Synthesis and Characterization of Lithium Bis(fluoromalonato)borate for Lithium‐Ion Battery Applications

A new orthochelated salt, lithium bis(monofluoromalonato)borate (LiBFMB), is synthesized and purified for application in lithium‐ion batteries. The presence of fluorine in the borate anion of LiBFMB increases its oxidation potential and also facilitates ion dissociation, as reflected by the ratio of ionic conductivity (σ_exp) and ion diffusivity coefficients (σ_NMR). Half‐cell tests using 5.0 V lithium nickel manganese oxide (LiNi_0.5Mn_1.5O₄) as a cathode and ethylene carbonate (EC)/dimethyl carbonate (DMC)/diethyl carbonate (DEC) as a solvent reveals that the impedance of the LiBFMB cell is much larger than those of LiPF₆‐ and lithium bis(oxalato)borate (LiBOB)‐based cells, which results in lower capacity and poor cycling performance of the former. X‐ray photoelectron spectroscopy (XPS) results for the cycled cathode electrode suggest that because of the stability of the LiBFMB salt, the solid electrolyte interphase (SEI) formed on the cathode surface is significantly different from those of LiPF₆ and LiBOB based electrolytes, resulting in more solvent decomposition and a thicker SEI layer. Initial results also indicate that using a high dielectric constant solvent, propylene carbonate, alters the surface chemistry, reduces the interfacial impedance, and enhances the performance of LiBFMB‐based 5.0 V cell.     

MnPO4‐Coated Li(Ni0.4Co0.2Mn0.4)O2 for Lithium(‐Ion) Batteries with Outstanding Cycling Stability and Enhanced Lithiation Kinetics

Herein, the successful synthesis of MnPO₄‐coated LiNi_0.4Co_0.2Mn_0.4O₂ (MP‐NCM) as a lithium battery cathode material is reported. The MnPO₄ coating acts as an ideal protective layer, physically preventing the contact between the NCM active material and the electrolyte and, thus, stabilizing the electrode/electrolyte interface and preventing detrimental side reactions. Additionally, the coating enhances the lithium de‐/intercalation kinetics in terms of the apparent lithium‐ion diffusion coefficient. As a result, MP‐NCM‐based electrodes reveal greatly enhanced C‐rate capability and cycling stability—even under exertive conditions like extended operational potential windows, elevated temperature, and higher active material mass loadings. This superior electrochemical behavior of MP‐NCM compared to as‐synthesized NCM is attributed to the superior stability of the electrode/electrolyte interface and structural integrity when applying a MnPO₄ coating. Employing an ionic liquid as an alternative, intrinsically safer electrolyte system allows for outstanding cycling stabilities in a lithium‐metal battery configuration with a capacity retention of well above 85% after 2000 cycles. Similarly, the implementation in a lithium‐ion cell including a graphite anode provides stable cycling for more than 2000 cycles and an energy and power density of, respectively, 376 Wh kg^?1 and 1841 W kg^?1 on the active material level.     

Utilizing Environmental Friendly Iron as a Substitution Element in Spinel Structured Cathode Materials for Safer High Energy Lithium‐Ion Batteries

Suppressing oxygen release from lithium ion battery cathodes during heating is a critical issue for the improvement of the battery safety characteristics because oxygen can exothermically react with the flammable electrolyte and cause thermal runaway. Previous studies have shown that oxygen release can be reduced by the migration of transition metal cations from octahedral sites to tetrahedral sites during heating. Such site‐preferred migration is determined by the electronic structure of cations. Taking advantage of the unique electronic structure of the environmental friendly Fe, this is selected as substitution element in a high energy density material LiNi_0.5Mn_1.5O₄ to improve the thermal stability. The optimized LiNi_0.33Mn_1.33Fe_0.33O₄ material shows significantly improved thermal stability compared with the unsubstituted one, demonstrated by no observed oxygen release at temperatures as high as 500°C. Due to the electrochemical contribution of Fe, the high energy density feature of LiNi_0.5Mn_1.5O₄ is well preserved.     

A Chemical Approach to Raise Cell Voltage and Suppress Phase Transition in O3 Sodium Layered Oxide Electrodes

Sodium ion batteries (NIBs) are one of the versatile technologies for low‐cost rechargeable batteries. O3‐type layered sodium transition metal oxides (NaMO₂, M = transition metal ions) are one of the most promising positive electrode materials considering their capacity. However, the use of O3 phases is limited due to their low redox voltage and associated multiple phase transitions which are detrimental for long cycling. Herein, a simple strategy is proposed to successfully combat these issues. It consists of the introduction of a larger, nontransition metal ion Sn⁴⁺ in NaMO₂ to prepare a series of NaNi_0.5Mn_{0.5? y} Sn _y O₂ (y = 0–0.5) compositions with attractive electrochemical performances, namely for y = 0.5, which shows a single‐phase transition from O3 ? P3 at the very end of the oxidation process. Na‐ion NaNi_0.5Sn_0.5O₂/C coin cells are shown to deliver an average cell voltage of 3.1 V with an excellent capacity retention as compared to an average stepwise voltage of ≈2.8 V and limited capacity retention for the pure NaNi_0.5Mn_0.5O₂ phase_. This study potentially shows the way to manipulate the O3 NaMO₂ for facilitating their practical use in NIBs.     

Prescribing Functional Additives for Treating the Poor Performances of High‐Voltage (5 V‐class) LiNi0.5Mn1.5O4/MCMB Li‐Ion Batteries

In this paper, tris(trimethylsilyl) phosphite (TMSP) and 1,3‐propanediolcyclic sulfate (PCS) are unprecedentedly prescribed as binary functional additives for treating the poor performances of high‐voltage (5 V‐class) LiNi_0.5Mn_1.5O₄/MCMB (graphitic mesocarbon microbeads) Li‐ion batteries at both room temperature and 50 °C. The high‐voltage LiNi_0.5Mn_1.5O₄/MCMB cell with binary functional additives shows a preponderant discharge capacity retention of 79.5% after 500 cycles at 0.5 C rate at room temperature. By increasing the current intensity from 0.2 to 5 C rate, the discharge capacity retention of the high‐voltage cell with binary functional additives is ≈90%, while the counterpart is only ≈55%. By characterizations, it is rationally demonstrated that the binary functional additives decompose and participate in the modification of solid–electrolyte interface layers (both electrodes), which are more conductive, protective, and resistant to electrolyte oxidative/reductive decompositions (accompanying active‐Li⁺ consuming parasitic reactions) due to synergistic effects. Specifically, the TMSP additive can stabilize LiPF₆ salt and scavenge erosive hydrofluoric acid. More encouragingly, at 50 °C, the high‐voltage cell with binary functional additives holds an ultrahigh discharge capacity retention of 79.5% after 200 cycles at 1 C rate. Moreover, a third designed self‐extinguishing flame‐retardant additive of (ethoxy)‐penta‐fluoro‐cyclo‐triphosphazene (PFPN) is introduced for reducing the flammability of the aforementioned binary functional additives containing electrolyte.     

High Voltage LiNi0.5Mn0.3Co0.2O2/Graphite Cell Cycled at 4.6 V with a FEC/HFDEC‐Based Electrolyte

A high voltage LiNi_0.5Mn_0.3Co_0.2O₂/graphite cell with a fluorinated electrolyte formulation 1.0 m LiPF₆ fluoroethylene carbonate/bis(2,2,2‐trifluoroethyl) carbonate is reported and its electrochemical performance is evaluated at cell voltage of 4.6 V. Comparing with its nonfluorinated electrolyte counterpart, the reported fluorinated one shows much improved Coulombic efficiency and capacity retention when a higher cut‐off voltage (4.6 V) is applied. Scanning electron microscopy/energy dispersive X‐ray spectroscopy and X‐ray photoelectron spectroscopy data clearly demonstrate the superior oxidative stability of the new electrolyte. The structural stability of the bulk cathode materials cycled with different electrolytes is extensively studied by X‐ray absorption near edge structure and X‐ray diffraction.     

Ambient‐Pressure Relithiation of Degraded LixNi0.5Co0.2Mn0.3O2 (0 < x < 1) via Eutectic Solutions for Direct Regeneration of Lithium‐Ion Battery Cathodes

With the rapid growth of the lithium‐ion battery (LIBs) market, recycling and re‐use of end‐of‐life LIBs to reclaim lithium (Li) and transition metal (TM) resources (e.g., Co, Ni), as well as eliminating pollution from disposal of waste batteries, has become an urgent task. Here, for the first time the ambient‐pressure relithiation of degraded LiNi_0.5Co_0.2Mn_0.3O₂ (NCM523) cathodes via eutectic Li⁺ molten‐salt solutions is successfully demonstrated. Combining such a low‐temperature relithiation process with a well‐designed thermal annealing step, NCM523 cathode particles with significant Li loss (≈40%) and capacity degradation (≈50%) can be successfully regenerated to achieve their original composition and crystal structures, leading to effective recovery of their capacity, cycling stability, and rate capability to the levels of the pristine materials. Advanced characterization tools including atomic resolution electron microscopy imaging and electron energy loss spectroscopy are combined to demonstrate that NCM523's original layered crystal structure is recovered. For the first time, it is shown that layer‐to‐rock salt phase change on the surfaces and subsurfaces of the cathode materials can be reversed if lithium can be incorporated back to the material. The result suggests the great promise of using eutectic Li⁺ molten–salt solutions for ambient‐pressure relithiation to recycle and remanufacture degraded LIB cathode materials.     

Reaching the Energy Density Limit of Layered O3‐NaNi0.5Mn0.5O2 Electrodes via Dual Cu and Ti Substitution

Although being less competitive energy density‐wise, Na‐ion batteries are serious alternatives to Li‐ion ones for applications where cost and sustainability dominate. O3‐type sodium layered oxides could partially overcome the energy limitation, but their practical use is plagued by a reaction process that enlists numerous phase changes and volume variations while additionally being moisture sensitive. Here, it is shown that the double substitution of Ti for Mn and Cu for Ni in O3‐NaNi_0.5?yCu_yMn_{0.5? z}Ti_zO₂ can alleviate most of these issues. Among this series, electrodes with specific compositions are identified that can reversibly release and uptake ≈0.9 sodium per formula unit via a smooth voltage‐composition profile enlisting minor lattice volume changes upon cycling as opposed to ΔV/V≈23% in the parent NaNi_0.5Mn_0.5O₂ while showing a greater resistance against moisture. The positive attributes of substitution are rationalized by structure considerations supported by density functional theory (DFT) calculations. Electrodes with sustained capacities of ≈180 mAh g^?1 are successfully implemented into 18 650 Na‐ion cells having greater performances, energy density‐wise (≈250 Wh L^?1), than today's Na₃V₂(PO₄)₂F₃/HC Na‐ion technology which excels in rate capabilities. These results constitute a step forward in increasing the practicality of Na‐ion technology with additional opportunities for applications in which energy density prevails over rate capability.     

The Fine Line between a Two‐Phase and Solid‐Solution Phase Transformation and Highly Mobile Phase Interfaces in Spinel Li4+xTi5O12

Phase transitions play a crucial role in Li‐ion battery electrodes being decisive for both the power density and cycle life. The kinetic properties of phase transitions are relatively unexplored and the nature of the phase transition in defective spinel Li_4+xTi₅O₁₂ introduces a controversy as the very constant (dis)charge potential, associated with a first‐order phase transition, appears to contradict the exceptionally high rate performance associated with a solid–solution reaction. With the present density functional theory study, a microscopic mechanism is put forward that provides deeper insight in this intriguing and technologically relevant material. The local substitution of Ti with Li in the spinel Li_4+xTi₅O₁₂ lattice stabilizes the phase boundaries that are introduced upon Li‐ion insertion. This facilitates a subnanometer phase coexistence in equilibrium, which although very similar to a solid solution should be considered a true first‐order phase transition. The resulting interfaces are predicted to be very mobile due to the high mobility of the Li ions located at the interfaces. This highly mobile, almost liquid‐like, subnanometer phase morphology is able to respond very fast to nonequilibrium conditions during battery operation, explaining the excellent rate performance in combination with a first‐order phase transition.     

Visualization of Light Elements using 4D STEM: The Layered‐to‐Rock Salt Phase Transition in LiNiO2 Cathode Material

The layered oxide LiNiO₂ (LNO) has been extensively investigated as a cathode active material for lithium‐ion batteries. Despite LNO's high gravimetric capacity, instability issues hinder its commercialization. It suffers from capacity loss during electrochemical cycling and is difficult to synthesize without defects. This is related to poor structural stability, leading to decomposition into the parent rock‐salt‐type oxide. In order to understand such phase transformations and to develop measures to inhibit them, the development of techniques able to image all atoms is crucial. In this study, the use of a fast, pixelated detector and 4D imaging in scanning transmission electron microscopy are explored to tackle this challenge. Selecting specific angular regions in the diffraction patterns and calculating virtual annular bright‐field images significantly enhances the contrast of the lithium atoms, such that all atoms are visible even in realistic samples. The developed technique is applied to image the layered‐to‐rock salt phase transition region. The data show that in this region, nickel atoms are in tetrahedral positions and the oxygen atoms are asymmetrically distributed. Taken together, the results shed light on the phase transformation mechanism at the atomic scale and can guide future research toward stabilizing LNO.