Lithium-ion batteries (LIBs) are a new type of rechargeable batteries that are characterized by high specific capacities and specific energies, small volumes, long life cycles, low cost, low energy consumption, low self-discharge efficiencies, small internal resistance, and high working voltages (Yoshio, 2001; Junmin et al., 2005; Dong-il and Han, 2006; Wang M. X. et al., 2019; Wang S. et al., 2019; Huang et al., 2020; Wang et al., 2020; Yi et al., 2020a), and they have wide applications, including in mobile phones, laptops, cameras, digital cameras, electric automobiles, energy storage, aerospace, and space exploration (Tarascon and Armand, 2001; Xiao et al., 2018, 2019; An et al., 2019; Hong et al., 2019). The selection of suitable anode materials for LIBs is extremely important for the performance of these batteries to have excellent life cycles and charge/discharge rate characteristics.

Recently, there have been numerous studies of anode materials, including LiTi₂O₄, Li₂Ti₃O₇, Li₂Ti₆O₁₃, Li₄Ti₅O₁₂, Na₂Li₂Ti₆O₁₄, TiNb₂O₇, and Li₂ZnTi₃O₈ (Tang et al., 2014a; Chen B. K. et al., 2015; Li G. H. et al., 2016; Liu et al., 2016; Li Z. F. et al., 2016; Yi et al., 2020b). Li₂ZnTi₃O₈ with the cubic spinel structure has been considered as a promising material because of its lack of toxicity, low cost, relatively high theoretical capacity of 227 mA h g⁻¹, and its low discharge voltage plateau of ~0.5 V (vs. Li/Li⁺) (Jović et al., 2009; Chen B. K. et al., 2015; Chen W. et al., 2015). However, compared with other anode materials, Li₂ZnTi₃O₈ suffers from low electronic conductivity and an even worse rate performance, which means that its performance in practice is lower than the theoretical value (Tang and Tang, 2014; Chen B. K. et al., 2015). Hence, by increasing its electronic conductivity, its use can be extended to a wider range of applications.

Until now, this problem has been solved by applying a synthesized coating containing conductive species, nano-sized particles, and doping ions (Shenouda and Murali, 2008; Qi et al., 2009; Tian et al., 2010; Lin et al., 2011, 2013; Yu et al., 2011; Zhang et al., 2011; Bai et al., 2012; Jhan and Duh, 2012; Wu et al., 2013; Xu et al., 2013; Mani et al., 2014; Tang et al., 2014a; Yi et al., 2020c). As is widely known, by using a synthesized coating with conductive species on Li₂ZnTi₃O₈, the transportation of electrons can be improved while decreasing the particle size, which further accelerates ionic transportation. Moreover, it has been shown that doping ions into the material can increase its internal electronic conductivity (Lee et al., 2010; Fang et al., 2013; Lin et al., 2014). According to previous studies, the introduction of Al (Tang et al., 2014b), Ag (Tang et al., 2014a), Mo (Wang M. X. et al., 2019), and Ti (III) (Chen et al., 2018) to Li₂ZnTi₃O₈ greatly improved the electrochemical properties of these materials. For example, Tang et al. (2014b) partially replaced Ti with Al, and the resulting Li₂ZnTi_2.9Al_0.1O₈ composite showed a given capacity of 223.1 mA h g⁻¹ at 0.1 A g⁻¹ (Tang et al., 2014b). Wang M. X. et al. (2019) improved the electronic conductivity of the Li₂ZnTi₃O₈ compound by introducing Mo, and Li₂Zn_0.93Mo_0.07Ti₃O₈@graphene showed a high cycle capacity at high current densities of 5 A g⁻¹, with a power rating of 153 mA h g⁻¹ still being delivered on the 200th cycle (Wang M. X. et al., 2019). Recently, researchers have paid close attention to the use of Cr doping, which, according to one study (Ruan et al., 2017), can further enhance the electronic conductivity. Pan et al. (2018) have successfully prepared Cr-doped γ-Fe₂O₃/rGO cathode material by microwave method. The as-obtained 4.0 at% Cr-doped γ-Fe₂O₃/rGO sample exhibits the capacity of 1,060 mAh g⁻¹ after 100 cycles at 100 mA g⁻¹. Liu et al. (2018) have prepared Fe_1.95Cr_0.05F₅·H₂O material, which retains a discharge capacity of 171 mAh g⁻¹ after 100 cycles at 0.1 C (1 C = 200 mAh g⁻¹). Feng et al. (2009) have successfully synthesis Cr-LiV₃O₈ cathode material, and it showed an excellent electrochemical performance, with the retention of 94.4% after 100 cycles. To the best of our knowledge, the introduction of Cr³⁺-doped ions into Li₂ZnTi₃O₈ has not yet been reported. In this paper, we describe the improvement in electrochemical performance of Li₂ZnTi₃O₈ anode material as a result of Cr doping.

Li₂ZnTi_2.9Cr_0.1O₈ was prepared by the liquid phase method. Stoichiometric amounts of TiO₂, (CH₃COO)Li, (CH₃COO)₂Zn, and Cr(NO₃)₃ were mixed with anhydrous ethanol as solvent. The mixture was dried at 80°C for 2 h and then roasted in a muffle furnace at 700°C for 10 h to obtain the final Li₂ZnTi_2.9Cr_0.1O₈ compound. For comparison, Li₂ZnTi₃O₈ was also synthesized without using Cr(NO₃)₃ as the raw material.

X-ray diffraction (XRD, Brook AXS's D2 PHASER) was used to examine the crystalline phase of both Li₂ZnTi₃O₈ and Li₂ZnTi_2.9Cr_0.1O₈, which was recorded within the range of 10–70° (2θ). Scanning electron microscopy (SEM, TESCAN VEGA3) was used to examine the morphology of the samples. X-ray photoelectron spectroscopy (XPS) with Cr-Kα radiation was used to monitor the surface electronic states of the elements that were the sources of X-rays.

To construct the working electrodes, the as-prepared material, Super-P and LA-132 were mixed in weight ratios of 80:10:10 with water as solvent, and the prepared slurry was then pasted onto copper foil and dried at 100°C for 10 h in a vacuum oven. The working electrode, lithium metal, Celegard 2400 separator, and 1 M LiPF₆ electrolyte solution containing a 1:1:1 mixture of ethylene carbonate (EC), dimethyl carbonate (DMC), and ethylmethyl carbonate (EMC) were used to prepare CR2032 coin cells in an Ar-filled glovebox. All electrochemical performances were tested in the voltage range of 0.05–3.0V at room temperature.

Figure 1 shows the XRD patterns of Li₂ZnTi₃O₈ and Li₂ZnTi_2.9Cr_0.1O₈. It is clear from the figure that the diffraction peaks of the samples conform to the standard diffraction peaks of cubic spinel Li₂ZnTi₃O₈, which demonstrates that the presence of Cr(NO₃)₃ does not obviously affect the structure of cubic spinel Li₂ZnTi₃O₈. The crystal lattice constants calculated from the recorded XRD data are listed in Table 1, and the lattice parameters of Li₂ZnTi₃O₈ and Li₂ZnTi_2.9Cr_0.1O₈ were estimated to be a = 8.4305 and 8.4340 Å, respectively. The lattice parameters increased following introduction of Cr³⁺ into the Li₂ZnTi₃O₈ crystal structure due to the larger radius of Cr³⁺ (0.062 nm) compared with that of Ti⁴⁺ (0.061 nm) (Novikova et al., 2018), which promotes the transportation of lithium ions and further enhances the electrochemical performance (Tai et al., 2020).

Additional information about the surface electronic states of the elements was gained from XPS measurements. Figure 2A shows the XPS survey spectra of Li₂ZnTi_2.9Cr_0.1O₈. As can be seen from the figure, the peaks of Li 1s, C 1s, O 1s, Ti 2p, Zn 2p, and Cr 2p are clearly visible. Figure 2B reveals that two peaks centered at approximately 576.36 and 585.60 eV correspond well to the Cr 2p_3/2 and Cr 2p_1/2 peaks, demonstrating that Cr is present as Cr³⁺ ions in the as-synthesized Li₂ZnTi_2.9Cr_0.1O₈. The two peaks shown in Figure 2C centered at approximately 458.4 and 464.40 eV are detected for Li₂ZnTi₃O₈ and Li₂ZnTi_2.9Cr_0.1O₈, which can correspond to the peaks of Ti (IV) 2p_3/2 and Ti (IV) 2p_1/2.

Figure 3 shows the SEM images of the Li₂ZnTi₃O₈ and Li₂ZnTi_2.9Cr_0.1O₈ samples. It can be clearly seen that both samples are well-crystallized, with a small grain size distribution. It should be noted that the morphology of the particles did not appreciably change following doping with minute amounts of Cr³⁺. While increasing the contact area between active particles and the electrolyte, a good dispersion can narrow the transmission distance between Li⁺ and the electrons and increase the high-rate performance of 5C.

The initial charge/discharge curves of Li₂ZnTi₃O₈ and Li₂ZnTi_2.9Cr_0.1O₈ are shown in Figure 4, which shows that the charge/discharge curves of Li₂ZnTi_2.9Cr_0.1O₈ are similar to those of Li₂ZnTi₃O₈, suggesting that Cr³⁺ doping exerts an effect on the electrochemical reaction. The specific capacities of Li₂ZnTi₃O₈ and Li₂ZnTi_2.9Cr_0.1O₈ at rates of 1 A g⁻¹ after the first cycle were 130.5 and 166.8 mA h g⁻¹, respectively. As can clearly be seen, Li₂ZnTi_2.9Cr_0.1O₈ demonstrates a higher specific capacity than Li₂ZnTi₃O₈. This can be explained by the fact that Cr doping can enlarge the transport tunnel of lithium ions, which further increases lithium ion transportation and electron transfer, thus enhancing the electronic conductivity (Zhang et al., 2018). In addition, Li₂ZnTi_2.9Cr_0.1O₈ appears to have the least voltage platform difference, indicating that a lower electrode polarization was obtained following Cr³⁺ doping and Li⁺ transport was improved.

The rate performances of Li₂ZnTi₃O₈ and Li₂ZnTi_2.9Cr_0.1O₈ are compared in Figure 5. Li₂ZnTi_2.9Cr_0.1O₈ delivered maximum discharge capacities of 221.7, 210.3, 185.1, 166, 156.7, and 107.5 mA h g⁻¹ at 0.1, 0.2, 0.5, 1, 2, and 5 C, respectively, whereas Li₂ZnTi₃O₈ delivered 193.4, 170.2, 147.8, 131, 98.3, and 43.5 mA h g⁻¹ at the same rates, respectively. As can be seen, Li₂ZnTi_2.9Cr_0.1O₈ showed better rate performance compared with Li₂ZnTi₃O₈, which may be due to either or both of the following reasons: (1) Cr doping improves the electronic conductivity, which leads to Li₂ZnTi_2.9Cr_0.1O₈ demonstrating better electrochemical performance than Li₂ZnTi₃O₈; (2) the improved cell volume following Cr doping enhances lithium ion diffusion and electron transfer (Chen et al., 2009; Nie et al., 2020).

The cycling performance of Li₂ZnTi₃O₈ and Li₂ZnTi_2.9Cr_0.1O₈ was examined at a rate of 1 A g⁻¹, and the results are shown in Figure 6. As seen, the coulombic efficiency of Li₂ZnTi₃O₈ and Li₂ZnTi_2.9Cr_0.1O₈ were 80.2 and 84.8% in the first cycle, indicating that the coulombic efficiency can be enhanced after Cr doped. After several cycles, the coulombic efficiency of two samples is close to 100%. After 200 cycles, the reserving ratios were 90.2 and 97.2% for Li₂ZnTi₃O₈ and Li₂ZnTi_2.9Cr_0.1O₈, respectively. Therefore, Li₂ZnTi_2.9Cr_0.1O₈ possesses the better cycling performance. These results indicate that Li₂ZnTi_2.9Cr_0.1O₈ shows better capacity retention than Li₂ZnTi₃O₈, which can probably be explained by assuming that moderate Cr doping can enlarge the transport tunnel of lithium ions, which further increases lithium ion transportation and electron transfer.

Electrochemical impedance spectroscopy (EIS) measurements for Li₂ZnTi₃O₈ and Li₂ZnTi_2.9Cr_0.1O₈ were carried out to probe the kinetic properties of these anode materials, and the results are shown in Figure 7. Similar patterns are displayed by the impedance spectra of the two samples, which formed a semicircle in the high-frequency regions and a straight line in the low-frequency regions. The semicircle in the high-frequency regions is associated with charge transfer resistance on the electrode/electrolyte interface, while the straight line in the low-frequency regions is ascribed to the diffusion of Li⁺ into the Warburg resistance (Long et al., 2011; Tang et al., 2019), which constitutes the bulk of the electrode materials. It is clear that the charge transfer resistance of the Li₂ZnTi_2.9Cr_0.1O₈ electrode material is lower than that of Li₂ZnTi₃O₈, showing that a certain small amount of Cr³⁺ doping of Li₂ZnTi₃O₈ is useful for enhancing the electronic conductivity (Li Z. F. et al., 2016; Kou et al., 2020). In addition, the slope for the Li₂ZnTi_2.9Cr_0.1O₈ electrode material in the low-frequency regions is slightly higher than that for Li₂ZnTi₃O₈ because Cr doping can strengthen lithium ion migration through Li₂ZnTi₃O₈.

This study successfully prepared Li₂ZnTi₃O₈ and Li₂ZnTi_2.9Cr_0.1O₈ samples with the cubic spinel structure by the sol-gel method. XRD revealed that the element Cr successfully doped into the Li₂ZnTi₃O₈ exerted no effect on the spinel structure of Li₂ZnTi₃O₈. SEM (Chen B. K. et al., 2015) results demonstrated that the morphology of the particles did not obviously change following Cr³⁺ doping. The measured electrochemical properties indicated that Li₂ZnTi_2.9Cr_0.1O₈ shows a better rate performance and excellent cyclic reversibility than Li₂ZnTi₃O₈. The electrochemical performance might be significantly enhanced as a result of the higher electronic conductivity following Cr³⁺ doping, demonstrating that Li₂ZnTi_2.9Cr_0.1O₈ is a promising anode material for high-rate LIBs.

All datasets presented in this study are included in the article/supplementary material.

XZ and JP contributed conception and design of the study. HZ and YG organized the database. JP and XH wrote the first draft of the manuscript. XZ revised the whole manuscript. All authors contributed to the article and approved the submitted version.

HZ was employed by the company Langxingda Technology Co, Ltd. The remaining 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.