Thermoplastic Polyurethane Derived from CO2 for the Cathode Binder in Li-CO2 Battery

High-energy-density Li-CO2 batteries are promising candidates for large-capacity energy storage systems. However, the development of Li-CO2 batteries has been hindered by low cycle life and high overpotential. In this study, we propose a CO2-based thermoplastic polyurethane (CO2-based TPU) with CO2 adsorption properties and excellent self-healing performance to replace traditional polyvinylidene fluoride (PVDF) as the cathode binder. The CO2-based TPU enhances the interfacial concentration of CO2 at the cathode/electrolyte interfaces, effectively increasing the discharge voltage and lowering the charge voltage of Li-CO2 batteries. Moreover, the CO2 fixed by urethane groups (-NH-COO-) in the CO2-based TPU are difficult to shuttle to and corrode the Li anode, minimizing CO2 side reactions with lithium metal and improving the cycling performance of Li-CO2 batteries. In this work, Li-CO2 batteries with CO2-based TPU as the multifunctional binders exhibit stable cycling performance for 52 cycles at a current density of 0.2 A g−1, with a distinctly lower polarization voltage than PVDF bound Li-CO2 batteries.


Introduction
The escalating emissions of CO 2 due to the excessive utilization of fossil fuels underscore the pressing need for the capture and utilization of CO 2 as a crucial aspect of sustainable development [1,2].Li-CO 2 batteries, an electrochemical device that converts the chemical energy from the reaction between lithium metal and CO 2 into electrical energy, characterized by notable theoretical specific capacity (1876 Wh kg −1 ) [3] and elevated discharge voltage (2.8 V) [4], emerge as promising candidates for high-capacity battery applications.The operational principle of Li-CO 2 batteries is as follows [5,6]: (1) where ϕ + represents the electric potential of the battery, T signifies temperature, R denotes the gas constant, F represents the Faraday constant, [CO 2 ] indicates the concentration of dissolved carbon dioxide, and [Li + ] represents the concentration of lithium ions.Specifically, the concentration should be the interfacial concentration at cathode/electrolyte interfaces.
The discharge product of Li-CO 2 battery cathode is thermodynamically stable and poorly conductive Li 2 CO 3 [7][8][9][10], leading to the challenges of the high charging voltage, low cycling life, and poor rate performance of Li-CO 2 battery [11][12][13].Consequently, the cathode's performance assumes paramount importance in determining the overall efficacy of Li-CO 2 batteries [14,15].The polymer binder of the cathode plays a pivotal role in mitigating volume fluctuations of the cathode material during charge and discharge cycles, preventing material detachment and reducing internal resistance [16][17][18].Conventional PVDF binder in batteries suffers from poor electrical conductivity and ionic conductivity [19][20][21], resulting in significant overpotentials in Li-CO 2 batteries.Additionally, PVDF lacks catalytic activity for cathodic reactions, hindering the decomposition of Li 2 CO 3 discharge products and compromising cycling reversibility [22]; moreover, PVDF lacks self-healing properties, and during the cycling process of Li-CO 2 , the cathode is prone to failure and powder detachment due to the volume expansion caused by the formation and decomposition of Li 2 CO 3 , hindering the improvement of Li-CO 2 battery cycling performance [23,24].Therefore, the development of binders with high conductivity conducive to enhanced cycling performance of Li-CO 2 batteries, and possessing self-healing properties, assumes paramount importance for their advancement.
In this study, CO 2 -based thermoplastic polyurethane (TPU), synthesized from CO 2 , propylene oxide and isocyanate, etc. [25][26][27], was employed to replace conventional polyvinylidene fluoride (PVDF) as the binder of CO 2 cathode.CO 2 -based TPU possesses the following advantages: (1) CO 2 -based TPU exhibits excellent CO 2 adsorption characteristics, effectively enhancing the CO 2 adsorption capacity of the cathode in Li-CO 2 batteries and increasing the discharge voltage.(2) CO 2 -based TPU can increase the interfacial concentration of CO 2 at cathode/electrolyte interfaces, ensuring uniform formation of Li 2 CO 3 as a discharge product on the cathode surface during discharge; as a result, the particle sizes of Li 2 CO 3 are much smaller, lowering the energy barrier of decomposition and reducing the charging voltage of Li-CO 2 batteries.(3) Due to the excellent CO 2 adsorption characteristics of CO 2 -based TPU with urethane groups (-NH-COO-), the dissolution of CO 2 into the electrolyte and its shuttle to the anode during cycling are effectively reduced, protecting the lithium metal of anode and improving the cycling performance of Li-CO 2 batteries.(4) CO 2 based TPU exhibits excellent self-healing properties.During the cycling process of Li-CO 2 batteries, the volume expansion caused by the formation and decomposition of Li 2 CO 3 easily leads to cathode failure and powder detachment.CO 2 -based TPU binder possesses self-healing properties, effectively resisting binder damage and failure caused by volume expansion, thereby improving the cycling performance of Li-CO 2 batteries.As a result, Li-CO 2 batteries based on CO 2 -based TPU binder achieved a discharge voltage of 2.0 V at a current density of 200 mA g −1 , a charging voltage of 3.5 V in the first cycle, and a cycling life of 52 cycles.

Synthesis of CO 2 Based TPU
CO 2 -based TPU was synthesized by a one-pot two-step method [28][29][30] (Figure 1).The oligocarbonate diols (PPCDLs) were synthesized by the direct copolymerization of propylene oxide (PO) and carbon dioxide (CO 2 ) with 1,4-butanediol (BDO) as the proton exchange agent.The pre-polymer of PU with terminal isocyanate groups was prepared by reacting the prepared PPCDLs with excess hexamethylene diisocyanate (HMDI) and polypropylene glycol (PPG).The calculated amounts of polycarbonate diols were introduced into a three-necked flask equipped with a vacuum source, a thermometer, and a nitrogen inlet.After adding hexamethylene diisocyanate, the temperature was gradually increased to 80 • C.After 2 h of reaction, the pre-polymer with terminal -NCO groups was obtained.Then, the calculated amount of 1,4-butanediol (BDO) was slowly added to the system as a chain extender.Solvent N,N-dimethylformamide (DMF) and catalyst tin octoate were added sequentially.The reaction was maintained at 80 • C for another 2 h.After cooling to room temperature, the mixture was slowly poured into methanol, and the product was precipitated at 80 • C under vacuum and dried for 24 h.exchange agent.The pre-polymer of PU with terminal isocyanate groups was prepared by reacting the prepared PPCDLs with excess hexamethylene diisocyanate (HMDI) and polypropylene glycol (PPG).The calculated amounts of polycarbonate diols were introduced into a three-necked flask equipped with a vacuum source, a thermometer, and a nitrogen inlet.After adding hexamethylene diisocyanate, the temperature was gradually increased to 80 °C.After 2 h of reaction, the pre-polymer with terminal -NCO groups was obtained.Then, the calculated amount of 1,4-butanediol (BDO) was slowly added to the system as a chain extender.Solvent N,N-dimethylformamide (DMF) and catalyst tin octoate were added sequentially.The reaction was maintained at 80 °C for another 2 h.After cooling to room temperature, the mixture was slowly poured into methanol, and the product was precipitated at 80 °C under vacuum and dried for 24 h.

Characterization
The successful synthesis of TPU was characterized by 1 H nuclear magnetic resonance ( 1 H-NMR, Bruker AVANCE 500, Billerica, MA, USA) and Fourier-transform infrared spectroscopy (FTIR, Thermo Fisher Nicolet 6700, Waltham, MA, USA, Frontier resolution: 0.15 cm −1 ).The CO2-based TPU was investigated using differential scanning calorimetry (DSC, DSC200PC, Netzsch, Selb, Germany) from −30 to +120 °C performed at a heating rate of 10 °C min −1 , a cooling rate of 10 °C min −1 , and a holding time of 3 min in a flow of N2.The mechanical properties of TPU samples were tested using an auto tensile tester (C610) at a tensile rate of 50 mm min −1 , room temperature of 25 °C, and a relative humidity of 45%.

Characterization
The successful synthesis of TPU was characterized by 1 H nuclear magnetic resonance ( 1 H-NMR, Bruker AVANCE 500, Billerica, MA, USA) and Fourier-transform infrared spectroscopy (FTIR, Thermo Fisher Nicolet 6700, Waltham, MA, USA, Frontier resolution: 0.15 cm −1 ).The CO 2 -based TPU was investigated using differential scanning calorimetry (DSC, DSC200PC, Netzsch, Selb, Germany) from −30 to +120 • C performed at a heating rate of 10 • C min −1 , a cooling rate of 10 • C min −1 , and a holding time of 3 min in a flow of N 2 .The mechanical properties of TPU samples were tested using an auto tensile tester (C610) at a tensile rate of 50 mm min −1 , room temperature of 25 • C, and a relative humidity of 45%.The 180 • peel test was conducted using an auto tensile tester (C610) at a tensile rate of 300 mm min −1 , a room temperature of 25 • C, and a relative humidity of 60%.The surface morphologies of the cathode and Li metal samples were observed by scanning electron microscopy (SEM, TESCAN ClARA, 500-5000×, Brno, Czech Republic).The presence of Li 2 CO 3 after charging and discharging was detected using an X-ray diffractometer (XRD, Malvern Panalytic, Empyrean, Malvern, UK, 5 • min −1 , Mo K α = 0.71073 Å) and Raman spectroscopy (Renishaw inVia, Wotton-under-Edge, UK, 532 nm laser, resolution < 1 cm −1 ).The CO 2 adsorption capacities of PVDF and TPU were tested using specific surface area and porosity analyzer (BET, Micromeritics ASAP 2460, Malvern, UK) at 0 • C. The characterization of surface of Li metal was performed using X-ray photoelectron spectroscopy (XPS, Thermo Fisher ESCALAB Xi+, Waltham, MA, USA).

Preparation of CO 2 Cathode
Graphene (80%), CNT (10%), and PVDF(HSV-900+) or TPU binders (10%) were mixed and grinded in NMP to form a uniform slurry.The prepared slurry was scraped on carbon paper by a blade and dried in vacuum oven at 60 • C for 24 h.After drying, the material-loaded carbon paper was punched into 10 mm-diameter circular discs as working electrodes, with each containing 0.09~0.12mg of active material (the mass of graphene and CNTs).

Fabrication of Li-CO 2 Batteries
In this study, 2032-type coin cells with holes at the positive side were used.Polytetrafluoroethylene (PTFE) was applied on cathode to reduce solvent volatilization.The cell fabrication was conducted in an argon (Ar)-filled glovebox.A Li metal anode (15.8 mm in diameter) and glass fiber separator (Whatman, Maidstone, UK) were employed.A total of 1 M LiTFSI in TEGDME was used as the electrolyte.

Linear Sweep Voltammetry (LSV)
LSV was employed to test the electrochemical window of TPU binder.The scanning range was set at 0-5 V, with a scanning speed of 1 mV s −1 , utilizing a CHI604D electrochemical workstation (CH Instruments, Shanghai, China).

Electrochemical Impedance Spectroscopy (EIS)
An EIS test was conducted on the Li-CO 2 battery after 30 cycles.The test employed a 10 mV amplitude, with a frequency range of 0.1-10 6 Hz, and was performed at a temperature of 28 • C using a CHI604D electrochemical workstation.

Battery Performance Test of Li-CO 2 Batteries
The Li-CO 2 battery was placed inside a test chamber in an Ar atmosphere.All Ar gas was evacuated using a mechanical pump, followed by immediate injection CO 2 at 0.12 MPa.Constant current charge-discharge cycles were conducted under conditions of 0.12 MPa of CO 2 , a room temperature of 28 • C in a constant temperature incubator (Yuejin SPX-2500-II, Shanghai, China), a current density of 0.2 A g −1 , and a cut-off capacity of 1000 mAh g −1 (20 µA for 0.1 mg of active material, for example).

Open Circuit Voltage Test
The Li-CO 2 battery was placed inside a test chamber in an Ar atmosphere that was replaced with CO 2 at 0.12 MPa.The Li-CO 2 batteries were statistically placed under 0.12 MPa CO 2 and 28 • C for 90 h, and the open circuit voltage was tested.

Deep Discharge Test
The Li-CO 2 battery was placed inside a test chamber in Ar atmosphere and replaced by CO 2 of 0.12 MPa.The battery was continuously discharged at 28 • C, 0.2 A g −1 until the discharge voltage reached 1.7 V.

Results and Discussion
The structural schematic of TPU and the corresponding 1 H-NMR peaks with their respective chemical bonds are shown in Figure 2a.Successful synthesis of CO 2 -based TPU is validated through 1 H-NMR (Figure 2b), 13 C-NMR (Figure 2c), H-H COSY (Figure 2d), and FTIR (Figure 2e).A small characteristic signal appears at 6.95 ppm, attributed to the amino group in the amine-terminated polyester segment.The peaks at 1.24, 4.3, and 5.0 ppm correspond to the hydrogen atoms in the poly(propylene carbonate) diol (PPCDL) segment [28].The peaks at 1.1 and 3.5 ppm are assigned to the hydrogen atoms in the poly(propylene glycol) (PPG) segment [31,32].Additionally, the peak at 1.20 ppm is attributed to the hydrogen atoms in the hexamethylene diisocyanate (HMDI) segment [33,34].Based on the integration of 1 H-NMR, the molar ratios of PPCDL, PPG, BDO, and HMDI in TPU can be calculated to be 60.59%, 14.22%, 9.82%, and 15.36%, respectively.The peaks at 154.9, 75.4,72.9, and 29.6 ppm correspond to the carbon atoms in the PPCDL segment.The peaks at 32.0, 25.The bonding strength of CO 2 -based TPU was slightly weaker than that of PVDF.The adsorption capacity of PVDF and TPU powders for CO 2 was tested by BET in a mixed ice-water bath at 0 • C [35] (Figure 3d).PVDF exhibits almost no CO 2 adsorption capacity at all partial pressures, whereas CO 2 -based TPU displays significant adsorption capacity due to the presence of urethane groups (-NHCOO-).This uniform absorption and enrichment of CO 2 at cathode/electrolyte interfaces contribute to an increase in the discharge voltage in Li-CO 2 batteries.Moreover, the resultant discharge product Li 2 CO 3 exhibits smaller particle sizes, thereby reducing the energy barrier of decomposition and enabling lower charge voltages.Consequently, the electrolytes remain more stable at lower voltages, thereby enhancing the cycle life of Li-CO 2 batteries.Subsequently, linear sweep voltammetry (LSV) is employed to characterize the electrochemical stability of CO 2 -based TPU (Figure 3e).Within the range of 0-5 V, the current remained within 0.3 µA, with no significant increase in current, indicating that TPU did not undergo electrochemical decomposition.Therefore, it can be concluded that TPU is electrochemically stable within the working voltage range of Li-CO 2 batteries (1.8-4.4V) without decomposition.The CO2-based TPU exhibited excellent self-healing properties.Following incisions cut by a blade, the CO2-based TPU samples before and after self-healing at 60 °C were observed under an optical microscope at a magnification of 10× [36].The scratches were The CO 2 -based TPU exhibited excellent self-healing properties.Following incisions cut by a blade, the CO 2 -based TPU samples before and after self-healing at 60 • C were observed under an optical microscope at a magnification of 10× [36].The scratches were significantly restored after 60 min of self-healing (Figure 4a-c).After 10 min of self-healing at 60 • C, the scratches show obvious recovery; after 60 min of healing, the scratches are essentially completely gone.The tensile strength of the TPU before cutting and after healing was measured, indicating a healing efficiency of 72.8% at 32 • C (Figure 4d).The self-healing mechanism of TPU involves the mobility of its chain segments.As temperature increases, the mobility of chain segments is enhanced, facilitating the reestablishment of hydrogen bonds between molecules.The outstanding self-healing performance of TPU can effectively mitigate powder shedding attributed to cathode volume variation.
essentially completely gone.The tensile strength of the TPU before cutting and after healing was measured, indicating a healing efficiency of 72.8% at 32 °C (Figure 4d).The selfhealing mechanism of TPU involves the mobility of its chain segments.As temperature increases, the mobility of chain segments is enhanced, facilitating the reestablishment of hydrogen bonds between molecules.The outstanding self-healing performance of TPU can effectively mitigate powder shedding attributed to cathode volume variation.We assembled Li-CO 2 batteries utilizing PVDF and CO 2 -based TPU as binders, respectively, and conducted charge-discharge cycles at a constant current of 0.2 A g −1 and a cut-off capacity of 1000 mAh g −1 in CO 2 atmosphere.The charge-discharge curves for the 1st cycle, 5th cycle, and final cycle are shown in Figure 5a,b.From the charging platform, the charging voltage of PVDF battery is around 3.8 V, while that of TPU battery is around 3.5 V, indicating that TPU binders reduces the charging voltage of Li-CO 2 batteries.The discharge voltage of the first cycle of the PVDF battery is around 1.8 V, while that of the CO 2 -based TPU battery is around 2.0 V.This discrepancy arises due to the CO 2 adsorption capacity of CO 2 -based TPU, which elevates the partial pressure of CO 2 on the surface of the electrode's active material; according to the Nernst equation (Equation ( 2)), the absorption characteristics of CO 2 -based TPU for CO 2 enhance the discharge voltage of Li-CO 2 batteries.Meanwhile, from Figure 5a,b, it can be seen that the cycle life of Li-CO 2 batteries with CO 2 -based TPU as the binder is significantly higher than that of Li-CO 2 batteries with PVDF.The cycle life of Li-CO 2 batteries with PVDF and CO 2 -based TPU binder at 0.2 A g −1 at a cut-off capacity of 1000 mAh g −1 is shown in Figure 5c.The lifespan of Li-CO 2 batteries with PVDF is only 400 h, while the lifespan for batteries with CO 2 -based TPU reaches 550 h, demonstrating the significant role of CO 2 -based TPU in extending the battery cycle stability of Li-CO 2 batteries.
Electrochemical impedance spectroscopy (EIS) was conducted on Li-CO 2 batteries with different cycles, as shown in Figure 6a.After the first cycle, both the EIS spectra of the Li-CO 2 batteries with PVDF and TPU binders exhibited two semicircles, indicating that the SEI had not yet stabilized on either the cathode or the anode interfaces.After the fifth cycle, only one semicircle appeared, indicating that a stable SEI was formed on both the cathode and anode interfaces, resulting in similar interfacial characteristics.The Li-CO 2 battery with PVDF as the binder exhibited a higher initial impedance of charge transfer resistance (R ct1 ) of 231.3 Ω and R ct2 = 386.4Ω.In contrast, the Li-CO 2 battery with CO 2 -based TPU as the binder showed a lower initial impedance of only R ct1 = 158.1 Ω and R ct2 = 362.2Ω.The lower initial impedance of the TPU binder battery compared to the PVDF binder battery indicates that the Li-ion transfer impedance is smaller with CO 2 -based TPU as the binder.During the 5th and 10th cycles, the R ct of the Li-CO 2 batteries with PVDF binder increased to 638.3 Ω and 674.8 Ω, respectively, while the R ct of the Li-CO 2 batteries with TPU binder was only 504.2 Ω and 561.9 Ω, respectively.The Li-CO 2 batteries with PVDF binder exhibited higher solution resistance (R s ) values at the 1st, 5th, and 10th cycles of 26.9 Ω, 65.8 Ω, and 98.9 Ω, respectively.In contrast, the Li-CO 2 batteries with TPU binder showed lower R s values of 9.9 Ω, 57.81 Ω, and 45.2 Ω, respectively, suggesting that TPU plays a role in reducing the electrolyte consumption during SEI formation.The Li-CO 2 batteries with PVDF binder exhibited a Warburg impedance of 660 Ω, 640 Ω, and 653 Ω at the 1st, 5th, and 10th cycles, respectively.In contrast, the Li-CO 2 batteries with TPU binder showed a Warburg impedance of 652 Ω, 642 Ω, and 649 Ω at the same cycles.The nearly constant diffusion impedance indicates that TPU does not affect the lithium-ion conduction capability of the electrolyte.After 10 cycles, the Li-CO 2 battery with CO 2 -based TPU binder exhibited significantly lower electrochemical impedance than the PVDF binder battery, suggesting the formation of a better solid electrolyte interface (SEI) with CO 2 -based TPU as the binder, which reduces the corrosion of the anode, thus preventing a sharp increase in impedance.We conducted rate performance tests on Li-CO 2 batteries with PVDF and CO 2 -based TPU binders at a cut-off capacity of 1000 mAh g −1 .The batteries underwent five cycles each at 0.2, 0.5, 1, and 2 C, with the rate performance of the Li-CO 2 batteries assessed based on the overpotential (Figure 6b).The overpotential of the Li-CO 2 batteries with CO 2 -based TPU was significantly smaller than those with PVDF, attributed to the CO 2 adsorption capability of TPU, which increases the CO 2 partial pressure on the surface of cathode, resulting in the higher discharge plateau of the battery.Additionally, the smaller Li 2 CO 3 grains generated under higher CO 2 partial pressure facilitate easier decomposition during charging.As a result, the Li 2 CO 3 battery with a CO 2 -based TPU binder displayed a lower charging plateau, leading to reduced overpotential and enhanced rate performance.to the CO2 adsorption capability of TPU, which increases the CO2 partial pressure on the surface of cathode, resulting in the higher discharge plateau of the battery.Additionally, the smaller Li2CO3 grains generated under higher CO2 partial pressure facilitate easier decomposition during charging.As a result, the Li2CO3 battery with a CO2-based TPU binder displayed a lower charging plateau, leading to reduced overpotential and enhanced rate performance.Li-CO2 batteries with PVDF and CO2-based TPU binders were subjected to 0.12 MPa of CO2, respectively, for 90 h, with the open circuit voltage measured under a CO2 atmosphere (Figure 6c).Under a 0.12 MPa CO2 atmosphere, the open circuit voltage of the Li-CO2 batteries with PVDF binder reached a maximum of 2.6 V, stabilizing at around 2.5 V whereas the Li-CO2 battery with the CO2-based TPU binder reached a maximum of 2.77 V, stabilizing at around 2.7 V. The higher open circuit voltage of the TPU bound Li-CO2 battery in the static state is attributed to the higher CO2 affinity of CO2-based TPU, resulting in a higher CO2 concentration on the surface of cathode, leading to a higher discharge plateau of the battery.Therefore, the open circuit voltage of the TPU binder Li-CO2 battery exceeded those of PVDF.The adsorption capacity of TPU for CO2 is stronger than that of PVDF, resulting in higher surface CO2 concentration on the cathode, which has a Li-CO 2 batteries with PVDF and CO 2 -based TPU binders were subjected to 0.12 MPa of CO 2, respectively, for 90 h, with the open circuit voltage measured under a CO 2 atmosphere (Figure 6c).Under a 0.12 MPa CO 2 atmosphere, the open circuit voltage of the Li-CO 2 batteries with PVDF binder reached a maximum of 2.6 V, stabilizing at around 2.5 V whereas the Li-CO 2 battery with the CO 2 -based TPU binder reached a maximum of 2.77 V, stabilizing at around 2.7 V. The higher open circuit voltage of the TPU bound Li-CO 2 battery in the static state is attributed to the higher CO 2 affinity of CO 2 -based TPU, resulting in a higher CO 2 concentration on the surface of cathode, leading to a higher discharge plateau of the battery.Therefore, the open circuit voltage of the TPU binder Li-CO 2 battery exceeded those of PVDF.The adsorption capacity of TPU for CO 2 is stronger than that of PVDF, resulting in higher surface CO 2 concentration on the cathode, which has a significant impact on the morphology of discharge products.Deep discharge tests were conducted on Li-CO 2 batteries with PVDF and CO 2 -based TPU binders at a current density of 0.2 A g −1 (Figure 6d).The discharge plateau of the CO 2 -based TPU Li-CO 2 battery reached 2.0 V, surpassing the 1.8 V of the PVDF battery, while the discharge capacity of TPU binder Li-CO 2 batteries reached 8654 mAh g −1 , higher than that of the PVDF battery (6173 mAh g −1 ).These results underscore the fact that the CO 2 adsorption capacity of TPU increases the reaction concentration of CO 2 on the cathode.
In order to investigate the influence of CO 2 -based TPU binder on the discharge products, we disassembled cycled Li-CO 2 batteries after 30 cycles at 0.2 A g −1 and analyzed the cathode of PVDF and CO 2 -based TPU using an X-ray diffractometer (XRD) after sealing with vacuum mud.The XRD patterns of discharged PVDF and TPU binder Li-CO 2 batteries are shown in Figure 7a, which exhibit obvious Li 2 CO 3 peaks [37,38], which are the discharge product of Li-CO 2 batteries.However, charged PVDF binder Li-CO 2 batteries still exhibited distinct Li 2 CO 3 peaks with lower intensities compared to the discharged state, indicating the insufficient reversibility of Li 2 CO 3 at the PVDF binder CO 2 cathode; During charging, incomplete decomposition of Li 2 CO 3 into CO 2 occurs, resulting in continuous accumulation of Li 2 CO 3 at the cathode during cycling.This accumulation compromises the active material and ultimately leads to Li-CO 2 battery failure.In contrast, the XRD patterns of CO 2 -based TPU binder Li-CO 2 batteries show no peaks of Li 2 CO 3 post-charging, signifying complete decomposition of Li 2 CO 3 into CO 2 in the presence of TPU binder.Therefore, Li 2 CO 3 does not accumulate on cathode active sites, prolonging the cycle life for TPU binder Li-CO 2 batteries, in line with the results in Figure 5. Similarly, the charged and discharged PVDF and CO 2 -based TPU binder Li-CO 2 batteries were subjected to Raman tests under air-isolated conditions.The Raman test results of Li metal are shown in Figure 7b and are consistent with the results of X-ray diffraction.PVDF binder Li-CO 2 batteries retained distinct Li 2 CO 3 Raman peaks post-charging, whereas the Li 2 CO 3 Raman peaks [39,40] vanished completely post-charging in TPU binder Li-CO 2 batteries, underscoring the role of TPU in promoting the reversible decomposition of Li 2 CO 3 and facilitating the reversible cycling of Li-CO 2 batteries.Subsequently, the cathode sheets of TPU and PVDF binder Li-CO 2 batteries after 30 cycles at 0.2 A g −1 were characterized via SEM.In the discharged state, the surfaces of PVDF (Figure 7e) and TPU (Figure 7c) cathode sheets exhibit notable differences.Li 2 CO 3 clusters on the PVDF sheet are large and clustered, with clusters reaching 10 µm in diameter, whereas Li 2 CO 3 on the TPU sheet is uniformly distributed on carbon fibers without clustering.In the charged state, Li 2 CO 3 residues persist on the PVDF sheet (Figure 7f) albeit with smaller clusters compared to the discharged state, approximately 8 µm in size, whereas Li 2 CO 3 is nearly absent on the CO 2 -based TPU sheet (Figure 7d) in the charged state.This further underscore CO 2 -based TPU's efficacy in promoting reversible Li 2 CO 3 decomposition, effectively mitigating the loss of active sites due to Li 2 CO 3 deposition on the cathode, confirming its pivotal role in Li-CO 2 battery cycling stability.This observation aligns with the TPU's superior cycling performance compared to PVDF, as shown in Figure 5.The SEM images confirm that Li 2 CO 3 in PVDF binder cathode cannot be completely reversibly decomposed during cycling, while Li 2 CO 3 in TPU binder cathode sheets can maintain good reversibility during cycling.
To assess the impact of CO 2 -based TPU binder on the anode corrosion during cycling, PVDF and CO 2 -based TPU binder-based Li-CO 2 batteries were disassembled after 30 cycles in an Ar atmosphere glove box, and the composition of the anode surface was characterized by XPS.From the XPS carbon spectra, Li 2 CO 3 peaks [41,42] were observed on the surface of the anode in the PVDF binder-based Li-CO 2 battery (Figure 8d), indicating anode corrosion.In contrast, no Li 2 CO 3 peak was observed on the surface of the anode in the TPU binder-based Li-CO 2 battery (Figure 8a), indicating that the anode remained intact without corrosion.Meanwhile, peaks corresponding to organic nitrides and organic fluorides were observed on the TPU binder-based battery, indicating the presence of organic fluoride and nitride compounds in the SEI layer.These components play an important role in protecting the lithium metal from corrosion, enhancing the cycling stability and lifespan of Li-CO 2 batteries.Furthermore, from the XPS oxygen spectra, it is evident that the Li 2 CO 3 signal peaks appeared prominently on the surface of the anode of PVDF binder-based Li-CO 2 battery (Figure 8e), while no Li 2 CO 3 signal peak was observed on the surface of anode of the TPU binder Li-CO 2 battery (Figure 8b), further confirming the ability of CO 2based TPU binder to protect lithium metal from corrosion.This is attributed to the strong affinity of TPU towards CO 2, which effectively reduces the dissolution of CO 2 into the electrolyte, preventing CO 2 diffusion to the lithium metal during cycling, thereby reducing corrosion of CO 2 and lithium metal at the anode surface, ultimately achieving the goal of protecting the lithium metal and enhancing the cycling stability and lifespan of Li-CO 2 batteries.Moreover, from the XPS fluorine spectra, it can be observed that both LiF [43] and LiTFSI [44] are the main forms of fluorine present at the surface of anode of PVDF (Figure 8f) and TPU (Figure 8c) binder-based Li-CO 2 batteries.However, the ratio of LiF peak area to LiTFSI peak area is higher for TPU binder-based batteries, indicating that a higher content of LiF protects the lithium metal in the SEI layer of TPU binder-based batteries.As a result, TPU binder-based Li-CO 2 batteries have better cycling stability than PVDF binder-based batteries.To assess the impact of CO2-based TPU binder on the anode corrosion during cycling, PVDF and CO2-based TPU binder-based Li-CO2 batteries were disassembled after 30 cycles in an Ar atmosphere glove box, and the composition of the anode surface was characterized by XPS.From the XPS carbon spectra, Li2CO3 peaks [41,42] were observed on the surface of the anode in the PVDF binder-based Li-CO2 battery (Figure 8d), indicating anode corrosion.In contrast, no Li2CO3 peak was observed on the surface of the anode in the TPU binder-based Li-CO2 battery (Figure 8a), indicating that the anode remained intact without corrosion.Meanwhile, peaks corresponding to organic nitrides and organic fluorides were observed on the TPU binder-based battery, indicating the presence of organic fluoride and nitride compounds in the SEI layer.These components play an important role in protecting the lithium metal from corrosion, enhancing the cycling stability and lifespan of Li-CO2 batteries.Furthermore, from the XPS oxygen spectra, it is evident that the Li2CO3 signal peaks appeared prominently on the surface of the anode of PVDF binder-based Li-CO2 battery (Figure 8e), while no Li2CO3 signal peak was observed on the surface of anode of the TPU binder Li-CO2 battery (Figure 8b), further confirming the ability of CO2-based TPU binder to protect lithium metal from corrosion.This is attributed to the strong affinity of TPU towards CO2, which effectively reduces the dissolution of CO2 into the electrolyte, preventing CO2 diffusion to the lithium metal during cycling, thereby reducing corrosion of CO2 and lithium metal at the anode surface, ultimately achieving the goal of protecting the lithium metal and enhancing the cycling stability and lifespan of Li-CO2 batteries.Moreover, from the XPS fluorine spectra, it can be observed that both LiF [43] and LiTFSI [44] are the main forms of fluorine present at the surface of anode of PVDF (Figure 8f) and TPU (Figure 8c) binder-based Li-CO2 batteries.However, the ratio of LiF peak area to LiTFSI peak area is higher for TPU binder-based batteries, indicating that a higher content of LiF protects the lithium metal in the SEI layer of TPU binder-based batteries.As a result, TPU binder-based Li-CO2 batteries have better cycling stability than PVDF binder-based batteries.To visually assess the differences in anode morphology of Li-CO2 batteries with PVDF and TPU binders after 30 cycles at 0.2 A g −1 , the cycled Li-CO2 batteries were disassembled inside a glovebox and the anode surfaces were examined via SEM.The surface of the lithium metal in the TPU binder batteries (Figure 9c,d) displayed a smooth and uniform morphology without noticeable defects.In contrast, the surface of the lithium To visually assess the differences in anode morphology of Li-CO 2 batteries with PVDF and TPU binders after 30 cycles at 0.2 A g −1 , the cycled Li-CO 2 batteries were disassembled inside a glovebox and the anode surfaces were examined via SEM.The surface of the lithium metal in the TPU binder batteries (Figure 9c,d) displayed a smooth and uniform morphology without noticeable defects.In contrast, the surface of the lithium metal in the PVDF binder batteries (Figure 9a,b) exhibited severe corrosion and the formation of prominent dendrites.This observation aligns with the conclusions drawn from XPS spectra, suggesting that the CO 2 -based TPU binder promotes the formation of an SEI passivation layer rich in LiF, which effectively shields the lithium metal from corrosion by CO 2 during cycling.Moreover, the SEI facilitates a more uniform distribution of lithium ions reduced to lithium metal on the lithium metal surface, effectively reducing dendrite growth.This significantly contributes to the lower impedance observed in TPU binder Li-CO 2 batteries compared to PVDF binder Li-CO 2 batteries.CO 2 -based TPU hinders the dissolution of 2 into the electrolyte, preventing corrosion of the lithium metal, and facilitates the formation of a more effective SEI protective layer.These combined effects enhance the cycling stability and lifespan of Li-CO 2 batteries.This also explains the longer cycle life observed in TPU binder Li-CO 2 batteries (Figure 5), as well as the lower impedance observed in TPU binder Li-CO 2 batteries (Figure 6a), with impedance growth during cycling that is significantly lower than that of PVDF binder batteries.

Conclusions
In summary, by replacing the commonly used commercial polyvinylidene fluoride (PVDF) binder with a CO2-based thermoplastic polyurethane (TPU) binder, the stable cycling life of Li-CO2 batteries has been increased from 40 cycles with PVDF binder to 52 cycles with TPU binder, which reach a high level for catalyst-free Li-CO2 batteries [45][46][47].This enhancement can be attributed to several key factors: (1) TPU, characterized by -NHbonds, exhibits Lewis basicity, possessing stronger CO2 adsorption capability.This enhances the adsorption of CO2 at the cathode, reducing the diffusion of CO2 to the anode and its side reactions with lithium metal.(2) The CO2-based TPU binder enhances the cycling stability and lifespan of Li-CO2 batteries.TPU reinforces the CO2 adsorption capacity of the cathode, elevating the discharge voltage plateau of Li-CO2 batteries.Additionally, the generated Li2CO3 particles are smaller, facilitating more reversible cycling during charging and reducing the charging plateau of Li-CO2 batteries.(3) TPU promotes the formation of a dense SEI rich in LiF on the anode.This SEI layer enables uniform deposition and stripping of lithium ions at the anode during cycling, effectively reducing dendrite formation.Moreover, the dense SEI layer prevents direct contact between dissolved CO2 in the electrolyte and the lithium metal, thereby reducing the side reactions between CO2

Conclusions
In summary, by replacing the commonly used commercial polyvinylidene fluoride (PVDF) binder with a CO 2 -based thermoplastic polyurethane (TPU) binder, the stable cycling life of Li-CO 2 batteries has been increased from 40 cycles with PVDF binder to 52 cycles with TPU binder, which reach a high level for catalyst-free Li-CO 2 batteries [45][46][47].This enhancement can be attributed to several key factors: (1) TPU, characterized by -NHbonds, exhibits Lewis basicity, possessing stronger CO 2 adsorption capability.This enhances the adsorption of CO 2 at the cathode, reducing the diffusion of CO 2 to the anode and its side reactions with lithium metal.
7, and 25.0 ppm are assigned to the carbon atoms in the PPG segment.The peak at 73.3, 72.4,69.0, 17.3, and 16.3 ppm is attributed to the carbon atoms in the HMDI segment.The test result of H-H COSY is shown in Figure 2d.J 67 and J 78 are the couple effect of the hydrogen of the PPCDL segment.J 13 is the couple effect of the hydrogen of the PPG segment.J 910 is the couple effect of the hydrogen of the BDO segment.The infrared absorption spectrum (FTIR) of TPU is depicted in Figure 2e.Strong absorption peaks appear at 1260 and 1750 cm −1 , corresponding to the stretching vibrations of the O-C=O and C-O groups, respectively.Peaks at 2900 and 2875 cm −1 are attributed to the stretching vibrations of -CH 3 and -CH 2-groups, respectively.Furthermore, peaks at 3340 and 1540 cm −1 are assigned to the stretching vibrations of the -NH-and C-NH bonds, respectively.To verify the thermal stability of CO 2 -based TPU, we conducted DSC testing on CO 2 -based TPU.The DSC curve of CO 2 -based TPU is shown in Figure 2f.The black curve is the differential curve of DSC curve, and the peak of it represents the temperature of glass transition temperature (T g ).No crystallization or melting peaks are observed for CO 2 -based TPU in the temperature range of −30 to +120 • C, demonstrating its stability under working conditions of Li-CO 2 batteries.The T g of CO 2 -based TPU is measured at 23.5 • C, suggesting its elasticity and bonding capability under normal working conditions.Nanomaterials 2024, 14, x FOR PEER REVIEW 6 of 15decomposition.Therefore, it can be concluded that TPU is electrochemically stable within the working voltage range of Li-CO2 batteries (1.8-4.4V) without decomposition.

Figure 2 .
Figure 2. (a) Structure of TPU with labels for 1 H-NMR (blue) and 13 C-NMR (black); (b) 1 H-NMR spectrum of TPU with DMSO-d6; (c) 13 C-NMR spectrum of TPU with CDCl 3 ; (d) H-H COSY spectrum of TPU; (e) FTIR spectrum of TPU; and (f) DSC curves of TPU recorded from −30 to +120 • C. The tensile test results of CO 2 -based TPU are shown in Figure 3a.The tensile strength of CO 2 -based TPU reaches 19 MPa, and the elongation at break achieves 427%, indicating outstanding mechanical properties.The 180 • peel strength tests of the cathode with PVDF and TPU as the binders are, respectively, shown in Figure 3b,c.The peel strength of the cathode with PVDF reached 7.3 N m −1 , whereas that with CO 2 -based TPU was 4.7 N m −1 .The bonding strength of CO 2 -based TPU was slightly weaker than that of PVDF.The

Figure 3 .
Figure 3. (a) Tensile strength-strain curves of TPU at 25 °C and 50 mm min −1 ; (b) 180° peel test of PVDF and TPU cathode at 25 °C and 300 mm min −1 ; (c) peel strength of PVDF and TPU; (d) CO2 absorption of PVDF and TPU powder tested with BET at 0 °C; (e) LSV curve of TPU in 0-5 V at 1 mV s −1 .

Figure 3 .
Figure 3. (a) Tensile strength-strain curves of TPU at 25 • C and 50 mm min −1 ; (b) 180 • peel test of PVDF and TPU cathode at 25 • C and 300 mm min −1 ; (c) peel strength of PVDF and TPU; (d) CO 2 absorption of PVDF and TPU powder tested with BET at 0 • C; (e) LSV curve of TPU in 0-5 V at 1 mV s −1 .

Figure 4 .
Figure 4. Optical microscopy images of TPU self-healed at 60 °C; the scratches healed gradually in (a) 0 min, (b)10 min, and (c) 60 min.(d) Tensile strength of pristine TPU and self-healed TPU.We assembled Li-CO2 batteries utilizing PVDF and CO2-based TPU as binders, respectively, and conducted charge-discharge cycles at a constant current of 0.2 A g −1 and a cut-off capacity of 1000 mAh g −1 in CO2 atmosphere.The charge-discharge curves for the 1st cycle, 5th cycle, and final cycle are shown in Figure5a,b.From the charging platform, the charging voltage of PVDF battery is around 3.8 V, while that of TPU battery is around 3.5 V, indicating that TPU binders reduces the charging voltage of Li-CO2 batteries.The discharge voltage of the first cycle of the PVDF battery is around 1.8 V, while that of the CO2-based TPU battery is around 2.0 V.This discrepancy arises due to the CO2 adsorption capacity of CO2-based TPU, which elevates the partial pressure of CO2 on the surface of the electrode's active material; according to the Nernst equation (Equation (2)), the absorption characteristics of CO2-based TPU for CO2 enhance the discharge voltage of Li-CO2 batteries.Meanwhile, from Figure5a,b, it can be seen that the cycle life of Li-CO2 batteries with CO2-based TPU as the binder is significantly higher than that of Li-CO2 batteries with PVDF.The cycle life of Li-CO2 batteries with PVDF and CO2-based TPU binder at 0.2 A g −1 at a cut-off capacity of 1000 mAh g −1 is shown in Figure5c.The lifespan of Li-CO2 batteries with PVDF is only 400 h, while the lifespan for batteries with CO2-based TPU reaches 550 h, demonstrating the significant role of CO2-based TPU in extending the battery cycle stability of Li-CO2 batteries.

Nanomaterials 2024 , 15 Figure 5 .
Figure 5. Voltage-specific capacity curves of Li-CO2 batteries at 0.2 A g −1 and 1000 mAh g −1 with (a) PVDF and (b) TPU binder.(c) The lifespan curves of Li-CO2 batteries at 0.2 A g −1 and 1000 mA hg −1 with PVDF and TPU binders.Electrochemical impedance spectroscopy (EIS) was conducted on Li-CO2 batteries with different cycles, as shown in Figure 6a.After the first cycle, both the EIS spectra of the Li-CO2 batteries with PVDF and TPU binders exhibited two semicircles, indicating that the SEI had not yet stabilized on either the cathode or the anode interfaces.After the fifth cycle, only one semicircle appeared, indicating that a stable SEI was formed on both

Figure 5 .
Figure 5. Voltage-specific capacity curves of Li-CO 2 batteries at 0.2 A g −1 and 1000 mAh g −1 with (a) PVDF and (b) TPU binder.(c) The lifespan curves of Li-CO 2 batteries at 0.2 A g −1 and 1000 mA hg −1 with PVDF and TPU binders.

Figure 6 .
Figure 6.(a) Electrochemical impedance spectra (EIS) of Li-CO2 battery with PVDF and TPU after 1, 5, 10 cycles at the current density of 0.2 A g −1 and the cut-off capacity of 1000 mAh g −1 .(b) Rate performance of Li-CO2 battery with TPU (red) and PVDF (blue) as binder at 0.2, 0.5, 1 and 2 C, the current density of 0.2 A g −1 and the cut-off capacity of 1000 mAh g −1 .(c) The open voltage of Li-CO2 batteries with TPU (yellow) and PVDF (blue) statically placed under a CO2 atmosphere for 90 h.(d) The voltage-specific capacity profiles of a Li-CO2 battery with PVDF or TPU discharged to 1.7 V at 0.2 A g −1 .

Figure 6 .
Figure 6.(a) Electrochemical impedance spectra (EIS) of Li-CO 2 battery with PVDF and TPU after 1, 5, 10 cycles at the current density of 0.2 A g −1 and the cut-off capacity of 1000 mAh g −1 .(b) Rate performance of Li-CO 2 battery with TPU (red) and PVDF (blue) as binder at 0.2, 0.5, 1 and 2 C, the current density of 0.2 A g −1 and the cut-off capacity of 1000 mAh g −1 .(c) The open voltage of Li-CO 2 batteries with TPU (yellow) and PVDF (blue) statically placed under a CO 2 atmosphere for 90 h.(d) The voltage-specific capacity profiles of a Li-CO 2 battery with PVDF or TPU discharged to 1.7 V at 0.2 A g −1 .

Nanomaterials 2024 , 15 Figure 7 .
Figure 7. (a) XRD patterns of charged and discharged CO2 cathodes in Li-CO2 batteries with PVDF and TPU binders.(b) Raman spectra of charged and discharged CO2 cathodes in Li-CO2 batteries with PVDF and TPU binders.SEM images of (c) discharged Li-CO2 battery with TPU binders, (d) charged Li-CO2 battery with TPU binders, (e) discharged Li-CO2 battery with PVDF binders, and (f) charged Li-CO2 battery with PVDF binders after 30 cycles at 0.2 A g −1 .

Figure 7 . 15 Figure 8 .
Figure 7. (a) XRD patterns of charged and discharged CO 2 cathodes in Li-CO 2 batteries with PVDF and TPU binders.(b) Raman spectra of charged and discharged CO 2 cathodes in Li-CO 2 batteries with PVDF and TPU binders.SEM images of (c) discharged Li-CO 2 battery with TPU binders, (d) charged Li-CO 2 battery with TPU binders, (e) discharged Li-CO 2 battery with PVDF binders, and (f) charged Li-CO 2 battery with PVDF binders after 30 cycles at 0.2 A g −1 .Nanomaterials 2024, 14, x FOR PEER REVIEW 12 of 15

Figure 9 .
Figure 9. SEM images of anode of Li-CO 2 batteries with PVDF at (a) 500× and (b) 5000× and TPU at (c) 500× and (d) 5000× after 30 cycles at 0.2 A g −1 .(e) Schematic diagram of CO 2 shuttle effect in Graphene@PVDF cathode.(f) Schematic diagram of CO 2 shuttle effect reduced by Graphene@TPU cathode.(g) Schematic diagram of interaction between CO 2 and -NH-of TPU.
(2) The CO 2 -based TPU binder enhances the cycling stability and lifespan of Li-CO 2 batteries.TPU reinforces the CO 2 adsorption capacity of the cathode, elevating the discharge voltage plateau of Li-CO 2 batteries.Additionally, the generated Li 2 CO 3 particles are smaller, facilitating more reversible cycling during charging and reducing the charging plateau of Li-CO 2 batteries.(3) TPU promotes the formation of a