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Posted: Jun 18, 2014

The promise of nanotechnology for the next generation of lithium-ion batteries (page 2 of 3)

Safety
The demand for the LIBs with increased power/energy density (P/E) ratio is accompanied by the greater safety risk of the battery. Preferably, a P/E ratio of roughly 0.5 along with uncomplicated heat management is proposed for the next generation LIBs. In order to avoid fire hazards, heat generated during the charging and discharging of the battery should be dissipated quickly and non-combustible materials should be used in LIBs. In case of the LIBs with lithium metal as anodes, the so-called dendrite problem (growth of microscopic fibers of lithium across the electrolyte that leads to short circuits and overheating) remains to be solved.
Separators with nanoporous structures can prevent the spreading of dendtrites by acting as a mechanical barrier without hindering the ion-transport during charging and discharging cycles. Recently, a nanoporous polymer-ceramic composite separator that could prevent the spreading of dendrites has been reported. This novel separator consist of a laminated nanoporous gamma alumina sheet (pore size of 100 nm) sandwiched between macroporous polymer membranes. The nanoporous alumina in this layered composite could effectively impede the proliferation of dendrites and prevent cell failure that are caused by short circuits13.
Thermally stable electrolytes, for example, nanoarchitectured plastic crystal polymer electrolytes (N-PCPE) can facilitate the development of safe LIBs. Owing to its nanoarchitectural structure, N-PCPE is flexible while maintaining high ionic conductance and thermal stability. This makes the material to perform well with high electrochemical stability even in a wrinkled state. As it suffers no internal short-circuit problems even under severely deformed state, N-PCPE can be used in place of currently used flammable carbonate-based liquid electrolytes and polyolefin separator membranes to improve the safety of the LIBs14.
In another context, it can be said that nanotechnology, in a way helps to use thermally stable advanced new materials as electrodes. For example, Li4Ti5O12 spinel, which is a state-of-the-art anode material for LIBs has excellent safety and structural stability during cycling, but suffer from low ionic and electronic conductivities (in bulk form) that hampers the wide-spread use of this material. By making anodes with nanosized Li4Ti5O12 spinel and Li4Ti5O12/carbon nanocomposites, the safety as well as the electrochemical performance of the battery can be improved15.
Also, nano-enabled separators with improved stability and low shrinkage properties at high temperatures have proved to improve the safety aspects as well as the performance of the LIBs16.

For example, separators made of polymeric nanofibers (DuPont™ Energain™ battery separators) can allow automobile LIBs to accelerate quickly but safely due to their excellent stability at high temperatures.

Durability

The cycle life (number of times the LIB can be charged and discharged (one cycle together) by maintaining up to 70-80% of its original capacity) can be improved by the use of nanostructured electrodes.

New nanostructures like mesoporous CNT@TiO2-C nanocable having an inner core of carbon nanotubes encapsulating TiO2 nanoparticles, which are further covered by an outer carbon layer with mesoporous architectures provided superior electrochemical performance as anodes, hence achieving long-term cycling stability at high rates17. A high charge of 122 mA h g-1 even after 2000 cycles at 50 C could be achieved using this material.

Durable high rate LIB anodes, namely, carbon-encapsulated Fe3O4 nanoparticles homogeneously embedded in 2D porous graphitic carbon nanosheets present an excellent cycling performance (a capacity-loss of just 3.47% after 350 cycles at a high rate of 10 C). This is the highest among other conventional as well as nanostructured Fe3O4-based electrodes. Here, Fe3O4 nanoparticles of size of about 18.2 nm were homogeneously coated with conformal and thin onion-like carbon shells and embedded into 2D carbon nanosheets (thickness <30 nm). The carbon shells prevent the exposure of Fe3O4 nanoparticles to the electrolyte and stabilize the electrode-electrolyte interface18.
New 2D and 3D battery designs like forest of nanowires/rods on a thin film electrode and stacked nanorods in a ‘truck bed’ are also being explored to accommodate the volume expansion of new electrode materials and hence improve their stability.
Cost
By the year 2020, the cost of the LIBs for automotive applications are expected to come down by half19 and almost 70% reduction in the lifetime cost of the LIBs (which brings down the cost of a battery by three times)20 would be achieved by using nanomaterials (graphene coated silicon) for fabricating the LIB electrodes. In terms of using high energy electrode materials in a minimal quantity, nanotechnology can help reducing the cost of the next generation LIBs. Also, improvement in the durability (cycle life) of the LIBs using nanostructured components can improve their cost- benefit aspects. Recent advances in paper-based batteries are attractive for consumer electronics as they enable low cost manufacturing of devices like transistors, smart displays, etc21.
Nanotechnology and nanomanufacturing techniques are expected to open up possibilities of low-energy processing methods for fabricating and stacking of the LIB components.
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