Stanford Research Team Uses Lithium Titanium Phosphate Anode in Li-ion Batteries For Good Cycle Life and Efficiency

Specific capacity and coulombic efficiency of LTP at a C/5 rate in aqueous electrolyte. Source: Wessells et al. Click to enlarge.

A team from Stanford University has shown a lithium titanium phosphate (LTP) material as an “excellent” candidate for anode material for use in aqueous Li-ion batteries. A paper describing their work was published in the Journal of the Electrochemical Society.

Most commercial lithium-ion cells use a highly flammable organic electrolyte. The replacement of the organic electrolytes found in these commercial lithium-ion cells with an aqueous electrolyte would resolve the safety concerns surrounding these devices, note Yi Cui and his colleagues, as well as lowering the cost of Li-ion batteries. Aqueous lithium-ion batteries could therefore be used for applications that require excellent safety, they write.

Several research groups have studied aqueous lithium-ion batteries using a variety of electrode materials including common cathode materials such as LiMn₂O₄, LiFePO₄, and LiCoO₂. The Stanford team recently demonstrated a favorable performance of LiCoO₂ in aqueous lithium nitrate. However:

The narrow electrochemical stability range of water limits the choice of lithium intercalation materials that may be used as electrodes in aqueous cells. LiMn₂O₄ and LiCoO₂ react with lithium at potentials near the upper limit of the electrochemical stability range of aqueous electrolytes. This allows their successful use as cathode materials in aqueous cells. However, the choice of a suitable anode material for an aqueous lithium-ion battery is more difficult. At a pressure of 1 atm, ambient temperatures, and a neutral pH, aqueous electrolytes will start to decompose below potentials of about -0.4 V with respect to the standard hydrogen electrode (SHE) or 2.6 V with respect to metallic lithium.

It has been shown that LiTi₂(PO₄)₃ (LTP) reacts with lithium at an open-circuit potential of 2.5 V with respect to lithium. Thus it is among only a handful of materials currently known to intercalate lithium at potentials near the lower limit of the electrochemical stability range of pH-neutral aqueous electrolytes.

—Wessells et al.

For the study, the team used partially delithiated Li_xFePO₄ as a reference electrode in aqueous cells or use with the LTP anode material. They constructed pouch cells using an organic electrolyte as well as an aqueous electrolyte containing pH-neutral 2 M Li₂SO₄ were also constructed.

LTP in the organic electrolyte showed a first-discharge capacity of 115 mAh/g at a cycling rate of 1 C—close to the maximum theoretical capacity, which is 138.8 mAh/g, based upon the insertion of two lithium ions into the crystal structure. 84% of this initial discharge capacity was retained after 100 cycles, while 70% was retained after 160 cycles. The coulombic efficiency remained above 0.99 throughout cycling.

In the aqueous electrolyte, the team found an initial discharge capacity of 113 mAh/g. 89% of the initial capacity was retained after 100 cycles at a C/5 rate.

It was found that the coulombic efficiency during cycling in the aqueous electrolyte was greater at higher charge and discharge rates, contrary to what is generally found in lithium-ion batteries. The reason for this is that there is some steady-state self-discharge in aqueous cells in which the electrode potential is beyond the equilibrium stability range of the electrolyte. The magnitude of this phenomenon varies with the extent of this deviation.

Slower experiments, during which the electrode potential was in this range for longer times, showed greater capacity losses upon cycling. This loss can be minimized by constraint of the low end of the potential range during cycling to that at which most of the capacity is found, rather than driving it down further to attain slightly greater apparent capacities.

—Wessells et al.

This work was performed with support from the King Abdullah University of Science and Technology (KAUST) and the Global Climate and Energy Project (GCEP) at Stanford.

Source: Green Car Congress