Despite the recently reported battery-flaming problem of lithium-ion batteries (LIBs) in Boeing’s 787 Dreamliners and laptops (in 2006), LIBs are now successfully being used in many sectors. Consumer gadgets, electric cars, medical devices, space and military sectors use LIBs as portable power sources and in the future, spacecraft like James Webb Space Telescope are expected to use LIBs.
The main reason for this rapid domination of LIB technology in various sectors is that it has the highest electrical storage capacity with respect to its weight (one unit of LIB can replace two nickel-hydrogen battery units). Also, LIBs are suitable for applications where both high energy density and power density are required, and in this respect, they are superior to other types of rechargeable batteries such as lead-acid, nickel-cadmium, nickel-metal hydride, nickel-metal batteries, etc.
However, LIBs are required to improve in the following aspects: (i) store more energy and deliver higher power for longer duration of time, (ii) get charged in shorter period of time, (iii) have a longer life-time and (iv) be resistant to fire hazards. Figure 1 depicts the basic LIB Characteristics required for different applications and the respective properties that need to be improved.
Fig. 1: Basic LIB characteristics required for different applications 1,2 (DOD: Depth of Discharge, SOC: State of Charge). (click on image to enlarge)
At present, there is a great deal of interest to upgrade the existing LIBs with improved properties and arrive at a battery technology that would permit smart-storage of electric energy. Futuristic smart electric grids that can provide an uninterruptible power supply to a household for 24 hours can replace the currently used lead acid battery systems by performing better in terms of longer back up time and reduced space requirements.
With the advent of next generation LIBs, electric vehicles are expected to cover longer distances with shorter charging times; mobile phones and laptops are expected to be charged within minutes and last longer.
What Nanotechnology can do to Improve the Performance of LIBs Nanotechnology has the potential to deliver the next generation LIBs with improved performance, durability and safety at an acceptable cost. A typical LIB consists of three main components: an anode (generally made of graphite and other conductive additives), a cathode (generally, a layered transition metal oxide) and electrolyte through which lithium ions shuttles between the cathode and anode during charging and discharging cycles.
On electrodes: The electrodes of LIB, both anode and cathode are made of materials that have the ability to be easily intercalated with lithium ions. The electrodes also should have high electrical conductivity so that the LIB can have high charging rates. Faster intercalation of Li ions can be facilitated by using nanosized materials for electrodes, which offer high surface areas and short diffusion paths, and hence faster storage and delivery of energy. One prominent example is the cathode material of A123 LIBs that use nanosized lithium iron phosphate cathode. Researchers have been trying to increase the electrical conductivity of lithium iron phosphate by doping it with metals.
However, without the need for doping, the conductivity and hence the performance of the cathode material could be improved significantly by using nano-sized lithium iron phosphate. One dimensional vanadium oxide materials, LiCoO2 nanofibers, nanostructured spinels (LiMn2O4) and phosphor-olivines (LiFePO4), etc., are being explored as cathode materials for the next generation LIBs. Similarly, nanosizing the anode materials can make the anode to have short mass and charge pathways (i.e allow easier transport of both lithium ions and electrons) resulting in high reverse capacity and deliver at a faster rate.
Nanostructured materials like silicon nanowires, silicon thin films, carbon nanotubes, graphene, tin-filled carbon nanotubes, tin, germanium, etc., are currently being explored as anode materials for the next generation LIBs.
On electrolyte: Electrolytes in LIB conduct lithium ions to and fro between two electrodes. Using solid electrolytes could render high-energy battery chemistries and better safety (avoids fire hazards) when compared to the conventionally used liquid electrolytes. However, achieving the optimal combination of high lithium-ion conductivity and a broad electrochemical window is a challenge. Also, reduction of interfacial resistance between the solid electrolyte and lithium based anodes also poses a formidable challenge3.
Nanostructuring of solid electrolytes has proven to improve the lithium ion conductivity, for example, when the conventional bulk lithium thiophosphate electrolyte was made nanoporous, it could conduct lithium ions 1000 times faster4. Another example is the nanostructured polymer electrolyte (NPE), which ensures safety. Main advantage of using this benign electrolyte is that it allows the use of lithium metal as anodes (instead of carbon based anodes) and contribute to the increase of energy density of the battery5.
On improving the performance of LIBs: The performance of the LIB is typically measured by its power and energy stored per unit mass or unit volume. The power density of the LIBs can be increased but often at an expense of energy density5. In order to achieve high power density as well as energy density, researchers are using nanotechnology to design electrodes with high surface area and short diffusion paths for ionic transport.
The high surface area provides more sites for lithium ions to make contact allowing greater power density and faster discharging and recharging. Another important parameter known as rate capability, indicates the maximum current output the LIB can provide and it plays an important role in deciding life-cycle of the LIB. In general, higher the rate capability, greater is the power density and longer the cycle-life.
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.
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.
By the year 2020, the cost of the LIBs for automotive applications are expected to come down by half 19] and almost 70% reduction in the lifetime cost of the LIBs (which brings down the cost of a battery by three times)  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, etc.
. Nanotechnology and nanomanufacturing techniques are expected to open up possibilities of low-energy processing methods for fabricating and stacking of the LIB components.
Challenges in Developing Nanoenabled LIBs
Though the LIB technology is about twenty years old now and even with the advent of nanotechnology, it is still a challenge to attain LIBs with optimal combination of energy, reliability, cost and safety. With regard to the anode materials, lithium suffers from the dentrite-formation (leading to an explosion of the battery), high reactivity, etc. Hence, nanostructures of tin, silicon, etc., are being used as new anode materials.
Fig. 2: Challenges in the development of nano-enabled LIBs.
Various strategies like (i) decreasing the particle size to nano-range (ii) employing hollow nanostructures (iii) making nanocomposites or nanocoatings with carbon and/or inert components, etc., are being used to achieve high capacity and stable cycle-life of electrodes.
However, these approaches reduce the overall energy density of the anode material due to the following reasons: (i) low packing -density of nanosized materials (ii) presence of large voids in the hollow structures (iii) increased weight -percentage of added carbon/or inert components. Lately, smartly-designed nanoparticle agglomerates in micron size range are proposed to be used to solve the above said technical drawbacks of using nano-enabled anodes and similar strategies can also be applied for designing efficient nano-sized cathode materials [23,24].
Other challenges such as lowering the high fabrication cost due to energy- consuming synthetic processes, avoiding undesired reactions at electrode/electrolyte interface that arise due to the large surface areas of nanomaterials, preventing the formation of agglomerates during the fabrication process, etc., can be overcome by careful selection of the fabrication procedure.
Commercialization of Nanoenabled LIBs: Current Scenario
LIBs have already penetrated the consumer electronics market and are now making the move into HEV/EV applications and grid-storage applications. By 2018, global market for LIBs is expected to grow strong and reach $24.2 billion. Unlike before, the industry is ready to develop improved LIBs for diverse and new applications, thanks to the growing knowledge on new materials/technologies.
At present, most of the research efforts to develop advanced electrodes, safe electrolytes, etc., employ nanomaterials/nanotechnology routinely. As discussed in the previous section, there are number of challenges that are yet to be met to achieve 100% reliability and the merit of using nanomaterials for next generation LIBs. Especially, in the case of LIBs for electric vehicles, which is considered as a golden ticket for the commercialization LIBs, some startup companies like A123, Ener1, etc., announced bankruptcies in the past few years in spite of receiving huge capital investment and producing batteries with exceptional properties.
Experts note that this downfall cannot be solely attributed to the new nanotech-enabled LIB technology but also to the issue of replacing internal combustion engine in vehicles [25,26]. At present, LIBs consume the 65% of the total cost of an electric vehicle, and hence in order to be cost-completive with gasoline, LIBs with twice the energy storage of state-of-art LIBs at 30 % of cost are required .
Thus, the successful commercialization of nano-enabled LIBs for all-electric vehicles depends on various factors as mentioned above. Apart from these automobile applications, nanoenabled LIBs for powering handheld gadgets and for stationary storage applications are more likely to depend on the improvement in the properties of the LIBs, volume production rates) and usage of abundant, low cost, high energy materials.
LIB technology is rapidly emerging as the most advantageous battery chemistry for transportation as well as consumer electronics. Various research efforts on nanotechnology based LIB technology has already led into the production and use of high performance LIBs (Toshiba, A123 Systems, Altair Nano, Next Alternative Inc., etc.) and yet more improvement with respect to the performance, durability and safety aspects, especially for automotive applications are more likely to be achieved in the future.
The author would like to thank Dr. Srinivasan Anandan of ARCI for the insightful discussions on the current research trends on LIBs and Dr. C.K. Nisha of CKMNT for her suggestions on enhancing the content of the article.
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The full article has appeared in the April 2014 issue of “Nanotech Insights” and the above article is an abridged and revised version of the same.
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