The question of how much Lithium or Lithium Carbonate is required per kWh of battery capacity has become a matter of some importance due to the limited availability of Lithium for EV applications. Questions as to the feasibility of establishing mass production of more than a few million PHEV battery packs per year are in part met with assurances that the quantity of Lithium required per kWh is low.
A range of figures for the quantity of Lithium required per unit battery storage capacity (kWh) have been published or quoted recently. Some of these figures quote the minimum theoretical quantity of Lithium per kWh as if this is achievable in a practical device. Other figures are also unrealistically low.
To address this issue objectively, we have produced a briefing paper to illustrate for strategic planners in the automotive industry how real world battery efficiency differs from theoretical.
In a recent report to investors (“Lithium Hype or Substance” 28/10/09), Dundee Capital Markets assume a Lithium Carbonate requirement of 425 grams LCE per kWh (80 g of Lithium metal). This is equal to the theoretical minimum amount of Lithium needed per nominal kWh in a 3.3 V (LiFePO4) system. It is unrealistic to expect to approach that capability in a real battery.
In a more detailed presentation from ANL (“Lithium Ion Battery Recycling Issues”, Linda Gaines, Argonne National Laboratory, 21/5/09), estimates are presented varying between 113 g and 246 g of Lithium (600 g and 1.3 kg LCE) per kWh for various cathode types of batteries all with a graphite anode; a Lithium titanate spinel anode battery is shown as having a high requirement of 423 g Li (2.2 kg LCE) per kWh.
This range of figures illustrates the difficulty that may exist in modelling LCE requirements for strategic planners. The briefing paper describes the main factors that occur in a real battery to reduce its effective capacity and estimates a realistic figure for the quantity of raw LCE that should be assumed to be required per kWh battery capacity.
In a real battery, the main factors which reduce capacity below the theoretical maximum are:
Irreversible capacity loss: when the battery is first charged, some of the Lithium becomes bound up in the anode and cathode and electrochemically inactive. This can be as high as 50% of the Lithium originally put into the cathode before the battery is charged for the first time. In particular Lithium forms a layer known as a Solid Electrolyte Interphase on the anode which increases the internal resistance in the battery and internal energy losses.
Discharge rate: this is the major variable which reduces day to day effective capacity while the battery is in use. The Energy batteries required for PH(EV) use are more sensitive to this than power batteries and the problem is further exacerbated by using small batteries in a PHEV. Up to 50% of the effective capacity could be lost at medium to high speeds. Manufacturer capacity figures that only apply at low discharge rates are of little use in determining a realistic benchmark for PHEV battery capacity, for which capacity at the 1C rate at least should be used for a realistic indication.