Synthesis and Electrochemical Properties of LiNi0.5Mn1.5O4 Cathode Materials with Cr3+ and F− Composite Doping for Lithium-Ion Batteries
© The Author(s). 2017
Received: 22 March 2017
Accepted: 30 May 2017
Published: 15 June 2017
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.
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 [1–3]. The cathode material is a key material in lithium-ion batteries, and research and development into high-potential cathode materials is one of the main ways to improve the energy density of lithium-ion batteries. Spinel LiNi0.5Mn1.5O4 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 LiNi0.5Mn1.5O4 is poor, and cycling of this material results in the Jahn-Teller effect and Mn dissolution [4–7].
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 LiNi0.5Mn1.5O4 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 [10–12]. Cation doping ( Na , Ru , Rh , Co , Al , Cr , Zn , Nd , Mg , Mo , Sm , Cu , etc.) and anion doping (S , P , and F ) have been applied to modify LiNi0.5Mn1.5O4. For instance, compared to pure LiNi0.5Mn1.5O4, Al-doped LiNi0.5Mn1.5O4 can effectively improve the discharge capacity (up to 140 mAh g−1) and cycling stability (70% capacity retention after 200 cycles) .
In this paper, F- and Cr3+ are selected to improve the rate capability via anion-cation compound substitution, and their doping amounts are optimized . In addition, the structure, morphology, and electrochemical properties of the samples were tested and analyzed.
The LiNi0.5Mn1.5O4 materials were synthesized by the solid-state method using Ni(CH3COO)2 · 4H2O、Mn(CH3COO)2 · 4H2O and Cr(CH3COO)3 · 6H2O as the starting materials.
Preparation of LiCrxNi0.5−0.5xMn1.5−0.5xO3.95F0.05
The LiNi0.5Mn1.5O4 materials were synthesized by the solid-state method using Ni(CH3COO)2 · 4H2O、Mn(CH3COO)2 · 4H2O and Cr(CH3COO)3 · 6H2O as the starting materials. The materials were fully mixed by ball-milling for 2 h using stoichiometric amounts of LiCrxNi0.5−0.5xMn1.5−0.5xO3.95F0.05 (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 (Li2CO3 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 Cr3+ and F− composite doping amounts, LiCrxNi0.5−0.5xMn1.5−0.5xO3.95F0.05 (x = 0.025, 0.05, 0.075), were obtained after natural cooling in a muffle furnace.
The crystal structures of the samples were identified by X-ray 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 LiCrxNi0.5−0.5xMn1.5−0.5xO3.95F0.05 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 LiCrxNi0.5−0.5xMn1.5−0.5xO3.95F0.05 electrode as the cathode electrode, Li metal foil as the anode electrode, American Celgard2400 as the separator and 1 mol/L LiPF6 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 LiCrxNi0.5−0.5xMn1.5−0.5xO3.95F0.05/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 ).
Results and Discussion
Refinement results for the samples
Fitting parameters of the Nyquist plots for the samples
1.05 × 10−11
5.48 × 10−11
1.51 × 10−10
4.49 × 10−11
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 LiCr0.05Ni0.475Mn1.475O3.95F0.05 decreased greatly. The decrease in the R s value indicates that Cr3+, 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. LiCr0.05Ni0.475Mn1.475O3.95F0.05 exhibited the lowest R ct value (24.9 Ω) and the highest lithium diffusion coefficient (1.51 × 10−10 cm2 s−1) among all the samples, indicating that its electrochemical polarization is the lowest and the lithium-ion mobility of LiNi0.5Mn1.5O4 can be effectively improved by anion-cation compound substitution. EIS also can be used to compare the size of the electronic conductivity. The smaller charge-transfer resistance of the Cr3+ and F− co-doping LiNi0.5Mn1.5O4 indicates a larger electronic conductivity than that of pristine LiNi0.5Mn1.5O4. The electronic conductivity of LiNi0.5Mn1.5O4 is about 3.88 × 10−5 S cm−1, while the electronic conductivities of LiCrxNi0.5−0.5xMn1.5−0.5xO3.95F0.05 (x = 0.025, 0.05, 0.075) samples were 6.19 × 10− 5 S cm-1, 1.25 × 10-4 S cm−1, and 5.98 × 10−5 S cm−1, respectively. In fact, LiCr0.05Ni0.475Mn1.475O3.95F0.05 has the best electrochemical performance among all four samples. The decrease in R ct and the increase in D indicate that the proper amount of Cr3+, F− co-doping has a positive effect on the electrochemical reaction activity of the material.
The Cr3+, F− co-doped analog of LiNi0.5Mn1.5O4 (LiCrxNi0.5−0.5xMn1.5−0.5xO3.95F0.05 (x = 0.025, 0.05, 0.075)) was synthesized by the high-temperature solid-state method. The materials’s XRD patterns showed that Cr3+ and F− successfully substituted some of the Ni2+, Mn4+, Mn3+, and O2- atoms in the spinel material, and no impurity peaks existed. The specific discharge capacities of LiCr0.05Ni0.475Mn1.475O3.95F0.05 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 LiCr0.05Ni0.475Mn1.475O3.95F0.05 was the largest among the samples. The materials had good crystallinity, and the largest number of octahedral spinel was well distributed. Cr3+, 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.
This work was supported by the National Natural Science Foundation of China (No. 20672023), the Science and Technology Plan Foundation of Guangzhou (201704030031) and the Science and Technology Plan Foundation of Guangdong (2015A050502046).
The idea is from JL, the manuscript was mainly written by SL and SH, and the figures were mainly drawn by SX and JZ. All authors read and approved the final manuscript.
The authors declare that they have no competing interests.
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