Active lithium loss (ALL) resulting in a capacity loss (Q), which is caused by lithium consuming parasitic reactions like SEI formation, is a major reason for capacity fading and, thus, for a reduction of the usable energy density of lithium-ion batteries (LIBs). Q is often equated with the accumulated irreversible capacity (Q). However, Q is also influenced by non-lithium consuming parasitic reactions, which do not reduce the active lithium content of the cell, but induce a parasitic current. In this work, a novel approach is proposed in order to differentiate between Q and Q. The determination of Q is based on the remaining active lithium content of a given cell, which can be determined by de-lithiation of the cathode with the help of the reference electrode of a three-electrode set-up. Lithium non-consuming parasitic reactions, which do not influence the active lithium content have no influence on this determination. In order to evaluate this novel approach, three different anode materials (graphite, carbon spheres and a silicon/graphite composite) were investigated. It is shown that during the first charge/discharge cycles Q is described moderately well by Q. However, the difference between Q and Q rises with increasing cycle number. With this approach, a differentiation between "simple" irreversible capacities and truly detrimental "active Li losses" is possible and, thus, Coulombic efficiency can be directly related to the remaining useable cell capacity for the first time. Overall, the exact determination of the remaining active lithium content of the cell is of great importance, because it allows a statement on whether the reduction in lithium content is crucial for capacity fading or whether the fading is related to other degradation mechanisms such as material or electrode failure.

Nickel-rich layered oxide materials (Li-Ni x Mn y Co 1−x−y O 2 , x ≥ 0.8, LiNMC) attract great interest for application as positive electrode in lithium ion batteries (LIBs) due to high specific discharge capacities at moderate upper cutoff voltages below 4.4 V vs Li/Li + . However, the comparatively poor cycling stability as well as inferior safety characteristics prevent this material class from commercial application so far. Against this background, new electrolyte formulations including additives are a major prerequisite for a sufficient electrochemical performance of Ni-rich NMC materials. In this work, we introduce triphenylphosphine oxide (TPPO) as electrolyte additive for the application in graphite/LiNi 0.8 Mn 0.1 Co 0.1 O 2 (NMC811) cells. The addition of only 0.5 wt % TPPO into a carbonate-based electrolyte (LiPF 6 in EC:EMC) significantly increases the first cycle Coulombic efficiency as well as the reversible specific capacity and improves the capacity retention of the LIB full cell cycled between 2.8 and 4.3 V. Electrochemical results indicate that the full cell capacity fade is predominantly caused by active lithium loss at the negative electrode. In this contribution, X-ray photoelectron spectroscopy and inductively coupled plasma-mass spectrometry analysis confirm the participation of the electrolyte additive in the solid electrolyte interphase formation on the negative electrode as well as in the cathode electrolyte interphase formation on the positive electrode, thus, effectively reducing the active lithium loss during cycling. Furthermore, the performance of the TPPO additive is compared to literature known electrolyte additives including triphenylphosphine, vinylene carbonate, and diphenyl carbonate demonstrating the outstanding working ability of TPPO in graphite/NMC811 cells.

Micron-sized truncated octahedral LiNi 0.5 Mn 1.5 O 4 (LNMO) samples with different degrees of Ni/Mn disordering have been obtained by controlling the synthesis conditions, such as calcination atmosphere (O 2 and air), cooling rate or additional annealing step. The influences of Ni/Mn disordering on the physical properties and electrochemical performance of the truncated octahedral LNMO samples have been systematically investigated. The analyses of thermogravimetry, X-ray photoelectron spectroscopy, X-ray diffraction, powder neutron diffraction, Raman spectroscopy and X-ray absorption spectroscopy reveal that the occurrence and degree of Ni/Mn disordering are closely related with the formation of oxygen vacancies and presence of Mn 3+ . Slow cooling rate and post-annealing can result in low degrees of Ni/Mn disordering and oxygen vacancies. Electrochemical measurements show that Ni/Mn disordering and oxygen vacancies have no obvious effect on the rate capability since all LNMO samples share a truncated octahedral morphology with the exposed {100} surfaces. However, they play significant roles in improving long-term cycling stability, especially at the elevated temperature of 60 • C. This work suggests that the electrochemical performance of LNMO with optimized truncated morphology can be further enhanced through tuning the degrees of Ni/Mn disordering and oxygen vacancies.