Guangchuan Liang

Synthesis of porous peanut-like LiNi0.5Mn1.5O4 cathode materials through an ethylene glycol-assisted hydrothermal method using urea as a precipitant

The high voltage spinel LiNi0.5Mn1.5O4 with a peanut-like morphology and porous structure was synthesized by an ethylene glycol-assisted hydrothermal method using urea as a precipitant followed by high-temperature calcination. For comparison, the LiNi0.5Mn1.5O4 sample was also synthesized in the aqueous solution in the absence of ethylene glycol (EG). The as-prepared materials were characterized by XRD, SEM, TEM, FT-IR, CV, EIS and galvanostatic charge/discharge tests. The presence of EG in the hydrothermal process improves the dispersity and decreases the particle size of the final LiNi0.5Mn1.5O4 product, thus leading to its better rate capability, whose discharge capacity at a 10C rate could reach as high as 121.4 mA h g−1. On the other hand, the presence of EG in the hydrothermal process could make the reagents mix more homogeneously, thus leading to higher crystallinity, lower impurity and Mn3+ contents, which are advantageous to the cycling performance. Furthermore, the porous structure of the LiNi0.5Mn1.5O4 material could effectively mitigate the volume change caused by the repeated Li+ insertion/extraction process, which is also conducive to the cycling stability. The LiNi0.5Mn1.5O4 cathode material synthesized by the EG-assisted hydrothermal process shows a capacity retention rate of 96.4% after 100 cycles at a 1C rate. Additionally, a possible formation mechanism for the Ni0.25Mn0.75CO3 precursor with a peanut-like morphology was also proposed.

Improved structural and electrochemical performances of LiNi0.5Mn1.5O4 cathode materials by Cr3+ and/or Ti4+ doping

High voltage LiNi0.45M0.05Mn1.5O4 (M = Ni, Cr, Ti, Cr0.5Ti0.5) cathode materials were synthesized by solid-state method, and the effects of Cr/Ti doping alone and co-doping on the crystalline structure, Mn3+ content, particle morphology and electrochemical performance of LiNi0.5Mn1.5O4 cathode materials were systematically investigated. The as-prepared samples were characterized by XRD, FT-IR, SEM, CV, EIS and galvanostatic charge/discharge cycling tests. XRD results show that both pristine and doped materials have cubic spinel structure with Fd3m space group, and the Cr and/or Ti doping can effectively prevent the formation of LiyNi1−yO impurity phase. FT-IR spectra indicate that the Cr and/or Ti doping increases the disordering degree of Ni/Mn ions in 16d octahedral sites. SEM observation discloses that the Cr and/or Ti doping increases the particle size distribution homogeneity and decreases the average primary particle size. EIS analysis illustrates that the Cr and/or Ti doping decreases the charge transfer resistance and increases the Li+ ion diffusion coefficient. All of the above-mentioned factors are believed to be advantageous to the cycling stability and rate capability. Among which, the Cr and Ti co-doped sample LiNi0.45Cr0.025Ti0.025Mn1.5O4 exhibits optimal cycling performance with a capacity retention rate of 102.1% after 100 cycles at 1C rate, and optimal rate capability with a discharge capacity of 118.7 mA h g−1 at 10C rate, which is 96.1% of its capacity at 0.2C rate. The excellent electrochemical performance of LiNi0.45Cr0.025Ti0.025Mn1.5O4 cathode material may be mainly attributed to the presence of appropriate Mn3+ content and higher Li+ ion diffusion coefficient.

Synthesis and characterization of Al-substituted LiNi0.5Co0.2Mn0.3O2 cathode materials by a modified co-precipitation method

An Al-substituted LiNi0.5Co0.2Mn0.3O2 (Al-NCM) cathode material was synthesized by a modified co-precipitation method followed by heat treatment process. The obtained materials were extensively characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), galvanostatic charge/discharge tests, cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). The results indicated that the as-prepared Al-NCM sample exhibited a well hexagonal α-NaFeO2 structure, lower cation mixing, higher crystallinity, a more homogeneous spherical structure, lower charge transfer resistance and a higher lithium ion diffusion coefficient, thus leading to enhanced electrochemical performance. The effects of Al substitution on rate capability, cycling stability and low-temperature performance of LiNi0.5Co0.2Mn0.3O2 (NCM) were evaluated by coin cells and 14500 full batteries. Specifically, the Al-NCM sample delivered discharge specific capacities of 159.7, 147.4, 139.8, 135.7, 126.9 and 111.3 mA h g−1 at 0.5C, 1C, 2C, 3C, 5C, and 10C rates, respectively, with capacity retention ratio of 86.6% after 100 cycles at 1C rate under the voltage range of 3.0–4.3 V. The 14500 battery utilizing Al-NCM material as the cathode delivered a discharge capacity of 803.55 mA h at 1C rate in the potential range of 2.7–4.2 V, with capacity retention of 89.30% after 200 cycles. Moreover, the 14500 battery fabricated with Al-NCM as the cathode delivered a discharge capacity of 590.06 mA h at 1C rate at −20 °C, much higher than 471.30 mA h of the battery assembled with NCM as the cathode.

Effects of Na + doping on crystalline structure and electrochemical performances of LiNi 0.5 Mn 1.5 O 4 cathode material

Abstract Pristine LiNi0.5Mn1.5O4 and Na-doped Li0.95Na0.05Ni0.5Mn1.5O4 cathode materials were synthesized by a simple solid-state method. The effects of Na+ doping on the crystalline structure and electrochemical performance of LiNi0.5Mn1.5O4 cathode material were systematically investigated. The samples were characterized by XRD, SEM, FT-IR, CV, EIS and galvanostatic charge/discharge tests. It is found that both pristine and Na-doped samples exhibit secondary agglomerates composed of well-defined octahedral primary particle, but Na+ doping decreases the primary particle size to certain extent. Na+ doping can effectively inhibit the formation of LixNi1–xO impurity phase, enhance the Ni/Mn disordering degree, decrease the charge-transfer resistance and accelerate the lithium ion diffusion, which are conductive to the rate capability. However, the doped Na+ ions tend to occupy 8a Li sites, which forces equal amounts of Li+ ions to occupy 16d octahedral sites, making the spinel framework less stable, therefore the cycling stability is not improved obviously after Na+ doping.

Improved conductivity and electrochemical properties of LiNi0.5Co0.2Mn0.3O2 materials via yttrium doping

A series of LiNi0.5Co0.2−xMn0.3YxO2 (x = 0, 0.01, 0.02, 0.03) materials was synthesized using a co-precipitation method, and the impact of yttrium doping on the crystalline structure, particle morphology, particle size, electronic conductivity and electrochemical performances was investigated. The PXRD refinement, SEM, EDS and XPS results indicate that yttrium ions have been successfully incorporated into the matrix structure of the materials, and that the crystalline structure and particle morphology of the LiNi0.5Co0.2Mn0.3O2 cathode material is not changed after yttrium doping. Electrochemical test results show that the rate capability, cycling stability and low-temperature performance of the yttrium doped materials are remarkably improved. Especially, the LiNi0.5Co0.18Mn0.3Y0.02O2 sample exhibits the best electrochemical performance, delivering a high discharge capacity of 125.6 mA h g−1 at a rate of 10C, with a capacity retention of 83.0% after 150 cycles at a rate of 1C in the voltage range of 3.0–4.5 V. The material also shows a discharge capacity of 148.9 mA h g−1 at −20 °C at a rate of 0.2C, which is 81.0% of the same discharge rate at 25 °C, with a capacity retention of 81.5% after 50 cycles. CV and EIS measurements show that LiNi0.5Co0.18Mn0.3Y0.02O2 exhibits lower polarization, lower charge transfer resistance and a larger lithium ion diffusion coefficient than LiNi0.5Co0.2Mn0.3O2. Additionally, the electronic conductivity of the LiNi0.5Co0.18Mn0.3Y0.02O2 sample is 2.69 × 10−2 S cm−1, which is fourteen times higher than that of LiNi0.5Co0.2Mn0.3O2. Therefore, both the electronic conductivity and ionic conductivity of LiNi0.5Co0.2Mn0.3O2 are improved by yttrium doping, which are beneficial to enhancing electrochemical performance.

Urea-assisted hydrothermal synthesis of a hollow hierarchical LiNi0.5Mn1.5O4 cathode material with tunable morphology characteristics

A hollow hierarchical LiNi0.5Mn1.5O4 cathode material has been synthesized via a urea-assisted hydrothermal method followed by a high-temperature calcination process. The effect of reactant concentration on the structure, morphology and electrochemical properties of the carbonate precursor and corresponding LiNi0.5Mn1.5O4 product has been intensively investigated. The as-prepared samples were characterized by XRD, FT-IR, SEM, CV, EIS, GITT and constant-current charge/discharge tests. The results show that all samples belong to a cubic spinel structure with mainly Fd3m space group, and the Mn3+ content and impurity content initially decrease and then increase slightly with the reactant concentration increasing. SEM observation shows that the particle morphology and size of carbonate precursor can be tailored by changing reactant concentration. The LiNi0.5Mn1.5O4 sample obtained from the carbonate precursor hydrothermally synthesized at a reactant concentration of 0.3 mol L−1 exhibits the optimal overall electrochemical properties, with capacity retention rate of 96.8% after 100 cycles at 1C rate and 10C discharge capacity of 124.9 mA h g−1, accounting for 99.9% of that at 0.2C rate. The excellent electrochemical performance can be mainly attributed to morphological characteristics, that is, smaller particle size with homogeneous distribution, in spite of lower Mn3+ content.

Blended spherical lithium iron phosphate cathodes for high energy density lithium–ion batteries

Blended spherical cathodes of lithium iron phosphate with different particle sizes were prepared using a physical mixing method. The processability and electrochemical properties of blended spherical cathodes were systematically investigated. The characterization results suggest that the blended spherical cathodes contain two different-sized particles, and smaller particles can fill in the gaps created by larger ones. This structural feature causes an increase in both the connect area of the particles and the compaction density of the blended spherical cathodes, which can enhance the processability of the materials. Increasing the difference in particle sizes between the two groups of blended spherical cathodes is beneficial to obtain an enhanced overall performance, as tested with 14,500 cylindrical batteries, such as higher compaction density, lower resistance, superior high-rate capacity, excellent cycle stability, and higher volume energy density. Therefore, lithium iron phosphate batteries fabricated using the blended spherical cathode with two particle groups that differ significantly in size have a good application prospect.