Integral bi-layer separator-electrode construction for lithium-ion batteries

Abstract

A porous bi-layer separator composed of a first layer with a contacting array of non-conducting particles overlaid with a second layer of a microporous polymer layer, may be fabricated on the electrode surface of the anode of a lithium-ion battery to form an integral electrode-separator construction. The bi-layer separator may prevent development of a direct electronic path between the anode and cathode of the battery while accommodating electrolyte solution and enabling passage of lithium ions. Such an integral separator should be mechanically robust and tolerant of elevated temperatures. Exemplary bi-layer separators may be fabricated by sequential deposition of solvent-containing slurries and polymer solutions with subsequent controlled evaporation of solvent. The elevated temperature performance of lithium-ion battery cells incorporating such integral electrode-bi-layer separators was demonstrated to exceed the performance of similar cells using commercial and experimental single layer polymer separators.

Description

TECHNICAL FIELD[0001]This invention pertains to bi-layer porous separators integral with an electrode of a lithium-ion cell to assure electrical separation between the anode and cathode of the cell. More specifically, this invention pertains to the sequenced deposition of conformal, electronically non-conductive, porous ceramic and polymer coatings on one of the cell electrodes to form a cooperating electrode-separator construction.BACKGROUND OF THE INVENTION[0002]Lithium-ion secondary batteries are common in portable consumer electronics because of their high energy-to-weight ratios, lack of memory effect, and slow self-discharge when not in use. Rechargeable lithium-ion batteries are also being designed and manufactured for use in automotive applications to provide energy for electric motors to drive vehicle wheels.[0003]The basic unit of a lithium-ion battery is an individual cell which includes an anode, a cathode and a liquid, non-aqueous electrolyte suitable for carrying and c...

AI Technical Summary

Benefits of technology

[0014]The two layers of the bi-layer separator cooperate to provide superior performance than may be achieved with single layer separators. The first, particle layer, comprising overlapping hard ceramic particles resistant to penetration offers improved resistance to mechanical breaching of the separator, for example by dendrites, metal fines, or detached electrode particles. The overlying, and at least coextensive polymer layer, supports and retains the particle layer by improving the adhesion between the coating layer and the counter electrode. The flexible and compliant polymer layer is effective in accommodating volume changes occurring in the electrode as lithium is inserted into and removed from the electrode during charge-discharge cycles,.

[0017]In service, the integral combination of the electrode and bi-layer separator cooperates to enhance cell performance. The layer of ceramic particles is in intimate contact with the electrode surface and so minimizes the extent of any dendrites which form and reduce the interfacial resistance. This extends the useful working life of the cell improves the ability of the battery to tolerate abuse, and conveys good temperature resistance. The polymer layer provides additional electronic insulation and serves to retain the ceramic layer in intimate contact with the electrode and to suppress spalling or flaking of the ceramic layer. Absent the polymer layer, spalling or flaking of the ceramic particles may result from the cyclic growth and shrinkage of the electrode resulting from insertion and removal of elemental lithium, handling damage during cell assembly, or in-service thermal or vibratory stresses. The polymer layer may adhere to any one or more of the ceramic layer, the electrode or the current collector.

[0020]The particle slurry may be applied to the cell anode in any convenient manner including a doctor blade, a slot die coater or a comma bar coater each of which is effective in applying a layer of generally uniform thickness coextensive with the electrode. The thickness of the applied slurry may be selected to ensure that the resulting particle layer thickness ranges from about 5 micrometers to about 45 micrometers. As deposited, the pores formed between abutting particles will be filled with the dilute binder solution. However, evaporation of the solvent will reduce the volume of the solution, causing it to shrink and leave a series of interconnected voids or pores behind. A volume fraction of voids of about 30% may be anticipated. Upon complete evaporation of the solvent the binder material will be left connecting all the particles and binding them together. The binder will also be effective in binding the particles to the graphite-based anode.

[0027]Thus, this invention provides a porous bi-layer separator membrane with an inner ceramic layer which offers increased temperature resistance and increased resistance to penetration, whether by dendrites, metal fines or electrode particles. The ceramic layer, which may be somewhat brittle and prone to spall under vibratory loads, is supported by the more compliant, adherent microporous polymer layer. In turn, the ceramic layer restrains shrinkage of the polymer layer and will continue to serve as an electrode separator even if the polymer layer softens or melts.

Problems solved by technology

Lithium-ion secondary batteries are common in portable consumer electronics because of their high energy-to-weight ratios, lack of memory effect, and slow self-discharge when not in use.

But, under normal operating conditions, battery life is affected by the degradation of the active materials (e.g. anode, cathode and electrolyte) and abnormal operation can induce the formation of lithium dendrites, surface deposits of lithium on the anode.

Even if penetration of the separator does not occur, any prolonged exposure to temperatures in excess of 130° C. may result in shrinkage or even melting of the separator.

Clearly, any of these behaviors, shrinkage, softening or melting, will diminish the separators ability to provide electrical insulation between the battery anode and cathode to prevent internal short circuits.

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