Multifunctional Manganese Ions Trapping and Hydrofluoric Acid Scavenging Separator for Lithium Ion Batteries Based on Poly(ethylene‐alternate‐maleic acid) Dilithium Salt

Abstract

of the PF 6 − anion and solvent molecules, and lithium consumption, all of which adversely affect the solid electrolyte interphase (SEI) that passivates the negative electrode in a LIB and increase the cell impedance. [1] Simultaneously, the irreversible loss of Mn disrupts the structural integrity of the active material in the positive electrode. As a consequence, the cell experiences performance degradation and a reduced useful life. It is well documented that the Mn dissolution rate increases drastically at elevated temperatures and high voltage due to increased amounts of hydrofluoric acid (HF) formed under such conditions. [2] Direct HF attack on the positive electrode active material thus becomes the main driver of Mn dissolution. [3] It has been proposed that HF also catalyzes the disproportionation of Mn 3+ species in case of spinel LiMn 2 O 4 (LMO) and produces electrolyte soluble Mn 2+ species. [4] Furthermore, has been reported in the literature that the structural instability of LMO due to Jahn-Teller distortion, relevant mainly at low Mn oxidation states (<3.0 V vs Li/Li + ), may be also responsible for Mn dissolution. [5] Finally, it should be noted in this context that mounting evidence points to Mn 3+ and not Mn 2+ as the dominant (in 70% to 80% amounts) manganese solution species in LMO-graphite cells. [6,7] The extant controversy regarding the oxidation state of dissolved manganese ions notwithstanding, several approaches for reducing the Mn dissolution from positive electrode materials and its consequences have been investigated over the past two decades: [4,8] elemental substitutions in the bulk of the active material (to improve the structural integrity of the positive electrode), [9] surface coatings on active materials (to avoid direct contact between the positive electrode and the electrolyte solution, and thus reduce HF attack), [10] and passivating electrolyte solutions additives for both positive and negative electrodes. [11] Unfortunately, it is not possible to completely prevent Mn dissolution with any single or even combination of the previously proposed mitigation measures without adversely affecting the electrochemical performance of the cells. Therefore, a new mitigation measure, that of stopping -or at least impeding -the Manganese dissolution from positive electrodes seriously reduces the life of Li-ion batteries, due to its detrimental impact on the passivation of negative electrodes. A novel multifunctional separator incorporating inexpensive mass-produced polymeric materials may dramatically increases the durability of Li-ion batteries. The separator is made by embedding the poly(ethylenealternate-maleic acid) dilithium salt polymer into a poly(vinylidene fluoridehexafluoropropylene) copolymer matrix. LiMn 2 O 4 -graphite cells comprising a 1 m LiPF 6 solution in ethylene carbonate plus dimethyl carbonate (1:1 v/v) andthe functional separator retain 31% and 100% more capacity than baseline cells with plain commercial separators after 100 cycles at C/5 rate, respectively, at 3...

Review—Multifunctional Materials for Enhanced Li-Ion Batteries Durability: A Brief Review of Practical Options

“…61,62 Galvanostatic cycling data for cells containing inhouse fabricated separators functionalized with poly-BPA, poly-PEI, and Na 4 EDTA and 1 M LiPF 6 /EC:DMC (1:1 v/v) recorded at 55…”

“…We have recently shown that Mn trapping and the associated benefits for capacity retention in LMO-graphite cells upon high-temperature cycling can be obtained with several inexpensive commercially available polymeric or molecular ion-trapping materials. [61][62][63] Furthermore, we have also demonstrated that besides trapping Mn ions, these materials can scavenge acidic species (HF, POF 3 , etc.) and also act as a source of sacrificial Li + or Na + ions.…”

“…All other functionalized separators were fabricated in-house by a phase-inversion technique, 79,80 using PVdF-HFP as matrix and various chelating materials as filler. [61][62][63] Typical PVdF-HFP copolymer to filler ratios in the dry separator membranes were 1:2. The chelating separators were tested in both LMO-Li and LMO-graphite cells, having both coin (model: 2325, CNCR, Canada, with 14 mm diameter electrodes) and pouch cell (30 mm × 35 mm electrode dimensions) formats.…”

“…Impedance spectra were recorded both after cell formation and after the galvanostatic cycling. [61][62][63] Cells were disassembled immediately subsequent to the electrochemical testing and their components were harvested for further analyses. Structural changes in the LMO positive electrodes upon cycling were identified by X-ray diffraction (XRD).…”

“…[43][44][45][46][47][48][49][50] We and other researchers focused our efforts on the benefits afforded to battery performance by separators made of polymers capable of trapping Mn ions or/and scavenge HF. 24,[53][54][55][56][57][58][59][60][61][62][63] We chose separators instead of binders as test platform for our proof-of-concept experiments due to the ease of analysis and clarity of interpretation they offer during the post-mortem characterization of components. We will henceforth mostly discuss, in the remainder of our paper, the benefits afforded to cell performance by functionalized separators.…”