Layered oxides, particularly including Li[NixCoyMnz]O2 (NCMxyz) materials, such as NCM523, are the most promising cathode materials for high‐energy lithium‐ion batteries (LIBs). One major strategy to increase the energy density of LIBs is to expand the cell voltage (>4.3 V). However, high‐voltage NCM∥ graphite full cells typically suffer from drastic capacity fading, often referred to as “rollover” failure. In this study, the underlying degradation mechanisms responsible for failure of NCM523∥ graphite full cells operated at 4.5 V are unraveled by a comprehensive study including the variation of different electrode and cell parameters. It is found that the “rollover” failure after around 50 cycles can be attributed to severe solid electrolyte interphase growth, owing to formation of thick deposits at the graphite anode surface through deposition of transition metals migrating from the cathode to the anode. These deposits induce the formation of Li metal dendrites, which, in the worst cases, result in a “rollover” failure owing to the generation of (micro‐) short circuits. Finally, approaches to overcome this dramatic failure mechanism are presented, for example, by use of single‐crystal NCM523 materials, showing no “rollover” failure even after 200 cycles. The suppression of cross‐talk phenomena in high‐voltage LIB cells is of utmost importance for achieving high cycling stability.

Lithium ion battery cells operating at high‐voltage typically suffer from severe capacity fading, known as ‘rollover’ failure. Here, the beneficial impact of Li2CO3 as an electrolyte additive for state‐of‐the‐art carbonate‐based electrolytes, which significantly improves the cycling performance of NCM523 ∥ graphite full‐cells operated at 4.5 V is elucidated. LIB cells using the electrolyte stored at 20 °C (with or without Li2CO3 additive) suffer from severe capacity decay due to parasitic transition metal (TM) dissolution/deposition and subsequent Li metal dendrite growth on graphite. In contrast, NCM523 ∥ graphite cells using the Li2CO3‐containing electrolyte stored at 40 °C display significantly improved capacity retention. The underlying mechanism is successfully elucidated: The rollover failure is inhibited, as Li2CO3 reacts with LiPF6 at 40 °C to in situ form lithium difluorophosphate, and its decomposition products in turn act as ‘scavenging’ agents for TMs (Ni and Co), thus preventing TM deposition and Li metal formation on graphite.

Intermediate term discharge experiments were performed for Si-air full cells using As-, Sb-and B-doped Si-wafer anodes, with 100 and 111 orientations for each type. Discharge characteristics were analyzed in the range of 0.05 to 0.5 mA/cm 2 during 20 h runs, corrosion rates were determined via the mass-change method and surface morphologies after discharge were observed by laser scanning microscopy and atomic force microscopy. Corresponding to these experiments, potentiodynamic polarization curves were recorded and analyzed with respect to current-potential characteristics and corrosion rates. Both, discharge and potentiodynamic experiments, confirmed that the most pronounced influence of potentials -and thus on performance -results from the dopant type. Most important, the corrosion rates calculated from the potentiodynamic experiments severely underestimate the fraction of anode material consumed in reactions that do not contribute to the conversion of anode mass to electrical energy. With respect to materials selection, the estimates of performance from intermediate term discharge and polarization experiments lead to the same conclusions, favoring 100 and 111 As-doped Si-wafer anodes. However, the losses in the 111 As-doped Si-anodes are by 20% lower, so considering the mass conversion efficiency this type of anode is most suitable for application in non-aqueous Si-air batteries. One line of development in technologies for electrical energy storage is metal-air batteries, which provide high specific energies and -when referring to Zn, Al, Fe, or Si -are at the same time resource effective with respect to the availability and price of the anode materials. The theoretical specific energy of a Si-air cell, related to the anode mass only, is 8470 Wh/kg. Using Si material in aqueous alkaline solutions, however, results in a severe corrosion reaction which is accompanied by intense hydrogen evolution.1-3 Despite the corrosion reaction, it is still feasible to build an alkaline Si-air cell at a discharge potential around 1.1 V, however, with sacrifice of huge amount of Si anode to corrosion. [4][5][6] Therefore, new approaches to establish batteries on silicon materials have been put forward using ionic liquid electrolytes. One of the possible approaches is the usage of EMIm(HF) 2.3 F electrolyte which possesses high conductivity, low viscosity and chemical stability in air.7-10 The proof of concept, that substantial discharge was possible when using EMIm(HF) 2.3 F electrolyte, was proposed in 2010 according to the following reactions: Additionally, a screening of several anode materials -As-, Sband B-doped Si wafers -was performed, in which the cell potential at intermediate current densities as determined from potentiodynamic polarization measurements, was set as major criterion. The corrosion current densities as obtained by the Tafel fits from the polarization experiments for the different wafer types were also considered for the material selection. However, owing to the low corrosion rates, it played a minor ro...

Li/Mn-rich positive electrode materials are widely known for their high energy content, which exceeds that of current cathodes significantly. Their implementation in practical high-energy lithium ion batteries has to date been hindered by various technical challenges, most of all, their insufficient long-term charge/discharge cycling performance. Here, a new concept for designing core–shell (CS) materials for this class is introduced, which is based on cathode particles, which are specifically designed to have an anionic redox-rich core and a shell with reduced anionic redox. This material design approach aims to overcome the drawback of having the sensitive anionic redox at the electrolyte/active material interface, and, therefore, extenuating the phase degradation from layered- to spinel- or even rock salt-structure. In this work, the effect of a CS particle design with a Co-free, Mn-rich core and a Co-containing shell with lower Mn content is studied. The spherical cathode particles, produced by a Couette Taylor Flow Reactor, enable a superior electrochemical performance with excellent initial Coulombic efficiencies of 90%–95% and improved long-term cycling performances compared to the reference materials, consisting of the pure core or pure shell composition.

Voltage decay during cycling is still a major issue for Li-rich cathodes in lithium ion batteries. Recently, the increase of Ni content has been recognized as an effective way to mitigate this problem, although it leads to lower-capacity materials. To find a balance between voltage decay and high capacity, particles of Lirich materials with concentration gradients of transition metals have been prepared. Since voltage decay is caused by oxygen loss and phase transition that occur mainly on the particle surface, the Ni content is designed with a negative gradient of concentration from the surface to the bulk of particles. To do so, microsized Li 1.20 Ni 0.13 Co 0.13 Mn 0.54 O 2 particles are mixed with much smaller LiNi 0.8 Co 0.1 Mn 0.1 O 2 particles to form deposits of small particles onto larger particles. The concentration gradient of Ni is achieved as the Ni ions in LiNi 0.8 Co 0.1 Mn 0.1 O 2 penetrate into Li 1.20 Ni 0.13 Co 0.13 Mn 0.54 O 2 during a calcination post-treatment. Gradient samples show superior cycling performance and voltage retention as well as improved safety. This systematic study explores a material model combining Li-rich and high-Ni layered cathodes that is shown to be effective in creating a balance between mitigated voltage decay and high energy density.

Porosity is frequently specified as only a value to describe the microstructure of a battery electrode. However, porosity is a key parameter for the battery electrode performance and mechanical properties such as adhesion and structural electrode integrity during charge/discharge cycling. This study illustrates the importance of using more than one method to describe the electrode microstructure of LiNi0.6Mn0.2Co0.2O2 (NMC622)-based positive electrodes. A correlative approach, from simple thickness measurements to tomography and segmentation, allowed deciphering the true porous electrode structure and to comprehend the advantages and inaccuracies of each of the analytical techniques. Herein, positive electrodes were calendered from a porosity of 44–18% to cover a wide range of electrode microstructures in state-of-the-art lithium-ion batteries. Especially highly densified electrodes cannot simply be described by a close packing of active and inactive material components, since a considerable amount of active material particles crack due to the intense calendering process. Therefore, a digital 3D model was created based on tomography data and simulation of the inactive material, which allowed the investigation of the complete pore network. For lithium-ion batteries, the results of the mercury intrusion experiments in combination with gas physisorption/pycnometry experiments provide comprehensive insight into the microstructure of positive electrodes.

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