We demonstrate herein that Mn and not Mn, as commonly accepted, is the dominant dissolved manganese cation in LiPF-based electrolyte solutions of Li-ion batteries with lithium manganate spinel positive and graphite negative electrodes chemistry. The Mn fractions in solution, derived from a combined analysis of electron paramagnetic resonance and inductively coupled plasma spectroscopy data, are ∼80% for either fully discharged (3.0 V hold) or fully charged (4.2 V hold) cells, and ∼60% for galvanostatically cycled cells. These findings agree with the average oxidation state of dissolved Mn ions determined from X-ray absorption near-edge spectroscopy data, as verified through a speciation diagram analysis. We also show that the fractions of Mn in the aprotic nonaqueous electrolyte solution are constant over the duration of our experiments and that disproportionation of Mn occurs at a very slow rate.

Li-ion batteries (LIBs) today face the challenge of application in electrified vehicles (xEVs) which require increased energy density, improved abuse tolerance, prolonged life, and low cost. LIB technology can significantly advance through more realistic approaches such as: i) stable high-specific-energy cathodes based on Li Ni Co Mn O (NCM) compounds with either Ni-rich (x = 0, y → 1), or Li- and Mn-rich (0.1 < x < 0.2, w > 0.5) compositions, and ii) chemically active separators and binders that mitigate battery performance degradation. While the stability of such cathode materials during cell operation tends to decrease with increasing specific capacity, active material doping and coatings, together with carefully designed cell-formation protocols, can enable both high specific capacities and good long-term stability. It has also been shown that major LIB capacity fading mechanisms can be reduced by multifunctional separators and binders that trap transition metal ions and/or scavenge acid species. Here, recent progress on improving Ni-rich and Mn-rich NCM cathode materials is reviewed, as well as in the search for inexpensive, multifunctional, chemically active separators. A realistic overview regarding some of the most promising approaches to improving the performance of rechargeable batteries for xEV applications is also presented.

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...

Manganese dissolution from positive electrodes significantly reduces the durability of lithium-ion batteries. Knowledge of dissolution rates and oxidation states of manganese ions is essential for designing effective mitigation measures for this problem. We show that electron paramagnetic resonance (EPR) combined with atomic absorption spectroscopy (AAS) or inductively coupled plasma (ICP) can determine both manganese dissolution rates and relative Mn(3+) amounts, by comparing the correlation between EPR and AAS/ICP data for Mn(2+) standards with that for samples containing manganese cations dissolved from active materials (LiMn2O4 (LMO) and LiNi(0.5)Mn(1.5)O4 (LNMO)) into the same electrolyte solution. We show that Mn(3+), and not Mn(2+), is the dominant species dissolved from LMO, while Mn(2+) is predominant for LNMO. Although the dissolution rate of LMO varies significantly for the two investigated materials, due to particle morphology and the presence of Cr in one of them, the Mn speciation appears independent of such details. Thus, the relative abundance of dissolved manganese ions in various oxidation states depends mainly on the overall chemical identity of the active material (LMO vs LNMO). We demonstrate the relevance of our methodology for practical batteries with data for graphite-LMO cells after high-temperature cycling or stand at 4.2 V.

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