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Macro-porous, nanocrystalline silicon layer for lithium-ion batteries

Abstract

For the production of rechargeable batteries, it is desirable to use silicon as anode material in Li-ion batteries. The use of silicon anodes theoretically increases battery capacity tenfold compared to conventional graphite anodes. However, the attempt had previously failed, since the layers would expand by 300 to 400 % due to the storage of lithium ions in the Si bulk material. This induces a high residual strain and can destroy the bulk Si after only a few charge cycles. In addition, as a consequence of the irreversible reaction between the Si anode and electrolyte a layer of solid electrolyte interphase (SEI) can develop and lead to a low coulombic efficiency.
Scientists of the University of Stuttgart now succeeded in developing a porous semiconductor layer, which displays a pore distribution from 50 to 3000 nm and eliminates the residual strain. It can be manufactured in a continuous process.

Background

For the production of rechargeable batteries, it is desirable to use silicon as anode material in Li-ion batteries. Until recently, however, this was not feasible.

Problem

The use of silicon anodes theoretically increases battery capacity tenfold compared to conventional graphite anodes.
However, the attempt had previously failed, since the layers would expand by 300 to 400 % due to the storage of lithium ions in the Si bulk material. This induces a high residual strain and can destroy the bulk Si after only a few charge cycles. In addition, as a consequence of the irreversible reaction between the Si anode and electrolyte a layer of solid electrolyte interphase (SEI) can develop and lead to a low coulombic efficiency.

Solution

Scientists of the University of Stuttgart succeeded in developing a structure that eliminates the residual strain and can be manufactured in a continuous process. By selecting the optimal combination of process parameters (deposition process, annealing and chemical treatment) a porous semiconductor layer can be formed, which displays a pore distribution from 50 to 3000 nm.
According to the invention a semiconductor layer (e.g. silicon, germanium or their alloys) is deposited on the substrate (e.g. glass, silicon dioxide, titanium, nickel or ceramics). Onto this semiconductor layer, an additional metallic layer, e.g. aluminum, is deposited. The following heat treatment with an optimized combination of temperature and duration produces an incomplete interdiffusion between the two layers. Spontaneously, the amorphous semiconductor layer transforms at least partially into a crystalline state. In the last step, the metallic phase is removed by selective etching. A conformal layer of Al oxide can prevent the formation of an SEI layer. The porous semiconductor layer with a thickness of 300 nm to 5 µm is doped and capable of absorbing lithiation-induced strain without breaking.
Initial experiments with a laboratory model of a Li-ion battery have already shown that even after 500 charge/ discharge cycles the capacity remains stable at approx. 1650 mAh/g, without any major optimization efforts.

Cross-sectional images created via FIB (focused ion beam): a) As-deposited status, Al/a-Si bilayers constructed onto a flexible conductive substrate; (a-Si=amorphous silicon) b) After removal and etching, AlOx-coated, crystalline Si nanocolumns formed. (AlOx=aluminum oxide), (c-Si=crystalline silicon)

Advantages

  • Method for creating a mechanically stable, porous semiconductor layer for Li-ion batteries
  • Stable charge capacity of 1650 Ah/g already shown in laboratory experiments
  • Simple, inexpensive layer fabrication in continuous process, also for larger surface areas
  • Wide range of options for controlling the production process in terms of surface topology and pore morphology
  • Conformal Al-oxide layer to insure the high coulombic efficiency all the time
  • Very good adhesion between the porous semiconductor layer and conductive substrate

Application

The method according to the invention is a special 'all-in-one' processing technology for the production of anode material, made of semiconductor materials (silicon (Si) or/ and germanium (Ge)) and used in Li-ion batteries with a high capacity and long lifespan.

Exposé
Contact
Dr.-Ing. Hubert Siller
Technologie-Lizenz-Büro (TLB)
Ettlinger Straße 25
76137 Karlsruhe | Germany
Phone +49 721-79004-0
siller(at)tlb.de | www.tlb.de
Development Status
Concept / TRL2
Patent Situation
EP 3087629 B1 granted,
DE, FR, GB validated
Reference ID
13/085TLB
Service
Technologie-Lizenz-Büro GmbH is responsible for the exploitation of this technology and offers companies the possibility of cooperation and/or assists them in obtaining licences.