Colloidal sphere templates and sphere-templated porous materials

Abstract

A method of making colloidal sphere templates and the sphere-templated porous materials made from the templates. The templated porous materials or thin films comprise micron and submicron-scaled spheres in ordered, disordered, or partially ordered arrays. The invention is useful in the synthesis of submicron porous, metallic tin-based and other high capacity anode materials with controlled pore structures for application in rechargeable lithium-ion batteries. The expected benefits of the resulting nanostructured metal films include a large increase in lithium storage capacity, rate capability, and improved stability with electrochemical cycling.

Description

BACKGROUND OF THE INVENTIONDescription of the Related ArtLithium-Ion Anode Materials[0001]The commercial requirements for low cost, safety, and high energy have driven the development of anode materials for lithium batteries (See references 1-8). Lithium metal itself has an exceptionally high specific capacity and provides the minimum anode potential of 0 V vs. Li / Li+. However, several reliability and safety issues limit its application in rechargeable battery systems, including the growth of dendrites that cause short circuits. Furthermore, lithium metal electrodes require a four-fold excess of metal for reversibility (See references 7-8). Table 1 shows the performance comparison of candidate lithium-ion anode materials relative to lithium metal (* theoretically calculated values; 64 volume % from randomly packed spherical pores). The lithium-ion anode materials show a theoretical reversible specific capacity that is a quarter that of the primary capacity of lithium metal, since it...

AI Technical Summary

Benefits of technology

[0010]The present invention is useful in the synthesis of submicron porous, metallic tin-based and other high capacity anode materials with controlled pore structures for application in rechargeable lithium-ion batteries. The expected benefits of the resulting nanostructured metal films include a large increase in lithium storage capacity, rate capability, and improved stability with electrochemical cycling as compared with the existing alternatives. The various sources of capacity loss (electrode fragmentation, irreversible formation of Li2O, electrode surface passivation, electronic isolation of active material, etc.) will be minimized, accommodated, or entirely avoided by the disclosed material and method of the present invention, in order to maximize the energy density of portable power supplies. Furthermore, the porous metal fabrication methods of the present invention can be extended to other electrochemical and energy storage material applications.

[0015]The templates and porous films and the methods for making them described herein make possible the production of superior electrode technologies for lithium ion batteries including anodes that outperform graphite and TCO materials in terms of reversible capacity, rate capability, and cycling efficiency. The inherent specific capacity of tin metal for lithium storage is a factor of several times higher than that of graphitic carbon (Table 1). Furthermore, reversibility is enhanced by the use of metallic tin instead of tin oxide-based material systems because the irreversible formation of Li2O is avoided. Charge and discharge rates and cycling efficiency are improved by the sub-micron scaled morphology of the metal network. The size of the original spheres dictate several important structural parameters of the electrode, especially (a) the amount of interfacial area per volume between the electrode and the electrolyte and (b) the size scale of the metal skeleton (i.e. the maximum distance from any interior point in the metal to the surface).

[0016]Creating very small metal structures that alloy electrochemically with lithium minimizes the absolute size changes during swelling (i.e., due to a typically large decrease in density from the base metal to the alloy), thus reducing the tendency towards material fracture. Another benefit of creating very small metal structures that alloy electrochemically with lithium is that the distance over which the diffusion of lithium must occur during charging and discharging of a lithium ion battery is also minimized. If lithium diffusion distances are high, a concentration gradient will develop and several lithium-metal intermetallic phases will be present simultaneously. Mechanical strains and density differences between these phase-separated domains contribute to the “pulverization” or “fragmentation” problem that plagues metal anodes in lithium-ion batteries and causes the loss of electrochemical contact between anode material and the remaining electrode. By limiting diffusion distances to several nanometers or tens of nanometers, lithium can be made to leave the alloy more homogeneously, avoiding the deleterious effects of phase separation.

[0018]The present invention describes methods for the assembly of sphere films, which make wide areas of crack free template by forgoing the objective of perfectly ordered spheres. Thus, cracking of dried films can be dramatically decreased by intentionally introducing disorder in the arrangement of the sphere packing.

[0019]The present invention also provides: 1) The creation of metallic tin or tin-based alloy films containing an interconnected network of micron or submicron sized pores derived from electrochemical deposition though a sacrificial template of silica or polymer spheres; 2) The intentional use of micron or submicron sphere populations with naturally occurring or artificially imposed size polydispersity in order to create disordered (or partially disordered) crack-free sacrificial template films over relatively wide areas compared to well-ordered films obtained from sphere populations having low polydispersity; 3) The intentional use of mixtures of micron or submicron sphere populations in order to create artificially imposed multi-modal size distributions in order to create disordered (or partially disordered) crack-free sacrificial template films over relatively wide areas compared to well-ordered films obtained from sphere populations having low polydispersity; 4) The application of so-obtained porous tin or tin-based alloy films as the lithium storage anode in a lithium-ion secondary (rechargeable) battery in order to obtain improvements in (a) specific charge capacity and charge / discharge rate capability compared with currently available commercial anodes and (b) cycling efficiency compared with bulk tin metal or tin-based alloys; and 5) The ability to engineer the network structure of the porous metal electrode for optimal mechanical and electrochemical performance by simply changing the average sphere size(s), polydispersity, and relative weight fractions (in the case of multi-modal distributions) contained within the sacrificial template.

Problems solved by technology

However, several reliability and safety issues limit its application in rechargeable battery systems, including the growth of dendrites that cause short circuits.

Research in this area diminished significantly when it was learned that the lithium-metal alloy systems had very poor cycling characteristics.

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