Section: Batteries & Fuel Cells

Battery Anodes

Introduction
The anode is the negative electrode of a primary cell and is always associated with the oxidation or the release of electrons into the external circuit. In a rechargeable cell, the anode is the negative pole during discharge and the positive pole during charge. 

Lithium Anode
The anode in the battery deserves an equal say in the overall performance of a battery. For an effective development of a high energy density battery, the use of high capacity electrode materials (anode & cathode) is an essential factor.  For such systems, alkali metals are perhaps the obvious choice. The most promising types of advanced batteries currently under production are based on lithium anodes.  The choice of the anode material is very much restricted by the need for a high energy content, which is unavoidably linked, to the use of an alkali metal as the main anode material.Lithium is generally preferred, since it can be more easily handled (though with care) than other alkali metals and more significantly, the lightest and the most electropositive among the alkali metal family.  Also, the low density of lithium metal (0.534g/cc) leads to the highest specific capacity value of 3.86Ah/g, which stands exceptional. Therefore, lithium batteries possess the highest voltage and energy density of all other rechargeable batteries and are therefore favored in applications related to portable appliances where low weight and small volume are the major constraints. The advantages of using lithium metal as the anode are as follows:

  • Good reducing agent 
  • Highly electropositive(so higher voltage is obtained depending upon the cathode used)
  • High electrochemical equivalence High capacity (3.82Ah/g) and energy density (1470Wh/Kg)   
  • Good conducting agent 
  • Good mechanical stability 
  • Ease of fabrication/compact design 

The essential reaction of metallic lithium anode is very simple:

 

 

 

But, in spite of this simplicity, the practical application of Li metal to a rechargeable anode has been very difficult due to some crucial issue. The most important one is that Li metal usually will tend to deposit as a dendrite or mossy structure during charge, and the disordered metallic deposit gives rise to a poor coulombic efficiency. This happens because such a fine Li metal often acts as an active site inducing reductive decomposition of electrolyte components. Part of the deposit may become electrically isolated and shedding may also occur. Furthermore, the fine metallic lithium may easily penetrate into the separator and eventually cause internal short, this resulting in heat generation and contingent ignition. One of the main reasons for the failure of rechargeable lithium systems lies in the reactivity of lithium with electrolytes].  Hence the hazardous nature of Li has paved way to identify some other safer anode materials, possessing comparably the same electrochemical features as that of lithium.   

Alternate anodes for lithium batteries 
Carbonaceous materials, which allow the intercalation of Li within the layers, are clearly the most suitable candidates, leading to the popularly known lithium-ion or shuttlecock or Lithium Rocking Chair Batteries (RCB). Most carbon varieties including graphite are gaining importance as attractive candidates of anode materials for rechargeable lithium batteries, because they can accommodate lithium reversibly and offer high capacity, good electronic conductivity and low electrochemical potential (with respect to Li metal). The maximum amount of lithium that can be intercalated within the graphite structure is 1 per 6 carbon atoms, yielding a specific capacity of 372mAh/g. The cost, availability, performance and potential (vs. Li metal) of carbon-based materials are all acceptable and even preferable when compared to lithium metal anode for practical cells.  An important evidence for this is the commercial availability of LiCoO2/carbon cells manufactured by Sony Inc. There is no significant swelling of, or stack pressure generation by the carbon electrode on prolonged cycling and therefore Li-ion cells can be constructed as flat or prismatic cells with thin-walled cases or in any other cell configurations. The shortcomings on the deployment of different types of anode materials are displayed in Table 1 

 
MATERIAL

REMARK
LITHIUM
DENDRITE GROWTH, EXPENSIVE, TOXIC
CARBON
IRREVERSIBLE CAPACITY LOSS
TIN
INCLUSION OF SOLID ELECTROLYTE PHASE IN THE ELECTRODE
ATCO
COMPLICATED LITHIUM UPTAKE/REMOVEL
M-M ALLOY
LARGER VOLUME CHANGES (MECHANICAL DECRIPITATION)
TERNARY METAL VANADATES
ARGUABLE Li  DIFFUSION MECHANISM
METALLOIDS
MOISTURE SENSITIVE


Hollow Fe3O4 Nano materials as anodes
Figure 1.The current work investigates the potential for hollow nanostructures to mitigate the pulverization problem and fast capacity fading for anode materials in lithium-ion batteries (LIBs). Hollow Fe3O4 nanoparticles are synthesized via a template-free solvothermal method using FeCl3, urea and ethylene glycol as starting materials. Temporal XRD and TEM (Figure 1) studies indicate that the growth follows an inside-out Ostwald ripening mechanism. Higher concentrations of urea in the starting material result in lower percentages of hollow particles (phi) and this observation is consistent with the proposed growth mechanism. The performance of the hollow particles as anode materials in LIBs is tested and shown to be superior to their solid counterparts, Figure 2.with higher percentages of hollow particles giving better performance (Figure 2), which provides evidence for the hypothesis that hollow structures are able to alleviate the pulverization problem. Cyclic voltammograms of Fe3O4 nanoparticles are analyzed which provides some insight into the reaction mechanism of the lithium-ion insertion/deinsertion process.