THE SOL-GEL GROUP

Nathalie Pereira, Ph.D. 

"New Materials Used as Anodes in Lithium Ion Batteries" 

  Rechargeable batteries have become essential with the development growth and use of a variety of portable electronic devices. Rechargeable Li-ion batteries are not only light but they also have a high energy density and a long life. There is now a great opportunity for the development of large batteries with electric vehicles and also battery energy storage systems in order to decrease the consumption of fossil fuels. Research on Li-based batteries has become a priority.

  A problem with the use of metallic lithium as an anode in lithium batteries is the growth of dendrites at the interface with electrolyte. This phenomenon increases the probability of internal shorting and thus prevents its use in commercial batteries.

  Carbonaceous materials are currently used as negative electrodes in commercial batteries to overcome the dendrite problem. The Li ions reversibly insert into the carbon structure via an intercalation reaction with only 10% volume change. However, since its gravimetric capacity is relatively low, 350 mAh/g, as anode material, the attention has been focused on the search of alternative materials to be used as anodes in Li-ion batteries.

  Alloying reactions are an alternative way to reversibly accommodate Li-ions in a structure. Lithium alloys present a higher capacity than carbonaceous materials but the large volumetric changes resulting from the reversible insertion of lithium are responsible for poor cyclability, unacceptable in commercial batteries. To minimize this detrimental effect, the concept of multiphase materials where an inert matrix surrounds the active material reacting with lithium has been introduced by Huggins et al. Another option has been presented by Besenhard et al., the SnSb matrix surrounding metallic tin reacts with lithium to get metallic Sn and Li₃Sb during the first cycle. The latter then forms the inert matrix dispersing the active material, metallic tin in this case.

  An alternative way to get an active material dispersed in an inert matrix has been introduced by Idota et al. They used tin oxide composites as anode materials, which react with lithium in a two-step process according to Dahn et al. The electrochemical reactions can be expressed as follow:

2x Li⁺+ MO_x + 2x e^-   x Li₂O +M

M + y Li⁺ + y e^-  ÷  Li_yM

  The oxide first reacts with lithium to form nanodomains of metallic tin dispersed in lithia (Li₂O) (1). The tin further alloys with in-coming lithium ions (2) and is responsible for the large reversible capacity of the electrode, 991 mAh/g. The conversion reaction of the tin oxide into lithia is responsible for a high irreversible capacity loss occurring during the first cycle. The extent of this capacity loss depends on the oxygen content of the initial tin oxide.

  Dahn et al. have also shown that the use of small particle size materials compared to coarse materials can improve the cyclability. The grain size of the material but also the size of the active tin domains have to be small to avoid the coexistence of two phases of different compositions and thus having different volume changes. This mismatch would lead to rapid failure.

  The different attempts at improving the cyclability by means of a matrix addition or a decrease in particle size haven't been successful in the realization of a negative electrode for practical applications: a solution has yet to be found.

  We have just mentioned that both smaller particle size and multiphase materials improve capacity retention. The purpose of this study was to combine and quantify the effect of these two factors on cyclability using tin powders. We first quantify the particle size effect by comparing two tin powders of different particle size. The second step of the study was to anneal in air the initial tin powders for increasing periods of time in order to quantify the multiphase effect. An alternative material is also provided to increase both capacity and cyclability. 

This page is maintained by Manuel A. Alvarez.
Last updated March 28, 2002.
For any comments or questions please send e-mail to: solgel@rci.rutgers.edu