Synergistic Effects of Stabilizing the Surface Structure and Lowering the Interface Resistance in Improving the Low-Temperature Performances of Layered Lithium-Rich Materials

The layered lithium-rich cathode material, Li_1.2Ni_0.2Mn_0.6O₂, was successfully synthesized by a sol–gel method followed by coating with different amounts of Li₂O-2B₂O (LBO, 1, 3, and 5 wt %). The effects of LBO-coating layer on the structure, morphology, and low-temperature (−30 °C) electrochemical properties of these materials are investigated systematically. The morphology, crystal structure, and grain size of the Li-rich layered oxide are not essentially changed after surface modification; according to the TEM results, the Li–B–O coating layer exists as an amorphous layer with a thickness of 5–8 nm when the amount is 3 wt %. Electrochemistry tests reveal that 3 wt % LBO-coated samples present the best electrochemical capability at low temperature. At −20 °C, the 3 wt % LBO-coated sample could retain 45.7% of the initial discharge capacity (131.7/288.0 mAh g^–1) of that at 30 °C, while the pristine material could only retain 22.5% (57.5/256.0 mAh g^–1). XPS spectra and EIS results reveal that such an enhancement of low-temperature discharge capacity should be attributed to the proper LBO-coating layer, which not only endows the modified materials with more stable surface structure but also lowers the interface resistance of Li⁺ diffusion through the interface and charge transfer reaction

Effect of Ni²⁺ Content on Lithium/Nickel Disorder for Ni-Rich Cathode Materials

Li excess LiNi_0.8Co_0.1Mn_0.1O₂ was produced by sintering the Ni_0.8Co_0.1Mn_0.1(OH)₂ precursor with different amounts of a lithium source. X-ray photoelectron spectroscopy confirmed that a greater excess of Li⁺ leads to an increase in the number of Ni²⁺ ions. Interestingly, the level of Li⁺/Ni²⁺ disordering decreases with an increase in Ni²⁺ content determined by the <i>I</i>₀₀₃/<i>I</i>₁₀₄ ratio in the X-ray diffraction patterns. The electrochemical measurement shows that the cycling stability and rate capability improve with an increase in Ni²⁺ content. After cycling, electrochemical impedance spectroscopy shows decreased charge transfer resistance, and the XRD patterns exhibit an increased <i>I</i>₀₀₃/<i>I</i>₁₀₄ ratio with an increase in Ni²⁺ content, reflecting the decrease in the level of Li⁺/Ni²⁺ disorder during cycling

High-Rate and Cycling-Stable Nickel-Rich Cathode Materials with Enhanced Li⁺ Diffusion Pathway

The nickel-rich LiNi_0.7Co_0.15Mn_0.15O₂ material was sintered by Li source with the Ni_0.7Co_0.15Mn_0.15(OH)₂ precursor, which was prepared via hydrothermal treatment after coprecipitation. The intensity ratio of I₍₁₁₀₎/I₍₁₀₈₎ obtained from X-ray diffraction patterns and high-resolution transmission electronmicroscopy confirm that the particles have enhanced growth of (110), (100), and (010) surface planes, which supply superior inherent Li⁺ deintercalation/intercalation. The electrochemical measurement shows that the LiNi_0.7Co_0.15Mn_0.15O₂ material has high cycling stability and rate capability, along with fast charge and discharge ability. Li⁺ diffusion coefficient at the oxidation peaks obtained by cyclic voltammogram measurement is as large as 10^–11 (cm² s^–1) orders of magnitude, implying that the nickel-rich material has high Li⁺ diffusion capability

Ni-Rich LiNi_0.8Co_0.1Mn_0.1O₂ Oxide Coated by Dual-Conductive Layers as High Performance Cathode Material for Lithium-Ion Batteries

Ni-rich materials are appealing to replace LiCoO₂ as cathodes in Li-ion batteries due to their low cost and high capacity. However, there are also some disadvantages for Ni-rich cathode materials such as poor cycling and rate performance, especially under high voltage. Here, we demonstrate the effect of dual-conductive layers composed of Li₃PO₄ and PPy for layered Ni-rich cathode material. Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy show that the coating layers are composed of Li₃PO₄ and PPy. (NH₄)₂­HPO₄ transformed to Li₃PO₄ after reacting with surface lithium residuals and formed an inhomogeneous coating layer which would remarkably improve the ionic conductivity of the cathode materials and reduce the generation of HF. The PPy layer could form a uniform film which can make up for the Li₃PO₄ coating defects and enhance the electronic conductivity. The stretchy PPy capsule shell can reduce the generation of internal cracks by resisting the internal pressure as well. Thus, ionic and electronic conductivity, as well as surface structure stability have been enhanced after the modification. The electrochemistry tests show that the modified cathodes exhibited much improved cycling stability and rate capability. The capacity retention of the modified cathode material is 95.1% at 0.1 C after 50 cycles, whereas the bare sample is only 86%, and performs 159.7 mAh/g at 10 C compared with 125.7 mAh/g for the bare. This effective design strategy can be utilized to enhance the cycle stability and rate performance of other layered cathode materials

Role of Cobalt Content in Improving the Low-Temperature Performance of Layered Lithium-Rich Cathode Materials for Lithium-Ion Batteries

Layered lithium-rich cathode material, Li_1.2Ni_{0.2–<i>x</i>}Co_2<i>x</i>Mn_{0.6–<i>x</i>}O₂ (<i>x</i> = 0–0.05) was successfully synthesized using a sol–gel method, followed by heat treatment. The effects of trace amount of cobalt doping on the structure, morphology, and low-temperature (−20 °C) electrochemical properties of these materials are investigated systematically. X-ray diffraction (XRD) results confirm that the Co has been doped into the Ni/Mn sites in the transition-metal layers without destroying the pristine layered structure. The morphological observations reveal that there are no changes of morphology or particle size after Co doping. The electrochemical performance results indicate that the discharge capacities and operation voltages are drastically lowered along with the decreasing temperature, but their fading rate becomes slower when increasing the Co contents. At −20 °C, the initial discharge capacity of sample with <i>x</i> = 0 could retain only 22.1% (57.3/259.2 mAh g^–1) of that at 30 °C, while sample with <i>x</i> = 0.05 could maintain 39.4% (111.3/282.2 mAh g^–1). Activation energy analysis and electrochemical impedance spectroscopy (EIS) results reveal that such an enhancement of low-temperature discharge capacity is originated from the easier interface reduction reaction of Ni⁴⁺ or Co⁴⁺ after doping trace amounts of Co, which decreases the activation energy of the charge transfer process above 3.5 V during discharging

Layer-by-Layer Assembled Architecture of Polyelectrolyte Multilayers and Graphene Sheets on Hollow Carbon Spheres/Sulfur Composite for High-Performance Lithium–Sulfur Batteries

In the present work, polyelectrolyte multilayers (PEMs) and graphene sheets are applied to sequentially coat on the surface of hollow carbon spheres/sulfur composite by a flexible layer-by-layer (LBL) self-assembly strategy. Owing to the strong electrostatic interactions between the opposite charged materials, the coating agents are very stable and the coating procedure is highly efficient. The LBL film shows prominent impact on the stability of the cathode by acting as not only a basic physical barrier, and more importantly, an ion-permselective film to block the polysulfides anions by Coulombic repulsion. Furthermore, the graphene sheets can help to stabilize the polyelectrolytes film and greatly reduce the inner resistance of the electrode by changing the transport of the electrons from a “point-to-point” mode to a more effective “plane-to-point’’ mode. On the basis of the synergistic effect of the PEMs and graphene sheets, the fabricated composite electrode exhibits very stable cycling stability for over 200 cycles at 1 A g^–1, along with a high average Coulombic efficiency of 99%. With the advantages of rapid and controllable fabrication of the LBL coating film, the multifunctional architecture developed in this study should inspire the design of other lithium–sulfur cathodes with unique physical and chemical properties

Enhanced Electrochemical Performance of Layered Lithium-Rich Cathode Materials by Constructing Spinel-Structure Skin and Ferric Oxide Islands

Layered lithium-rich cathode materials have been considered as competitive candidates for advanced lithium-ion batteries because they are environmentally benign, high capacity (more than 250 mAh·g^–1), and low cost. However, they still suffer from poor rate capability and modest cycling performance. To address these issues, we have proposed and constructed a spinel-structure skin and ferric oxide islands on the surface of layered lithium-rich cathode materials through a facile wet chemical method. During the surface modification, Li ions in the surface area of pristine particles could be partially extracted by H⁺, along with the depositing process of ferric hydrogen. After calcination, the surface structure transformed to spinel structure, and ferric hydrogen was oxidized to ferric oxide. The as-designed surface structure was verified by EDX, HRTEM, XPS, and CV. The experimental results demonstrated that the rate performance and capacity retentions were significantly enhanced after such surface modification. The modified sample displayed a high discharge capacity of 166 mAh·g^–1 at a current density of 1250 mA·g^–1 and much more stable capacity retention of 84.0% after 50 cycles at 0.1C rate in contrast to 60.6% for pristine material. Our surface modification strategy, which combines the advantages of spinel structure and chemically inert ferric oxide nanoparticles, has been shown to be effective for realizing the layered lithium-rich cathodes with surface construction of fast ion diffusing capability as well as robust electrolyte corroding durability

Ultrathin Spinel Membrane-Encapsulated Layered Lithium-Rich Cathode Material for Advanced Li-Ion Batteries

Lack of high-performance cathode materials has become a technological bottleneck for the commercial development of advanced Li-ion batteries. We have proposed a biomimetic design and versatile synthesis of ultrathin spinel membrane-encapsulated layered lithium-rich cathode, a modification by nanocoating. The ultrathin spinel membrane is attributed to the superior high reversible capacity (over 290 mAh g^–1), outstanding rate capability, and excellent cycling ability of this cathode, and even the stubborn illnesses of the layered lithium-rich cathode, such as voltage decay and thermal instability, are found to be relieved as well. This cathode is feasible to construct high-energy and high-power Li-ion batteries

Exposing the {010} Planes by Oriented Self-Assembly with Nanosheets To Improve the Electrochemical Performances of Ni-Rich Li[Ni_0.8Co_0.1Mn_0.1]O₂ Microspheres

A modified Ni-rich Li­[Ni_0.8Co_0.1Mn_0.1]­O₂ cathode material with exposed {010} planes is successfully synthesized for lithium-ion batteries. The scanning electron microscopy images have demonstrated that by tuning the ammonia concentration during the synthesis of precursors, the primary nanosheets could be successfully stacked along the [001] crystal axis predominantly, self-assembling like multilayers. According to the high-resolution transmission electron microscopy results, such a morphology benefits the growth of the {010} active planes of final layered cathodes during calcination treatment, resulting in the increased area of the exposed {010} active planes, a well-ordered layer structure, and a lower cation mixing disorder. The Li-ion diffusion coefficient has also been improved after the modification based on the results of potentiostatic intermittent titration technique. As a consequence, the modified Li­[Ni_0.8Co_0.1Mn_0.1]­O₂ material exhibits superior initial discharges of 201.6 mA h g^–1 at 0.2 C and 185.7 mA h g^–1 at 1 C within 2.8–4.3 V (vs Li⁺/Li), and their capacity retentions after 100 cycles reach 90 and 90.6%, respectively. The capacity at 10 C also increases from 98.3 to 146.5 mA h g^–1 after the modification. Our work proposes a novel approach for exposing high-energy {010} active planes of the layered cathode material and again confirms its validity in improving electrochemical properties