Currently lithium iron orthosilicate (Li2FeSiO4) attracts considerable attention as next-generation cathode material for high-energy density Li-ion batteries because of its high theoretical capacity of 330 mAh/g and its inherent safety arising from the polyanionic framework. The challenge however lies in realizing its high specific energy in a multi-cycle reversible stable mode and at high charge and discharge rate. So far most of the studies report only one Li extraction and at relatively low rate associated with significant capacity fade. The majority of these studies have focused though on the high-T (typically obtained at 700 C) monoclinic phase, P21/n synthesized in different nanoparticle morphologies and coated with carbon to improve its ionic and electronic conductivities. However, this phase is metastable undergoing phase transition to the inverse Pmn21 orthorhombic phase upon electrochemical cycling, this phase transition appearing to be driven by Li-Fe antisite defects. Recently we reported that monoclinic LFS nanocrystals obtained at lower annealing temperature (400 C) exhibit different Li-ion storage behavior than the equivalent material obtained at 700 C owing to its nanograined structure. Since, there is significant evidence of cycling-induced transition from P21/n to inverse Pmn21, the latter crystal phase becomes critical to understand and properly engineer in our quest to develop high-energy density LFS cathodes. Herein, we report highlights from our recent LFS research spanning from the synthesis (@200 C) of defect-free Pmn21 particles to their doping, mechanochemical activation, and their non-C coating leading to improved performance. We have discovered non-stoichiometric LFS to exhibit higher reversible capacity than stoichiometric LFS; cation doping to enhance the stability of the cycled crystal structure; high-energy milling to lead to crystal changes analogous to thermal annealing; and conductive polymer-coated LFS to exhibit fast and stable cyclability. Complementary ex-situ and in-situ characterization results are presented helping us shed light into the cycled LFS material changes opening new pathways towards thair further development.

Nanocrystal engineering of lithium iron orthosilicate cathodes for improved reversible-stable storage and rate capability

256th National Meeting (2018)

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