Rechargeable lithium batteries have reformed the portable electronic industry. This is due to their superior energy density (they can store 2-3 times more energy per unit weight and volume in comparison with conventional rechargeable batteries).
Lithium ion batteries involve generally a graphite negative electrode (anode), a non-aqueous liquid electrolyte and a positive electrode (cathode) of a spinel type oxide LiMO2 (where M = Co and sometimes Mn and Ni). On charging, lithium ions are desintercalated from the layered LiMO2 intercalation host, pass across the electrolyte and intercalated between the graphite layers in the anode. Discharge reverses the process. The electrons of course pass around the external circuit. Research in the field on Li-ion batteries aims at improving the energy density of such devices which relates to the discovery of novel electrode materials able to provide an improved performance.
The storage of one Li atom between every six carbon atoms is only valid for graphite. This LiC6 stoichiometry permits to graphite a storage capacity of 372 mAhg-1; a value lower that it can be obtained with other materials. However, this low storage capacity results in only a small volumetric change of about 10% and allows for a life of at least 500 cycles depending on the current rate used. The anode reaction based on intercalation and desintercalation is:
yC + xLi + e = LixCy
Disordered carbons (the so called hard carbons) can easily exceed this theoretical value reported for graphite. In our group we are exploring the performance of various porous disordered carbon materials based on biomass precursors ( glucose, rice straw, bacterial cellulose) as negative electrodes in Li-Ion Batteries. The main problem associate with carbon materials is that the values for higher capacities are obtained when the potential is close to 0V vs Li/Li+, which is not safe especially for high power applications such as for example electrical vehicles.
We also explore other options such as hybrid materials based on Si/C or SnO2/C. Such semiconductors can react with Li forming alloys according to LixMy through electrochemical processes. The reaction is partially reversible and involves a large number of atoms per formula unit. The specific volumetric and gravimetric capacities exceed those of graphite. For example Li 4.4Sn has a gravimetric capacity of 993 mAh/g versus 372 mAh/g for graphite. The corresponding values for Li4.4 Si are 4200 mAh/g. However, such alloying reactions are normally associated with severe volume changes during Li insertion/ desinsertion leading to a very fast decay in capacity with cycling. Another associated problem is the low conductivity of such materials. To take advantage of the high energy density offered by such compounds but overcome the problems associated with their low conductivity and poor cycle performance we have developed easy processes for coating such nano structured semiconductors with a thin layer of carbon.
We are also interested in cathode materials, especially LiFePO4 as a benign and abundant material choice. We have synthesized a new class of mesocrystallyne LiFePO4 and we are currently study in detail the charge-discharge mechanism.
There has been recent concern that the amount of the Li resources that are buried in the earth would not be sufficient to satisfy the increased demands on Li ion batteries. While there is ample evidence that this is no cause for immediate concern, very large market share of electric vehicles can put a strain on Li production capability and its price. Sodium (Na) is located below Li in the periodic table and they share similar chemical properties in many aspects. The fundamental principles of the Na-ion batteries and Li-ion batteriesare identical; the chemical potential difference of the alkali-ion (Li or Na) between two electrodes (anode and cathode) creates a voltage on the cell. In charge and discharge the alkali ions shuttle back and forth between the two electrodes.
There are several reasons to investigate Na-ion batteries. Recent computational studies on voltage, stability and diffusion barrier of Na-ion and Li-ion materials indicate that Na-ion systems can be competitive with Li-ion systems. In any case, Na-ion batteries would be interesting for very low cost systems for grid storage, which could make renewable energy a primary source of energy rather than just a supplemental one. As battery applications extend to large-scale storage such as electric buses or stationary storage connected to renewable energy production, high energy density becomes less critical. Moreover, the abundance and low cost of Na in the earth can become an advantage when a large amount of alkali is demanded for large-scale applications, though at this point, the cost of Li is not a large contribution to the cost of Li-ion batteries. But most importantly, there may be significant unexplored opportunity in Na-based systems, Na-intercalation chemistry has been explored considerably less than Li-intercalation, and early evidence seems to indicate that structures that do not function well as Li-intercalation compounds may work well with Na. Hence, there may be opportunity to find novel electrode materials for Na Ion Batteries.
Graphite, the standard anode material in current lithium-ion batteries, seems not suited for a sodium based system, as Na hardly forms staged intercalation compounds with graphite. The situation is different when amorphous carbons are used as intercalation media and it is suggested that the storage mechanisms for Na and Li are similar, although the capacity is smaller in the case of sodium.
We are exploring porous amorphous carbons, heteroatom doped or un-doped as negative electrodes for room temperature Na-ion batteries.
Check out some of our papers related to sustainable carbon materials for rechargeable batteries applications: