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.
performance in high-voltage LIB cells. Our results point out the importance to thoroughly evaluate the impact of the separator on cell performance, especially when it comes to comparison of electrochemical data within the scientific community.
Results and Discussion2.1. Impact of PP Membrane and PP Fiber Separators with Different Thicknesses on the "Rollover" Failure
In this study, laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) was applied to previously aged carbonaceous anodes from lithium ion batteries (LIBs). The electrodes were treated by cyclic aging in a lithium ion cell set-up with LiNiMnO (LNMO) cathodes and hard carbon (HC)/mesocarbon microbead (MCMB) anodes. An inhomogeneous transition metal deposition pattern could be induced by replacing the spacer in a standard coin cell set-up with a washer. The inhomogeneity pattern matched the dimension of the washer depicted by the hole in the center. These transition metal (TM) patterns were used to optimize higher lateral scanning speeds and frequencies on the spatial resolution of the mapping experiments using LA-ICP-MS. Higher scanning speeds had an observable influence on the resolution of the obtained image and an overall saving of 60% with regard to time and gas consumption could be achieved. Additionally, the optimized method was applied to the cathode and separator in order to visualize the distribution and deposition pattern, respectively.
High-voltage Li ion batteries are compromised by lower cycle life due to enhanced degradation of cathode material, for example LiNi 0.5 Co 0.2 Mn 0.3 O 2 (NCM523). Crucial part is the initiated electrode crosstalk, that is transition metal (TM) dissolution from the cathode and subsequent deposition on the anode, as it forces formation of high surface area lithium, capacity losses and risk of Li dendrite penetration, finally leading to an abrupt end-of-life (= rollover failure). Hence, suppression of this failure cascade is the pivotal strategy to prolong cycle life. A pragmatic approach was presented: the electrolyte manipulation towards formation/presence of fluorophosphates, as they effectively suppressed electrode crosstalk through TM scavenging. Either, they could be intrinsically formed, e. g. by elimination of ethylene carbonate (EC) solvent (= EC-free electrolyte), or simply externally added, e. g. using (good-soluble) lithium difluorophosphate electrolyte additive. Their effectiveness was demonstrated for conventional ECbased and EC-free electrolytes at limiting conditions (4.5 and 4.6 V, respectively). In parallel to supportive approach combinations (e. g. coating), also destructive combinations were highlighted, that is approaches, which even decrease the fluorophosphate content, e. g. vinylene carbonate additive in EC-free electrolytes. Finally, by demonstrating the value of (concentration-optimized) fluorophosphates, appropriate benchmark electrolyte formulations for high-voltage LIBs were discussed.
The chemical and structural complexity of lithiumion battery electrodes and their constituting materials requires comprehensive characterization techniques to reveal degradation phenomena at the mesoscale. For the first time, application of single-particle inductively coupled plasma-optical emission spectroscopy enables the investigation of the chemomechanical interplay on the particle level of lithium transition-metal oxide [e.g., Li(Ni 1/3 Co 1/3 Mn 1/3 )O 2 ] cathode materials. The sampleinherent polydisperse size distribution of particles ranging up to 10 μm was effectively restricted with the use of a custom-made gravitational-counter-flow classifier to facilitate complete particle vaporization and excitation. After classification, the particles were transported directly to the plasma by means of an argon flow to prevent chemical alterations in aqueous media due to potentially occurring Li + −H + exchange reactions. The size-separated particles were monitored online by flow cell particle analysis (FPA). The influence of different gas flow settings and plasma parameters on the peak emission intensity of Li and Mn was evaluated. A particle size detection limit of ∼0.5 μm was estimated based on the 3σ criterion of the baselines and the measured peak intensities for Li and Mn considering the particle size distribution as obtained by FPA. The corresponding analyte masses at the detection limits were ∼30 and ∼180 fg for Li and Mn, respectively. Furthermore, an approach for a matrix-matched external calibration with electrochemically delithiated lithium transition-metal oxides is presented.
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