Lithium based batteries are a hot research field with lithium air and lithium sulfur compounds at the top of research listings.  Stanford’s Yi Cui, builds the battery anode in the form of silicon nanowires, giving the silicon room to grow and shrink without damage.  That neatly solves silicon’s swelling when charged with positively charged lithium and shrinks during discharge such that the silicon has a tendency to self “pulverize.”

Compared to present lithium-ion batteries, Stanford’s design is “significantly safer” and currently achieves 80 percent more capacity, but it’s nowhere near being commercially viable with just 40 to 50 charge cycles (conventional Li-ion does “300 to 500”) due to the compound’s still rapid degradation. There is still a theoretical quadruple boost in capacity as the technology matures.

Stanford Nanosilicon Wire and Sulfur Battery Electrodes Graphic. Click image for the largest view.

Yi Cui has announced a new cathode consisting of a similarly high capacity lithium sulfide nanostructure.  The cathode is mesoporous carbon/Li2S nanocomposite. A company has formed based on the technology, but the cycle life remains a major problem.

The Stanford team thinks they have found their answer: a proof-of-concept lithium-sulfide cathode with 10 times the power density of conventional lithium-ion cathodes. Together, the anode and cathode could yield a battery that lasts four times as long and is significantly safer than existing lithium-ion batteries. The new battery cannot achieve the 10 times the energy storage capacity because the new cathode has significantly lower conductivity than the lithium metals used in conventional batteries.

All this represents an approximately 80 percent increase in the energy density over commercially available lithium-ion batteries.  But by the fifth cycle the capacity is down by a third and the batteries are, well, dead by the 50th cycle.  There’s lots of hope here though.  Just getting the silicon to work at all is a major achievement. Cui is said to be thinking the capacity loss is likely due to polysulfides, chemicals that form during normal discharging and recharging. If allowed to dissolve into the battery’s liquid electrolyte, polysulfides can poison the battery by blocking future charging and discharging.

The past weekend saw researchers at the Università degli Studi di Roma La Sapienza announce development of a novel polymer tin sulfur lithium-ion battery that takes advantage of the high theoretical specific energy and energy density of the lithium-sulfur battery chemistry.  It’s a whole new take – rather than taking the more conventional approach of using a sulfur cathode and a lithium metal anode, Jusef Hassoun and Bruno Scrosati have developed a lithium-metal-free battery, using a carbon lithium sulfide composite as the cathode and a tin carbon composite anode.

The difference here is Li-ion batteries use a process called intercalation to store lithium ions by inserting the ions between molecules in the electrode, while lithium-sulfur batteries rely on a multi-step redox reaction with sulfur that results in a number of stable intermediate sulfide ions. This storage process, in theory, reduces limitations of electrode structure – thus enabling higher capacity in similar volumes.

“(One) major hurdle is the high solubility in the organic electrolyte of the polysulfides Li2Sx (1≤x≤8) that form as intermediates during both charge and discharge processes. This high solubility results in a loss of active mass, which is reflected in a low utilization of the sulfur cathode and in a severe capacity decay upon cycling. The dissolved polysulfide anions, by migration through the electrolyte, may reach the lithium metal anode, where they react to form insoluble products on its surface; this process also negatively impacts the battery operation,” Hassoun and Scrosati say in their paper.

The pair goes on to explain, “The key challenge is then to totally renew the chemistry of this battery such as to achieve an advanced configuration that can consistently provide high capacity, a long cycle life, and safe operation. Herein, we report an example of a lithium metal- free new battery version and demonstrate that, to a large extent, it can effectively meet these targets. In contrast to most of the Li–S batteries proposed to date, which are fabricated in the “charged” state, that is, using a carbon–sulfur composite cathode that necessarily requires a lithium metal counter electrode (anode) to assure the 16Li+S8→8Li2S discharge process, we propose to fabricate the battery in the “discharged” state by using a carbon lithium sulfide composite as the cathode.”

Polymer Tin and Sulfur Battery Principle Graphic. Click image for more info.

The pair has also replaced the common liquid organic solutions with a gel-type polymer membrane. Since the lithium ions necessary to drive the electrochemical process are provided by the Li2S/C cathode, any material capable of accepting and releasing lithium ions can be chosen as anode to replace lithium metal, they said. They chose a tin/carbon nanocomposite, with Sn/C at 1:1 by weight. The specific capacity of the improved Sn/C electrode matches that of the Li2S/C electrode, and the Sn/C has high chemical stability.

It’s a slick take on the problem – the chemical process goes from the conversion of lithium sulfide into sulfur with the release of lithium ions, which travel through the electrolyte to reach the anode where they form an alloy with the tin.  Completing the process cycle is the reversible reaction of the lithium – tin alloy with elemental sulfur to form tin metal and lithium sulfide.

The pair is saying the results are effective in controlling most of the sulfur based lithium technology problems.  Their calculation shows the innovation could drive specific energy to the order of 1100 Wh kg-1, a value not previously achieved for a lithium metal-free battery.

Hassoun and Scrosati note the “the road to a practical lithium-sulfide battery is still long,” with optimization of the electrode morphology and cell structure needed to further improve the cycle life and the rate capability.

This is progress, offered in multiples of capacity that should excite the electron storage folks quite nicely. Lets hope the puzzle of cycle life and costs come up solved and low cost.


Comments

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