1.   Introduction

The depletion of traditional fuel resources and aggravation of environmental challenges have recently led to a boost in the development of new energy. The development of lithium ion batteries (LIBs), which possess a high specific capacity and working voltage, an excellent cycle stability, and environmental friendliness, has led to unprecedented opportunities for a new generation of electric vehicles and plug-in hybrid electric vehicles [1–6]. The capacity of cathodes has always been the decisive factor that hinders further improvement in the capacity of LIBs. In all types of current cathodes, the layered Ni-rich cathode LiNi_0.8Co_0.15Al_0.05O₂ (NCA) has attracted the attention of researchers because of its high specific power density, low cost, and toxicity, and it has gradually become the most promising cathode material [7–11]. However, the capacity attenuation caused by the poor structural stability of the material during cycling has hindered the further commercialization of NCA cathodes. The capacity loss is derived from the structure degradation from the layered structure (R-3m space group) to the spinel structure (Fd3m) and ultimately to the rock-salt structure (Fm-3m) [12–16]. The generation of the rock-salt phase is considered a severe factor resulting in a drastic performance fading. Meanwhile, the generation of oxygen gas from the surface of NCA directly accelerates the degradation of the structure and increases the explosibility [17].

The transportation of electrons and Li ions plays a crucial role during the process of establishing a fluent cycling from LiNiO₂ to Li_1−xNi^3+/2+O_2−y [18–19]. During the delithiation process, Ni ions would be oxidized from Ni³⁺ to Ni⁴⁺, which is thermodynamically metastable and could be easily reduced to Ni²⁺ [20−21]. The reduced Ni²⁺ would slide to the interstitial sites of NCA, resulting in cation mixing [22−23]. During the lithiation process, the localized cation mixing is reversible with the intercalation of Li⁺ and oxidation of Ni²⁺. If the transportation of Li⁺ or electrons is blocked, the transition of Li_1−xNi^3+/4+O₂ to Li_1−xNi^3+/2+O_2−y would not be fully reversible [19,24]. In such a case, the accumulated cation mixing will cause the generation of the rock-salt structure, which accounts for the capacity loss of NCA upon cycling [25]. The evolution of oxygen from the NCA lattice was caused by the insufficient compensation of Li ions and charge [17]. To address these problems, fast ion conductors have been used as active additions in the cathode. Coating methodologies are commonly adopted to enhance the performance of NCA. Various solid electrolytes or oxides, such as LiZr₂(PO₄)₃ [26], Li_0.5La_0.5TiO₃ [27], LiPON [28], Li₂O–ZrO₂ (LZO) [29], LiMnPO₄ [30], Li₂SiO₃ [31], Li₂MnO₃ [21], CeVO₄ [32], Co₃O₄ [33], Al₂O₃ [34], and SnO₂ [35] have been intensively applied to construct core-shell structures to provide a sufficient Li⁺ diffusion “shell” and solid “clothes” to support active materials. Alternatively, a full gradient cathode was developed by doping Li_6.75La₃Zr_1.75Ta_0.25O₁₂ (LLZTO), and a localized cation migration region was generated in NCA, which hindered the further distortion of the crystal structure and enhanced the capacity retention [36]. The construction of a full gradient cathode also proved the significance of optimizing the kinetics environment of the entire NCA cathode [27]. Lithium lanthanum titanate (Li_3xLa_2/3–xTiO₃, denoted LLTO) exhibits excellent Li⁺ conductivity, a wide electrochemical window and good thermal stability. Therefore, it has been investigated as a fast ion conductor oxide solid electrolyte. At the same time, it is a modifying material for enhancing the Li transport kinetics of the cathode [27,36]. The straight LLTO nanofiber can provide a continuous and rapid conductivity path of lithium ions. Theoretically, it is not sufficient to construct only a Li ion diffusion path in a cathode. However, reports on the coupling of ion and electron transport in a cathode are few. We are motivated to propose the application of LLTO@C nanofiber in the NCA cathode, which can achieve a synergy of Li⁺ and e− conductivity.

However, as indicated by the reaction process of Ni ions, similar to both sides of a coin, the reversibility is not only determined by the ion transport process but also by the electron process [18–19,37−39]. The investigation of the coupling mechanism between the transport of Li ions and electrons was not as sufficient as that of some novel types of batteries [40]. In this work, we propose a prototype cathode configuration to demonstrate the coupling mechanism of Li ions and electrons to enhance the reversibility via the addition of Li_0.35La_0.55TiO₃@C (LLTO@C) nanofibers to the cathode of NCA, by which the kinetics condition and phase transition process are regulated. As expected, the modified NCA cathode presents a profoundly enhanced electrochemical performance during high-rate cycling.

2.   Experimental

2.1   Synthesis of LLTO@C nanofibers

The LLTO nanofibers were prepared by electrospinning and subsequent calcination processes. A precursor consisting of lithium nitrite (LiNO₃, 2.01 mmol), lanthanum nitrite hexahydrate (La(NO₃)₃·6H₂O, 2.75 mmol), tetrabutyl titanate (Ti(OC₄H₉)₄, 5 mmol), and an appropriate amount of polyvinylpyrrolidone (PVP) was dissolved in a uniform solution containing a mixture of N, N-dimethylformamide (DMF, 12 mL), ethanol (C₂H₅OH, 10 mL), and glacial acetic acid (CH₃COOH, 4 mL). In particular, an excess of 15wt% LiNO₃ was provided to compensate for the Li loss during the sintering process. The precursor solution was stirred for 12 h until a pale yellow homogeneous solution was obtained. Then, the precursor solution was loaded into a 10 mL hypodermic syringe and injected from a stainless steel needle to a rolling collector (50 r/min) at a −5 kV voltage. A positive voltage of 15 kV was applied to the stainless steel needle. The syringe piston was pushed ahead at a rate of 1 mm/min. The spinning needle was controlled at a distance of 15 cm from the collecting drum. The as-spun nanofibers were collected and sintered at 850°C in air for 3 h at a heating rate of 5°C/min to remove the organic components and crystallize the LLTO. A typical example of this electrostatic spinning process is provided in the literature [41−43].

The prepared nanofibers and glucose (C₆H₁₂O₆) in a mass ratio of 1: 5 were placed in a Teflon-lined stainless steel autoclave with a 100-mL capacity that contained 60 mL of deionized water (DW). After vigorous stirring for 10 min, a suspension was obtained. The autoclave was sealed and treated at 180°C for 5 h in an oven. After heating, the suspension was centrifuged and cleaned with DW four times and desiccated at 80°C for 8 h. Finally, the as-obtained sample was sintered at 600°C for 2 h in a vacuum furnace (0.1 Pa) to achieve carbonization. The sample was denoted LLTO@C.

2.2   Battery assembly and electrochemical measurements

The electrochemical performance was measured in a CR2025-type coin cell. The as-prepared LLTO@C nanofibers were added to the polyvinylidene difluoride (PVDF) binder at a weight ratio of 0, 10wt%, or 20wt%. To prepare the cathode slurry, the active material LiNi_0.8Co_0.15Al_0.05O₂ (NCA, MTI Corporation, Hefei), carbon black (Super C65, TIMACAL), and the PVDF binder (Arkema, Kynar 761) were mixed uniformly in an N-methyl-2-pyrrolidone (NMP, Aladdin) solvent in a weight ratio of 8:1:1. The slurry was vigorously stirred for 12 h to guarantee its uniformity, and was cast onto an aluminum foil (MTI Corporation, Hefei). After that, the electrodes were desiccated in a vacuum at 80°C for 12 h to remove the residual NMP. The electrode films were punched into disks with a diameter of 12 mm, and further desiccated at 100°C for 12 h in a vacuum. For convenience, the as-prepared cathodes were named according to the weight ratio of LLTO@C in the PVDF binder: NCA-pristine (0), NCA-10L@C (10wt%), and NCA-20L@C (20wt%). Cathodes with 20 wt% LLTO nanofibers in PVDF without carbon layers were also prepared and were denoted NCA-20L. The CR2025-type coin cells were assembled in a glove box filled with argon. The as-prepared cathode, Celgard 2300 film as the separator, and lithium metal anode were pressed together in that order. An electrolyte solution with 1 mol of LiPF₆ dissolved in ethylene carbonate and ethylmethyl carbonate (EMC) (3:7 in volume) was used. Electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV) were performed with a VMP3B-20 electrochemical workstation. The EIS measurement frequency ranged from 100 to 100 mHz with an ac amplitude of 5 mV at room temperature (RT). CV tests with different scan rates from 0.02 to 2 mV/s were performed. The charge and discharge profiles were recorded in the windows of 2.7–4.3 V and 2.7–4.5 V, respectively, vs Li/Li⁺ at RT with a LANHE CT2001A battery system. All tested cells were activated at 0.1 C for 5 cycles (1 C = 200 mA·g⁻¹).

3.   Results and discussion

3.1   Characterization of the as-prepared LLTO@C nanofiber

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 Li_0.35La_0.55TiO₃ 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.

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3.2   Coupling conductive effect in the LiNi_0.8Co_0.15Al_0.05O₂ cathode

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 [26]. 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 (R_ct) of the electrochemical reaction, and the slope in the low-frequency region refers to the Warburg impedance (Z_w) 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 R_ct 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 [26]. 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.

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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.

Table  1.  Values of the potential interval for the NCA-pristine and NCA-20L@C samples V

Number	NCA-pristine	NCA-20L@C
∆1	0.22	0.19
∆2	0.03	0.02
∆3	0.04	0.01

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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 (I_p) and v^1/2 of NCA-pristine and NCA-20L@C.

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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 (v^1/2) and can be expressed by the Randles–Sevcik equation, from which the diffusion coefficient (D_Li+) of NCA-pristine and NCA-20L@C can be estimated [26,30,51]. The linear relationship between I_p and v^1/2, shown in Fig. 3(e), indicates that the reaction kinetics of the NCA cathode is controlled by the diffusion process [30]. As presented in Table 2, the calculated D_Li+ 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.

Table  2.  CV testing detail parameters used for NCA-pristine and NCA-20L@C samples

Sample	DLi+ (charge) / (cm2·S−1)	DLi+ (discharge) / (cm2·S−1)
NCA-pristine	1.714 × 10−10	2.079 × 10−11
NCA-20L@C	7.661 × 10−10	3.626 × 10−10

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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).

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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 2p_3/2 and Ni 2p_1/2. The Ni 2p_3/2 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 2p_3/2 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.

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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 [16]. This region is also a so-called “broken atomic region” according to Yang and Xia [21]. 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 [22]. 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 “d_l” refers to the lattice distance of NCA.

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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 [60]. 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 “d_l” refers to the lattice distance of NCA.

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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 Ni^2+/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 [61]. 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 [27]. 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.

Figure  8.  Schematic of the structure of the coupling mechanism of the co-conduction of Li ions and electrons in the NCA cathode.

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4.   Conclusion

The coupling effect of the Li cation and electron conductivities was revealed by introducing LLTO and LLTO@C into the PVDF binder of an NCA cathode. The EIS measurements before and after cycling demonstrate the promotion of the Li conductivity. Interestingly, the electrical conductivity of the NCA-20L@C cathode before cycling was almost identical to that after cycling. The surface diffusion coefficient of Li⁺ was improved. The EIS and CV curves verified the decreased polarization and enhanced diffusivity. By modification with LLTO@C, the capacity retention of the NCA-20L@C cell increased from 40.2%, which is the capacity retention of pristine NCA, to 68.9% at 5 C after 200 cycles. The XPS results confirm the suppression of the formation of Ni²⁺ on the surface of NCA in NCA-20L@C. The TEM images of NCA in NCA-20L@C demonstrate that cation mixing was alleviated and the structural deterioration was inhibited. The irreversible phase transition only occurred locally on the surface. In contrast, the pristine NCA showed a continuous rock-salt layer on the surface. In the interior of NCA, a broken lattice region formed, which was accompanied by the formation of dense dislocation walls, indicating a severe phase transition between H₂ and H₃. The coupling effect was emphasized by the change in the cathode’s electrochemical performance with the addition of LLTO@C and LLTO. The coupling of the Li ions and electrons in NCA cathodes provides a new approach for optimizing the kinetics of microenvironments, which is a promising parallel strategy to the “core-shell” structure.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (Nos. 51571182 and 51001091), the Fundamental Research Program from the Ministry of Science and Technology of China (No. 2014CB931704), the Program for Innovative Research Team (in Science and Technology) in University of Henan Province (No. 21IRTSTHN003). This work was also partially supported by the Provincial Scientific Research Program of Henan (No. 182102310815).