First-principle Studies On Electrochemical Properties Of Li-ion Battery Cathode Material Li₂MSiO₄ （M=Mn, Fe） And Its O Site N Substituted Structure

In last decade, a kind of high performance cathode material Li2MSiO4 (M=Fe and Mn) has been investigated experimentally and theoretically. The Li2MSiO4 family is attractive due to its theoretical possibility to reversibly deintercalate two lithium ions per formula unit, which brings a high theoretical capacity up to 330mAh/g. But in practice, it has never been implemented. In this paper we use first principles calculation to study the causes:(1) The site-exchange (SE) of Li and M ions in charge-discharge cycles was experimentally observed and deinsertion voltage changes with it. Recent studies focus on whether SE blocks Li ions diffusion. We study its electrochemical properties and find the SE structure brings a large cell expand. This expansion influences the cycle stability severely.(2) The deinsertion voltage is one of the factors for theoretical capacity of cathode material. Li2FeSiO4 can’t deintercalate the second lithium ion because its deinsertion voltage is higher than anodic stability of the electrolyte. Former research found that N substitution lowers the deinsertion voltage and our research find the SE structure will aggravate this change.(3) Cycle stability is another factor for theoretical capacity. Secondary battery needs to charge and discharge as many times as possible therefore it is important to be cycle stable. The trouble in Li2MnSiO4 is the volume expansion in charge-discharge cycles. The large and repeated volume expansion requires an excellent ductility. Thus we calculated a factor B/G to represent the ductility. A B/G larger than 1.75 means ductility, lower than 1.75 means brittleness. Using this we successfully explain the capacity fading of Li2MnSiO4, and predict the same phenomenon will happen in Li2FeSiO4. N substitution improves the ductility but the material is still brittle therefore its capacity will fade.