ABSTRACT
High voltage spinel is one of the most promising next-generation cobalt-free cathode materials for lithium ion battery applications. Besides the typically utilized compositional range of LixNi0.5Mn1.5O4 0 < x < 1 in the voltage window of 4.90-3.00 V, additional 1.5 mol of Li per formula unit can be introduced into the structure, in an extended voltage range to 1.50 V. Theoretically, this leads to significant increase of the specific energy from 690 to 1190 Wh/kg. However, utilization of the extended potential window leads to rapid capacity fading and voltage polarization that lack a comprehensive explanation. In this work, we conducted potentiostatic entropymetry, operando XRD and neutron diffraction on the ordered stoichiometric spinel LixNi0.5Mn1.5O4 within 0 < x < 2.5 in order to understand the dynamic structure evolution and correlate it with the voltage profile. During the two-phase reaction from cubic (x < 1) to tetragonal (x > 1) phase at â¼2.8 V, we identified the evolution of a second tetragonal phase with x > 2. The structural evaluation during the delithiation indicates the formation of an intermediate phase with cubic symmetry at a lithium content of x = 1.5. Evaluation of neutron diffraction data, with maximum entropy method, of the highly lithiated phase LixNi0.5Mn1.5O4 with 2 < x < 2.5 strongly suggests that lithium ions are located on octahedral 8a and tetrahedral 4a positions of the distorted tetragonal phase I41amd. Consequently, we were able to provide a conclusive explanation for the additional voltage step at 2.10 V, the sloping voltage profile below 1.80 V, and the additional voltage step at â¼3.80 V.
ABSTRACT
The combination of two active materials into one positive electrode of a lithium-ion battery is an uncomplicated and cost-effective way to combine the advantages of different active materials while reducing the disadvantages of each material. In this work, the concept of binary blends is extended to ternary compositions. The combination of three different active materials provides high versatility in designing the properties of an electrode. Therefore, the unique properties of a layered oxide, phospho-olivine, and spinel type material are mixed to design a high-energy cathode with improved environmental friendliness. Four different compositions of blend electrodes are investigated, each with individual benefits. Synergistic effects improved the rate capability, power density, thermal and chemical stability simultaneously. The blend electrode consisting of 75 % NMC, 12.5 % LMFP and LMO provides similar energy and power density as a pure NMC electrode while economizing 25 % cobalt and nickel.
ABSTRACT
The addition of Si compounds to graphite anodes has become an attractive way of increasing the practical specific energies in Li-ion cells. Previous studies involving Si/C anodes lacked direct insight into the processes occurring in full cells during low-temperature operation. In this study, a powerful combination of operando neutron diffraction, electrochemical tests, and post-mortem analysis is used for the investigation of Li-ion cells. 18650-type cylindrical cells in two different aging states are investigated by operando neutron diffraction. The experiments reveal deep insights and important trends in low-temperature charging mechanisms involving intercalation, alloying, Li metal deposition, and relaxation processes as a function of charging C-rates and temperatures. Additionally, the main aging mechanism caused by long-term cycling and interesting synergistic effects of Si and graphite are elucidated.
