The as-fabricated LLTO has a morphology of continuous fibers having a diameter of less than 100 nm (Fig. 1(a)). After the fibers are coated by carbon layers (Fig. 1(b)), the increasing diameter of the fibers indicates the uniformity of the carbon coating. LLTO and LLTO@C have similar ionic conductivity to Li0.35La0.55TiO3 with cubic-phase primitive perovskite structure, which is more than 1 × 10−4 S·cm−1 at RT [44–47]. The TEM image of LLTO@C in Fig. 1(c) depicts a coaxial structure consisting of an inner LLTO nanofiber and an outer carbon layer. The thickness of the carbon layer is shown in the inset of Fig. 1(c). The carbon layer maintains a continuous and sharp interface with LLTO. In addition, the distribution of the elements in the LLTO@C nanofiber is analyzed by EDS mapping, as shown in Figs. 1(e)−1(i). This EDS mapping clearly shows that the C, La, Ti, and O elements are distributed uniformly. The distribution of C (Fig. 1(f)) is wider than that of the other elements, indicating that the C coating is uniform.
Figure 1. Scanning electron microscope (SEM) images of the (a) as-fabricated LLTO nanofibers and (b) LLTO@C nanofibers. (c) TEM images of the LLTO@C nanofibers. The inset shows the magnified carbon layer and the LLTO@C interface. (d) High-angle annular dark field image of a selected LLTO@C nanofiber. (e) Overlapping image of the EDS mapping and corresponding distribution of C (f), La (g), Ti (h) and O (i). The “d” in (f) and (h) refers to the diameter of LLTO nanofiber.
The CR2025-type coin cells were assembled by coupling composite NCA cathodes with a Li anode. Fig. 2(a) compares their cycling performances for 200 cycles at 1 C in the voltage range of 2.7−4.3 V. The discharge capacity of NCA-pristine, NCA-10L@C, and NCA-20L@C is 101.3, 114.5, and 142.0 mAh·g−1 after 200 cycles, respectively. The NCA-20L@C cell possesses the highest capacity retention rate (77.2%), followed by NCA-10L@C (62.7%) and NCA-pristine (56.2%). Fig. 2(b) shows the variation trend of the medium voltage at the discharge plateau within 200 cycles. The NCA-20L@C maintains the highest discharge median voltage of ~ 3.698 V, while the NCA-pristine cell presents a more sluggish voltage drop (3.592 V) after 200 cycles. The NCA-10L@C cell is less promoted than NCA-20L@C. The drop in the voltage (Fig. 2(b)) is widely regarded to be closely related to the transition from the spinel phase to the layered phase by the migration of transition metal (TM) ions . The voltage drop is effectively mitigated with the addition of LLTO@C. The inhibition of the voltage fading of NCA-20L@C is ascribed to the structural stability obtained by incorporating LLTO@C nanofibers. To better comprehend the remarkable cycling degradation that occurred during the long-term cycling of the NCA cathode, EIS before and after 200 cycles was additionally recorded, as shown in Figs. 2(c) and 2(d), respectively. In addition, the illustration in Fig. 2(c) shows the equivalent circuit T-model derived from the Nyquist plots. The EIS plots include a semicircle in the high- to intermediate-frequency region, which refers to the charge-transfer resistance (Rct) of the electrochemical reaction, and the slope in the low-frequency region refers to the Warburg impedance (Zw) of surface Li+ ions diffusion [28,48]. The three cells present low resistance values before cycling (inset in Fig. 2(c)), which indicates a good contact between the interface and assembly. After 200 cycles, the impedance value Rct of NCA-pristine increases from 89.72 (1st) to 213.29 Ω (200th), suggesting that the resistance increases upon cycling. In contrast, the NCA-20L@C cell presents the lowest impedance change, indicating that the NCA-20L@C cathode can maintain the original structure and interface contact . The inset of Fig. 2(d) presents the suppressive effect on the increasing electrochemical impedance in the high-frequency region for NCA-20L@C. The enhancement of the conductivity of Li+ and electrons with the addition of LLTO@C was determined by the EIS evolution in the low- and high-frequency regions, respectively. In addition to the carbon layer on the fibers, the improvement is also related to the structural integrity, which well maintains electrical contact in the NCA particles.
Figure 2. Electrochemical performance of NCA-pristine, NCA-10L@C, and NCA-20L@C: (a) cycle performance at 1 C; (b) the variation trend of the medium voltage at the discharge plateau of the 1st, 50th, 100th, and 200th cycles within 200 cycles; EIS plots (c) before cycling and (d) after 200 cycles. The insets in (c) and (d) show the corresponding magnifications of the EIS plots in the high-frequency (100 kHz) region.
CV curves were studied and the detailed information on the redox behavior correlating with the phase transition was obtained. Figs. 3(a) and 3(b) show the initial three cycles performance at 2.7–4.3 V for the NCA-pristine and NCA-20L@C samples, respectively. In the positive scan of the first cycle, both samples show three anodic peaks, which are caused by the phase transitions from hexagonal to monoclinic (H1 to M), monoclinic to hexagonal (M to H2), and hexagonal to hexagonal (H2 to H3) during the Li ions extraction process [22,30]. The locations of the first anodic peaks for NCA-pristine and NCA-20L@C are 3.65 and 3.71 V, respectively. An obvious decrease in polarization can be observed after the modification of NCA-20L@C. In particular, it was generally accepted that the phase transition from H2 to H3 at approximately 4.20 V would generate a large volume variation in the NCA cathode because of the significant difference in their lattice parameters [21,49]. In this phase transition process, a NiO2 microregion would be generated, which is thermodynamically metastable and easily gives rise to the release of oxygen gas, ultimately leading to structural damage and capacity loss [21,50]. Comparing Fig. 3(a) with Fig. 3(b), it shows that the third anodic peak (4.20 V) assigned to the H2–H3 transition is suppressed distinctly. Moreover, the differences between the oxidation peak and reduction peak during the initial 3 cycles are quantified and shown in Table 1, which illustrates the degree of polarization. During the first cycle of CV, the difference between the main redox peaks (Δ1) of NCA-20L@C is 0.19 V, which is smaller than that of NCA-pristine (0.22 V). Notably, the potential differences between the redox peaks of NCA-20L@C during the second and third cycles drastically decrease to 0.02 and 0.01 V, respectively, compared with those of NCA-pristine. The further separation of the oxidation peaks shown in Fig. 3(a) suggests that the structure degradation has begun to accumulate in the NCA cathode without LLTO@C.
Number NCA-pristine NCA-20L@C ∆1 0.22 0.19 ∆2 0.03 0.02 ∆3 0.04 0.01
Table 1. Values of the potential interval for the NCA-pristine and NCA-20L@C samples
Figure 3. CV curves of (a) NCA-pristine and (b) NCA-20L@C during the initial three cycles performed at a rate of 0.05 mV/s. CV curves of (c) NCA-pristine and (d) NCA-20L@C obtained with different scan rates. (e) The relationship between the peak current (Ip) and v1/2 of NCA-pristine and NCA-20L@C.
Additionally, to further investigate the promotion effect on the Li conductivity by LLTO@C addition to the cathode, a series of CV curves were obtained at scan rates ranging from 0.2 to 2 mV·s−1 for NCA-pristine and NCA-20L@C. As shown in Figs. 3(c) and 3(d), with increasing scan rate, the cathodic peaks shift to the lower potentials, and the anodic peaks shift to the higher potentials. Meanwhile, it shows the degree of polarization at different scan rates, in which the suppressed polarization by the modification of LLTO@C is apparent. The peak current of the CV curve is proportional to the square root of the scan rate (v1/2) and can be expressed by the Randles–Sevcik equation, from which the diffusion coefficient (DLi+) of NCA-pristine and NCA-20L@C can be estimated [26,30,51]. The linear relationship between Ip and v1/2, shown in Fig. 3(e), indicates that the reaction kinetics of the NCA cathode is controlled by the diffusion process . As presented in Table 2, the calculated DLi+ value of NCA-20L@C (7.661 × 10−10 cm2·S−1) is higher than that of NCA-pristine (1.714 × 10−10 cm2·S−1), indicating the enhanced Li+ diffusion coefficient and kinetics resulting from modification with LLTO@C, which is consistent with the results of EIS.
Sample DLi+ (charge) /
DLi+ (discharge) /
NCA-pristine 1.714 × 10−10 2.079 × 10−11 NCA-20L@C 7.661 × 10−10 3.626 × 10−10
Table 2. CV testing detail parameters used for NCA-pristine and NCA-20L@C samples
To verify the coupling effect between LLTO and the carbon layer, the cycling at 5 C and the rating performance of NCA-20L and NCA-20L@C were compared. As shown in Fig. 4(a), the NCA-20L@C cell maintains the highest capacity, 102.7 mAh·g−1, and a capacity retention of 68.9% after 200 cycles, which is much higher than that of pristine NCA (only 40.2%). The NCA-20L sample, modified with bare LLTO fibers, delivers a moderate cycling performance with a capacity retention of 51.4%. The superior high-rate tolerance of NCA-20L@C is demonstrated through the long-term cycling at 5 C. Meanwhile, the rate capability of the three samples was also investigated and is shown in Fig. 4(b). The rates were increased from 0.1 to 5 C (1 C = 200 mA·g−1), then decreased to 0.1 C. The increasing rates give rise to a gradually fading capacity because of the electrode polarization. It is apparent that the NCA-pristine cell exhibits an inferior rate performance, with a sharp decrease in the reversible capacity, especially at the high rate of 5 C, which results from the degraded structure and increased impedance (as shown in Figs. 2 and 3). The NCA-20L@C sample presents a superior rate ability and a stable discharge capacity (136.1 mAh·g−1) at 5 C. When the rate gradually returns to 0.1 C, the capacity of NCA-20L@C recovers to the initial capacity level of 206.1 mAh·g−1.
Figure 4. (a) Cycling performance of NCA-pristine, NCA-20L, and NCA-20L@C at 5 C; (b) rating performance from 0.1 up to 5 C and down to 0.1 C. The voltage ranged from 2.7 to 4.3 V (1 C = 200 mA·g−1).
To explain the enhancement of the cell performance, the structure and chemical state evolutions were analyzed. An ex situ XRD test was conducted after 200 cycles at 5 C, as shown in Fig. 5(a). The NCA-pristine cathode shows an evident co-existence of the H2 and H3 phases after cycling. Meanwhile, the NCA-20L@C structure is most similar to the NCA structure before cycling [52–53]. XPS was used to analyze the chemical state of Ni in the NCA particle after cycling. In Figs. 5(b)–5(d), the two main peaks correspond to Ni 2p3/2 and Ni 2p1/2. The Ni 2p3/2 peaks of NCA-pristine, NCA-20L, and NCA-20L@C are located at 854.9, 855.1, and 855.4 eV, respectively. The higher binding energy indicates an enhancement of the nickel-oxygen bond. When the Ni cations slide to the Li sites, they are reduced and show a decrease in the Ni–O bond length [36,54–55]. In the deconvoluted Ni 2p3/2 peaks, the Ni2+ content of 15.80% for NCA-20L@C is much lower than those of NCA-20L and NCA-pristine, which are 18.95% and 25.12%, respectively. The modification by LLTO@C sustained the spinel structure and suppressed the cation mixing [32,56].
Figure 5. (a) Evolution of the (003) diffraction peak of the cathodes before and after 200 cycles at 5 C. Deconvoluted XPS spectra of Ni 2p taken from the surface of (b) NCA-pristine, (c) NCA-20L, and (d) NCA-20L@C after 200 cycles at 5 C.
The SEM and HRTEM images of the NCA cathodes after cycling at 5 C were analyzed. Figs. 6(a) and 6(b) show the HRTEM image and corresponding fast Fourier transform (FFT) patterns of the regions in the bulk and surface of NCA, respectively, after 200 cycles at 5 C. In Fig. 6(a), the phase transition of NCA-pristine caused severe lattice damage that is continuous and forms a domain-boundary-like region. The spinel phase is maintained with a distinct lattice fringe spacing of 0.46 nm, which corresponds to the (111) crystalline plane of the H2 phase. The representative regions in Fig. 6(a) were observed (Fig. 6(a1)) and denoted regions I, II, and III. The damaged region (II) consists of serial edge dislocations (⊥) that form a dislocation wall. The formation of a dense dislocation wall is considered an important cause of the domain boundary propagation. The formation of domains in the ternary cathode materials remains under much investigation . This region is also a so-called “broken atomic region” according to Yang and Xia . This phenomenon originated from the accumulation of contraction and expansion of the unit cell volume, which is induced by the phase transition between H2 and H3 during repeated delithiation and lithiation. In Region III, faint bright spots, as shown by the yellow circles, are apparent between the crystalline planes, indicating that a large amount of TM ions (Ni2+) has diffused to the Li+ vacancy . Severe cation mixing leads to aggravated lattice distortion and structure degradation, which indicates the occurrence of the irreversible phase transition of H3. The lattice spacing narrows because of the insertion of the ordered TM atom layers in the interstitial Li sites, which is the reason for the generation of the rock-salt phase. As a result, in the surface region of NCA (Fig. 6(b)), the irreversible rock-salt phase (NiO) is formed. The formation of the NiO phase is a direct result of highly ordered cation mixing (III in Fig. 6(a1). The lattice distance is 0.21 nm, which corresponds to the (200)C crystalline plane of the rock-salt phase (Fm-3m). A continuous NiO phase “shell” on the NCA particles results in severe irreversible capacity loss in the NCA cathode, as reported previously [57–59].
Figure 6. HRTEM images and fast Fourier transform (FFT) patterns (insets) of NCA-pristine after cycling: (a) HRTEM image from a region inside the NCA particle; (a1) magnified image of the selected region of the rectangle in (a); (b) HRTEM image from the surface region of NCA. The “dl” refers to the lattice distance of NCA.
The structure of NCA in NCA-20L@C after high-rate cycling was observed using HRTEM and is shown in Fig. 7. Figs. 7(a) and 7(b) show the two typical regions from the interior of the NCA particle (Fig. 7(a)) to the surface (Fig. 7(b)). In Fig. 7(a), the well-ordered layered structure with a lattice distance of 0.47 nm is maintained, and no structural damage occurs in the bulk, which differs from the localized structure degradation of NCA-pristine (Fig. 6). The layered crystalline plane (003)R with an R-3m symmetry can be further verified by the corresponding FFT shown in the inset of Fig. 7(a). Fig. 7(b) shows the structure on the surface region of NCA in NCA-20L@C. The clear lattice fringes corresponding to the (111) plane of the spinel phase demonstrate that most regions maintain the spinel structure on the surface. The two broken atomic planes in Fig. 7(b1) resulted in the formation of adjacent edge dislocations (⊥). The lattice damage is limited to the scale of two atom layers. As seen in Figs. 6(a) and 6(a1), the phase transition (H2 and H3) in NCA-pristine caused severe lattice damage, where the dense edge dislocations form a domain boundary and propagate inside the NCA bulk. The cation mixing of Li–Ni is shown in Fig. 7(b2), as indicated by the yellow circles. The formation of Li+/Ni2+ mixing is limited within a scale of several sub-cells, and the resultant change in the lattice spacing occurs accordingly . A highly ordered cation mixing region is formed locally in the surface region (2−4 nm), as shown in Fig. 7(b3). In the selected region of Fig. 7(b3), the highly ordered occupation generates distorted rock-salt phase “pitting.” The lattice spacing varied from 0.22 to 0.24 nm, which corresponds to the (111) plane spacing of NiO. With the modification of LLTO@C, the generation of the rock-salt phase was limited and localized on the surface.
Figure 7. HRTEM images and the corresponding FFT (inset) obtained from the bulk (a) to the surface (b) of NCA in NCA-20L@C. Images (b1), (b2), and (b3) present magnifications of the three regions shown in image (b). The “dl” refers to the lattice distance of NCA.
A mechanism based on the coupling effect of the Li ions and electron conductivities is illustrated in Fig. 8. As required by the typical cycling reaction of Ni ions, the transport of Li+ and electrons is necessary for obtaining the full reversibility of Ni2+/3+. However, the transport of Li cations and electrons relies on different systems. In the cathode, PDVF binder provides a low conductivity to Li+, while the electrical conductivity depends on the discontinuous carbon components . The modification of the LLTO@C core-shell nanofibers in PVDF can set up a series of coupling pathways for Li+ and e−, as illustrated in Fig. 8. The blue and purple arrows indicate the conductivities of Li+ and electrons in the core and sheath of LLTO@C, respectively. During the lithiation, the supply of Li ions is boosted by LLTO, which helps Li+ to fully occupy the interstitial sites and promotes Ni cation migration back to the lattice sites . The charge of the Li ions can also be compensated by the external circuit because of their “rocking chair” effect. As indicated in Figs. 2 and 3, the LLTO@C improves the diffusion kinetics in the NCA cathode, which can promote the stabilization of the layer structure, as previously reported [39,61–63]. However, the TM cations must obtain charge compensation from the local environment. The carbon layer on LLTO fibers provides a continuous, electrically conductive path beyond dispersed carbon black, which helps to promote the Ni transition from the +2 state to the +3 state. As a result, NCA-20L@C tends to retain its initial layer structure, rather than the rock-salt state.