Abstract:In order to meet the sophisticated demands for large-scale applications such as electro-mobility, next generation energy storage technologies require advanced electrode active materials with enhanced gravimetric and volumetric capacities to achieve increased gravimetric energy and volumetric energy densities. However, most of these materials suffer from high 1st cycle active lithium losses, e.g., caused by solid electrolyte interphase (SEI) formation, which in turn hinder their broad commercial use so far. In general, the loss of active lithium permanently decreases the available energy by the consumption of lithium from the positive electrode material. Pre-lithiation is considered as a highly appealing technique to compensate for active lithium losses and, therefore, to increase the practical energy density. Various pre-lithiation techniques have been evaluated so far, including electrochemical and chemical pre-lithiation, pre-lithiation with the help of additives or the pre-lithiation by direct contact to lithium metal. In this review article, we will give a comprehensive overview about the various concepts for pre lithiation and controversially discuss their advantages and challenges. Furthermore, we will critically discuss possible effects on the cell performance and stability and assess the techniques with regard to their possible commercial exploration.

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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.

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In this work, carbon nanospheres (CS) are prepared by hydrothermal synthesis using glucose as precursor, followed by a subsequent carbonization step. By variation of the synthesis parameters, CS particles with different particle sizes are obtained. With particular focus on the fast charging capability, the electrochemical performance of CS as anode material in lithium ion batteries (LIBs) is investigated, including the influence of particle size and carbonization temperature. It is shown that CS possess an extraordinary good long-term cycling stability and a very good rate capability (up to 20C charge/ discharge rate) at operating temperatures of 20 and 0 °C compared to graphitic carbon and Li 4 Ti 5 O 12 (LTO)-based anodes. One major disadvantage of CS is the very low first cycle Coulombic efficiency (C eff ) and the related high active lithium loss, which prevents usage of CS within LIB full cells. Nevertheless, in order to overcome this problem, we performed electrochemical pre-lithiation, which significantly improves the first cycle C eff and enables usage of CS within LIB full cells (vs NMC-111), which is shown here for the first time. The improved rate capability of CS is also verified in electrochemically prelithiated NMC-based LIB full cells, in comparison to graphite and LTO anodes. Further, CS also display an improved specific energy (at ≥5C), energy efficiency (at ≥2C), and energy retention (at ≥2C) compared to graphite and LTO-based LIB full cells.

In this work, silicon/carbon composites are synthesized by forming an amorphous carbon matrix around silicon nanoparticles (Si-NPs) in a hydrothermal process. The intention of this material design is to combine the beneficial properties of carbon and Si, i.e., an improved specific/volumetric capacity and capacity retention compared to the single materials when applied as a negative electrode in lithium-ion batteries (LIBs). This work focuses on the influence of the Si content (up to 20 wt %) on the electrochemical performance, on the morphology and structure of the composite materials, as well as the resilience of the hydrothermal carbon against the volumetric changes of Si, in order to examine the opportunities and limitations of the applied matrix approach. Compared to a physical mixture of Si-NPs and the pure carbon matrix, the synthesized composites show a strong improvement in long-term cycling performance (capacity retention after 103 cycles: ≈55% (20 wt % Si composite) and ≈75% (10 wt % Si composite)), indicating that a homogeneous embedding of Si into the amorphous carbon matrix has a highly beneficial effect. The most promising Si/C composite is also studied in a LIB full cell vs a NMC-111 cathode; such a configuration is very seldom reported in the literature. More specifically, the influence of electrochemical prelithiation on the cycling performance in this full cell set-up is studied and compared to non-prelithiated full cells. While prelithiation is able to remarkably enhance the initial capacity of the full cell by ≈18 mAh g−1, this effect diminishes with continued cycling and only a slightly enhanced capacity of ≈5 mAh g−1 is maintained after 150 cycles.

We report on interphase modification by in situ generated protons (H + ) via electropolymerization. The protons are released from oxidative electropolymerization of indole-3-carboxylic acid (InAc) used as an additive in a LiNi 0.8 Co 0.15 Al 0.05 O 2 (NCA) based cathode during cycling against lithium metal and silicon−graphite composite (Si−Gr) electrodes. Electrochemical data supported by ex situ NMR spectroscopy and X-ray photoelectron spectroscopy (XPS) prove that the H + produced in the lithium metal cell are used for cathode interphase formation and thereby improve the Coulombic efficiency during cycling. With the help of Li 3 PO 4 , which scavenges H + by the release of Li + , it can be established that H + is reduced at the cathode at a potential of ∼3 V vs Li/Li + to form H 2 without large-capacity fade. The H + have a more pronounced effect on the anode side when replacing lithium metal by Si−Gr. This is due to the facile H + reduction at the anode during charge which modulates the solid electrolyte interphase (SEI) as well as the Si surface which is proven by ex situ XPS. The reduction of H + at the anode is found to have a positive effect in mitigating the irreversible Li + loss (10% capacity gain with 10% InAc) at the Si−Gr electrode, which was maintained over 100 cycles.