Hydrothermally Synthesized Mg-Doped LiMn_1-XFe_xPO₄/Li₄Ti₅O₁₂ Li-Ion Batteries for Stationary Power Applications

Abstract

Li-ion batteries using Li4Ti5O12(LTO) anode have been developed for not only automotive but also stationary power applications due to its long-life, safety, high-power performance[1]. Considering cathodes practically used, LiFePO4(LFP) shows excellent life and safety properties compared to 4V-class cathodes such as LiCoO2, Li(NiCoMn)O2, and LiMn2O4, although LiFePO4 has low working potential of 3.4V. In order to enhance the energy density, long-life, and safety performance of Li-ion battery using the LTO anode, LiMnPO4(LMP) has been developed as a novel 4V-class cathode. It has been recently suggested that LMP/LTO battery system is promising for load-leveling application[2]. However, the electrochemical kinetics of LMP is slower than that of LFP due to its low electronic and ionic conductivity. It has been previously reported that Fe and Mg co-substituted LMP cathode improved the electrochemical properties[3]. This paper reports on hydrothermally synthesized Mg doped LiMn0.85Fe0.1Mg0.05PO4(Mg-LMFP) as the cathode and the performance of Mg-LMFP/LTO batteries for stationary power applications. Mg-LMFP and LiMn0.85Fe0.15PO4(LMFP) were synthesized by using a hydrothermal process. Lithium, manganese, iron, and magnesium sulfates, diammonium hydrogen phosphate, and carboxymethylcellulose(CMC) were used as starting materials. The materials with purified water in the autoclave were heated at 200°C for 3 hours. The obtained powder was pulverized by a ball-milling process. The milled powder was heated at 700°C under a flow of Ar containing 3vol % H2 for 1 hour. The hydrothermal process is a promising method for synthesizing high crystalline and fine particles. The Mg-LMFP and LMFP electrodes after charging at cut-off voltage of 4.25V vs. Li/Li+ showed the discharge capacity of 158 and 145 mAh/g at 0.1 C rate at 25°C, respectively. Figure 1 shows I-t curves obtained from potential step chronoamperometry(PSCA) of Mg-LMFP and LMFP electrodes on the anodic step from 80% state of charge(SOC) to 4.25V. Open circuit voltages of Mg-LMFP and LMFP electrodes at 80% SOC were about 4.1V. Current response of Mg-LMFP was larger than that of LMFP, although particle size of both samples was almost same at 80 nm. The fast charging kinetics of Mg-LMFP would be attributed to fast phase boundary movement due to reduction of the lattice mismatch between lithiated and delithiated phases of Mg-LMFP owing to non-active Mg2+. Laminated Mg-LMFP/LTO batteries were constructed to demonstrate the performance of discharge, cycle life, and safety. A mixture of propylene carbonate(PC) and diethyl carbonate(DEC) solvent containing LiPF6 was used as the electrolyte. The nominal capacity, the nominal voltage, and the energy density of 3Ah-class Mg-LMFP/LTO battery were 3Ah, 2.5V, and 90Wh/kg, respectively. The energy density of a large-scale Mg-LMFP/LTO battery with 25Ah was estimated to be 100Wh/kg comparable to that of conventional LiMn2O4/carbon Li-ion batteries. The Mg-LMFP/LTO battery showed high-rate discharge performance at 25°C as shown in Figure 2. The capacity retention of the battery was 96% even at 10 C rate. Figure 3 shows discharge curves of the Mg-LMFP/LTO battery at various cycle numbers under a high temperature condition of 60°C. We noted almost no increase in the overpotential and a high capacity retention of 95% at 500 cycles during the high temperature cycling. Little amount of Mn and Fe was deposited on the LTO anode after the cycle test. Mg could suppress Mn and Fe dissolution from Mg-LMFP. Large-scale Mg-LMFP/LTO batteries can be applied for stationary power applications because of the long-life, high rate, and safety performance. The results of abuse tests of Mg-LMFP/LTO batteries will be also reported. References [1]N. Takami et al, J. Power Sources, 244, 469 (2013) [2]S. K. Martha et al, J. Electrochem. Soc., 158, A790 (2011) [3]C. Hu et al, Electrochem. Commun., 12, 1784 (2010)

12 V-Class Bipolar Lithium-Ion Batteries Using Li4Ti5O12Anode for Low-Voltage System Applications

“…5 The LMFP cathode will be suitable for large-capacity battery systems rather than high-power battery systems. 8,9 Several cathodes in a number of series-connected cells may deeply discharge by an increase in the capacity difference of cells as the charge-discharge cycling proceeds. Over-discharge cycle performance of 4 V-class cathodes is shown in Figure 3.…”

“…Mg-doped LiMn 0.85 Fe 0.1 Mg 0.05 PO 4 was used as the LMFP cathode because the discharge capacity was 158 mAh g −1 compared to 145 mAh g −1 for LiMn 0.85 Fe 0.15 PO 4 . 8 LiMn 0.85 Fe 0.1 Mg 0.05 PO 4 powder was synthesized by using a hydrothermal process. Lithium, manganese, iron, and magnesium sulfates, diammonium hydrogen phosphate, and carboxymethylcellulose (CMC) were used as starting materials.…”

“…The milled powder was heated at 700 • C under a flow of Ar containing 3 vol % H 2 for 1 hour. 8,11 The hydrothermal process is a promising method for synthesizing highly crystalline particles. LMFP secondary particles with an average single particle size of about 80 nm and no impurity phase were obtained.…”

“…5 We have been developing olivine LiMn 1-x Fe x PO 4 (LMFP) materials as promising cathodes for large-scale 12 V-class batteries in view of the excellent safety and life performance because the LMFP has high thermal stability compared to that of 4 V-class cathodes such as LiCoO 2 , LiNi 1/3 Co 1/3 Mn 1/3 O 2 , and LiMn 2 O 4 . 8 Lithium-ion cells with a combination of the LMFP cathode and the LTO anode exhibited a voltage plateau of about 2.5 V for Mn 3+ /Mn 2+ redox, excellent safety, and high-energy density compared to LTO/LMO cell, indicating the possibility of promising batteries for 12 V lead-acid replacement batteries and stationary power sources. 8,9 We have focused recently on bipolar battery technologies in order to make 12 V battery systems simple and low-cost compared with the battery module.…”

“…8 Lithium-ion cells with a combination of the LMFP cathode and the LTO anode exhibited a voltage plateau of about 2.5 V for Mn 3+ /Mn 2+ redox, excellent safety, and high-energy density compared to LTO/LMO cell, indicating the possibility of promising batteries for 12 V lead-acid replacement batteries and stationary power sources. 8,9 We have focused recently on bipolar battery technologies in order to make 12 V battery systems simple and low-cost compared with the battery module. Therefore, we conducted a preliminary study of a hybrid solid electrolyte based on lithium-ion conducting ceramics with a gel polymer electrolyte in order to overcome the issues of all-solid electrolyte batteries and we fabricated 12 V-class bipolar LTO/LMFP batteries.…”