BACKGROUND OF THE INVENTION

Rapid increases in popularity and availability of battery-powered devices, such as mobile electronics and electrical vehicles, are catalyzed by breakthroughs in battery technology. These breakthroughs have dramatically lowered the price of batteries, expanded material options, and opened doors for new applications and markets. For example, world-wide registration of new plug-in hybrid electric and battery-only electric vehicles (PHEV and BEV) have increased dramatically in recent years. Advancing energy and power density of batteries is critical to further expansion of this market. Extending operating life of batteries is another important aspect, particularly when operating under aggressive duty cycles common for mobile electronics and electrical vehicles.

Longevity of batteries has been a serious concern, particularly for electrical vehicles that utilize large and expensive batteries. Range reduction stemming from premature capacity fade is viewed as a major barrier for rapid growth of this market segment. Capacity fade and associated rising cell impedance has its origins in a mechanism associated with contamination of negative electrodes by transition metals released from positive electrodes. For example, in a typically lithium ion battery, these metals leach out of the positive electrode, migrate across the separator, and perturb the protective surface film on the negative electrode. This capacity fade is most severe when using manganese-rich materials, such as spinel and layered lithium manganese oxide (LMO) and high-voltage nickel manganese oxide (NMO), for positive electrodes. Other materials, such as nickel manganese cobalt oxide (NMC or NCM), lithium cobalt oxide (LCO), and lithium iron phosphate (LFP), are similarly susceptible to this fade mechanism. Conventional separators do not block the migration of transition metals to the negative electrode. New materials and methods are needed for protecting electrochemical cells from the capacity fade and, more generally, for preventing selected species from freely migrating from one electrode to another.

BRIEF SUMMARY OF THE INVENTION

Provided are electrochemical cells including separators permeable to some materials and impermeable to other materials in electrolytes. Also provided are methods of forming such separators. The selective permeability of a separator is achieved by its specific pore diameter and its narrow pore-size distribution around this diameter. Specifically, a species responsible for desirable ion transport in an electrochemical cell is allowed to pass through the separator, while another species is blocked thereby preventing degradation of the cell. For example, a species including alkali metal cations, such as lithium ions or sodium ions, is allowed to pass through the separator in rechargeable cells, while one or more species containing a transition metal are blocked. The membrane layer may be a standalone layer, supported by a membrane support, or supported by one of the electrodes.

In some embodiments, an electrochemical cell comprises a positive electrode, a negative electrode, a separator, and an electrolyte. The separator is disposed between the positive electrode and the negative electrode. The separator provides electronic isolation between the positive electrode and the negative electrode. The separator comprises a first membrane layer. The electrolyte comprising a first species and a second species. At least a portion of the electrolyte is disposed within the separator. The separator, with at least the portion of the electrolyte is disposed within the separator, is permeable to the first species providing ionic communication between the positive electrode and the negative electrode. The separator, with at least the portion of the electrolyte is disposed within the separator, is substantially impermeable to the second species comprising the element of the electrochemical active material of the positive electrode. The first species is selected from group consisting of H+, Li+, Na+, K+, Rb+, Cs+, F−, Cl−, Br−, I−, BF4−, PF6−, AsF6−, ClO4−, methanesulfonate, trifluoromethanesulfonate, tris(pentafluoroethyl)trifluorophosphate, bis(fluorosulfonyl)imide, cyclo-difluoromethane-1,1-bis(sulfonyl)imide, cyclo-hexafluoropropane-1,1-bis(sulfonyl)imide, bis(trifluoromethanesulfonylimide), bis(perfluoroethanesulfonyl)imide, bis(oxalate)borate, difluoro(oxalate)borate, bis(fluoromalonato)borate, tetracyanoborate, dicyanotriazolate, dicyano-trifuloromethyl-imidazolate, dicyano-pentafuoroethyl-imidazoleate, and ion pairs thereof. The second species comprises at least one of an atom, an ion, an ion pair, and a molecule comprising Be, Mg, Ca, Sr, Ba, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Y, Zr, Nb, Mo, Tc, Ru, Rh, Pd, Ag, Cd, Lu, Hf, Ta, W, Re, Os, Ir, Pt, Au, Hg, B, C, N, O, Al, Si, P, S, Ga, Ge, As, Se, In, Sn, Sb, Te, Ti, Pb, Bi, Po, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Ac, Th, Pa, U, methanesulfonate, trifluoromethanesulfonate, tris(pentafluoroethyl)trifluorophosphate, bis(fluorosulfonyl)imide, cyclo-difluoromethane-1,1-bis(sulfonyl)imide, cyclo-hexafluoropropane-1,1-bis(sulfonyl)imide, bis(trifluoromethanesulfonylimide), bis(perfluoroethanesulfonyl)imide, bis(oxalate)borate, difluoro(oxalate)borate, bis(fluoromalonato)borate, tetracyanoborate, dicyanotriazolate, dicyano-trifuloromethyl-imidazolate, dicyano-pentafuoroethyl-imidazoleate, acetylacetonate, acetylacetone, a β-ketoester of the formula RC═OCH2COOR′, where R and R′ are each independently C1-6alkyl, C6-12aryl, or C1-6alkoxy-C1-6alkyl or a combination of, and ion pairs thereof.

In some embodiments, the first membrane layer comprises a planar species and a linker having a site of contortion. The linker may be selected from the group consisting of a spiro group, a bridged ring moiety, and a sterically congested single covalent bond. The planar species may be represented by the following formula:



In some embodiments, the separator further comprises a membrane support laminated to the first membrane layer. The membrane support being permeable to the second species. The membrane support may be a porous polymer selected from the group consisting of polypropylene and polyethylene. The average pore diameter of the membrane support is at least about 10 nanometers. The first membrane layer may be disposed between the membrane support and the positive electrode.

In some embodiments, the separator further comprises a second membrane layer laminated to the membrane support. The membrane support may be disposed between the first membrane layer and the second membrane layer. The second membrane layer may be permeable to the first species providing ionic communication between the positive electrode and the negative electrode, and wherein the second membrane layer is substantially impermeable to the second species comprising the element of the electrochemical active material of the positive electrode. In some embodiments, the second membrane layer comprises a ceramic material selected from the group consisting of aluminum oxide, silicon oxide, silicon carbide, titanium dioxide, magnesium oxide, tin oxide, cerium oxide, ziconium oxide, barium titanite, yttrium oxide, boron nitride, and an ion conducting ceramic.

In some embodiments, the first membrane layer directly interfaces the negative electrode and the positive electrode.

These and other embodiments are described further below with reference to the figures.

