Manganese dissolution is the root-cause for a main degradation mode in Li-ion batteries with Mn-based active materials in their positive electrodes. Manganese cations dissolved from the positive electrode migrate through the electrolyte solution and deposit on the negative electrode, leading to reductions in both power performance and useful life. [1-4] Several methods were proposed over the years for mitigating the manganese dissolution or its consequences, such as elemental substitutions in positive electrode active material, surface coatings, inorganic barrier coatings by atomic layer deposition, passivating electrolyte solution additives, [5, 6] but none proved completely successfully so far. A different and complementary approach is that of using a separator containing manganese ion chelating agents, to trap the manganese ions in the inter-electrode space and prevent their migration to and deposition at the negative electrode. We showed in previous work that polymers functionalized by macrocycles (crown ethers) can both prevent manganese deposition on graphite negative electrodes and improve the capacity retention of lithium manganate spinel (LMO) - graphite cells during high-temperature (50 ºC) cycling. [7, 8]

In the present work we show that the concept of Mn trapping separators has a wider applicability, beyond the class of macrocycles, i.e., that it can be extended to chelating agents with an open chemical structure, as exemplified by iminodiacetic acid  disodium salt functional groups attached to styrene divinylbenzene polymeric particles (poly-IDANa₂, for short). We demonstrate that separators functionalized with poly-IDANa₂ perform as well as polymeric azacrowns with respect to the ability to chelate Mn ions and lead to similar improvements in the capacity retention of LMO-graphite cells during cycling at ambient and at elevated temperatures.

The Mn cation trapping separators were fabricated in-house. A mixture of commercial poly-IDA resin and PVdF-HFP was either fabricated into poly-IADNa₂ filled separator or coated onto a commercial separator by means of a phase-inversion method. We determined and compared the physical properties (surface and cross-section morphologies, electrolyte uptake, porosity, ionic conductivity, and electrochemical stability) of the Mn trapping separators with those of a plain commercial separator. Poly-IDANa₂ filled separators had a McMullin Number (or conductivity derating divisor) of 2.7 at an overall porosity of 40%, compared with a McMullin Number of 8 - 12 for commercial polyolefin separators with the same porosity. Linear sweep voltammograms with cells containing poly-IDANa₂ filled separators show onset of oxidative degradation near 5 V vs. Li/Li⁺; demonstrating that the electrochemical stability of these separators matches the stability limit of the 1M LiPF₆/EC:DMC (1:1) electrolyte. Hence such filled separator should be suitable for use in present day commercial lithium-ion batteries and also enable improved low temperature power performance.

Cycling tests showed that LMO-graphite cells containing poly-IDANa₂ filled separators display improved capacity retention over LMO-graphite cells containing commercial polyolefin separators. After 100 constant current cycles at C/5 rate, cells with poly-IDANa₂ filled separators displayed (a) smaller increases in interfacial film and charge transfer resistances (by factors of 1.5 to 2.3 at 30 ºC and of 1.9 to 4.4 at 55 ºC, compared, respectively, with factors of 2.2 - 4.4 and 2.8 - 11.8 for cells with commercial polyolefin separators); (b) significantly reduced Mn poisoning of graphite electrodes (Mn amounts on the graphite electrodes from pouch cells with poly-IDANa₂ filled separators were 8.3x and 5.5x smaller than Mn amounts on graphite electrodes from cells with commercial polyolefin separators, at 30 ºC and 55 ºC, respectively); and (c) significantly increased cycle life for LMO-graphite cells (cells with poly-IDANa₂ separators retained 20% and 55% more capacity than cells with commercial polyolefin separators at 30 and 55 °C, respectively).

References

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