During a discharge process, a lithium metal anode provides several electrons and produces lithium ions that are then transported to the CO₂ cathode side. The CO₂RR usually occurs at the gas-liquid-solid interface among CO₂/aprotic electrolyte/electrocatalyst, where the CO₂ is able to be transformed to discharge products, including Li₂CO₃/C, Li₂CO₃/CO, Li₂C₂O₄, and HCOOH, with the involvement of Li⁺ ion and electrons.

The Li₂CO₃/C is generally accepted as a discharge product in rechargeable aprotic Li-CO₂ batteries^[13].

The peroxide-type intermediate could be preferred on carbon-based cathodes. For example, a one-electron reduction of CO₂ generates C₂O₄^2- [Figure 2A], resulting in an open circuit potential (3.0 V)^[19]. The intermediate C₂O₄^2- disproportionate constantly in the successive two steps [Figure 2A], generating stable crystalline Li₂CO₃. Specifically, the formation of Li₂CO₃/C is impacted by the adsorption energy between intermediates and electrocatalysts [Figure 2B]^[20]. If the binding energy between carbonaceous intermediates and the cathode is relatively weak, the Li₂CO₃ preferentially generates in the aprotic electrolyte close to the CO₂ cathode surface via a solvation-mediated growth process. On the contrary, if a sufficiently strong binding energy exists between the carbonaceous and catalyst surface, the Li₂CO₃ layer is inclined to form directly on CO₂ cathode surface by surface-adsorption interaction to generate homogeneous and conformal microstructure.

Figure 2. (A) Formation mechanism of Li₂CO₃. (B) Growth mechanisms for Li₂CO₃/C products. (Reproduced with permission^[3]. Copyright 2023, Wiley-VCH). (C) Formation mechanism of Li₂O. (D and E) Galvanostatic discharge curves and corresponding product of the Li-CO₂ battery under different capacities. (Reproduced with permission^[21]. Copyright 2017, Elsevier). (F) Formation mechanism of Li₂C₂O₄. (G) Solid RM(II)-BTC-catalyzed reaction mechanism towards Li₂C₂O₄ pathway. (H) The CV curves of solid RM(II)-BTC and CNTs cathode in CO₂ and Ar atmosphere. (I) Ex-situ FTIR spectra of solid RM(II)-BTC cathode. (Reproduced with permission^[24]. Copyright 2024, Springer Nature). (J) Formation mechanism of HCOOH. (K) Comparison between aqueous and aprotic Li-CO₂ batteries. (Reproduced with permission^[27]. Copyright 2021, Wiley-VCH).

Another disproportionation reaction occurs in aprotic Li-CO₂ batteries on the basis of Li₂CO₃ and CO, which facilitates batteries with low overpotential and highly reversible [Figure 2C]. In 2013, Xu et al. first reported that Li₂C₂O₄ intermediates could be transformed into CO and Li₂CO₃ at high temperatures^[13]. Qiao et al. used in-situ Raman spectroscopy to observe intermediate evolution during discharge process on an Au substrate^[21]. Li₂CO₃ and C species features can be obviously observed [Figure 2D]. Interestingly, a smaller discharge plateau was monitored at about 1.8 V at 20 μAh. This discharge plateau can also be attributed to the presence of Li₂O species [Figure 2E]. The electrochemical redox reaction was expressed as follows:

The calculated standard electrode potential of 1.89 V refers to the discharge plateau. The low discharge plateau is attributed to the ultralow CO₂ content and dynamic equilibrium on the electrode surface. Nevertheless, Li₂O is inclined to transform into Li₂CO₃ in an atmosphere including CO₂.

Recently, research has reported that thermodynamically unstable Li₂C₂O₄ as the ultimate discharge product can decrease the charge voltage to 3.8 V utilizing specific cathode catalysts in Li-CO₂ batteries^[22,23](Equation 3).

An oxalate-based (C₂O₄^2-) pathway is inclined to be promoted by metal-rich cathodes. For example, Li et al. analyzed and proved the electrocatalytic CO₂RR via solid redox mediator Cu (II)-MOF (denoted as RM(II)-BTC) during the discharging process^[24]. Firstly, during the process of discharge, solid RM(II)-BTC loaded onto the carbon nanotube (CNT) surface is reduced to low-valence Cu(I)-MOF (denoted as RM(I)-BTC). Subsequently, RM(I)-BTC interacts with captured CO₂ to successively generate Cu(II)-CO₂^- and Cu(II)-C₂O₄^2--Cu(II) intermediates through the chemical reactions. Finally, Li ions combine with the Cu(II)-C₂O₄^2--Cu(II) intermediates, evolving Li₂C₂O₄ product on the surface of solid RM(II)-BTC [Figure 2F and G]. Interestingly, solid RM(II)-BTC cathodes show different redox peaks at 3.34 V(E_a1) and 2.81 V (E_c1), respectively, revealing the reversible oxidation and reduction reaction of Cu(II)/Cu(I) in solid RM(II)-BTC. In comparison with the oxidation peak at (E_a1) in Ar atmosphere, two new oxidation peaks occurred in CO₂ atmosphere, resulting from the oxidation of trace Cu(I) (E_c2) and corresponding to the decomposition of the Li₂C₂O₄ discharge product, respectively [Figure 2H]. Besides, they found that noticeable characteristic peaks of Li₂C₂O₄ are observed in the discharged cathode by ex-situ Fourier transform infrared (FTIR) spectroscopy [Figure 2I]. Similarly, Wang et al. synthesized a soluble redox mediator (2,2,6,6-tetramethylpiperidoxyl, denoted as TEM RM) to facilitate selective transformation of CO₂ into Li₂C₂O₄^[25]. The soluble TEM RM first combined CO₂ molecules from the electrolyte, generating TEM^--Li⁺-CO₂ intermediates that promoted electron transport during the discharge process. Subsequently, the original TEM was liberated and Li₂C₂O₄ layers were precipitated regularly, which improves the redox kinetics and rechargeability. Moreover, the donor number (DN) of solvents also possesses a significant impact on the discharge process. In high-DN solvents, the generation of reaction intermediates (C₂O₄^2- and C₂O₆^2-) in Li-CO₂ batteries proceeded mainly by an “electrochemical solution pathway (high capacity but poor stability)”. In contrast, peroxide-type intermediate (Li₂O₂) reacted with CO₂ to generate Li₂CO₃ by a "chemical surface pathway (low capacity but high stability)" in low-DN solvents^[26].

Apart from solid discharge products, soluble liquid-phase products could be optimal because they can efficiently desorb from the active site, and adequate contact between catalysts and intermediates ensures completed decomposition. In particular, formic acid (HCOOH) is appealing and practical due to its high kinetical accessibility. For example, Xue et al. employed a Li_1+xAl_xGe_2-x(PO₄)₃ (LAGP) as the separator which only provides the transport of lithium ions^[27]. They found that Pd substrates served as cathode electrocatalysts for Li-CO₂ batteries (Equation 3) in which potential for HCOOH formation and dissolution is 2.80 V [Figure 2J]. As a result, HCOOH was observed with excellent selectivity and thus active sites are efficiently exposed on the surface of electrocatalysts. The Li-CO₂ battery delivered excellent cycling stability over 300 h with a lower charging voltage (2.99 V).

The Pd with LAGP can facilitate the CO₂RR in the aqueous electrolyte system without impacting the lithium-ion deposition and strip at lithium metal anodes [Figure 2K]. During discharging process, metallic Li releases electrons and the produced Li⁺ ions are transported across the LAGP separator, resulting in the formation of HCOOH from CO₂ on cathodic Pd films. Nevertheless, side reactions, such as CO evolution reaction (COER) and hydrogen evolution reaction (HER), can decrease selectivity and induce catalyst degradation. In addition, the local pH alteration during electrocatalysis process resulted from the depletion of protons or the generation of OH^-, which could influence the HCOOH selectivity^[28].

Mechanisms of CO₂ER

Theoretically, CO₂ER during charge possesses can be regarded as the reverse reaction of CO₂RR, in which CO₂ at the cathode side and metallic Li at the anode side are respectively regenerated [Figure 3A]. As a typical example, Qiao et al. found that the discharging voltage plateau of Ru-based cathode [Figure 3B] presented an obvious decrease compared with Au-based cathodes^[21]. The reduction of both carbon species and Li₂CO₃ content was detected on the metal Ru substrate [Figure 3C], revealing that the dissolution of both discharge intermediates with the metal Ru catalyst occurred via a one-step reaction as follows:

Figure 3. (A) Decomposition mechanism of Li₂CO₃/C. (B) Galvanostatic charge/discharge profile of the Ru/Au-based cathode. Linear potential scan curves at both the Ru (C) and Au (D)cathodes. (E) Decomposition mechanism of Li₂CO₃. (F) Voltage profile of a Li-CO₂ battery. (G) The DEMS figures of discharging to point-B and (H) discharging to point-C. (Reproduced with permission^[21]. Copyright 2017, Elsevier). (I) Decomposition mechanism of Li₂C₂O₄. (J) Galvanostatic charge-discharge profiles of different electrodes. (K) Schematic diagram of reactions during charge and discharge of Mo₂C/CNT. (Reproduced with permission^[29]. Copyright 2017, Wiley-VCH).

In contrast, the characteristic peaks of carbon species remain unchanged in Au-based Li-CO₂ batteries through in-situ Raman, indicating that Au-based catalysts only allow the self-decomposition of Li₂CO₃ [Figure 3D]. Furthermore, they proposed the specific charging products of oxygen species at the corresponding discharge-charge stages [Figure 3E and F]. Differential electrochemical mass spectrometry (DEMS) measurements indicated that the solo decomposition of Li₂CO₃ at a low current density followed the reaction step [Figure 3G]:

Increasing the battery capacity limitation to the discharge plateau of Li₂O (1.0 mAh), the emergence of O₂ was detected [Figure 3H]. Moreover, according to the obtained charge-to-mass ratio of 4e^-/O₂ at the specific stage, the solo dissolution mechanism of the Li₂O (2Li₂O → O₂ + 4Li⁺ + 4e^-) could be further affirmed.

In addition to Li₂CO₃ and Li₂CO₃/C, Hou et al. proposed that charging products of Li₂C₂O₄ in Mo₂C-CNT-based Li-CO₂ batteries [Figure 3I]^[29]. In contrast to CNT-based cathodes, the amorphous Li₂C₂O₄-Mo₂C discharge product can be decomposed below 3.5 V on charge [Figure 3J]. The Mo₂C possesses stabilization function owing to the low valence of Mo in Mo₂C, which facilitates the transport of outer electrons to oxygen within Li₂C₂O₄ intermediate product and impedes its disproportionation to Li₂CO₃. Subsequently, only the amorphous Li₂C₂O₄-Mo₂C liberates CO₂ and Li⁺ below 3.5 V during charging process [Figure 3K]. Consequently, Mo₂C is expected to play a crucial role in stabilizing C₂O₄^2- species and prevent its subsequent disproportionation reaction.

In addition, it has been demonstrated that metal-based cathodes could stabilize the intermediate Li₂C₂O₄ using cobalt phthalocyanine and copper complex mediators^[18,22]. Moreover, researchers have not yet elucidated the dissolution mechanisms of other reaction intermediates, such as HCOOH. More investigations are required to explore the reversibility and rechargeability of Li-CO₂ batteries.

Shaped nanomaterials

Electrode materials can exhibit greatly enhanced electrochemical properties after reducing their sizes, since the nanoscale effect often renders these materials with higher electronic conductivity and robust structural stability^[11,23]. The nanoscale materials are anticipated to promote the round-trip conversion of CO₂RR and CO₂ER. Because of different synthetic methods, these nanosized materials usually display various shapes, such as spheres, polyhedrons, nanosheets, and nanorods. In this section, we discuss the rational design and potential applications of these shaped nanoparticles in Li-CO₂ batteries by utilizing typical instances of nanosheets and nanorods.

The 1D materials with interconnected fibrous networks are demonstrated to afford favoring pathways for rapid electron transfer and enhance interface interaction between fiber supports and electrolytes^[30,31]. Besides, many functional modifiers are added into these 1D fibrous substrates including nanotubes^[32], nanofibers^[33], and nanorods^[34] to improve their overall characteristics, such as reaction intermediates and electrocatalysts.

In terms of study of fibrous-based materials for Li-CO₂ batteries, a representative example is one in which Zhou et al. successfully synthesized unusual phase 4H/fcc Ir microstructures by the formation of Ir on the surface of 4H/fcc Au nanorods [Figure 4A and B]^[35]. High-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) technique revealed that 4H and fcc phases were clearly observed [marked regions of Figure 4C]. Furthermore, after the first discharge state, large-sized aggregates of discharge products were dispersedly distributed in the Au cathode; in contrast, the discharge products existed in the form of a continuous thin film covering electrocatalysts in the Au@4H/fcc-Ir0.14 cathode [Figure 4D]. As a result, 4H/fcc Ir showed superior electrochemical performance than fcc Ir in enhancing the cyclic redox kinetics of CO₂, realizing a low charging plateau below 3.61 V and excellent energy efficiency of 83.8%. The as-fabricated pouch cell can deliver practical cycling stability for a clock even under all kinds of bending angles [Figure 4E]. In-situ/ex-situ tests and theoretical calculations indicated that the improved reaction kinetics attributed to the highly reversible formation of low-crystalline discharge products on the surface of 4H/fcc Ir through the Ir-O coupling effect.

Figure 4. (A) SEM, (B) TEM and (C) HAADF-STEM images of the as-synthesized Au@4H/fcc-Ir0.14 nanostructures. (D) SEM images of discharge products generated from the Au@4H/fcc-Ir0.14. (E) Discharge-charge profiles of a Li-CO₂ pouch cell utilizing Au@4H/fcc-Ir0.14 (Reproduced with permission^[35]. Copyright 2022, National Academy of Sciences of the United States of America). (F) STEM-EELS elemental mapping of triangular RuRh nanosheet. (G) Schematic Illustration of the reaction mechanism on a single triangular RuRh alloy nanosheet. (Reproduced with permission^[44]. Copyright 2020, Cell Press).

In addition to 1D nanomaterials, 2D materials such as graphene, noble metal alloy, transition metal dichalcogenides, and MXene have displayed promising application prospects in Li-CO₂ batteries because of their unique structures and electronic properties^[9,36-43]. As a typical example, a kind of 2D triangular RuRh alloy was synthesized as a bifunctional electrocatalyst for tremendously accelerating the CO₂RR and CO₂ER and realizing excellent performances of Li-CO₂ batteries [Figure 4F]^[44]. The scanning electron microscopy (SEM) images exhibited that the Li₂CO₃ discharge products regularly grew on the RuRh/VC72 surface and reduced in size with improving current density. Nevertheless, Li₂CO₃ particles cannot adequately covered on the VC72 cathode surface at large current density, leading to obviously decreased capacity. Projected density of states (PDOS) calculation revealed that the 4d band exhibits more electron-rich at the Ru-db active sites with 2.2 eV lower than that of the Ru-sf active sites. The Ru-sf sites function as an ‘‘electron-depleting center’’ to facilitate the electron transport toward the adsorbing species. Experiments and calculations revealed that ultrathin RuRh alloy nanosheets are able to not only significantly promote Li₂CO₃ decomposition but also act as the bifunctional active sites for CO₂RR and Li₂CO₃ nucleation. As a result, the RuRh-based Li-CO₂ battery delivered the smallest potential gap (1.35 V) during the discharge-charge process and long cycle life (180 cycles) with a limited capacity of 1,000 mAh g^-1 at 1,000 mA g^-1[Figure 4G].

Porous framework

Organic crystal materials with elaborately designed porous structures and versatile sites, including hydrogen-bonded organic frameworks (HOFs), covalent-organic frameworks (COFs), and metal-organic frameworks (MOFs), offer opportunity for the development of high-performance Li-CO₂ batteries^[45,46]. Their functional porous architecture can generate strong binding with CO₂ and O₂. In addition, binding near the catalytic site can effectively enhance the electrocatalytic efficiency. The pore size of organic frameworks can not only satisfy the fast transport of CO₂, aprotic electrolyte and the reaction intermediate species, but also satisfy the requirement that the proper distance between adsorption and electrocatalytic sites, which should choose the various ligands to improve the pore size to enhance the catalytic efficiency. Therefore, the unique advantages of a porous framework are shorter diffusion pathways, easier near active sites for conversion reactions, and various functionalized inorganic components.

In 2018, Li et al. utilized MOFs as an electrocatalyst of the cathode for aprotic Li-CO₂ batteries^[47]. Various nonporous and porous MOFs were investigated in the Li-CO₂ batteries. Experiment results demonstrated that the MOFs with Mn sites can tremendously decrease the charging overpotential, and the great CO₂ absorption ability of MOFs tends to enhance the discharge ability [Figure 5A-C]. In-situ DEMS results demonstrated that only CO₂ was formed in a Mn₂(bodbc) cathode during the recharging process, which further indicated the reversible reactions of CO₂ conversion on the MOFs. Recently, the phthalocyanine-based MOF nanosheets (CoPc-Mn-O) have been prepared via the construction of Mn and Co-phthalocyanine, which can act as photocathode catalysts toward light-assisted Li-CO₂ batteries^[48]. Density functional theory (DFT) calculations and advanced characterizations confirmed that the existence of photosensitive CoPc-Mn-O with double metal active sites is conducive to both the CO₂ER and CO₂RR processes under light-assisted condition. The resulting battery delivered a large round-trip efficiency (98.5%) with a very low overpotential (0.05 V) and excellent cycling performance (81.3%) for 60 h at a current density of 0.02 mA cm^-2.

Figure 5. (A) Crystal structures (B) CO₂ adsorption isotherms at a temperature 298 K, and (C) discharge voltage curve at 50 mA g^-1 with a limited voltage of 2.0 V of different MOF samples (Reproduced with permission^[47]. Copyright 2018, The Royal Society of Chemistry). (D) Illustration of the TTCOF-2H catalysts in Li-CO₂ batteries. (E) Long-term cycling stability of different cathode catalysts at 300 mA g^-1. (F) CO₂ binding energy on TAPP-M (M = Cu, Ni, Co, Mn) molecules. (G)Free energy profiles (ΔG) for Li₂CO₃ nucleation and C_n generation with the different reaction pathways on the TAPP-Mn molecule at U = 0 V (Reproduced with permission^[49]. Copyright 2021, American Chemical Society). (H) Crystal texture of HOF-FJU-1. (I) Adsorption energies for Li⁺ and TFSI^- with HOF-FJU-1, and schematic illustration of CO₂ and Li⁺. (J) Ex-situ FTIR spectra of HOF-FJU-1 based cathode (Reproduced with permission^[53]. Copyright 2023, Wiley-VCH).

Recently, researchers found that the COFs composed of an electron donor and electron acceptor can improve the electron efficiency and promote the transport of the intermolecular charge-electron^[19]. As a typical example, a porphyrin-based COF (TTCOF-Mn) prepared through the covalent interaction between the electron-donating tetrathiafulvalene (TTF) and electron-accepting tetrakis (4-aminophenyl)-porphinato manganese(II) (TAPP-Mn) was fabricated as a high-efficiency cathode catalyst for Li-CO₂ batteries [Figure 5D]^[49]. The unique characteristics of TTCOF-Mn consisting of uniform single-Mn(II)-sites, rapid Li⁺ transfer pathways, and excellent electron transport efficiency are beneficial to excellent stability of 180 cycles at 300 mA g^-1 with a limited 1,000 mAh g^-1 capacity [Figure 5E]. The TAPP-Mn site showed a stronger binding capability toward CO₂ during the aprotic CO₂RR than other metal sites [Figure 5F]. DFT calculations indicated that Pathway 1 is the most thermodynamically favorable among the three possible reaction pathways [Figure 5G]. Huang et al. prepared the graphene@COF by uniformly coating graphene with imine COF, using it as a CO₂ catalyst for Li-CO₂ batteries^[50]. CO₂ can be confined in the micropores structures of the imine-COF which can optimize the diffusion pathway of CO₂ and improve the conductivity of electrons and Li⁺. Besides, the regular structure frameworks of COFs can avoid the accumulation and agglomeration of discharge products resulting from high CO₂ concentrations. Moreover, Li et al. synthesized a composite material via hybridizing hydrazine-linked COF and CNT covered with Ru nanoparticles (COF-Ru@CNT) as a high CO₂ efficiency catalyst^[51]. The COFs can significantly enhance the capacity of capturing CO₂ in discharge process and accelerate the decomposition of product during recharging. In addition, the stable 1D channels in the COF can be utilized as channels for CO₂^- and Li⁺ diffusion and facilitate the kinetics of electrochemical reactions, thereby improving the rate capability. Consequently, the Li-CO₂ batteries delivered an excellent rate capability and operated steadily for 200 cycles at 1,000 mA g^-1.

HOFs are generated through hydrogen bonding between organic molecules, which has the benefits of mild preparation processes, low cost and recyclability. Therefore, HOFs hold promising high-efficiency cathodes of rechargeable batteries^{[13,21,27,52]}. Very recently, the robust HOF (HOF-FJU-1) [Figure 5H] is generated via self-assembly of 3,3′,6,6′-tetracyano-9,9′-bicarbazole and serve as a CO₂ cathode via composing with Ru0 nanoparticle-modified CNT (Ru@CNT)^[53]. The adsorption energies of lithium ions and bis[(trifluoromethyl)sulfonyl]imide anions (TFSI^-) on the surface of HOF-FJU-1 were -1.84 and 2.12 eV, revealing that HOF-FJU-1 has a stronger attraction to lithium ions and repels TFSI^- [Figure 5I]. In addition, the production and dissociation of Li₂CO₃ on the surface of HOF-FJU-1-Ru@CNT catalyst during the charge/discharge were detected by the ex-situ FTIR spectrum [Figure 5J], revealing the reversible behavior and dissociation of Li₂CO₃ during charge/discharge. The HOF-FJU-1 with regularly arranged open channels is helpful for CO₂ and Li⁺ transmission, realizing fast redox kinetic conversion of CO₂. Thus, the Li-CO₂ batteries with HOFs can operate stably at 400 mA g ^-1 for 1,800 h and remain a small overpotential (1.96 V) at a high current density of 5 A g^-1. Despite these promising benefits, the development of HOFs for high-efficiency cathodes in advanced Li-CO₂ batteries has rarely been reported.

0D-1D hybridization

Generally, 0D components applied in CO₂ cathodes refer to ultrafine adsorptive/electrocatalytic nanoparticles and even much smaller clusters/single atoms in support materials^[54,55]. Thus, a typical hybridization form of 0D-1D is loading of nanoscale catalysts onto 1D fibrous networks, including nanotubes^[56,57], nanofibers^[58,59] and nanowires^[60]. These nanosized catalysts can expose ample active sites to facilitate the transformation of intermediates; meanwhile, those carbon fibers can offer rapid electron transport pathways and substantial accommodation space for CO₂ species. In this regard, W₂C nanoparticles supported by CNTs (W₂C-CNTs) are successfully synthesized using a two-step carburization approach [Figure 6A and B]^[61]. In comparison with the pure CNTs, the Li-CO₂ battery catalyzed by W₂C-CNTs displayed a large discharge capacity (10,632 mAh g^-1) and low polarization voltage [Figure 6C]. The small polarization results from electron-rich W atoms, which break the stable triangle of CO₃^2- through W-O bonds [Figure 6D].

Figure 6. (A) Schematic illustration of synthesis process of the W₂C-CNTs. (B) SEM image of W₂C-CNTs. (C) Deep discharge-charge curves of CNTs and W₂C-CNTs. (D) WL₃-edge EXAFS spectra for W₂C-CNTs. (Reproduced with permission^[61]. Copyright 2021, American Chemical Society). TEM images of NCNS (E) and TeAC@NCNS (F) cathodes at different charge and discharge states. (G) Typical first charge-discharge curves of Li-CO₂ batteries. (H) Corresponding ex-situ XRD patterns at distinct discharge and charge states. (I) Schematic Illustration of reaction mechanism on multiple catalysts. (Reproduced with permission^[68]. Copyright 2023, Wiley-VCH).

Apart from CNTs, the nanofiber is also a promising alternative to 1D skeletal support for embedding of 0D units, which often serves as a substrate to construct flexible freestanding electrodes^[59-62]. For example, Cheng et al. reported a bifunctional Mo₂N-ZrO₂ heterostructure in conductive self-standing carbon nanofibers (Mo₂N-ZrO₂@NCNF)^[58]. The Mo₂N-ZrO₂ nanoparticles in conductive carbon nanofibers possess efficient catalytic transformation ability for CO₂ (ZrO₂) and stabilizing performance toward Li₂C₂O₄ (Mo₂N), which simultaneously facilitate CO₂RR and CO₂ER. Therefore, the as-synthesized Li-CO₂ batteries could realize a small overpotential (0.32 V), an ultrahigh energy efficiency (89.8%), and excellent cyclic performance over 400 cycles even at a large current density of 50 µA cm^-2. With synergistic effect by such a 0D-1D hybridization strategy, an optimal 1D construction with interlinked architectures would be constructed, which will undoubtedly enhance the Li-CO₂ battery performance. In the future research, many other 0D-1D interlinked architectures are expected to be studied as CO₂ cathode materials for Li-CO₂ batteries.

0D-2D hybridization

The aforementioned 0D nanomaterials can also interact with 2D support materials, such as MXenes^[63], carbon nanosheets^[64,65], and graphene^[66,67]. As a superior 2D material, graphene is often deployed as a desirable 2D template or skeleton to construct the 0D-2D hybridization materials. For instance, tiny-sized ZnS quantum dots (≈2.5 nm) were synthesized successfully and anchored on the surface of N-doped reduced graphene oxide (ZnS QDs/N-r GO) as a CO₂ host for Li-CO₂ batteries^[67]. It should be mentioned that the electronegative ZnS QDs can offer a large amount of growth sites for discharge products owing to their high CO₂ binding energy and rapid electron transfer, leading to the generation of a thin Li₂CO₃/C layer on the catalyst surface. The average charge potential of Li-CO₂ batteries with ZnS-based catalyst increases to 4.40 V rapidly, while the one with ZnS QDs/N-r GO merely rises from 4.13 to 4.26 V after 190 cycles, indicating excellent catalytic activities and stability for ZnS QDs/N-r GO.

Except for natural 2D graphene, carbon nanosheets can also be easily prepared by rational chemical fabrication methods to serve as a substrate of 0D materials. Recently, Wang et al. designed a heterostructure with Te atomic clusters embedded with N-doped carbon nanosheets (TeAC@NCNS)^[68]. Distinct from N-doped carbon nanosheets (NCNS) cathodes, high-area and amorphous discharge products are produced on the TeAC@NCNS cathode surface during the discharge process [Figure 6E and F]. Specifically, on the NCNS cathode, some bulk-like discharge product is discretely distributed on the surface of the electrode after discharging the battery to 2 mAh (state 1). Meanwhile, the NCNS nanosheet architecture can still be observed. It was further discharged to 4 mAh (state 2); the densely stacked discharge product fully covered the electrocatalyst surface. A minor number of discharge products remain on the nanosheet surface after recharging (state 3). On the contrary, for the TeAC@NCNS cathode, the film-like discharge products were homogeneously covered on the cathode surface (state 1′). On subsequent discharge to 4 mAh (state 2′), it is easy to observe the larger-area film-like morphology along the nanosheets. The adequate contact interface between catalysts and discharge products can fully utilize the active sites of TeAC@NCNS, ensuring that the charging process was energetically more favorable. Furthermore, distinct diffraction peaks of Li₂CO₃ species were observed on the NCNS cathode discharged to 2.0 mAh [Figure 6G and H]. Meanwhile, for the TeAC@NCNS cathode, no peaks related to Li₂CO₃ could be detected under the same measurement conditions, indicating the amorphous characteristic of the final discharge product. The TeAC@NCNS cathode with Te atomic clusters provided more CO₂ catalytic sites and enhanced CO₂RR kinetics. Such a uniformly distributed Li₂CO₃ film can be easily decomposed by a Te atomic cluster catalyst, significantly facilitating Li₂CO₃ decomposition kinetics [Figure 6I]. The as-fabricated battery exhibited an ultrahigh discharge capacity (28.35 mAh cm^-2 at 0.05 mA cm^-2) and excellent cycling performance (1,000 h at an ultrahigh cut-off capacity of 1 mAh cm^-2).

0D-3D hybridization

In comparison with 0D-1D and 0D-2D hybridization, more 3D nanomaterials can be chosen as supports of 0D materials for the 0D-3D hybridization model because of their unique 3D nanostructured frameworks^[69]. For example, Hu et al. successfully anchored single Fe atoms into a 3D N, S-co-doped holey graphene architecture (Fe-ISA/N, S-HG) by a balanced force between the π-π stacking of oxidized holey graphene (HGO) nanosheets and repulsion interaction of their negative charge carboxylate group [Figure 7A]^[70]. The synthesized Fe-ISA/N, S-HG catalyst, with its ample surface area and sufficient space within the crisscrossing holey graphene (HG) network, can not only promote electron transfer and CO₂/lithium-ion transport, but also realize high uptake of Li₂CO₃ to deliver a large capacity. In addition, its excellent catalytic activity and the absence of Fe single-atom aggregation after cycling result in the generation of small dense Li₂CO₃ nanoparticles loading on the Fe-ISA/N, S-HG electrocatalyst, which can be rapidly dissolved during the charge process [Figure 7B and C]. Consequently, Li-CO₂ batteries with Fe-ISA/N, S-HG cathode showed a high discharge capacity (23,174 mAh g^-1_{Fe-ISA/N, S-HG}), low voltage polarization (1.17 V at 100 mA g^-1), superior rate capability and cycling performance even at a high current density of 1.0 A g^-1 [Figure 7D].

Figure 7. (A) Schematic illustration of the synthesis process of Fe-ISA/N, S-HG. (B) SEM and (C) Atomic-resolution STEM images of Fe-ISA/N, S-HG. (D) Full charge/discharge profiles. (Reproduced with permission^[70]. Copyright 2020, Wiley-VCH). (E) TEM and HRTEM images of ICS. (F) Band diagram of the ICS. (G) Photocurrent response to UV light. (H) Illustrations of the working principle for the light-induced discharging process. (Reproduced with permission^[82]. Copyright 2020, Wiley-VCH).

In addition to HG, the cellulose carbon aerogel (CCA), graphene aerogel^[71], and porous carbon foam^[72] are also promising alternatives to 3D skeletons for embedding of 0D units^[73]. A typical case study by Liu et al. successfully synthesized a 3D free-standing CCA with elaborately designed Ru/C heterointerfaces (Ru@CCA) as a promising CO₂ cathode^[74]. The CCA support can offer a rapid mass (Li⁺, CO₂, etc.) and electron transport path and, meanwhile, avoid the loss of active sites induced by stacking of carbon-based substrates. Furthermore, due to the reduced reaction barrier of reaction intermediates and the excellent electronic configuration, the interface of Ru/C exhibited the optimal electrocatalytic activity for CO₂ER and CO₂RR during charge-discharge processes. Consequently, the as-obtained Li-CO₂ batteries significantly enhance energy efficiency to 80% with an ultrahigh discharge capacity (10.71 mAh cm^-2 at 20 µA cm^-2) and long cyclic performance (421 cycles at 100 µA cm^-2). These works offer guidelines for the rational design of electrocatalysts with excellent catalytic activity and stability for advanced Li-CO₂ batteries.

1D-2D hybridization

Compared with 0D-3D nanomaterials, the specific surface area of 1D-2D nanomaterials is larger, leading to an ultrahigh density of active sites. These advantages ensure the 1D-2D structure to possess a better synergistic effect^[75]. For example, a novel composite of 2D nanosheet NiO distributed on CNTs was synthesized by a solvothermal approach^[76]. The composite served as a high-efficiency CO₂ cathode electrocatalyst with ultrahigh coulombic efficiency (97.8%) and excellent cycling performance (40 cycles). The NiO-CNT with highly covered NiO nanosheets on CNTs can offer superior electrocatalytic activity sites for the dissolution of product Li₂CO₃ during charging process.

In addition, the 2D metal sulfides can also form composites with 1D nanomaterials as promising cathode hosts for Li-CO₂ batteries. For example, Chen et al. successfully combined MoS₂/CNT with different ratios serving as a CO₂-cathode electrocatalyst^[77]. Consequently, the ideal MoS₂/CNT realized a maximum discharge capacity (8,551 mAh g^-1) with a high coulombic efficiency (96.7%), a charge plateau (3.87 V) at 100 mA g^-1 with a limited capacity (500 mAh g^-1) and superior electrochemical stability.

2D-3D hybridization

The 2D materials primarily adopt a hierarchical architecture to grow vertically on the 3D substrate surface, promoting electrons to be transported rapidly in the 2D plane^[78-81]. For instance, MnOOH arrays in-situ grown on the stainless-steel (SS) network have been reported and employed as a cathode in Li-CO₂ batteries^[20]. Experiments and calculations confirmed that the generation of reaction intermediates (LiCO₂, LiCO and Li₂CO₃) interacting with MnOOH by Li⁺ modulated the formation of Li₂CO₃ and C, promoting their growth on MnOOH uniformly. The ultrafine Li₂CO₃ particles embedded into the carbon substrate significantly enlarge the contact interface, thus promoting the transport of electrons via the discharge products and improving CO₂ evolution activity.

Recently, Guan et al. reported a photo-assisted Li-CO₂ battery with a unique characteristic, hierarchical porous and free-supporting In₂S₃@CNT/SS (ICS) as a dual-functional electrode for both CO₂ER and CO₂RR [Figure 7E]^[82]. The 2D In₂S₃ is homogeneously covered on the CNT forest surface, which is helpful to expose abundant active sites and shorten the transport length of photo-holes and photoelectrons [Figure 7F]. Besides, the ICS 2D-3D nanostructure can also promote the rapid transfer of the electron, CO₂, and Li⁺, and expand the electrolyte-electrode contact area. Therefore, the battery showed a high discharge voltage (3.14 V) and ultralow charge voltage (3.20 V), an excellent rate performance and cycle stability, and especially a record superior round trip efficiency (98.1%) [Figure 7G]. These excellent performances can be attributed to the hierarchical porous and self-standing architectures of ICS, and the significant role of photo-assisted electrons and photo-holes during charging-discharging processes [Figure 7H].

Defect construction

The utilization of defect engineering has significantly improved the electrochemical performance of various electrochemical energy storage devices^[83]. Among these defect strategies, vacancy is considered a fundamental defect engineering approach for the electronic and geometrical configuration of compounds. For instance, oxygen vacancies (Vo) were introduced into the 2D NiO arrays on the carbon cloth surface (NiO-Vo NAs/CC) using an argon plasma engraving approach^[84]. DFT calculations demonstrated that the Vo causes the appearance of new electronic states near the Fermi level (E_f), resulting in the up-shift of the d-band center. As a result, the NiO-Vo NAs/CC cathode-based Li-CO₂ battery showed higher capacity, more excellent round-trip efficiency, and ultra-long cycle life compared with its counterpart without Vo.

Heteroatom doping of target materials represents another crucial strategy of defect engineering. As an example, Li et al. reported vertically aligned N-doped CNT (VA-NCNT) arrays grown on Ti wires free-supporting, binder-free for quasi-solid-state flexible Li-CO₂ batteries^[85]. The VA-NCNT arrays possess a large amount of vertically aligned CNT with high N content, void space and abundant defects, which promote the diffusion of CO₂ and permeation of electrolyte, provide extra space and exposed active sites for the accommodation and generation of Li₂CO₃, and facilitate the dissolution of Li₂CO₃ and amorphous carbon during the charge process.

Recently, Chen et al. prepared modified ReS₂ material with a large amount of nucleophilic N dopants and electrophilic S vacancies within its basal plane [Figure 8A]^[86]. The electrophilic and nucleophilic dual centers exhibited suitable adsorption for all intermediates during charge-discharge processes, leading to a low energy barrier in the rate-limiting step. In particular, DEMS measurements confirmed that the mass-to-charge ratios of CO₂ for ReS₂/CP, NS_V-ReS₂(8/7)/CP, and NS_V-ReS₂(5)/CP-based cathodes respectively are 0.63, 0.72, and 0.75 [Figure 8B-D]. Thus, NS_V-ReS₂(5)/CP exhibited a 100% faradic efficiency of 3CO₂/4e^- process, having higher CO₂RR activity, while ReS₂/CP cathodes possessed the lowest CO₂RR activity. Consequently, the optimum catalyst delivered an ultrasmall voltage gap (0.66 V) and ultrahigh energy efficiency (81.1%) at 20 μA cm^-2. The introduction of dual defect centers offers novel opportunities to design excellent bifunctional catalysts of Li-CO₂ batteries.

Figure 8. (A) Diagram of the CO₂ER and CO₂RR on the different substrates. DEMS curves with discharge profiles for (B) ReS₂/CP, (C) NSV-ReS₂(8/7)/CP, and (D) NSVReS₂(5)/CP cathodes. (Reproduced with permission^[86]. Copyright 2021, American Chemical Society). (E) The HAADF-STEM image and (F) the fitting EXAFS profiles of MCs- (N, O). (G) Gibbs free energy profiles for the conversion of CO₂ into CO on the different substrates. (H) The adsorption configurations for Li₂C₂O₄ on the various catalysts. (I) Time-voltage curves at 0.02 mA cm^-2. (Reproduced with permission^[95]. Copyright 2023, Wiley-VCH).

Single-atom engineering

Among various catalysts, single-atom catalysts (SACs) with quantum size effect, unique coordination structure, and metal-carrier interactions show the advantages of maximized metal utilization, excellent tunability of electrocatalytic sites, and cost-effectiveness. They have been commonly used in many fields such as electrocatalysis, photocatalysis and energy storage devices^[87-90]. Recently, researchers have extended their application to aprotic Li-CO₂ batteries to study their catalytic performance in this emerging field^[91-94].

The well-defined and uniformly dispersed active sites in SACs provide excellent selectivity and activity for electrochemical CO₂ER and CO₂RR in Li-CO₂ batteries^[68]. For example, Mn-N₄ atomic sites [Figure 8E] are encapsulated into a mesoporous carbon microsphere with the epoxy functional groups by Wang et al.^[95]. As a CO₂ electrocatalyst of aprotic Li-CO₂ batteries, the Mn-SACs with N and O-doped electrocatalyst [MCs- (N, O)] with special bowl-like mesoporous morphology and well-regulated active sites optimize the nucleation growth process of discharge products. The ex-situ X-ray absorption fine-structure (EXAFS) fitting results indicated that Mn atoms were coordinated with four N atoms [Figure 8F]. DFT calculations demonstrated that an energy barrier of 0.30 eV for Mn-N₄(2O) is significantly lower than other theoretical models of the CO desorption, indicating the better energetics for CO₂ reduction [Figure 8G]. In addition, the optimized model of Mn-N₄(2O) possesses a higher adsorption energy (-1.97 eV) for Li₂C₂O₄ compared with N, O-doped graphene (-1.46 eV) [Figure 8H]. The "surface-adsorption growth pathway" is more favored when the binding energy is higher, leading to the direct formation of a thin layer of Li₂CO₃ products on the CO₂ cathode surface. The results indicated that the Li-CO₂ cell using the MCs-(N, O) cathode can achieve a low voltage gap of about 1.3 V while working stably for 1,000 h [Figure 8I]. In addition to the Mn single atoms mentioned above, transition metal SACs, including Fe, Co, Ni, Cu, Zn, and Cd, have also been reported as high-efficiency electrocatalysts in Li-CO₂ batteries^[94,96-99].

According to a past report, the defective areas on the graphene substrate with a smaller work function and a superior reducing ability could form atomic metal clusters resulting from the electroless deposition of metal ions. In this regard, Xu et al. synthesized copper polyphthalocyanine-CNTs composite materials (CuPPc-CNTs) via in-situ polymerization of CuPPc on the surface of CNTs as the CO₂ host for reversible Li-CO₂ batteries^[100]. CuPPc possesses very efficient Cu SA electrocatalytic sites with good CO₂ activation and adsorption, while CNTs offer a conductive network. Benefiting from the cooperative effect of the two compounds facilitates Li-CO₂ batteries to achieve excellent discharge capacity (18,652.7 mAh g^-1) at 100 mA·g^-1 and 1.64 V polarization at a current density of 1,000 mA g^-1. Similarly, Zheng proposed molecularly dispersed catalysts [molecularly dispersed electrocatalysts (MDEs)] of nickel phthalocyanine (NiPc) confined on CNTs as the CO₂ electrocatalyst for Li-CO₂ batteries^[93]. The dispersed NiPc efficiently facilitates CO₂ reduction, while the porous and conductive CNT networks promote CO₂ER, leading to improved charging and discharging performance compared with the CNTs and NiPc mixture.

Heterostructure

In order to achieve material multifunctionalities, a kind of common strategy is to prepare chemical composite materials by combining various material components so as to obtain an integration of multifunctional nature. This material design tactic has been widely applied into electrode materials for Li-CO₂ batteries^{[67,71,101-103]}. For example, a RuAl/Ru heterostructure (RuAl/Ru) material was synthesized by the microstructure and phase reconstruction of a RuAl alloy^[104]. Attributed to multiple benefits of the optimized electronic structure, the multilevel porous structure, and the rich RuAl/Ru interface, RuAl/Ru exhibited much superior stability and electrocatalytic activity than its counterpart. DFT calculations demonstrated the important role of the intrinsically superior activity of RuAl heterointerface to adjust the adsorption of intermediates, thereby reducing the decomposition barrier of the Li-decomposition steps during the CO₂ER.

It is worth noting that the in-built electric field (BEF) based on heterostructure with different Fermi levels has been a highly effective tactic for optimizing interface charge transfer and distribution, thereby adjusting the micro-environment of catalytic sites of catalyst surface and lowering the free energy barriers for the generation of reaction intermediates^[27,105,106]. Lu et al. proposed a BEF-driven oriented electron conduction tactic for electrocatalytic redox of CO₂ based on a tandem electrocatalyst, Fe₂O₃@CoS, which is derived from spent lithium-ion batteries [Figure 9A]^[107]. The tandem electrocatalyst exhibited excellent catalytic activities for CO₂ER and CO₂RR in the Li-CO₂ batteries with an extremely high energy efficiency (92.4%), an extremely small voltage gap (0.26 V), and a long cycle life of over 550 h at 20 μA cm^-2. The differential charge density analysis suggested that Li₂CO₃ adsorbed on the surface of Fe₂O₃@CoS showed more interface electron transfer, inducing more active interactions compared with CoS and Fe₂O₃ [Figure 9B]. The well-defined heterointerface-induced BEF adjusted the adsorption and dissolution of the species during CO₂ER and CO₂RR, empowering the recycled tandem electrocatalyst with remarkable bidirectional catalytic activities.

Figure 9. (A) Schematic illustration of the BEF generated on the Fe₂O₃@CoS. (B) The charge density difference of different systems. (Reproduced with permission^[107]. Copyright 2023, Wiley-VCH). (C) The double- spherical-aberration-corrected TEM image of Vs-Co₂CuS₄. (D) Cycling performance of different cathode materials. (E) Adsorption models and Li-O bond length of Li₂CO₃. (Reproduced with permission^[110]. Copyright 2023, Wiley-VCH).

Synergy engineering

The creation of interacting sites with synergy between multiple active sites is superior to the one with only solo active sites in enhancing Li-CO₂ batteries^[108]. In this regard, incorporating the single atoms into the metal clusters or metal compounds can synergistically enhance the electrocatalytic activity toward CO₂ redox reactions in the Li-CO₂ battery. Lian et al. introduced Ru atoms into the Co₃O₄ nanosheet arrays grown on the surface of carbon cloth (SA Ru-Co₃O₄/CC), and applied successfully in Li-CO₂ batteries^[65]. The Co₃O₄ nanosheet arrays with 3D structure have a high specific surface area and suitable pore volume/size to promote the mass transfer of reactants and electrolytes, and provide sufficient space to accommodate the discharge products of C and Li₂CO₃. In addition, uniformly dispersed Ru atoms as active sites regulated the nucleation process of discharge products, therefore optimizing their ultimate distribution, which is beneficial for redecomposition behavior during the charging process. Lately, Lin et al. fabricated a new electrocatalyst comprising well-defined single-atom Ru-N₄ (Ru_SA) and atomic clusters (Ru_AC) active sites anchored on carbon nanobox substrates (Ru_SA+AC@NCB) as electrocatalytic cathodes for Li-CO₂ batteries^[109]. The electronic synergistic effect between Ru_SA and Ru_AC empowers the Ru_SA+AC composite site with enhanced electrocatalytic activity for CO₂RR and CO₂ER in the Li-CO₂ cell, as proved by the much smaller charge/discharge overpotential of the Ru_SA+AC@NCB-based cathode than those of the Ru_AC@NCB, Ru_SA@NCB and NCB-based cathodes.

Very recently, Lu et al. found that the tuned p-band and d-band centers of Co₃S₄ by virtue of Cu and S vacancies were hybridized between Li species and CO₂, separately, which is beneficial for the dissolution of Li₂CO₃ and the adsorption of intermediates^[110]. Double-spherical-aberration-corrected transmission electron microscopy (TEM) demonstrated that Cu, S and Co atomic columns are observed clearly [Figure 9C]. The intensity of S active sites in the marked area is obviously lower than that of other active sites, indicating the presence of S vacancies. The Vs-Co₂CuS₄ cathode exhibited outstanding electrochemical performance with an extremely low voltage gap (0.73 V), as well as a lower charge voltage (3.88 V) and a higher discharge voltage (2.73 V) after 600 h of cycling at 20 μA cm^-2 [Figure 9D], which is obviously better than those of CuS, Co₃S₄, Co₂CuS₄ cathodes. The density of states calculation results indicated that compared with Co₂CuS₄ and Co₃S₄, d-band and p-band centers of Vs-Co₂CuS₄ obviously up-shift after the introduction of sulfur vacancies and Cu in Co₃S₄. In addition, the Li-O bond length of Li₂CO₃ adsorbed on the catalyst surface of Vs-Co₂CuS₄ became longer than those of Co₂CuS₄ and Co₃S₄ [Figure 9E]. Thus, when the adsorbed substrate was converted from Co₃S₄ to Vs-Co₂CuS₄ and Co₂CuS₄, the decomposition behavior of Li₂CO₃ became easier. To date, despite the inspiring progress, more efforts still need to be made to explore a better synergistic effect between the electrocatalyst activity and catalytic cathode structure being devoted to achieving excellent battery performance.

Identification and reconstruction of active sites

The identification and reconstruction of the active sites in a working electrocatalyst play a vital role in regulating the catalytic activity and battery reaction kinetics. Considering catalysts with various oxidation states and coordinated configurations, identification of the active reaction sites and their interaction mechanism with the battery reaction is highly required. For example, Liu et al. found that Co-O₆ octahedra are the primary active motifs in Co₃O₄^[111]. They utilized an ion-substitution method involving catalytically inert Al³⁺ and Zn²⁺ with d⁰ and d¹⁰ configurations substituting Co ions in octahedral and tetrahedral sites, which Co²⁺_Td and Co³⁺_Oh retain in synthesized CoAl₂O₄ and ZnCo₂O₄ [Figure 10A]. As a result, the Co₃O₄ showed the highest capacity (2,407 μAh cm^-2 with a high -coulombic efficiency (86.3%). The ZnCo₂O₄ exhibited a capacity (1,995 μAh cm^-2) with Coulombic efficiencies of 85.6% [Figure 10B]. In contrast, CoAl₂O₄ had nearly no capacity, indicating its inertness in Li-CO₂ batteries. The finite element method (FEM) simulation demonstrated that the majority of the surface of CoAl₂O₄ exhibited sluggish CO₂ transfer ability owing to the obstruction by the large area of Li₂CO₃ product, while the transport ability of Co₃O₄ and ZnCo₂O₄ exhibited faster after discharge [Figure 10C]. Consequently, the high catalytic activity of octahedral Co facilitates uniform and vertical Li₂CO₃ growth during discharge, promoting fast mass transport and charge transfer during battery operation. In contrast, the large Li₂CO₃ particles on the surface of the catalyst, which contains only tetrahedral Co, obstruct the CO₂ diffusion, and therefore, CO₂RR is gradually substituted by Li-ion intercalation reaction during discharge, resulting in a large overpotential [Figure 10D]. Subsequently, DFT calculations indicated that the e_g orbitals in an octahedral structure play a crucial role in CO₂RR activity, in which lower e_g filling and O charge promotes the CO₂ and Li adsorption [Figure 10E].

Figure 10. (A) Structure illustration of Co₃O₄. (B) Galvanostatic charge-discharge profiles of different electrodes. (C) Schematics diagram of the discharge product (Li₂CO₃) generation morphology as well as its impact on CO₂ transport on the different cathode catalysts in the FEM simulation. (D) Schematic illustration of discharge reaction on tetrahedral and octahedral Co. (E) Schematic of the correlation between M-O states and CO₂RR activity. (Reproduced with permission^[111]. Copyright 2024, American Chemical Society). (F) Schematic illustration of structural evolution of CoS₂. (G) Co K-edge FT-EXAFS of CoS₂ at different stages. (H) The projected density of states (PDOS) of CoS₂ and O-CoS₂. (Reproduced with permission^[112]. Copyright 2024, Springer Nature). (I) The EXAFS spectra for Mn K-edge. (J) The optimal routes for Mn(II) and Mn(III) sites during discharge and charge processes. (Reproduced with permission^[113]. Copyright 2024, Wiley-VCH).

Compared with transition metal oxide catalysts, metal sulfides exhibit superior electrocatalytic abilities in Li-CO₂ batteries. However, the thermodynamically unstable sulfides are prone to irreversible reconstruction, especially oxidation, due to the aggressive oxygen species resulting from Li₂CO₃ decomposition. Their reconstructed structures and different oxidized states can regulate the activity of sulfides. For instance, Liu et al. explored the structural evolution and electrochemical performance of three varieties of cobalt (II) sulfide (CoS_x, x = 8/9, 1.097, and 2) pre-electrocatalysts in Li-CO₂ batteries^[112]. X-ray absorption spectroscopy analysis suggested the oxidation of CoS₂ with Co-S₄-O₂ motifs [Figure 10F and G], while Co₉S₈ and CoS_1.097 are completely oxidized to CoO structure in Li-CO₂ batteries. DFT calculations indicated that partial oxygen substitution regulated the electronic structure and moved the d-band center of Co to a higher energy level [Figure 10H], thereby enhancing the catalytic performance of CoS₂. As a result, reconstructed CoS₂ with Co-S₄-O₂ motifs exhibited high energy conversion efficiency (> 80%) and excellent cycling stability with a low overpotential of 0.43 V after 400 h. Recently, Liu et al. reported novel Mn dual-active sites (MOC) anchored on N-doped carbon nanofibers^[113]. They observed that MOC underwent significant electrochemical reconstruction during battery cycling due to the in-situ formation and stable presence of Mn(III). Mn(III) is reduced to Mn(II) during the discharge process, while Mn(II) was electrochemically oxidized to Mn(III) during subsequent charging process. EXAFS analysis exhibited the intensity and coordination number of Mn-O and Mn-Mn peaks significantly changes during the discharge and charge proceeds, demonstrating the electrochemical reconstruction appearing during CO₂RR and CO₂ER possesses [Figure 10I]. In the charged state, in particular, the peak intensity of the Mn-Mn and MnC/ON enhanced, further revealing the occurrence of Mn(III). DFT calculations exhibited the Gibbs free energy for the charge/discharge processes, indicating that Mn(II) is more advantageous for CO₂RR, while Mn(III) is more conducive for CO₂ER. The crystal occupation Hamiltonian population (COHP) results indicated that the Mn(III) sites possess a more conducive electronic structure for breaking the Li-O bond of the intermediate (*Li₂C₂O₄) during the charging process, thereby facilitating the dissociation of Li₂C₂O₄ and promoting the kinetics of the CO₂ER process [Figure 10J].