Evaluation of Alternative Lithium Salts for High-Voltage Lithium Ion Batteries: Higher Relevance of Plated Li Morphology Than the Amount of Electrode Crosstalk

Abstract

Increasing the upper cut-off voltage (UCV) enhances the specific energy of Li-ion batteries (LIBs), but is accompanied by higher capacity fade as a result of electrode cross-talk, i.e., transition metals (TM) dissolution from cathode and deposition on anode, finally triggering high surface area lithium (HSAL) formation due to locally enhanced resistance. Here, LiPF₆, LiBF₄, lithium difluoro(oxalate)borate (LiDFOB), lithium bis(oxalate)borate (LiBOB), lithium bis(fluorosulfonyl)imide (LiFSI), and lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) in carbonate-based solvents are investigated in LiNi_0.6Co_0.2Mn_0.2O₂ (NCM 622) || graphite pouch cells with 4.5 V UCV. Despite the lower oxidative stabilities of LiBF₄ and LiDFOB, thus enhanced HF formation, TM dissolution, and consequently electrode cross-talk, higher capacity retention is observed compared to the case of LiPF₆ electrolyte. Counterintuitively, it is not the TM deposit amount but rather the Li plating morphology that governs capacity fade, as these salts cause more uniform and compact lithium plating, i.e., lower surface area. In contrast, the dendritic HSAL with LiPF₆ has a higher surface area, and more parasitic reactions, thus active Li ("Li inventory") losses and capacity fade. Although NCM initiates the failure cascade, the capacity losses and cycle life of high-voltage LIBs are predominantly determined by the anode, in particular the Li plating morphology and the corresponding surface area.

Keywords: cross‐talk; full‐cell; high surface area lithium (HSAL) plating; high‐voltage operation; lithium battery; lithium conducting salts; lithium ion battery; rollover failure; transition metal dissolution.

Figure 1

a) Discharge capacity versus cycle number of cells with 1 m LiPF₆ ‐based electrolytes charged to UCVs of 4.3 and 4.5 V and compared with 1 m LiBF₄, 1 m LiDFOB, and 1 m LiPF₆ + 2 wt% LiDFP as well as b) 0.6 m LiBOB, 1 m LiBOB, 1 m LiFSI, and 1 m LiTFSI. c) Charge and discharge capacities in the 0th and 1st cycle. d) 0th and e) 1st cycle differential capacity versus voltage plot. f) Overcharge experiments for various electrolytes in LNMO || Li cells to establish anodic stability limit. e) Ionic conductivity of 1 m LiPF₆, 0.6 m LiBOB, 1 m LiDFOB, and 1 m LiBF₄ at a temperature range of −20–60 °C. f) Normalized voltage profile, g) polarization growth of cells with LiPF₆, LiBOB (1 m), LiDFOB, and LiBF₄

Figure 2

Evolution of voltage profiles of cells with a) LiPF₆, b) LiBF₄, and c) LiDFOB, from cycle 3 to 102. g) Endpoint capacity versus cycle number plot, h) charge endpoint slippage slope, and f) discharge endpoint slippage slope of cells with LiPF₆, LiDFOB, and LiBF₄.

Figure 3

Cycled NCMs from graphite‐based cells reassembled in fresh lithium cells. a) Cell voltage versus capacity, b) differential capacity versus voltage, and c) DCIR versus SoC plots of NCM 622 || Li cells with NCM 622 extracted from NCM 622 || graphite cells cycled with LiPF₆, LiBF₄, and LiDFOB‐based electrolytes.

Figure 4

a) Anion‐separated chromatography for electrolytes with LiPF₆, LiBF₄, and LiDFOB as conducting salts. The initially high amount of F^‐ in the pristine LiDFOB electrolyte likely stems from the thermodynamically favorable LiDFOB hydrolysis.^{[ 61 ]} LA‐ICP‐MS element mapping results (Li, Ni, Co, Mn) for cells with b) LiPF₆, c) LiBF₄, d) LiDFOB of selected negative electrodes after 102 charge/discharge cycles. e) Quantification of TM content on the negative electrodes after 102 cycles obtained via ICP‐OES.

Figure 5

(a,c,e) Low and (b,d,f‐h) high magnification SEM images of fully charged negative electrodes obtained from cells with electrolytes containing LiPF₆, LiBF₄, and LiDFOB at cycle #44. The yellow circles in the insets of (a,c,e) indicate the region where the respective SEM images are taken from. (g) and (h) are taken from areas not shown in (e). NMR spectra of metallic Li on fully charged graphite negative electrodes from cells with (i) LiPF₆, (j) LiBF₄, and (k) LiDFOB, fitted with three peaks arising from different Li morphology.

Figure 6

The lower oxidative stability of LiBF₄ and LiDFOB compared to LiPF₆‐based electrolytes in EC‐EMC solvent mixture (3:7 by wt), leads to higher HF formation, TM dissolution, and electrode cross‐talk. b) Despite higher electrode crosstalk, the cycle life and capacities are counterintuitively improved for high voltage NCM || graphite cells (4.5 V UCV) compared to LiPF₆. A more uniform and compact Li plating morphology (low surface area) for cells with LiBF₄ and LiDFOB, compared to dendritic Li plating (high surface area) for cells with the LiPF₆ electrolyte, results in less parasitic reactions (e.g., with the electrolyte), less active Li loss, and consequently less capacity fading. The morphology and surface area of plated Li is obviously more crucial for high‐voltage applications than the amount of TM deposits. Partially redrawn from a previous report.^{[ 18 ]}