Mo⁶⁺–P⁵⁺ co-doped Li₂ZnTi₃O₈ anode for Li-storage in a wide temperature range and applications in LiNi_0.5Mn_1.5O₄/Li₂ZnTi₃O₈ full cells

Abstract

A one-step solid-state method has been used to synthesize Li₂ZnTi₃O₈ (LZTO) co-doped with Mo⁶⁺ and P⁵⁺ ions (LZM7TP3O). The structural stability, pore size and specific surface area have been improved via this strategy. In addition, LZM7TP3O has high ionic and electronic conductivities, and small transfer resistance. So, this co-doping strategy tremendously enhances the electrochemical performance of LZM7TP3O in a wide temperature range. No capacity fading occurs for LZM7TP3O after cycling for 600 cycles at 25 °C, based on the 2nd cycle at 1 A g⁻¹. 180.5 mA h g⁻¹ is still kept at 3 A g⁻¹ for the 120th cycle. No capacity decay occurs for LZM7TP3O after cycling for 100 cycles at 55 °C, relative to the 2nd cycle at 1 A g⁻¹. At 0 °C, there is no capacity lost after cycling for 1000 cycles at 0.5 A g⁻¹, relative to the 2nd cycle. At 0.5 C, the initial discharge specific capacity of the LiNi_0.5Mn_1.5O₄/LZM7TP3O full cell can reach 214.3 mA h g⁻¹ in 2–4.55 V, when LZM7TP3O is used as the anode of the full cell. Moreover, the full cell can power red, green and blue light emitting diode (LED) bulbs. The results indicate that LZM7TP3O is an appealing anode for lithium-ion batteries.

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Introduction

Various electrochemical energy storage devices have been developed due to the increasing demand for sustainable, environmental and efficient energy supplies. Among these devices, lithium-ion batteries (LIBs) have been widely researched and applied owing to their exceptional performance.^1–3 Graphite is the typical anode in traditional LIBs. However, the application of graphite is limited in high-power LIBs due to its poor safety and rate capability.^4,5 Subsequently, Li₄Ti₅O₁₂ (LTO), with a spinel structure, has been developed as a candidate because it can overcome the shortcomings of graphite. However, the energy densities of LIBs are limited when LTO with a low specific capacity in the 1–3 V range is used as the anode.^6,7 Although a higher capacity can be obtained for LTO in the 0–3 V range, the cycling performance and coulombic efficiency (CE) at the 1st cycle will be worsened.^8,9 Therefore, the electrochemical performance of LTO has mainly been researched in the 1–3 V range.^10,11

Li₂ZnTi₃O₈ (LZTO), with a spinel structure, has been researched as an anode material for LIBs since 2010.¹² Zn ions occupy the tetrahedral sites, and the octahedral sites are occupied by Li and Ti ions with 1 : 3 cation ordering in the LZTO crystal structure. LZTO has aroused great interest from researchers as a new Ti-based anode as it has many advantages:^12–22 (1) the insertion potential of Li⁺ ions is high for LZTO, which can guarantee the good safety of the LIBs; (2) compared with LTO, a relatively high theoretical specific capacity can be obtained for LZTO; (3) the zero-strain characteristic of LZTO guarantees the good cycling performance; (4) LZTO has wide channels for the migration of Li⁺ ions; (5) generally speaking, the sintering temperature of LTO is 800–900 °C and the sintering time is ca. 10 h in a solid-state route.^23–25 The successful synthesis of LZTO is usually below 800 °C and the synthetic time is short, at 3–5 h.^12,13 So, compared with LTO, the synthetic technology is simple and easy to industrialize for LZTO.

Even so, the low electrical conductivity is the main drawback of LZTO.^26,27 Coating with conductive materials, metal ion doping and morphology designing are the major modification methods.^28–33 Among these strategies, doping can not only enhance the electrical conductivity but can also stabilize the structure of LZTO. For LZTO, the whole electrochemical performance has been highly enhanced via Mo⁶⁺-,³⁴ Fe³⁺-,³⁵ Ce⁴⁺-,³⁶ or Ti³⁺-doping.¹³ Chen et al. synthesized Ce-doped LZTO via a solid-state route.³⁶ The doping strategy significantly improved the overall electrochemical performance. When the current density was 20 C, 178.4 mA h g⁻¹ was maintained for Ce-doped LZTO. Compared with LZTO, the value improved by 27.5 mA h g⁻¹ for Ce-doped LZTO. At 10 C cycling for 500 cycles, the capacity retention reached 75% relative to the 2nd cycle for Ce-doped LZTO. However, the value was only 22.6% for LZTO, which was remarkably lower than that of Ce-doped LZTO. To some extent, the rate capability or cycling performance of LZTO have been improved via Zr⁴⁺-,³⁷ Ag⁺-,³⁸ Cu²⁺-,³⁹ Al³⁺-,⁴⁰ or Na⁺-doping.⁴¹ Yang et al. synthesized Zr-doped LZTO (LZTO/LZO) via a two-step route.³⁷ The cycling performance was significantly improved. At 0.5 A g⁻¹, 199.2 mA h g⁻¹ was retained after 600 cycles with a retention of 99.1% for Zr-doped LZTO. However, when LZTO/LZO was cycled at 1.6 A g⁻¹, only 109.8 mA h g⁻¹ was maintained. The rate capability of LZTO/LZO was not perfect. Chen et al. synthesized Na-doped LZTO via a solid-state method.⁴¹ The discharge specific capacities of LZTO at high current densities were greatly enhanced via the strategy. When the current density was 2 C, 103.2 and 162.3mA h g⁻¹ were obtained for LZTO and Na-doped LZTO after 80 cycles, respectively. At 0.1 A g⁻¹, 571.9 and 616.3 mA h g⁻¹ were delivered at the initial cycle for LZTO and Na-doped LZTO, respectively. Nevertheless, after 50 cycles, the values were only 171.2 and 267.2 mA h g⁻¹ for LZTO and Na-doped LZTO, respectively. The capacity losses were severe for the two samples. Therefore, the selection of dopants is important. It has been reported that the substitution of ions with high valences for ions with low valences will introduce some Ti³⁺ ions due to the charge balance. The electronic conductivity of LZTO will increase, owing to the existence of the mixed Ti⁴⁺/Ti³⁺ states. Substitution of Mo⁶⁺ for Zn²⁺ improved the whole electrochemical performance of LZTO in our previous reports.³⁴ It is feasible to replace Ti⁴⁺ with P⁵⁺ due to the high valence of P⁵⁺. Yan et al. synthesized P⁵⁺-doped Li₄Ti₅O₁₂.⁴² Li₄Ti_4.8P_0.2O₁₂ showed good cycling performance with an increase in the electronic conductivity due to the presence of the mixed Ti³⁺/Ti⁴⁺ states. Long et al. successfully introduced P into a Si anode for LIBs.⁴³ At 0.2 A g⁻¹, the P-doped Si anode delivered a large specific capacity of 2460.4 mA h g⁻¹. 911 mA h g⁻¹ was still delivered even at a high current density of 16 A g⁻¹. In addition, after cycling for 100 cycles at 2 A g⁻¹, 1564 mA h g⁻¹ was obtained. P-doping improved the redox kinetics of the Si anode.

The radii of Mo⁶⁺ and P⁵⁺ are smaller than those of Zn²⁺ and Ti⁴⁺, respectively. Therefore, Mo⁶⁺ and P⁵⁺ ions will easily enter the crystal lattice of LZTO. Moreover, the substitution of Mo⁶⁺ and P⁵⁺ with high valences for Zn²⁺ and Ti⁴⁺ with low valences will introduce some Ti³⁺ ions due to the charge balance, which will then increase the electronic conductivity of LZTO. Therefore, it is expected that Mo⁶⁺-P⁵⁺ co-doped LZTO will show a remarkably improved electrochemical performance. However, the application of Mo⁶⁺–P⁵⁺ co-doped LZTO in LIBs has not been seen in previous reports. In this work, a Mo⁶⁺–P⁵⁺ co-doping strategy was explored to enhance the overall electrochemical performance of LZTO (Scheme 1) for the first time. In addition, the Li-storage of Mo⁶⁺–P⁵⁺ co-doped LZTO was researched in half and full cells.

Scheme 1 The synthetic process of Mo⁶⁺–P⁵⁺ co-doped LZTO.

Experimental

Materials synthesis

Li₂Zn_1−xMo_xTi₃O₈ (x = 0, 0.05, 0.07, 0.09), Li₂ZnTi_3−yP_yO₈ (y = 0.01, 0.03, 0.05) and Li₂Zn_1−xMo_xTi_3−yP_yO₈ (x = 0.07, y = 0.03) anodes were fabricated via a one-step solid-state route. The synthetic details are given in the ESI†.

Physical and electrochemical measurements

The synthetic details are given in the ESI†.

Results and discussion

The rate capabilities of LZTO and Mo-doped LZTO are shown in Fig. 1a. 129.3, 154.7, 174.0 and 132.3 mA h g⁻¹ (the 110th cycle) are obtained at 3 A g⁻¹ for LZTO, LZM5TO, LZM7TO and LZM9TO, respectively. Therefore, LZM7TO has the best rate capability. At 1 A g⁻¹, the cycling results of LZTO and P-doped LZTO (Fig. 1b) show that P-doping enhances the discharge specific capacity of LZTO. At the 200th cycle, 159.8, 168.2, 186.6 and 178.6 mA h g⁻¹ are maintained for LZTO, LZTP1O, LZTP3O and LZTP5O, respectively. LZTP3O has the highest values in the whole cycling process. So, the optimal amounts of the Mo element and P element are selected as 0.07 and 0.03, respectively, in the Mo–P co-doped LZTO (LZM7TP3O). The theoretical fractions of Mo and P are 1.90 wt% and 0.26 wt% in LZM7TP3O, respectively. The actual values are 1.77 wt% and 0.25 wt%, respectively, tested by the ICP (inductively coupled plasma emission spectroscopy) technique. The actual fractions are close to the theoretical values for the Mo and P elements. Fig. S1† shows the thermogravimetric and differential thermogravimetric (TG-DTG) curves of the precursors for LZTO and Mo–P co-doped LZTO. No weight loss occurs over 530 °C, suggesting that the LZTO or LZM7TP3O phase forms at this temperature. So, 700 °C is selected as the calcination temperature in the work.

Fig. 1 (a) Rate capability of LZTO and Mo-doped LZTO. (b) Cycling performance of LZTO and P-doped LZTO.

Fig. 2a shows the X-ray diffraction (XRD) patterns of LZTO and LZM7TP3O. All the diffraction peaks belong to LZTO (JCPDS# 86-1512) for the two samples. No other phases are detected from the spectrum of LZM7TP3O, indicating that Mo and P elements have entered the crystal lattice of LZTO. X-ray photoelectron spectroscopy (XPS) results show that Mo and P in the states of Mo⁶⁺ and P⁵⁺ exist in LZM7TP3O, respectively, and there are some Ti³⁺ ions on the surface of LZM7TP3O (Fig. S2†). Therefore, the extra charges of Mo⁶⁺ and P⁵⁺ should be compensated by the existence of Ti³⁺, indicating that Mo and P elements have been doped into LZTO. It has been reported that the mixed valences of Ti⁴⁺/Ti³⁺ can enhance the electron conduction of LZTO.^13,30,44 On one hand, for LZTO, the lattice parameters will shrink via the substitution of the ions with small radii for the ions with large radii. The radii of Mo⁶⁺, Zn²⁺, P⁵⁺ and Ti⁴⁺ are 62, 74, 34 and 61 nm, respectively. On the other hand, for LZTO, the parameters will increase due to the appearance of some Ti³⁺ ions. The radius of Ti³⁺ (r = 67 nm) is larger than that of Ti⁴⁺ (r = 61 nm). The enlarged (311) plane slightly shifts to a lower angle (Fig. 2b), indicating that the lattice constants increase, and that the latter is the main factor. In order to investigate the sites of Mo and P in LZTO, the X-ray diffraction pattern of LZM7TP3O was refined using the Rietveld refinement method. The refinement results (Tables 1 and 2, and Fig. 2d) indicate that the substitution of Mo⁶⁺ ions into the 8c site for some Zn²⁺ ions and of P⁵⁺ into the 12d site for some Ti⁴⁺ ions are the best and most reasonable. In addition, the refinement results further show the increase of the lattice constants (Table 2). Moreover, the dopants of Mo and P are uniformly dispersed in LZM7TP3O (Fig. S3†).

Fig. 2 (a) XRD patterns of LZTO and LZM7TP3O, and (b) magnified (111) peaks of LZTO and LZM7TP3O. XRD refinement profiles of (c) LZTO and (d) LZM7TP3O.

Table 1 Rietveld refinement data for LZM7TP3O

Site	Atom	x	y	z	Occupancy
4b	Li	0.625	0.625	0.625	1
8c	Li	−0.00160	−0.00160	−0.00160	0.5
8c	Zn	−0.00160	−0.00160	−0.00160	0.465
8c	Mo	−0.00160	−0.00160	−0.00160	0.035
8c	O	0.39203	0.39203	0.39203	1
12d	Ti	0.36759	0.88241	0.125	0.99
12d	P	0.36759	0.88241	0.125	0.01
24e	O	0.10668	0.12222	0.39118	1

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Table 2 Evaluation and lattice parameters of LZTO and LZM7TP3O

Sample	a = b = c (Å)	V (Å^3)	R wp (%)	R p (%)	χ ^2
LZTO	8.369	586.176	11.8	8.75	2.00
LZM7TP3O	8.369(2)	586.203	11.6	8.41	1.89

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The morphologies of LZTO and LZM7TP3O are shown in Fig. 3a and b. It can be seen that the two samples consist of nanoparticles and no obvious difference in the morphologies is found between the two samples. The particle sizes are 38 and 28 nm for LZTO and LZM7TP3O (Fig. 3c and d), respectively. Therefore, Mo–P co-doping can reduce the particle size and increase the pore size, as well as the specific surface area (Fig. S4 and Table S1†), which will provide many active sites for the insertion and de-insertion of Li⁺ ions and promote the fast diffusion of Li⁺ ions. Heterogeneous Mo and P in the LZTO lattice may inhibit the overgrowth of the LZTO crystals and then the particle size of LZM7TP3O is reduced. In addition, the two samples have good crystallinity according to the high-resolution transmission electron microscopy (HR-TEM) images (Fig. 3e and f), which is advantageous to the electrochemical performance.

Fig. 3 SEM images of (a) LZTO and (b) LZM7TP3O. TEM images of (c) LZTO and (d) LZM7TP3O (insets: histograms of the particle size distribution). HR-TEM images of (e) LZTO and (f) LZM7TP3O.

Charge–discharge and cyclic voltammetry (CV) techniques were used to research the electrochemical reaction mechanism (Fig. 4). During the insertion of Li⁺ ions, two discharge plateaus appear in each discharge curve at ca. 1.06 and 0.46 V, and accordingly, two cathodic peaks appear in every CV curve. During the de-insertion process of Li⁺ ions, a charge plateau appears in each charge curve at ca. 1.47 V, and accordingly, one anodic peak appears in every CV curve. The insertion and de-insertion of Li⁺ ions are assigned to the redox couple of Ti⁴⁺/Ti³⁺. The plateau/peak at ca. 0.5 V may be related to the multiple restorations of Ti⁴⁺.^45,46 From the 2nd cycle, it is obvious that the two cathodic peaks move to high potentials, which may originate from the irreversible lithiation at the 1st cycle.^47,48Fig. 4d makes comparisons of CV curves between LZTO and LZM7TP3O for the initial cycle. It is obvious that Mo–P co-doping can improve the reversibility of insertion and de-insertion for Li⁺ ions (Fig. 4d and Table 3).

Fig. 4 (a) Voltage-capacity curves of LZTO and LZM7TP3O at 1 A g⁻¹ for the 1st cycle. CV curves of (b) LZTO and (c) LZM7TP3O, and (d) comparison of cyclic voltammograms between LZTO and LZM7TP3O for the 1st cycle from 0.02 to 3 V at 0.5 mV s⁻¹.

Table 3 Potentials of the CV peaks for the LZTO and LZM7TP3O electrodes during the initial cycle

Sample	φ pa (V)	φ pc (V)	φ p (V) = φpa − φpc
LZTO	1.561	1.087	0.474
LZM7TP3O	1.537	1.118	0.419

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The cycling performance of LZTO and LZM7TP3O at 1 A g⁻¹ is shown in Fig. 5a. For LZTO and LZM7TP3O, 113.8 and 208.1 mA h g⁻¹ are still obtained for the 600th cycle, respectively. The capacity retention is only 69% for LZTO relative to the specific capacity of the 2nd cycle. However, there is no capacity fading for LZM7TP3O. Obviously, the cycling performance is greatly improved for LZTO via Mo–P co-doping. 209.3 and 237.7 mA h g⁻¹ are delivered and the CE reaches 73.2% and 87.3% for LZTO and LZM7TP3O during the initial cycle, respectively. For the two samples, the CE is close to 100% after three cycles, and is higher for LZM7TP3O (Fig. 5b), indicating the high reversibility of insertion and de-insertion for Li⁺ ions. For the two electrodes, the high irreversible initial capacity losses originate from the formation of a solid electrolyte interface (SEI) layer, which will reduce the side reactions between LZTO/LZM7TP3O and the electrolyte, and then help the subsequent cycling performance. When the current densities are increased, LZM7TP3O still exhibits good cycling performance. After 500 cycles at 1.5 A g⁻¹, no capacity loss occurs relative to the 2nd cycle. After cycling for 500 cycles at 2 A g⁻¹, 98.2% of the specific capacity at the 2nd cycle is retained (Fig. 5c). For LZM7TP3O, its cycling performance exceeds those of many reports in literatures (Table S2†). It has been found in previous reports that the capacities of LZTO electrodes continuously increase during the initial several cycles, owing to the continuous decomposition of the electrolyte.^44,47 A similar phenomenon appears in other Ti-based anodes, such as the LTO anode.^49,50 As previously reported, coating can reduce the decomposition of the electrolyte on the Ti-based anodes. Therefore, it is expected that the combination of coating and doping can further enhance the cycling performance of Ti-based anodes. In addition, when the current densities are increased, the initial CE of LZM7TP3O slightly decreases, owing to the increase of the polarization for the electrode. After several cycles, the CE is close to 100%.

Fig. 5 (a) Cycling performance and (b) coulombic efficiency of LZTO and LZM7TP3O at 1 A g⁻¹. (c) Cycling performance of LZM7TP3O at 1.5 and 2 A g⁻¹ and corresponding coulombic efficiency. (d) Cycling performance of LZTO and LZM7TP3O at different current densities and corresponding coulombic efficiency.

The rate capabilities of LZTO and LZM7TP3O were measured at 0.3–3 A g⁻¹, and the results are shown in Fig. 5d. It is obvious that the rate capability of LZM7TP3O is greatly improved via Mo–P co-doping. At 2, 2.5 and 3 A g⁻¹, 201.8, 191.9 and 180.5 mA h g⁻¹ are delivered, respectively. The performance is better than that of LZTO and some other anodes reported in the literature (Tables S3 and 4†). The enhanced rate capability and cycling performance of LZM7TP3O may be ascribed to the following reasons: (1) the internal resistance can be greatly reduced via Mo–P co-doping (Fig. S5†); (2) the structural stability of LZM7TP3O can be improved via Mo–P co-doping (Fig. S6†). At 1 A g⁻¹, the LZTO and LZM7TP3O electrodes were cycled for 200 cycles and the XRD patterns were collected for the two electrodes. The diffraction peaks of the LZM7TP3O electrode are still sharp after cycling, suggesting that the structure of LZM7TP3O retains remarkable stability during the insertion and de-insertion of Li⁺ ions. However, some of the diffraction peaks for the LZTO electrode are blurry after cycling, indicating that the structure of LZTO is partly changed during the cycling process; (3) surface integrity and good electrical contact are obtained in the cycling process for the LZM7TP3O electrode (Fig. S7†); (4) more active sites for the insertion and de-insertion of Li⁺ ions will be provided due to the large specific surface area of LZM7TP3O (Fig. S4 and Table S1†). The specific surface areas are 21.4 and 23.4 m² g⁻¹ for LZTO and LZM7TP3O, respectively; (5) the large pore size of LZM7TP3O can quicken the diffusion of Li⁺ ions (Fig. S4 and Table S1†); (6) small R_ct and high D_Li⁺ and electronic conductivity (Fig. S8 and Table S5†) can be in favour of the rate capability of LZM7TP3O. The R_ct are 155 and 28.73 Ω for the LZTO and LZM7TP3O electrodes cycling for cycles at different current densities (Fig. 5d), respectively. The D_Li⁺ are 1.12 × 10⁻¹⁰ and 1.47 × 10⁻⁹ cm² s⁻¹ for the LZTO and LZM7TP3O electrodes, respectively.

It is important for a cathode or an anode to have good electrochemical performance in wide temperature range. However, only few studies have reported the electrochemical performance of LZTO at high and low temperatures. Fig. 6a shows the cycling performance of LZTO and LZM7TP3O at 55 °C. At the 100th cycle, the capacity retention of LZTO and LZM7TP3O reaches 89.6% and 115.8% relative to the 2nd cycle, respectively. It is well known that the side reactions between electrolytes and active materials will intensify at high temperatures, and then the cycling performance of the electrodes will worsen. However, LZM7TP3O shows excellent cycling performance at 55 °C, which can exceed that of the previous reports,^47,51 indicating that Mo–P co-doping has greatly stabilized the structure of LZTO. In addition, at a low temperature of 0 °C, LZM7TP3O still has excellent cycling performance and rate capability. At 0.6 A g⁻¹, 196.2 mA h g⁻¹ is maintained at the 60th cycle (Fig. 6b). Even after 1000 cycles at 0.5 A g⁻¹, no capacity fading occurs for LZM7TP3O (Fig. 6c). When the temperature increases, the side reactions intensify and then the CE decreases. Therefore, LZTO and LZM7TP3O have higher initial CE at the low temperature of 0 °C than at 55 °C.

Fig. 6 Cycling performance of LZTO and LZM7TP3O at (a) 1 A g⁻¹ at 55 °C and (b) 0.5 A g⁻¹ at 0 °C, and (c) rate capability of LZTO and LZM7TP3O at 0 °C and the corresponding coulombic efficiency.

If LZM7TP3O is to be commercialized, it should have good electrochemical performance in full cells, which is evaluated in LiNi_0.5Mn_1.5O₄/LZM7TP3O using LiNi_0.5Mn_1.5O₄ as the cathode. In this study, LiNi_0.5Mn_1.5O₄ was purchased from Shenzhen Biyuan Electronics Co., Ltd. As is usually reported, there is a very small amount of rock salt phase in LiNi_0.5Mn_1.5O₄ (Fig. 7a). LiNi_0.5Mn_1.5O₄ shows an irregular morphology (Fig. 7b).

Fig. 7 (a) XRD pattern and (b) SEM image of LiNi_0.5Mn_1.5O₄.

Usually, the design of a full cell is according to the capacities of the cathode and anode electrodes at 0.5 C. So, the discharge specific capacities were tested for the LiNi_0.5Mn_1.5O₄ cathode and the LZM7TP3O anode at 0.5 C, and the values are ca. 130 and 280 mA h g⁻¹, respectively (Fig. 8a and b). It is noted that the discharge specific capacity of LZM7TP3O is over the theoretical value. This may be related to the side reactions between LZM7TP3O and the electrolyte.^44,47 It is well known that the capacity of the carbon anode is slightly in excess to prevent lithium deposition in commercial LIBs. That is to say, the capacity of the cell is limited by the positive electrode. When the lithiation potential is high for an anode, the limiting of the cell capacity by the positive electrode is unnecessary. As for the LiNi_0.5Mn_1.5O₄/LTO full cell, it is necessary to design the cell with excess positive capacity to alleviate the decomposition of the electrolyte at the high potential region by keeping a relatively large value of x in Li_xNi_0.5Mn_1.5O₄ when the cell is near or at its fully charged state.^52,53 The lithiation potential is high for the LZTO anode. Therefore, the capacity of the positive electrode should be slightly in excess when the full cell consists of a LiNi_0.5Mn_1.5O₄ cathode and LZTO anode. The electrochemical performance of the LiNi_0.5Mn_1.5O₄/LZM7TP3O full cells was assessed in the range of 2.0–4.55 V. The selection of the voltage range was based on the CV results (Fig. S9†). Fig. S9† shows the CV curves of LiNi_0.5Mn_1.5O₄/Li, LZM7TP3O/Li and LiNi_0.5Mn_1.5O₄/LZM7TP3O in 3.5–5.0 V, 0.02–3 V and 2–4.55 V, respectively. For LiNi_0.5Mn_1.5O₄/Li (Fig. S9a†), the peaks at 4.03/3.97 V are from the redox couple of Mn⁴⁺/Mn³⁺, and the peaks at 4.75 (4.80)/4.61 V originate from the redox couples of Ni³⁺/Ni²⁺ and Ni⁴⁺/Ni³⁺, respectively. For LZM7TP3O/Li (Fig. S9b†), the pair of redox peaks at 1.24/1.55 V is due to the redox couple of Ti⁴⁺/Ti³⁺. In addition, a cathodic peak at 0.5 V is detected. There are three oxidation peaks at 2.67, 3.46 and 4 V, and one sharp reduction peak at 3.24 V for the LiNi_0.5Mn_1.5O₄/LZM7TP3O full cell (Fig. S9c†). Li⁺ ions can be inserted/de-inserted to the maximum extent in the range of 2.0–4.55 V for the LiNi_0.5Mn_1.5O₄/LZM7TP3O full cell. In addition, the potential window will not cause many side reactions between the electrolyte and active materials.

Fig. 8 Charge–discharge curves of (a) LiNi_0.5Mn_1.5O₄/Li and (b) LZM7TP3O/Li. (c) Cycling performance of the LiNi_0.5Mn_1.5O₄/LZM7TP3O full cells with different P/N ratios at 0.5 C. (d) Impedance spectra of the LiNi_0.5Mn_1.5O₄/LZM7TP3O full cells with different P/N ratios after cycling for 30 cycles at 0.5 C.

When the capacity ratios of the cathode to the anode (P/N) are 1.1 : 1, 1.2 : 1 and 1.3 : 1 in the LiNi_0.5Mn_1.5O₄/LZM7TP3O full cells, the discharge specific capacities of the full cells are 202.8, 214.3 and 207.4 mA h g⁻¹ at the initial cycle, respectively. When cycled for 30 cycles, the capacity retention is 83.4%, 86.8 and 85%, respectively. Therefore, when the capacity ratio of the cathode to the anode is 1.2 : 1, the LiNi_0.5Mn_1.5O₄/LZM7TP3O full cell possesses the largest discharge capacity and best cycling performance (Fig. 8c) at 0.5 C. Fig. 8d shows the impedance spectra of the LiNi_0.5Mn_1.5O₄/LZM7TP3O full cells with different P/N ratios after cycling for 30 cycles at 0.5 C. The semicircles are attributed to the charge transfer resistances of the full cells. It is known that a small charge transfer resistance is advantageous to the electrochemical performance of a cell. Among the three types of full cells, the LiNi_0.5Mn_1.5O₄/LZM7TP3O full cell with the P/N of 1.2 : 1 has the smallest charge transfer resistance, which guarantees the large specific capacity and good cycling performance of the full cell. However, the cycling performance of the full cells need be further improved in our subsequent work.

When the driving voltages are 1.8–2.2, 2.9–3.1 and 3.0–3.2 V, light emitting diode (LED) bulbs can emit red, green and blue lights, respectively. The working voltage is over 3.0 V for the LiNi_0.5Mn_1.5O₄/LZM7TP3O full cell. So, it can successfully power LED bulbs that emit different colors of light (Fig. 9).

Fig. 9 (a) Schematic illustration of the full cell of LiNi_0.5Mn_1.5O₄/LZM7TP3O. Applications of the full cell for LiNi_0.5Mn_1.5O₄/LZM7TP3O to power (b) a red LED bulb, (c) a green LED bulb and (d) a blue LED bulb.

Conclusions

Mo⁶⁺–P⁵⁺ co-doped Li₂ZnTi₃O₈ (LZM7TP3O) has been successfully structured via a one-step solid-state route. The modification enhances the structural stability, pore size and specific surface area, and gives LZM7TP3O a high ionic conductivity and small transfer resistance. The electrochemical performance of LZM7TP3O is greatly improved in a wide temperature range via the co-doping strategy. No capacity fading occurs for LZM7TP3O after cycling for 600 cycles at 25 °C, relative to the 2nd cycle at 1 A g⁻¹. 180.5 mA h g⁻¹ is still maintained at 3 A g⁻¹ for the 120th cycle. No capacity decay occurs for LZM7TP3O after cycling for 100 cycles at 55 °C, relative to the 2nd cycle. When the current density is 0.5 A g⁻¹, there is no capacity loss after 1000 cycles at 0 °C (relative to the 2nd cycle). When LZM7TP3O is used as the anode of the LiNi_0.5Mn_1.5O₄/LZM7TP3O full cell, the discharge specific capacity of the full cell can reach 214.3 mA h g⁻¹ at 0.5 C in the voltage range of 2–4.55 V for the 1st cycle. The working voltage of the full cell is over 3 V and it can power red, green and blue light emitting diode (LED) bulbs. In view of the good electrochemical performance in the wide temperature range and simple synthetic route, it is likely that the Mo⁶⁺–P⁵⁺ co-doped LZTO structure can be used as the anode of LIBs.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (U1504532), the Liaoning Revitalization Talents Program (XLYC1907025), the Liaoning Province Project Education Fund (LJKZ0408) and the Natural Science Foundation of Liaoning Shihua University (2018XJJ-012).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/d1qi01077h

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