Rb-doped lithium titanates, Li_4-xRb_xTi₅O₁₂ (x = 0.010, 0.015, 0.020), were synthesized through a solid-state reaction using stoichiometric amounts of TiO₂ (≥99.8%, Aladdin Shanghai China), Rb₂CO₃ (≥99.8%, Aladdin Shanghai China) and Li₂CO₃ (≥ 99%, Aladdin Shanghai China) as Ti, Rb, Li sources. In a typical procedure, starting materials were mixed by ball milling (8 h, 350 r/min) in water/ethanol slurry and then dried by vacuum spinning at 60°C. The collected powder was further dried in air at 60°C overnight and calcined in muffle oven at 400°C for 2 h; then the temperature was further increased to 850°C for 4 h. The heating rate was kept at 5°/min. Neat Li₄Ti₅O₁₂ (x = 0) was prepared under the same conditions for comparison.

Crystal structure and chemical composition of the samples were studied by X-ray diffraction (XRD PANALITICAL, 2θ = 5-80°, V = 40 kV, I = 40 mA) and X-ray photoelectron spectroscopy (XPS ESCALAB 250 Thermo Fishing Scientific). Morphology and microstructure were observed using scanning electron microscopy (SEM Hitachi S-4800). The electronic conductivity of pristine powders was measured by a powder conductivity measurement system (FD-300) at 25°C.

The anodes were made by mixing Li_4-xRb_xTi₅O₁₂ (x = 0, 0.010, 0.015, 0.020), acetylene black and polyvinylidene fluoride (PVDF5130, Shenzhen Micro Electron Co., China) in a weight ratio of 8:1:1, using a suitable amount of N-methyl-2-pyrrolidone solvent (99.99%, Aladdin Industrial Corporation, China) to obtain a homogeneous slurry. The slurry was coated on a Cu foil substrate, dried at 60 °C, cut into 14 mm diameter circular disks and further dried at 110°C overnight. The active material loading level was 1.8 - 2 mg/cm². Coin half-cells (CR 2025) were assembled in Ar-filled glove box (H₂O, O₂ ≤ 0.1 ppm) using Celgard 2400 as separator, pure lithium foil as a counter electrode, and 1 M LiPF₆ EC:DMC:EMC v/v/v = 1:1:1 (Zhangjiagang Guotai-Huarong New Chemical Materials Co., China) as electrolyte. Full cells were assembled using commercial LiFePO₄ as cathode and Li_4-xRb_xTi₅O₁₂ (x = 0.010) as anode. The capacity of the full cell was calculated based on the LiFePO₄ mass. The mass of Li_3.99Rb_0.01Ti₅O₁₂ was adjusted to be slightly higher than LiFePO₄ mass. Galvanostatic charge/discharge was conducted at a Shenzhen Neware battery cycler (China) at various current rates in a voltage window between 1.0-2.5 V (vs. Li/Li⁺) for the half cells and 1.0-3.0 V for the full cells. Electrochemical impedance spectroscopy (EIS) was measured for the half cells using an IM6e electrochemical workstation (Zahner, Germany) in a frequency range between 10 mHz and 100 kHz with an amplitude of 5 mV.

Rb-doped LTOs with various doping levels (Li_4-xRb_xTi₅O₁₂; x = 0.010, 0.015, 0.020) were prepared via ball milling and subsequent solid-state reaction at 850°C. For comparison, neat Li₄Ti₅O₁₂ (x = 0) was synthesized under the same conditions. XRD patterns of Li_4-xRb_xTi₅O₁₂ (x = 0, 0.010, 0.015, 0.020) are displayed in Fig. 1a. Diffraction peaks at 2θ = 18.4°, 35.6°, 43.2°, 47.4°, 57.2°, 62.8°, 66.1°, 74.3°, 79.3° are attributed to (111), (311), (400), (331), (333), (440), (531), (533), (444) planes of Li₄Ti₅O₁₂ spinel (JCPDS#49-0207)^[5,42]. The magnification of (111) crystal panel peak at 18.4° shows the shift of the peak to the lower angle with the increase of doping level, indicating that Rb successfully incorporated into the LTO lattice, occupying Li 8a site, and increased its lattice parameter (Fig. 1b). XPS survey spectra of Li_4-xRb_xTi₅O₁₂ (x = 0, 0.010, 0.015, 0.020) identify O 1s, Ti 2p, Li 1s peaks of main elements (Fig. 1c). In addition to these elements, Rb 3d peak is detected for all Rb-doped samples Li_4-xRb_xTi₅O₁₂ (x = 0.010, 0.015, 0.020), indicating the successful doping of Rb⁺ in the structure^[23]. Fig. 1d shows the two peaks at 109.9 eV and 111.4 eV corresponding to Rb 3d_5/2 and Rb 3d_3/2. SEM shows that Li_4-xRb_xTi₅O₁₂ (x = 0, 0.010, 0.015, 0.020) particles all have similar morphologies with almost cubic shape and the particle size is in the range of 0.2-1.5 μm (Fig. 2). The electronic conductivity was measured using a powder conductivity apparatus at 25°C. The obtained values are 3×10^-8 S/cm, 11.2×10^-8 S/cm, 12.5×10^-8 S/cm, 17.5×10^-8 S/cm for Li₄Ti₅O₁₂, Li_3.99Rb_0.01Ti₅O₁₂, Li_3.985Rb_0.015Ti₅O₁₂, and Li_3.98Rb_0.02Ti₅O₁₂, respectively. Obviously, Rb doping enhances the electronic conductivity of LTO, which can be attributed to the increase of lattice parameter due to the replacement of Li⁺ atom by Rb⁺ with larger ionic radius^[12,37].

The initial charge-discharge curves of Li_4-xRb_xTi₅O₁₂ (x = 0, 0.010, 0.015, 0.020) were measured at a rate of 1 C in a voltage range of 1-2.5 V (Fig. 3a). All samples show a flat charge plateau at ~1.6 V. However, the polarization of Li_3.99Rb_0.01Ti₅O₁₂, characterized by the voltage difference ΔV between charge (V_ch) and discharge (V_dsch) plateaus, is smaller than Li₄Ti₅O₁₂, Li_3.985Rb_0.015Ti₅O₁₂, and Li_3.98Rb_0.02Ti₅O₁₂ (Table 1). Between all samples, Li_3.99Rb_0.01Ti₅O₁₂ (x = 0.010) delivers the initial charge/ discharge capacity of 161.2/175.9 mA∙h/g, which exceeds the capacity of neat LTO and Rb-doped samples with higher doping levels (x = 0.015, 0.020). To demonstrate the advantage of Rb doping on enhancing rate capability, samples were tested at different current rates from 1 C to 10 C (Fig. 3b). At the current rate of 1 C, the charge capacity is 150.0 mA∙h/g for Li₄Ti₅O₁₂ and 159.1 mA∙h/g, 133.4 mA∙h/g, 121.8 mA∙h/g for Li_3.99Rb_0.01Ti₅O₁₂, Li_3.985Rb_0.015Ti₅O₁₂, and Li_3.98Rb_0.02Ti₅O₁₂, respectively. With the increase of current density, the capacity decreases significantly to only 57.2 mA∙h/g, 62.3 mA∙h/g, 49.6 mA∙h/g, 45.2 mA∙h/g at 10 C, which corresponds to the capacity retention of 38.1%, 39.2%, 37.2% and 37.1% for Li₄Ti₅O₁₂, Li_3.99Rb_0.01Ti₅O₁₂, Li_3.985Rb_0.015Ti₅O₁₂, and Li_3.98Rb_0.02Ti₅O₁₂, respectively. The long-term cycling of Li_4-xRb_xTi₅O₁₂ (x = 0, 0.10, 0.015, 0.020) was performed at 1 C in a voltage range of 1-2.5 V (Fig. 3c). After 1 000 cycles, Li_4-xRb_xTi₅O₁₂ still maintained 97.5% (x = 0), 90.9% (x = 0.010), 83.9% (x = 0.015) of its initial capacity, indicating an excellent reversibility (vs. 2^nd cycle).

EIS can further clarify the effect of the doping level on the charge-transfer resistance (R_CT) and lithium diffusion coefficient (${{D}_{\text{L}{{\text{i}}^{+}}}}$). Measurements were carried out after 10 cycles (Fig. 3d). Nyquist plots consist of a semicircle in a high-frequency region and inclined line in the low-frequency region, corresponding to charge transfer resistance (and surface film formation) R_CT and lithium-ion transfer resistance, respectively.

Li_3.99Rb_0.01Ti₅O₁₂ exhibits the smallest semicircle and thus the lowest R_CT between all samples, inferring the smallest electrochemical polarization of Li_3.99Rb_0.01Ti₅O₁₂, which leads to the best cycling and rate performance.

The lithium diffusion coefficient ${{D}_{\text{L}{{\text{i}}^{+}}}}$ can be calculated from on EIS results in low-frequency region based on equations (1) and (2):

${{D}_{\text{L}{{\text{i}}^{+}}}}=\frac{{{R}^{2}}{{T}^{2}}}{2{{A}^{2}}{{n}^{4}}{{F}^{4}}{{\sigma }^{2}}C_{\text{L}{{\text{i}}^{+}}}^{2}}$ (1)

${Z}'={{R}_{\text{s}}}+{{R}_{\text{CT}}}+\sigma {{\omega }^{-0.5}}$ (2)

where R-gas constant, T-absolute temperature, A-contact area of the electrode, n-the amount of electrons per molecule, F-Faraday constant, ${{C}_{\text{L}{{\text{i}}^{+}}}}$-concentration of Li-ions, σ-Warburg coefficient associated with real impedance (${Z}'$) in low frequency (ω) region by equation (2), where R_s - electrolyte resistance. ${{D}_{\text{L}{{\text{i}}^{+}}}}$ is calculated to be 2.17×10^-9 cm²/s, 1.81×10^-9 cm²/s, 2.36×10^-10 cm²/s, 5.17×10^-10 cm²/s for Li₄Ti₅O₁₂, Li_3.99Rb_0.01Ti₅O₁₂, Li_3.985Rb_0.015Ti₅O₁₂, Li_3.98Rb_0.02Ti₅O₁₂, respectively. Li_3.99Rb_0.01Ti₅O₁₂ has the highest lithium diffusion coefficient, which is favorable for electrode kinetics, while the values for Li_3.985Rb_0.015Ti₅O₁₂ and Li_3.98Rb_0.02Ti₅O₁₂ are even lower than for neat Li₄Ti₅O₁₂.

Li_3.99Rb_0.01Ti₅O₁₂ shows the highest specific capacity, smallest polarization, and best rate performance among all LTO samples. This indicates that the optimal level of Rb-doping is x = 0.010, while the effect of higher doping amounts (x = 0.015, 0.020) is adverse. Since the capacity of the cell is coming from the number of lithium ions that are involved in charge/discharge process, it can be seen that at higher Rb-doping levels, a significant amount of lithium ions are eliminated from cycling, which affects the specific capacity values. On the top over that, the excessive doping hampers lithium ions transport^[18].

The fundamental difference between monovalent and heterovalent atom doping is enclosed in the charge distortion, which it introduces into lithium titanate structure. Atoms with the valence if II, III, V, VI cause a partial transition of insulating Ti⁴⁺ to conductive Ti³⁺, enhancing the electronic conductivity. At the same time, the majority of investigated dopants have larger ionic radii, than Li⁺ or Ti⁴⁺, those sides they occupy. It results in the expansion of the lattice parameter, and thus, facilitates lithium ion transport. Dopants with the valence of I can not introduce the charge distortion. Thus, the enhancement in electrochemical performance for Li_3.99Rb_0.01Ti₅O₁₂ is mainly attributed to the improvement of lithium diffusivity due to the increased lattice parameter. Regarding to higher doping levels (x = 0.015, 0.020), lithium transport can be hampered due to the excessive amount of Rb forming impurities in the structure of LTO.

To evaluate the potential application of Li_3.99Rb_0.01Ti₅O₁₂ electrode, a full cell with a Li_3.99Rb_0.01Ti₅O₁₂ as anode and commercial LiFePO₄ as cathode was tested. Cells were cycled between 1 and 3 V at the current density of 0.1 C (1^st, 2^nd cycle), 0.2 C (3^rd, 4^th cycle), and 0.5 C (from 5^th cycle onwards). Galvanostatic charge-discharge curves of Li_3.99Rb_0.01Ti₅O₁₂//LiFePO₄ have a very flat discharge plateaus at 1.8 V at various current rates (1^st - 0.1 C, 3^rd - 0.2 C, 5^th - 0.5 C), implying an ideal battery system^[15,43] (Fig. 4a). Li_3.99Rb_0.01Ti₅O₁₂//LiFePO₄ cell delivers an initial charge/discharge capacity of 167.0/145.7 mA∙h/g with a coulombic efficiency of 87.2%. The plateau specific discharge capacity in the 1^st cycle (at 0.1 C) is 139.7 mA∙h/g, which is 95.9% of its total initial discharge capacity (145.7 mA∙h/g). In the 3^rd cycle (at 0.2 C) plateau discharge capacity retains at 139.7 mA∙h/g, maintaining 96.5% of total discharge capacity (144.7 mA∙h/g). When the current density reaches 0.5 C, the plateau discharge capacity decreases to 126.4 mA∙h/g, still delivering 87.8% of 5^th discharge capacity (144.0 mA∙h/g). Fig. 4b demonstrates a long term cycling performance of Li_3.99Rb_0.01Ti₅O₁₂//LiFePO₄ at 0.5 C. After 150 cycles Li_3.99Rb_0.01Ti₅O₁₂//LiFePO₄ still delivers 113.8/113.5 mA∙h/g, corresponding to the capacity retention of 78.8%.