The nanostructured design of both electrodes assists in overcoming issues associated with using sulfur compounds and silicon in lithium-ion batteries, including poor electrical conductivity, significant structural changes, and volume expansion. A paper on the novel battery was published online 25 February in the ACS journal Nano Letters.

Comparison of theoretical specific energy for different types of Li-ion batteries, calculated based on the theoretical capacities of the active materials in the electrodes and the average operating voltage of the battery. Credit: ACS, Yang et al. Click to enlarge.

A great deal of research has gone into developing silicon as an anode material, including earlier work by Cui and his colleagues, due to its high theoretical charge capacity (4,200 mAh g^-1—more than 10 times that of graphite anodes and much larger than various nitride and oxide materials) and low discharge potential. The development of various Si nanostructures is targeted to address the material’s large volume changes (by up to 400%) upon the insertion and extraction of lithium ions during charge/discharge cycles, which results in pulverization and capacity fading.

However, Yang et al. note in this new paper, despite the progress on the silicon anode side, for the cell as a whole:

...the relatively low charge capacity of cathodes remains the limiting factor preventing higher energy density. Current cathode materials, such as those based on transition metal oxides and phosphates, have an inherent theoretical capacity limit of ~300 mAh g^-1, and a maximum practically usable capacity of only ~210 mAh g^-1 has been reported.

The lithium/sulfur system, which during the redox process behaves according to the reaction 2Li + S → Li₂S, has the potential to overcome these capacity limitations. Although the system has an average voltage of ~2.2 V vs Li/Li⁺ (about 60% of the voltage of conventional Li-ion batteries), the theoretical capacity of sulfur is 1,672 mAh g^-1, which leads to a theoretical specific energy of ~2,600 Wh kg^-1 for the lithium/sulfur battery. However, sulfur-based cathodes present a variety of problems, including low electronic conductivity, significant structural and volumetric changes during reaction, and dissolution of lithium polysulfides in the electrolyte. Much effort has been dedicated to improving this system, including the development of electrode coatings, conductive additives, and novel electrolytes.

Recently, cells utilizing a sulfur/mesoporous carbon nanocomposite exhibited capacity exceeding 1,000 mAh g^-1 and moderate cycle life. Despite these advances, the use of elemental lithium as the anode in lithium/sulfur batteries remains a major problem due to safety concerns arising from the formation of lithium dendrites during cycling, which can penetrate the separator and lead to thermal runaway. Even though much research has been dedicated to solving this problem, an elemental lithium anode has not yet been commercialized for use in secondary batteries with a liquid electrolyte.

—Yang et al.

One way around the safety issue in the lithium/sulfur is to use a high-capacity anode material in combination with sulfur’s lithiated counterpart, lithium sulfide, in the cathode. However, despite a theoretical capacity of 1,166 mAh g^-1, lithium sulfide’s poor electrical conductivity restricts its actual capacity to much lower values that are not competitive with current materials.

Cui and his group combined their earlier work on silicon nanowire anodes with a cathode comprising a nanocomposite in which Li₂S fills the pores of CMK-3 mesoporous carbon particles. CMK-3 carbon is made up of hexagonally arranged 7-8 nm thick carbon nanorods separated by 3-4 nm pores. The interconnected carbon rods act as conductive pathways to provide electronic access to insulating Li₂S within the pores, while the submicrometer size of the carbon particles helps to shorten lithium diffusion paths. As a result, the researchers note in their paper, “the problems associated with the slow kinetics of Li₂S-based cathodes can be solved.”

Although the Li₂Si system shows high specific energy, the specific capacity of the full cell decays faster than the specific capacity of the half-cell. This could be caused by several factors, Yang et al. suggest, including a limited supply of lithium ions in a full cell, and that the voltage of each electrode is not separately controlled in full cells. The deep discharge or overcharge of Li₂S or silicon is detrimental to cycling performance, they note, and suggest and this might occur during cycling since they only control the voltage of the full cell.

Although the capacity decay from the first to the 20^th cycle from our proof-of-concept Li₂S/Si battery is better or comparable to many other reports on Li₂S or sulfur-lithium batteries, more research is required to overcome these issues and compete with well-developed Li-ion battery systems. In addition to these concerns, the volumetric energy density of our current cell is not as high as the LiCoO₂/graphite system, even though the theoretical volumetric energy density of the Li₂S/Si system is about twice that of the LiCoO₂/graphite system.

...The development of this novel battery system will have a significant impact on applications that require high specific energy, such as batteries for electric vehicles and portable electronics.

—Yang et al.

Resources

• Yuan Yang, Matthew T. McDowell, Ariel Jackson, Judy J. Cha, Seung Sae Hong and Yi Cui (2010) New Nanostructured Li₂S/Silicon Rechargeable Battery with High Specific Energy. Nano Lett., Article ASAP doi: 10.1021/nl100504q