Charge and discharge properties of spinel LiNi 0.5 Mn 1.5 O 4 positive electrodes were investigated using propylene carbonate (PC)-based concentrated electrolyte solutions in the range of 0.83 (ca. 1 mol dm-3) to 4.45 (nearly saturated) mol kg-1. The solvation state of PC in the concentrated electrolyte solution was studied by Raman spectroscopy. The HOMO energy of PC in Li + (PC) n was evaluated by first-principle calculation to assess the oxidative stability of PC molecules solvating lithium ion. Charge and discharge measurements revealed that the irreversible capacity due to solvent decomposition decreased with increasing the concentration of LiPF 6 , while discharge capacity increased. In addition, cycle performance of LiNi 0.5 Mn 1.5 O 4 positive electrodes was remarkably improved by the use of graphitized Ketjenblack as a conductive additive.
Highly concentrated electrolytes have many unique properties,1-4 such as Li+-intercalation into graphite anode without EC,1,3 suppression of S8 2- ion dissolution in Li-S batteries, etc., and is attracting much attention of many researchers. In highly concentrated electrolytes, all solvent molecules are strongly coordinated with Li+ ions, and hence the stability of the electrolytes against oxidation is improved significantly. Yoshida et al. reported that LiCoO2, which is a 4 V-class cathode, can be charged and discharged in a LiTFSI/triglyme (1:1) electrolyte with good cycleability,3 though triglyme is an ether compound and usually cannot be used as solvent. 5 V-calss cathodes, e.g. LiNi0.5Mn1.5O4 and LiCoPO4, are promising for the next-generation LIBs with high energy densities. Unfortunately no electrolyte systems that tolerate the highly oxidative 5 V-class cathode have been reported so far. In the present study, we investigated the effect of concentration on the stability of highly concentrated electrolytes, LiPF6/PC and LiBF4/PC, against a 5 V-class cathode, LiNi0.5Mn1.5O4 to realize 5 V-class LIBs with high energy densities. Figure 1 compares charge/discharge curves of a 5 V class spinel LiNi0.5Mn1.5O4 in standard (0.83 mol kg-1, Li/PC = 1/11.8) and highly concentrated (4.9 mol kg-1, Li/PC = 1/2) LiBF4/PC at 30°C. The charge/discharge rate of C/10 was employed to emphasize electrolyte decomposition. Thought the LiNi0.5Mn1.5O4 can be charged and discharged in both electrolytes, the irreversible capacity (Qirr) in 1 M LiBF4/PC was high (76 mAh g-1) because of vigorous electrolyte decomposition. In contrast, Qirr was significantly reduced in the concentrated electrolyte, which indicated that stability against oxidation was improved in the highly concentrated electrolyte. Similar tendency was also observed in highly concentrated LiPF6/PC electrolytes, though polarization on charging and discharging were much higher. A fresh half-cell was fully charged to 5.0 V, and kept at 60°C for 3 days in LiPF6/PC electrolytes. The amount of Mn deposited on lithium counter electrode was evaluated by ICP as a measure of dissolved Mn ions. The amount of dissolved Mn ions decreased with increasing Li/PC ratio, which indicated that the use of highly concentrated electrolytes is also effective for suppressing Mn ion dissolution. This work was supported by a Kyoto Area Super Cluster Program, Japan Science and Technology Agent (JST), Japan. References:[1] S.-K. Jeong et al., J. Power Sources, 175, 540 (2008).; [2] K. Dokko, et al., J. Electrochem. Soc., 160, A1304 (2013).; [3] K. Yoshida, et al., J. Am. Chem. Soc., 133,13121 (2011).; [4] Y. Yamada, et al., ACS Appl. Mater. Interfaces, 6, 10892 (2014). Figure 1
Introduction There are great needs for lithium-ion batteries with higher energy density for realizing electric vehicles and hybrid electric vehicles. The energy density of lithium-ion batteries can be increased by using positive electrode materials with high capacities and/or high working potentials. Several kinds of active materials with working potentials higher than that of LiCoO2 have been reported. Among them, spinel LiN0.5Mn1.5O4 exhibits extremely positive potentials at ca. 4.7 V vs. Li/Li+, and is a promising positive electrode material. However, almost all electrolyte solutions oxidatively decompose at such high electrode potentials, which results in a loss of reversible capacity and lowers the Coulombic efficiency. A number of studies have been so far done on the surface coating of LiN0.5Mn1.5O4 with metal oxides. Li-rich solid-solution layered oxides, such as Li2MnO3-LiMO2 (LMNC, M=Ni, Co, Mn), are also promising active materials with high working potentials (ca. 4.8-2.0 V) and high specific capacity (ca. 320 mAh g-1). However, LMNC is known to deteriorate upon charge/discharge cycles due to the layered-to-spinel phase transformation, in addition to the irreversible electrolyte decomposition. In this work, highly concentrated electrolytes [1] were used for LiNi0.5Mn1.5O4 and LMNC positive-electrodes and the effects of lithium-salt concentration on the charge-discharge performance were studied. Experimental The active material of LiNi0.5Mn1.5O4 or LMNC (80 wt%) was mixed with acetylene black (10 wt%) as a conductive additive and poly(vinylidene fluoride) (10 wt%) as a binder as a binder using 1-methy-2-pyrrolidone to form a slurry. The slurry was spread out onto Al foil, and then dried at 80oC for 18 h under vacuum. The electrode sheets were punched out into disks of 13 mm in diameter. Charge-discharge tests were performed at a constant current mode (30 mA cm-2, C/10 rate) using two-electrode coin-type cells at 30 oC. The test cells were assembled in a glove box filled with Ar. Li foil served as a counter electrode. The electrolyte solutions used were 0.833 and 4.45 mol kg-1 LiPF6 dissolved in propylene carbonate (PC). Results and discussion Fig. 1 shows the initial charge-discharge curves for Li | LiNi0.5Mn1.5O4 cell with 0.833 mol kg-1 LiPF6/PC. The charge capacity (164 mAh g-1) was higher than the theoretical capacity (148 mAh g-1), and a high irreversible capacity (46 mAh g-1) was observed. The irreversible capacity was mainly due to the oxidative decomposition of the electrolyte solution. On the other hand, the initial charge capacity was 144 mAh g-1 when highly concentrated (4.45 mol kg-1) LiPF6/PC was used as an electrolyte (Fig. 2), which is close to theoretical capacity. The discharge capacity in the 1st cycle was 130 mAh g-1, and hence the irreversible capacity was 14 mAh g-1. The discharge capacity increased by 12 mAh g-1 higher, while the irreversible capacity decreased by 32 mAh g-1 by using the concentrated (4.45 mol kg-1) LiPF6/PC system. These results suggest that oxidative decomposition of the electrolyte solution was suppressed by increasing the concentration of Li salts. Similar suppression of the electrolyte decomposition was also observed for LMNC positive-electrodes. Therefore, the use of highly concentrated electrolytes is effective for the high-potential positive-electrodes in lithium ion batteries. However, the overpotential in charge/discharge reactions, which was estimated from the difference between the potentials at 50% state of charge (SOC) and 50% depth of discharge (DOD), was as high as 0.205 V for the concentrated electrolyte of 4.45 mol kg-1 LiPF6/PC, as shown in Fig.1. This value was much higher than that for 0.833 mol kg-1 LiPF6/PC (0.030 V). The large overpotential is likely due to a poor ionic conductivity of the concentrated electrolyte solution with a high viscosity. This issue remains to be solved before practical application, and is currently under investigation. Reference 1) S. Jeong et al., Electrochem. Solid-State Lett., 6(1) A13-A15 (2003). Figure 1
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