Better Batteries with a Conducting Polymer Binder
Lawrence Berkeley National Laboratory (Berkeley Lab) scientists have invented a new material for use in rechargeable batteries that can boost power storage capacity by 30 percent, a dramatic improvement in a field marked by little progress for more than a decade. It is called a Conducting Polymer Binder, literally a kind of flexible plastic glue that holds electrode materials together while facilitating the shuttling of electrons and positively charged lithium ions.
Today's ongoing revolution in electronics is driven by something engineers call "Moore's Law," the ability to double every 18 months the number of transistors on a silicon chip. It is the reason why computers that once took up an entire building can now fit inside a smartphone.
Unfortunately, there is no Moore's Law for batteries.
The amount of power they can store has increased at a plodding five percent per year. As a result, conventional batteries are becoming the largest and heaviest component of all things mobile and electronic; and in electric cars, they are the costliest. Breaking this bottleneck is enormously important. Cars and trucks account for 30 percent of our nation's carbon dioxide emissions, and wider use of electric vehicles could reduce fossil fuel use and the release of greenhouse gases.
In their effort to make smaller, lighter, and cheaper batteries, a Berkeley Lab team led by Gao Liu of the Environmental Energy Technologies Division (EETD) focused on improving the negative electrode, or anode. During charging of any lithium battery, lithium ions are driven to the anode, causing electrons to build up potential energy at the anode. Complete a circuit by turning on a switch, and those electrons start flowing.
The anodes of conventional lithium-ion batteries are made from a composite of carbon particles, or graphite. Engineering has improved these graphite anodes, but they are rapidly approaching their power storage design limit. In theory, anodes made from lighter-weight silicon could store ten times more energy. But there is a catch: During charging, when silicon absorbs lithium, a silicon anode literally swells to four times its size. On discharge, it contracts. After a just few cycles, this breath-like expansion and contraction causes silicon anodes to break down.
The key to building a better battery, it turns out, is to build a better binder.
The Berkeley Lab technology directly addresses the "volume-expansion" problem by engulfing silicon particles in a rubbery polymer to form an anode composite. Four features make this binder unusually attractive for battery designers: it is strong, elastic, porous, and highly conductive. The elastic material stretches during the expansion of silicon particles as the battery charges and contracts during discharge—giving silicon anodes the flexibility to "breathe." The binder's porosity permits the passage of lithium ions, while its conductivity handles the transit of electrons. Other researchers have tried the flexible-binder approach, but their polymers lose their conductivity in the hostile chemical environment inside a battery. Much of Liu's development effort focused on modifying the polymer binder to maximize its conductivity under a battery's actual operating conditions.
The Berkeley Lab team used a soft X-ray beamline at the Advanced Light Source to analyze and solve the loss-of-conductivity problem. These observations were subsequently used to choose molecular structures on binder candidates that become even more electrically conductive in the electron-rich, highly reductive environment of an anode. The optimal design determined by the research is the Conducting Polymer Binder Poly(9,9-dioctylfluorene-co-9-fluorenone-co-methybenzoic ester), or PFM. Subsequent research using this same approach has yielded new binder candidates that offer comparable performance at lower cost.
The promise of silicon lies in its theoretical specific capacity of 4,200 milliamp hours per gram (mAh/g). The theoretical limit for graphite is about one-tenth of that, or 372 mAh/g. The Berkeley Lab team has successfully fabricated silicon composite anodes with a specific capacity of 1,400 mAh/g. Batteries built with this first-generation silicon anode therefore have demonstrated an immediate 30 percent improvement in storage capacity over comparable commercial-grade graphite anode-based cells.
Improved charge capacity is not enough, however. A practical silicon anode design must also be economical. To keep costs down, the Berkeley Lab team designed its binder to be compatible with today's battery manufacturing technology. Graphite electrodes, for example, are made using a slurry of carbon particles, binder, and a solvent, which is spread onto the terminal contact surface and dried like paint. That same strategy can be used to build the silicon composite electrode from a mixture of solvent and silicon particles held together by the Conducting Polymer Binder.
This technology is available for licensing.
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Shidi Xun, Xiangyun Song, Vincent Battaglia, and Gao Liu. "Conductive Polymer Binder-Enabled Cycling of Pure Tin Nanoparticle Composite Anode Electrodes for a Lithium-Ion Battery," Journal of The Electrochemical Society 160 (6) A849–A855 (2013) A849.