Synthesis Optimization of LiNi0.5 Mn1.5 O4 for Long Cycle Life Li4Ti5O12/LiNi0.5Mn1.5O4

Abstract

Li-ion batteries (LIBs) have revolutionized the market of portable consumer electronics due to their high energy density (200 – 250 Wh Kg-1). However, most Li-ion chemistries employ expensive cobalt-based cathode active material. In addition, these cells employ a graphitic negative electrode which has a Li-intercalation potential that is close to the Li-plating potential. Therefore, lithium plating-induced cell short-circuiting leading to cell fires is a possibility in commercially available LIBs that employ graphite negative electrodes and /NiMnCo(NMC)- based oxide positive electrodes. The use of Li4Ti5O12 (LTO) as a negative electrode avoids lithium dendrite growth. However, the Li-intercalation potential in LTO is at 1.5 V vs Li+/Li which decreases the cell potential of LTO/NMC cells to < 2.5 V. Therefore, the increased safety comes at the expense of decreased energy density of an LTO/NMC cell. A Li-ion battery with LTO as a negative electrode and a 5V-class cobalt-free LiNi0.5Mn1.5O4 cathode could charge more safely while also providing excellent energy density (with a cell potential exceeding 3 V). The critical impediment to the commercialization of LTO-LNMO batteries is their inferior cycle life, which is primarily associated with the degradation of LNMO cathodes during electrochemical cycling. This short cycle life exacerbates especially (a) at active LNMO loading ≥ 1 mAh cm-2, (b) in the presence of even trace amounts of Mn3+ (disproportionation followed by migration and deposition), and (c) with the oxidation of LiPF6-carbonate solvents’ mixture beyond a positive electrode potential of 4.3 V vs Li+/Li. Herein, we have optimized the annealing temperature, particle morphology, and electrode fabrication method to achieve a cycle life = 450 cycles (80 % DoD) at an active LNMO loading of ≈ 7 mg cm-2 (1 mAh cm-2) and a charge/discharge rate of 1C in the presence of LiPF6- carbonate based electrolyte. Furthermore, it was determined that most of the cycle-life degradation stemmed from a composite LNMO electrode rather than an LTO.