The inexorable depletion of fossil fuels and several environmental issues such as global warming have ignited the research efforts in the field of nonconventional energy resources. The non-conventional energy resources such as solar energy, wind energy and hydro energy could be the potential and promising replacements of the conventional energy resources. These are sustainable, renewable and environment friendly, but most of these are intermittent and periodic in nature. Therefore, to ensure the continuous supply of energy, electrical energy storage devices, such as batteries, are becoming essential. The term battery was first coined by Benjamin Franklin in 1748 to report an array of charged glass plates.1 Basically a battery is an electrochemical storage device which can store energy in the form of chemical potential that can be used later. Secondary or rechargeable batteries appear more attractive, as they can be charged and discharged for a number of times. Li-ion battery technology has emerged as one of the most successful rechargeable battery technologies for consumer electronics. It offers highest specific energy among all the rechargeable battery technologies. Figure-1 gives a comparison between the specific energy (Wh/kg) and cost ($) (cost specified is of a 7.2V battery pack for Li-ion, Ni-Cd, Ni-MH battery and a 6V battery pack for Lead-acid battery) of different existing secondary battery technologies.2
Figure-1: Comparison between specific energy and cost of different battery technologies
The Li-ion battery market is expected to witness a robust growth in the coming years. This is imputable to the various advantages offered by the Li-ion battery such as higher capacity, lighter weight and longer lifespan. The Li-ion battery has shown a broad scope of applications ranging from hybrid and fully electric vehicle, mobile electronic devices to electric storage systems. Increasing demand of lithium-ion battery in the automotive sector, like in running electric vehicles (EV), is expected to fuel the growth of Li-ion battery technology in near future. Li-ion battery has already proved its usefulness in hybrid electric vehicle. Whereas in the case of fully electric vehicle two of the major automobile companies, Volkswagen and BMW group have recently launched some fully electric vehicles. On the other hand, GM’s fully electric vehicle, Chevy Volt, has also grabbed a lot of news highlights last year. Other giants of the automobile sector, such as Mercedes, Porsche, Audi and Bentley are all set to embark into the market in coming years.
Figure 2: Application of Li-ion battery in consumer electronics.
Figure 3: Electric vehicle (Source:http//usahitman.com/wscecof).
Different consultants and market research groups have forecasted about the Li-ion battery market development in the coming future. Navigant Research group says “Demand for lithium-ion batteries will surge in the coming decade with the market growing to a value of $26 billion annually by 2023”. Whereas another parallel research group Transparency Market Research group claims a bit higher value of $33.11 billion for Li-ion battery global market by 2019. The group has also claimed that military and consumer sectors could be other major driving sectors, alongside the automotive sector.3
The mobile electronic device industry, including cell phones, laptops, notebooks and other mobile electronic gadgets, is another sector that occupies nearly 50% of the Li-ion battery market share. According to the analysis given by Roland Berger strategy consultants, this is going to grow by 76% from 2015 to 2020. Whereas another emerging sector, Electric Storage System (ESS), is expected to grow by 35% annually on an average between 2015 and 2020. In spite of this expected growth of the Li-ion battery market, cost of production is going to as a major hindrance. However, it is predicted that the cost factor between lead-acid and Li-ion batteries will drop down from 1:3 as on today to 1: 1.5 by 2020.4
Essentially, a Li-ion battery is comprised of three main components: a cathode, an electrolyte and an anode. Conventionally the two electrodes, cathode (mostly lithium metal based compounds, e.g. LiCoO2) and anode (mostly a carbonaceous material, e.g. graphite) are separated by an organic electrolyte having lithium salt (e.g. LiPF6) in it, which enables transport of Li-ions during the charging and discharging process. A separator (polymeric membrane) is also being used to prevent short circuit between the two electrodes. A Li-ion battery works on the principle of reversible shuttling of ions. In short, when a voltage is applied across the electrodes in order to charge the battery, the metal oxide at the cathode terminal gets reduced and generates a Li-ion (Li+) and an electron. Due to high ionic conductivity of the electrolyte, it allows the flow of Li-ions towards the anode (negative electrode). And the electrons generated during the reduction process move out of the battery through the externally connected circuitry. Now these Li-ions reach to the anode (graphite) and get intercalated into the layered structure of graphite and create a potential difference between the two electrodes which further can be used to supply power to any given closed circuit. Whereas, in the case of discharging, the process gets reversed and ions move from the anode to the cathode.
Ability of a material to accommodate Li-ions within its structure determines its storage capacity and feasibility as an anode material. Conventionally, graphite is being employed as an anode material that offers theoretical specific capacity of 372mAh/g which is relatively low. In graphite, six carbon atoms together bonds with only one Li-ion via formation of an intercalation compound LiC6.5 Still, graphite is common among the commercial Li-ion batteries, because of its low volume expansion during the lithiation process. This low volume expansion improves the stability of the battery. But the specific capacity offered by these batteries is not high enough for the upcoming applications such as electric vehicles. In order to cope up with rapidly increasing market and wide range of applications, researchers have come up with a number of novel nanomaterials based anodes, showing very high specific capacity and good stability. The elements of group III, IV, and V were examined extensively because of their capability to store lithium at low voltage, e.g. Silicon, Tin, Germanium, Aluminium, and different Carbon allotropes. Silicon attracted greatest attention because of its highest theoretical specific capacity of 4200 mAh/g for Li4.4Si. Furthermore, other materials such as Sn (981 mAh/g for Li22Sn5), Sb (536mAh/g for Li3Sb), Al (2234mAh/g for Al4Li9) and Ge (1600mAh/g for Li4.4Ge) are also known for their very high theoretical specific capacity.5 But pulverization is the primary issue with most of these materials, hampering their use at the commercial level. The volume of these materials expands by 100-400% when subjected to lithiation and contracts after de-lithiation (6-8). This huge expansion and contraction in volume leads to issues like mechanical fracture and dissociation of active material from the current collector and electrolyte. To overcome this issue, different techniques have been employed such as use of nano-sized particles, preparation of composites with carbon, high binder content and thin film deposition. All of these techniques were able to solve the issue to a certain extent. Carbon nanotubes (CNT) were also investigated intensively as an anode material. CNT offers several advantages over the conventionally used graphite. Its hollow cylindrical structure facilitates the Li-ion insertion and provides a conducting path for easy flow of electrons. Its porous structure enables the fast movement of ions and also leads to flexibility. In case of composite anodes, CNT acts as a buffer and accommodates the strain generated by other materials (Si, SnO2), which has been successfully established with silicon nanowire based anodes.6
Nanomaterials based anodes offer several advantages over anodes based on bulk materials. The nano materials provide a smaller diffusion length to the intercalating/alloying Li-ions, which is directly related to the diffusion time of the ions. It reduces the diffusion time significantly and therefore increases the power density and improves the rate performance of the battery.7 “The big thing in these batteries is the compromise between how much you can store and how quickly you can release the energy,” said Reilly Brennan (Executive Director of the Revs Automotive Research Program at Stanford). But the use of nanomaterials has made it possible to store high amount of energy with a decent energy releasing rate. On the other hand, nanomaterials provide better electron transport, which eventually leads to better charging, discharging kinetics, cyclic and rate performance. Whereas, the very large surface area of nanomaterials provides more reaction sites for the Li-ions to react, thus improving the gravimetric capacity of the battery. Furthermore, the high surface area also provides high contact area with the electrolyte which eventually leads to high Li-ion flux at the junction. Although nanomaterials based electrodes exhibit numerous promising properties and advantages over the conventional electrodes, but some bottlenecks are hindering commercialization of nanomaterials based electrodes. One of the bottlenecks is formation of thick solid electrolyte interface (SEI) due to large surface area of nanomaterials. Although the large surface area facilitates the high intake of Li-ion and provides very high initial specific capacity, but also leads to formation of SEI which restricts mobility of Li-ions and leads to reversible capacity loss. The use of nanomaterials also increases overall cost of the product.7
The scientific community has the responsibility to overcome the issues hindering the commercialization of nanomaterials based Li-ion batteries. The major issues hindering the commercialization can broadly be classified into two categories. I). Capacity: The actual capacity offered by the batteries is way less than the theoretical capacity claimed. It implies that in order to attain the claimed theoretical values, we need to use new materials and novel manufacturing technologies which can bring on the full potential and can also reduce the overall cost of the product. II). Cost: The ingredient of cost should always be remembered because economic gravity is the factor matters the most for any industry and in market the cheapest ends up bringing home the bacon (7). Every battery needs a safety circuitry that does not allow the battery to get over-charged and also prevents it to get fully discharged. This additional safety circuitry again adds to the cost of the battery. The use of flammable liquid electrolyte also increases the risk. Researchers are trying to overcome this issue by using solid electrolyte.
Automotive Energy Supply Corporation (AESC), TDK Corporation, LG, Samsung, Hitachi and Panasonic are some of the biggest players of the battery industry. Whereas, the major percentage of the battery market is occupied by China, South Korea and Japan. Owing to the dream of zero carbon footprints and a clean, renewable energy resource, the development of highly efficient energy storage devices is essential. The nanomaterials based anodes have shown a great promise as future battery technology. The battery industry is progressing with a great pace and the amount of investment in this field is clearly visible. This is the time for the scientific community to accept the responsibility to sweep over the related issues and to come forward with novel solutions. At the same time, attention should be given to bring down the cost of the final product, so that the technology can be accepted more widely in the marketplace and can sustain for a longer time.
References:
1. Benjamin Franklin: Philadelphia, Serendipity, and a Summer Storm
By Dr. Bryen E. Lorenz, Pennsylvania Iota ’76
2. Battery Primer, A short battery primer, Handbook of batteries By Linden and Reddy
3. http://ecomento.com/2014/04/07/electric-cars-to-drive-26-billion-demand-for-lithium-ion-batteries/
4. Technology & Market Drivers for Stationary and Automotive Battery Systems By Roland Berger Strategy Consultants
5. A review of application of carbon nanotubes for lithium ion battery anode material, Charles de las Casas, Wenzhi Li. Journal of Power Sources 208 (2012) 74–85
6. High-performance lithium battery anodes using silicon nanowires, CANDACE K. CHAN1, nature nanotechnology | VOL 3 | JANUARY 2008
7. Carbon Nanostructures in Lithium Ion Batteries: Past, Present, and Future, Indranil Lahiri, Solid State and Materials Sciences, 38:128–166, 2013
8. Review on recent progress of nanostructured anode materials for Li-ion batteries, Subrahmanyam Goriparti, Journal of Power Sources 257 (2014) 421e443
Author:
Sameer Chouksey
M.Tech. (Nanotechnology)
Indian Institute of Technology Roorkee, India.
Contact: +91-8126365050
E-mail: sameer.chouksey@gmail.com
