High-Voltage Lithium Metal Phospho-Olivines for Li-Ion Batteries

Abstract

There is an ever-growing need for higher energy Li-ion batteries. Lithium metal phospho-olivines based on LiCoPO4 are promising cathode materials to help fulfill this need owing to their relatively high discharge capacity of up to 167 mAh g-1 at a discharge potential of 4.8V [1]. However, LiCoPO4 based Li-ion cells show a severe loss of discharge capacity upon multiple charge – discharge cycles owing to structure deterioration and electrolyte decomposition [2]. Partial substitution of Co by Fe significantly improves the cycle life [3] and increases Li+ ion and electrical transport [4]. Our most recent studies show that multi-element substitution for Co in LiCoPO4 further reduces capacity fade, substantially improves discharge capacity and reduces reactivity of the cathode with the electrolyte. Currently, we have achieved an energy storage capacity of 670 Wh kg-1 of cathode material (84% of LiCoPO4 theoretical) which can be compared to a maximum theoretical 576 Wh kg-1 for commercialized LiFePO4. The capacity fade of Li / LiCo1-xMxPO4 half cells over 500 cycles is less than 6%. This paper will report on our most recent results. We will discuss synthesis methods, transport measurements, electronic structure[5], the most promising substitutional chemistry and the structure of substituted LiCoPO4 as well as performance enhancements that result from improving the electrolyte high voltage stability [6]. References K. Amine, H. Yasuda, M. Yamachi, “Olivine LiCoPO4 as 4.8 V electrode material for lithium batteries,” Electrochem. and Solid State Letters, 3 (2000) 178.J. Wolfenstine, U. Lee, B. Poese, J.L. Allen, “Effect of oxygen partial pressure on the discharge capacity of LiCoPO4,” J. Power Sources 144 (2005) 226.J.L. Allen, T.R. Jow, J. Wolfenstine, “Improved cycle life of Fe-substituted LiCoPO4,” J. Power Sources 196 (2011) 8656.J.L. Allen, T. Thompson, J. Sakamoto, C.R. Becker, T.R. Jow, J. Wolfenstine, “Transport Properties of LiCoPO4 and Fe-substituted LiCoPO4,” J. Power Sources 254 (2014) 204.M.D. Johannes, K. Hoang, J.L. Allen, K. Gaskell, “Hole polaron formation and migration in olivine phosphate materials,” Phys. Rev. B 85 (2012) 115106.J.L. Allen, J.L. Allen, S.A. Delp, T.R. Jow, “Electrolyte additives for reducing the irreversible capacity loss, impedance and polarization of a doped LiCoPO4 cathode,” ECS Transactions 66 (2015) 149. Figure 1