Enhancement of the electrochemical performance of LiNi0.5Mn1.5O4 cathode materials for Li-ion battery by Mo-Fco-doping

doi:10.21203/rs.3.rs-3457578/v1

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Enhancement of the electrochemical performance of LiNi_0.5Mn_1.5O₄ cathode materials for Li-ion battery by Mo-Fco-doping

https://doi.org/10.21203/rs.3.rs-3457578/v1

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The pristine LiNi_0.5Mn_1.5O₄ (LNMO) and Mo-F co-doped LiNi_0.5Mn_1.5O₄ spinel materials were prepared via a rheological phase method. The four samples were analyzed by X-ray diffraction (XRD), Fourier transform infrared spectrometer (FTIR), scanning electron microscopy (SEM), energy dispersive spectrometer (EDS), and X-ray photoelectron spectroscopy (XPS). Compared with the pristine LNMO sample, Mo-F co-doped LNMO materials could increase the lattice parameters, reduce particle sizes, increase Mn³⁺ contents, and significantly improve the electrochemical performances of LNMO. The doped material exhibited optimum electrochemical properties when the Mo and F doping amounts are 1% and 3% ,respectively, denoted as Mo/F-2. The discharge capacity retention of Mo/F-2 is 95.6%, which is higher than the pristine sample (87.7%) after 100 cycles at 1C and room temperature. Furthermore, the discharge-specific capacity of the Mo/F-2 sample reaches 113.4 mAh g^− 1 at 5C, while the pristine sample reaches only 61.9 mAh g^− 1. After CV and EIS analysis, it was found that the Mo-F co-doped LNMO materials had better Li⁺ diffusion kinetics than the pristine LNMO sample. Thus, Mo-F co-doping is considered an effective modification method for LNMO cathode material.

Li-ion battery

LiNi0.5Mn1.5O4

Mo6+ and F− co-doping

disordered phase

electrochemical performance

Nowadays, energy shortage and environmental pollution problems are increasingly prominent, and the need to explore new environment friendly energy is imminent. In this context, people vigorously seek and develop sustainable and renewable energy, and reusable batteries have become the main trend of energy development. Among many energy storage devices, lithium-ion battery (LIB) stands out with its advantages of high energy density, long cycle life, high rate performance, and good safety, and is considered to be a new large-scale power source for electric vehicles (EV) and hybrid electric vehicles (HEV) [1]. In addition, the cost of positive electrode materials accounts for 30% of the entire battery cost [2], and the reduction of the cost of positive electrode materials directly determines the reduction of the battery cost, so the research and development of high energy density positive electrode materials are very important. Among the lithium-ion cathode materials, the cobalt-free cathode material LiNi_0.5Mn_1.5O₄ (LNMO) is one of the promising cathode materials due to its high voltage plateau (4.7V vs Li⁺/Li), superior specific theoretical capacity (147 mAh g^− 1), high energy density (650 Whk g^− 1), low cost and less environmental impact. It is now being actively considered for use in the manufacture of all-solid-state batteries such as 5 V lithium metal batteries [3–6].

It has been reported that LNMO has two space groups which are the Fd3m space group of Ni/Mn-disordered face-centered cubic and the P4₃32 space group of Ni/Mn-ordered sample cubic [6]. When calcined below 700°C, the ordered structure could be synthesized, and when calcined at a higher temperature, the structure changes from ordered structure to disordered structure. The process of structural transformation is often accompanied by oxygen deficiency, the formation of Li_xNi_1−xO rock-salt type impurities, and the generation of Mn³⁺ [7–8]. This structural transition is usually reversible. P4₃32 space group can increase by using synthesis processes under oxygen or post-annealing in air below 700°C, and Mn³⁺ can be oxidized to Mn⁴⁺ [9].

The ordered P4₃32 phase and the disordered Fd3m phase generally exist together in LNMO, and the electrochemical performance of LNMO composed of different phases is also different [10]. However, the Fd3m phase has higher ionic and electronic diffusion coefficients than the P4₃32 phase which also leads to better electrochemical properties of the disordered structure, Therefore, the disordered phase lithium nickel-manganate is more inclined to be synthesized [11–13]. However, disordered structures are often accompanied by problems such as the formation of impurity phases and the dissolution of Mn³⁺ [14, 15], which affects its electrochemical performance. In addition, the higher voltage will aggravate the decomposition of electrolytes, side reactions between the electrolyte and battery active material, and the formation of thick SEI film, which results in difficulties in the intercalation and deintercalation of lithium-ions [16]. These issues limit the further application and commercialization of LNMO [17].

To solve these issues, it is well known that elemental doping is an effective modifying method to enhance the electrochemical performances of LNMO. According to researches, elemental doping mainly includes cation doping( Al³⁺ [18], Nb⁵⁺ [19], Ti⁴⁺ [20], La³⁺ [21], Sr²⁺ [22], Si⁴⁺ [23], etc.), anion doping (Cl⁻ [24], F⁻ [25], S²⁻ [26], etc.), and multi-ion doping (Cr-F [27], Y-Ti [28], etc.). The multi-ion doped cathode materials usually have superior electrochemical properties with the synergistic of different elements [29].

Zeng et al. found that the doping of Mo⁶⁺ in to LNMO could favor Li⁺ diffusion and increase electronic conductivity [30]. A very small amount of doping of Mo⁶⁺ was shown to improve the electrochemical performance of LNMO, including its rate and cycling performance and the Mo-doped material showed superior electrochemical performance when the doping amount is 1%. The F⁻ ion is usually substituted for O²⁻ to inhibit the dissolution of Mn³⁺, to inhibit the side reactions between the electrolyte and the active material, to reduce the resistance, and to prevent the corrosion of HF in the electrolyte [25, 31]. So it is our hope and speculation that the Mo⁶⁺ and F⁻ co-doping could improve the electrochemical performance of LNMO in a great deal. In this work, we prepared a series of samples with varying F content but fixed Mo content (x = 0.01) to investigate the synergistic effect of Mo⁶⁺ and F⁻ on the electrochemical performance of LNMO.

Synthesis and characterization of materials

The undoped and doped LiNi_0.5Mn_1.5O₄ samples were prepared by the rheological phase method. The materials were prepared according to the element molar ratio (Ni_0.5 Mn_1.5): Mo = 1: 0.01, and O: F = 4-x: x (x = 0, 0.01, 0.03, 0.05). According to the element molar ratio to weigh Ni(CH₃COO)₂·4H₂O (AR, 99%), Mn(CH₃COO)₂·4H₂O (Sinopharm Chemical Reagent Co., AR, 99%), CH₃COOLi (Sinopharm Chemical Reagent Co., AR, 99%), NH₄F (AR, 99%) by exceeding 5%, and (NH₄)₆Mo₇O₂₄·4H₂O (Sinopharm Chemical Reagent Co., AR, 99%). Mixed and ground the above raw materials thoroughly. An appropriate amount of distilled water/ethanol solution was added to form the paste, and an appropriate amount of ammonia (inopharm Chemical Reagent Co.,AR, 99%) was added to form a brown paste. Dried at 120°C for 12 h, pre-calcined at 500°C for 6h in an air atmosphere, and further calcined at 850°C for 12 h. The undoped and doped LiNi_0.5Mn_1.5O₄ samples are referred to as Pristine, Mo/F-1, Mo/F-2, and Mo/F-3, respectively.

The samples were characterized by X-ray diffraction (XRD, Bruker D8 Advance) with Cu Kα radiation at a scan rate of 4° min^− 1 in the 2θ range of 10–90°. Fourier transform infrared spectrometer (FT-IR, Thermo Fisher iS50, USA) was used to distinguish between the ordered and disordered phases. Scanning electron microscopy (SEM, S-4800, Hitachi, Japan) and energy dispersive spectrometer (EDS) were used to characterize the morphologies and elemental distribution. The surface chemical states were tested by X-ray photoelectron spectroscopy (XPS, Thermo K-alpha).

Electrochemical measurement

The button cells (CR-2032) were assembled to study the electrochemical performances of cathode materials. The active material (80 wt%), acetylene black (12 wt%), and polyvinylidene fluoride (PVDF, 8 wt%) dissolved in NMP solvent were mixed by enough stirring to obtain a slurry which was coated on aluminum foil with a Doctor blade technique. Dried the slurry in a vacuum oven at 80°C for 12 hours. Then, the foil is punched into a round electrode with a diameter of 14 mm. The CR2032 coin cells were assembled in an argon-filled glove box (SUPER 1220/750, made in Shanghai, China), using lithium metal as the counter electrode, Celgard 2325 as the separator, and 1 M LiPF₆ in EC: DMC: EMC (v: v: v = 1:1:1) solution as the electrolyte.

To evaluate the electrochemical performances of the prepared samples, The constant current charge-discharge cycle test was carried out by using the LAND battery test system (CT2001A, Wuhan in China) at 25°C. Cells were tested at different current densities (1C = 147 mAh g^− 1) in the voltage range of 3.5–4.95 V. The cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) measurements were carried out at an electrochemical workstation (IM6, Zahner, Germany) within the voltage range of 3.5–5.1 V. CV tests were conducted at the scan rate of 0.1 mV s^− 1. EIS tests were performed in the frequency range of 0.01–100 kHz and amplitude of 5 mV.

3.1. Morphology and structural analysis

Figure 1a exhibits the XRD patterns of pristine LNMO and LiNi_0.4975Mn_1.4925Mo_0.01O_4−xF_x (x = 0.01, 0.03, 0.05) samples. The diffraction peaks of all samples match the patterns of standard PDF card No. 80-2162, and the sharp diffraction peaks indicate good crystallinity of the materials, which infers that the low dose of Mo-F co-doping will not change the basic structure of the material. The absence of the (220) diffraction peak indicates that the transition metal ions do not occupy position 8a [32]. Located at 2θ = 37.6° and 43.7° are the weak peaks of the Li_xNi_1−xO impurity phase. With the preparation of the Fd3m structure of the LNMO sample at a high synthesis temperature, the oxygen deficiency formed, and the nickel departed from the spinel phase to form the rock salt impurities. Such a reaction was proposed [8]:

LiNi_0.5Mn_1.5O₄ ↔αLi_xNi_1−xO + βLiNi_0.5−xMn_1.5+x + γO₂. (1)

The impurity phase can decrease the content of the active material and reduce the specific capacity of the material, but there is no obvious significant negative effect on the cycling performance of the product [33, 34]. The Rietveld refinement profiles of pristine, Mo/F-1, Mo/F-2, and Mo/F-3 are shown in Fig. 1b-e. The lattice parameters obtained from Rietveld refinement patterns for all samples are shown in Table 1. Compared with the bare LNMO, the lattice parameters of the Mo-F doped samples have a little increase. This can be attributed to an increase in the interlayer distance as Mo⁶⁺ is doped [35]. Furthermore, it also can be attributed to the increase in the content of Mn³⁺ in the material due to the doping of F⁻ and Mo⁶⁺. The larger ion radius (0.58 Å for low and 0.65 Å for high spin manganese) of Mn³⁺ compared with Mn⁴⁺ (0.53 Å) makes the lattice parameters of the material expand, which is more conducive to the transport of Li⁺ [30, 36].

Table 1

Lattice parameters of all samples
Samples	Lattice parameter		Inter	R_wp/%
Samples	α/Å	V/Å³	d₍₁₁₁₎/ Å	R_wp/%
Pristine	8.17823	546.988	4.7217	3.47
Mo/F-1	8.17852	547.047	4.7219	3.77
Mo/F-2	8.18216	547.778	4.7240	3.33
Mo/F-3	8.18707	548.765	4.7268	3.57

LNMO can be divided into ordered P4₃32 and disordered Fd3m phases due to the different order of Ni/Mn cations in the spinel structure. Because the scattering factors of Ni and Mn are very similar, XRD is hard to identify the two space groups accurately[37]. FT-IR spectroscopy can be used to analyze the degree of ordering of samples. It is reported that the P4₃32 space group of LNMO material has 8 infrared absorption bands, while the Fd-3m space group has only 5 bands between 400 cm^− 1 and 700 cm^− 1 [38]. As shown in Fig. 1f, five infrared absorption peaks can be seen at 621, 581, 555, 505, and 469 cm^− 1. Among them, the absorption peaks at 621 cm^− 1 and 555 cm^− 1 were related to the vibration of Mn-O, while the absorption peaks at 581 cm^− 1 and 505 cm^− 1 were related to the vibration of Ni-O. The intensity ratio of Mn-O at 621 cm^− 1 and Ni-O at 581 cm^− 1 (I₍₆₂₁₎/I₍₅₈₁₎) can be used qualitatively to assess the percentage of ordering in spinel [39]. The intensity of 621 cm^− 1 is higher than the peak intensity at 581 cm^− 1 for all samples, indicating that all samples are predominantly disordered Fd3m phases. The I₍₆₂₁₎/I₍₅₈₁₎ ratios of the pristine, Mo/F-1, Mo/F-2, and Mo/F-3 samples were 1.073, 1.175, 1.188, and 1.196, respectively, so the Mo-F co-doped samples show a higher degree of disorder than the pristine LNMO. Compared with the ordered P4₃32 phase, the disordered Fd3m phase has better rate capability [40].

The morphologies of the four materials were characterized by SEM, as shown in Fig. 2a-d. After the doping of Mo⁶⁺ and F⁻, the LNMO morphology changed from a truncated octahedral morphology to a standard octahedral structure. The (111) facet is conducive to the formation of SEI films, which makes the (111) facet more stable than (100) and (110) facets, The doped LNMO materials with the dominant (111) surfaces exhibit a superior cycle life compared to pristine LNMO [41]. It can be easily seen from Fig. 2a-d that compared to pristine LNMO, the doped LNMO samples have relatively smaller particle sizes. The smaller particle size will shorten the Li⁺ diffusion path, which improves the rate capability of the material [30]. Among doped LNMO materials, the Mo/F-2 sample has a smooth surface, sharper crystal edges, and uniform particle size. When the doping content continues to increase, as shown in Fig. 2d, small particles appear on the large particle surface, which will be detrimental to the electrochemical performance of the material.

EDS scanning photographs of Mn, Ni, O, Mo, and F elements in the Mo/F-2 sample are shown in Fig. 2e-j. The distribution of each element is uniform, indicating that the Mo⁶⁺ and F⁻ were successfully introduced into the LNMO structure.

XPS was used to further analyze the elemental composition and chemical valence states of the samples. Figure 3a-b shows the XPS spectrums of Mn 2p for pristine and Mo/F-2 LNMO samples, where the peaks of Mn 2p_1/2 and Mn 2p_3/2 are located at ~ 654 eV and ~ 642 eV, respectively. The Mn 2p_3/2 is divided into Mn⁴⁺ and Mn³⁺, whose peaks are located at ~ 643eV and ~ 642 eV respectively. This indicates that Mn³⁺ and Mn⁴⁺ both exist on the surface of the two samples, which once again proves the existence of the disordered Fd3m phase in the samples. By calculating the ratio of Mn³⁺/Mn⁴⁺ peak areas in the two samples, it is found that the content of Mn³⁺ in the Mo/F-2 LNMO sample is significantly higher than in the pristine LNMO sample. It was shown that an appropriate amount of Mo-F co-doping could increase the Mn³⁺ content in the LNMO material which is good for improving the rate capability of the LNMO material. Figure 3c shows the XPS pattern of Mo 3d of the Mo/F-2 sample. The Mo 3d peak shape is symmetrical, and the peaks of Mo 3d_5/2 and Mo 3d_3/2 are located at 232.16 eV and 235.37 eV, respectively, corresponding to the binding energy of MoO₃. The XPS spectrum of F 1s is shown in Fig. 3d, and a peak at 684.39 eV can be observed. The results showed that Mo⁶⁺ and F⁻ existed on the crystal surface of Mo/F-2-doped materials.

3.2. Electrochemical test

A series of electrochemical tests were performed on the four samples to explore the effect of Mo-F co-doping on the electrochemical properties of the LNMO material. Figure 4a shows the initial charge-discharge profiles for all samples at 0.2 C. It can be seen from the profiles that all samples have a short plateau at around 4.0 V and a long plateau at around 4.7 V. The former is derived from the Mn³⁺/Mn⁴⁺ redox couple, and the latter is derived from the Ni²⁺/Ni⁴⁺ redox couple. All samples have a short stage at 4.0 V, proving that there were disordered Fd3m phases in all samples. The first charge-discharge coulombic efficiency of all samples is poor, which can attributed to the decomposition of the electrolyte at high voltage, the formation of a solid electrolyte interface (SEI) layer onto the spinel, and the formation of Mn₃O₄-like structure and the rocksalt-like structure [42–44]. The initial discharge capacity of the pristine LNMO sample is 129.5 mAh g^− 1, while the discharge capacities of the doped samples of Mo/F-1, Mo/F-2, and Mo/F-3 were 132.8 mAh g^− 1, 136.4 mAh g^− 1 and 134.7 mAh g^− 1, respectively. The Mo-F co-doped LNMO materials have better initial charge-discharge features than pristine LNMO, and the Mo/F-2 sample has the highest initial discharge capacity. The discharge capacity of Mo/F-3 is lower than Mo/F-2. It can be attributed to increased Mn³⁺ which can be seen from the platform at 4 V. The Mn dissolution increased, and more Mn²⁺ was deposited on the anode, hindering the extraction of Li⁺ and also consuming part of Li⁺, resulting in a decrease in material capacity [15].

Figure 4b shows the discharge capacities of all samples at different current densities at 0.2 C, 0.5 C, 1 C, 2 C, and 5 C, and then again to 0.2 C. The rate capacities of all the doped LNMO samples are significantly better than the pristine LNMO sample. The Mo/F-2 sample exhibits the optimal rate capability for 136.2, 135.1, 130.9, 126.0, and 113.4 mAh g^− 1 at 0.2 C, 0.5 C, 1 C, 2 C and 5 C, respectively. However, the discharge capacities of the pristine LNMO sample at the same current densities were 130.7, 126.1, 118.6, 106.1, and 61.9 mAh g^− 1. It indicates that the doping of Mo⁶⁺ and F⁻ can improve the rate capability of LNMO materials.

The improvement of its rate capability can be attributed to the following reasons: (i) Mo-F co-doping increases the lattice parameters of the material which facilitates the Li⁺ diffusion (ii) All the particle sizes of the doped materials are smaller than pristine LNMO sample, which shortens the Li⁺ diffusion path and is beneficial to Li⁺ diffusion. (iii) The higher binding energy of Mo-O compared to Ni-O makes the spinel frame structure more stable and is conducive to the extraction and insertion of Li⁺ [30] (iv) F⁻ can inhibit the dissolution of the electrolyte, reduce Mn dissolution, and suppress the polarization [31]. When the discharge capacity returns from 5C to 0.2C, the discharge capacity of all samples almost returns to the original value, indicating that all samples have good structural stability after a rapid lithium-ion insertion/extraction process [45].

Figure 4c-d shows the rate cycle performance curves of the pristine LNMO sample and the Mo/F-2 LNMO sample at 0.2-5 C. At the lower rates, the curves of the two samples are similar, with high electrode potential, wide discharge plateau, and two discharge plateaus at 4 V and 4.7 V. As the discharge current increased, the electrode potential of the pristine sample decreased significantly, the discharge plateau narrowed, and the 4.0 V plateau gradually disappeared. In contrast, the change in the discharge platform and electrode potential of the Mo/F-2 sample was relatively small, which indicated that Mo-F co-doping inhibited electrochemical polarization and ohmic polarization at high rates. The results indicate that the appropriate amount of Mo-F co-doping is favorable to reduce the polarization which improves the rate capability of the LNMO.

Figure 4e shows the cyclic volts curve of the pristine and Mo/F-2 materials at 0.1 mV s^− 1 with the scan voltage range of 3.5 to 5.1 V. The CV curves of pristine and Mo/F-2 samples are similar, with a peak around 4V, corresponding to the Mn³⁺/Mn⁴⁺ redox couples, and one peak at about 4.7 V, corresponding to the Ni²⁺/Ni⁴⁺ redox couples [46]. The larger the peak area at 4 V, the greater the Mn³⁺ there are [45]. It can be seen from the graph that the Mn³⁺ in the Mo/F-2 sample has increased compared with the pristine sample. The Ni²⁺/Ni⁴⁺ anodic and cathodic peaks correspond to 4.911 V and 4.563 V, respectively, and the voltage difference (ΔE) is 0.348 V. The Ni²⁺/Ni⁴⁺ anodic and cathodic peaks of the Mo/F-2 sample correspond to 4.879 V and 4.578 V, respectively, and the ΔE value is 0.301 V. Typically, the potential difference (ΔE) between the anode peak and the cathode peak reflects the electrochemical polarization [47]. The ΔE value of Mo/F-2 sample is smaller than the pristine sample, indicating faster lithium insertion/extraction kinetics in Mo/F-2 sample. The result is consistent with the above rate performance test results, indicating that an appropriate amount of Mo-F co-doping can help reduce polarization which improves the rate capability of the LNMO material.

Figure 4f shows the cycle performances of all samples after 100 cycles at 1 C and 25°C. After 100 cycles, the discharge capacities of Pristine LNMO, Mo/F-1, Mo/F-2, and Mo/F-3 samples change from 117.6, 122.3, 130.5, and 126.7 mAh g^− 1 to 103.2, 111.6, 124.8, and 119.5 mAh g^− 1, with the capacity retention rates of 87.7%, 91.3%, 95.6%, and 94.3%, respectively. The capacity retention rates of the doped samples are all higher than the undoped sample and the Mo/F-2 sample had optimal cycle stability which indicates that a proper amount of Mo-F co-doping is beneficial to the cycle performance. The result can be attributed to that the Mo⁶⁺ and F⁻ can make the structure of LNMO more stable which inhibits the contraction and expansion of the unit cell during the cycle. Furthermore, the doping of F⁻ can reduce the side reaction between the cathode material and the electrolyte, and inhibit the dissolution of Mn³⁺, so that improve the cycling performance of the material. The capacity retention rate of the Mo/F-3 sample is slightly lower than that of the Mo/F-2 sample because there are some small particles on the surface of the Mo/F-3 sample, which affects the cycling performance of the material.

As shown in Fig. 5a, EIS was used to further analyze the electrochemical kinetic properties of the undoped and doped samples after 3 cycles in the frequency range of 0.01 Hz ~ 100 kHz. The impedance spectrum comprises a semicircular section followed by a linear segment in each curve. The point where the curve first intersects with Z' denotes the solution impedance (R_s), encompassing the ohmic resistance found within the electrolyte, porous membrane, wire, and active material particles. The semicircular shape observed within the mid-to-high frequency range signifies charge transfer resistance (R_ct), which represents the charge transfer of Li⁺ between the electrolyte and the electrode. The oblique line in the low-frequency region corresponds to the Warburg impedance related to the diffusion impedance of Li⁺ in the electrode [48]. According to the equivalent circuit diagram of Fig. 5c, the impedance values calculated using the Z-view software are presented in Table 2. The R_s of Pristine, Mo/F-1, Mo/F-2, and Mo/F-3 are 6.15, 5.91, 4.45, and 5.11 Ω, respectively, the R_s values of all materials are similar. In contrast, the R_ct values of the four samples are more different, its values are 96.82, 71.57, 37.84, and 47.51 Ω, respectively. The R_ct values of the Mo-F co-doped samples were all smaller than those of the pristine LNMO sample. This is because the doping of Mo⁶⁺ and F⁻ increased the content of Mn³⁺ which improved the conductivity of the material. Furthermore, F⁻ can reduce the oxidative decomposition of the electrolyte and the corrosion of HF in the electrolyte, thereby, reducing the side reactions on the surface and impedance caused by the decomposition products [31]. Among them, Mo/F-2 has the smallest resistance and has the best electrochemical kinetics, while Mo/F-3 resistance has increased, which may be because excess F⁻ interferes with the migration of Li⁺. The results indicated that the proper amount of Mo-F co-doping could reduce the charge transfer resistance and improve the electronic conductivity of the LNMO material.

Table 2

Impedance and Li⁺ diffusion coefficient of all samples after the 3rd cycles
Samples	R_s (Ω)		σ	D_Li+ (cm² s^− 1)
Pristine	6.15	96.82	103.93	1.1×10^− 13
Mo/F-1	5.91	71.57	76.81	2.01×10^− 13
Mo/F-2	4.45	37.84	39.98	7.44×10^− 13
Mo/F-3	5.11	47.51	62.55	3.04×10^− 13

The diffusion coefficient of Li⁺ can be calculated from the slope of the impedance low-frequency area in Fig. 5b, and the formulas are as follows [49]:

$${\text{D}}_{{\text{Li}}^{\text{+}}}\text{=}\frac{{\text{R}}^{\text{2}}{\text{T}}^{\text{2}}}{\text{2}{\text{n}}^{\text{4}}{\text{F}}^{\text{4}}{\text{C}}_{\text{Li}}^{\text{2}}{{\text{A}}_{\text{c}}^{\text{2}}\sigma }^{\text{2}}}$$

$${\text{Z}}^{{\prime }}\text{=}{\text{R}}_{\text{s}}\text{+}{\text{R}}_{\text{ct}}\text{+σ}{\text{ω}}^{\text{-0.5}}$$

In Eq. (2), D_Li+, R, T, n, F, C_Li, and A_c represent the Li⁺ diffusion coefficient, gas constant (8.314 J mol^− 1 K^− 1), the absolute temperature of the test environment, the number of transfer electrons, the Faraday constant, the molar concentration of Li⁺ in the cathode material and the contact area between the electrode and the electrolyte, respectively. In Eq. (3), ω is the angular frequency and σ is the Warburg impedance scale factor fitted by ω^−0.5 and Z′. The Li⁺ diffusion coefficients of the four samples are shown in Table 2. The D_Li+ of Pristine, Mo/F-1, Mo/F-2, and Mo/F-3 are 1.1×10^− 13, 3.04×10^− 13, 7.44×10^− 13 and 2.01×10^− 13 cm² s^− 1, and the Li⁺ diffusion coefficients of all doped samples were higher than the pristine sample and Mo/F-2 had the highest D_Li+. The result can be attributed to that the introduction of Mo⁶⁺ and F⁻ made the material structure more stable, expanded the unit cell parameters of the material, and shortened the Li⁺ diffusion path, which increased the Li⁺ diffusion coefficient.

In this work, the synergistic effect of Mo⁶⁺ and F⁻ on the electrochemical performance of LNMO was explored. Through various characterization and electrochemical tests, it is found that the introduction of Mo⁶⁺ and F⁻ is beneficial to stabilize the material structure, increase Mn³⁺ content, reduce the polarization, reduce the charge transfer resistance, and improve the electronic conductivity and Li⁺ diffusion coefficient of the LNMO cathode material, and so that improve the rate capability and cyclic stability of LNMO cathode material. The Mo/F-2 sample showed the optimal electrochemical performance, with a capacity retention rate of 95.6% after 100 cycles at 1 C and a discharge capacity of 113.4 mAh g^− 1 at 5 C. In summary, Mo-F co-doping is an effective modification method for LNMO, which is of great significance for improving the electrochemical performance and commercial application of LNMO.

Ethical Approval

No human and/or animal studies have been include in this paper. Also no any other ethical problems are involved.

Competing interests

I declare that the authors have no competing interests, or other interests that might be perceived to influence the results and/or discussion reported in this paper.

Authors' contributions

All the three authors have contributed to the work in this paper. But the first author has contributed the most. This manuscript contains contributions of all authors, and all authors have approved of the final version of the manuscript.

Funding

No funding for this paper.

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No competing interests reported.

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Enhancement of the electrochemical performance of LiNi_0.5Mn_1.5O₄ cathode materials for Li-ion battery by Mo-Fco-doping

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Abstract

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Introduction

Experimental section

Synthesis and characterization of materials

Electrochemical measurement

Result and discussion

3.1. Morphology and structural analysis

3.2. Electrochemical test

Conclusion

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References

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