A high‐energy functional cathode material with an average composition of Li[Ni0.72Co0.18Mn0.10]O2, mainly comprising a core material Li[Ni0.8Co0.2]O2 encapsulated completely within a stable manganese‐rich concentration‐gradient shell is successfully synthesized by a co‐precipitation process. The Li[Ni0.72Co0.18Mn0.10]O2 with a concentration‐gradient shell has a shell thickness of about 1 µm and an outer shell composition rich in manganese, Li[Ni0.55Co0.15Mn0.30]O2. The core material can deliver a very high capacity of over 200 mA h g−1, while the manganese‐rich concentration‐gradient shell improves the cycling and thermal stability of the material. These improvements are caused by a gradual and continuous increase of the stable tetravalent Mn in the concentration‐gradient shell layer. The electrochemical and thermal properties of this cathode material are found to be far superior to those of the core Li[Ni0.8Co0.2]O2 material alone. Electron microscopy also reveals that the original crystal structure of this material remains intact after cycling.

Expanding the performance limit of current Li-ion batteries requires ion−ion and ion−solvent interaction, which governs the ion transport behavior of the electrolytes, to be fully understood as a matter of crucial importance. We herein examine the ionic speciation and conduction behavior of propylene carbonate (PC) electrolytes of 0.1−3.0 M LiPF 6 and LiBF 4 using Raman spectroscopy, dielectric relaxation spectroscopy (DRS), and pulsed-field gradient NMR (PFG-NMR) spectroscopy. In both LiPF 6 − PC and LiBF 4 −PC, free ions and a solvent-shared ion pair (SIP) are dominant species at dilute salt concentrations (<0.8 M), and SIP becomes dominant at intermediate concentrations (0.8−1.5 M). At higher concentrations (1.5−3.0 M), the solvent-shared dimer (SSD) and contact dimer (CD) are dominant in LiPF 6 −PC, whereas the contact ion pair (CIP), CD, and agglomerate (AGG) prevail in LiBF 4 −PC. Ionic conduction in 0.1−1.5 M LiPF 6 −PC and LiBF 4 −PC is governed by the migration of free ions and SIP. Notably, above 1.5 M of the two PC electrolytes, SSD participates in ionic conduction via the migration mode as well. Furthermore, it is suggested that the large number of CIPs present in LiBF 4 −PC may contribute to ionic conduction via a Grotthuss-type mechanism.

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Highly crystalline Li[Ni1−x−yCoxMny]normalO2 (x+y⩽0.5) (Li[Ni0.6Co0.2Mn0.2]normalO2 , Li[Ni0.55Co0.15Mn0.3]normalO2 , and Li[Ni0.5Co0.25Mn0.25]normalO2 ) were synthesized through a coprecipitation method. The capacities of the prepared samples were proportional to the amount of Ni in the host structure. The thermogravimetric analysis (TGA) and in situ high-temperature–X-ray diffraction (HT-XRD) analysis revealed that changes in the amount of manganese ions in the host structure profoundly affect the structural stability of the samples with x+y⩽0.5 . Li[Ni0.55Co0.15Mn0.3]normalO2 , containing the highest manganese content (y=0.3) , showed the most stable structural integrity among the samples as confirmed by in situ HT-XRD. The electrochemical performances of the samples in Ni amount (0.5⩽1−x−y⩽0.6) with the variation of Co (0.15⩽x⩽0.25) did not significantly vary under the test conditions (3.0–4.3V) . The small increase of Mn ions plays an important role in preservation of its initial structural symmetry during the high-temperature heating as well as electrochemical cycling. Furthermore, the structural stability has a relationship with the thermal stability and the electrochemical stability, especially at an elevated temperature (55°C) . On the basis of the differential scanning calorimetry and TGA results, the Li[Ni0.55Co0.15Mn0.3]normalO2 sample demonstrated improved thermal stability compared to the other samples.