Exploiting the Degradation Mechanism of NCM523Graphite Lithium‐Ion Full Cells Operated at High Voltage

Abstract

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.

Understanding the Outstanding High‐Voltage Performance of NCM523||Graphite Lithium Ion Cells after Elimination of Ethylene Carbonate Solvent from Conventional Electrolyte

“…These domains are validated in our previous publication and could be attributed to Li dendrites by means of, for example, microscopic techniques and Li NMR. [ 20 ] Moreover, these Li metal dendrites are obviously formed at the same spots, where TM previously deposit, as they are located at identical spots as shown in the SEM‐EDX analysis for Co, Mn, and Ni. Interestingly, the graphite‐based anode cycled in the EC‐free electrolyte is free of these spots based on TMs and Li metal dendrites.…”

“…It is known, that the roll‐over fading is the result of the electrode cross‐talk, that is, TM dissolution from the cathode and transport to and deposition on the anode. [ 20 ] The cathode, as obvious initiator of this failure, can be modified by, for example, literature well‐known coating with Al 2 O 3 . [ 41–43 ] Indeed the coating can prolong the cycle life but still cannot prevent the sudden roll‐over fading (here: cycle no.…”

“…[ 8–16 ] In the worst case, high‐voltage operation of NCM‐based LIB cells (e.g., at cell voltages of 4.5 V) results in fast and significant capacity fading, which is often referred to as “rollover” failure, and has been observed in different studies. [ 9–11,17–19 ] In our previous publication, [ 20 ] we reported on the underlying mechanism for the rapid cell failure of high‐voltage LIB cells, which can be attributed to severe SEI alteration at the graphite anode, due to deposited TMs migrating from the cathode to the anode, which in turn induce the formation of Li metal dendrites.…”

On the Beneficial Impact of Li2CO3 as Electrolyte Additive in NCM523 ∥ Graphite Lithium Ion Cells Under High‐Voltage Conditions

“…The cells operated at 4.5 V show an early induced ‘rollover’ failure after ≈50 cycles, caused by TM deposition (Ni, Mn, Co), significant SEI growth and Li metal dendrite formation, which subsequently results in the generation of (micro‐)short‐circuits, as discussed in a previous work. [ 11 ] In contrast, the LIB cells operated at 4.3 V do not show any rollover failure and a rather stable capacity over 100 cycles.…”

Understanding the Effectiveness of Phospholane Electrolyte Additives in Lithium‐Ion Batteries under High‐Voltage Conditions

“…This surface layer is clearly the reason for the poor performance even compared to the reference electrolyte. A recent study revealed impedance growth on the positive electrode and especially transition metal dissolution from its active material to be the predominant reasons for capacity fading and in some cases even rapid cell failure, known as “rollover” failure, in NMC532‖graphite cells under high voltage conditions [34] . In line with this, the formation of an effective CEI by the studied phospholane functional additives evidently alters these processes and is thus beneficial for an enhanced battery performance.…”