3M Develops Li-Ion Battery Alloy Technology

3M has developed technology that enables the use of alloy anode materials in commercial lithium ion batteries. Cells utilizing this technology can be made by conventional means and are expected to have an improvement of in energy density of 20% over conventional lithium ion cells. 3M’s alloy materials are now in the process of scale-up for commercialization: Researchers from 3M were awarded a U.S. patent, 7,851,085, for lithium ion (Li-Ion) battery anodes alloys that include a composition of silicon, aluminum, a transition metal, tin, and a fifth element that contains yttrium, a lanthanide element, an actinide element, or a combination thereof. The alloy composition is a mixture of an amorphous phase and a nanocrystalline phase.

The technology includes:

• silicon-based nanocrystalline alloys with low raw
  materials costs
  
• a water-based binder system utilizing conventional
  coating methods
  
• an electrode formulation that increases energy
  density, while significantly reducing volume
  expansion
  
• an electrolyte system that forms a stable SEI-layer
  on alloy materials
  

According to 3M, new chemistries of high capacity anode and cathode materials are required to significantly increase the energy density of today’s lithium ion cells. It has long been known that elements that alloy with lithium have significantly higher volumetric and gravimetric capacities than graphite. However the implementation of alloy materials in commercial cells is challenging for a number of reasons. These challenges include maintaining the integrity of the alloy particles and the composite coating during cycling, forming a stable SEI layer on the alloy surface to avoid degradation of the electrolyte, ensuring good rate capability, thermal stability and accommodation of the alloy volume expansion to avoid cell swelling and electrode distortion or tearing. Furthermore, to be
commercially viable an alloy anode needs to be made from low cost raw materials and utilize practical manufacturing methods.

This figure shows a coin cell test of a 3M alloy in a coating formulated for low volume expansion and a high loading coating for an 18650 energy cell. Although the alloy itself has a volume expansion of 115%, the volume expansion of the coating is only 50%. The volumetric capacity of the coating is about twice that of a graphite
coating.

According to the patent filing, most commercially available lithium ion batteries have anodes that contain materials such as graphite that are capable of incorporating lithium through an intercalation mechanism during charging. Such intercalation-type anodes generally exhibit good cycle life and coulombic efficiency. However, the amount of lithium that can be incorporated per unit mass of intercalation-type material is relatively low.

A second class of anode material incorporates lithium through an alloying mechanism during charging. Although these alloy-type materials can often incorporate higher amounts of lithium per unit mass than intercalation-type materials, the addition of lithium to the alloy is usually accompanied with a large volume change. Some alloy-type anodes exhibit relatively poor cycle life and coulombic efficiency. The poor performance of these alloy-type anodes may result from the formation of a two-phase region during lithiation and delithiation. The two-phase region can create internal stress within the alloy if one phase undergoes a larger volume change than the other phase. This internal stress can lead to the disintegration of the anode material over time.

Further, the large volume change accompanying the incorporation of lithium can result in the deterioration of electrical contact between the alloy, conductive diluent (e.g., carbon) particles, and binder that typically form the anode. The deterioration of electrical contact, in turn, can result in diminished capacity over the cycle life of the anode.

The 3M patent describes a lithium ion battery that contains a cathode, an anode, and an electrolyte that is in electrical communication with both the anode and the cathode. The anode includes an alloy composition that contains (a) silicon in an amount of 35 to 70 mole percent, (b) aluminum in an amount of 1 to 45 mole percent, (c) a transition metal in an amount of 5 to 25 mole percent, (d) tin in an amount of 1 to 15 mole percent, and (e) a fifth element that includes yttrium, a lanthanide element, an actinide element, or a combination thereof in an amount of 2 to 15 mole percent. Each mole percent is based on a total number of moles of all elements except lithium in the alloy composition. The alloy composition is a mixture of an amorphous phase that includes silicon and a nanocrystalline phase that includes tin and the fifth element.

A method of making a lithium ion battery is also described that includes preparing an anode that contains an alloy composition, providing a cathode, and providing an electrolyte that is in electrical communication with both the anode and the cathode. The alloy composition contains (a) silicon in an amount of 35 to 70 mole percent, (b) aluminum in an amount of 1 to 45 mole percent, (c) a transition metal in an amount of 5 to 25 mole percent, (d) tin in an amount of 1 to 15 mole percent, and (e) a fifth element that includes yttrium, a lanthanide element, an actinide element, or a combination thereof in an amount of 2 to 15 mole percent. Each mole percent is based on a total number of moles of all elements except lithium in the alloy composition. The alloy composition is a mixture of an amorphous phase that includes silicon and a nanocrystalline phase that includes tin and the fifth element.

The alloy composition contains (a) silicon in an amount of 35 to 70 mole percent, (b) aluminum in an amount of 1 to 45 mole percent, (c) a transition metal in an amount of 5 to 25 mole percent, (d) tin in an amount of 1 to 15 mole percent, and (e) a fifth element that includes yttrium, a lanthanide element, an actinide element, or a combination thereof in an amount of 2 to 15 mole percent. Each mole percent is based on a total number of moles of all elements except lithium in the alloy composition. The alloy composition is a mixture of an amorphous phase that includes silicon and a nanocrystalline phase that includes tin and the fifth element.