Balancing the Ion/Electron Transport of Graphite Anodes by a La-Doped TiNb₂O₇ Functional Coating for Fast-Charging Li-Ion Batteries

Abstract

A functional coating layer (FCL) is widely applied in fast-charging lithium-ion batteries to improve the sluggish Li⁺ transport kinetics of traditional graphite anodes. However, blindly increasing the Li⁺ conductivity for FCL reduces the overall electron conductivity of the anodes. Herein, we decoupled the effect of La-doping on TiNb₂O₇ (TNO) in terms of the phase evolution, Li⁺/electron transport, and lithiation behavior, and then proposed a promising La_0.1TNO FCL with balanced Li⁺/electron transport for a fast-charging graphite anode. By optimizing the doping concentration of La, more holes are introduced into the TNO as electron carriers without causing lattice distortion, thus maintaining the fast Li⁺ diffusion channel in TNO. As a result, the graphite with La_0.1TNO FCL delivers an excellent capacity of 220.2 mAh g^–1 (96.3% retention) after 450 cycles at 3 C, nearly twice that of the unmodified one.

The widespread application of electric vehicles (EVs) with lithium-ion batteries (LIBs) has been restricted by their time-consuming charging process.^1,2 Anode polarization, electrolyte side reaction, and mechanical cracks significantly increase the risk of battery degradation under high-rate charging.³⁻⁵ Meanwhile, unsafe Li plating will occur due to the low operating potential (0.1 V vs Li/Li⁺) and high energy barrier of Li-ion (Li⁺) intercalating and diffusing of the graphite (Gr) anodes.⁶ Therefore, rapid capacity and lifetime fading are generally inevitable and irreversible at a high charging rate.^7,8

Numerous strategies for overcoming the fast-charging issue of Gr have been proposed, such as shortening the diffusion path,^9,10 expanding the graphite layer,^11,12 and interface modification.¹³⁻¹⁵ Among them, interface modification via a functional coating layer (FCL) effectively improves the operating potential and reduces the Li⁺ transport barrier of the anodes. Meanwhile, it is necessary to isolate the anode material from electrolytes and regulate interface reactions effectively. Sun et al. construct an ultrathin S-bridged phosphorus layer on a Gr surface, which in situ converts to crystalline Li₃P-based FCL with a fast Li⁺ diffusion capability and lower Li⁺ desolvation barrier.¹⁶ Besides, Zhang et al. improved the Coulomb efficiency of spent graphite by the surface modification of a TiNb₂O₇ (TNO) nanolayer.¹⁷ However, previous studies mainly focused on enhancing the Li⁺ conductivity of the coating layers, while ignoring the importance of electronic conductivity.

Maximizing the Li⁺ conductivity often is accompanied by sacrificing the intrinsic electronic conductivity of the coating materials. Due to the unbalanced Li⁺/electron transport, excess Li⁺ or electrons will accumulate at the Gr surface, leading to the reduction of electrolytes, enlarged polarization, and Li plating, eventually causing capacity fading.¹⁸ Therefore, the critical challenge in designing an FCL of Gr anodes is to have high compatibility in both Li-ion and electron transport behavior.

In this work, we proposed a La-doped TNO (La_xTNO) as an FCL with decoupled Li⁺/electron transport to broaden the Li⁺ diffusion channels and maintain an optimized electron conductivity for a fast-charging Gr anode. The results indicate that the La_0.1TNO FCL started the Li⁺ intercalation at a higher potential (∼1.6 V vs Li⁺/Li), accelerating the Li⁺ diffusion toward the surface of Gr. Then, the pervasive electron conductive network facilitates electrical contact among the Gr particles inside the anode, thereby completing the fast intercalation and transfer of Li⁺ into the bulk phase of Gr. This work employed nanoengineering technology to better advance the usefulness of nano-TNO materials as an FCL, which provides a new idea for the fast charging of graphite.

Theoretical Simulations of M_x-TNO

TiNb₂O₇ (TNO) with fast Li⁺ diffusion channels along the b-axis direction can be deemed as an FCL material for fast-charging graphite (Gr),¹⁹ but the electronic insulation of TNO needs to be overcome. Principle-based computation indicates that pristine TNO is electronically insulated with a wide bandgap of 1.81 eV. To improve its electron conductivity, different elements with valences varying from +1 to +6 are selected as dopants to replace the Ti element (Figures 1a, b, and S1). Based on the balance between Li⁺ and electron conductance, La is chosen as a promising candidate for the chemical-modified TNO. Replacing Ti⁴⁺ with La³⁺ in the TNO framework triggers the generation of holes. It introduces intermediate states in between the band gaps, indicating the enhancement of their electron conductance.^20,21

Figure 1.

(a) Calculated bandgap width of TNO doped with different elements. (b) Ionic radii of TNO component and doping elements. (c–f) Calculated TDOS-PDOS and (g–j) optimized structure models of La_x-TNO (x = 0, 0.05, 0.1, and 0.2).

These intermediate states reduce the band gap into two small energy gaps, reducing the required energy for the electron hooping from the valence band to the conduction band (Figure 1c–f). The comparison in the first/second energy gaps indicates that the electron conductivity reaches the maximum when x of the La_xTNO is 0.1. The doping concentration also influences the Li⁺ conductance in TNO. With a much larger ionic radius of La³⁺ (1.032 Å) than that of Ti⁴⁺ (0.605 Å), La atoms help to expand the lattice spacing of TNO and reduce the resistance of Li⁺ intercalation and deintercalation.²² Meanwhile, the Li⁺ conductivity in TNO severely declines once the lattice distortion is too large. Higher La doping concentration (x = 0.2) severely disturbs the structural integrity (Figure 1g–j), narrowing the Li⁺ diffusion channel in TNO. Therefore, doping La in TNO and optimizing the doping amount are expected to obtain an FCL to balance the Li⁺/electron transport for fast-charging batteries.

Structure of Gr@La_xTNO

Mesocarbon microbeads (MCMBs), an artificial Gr,²³⁻²⁵ are used in this work to explore the ion/electron transport balance of the La_xTNO FCL. The Gr@La_xTNO (x = 0, 0.05, 0.1, and 0.2) composites were synthesized by a one-step hydrothermal method (Figure S2), such as building an FCL to improve the rate and cycling performance of the graphite. The micromorphologies of the bare Gr (Bare-Gr) and Gr@La_xTNO were investigated to confirm the structure of FCL on the Gr. As shown in Figures 2a, b, and S3, dense La_xTNO particles are covered on the Gr surface, and there is no particle shedding after coating on the Cu current collector, exhibiting excellent stability of the La_xTNO FCL. Moreover, the transmission electron microscopy (TEM) image of Gr@La_xTNO exhibits highly dispersed ultrafine nanoparticles with an average size of ∼30 nm (Figure 2e and S4).

Figure 2.

(a, b) SEM images of the Gr@La_0.1TNO. (c, d) HR-TEM images of the Gr@La_0.1TNO. (e) TEM image and (f–h) corresponding EDX mapping of the Gr@La_0.1TNO. (i) XRD patterns of Bare-Gr and Gr@La_xTNO (x = 0, 0.05, 0.1, and 0.2). (j) TG-DSC curves of Gr@La_0.1TNO. (k) High-resolution XPS spectra of the Bare-Gr and Gr@La_xTNO (x = 0, 0.05, 0.1, and 0.2).

The high-resolution TEM (HR-TEM) image reveals that La_0.1TNO nanoparticles have high crystallinity and noticeable lattice fringes with the interplanar spaces of 0.404 nm, corresponding to the (110) plane of TNO (Figure 2c and d). Compared with pristine TNO, La doping expands the lattice spacing of the TNO, and Gr@La_0.2TNO has the largest distortion among all La-doped FCL (Figure S5), which is in line with the DFT results. Unexpectedly, a strain effect at the interface between La_0.1TNO and Gr results in lattice stretching of the Gr(002) plane for 0.336 to 0.363 nm.²⁶ This impact will somewhat facilitate initial Li⁺ intercalation on the graphite surface. In addition, the energy-dispersive X-ray spectroscopy (EDX) mapping confirms the existence of La. It shows that Nb, Ti, and La are homogeneously distributed on the Gr@La_xTNO surface (Figures 2f–h and S4).

X-ray diffraction (XRD) measurements were performed to reveal the crystal structures of Bare-Gr and Gr@La_xTNO (Figure S6). Due to the ultrathin La_xTNO FCL on the Gr surface, all XRD peaks of both the Bare-Gr and Gr@La_xTNO are in accordance with the standard pattern of graphite (JCPDS 41-1487). By limiting the 2-theta at the range of 30–50° (Figure 2i), the peaks of monoclinic crystalline TiNb₂O₇ (JCPDS 77-1374) appear in Gr@La_xTNO samples. The peaks of TNO shift to lower angles with an increase in La-doped concentration, corresponding to the expansion of crystalline lattices. Raman spectra of TNO, La_0.1-TNO, and Gr@La_xTNO were also performed to analyze the structure of the La_xTNO FCL (Figure S7). The peaks at 999/894 cm^–1 and 652/544 cm^–1 are attributed to the vibrations of the edge/corner-shared NbO₆ and TiO₆ octahedrons, respectively. Based on these results, the La³⁺ could be confirmed to be in situ-doped into the TNO lattice. With the combination of the thermogravimetric (TG) and inductively coupled plasma (ICP) analysis (Figures 2j and S8 and Table S1), it was found that the mass loading of La_xTNO is about 2 wt %, and the content of each element is consistent with the material ratio. Accordingly, a thin and uniform FCL consisting of La_xTNO was successfully coated onto the Gr particles.

The chemical environment and valence states of elements in La_xTNO are studied by X-ray photoelectron spectroscopy (XPS). In Figure S9, the peaks located at 465.2 and 459.4 eV contribute to Ti 2p 1/2 and 3/2, respectively. The peaks of Nb 3d 3/2 and 5/2 are located at 210.4 and 207.7 eV, respectively. These peaks shift to lower binding energies in Gr@La_xTNO, indicating that the Ti⁴⁺/Nb⁵⁺ bonds are reduced by the introduction of La. The high-resolution XPS spectra of O 1s (Figures 2k and S10) show that the peak located at ∼530.8 eV contributes to the metal–oxygen bond in Gr@La_xTNO, and the broad peak at ∼532.2 eV should be assigned to the oxygen-deficient region.²⁷ As the La doping occurs, the oxygen vacancy concentration increases gradually. The electron paramagnetic resonance spectra display a much stronger symmetrical signal at g = 2.003 (Figure S11), corresponding to the unpaired electrons at oxygen vacancy sites. According to the formula of mobility (eq S1), the increase of hole concentration promotes electron conductivity in Gr@La_xTNO.

Electrochemical Performance of Gr@La_xTNO

CR-2032-type half-cells were used to study the electrochemical performances of the Gr@La_xTNO. The rate performances of Bare-Gr and Gr@La_xTNO are shown in Figure 3a. The Bare-Gr cell shows average capacities of 330.3, 263.0, 192.4, 134.1, 81.4, and 63.5 mAh g^–1 as the rate rises from 0.2 to 5 C. By contrast, the average capacity of the Gr@La_0.1TNO under the same rates is all higher than that of the Bare-Gr. The capacity retention of the Gr@La_0.1TNO during high-rate cycling is nearly twice that of Bare-Gr, exhibiting the effective enhancement of fast-charging performance of the Gr anode with La_0.1TNO FCL. Moreover, the Gr@La_xTNO doped with different amounts of La shows rate performance between Gr@La_0.1TNO and Bare-Gr, whereas the Gr@TNO has the lowest capacity output (Figure S12).

Figure 3.

(a) Rate performance and (b) EIS of the half-cells using Bare-Gr and Gr@La_xTNO (x = 0, 0.05, 0.1, and 0.2). (c) CV curves of the Gr@La_0.1TNO at different sweep rates. (d) Li⁺ diffusion coefficient of the Bare-Gr and Gr@La_xTNO (x = 0, 0.05, 0.1, and 0.2) obtained via GITT tests. Insets show the structures and calculated E_f of one Li-atom insertion into La_x-TNO (x = 0 and 0.1). (e) E_a derived from R_ct and R_SEI of the half-cells using Bare-Gr and Gr@La_xTNO (x = 0, 0.05, 0.1, and 0.2). (f) Cycling stability of the half-cells using Bare-Gr and Gr@La_xTNO (x = 0, 0.05, 0.1, and 0.2). (g) Voltage profiles of the half-cells using Gr@La_0.1TNO at 1 C.

The electrochemical impedance spectra (EIS) of the half-cells were collected, as shown in Figure 3b. The interfacial resistance of the Gr@TNO (154.3 Ω) is larger than that of the Bare-Gr (123.6 Ω), as the insulation of TNO FCL enhances the electron transfer barrier among the Gr particles. With the La doping, the resistance of the cell decreases rapidly due to the improved electron conductivity of Gr@La_xTNO. The resistances for Gr@La_0.05TNO, Gr@La_0.2TNO, and Gr@La_0.1TNO are 113.6, 93.4, and 82.4 Ω, respectively, indicating that 10% La doping is the most optimized to the Li⁺ kinetics under fast-charging.

The initial cyclic voltammetry (CV) curves of the Bare-Gr and Gr@La_xTNO between 0.01 and 3 V (vs Li⁺/Li) with variable sweep rates are shown in Figures 3c and S13. For Gr@La_xTNO, the typical pair of reduction/oxidation peaks at ∼1.60/1.72 V can be assigned to the redox pair of Nb⁵⁺/Nb⁴⁺. The peaks with broad shoulders at ∼1.8 and 1.9 V correspond to the Ti⁴⁺/Ti³⁺ redox couple. Therefore, the lithiation of La_xTNO FCL can occur at an operating potential higher than that of Bare-Gr. In Figure S14, the La_xTNO FCL released a small capacity at ∼1.6 V. In addition, both the Bare-Gr and Gr@La_xTNO have significant redox peaks at a lower potential, corresponding to the Li⁺ de/intercalation in Gr. Meanwhile, Gr@La_0.1TNO exhibits a small profile change when the peak potential ranges from 0.2 to 1.0 mV S^–1, showing low electrochemical polarization.

The value of D_Li+ in different anodes is obtained by galvanostatic intermittent titration technique (GITT) results (Figure S15) and Fick’s Second Law (eq S3). At the beginning of discharging (Figure 3d), the capacity is mainly provided by the lithiation of the Gr@La_xTNO. During this time, the D_Li+ of Gr@La_0.2TNO is greater than other Gr@La_xTNO samples at high voltage due to the wider lattice spacing of Gr@La_0.2TNO, which can absorb Li⁺ easily. However, the D_Li+ of Gr@La_0.2TNO gradually reduced as the discharge reduced, which can be attributed to structural distortion of the TNO after La overdoping. The high formation energy (E_f) of a single Li atom inserting into the TNO also reflects that the Li ions are strongly bound by the crystal lattice; thus, the lithiated TNO has a relatively low D_Li+ (eq S2). When at 15–30% state of charge (SOC), Gr@La_0.1TNO has the highest D_Li+ as 5.5 × 10^–10 cm² s^–1, echoing the smaller E_f for the lithiated La_0.1-TNO. The following are Gr@La_0.05TNO, Gr@La_0.2TNO, Gr@TNO, and Bare-Gr, indicating that the lithiated La_0.1TNO with high Li⁺ flux greatly enhances the Li⁺ transport kinetics on the Gr interface. It is worth mentioning that nano La_x-TNO particles have a high crystallinity, making it difficult to release Li capacity (Figure S16).

The activation energies (E_a) for Li⁺ across the solid electrolyte interphase (SEI) and Li⁺ desolvation process are calculated by temperature-dependent EIS tests (Figure S17). Based on the Arrhenius eq (eq S4), the two kinds of E_a in Gr@La_0.1TNO are close and have the lowest values (54.2 and 52.7 kJ mol^–1, respectively) (Figures 3e and S18). This indicates that La_0.1TNO FCL not only harmonizes the competition of Li⁺/electron transport inside the anode but also facilitates the formation of a high-performance SEI, strongly supporting the fast charging of Gr@La_0.1TNO.

Figure 3f shows the cycling performance of the Gr@La_xTNO at 1 C. With the La_xTNO FCL, the Gr delivers a high reversible capacity (300.6 mAh g^–1) and reaches a 97.7% capacity retention after 300 cycles, outperforming the counterparts. The polarization of the Gr@La_0.1TNO cell decreases with cycles (Figure 3g) since the La_0.1TNO FCL featured by the balanced Li⁺/electron transport and the hole-type conduction greatly improve the interface stability of the graphite. Therefore, when the rate increases to 3 and 5 C, the long-term cycling of Gr@La_0.1TNO is more excellent than that of the other control anodes (Figures S19 and 20).

Analysis of Li⁺/Electron Transport

The morphology and surface chemistry of the anodes before and after 5 C rate cycling were investigated. In Figures 4a–d and S21, the electrolyte-derived SEI on the Gr@La_0.1TNO surface after 100 cycles at 5 C is more uniform than that on other samples, indicating that there is no obvious local current density formed on La_0.1TNO under fast-charging. In-depth XPS spectra were further conducted to determine the SEI under La_0.1TNO FCL. From the O 1s XPS spectra (Figure 4e and g), the reduced intensity of the C–O peak at 532.5 eV accompanied by the emerged metal–O (La_0.1TNO) peak at 529.6 eV demonstrates the lower thickness of the SEI formed on Gr@La_0.1TNO, as compared to the SEI on Bare-Gr. Similarly, the XPS spectra of the cycled Gr@La_0.1TNO also show a lower F concentration compared to the cycled Bare-Gr (Figure 4f and h), further proving the inhibition of the La_0.1TNO FCL on electrolyte decomposition (Figure S22).

Figure 4.

SEM images of Gr@La_0.1TNO (a) before and (b) after 100 cycles. The corresponding EDX mapping of (c) C and (d) F on a cycled Gr@La_0.1TNO surface. High-resolution XPS spectra of (e) the spectral value of O 1s and (f) the value of F 1s for cycled Bare-G. High-resolution XPS spectra of (g) O 1s and (h) F 1s for cycled Gr@La_0.1TNO. 3D TOF-SIMS images of (i) C^– and (j) F^– for cycled Bare-G. 3D TOF-SIMS images of (k) C^– and (l) F^– for cycled Gr@La_0.1TNO. In situ XRD patterns of the Gr@La_0.1TNO during the initial discharging/charging process at (m) 0.1 and (n) 1 C.

Detailed information about SEI distribution can be seen in the time-of-flight secondary ion mass spectrometry (TOF-SIMS) results. Figure 4i–l recorded the 3D (70 × 70 μm) images of the anodes after 4 cycles at 3 C. Compared to the Bare-Gr, the distribution of F on the Gr@La_0.1TNO surface is more uniform and mainly concentrated in the inner SEI. But for Bare-Gr, its thermodynamically unstable SEI is always in a cycle of dissolution and regeneration during the high-rate operations, thus exposing more obvious LiF on the SEI surface, instead of the solvent-derived organic components, as shown in the 3D TOF-SIMS image. In addition, due to the unbalanced Li⁺/electron conductivity of the TNO FCL, the SEI formed on Gr@TNO is also uneven and fragile (Figure S23).

The in situ XRD characterization was applied to verify the structure evolution of graphite with and without La_0.1TNO protection. As displayed in Figure 4m, the XRD pattern of the Gr@La_0.1TNO presents a good reversible phase evolution of graphite in the initial cycle at 0.1 C, in which the lithiation process can be clearly divided into four typical stages containing LiC₂₄–LiC₁₈–LiC₁₂–LiC_6.²⁸ Because the sluggish Li⁺ reaction kinetics stemmed from the lack of the electron transport network among each Gr@TNO particle, the phase evolution of Gr@TNO is slower than that of Bare-Gr and Gr@La_0.1TNO (Figure S24). Meanwhile, a diffraction peak attributed to Li_y-TNO appears in the XRD pattern of Gr@TNO, indicating that the lithiation of TNO occurs easily but is irreversible. Furthermore, the phase evolution of Gr@La_0.1TNO under 1 C-rate cycling was also detected in Figure 4n. Although both Bare-Gr and Gr@La_0.1TNO exhibit excellent phase reversibility, the faster phase evolution of Gr@La_0.1TNO from LiC₂₄ to LiC₁₂ in the voltage range of 0.1–0.2 V (vs Li/Li⁺) indicates its higher rate response and performance.

Rate Performance of Full-Cells

The full-cells using the LiNi_0.6Co_0.2M_0.2O₂ (NCM622) cathode and different anodes assembled at a negative to positive areal capacity (N/P) ratio of 0.5. A low N/P ratio can compensate for the slow Li⁺ kinetics of the cathode. The calculation of the specific capacity of the cells is based on the mass loading of the NCM622. In Figure 5a, the Gr@La_0.1TNO||NCM622 cell presents average capacities of 126.6, 108.6, 91.9, 77.4, and 63.5 mAh g^–1 at 1, 2, 3, 4, and 5 C, respectively. As to the Bare-Gr||NCM622 cell, the average capacity delivered at the same rate is lower than that of the cell using Gr@La_0.1TNO. The long-term cycling of the different full-cells is compared in Figure 5e. At 3 C, the Gr@La_0.1TNO cell shows a 76.3% capacity retention after 200 cycles, while the capacity retention for the Bare-Gr cell over 100 cycles is merely 38.2%.

Figure 5.

(a) Rate performance of the Bare-Gr||NCM622, Gr@TNO||NCM622, and Gr@La_0.1TNO||NCM622 full-cells under a N/P ratio of 0.5. (b) Potential changes of the Bare-Gr and Gr@La_0.1TNO in the three-electrode cell testing. SEM images of (c) Bare-Gr and (d) Gr@La_0.1TNO after 100 cycles in the full-cells. (e) Long-term cycling of the NCM622 full-cells using Bare-Gr, Gr@TNO, and Gr@La_0.1TNO at 3 C.

A three-electrode cell was assembled to monitor the potential changes in the anodes working in the full-cells (Figure 5b). The results demonstrate that under the same conditions, the La_0.1TNO FCL can delay the occurrence of Li plating until the graphite is completely lithiated. By contrast, the Li plating occurs more easily on the Bare-Gr surface due to the larger polarization (Figure 5c–d). The SEM images of the cycled anode obtained from the full-cells show that the interface of Gr@La_0.1TNO is clearer than that of Bare-Gr, whose interface was covered by a large amount of dendrite-like dead Li after 100 cycles. The above results further demonstrate the beneficial effects of the La_0.1TNO FCL on balancing the ion/electron transport and promoting the charge exchange process of Li⁺, thus improving the fast-charging performance and protecting the anode without Li plating hazards.

In this work, the La_0.1TNO FCL with balanced transport of Li⁺/electron on the graphite anode can greatly improve the fast-charging performance and cycling stability of the LIBs. According to the guidance of theoretical calculations, the TNO was doped by 10% La, which not only improved its electronic conductivity but also expanded the Li⁺ diffusion channels with the large ionic radius. Moreover, the La_0.1TNO FCL improved the kinetics of Li⁺ intercalation in the graphite interface and interparticle, effectively suppressing the capacity attenuation arising from enhanced polarization during the fast-charging process. Therefore, Gr@La_0.1TNO exhibits a reversible capacity of 220.2 mAh g^–1 and 96.3% capacity retention after 450 cycles at 3 C, outperforming the control anodes. The conception of La_0.1TNO FCL on tuning the Li⁺/electron transport is also applicable to other anode or cathode materials when they are facing the fast-charging challenge.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.nanolett.3c05151.

• Materials preparation, characterization, cell preparation, electrochemical measurement, theoretical calculation details, and additional details on XPS spectra, voltage profiles, EIS curves, in situ XRD spectra, TOF-SIMS depth profiles, TEM images, and SEM images (PDF)

Author Contributions

^# Yeliang Sheng, Xinyang Yue, and Wei Hao contributed equally to this work. Yeliang Sheng, Yakun Liu and Zheng Liang designed the research; Yeliang Sheng performed the research with assistance from Wei Hao and Yongteng Dong; Wei Hao analyzed the data; Yeliang Sheng and Xinyang Yue wrote the paper with the assistance from all authors; and Zheng Liang conceived and supervised the research.

This work was supported by the National Natural Science Foundation of China (NSFC) under Grant Nos. 52102282 and 22379093, the Fundamental Research Funds for the Central Universities (22X010201631 and 23X010301599), and the funds from CATL Future Energy Research Institute under Grant No. 22H010102023.

The authors declare no competing financial interest.