Lithium batteries improve

Innovative materials remedy dendrite bug

Recently researched at Rensselaer, a new type of a battery is in the works, specifically, an all-carbon lithium battery. Current batteries in use, like the ones in cellphones, tablets, and laptops, are lithium-ion batteries. Lithium is a light metal, which is necessary in designing light, portable devices.

The basic structure of a lithium-ion battery has not changed much since 1991; the basic structure contains a source of lithium-cobalt oxide. When charging a battery, voltage is applied and breaks the bond between lithium and cobalt. The lithium diffuses across a barrier from the cathode to the anode. At the same time, electrons move from one side to the other. Once the battery is fully charged, it can be used. As a battery is discharged, the Lithium ions go from the anode to cathode and the electrons move, powering the device. For these types of batteries, lithium metal is not used, but rather lithium atoms bonded to cobalt on the cathode side and carbon on the anode side.

Dr. Nikhil Koratkar, a professor of engineering at RPI, looked at using lithium metal instead of lithium bonded to cobalt. The process would be exactly the same, yet without cobalt.

According to Koratkar, in the 1970s Exxon first tried to create an all lithium-carbon battery; however, there were major safety concerns. When the lithium diffuses across the barrier membrane from the anode to cathode (as the battery discharges), small, sharp structures, known as dendrites, are created on the cathode side when the lithium does not re-plate uniformly. The dendrites that form as a result from non-uniform bonding pierce the barrier between the cathode and anode side causing the battery to overheat, ignite, and fail. Due to the failure, lithium metal was not used, cobalt was added to the battery and the lithium ions would bond uniformly to the cobalt, thus preventing the dendrites and the failure that occurred because of them.

To fix the dendrite problem, Koratkar has been working on lithium batteries since 2009, first looking at using silicon. Recently, in 2012, Koratkar thought of another solution: thermally shocking graphene. When Koratkar and his team created thermal shocked graphene plates, they found the sheets had vacancies and large pores. These sheets differ from the tightly packed stacked graphite sheets normally used in lithium-ion batteries. The defect sites in the thermal shocked graphene sheets attracted the lithium because the defects were sites of high energy. The lithium wants to get there in order to lower the total energy of the system; as a result, lithium clusters form around the defect sites. Once a large number of lithium ions cluster in the sites, lithium metal is created. When checked for dendrites, Koratkar and his team found none. Koratkar believes that the chunks of lithium metal form are very small in volume, so small, that the do not form a dendrite or if they do form a dendrite, it is contained in the holes in the thermal shocked graphene plates. According to him, “The graphene acts a cage that restricts the dendrite and prevents it from breaking loose.”

Koratkar also found there to be high values of energy density, which means several things: the battery lasts longer and has the potential for a faster recharge rate. Furthermore, cobalt is no longer needed thus creating a non-toxic battery. This creates a lower cost, more environmentally friendly, higher energy battery. However, it will still be some time before these batteries will be seen on the market. The next step is to scale the battery up and see the effects and changes. Furthermore, the next process involves funding from investors. Still, Koratkar believes there should not be too many problems when scaling up and expects these batteries to be on the market someday.

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