Effects of Co/Mn Content Variation on Structural and Electrochemical Properties of Single-Crystal Ni-Rich Layered Oxide Materials for Lithium Ion Batteries.

The development of single-crystal nickel-rich layered LiNixCoyMn1-x-yO2 materials (S-NCMs) represents the most significant progress for the electrification applications of nickel-rich ternary materials. There has been prior research on the important role of transition metal elements in agglomerated materials, supplemented by surface and internal lattice optimization to drive the performance improvements. However, studies on S-NCMs, especially on the role of transition metals (TM, i.e., Co and Mn), have not been reported. In this study, we synthesized four kinds of S-NCMs with different Co/Mn contents and studied their structural, electrochemical, kinetic, and thermodynamic properties with different Co/Mn contents. The results were as follows: (1) Electrochemically, Co was more effective than Mn at 25 °C at enhancing the intercalation/deintercalation kinetics, which resulted in an increased discharge capacity, an improved rate capability, and a reduced energy loss. (2) Thermodynamically, Mn was more effective at maintaining a higher thermal stability than Co, especially at a low cutoff voltage, but at a high cutoff voltage, the difference between the action of Co and Mn decreased. The main finding of this work was the enhanced structural stability provided by Co, which could be attributed to the following: (i) the absence of the H2/H3 phase transformation when Co exceeded 15%, which inhibited the irreversible phase transformation and reduced the volume strain, and (ii) the lower degrees of decrease in the cell parameters a and c with higher contents of Co, which contributed to a low cracking degree along the (003) crystal plane. The current work provides an important reference for the single-crystallization strategy of nickel-rich materials.

Ultrathin Li-Si-O Coating Layer to Stabilize the Surface Structure and Prolong the Cycling Life of Single-Crystal LiNi0.6Co0.2Mn0.2O2 Cathode Materials at 4.5 V.

Single-crystal LiNi1-x-yCoxMnyO2 cathode materials can effectively suppress intergranular cracks that usually is seen in commercial polycrystal LiNi1-x-yCoxMnyO2 cathode materials. However, the surface structure degradation for single-crystal LiNi1-x-yCoxMnyO2 cathode materials is still aggravated at a higher cutoff voltage (over 4.5 V). In this work, we prepare single-crystal LiNi0.6Co0.2Mn0.2O2 cathode materials via a solid-state method and then coat an ultrathin Li-Si-O layer on their surface by a wet coating method. The results show that the single-crystal LiNi0.6Co0.2Mn0.2O2 cathode materials with a Li-Si-O coating layer deliver excellent cycling performance even at a higher cutoff voltage of 4.5 V. The optimized Li-Si-O-modified sample displays a capacity retention of 90.6% after 100 cycles, whereas only 68.0% for unmodified single-crystal LiNi0.6Co0.2Mn0.2O2. Further analysis of the cycled electrodes reveals that the surface structure degradation is the main reason for the decrease of electrochemical performance of single-crystal LiNi0.6Co0.2Mn0.2O2 at a high voltage (4.5 V). In contrast, with Li-Si-O coating, this phenomenon can be suppressed effectively to maintain interfacial stability and prolong the cycling life.

Revealing the Role of W-Doping in Enhancing the Electrochemical Performance of the LiNi0.6Co0.2Mn0.2O2 Cathode at 4.5 V.

More and more attention has been focused on Ni-rich ternary materials due to their superior specific capacity, but they still suffer inherent structural irreversibility and rapid capacity degradation under a high voltage. Oxidation of unstable oxygen will lead to the irreversible transformation of the structure. Taking into account the strong W-O bond, an appropriate amount of W-doping is studied to reinforce the thermal stability and electrochemical performance of LiNi0.6Co0.2Mn0.2O2 (NCM622) at 4.5 V. Combining experiments and theoretical calculations, it can be found that W-doping is most preferred at Co sites, and the average charge around O in the NiO6 octahedron becomes more negative after W-doping, which can successfully restrain the release of oxygen, thereby improving the stability of the crystal structure during deep delithiation. In addition, W-doping decreases the energy barrier of the Li+ migration slightly and boosts the kinetic diffusion of lithium ions. As a result, NCM622 doped with 0.5% W boasts an outstanding capacity retention of 96.7% at 1 C after 100 cycles and a discharge specific capacity of up to 152.8 mA h g-1 at 5 C between 3.0 and 4.5 V. Furthermore, analysis of the cycled electrodes indicates that the lattice expansion and the formation of microcracks during long cycling are suppressed after W-doping, thereby elevating the structure and interface stability. Therefore, doping an appropriate amount of W via simple methods is helpful to obtain Ni-rich cathode materials with admirable performance.

Effect of Organic Electrolyte on the Performance of Solid Electrolyte for Solid-Liquid Hybrid Lithium Batteries.

The interface problem caused by the contact between the electrodes and the solid electrolyte was the main factor hindering the development of solid-state batteries. To enhance the electrode|solid electrolyte interface property, we designed a hybrid electrolyte, the combination of x vol % Li1.3Al0.3Ti1.7(PO4)3 (LATP) inorganic solid electrolyte and 1 - x vol % liquid organic electrolyte (LE). In this work, the 1 - x vol % LE was dropped between the electrode and the solid electrolyte, and it is found that the electrochemical performance of the LiFePO4|Li solid-liquid hybrid battery is significantly improved. At the current density of 0.1 and 0.5 C, the LATP with 15% liquid organic electrolyte could deliver a specific capacity of 160.5 and 124.3 mAh g-1, respectively; moreover, the specific discharge capacity remained as high as 111 mAh g-1 at 0.5 C after 100 cycles, indicating that the larger interface impedance was eliminated. The LE may have three functions: (1) forming a solid-liquid electrolyte interphase on the surface of the LATP particles to prevent further reduction of LATP, (2) wetting the electrode and solid electrolyte to reduce the interface resistance, and (3) improving interfacial Li-ion transport.

LiFePO4-coated LiNi0.6Co0.2Mn0.2O2 for lithium-ion batteries with enhanced cycling performance at elevated temperatures and high voltages.

LiNi0.6Co0.2Mn0.2O2 (NCM622) is a highly promising cathode material owing to its high capacity; however, it is characterized by inferior cycling performance and safety problems. We report a novel strategy to improve electrochemical characteristics and safety issues of NCM622 by coating it with LiFePO4 (LFP). Although having a lower capacity, LFP is a safe and long-cycle cathode material; it is more chemically and thermally stable than NCM622 when exposed to common electrolytes. The LFP-coated NCM622 (NCM@LFP) showed similar rate performance and cycling performance at room temperature compared with the pristine NCM622 under the same conditions. However, significant differences between the NCM622 and NCM@LFP began to emerge at high temperatures. During cycling at 1C for 100 cycles at 55 °C, NCM@LFP showed much improved specific discharge capacity retentions of 92.4%, 90.9%, and 88.2% in the voltage ranges of 3-4.3 V, 3-4.4 V and 3-4.5 V, respectively. The NCM622 suffered significant discharge specific capacity decay under the same condition. In addition, as demonstrated by the delayed exothermic peak in the differential scanning calorimetry (DSC) test, NCM@LFP exhibited excellent thermal stability compared with NCM622, which is critical to battery safety.

Novel Nonconjugated Polymer as Cathode Buffer Layer for Efficient Organic Solar Cells.

A novel nonconjugated polymer named poly(2-acrylamido-2-methyl-1-propanesulfonic acid sodium salt) (PAMPS-Na) was designed and synthesized. The PAMPS-Na has good solubility in polar solvents, such as water, methanol, and ethanol, which can be used as the cathode buffer layer in organic solar cells (OSCs) through solution processing without damaging the underlying active layer. Moreover, it was found that PAMPS-Na can significantly decrease the Al work function when it was modified with Al. To reveal its universal application in organic photovoltaic devices, a variety of photovoltaic donor materials, including two medium-band gap polymers, a wide-band gap polymer, and a small molecule donor were employed to fabricate OSCs. Compared with OSCs with Ca/Al electrode, the devices based on PAMPS-Na/Al exhibited higher photovoltaic performance, mainly because of the increased short-circuit current. Additionally, OSCs with PAMPS-Na/Al displayed better ambient stability than devices with Ca/Al. It is also interesting to find that the performance of the devices can tolerate a wide change of PAMPS-Na's thickness, enabling the potential for large-scale fabrication of OSCs. The results suggest that PAMPS-Na is a promising candidate as the cathode buffer layer to improve the efficiency and stability of OSCs.

Organic Solar Cells Based on WO2.72 Nanowire Anode Buffer Layer with Enhanced Power Conversion Efficiency and Ambient Stability.

Tungsten oxide as an alternative to conventional acidic PEDOT:PSS has attracted much attention in organic solar cells (OSCs). However, the vacuum-processed WO3 layer and high-temperature sol-gel hydrolyzed WOX are incompatible with large-scale manufacturing of OSCs. Here, we report for the first time that a specific tungsten oxide WO2.72 (W18O49) nanowire can function well as the anode buffer layer. The nw-WO2.72 film exhibits a high optical transparency. The power conversion efficiency (PCE) of OSCs based on three typical polymer active layers PTB7:PC71BM, PTB7-Th:PC71BM, and PDBT-T1:PC71BM with nw-WO2.72 layer were improved significantly from 7.27 to 8.23%, from 8.44 to 9.30%, and from 8.45 to 9.09%, respectively compared to devices with PEDOT:PSS. Moreover, the photovoltaic performance of OSCs based on small molecule p-DTS(FBTTh2)2:PC71BM active layer was also enhanced with the incorporation of nw-WO2.72. The enhanced performance is mainly attributed to the improved short-circuit current density (Jsc), which benefits from the oxygen vacancies and the surface apophyses for better charge extraction. Furthermore, OSCs based on nw-WO2.72 show obviously improved ambient stability compared to devices with PEDOT:PSS layer. The results suggest that nw-WO2.72 is a promising candidate for the anode buffer layer materials in organic solar cells.