To determine the phase evolution at the electrochemical interface at various relaxation times, we conduct the in situ XRD analysis of conversion electrodes to observe the significant changes in phase composition during relaxation (Figure S9). For CuF2 electrode, a structural transition from the Cu(II) phase (CuF2) to a new Cu(I) phase can be clearly identified (Figure S10 and Figure S11d,e). Interestingly, the equilibrium potential after relaxation reaches a high value of 3.24 V close to that of the CuF/Cu redox couple(11). This result indicates that the Cu(I) phase-dominated electrochemical interface can effectively eliminate the voltage hysteresis of CuF2 electrode.
We next introduce a polymer interface layer (PIL) (Figure S12) to induce and stabilize the meta-stable Cu(I) state phase (Figure S11b,d) through the adsorption and charge transfer between the polymer functional group and the electrode particles(32, 33) (Figure S13 and S14). Meanwhile, the solid state electrolyte is used to inhibit Cu dissolution issues (Figure S15 and Figure S16). As shown in Fig. 2a,b, the CuF2 electrode in CuF2−LIB delivers a specific discharge capacity of 468 mAh g− 1 with a discharge voltage below 2.9 V and a specific charge capacity of 261 mAh g− 1 with a charge voltage higher than 3.5 V at 0.1 C. When introducing PIL-PVDF-HFP to CuF2 electrode (named as CuF2-PPH), the CuF2-PPH delivers a specific discharge capacity of 462 mAh g− 1 with a discharge voltage of 3.20 V and a specific charge capacity of 466 mAh g− 1 with a charge voltage of 3.25 V, leading to a tiny hysteresis of only 50 mV -almost “voltage hysteresis free”!
To unveil the cause of the voltage hysteresis drop from over 700 mV to 50 mV, we first employ the DFT calculations. It is found that that among CuF2 (011), CuF (011), and Cu (011) surfaces, PIL-PVDF-HFP exhibit higher adsorption energy on CuF (011) surface (Fig. 2c and Table S1), suggesting that PIL-PVDF-HFP will preferentially bind to CuF surface and stabilize it. It is worth noting that the strong adsorption of PIL-PVDF-HFP on the CuF (011) surface has little influence on the Cu-F coordination at the surface, which is distinctly different from the adsorption structure of PIL-PEI (Table S2)(34).
Moreover, the Bader charge of surface Cu atoms on the Cu (011) slab exhibits a significant surface charge transfer for the sample with PIL-PVDF-HFP. The surface Cu atoms on Cu (011) slab tend to donate charge, eventually resulting in the valence states of the Cu element approaching to Cu(I). The situation is opposite to the case with PIL-PEI, in which surface Cu atoms on the Cu (011) slab withdraw charge (Fig. 2c and Table S3). This may be due to the protonated PIL-PEI(35) inducing the aggregation of electron density on the Cu (011) surface (Figure S13f and S14f), thereby reducing the valence state of surface Cu. Consistent with the opposite surface charge transfer of PIL-PEI and PIL-PVDF-HFP, the OCV of corresponding experimental half-cells shows a contrary trend of increasing (with PIL-PVDF-HFP) and decreasing (with PIL-PEI) as compared to that without PIL (Fig. 2d). The CuF2-PPH exhibits the strongest Cu(I) signal and a stable discharge plateau of ~ 3.2 V, while CuF2 with PIL-PEI has a strong Cu(0) signal and the lowest discharge voltage of ~ 1.9 V (Fig. 2e, f and Figure S17).
We further validate the PIL-PVDF-HFP to induce and stabilize the Cu(I) phase through different characterizations on pristine CuF2-PPH. XRD and Neutron powder diffraction patterns indicate no structural changes for CuF2-PPH compared to pure CuF2 particles (Figure S18). Nevertheless, upon the introduction of PIL-PVDF-HFP, electron paramagnetic resonance (EPR) spectra show that the g-value decreases significantly from 2.19 to 2.07 with a signal intensity decreasing, indicating the reduction of the partially paramagnetic Cu(II) phase to the diamagnetic Cu(I) phase(36, 37) (Figure S19a). It can also be verified by the new peak at 216 cm− 1 corresponding to the 2Γ12− signal of Cu(I) in Raman(38) spectra as well (Figure S20a). Besides, the XPS peak shifts of Cu binding energy demonstrate that a stronger Cu(I) signal is observed either for CuF2 or Cu binding to PIL-PVDF-HFP (Figure S20b,c). The above results evidence that the Cu(I) state can be induced and stabilized by PIL-PVDF-HFP through the synergistic regulation of strong adsorption and charge transfer.
To elucidate the mechanism of "voltage hysteresis free" with the introduction of PIL-PVDF-HFP on the CuF2, we systematically investigate the discharge/charge processes in the lithium semi-solid-state battery (LSSB). The new XRD peaks assigned to the CuF and Li3CuF4 phases appear (Figure S11d,e), with intensity first increases then decreases during discharging (Figure S21). The new Cu(I) phase-dominated reaction path of the CuF2 electrode is facilited by PIL. When combining with PIL-PVDF-HFP, the Cu(I) phase-dominated suface of CuF2 can be incudced and stabilized. Furthermore, more new CuF phases combined with PIL-PVDF-HFP can be formed due to the grain refinement during discharging (39). From the EDS mapping, there is a significant increase in the content of element C within the CuF2-refined particles (Figure S22 and Figure S23). Subsequently, the entry of Li replaces Cu+ in meta-stable CuF to form a thermodynamically stable Li3CuF4 phase from phonon spectra calculations (Figure S11c,e), supported by the newly generated peak at 3443 G(40) in EPR (due to reduction of paramagnetic Cu(II) to diamagnetic Cu(I)).
Meanwhile, the enhancement of the NMR peaks at ~-74 ppm and a new peak at -184 ppm can be observed in CuF2-PPH in the half-discharged state, exhibiting Cu(I) bonding to the group of CF3 in PVDF-HEP and CuF, respectively(41, 42)(Figure S19b,c). The coexistence of LiF, Cu (cubic phases), CuF2 (monoclinic phases), Cu(I) phase (consistent with the calculated cubic phase (space group of F\(\stackrel{\text{-}}{\text{4}}\)3m)) (Figure S11d) can be observed from HRTEM in the half-discharge state (~ Li-1.0), which also support the generation of CuF/Li3CuF4 (Figure S24). The etching results of XPS and EELS mappings (Fig. 3b, Figure S25 and Figure S26) for CuF2-PPH in the half-discharged state show that the Cu(I) valence state is enriched on the surface and extends inward. Therefore, a voltage plateau switching from 2.9 V to 3.2 V reflects a stable Cu(I)-dominated electrochemical interface (Figure S27).