K-doped Li₂ZnTi₃O₈/C as an efficient anode material with high performance for Li-ion batteries

Abstract

Li₂ZnTi₃O₈/C and Li_1.9K_0.1ZnTi₃O₈/C were successfully synthesized using the sol–gel method. Doping K apparently yielded a wider tunnel, helpful for increasing the rate of transport of lithium ions, and furthermore yielded excellent electrochemical properties. The first discharge capacity for Li_1.9K_0.1ZnTi₃O₈/C was 352.9 mA h g⁻¹ at a current density of 200 mA g⁻¹. Li_1.9K_0.1ZnTi₃O₈/C also performed stably, retaining a capacity of 323.7 mA h g⁻¹ at the 100th cycle, indicative of its excellent cycling properties. In the rate performance test, Li_1.9K_0.1ZnTi₃O₈/C showed at the first cycle a high discharge capacity of 379.5 mA h g⁻¹ for a current density of 50 mA g⁻¹ and a capacity of 258.9 mA h g⁻¹ at 1000 mA g⁻¹. The results indicated that K-doping should be considered a useful method for improving electrochemical performances.

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1 Introduction

Lithium-ion batteries (LIBs) are effective batteries because of their high specific energy levels, small volumes, long cycling lifetimes and high working voltages,^1–3 and have been used in diverse applications. The anode material, as one of the important components of the LIB, directly affects the electrochemical performance of the battery. Cubic Li₂ZnTi₃O₈ (LZTO) has been investigated as a potential anode material due to its lack of toxicity, high safety, low cost, relatively high theoretical capacity and low discharge voltage plateau of approximately 0.5 V (vs. Li/Li⁺).^4–6 But its poor electronic conductivity has restricted its practical applications.^7–10 Therefore, finding methods to improve the electronic and ionic conductivities of LZTO is extremely important.

To overcome such defects, methods such as carbon coating, improving the preparation process, and doping diverse ions have proved effective.^11–22 Carbon materials have great advantages, such as low price, high natural abundance, excellent conductivity, low amounts required for using them to modify other materials, stable physical and chemical properties, etc., which could not only endow the matrix materials with high conductivity, but also reduce the amounts of side reactions between electrode materials and electrolyte. Therefore, carbon inclusion is considered a convenient modification method to improve the electrical conductivity of electrode materials.^20,23,24 Producing small, i.e., nano-sized, particles of lithium ions can promote the increase of charge–discharge time, and is also conducive to the electrolyte infiltration of the material.²⁵ The ionic conductivity and electronic conductivity of LZTO can be improved by doping it with ions, and the location and amount of doping affects the electrochemical properties of electrode materials.^11,26–28 The ions currently used for doping lithium zinc titanate mainly include Na⁺,²⁹ Ag⁺,³⁰ Al³⁺,³¹ Ce⁴⁺,¹⁰ and V⁵⁺.³²

Na⁺ is considered a promising doping element due to its high abundance and environmental friendliness. Chen et al.²⁹ has synthesized Na⁺-doped Li₂ZnTi₃O₈ using the solid state method. Their experiments showed the capacity of Li_1.95Na_0.05ZnTi₃O₈ reaching as high as 267.3 mA h g⁻¹ at the 50th cycle when using a current density of 0.1 A g⁻¹. Of the materials they tested, it was shown to display the highest reversible capacity, mainly due to the diameter of Na⁺ being greater than that of Li⁺, thus enlarging the diffusion channel of Li⁺.

Tang et al.³¹ used the solid phase method to add Al₂O₃ into Li₂ZnTi₃O₈ to realize the doping with Al³⁺. Their experimental results showed capacity retentions of 99.8%, 91.6%, 86.6% and 93.4% obtained for Li₂ZnTi_2.9Al_0.1O₈ at the 100th cycle for current densities of 0.5 A g⁻¹, 1.0 A g⁻¹, 2.0 A g⁻¹ and 3.0 A g⁻¹, respectively. Therefore, the doping of Al³⁺ was found to significantly improve the capacity and rate performance of Li₂ZnTi₃O₈.

Yi et al.³² used V₂O₅ as a raw material to synthesize V⁵⁺-doped Li₂ZnTi₃O₈ by carrying out a simple solid-phase method in one step. V⁵⁺ was found to replace some of the Li⁺ ions and in this way improve the diffusion rate of lithium ions. The reversible capacities of Li_1.95V_0.05ZnTi₃O₈ at rates of 0.2, 1.0, 2.0 and 5.0C were measured to be, respectively, 213.3, 171.2, 132.5 and 84.7 mA h g⁻¹ values higher than those measured for pure Li₂ZnTi₃O₈ (184.5, 129.5, 107.3 and 24 mA h g⁻¹). Qie et al.³³ studied the influence of Cu²⁺ doping on the electrochemical properties of Li₂ZnTi₃O₈ and found that Li_1.95Zn_0.9Cu_0.1Ti₃O₈ displayed the best electrochemical performance.

K is similar to Li in chemical properties and is a promising doping atom. However, to the best of our knowledge, it has not been used to dope Li₂ZnTi₃O₈. Therefore, the electrochemical reaction of Li_1.9K_0.1ZnTi₃O₈/C was studied using the sol–gel method in the current work, and the results showed its excellent electrochemical performance, demonstrating that K-doping should in general be considered a useful method for improving electrochemical performances.

2 Materials and methods

Li_1.9K_0.1ZnTi₃O₈/C (0.02 mol) was synthesized using the sol–gel method. To do so, 4.7922 g TiO₂ (99.8%, Shanghai Maclin Biochemical Technology Co., Ltd), 4.39 g (CH₃COO)₂Zn·2H₂O (AR, Chengdu Cologne Chemical Reagent Co., Ltd), 2.5076 g (CH₃COO)Li (99.9%, Shanghai Maclin Biochemical Technology Co., Ltd), 4 g polyvinylpyrrolidone K30 (PVP K30) (AR, Chengdu Cologne Chemical Reagent Co., Ltd), and 0.1491 g KCl (AR, Chengdu Cologne Chemical Reagent Co., Ltd) were added into a sample of anhydrous ethanol. Then the resulting mixture was dried at 80 °C for 4 h, and the dried material was heated at 750 °C for 15 minutes in a microwave sintering furnace to achieve the final Li_1.9K_0.1ZnTi₃O₈/C electrode material. For purposes of comparison, Li₂ZnTi₃O₈/C was prepared using the same protocol except that KCl was not included.

X-ray diffraction (XRD) patterns were acquired using a Brook AXS's D2 PHASER, with a 2θ range of 10–80°. Scanning electron microscopy (SEM) images were acquired using a TESCAN VEGA3 apparatus. X-ray photoelectron spectroscopy (XPS) was carried out using K-Kα radiation as the excitation source. Data for high-resolution transmission electron microscopy (HR-TEM) and TEM-EDX mapping were collected using an FEI Talos F200X apparatus. Raman spectra were recorded using a DXR Raman spectrometer (Thermo Scientific) at room temperature with a 780 nm excitation line using an Ar⁺ laser.

The electrochemical performance of Li_1.9K_0.1ZnTi₃O₈/C was assessed with lithium metal as the counter electrode. Li_1.9K_0.1ZnTi₃O₈/C, Super-P and sodium carboxymethylcellulose (CMC) with a weight ratio of 80 : 10 : 10 were added into water, and the resulting electrode slurry was spread on a copper current collector and dried at 80 °C for 8 h in a vacuum oven. The working electrode was prepared by following the above steps. CR2032 coin cells were prepared in an Ar-filled glovebox. All electrochemical properties were characterized between 0.05 and 3.0 V at 25 °C, and the EIS data were collected at frequencies ranging from 0.1 Hz to 100 kHz.

3 Results and discussion

Fig. 1(A) shows the XRD patterns of the materials investigated. All of the samples were determined using these patterns to have formed the spinel LZTO structure, indicating that the low dosage of K⁺ did not alter the structure. As shown in Table 1, the cell parameters of Li₂ZnTi₃O₈/C and Li_1.9K_0.1ZnTi₃O₈/C were determined to be, respectively, a = 8.4112 and 8.5032 Å with the greater cell length for Li_1.9K_0.1ZnTi₃O₈/C due to the introduction of K⁺ into the crystal structure resulting in a replacement of some of the Li⁺ ions (ionic radius 0.076 Å) with the larger K⁺ ions (ionic radius 1.38 Å).^34,35 This difference was perhaps associated with a wider tunnel for Li_1.9K_0.1ZnTi₃O₈/C, a feature apt to increase the speed of Li⁺ transport and further improve the electrochemical properties of the resulting electrode material.³⁶ Fig. 1(B) shows the XRD patterns of Li_1.9K_0.1ZnTi₃O₈/C before and after 100 cycles. In these patterns, the diffraction peaks were essentially identical, demonstrating that the structure of Li_1.9K_0.1ZnTi₃O₈/C was very well maintained after 100 cycles.

Fig. 1 (A) XRD patterns of Li₂ZnTi₃O₈/C and Li_1.9K_0.1ZnTi₃O₈/C. (B) XRD patterns of Li_1.9K_0.1ZnTi₃O₈/C before and after 100 cycles. (C) XPS survey spectra of Li₂ZnTi₃O₈/C and Li_1.9K_0.1ZnTi₃O₈/C. (D) Li_1.9K_0.1ZnTi₃O₈/C XPS spectrum in the K 2p region. (E) Li₂ZnTi₃O₈/C and Li_1.9K_0.1ZnTi₃O₈/C XPS spectra in the Li 1s region.

Table 1 Crystal lattice parameter values for Li₂ZnTi₃O₈/C and Li_1.9K_0.1ZnTi₃O₈/C

Samples	Lattice parameters
	a (Å)	V (Å^3)
Li1.9K0.1ZnTi3O8/C	8.5032	614.8189
Li2ZnTi3O8/C	8.4112	595.0780

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More information was obtained by taking XPS measurements. Fig. 1(C) shows the XPS survey spectrum of Li_1.9K_0.1ZnTi₃O₈/C; peaks corresponding to Li 1s, O 1s, C 1s, Zn 2p, Ti 2p, and K 2p were identified in this spectrum. Peaks observed at about 292.78 and 295.58 eV (Fig. 1(D)) appeared to correspond well to K 2p_3/2 and K 2p_1/2,³⁷ verifying the presence of K⁺ in the Li_1.9K_0.1ZnTi₃O₈/C sample tested. The peak observed at about 54.48 eV in the spectrum of the Li_1.9K_0.1ZnTi₃O₈/C sample (Fig. 1(E)) can be attributed to Li 1s.³⁷

The SEM images (Fig. 2(A) and (B)) acquired of the Li₂ZnTi₃O₈/C and Li_1.9K_0.1ZnTi₃O₈/C samples indicated that they were well crystallized. Note that the particle morphology did not obviously change after the doping with the low dose of K⁺. More detailed structural information was gained from the TEM images of the Li₂ZnTi₃O₈/C and Li_1.9K_0.1ZnTi₃O₈/C composites (Fig. 2(C) and (D)). And as seen in Fig. 2(E) and (F), no obvious difference between the lattice fringes of these composites was observed, consistent with the XRD results. The results demonstrated that the crystal structure of the composite did not change upon it being doped with small amounts of K. To further investigate the doping into Li₂ZnTi₃O₈/C, Li_1.9K_0.1ZnTi₃O₈/C was examined using energy dispersive spectroscopy (EDS) mapping. From the EDS mappings, the elements C (Fig. 2(G)), Ti (Fig. 2(H)), Zn (Fig. 2(I)) and K (Fig. 2(J)) were seen on the surface of Li_1.9K_0.1ZnTi₃O₈/C.

Fig. 2 (A and B) SEM images of (A) Li₂ZnTi₃O₈/C and (B) Li_1.9K_0.1ZnTi₃O₈/C. (C and D) Typical TEM images of (C) Li₂ZnTi₃O₈/C and (D) Li_1.9K_0.1ZnTi₃O₈/C. (E and F) HRTEM images of (E) Li₂ZnTi₃O₈/C and (F) Li_1.9K_0.1ZnTi₃O₈/C. (G–J) EDS mappings of Li_1.9K_0.1ZnTi₃O₈/C for the elements (G) C, (H) Ti, (I) Zn, and (J) K.

The pore size distributions determined using the Barrett–Joyner–Halenda (BJH) method are shown in Fig. 3(A) and (B) (inset). The average diameters of the pores of Li₂ZnTi₃O₈/C and Li_1.9K_0.1ZnTi₃O₈/C were found to be 10.26 nm and 11.13 nm, respectively. From the data obtained, more pores were identified in the Li_1.9K_0.1ZnTi₃O₈/C material than in the Li₂ZnTi₃O₈/C material, which indicated that the degree of agglomeration was reduced and that more tunnels for Li⁺ conduction were provided by Li_1.9K_0.1ZnTi₃O₈/C.

Fig. 3 Nitrogen adsorption–desorption isotherms of (A) Li₂ZnTi₃O₈/C and (B) Li_1.9K_0.1ZnTi₃O₈/C with the corresponding pore diameter distributions in the insets.

To verify the presence of carbon in the synthesized Li₂ZnTi₃O₈/C and Li_1.9K_0.1ZnTi₃O₈/C samples, Raman spectra were acquired, as shown in Fig. 4. From the spectra of Li₂ZnTi₃O₈/C and Li_1.9K_0.1ZnTi₃O₈/C, the D band (at ca.1330 cm⁻¹) and G band (at ca.1560 cm⁻¹) were identified, demonstrating the presence of carbon in the Li₂ZnTi₃O₈/C and Li_1.9K_0.1ZnTi₃O₈/C samples.³⁸

Fig. 4 Raman spectra of Li₂ZnTi₃O₈/C and Li_1.9K_0.1ZnTi₃O₈/C.

The electrochemical reaction mechanisms for Li₂ZnTi₃O₈/C and Li_1.9K_0.1ZnTi₃O₈/C were investigated using cyclic voltammetry (CV). These tests were performed using a scan rate of 0.1 mV s⁻¹ and potentials between 0.05 V and 3 V (Fig. 5 (A) and (B)). The anodic peak potential (φ_Pa), cathodic peak potential (φ_Pc) and the difference between them (Δφ_P) are shown in Table 2. Li₂ZnTi₃O₈/C and Li_1.9K_0.1ZnTi₃O₈/C showed cathodic and anodic peaks at potentials between 1 V and 2 V, which we interpreted as involving the participation of the reactions of the Ti⁴⁺/Ti³⁺ redox couple, and indicated that applying a low dose of K-doping did not change the Li₂ZnTi₃O₈/C electrochemical process.³⁹ As shown in Table 2, the difference between the potentials of the oxidation and reduction peaks was 151 mV for the Li₂ZnTi₃O₈/C sample, but considerably less, at 97 mV, for the Li_1.9K_0.1ZnTi₃O₈/C sample. Compared with Li₂ZnTi₃O₈/C, Li_1.9K_0.1ZnTi₃O₈/C has been shown to display higher lithium-ion diffusion rates and lower polarization.⁴⁰ Therefore, the reversibility and cycling abilities of the electrode materials were enhanced by the low-dose doping of Li₂ZnTi₃O₈/C.

Fig. 5 (A and B) CV curves of (A) Li₂ZnTi₃O₈/C and (B) Li_1.9K_0.1ZnTi₃O₈/C from 0.05 V to 3.0 V. (C) Initial charge–discharge curves of Li₂ZnTi₃O₈/C and Li_1.9K_0.1ZnTi₃O₈/C. (D) Cycling capacity of Li₂ZnTi₃O₈/C and Li_1.9K_0.1ZnTi₃O₈/C for 100 cycles. (E) Rate performances of Li₂ZnTi₃O₈/C and Li_1.9K_0.1ZnTi₃O₈/C at different current densities. (F) The composition of the CR2032 button battery.

Table 2 Electrochemical parameters of investigated samples, including the difference between the potentials of the anodic and cathodic peaks from CV

Sample	φPa (V)	φPc (V)	ΔφP (mV)
Li1.9K0.1ZnTi3O8/C	1.592	1.495	97
Li2ZnTi3O8/C	1.579	1.428	151

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The CV curves for the first cycle at 200 mA g⁻¹ for potentials of 0.05–3 V are shown in Fig. 5(C). Here, the discharge capacity of Li_1.9K_0.1ZnTi₃O₈/C was higher than that of Li₂ZnTi₃O₈/C at the 1st cycle. The voltage of the discharge platform was 0.57 V (vs. Li⁺/Li), the voltage of charging platform was 1.48 V (vs. Li⁺/Li), and the voltage difference between charge and discharge platform was the smallest, indicating that the polarization of the synthesized material was small during Li⁺ disintercalation.⁴¹

The cycling properties of Li₂ZnTi₃O₈/C and Li_1.9K_0.1ZnTi₃O₈/C were characterized at 200 mA g⁻¹ (Fig. 5(D)). At the 100th cycle, 75.06% and 91.73% (323.7 mA h g⁻¹) of the initial capacities were retained for Li₂ZnTi₃O₈/C and Li_1.9K_0.1ZnTi₃O₈/C, respectively. Chen et al.²⁹ synthesized Li_1.95Na_0.05ZnTi₃O₈, and it reached 267.3 mA h g⁻¹ at the 50th cycle at 0.1 A g⁻¹, indicating a better cycling performance resulting from K⁺ doping than from Na⁺ doping. Therefore, Li_1.9K_0.1ZnTi₃O₈/C was indicated to demonstrate a better capacity retention and cycling performance than Li₂ZnTi₃O₈/C, which could be attributed to the low-dose K⁺ doping enlarging the Li⁺ transport tunnels, and hence increasing the rates of Li⁺ transport and electron transport.³⁶

The rate properties of Li₂ZnTi₃O₈/C and Li_1.9K_0.1ZnTi₃O₈/C are compared in Fig. 5(E). Li_1.9K_0.1ZnTi₃O₈/C delivered a maximum discharge capacity of 379.5 mA h g⁻¹ at the 1st cycle for a current density of 50 mA g⁻¹, and discharge capacities of 322.9, 305.5, 284.5, and 258.9 mA h g⁻¹ for 100, 200, 500, and 1000 mA g⁻¹, respectively, whereas Li₂ZnTi₃O₈/C delivered capacities of 260.6, 226.2, 166.9, and 111.4 mA h g⁻¹ at these current densities, respectively. Li_1.9K_0.1ZnTi₃O₈/C showed better rate properties than did Li₂ZnTi₃O₈/C, which may be attributed to (1) doping K possibly having enhanced the electronic conductivity and yielding a good rate performance for Li_1.9K_0.1ZnTi₃O₈/C, and (2) the increased cell volume after low-dose K doping possibly increasing the rates of Li⁺ transport and electron transport.^42,43

EIS measurements for the samples were taken as shown in Fig. 6. The corresponding plots were composed of a semicircle at high frequency and an inclined line at the low-frequency region, indicating the charge transfer resistance and Li⁺ diffusion process within the electrodes, respectively.^44,45 As shown in Fig. 6, the charge-transfer resistance of the Li_1.9K_0.1ZnTi₃O₈/C was found to be lower than that of Li₂ZnTi₃O₈/C, demonstrating low-dose K⁺ doping to be a useful method for enhancing the electronic conductivity.^46,47 In addition, according to the data in the low-frequency regions, the Li⁺ diffusion rate of the Li_1.9K_0.1ZnTi₃O₈/C sample was slightly higher than that of the Li₂ZnTi₃O₈/C sample.

Fig. 6 EIS results for Li₂ZnTi₃O₈/C and Li_1.9K_0.1ZnTi₃O₈/C.

4 Conclusions

In this study, an XRD analysis indicated no change in the structure of Li₂ZnTi₃O₈/C resulting from introducing into it a low dose of K. In addition, a low dose of K doped into Li₂ZnTi₃O₈/C was good for enhancing the electronic and ionic conductivity levels, which could improve the electrochemical performance. The initial discharge capacity was 352.9 mA h g⁻¹ (Li_1.9K_0.1ZnTi₃O₈/C) at 200 mA g⁻¹. The sample performed stably, with a capacity of 323.7 mA h g⁻¹ retained for Li_1.9K_0.1ZnTi₃O₈/C after 100 cycles. In the rate test, Li_1.9K_0.1ZnTi₃O₈/C showed a high discharge capacity of 379.5 mA h g⁻¹ at the 1st cycle for a current density of 50 mA g⁻¹ and a capacity of 258.9 mA h g⁻¹ for 1000 mA g⁻¹. Of the various materials tested, Li_1.9K_0.1ZnTi₃O₈/C exhibited the best rate properties and excellent cycling stability. Therefore, K-doping should be considered a useful method for improving electrochemical performances.

Funding

This work was financially supported by the Demonstration Project of Scientific and Technological Achievements Transfer and Transformation of Sichuan Province (2021ZHCG0040).

Author contributions

X.-G. Zeng and J. Peng contributed to the conception and design of the study. H.-F. Zhu and K. Xia organized the database. J. Peng and J. Gong wrote the first draft of the manuscript. X.-G. Zeng revised the whole manuscript.

Conflicts of interest

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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