Electrochemical performance of spherical Li-rich LMNCO cathode materials prepared using a two-step spray-drying method

In this study we synthesized Li-rich Li_1.2Ni_0.13Mn_0.54Co_0.13O₂ (LMNCO) as a composite cathode material through a two-step spray-drying method, using transition metal (TM) acetates and citric acid (CA, as a chelating agent) at various molar ratios and then calcining at various temperatures for various periods of time. This two-step spray-drying method created hierarchical nano/micro-sphere structures of the LMNCO cathode material. The LMNCO cathode exhibited the best electrochemical performance when synthesized with a TM:CA ratio of 3:2, a calcination temperature of 900 °C, and a calcination time of 5 h. This as-prepared LMNCO composite was then modified with polyimide (PI) at various weight ratios (PI/LMNCO = 0.5, 1.0, and 1.5 wt%) to improve its electrochemical properties. Among the various structures, the LMNCO cathode material coated with 1.0 wt% of PI at a layer thickness of approximately 1.88 nm achieved the best initial discharge capacities. This modified electrode also displayed enhanced cycle stability, with over 93.3 and 87.9% of the capacity retained after 30 cycles at 0.1C and 100 cycles at 1C, respectively. In comparison, the capacity retention of the unmodified LMNCO electrode measured under the same conditions was no more than 91.3% at 0.1C and 70.1% at 1C. Thus, surface modification with PI was an effective method for improving the coulombic efficiency, discharge capacity, and long-term cycling performance of the LMNCO cathode. Such PI-coated LMNCO composite cathode materials appear to be potential candidates for use in next-generation high-performance lithium-ion batteries.     

Rational design of thermally stable polymorphic layered cathode materials for next generation lithium rechargeable batteries

Classical layered transition metal oxides have remained the preferred cathode materials for commercial lithium-ion batteries. Variation in the transition metal composition and local ordering can greatly affect the structure stability. In classical layered cathodes, high concentrations of electrochemically inert Mn elements usually act as a pillar to stabilize the structure. When excess amount of Li and Mn are present in the layered structure, the capacity of the Li-rich layered oxide (molar ratio of lithium over transition metal is larger than one by design) can exceed that expected from transition metal redox. However, the over lithiation in the classical layered structure results in safety issues, which remains challenging for the commercialization of Li-rich layered oxides. To characterize the safety performance of a series of Li-rich layered cathodes, we utilize differential scanning calorimeter and thermal gravimetric analysis; this is coupled with local structural changes using in situ temperature dependent synchrotron X-ray diffraction and X-ray adsorption spectroscopy. These methods demonstrate that the gradual decrease of the Mn–M (M = Ni, Co, Mn and Li) coordination number directly reduces structural stability and accelerates oxygen release. For safety characterization tests in practice, we evaluate the thermal runaway process through accelerating rate calorimeter in 1.0 Ah pouch cells to confirm this trend. Using the insights obtained in this work, we design a polymorphic composition to improve the thermal stability of Li-rich layered cathode material, which outperforms Ni-rich layered oxides in terms of both electrochemical and safety performances.     

Mg和Na共掺杂对富锂正极材料结构和电性能影响研究

采用溶胶—凝胶法制备了Na和Mg共掺杂的无钴富锂锰基正极材料,通过透射电子显微镜(HRTEM)、X射线衍射仪(XRD)、X射线光电子能谱仪(XPS)、充放电测试仪和电化学工作站对所制备样品的形貌、结构和电化学性能进行了测试。结果表明,所有样品颗粒大小约100nm,呈近似球形,样品均无杂相峰出现,都具有良好的层状结构,Na~+和Mg~(2+)对Ni和Mn离子的氧化状态没有影响。LNi018具有最好的电化学性能,初始放电容量为225.5mAh/g,库伦效率为78.0%,循环250周后容量保持率为83.1%。LNi018样品中Ni含量相对于其它样品较少,Ni ~(2+)和Mg~(2+)的价态相同,离子半径近似(Ni ~(2+)6.9nm,Mg~(2+)7.2nm),在晶格中Mg~(2+)取代了Ni~(2+)的位置,同时较大半径的Na~+(10.2nm)增大锂层间距,使晶体结构在充放电循环过程中更加稳定,因而能保持较高的放电电压和容量。     

导电剂形貌对富锂正极材料性能发挥的影响

研究了纳米颗粒状的Super P、气相生长的碳纤维(VGCF)、片状的KS6和石墨烯四种不同的导电剂对富锂正极材料Li_(1.2)Mn_(0.54)Ni_(0.13)Co_(0.13)O_2(LR-MNC)的电化学性能发挥的影响。研究结果表明,当导电剂的添加量为10%(质量分数),以Super或VGCF的导电剂在活性材料的表面形成了完整的导电通路,电极的表面电阻最小,从而有利于电子的传输,因此,正极活性物质表现出优异的倍率放电性能和循环性能。其中,以Super P为导电剂的电极性能最优,3 C放电比容量为164.4 mAh/g,1 C循环100周,容量保持率为82.3%。而以片状的KS6或石墨烯单独作为导电剂,在电极中没有形成完整的导电通路,不利于正极活性物质的大倍率放电性能的发挥。     

The effects of multifunctional coating on Li-rich cathode material with hollow spherical structure for Li ion battery

Structural stability and high ionic and electronic conductivity are crucial for the performance of Li-ion batteries. To improve the electrochemical performance of Li-rich Mn-based cathode material, we have adopted a combination of bulk doping and surface coating rather than a single modification, thereby enhancing structural stability and conductivity. This combination is achieved tactfully by one-step method using surface coating on hollow material. After coating and treatment at high temperature, elements from coating layer substitute transition metal elements and distribute gradiently throughout the entire shell of cathode material, showing collective advantages of element doping and surface coating. The modified samples show enhanced rate capability and cycling stability. We discuss the relationship between battery performance and structural improvement.     

Enhanced electrochemical performance in lithium ion batteries of a hollow spherical lithium-rich cathode material synthesized by a molten salt method

A high voltage layered Li1.2Ni0.16Co0.08Mn0.56O2 cathode material with a hollow spherical structure has been synthesized by molten-salt method in a NaCI flux. Characterization by X-ray diffraction and scanning electron microscopy confirmed its structure and proved that the as-prepared powder is constituted of small, homogenously sized hollow spheres （1-1.5 μm）. The material exhibited enhanced rate capability and high first cycle efficiency due to the good dispersion of secondary particles. Galvanostatic cycling at different temperatures （20, 40, and 60 ℃） and a current rate of 2 C （500 mA.g-1） showed no significant capacity fade.     

Investigation on performances of Li1.2Co0.4Mn0.4O2 prepared by self-combustion reaction as stable cathode for lithium-ion batteries

Poor rate capability and cycling performance are the major barriers for Li-rich layered cathode materials to be applied as the next generation cathode materials for lithium-ion batteries. In our work, Li_1.2Co_0.4Mn_0.4O₂ has been successfully synthesized via a self-combustion reaction (SCR) and a calcination procedure. Compared with the material produced by the solid state method (SSM), the one by SCR exhibits both better rate capability and cycling performance. Its initial discharge capacity is 166.01 mA h g⁻¹ with the capacity retention of 85.98% after 50 cycles at a current density of 200 mA h g⁻¹. Its remarkable performance is attributed to a thin carbon coating layer, which not only slows down the transformation rate of layered to spinel structure, but provides a good electronic pathway to increase the Li⁺ diffusion coefficient.     

Structural insights into composition design of Li-rich layered cathode materials for high-energy rechargeable battery

The Li-rich layered oxide is considered as one of the most promising cathode materials for high energy density batteries, due to its ultrahigh capacity derived from oxygen redox. Although incorporating over-stoichiometric Li into layered structure can generate Li₂MnO₃-like domain and enhance the oxygen redox activity thermodynamically, the fast and complete activation of the Li₂MnO₃-like domain remains challenging. Herein, we performed a systematic study on structural characteristics of Li-rich cathode materials to decipher the factors accounting for activation of oxygen redox. We reveal that the activation of Li-rich cathode materials is susceptible to local Co coordination environments. The Co ions can intrude into Li₂MnO₃-like domain and modulate the electronic structure, thereby facilitating the activation of Li-rich layered cathode materials upon first charging, leading to higher reversible capacity. In contrast, Li₂MnO₃-like domain hardly contains any Ni ions which contribute little to the activation process. The optimum composition design of this class of materials is discussed and we demonstrate a small amount of Co/Mn exchange in Li₂MnO₃-like domain can significantly promote the oxygen redox activation. Our findings highlight the vital role of Co ions in the activation of oxygen redox Li-rich layered cathode materials and provide new insights into the pathway toward achieving high-capacity Li-rich layered cathode materials.     

Fast Ion Transport in Li-Rich Alloy Anode for High-Energy-Density All Solid-State Lithium Metal Batteries

All-solid-state Li batteries (ASSLBs) with solid-polymer electrolytes are considered promising battery systems to achieve improved safety and high energy density. However, Li dendrite formation at the Li anode under high charging current density/capacity has limited their development. To tackle the issue, Li-metal alloying has been proposed as an alternative strategy to suppress Li dendrite growth in ASSLBs. One drawback of alloying is the relatively lower operating cell voltages, which will inevitably lower energy density compared to cells with pure Li anode. Herein, a Li-rich Li₁₃In₃ alloy electrode (LiRLIA) is proposed, where the Li₁₃In₃ alloy scaffold guides Li nucleation and hinders Li dendrite formation. Meanwhile, the free Li can recover Li's potential and facilitate fast charge transfer kinetics to realize high-energy-density ASSLBs. Benefitting from the stronger adsorption energy and lower diffusion energy barrier of Li on a Li₁₃In₃ substrate, Li prefers to deposit in the 3D Li₁₃In₃ scaffold selectively. Therefore, the Li–Li symmetric cell constructed with LiRLIA can operate at a high current density/capacity of 5 mA cm⁻²/5 mAh cm⁻² for almost 1000 h.     

Voltage Decay of Li-Rich Layered Oxides: Mechanism,Modification Strategies,and Perspectives

Li-rich layered oxides (LLOs) have been considered as the most promising cathode materials for achieving high energy density Li-ion batteries. However, they suffer from continuous voltage decay during cycling, which seriously shortens the lifespan of the battery in practical applications. This review comprehensively elaborates and summarizes the state-of-the-art of the research in this field. It is started from the proposed mechanism of voltage decay that refers to the phase transition, microscopic defects, and oxygen redox or release. Furthermore, several strategies to mitigate the voltage decay of LLOs from different scales, such as surface modification, elemental doping, regulation of components, control of defect, and morphology design are summarized. Finally, a systematic outlook on the real root of voltage decay is provided, and more importantly, a potential solution to voltage recovery from electrochemistry. Based on this progress, some effective strategies with multiple scales will be feasible to create the conditions for their commercialization in the future.