1 Introduction

With the energy and environment challenges, the lithium-ion battery as a preferred second battery is highly demanded for the portable electronic devices and even the large-scale applications due to its higher power density, safety, low cost and environmental friendliness [1,2,3]. Moreover, with the rapid development of the electronic devices and automobile industry, the lithium batteries are required to supply a longer cycling life, higher energy density and superior security. In this case, the 5 V spinel LiNi0.5Mn1.5O4 cathode comes into the sight of people and also attracts much more attentions for its high energy density of 640 W h kg−1, good cycling capability and rate performance at room temperature [4,5,6,7,8].

However, the shortcomings of the spinel LiNi0.5Mn1.5O4 material seriously restrict its commercial applications, which include the decomposition of electrolyte at high working voltage, the sharp capacity fading at high temperature (50–60 °C), and poor rate performance [6, 8, 9]. Partial substitutions at Ni site or Mn site in LiNi0.5Mn1.5O4 with different metal ions such as Fe, Co, Cr, Al, Mg, Cu, Ru, Ti, Nb etc. [4, 10,11,12,13,14,15,16,17,18,19,20,21] have proved to be an effective way to improve its electrochemical properties. On the other hand, the structures of spinel LiNi0.5Mn1.5O4 belong to two different space groups, a face-centered cubic Fd3m space group with Ni2+ and Mn4+ ions randomly distributing on 16 sites and a primitive cubic P4332 space group with the two ions orderly occupying 12d and 4b sites, respectively [16, 22,23,24,25]. Moreover, the sample with Fd3m space group is widely believed to have better electrochemical performance due to the higher content of Mn3+ ions [22, 24, 26,27,28]. In our previous work [12], a thermopolymerization method was employed to synthesize the divalent-metal-ions doped samples. Among these samples, LiNi0.45Cu0.05Mn1.5O4 with P4332 space group delivered excellent rate performance and good cycling stability even at elevated temperature. However, the improvement is unsatisfactory. So in this work, we aim to prepare LiNi0.45Cu0.05Mn1.5O4 with Fd3m space group and look forward to some surprises. According to literature work [16, 29,30,31,32], a high-temperature treatment (> 700 °C) can lead to the formation of disordered LiNi0.5Mn1.5O4 with the space group Fd3m due to the release of a tiny amount of oxygen, but after the annealing treatment at 700 °C in air or oxygen for a long time, the spinel can transform its crystal structure from Fd3m to P4332 space group. Such a transformation is also observed in Cu-doped composition LiNi0.45Cu0.05Mn1.5O4 [12]. Therefore, in this work, we keep the calcination process at high temperature (900 °C) to synthesize LiNi0.45Cu0.05Mn1.5O4 samples with Fd3m space group. The electrochemical testing results prove that, compared with LiNi0.45Cu0.05Mn1.5O4 with P4332 space group, the disordered LiNi0.45Cu0.05Mn1.5O4 (Fd3m) exhibits excellent rate performance and cycling stability at the room temperature as well as the elevated temperature (55 °C).

2 Experimental

The LiNi0.5Mn1.5O4 and LiNi0.45Cu0.05Mn1.5O4 samples with P4332 space group were synthesized via a thermopolymerization method as described previously [12]. Lithium nitrate (LiNO3, 5% excess), copper nitrate (Cu(NO3)2·3H2O), nickel nitrate (Ni(NO3)2·6H2O) and manganese acetate (Mn(CH3COO)2·4H2O) were mixed in stoichiometric molar ratio with 100 ml deionized water. Secondly, 50 ml acrylic acid (AA) was added to form a mixed solution, which was then kept in an oven at 150 °C for 10 h to proceed the thermopolymerization reaction and get a fluffy powder product. This powder was grinded and calcined at 500 °C for 5 h and then naturally cooled to room temperature. After another grinding, the intermediate products were further sintered at 900 °C for 15 h and subsequently annealed at 700 °C for 48 h. All these heat treatment processes were carried out in air atmosphere. For simplicity, LiNi0.5Mn1.5O4 and LiNi0.45Cu0.05Mn1.5O4 synthesized by the above process were respectively referred to as LNM and LNM-Cu-9-7. The disordered LiNi0.45Cu0.05Mn1.5O4 materials were also prepared by a process similar to the above but without the annealing treatment at 700 °C. Therefore they were named as LNM-Cu-9.

Powder X-ray diffraction (XRD) was carried out by a diffractometer (Rigaku TTRIII, Cu K-alpha radiation) with a scanning rate of 1° min−1 in the range 2θ = 10°–80°. The morphologies of the as-prepared samples were observed under a scanning electron microscope (SEM, JSM-6390LA, JEOL). A Nicolet 8700 infrared spectrometer (Thermo Scientific Instrument Co. USA) was also used for Fourier transformed infrared (FTIR) study. An inductively coupled plasma atomic emission spectrometer (ICP) (Optima 7300 DV, Perkin-Elmer Co., USA) was introduced to measure the concentration of the soluble Mn2+ ions.

For electrochemical testing, the active materials (LNM, LNM-Cu-9-7 and LNM-Cu-9), acetylene black and PVDF (80:10:10, w/w/w) with N-methyl-2-pyrrolidone were first mixed to prepare homogenous slurries. Then they were separately dispersed on an aluminum foil with a doctor blade and dried at 70 °C overnight. The individual electrode discs with 14 mm diameter were punched out and dried at 70 °C for 2 h in a vacuum oven,and the average loading density is about 3.2 mg cm−2. The CR2032 half cells were assembled in an argon-filled glove box (MBraun Labmaster 130) and the electrolyte used in the cells was 1.0 M LiPF6 solution in ethylene carbonate (EC)–dimethyl carbonate (DMC) (1:1 w/w, Guotai Huarong New Chemical Materials Co.). Galvanostatic charge/discharge was carried out by a multi-channel battery cycler (Neware BTS2300, Shenzhen) with a voltage window of 3.5–4.9 V at room temperature (25 °C, RT). The cells were also cycled at 55 °C by keeping them in an oven.

The cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) of the cells were both conducted on a CHI 604A electrochemical instrument. The EIS tests and the CV tests were carried out in the frequency range of 0.01–105 Hz and in the potential window of 3.0–5.1 V at a scan rate of 0.1 mV s−1, respectively. To evaluate the electrical conductivity of LNM-Cu-9 [12], the intermediate precursor of LNM-Cu-9 was pressed into discs with a diameter of 16 mm and then sintered at 900 °C for 15 h (The weight of the powder of the three samples is 1.000 g.). The sintered pellet was coated with the silver paste as conductive adhesive on top and bottom sides, then measured their size which was illustrated in Table 1 in the supporting information. Finally, they are tested on the CHI 604A electrochemical workstation.

Table 1 ICP results of the dissolution of Mn2+ ions after 500 cycles
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3 Results and discussion

3.1 Crystal structures and particle morphology

The SEM images of LNM, LNM-Cu-9-7 and LNM-Cu-9 are displayed in Fig. 1. It is obvious that the particles of the three samples own well-crystallized octahedral morphologies and similar particle size of 1–3 μm.

Fig. 1
figure 1

SEM images of LNM (a), LNM-Cu-9-7 (b) and LNM-Cu-9 (c) samples

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The XRD patterns of LNM, LNM-Cu-9-7 and LNM-Cu-9 samples are compared in Fig. 2a. All the samples reveal well-defined spinel structure without any impurities (LixNi1−xO etc.) at 37.3°, 43.3° and 63.2° [16, 22]. Rietveld refinements were performed to obtain the lattice parameter of the LNM-Cu-9 (8.1774 Å). Compared to the lattice parameter of LNM (8.1703 Å) and LNM-Cu-9-7 (8.1700 Å) [12], the slightly increased lattice parameter of LNM-Cu-9 is due to the combined effect of the formation of Mn3+ (r 3+Mn  = 0.65 Å, > r 4+Mn  = 0.53 Å) and Cu3+ (r 3+Cu  = 0.54 Å, < r 2+Cu  = 0.73 Å) during the synthesis process.

Fig. 2
figure 2

XRD (a) patterns and FTIR spectra (b) of the samples: LNM, LNM-Cu-9-7 and LNM-Cu-9. Crystal structure of LNM-Cu-9-7 with P4332 space group (c) and LNM-Cu-9 with Fd3m space group (d)

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Except for the XRD analysis, the FTIR spectra are also considered to be an effective way to distinguish the space group between Fd3m and P4332. For the ordered P4332 space group, there are additional three peaks separately distributed at about 646, 464 and 430 cm−1, which are normally absent in the disordered Fd3m space group [33, 34]. As can be seen from Fig. 2b, LNM and LNM-Cu-9-7 samples are assigned to be the ordered P4332 crystalline structure with the obvious adsorption at 649, 469 and 431 cm−1. LNM-Cu-9 sample nearly has no adsorption at these three wavenumbers, suggesting that it is with disordered Fd3m space group. As shown in Fig. 2b, c, Ni2+, Cu2+ and Mn4+ ions in LNM-Cu-9-7 sample with P4332 space group separately occupy 4b and 12d octahedral sites. While for the LNM-Cu-9 sample with Fd3m space group, they are randomly located at the octahedral 16d sites.

3.2 Electrochemical performance at room temperature (RT)

Cyclic voltammograms for half cells of Li/LNM, Li/LNM-Cu-9-7 and Li/LNM-Cu-9 are depicted in Fig. 3. It is well known that the peaks at around 4.0 V are assigned to the redox couple Mn3+/Mn4+ and the peaks at around 4.7 V are due to Ni2+/Ni4+ [15,16,17,18, 35]. The weak peaks between 4.2 and 4.4 V are ascribed to the redox couple Cu2+/Cu3+ [17, 36, 37]. As shown in Fig. 3, the voltage difference (ΔV) of the Ni2+/Ni4+ redox peaks just as the dotted line for LNM-Cu-9 sample is smaller than the other two samples,which implying its lower polarization [17, 35, 38, 39]. There two couples of mixed valences (Cu2+/Cu3+ and Mn3+/Mn4+) can be observed on the curve of LNM-Cu-9 sample, which are helpful to increase the electronic conductivity of LNM-Cu-9 sample.

Fig. 3
figure 3

CV plots of LNM (a), LNM-Cu-9-7 (b) and LNM-Cu-9 (c) samples between 3.0 and 5.1 V at RT

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The initial charge and discharge profiles and long cycling measurements are shown in Fig. 4. Figure 4a presents the initial charge/discharge profiles of LNM, LNM-Cu-9-7 and LNM-Cu-9 samples. The weak plateau at 4.2–4.4 V ascribed to the redox couple Cu2+/Cu3+ can be observed on the charge and discharge curves of the Cu-doping LNM samples [12, 17, 18]. The voltage plateau at 4.0 V assigned to the Mn3+/Mn4+ redox couple is only visible on the curve of LNM-Cu-9 sample, suggesting that the synthesis process without the annealing treatment leads to the formation of Mn3+ [15,16,17]. The observed charge/discharge voltage plateaus are consistent to the above CV results. The initial discharge capacity of the three samples nearly reaches 128 mAh g−1. Moreover, we also have a comparison of the IR drops between the charge and discharge curves at the semi-full-charged state for the three half cells. As a result, the IR drops are 0.1 V (LNM), 0.06 V (LNM-Cu-9-7) and 0.0 V (LNM-Cu-9), respectively, which indicates the reduced polarization and impedance after Cu doping especially the Cu-doped sample with Fd3m space group. Besides, as as shown in Fig. 5b, LNM-Cu-9 exhibits much better cycling stability than LNM and LNM-Cu-9-7. LNM-Cu-9 shows a capacity retention of 100% after 300 cycles, and 91% after 500 cycles, while the capacity retentions of LNM and LNM-Cu-9-7 samples are separately 65% and 77% after 500 cycles.

Fig. 4
figure 4

The initial charge–discharge curves at 0.2 C (a) and cycling stability (b) of LNM, LNM-Cu-9-7 and LNM-Cu-9 samples at 1 C and RT

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Fig. 5
figure 5

SEM images of the electrodes before and after 500 cycles: LNM (a, d), LNM-Cu-9-7 (b, e) and LNM-Cu-9 (c, f) samples

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After 500 cycles, the cells of Li/LNM, Li/LNM-Cu-9-7 and Li/LNM-Cu-9 are disassembled and soaked in 40 ml distilled water to measure the amounts of the dissolution of Mn2+ ions by ICP. The results of the dissolution of Mn2+ are listed in Table 1, After 500 cycles, LNM-Cu-9 exhibits the lowest amounts of soluble Mn2+ ions produced by the disproportionated reaction of Mn3+, while the undoped LNM gives the largest dissolution of Mn2+ ions. Apparently, the Cu2+ doping can effectively prevent the dissolution of Mn2+ ions and this effect is more obvious with the increase of Mn3+ ions.

The morphologies of LNM, LNM-Cu-9-7 and LNM-Cu-9 electrodes before and after 500 cycles at RT are shown in Fig. 5. Compared to the Cu-doping samples, the surface of the LNM electrode is completely covered by the products of the side reactions occurring on the interface of the electrolyte and electrode after 500 cycles, and is partly covered for LNM-Cu-9-7 electrode. However, the particles of LNM-Cu-9 after 500 cycles are mostly exposed and remained well. The combined contribution of few soluble Mn2+ ions and the seldom side reaction during the repeated charge and discharge process collectively result in the excellent cycling stability of LNM-Cu-9.

To investigate the rate capability of LNM, LNM-Cu-9-7 and LNM-Cu-9, the cells were charged and discharged at the current densities from 70 mA g−1 (0.5 C rate) to 1400 mA g−1 (10 C rate). In Fig. 6a, it is obvious that LNM-Cu-9-7 and LNM-Cu-9 both deliver higher discharge capacity than LNM. Compared to LNM-Cu-9-7, LNM-Cu-9 exhibits higher discharge capacity at 8 C and 10 C, separately reaches 118.7 and 110.4 mAh g−1. Also, at a current density of 1400 mA g−1 (10 C), as shown in Fig. 6b, the capacity retentions of LNM-Cu-9 and LNM-Cu-9-7 remain 91.2% and 80.5%, respectively after 300 cycles. Based on the above results, it can be concluded that LNM-Cu-9 owns much better electronic conductivity and cycling stability than LNM-Cu-9-7. As mentioned in Ref. [12], LNM-Cu-9-7 sample shows better electronic conductivity due to the appearance of the mixed-valence Cu2+/Cu3+ couple and the mixed-valence couple facilitating electron hopping has been reported in literatures [18, 22]. Hence, for LNM-Cu-9, the additional mixed-valence Mn3+/Mn4+ can further improves the electronic conductivity, which results in much better rate performance than LNM-Cu-9-7.

Fig. 6
figure 6

The rate performance (a) at different current densities of LNM, LNM-Cu-9-7 and LNM-Cu-9 samples at RT. The cycling stability of the Cu-doping samples at a large current density 1400 mA g−1 (b)

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EIS measurements are adopted to test the half-cells of Li/LNM, Li/LNM-Cu-9-7 and Li/LNM-Cu-9 at the state-of-charge (SOC) of 50%. The EIS spectra, the equivalent circuit and the fitting results are shown in Fig. 7. In the equivalent circuit, R1 is ascribed to the ohmic resistance and R2 is assigned to the charge transfer resistance at the interface between the electrode and the electrolyte. Based on the fitting results shown in the inserted table in Fig. 7, LNM possesses the largest R2 (161.2 Ω), while LNM-Cu-9-7 and LNM-Cu-9 own smaller R2 (59.3 Ω and 54.6 Ω, respectively), which are just consistent with their rate performances in Fig. 6a.

Fig. 7
figure 7

EIS spectra of Li/LNM, Li/LNM-Cu-9-7 and Li/LNM-Cu-9 half-cells at RT

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Meanwhile, the electrical conductivities of LNM, LNM-Cu-9-7 and LNM-Cu-9 are also measured based on the EIS measurement of their sintered pellets (Fig. S1). The results are listed in Table 2. Undoubtedly, LNM-Cu-9 has the highest electrical conductivity (3.58 × 10−5 S cm−1) which is due to the existences of mixed-valences Cu2+/Cu3+ and Mn3+/Mn4+ couples, which is consistent to its best rate performance shown in Fig. 6.

Table 2 The electrical conductivity of LNM, LNM-Cu-9-7, and LNM-Cu-9 samples at RT
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3.3 Cycling behavior of LNM, LNM-Cu-9-7 and LNM-Cu-9 at 55 °C

Figure 8 shows the electrochemical performances of LNM, LNM-Cu-9-7 and LNM-Cu-9 at 55 °C. It is noticed that the Cu-doping samples both exhibit better cycling stability than LNM sample at the elevated temperature (55 °C). Moreover, the discharge capacity of LNM-Cu-9 has nearly no capacity loss after 100 cycles, i.e. with a capacity retention of 100%. Thus LNM-Cu-9 shows outstanding cycling stability at 55 °C. Therefore, LNM-Cu-9 surely further improves the electrochemical performance at room temperature and the elevated temperature, which just realizes our anticipated objective in the beginning.

Fig. 8
figure 8

The electrochemical cycling performances of LNM, LNM-Cu-9-7 and LNM-Cu-9 at 55 °C and 1 C

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Finally, Table 3 compares the electrochemical performance of LNM-Cu-9 with those of other LiNi0.5Mn1.5O4 samples reported in literature. Undoubtedly, LNM-Cu-9 sample shows superior electrochemical properties both in cycling stability and rate capability.

Table 3 Comparison of the electrochemical performance for the divalent-cation-doped LiNi0.5Mn1.5O4 spinel with our sample
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4 Conclusion

In summary, the results of physical properties analysis demonstrate that LNM-Cu-9 sample without the annealing treatment at 700 °C is indeed with the disordered Fd3m space group. During the electrochemical measurements, LNM-Cu-9 sample delivers the initial discharge capacity 127.2 mAh g−1 and the capacity retention 91% after 500 cycles at RT. Moreover, at the elevated temperature (55 °C), the capacity retention can keep 100% after 100 cycles. Besides, due to it high electrical conductivity (3.58 × 10−5 S cm−1), LNM-Cu-9 sample exhibits the discharge capacity 110.4 mAh g−1 at the large current density (10 C rate). The LNM-Cu-9 cathode material with outstanding rate capability and cycling stability at the room and elevated temperature, is anticipated to provide new opportunities for the high power and high energy density LIBs in the commercial applications.