[Nanosized Insertion Materials for Li-Ion Batteries]LixTiO2(Anatase, Rutile, TiO2(B), and Brookite)

Advantage: inherent safety and stability of titanium oxides  working at potentials around 1.5 V

• Nanosize
• Increased reaction areas
• Shortened Li diffusion paths
• Enhanced Li solubility and capacity

Voltage profiles of different particle sizes 

(b) Solubility limits in anatase LixTiO2 where α,β, and γ represent anatase, lithium-titanate, and LiTiO2 respectively. (α)+(β) and (α+β) refer to the situation that each particle either has phase α or β and that both phases coexist within one particle, respectively.

 * the thermodynamics of insertion in anatase is strongly affected by the crystal particle size
 * The increase of the surface area has profound impact on the storage properties

a particle size of 7 nm can completely be transformed toward tetragonal LiTiO₂. Down to ∼3 nm deep, the surface allows lithium storage exceeding the orthorhombic Li_∼0.5TiO₂ composition, which is responsible for the larger reversible (dis)charge capacities observed

the storage capacity increases with decreasing particle size, suggesting similar surface environment enhanced Li storage
 - The region where the voltage is constant reflects the first-order phase transition from Li-poor anatase Lix_a≈0.025TiO₂ to Li-rich lithium-titanate Lix_b≈0.5TiO₂
 - A remarkable observation is that the Li-ion solubility in the various phases depends systematically
on the crystal particle size, shifting the miscibility gap rather than decreasing it

• The 120 nm anatase crystals can host approximately Li/Ti = 0.03
• the 7 nm particles are able to host up to Li/Ti = 0.21 while maintaining the anatase structure.

The disappearance of the voltage plateau for smaller particle sizes has been related to these changing solubility limits

Another interesting observation is the different phase behavior in particle sizes above and below ∼80 nm referred to as (α +β) and (α) + (β)

•  Large particles appear to be able to host both phases within one crystallite
• small particles have either the Li-poor anatase or lithium-titanate phase

 - The origin of this was suggested to be the prevention of intraparticle coexisting phases and the associated phase boundary, that is, preventing the resulting energy penalty due to interface energy and strain
 - The absence of the interface rules out the interface energy effects on the solubility limits as discussed for LiFePO4

Upon nanosizing, all TiO2 polymorphs suffer from a substantial irreversible capacity loss on the first cycle that appears to scale with the surface area, compromising the use of nanostructured materials. Generally, the irreversible capacity loss is attributed to trapped lithium in the host structure or decomposition of the electrolyte and SEI formation. However, titanium oxide surfaces are wellknown for H2O and OH physisorption and chemisorption, forming strong Ti -O- H type bonds. We have shown that this explains the irreversible capacity loss by the formation of Ti -O- Li at the surface of amorphous TiO2 where Li+ exchanges with H+, which reduces the electrolyte