No abstract

Ni-rich NCM-based positive electrode materials exhibit appealing properties in terms of high energy density and low cost. However, these materials suffer from different degradation effects, especially at their particle surface. Therefore, in this work, tungsten oxide is evaluated as a protective inorganic coating layer on LiNi 0.8 Co 0.1 Mn 0.1 O 2 (NCM-811) positive electrode materials for lithium-ion battery (LIB) cells and investigated regarding rate capability and cycling stability under different operation conditions. Using electrochemical impedance spectroscopy, the interfacial resistance of uncoated and coated NCM-811 electrodes is explored to study the impact of the coating on lithium-ion diffusion. All electrochemical investigations are carried out in LIB full cells with graphite as a negative electrode to ensure better comparability with commercial cells. The coated electrodes show an excellent capacity retention for the long-term charge/ discharge cycling of NCM-811-based LIB full cells, i.e., 80% state-of-health after more than 800 cycles. Furthermore, the positive influence of the tungsten oxide coating on the thermal and structural stability is demonstrated using postmortem analysis of aged electrodes. Compared to the uncoated electrodes, the surface-modified electrodes show less degradation effects, such as particle cracking on the electrode surface and improvement of the thermal stability of NCM-811 in the presence of electrolyte.

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.

The increase of specific energy of current Li ion batteries via further increase of the cell voltage, for example, to 4.5 V is typically accompanied by a sudden and rapid capacity fade, known as “rollover” failure. This failure is the result of Li dendrite formation triggered in the course of electrode cross‐talk, that is, dissolution of transition metals (TMs) from the cathode and deposition on the anode. It is shown herein, that the elimination of ethylene carbonate (EC) from a state‐of‐the‐art electrolyte, that is, from 1.0 m LiPF6 in a 3:7 mixture of EC and ethyl methyl carbonate prevents this failure in high‐voltage LiNi0.5Co0.2Mn0.3O2||graphite cells, even without any electrolyte additives. While the oxidative stability on the cathode side is similar in both electrolytes, visible by a decomposition plateau at 5.5 V versus Li|Li+ during charge, the anode side in the EC‐free electrolyte reveals significantly less TM deposits and Li metal dendrites compared to the EC‐based electrolyte. The beneficial effect of EC‐free electrolytes is related to a significantly increased amount of degraded LiPF6 species, which effectively trap dissolved TMs and suppress the effect of detrimental cross‐talk, finally realizing rollover‐free performance under high voltage conditions.

The decomposition of state‐of‐the‐art lithium ion battery (LIB) electrolytes leads to a highly complex mixture during battery cell operation. Furthermore, thermal strain by e.g., fast charging can initiate the degradation and generate various compounds. The correlation of electrolyte decomposition products and LIB performance fading over life‐time is mainly unknown. The thermal and electrochemical degradation in electrolytes comprising 1 m LiPF6 dissolved in 13C3‐labeled ethylene carbonate (EC) and unlabeled diethyl carbonate is investigated and the corresponding reaction pathways are postulated. Furthermore, a fragmentation mechanism assumption for oligomeric compounds is depicted. Soluble decomposition products classes are examined and evaluated with liquid chromatography‐high resolution mass spectrometry. This study proposes a formation scheme for oligo phosphates as well as contradictory findings regarding phosphate‐carbonates, disproving monoglycolate methyl/ethyl carbonate as the central reactive species.

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.

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.

Compared to conventional poly-crystal NCM, single-crystal NCM reveals significantly improved particle stability and less electrode cross-talk, which not only suppresses rollover fading but also improves performance aspects and the cycle life up to 4.7 V.