Hydrothermally Synthesized Mg-Doped LiMn1-XFexPO4/Li4Ti5O12 Li-Ion Batteries for Stationary Power Applications

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 LiMn_0.85Fe_0.1Mg_0.05PO₄(Mg-LMFP) as the cathode and the performance of Mg-LMFP/LTO batteries for stationary power applications.

Mg-LMFP and LiMn_0.85Fe_0.15PO₄(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)