Lithium-ion cells containing lithium– and manganese– rich layered-oxides (LMR-NMC) have gained significant attention in recent years because of their ability to deliver high energy densities. In this article we report on a comprehensive performance and degradation study of cells, containing Li1.2Ni0.15Mn0.55Co0.1O2–based positive electrodes and graphite–based negative electrodes, on extended cycling. In addition to electrochemical measurements on full cells, characterization data on harvested electrodes by techniques that include scanning electron microscopy (SEM), X-ray diffraction (XRD), X-ray absorption spectroscopy (XAS), Raman spectroscopy, and secondary ion mass spectrometry (SIMS) are discussed. Our data show that cell capacity fade mainly results from lithium trapping in the solid electrolyte interphase (SEI) of the negative electrode. In addition, cell impedance rise and voltage fade mainly arise at the positive electrode and result from degradation processes in its oxide and carbon constituents. Processes that include the accumulation of transition metal elements at the negative electrode, and increasing misalignment of electrode capacity windows on extended cycling, also have a deleterious effect on cell performance. Identifying sources of performance degradation has enabled strategies to extend cell life, which include improved cell fabrication protocols, positive electrode coatings, and bifunctional electrolyte additives.
Improving the stability of Li ion electricity storage devices is important for practical applications, including the design of rechargeable automotive batteries. Many promising designs for such batteries involve positive electrodes that are complex oxides of transition metals, including manganese. Deposition of this Mn on the graphite negative electrode is known to correlate with gradual capacity fade [by increasing retention of lithium cations in the solid electrolyte interphase (SEI)] in Li ion batteries. This SEI contains partially reduced and fully mineralized electrolyte, in the outer (organic) and inner (mineral) layers. In this study, we explore structural aspects of this Mn deposition via a combination of electrochemical, X-ray absorption, and electron paramagnetic resonance experiments. We confirm previous observations that suggest that on a delithiated graphite electrode Mn is present as Mn2+ ion. We show that these Mn2+ ions are dispersed: there are no Mn-containing phases, such as MnF2, MnO, or MnCO3. These isolated Mn2+ ions reside at the surface of lithium carbonate crystallites in the inner SEI layer. For a lithiated graphite electrode, there is reduction of these Mn2+ ions to an unidentified species different from atomic, nanometer scale or mesoscale Mn(0) clusters. We suggest that Mn2+ ions are transported from the positive electrode to the graphite electrode as complexes in which the cation is chelated by carboxylate groups that are products of electrolytic breakdown of the carbonate solvent. This complex is sufficiently strongly bound to avoid cation exchange in the outer SEI and thereby reaches the inner (mineral) layer, where the Mn2+ ion is chemisorbed at the surface of the carbonate crystallites. We conjecture that stronger chelation can prevent deposition of Mn2+ ions and in this way retard capacity fade. This action might account for the protective properties of certain battery additives.
Concentrated electrolytes usually demonstrate good electrochemical performance and thermal stability, and are also supposed to be promising when it comes to improving the safety of lithium-ion batteries due to their low flammability. Here, we show that LiN(SO2F)2-based concentrated electrolytes are incapable of solving the safety issues of lithium-ion batteries. To illustrate, a mechanism based on battery material and characterizations reveals that the tremendous heat in lithium-ion batteries is released due to the reaction between the lithiated graphite and LiN(SO2F)2 triggered thermal runaway of batteries, even if the concentrated electrolyte is non-flammable or low-flammable. Generally, the flammability of an electrolyte represents its behaviors when oxidized by oxygen, while it is the electrolyte reduction that triggers the chain of exothermic reactions in a battery. Thus, this study lights the way to a deeper understanding of the thermal runaway mechanism in batteries as well as the design philosophy of electrolytes for safer lithium-ion batteries.
A neutral C4 cumulene 1 that includes a cyclic alkyl(amino) carbene (cAAC), its air-stable radical cation 1(·+) , and dication 1(2+) have been synthesized. The redox property of 1(·+) was studied by cyclic voltammetry. EPR and theoretical calculations show that the unpaired electron in 1(·+) is mainly delocalized over the central C4 backbone. The commercially available CBr4 is utilized as a source of dicarbon in the cumulene synthesis.
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