Not all lithium phosphate (olivine) cathodes materials are (thermally) the same.....

LiFePO4 (olivine) is now well established in the industry as a safe cathode material for large format Li-ion batteries. Due to its excellent rate properties it is also useful for some power applications. The inherent safety behavior of the de-lithiated (charged) Li1-xFePO4 phase results from the thermal stability of the Fe-O-P bond configuration.

The one major drawback with the use of LiFePO4 in Li-ion systems is the modest energy density and specific energy performance. These characteristics are driven primarily by the relatively low operating voltage of the lithium insertion/extraction reactions in LiFePO4 - typically around 3.4 V vs Li.

The Mn olivine analogue, LiMnPO4, presents some important advantages over LiFePO4. Most notably, the operating voltage of around 4.2 V vs Li, suggests that the energy density of Li-ion cells based on this material should be significantly improved. Until recently the poor electronic conductivity of the LiMnPO4 phase has meant that the material utilization has been disappointing, even at relatively low discharge rates. Recent optimization work, particuarly at the Swiss-based company, High Power Lithium (HPL), has led to some fairly impressive specific capacity figures (around 150 mAh/g) at moderate discharge drain rates. This type of material performance has promised improved Li-ion energy density. So far so good........

But here's the rub.....recent studies have suggested that the thermal stability of the de-lithiated Li1-xMnPO4 is actually quite poor. To compound the problem, the thermal decomposition reaction actually results in the release of oxygen:

2 MnPO4 --> Mn2P2O7 + 0.5 O2

This is exactly what we do not want!! In addition, the reaction may occur at temperatures as low as 210oC. This is a quite a surprise, since most workers in the field (including me!!) expected the Mn phase to behave (thermally) similarly to the LiFePO4.

This work has been reported by S-W. Kim and co-workers in a recent paper published in the Journal of the Electrochemical Society (156, A635, (2009)). Similar results have also been reported by T. Richardson from the Lawrence Berkeley National Labs in the US.

In summary, these safety data may have a profound negative impact on the safety performance of commercial Li-ion cells and batteries made using LiMnPO4.

So what next? If we really want next generation of Li-ion cells to offer the holy grail of both high energy density and excellent safety performance perhaps we should consider the lithium vanadium fluorophosphate, LiVPO4F (for details, see below). This material offers both.