Lithium-rich Layered Oxide Research Articles

Ionic liquid electrolytes for high-voltage, lithium-ion batteries

Ionic liquid electrolytes based on imidazolium and tetra-alkyl-ammonium cations, coupled with bis(perfluroalkylsulfonyl)imide anions, are specifically tailored for lithium battery systems operating up to almost 5 V. Their ion transport properties and the electrochemical stability are herein reported and compared. Specifically, the effect of temperature and lithium salt incorporation as well as the nature of the main cation side group are discussed. Preliminary battery cycling tests of the selected ionic liquid electrolytes employing high-voltage, lithium-rich layered oxide positive electrodes, (i.e., Li1·2Ni0·2Mn0·6O2) are also reported. Also, potential anodic dissolution phenomena of the aluminum current collector, due to the presence of the ionic liquid electrolytes, were preliminarily investigated.

Microwave-assisted hydrothermal synthesis of lithium-rich layered oxide cathode materials with high stability

Lithium-rich layered oxides have the advantages of high specific capacity (> 250 mAh g−1) and high energy density, which make them highly competitive in the lithium-ion cathode material market. However, low efficiency, poor cycle stability, and poor rate performance severely constrained their development. In this paper, the spherical lithium-rich Li1.2Mn0.54Ni0.13Co0.13O2 compound was rapidly synthesized by microwave hydrothermal method, and the microwave hydrothermal time was optimized. The samples were characterized and analyzed by scanning electron microscopy (SEM), X-ray diffraction (XRD), Raman spectroscopy (Raman), and X-ray photoelectron spectroscopy (XPS). The prepared lithium-rich compound is porous spherical secondary particles, of which the size distributed between 2.5 and 3.5 μm, which makes materials have larger specific surface area and active potential and promotes the improvement of electrochemical kinetics. The results of 100 charge-discharge cycles show that the microwave hydrothermal 40-min cathode has good stability, and the capacity retention rate can reach 91.4%. Furthermore, it can still provide a discharge capacity of 160.5 mAh g−1 at a current density of 5 C.

Synergistic effect of uniform lattice cation/anion doping to improve structural and electrochemical performance stability for Li-rich cathode materials

There has been extensive research into lithium-rich layered oxide materials as candidates for the nextgeneration of cathode materials in lithium-ion batteries, due to their high energy density and low cost; however, their poor cycle life and fast voltage fade hinder their large-scale commercial application. Here, we propose a novel cation/anion (Na+/PO43–) co-doping approach to mitigate the discharge capacity and voltage fade of a Co-free Li1.2Ni0.2Mn0.6O2 cathode. Our results show that the synergistic effect of cation/anion doping can promote long cycle stability and rate performance by inhibiting the phase transformation of the layered structure to a spinel or rock-salt structure and stabilizing the well-ordered crystal structure during long cycles. The co-doped sample exhibits an outstanding cycle stability (capacity retention of 86.7% after 150 cycles at 1 C) and excellent rate performance (153 mAh g−1 at 5 C). The large ionic radius of Na+ can expand the Li slab to accelerate Li diffusion and the large tetrahedral PO43– polyanions with high electronegativity stabilize the local structure to improve the electrochemical performance.

Structural Distortion Induced by Manganese Activation in a Lithium-Rich Layered Cathode.

The search for batteries with high energy density has highlighted lithium-rich manganese-based layered oxides due to their exceptionally high capacity. Although it is clear that both cationic and anionic redox are present in the charge compensation mechanism, the microstructural evolution of the Li2MnO3-like phase during anionic redox and its role in battery performance and structural stability are still not fully understood. Here, we systematically probe microstructural evolution using spatially resolved synchrotron X-ray measurements and reveal an underlying interaction between the Li2MnO3-like domains and bulk rhombohedral structure. Mn ion activation and a previously unobserved structural distortion are discovered at high voltages, and can be related to structural strain present in the Li2MnO3-like phase upon substantial lithium ion extraction. Moreover, we elucidate a correlation between this structural distortion and irreversible phase transitions by thermally perturbing delithiated samples. These insights highlight a pathway toward achieving high capacity cathode materials required for future commercial applications.

Achieving high structure and voltage stability in cobalt-free Li-rich layered oxide cathodes via selective dual-cation doping

Lithium-rich layered oxides (LLOs) are regarded as one of the most promising cathode materials for next generation Li-ion batteries (LIBs) due to their high energy density. However, the associated oxygen release and structure collapse resulting from the intrinsic anion and cation redox reactions lead to performance degradation, particularly the characteristic voltage fading which has prohibited the commercialization of LLOs for more than a decade. Herein, we have developed a dual-doping technique to overcome the longstanding structure and voltage instabilities of Co-free Li1.2Mn0.533Ni0.267O2, through the concurrent introduction of neodymium (Nd) and aluminum (Al) ions. Selective atomic substitution of Ni/Mn with Nd/Al ions and the preconstructed heteroepitaxial interface significantly enhance the voltage and capacity retention by regulating Ni ion activity and suppressing the phase transformation and Mn dissolution, thereby improving rate performance through tuning the electronic structure and promoting Li+ migration. The dual-doped material exhibits a superior cycling stability, with over 90% voltage retention and 82% capacity retention after 200 cycles, and excellent rate performance (150 mAh g−1 at 10 C).

Highly reversible oxygen redox in layered compounds enabled by surface polyanions

Oxygen-anion redox in lithium-rich layered oxides can boost the capacity of lithium-ion battery cathodes. However, the over-oxidation of oxygen at highly charged states aggravates irreversible structure changes and deteriorates cycle performance. Here, we investigate the mechanism of surface degradation caused by oxygen oxidation and the kinetics of surface reconstruction. Considering Li2MnO3, we show through density functional theory calculations that a high energy orbital (lO2p’) at under-coordinated surface oxygen prefers over-oxidation over bulk oxygen, and that surface oxygen release is then kinetically favored during charging. We use a simple strategy of turning under-coordinated surface oxygen into polyanionic (SO4)2−, and show that these groups stabilize the surface of Li2MnO3 by depressing gas release and side reactions with the electrolyte. Experimental validation on Li1.2Ni0.2Mn0.6O2 shows that sulfur deposition enhances stability of the cathode with 99.0% capacity remaining (194 mA h g−1) after 100 cycles at 1 C. Our work reveals a promising surface treatment to address the instability of highly charged layered cathode materials.

Lithium-Rich Cobalt-Free Manganese-Based Layered Cathode Materials for Li-Ion Batteries: Suppressing the Voltage Fading

Lithium-rich layered oxides are recognized as promising materials for Li-ion batteries, owing to higher capacity than the currently available commercialized cathode, for their lower cost. However, their voltage decay and cycling instability during the charge/discharge process are problems that need to be solved before their practical application can be envisioned. These problems are mainly associated with a phase transition of the surface layer from the layered structure to the spinel structure. In this paper, we report the AlF3-coating of the Li-rich Co-free layered Li1.2Ni0.2Mn0.6O2 (LLNMO) oxide as an effective strategy to solve these problems. The samples were synthesized via the hydrothermal route that insures a very good crystallization in the layered structure, probed by XRD, energy-dispersive X-ray (EDX) spectroscopy, and Raman spectroscopy. The hydrothermally synthesized samples before and after AlF3 coating are well crystallized in the layered structure with particle sizes of about 180 nm (crystallites of ~65 nm), with high porosity (pore size 5 nm) determined by Brunauer–Emmett–Teller (BET) specific surface area method. Subsequent improvements in discharge capacity are obtained with a ~5-nm thick coating layer. AlF3-coated Li1.2Ni0.2Mn0.6O2 delivers a capacity of 248 mAh g−1 stable over the 100 cycles, and it exhibits a voltage fading rate of 1.40 mV per cycle. According to the analysis from galvanostatic charge-discharge and electrochemical impedance spectroscopy, the electrochemical performance enhancement is discussed and compared with literature data. Post-mortem analysis confirms that the AlF3 coating is a very efficient surface modification to improve the stability of the layered phase of the Li-rich material, at the origin of the significant improvement of the electrochemical properties.

Niobium doping of Li1.2Mn0.54Ni0.13Co0.13O2 cathode materials with enhanced structural stability and electrochemical performance

Lithium-rich layered oxides with high energy density have been intensively investigated as advanced lithium-ion batteries cathode materials. However, capacity degradation and voltage decay caused by irreversible lattice oxygen loss and structural transformation during cycling restrict their application. Herein, we proposed a high valance cations Nb5+ doping strategy and synthesized a series of Li1.2Mn0.54-x/3Ni0.13-x/3Co0.13-x/3NbxO2 (x = 0, 0.01, 0.02 and 0.03) cathode materials. The effects of Nb5+ doping on crystallographic structure and electrochemical property were systematically studied. In virtue of the large ionic radii and strengthened Nb–O bonds, the doped samples present commendable structural stability and expanded interlayer spacing for Li-ions migration, which ensures the upgraded cyclic stability and rate performance. In particular, the electrode with x = 0.02 delivers a discharge specific capacity of 265.8 mAh g-1 at 0.2 C with decelerated voltage decay, while 86.9% capacity are remained after long-term cycles. Moreover, excellent discharge specific capacity of 153.4 mAh g−1 is still attained at 5 C accompanied with enhanced Li-ion diffusion kinetics.

Does Spinel Serve as a Rigid Framework for Oxygen Redox?

The oxygen-redox chemistry of lithium-rich layered transition-metal oxides has recently paved a path for the development of large-capacity positive electrode materials. Although oxygen-redox activi...

Nanolaminated ZnO–TiO2 coated lithium-rich layered oxide cathodes by atomic layer deposition for enhanced electrochemical performances

The surfaces of the lithium-rich layered oxide cathodes (Li 1.2 Mn 0.6 Ni 0.2 O 2 , LLO) were modified with nanolaminated ZnO–TiO 2 ultra-thin films (1.7 ± 0.4 nm) by atomic layer deposition (ALD) with various coating sequences. The layered oxide cathodes were coated with (I) ALD TiO 2 , followed by ZnO (P@T@Z), (II) ALD ZnO, followed by TiO 2 (P@Z@T), and (III) ALD TiO 2 , followed by ZnO and TiO 2 (P@T@Z@T). Among the nanolaminated coatings, a uniform coating was only obtained for the P@Z@T samples due to the outer layer-like growth of ALD TiO 2 . Surprisingly, the P@Z@T samples exhibited superior electrochemical performances and thermal stabilities to the other samples. The samples possessed a higher initial specific discharge capacity of 123 mAh g −1 at 1 C, and a capacity retention of 97% after 80 charge-discharge cycles, in comparison with the pristine samples, which were 100 mAh g −1 and 68%, respectively. When the current density increased to 5 and 10 C rate, the samples possessed the specific discharge capacities of 134 and 103 mAh g −1 , but the capacities were absent for the pristine samples. The samples charging at 4.8 V withstood a higher decomposition temperature of 267 °C and a lower heat release of 62.4 J/g compared to the pristine samples, 260 °C with 253.6 J/g. The enhanced rate capability and cyclability of the P@Z@T samples were related to the decreased resistance from 1894 to 847 Ωcm −2 and less side reactions. This was due to the uniform surface and complete protection of the nanolaminated film. • The surface of the LLO cathodes can be modified with ALD nanolaminated ZnO–TiO 2 . • The P@Z@T samples exhibit superior electrochemical performances. • The surface morphology plays a crucial role in the enhanced electrochemical performances.

Suppression of voltage decay through adjusting tap density of lithium-rich layered oxides for lithium ion battery

The voltage decay of lithium-rich layered oxides (LLOs) is still one of the key challenges for their application in commercial battery although these materials possess the advantages of high specific capacity and low cost. In this work, the relationship between voltage decay and tap density of LLOs has been focused. The voltage decay can be significantly suppressed with the increasing tap density as well as the homogenization of the primary or secondary particle size of agglomerated spherical LLOs. Experimental results have shown that an extreme small voltage decay of 0.98 mV cycle−1 can be obtained through adjusting the tap density of agglomerated spherical LLOs to 1.99 g cm-3, in which the size of primary and secondary particles are uniform. Our work offers a new insight towards the voltage decay and capacity fading of LLOs through precursor preparation process, promoting their application in the real battery in the future.

Improving electrochemical performances of Lithium-rich oxide by cooperatively doping Cr and coating Li3PO4 as cathode material for Lithium-ion batteries

Lithium-rich layered oxides exhibit one of the highest reversible discharge capacities among cathode materials for lithium-ion batteries. However, their voltage decay and poor cycle stability severely restrict their use as a commercial cathode material. In this work, a novel approach of that combines Cr doping and a Li3PO4 coating was designed to address the problems associated with lithium-rich Li1.2Mn0.54Ni0.13Co0.13O2 materials. The synergistic method not only increases the discharge capacity and cycle stability but also decreases the voltage decay. The 1.0 wt% Li3PO4 coating and 0.08 Cr doping on Li1.2Mn0.54Ni0.13Co0.13O2 cathode shows a capacity retention of 76.5% compared to the 59.0% capacity retention for the pristine electrode after 200 cycles. The initial discharge capacity is also increased from 255.8 mAh·g−1 to 265.2 mAh·g−1. In addition, the discharge voltage decay decreases from 0.84 V to 0.39 V after 200 cycles as a result of the Cr doping and Li3PO4 coating. These enhanced electrochemical properties are attributed to the fact that the Cr doping stabilized the layered structure and inhibited its phase transformation to the spinel phase, and the Li3PO4 coating confined the interfacial side reactions between the electrode and electrolyte. This work may provide a new method to solve the subsistent problems of lithium-rich cathode materials.

Synthesis and Performance Evaluation of SiO2 Coated Li-Rich Li1.2Ni0.13Mn0.54Co0.13O2 Cathode Materials for Li-Ion Batteries

Lithium rich layered oxides (LLO) are one of the potential candidates for Li-ion batteries. however, LLO materials have serious drawbacks such as poor cyclic stability, poor rate capability and rapid voltage decay upon cycling, which needs to be overcome in order to successfully commercialize them. In this work, SiO2 coating is employed (1.0%, 1.5%, and 2.0 wt. %) to combat these issues. Li1.2Ni0.13Mn0.54Co0.13O2 synthesis was carried out by sol-gel process and SiO2 coating was done using dry ball milling method. XRD analysis confirmed the synthesis of phase pure materials without the presence of any impurity phases. SEM and TEM analysis showed the synthesis of nanometric sized particles. Furthermore, TEM elemental mapping displayed the homogeneous distribution of SiO2 on the cathode particles. XPS was further employed to confirm the SiO2 on the cathode material. The electrochemical analysis showed that 1.0 wt.% SiO2 displayed the best performance in terms of rate capability. Uncoated Li1.2Ni0.13Mn0.54Co0.13O2 displayed a fast capacity fade, especially at higher C-rate. On the other hand, SiO2 coated Li1.2Ni0.13Mn0.54Co0.13O2 materials showed improved rate capability. Coated materials also showed improved cycling performance when compared with bare Li1.2Ni0.13Mn0.54Co0.13O2. The improved electrochemical performance can be attributed to the positive impact of SiO2 coating, which impedes the electrolyte oxidation by preventing the direct contact of cathode active material and electrolyte. Overall, the SiO2 coating had a positive impact on the electrochemical performance of Li1.2Ni0.13Mn0.54Co0.13O2 and can pave the way for commercialization of LLO materials.

High-Temperature Electrochemical Performance Enhancement of Lithium-Rich Layered Oxides by Surface Modification

Lithium-rich layered oxides (LLOs) are one of the attractive cathodes for high-energy lithium-ion batteries (LIBs). However, their rate performance and cycling stability, especially at high tempera...

Clearing surficial charge-transport obstacles to boost the performance of lithium-rich layered oxides

The interfacial property of cathode materials in Li-ion batteries plays a vital role for the mass transport in electrochemical process. In this paper, Li2RuO3 coating layer, acting as an electronic/ionic conductive network, is introduced to lithium-rich Mn-based material that is known for its low rate capability and quick capacity fading. The reaction kinetics are promoted due to the depolarizing effect of this modified layer. The target material achieves a discharge capacity as high as 195.4 and 150.6 mAh g−1 at 2C and 8C, respectively. The conductive Li2RuO3 coating layer helps to accelerate the transport of Li ion and electron at grain surface and relieve phase transformation, ultimately reducing the reaction resistance. Cycle stability is also enhanced (capacity retention of 92% after 100 cycles) by mitigating side reactions between cathode and electrolyte. This study provides an effective approach to ameliorate the sluggish Li-ion transport of lithium-rich Mn-based material and other analogous cathode materials.

Enhanced electrochemical performance of lithium-rich layered oxide cathodes by a facile self-template method for lithium-ion batteries

Lithium-rich layered oxide cathode material Li1.2Ni0.13Co0.13Mn0.54O2 has been successfully synthesized via a self-template method followed with sol-gel reaction, microspheres consisting of three-dimensional (3D) nanothorn MnO2 are used as the template and Mn source. The as-prepared sample (denoted as LNCM-T) exhibits well-layered structure and high crystallinity, which is confirmed by XRD and HRTEM. The results of electrochemical performances show that the LNCM-T sample has excellent high-rate capability with a discharge capacity of 159.7 mA h g−1 at a current density of 1250 mA g−1. Besides, it delivers a high initial Coulombic efficiency of 84.3% with a discharge capacity of 243.8 mA h g−1 at room temperature. The remarkable cycling stability and excellent rate capability of LNCM-T sample are ascribed to the enhanced layered structural stability, short lithium-ion diffusion path and reduced charge transfer resistance.

Nonstoichiometry of Li-rich cathode material with improved cycling ability for lithium-ion batteries

Lithium-rich layered oxides are considered as promising cathode materials for lithium-ion batteries due to its high capacity, but the rapid decay of capacity and operating voltage are great challenges to achieve its commercial application. In this work, the nonstoichiometry of Li-rich layered oxide Li1.2Mn0.6Ni0.2O2 was designed by directly declining the Mn amounts in the form of Li1.2MnxNi0.2O2 (x = 0.59, 0.57, 0.55). The nonstoichiometric sample Li1.2Mn0.55Ni0.2O2 exhibits a capacity of 170.73 mAh g−1 at 0.5 C, a little lower than 187.29 mAh g−1 of Li1.2Mn0.6Ni0.2O2, however, better cycling stability of operating voltage and capacity is attained with the reduction of Mn amounts, compared to that of Li1.2Mn0.6Ni0.2O2. The capacity retention of Li1.2Mn0.55Ni0.2O2 is enhanced to 88.7% via 74.7% of Li1.2Mn0.6Ni0.2O2 after 100 cycles at 0.5 C. The declining value of operating voltage for Li1.2Mn0.55Ni0.2O2 is 0.200 V as compared to 0.559 V for Li1.2Mn0.6Ni0.2O2. X-ray photoelectron spectra (XPS) was employed to confirm the existence of Ni3+ in the nonstoichiometric samples, and the amounts of Ni3+ increase along the Mn contents decrease. The improvement of electrochemical properties for nonstoichiometric samples is attributed to the presence of Ni3+ due to Ni3+ can defer the transition of layered-to-spinel structure through decreasing the Li/Ni mixing.

L-Tryptophan: Antioxidant as a Film-Forming Additive for a High-Voltage Cathode.

l-tryptophan (TrP) was investigated as a functional film-forming additive on a lithium-rich layered oxide cathode because it has a much lower oxidation potential than other common carbonate-based electrolytes. Owing to its prior oxidation to a base electrolyte, an artificial cathode-electrolyte interphase (CEI) was formed on the cathode surface, which could be confirmed via X-ray photoelectron spectroscopy and scanning electron microscopy and verified through density functional theory calculations. The functional film formed on the cathode surface suppressed the side reactions between the cathode and electrolyte during cell cycling. As a result, the film prevented CEI thickening and performance deterioration. The optimum weight of TrP was determined to be 0.4 wt % for obtaining the best performance.

Improved low-temperature performance of surface modified lithium-rich Li1.2Ni0.13Co0.13Mn0.54O2 cathode materials for lithium ion batteries

High capacity lithium-rich layered oxide Li1.2Ni0.13Co0.13Mn0.54O2 cathode material is considered as a most promising cathode material for lithium ion batteries. However, its application is greatly limited due to the characteristics of low initial coulombic efficiency and poor low-temperature performance. In this study, AlF3-coated Li1.2Ni0.13Co0.13Mn0.54O2 has been prepared via a facile wet chemical process. It is found that a dense AlF3 layer with a nanoscale thickness is covered on the surface of Li1.2Ni0.13Co0.13Mn0.54O2 particles, building a rapid lithium transport bridge for adjacent active materials and reducing side reaction between the electrolyte and active material. Compared to the pristine Li1.2Ni0.13Co0.13Mn0.54O2, 2% AlF3-coated sample exhibits higher discharge capacity and initial coulombic efficiency (86.7%). Meanwhile, 2% AlF3-coated sample displays obviously superior rate capability at 0 °C and higher capacity retention at −10 and −20 °C. The improvement of low-temperature performance can reasonably be attributed to the formation of the spinel structure and the LiAlO2 on the surface of the active materials, which can improve the electronic conductivity and lithium ion conductivity of the cathode material, respectively.

Surface substitution of polyanion to improve structure stability and electrochemical properties of lithium-rich layered cathode oxides

Lithium-rich layered cathode oxides attract enormous attention due to their superior capacity, high voltage and environmentally friendliness. However, it is considerable that they suffer from serious capacity fade, voltage attenuation and phase transition deriving from inevitable oxygen evolution. In this work, polyanions are induced on the surface of the oxides via the method of surface substitution, considering synergy effects of bulk substitution and surface modification to stabilize crystal structure and promote electrochemical properties. The material doped with 5 wt% PO43− exhibits excellent specific capacity of 217.1 mAh g−1 with capacity retention of 88.43% and average discharge voltage drop of 0.4846 V after 200 cycles at 0.5 C significantly superior to the pristine material. And the discharge capacity of the material can reach 105.6 mAh g−1 at 10 C owing to enlarged interlayer spacing and improved ionic conductivity. Transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS) and electrochemical impedance spectroscopy (EIS) of the material before and after cycle verify that polyanions are beneficial to supply more active oxygen, restrain transition metal migration, reduce surface microstrain, and enhance electronic and ionic conductivities. All above indicate that it is effective for polyanions to promote lithium-rich cathode oxides to give full play to their electrochemical advantages.