Synthesis and Electrochemical Properties of LiNi0.5Mn1.5O4 Cathode Materials with Cr3+ and F− Composite Doping for Lithium-Ion Batteries

A Cr3+ and F− composite-doped LiNi0.5Mn1.5O4 cathode material was synthesized by the solid-state method, and the influence of the doping amount on the material’s physical and electrochemical properties was investigated. The structure and morphology of the cathode material were characterized by XRD, SEM, TEM, and HRTEM, and the results revealed that the sample exhibited clear spinel features. No Cr3+ and F− impurity phases were found, and the spinel structure became more stable. The results of the charge/discharge tests, cyclic voltammetry (CV), and electrochemical impedance spectroscopy (EIS) test results suggested that LiCr0.05Ni0.475Mn1.475O3.95F0.05 in which the Cr3+ and F− doping amounts were both 0.05, had the optimal electrochemical properties, with discharge rates of 0.1, 0.5, 2, 5, and 10 C and specific capacities of 134.18, 128.70, 123.62, 119.63, and 97.68 mAh g−1 , respectively. After 50 cycles at a rate of 2 C, LiCr0.05Ni0.475Mn1.475O3.95F0.05 showed extremely good cycling performance, with a discharge specific capacity of 121.02 mAh g−1 and a capacity retention rate of 97.9%. EIS test revealed that the doping clearly decreased the charge-transfer resistance.


Background
The increasing demand for electric vehicles (EV), hybrid electric vehicles (HEV), and high-capacity storage batteries requires higher performance lithium-ion batteries with improved energy density and power density [1][2][3]. The cathode material is a key material in lithium-ion batteries, and research and development into highpotential cathode materials is one of the main ways to improve the energy density of lithium-ion batteries. Spinel LiNi 0.5 Mn 1.5 O 4 has the advantage of discharge voltage plateaus at approximately 4.7 V: low cost, excellent structural stability, and heat stability, and is considered one of the most promising cathode materials for lithium-ion batteries. However, the cycling stability of LiNi 0.5 Mn 1.5 O 4 is poor, and cycling of this material results in the Jahn-Teller effect and Mn dissolution [4][5][6][7].
Modification of the material by doping and coating has been applied to suppress the Jahn-Teller effect and to reduce Mn loss in order to improve the electrochemical properties of the material. Doping modification is a very effective approach that can not only enhance the stability of the crystal structure but also improve the rate capability of the material [8,9]. During charging, 4.7% of the volume of LiNi 0.5 Mn 1.5 O 4 is maintained when going from the lithium-rich phase to the lithium-poor phase. The volume change in the material during the insertion/ extraction process of Li ions can be effectively suppressed by applying a small amount of doping and surface coating, and furthermore, doping can improve the rate capability and cycling performance of the material [10][11][12]. Cation doping ( Na [13], Ru [14], Rh [15], Co [16], Al [17], Cr [18], Zn [19], Nd [20], Mg [21], Mo [22], Sm [23], Cu [24], etc.) and anion doping (S [25], P [26], and F [27]) have been applied to modify LiNi 0.5 Mn 1.5 O 4. For instance, compared to pure LiNi 0.5 Mn 1.5 O 4 , Al-doped LiNi 0.5 Mn 1.5 O 4 can effectively improve the discharge capacity (up to 140 mAh g −1 ) and cycling stability (70% capacity retention after 200 cycles) [28].
In this paper, Fand Cr 3+ are selected to improve the rate capability via anion-cation compound substitution, and their doping amounts are optimized [29]. In addition, the structure, morphology, and electrochemical properties of the samples were tested and analyzed.

Experimental
Preparation of LiCr x Ni 0.5−0.5x Mn 1.5−0.5x O 3.95 F 0.05 The LiNi 0.5 Mn 1.5 O 4 materials were synthesized by the solid-state method using Ni(CH 3 COO) 2 · 4H 2 O、Mn(CH 3-COO) 2 · 4H 2 O and Cr(CH 3 COO) 3 · 6H 2 O as the starting materials. The materials were fully mixed by ball-milling for 2 h using stoichiometric amounts of LiCr x Ni 0.5 −0.5x Mn 1.5−0.5x O 3.95 F 0.05 (x = 0.025, 0.05, 0.075), and the dry mixture was heated at 400°C in air for 5 h. The Ni-Mn-Cr complex oxide formed after natural cooling in a muffle furnace. The obtained complex oxide and lithium source (Li 2 CO 3 and LiF) were mixed by ball-milling for 4 h using anhydrous alcohol as a dispersant, and the mixture was then heated at 850°C in air for 12 h to strengthen its crystallization in a muffle furnace. After being reduced at 650°C in air for 12 h, materials with different Cr 3+ and F − composite doping amounts, LiCr x Ni 0.5−0.5x Mn 1.5−0.5x O 3.95 F 0.05 (x = 0.025, 0.05, 0.075), were obtained after natural cooling in a muffle furnace.

Characterization
The crystal structures of the samples were identified by Xray diffraction (XRD, UltimaIII, diffractometer Cu-Kα radiation, 40 kV, 40 mA, Rigaku, Japan) at room temperature over a 2θ range of 10°to 80°with a scanning speed of 8°min −1 . The morphology of the LiCr x Ni 0.5 −0.5x Mn 1.5−0.5x O 3.95 F 0.05 samples was measured by a scanning electron microscopy (SEM, Hitachi, S-3400N, Japan). The microstructure and elemental composition of the obtained materials were observed by transmission electron microscopy (TEM, Tecnai G2 F20, FEI) equipped with energy dispersive spectroscopy (EDS).

Electrochemical Performance Test
The electrochemical properties were assessed with CR2032 coin cells, and the cells consisted of the LiCr x Ni 0.5 −0.5x Mn 1.5−0.5x O 3.95 F 0.05 electrode as the cathode electrode, Li metal foil as the anode electrode, American Celgard2400 as the separator and 1 mol/L LiPF 6 in EC/EMC/DMC (1:1:1 in volume) as the electrolyte. The cathode was synthesized by mixing the active material, carbon black, and polyvinylidene fluoride (PVDF) at a weight ratio of 8:1:1 in the N-methyl pyrrolidinone (NMP) to form a homogeneous slurry, which was then coated on Al foil by a doctor blade coater and subsequently dried in a vacuum oven at 120°C for 24 h to remove NMP and residual water. The coin cells were assembled in an argon-filled glove box (MBRAUN PRS405/W11006-1, Germany).
The electrochemical performance of LiCr x Ni 0.5−0.5x Mn 1.5 −0.5x O 3.95 F 0.05 /Li coin cells was evaluated by charging and discharging over 3.5-5.0 V using a CT-300-1A-SA tester (Neware Technology Ltd.). Cyclic voltammograms (CV) tests (the cathode was the working electrode and Li metal foil was both the counter and reference electrode) were carried out using an electrochemical work station (Metrohm Co., Autolab PGSTAT302N, Netherlands) with a scanning rate of 0.1 mV/s and a scanning frequency of 0.5 Hz between 3.5 and 5.0 V. Electrochemical impedance spectroscopy (EIS) was conducted on an electrochemical work station with an AC amplitude of 5 mV in the scanning frequency range of 0.01 to 100 kHz (the cathode was the working electrode and Li metal foil was both the counter and reference electrode ).  Table 1.  . However, at other high rates of 0.5, 2, 5, and 10 C, the specific discharge capacities of the LiCr 0.05 Ni 0.475 Mn 1.475 O 3.95 F 0.05 were the highest, which were 128.70, 123.62, 119.63, and 97.68 mAh g −1 , respectively. When undoped LiNi 0.5 Mn 1.5 O 4 is discharged at a rate of 2 C , the attenuation of its specific discharge capacity is more obvious. At a discharge rate of 5 C , the structure of the materials may be severely damaged. As the doping amount and substitution of Cr 3+ increases, the cycling stability increases. A higher doping amount will reduce the specific discharge capacity of the material, making the 4.1 V plateau more obvious and decreasing the energy density of the batteries. On one hand, due to the small polarization at low rate, the polarization effect showed little differences before and after doping. However, the amount of active material decreased after doping, resulting in lower specific capacity. On the other hand, owing to the large polarization at high rate and improved lithium-ion diffusion coefficient, the doped LiNi 0.5 Mn 1.5 O 4 cathode exhibited higher specific capacity. This result indicates that an appropriate amount of Cr 3+ , F − co-doping can lead to excellent cycling stability and rate capacity.     oxidation peak potential was 4.861 V, while the reduction peak potential was 4.563 V. The oxidation peak current (I Pa ) was 2.242 mA, and the reduction peak current (I Pc ) was 2.288 mA, and thus the I Pa /I Pc ratio was 0.9799. The LiCr 0.05 Ni 0.475 Mn 1.475 O 3.95 F 0.05 oxidation peak potential was 4.864 V, the reduction peak potential was 4.578 V, and the potential difference was 0.286 V. The I Pa was 1.273 mA, the I Pc was 1.277 mA, and the I Pa /I Pc ratio was 0.9968 (approximately 1). The above results indicated that the co-doped materials had good reversibility of lithium ions intercalation/deintercalation and improved coulombic efficiency. Figure 9 shows the EIS patterns of the samples. All the EIS spectra in the figure consist of two semicircles in the high-to-medium frequency region and a sloping line in the low frequency region. The semicircle in the high-frequency region corresponds to lithium ions passing through the electrolyte surface film resistance (R s ). The semicircle in the medium frequency region corresponds to the charge-transfer resistance (R ct ), and the sloping line in the low frequency region is the Warburg impedance (Z w ), which is related to lithium ion diffusion in the materials. As seen in Fig. 7, doping decreased the R s , which contributed to improving the diffusivity of lithium ions, the conductivity, and the rate capability of the materials.

Results and Discussion
The Nyquist plot of the equivalent circuit analog fitted by the ZsimpWin software is shown in Fig. 10. In this circuit, R e and R s represent the electrolyte resistance and the particle-to-particle interfacial contact resistance of the SEI film. R ct is the charge-transfer resistance, and Z w stands for the Warburg impedance caused by diffusion of lithium ions. CPEs and CPEdl  are constant phase elements of the solid electrolyte membrane and the double-layer capacitance of the electrode-electrolyte interface, respectively [32]. The fitting parameters of the equivalent circuit analog are summarized in Table 2.
Previous studies have suggested that the diffusion coefficient of lithium ions is associated with the Warburg factor, which can be calculated by the sloping line in the low frequency region. The lithium-ions diffusion coefficient was calculated by Fick's rule using the following equation: [33] where D is the lithium-ion diffusion coefficient, T is the absolute temperature, R is the gas constant, A is the surface area of the electrode, n is the electron transfer number, F is the Faraday constant, C is the molar concentration of lithium ions, and σ is the Warburg factor, which is the slope of the sloping line in Fig. 7. As seen in Table 2, the R s values of the doped samples were greatly decreased compared with the undoped sample, and the R s value of LiCr 0.05 Ni 0.475 Mn 1.475 O 3.95 F 0.05 decreased greatly. The decrease in the R s value indicates that Cr 3+ , F − co-doping can inhibit the growth of the SEI film to some extent, which may be due to the F − side reactions between the electrode material and the electrolyte solution. A lower charge-transfer resistance value indicates lower electrochemical polarization, which will lead to higher rate capability and cycling stability.

Conclusions
The Cr 3+ , F − co-doped analog of LiNi 0.5 Mn 1.5 O 4 (LiCr x Ni 0.5 −0.5x Mn 1.5−0.5x O 3.95 F 0.05 (x = 0.025, 0.05, 0.075)) was synthesized by the high-temperature solid-state method. The materials's XRD patterns showed that Cr 3+ and F − successfully substituted some of the Ni 2+ , Mn 4+ , Mn 3+ , and O 2atoms in the spinel material, and no impurity peaks existed. The specific discharge capacities of LiCr 0.05 Ni 0.475 Mn 1.475 O 3.95 F 0.05 at 0.1, 0.5, 2, 5, and 10 C were 134.18, 128.70, 123.62, 119.63, and 97.68 mAh g −1 , respectively. The specific discharge capacity was 121.02 mAh g −1 after 50 cycles at 2 C, which is of 97.9% the initial discharge capacity. The capacity retention rate of LiCr 0.05 Ni 0.475 Mn 1.475 O 3.95 F 0.05 was the largest among the samples. The materials had good crystallinity, and the largest number of octahedral spinel was well distributed. Cr 3+ , F − co-doped of the materials significantly improved the specific discharge capacity at higher rate, improved the cycling stability, enhanced the reversibility of lithium ions, and reduced the impedance value.