ABSTRACT
Coating and single crystal are two common strategies for cobalt-free nickel-rich layered oxides to solve its poor rate performance and cycle stability. However, the action mechanism of different modification protocols to suppress the attenuation are unclear yet. Herein, the Li2MoO4 layer-coated polycrystalline LiNi0.9Mn0.1O2 (1.0 %-Mo + NM91) and single crystal LiNi0.9Mn0.1O2 (SC-NM91) are prepared to investigate this difference, respectively. By focusing on the interior of particles, the relationship between structure evolution and electrochemical behavior is systematically studied, and the intrinsic mechanism of coating/single-crystallization modifications on suppressing the attenuation is clarified. The results show that microcracks in LiNi0.9Mn0.1O2 (NM91) are the main culprit leading to the rate capability decay, and the coating can effectively prevent the radial diffusion of microcracks from the center to surface, inhibiting the generation of surface side reactions. Therefore, the coating has a more advantage in improving the rate performance at 5.0C, the discharge capacity of 1.0 %-Mo + NM91 (130.6 mAh/g) is 7.9 % higher than that of SC-NM91 (121.0 mAh/g). In contrast, the single-crystallization can effectively prevent the formation of intergranular cracks arising from the anisotropic stress in NM91, which causes the severe cycle degradation. Correspondingly, the grain boundary-free SC-NM91 shows superior cyclability. The capacity retention rate of SC-NM91 (80.8 %) at 0.2C after 100cycles is 6.3 % higher than that of 1.0 %-Mo + NM91 (74.5 %). This work concludes the effect difference of different modification methods on enhancing the electrochemical performance, which provides theoretical and technical guidance for the optimized and targeted modification design in the cobalt-free high nickel cathode materials.
ABSTRACT
The design of a functional electrolyte system that is compatible with the LiNi0.8Co0.15Al0.05O2 (LNCA) cathode is of great importance for advanced lithium-ion batteries (LIBs). In this work, chelated lithium salts of lithium difluoro(bisoxalato) phosphate (LiDFBOP) and lithium tetrafluoro(oxalate) phosphate (LTFOP) are synthesized by a facile and general method. Then, the complexes of LiDFBOP, LTFOP, and lithium difluorophosphate (LiDFP), all of which have a central phosphorus atom, were selected as the salt-type additives for the LiPF6-based electrolyte to improve the electrochemical performances of LNCA/Li half-batteries, respectively. The results of electrochemical tests, quantum chemistry calculations, potential-resolved in situ electrochemical impedance (PRIs-EIS) measurements, and surface analyses show that the interface property and the battery performance are closely associated with molecular structures of phosphorus-centered complex additives. It indicates that LiDFP with the PâO bond can significantly reduce the interfacial impedance of LNCA/Li half-batteries due to the increase of Li3PO4 and the decrease of Li2CO3 in the cathode electrolyte interface (CEI). While in LiDFBOP, according to the calculated vertical ionization potential (VIP), the two oxalate-chelated ligands bring about a bidirectional cross-linking reaction, which makes it preferential to be oxidized. This process is self-healing and can form a dense and stretched CEI, which is favorable to the cycle performance at the late stage. In contrast, the polymerization reaction will occur in one direction for LTFOP due to its lone oxalate ligand. Additionally, an unfavorable side reaction between LTFOP and EC has been proposed by the aid of Gibbs free energy calculation. This is a good explanation for the formation of the uneven and unstable CEI, as well as the continuous decomposition of the electrolyte in PRIs-EIS measurement. This work has an extensive applicability and practical significance not only for molecular designing of novel lithium salts, but also for the construction of a functional electrolyte system that is compatible with different electrode materials.
