Nanoscale Engineering for the Mechanical Integrity of Li‐Ion Electrode Materials

Abstract

Portable power technology plays a key role in the advancement of electronic devices required by modern civilization. Lap-top computers, cellular phones and most portable electronic memory devices require the use of rechargeable batteries. Such batteries have also found use in biomedical devices, including pacemakers and implantable defibrillators. Thus, research focused on the development of rechargeable, high-energy density power sources continues to be driven by technological and commercial applications.Although various types of battery chemistries exist -such as nickel-metal hydride (Ni-MH) and nickel-cadmium (Ni-Cd) -lithium (Li) ion batteries are the dominant force as they have significant advantages in energy density. The large volumetric and gravimetric energy densities exhibited by Li-ion batteries allow their volume and mass to be reduced by 20% and 50%, respectively, as compared to other battery chemistries; in fact, a Li battery can provide three times the voltage of a Ni-Cd or Ni-MH battery. Furthermore, the self-discharge rate of Li batteries is very small over a long period of time, which makes them extremely reliable, while their operating voltage allows a reduction in the number of batteries required to operate a device. All of the aforementioned properties contribute to the miniaturization of electronic devices. Finally, it should be mentioned that -unlike other battery base materials (Cd, Ni) -Li is non-toxic and, as opposed to Ni-Cd batteries, Li-batteries have no memory effect. Moreover, the charging capacity (total time integrated battery current per mass) is not reduced by repeatedly charging and discharging to insufficient levels, and therefore partial charging is possible (the discharging process takes place during the operation of the respective electronic device).One major technological barrier to improvements in energy density and reliability of the Li-ion battery systems is related to the stability of the anode and cathode materials. One of the reasons that Li-ion chemistries exhibit high energy densities is because of the relatively high cell voltage. This means that there is a large j319 Nanostructured Materials in Electrochemistry. Edited by Ali Eftekhari

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“…Thin silicon films have demonstrated the highest capacities and longest cycle lives due to the small amount of active material limiting total volumetric expansion. The performance of thin films improves as film thickness decreases, consistent with the Griffith-Irwin relation, which states that the critical fracture stress increases as film thickness decreases: 126,127…”

“…Comparison of the calculated misfit stress energy in partially delithiated particles (consisting of a lithiated core and delithiated shell) suggests fracture should not occur during cycling for particles having diameters of 10 nm or less. 126 Other calculations focusing on the Li-Sn system, which undergoes a similar volume expansion during electrochemical alloying, suggest that particles with diameters less than 200 nm will resist fracture. 127 Indeed, a reduction in particle diameter is correlated with an increase in cycling stability.…”

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