Rapid Synthesis and Charge–Discharge Properties of LiMnPO4 Nanocrystallite-embedded Porous Carbons

Abstract

LiMnPO 4 nanocrystallite-embedded porous carbons were successfully synthesized within a few minutes by a microwaveheating process. The nanocomposites showed higher charge discharge capacity and better rate capability than bulk-LiMnPO 4 particles synthesized in a similar manner without porous carbons.Olivine-type lithium metal phosphates have attracted much attention as a potential cathode material for secondary lithiumion batteries due to the relatively high theoretical capacity (ca. 170 mA h g ¹1 ) as well as thermal and electrochemical stabilities. However, these materials have intrinsically poor electronic conductivity and Li-ion diffusivity, and hence poor rate capability.1 Down sizing Li host particles is effective to overcome the latent problem of LiFePO 4 2,3 but not enough for LiMnPO 4 having much lower conductivity of than LiFePO 4 . 4,5 It was reported that the poor rate capability of LiMnPO 4 is ascribable to a large kinetic barrier at the mismatched interface of MnPO 4 / LiMnPO 4 during Li insertion/extraction processes and that down sizing the olivine particle to nanometer level would be effective to lower the kinetic barrier.6 Since LiMnPO 4 is expected to have higher energy density than LiFePO 4 due to its higher redox potential of 4.1 V vs. Li/Li + (3.5 V vs. Li/Li + for LiFePO 4 ), various methods to synthesize nanosized LiMn-PO 4 particles have been developed in recent years for achieving reasonable capacity and rate. However, there is still a problem of poor electronic conductivity in the electrode because strong agglomeration of nanosized particles prevents homogeneous mixing with a conductive additive such as carbon black. The present study shows the first attempt for synthesis of LiMnPO 4 nanocrystallites in the nanopores or on the pore walls of nanoporous carbons. Here the nanoporous carbon provides electron-conduction and ion-transport paths in the composite. Indeed it has been recently reported that nanoporous composites of TiO 2 /CNTs and V 2 O 5 /porous carbon exhibited excellent rate of Li insertion/extraction.79 Therefore, the present composite is expected to facilitate electrochemical reactions of LiMnPO 4 nanocrystallites. In addition, the present study succeeded in a rapid synthesis of LiMnPO 4 nanocrystallites within a few minutes through a local heating of carbon framework using a microwave irradiation, contrasting the reported synthetic processes such as a conventional solid-state, 1 solgel, 5 and polyol 10 methods requiring several hours to produce LiMnPO 4 crystals. Although colloidal crystal-derived porous carbons were employed here as a representative carbon framework, the present methodology is applicable in principle to various porous carbons such as activated carbons which can be easily obtained. Nanoporous carbons with an average pore diameter of 110 or 450 nm were obtained by a SiO 2 -opal template process as previously reported.11,12 An inorganic source solution for LiMnPO 4 synthesis was prepared by dissolving LiH 2 PO 4 and Mn(CH 3 COO) 2 ¢4H 2 O in diethylene glyco...

Olivine Positive Electrodes for Li-Ion Batteries: Status and Perspectives

“…Used as a cathode, the product delivered a capacity of 168 mAh g −1 at 0.1 C and 105 mAh g −1 at 5 C. At 1 C, the initial capacity was 135 mAh g −1 with, however, a capacity retention of 81% at the 50th cycle [126]. These results were actually an improvement with respect to prior reports on C-LMP prepared by high-energy milling [127], solid-state reaction [128], microwave heating [129] (capacity of 140 mAh g −1 at 0.05 C), or even spray pyrolysis [130] or chemical vapor deposition [131] (147 mAh g −1 at 0.05 C in both cases). Since then, better rate capabilities have been obtained with almost spherical C-LMP particles of very small size (8-12 nm) by a solvothermal method using sucrose as the carbon precursor [132].…”

Effect of different carbon sources on the electrochemical properties of rod-like LiMnPO4–C nanocomposites

“…The underlying issues come from its inferior electronic conductivity (,10 210 S cm 21 ) and poor ionic conductivity (,10 216 -10 214 cm 2 s 21 ) 2 caused by the polaronic holes localized on the Mn 3+ sites and the strain at the MnPO 4 -LiMnPO 4 interface. [3][4] These issues could be addressed by reducing particle size to the nanoscale [5][6][7][8][9][10] and coating the surface with a carbon layer, [11][12][13][14][15][16][17][18][19][20][21][22][23][24][25][26][27] both of which could be assembled as LiMnPO 4 -C nanocomposites.…”

“…LiMnPO 4 -C powders with a specific capacity of 68 mA h g 21 at 0.05 C were obtained by highenergy milling. 22 A flower-like LiMnPO 4 -C composite prepared by a solid-state reaction showed a reversible capacity of 85 mA h g 21 at a rate of 0.05 C. 23 Aono et al synthesized a LiMnPO 4 -C nanocomposite by a microwave-heating process that showed a discharged capacity of 140 mA h g 21 at a rate of 0.05 C. 24 LiMnPO 4 -C nanocomposites prepared by spray pyrolysis followed by a heat treatment exhibited an initial discharge capacity of 147 mA h g 21 at 0.05 C. 25 The chemical vapor deposition (CVD) method was developed for LiMnPO 4 -C powders with a reversible capacity of 147 mA h g 21 at a rate of 0.05 C. 26 Later, the effect of different carbon sources on the electrochemical property of a LiMnPO 4 -C composite was also studied. 27 The differences of the composites in electrochemical performance were assigned to the graphitization degree of the carbon layer.…”

Nanostructure-controlled Materials for Electrochemical Charging–Discharging

“…13,14 Enhanced chargedischarge properties were also reported on LiMn 2 O 4 , of which redox reactions are generally observed around 4 and 3 V vs. Li/Li + . 15 The latter redox is accompanied with rearrangement of atoms and large lattice volume change by the phase transition between cubic and tetragonal crystal phases; thus its capacity is very small for bulk LiMn 2 O 4 .…”

“…Similar behavior was also observed on LiMnPO 4 -embedded nanoporous carbons. 13 A more suitable post modification should be coating with host nanolayer on the inner surface of porous carbons with appropriate pore size so as to provide nanopore channels for electrolyte transport. The possibility was demonstrated by a simulation, and actually prepared V 2 O 5 nanolayer-coating of CCT-derived porous carbons maintained a high charge discharge capacity up to extremely high current densities due to the rapid Li intercalation into whole V 2 O 5 nanophases.…”