Solution-Based Synthesis of Cathode Materials for Lithium Ion Batteries

Abstract

Electrochemical energy storage is expected to make an important contribution to combating global climate change, e.g. via electric vehicles and stationary storage. Currently, lithium-ion batteries are the most prominent candidate for rechargeable batteries to be used in this respect. However, performance of lithium-ion batteries is strongly influenced by the active material at the cathode side, which is often a complex multi-metal oxide. Such a material's properties are dictated by the fabrication method. The design of an appropriate synthesis strategy is then logically also aimed at optimal material properties. However, the design strategy should also take into account green chemistry principles, such as atom economy and energy efficiency. Solution-based routes towards (multi)metal oxide synthesis provide a large diversity of routes and therefore a strong versatility in their design. In this contribution, two groups of routes will be discussed: aqueous solution-gel and coprecipitation routes. The aqueous solution-gel route encompasses three stages: precursor solution synthesis, in which the metal ions are stabilized by complexation with carboxylate ligands, gelation to form an amorphous precursor gel in which the metal ions are homogeneously mixed, and decomposition into the final metal oxide. In this route, precipitation is to be avoided, since it would introduce inhomogeneities. In contrast, the coprecipitation route is entirely based on the formation of insoluble solid precipitates, which can be further transformed in a solid state reaction into lithiated compounds. Whereas solution-gel is ideally suited for complex metal oxides with a large number of metal ions, coprecipitation of many different metal ions at the same time can be challenging. On the other hand, the atom efficiency of the latter can be very high which is attractive for upscaling. Certainly, both of these routes have a high versatility because of the flexibility in compositions (starting products, solvents, ...) and conditions (temperature, time, ...) during the synthesis.In this presentation, an overview will be given of the application of these synthesis routes to the synthesis and/ or surface modification of various cathode materials for lithium ion batteries, which are of general interest and are ranging from Ni-rich compositions of NMC (NMC622), to lithium manganese rich NMC and to Co-free compositions such as LNMO and Li rich LMO. First, the classic NMC 622 is surface modified by a TiOx via a sol-gel route, leading to the formation of core-shell structured particles. The presence of TiOx improved the rate performance and electronic conductivity of the cathode material, and led to a higher initial coulombic efficiency and lower ageing rates in full cells with LTO. Furthermore, layered Li-rich/Mn-rich (LMR-)NMC, which is interesting because of its remarkably high specific discharge capacity was synthesized by two different routes, solution-gel and coprecipitation. It was shown that the solution-gel material in this case had a retarded voltage fade but larger capacity fade than the coprecipitated material. This was correlated to differences in particle morphology and the presence of Ni-enriched spinel-type surface layer. Furthermore, the effect of Sn4+ substitution in the LMR-NMC was investigated. The solubility of Sn4+ substituting for Mn4+ turned out to be a limiting factor here, and could not prevent the voltage fade sufficiently. Finally, we turned to Co-free cathode compositions. LiNi0.5Mn1.5O4-delta was prepared by an aqueous solution-gel route, which allowed to tune the particle size (a few µm versus 70 µm) and morphology. This led to optimized initial discharge capacity and its retention. Finally, Ti4+ substituted Li2MnO3 was synthesized as well via an aqueous solution-gel method. The substitution was shown to improve the structural stability, and galvanostatic cycling hinted at a delayed oxygen release during the first 10 cycles, however, followed by pronounced oxygen redox activity.In conclusion, solution-based routes are highly versatile and provide many opportunities to synthesize cathode materials for lithium ion batteries.