Can LiMnPO4 Compete with LiFePO4

In contrast to LiFePO₄ (LFP), its chemical and structural analogue, LiMnPO₄(LMP), has long been considered a low priority material - not viable for practical applications [1,2]. Despite continuous optimization it has showed relatively low reversible capacities and low rate capability. The interest in this material has grown considerably after demonstration of much improved performance when used in mixtures with LFP [3]. In parallel to that, improvements of the performance of pure LMP have also been occasionally reported.[4]

As indicated in our previous work [5], a precondition for increased performance of LMP is creation of small particle size (on the order of 10-20 nm or smaller) whereby agglomeration of particles needs to be prevented as much as possible. A convenient way to separate particles is their embedment into pyrolytic carbon, as known from many other studies. In addition to the small, well separated particles, the improved performance of our LMP material could partly be due to slightly changed structural features; (in our case the lattice volume was 301.5(6) ųwhich is about 0.3-0.5 % smaller than reported for other synthetic procedures)[5].

In the present contribution we analyze carefully the behaviour of LMP and compare it to the performance of LFP. First we show that the capacity of LMP can approach to its theoretical limitation, 170 mAh/g. Secondly, we show that the power performance can also be greatly increased if appropriate charge-discharge conditions are taken into account. At high rates the energy efficiency of LMP may even exceed the efficiency of LFP. We explain in some detail the role of inherent hysteresis of LMP in the light of our recent model [6]. In particular, we show that the rate is decreased due to slow redistribution of charge inside the cathode matrix which leads to relatively high polarization at usual rates. In this context we propose directions for further systematic improvement of the electrochemical behaviour of LMP. Finally, we show for the first time, many hundreds of cycles of LMP without significant loss of capacity (see Fig.). We also perform a post-mortem analysis and analyse the reasons for the failure. We point at the problem of electrolyte degradation (causing problems in the charge step).

References

1. A. Yamada, Y. Kudo, K. Y. Liu, J. Electrochem. Soc. 2001, 148, A747.

2. C. Delacourt, L. Laffont, R. Bouchet, C. Wurm, J.-B. Leriche, M. Morcrette, J.-M. Tarascon, C. Masquelier, J. Electrochem. Soc. 2005, 152, A913.

3. S. K. Martha, J. Grinblat, O. Haik, E. Zinigrad, T. Drezen, J. H. Miners, I. Exnar, A. Kay, B. Markovsky, D. Aurbach, Angew. Chem. Int. Ed. 2009, 48, 8559.

4. T. Drezen, N.-H. Kwon, P. Bowen, I. Teerlinck, M. Isono, I. Exnar, J. Power Sources 2007, 174, 949.

5. M. Pivko, M. Bele, E. Tchernychova, N. Zabukovec Logar, R. Dominko, M. Gaberscek,  Chem. Mater. 2012, 24, 1041-1047.

6. W. Dreyer, J. Jamnik, C. Guhlke, R. Huth, J. Moškon, M. Gaberšček, Nat. Mater. 2010, 9, 448.